HEAT ENGINES iBEING A NEW EDITION OF STEAM) RIPPER Longmans' Elementary Science Manuals. BOUGHT WITH THE INCOME FROM THE SAGE ENDOWMENT FUND THE GIFT OF Denrg W. Sage 1891 Engineering '357 The date shows whdR irHTWIiHYNirWas taken. To renew this book copy the call No. and give to the librarian . HOME USE RULES^ All Bookt subject to Recair. Books not used for jnstruction or research are returnable within 4 weeks. Volume of periodi- cals and of pamphlets are held in the library as much as possible. For special purposes they are given out for a limited time. Borrowers should not use their library privileges for the bene- fit of other persons. Books not needed during recess periods should be returned to the library, or arrange- ments made for their , return during borrow- er's absence, if wanted. Books needed by more than one person are held on the reserve list. / Books of special value and gift books, when the giver wishes it, are not allowed to circulate. Readers are asked to report all cases of books marked or muti- lated. Do not deface bovici by marks and writiBg. Cornell University Library TJ 275.R59 1909 Heat engines. (Being a new edition of "St 3 1924 003 958 893 The original of tliis book is in tine Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31924003958893 ELEMENTARY SCIENCE MANUALS Written specially to meet the requirements of Stage I. of Science Subjects as laid down in the Syllabus of the Board of Education. Practical Plane and Solid Gp-OMETiiT. By I. H. MoRRrs and Josepu HUSBAKD, A.R.C.S. With 322 Worked Problems with Figures. 2s. 6d. Geometrical Drawino for Art Students. By I. H. Morris. 2s. Text-Book on Practical, Solid, OR Descriptive Geosietrt. By David Allan Low. Part I., 2s. Part II., 3s. An Introduction to Machine Drawing and Desigii. By Davio Allan Low. With 153 Illustrations and Dia- grams. 25. 6(Z. 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WITH DIAGRAMS LONGMANS, GREEN, AND CO. 39 PATERNOSTER ROW, LONDON NEW YORK, BOMBAY, AND CALCUTTA igog All rights reserved I. RIPPER'S ELEMENTARY STEAM BIBLIOGRAPHICAL NOTE First printed October, iSSg Reprinted January, 1892, May, 1893, and Septemier, 1895 Ne^v Edition, January, 1897 Reprinted September, 1898, and December, 1899 A''^zo Edition, February, 1902 Reprinted January, 1903, and May, 1905 jV^f Edition, July, 1907 iV^» Edition tinder the title of" Heat Engines," October, 1909 PREFACE TO THE EDITION OF 1902 This book has been written as an introductory text-book for the use of engineering students, and more especially for those who already have some practical acquaintance with the manu- facture or use of steam machinery. It deals in an elementary manner with the various subjects included in the practice of steam engineering, but particular attention has been given throughout to the explanation and enforcement of those principles upon which steam-engine and boiler efficiency and economy depend. Several additions have been made to the book, including chapters on Corliss valve gear, water-tube boilers, the manage- ment of boiler furnaces, and the science of fuel combustion — a subject which will demand much more serious attention in the future than it has received in the past. The series of questions at the end of the book has also been revised. W. RIPPER. Sheffield, January, 1902. PREFACE TO THE EDITION OF 1909 Chapters have now been added on the Steam Turbine and on Internal Combustion Engines, and the title of the work has been altered from that of " Steam " to " Heat Engines." W. RIPPER. August, 1909. CONTENTS CHAPTER I TACK Introduction — Heat, its nature and effects — Temperature — Ther- mometers — Specific heat — Absolute temperatures i CHAPTER n Unit of heat and unit of work — Hcrse-power — Mechanical equivalent of heat . . .... 7 CHAPTER HI Transfer of heat — Radiation — Conduction — Convection CHAPTER IV Application of heat to solids — Application of heat to gases — Pressure of the air — Absolute pressure — Application of heat to vcater — Boiling — Condensation of steam — Vacuum — The Pulsometer — Newcomen's atmospheric engine . . i6 CHAPTER V Action of heat in the formation of steam — Work done by steam during formation at low- and high-pressure respectively — Efficiency of the steam — Heat rejected by steam to condenser — Sensible heat — Latent heat — Total heat of evaporation ... 28 viii Contents CHAPTER VI PAGE Saturated steam — Table of properties — Water heated in a closed vessel — Temperature of mixtures — Condensing water .... 38 CHAPTER Vn Relation between the pressure and volume of gases — The hyperbolic curve 42 CHAPTER VIII Expansive working — Work done by steam used expansively — Back pressure — Mean pressure — Indicated horse-power — Examples illustrating economy of expansive working — Limit of useful expansion — Clearance in the cylinder — Priming — Cylinder condensation ... . . ..... 48 CHAPTER IX The steam engine — Non-condensing engines — Engine details. The cylinder — Cylinder liner — Steam jacket — -Escape valve — Relief cocks. Pistons — Piston speed — Piston displacement — Piston rods — Crossheads and guide blocks — The connecting rod — Relative positions of piston and crank pin — Rotary engines . . 69 CHAPTER X The slide valve — Lap, lead, angular advance — Piston valves — The double-ported slide valve — To set a slide valve — Eccentrics — Reversing gear — The link motion CHAPTER XI Corliss valve gear jqi CHAPTER XII Cranks and crank shafts — Tangential pressure on crank pin — Shaft couplings — Journals — Bearings ^oi Contents ix CHAPTER XIII PAGE Condensers — The jet condenser — The air-pump — The surface con- denser — The vacuum gauge — Pumps IIS CHAPTER XIV Governors — The Watt governor — The Porter governor with auto- matic expansion gear — Fly wheels — The locomotive engine, arrangement and construction of 125 CHAPTER XV The Indicator . . 135 CHAPTER XVI Compound engines — compared with single-cylinder engine — The two-cylinder compound engine illustrated 141 CHAPTER XVII Types of compound engines — The Woolf engine — Distribution of steam — The range of temperature — The distribution of stresses — The Receiver engine — Distribution of the steam — Triple and Quadruple expansion engines. Economy due to compounding . 152 CHAPTER XVIII Boilers — Resistance of cylindrical vessels — Descriptions of boilers — The Cornish boiler — The Lancashire boiler — The vertical boiler — Marine boilers — The economizer — The locomotive boiler — Heating surface of tubes — Water-tube boilers — Safety valves — To graduate the lever 168 CHAPTER XIX The Furnace . . , .... 203 CHAPTER XX Steam generation 212 ^ Contents CHAPTER XXI PAGE Combustion of fuel . . . . , 215 CHAPTER XXn Practical notes on the care and management of engines and boilers — Annual inspection of engines and boilers > . , , , . 228 CHAPTER XXni The Steam Turbine . . . 233 CHAPTER XXIV Internal combustion engines . . . 249 APPENDIX :— Questions and Exercises . . . . 269 INDEX 309 STEAM CHAPTER I INTRODUCTION The object of the study of steam and its applications is to learn what are the conditions which tend to the highest efficiency, so as to obtain from a steam-power plant the greatest possible amount of useful work for the least possible expenditure of fuel. In order to understand the principles which underlie the economical production and use of steam, we shall consider the following subjects, and in the order given, viz. : 1. Heat. 2. Steam. 3. Engines. 4. Boilers. Heat, its Nature and Effects If I lb. of cold water be heated in a closed vessel till the water becomes warm, although the temperature of the water has changed, its weight remains the same ; and if the heat be continued until all the water is converted into steam, pro- vided none of the steam can escape, the total weight of the steam is still exactly the same as that of the water from which it was produced. It is evident, therefore, that the heat which produced these changes is without weight. Heat cannot, therefore, be a material substance. It was formerly thought to be some kind of subtle fluid, which flowed from hot bodies into colder ones ; B 2 Steam but this theory is now no longer accepted, because it was found that heat could be developed to an unlimited extent from cold bodies merely by rubbing them together. A piece of cold iron can be made red hot by hammering it. A carpenter's saw, an engineer's chisel, or turning tool, soon get hot when a rubbing action, or friction, is set up between the tool and the work, although they are all quite cold to begin with. Sir Humphry Davy melted' two blocks of ice by rubbing them one upon another, from which he concluded that 'the immediate cause of the phenomenon of heat is motion ' ; and this is now the generally accepted view of the nature of heat. Still we know that things may be hot without being visibly in motion ; hence, if heat is motion, the motion must exist in parts of the body too minute to be seen. All bodies are assumed to be composed of minute particles called molecules, held together by mutual attraction or cohesion, and these molecules are in a state of continual agitation or vibration. The hotter the body the more vigorous the vibra- tions of its constituent particles. In solid bodies the vibrations are limited in extent. If this limit is exceeded, owing to addi- tion of heat, cohesion is sufficiently overcome to enable the particles to move about freely and without restriction, and the solid has now become a liquid. On still continuing the heat, further separation of the molecules takes place, cohesion is completely overcome, and they fly off in all directions. The liquid has now become a gas. The pressure exerted by the gas on the interior surface of the vessel in which it is confined is due to the collision of the molecules with the sides of the vessel. The greater the intensity of the heat the more violent the impact, and there- fore the greater the pressure exerted. This is the condition of things in the interior of a steam boiler. If a part of the enclosing vessel were movable, it would evidently be pushed backward and outward. This is what happens to the piston of the steam engine. From what has been just stated, we see that heat is a form of energy, and that heat and mechanical work are mutually Temperature 3 convertible the one into the other. We shall presently show that an exact and invariable relation exists between heat and work. Temperature The temperature of a body indicates how hot or how @ cold the body is, or the intensity of the heat of the body. The temperature of a body should be distinguished from the quantity of heat in the body. For example, if a cup of water be dipped out of a pailful of water, the temperature of the water is the same throughout, but the quantity of heat varies as the weight of water in each vessel. Thermometers are used to indicate temperature, and they do so by the rise or fall of a little column of mer- cury enclosed in a tube of very fine bore, and having a small bulb at the bottom containing a store of mercury. If the thermometer be warmed, the mercury ex- pands or tends to occupy a larger volume, and the column therefore rises in the stem of the tube ; or, if the thermometer be cooled, the mercury will con- tract or diminish in volume, and the column will shorten or fall. A graduated numbered scale is affixed, and the smallest change of temperature, shown by the movement of the surface of the column, is thus easily detected. To graduate the scale the thermometer is placed in melting ice, and the point to which the mercury in the stem has fallen is marked, and called the freezing point. It is then placed in boiling water at the pressure of the atmosphere, and the level of the column of mercury is again marked, and called the boiling point. The distance between these two marks is divided on the Fahrenheit thermometer into 180 equal parts or degrees ; on the Centigrade thermometer the distance between the two marks is divided into 100 equal parts or degrees ; and on the Reaumur scale the same distance is divided into 80 equal parts or degrees. English engineers mostly use the Steam Fahrenheit scale. The following sketch will show that the difference between the various thermometers is not in the height of the mercury, but in the scales of degrees by which the height is expressed. It will be seen from the fig. that the scales are marked as follows: j,^g^i„g B„;,i^g point. point. 32 212 o 100 80 I atmosphere 212° , 2 atmospheres 249" , 3 )T 273° , 4 )) 291" 5 53 306° , 10 J, 357° , The presence of solid bodies such as salt dissolved in the water raises the temperature of the boiling point. Thus the boiling point of sea water under atmospheric pressure is 2I3'2° F. Condensation of Steam — A'acuum Steam is water in the gaseous condition, and when the steam is cooled, it again re- to the liquid and becomes Fig. 15. turns state water. Thus, let a flask A contain a known weight of pure water. Fit a cork and glass tube to it as shown, and connect with a spiral tube surrounded by flowing cold water ; let the lower end of the tube pass into a vessel B. Boil the water in A. It will pass off" as steam by the tube C to the spiral; and if the spiral be sur- rounded by a stream of cold water, the steam will be condensed to water, which will drop from the end of the tube. At the end of the operation the loss of weight by A is equal Condensation 23 ~U^ i^ A\ I to the gain by B. This illustrates the process of distillation, and by this method pure water may be obtained from water containing impurities. Advantage was taken by the early engineers of the property possessed by steam of being easily condensed. They valued steam not so much for its own sake, but because by condensa- tion they were able to call to their aid the pressure of the atmosphere in the performance of work. A vacuum is literally an empty space — that is, a space abso- lutely free from air or vapour of any kind capable of exerting pressure. Vapour arises from water at all temperatures, and exerts an appreciable pressure. And the lowness to which the pressure can be reduced in condensers depends on the temperature of the condensed steam, and this temperature in practice cannot economically be reduced below about 102° F., at which tempe- rature the vapour of water exerts a pressure of i lb. per square inch. But, further, the condensed steam, vapour, and air in engine condensers are removed by a pump called an air-pump, as in fig. 16. Now, when the plunger or pump bucket P is lifted, the valve V will lift by virtue of the diiiference in pressure on the two sides of the valve. Assuming that we could obtain a perfect vacuum in the pump chamber, yet the pressure per square inch in the condenser C can never fall below that necessary to lift the valve V. Experiment— Takt a thin tin cylinder closed at both ends, having a tap, /, at one end. Pour a little water into the cylinder by the tap. The vessel now contains air and water. Boil the water till the steam escapes from / and has driveh most of the air out. Now the vessel contains steam and very little air. Fig. 16. 24 Steam Fig. 17. Close the tap and pour cold water on the vessel. The steam is immediately condensed to water ; and since water occupies only about ytso o^ 'he space of the steam at atmo- spheric pressure, a partially empty space has been formed inside the vessel, and the external pressure of the atmosphere will collapse or crush the vessel. If the cylinder had been made strong enough to resist the excess of external pressure over in- ternal pressure, and a tube had been led from the cylinder into water some depth below it, then the water would be forced up the tube into the cylinder by the pressure of the atmosphere, till the pressure on the inside of the cylinder is the same as the atmospheric pres- sure outside. Here, then, useful work would be done in lifting water from a low level to a higher level, and this was the principle of the early pumping engines as made by Savery. Again, if the top of the cylinder bad been movable, then it would act like a piston, and be forced towards the bottom. This was the principle of Newcomen's engine, which was called the ' atmospheric ' engine, because the work was really done by the atmosphere on the piston after a vacuum had been formed in the cylinder by the con- densed steam. Sa very 's- engine, having drawn its water from a low level into a chamber, as previously explained, delivered it to a still higher level above the chamber b}- introducing steam to the same chamber, and forcing the water up the delivery pipe to a higher level by the pressure of the steam upon the surface of the water. Fig. 18. Condensation 25 This principle has been again revived in the Pulsometer and similar pumps. The Pulsometer is illustrated in fig. 19, and consists of a single casting called the body, composed of two chambers A, A joined side by side with tapering necks, the two passages terniinating in a common steam chamber, wherein the ball valve I is fitted so as to oscillate be- tween the seats at the opening to each chamber. Between the chambers A, A and the suction pipe C are the suction valves E, E as shown. A discharge chamber common to both working chambers, and lead- ing to the discharge pipe G, is also provided, and this contains de- livery valves F, F. The air- chamber B communicates with the suction. The pump being first filled with water, steam is admitted by the steam pipe K, passes down that side of the steam neck which is left open to it by the position of the steam ball, and presses upon the surface of the water in the chamber, depressing it without agitation of the water, and there- fore without much condensation of steam, and forcing the water up the delivery pipe. The moment that the level of the water is as low as the horizontal orifice which leads to the discharge pipe, the steam flows through with a certain amount of violence, causing agitation and instantaneous condensation of the steam in the chamber, a vacuum is formed, and the steam ball falls over, closing the mouth of the chamber. This prevents further admission of steam, and allows the vacuum to be completed; 26 Steam meantime water rises through the suction valve, and rapidly fills the empty chamber. The same operations are repeated in the other chamber, and proceed alternately in the two chambers, one delivering while the other is being filled. Newcomen's " Atmospheric " Engine This engine was devised by Newcomen, a blacksmith of Dartmouth, in 1705, and though of the crudest design and construction, this type of engine served a useful purpose for many years as a pumping engine for mines, imtil it was dis- placed by the greatly improved engine introduced by James Watt. The Newcomen engine is illustrated in fig. 20. Steam was admitted in the first place from the boiler B to the cylinder D. The steam below the piston was only at about atmospheric pressure, and the piston, being in equilibrium (having equal pressure above and below it), was raised from the bottom to the top of the cylinder by the greater weight of the pump- rod H suspended from the opposite end of the main beam G, and acting as a counterpoise. When the piston reached the top of the cylinder the steam was shut off, and a j«t of cold water was sprayed into the cylinder, condensing the steam, and thereby forming a partial vacuum under the piston. The atmospheric pressure then forced the piston downwards, and through the medium of the beam the pump-rod was raised. On the next steam admission the water in the cylinder was expelled, and the operations above described repeated. It was while experimenting with a model of this engine that James Watt, in 1763, discovered how large a waste of steam was going on in the cylinder, owing to condensation of the steam through contact with the cold wet walls of the cylinder. It was this waste by condensation that led W'att to the invention of the separate condenser, the steam being passed from the cylinder into a second chamber called the condenser, Condensation 27 where it might be condensed by contact with cold water without the need of cooling the cylinder itself (see fig. loi). Fig. 20 A = the boiler furnace B = the steam boiler C = the steam valve D = the engine cylinder E = the piston F = the piston-rod G = the main beam H = the heavy pump-rod J = the mine pump K = pump for condensing water L = pipe leading to condensing water- tank M = condensing water-pipe N = injection cock to cylinder = water-tap to top of piston P ■=■ relief or snifting valve Q = eduction-pipe with non-return valve at end R = feed-water tank This invention by Watt of the separate condenser, though apparently so simple, was the secret of the great success which followed the steam engine from this time forward. 28 Steam CHAPTER V ACTION OF HEAT IN THE FORMATION OF. STEAM ' The action of heat in the formation of steam from water may be illustrated by the following diagrams. (i) Let the cylinder (stage i, fig. 21) contain i lb. of water at 32° F., and let the pressure of the atmosphere be Stage I. Stage Stage 3. Stage 4. Fig. represented by a weighted piston. Then, if heat be applied to the water, the temperature will rise higher and higher, though the piston will remain stationary, except for the small e.xpansion of the water, until the temperature of the water reaches 212°- (2) On continuing the heat the water shows no further in crease of temperature by the thermometer, but steam begins to form and the piston commences to ascend in the cylinder (stage 2), Work done by Steam 29 rising liigher and higher as more and more steam is formed, until the whole of the water is converted into steam. In stage i the steam did not begin to form until the temperature reached 212°. Evidently, therefore, this is the lowest temperature at which steam can exist under atmospheric pressure. Again, in stage 3, as soon as the last drop of water dis- appears, we have i lb. of steam occupying the least possible volume at the given pressure ; the steam in this condition is termed saturated steam. (3) If the heat is continued the steam will become super- heated — that is, its temperature will rise above that of saturated steam, and the piston will continue to rise. (4) If the steam be surrounded by a vessel containing an in- definite supply of cold water (stage 4), then the heat will be extracted from the steam by the surrounding water, and the steam will be condensed to water, the same in every particular as to weight and properties as the water with which we started ; and if the temperature of the water is now the same as its temperature before starting, then the whole heat -taken away when the steam is condensed is equal to the whole heat added during the operation. The series of changes have, therefore, been brought about by the addition or subtraction of heat only. We have so far been content with a general statement of the action of heat in the formation of steam ; we will now con- sider what quantities of heat are required to perform the several stages of the operation. Work done by Steam during Formation Referring to fig. 22, let i lb. of water at 32° F. be contained at the bottom of a cylinder i sq. ft., or 144 sq. ins., in sectional area. Then, first to find the height of the water in the cylinder ; since the area of the vessel is i sq. ft., and the weight of I cubic foot of water is 62-5 lbs., 62-5 lbs. of water will stand i ft. high, lib. „ „ J-ft. 62-5 = 016 ft 30 Steam HI Mil 31 ^ « S3 Let the pressure of the atmosphere be represented by a piston resting on the surface of the water loaded with a weight of 147 lbs. per sq. in. The area of the piston being i sq. ft., the total weight on the piston is therefore 147 x i44=2ii6'8 lbs. (1) On applying heat to the water, it will at first gradually rise in temperature from 32° to 212° before evaporation com- mences, as explained on page 18. Then, 212 — 32 = 180 = the number of heat units required to raise water from 32° to boiling tem- perature at atmospheric pressure, and this represents the heat units expended in stage i, fig. 21. (2) Steam now begins to form and the piston to rise ; and, on continuing the heat, the water is eventually all converted into steam at 212", and the piston continues to rise till the steam occupies a volume, under the pressure of the atmosphere of 26-36 cub. ft. as found by experiment (see Reg- nault's Tables, p. 39). The heat expended in evapo- rating the I lb. of water at 212" into I lb. of steam at 2 12° is found to be 966 units. Hence the total heat required, first to raise water from 32° to 212", and then to convert it into steam at the same temperature under atmospheric pressure, =i8o-f966 = 1,146 units. Now, in stage i, fig. 21, it is quite evident how the heat has been expended, namely, in raising the temperature of the water ; but in converting the water into steam, though 966 units of heat have been expended, there is no increase in temperature, and it is not at first quite clear what has become of this heat ; hence it was called latent or hidden heat. What has become of this heat, however, will be understood from the following explanation : Fig. Work done by Steam 31 It will be noticed that in this operation two things have happened : firstly, the water has all been converted into steam, which occupies a greatly increased volume (1,644 times, at at- mospheric pressure) as compared with the water from which it was generated ; and, secondly, the piston has been raised from the surface of the water in stage i to that of the steam in stage 3. The heat, usually called latent heat, has been expended, then, in two ways : firstly in overcoming the internal molecular resistances of the water in changing its condition from water to steam ; and, secondly, in overcoming the external resistance of the piston to its increasing volume during formation. The first of these effects of ' latent ' heat is called internal work, because the changes have been wrought within the body itself ; and the second is called external -vior^i, because the work has been done on bodies external to itself ; and these two kinds of work must be carefully distinguished. The first represents energy contained in the steam ; the second represents energy which has passed out of it, having been expended in doing work on the piston. We will now consider what share of the heat has been expended on each operation respectively. The heat expended in doing the external work of raising the piston under a pressure of 2,116-8 lbs. through a height of 26-36 ft. = 2,1 i6-8x 26-36 = 55,799 foot lbs. ; or, 5S,799-^772 = 72-3 units of heat. Now, the total heat applied to the water, as we have seen, is 1,146 units; and we have so far accounted for 180 +72-3 = 252-3 units, leaving a difference of 1,146 — 252-3=893-7 units, and this difference represents the heat absorbed in doing the internal work of converting the water into steam. The distribution of the heat may be summarised as follows : units, (x) In raising temp, of water from 32° to 212° =180 (2) In overcoming internal resistance = 893-7 (3) In raising piston against external resistance = 72-3 Total heat = 1,146-0 Now, the external work done per lb. of steam during its formation may be represented by an area. For the pressure P 32 Steam per square foot multiplied by the area of the piston in square feet gives the load on the piston, and this multiplied by the height / through which the piston moves in feet gives the work done ; or External work = P x « X /. But a X / = v = the volume occu- pied by the i lb. of steam ; there- fore External work = P x w. en ^* ex n ySY^jq ft. Us. V-=:r 26- % Cui.fi- Fig. 23. If, then, a rectangle be constructed, as in fig. 23, having one side=P, and an adjacent side=z', to any convenient scale, the area of the rectangle will equal the work done. ^^2fOl2:iil ^ Similarly, the proportion which -^ .c J- I Jntcvual Work m Converting the heat converted into external or useful work bears to the whole heat expended may be shown by the aid of rectangular areas. From the above summary of re- sults we see that the ratio of the ther- mal units expended as described is as 180 : 8937 : 72-3 ; or, dividing each of the numbers by 72 "3, we have 2'48 : i2'36 : i. Draw the rectangle A B^a; (fig. 24), making A B = pressure and B ^ = volume to any scale to represent the external work done by the steam. To the base 'Bb add the rectangle BCcb = 1 2 '36 times the rectangle A 1^. This is done by making B C = t2'36 times A B. Make also C D = 2 -48 times A B and complete the rectangle. Then the total heat required to heat I lb. of water from 32° to 212", and to convert it into steam at the same temperature, is given by the rectangle AT) da, and the share of this which goes to perform useful work is represented wciXci from, .32' to 212' Fig. 24. Work done by Steam 33 by the remarkably small area given by the rectangle A'Bba. But the ratio which the useful work done bears to the total heat expended is called the efficiency of the steam. Hence, in this „„^„ tu„ „£c • area A B /J a i u ^ i case, the efficiency= = or about — . areaADrfiS! i5"84 16 In other words, in such an engine as this, taking steam at full pressure throughout the whole stroke, only yV of the heat is usefully employed, while the remainder escapes into the air or condenser in the exhaust steam, except the small part which is wasted by radiation and conduction. Hence, for every 16 lbs. of coal consumed, the heat from I lb. only is converted into work, or ^ig^x 100 = 6-25 per cent. And this is better than would be the case in practice under the same circumstances, because we have neglected the many sources of loss which will be described hereafter. We may now consider the effect of using steam at a higher pressure than that of the atmosphere. Take, for example, steam at 100 lbs. per square inch absolute. The external work done by i lb. of steam at 100 lbs. pressure per square inch absolute, having given that i lb. of steam at 100 lbs. pressure occupies 4-33 cubic feet, is found as follows : P =: 100 X 144 = 14,400 lbs. and V = 4'33 cub. ft. Then total external work of steam during formation = P x » = 14,400 X 4-33 = 62,352 ft. lbs. Comparing this with the external work done by i lb. of steam at atmospheric pressure, we have external work in ft. lbs. I lb. Steam at 100 lbs. pressure = 62,352 1 lb. „ 147 » = 55,799 and these numbers do not differ very greatly. From this we see that, when steam is noi used expansively — that is, when it is supplied at full pressure throughout the stroke — i lb. of high-pressure steam is not capable of doing much more useful work than the same weight of low-pressure steam. D 34 Steam In comparing the work done by high- and low-pressure steam, it will be noticed we have taken the work done by equal weights and no expansion. The same would not be true of equal volumes, for evidently if the cylinder were sup- plied with high-pressure steam, it would do more work on the piston than the same volume of steam at a lower pressure ; but then there would be a proportionally greater weight of steam used, and, therefore, a greater quantity of fuel consumed ; and the object of the engineer is to get the greatest amount of work from the least consumption of fuel. Thus, if a cylinder is filled at each stroke with steam at loo lbs. pressure per square inch throughout, then, assuming there is no back pressure, this steam would do twice as much work as steam at 50 lbs. ,• but the weight of each cylinder full at 100 lbs. pressure is approximately twice that of the cylinder full at 50 lbs. Hence, though we have done twice the work, we have used twice the weight of steam, and, therefore, weight for weight, the work done in both cases is equal. To find the work done per lb. of steam during formation, without expansion, at any given absolute pressure per square inch p : — Find by Table III. (p. 39) the given pressure, the volume V per lb. in cubic feet ; then/ x 144 x » = work done. Example. — Find the external work done per i lb. of steam at 60 lbs. pressure absolute ; then by Table III., vol. per lb. of steam at 60 lbs. pressure = 7-01 cub. ft., and 60 x 144 x 7-01 =60,566-4 foot lbs. per lb. To find the weight of steam required per horse-power per hour : Divide work done per horse-power per hour by work done per lb. of steam. The work done per horse-power per hour = 33,000 x 60 = 1,980,000 ft. lbs. The work done per lb. of steam at 100 lbs. pressure absolute without expansion = 62,352 ft. lbs. Therefore, the number of pounds of steam required per horse-power per hour under the above conditions 1,980,000 62,352 317 lbs. Heat rejected to Condenser 35 Heat rejected by Steam to Condenser When steam is condensed, the heat rejected by it to the condensing water is not always the same, but depends upon the conditions under which it is condensed. If it is con- densed under the same constant pressure at which it was formed, the heat given out will be the same as the total heat supplied ; in other words, the heat rejected is the same as its total heat of formation ; but if it be condensed under any other conditions, the heat rejected by the steam to the con- densing water will be different. This statement may be illus- trated by taking three cases : 1st case. — Referring again to fig. 21, stage 4, suppose that when the last particle of water is evaporated, we now com- mence to cool down the cylinder till the steam is condensed, and converted finally to water at 32", the piston having fallen to its first position. Now, it will be evident that just as the formation of steam took place under the constant pressure of the weighted piston, so condensation has here been carried on under the same constant pressure, and the whole of the process of formation has been exactly reversed. Hence, heat rejected by water in falling from 212" to 32"= 180 units ; heat rejected by steam = heat absorbed in internal work := 8937 units ; and, lastly, heat expended in raising piston which has been re- stored to steam by piston compressing it back to original volume as water = 7 2 -3 units ; and, therefore, Heat rejected = 180 -|- 8937 -l-72'3 = 1,146 units. = total heat supplied. 2nd case. — Suppose, in fig. 25, that, when the cooling commenced, the piston had been secured so that it could not fall as the volume of the steam de- creased. Then evidently the heat rejected would be less than in the previous case by the amount of work done on the steam by the falling piston under atmospheric pressure ; or, Fig. 25. 36 Steam Heat rejected= total heat— external work = 1,146-72-3 = 1,0737 units for this particular case. This corresponds to the amount of heat rejected when the steam is exhausted to a condenser without back pressure. 3r(/iraj-e.— Suppose now that the steam is exhausted into a condenser against a back pressure of say one-third of the pres- sure of the atmosphere. Then the effect is the same as though, when the piston had arrived at the extreme height due to the volume of i lb. of steam at 2 1 2 ° under the pressure of the atmo- sphere, the piston is secured, the weight representing the atmo- spheric pressure slipped off, and a weight one-third this size placed on the piston (fig. 26). Then, when the steam has been cooled till it only exerts a pressure of 5 lbs. (ft ^ per square inch, the piston will begin to ■ ^ '" ^ fall, and, on continuing the cooling ope- y^ ration, the steam is condensed to water, ^ and the water falls to 32"- Here the stages ^ during the formation of steam have been -^ reversed, except that the work done on the steam by the falling piston will be only ^ of that done on the piston by the steam; hence Heat rejected=heat of water from 212° to 32''=i8o + internal latent heat =8937 Fig. 26. -(- -|external work=-i- of 72'3 = 24*1 1,097-8 A\'e shall now be able more fully to appreciate the meaning of the following definitions : Sensible heat is the heat added to the water which changes its temperature, and the term is used to denote the heat re- quired to raise the temperature of i lb. of water from 32° to the ■given temperature. Thus for water at boiling temperature under atmospheric pressure the sensible heat==2i2 — 32 = 180. If the temperature of the water to begin with is, say, 50° F. Heat rejected to Condenser 37 instead of 32°, then the number of thermal units required to raise water at 50° to water at 2i2"=2i2 — 50=162. Tfie. latent heat of steam is defined as the amount of heat required to convert i lb. of water at a given temperature into steam at the same temperature. The total heat of evaporation is the sum of the latent and sensible heats and is defined as the quantity of heat required to raise i lb. of water from 32° to the temperature of evaporation, and to convert it into steam at that temperature. The total heat of evaporation for steam at any particular temperature, t, may be obtained approximately from the follow- ing formula : Total heat=i,o82 + '3 t. The latent heat may be obtained by subtracting / — 32 from the total heat found as above ; or from the following formula : Latent heat=i,ii4--7 /. Example. — Find the latent heat of steam at 120 lbs. pressure absohite, given that the temperature of steam at this pressure is 341° F. Then , latent heat = l, 114-7^ = 1,114-7x341 = 87S-3- The internal latent heat is that portion of the latent heat which is contained in the i lb. of steam after formation ; thus — Internal latent heat = (latent heat) - (heat absorbed in doing external work during formation). The internal or intrinsic energy of the steam includes the in- ternal latent heat and the sensible heat reckoned from 32°; or — Intrinsic energy = (total heat) — (heat absorbed in doing external work during formation). From the formulae given above for the total and latent heats of steam, it will be evident that the total heat increases as the temperature of the steam increases, while the latent heat decreases as the temperature increases. 38 Steam CHAPTER VI SATURATED STEAM— TABLE OF PROPERTIES Steam in contact with the water from which it is generated is said to be saturated. It is then at its maximum density and pressure for the given temperature. From the following table, p. 39, it will be seen that saturated steam under a given pressure has a fixed temperature, also that the temperature and density increase with the pressure. But it will be further noticed that the total heat increases in a very slow ratio compared with the pressure and temperature, there being only a very small increase of total heat per lb. of steam as the pressure increases. This is an important point in practice when considered in reference to coal consumption, for it shows that it is not much more costly in fuel to generate high-pressure steam than low-pressure steam, weight for weight ; but we shall see further on that far more work can be obtained from high-pressure steam when used expansively than from the same weight of low-pressure steam, and hence the economy of high-pressure steam. Example.--K cylinder contains 15 cub. ft. of steam at 40 lbs. absolute pressure : find the weight of this volume of the steam. By Table III. steam at 40 lbs. absolute pressure occupies 10'28 cub. ft. per lb. Then, 10-28 cub. ft. of steam at 40 lbs. pressure weigh i lb. I >) >> ,, „ lbs. 10-28 '5 ^> » „ 15 X i lbs. 10-2S = I -46 lbs. Properties of Saturated Steam 3$ to 13 to f? OJ.S « u^ OMO ro tJ- t^ u^CO \0 VD d f*^ ON t^-sO m Th r^OO p t^ y^ fo i-« pNcp \o fo •-' OS t^vo ■^ rri N 00 lO r^j •-• Total heat of evapora- tion from water at 32° F. t^t^oo oooocoooooQooo 1-1 •- 2-§ S.t2 1" h2 •-< r/^ -c t^ m\0 '-' N 0\fO-<*-Pr)0 "^00 00 HH 1-1 'O "^ t^ '-' ■^oo i-« ^-% N 00 moo ro t^ "-h o *■>■ O "^ fomr^mf^mt^(nmc^fOfOMrorr)rO'^'^'^'0 p N t^pr^^ Absolute pressure Tempera- in lbs. per ture Fah. sq. in. 1^ pop M y~i *^Qp op t-Ninpp TfO «vp pep iohh ro inOO N 'it'OOO OM-^.'^'-' t^NOO N tv.N r^r«- Nvi) Cnt^^^hH ONvb in Tj- N .-. Ov ror^ ►-• 00 t^vO "-i^Tj-rorOfONNMNWMN'-' Total heat of evapora- tion from water at 32° F. P in t-i VD ^Op pN r-. ^ On p m t^\p On ON pNOO "P m rn b inob N- rnior^bib w r^ V^ "O t^oo On O "- 1 ^3 Tf^O i-i m NH On ^ POOO OnO w fimm'^O N t|- lO^O t-^ i-%00 OOONONOO'-'"-''-''-'t^nN Absolute pressure in lbs. per sq. in. 11 N fO ■^ u^NO l>.CiO ON t-i M rn V m\D t^OO On O 40 Steam Water heated in a Closed Vessel Let water at 32° be heated in a closed vessel, such as an ordinary steam boiler, containing space for the accumulation of steam, and let heat be gradually applied. Then the temperature of the water will gradually rise to that corresponding to the pressure within the vessel, after which evaporation commences and steam is formed. As the heat is increased, the temperature, pressure, and density, or weight per cubic foot, of the steam increase indefi- nitely, so long as the strength of the boiler is not exceeded; and the relation between the temperature, pressure, and density always bears a certain fixed relation, as given by Regnault's Tables, p. 39. If just sufficient heat is supplied as to maintain the tempera- ture constant, the pressure and density remain constant also, and evaporation ceases. If a communication be opened between the boiler and engine, on escape of steam from the boiler the pressure is momentarily reduced and re-evaporation commences rapidly. So long as the temperature is maintained, no sensible variation of pressure is noticeable in a boiler supplying steam to an engine. Temperature of Mixtures— Condensing Water Exatnple I. — If i lb. of water at 212° F. be mixed with 5 lbs. of water at 50° F., find the temperature of the mixture. Note. — In order to avoid confusion in problems of this kind, it is necessary to remember that the total heat in water or steam is always reckoned from 32° F. or 0° C. Hence it is necessary to subtract 32 from the temperature given in Fahrenheit degrees. Let < = temperature required. Then Total heat in i lb. of Total heat in 5 lbs. of _ Total heat in 6 lbs. of water at 2 1 2° water at 50° water at <°. 1(212-32)+ 5(50-32) ='6(/-32) 180+ 90 =6^—192 6^ = 462 ^=77° F. Condensing Water 41 Example 2. — How much water at 55° F. must be mixed with i lb. of water at 212° F. so that the resulting temperature of the mixture may be 105° F. ? Let W = weight of water required ; then Total heat in i lb. Total heat in W lbs. ^ Total heat in (W + i) lbs of water at 212° of water at 55° of the mixture at 105° 1(212-32)+ W(55-32) =(W+ i)(ios-32) 180+ 23W =73W + 73 5oW= 107 W = 2-i4 lbs. In this connection it is interesting and important to compare the difference in the weight of water required to cool a given weight of water, with that required to cool the same weight of steam at the same temperature. In the following example it is shown that it takes ten times as much water to cool i lb. of steam at 212" as it takes to cool the same weight of water at 212° to the same final tempera- ture of 105°. Example 3. — How much water at 55° F. will be necessary to condense I lb. o{ steam at 212° so that the resulting temperature in the vessel shall be 105° F., assuming condensation takes place at the pressure due to the temperature of the steam ? Let W = weight of water required ; then Total heat of i lb. Total heat in W lbs. ^ Total heat in (\V+ i) lbs. of steam at 212° of water at 55° of water at 105° 1146+ W(S5-32) =(VV+ 1) (105-32) 1146+ 23W =73W+73 5oW=io73 W = 2i-46 lbs. Compare this answer with that in Ex. 2 above. Example 4. — Find the temperature of the mixture when 21 -5 lbs. of con- densing water at 55° F. are used per lb. of steam at atmospheric pressure. Let t = the temperature required ; then Total heat in i lb. Total heat in 21-5 lbs. of_Total heat in 22-5 lbs. of steam at 212° condensing water at 55° ~ of mixture. 1146+ 2i'5(SS-32) =22-S(/-32) 1146+ 494'5 =22-5/-720 22-5 ; = 2360-5 /= 104-9° F. 42 Steam CHAPTER VII RELATION BETWEEN PRESSURE AND VOLUME OF GASES Let a portion of gas be introduced into a cylinder which is closed at one end and fitted with a movable piston. Then the gas will fill every part of the space beneath the piston, and exert a uniform pressure on each square inch of surface with which it is in contact. If the internal volume of the cylinder be in- creased, by lifting the piston, the gas will still completely fill the space, but it will be less dense— that is, it will weigh less per cubic foot — and it will exert less pressure per square inch of surface with which it is in contact. If the gas be compressed into a smaller space, it will become more dense, and it will exert a greater pressure per square inch. The relation between the volume and pressure of a perfect gas at constant temperature is expressed in the following terms, known as Boyle's Law : ' The volume of a given portion of gas varies inversely as the pressure, the temperature remaining constant.' This may be illustrated as follows : Let a cylinder (fig. 27 ) be closed at one end and contain a movable piston, and let the piston, when in position a, enclose one cubic foot of gas under atmospheric pressure, or, say, 1 5 lbs. per square inch. Suppose, now, that weights be added to the piston till the pressure on the enclosed gas is equal to 30 lbs. per square inch, or two atmospheres. Then, by the law just stated, the pressure Boyle's Law 43 SO^ — on the gas being doubled, the volume will be reduced one-half. Hence the piston now occupies position b, so that e b—\ e a. Again, apply to the piston a pressure equal to 60 lbs. per sq. iSr in., or four atmospheres. The pressure on the gas being now four times the original pressure, its volume is one-fourth of its original volume, and the piston now falls to c, so that e c^^ e a. Again, apply to the piston a pressure equal to 120 lbs. per sq. in., or eight atmospheres. The pressure on the gas being eight times the original pressure, the volume is now one-eighth of the original volume, and the piston /qq. ^. falls to d, so that e d=^e a. If 1 now horizontals be drawn from • the respective piston positions the length of which is equal to the pressure at these positions to any scale, and a curve be drawn through the extremities of the lines, the student will recognise the curve as being similar to that of an engine indicator diagram if the book be held so that the cylinder is horizontal. This curve is called a rectangu- lar hyperbola. Boyle's Law may also be expressed thus : If V is the volume at pressure P 2 P '60 i- e Fig. 37. then \ V and so on ; or. 2 V 4V 3P 4P iP ^P J-P From which it is evident that in each case, if the pressure be multiplied by the volume, the result is a constant number. 44 Steam The Hyperbolic Curve Boyle's Law and the properties of the hyperboHc curve may be further illustrated as follows : Suppose that one cubic foot of gas at 1 20 lbs. pressure is enclosed in a cylinder and expanded to eight times its original volume. Then the successive changes of volume and pressure may be represented by lines, thus : Draw two lines O M, O N from a common point O at right angles to one another. Let the vertical line O M be called the line of pressures, and the horizontal line O N the line of volumes. *- VOL. AT 60-^l Y VOL AT 30- VOL * Volumes. Fig. z8. Mark off on the line of pressures to a scale of, say, ^^ inch=: 15 lbs., a series of divisions, and on this scale place the figures 15, 30, 60, 120, &c., opposite the points representing these pressures. On the line of volumes take, say, | inch=i cub. ft., and mark i, 2, 4, 8, &c., opposite their respective positions. "From these points raise verticals to represent to scale the pressure of the gas at the various volumes. Thus i a = 120 = the initial pressure of the i cub. ft. of gas. The gas is now expanded to 2 cub ft., the temperature meanwhile being supposed to be kept constant ; and the pressure 2 b will now have fallen to ^ of 120 =i 60 ; at 4 cub. ft. the pressure 4^= ^ of 120 = 30 ; The Hyperbolic Curve 45 and at 8 cub. ft. the pressure 8 ^^ = ^ of 120 = 15. If the free ends a, b, c, d (fig. 22) of the verticals are now joined, the curve formed is called a rectangular hyperbola, fig. 23. Then the four rectangles Oa, Ob, Of, Od represent the conditions of the gas as to volume and pressure for the respec- tive piston positions, and these rectangles are all equal in area : for, 120 X r =60 X 2 = 30 X4=: 15x8:= 120. In other words, pressure x volume = a constant, namely, in the present ex- ample, 120. This curve represents the relative changes in volume and pressure for a perfect gas when the temperature is kept the n _..JliJ 3 % VJ u Xt a u z ! _... \ . r^t^:r^:,.c^^^^ i -1 ' <^ , i 1 " •—■^ Q 2 * s Line or Volumes Fig. 29. same throughout, hence it is called an isothermal curve, mean- ing the curve formed when the gas expands at equal or uniform temperature. The curve also describes fairly accurately the relation between the varying pressures and volumes of expand- ing steam in an engine cylinder. It is not, however, an ' iso- thermal ' for steam, because the tempsrature of saturated steam varies with the pressure (see Table III., p. 39), unless it be superheated, which is not usually the condition of steam in a steam-engine cylinder. Example. — Steam at 85 lbs. boiler pressure, or 100 lbs. pressure per square inch absolute, is admitted to a cylinder 5 ft. long, and cut off at \ 46 Steain of the stroke. Draw the theoretical indicator diagram, assuming that the hyberbolic curve is sufficiently accurate. Note. — In drawing theoretical indicator diagrams, always use absolute pressures. Let O M = line of pressures, and on it mark a scale of pressures, say ^ inch = 5 lbs. Let O N = line of volumes to scale of, say, | inch = i ft. of stroke of piston, and divide this line into ten equal parts. Com- plete the rectangle O M o A. Then O A = the volume of the steam and A a the pressure, at the point where the steam is cut off. To find the pressures B^, Cc, &c,, at B, C, D, &c., corresponding to the successive volumes O B, O C, O D, &c., advancing by distances along O N of 0-5 ft. Since the pressure at any point is inversely as the volume : Fig. 30. Pressure at B= O A.., _..,,_, 2 — -" OB "' 3 P OA "- OC " ,, =?x 100=50-00 4 OD " 2 ,, =- X 100 = 40-00 p. OA ^ OE '■ „ =?x 100 = 33-33 F OA OF " 2 ,, =-x ioo = 28-i;7 7 f. OA ,, =?x 100= 25-00 „ OA ,, =-X 100 =22-22 9 ON " 2 ,, = — X 100=20-00 10 The Hyperbolic Curve 47 The above method of finding the pressure at any point during the expansion when the initial pressure is given may be ex- pressed as follows : Multiply the initial pressure in lbs. per sq. in. by the length of stroke to point of cut-off, and divide by the distance of the given point from the beginning of the stroke. The hyperbolic curve may be described without any calcu- lation by the following simple geometrical method. Draw the lines O M and O N as before. (Note. — The point O in the line O M is the zero of pressure, and not the point through which the line of atmospheric pressure passes.) Complete the parallelogram O M a A as in fig. 31. Produce M a parallel to O N. To find the pressure at any point B Fig. 31- corresponding to the volume O B, draw the vertical B b and join O b, cutting A a in b' . Then the horizontal through b' to cut the vertical B b gives a point / in the curve. Any number of other points may be obtained in the same way, and the curve drawn through the points may be completed. This curve describes the relation between the varying pressures and volumes of a gas, whether the gas is expanding or being compressed, provided the temperature remains constant. Boyle's Law, however, is not absolutely obeyed by any known gas, and less so by steam ; but the knowledge of the law is of great value in enabling us to obtain results which, for ordinary purposes, correspond with sufficient accuracy to the behaviour of the steam expanding in the cylinder. 48 Steam CHAPTER VIII EXPANSIVE WORKING When engines are required to exert their full power for a short period — as .happens, for example, with the locomotive in mounting an incline — steam is admitted to the cylinder at full pressure through the greater part of the stroke, without regard to economy in the consumption of steam or fuel. But this is not the way in which steam is used for any length of time in well-constructed and well-managed engines ; and although extra work is obtained from the engine by neglecting to use the steam expansively, it is being very dearly paid for in the excessive proportion of steam and fuel consumed compared with the extra work done, as we shall now proceed to show. Work done by Steam used expansively We have seen that the work done per lb. of steam without expansion at high pressures only slightly exceeds that done by the same weight of steam at low pressures. We will now call attention to the increased work which may be ob- — tained from high- pressure steam when advantage is taken of its expan- sive properties. Let I lb. of steam at loo lbs. per sq. in. absolute be admitted to a cyhnder (fig. 32), when the piston P is at the end S.^M.■>^^w.w■«■■>w^^■',>■'^^■^.ww^■' .W\SS\\\S\^V*.k'^VVkk^'.'v'\^Vk'^<.'.^.'vUv\V^NVNSNNC K Expansion of Steam 49 Fig. 33. a of the cylinder, and let the supply of steam be continued for one-fourth of the stroke, namely, till the piston reaches b, when we will suppose it just contains i lb. of steam. The supply is now cut off and the piston is driven for the remainder of the stroke by the expansive force of the steam thus enclosed. At the end of the stroke , the steam occupies " four times its original volume, and its pres- sure is now one-fourth its original or initial pressure, and the work done by the i lb. of steam will be clearly shown by the aid of a diagram. Thus let ap (fig- 33) be drawn to cT any convenient scale of pressures to equal 100 lbs.; and make av■^ equal to 4'33 to any other convenient scale. {Note: i lb. of steam at 100 lbs. pressure absolute occupies 4'33 cub. ft., and if we assume the area of the piston = 1 sq. ft, then length a e/i =4-33 ft.) Produce the line to av^, making ia!»4=4 times av^ Complete the figure by the graphical method (p. 47). Now the whole work done by the steam is equal to the area of the figure/ a v^fr, and this whole area is made up of two parts, namely : (i) area/flZ'i r=work done during admission ; (2) arear z;, z'4/=work done during expansion. Hence, by making use of the expansive properties of steam, we obtain the additional work out of it represented by the latter area. To find the area of the figure would be a simple process if the line rfVag. $5) had been a straight line instead of a curve, for then the area of the admission portion=a/ xa Wj ; and the area of the expansion portion=w,z'4 multiplied by the mean height m m. But the curve falls below this line, hence the E 50 Steam area thus obtained is too large, and the greater the expansion the greater the error. Much greater accuracy, however, is secured by this method if several divisions are taken, as shown by the dotted lines re, en, nf, in fig. 33, and the greater the number of divisions taken the greater the accuracy of the result. This is practically the method used by engineers in finding the area of the indicator diagram, the figure being divided into ten equal portions, as explained on p. 55. The exact value of the expansion portion of a theoretical diagram may be readily obtained by referring to a table of hyperbolic logarithms; for, when the curve rf is hyperbolic, the hyperbolic logarithm expresses the relation between the area during expansion and the area during admission. Thus, if the steam is cut off at half-stroke, it is expanded to twice its original volume ; and if the area during admission=i, then the area during expansion =the hyperbolic logarithm of 2 ; hence total area=i+hyp. log. 2. Now, on turning to a table (given in most engineers' pocket- books), the hyp. log. of 2 is '693 ; then total area= i + -693 = i'693 ; and for a general case, if R=the ratio of expansion, or the volume of the steam at end of stroke divided by the volume at point of cut-off, then area during expansion =hyp. log. R, and total area=i +hyp. log. R. Thus, in the example (fig. 33), the steam was cut off at one- fourth of the stroke, therefore at the end of the stroke the volume occupied by the steam was four times its volume at the point of cut-off; in which case R=4, and the whole area = i-f-hyp. log. 4, but by table (p. 51), hyp. log. of 4=1-386 ; therefore whole area=i -1- 1-386=2-386. That is to say, if the area/a;-, r=i, then the area rv^v^f ■=i'386, and the whole area=2-386. Expansion of Steam 51 To express the work done in foot lbs. : Work done during admission =^ v = ioox 144x4-33 =62,352 ft. lbs. Total work done during admission and expansion to four volumes=62,3S2 x 2-386 = 148,771-872 ft. lbs. Example. — Find the weight of steam required per indicated horse- power per hour, working at a pressure of 100 lbs. per sq. in. absolute, with a cut-off at one-fourth of the stroke, assuming there is no back pressure or loss from other causes. ._, Work per I.H.P. per hour Work per lb. of steam = '■98°'°°° ^ 13-3 lbs. 148,772 The following table gives the proportional values of the work done for various degrees of expansion. The work done by the steam during admission is taken as I, and corresponds with the area aprvy (fig. 33). Ratio of Work dene Work done Steam in cylinder expansion during during expansion Total work done R admission = hyp. log. R Cut off at i stroke a I 0-262 1-262 „ r .- 2' I 0-693 1-693 )» 3 I 1-098 2-098 4 I 1-386 2-386 >. * 5 I 1-609 2-609 ., * 8 I 2 -079 3-079 >. ■ i 9 I 2-197 3-197 .. I's 10 I 2-302 3-302 From this table we see that, if steam is cut off at one-third of the stroke, and expanded to the end, the work done is about twice that done by the steam during admission. (The exact proportion is r : 2-098.) Now, if the steam had been admitted at initial pressure throughout the whole stroke, then three tiines the weight of steam would have been used, and the proportion of work then 52 Steam done in the two cases, namely, supplying steam through the whole length of the stroke, or cutting off at one-third and ex- panding, would be as 3 : 2-098 ; in other words, to get half as much work again out of the engine, three times the weight of steam, and therefore also weight of fuel, is consumed in the first case as in the second. The principle of the increased efficiency of steam with in- creased pressures and increased degrees of expansion may be further shown by the aid of the following diagram. LtME OF Volumes Fig. 34. On the diagram (fig. 34) let oV,, oVj, &c., represent the volume occupied by i lb. of steam at pressures varying from 15 lbs. to 100 lbs. per sq. in. absolute. Now, suppose in each case the steam is used non-expan- sively, that is, is supplied at full pressure throughout the stroke, and then allowed to escape into the air or condenser.. Then, neglecting back pressure, the effect of which will .be considered presently, the work done by the i lb. of steam under each of the several conditions is represented by an area as follows : Expansion of Steam 53 (i) Work done by steam at ioolbs.=area o P, ; (2) „ „ 6olbs.=area 0P2 ; (3) „ „ 3olbs.=area 0P3 ; (4) ,, „ 15 lbs.=area o P4. But, assuming that steam obeys the law of Boyle, which is sufificiently accurate for our present purpose, these areas are all equal ; hence the work done in each case is the same. If now advantage be taken of the expansive power of steam, then with steam at 100 lbs. absolute, expanded down to 15 lbs., without back pressure, we are able to add to the area o P , the further area P, P4 V4 V,, which shows a very considerable in- crease in the work done. If the area oP, = i, then the area Pi P4V4 ¥, = 1-896. For the steam is expanded 6 '66 times, hence area of whole figure = I + hyp. log. 6-66 = 1 + 1-896 = 2-896. That is, if the work done by i lb. of steam at 1 5 lbs. pressure := I, then by using the same weight of steam at 100 lbs. pressure and expanding down to 15 lbs. without back pressure, nearly three times the amount of work is done per pound of steam used, and practically also per pound of fuel consumed, for, as has been already shown, the consumption of fuel depends upon the weight of steam used, and is nearly independent of the pressure of the steam, owing to the fact that the total heat in steam at high pressures is only a very little greater than the total heat in steam of lower pressures (see table, p. 39). Back Pressure Back pressure has a considerable influence on the total work done by a given weight of steam. Suppose the piston of a steam engine to be acted upon on one side by steam of 45 lbs. pressure absolute, and, if it be possible, let there be no pressure at all acting on the other side. Then, if the pressure of the steam were maintained uniform throughout the stroke, the diagram of pressures and volumes. 54 Steam or, in other words, the diagram of work, would be a simple rectangle, thus (fig. 35) : US ■ 30 ■ If ^H ^<:^^^^^^ Fig. 35- Fig. 36- Fig. 37. But in ordinary engines without a condenser, as the loco- motive and most small factory engines, when the steam acts on one side of the piston, communication is open with the atmosphere through the exhaust passage on the other side, and it is therefore exposed to a back pressure of 15 lbs. per sq. in. (fig. ^6). The effective pressure is therefore 45 — 15 = 30 lbs. per sq. in. ; and the effect on the diagram is to remove all the lower part from zero to 15 lbs., and thus reduce the area of the figure, and therefore also the effective work done. In practice there is an additional back pressure of 2 to 4 lbs., due to incompleteness of exhaust, making a total back pressure of 17 lbs. to 19 lbs. per sq. in. It may be much more than this with high-piston speeds. If, however, the cylinder were put, during exhaust, into communication with a condenser, then a large portion of the atmospheric pressure is removed, and a back pressure of not more than about 3 lbs. absolute will now oppose the motion of the piston. In this case the area of the figure representing the effective work done will be extended down to within about 3 lbs. of the zero line (fig. 37) ; the gain of work being proportional to the gain of area ; while the weight of steam used in each case is clearly the same. Effective pressure = Difierence between pressures on each side of piston. Me.\n Pressure To find the mean effective pressure of steam per square inch on the piston, by measurement from the indicator diagram : Mean Pressure 55 (i) Divide the line of volumes into ten equal parts. (2) Measure the height of the figure at the centre of each division by the scale of pressures. (3) Add the measurements together, and divide the sum by ten. The result gives the mean effective pressure per square inch on the piston. To find the total mean pressure on the piston, multiply the mean pressure per square inch by the area of the piston in square inches. Then the mean pressure on the piston in lbs., multiplied by the length of stroke in feet, gives the area of the figure, or the work done per stroke in foot lbs. Example. — Find the mean effective pressure in the cylinder of a con- densing steam engine when the pressure of steam on admission is 80 lbs. absolute, cut off at one-fourth of the stroke. Back pressure 3 lbs. per square inch. The same result might have been obtained for the theo- retical diagram by using the following formula : Let / = mean pressure of steam per sq. in. P = initial pressure, or pressure on admission to cylinder. R = range of expansion, or ratio of volume at end of stroke to volume at point of cut-off. Then/ = P X r + hyP-logi^_ back pressure. R 56 Steam Thus, for steam at 80 lbs. per sq. in. absolute, cut off at one-fourth of the stroke, o i + i'386 / = 80 X — ! — ^ 3 4 = 4772-3 = 4472 lbs. per sq. in. The following is useful for reference in obtaining the theoretical mean pressure : IV. Table of Mean Pressures Number of times steam is expanded final volume Mean pressure throughout stroke. Initial Number of times steam is expanded final volume Mean pressure throughout stroke. Initial initial volume pressure = i initial volume pressure = i I* •964 6 '465 4 2 •937 •846 7 8 •421 ■38s 3 •699 9 ■355 4 ■596 10 •330 5 •522 For back pressure, subtract from result obtained by above table, 3 lbs. for condensing engines and 17 lbs. for non- condensing engines, and the remainder will give the mean effective pressure. Example. — Steam at 100 lbs. absolute is expanded down to 20 lbs., back pressure 17 lbs. ; find the mean effective pressure. Here : 5 = number of expansions mean pressure (by table) = '522 100 X -522 = 52 '2 lbs. mean absolute pressure, or, 52-2— 17 = 35-2 lbs. per sq. in. mean effective pressure. Indicated Horse-power The diagram representing the work done on the piston has been called an indicator diagram. From this diagram, having obtained the mean pressure, the work done per stroke may be found. The work done per minute = the work done per Indicated Horse-power 57 stroke x number of strokes per minute. The result may be expressed in horse-power by dividing the work done per minute by 33,000. The horse-power obtained from the indi- cator diagram in this way is called the Indicated Horse-power. It represents the effective work done on the piston by the steam. The formula for Indicated Horse-power (I.H.P.) may be written, so as to be easily remembered, as follows : — _ units of w ork done per minute _ PLAN ■ ■ ■ ~ 33.00° 33.000 where P = mean effective pressure in lbs. per sq. in. on piston. A = area of piston in sq. ins. = (diameter of cylinder in inches)^ X '7854. L = length of stroke in feet, or distance travelled by the piston from end to end of cylinder. N = no. of impulses per minute. = ,, strokes ,, for double-acting engines. = „ revolutions „ ,, single-acting engines. = „ explosions „ ,, gas and oil engines. For work is always estimated by a force of so many pounds acting through so many feet, and (P x A) pounds pressure on piston, acting through (L x N) feet per minute passed through ^= work done on piston per minute in foot lbs., which, divided by 33,000 = work done expressed in horse- power. This formula will repay for careful study. It shows that a given indicated horse-power can be obtained by a variety of conditions, providing that the product P x L x A x N remains constant. Thus P, the mean pressure, may be made up by high-pressure steam cut off at an early point in the stroke, or by low-pressure steam acting through the greater part of the stroke. If P is increased by substituting high -pressure steam for low pressure, then A, the area of the piston, may be less, which means that the engine may be made smaller. If N, the number of revolutions, be increased, then L, the length of the stroke, may be decreased. As a matter of fact, this is what has taken place in the development of the steam engine, namely, increased steam 58 Steam pressures and higher piston speeds, which has resulted in a smaller, and therefore cheaper, type of engine. The early engines using low-pressure steam and running at a comparatively small number of revolutions assumed the type of the massive beam engine. The modern engine develops the same power with high pressures and high-piston speeds, and its dimensions are therefore proportionally decreased. PLAN In the equation I.,H. P. = we have five indefinite terms, any one of which may be found when values are substi- tuted for the remaining terms. Example I. — Find the indicated horse-power of an engine with a cylinder 12 ins. diameter, length of stroke 18 ins., number of revolutions per minute 90, mean effective pressure per square inch on piston 40 lbs. Then I. H. P. = -—^— 33,000 _ (P X A) lbs. X (L X N) ft. per min. 33.000 _(40x 12 X 12 X 7854) lbs. X (1-5 X go X 2) ft. per min. 33.000 ' _ 4, 520 Ite. X 270 ft. per min. 33.000 = 37 nearly. Example 2. — An engine is required to indicate 37 horse-power with a mean effective pressure on piston of 40 lbs. per sq. in. , length of stroke 18 ins. , number of revolutions per minute 90 ; find the diameter of the cylinder. First find the area from the formula : PLAN I.H.P. =' 33.000 . 33,000 I.H.P. PxLxN ^ 33,000 X 37 40 X I -5 X 90 X 2 A, or area of piston = 113 sq. ins. From which the diameter may be obtained thus : Diameter^ a / _i£^= a / Ji^= -^1^= 12 inches. V -7854 V 7854 The horse-power of a compound engine is obtained in practice by finding the horse-power exerted in each cylinder separately from the indicator diagrams by the method above Expansion of Steam 59 explained, and adding the results together ; the sum then gives the total indicated horse-power. Or, its theoretical value may be obtained from an ideal diagram, by considering that the whole of the work is done in the low-pressure cylinder only, working with steam at the initial pressure of the high-pressure cylinder, expanding down to the terminal pressure of the low- pressure cylinder. Examples illustrating Economy of Expansive Working Example I. — A condensing engine works with steam at 30 lbs. boiler pressure, cut off in the cylinder at half-stroke. It is proposed to increase the boiler pressure to 60 lbs. and to cut off at one-fourth of the stroke. Compare the relative work done and weight of steam used in the two cases. Steam at 60 lbs. Steam at 30 lbs.- boiler pressure boiler pressure cut off at cut off at J stroke \ stroke Absolute initial pressure . . 75 lbs. 45 lbs. Theoretical mean pressure . 447 lbs. 38 lbs. Effective mean pressure, allow- ing 3 lbs. back pressure . . 417 lbs. 35 lbs. Relative density or weight of steam per cub. ft 75 45 Relative volumes used per stroke I vol. 2 vols. Relative weight used per stroke. ... ... 75x1=75 45x2=90 From which we gather that by using the higher pressure of steam with earlier cut-off there is a gain in effective mean pres- sure of 417 — 35=67 lbs. per sq. in.= -^x 100=19 pe'' cent., and a reduced consumption of steam =90 — 75 = 15 lbs., saving on each 90 lbs. formerly used, or a saving of —^ x 100= 16 -6 per cent. As formerly explained, there will be a saving in fuel con- sumption corresponding with the saving in the weight of steam. 1st case 2nd case 60 lbs 45 lbs. 75 lbs. 60 lbs. 10 bs. 10 lbs. 1 30 lbs. 27-9 lbs. 26 lbs. 23'9 lbs 60 Steam In practice it would be necessary to set against the above result — (i) Probable increased initial condensation of steam in the cylinder, with the higher pressure and greater expansion. (2) Increased initial stresses on the engine. Example 2. — Two engines, single cylinder condensing, have cylinders of equal dimensions, each works with steam having a terminal pressure of 10 lbs. absolute, back pressure 4 lbs. The boiler pressure by gauge for the first engine is 60 lbs. and for the second 45 lbs. Compare the result in the two cases. Boiler pressure . Absolute initial pressure . Terminal pressure Cut-off (neglecting clearance) Theoretical mean pressure Effective mean pressure, allow- ing 4 lbs. back pressure . The weight of steam used per stroke is the same in each case, for the cylinders are the same size, and the terminal pressures are equal. They, therefore, hold at the end of the stroke equal volumes of steam at the same pressure, and, there- fore, of the same weight. It is true that the total heat required to generate steam at 75 lbs. absolute is greater than that required for steam at 60 lbs. absolute, but this difference is so small that it may be neglected, and the consumption of fuel per lb. of steam generated may be assumed to be the same. There is, however, as seen by the table, a gain in mean effective pressure of 26 — 23'9=2-i lbs., or a gain of — x 100=9 P^r cent, in power exerted with the 23'9 higher pressure. To set against this we have greater initial stresses on the parts of the engine working with the higher pressure, in the proportion of llzA—ll or an increase of 60—4 56 27 per cent., requiring an engine in this case 27 per cent. stronger. To further illustrate the advantage of expansive working, Expansion of Steam 6i we will take another case from actual practice. A steamer of i,ooo I.H.P., having a pair of two-cylinder compound oscillat- ing paddle-wheel engine?, made by Messrs. Laird of Birkenhead, runs at the following speeds and coal consumption for varying degrees of cut-off in each cylinder : Point of cut-off. Knots. Coal per -consumption hour in cwts. Coal per l-consumption knot in cwts. 0-25 8 6 0-75 0-35 9 9 I'OO o'so 10 12 I'20 0-85 12 20 T-66 From this table it will be seen how the weight of steam sup- plied to the cylinder affects the speed and coal consumption. Exercise. — Plot the above results as curves, setting off the scales of values marked (a) along a horizontal line, and those marked [b) along a vertical line. i. (a) Point of cut-off; (i5) knots. 2. (a) Point of cut-off ; (b) coal consumption. 3. (a) Knots ; (b'\ coal consumption. 4. (a) Knots ; {b) coal consumption per knot. The effect of expansive working on the possible distance which a vessel can run with a given weight of fuel will also be evident ; for in the case we are considering the vessel would run 2'2 times the distance when cutting off at o"25 that she would run when cutting-off at 0-85 . The influence of this increased economy of expansive working on the power to run longer voyages where coal is not easily obtained has had im- mense influence on British commerce with distant parts of the globe. Limit of useful Expansion of Steam. The practical limit of expansion varies for different types and conditions of engines, and is the point beyond which no further reduction in weight of steam consumed, per unit of power, can be obtained. The gain by further expansion beyond this point is more than neutralized by loss from condensation in the cylin- der, and from work done by back pressure against the piston. It would evidently be useless to expand the steam to a pressure below that of the pressure at the back of the piston. Thus, let steam at 45 lbs. per sq. in. boiler pressure, or 62 Steam 60 lbs. per sq. in. absolute, be admitted to a cylinder, and cut off at one-fourth of the stroke of the piston. Let the engine be non-condensing with a back pressure of 15 lbs. per sq. in. Then, referring to fig. 39, the theoretical useful work done by the expanding steam is represented by the shaded portion from the commencement of the stroke to the point d on the line of volumes. Here the pressure of the steam {d t) and the back pressure are equal, and it will be evident that any further ex- tension of the diagram would be useless. In practice the ex- pansion cannot be carried with advantage so far as this. If the expansion be continued beyond four, to five or six expansions, the work done by the atmosphere against the piston in the later stages is greater than that done on it by the steam, to the extent represented by the area of the shaded part marked negative work. Hence, in an engine cylinder the steam should never be cut off so early as to cause it to expand to a pressure below that of the back pressure acting against the piston. Theoretical limit of number of expansions — '"itial pressure . back pressure Thus, in above case ^"'^"^ pressur e ^60^ expansions. In back pressure 15 ^ practice the maximum number of expansions should not exceed three-fourths of the theoretical limit. In condensing engines the steam is expanded down to a final pressure of about 10 lbs. per sq. in. absolute. And by dividing the known initial abso- Clearance 63 lute pressure by the known terminal pressure we determine the number of expansions required. Thus, for a condensing engine working with steam at an initial pressure of 150 lbs. absolute and expanding to a terminal pressure of 10 lbs. absolute, number of expansions =-™MP££!5Hie, =£Sf>=i. terminal pressure 10 Such a large number of expansions could not be economically carried out in one cylinder, but in practice would require three successive cylinders. Clearance in the Cylinder ^\^^en the piston in a cylinder is at the end of its stroke it does not touch the end or cover of the cylinder, but there is always a certain space left between them to prevent the danger of their coming into actual contact. In addition to this is the passage between the face of the slide valve and the cylinder by which the steam is conducted to the cylinder. These two spaces (marked c c, fig. 40), which make up the whole space between the face of the valve and the face of the piston when the piston is at the end of its stroke, are called the clearance volume. Let the volume displaced by the piston during its stroke = 9 cub. ft. ; and the volume of the clearance = i cub. ft. Then, when steam is admitted, i cub. ft. is used to fill the clearance space before the piston moves ; and if steam is used at full pressure throughout the stroke, 9 cub. ft. more is required to displace the piston. Thus i cub. ft. in every 10 cub. ft. of steam passes through the engine with- out doing any work, representing a loss of 10 per cent. But suppose the steam is cut off at -^th of the stroke. Then, during admission there is first i cub. ft. of steam to fill the clearance, and which so far does no work on the piston, and then I cub. ft. to displace the piston, when the steam is cut oif. There are now 2 cub. ft. of steam at initial pressure Fig. 40. 64 Steam enclosed in the cylinder. Expansion commences, and at the end of the stroke the volume occupied by the steam will evi- dently be 10 cub. ft. Hence, pressure of steam at end of stroke =y%ths, or -^th of initial pressure. If there had been no clearance, then we should have had I cub. ft. of steam in the cylinder at point of cut-off, which would expand to 9 cub. ft. with a terminal pressure of ^th, the initial pressure. Suppose the initial pressure had been 180 lbs. absolute. Then, neglecting the effect of clearance, the terminal pressure T Rr> = — = 20 lbs. per sq. in. absolute. 9 Including the effect of clearance, the terminal pressure = T 8n =36 lbs. per sq. in. absolute, or nearly twice the terminal pressure obtained neglecting clearance. I80r- Od a. Fig. 41. Although the steam required to fill the clearance space does no work on the piston during admission, yet when cut-off takes place the piston receives the advantage of the expansive force of this steam, and its effect in increasing the total work done is shown by the shaded part of the diagram (fig. 41). To draw the diagram, set off a (:=the stroke of the piston, to any scale and divide it into nine equal parts, construct the curve p, 20, by the graphical method from the point a, representing the expansion of steam of volume a b and pressure b p. To the Cylinder Condensation 65 left of a draw a d, making a d = |-th a c, that being the proportion of the volume of the cylinder occupied by the clearance space. Draw the curve /, 36, from the point d, representing the ex- pansion of steam of volume d b and pressure b f. The loss by clearance may be much reduced by closing the exhaust passage in the cylinder before the end of the stroke, so that the steam so enclosed may be compressed and fill the clearance space at a pressure and temperature approaching that of the newly entering steam. Cylinder Condensation When steam is admitted to a cylinder which is colder than the entering steam, the steam parts with some of its heat to the cylinder walls, a portion of the steam is condensed and deposited on the metallic surface, and more steam from the boiler enters the cylinder to take its place, while the tem- perature of the cylinder rises to that of the steam in contact with it. If the steam be supplied to the cylinder at the initial pres- sure and temperature throughout the whole stroke, and the exhaust port be then opened, the steam will escape into the air, and the pressure in the cylinder will fall to that of the atmosphere, or nearly so. But the water (which exists more or less as a film), being in contact with the metallic walls of the cylinder at the temperature of the initial steam, will evaporate immediately the pressure is reduced by the opening of the exhaust, and become reconverted into steam at the expense of the heat in the walls of the cylinder, thereby cooling them to the temperature of the steam during exhaust. The steam thus re-evaporated during exhaust not only absorbs heat, which will have to be made up again from the entering steam during the next stroke, but it passes away to the air without doing any useful work ; in fact, it acts rather as back pressure against the piston. When the piston reaches the end of its stroke, the boiler steam is readmitted into the cylinder and comes in contact with F 66 Steam the cooled surface of the cylinder cover, piston, and steam passages, which have been exposed to the temperature of the exhaust steam, and the same process of condensation and re- evaporation will be repeated. If the cylinder had been in communication with a condenser instead of with the air, the temperature of the cylinder during exhaust would have fallen still lower, namely, to that due to the decreased pressure in the condenser, and the condensation of the initial steam during admission would have been still greater. If the steam is cut off at an early point in the stroke, con- densation occurs, as before, during admission, while the steam is hotter than the cylinder; but as the expansion proceeds, a portion of the condensed steam is re-evaporated, the re- evaporation increasing as the pressure decreases towards the end of the stroke. On the opening of the port to exhaust, the pressure is still further reduced, and re-evaporation is com- pleted. Condensation, then, takes place during the early part of the stroke, while re-evaporation occurs partly towards the end of the stroke and partly during exhaust. The re-evaporation during expansion behind the piston helps the piston, and in- creases the total work done ; but the steam re-evaporated during exhaust in a single-cylinder engine passes away to waste. The loss due to condensation of steam in the cylinders of all engines varies from lo to 50 per cent., or more, of the whole steam consumed, the loss becoming greater as the mean temperature of the cylinder walls becomes less than the tem- perature of the initial steam. Condensation in the cylinder increases as the degree of expansion increases, because there is a decreasing mean temperature of the walls with a constant initial temperature of the steam. The economical advantage of using high-pressure steam is due to the power it possesses of doing work by expanding behind the piston after the supply is cut off from the boiler. But the temperature of saturated steam varies with the pressure, and, therefore, if, in a single cylinder, steam at high Cylinder Condensation 6^ pressure and temperature be admitted and expanded to a low pressure and temperature, the greater the degree of ex- pansion the greater the difference in temperature between the initial steam and the mean temperature of the cylinder walls. Hence there is a limit to the useful expansion of steam in a single cylinder, owing to the excessive condensation in the cylinder, with high degrees of expansion, resulting in increased consumption of fuel instead of a saving, and giving rise to the expression ' Expansive working is expensive working.' The secret of economy is to supply the cylinder with dry steam, and to maintain it as dry as possible throughout the stroke, and engineers from Watt's day to the present have striven to accomplish this result. The laws which govern the condensation of steam in the cylinder are not at present fully understood. The means adopted to reduce the amount of water of condensation in the cylinder are : (i) Obtaining the steam from the boiler as dry as possible, and maintaining it in the dry condition by carefully covering the parts traversed by the steam, on its way to the cylinder, with non-conducting material. (2) Placing a water-separator in the steam-pipe just before entering the engine. (3) Jacketing the cylinder with hot steam (an example of jacketed cylinders is given in figs. 126 and 127). The addition of the steam jacket reduces the amount of condensation in the cylinder. The jacket is the more necessary the greater the ■■ degree of expansion in one cylinder, and the slower the piston speed. (4) Compression of a portion of the exhaust steam by closing the exhaust port before the end of the stroke, and allowing the piston to compress the steam and thereby to raise its pressure and temperature, and therefore also the temperature of the cylinder cover, steam passage, and piston, before the new steam is admitted. (5) Compounding the cylinders, that is, adding one or more separate cylinders into which the steam may be successively 68 Steam expanded, and thereby reducing the variation of tempera- ture in each cyhnder. (6) Increasing the rotational speed of the engine. (7) Superheating the steam, that is, applying additional heat to the steam on its way Ijetvveen the boiler and the engine. 69 CHAPTER IX THE STEAM ENGINE Non-condensing Engines Engines which exhaust their steam into the air after it has done work in the cyUnder are called non-condensing engines. The locomotive and most factory and mill engines belong to this type. Non-condensing engines are known by the puffing of the escaping steam up the chimney, a phenomenon which is familiar to every reader. The puff of the exhaust steam occurs as the piston arrives at the end of its stroke, the escaping steam having previously driven the piston from one end of the cylinder to the other. In condensing engines there is no puffing of exhaust steam into the air, the steam being passed instead into a box or con- denser, where it is cooled and condensed by actual contact with a jet of cold water, or by contact with cold pipes through which cold water is flowing. The essential parts of all ordinary non-condensing engines, whether the engine be an horizontal or vertical one, are practi- cally the same, the difference in appearance among engines by different makers being due for the most part to a difference in shape or arrangement of the essential details. The following diagrams (figs. 42 and 43) give a front view and side view of a small vertical non-condensing steam engine, as in use for various kinds of factory and mill work. The pressure of steam used in such engines is about 60 lbs. per sq. in. above the atmosphere. The action of the parts is as follows : The steam is con- 70 Steam ducted from the boiler by the steam pipe to the slide jacket or chamber in which the slide valve S V works. Here, by a sliding motion of the slide valve on the face of the ports, the Fig. 42. C, cy inder ; P, piston ; P R, piston rod ; G, guides ; C R, connecting rod ■ C P r«nl, pin; E, eccentric; E R, eccentric rod; S V, slide valve; V #G v'aW ' ■ ji guide ; C S, crank shaft. =■»=> l*. valve spindle Non - Condensing Eng ines 71 steam passages or ports are alternately opened, admitting steam to one side of the piston, and allowing it to escape from the other side into the air ; or, if a condensing engine, into a con- FlG. 43. C H, cross ■head; C R, connecting rod ; B, bearings ; E P, exhaust pipe; S P, steam pipe ; B P, bed plate ; F W, fly-wheel. 72 Steam denser. (The exact action of the slide valve will be explained more fully presently.) The piston is thus made to move from end to end of the cylinder against the resistance due to the load which is communicated through the piston rod. Attached to the outer end of the piston rod is the crosshead, having a flat base called a slipper, which slides to and fro between guides, and compels the piston rod to move parallel to the axis of the cylinder, thus preventing the angular action of the connecting rod from bending the piston rod. The con- necting rod is attached at one end to the crosshead by a pin, sometimes called a gudgeon, which passes completely through the block and the fork end of the rod as shown, and at the other end to the crank pin. The reciprocating motion of the piston is by this means converted into the circular motion of the crank pin and shaft, and from the shaft by means of a pulley and belt, or by wheel gearing, the power of the engine is transmitted as required. See also figs. 125 and 26. Engine Details The Cylinder. — The cylinder, which is made of cast-iron, consists of the cylindrical chamber, bored out perfectly true, and of the slide jacket or valve box. The cylindrical chamber is connected at each end with the slide jacket by passages called steam- ports, S, through which steam passes to or from the cylinder. The pas- sage between the two steam ports leads to the air, or to a con- denser, and is called the exhaust port, E. This passage is put in Fig. 44. S P, steam pipe ; S, steam port ; SV, slide valve; P. piston, G, gland. PR, exhaust port ; piston rod ; The Cylinder 73 communication with either end of the cylinder as required by means of the slide valve. The ends of the cylinder are closed by covers bolted to the flanged ends. In the example (fig. 44) the bottom end is cast solid with the body of the cylinder. In order to make the hole in the cover through which the piston rod passes steam-tight, a stuffing box is used, the con- struction of which will be understood from the figure. The casting is so formed as to leave a small space around the rod, which is filled with some form of flexible material capable of making a steam joint round the piston-rod, and the packing is pressed down on the rod by means of a cover or gland fitted with two screwed bolts. A similar arrangement of stuffing box and gland is fitted to the slide valve rod ; it is also used for pump rods and other similar purposes. The steam passages should be made as short as possible, because at each stroke the passage must be filled with its own volume of steam before the steam acts upon the piston. The effect of this has been described under the heading of Cleat ance, on p. 63. The steam ports must be made large enough to admit sufficient steam to the cylinder during the instant the port is open, otherwise the steam will be wiredrawn. Wiredrawing is the gradual fall of pressure of the steam behind the piston, as it pro- ceeds on its stroke, owing to small and re- stricted steam passages. Its effect may be illus- trated by the diagram (fig. 45). If the pres- sure of the steam on ad- mission to the cylinder = O A, then the pres- sure, instead of being maintained at a pressure N E- to the point of cut-off, E, gradually falls from A to B. The stroke of the piston from end to end of the cylinder A E B X . V J f 1 SI 1 Fig. 45. 74 Steam (which is equal to the diameter of the crank-pin path) deter- mines the internal length of the cylinder from cover to cover, which must evidently be equal to the stroke of the piston, plus the thickness of the piston, plus twice the clearance allowed between the piston and cylinder cover, when the piston is at the end of its stroke. This clearance, which is kept as small as possible, varies from \ in. to \ in., according to the size of the cylinder. It will be noticed that the shape of the cylinder cover must be made to conform to that of the piston, otherwise a con- siderable volume of steam might be wasted at each stroke, in filling unnecessarily large clearance spaces. Cylinder liner. Steam jacket. — Cylinders are sometimes fitted with a separate internal barrel, called a cylinder liner, as shown in the sectional view of the compound engines (fig. 126), made of hard cast-iron or of steel. Between the liner and the body of the cylinder is a space called the steam jacket, which is filled with steam direct from the boiler. The depth of the jacket is about the same as the thickness of metal in the cylinder. Sometimes the cylinder covers are jacketed, as well as the body of the cylinder. Cylinder escape valves. — To avoid the danger of the piston bursting the cylinder cover as it approaches the end of its stroke, owing to the occa- sional presence of water through prim- ing or condensation, cylinder escape valves are often fitted on the cylinder covers. The diagram (fig. 46) will explain the construction of these valves. The ^"^- +^- valve is of the ordinary conical kind, kept in position by a spring loaded a little above the pressure in the boiler. Cylinder relief cocks (fig. 47) are also fitted to all cylinders to drain off the water, or to blow through the cylinder with the steam, and thus clear it of water, especially on starting the engine. The Cylinder 75 Exaviple I.— A cylinder is 15 ins. diameter, stroke of piston 25 ins. ; find the capacity of tlie cylinder, allowing an addition of 7 per cent, for clearance space. Ans. 47267 cub. ins. Note. — This represents the volume of steam in the cylinder at end of stroke ; the following example shows how to find the weight of this volume. Example 2.— Find the weight of 47267 cub. ins. of steam at 20 lbs. pressure per sq. in. absolute. By Table III. i lb. of steam at 20 lbs. pressure absolute occupies 197 cub. ft. Then 197 cub. ft. of steam at 20 lbs. pressure weigh I lb. 47267 1728 -^Ib. 197 47267 ^ lbs. 1728 197 = -y881b. The above two examples give the volume and weight of steam used per stroke in a cylinder of the above dimensions, CYLINDER Fig. 47. working with steam at 20 lbs. terminal pressure. To find the volume or weight of steam passing through the engine as steam vapour in a given time, multiply the above results by the number of times the cylinder is filled ; in other words, multiply by the number of strokes made by the piston in the given time. Example 3. — The engine in the above case runs at 100 revolutions per minute ; find the weight of steam used per hour. In I stroke the weight of steam used= '1388 lb. ,, I revolution ,, ,, =(-1388 x 2) lb. ,, I minute ,, ,, = (-1388 x 2 x 100) lbs. ,, I hour „ „ =('I388 X 2 X 100x60) lbs. = 1665 '6 lbs. 76 Steam Example 4. — Suppose it is known that the horse-power of the above engine, when working at 100 revolutions, is 90 ; find the number of lbs. of steam used per horse-power per hour. Ans. iS'S Fig. 48. Pistons The piston is the movable plug which moves from end to end of the cylinder, under the pressure of the steam, and through which the energy of the steam is converted into the motion of the mechanism. The piston must form a steam-tight division between the two ends of the cylinder. If it were possible to turn up a solid piston, which should so exactly fit the bore of the cylinder that it would be steam-tight, and at the same time move freely without friction, this would be a perfect piston. In the early days of the steam engine, when steam pressures were very low, pistons were made steam-tight by coiling rope or junk in a groove on the rim of the piston, and this method is still adopted for pump buckets which only require to be water- tight. But for the pistons of steam cylinders a more perfect ar- rangement was soon found necessary. As at present made, the body of the piston is turned to an easy fit in the cyhnder, and it is then made steam-tight by means of spring rings. A common and simple arrangement is that of Ramsbottom's spring rings, which are simple steel or gun-metal rings of \ in. to | in. square section (fig. 49). They are turned at first to a diameter a little larger than that of the cylinder they are required to fit ; and a small piece is then taken out to enable them to close up to the bore of the cylinder when in their place. They are then sprung over the piston and fitted into grooves turned in the piston rim (fig. 49). Pistons 77 Figs. 49 and 50 are types of locomotive pistons; fig. 50 is fitted with two cast-iron packing rings about | in. thick by f in. wide, turned, cut, and rTTv. sprung into position as i^^9^-^=F\ before. The rings are some- times placed in the same groove, and sometimes in separate grooves. For large low-pressure cylinders pistons of the type shown in fig. 51 are much used. The packing ring consists of one large cast-iron ring, PR, which is pressed outwards against the cylinder by J If PR .^a- r~r~i ^ «IT^^ Fig. 51. J R, junk ring ; P R, packing ring ; T P. tongue piece ; S, spring. means of a series of springs, S, placed behind the packing- ring. For horizontal cylinders, the bottom spring is removed and a cast-iron block is substituted, which takes the weight of 78 Steam Fig. 52, the piston. Instead of the small separate springs, various patent coiled springs are used in vertical engines. The packing ring is turned- a little larger (^ in. per foot diameter) than the bore of the cylinder ; it is then cut through by an oblique slit and tends to spring open as wear takes place. The steam is pre- vented from leaking through this opening by a brass tongue piece, T P, which is fitted in another groove cut across the slit as shown. The tongue piece is secured to a plate fastened to the back of the ring, and on one side of the slit (fig. 52) The packing ring is held in its place between two flanges, one of which is cast solid with the piston, the other being formed by a loose flat ring called the junk ring, J R. The junk ring is secured to the piston by screwed bolts which screw into brass nuts in- serted in a cavity left for the purpose in the body of the pis- ton. These bolts are prevented from slacking back by a guard ring or pieces of a ring fitted be- tween the heads, as Fig- 53. shown. Fig. 53 is a section of Buckley's Patent Piston. The packing consists of two separate rings with a continuous coiled spring behind it. The action of the coiled spring is to keep the rings steam-tight, not only against the cylinder, but against the junk ring and flange of the piston. Pistons 79 The friction between the packing ring lind the cylinder should be as little as possible consistent with steam-tightness, and the piston should be as light as possible consistent with strength. Steel pistons are now becoming common, and by using this material the weight of the piston can be considerably reduced. A fruitful and all too common source of loss of efficiency in steam engines is the presence of leaky pistons, the steam passing from one side of the piston to the other. Such steam is worse than wasted, as it not only does no work on the piston but acts as back pressure against it. The .pistons of locomotives are usually kept in good condition, and the short sharp exhaust of the locomotive is in striking contrast with the asthmatical exhaust of too many factory and mill engines. Piston speed. — The mean speed of the piston in feet per minute = length of stroke x number of revolutions per minute X 2. Example. — An engine with a 3-ft. stroke makes 80 revolutions per minute ; fiad the mean speed of the piston. 3 ft. X (80 X 2) strokes = 480 ft. per minute. The mean speed of the piston in practice varies from about 250 ft. per minute for small stationary engines, to from 500 to 750 ft. per minute for marine engines, and in some cases it exceeds 1,000 ft. per minute in the locomotive. There has been, and is, an increasing tendency towards high piston speeds and light moving parts. Piston displacement per minute is the space swept through by the piston at each stroke, multiplied by the number of strokes per minute ; = the area of the piston in square feet, multiplied by the speed of the piston in feet per minute. Example.— 'Y'm& the displacement of the piston per minute in an engine, diameter of cylinder 18 ins. and length of stroke 2 ft. ; revolutions per minute, 70. Then (i -5 x i -5 x 7854) sq. ft. x 2 ft. x (70 x 2) strokes = 49476 cub. ft. per minute. Piston rods are subjected to alternate pushing and pulling stresses which occur in rapid succession, and which must severely test the material of the rod, and they are now invari- 8o Steam ably made of steel. The weakest part of the rod is at the screwed end which takes the nut. This part, however, is only subject to tension, and not to alternate tension and compres- sion, for when the steam enters the cylinder underneath the piston (fig. 49) the whole load is carried by the screwed part of the piston rod ; but on the return stroke, when the piston is descending, the stress is removed from the screwed part and comes on the tapered part of the rod and the collar. The load to be carried by the piston rod equals the differ- ence between the pressure on the two sides of the piston. Thus, in a condensing engine the effective pressure per sq. in. on the piston equals the boiler pressure by gauge, plus 15 lbs. pressure of atmosphere, minus loss of pressure between boiler and cylinder, minus back pressure due to imperfect vacuum in condenser. Crossheads and Guide Blocks The crosshead forms a head at the outer end of the piston rod, to which the connecting rod is attached by a pin passing through the crosshead. It varies very con- siderably in design. Guide blocks are sometimes attached to each end of the pin, on either side of the cross- head, as in fig. 54. Another arrange- ment is to make a foot solid with the crosshead, which acts as a guide block, and works between guides, as shown in fig. 55. The blocks and guides prevent the oblique thrust or pull of the connecting rod from bending the piston rod. This can be seen by reference to fig. 56. When the piston P is being im- pelled forward, so that the rotation of the crank pin is in the direction of the arrow, the resistance at the crank pin c causes a downward thrust through the connecting rod eg, which may be resolved into two forces, one tending to compress the piston rod and the other to bend it in the direction T, causing a down- FiG. 54. Crossheads and Guides 8i ward thrust upon the guides. Again, when the piston is being driven back by the steam, the resistance of the crank pin at d causes a downward pull at the point g of the piston rod, the tendency again being to cause a downward thrust upon the guides. If the engines were reversed the whole of the condi- FlG. 55 tions would be reversed, and the thrust ^T would be upwards instead of downwards. Hence the prevailing direction in which horizontal engines should run is that shown by the arrow in the figure, so that the pressure on the guides should be upon 1 J^^, Fig. 56. the lower rather than upon the upper guide bar ; this is especially important for the sake of efficient lubrication. It should be noticed that when the crank pin drags the piston, as it does, for example, when steam is shut off while the engine continues to rotate, the direction of the thrust on the guides is G 82 Steam reversed ; hence the necessity for a top and bottom guide bar under all circumstances. The amount of the thrust on the guides varies according to the angularity of the connecting rod, being greatest when the crank is at right angles to the axis of the piston rod, and being reduced to nothing at each end of the stroke ; hence the guides wear hollow in the middle, and arrange- ments should exist for removing the guides and truing them up. The amount of the thrust on the guides in the middle of the stroke may be found from the following simple formula : Pressure on piston x radius of crank in ins. Maximum thrust Length of connecting rod in ins. Example. — Find the maximum thrust on the guides when pressure on piston at half-stroke = 20,000 lbs. ; radius of crank = 15 ins. ; length of connecting rod = 5 ft. Maximum thrust = — '- — ^-— = = 5,000 lbs. 60 Fig. 57- (Dotted circle=crank-pin path.) When engines are required to rotate in either direction equally, as the locomotive, the surfaces in contact between the block and the guide are made equally large, as is the case in fig- S^i with the top and bottom guide bar ; but when the engine is intended to rotate always in one direction, or nearly so, as in the marine engine and in factory engines, the surface on which the thrust comes is made sufficiently large, while the opposite surface may be much reduced, as is the case with the slipfier, or shoe guide (fig. 55), the prevailing direction of the thrust being taken on the largest surface of the block. The Connecting Rod The connecting rod connects the crosshead with the crank pin, and by its means the reciprocating or to-and-fro motion The Connecting Rod 83 of the piston is transformed into the rotatory or circular motion of the crank pin. The length of the connecting rod, which is measured from the centre of the crank pin to the centre of the crosshead pin, varies from two to three times the length of stroke of the engine. By the stroke is meant the distance travelled by the piston from one ^ ', 1 ( 1 1 . -> b Fig, 59- end of the cylinder to the other, which is equal to the diameter of the crank-pin path, or to twice the length of the crank arm. Fig. 59 is an illustration of the marine type of connecting rod 84 Steam Fig. 60 shows a 'strap, gib, and cotter' arrangement for connecting rod end. Fig. 60. S, strap ; G, gib ;C, cotter. Relative positions of piston and crank fin. — When the piston P is at either end of the stroke, the centre Hne of the fe^ ^^^^^^^ c' " \ Fig. 61. connecting rod B C and of the crank o C lie on the axis of the cyhnder produced (see figs. 61 and 62), and the crank is then said to be on its dead centre ; for if the engine come to rest in this position, it will remain at rest, even when steam is admitted to The Connecting Rod 85 the cylinder, because the pressure of the piston is felt merely as a thrust on the crank shaft main bearing, and it has no tendency to cause the crank to rotate. In such a case it is necessary to ' bar ' the engine round by the fly wheel till the crank has moved off the dead centre, before admitting steam against the piston. There are two ' dead centres ' in a revolution. !y//////////////M///h\ Fig. 62. Let the dotted circle, fig. 63, represent the path of the crank pin about the centre of the crank shaft. If the connect- ing rod were infi- nitely long, or if we neglect the obliquity of the connecting rod, then, when the crank pin is at any ,. position between 0° and 180°, the cor- responding position of the piston is found by dropping a perpendicular up- on the diameter as shown, which diameter may be taken to represent the stroke of the piston. In such a case, when the crank pin is at 90° or one-fourth of a revolution from 0°, the piston would be in the middle of its stroke ; but this is not the case in practice, because of the obliquity of the connecting rod, as will now be shown. Since the position of the crosshead corresponds exactly with the position of the piston, we may for the present purpose Fig. 63. 86 Steam suppose the connecting rod to take hold of the piston direct, without the intervention of the crosshead and piston rod. First, to obtain the position of the piston for a given position of the crank pin. In fig. 64 let C, C2 be the diameter of the crank-pin path, and let the length of the connecting rod be i| times the stroke, namely, i^ times C , Cj. From Cj, with radius equal to length of connecting rod, mark P , , the position of the piston when crank is at Ci ; also from Cj with the same radius mark P2, the position of piston when crank is at C2. From any intermediate position on the circular crahk-pin path, and with radius equal to length of connecting rod, cut the line of stroke Pi P._„ then the intersection will give the corresponding position of the piston. Thus, when I \-„-r^ I c\ — k1>) — y, \ / I I \ '■A . V ^ / Fig. 64. the crank pin is at C with the crank at right angles to the line of stroke, the piston position is not at half stroke, but at some position Po beyond half stroke ; and the shorter the connect- ing rod the greater the distance travelled by the piston beyond the centre of the stroke ; or, the longer the connecting rod the more nearly P^ would coincide with the middle of the stroke. From the figure it will be clearly seen that while the crank pin rotates at a uniform velocity through the first quarter of a revolution, the piston travels at the same time from rest at P, a distance Pi P„ greater than half the stroke, when its velocity is equal to that of the crank pin ; and, during the uniform rota- tion of the crank pin through the second quarter, the piston travels a distance P^, P2, or less than half the stroke, again coming to rest at P^. The Connecting Rod 87 Conversely, to obtain the .position of the crank pin for a given position of the piston. When the crank pin is at Ci (fig. 65), the piston is at Pi ; and when the crank p.'n is at C2 the piston is at Pj ; and if the connecting rod were loose from the crank pin, and held at the centre of the crank shaft Co the piston would be at P^, namely, at the middle of the stroke. Now let the piston end of the rod remain in this middle position and move the other end of the rod from C^ in an arc of a circle from centre P^ till it cuts the crank-pin path at C. Then C is the position of the crank pin when the piston is in the middle of the.stroke. Any other position of the crank pin for a given position of the piston may be similarly obtained. I— 7= 2 )^Z Fig. 65. By the term piston speed is meant the mean speed of the piston. This, however, is less than the mean speed of the crank pin ; for during one stroke of the piston the crank pin moves through a semicircular path, the length of which, com- pared with its diameter or the stroke of the piston, is as : I ; or as 1-5708 : i. 2 Thus, if the mean piston speed is 1,000 ft. per minute, the mean speed of the crank pin is 1000 x i'57o8=i57o'8 ft. per minute. By the principle of work, since the work done on the piston is the same as that done on the crank pin, and that the mean speed of the crank pin is i'57o8 times that of the piston, there- 88 Steam fore the mean pressure on the crank pin in the direction of its motion IS - 1-5708 of the mean pressure on the piston. Example. — In a direct acting engine (he diameter of the cylinder is 1 7 ins. , and the mean pretsure of the steam is 60 lbs. per sq. in. , the crank being 12 ins. long ; what is the mean pressure on the crank pin in the direction of its motion ? (Sc. and A., 1878.) Then mean pressure on piston =-- 1 7 x 1 7 ^ 7854 '^ 6° = 13614 and mean pressure on crank pin = 13614 x = 8670 lbs. Fig. 66 shows how the short connecting rod affects the position of the piston relatively to the crank pin at the two ends of the stroke. Thus crank positions A and B represent corresponding angular movements from the respective ' dead centres,' but on referring to the piston positions along the Fig. 66. piston path C F we find a great discrepancy ; the distance a from the end C of the stroke is much greater than the dis- tance 1^ from the end F. It is necessary to allow for this in designing valve gear to cut off the steam in the cylinder at equal fractions of the stroke on each side of the piston. 89 CHAPTER X THE SLIDE VALVE Before explaining the action of the valve, it will be helpful to the student to have a clear idea of the actual shape of the cylinder face. This is shown in the following diagram, fig. 67, Fig. 67. where the slide jacket cover and slide valve are removed so as to expose to view the long rectangular shaped ports in the cylinder face in the upper part of the diagram. Three rect- angular openings are shown ; the middle port, which is the 90 Steam exhaust port, is wider than the other two. It is a passage leading direct from the cylinder face to the outside of the cylinder, one end of the passage being the rectangular opening, called the exhaust port, and the other end the circular opening with a flange, shown in the figure, to which the exhaust pipe may be bolted. The two other ports are the steam ports — one leading to one end of the cylinder, and the other to the other end. The slide valve is shaped somewhat like a hollow, rectangu- lar, inverted dish ; the edges of the dish, constituting the face of the valve, are planed and scraped to a perfectly true plane surface, and this works on a similarly prepared surface on the cylinder face. The following diagram will explain the action of the slide valve. AVe will first take the simplest form of valve in which the edges of the valve are exactly the same width as that of the steam ports. Fig. 68 shows such a valve in its central position completely closing both steam ports. The position of the piston at the same moment is at the end of the stroke, ready to commence a new stroke. The piston is connected to the crank pin C of the crank O C moving about the centre O of the crank shaft (shown out of its correct position for the sake of convenience), and the slide valve is connected to the pin E of the smaller crank O E moving about the same centre. The centre E is really the centre of the eccentric ; but, as will be explained later on, the action is the same as though O E were a little crank. The dotted circles representing the paths of the crank pin C and of the centre of the eccentric E have their diameters equal to the stroke of the piston and valve respectively, and the posi- tions C and E of these centres are correctly placed relatively to the positions of the piston and valve in fig. 68. The smallest movement of the shaft about its centre O in the direction of the arrow will cause the valve by its connection with E to uncover the left-hand port and admit steam against the piston. Suppose the shaft to have described one-fourth of a revo- lution from the first position, then the new positions of the pins C and E, and of the piston and valve respectively, are The Slide Valve 91 shown in the fig. 69. The distribution of the steam may be also followed by referring to the arrows, the steam being admitted from the boiler on one side of the piston, and on the other side exhausted into the air, or a condenser, by pass- ing out through the hollow part of the valve into the exhaust passage. As the piston continues to travel towards the end of its stroke, it will be seen, by following the movements of C and E, that the valve returns to its middle position, and again just closes the port as the piston reaches the end of the cylinder. -f ^r - -7 FROM BOILER -(4e Fig. 69. The valve then uncovers the right-hand port and the distribu- tion of steam is reversed. The valve which we have so far described has two impor- tant disadvantages : (i) It admits steam to the cylinder throughout the whole length of the stroke of the piston. The waste of steam involved in not cutting off the supply at an early point of the stroke, and using it expansively, has been already pointed out. (2) It opens the ports to steam and exhaust just after the 92 Steam piston moves forward on its return stroke instead of just before it commences to return. These disadvantages are overcome in two ways : (i) by adding laj> to the valve - that is, by extending the width of its face — and (2) by giving it lead — that is, by causing it to move forward so as to open the port just before the piston reaches the end of its stroke. Definitions of lap and lead. — The amount by which the valve overlaps the edges of the steam port when at the middle of its stroke is called the lap of the valve. The amount by which it overlaps the outside edges is called the outside lap. The amount by which it overlaps the inside edges is called the inside lap. Thus, in fig. 70, the lightly shaded part shows the valve with no lap. The darker parts show the addition of outside lap c c and inside lap i i, by increasing the width of the face from the width of the port S to the width ci. The amount of opening of the port for the admission of steam w/ien the piston is at the beginning of its stroke is called the leadoi the valve (pronounced ked). Thus the opening b (fig. 71) is the lead of the valve, if the piston at this moment is at the beginning of its stroke. It will be noticed that, the inside lap i being less than the outside lap c, the lead to the exhaust port is greater than that to the steam port, which permits of a ready escape to ex- haust. When a valve has no lap, it moves on each side of its middle position, in order to open the steam port fully, a distance equal to the width of the port. In other words, the radius O E (fig. 68)= width of port. But, when lap is added to the valve (fig. 72), the distance moved on each side of its central position must be increased, if the port is to be fully opened, to the width of the port plus the lap. Hence the radius O E (fig. 72) repre- The Slide Valve 93 senting the eccentricity of the eccentric or the half travel of the valve = width of port + lap. Let the piston be situated at the beginning of the stroke (fig- 73); then, to admit steam to the cylinder, the valve must be moved forward from its middle position a, past the edge of the port b, until it has opened the port by a distance equal to the lead required, namely, b c. To accomplish this, the centre of the eccentric E must be moved forward to some position E', making an angle C O E' with the crank greater than a right _ Ci Fig. 72. ■i—i4--\ \ y Fig, 73. angle. To find this position : From the centre O on the centre line C D set off O « equal to a c — that is, equal to the lap plus the lead — and from n raise a perpendicular n E' to cut the circular path of the eccentric centre. Then E' is the position required, and O E' produced is the centre line of the eccentric (see also fig. 74). But the piston is assumed at the end of its stroke, therefore O C is the position of the crank, and we now have the relative positions of the crank and eccentric centre lines. 94 Steam The angle E O E' which the centre line O E' of the eccentric is moved through beyond 90° ahead of the crank is called the angular advance of the eccentric (see also figs. 74 and 79). Example. — The width of a steam port is I in. , the lap of the valve \ in., and lead ^ in. Find the eccentricity of the eccentric, and the angular advance of the eccentric. Eccentricity of eccentric = half travel of valve Half travel of valve = lap + port opening. = J+i in- = li in. Therefore, from centre 0, with radius O E= ij in., draw a circle represent- ing the path of the centre of the eccentric. (Fig. 74 half size. ) Let O C be the position of the crank, and _£' draw oE at right angles to Co. On C produced make a — \'vcv. and ab = \ in., and from b draw b E' perpendicular to C to cut the path of the eccentric centre in E'. Join O E'. E O E' is the angular advance of the eccentric. The action of the valve on the face of the ports may be easily follow^ed by drawing the ports, and marking off the "^' '■*■ valve on the edge of a piece of paper, and moving the valve on the ports as required. The effect of the addition of outside lap is : (i) To cut off the steam at some earlier point of the stroke ; (2) To require the eccentric to be moved forward on the shaft, which results also in an earlier opening of the exhaust port. The effect of the addition of inside lap is : (i) To close the exhaust port at an earlier point in the stroke, producing compression of the steam at the back of the piston; (2) To delay the opening to exhaust. To set a slide valve. — Put the crank alternately on its two dead centres. Measure the opening of the port to steam allowed by the valve at each end of the stroke. When these Piston Valve. 95 are equal to the lead allowed in each case, the valve is correctly set. The travel of a slide valve from end to end of its stroke is equal to twice the distance moved by it on each side of its middle position= 2 x (outside lap + maximum opening of steam port). Example. — Find the travel of a valve having \ in. outside lap, maximum opening of steam port \\ in. 2x(i+ii) = 3rins. Piston valves. — Fig. 75 illustrates the type of slide valve known as the piston valve, so called because it consists of two pistons, each work- ing in a short barrel, in which an opening extending right round the barrel acts as the steam port. The chief advantage of the piston valve is that it is in A equilibrium, there being no pressure of the valve against the cylinder face, as with the common flat or locomotive type of valve. It is therefore much used for the high- pressure cylinder of triple expansion engines. The pistons are each fitted with spring rings. In the example given in fig. 75, which is taken from a drawing kindly sup- plied to the author by Messrs. Bow, MacLachlan & Co., of Paisley, the piston rings are of phosphor bronze. There are two rings in each groove made eccentric, and one inside the other, as shown in the section A A. The rings are prevented from catching in the ports by diagonal bars across the ports, as Section at A A. Fig. 75. 96 Steam shown also in the section. The face of each piston is the same as the length of the face of the common valve, the inside and outside lap being also the same. The steam is admitted at the two ends of the valve, and exhausts into the space between the two pistons, and thence to the next cylinder. Double-ported slide valve. — For large cylinders the travel of the valve, in order to open the port to supply sufficient steam, would necessarily be large. To reduce the travel and thereby also to reduce the work to be done by the eccentric in moving the valve, the double-ported slide valve is used as shown in fig. 76. The steam passage C of the cylinder terminates in two ports instead of one, and the steam ports are each made one-half the width of a single port, and therefore the travel of the double-ported valve is only half that of the common valve. Fig. 76. The valve is so constructed that, when in the middle of its stroke, each of the four steam ports is covered by it, the inside and outside lap in each case being the same as with the simple valve having the same travel, and hence its action in the dis- tribution of the steam is exactly the same as with the simple valve. The arrangement is equivalent to two separate slide valves, the steam being supplied to the inner portion of the valve by steam passages in the sides of the valve. B B are the exhaust passages. In large engines with a single flat slide valve, the pressure of the steam on the back of the valve would be so excessive, unless reduced by some means, that the load thrown on the eccentrics and working parts of the valve gear, owing to the friction between the valve and cylinder faces, would be enormous. To prevent this a packing ring is often fitted to Eccentrics 97 the back of the valve, as shown in the drawing (fig. 76). The ring is fitted in a circular groove in the slide jacket cover E, and it is tightened against a planed surface on the back of the valves by set screws a a pressing against a spring or packing. The set screws permit of a careful adjustment of the ring, so as to work steam tight without being excessively tight. By this arrangement the steam pressure is removed from a large portion of the back of the valve, and the enclosed space is connected instead by a pipe with the condenser. Eccentrics Eccentrics are used when a very small to-and-fro motion is required to be derived from a revolving shaft. They are applied mostly to drive steam-engine slide valves, or pumps having a short stroke. The simplest form of eccentric is a circular solid disc called a sheave, secured to and revolving with the shaft, the centre of the disc being ' out of centre ' or ' eccentric ' with the centre of the shaft. This arrangement is equivalent to a small crank (fig 77), the length of whose arm r is the same as the distance c e between the centre of sheave and centre of shaft. This length ir^is called the eccen- tricity of the eccen- tric. The travel of the valve is equal to twice the eccentricity of the . .^ . eccentric. The \ /^^^^ / [ sheave is sur- jy/MO^/a, rounded by a thin metal hoop, or bands (fig. 78), called the strap, to which the eccentric rod is attached. The ro- tation of the sheave H about the centre of the shaft is trans- mitted through the strap and rod, and results in the to-and-fro motion of the valve. The sheave may be considered as a very large crank pin, and the eccentric rod and strap as an ordinary H X Fig. 77. 98 Steam connecting rod. The sheave rotates within the strap just as the crank pin rotates within the head of the connecting rod. In order to get the eccentric in its place on the shaft it is mostly necessary to make the sheave in halves. Thehalvesare secured together by two bolts, not shown, which are passed through holes drilled in the sheave and se- cured by split cotters. The strap is also made in halves, each half having lugstotakethe bolts which secure them together. A small oil cup L is cast solid with the strap. The sheave is secured to the shaft by a key fitting in a key- way cut in the shaft and in the sheave. o o K Fig. 78. Fig. 79. The Link Motion 99 The method of fixing the eccentric on the crank shaft so that it may have the correct angular advance relatively to the crank is shown in fig. 79, in which the letters of reference O /' and E' correspond with the same letters in fig. 74. O E' is the centre line of the eccentric sheave, which has been found as explained on p. 94, and, the key way having been marked on the shaft in the correct position, it is cut out to receive the rectangular key which secures the sheave to the shaft. Reversing Gear — The Link Motion Not the least important quality possessed by the steam engine is the ease with which it lends itself to the most perfect control. For by the movement of a handle the massive engines of a steamship running ai a high speed may be instantly stopped and as quickly reversed. The following simple diagram will explain the principle of reversing gears. It has been shown that when the crank is in some poeition Fig. 81. O C (fig. 80), the centre line of the eccentric will be in a direc- tion O E ahead of the crank, the direction of rotation being shown by the arrow. But suppose we wished to reverse the engine — in other words, to change the direction of rotation, as in fig. 81 — then, unless we have some means of shifting the eccentric from E to E', the engine will not reverse, but will only rotate one way. This difficulty is easily overcome by the link motion, which H 2 lOO Steam is one of the most common methods of reversing, and it is done in the following way : Two eccentrics are used, one having its centre at E, and the other at E' (fig. 8i), and by means of the link (fig. 82) we have the power to use which ec- centric we please, and to throw the other out of gear ; hence the engine can be made to rotate in either direction with the greatest ease. Each ec- centric is attached by a rod to one end of the slotted bar or link shown in fig. 82, and the link is moved transversely by the levers so as to bring the slide valve under the influence of either eccen- tric as required. The slide valve is attached by a rod to a little block which fits in the slot of the link, so that any movement of the hnk in the direction of the axis of the valve rod affects the position of the valve. When the block is in the middle of the link, the valve is influenced equally by both eccentrics, with the result that the engine will not run in either direction. The nearer the block is to its mid-position in the slot, the less is the travel of the valve and the earlier the steam is cut off in the cylinder. The link motion is therefore useful, not only for reversing, but as an arrangement for working the steam expansively in the cylinder by varying the point of cut-off. Fig. 82. S V, slide valve ; L, link S V R, slide valve rod . R L, reversing lever ; S H, starting handle ; W S, weigh shaft ; E, eccentric ; E R, eccentric rod. CHAPTER XI CORLISS VALVE GEAR The Corliss valve gear for regulating the distribution of the steam in engine cylinders was the invention of George H. Corliss, an American engineer. This valve gear consists of four separate valves, A, B, C, D (Fig. 83), one valve at each end for steam admission, namely A and B, and one valve at each end for exhaust, namely C and D. S is the steam-admission pipe, and E the exhaust Fig. 83. pipe. In horizontal cylinders the steam-admission valves are at the top corners, and the exhaust valves are at the bottom I02 Steam corners, thus permitting of easy drainage of the cylinders through the exhaust valves. This system of valves reduces clearance volume in the cylinders, and permits of a wide opening of the port during steam admission with a sudden cut-off. It also lends itself to easy regulation of the cut-off by a governor gear even for engines of the largest power, because the power required from the governor is only that necessary to trip out the gear. Corliss Valve Gear 103 - ^ 1 104 Steam Fig 84 shows the general arrangement of a Corliss engine, with a single eccentric for both admission and exhaust valves. It will be seen that motion is given to the valve gear in the first place from an eccentric on the crank shaft, which is con- nected by its rod to a vertical rocker-arm, Q R S. Attached to the rocker-arm is the hook rod or lever F S which drives the v/rist plate W, and causes it to oscillate about its centre of motion. Fig. 85 shows a part general arrangement of a Corliss engine with the Reynolds-Corliss type of gear. This gear is driven as before through a wrist-plate W by means of an eccentric and eccentric rod. Attached to the wrist plate W are four valve rods, two marked A, A attached to the upper or steam-admission valves S, S, and two marked B, B to the two lower or exhaust valves X, X. The exhaust-valve rods are connected directly to the exhaust-valve levers, but the admission-valve rods are connected to a curved bell-crank lever C, which works freely on a boss of the valve-spindle bracket, and has a movement of its own independent of the valve spindle. Figs. 86 and 87 show the details to a larger scale. For the purpose of engaging in gear and tripping out of gear the steam-admission valve, a " crab-claw " catch is carried on the pin D of the bell-crank lever C. This catch or fork, which is made in one piece, consists of two prongs, E and F. The prong E is used to catch the square block N on the ad- mission-valve lever G. (This lever stands in front of the gear, and is shown dotted to prevent confusion of lines.) When the block N and the fork E are engaged (Fig. 86), then both the "crab-claw" and valve spindle will be raised by the movement of the bell-crank lever C about the centre O ; the valve has a twisting movement on its own axis through O, and the steam port is opened for admission of steam. The prong F is used to trip the prong E out of gear, and to liberate the valve- spindle lever G, which suddenly drops to its lowest position by the pull of the dash-pot lever H, and cuts off the steam supply. Disengagement of the prong E is effected by means of a stop K, which is fixed on the boss of the lever L (Fig. 87). Corliss Valve Gear 105 io6 Steam The lever L rides loose on the valve spindle, and is dependent only upon the governor for its movement, the governor being connected with the lever L through the rod M. If the speed of the engine increases above the rated speed, the governor rises, and the lever M moves the lever L so that the disengagement stop K moves downwards and, coming into contact with the prong F earlier, trips out the lever E and cuts off the steam earlier. Fig. 86 shows the valve spindle lever G in its lowest position just engaged by the crab-claw, and ready to be raised so as to open the steam port. Fig. 87 shows the crab-claw in its top position, the prong E having just been disengaged by the stop K acting on the prong F. The valve- spindle lever block N has just been liberated, and is now in the act of suddenly descending by the pull of the dash-pot lever. The working faces of the trip-gear are made more durable by letting in pieces of hardened steel (shown black in the figure). The prong E of the crab-claw is kept close to the face of the square block N by the action of the spring P, except when pressed away by the stop K. The dash pot Q (fig. 85) secures a sudden descent of the valve lever by the formation of a vacuum under the dash-pot piston when it is raised, the pressure of the atmosphere forcing down the piston suddenly when the catch is liberated. I07 CHAPTER XII CRANKS AND CRANK SHAFTS Cranks are used to convert the reciprocating motion of the piston into circular motion. Fig. 88 shows two views of a simple overhanging crank. This crank consists of an arm with a boss at each end — one to take the main shaft, and the other the crank pin. The crank is secured firmly in its place on the shaft, either by keying alone, or by ' shrinking ' and key- f^ ing. The shrinking is done by boring out the hole a shade smaller than the shaft, then heating the crank round the hole, and thus causing the material to expand and the hole to become larger. The crank is then slipped in its place on the shaft, and on cooling' it contracts and grips the shaft tightly. Forc- ing on by hydraulic pressure is now frequently adopted in preference to shrinking on. The crank pin is shrunk in position or forced in by hydraulic pressure, and riveted over the end as shown. The radius of the crank arm is measured from the centre of the shaft to the centre of the crank pin. The throw of the crank is equal to the diameter of the crank-pin path, and to the stroke of the piston. The following is a crank axle for a locomotive with inside Fig. 88. A=crank shaft ; C=crank pin ; B - web : D, D'=bosses ; E=key. io8 Steam cylinders, showing the cranks at right angles. The webs are here shown strengthened by wrought-iron straps shrunk on. la^jM I Fig. 89- With such a shaft the engines will start in any position ; for, if one crank is on its ' dead centre,' the other is in the best possible position for starting. Examples of crank shafts are also given in figs. 42, 126, &c. Tangential Pressure on the Crank Pin The tangential pressure on the crank pin is that share of the total pressure on the pin which tends to turn it about the centre of the shaft. To present this subject in the simplest form we will suppose the pressure of the steam on the oiston uniform throughout the stroke, the cormecting rod to be length, or, words, to parallel to of infinite in other act always the centre line of the engme, and the moving parts to be without weight. In fig. 90, ABC Fig. jo. represents the path of the crank pin. Let P =the uniform pressure on the piston, and let O A the radius of the circle be chosen equal to P to any scale. When the Pressure on Crank Pin 109 crank is in the position O A, the pressure P acts towards the centre only, and there is no tendency to turn the crank about the centre, but only to press the shaft against the bearing ; hence in this position the tangential pressure is nothing. The same is true of the position O C, and O A and O C are termed the ' dead centres.' At the position O B of the crank at right angles to the direction of the force, the whole of the force is expended in turning the pin about O, and the tangential pres- sure on the pin is therefore equal to P. Between these two points A and B the tangential pressure varies from nothing at A to a maximum at B, and again falls to nothing at C. To find the tangential pressure at an intermediate point, as E, the force P may be resolved into two forces, one acting towards the centre of the shaft, and one perpendicular to it, or tangential to the crank-pin path at E. Thus at E draw the line E E' parallel to A C and equal to the force P to scale, and by the parallelogram of forces resolve it into two forces, T E tangentially and R E radially. Then the line T E measures to scale the share of the force P acting at E, tending to turn the pin about the centre of the shaft, and R E measures also the share of force pressing the crank on the bearing. But when OE=EE', the perpendicular E F is equal to T E for any position of E. Therefore for any point on the crank-pin path, under the condi- tions above described, a perpendicular let fall from it upon the diameter A C represents the tangential pressure on the pin at that point. The diagram (fig. 91) further illustrates the variable nature of the turning effort on the crank pin, the amount of the turn- ing effort at points i, 2, &c. being proportional to the perpendiculars i a}, 2 «^, &c., and varying from nothing at A, I a' at I and so on, to a maximum at B, and again gradually falling to nothing at C. Further, it will be seen that this variation of twisting stress in the crank shaft occurs twice in every revolution of the crank; no Steam also that by increasing the pressure on the crank pin, either by increasing the area of the piston or the pressure of steam, the variation in the twisting stress is also increased in the same proportion. If a pair of engines of equal power work on to one crank shaft, and the cranks are placed at angles of o° or i8o° with each other — that is, with the cranks together or exactly opposite — the twisting stresses on the shaft will be double those pro- duced by the single engine alone, also the maximum and mini- mum twisting stresses on each crank will occur at the same time. But if the cranks be placed at right angles with each other, then, with the same engines, the maximum stress due to one engine will occur at the same time as the minimum stress due to the other engine, so that the total maximum stress will be reduced, and there will also be a much more uniform distribution of the stresses in the shaft. This will be more clearly seen by re- ferring to the following figures. Fig. 92 is a continuous diagram of the turning effort on the crank pin for a single engine, the value of which at any point a', aP', &c., is given by the vertical ordinate a' ^', a^ c^, and so on, varying from nothing at c° to a maximum a^ c^ at a^. Fig. 93 shows the effect of the addition of another engine of equal power to the same shaft, when the cranks are at 0° or 180° apart. In this case the stresses are doubled throughout, varying from nothing to a maximum a' b'^, which is twice «' , injection water, air and vapour, are pumped into the hot-well H W, and thence to waste ; and from the hot-well the water is taken to feed the boiler. The suction valve of the air pump is called the foot valvi-, and the delivery valve is called the head valve. "-^QT "AST Fig. Fig. 102 is a more complete drawing of an air pump as applied to vertical marine engines. Another form of jet condenser and air pump used for horizontal en- gines, as made by Messrs. Tangye, of Birmingham, is shown in the diagram, fig. 1 03, the air-pump rod being an extension of the piston rod. The exhaust steam enters the condensing chamber C, where it meets with the cold-water spray J and is condensed. The condensed steam and injection water are removed from this chamber by the air pump A P, which draws it through the suction valve F V, and forces it forward through the deliver)' valve H V into the hot-well H W, from which the boiler feed may be taken. The remainder overflows. The surface condenser has now entirely superseded the jet 1 1 8 Steam condenser for marine engines as tlie natural consequence of the endeavour of marine engineers to increase the economy of their engines. In order to do this it was found necessary to increase the pressure of the steam used in the marine boiler, which up to i860 was only about 30 lbs. on the square inch. Up to this time the boiler feed, which was from the hot-well of the jet condenser, was practically as salt as sea water, owing to the fact that the spray of the jet condenser was a sea water injec- tion, the sea water and the condensed steam being in the pro- portion of about 30 to I. Even with the low boiler pressures the salt in the feed water was a serious drawback, for sea water contains ^V of its weight of solid matter dissolved in it, and, when evaporated, the solids are deposited on the boiler plates, forming a more or less thick solid incrustation. This incrustation is a bad conductor of heat, and, further, since it keeps the water from contact with the hot furnace plate, there was great danger of the plate getting red hot and the top of the furnace collapsing. To prevent the water in the boiler from becoming too much satu- rated with salt, it was necessary to ' blow off' a portion of the water from time to time, and to supply its place with a fresh supply of ordinary sea water. By thus blowing away to waste large quantities of hot water, a considerable waste of heat was evidently the result. But when the attempt was made to increase the pressure and temperature of the steam — now made possible by the introduction of steel plates for boiler construction — the difficulty arising from the presence of salt in the feed water became more serious, for with higher temperatures the solid matter is de posited much more readily, and its effects are far more mis- chievous. Hence the introduction of the surface condenser, which does away with the necessity of feeding the boiler with salt water ; the condensed steam itself being pumped back again to the boiler as a fresh-water feed. For the steam is here condensed, not by being mixed with large volumes of cold water, but by coming mto contact with cold metallic surfaces. The general arrangement of a surface condenser is shown in fig. 104. The cold metallic surface required, by which to condense Condensers 119 the steam, is provided by means of a large number of thin tubes, through which a current of cold water is circulated. This arrangement supplies a large cooling surface within compara- tively small limits of space. The tubes are made to pass right through the condensing chamber, and so as to have no connection with its internal space. The steam is passed into the condenser and there comes in contact with the cold external surface of the tube, and is condensed, and removed, as before, by the air pump. The condenser may be made of any convenient shape. It sometimes forms part of the casting supporting the cylinders of yVaUr from Ct^uxiaX\n4 Pump Fig. 104. vertical engines ; it is also frequently made cylindrical with flat ends, as in fig. 1 04. The ends form the tube plates to which the tubes are secured. The tubes are, of course, open at the ends, and a space is left between the tube plate and the outer covers, shown at each end of the condenser, to allow of the circulation of the water as shown by the arrows. The cold water, which is forced through by a circulating pump, enters at the bottom, and is compelled to pass forward through the lower set of tubes by a horizontal dividing plate ; it then returns through the upper rows of tubes and passes out at the overflow ; the tubes are thus maintained at a low temperature. The steam enters at the top of the condenser and fills the I20 Steam space surrounding the tubes. The tubes are made of brass, \ or % inch outside diameter, and ^V '"^h thick ; and, being thin and of good conducting material, the steam is readily condensed against the cold outer surface of the tube. CTP Fig, 105. C T P = condenser tube plate. The diagrams figs. 105 and 106 show two methods of con- necting the tubes to the tube plates so as to make them tight. Fig. 105 shows a little stuffing box and screwed gland, which is very generally used. W F ^^^^^^^^^^ 'Y\\Q stuffing box packed with tape cord packing. Fig. 106 is a wood ferrule W F made to fit the tube exactly, but a little too large for the hole. It is driven in between the tube and the hole in the tube moisture and swells, IS or Fig. jo6. plate. When in its place it absorbs forming a perfectly tight connection. Kg. 107 shows an enlarged view of a disc valve as used for air pumps. It consists of a grating covered by a circular disc of india-rubber, or, as in the figure, by a thin flat plate of phosphor-bronze (Coe & Kinghorn's patent). The water lifts the valve against the saucer-shaped guard, and passes through The Vacuum Gauge 121 the grating. When the water attempts to return, the valve closes down upon the grating and prevents it. The Vacuum Gauge is used to determine the de- gree of vacuum in the con- denser. It is graduated on the face from o to 30, and ^"'- '°'' the degree of vacuum is indicated by a movable index-finger which passes over the graduated scale. The construction of the gauge is similar in principle to the Bourdon's pressure gauge described under Boiler Fittings. In order to be clear as to the meaning of the figures on the face of the vacuum gauge, it should be remembered that the object of the condenser is to remove as far as possible the pressure of the atmosphere from the back of the piston, and that the gauge is intended to r Ik 20- 15- 10- S - 'jtlSSiX.ft show how far we are success- ful in doing this. The ordi- nary boiler pressure gauge indicates the pressure in the boiler above the atmosphere. If there were a partial vacuum in the boiler before the fires are lighted, the pressure gauge would not show it, and it only CUwosftA^flc begins to indicate pressure when the pressure of the . steam rises above the pres- sure of the atmosphere, this being the starting point or zero. The vacuum gauge also starts from the same zero, namely, the pressure of the atmosphere, and reckons backwards. But, further, the figures on the vacuum gauge are doubled, and to understand the reason of this it should be remembered that they represent not pounds pressure but inches of mercury by the barometer, every two inches of mercury being equivalent 10 5 -- /o 20 ■ /S-i-30 Fig. 108. 122 Steam approximately to i lb. pressu-e, the old original vacuum gauge being constructed like a barometer. Hence, when the vacuum gauge indicates 25, it means that the difference between the pressure of the atmosphere and the pressure in the condenser is equivalent to the weight of a column of mercury 25 inches high, which is equal to 25-^2 = 12^ lbs. ; that is, 25 by the vacuum gauge means that the pressure in the condenser is 12^ lbs. below the pressure of the atmosphere, or 15 — 12! = 2i lbs. absolute pressure opposing the piston, instead of 15 lbs. which would be the approximate back pressure due to the atmosphere if there were no condenser. The gain in horse- power by using the condenser may be calculated by the usual formula, H P= — ^ — where P is the gain of pressure by 33,000 using a condenser, namely, in the present case, 12^ lbs. Utile. — To convert the reading of the vacuum gauge into pounds per square inch of pressure measured from absolute zero : Take reading of vacuum gauge, subtract from 30, and divide by 2. Pumps The feed pump is used to feed the Doiler, and it is required to supply a quantity of water at least equal to that evaporated and passed forward to the engine, together with leakage at safety valve, &c. ; but to provide also for emergencies it is usually made capable of supplying from 2 to 2^ times this quantity. The feed pump is sometimes worked from the engme direct, or from the shaft by an eccentric attached to the plunger (see iig. 125). "When it is worked independently of the main engine it is called a 'donkey pump.' The following diagram, fig. 109, illustrates the construction of a simple feed pump. It consists essentially of a plunger P of a suction valve S and a delivery valve D. The same construction may be used for the bilge pump, which pumps water that accumulates in the bilge or bottom of the ship. The action of the pump may be explained as follows : Suppose the plunger P at the bottom of its stroke, and the Pumps 123 whole interior of the pump to be full of air. When the plunger is drawn outwards the pressure on the suction valve S will be Fig. 109. — Feed pump (Messrs. Ernest Scott and Mountain). reduced, and the air in the supply pipe will lift the valves and flow into the barrel. The pressure of the air in the supply pipe is now less than before, and accordingly the pressure of the atmosphere on the external surface of the water forces water 1 24 Steam up the pipe to such a height as to make the pressure inside the pipe balance the pressure outside. When the plunger returns the suction valve is closed by the pressure, and the air is forced out through the delivery valve D. Each time the stroke of the plunger is repeated, the water will rise in the supply pipe until at last it reaches and fills the barrel. Now, when the plunger returns, it forces water instead of air through the delivery valve. The height of the column of water which will balance the pressure of the atmosphere is 34 ft. ; that is, a column whose weight is about 15 lbs. per sq. inch. In practice, however, the supply can never be drawn from a depth greater than about 25 ft. The valves are prevented from rising above a certain height by the springs shown by dots in the figure. The lift of a valve should not exceed one-fourth of its diameter, for with this lift the whole of the water which passes through the valve seating can escape freely round the edge of the valve. Any further lift is therefore useless. Thus, when the area of opening round edge of valve and the area of the valve are equal, we have area round edge = area of valve ; dia. X 3"i4i6 X lift = dia.^ x 07854; dia. lift = 4 Large valves are prevented from lifting so much as this, because of the excessive knocking which would result. Air vessels A V are chambers fitted to pumps close to and beyond the delivery valve, fig. 109. The air in the water col- lects in this vessel and forms a cushion or spring which enables the water to be delivered in a continuous stream instead of intermittently. The capacity of a pump in cubic inches = area of end of plunger X length of stroke in inches. The weight of a cubic foot of fresh water = 62 '5 lbs., or rooo ounces. The weight of a cubic foot of salt water = 64 lbs. [ lb. of water occupies o'oi6 cub. ft. r gallon of water = ro lbs. 2 "3 feet of water head = r lb. pressure per sq. inch. 125 CHAPTER XIV GOVERNORS A GOVERNOR is fitted to an engine for the purpose of securing, as far as possible, a uniform rate of rotation, and preventing variation of the speed at every fluctuation in the load or the boiler pressure. None of the governors applied to steam engines are able to ac- complish this result perfectly ; for, being themselves driven by the engine, they cannot begin to act until a change of velocity has first occurred. In practice, however, the governor is an invaluable adjunct to the steam engine ; for, when any change of velocity does take place, the governor instantly acts and prevents anything more than a small alteration of speed. Any permanent adjustment of the speed is regulated by hand at the steam supply. The following is a description of the Watt Pendulum Governor. The study of this governor will serve to introduce the student to those principles of con- struction upon which this and most other governors are based. Fig. 1 26 Steam The central spindle S of the governor, fig. no, is made to rotate by means of a belt, or, better, by a small shaft driven from the engine shaft by bevel wheels communicating with the bevel wheels at the bottom of the spindle. The spindle, arms, and balls then all rotate together, and at the normal velocity of the engine the inclination of the arms is about 30° with the vertical. If the velocity of the engine increase, due to removal of load, the balls and arms open out from the spindle, and in doing so they lift the sleeve E, which slides up and down on the spindle. This movement is communicated by levers moving about the fixed fulcrum C, to the throttle valve, by which the passage for the supply of steam to the engine is contracted ; or to an expansion gear, which is also an arrangement for reducing the steam supply, and the increasing speed of the engine is thereby checked. A slot is cut in the central spindle through which a cotter or pin secured to the sliding sleeve passes. The length of this slot limits the travel of the sleeve. There are three forces acting on the governor balls during rotation, namely : the iveight of the ball which acts vertically downwards, the centrifugal force which acts horizontally out- wards, and the tension in the arm ; and these three forces are in equilibrium and are represented proportionally by the three Governors 127 sides A C, A B, and B C (fig. in), which are respectively parallel to the forces. The vertical distance C A is called the height oi the governor or the height of the cone of revolution, and this height is constant for a given number of revolutions pet minute. The revolutions of the governor obey the same law as the oscillations of the pendulum, namely : the number of revolu- tions is inversely proportional to the square root of the height of the cone of revolution. Thus, any change in the speed of the engine causes the governor balls to fly off from the centre, and a change in the ci Fic. height of the governor to take place, as from C A to C a, fig. 1 11, It is the raising of the sleeve A to « by which the governor is made to influence the throttle valve or expansion gear ; but, in order to close the throttle valve, it requires to be driven at an increased speed, and this is precisely what the governor is intended to check. Such a governor, therefore, evidently permits of a variation in the number of its revolutions, and, therefore, also of the revolutions of the engine, between the limits due to the vary- ing height C A of the cone of revolution. But a perfect governor would permit of no increase either in the number of 128 Steam its own revolutions or that of the engine ; and, although this ideal cannot be attained, still it is the aim of designers to reduce this variation in the height of the cone as much as possible ; or, in other words, to enable the governor to lift a sufficient distance to close the valve without going through a considerable variation in speed in rising from its lowest to its highest position. The effect of the movement of the balls on the height of the cone when the point of suspension of the arms is on the centre line of the spindle is shown in fig. iii. When, however, the arms are suspended from points E and F (fig. 1 1 2), not on the centre line of the spindle, and the balls rise from D to D', the height of the cone now varies between C B and C B', instead of between C A and C a as before, the effect being to still further increase the amount of variation in height, and, therefore, in revolutions of the engine, for a given lift of the sleeve. The points of suspension E and F should, therefore, be as near the centre of rotation of the spindle as possible. The speed of the governor is independent of the weight ot the balls, but the parts require to be sufficiently heavy to exercise proper control over the throttle valve or expansion gear. Various forms of ' parabolic ' governors have been intro- duced to give the necessary movement of the sleeve without the accompanying necessary increase of velocity. The Watt governor is a slow-speed governor, owing to its height. To run at a higher speed it must be made much smaller, and then it would not be sufficiently powerful to control the supply of steam to the cylinder. But the tendency 0/ engine building has long been towards higher speeds, and for quick-running engines a AVatt governor geared so as to run slower than the engine is not sufficiently sensitive. This governor is, therefoie, now largely superseded by various forms of high-speed governors, of which the 'Porter' governor, illustrated by fig. 113, is one of the most common. This governor consists of two small balls with arms as before, but the lower links are jointed direct to the balls by means of a pin through the centre, their Governors 729 fiC. 1,3, ijo Steam lower ends being connected with the sliding sleeve. Resting on the sleeve, and free to slide up and down the central spindle with it, is a weight W. This weight prevents any move- ment of the sleeve until the speed of the balls is such that their centrifugal force is sufficient to lift it. The governor has then the control of the engine. The heavier the central weight, and the smaller the balls, the higher the speed and the more sensi- tive the governor. The form of governor illustrated in fig. 113 is Tyrrel and Deed's Patent, made by Messrs Clayton and Shuttleworth of Lincoln. The special feature of this governor is the dash-pot put into the dead weight. The object of the dash-pot is to give steadiness to the governor. The form of valve adopted when the governor is used for throttling the steam — that is, contracting the opening for supply — is the double beat equilibrium disc valve, illustrated in fig. 175. In fig. 113 the governor is shown having an arrangement for regulating the travel of a cut-off valve on the back of the slide valve, instead of being connected with a throttle valve. The eccentric rod causes the link shown in the figure to oscil- late about the upper fixed centre. The valve rod is attached to a sliding block in the link. When the speed increases sufficiently to cause the rotating balls to lift the weight and sliding sleeve, the end of the valve rod is raised in the link, and the travel is reduced, thereby cutting off the steam at an earlier point in the stroke. Fly-vitheels The importance of a uniform velocity of the engine has been already pointed out. But the turning effort on the crank pin, as we have seen, varies very considerably during each revolution ; there is, there- fore, a constant tendency to fluctuation of speed. In order to counteract this tendency the fly-wheel is added to stationary engines. The driving wheels answer the same purpose in locomotives. When the turning effort on the crank pin during a portion of the revolution is greater than the resistance due to the load. The Locomotive 131 the speed of the engine is increased ; and, conversely, when the resistance is greater than the turning effort, the speed of the engine is retarded. The fly-wheel, owing to its great mass and to the distance of the mass from the centre of the shaft, resists very effectually all tendencies to changes of speed. For excess of turning effort, instead of causing an immediate and excessive change of velo- city, is absorbed in giving a relatively small additional velocity to the mass of the rim of the wheel, and the power thus absorbed is restored when the turning effort falls below the resistance, thus maintaining a practically uniform velocity of the crank pin. Locomotive Engine The figures on pp. 132 and 133 illustrate the general con- struction and arrangement of an express passenger locomotive engine. The references to the parts are given below the figure. It is necessary that the locomotive shall be self-contained — that is, it must consist of a boiler and an engine, and the whole machine must be placed upon one carriage. The problem for locomotive engineers is how to obtain the greatest possible power for the least possible weight. This is done by working at high steam pressures, using small boilers of great strength, and of high evaporative efficiency, and using the steam at high pressure in small cylinders in order to obtain a large amount of power with a comparatively light engine, economy in the use of steam being sacrificed in order to keep down the weight. The engine and boiler are each bolted independently to the frame of the carriage. The frame is self-contained, and through it the whole of the stresses due to the pressure on the pistons, and the pull on the draw-bar due to the load, are transmitted. The frame is carried on wheels, one arrangement of which is shown in the figure. It will be noticed that the axle of the trailing wheels is placed just behind the boiler, the axle of the driving wheels just in front of the fire box, leaving clearance for the cranks and connecting-rod heads, and the axles of the bogie (or small K 2 132 S a The Locomotive 133 auxiliary carriage which works on a pivot beneath the cylinders) are placed in front of and behind the cylinders. The bogie wheels guide the engine, and prepare the rail to receive the weight of the large driving wheels ; the hind or trailing wheels steady the engine, while the driving wheels transmit the power of the engine to the rail, and they are placed as nearly as possible under the centre of gravity of the whole. The locomotive boiler is described in detail under the heading of Boilers. The locomotive engine is similar in principle to that already described on p. 70, with the addition of the link motion for reversing. The common arrangement is to have two cylinders of equal diameters, both using steam direct from the boiler, and exhausting independently into the chim- ney through the exhaust or ' blast ' pipe, the cylinders having the several working parts of a complete engine, thereby form- ing a pair of engines acting on one crank shaft with the cranks at right angles. Compound locomotives are running on the lines of one or two English Railway Companies, and are said to give satis- factory results. The principle of the compound engine will be considered in Chapter XVI. The cylinders of locomotives are con- structed of the best, close-grained, hard and strong cold blast cast iron ; the pistons are made of good tough cast iron ; the piston-rods are best cast steel, tapered al the ends and secured to the piston by a gun-metal nut with a taper steel pin through the nut. 134 Steam The valve spindles are of best Yorkshire iron, working through gun-metal bushes and glands in the steam-chest. The crossheads are of the bestYorkshire iron, case-hardened ; the sleeves are of the best hard cast iron. The gudgeon pins are of wrought iron, case-hardened. The guide bars are of the best mild crucible cast steel. The eccentric sheaves are in two parts; the smaller being of Yorkshire iron, and the larger of hard cast iron ; the eccentric straps are of good tough cast iron ; the eccentric rods are of Yorkshire iron, and the working parts and pins are case- hardened. The connecting and coupling rods are of Yorkshire iron ; all cotters and bolts of mild steel. The crank pins are of Yorkshire iron, case-hardened. The following particulars of a compound locomotive goods engine were given in a paper read before the Institution of Mechanical Engineers by Mr. R. H. Lapage : — Cylinder, diameter Ratio of piston areas . Length of stroke Length of connecting rod Throw of eccentrics . Angle of advance, forward gear ,) >, back gear Travel of valve, full forward gear ,, full back gear . Lap of valve .... Steam ports .... Cut-off, ordinary running . Pressure of steam in boiler, 175 lbs. per sq. in. above the atmosphere. Exercise i. — Find the area of the steam ports in each of the above cylinders, and express the ratio of steam port area and piston area in the two cases. Ans. H.P. cylinder 1:11-5 or 87 per cent. L.P. cylinder i : 15 or 6-65 per cent. Exercise 2. — The coal consumed in a compound locomotive was 79 cwts. in a run of 300 miles. The water used was 7546 gallons. Find the eva- poration per lb. of coal. 7546 X 10 High pressure. Low pressure. 16 ins. 23 ins. I 2-1 24 ins. 24 ins. . 6 ft. 6 ft. . 6| ins. 6J ins. ■ • 4° 4° • ■ 14° 14° . 3^ ins. 3i ins. • • 3eins. 3| ins. I in. I in. . iixi4 ins. If X 17 ins. . 40 per cent. 50 per cent. Ans. •Jlj-X. l\2 8-5 lbs. of water per lb. of coal. 135 CHAPTER XV THE INDICATOR The indicator was originally invented by James Watt, and, although improved in points of detail, the main features of the instrument as devised by him are substantially retained at the present time by makers of indicators. The uses to which the indicator is chiefly applied are — 1. To obtain a diagram from which conclusions may be drawn as to the correctness, or otherwise, of the behaviour of the steam in the cylinder; the promptness of the steam admission ; the loss by fall of pressure between the boiler and the cylinder ; the loss by wiredrawing; the extent and character of the expansion ; the efficiency of the arrangements for exhaust, including the extent of the back pressure ; the amount of compression. 2. To find the mean effective pressure exerted by the steam upon the piston, from which to calculate the indicated horse-power of the engine. 3. To determine whether the valves are set correctly by taking diagrams from each end of the cylinder and observing and comparing the respective positions of the points of admission, cut-off, release, and compression. Description of the Itidicator. — The instrument, of which there are several different types, consists essentially of a small steam-cylinder, containing a piston and spring, to regulate the movement of the piston according to the pressure of the steam ; a pencil, carried by a system of light levers, constituting a parallel motion, by which the pencil reproduces the vertical movement of the indicator piston, but magnified four or fiva 136 Steam times ; and a drum, to which a paper, or " card," is attached, and which receives a backward and forward rotation on its own axis by a motion derived by a reducing gear from the crosshead or other suitable portion of the engine. By the combined vertical movement of the pencil, and horizontal movement of the paper, a closed figure is drawn, called the indicator diagram. The enclosed area represents the effective work done by the steam upon the piston ; the upper line of the diagram represents the varying pressure of the steam during the forward or driving stroke of the piston, and the lower line that during the backward or exhaust stroke. The diagram traced by the indicator pencil differs more or less considerably from the theoretical diagrams already considered ; but the actual diagram is usually considered the more perfect as it approaches the more closely to the theo- retical diagram. Fig. 116 illustrates the construction of the Tabor indi- cator, which consists of a steam-cylinder, A, containing a piston B and spring C. The spring is secured to the piston at one end and to the cover D at the other end, and the pressure of the steam which enters the indicator cylinder through the opening E compresses the spring by an amount depending on the pressure. The movement of the piston is transferred to the paper on the drum F, and multiplied five times by means of the arrangement of levers shown. The most noticeable feature of this indicator is the means employed to secure a straight-line movement of the pencil. A plate G containing a curved slot is fixed in an upright position, and a small roller, fixed to the pencil lever, is fitted so as to roll freely in the slot. The curve of the slot is so formed that it exactly neutralizes the tendency which the pencil has of describing a circular arc in the opposite direction, and the path of the pencil is a straight line when the drum is not in motion. The pencil movement consists of three pieces — the pencil-bar H, the back link K, and the piston-rod link L. The two links K and L are parallel to each other in all positions. The lower pivots of these links and the pencil-point are always in a straight line. The paper drum is attached by a cord S to a suitable reducing motion from the engine ; the cord pulls the drum round on its The Indicator 137 own axis with a motion corresponding to that of the engine piston, and the return movement of the drum is obtained by the internal coiled spring M. •iiiiiiiiiiiirTi >iiiiiii>i>iim f Fig ii6. The Indicator Diagram. — Fig. 117 is an example of a common form of diagram from a single cylinder non-con- J ^\^^ ^r^n-\Y^a vnrinivirr \-\r\Ac-r rrrtnrl u7r»rVinrr COnditlonS, 138 Steam The admission-line A B shows the rise of pressure of the steam as it enters the cylinder. The steam-line B C shows how nearly the steam pressure in the cylinder reaches that of the boiler. This difference is obtained by measuring the height of B above the atmospheric line X Y with a scale corresponding to the scale of the spring in the indicator, and afterwards drawing a horizontal line above B measured with the same scale from X Y to represent the pressure in the boiler. • There is always a certain fall of pressure between the boiler and the cylinder in Fig. 117. — X Y = atmospheric line ; A B = admission line ; B C = steamline ; C D = expansion line; D E = exhaust line; E F = back-pressure line; F A = com- pression line ; A = point of admission ; C = point of cut-off ; D = point of release ; F = point of compression. consequence of throttling of the steam in the ports and pas- sages, especially at high speeds, or with too long or too small diameter steam-pipes, or with steam-pipes having sharp bends. n = insufficient lead ; f = late exhaust ; jc = late compression. m = excess of lead ; ^ = wiredrawing ; s = early release ; r= early coinpres- FlG. n8. Fig. 118 shows how the full-line diagram may be distorted by various effects, as shown by the dotted lines and explained below the figure. The Indicator 139 Fig. iig. The effect on the steam-line of regulating the engine by a throttle valve, and thus varying the opening for the supply of steam, is shown by fig. 119, which was obtained by suc- cessively removing portions of the load on the engine, and maintaining the speed constant by partially closing the steam-supply valve. The forward pressure-line A for a heavy load fell to B for a medium load, and to C for a light load ; the points of cut-off, release, and compression remaining constant. ThQ point of cut-off C, fig. 117, is a more or less sharp and definite point with trip-gear valves, which cut off suddenly by the action of a strong spring (see figs. 120 and 121) ; but with the slide valve the cut-off is more gradual, the corner is rounder, and the exact point of cut-off is more difficult to locate (see fig. 122). In such a case the point of cut-off may be taken at the point where the concave curve of the expansion line meets the convex curve of the cut-off corner. The effect on the diagram of varying the point of cut-off is shown in fig. 120 for non-condensing engines, and in fig. 121 Fig. 120. Fic. 121. for condensing engines with a trip gear, the cut-off being fairly sharp. Fig. 122 shows the effect of regulating the power by varying the cut-off in a slide-valve high-speed engine. In the non-condensing diagram (fig. 120) with an early cut-off, it is seen that the expansion line falls below the atmospheric line and forms a loop at the end of the diagram. This is due to the pressure of the steam during expansion 140 Steam falling below atmospheric pressure, and hence, when the exhaust port opens, the pressure will rise, instead of fall, to the back-pressure line. This is a most wasteful form of diagram. Fig. 122. The expansion curves of indicator diagrams vary con- siderably, and they do not obey any very definite law. They are, in fact, the resultant effect of a variety of separate causes operating to a different extent in different engines, and even in the same engine by change of conditions. The release point D (fig. 117) occurs just before the end of the stroke. With high-speed engines it is important to have an early exhaust, as the trouble is usually not to get the steam into the cylinder, but to get it out. The exhaust line D E (fig. 117) represents the fall of pressure which occurs in the cylinder when the exhaust port opens. Fig. 118 shows early opening to exhaust at s, and late opening to exhaust at t. A late opening to exhaust, as shown at /, is a very grave defect in a diagram. The back-pressure line E F (fig. 117) shows the amount of the pressure against the piston during its return stroke. In non-condensing engines the back-pressure line coincides the more nearly with the atmospheric line, as the exhaust passages permit of a free exit for the steam ; in condensing engines this line coincides the more nearly with the zero line, as the con- densing water temperature is lower, and as air leaks are absent. The compression curve F A (fig. 117) commences from the point of closure F of the exhaust port. This point depends upon the amount of inside lap on the valve, and the angular advance of the eccentric, and the nature of the curve will depend upon the pressure of the steam trapped, and upon the volume of the clearance space. 141 CHAPTER XVI COMPOUND ENGINES Compound engines are those which have two or more cylinders of successively increasing diameters so arranged that the ex- haust steam from the first and smallest cylinder is passed for- ward to do work in a second, ahd sometimes a third or fourth cylinder, before escaping to the condenser. The compound engine enables the fullest advantage to be taken of the expansive power of high-pressure steam : (i) By reducing the range of temperature in any one cylinder, and thereby reducing initial condensation of the steam in the cylinder. (2) By taking advantage of the re-evaporation which accom- panies cylinder condensation. For, since the bulk of the re- evaporation in a cylinder takes place during exhaust, it is obvious that in a single-cylinder engine the steam formed by re-evaporation during exhaust passes away to the air or the con- denser to waste without serving any useful purpose. But when the steam is exhausted into a second or third cylinder, the steam formed by re-evaporation in one cylinder is utilised in doing useful work on the pistons of the succeeding cylinders. (3) By the ease with which it may be adapted to work on to two or more cranks, thereby reducing the excessive variation of stress which occurs in a single-cylinder engine when working with steam at a high initial pressure expanded to a greatly reduced terminal pressure. Fig. 123 shows a pair of compound cylinders for a vertical engine. The steam is admitted at A to the high-pressure cylinder. It is exhausted at B, and carried to the low-pressure cylinder through the dotted pipe to the opening C in the low pressure valve chest. It is exhausted at D to the condenser. The following diagram (fig. 124) illustrates the difference between the action of the steam in a simple engine and in a triple-expansion compound engine. 142 Steam -mm^ Compound Engines 143 Suppose I lb. of steam at 150 lbs. pressure absolute admitted to a single cylinder and expanded down to 1 2 lbs. pressure absolute and exhausted into a condenser, when the pressure averages 3 lbs. absolute. Then the action of the steam in the single cylinder is represented by the whole figure shown cross- lined. In such a case the temperature in the cylinder would vary from 358° F., the temperature of the steam at 150 lbs. pressure down to 142° F., the temperature of the steam at 3 lbs. pres- sure ; or a diiiference of 358— 142 = 216° F. between the initial ,2,58° )2qf 'J * 1 Fig. 124. and final temperature in the cylinder. And since cylinder condensation increases with the increase in the range of tem- perature, the loss of steam by initial condensation would here be very great. If now the expansion of the steam be spread over three cylinders (called respectively high, intermediate, and low) the range of temperature in each will be proportionally reduced. Thus in the high-pressure cylinder, working between I 50 lbs. and 60 lbs. pressure, there is a variation of 65° F. ; in the intermediate cylinder, working between 60 lbs. and 20 lbs. pressure, there is again a variation of 65° F. ; in the low-pressure M4 Steam cylinder, working between 20 lbs. and 3 lbs. pressure, there is a variation of 86° F. Again, the initial stress on the piston of the single-cylinder engine would be equal to forward pressure minus back pressure =(150— 3) X area of piston, while the terminal stress would be (12— 3) X area of piston ; and therefore the initial stress is H2:=i6-3 times the terminal stress. This would be a most 9 objectionable variation of stress on the working parts, and as the engine must be made strong enough to bear the maximum stresses due to the high initial pressure acting on a large piston area, a much heavier engine would be required than if the stresses were more judiciously distributed. If now the expan- sion of the steam, the range of temperature, the initial stresses, and the total work are distributed among three cylinders con- nected with three cranks, a much more economical and mechanically perfect engine is the result. The shaded parts marked high, intermediate, low, represent the distribution of the work among three separate cylinders. The diagram further illustrates the historical growth of the steam engine, for the bottom part of the figure represents the condition of the early engines working up to 20 lbs. pressure with a single cylinder ; then came higher pressures, higher rates of expansion, and two-cylinder compound engines, and later, with the introduction of steel for boilers, and surface condensa- tion, we have had a rapidly increased boiler pressure and rate of expansion, and the introduction of the three-cylinder or triple expansion compound engine. Pressures are still increasing, while the terminal pressure remains constant, and a fourth cylinder is in many instances now being added, forming a quadruple expansion engine. Figs. 125, 126, and 127 illustrate a two-cylinder compound mill engine, H P being the high pressure and L P the low- pressure cylinder. The steam passes from the boiler by the steam pipe S P into the valve chest of the high-pressure cylinder, where it is admitted to the cyhnder and cut off at about one-half or one-third of the stroke ; it is then exhausted by the pipe connecting the two cylinders, shown in fig. 127 from the Compound Engines I4S 146 Steam Compound Engines ^47 high-pressure into the low-pressure cyUnder, where it again does work by acting on the low-pressure piston. The steam is then exhausted, either into the air or into a condenser, by a pipe shown below the low-pressure cylinder. In a two-cylinder compound engine the steam exhausted from the high-pressure cylinder into the low-pressure acts as forward pressure in the low, and as back pressure in the high, Fig. 127. and the effective work done is due to the difference in area between the two pistons. Thus, suppose steam admitted between two pistons of equal area fixed on a rod, as shown in fig. T28. Here it is evident that the pressures on the inner faces of the two pistons being equal and opposite, the pistons will not move in either direction from this cause, and the effective pressure transmitted to the piston rod R by P is quite independent of the pressure between the pistons. But if the pressure acts on two pistons of uneqiiaL area, as L 2 148 Steam in fig, 129, the effective pressure transmitted by the pistons to the piston rod R is equal to the external pressure P on the small piston, plus the internal pressure on the difference of area between the large and small piston, less the back pressure/ on the large piston j from which it will be seen that the greater the initial pressure P of the steam on the small or high-pressure piston, and the greater the pressure between the two pistons, and the less Fig. 128. the back pressure / on the low-pressure piston, the greater the effective pressure transmitted. The volume of the low-pressure cylinder of a compound engine required for a given power is the same as if the whole of the work to be done, and the whole of the expansions, were per- formed in that cylinder alone ; and its size is therefore estimated Fig. 129. as for a single-cylinder engine, to exert the required power with the given initial pressure of steam of the high-pressure cylinder, admitted at once to the low-pressure cylinder and expanded down to the terminal pressure, the assumed point of cut-off being arranged to allow the same number of expansions as with the compound engine. Compound Engines 149 It will be evident that the volume of steam exhausted into the condenser at each stroke is the volume due to the capacity of the low-pressure cylinder ; and, provided the terminal pressure is constant, the volume and weight exhausted at each stroke is constant, whether the steam was admitted at boiler pressure direct to the low-pressure cyhnder and expanded in it down to the constant terminal pressure, or whether it has arrived there after passing through one, two, or more cylinders. The size of the low-pressure cylinder having been deter- mined, the remaining cylinder or cylinders are so proportioned as to equalise as much as possible the initial and mean stresses and the range of temperature. The ratios of the volumes of the cylinders, or of the piston areas (all being of equal stroke), are as the squares of their diameters. Thus, if the low-pressure cylinder diameter be made twice that of the high-pressure, then their areas or volumes are as i : 4. Or, again, if the cylinders of a triple expansion engine have their respective diameters in the proportion of 3, 5, and 8, then the areas of the successive pistons are to one another as 3- : f : 82=9 : 25 : 64=1 : 278 : 7-ii. The number of expansions of the steam in any engine, whether simple or compound, = . . . , , : and this is ap- mitial volume ^1 , ^ initial pressure , ^, proximately equal to , — f where the pressures are termmal pressure expressed in lbs. per sq. in. absolute. Thus, neglecting the effect of clearance spaces, number of expansions = volume of low-pressure cylinder divided by volume of high-pressure cylinder to point of cut-off. For example, suppose that in a two-cylinder compound engine the ratio of the piston dia- meters is as I : 2, then the areas of the pistons and volumes of the cylinders are as i : 4. If, then, the steam were supplied to the high-pressure cylinder throughout the whole stroke and then exhausted into the low-pressure cylinder, the number of expansions would be final vol. _voI. of L. P. cylinder_ initial vol. vol.ofH. P. cylinder I so Steam But if the steam is cut off in the high-pressure cylinder at one- third of the stroke, the number of expansions = final vol. vol. of L. P. cylinder _ 4 _ initial vol. |^(vol. of H.P. cylinder) ^ of i Or, again, if the initial pressure of the steam in the high-pres- sure cylinder is 90 lbs. absolute, and the terminal pressure in the low-pressure cylinder is 10 lbs. absolute, then the number c ■ initial pressure 00 or expansions = -. — f =<-=zg. termmal pressure 10 Suppose the ratio of the cylinder capacities is as i : 4, and we wish to expand the steam from 90 lbs. initial pressure abso- lute to 10 lbs. terminal pressure = 9 expansions. Here the steam must evidently be cut off at an early point in the stroke of the high-pressure cylinder, which point is found as follows : T .. -n vol. of L. P. cylinder LetR= 1 fTj p y A =4- vol. of H. p. cyhnder Then the point of cut-off in the high-pressure cylinder = R _4 number of expansions 9 of the stroke. Example. — The ratio of the cylinder volumes of a two-cylindei com- pound engine are as I : 3 ; the initial pressure by boiler gauge is 75 lbs. ; and the terminal pressure required is 10 lbs. absolute : find the point of cut off in the high-pressure cylinder. Am. \ of the stroke. A single cylinder is sufficient when steam is expanded not more than about 5 times ; for a greater number of expansions the compound engine is more economical. Thus, suppose the terminal pressure at which it is required to work is 10 lbs. absolute, using a condenser ; then, if the pressure of the steam at our command is only, say, 40 lbs. by boiler gauge, that is 55 lbs. absolute, the number of expansions = 55 = 5-5, or allowing for losses =5, which would only re- 10 quire a single-cylinder engine. If, however, the pressure of steam at command is go lbs. per sq. in. by boiler gauge, or 105 lbs. absolute, the number of expansions = — ? = io'5,or Compound Engines 151 allowing for losses =10, in which case a two-cylinder com- pound would be necessary. Suppose the engine required is to be non-condensing, then with a terminal pressure of, say, 5 lbs. above the atmosphere, or 20 lbs. absolute, and a boiler pressure at command of 80 lbs., or 95 lbs. absolute, the number of expansions = ^=475, or practically 4 '5, in which case it would be unnecessary to use a compound engine. The influence of clearance, and intermediate passages between the cylinders, will be considered presently. 152 Steam ftom BolUr CHAPTER XVII TYPES OF COMPOUND ENGINES Compound engines may be roughly divided into two classes : (i) those in which the pistons of each cylinder commence the stroke simultaneously. In such engines the cranks are either at o° or i8o° apart. These engines are known as the 'Woolf type. (2) Those in which the cranks are set at various angles other than 0° or 180°, and exhaust from one cylinder before the next cylinder is ready to receive It ; in which case the steam is re- tained, for a portion of the stroke, in a chamber or receiver between the two cylinders. These are termed ' receiver ' engines. The following are the most common arrangements of cylinders and cranks of -compound engines : I. The Tandem Compound En- gine with cylinders, as shown in fig. 130, the high-pressure cylinder being in line with the low-pressure cylinder, and the two pistons at- ~ tached to the same piston rod. In the fig. 130, H P is the high-pres- sure cylinder and L P the low-pressure. Steam is conducted 7b condenser Compound Engines 153 from the boiler direct to the high-pressure cylinder H. P., where it is admitted alternately at either end of the stroke, cut off at about one-half or one-third of the stroke, expanded nearly to the end of the stroke and then exhausted into the low-pressure cylinder L P, where it further expands, acting as back pressure on the high-pressure piston, and forward pressure on the low-pres- sure piston, and is finally exhausted into the air or a condenser. The distribution of the steam in the cylinders of the tandem engine at various points in the stroke may be clearly followed by the aid of an ideal diagram. In order to simplify the diagram we will neglect the effect of clearance at the end of the cylinders, the connecting passages between the cylinders, the friction of the steam in the passages, compression, &c., and suppose the vacuum perfect. In fig. 131, let the relative volumes of the high- and low- pressure cylinders be as i : 4, then make a (5 = i = volume of high -pressure cylinder, and a c = 4 = volume of low-pressure cylinder. From a set off a ^ = the initial absolute pressure of steam in the high-pressure cylinder, the horizontal through a being the zero of pressures. Then, if the steam be admitted to the high-pressure cylinder for one-third of the stroke, d e^\ a b is the line of admission, and e is the point of cut-off, and ef the curve of expansion to the end of the stroke of the high- pressure cylinder, the terminal pressure being bf^\ a d. The steam is now exhausted into the low-pressure cylinder at an 1 54 Steam initial pressure ag equal to the terminal pressure bf, and the two cylinders are now in direct communication. The volume of steam in the low-pressure cylinder increases as its piston moves forward, while at the same time the volume in the high- pressure cylinder decreases till its piston reaches the end of its stroke, and compresses the whole of the steam into the low- pressure cylinder. Here the volume of the steam is four times the volume of the high-pressure cylinder, and its pressure, therefore, falls to c k=^ ag or bf. During the time the cylinders are in communication, the pressure gradually decreases as the volume increases, but all the time it acts as back pressure on the high-pressure piston, and forward pressure on the low-pressure piston. The curve gnik represents the gradual fall of pressure as the volume of the low-pressure cylinder increases, and the curve fnh represents the decreasing back pressure on the high-pressure piston during the same period ; bf=ag ; p n^rvi ; and ck^a h. Then defh is the theoretical indicator diagram for the high-pressure cylinder, and ag kc iot the low-pressure cylinder, and the areas of these figures represent the work done in each cylinder Tespectively. These two diagrams may be combined by drawing horizontal lines as i, 2, 3, 4, and making 3, 4 ^ i, 2, &c., and completing the curve to s. The varying pressures and volumes throughout the stroke in compound engines, as in fig. 130, may be further illustrated by the aid of a numerical example, account being taken, in this instance, of the clearance spaces, and of the volume of the con- necting passage or ' receiver ' between the cylinders. Thus, take the case of a compound tandem engine, as in fig. 109, of the following dimensions : Volume of high-pressure cylinder. . =5 cub. ft. Clearance at each end of high-pressure ='35 „ Volume of low-pressure cylinder . , =20 „ Clearance at each end of low-pressure . = i -2 „ Volume of connecting passage . . ^ 2 -3 „ Cut-off at \ of the stroke in high-pressure cylinder ; low- pressure cylinder takes steam to end of stroke. Compound Engines 155 Initial steam pressure = 100 lbs. per sq. in. absolute. Then, the volume of steam admitted to high-pressure cylinder = \ volume of cylinder + clearance :=} of 5 + '35=2 02 cub. ft. The final volume of the steam is that contained by volume of low-pressure cylinder -|- clearance of low-pressure cyhnder+ clearance of high-pressure cylinder (omitting the steam in the intermediate chamber, which is a constant volume at the end of each stroke)=2o-|- i-2-|-*3S=2i-5S cub. ft. rr-x i .. 1 1- r • final volume Then total ratio of expansion =,^ — initial volume =££55 = 10-67, 2 '02 and the terminal pressure of steam in the low-pressure cylinder = 100 X =9'4 lbs. per sq. in. 21-55 We may now trace the varying pressures of the steam in passing from the high-pressure cylinder, through the receiver, to the end of the stroke of the low-pressure cylinder. Pressure at end of stroke of high-pressure cylinder 2"02 „ = 100 X—;— =3775 lbs. per sq. in. The steam is now exhausted at this pressure into the receiver. If there were no intermediate chamber between the two cylinders — the steam passing from one immediately to the other — and no clearance, then the terminal pressure of the high-pressure cylinder, namely, 3775 lbs., would be the initial pressure of the low-pressure cylinder (as in fig. 131) ; but when there is a connecting pipe- which answers the purpose of a receiver, and sometimes not an inconsiderable one — there is a fall or ' drop ' in pressure owing to the increased. volume now occupied by the steam. The receiver, however, is not empty when the high-pressure steam is exhausted into it, but it con- tains steam at a pressure, in the present case, equal to the terminal pressure of the low-pressure cylinder. When communi- 156 Steam cation opens between the high-pressure cylinder and the receiver, we have, therefore, two volumes of steam at different pressures, namely, 5-35 cub. ft. at 3775 lbs. pressure, in the high-pressure cylinder, and 2-3 cub. ft. in the receiver at 9-4 lbs. pressure (the terminal pressure in the low-pressure cylinder). The resulting pressure will therefore be equal to . (5-35X37-75) + (^-3X9-4 )^ .^3g ^^^_ 5'35 + 2-3 This is the pressure of the steam now occupying 7-65 cub. ft., namely, the volume of the high-pressure cylinder and receiver. On admission to the low-pressure cylinder, the volume is now increased by the clearance in the low-pressure cylinder, and it therefore now occupies 7'65-fi'2=8'85 cub. ft. Then assuming no back pressure against the low-pressure piston, the initial pressure on the low-pressure piston is, there- fore, 29'226 x^--^=25'26 lbs. per sq. in. To find the pressure at any intermediate point in the stroke, say :^th ; then the volume occupied by the steam will be : | volume of high-pressure cylinder -|- clearance in high-pressure cylinder -|- volume of receiver -I- clearance in low-pressure cylinder -f ^ volume of low-pressure cylinder, =1 of 5 -I- -35 -f 2 "3 -I- 1 -2 -I- J of 20=12-6 cub. ft., and the pressure of the steam at this point acting as forward pressure on the low-pressure piston, and back pressure on the high- pressure piston, will be 29-226 x '- — |=i774 lbs. per sq. in. 1 2 '6 The pressure at the end of the stroke may also be found from the same data ;.for volume of steam at end of stroke = volume of low-pressure cylinder + clearance of low-pressure cylin- der -f volume of receiver + clearance of high-pressure cylinder = 20 -f 1-2 -I- 2-3 + -35 = 23-85 cub. ft., and its terminal pressure 7-61; =: 29-226 X — :^ = 9-4 lbs. per sq. in., 23 °5 and this is the same result as we obtained before. Communication is now opened with the condenser, and the pressure falls to that in the condenser. Compound Engines 157 II. The Compound Engine, with the cylinders placed side by side, and with the cranks at right angles, as shown at fig. 132. In this engine the steam enters the high-pressure cyhnder L P m H P m s t=3 Fig. T32. H P direct from the boiler, is cut off at about one-half or one- third of the stroke, and expands to the end of the stroke of the high-pressure piston, when it is exhausted into the receiver. Retei v'ER. ^ ^-^ ) , . RECEIVER. a. 7 J < ■\ a. J III! 1 -t™i ^r III!! 1:111 b- t tr II! :.i mil. Mill b- r Ir Fig. 133. F-c. 134- In practice it is usually found unnecessary to have a separate special chamber for a receiver, as the exhaust pipe of the high- pressure cylinder and the valve chest of the low-pressure afford sufficient capacity for the purpose. Suppose the steam is cut off in both the high- and low- 158 Steam pressure cylinders at half stroke. Then, at the moment of ex- haust from the high-pressure cylinder, the low-pressure piston is only at half stroke (see fig. 133), and the low-pressure cylinder is therefore not yet ready to receive the steam. The slide valve of the low-pressure cylinder, fig. 133, covers the port for admission of steam from the receiver to the side a of the low-pressure cylinder, because it is at present connected with the condenser ; and as the cut-off in the low-pressure cylinder occurs at half stroke, the port will also be closed for admission of steam to the side b. The position of the pistons is now as shown in fig. 133. The low-pressure piston proceeds to the end of its stroke, and the high-pressure piston (fig. 134) also returns from the bottom of the cylinder against the back pressure of its exhaust steam which fills the high-pressure cylinder and receiver, thereby reducing its volume and increasing its pressure. This proceeds till the high-pressure piston reaches its half stroke, by which time the low-pressure piston has reached the end of its stroke, and its steam port opens for admission of steam from the receiver to the end a, fig. 134. The confined steam in the re ceiver and high-pressure cylinder now expands, driving the low- pressure piston forward, and acting as back pressure on the high- pressure piston, and forward pressure on the low-pressure piston. The initial pressure in the low-pressure cylinder (neglecting loss by friction in the passages) is equal to the pressure in the receiver when the high-pressure piston has reached the middle of its return stroke. This steam expands in the low-pressure cylinder till its piston reaches, say, half stroke, when its steam port closes. The supply of steam from the receiver being now cut off, it expands to the terminal pressure, and is exhausted into the condenser. These operations may also be readily traced from an ideal indicator diagram. Suppose the steam cut off in both the high- and low-pressure cylinders at half stroke. In fig. 135 let the relative volumes of the high- and low-pressure cylinders be as I : 3. Make ab = \ = volume of high-pressure cylinder, and « 20,000 II ,, 50 "1 „ 15,000 29 ,, 100 i9i .. 10,500 72 „ 300 30 ,. 7,500 I6IJ „ The Steam Turbine 239 The large number of revolutions of the De Laval turbine is one of its disadvantages, as these high speeds are not required in practice. It is usual to reduce the speed of the shaft which carries the driving pulley by spiral gearing, generally having a ratio of 10 to i. In the Curtis, Rateau, and Zoelly impulse turbines this high speed of rotation is avoided by using several impulse wheels placed in series and expanding the steam in stages. The shaft and wheel of a De Laval turbine should be balanced as accurately as possible. Absolute accuracy is impossible, and the centre of gravity of the whole will not coincide exactly with the geometric axis. Thus when the wheel rotates, a force tending to bend the shaft is produced, which rapidly increases in amount until a certain speed is reached, called the ' critical speed.' If the turbine is run for a short time at this speed, the shaft would be broken. Above this speed the wheel will run steadily. The shaft is made flexible, so as to allow the shaft to rotate about its centre of gravity. The shaft will thus be slightly eccentric at full speed. The diameter of the shaft for a 300 h.p. turbine is only about i^^ inches. The working speed of a De Laval turbine is always above the critical speed, and the vibration which might occur on passing the critical speed can be allowed for by having sufficient radial clearance. Radial clearance is unimportant in impulse turbines, as the pressure is the same on both sides of the wheel, and there is no tendency for the steam to leak past the wheel. These turbines have been largely used for driving centri- fugal pumps, ventilators, air compressors, and small electrical generators. Losses in a De Laval Turbine. — The following approximate numbers will serve to show what the losses in a De Laval turbine are, where they occur, and some idea of their relative magnitude. Suppose the available energy obtained by expand- ing steam between two given pressures be represented by 100 when no losses take place. If the expansion takes place in a De Laval nozzle, there will be a loss of about 15 per cent, due to the friction of the steam against the sides of the nozzle and 240 Steam to the energy dissipated through the formation of eddies in the steam. This loss will, of course, be greater if the nozzle is not properly designed. The friction between the steam and the blades causes a loss of about 10 per cent. The turbine wheel revolves in an atmosphere of steam, and the loss due to friction between the turbine disc and the steam is about 5 per cent. The energy left in the steam as it leaves the turbine is about 10 per cent. Radiation and mechanical friction together absorb about 5 per cent. The total loss is thus about 50 per cent. Fig. 193 shows a section of a Parsons turbine of a simple type. It consists of a rotor to which rings of blades are attached, and a casing having rings of stationary blades attached internally. The rotor is supported at each end by bearings, and the casing consists of two parts bolted together. The steam enters at A, at the boiler pressure, and is usually superheated. It passes alternately through the station- ary and moving blades. There is a continuous fall of pressure as the steam passes through the turbine, with a consequent increase of volume. It is obvious that if the steam velocity through the turbine is to be constant the area through which the steam passes must be increased as the volume increases. If the blades are all of the same shape, and the same number is used per ring, then the blades in each successive ring must be lengthened. For convenience of construction it is usual to have several rings of blades of the same height and spacing, and to increase the diameter of the rotor at intervals. At the exhaust end the volume of the steam rapidly increases as the pressure falls, and the blades whilst retaining the same height may be spaced farther apart, and also be made with a flatter curvature. The following example will illustrate why a change of area per stage is more necessary at the low-pressure end than at the high-pressure end. Suppose the pressure falls in the first five stages (a stage consists of one rotating and one stationary set of blades) from 150 lbs. to 140 lbs. ; the volume will increase from 3'oi to 3'2o cub. ft. per lb., assuming adiabatic expansion of the steam. This is an increase The Steam Turbine 241 242 Steam P\ F1 B of about 6 'per cent. If the pressure falls in the low- pressure stage from 4 lbs. to 3 lbs. per sq. in., the volume will increase from 90 cub. ft. per lb. to 116 cub. ft., allowing for wetness, due to expansion, or an increase of about 29 per cent. The cross-sectional area between the blades should be correctl)' designed, so as to allow for a small fall of pressure and an increased velocity of the steam. The calculation of the best exit area of the blades is difficult, as the conditions of working are imperfectly known. Experience is the best guide in the first instance, and after the blades are in position they may be opened out or closed up by special tools, as experi- ment shows to be desirable. The steam after passing through the turbine escapes to the condenser. Turbine Blades. — The blades have been made of brass, gun- metal, steel, and an alloy of 80 per cent, copper with 20 per cent, nickel. Fig. 194 illus- - I j ^ trates a blade, B, and packing EJ I' I I ! 'I piece, P, of a Parsons marine turbine. The slot S near the top of the blade is for the bind- ing wire, which passes from blade to blade, and is wired or soldered to each blade. The blades are often thinned at their lips to about ~ in. to prevent serious damage, if they should com^ in contact with the inside of the casing. They are secured in position by the packing piece, which is placed between each pair of blades, and caulked. The two small grooves at the bottom of the blade assist in holding the blade securely. The blades and packing pieces are usually made of brass. The blades in the turbines of the Lusitania vary in height from 2| ins. to 22 ins. The pressure of the steam on the blades produces an ^=5^ ^^ Fig. 194. The Steam Turbine 243 unbalanced axial pressure on the rotor, tending to force the rotor in the direction of the steam exit. To balance this axial pressure three dummy pistons, B, C, and D, are intro- duced, fig. 193, each of which is connected with one of the sections. The pressure on any one of the dummy or balance pistons is the same as that in the section to which it is con- nectedj but in an opposite direction. To prevent leakage of steam past the balance pistons, a large number of grooves is turned in their circumference. This is found to be very effective in preventing leakage. The pressure on the outside of the large piston is maintained at the condenser pressure by connecting the space in which the out- side face rotates with the exhaust. Care is required in the axial adjustment of the rotating blades to prevent them from coming in contact with the stationary blades. This is accomplished by having a thrust- block at E. A number of grooves is turned in the shaft, and a number of rings in the bearing project into these grooves. The upper and lower halves of the bearing are adjustable separately by micrometer screws with dials to show the exact position of the blades. If the upper half of the bearing tends to press the shaft to the right, then the lower half is adjusted to tend to press the shaft to the left. In this way the rotor is fixed in a definite position and the axial clearance is main- tained. The radial clearance is made as small as possible, as a high efificiency of the turbine depends largely on a small radial clearance. The pressure on the two sides of the moving blade being different, a large clearance causes considerable leakage of steam past the blades (see fig. 195, in which Cj represents the clearance (not to scale) between the moving blades and casing, and C2 the clearance between the rotor and the stationary blades). The actual clearance of a 48-in. rotor is about o'048 in., or about YTnjo '"• P^"^ '\nQkv of diameter. The bearings are lubricated by oil which is forced in at a pressure of from 5 to 10 lbs. per square inch. The bearings R 2 244 Steam are usually of white metal, with a safety brass bearing slightly below the level of the white metal. This safety bearing will support the rotor and prevent the blades coming in contact with the casing, in the event of the white metal becoming over- heated and being melted out of the bearing. The maintenance of a continuous flow of oil to the turbine bearings is of the utmost importance, in order to prevent the bearings from becoming overheated. The oil from large bear- FlG. 195. ings should be cooled before returning it to the bearings. The temperature of the bearing on the U.S.S. Chester was ordinarily from 100° Fahr. to 105° Fahr. Much higher temperatures are, however, not uncommon. Velocity Diagrams. These diagrams are essential aids to the correct design of steam turbines. It is necessary, however, to have a clear idea of the terms absolute and relative velocity before pro- ceeding. Absotitte velocity is the velocity as compared with stationary objects. Relative velocity is the velocity in relation to that of a moving body. Suppose a ship is travelling at the rate of 20 feet per second, and a ball be thrown from the ship The Steam Turbine 245 at right angles to the ship at 15 feet per second, then relative to the ship the velocity of the ball will be 15 feet per second at right angles to the ship. The absolute velocity relative to the earth is obtained by constructing a parallelogram as in fig. 196, in which ab represents the ship's velocity and ac the velocity of the ball. The absolute velocity and direc- ' a ' i ^^ •?» b tion of the ball will be ad. Let the velocity of the jet leaving a De Laval c nozzle be Vj feet per second ^'°' ''^' and made an angle with the wheel of 20° ; let the velocity of the wheel be V feet per second. Find the relative velocity of the steam and the correct entrance angle of the blade so that the steam may enter without shock. Construct the diagram of velocities by drawing AB = Vi, and making an angle of 20° with the plane of the wheel DB. Make BC = V the velocity of the wheel. Then AC is the relative velocity and direction of the steam with respect to the wheel. The angle ACD is the entrance angle for the blades. The exit angle is approximately the same for a De Laval turbine. There is no change of velocity in passing through the turbine, and therefore AC represents the velocity on leaving relative to the blade. If EF = AC represents the magnitude and direction of the velocity relative to the blade on leaving, then, if the diagram of velocities is constructed so that FG = V, the velocity of the wheel, EG is the absolute velocity of the steam on leaving. By making the inlet and outlet angles equal there is 246 Steam practically no end thrust on the spindle, as the pressure of the steam is the same on each side of the wheel. The steam turbine has been largely used for the propulsion of ships. It is displacing reciprocating engines for ships requiring a high speed ; but has not been found economical for speeds less than about 16 knots. It has also been largely adopted for the production of electrical energy in large power stations. As a heat engine the principal merit of the steam turbine lies in- its abiUty to make available for useful work the energy in that portion of the steam which has hitherto passed away to waste in the exhaust of the reciprocating engine. Thus in fig. 198, if pressure ed represents the pressure at Fig. 198. exhaust in the reciprocating engine, beyond which it would not be profitable to expand, it is possible to obtain the still further area edfg by continued expansion of the steam in the turbine. And this is further increased by the superior vacuum obtained in turbine plants, thereby giving a lower back pressure line ag. The mechanical advantages and disadvantages of steam turbines may be briefly stated as follows : — Advantages Less attention is required on the part of the engineer. Less oil used for lubrication, and no oil in the condensed steam. Less vertical space required. The Steam Turbine 247 Less vibration and lighter foundations. No danger from exposed moving parts. Less sliding parts, such as pistons, valves, etc. Less cost for repairs. Disadvantages Great care is required in moving the heavy main castings and rotor for examination. The working parts being hidden and the clearances very small, the blades may be seriously injured by causes which are not easily preventable by the engineer in charge. The blades are not easily repaired. Speed too high for many purposes. Steam Consumption of Parsons Turbines. — Small turbines of this type are not economical in steam consumption. They are usually made in large units from 300 horse-power to 17,000 horse-powet or more in one casing. The admission pressure of the steam entering the turbine is generally about 150 lbs. per sq. in. A large variation in the admission pressure is possible without much change in the steam consumption. It would seem that the gain from the higher initial pressures is to some extent lost by the extra leak- age past the blades at the high pressure end and by the friction of the wheels, rotating in the denser steam. Superheating the steam before admission to the turbine improves the economy very considerably. Approximately a gain of 15 per cent, is obtained for 120° Fahr. of superheat at full load. The amount of gain varies with the size of turbine and also with the vacuum in the condenser. About 150° Fahr. of superheat appears to give the maximum economy when everything is taken into account. The most important gain in economy is from the use of a condenser with a high vacuum. In the reciprocating engine it is not usually economical to have a higher vacuum than 26 ins. In the steam turbine it is found that an increase of vacuum from 25 to 26 ins. improves the economy about 4 per cent. ; an increase from 26 to 27 ins. gives about 5 per cent, more economy ; and a 28-in. vacuum is about 6 per cent, better than 248 Steam a 27-in. vacuum. An improvement of about 15 per cent, is obtained by increasing the vacuum from 25 to 28 ins. Losses in Parsons Turbine. — Suppose the available energy entering a large Parsons steam turbine be represented by 100. The losses which take place are very approximately as follows : — Friction of steam in passing over the blades and loss due to eddies formed in the steam . . -16% Leakage over the tips of the blades • • • 7% Leakage past dummy pistons and past the steam glands 4% Loss due to kinetic energy left in the steam . . 4% Loss due to mechanical friction • • • • 7% giving a total loss of 38% or an efficiency of 62%. Exhaust Steam Turbines. — The great economy of the turbine at low pressures has led to the use of the exhaust steam turbine to take the steam leaving the reciprocating engines at atmospheric pressure ; or one or more non-condensing engines have had their exhaust steam passed through an exhaust steam turbine. The clearances in these turbines do not require to be as fine as when high pressures are used, and the temperatures being low there is little distortion of the casing. An economy of about 20 per cent, is possible by substituting an exhaust turbine for the low-pressure cylinder. Comparison of De Laval and Parsons Turbines De Laval Turbine. Speed very high and reduced by gearing. Only one ring of blades required. Made in small sizes only. Small clearances unimportant. Critical speed below the working speed. No end thrust on the wheel. Parsons Turbine. Speed comparatively low and direct coupled. Many rings of blades required. Made chiefly in large sizes. Small clearances essential to economical working. Critical speed above the working speed. End thrust requires to be balanced by dummy pistons. ( 249 ) CHAPTER XXIV INTERNAL COMBUSTION ENGINES Internal combustion engines include all engines which burn their fuel in the working cylinder, as gas engines, oil engines, and petrol engines. They are usually worked on what is known as the four- cycle system. In a few cases, however, a two-cycle system is employed. The four-cycle system was advocated by Beau de Rochas in 1862 and introduced by Otto in 1876, and is usually called the Otto cycle. It requires four strokes of the piston to complete the cycle of operations in the cylinder. \st Stroke. — A charge of gas and air is drawn into the cylinder by the piston. ■itid Stroke. — This charge is compressed on the return of the piston. yd Stroke. — The charge is fired at the commencement of this stroke, and the increased pressure of the gas urges the piston forward. d,th Stroke. — The exhaust gases are expelled from the cylinder. It will thus be seen that there is only one working stroke in four strokes of the engine. The four strokes are illustrated by fig. 199. At I. the piston is just commencing to draw in a charge of gas and air through the open gas and air valves. At II. the piston is just commencing to compress the mixture, all the valves being closed. At III. the compressed mixture is being fired, all the valves being closed. At IV. the pis'ton is nearly at the end of the working stroke, 250 Steam Charging Stroke ^~.\~ *^*j ^— i--^' «> i2 §> I Compressing Strol• — -^^^Ss::^^^ m Exhausting Stroke Fig. 199. Internal Combustion Engines 251 and the exhaust valve is just opening. During the return of the piston the exhaust gases escape, and the cycle recommences by drawing in a fresh charge. Fig. 200 shows the approximafe positions of the crank when the various operations occur in 3.fottr-cycle gas or oil engine. The positions of the crank coinciding with these operations vary with the speed and size of the engine. It will be noticed that the gas and air valves close after the crank has passed the dead centre. By this means a stronger charge is detained. This is possible because the pressure of GAS & A lfl OffENA ^^ ^^ J 1 EXHAUSTCLpsE^,;-— — -' ^'=5;:-----JIll *" J IGNITIIONT' ^^^^^iTT^'^^ CLOSES \ \ TAIR/CLOSES the gas and air in the cylinder during the suction stroke is less than the atmospheric pressure, and the piston must compress the gas slightly before atmospheric pressure is attained. Also the velocity of the gas through the valves having some momentum is not quickly reversed in direction. In the two-cycle system the operations are as follows. Fig. 201 refers to the crank positions of a large Korting two- cycle, double-acting gas engine. The mixture of gas and air in the cylinder is fired, and the piston is urged forward. When the piston is near the end of its working stroke, the crank being in the position shown, the exhaust valve is opened. 252 Steam The pressure in the cylinder is rapidly reduced to atmospheric pressure. The air valve opens just before the crank reaches the outer dead centre, and admits air at about 9 lbs. pressure above the atmosphere. The effect of this scavenging air is to cool the burnt products, and so to minimise the danger of pre- ignition when the new charge is admitted, as well as to give a better burning mixture. When the crank has passed the dead centre, the gas valve opens and admits gas under pressure along with the air. The exhaust valve closes soon after the gas valve has opened, and lastly the gas and air inlet valves Fig. 201. both close. Compression of the mixture by the engine piston now takes place until the firing point is reached in the cylinder when the crank is near the inner dead centre. The indicator diagram from a gas engine may be obtained by an ordinary indicator. An outside spring should preferably be used owing to the high temperature of the gases. Fig. 202 shows the normal indicator diagram taken with a strong spring. A scale of pressures and the clearance has been added. Considering the case of a four-stroke cycle, from A to B the charge is drawn into the cyhnder, the pressure being slightly below the atmosphere. From B to C compression Internal Combustion Engines 253 takes place. At C the mixture is fired, and the pressure rises to D. From D to E expansion of the gas takes place, the exhaust valve opening at E just before the end of the stroke. EA. represents the exhaust -which takes place at a pressure slightly above the atmospheric pressure. Fig. 202. The suction line AB and exhaust pressure line appear as one line coinciding with the atmospheric pressure line when taken with the usual strong spring in the indicator. Fig. 203 shows the suction and exhaust pressure lines as taken by an indicator having a weak spring. The mean pressure on the piston is obtained, s^ saMm as in the steam-engine ^"^' ^°^- diagram (see p. 55), by dividing the indicator diagram into ten equal parts or by using a planimeter. The indicated horse-power is obtained from the formula plan 33>°oo , where/ is the mean pressure in lbs. per sq. in. on the piston. / is the length of the stroke mfeet. a is the area of the piston in square inches, n is the number of explosions per minute. Example. — The mean pressure on the piston is 80 lbs. per sq. in. ; strol, 50 c 40 Ul 30 10 _ ^, , _ , Vol. before Compression Ratio of Compression, or vol. after Compression Fig. 212. The valves are of the same size and interchangeable. They are placed on opposite sides of the engine, and worked by cams on two cam shafts. The cam shafts are in the crank chamber and worked by spur gearing from the main shaft. The oil is forced through pipes to the bearings by the pump H. A gauge on the dashboard indicates to the driver the 266 Steam state of the lubrication. Sufificient oil is carried in the crank chamber for a run of 200 to 400 miles. The great advance in the construction and use of internal combustion engines in recent years is due to their high thermal efficiency. They will convert more of the heat in a pound of coal or oil into work than any other heat engine at present known. The efficiency of the internal combustion engine depends very largely on the amount of compression given to the charge before ignition. The higher the compression, the greater the possible efficiency. Fig. 199 shows how the efficiency increases as the compression is increased from 2 to 10, or, in other words, the volume is reduced by compression to | or ^ before ignition. The curve is drawn for an engine working on the Otto cycle, and compressing air without loss of heat. It is usual in practice to reduce the volume of the charge by com- pression to about f or I the volume before compression. The effect of compressing the mixture to a higher pressure before ignition is shown by the dotted area in Fig. 213, which represents the added work from the higher compression, the clearance being reduced from a to b. The advantages of high compression may be briefly stated as follows : — • 1. A greater mean pressure on the piston every working stroke. 2. A less size of cylinder for the same horse power. 3. A weaker mixture may be used and fired with more certainty. Internal Combustion Engines 267 4. Less burnt gases will be left in the cylinder at the end of the exhaust stroke. 5. The engine will require less gas for the same power. The great objection to compressing an explosive mixture beyond a certain amount is that the mixture may explode before the end of the compression stroke, thus causing a loss of power and a heavy stress on the engine. The cause of the pre-ignition is due to the high temperature developed in a gas when it is compressed. Some gaseous mixtures fire at a lower temperature than others. A gas containing chiefly carbon monoxide (CO), such as producer gas, may be compressed to Fig. 214. a higher pressure without pre-ignition than a gas containing much hydrogen, like common coal-gas. In the Diesel oil engine the difficulty of pre-ignition is avoided by compressing pure air only and injecting the oil when the piston has reached the end of the stroke. In this way a very high compression is possible (500 lbs. per sq. inch), and the efficiency attained is very high. The indicator diagram of an internal combustion engine may be used to find the temperature of the gas at any part of the cycle, if the temperature at any one point is known Take an indicator diagram (fig. 214) having an atmospheric 268 Steam line CD, and draw OX, the zero line, to the same scale as the diagram, 14*7 pounds below the atmospheric Hne. If AB is the length of the diagram and the clearance is 20 per cent., make OA 20 per cent, of AB, and draw OE to repre- sent the clearance line. From the properties of a gas we know that PV = RT, where P, V, and T represent the pressure, volume, and absolute temperature of a gas, and R is a constant. Assume that the temperature at D at the commencement of the compression stroke is known. If the mixture did not receive any heat from the hot cylinder, and from the residue of gas left in the cylinder, the temperature would be about 60° Fahr. It receives, however, heat from the cylinder, and its temperature may be assumed to be, say, 200° Fahr. Measure DB, the pressure at D, and the volume DC. As these quantities are to be compared to similar quantities, any scale of measurement will suffice, as long as the remaining quantities are measured to the same scale. For instance, DB may be measured in pounds, or in inches, or in millimetres. Similarly, DC may be measured in inches or millimetres. Suppose DB = o'o7 in. and DC = 279 ins.; then, PV substituting in the equation 7p- = R, we have — 0-07 X 279 i — -^ = R, and R = o 000291; 200 + 461 ' ^-^ To find the temperature at any other point P during expan- sion Measure PF = i-o8 ins. Measure PG = i'o5 ins. As R is the same as before — i'o8 X I'oi; ^ a = 0-000295 .-. T = 3838° absolute, or 3377° Fahr. APPENDIX MENSURATION OF SURFACES AND VOLUMES 1. Area of rectangle 2. Area of triangle 3. Diameter of circle 4. Circumference of circle 5. Area of circle . 6. Area of sector of circle = length X breadth. = base X \ perpendicular height. = radius x 2. = diameter x 3"l4i6 = diam. x diam. x "7^54 _ area of circle x No. of degrees in arc. 360 7. Area of surface of cylinder = circumference x length + area of two ends. 8. To find diameter of circle having given area : Divide the area by •7854, and extract the square root. 9. To find the volume of a cylinder : Multiply the area of the section in square inches by the length in inches = the volume in cubic inches. Cubic inches divided by 1728 = volume in cubic feet. QUESTIONS AND EXERCISES 1. Explain the nature of the phenomenon which we call ' heat." 2. Distinguish between ' temperature' and 'quantity of heat.' 3. Convert 5°, 14°, 41°, 68°, 158°, 266° Fahrenheit to Centigrade ; and 1°, -30°, -25°, 90°, 120° Centigrade to Fahrenheit. 4. A Fahrenheit thermometer rises through 45° ; how many degrees would this rise indicate on the Centigrade thermometer ? 5. What is meant by the ' specific heat ' of a substance ? One ounce 27° Steam of copper at 212° F. is immersed in i lb. of water 55° F. ; find the increase of temperature of the water. 6. Express the following temperatures in degrees absolute : 60° F., 100° F., 247° F., and 0° C, 100° C. 7. Convert 338° F. and 165° C. into degrees Reaumur. 1. Define the 'unit of heat,' the 'unit of work,' 'horse-power,' and the ' mechanical equivalent of heat. ' 2. Find the units of heat required to raise I lb. of water from 55° F. to 212° F., and express the same in units of work. 3. One pound of water is heated from 60° F. to 100° F. ; find the units of heat absorbed by the water, and the equivalent units of work. 4. Five-and-a-half pounds of water are heated from 50° V. to 75° F. ; find the units of heat absorbed, and the equivalent units of work. 5. A weight of a ton is lifted by a steam engine to a height of 400 ft. ; what amount of heat is consumed usefully in the act ? 6. Describe the experiment conducted by Joule to determine the mechanical equivalent of heat. Ill 1. Give examples of good and bad conductors of heat. 2. Explain the process of heating water by 'convection,' and illustrate by sketches. IV 1 . Give what practical illustrations you can of the expansion of metals by heat, and sketch two forms of expansion joints for steam pipes. 2. What is ' the law of Charles ' ? 3. A gas occupies 5'5 cub. ft. at 32° F. ; what volume will it occupy at 212° F. under constant pressure? 4. A volume of air at 350° F. exerts a pressure of 60 lbs. ; find its pressure when the temperature is reduced to 32° F. 5. A gas occupies I5'5 cub. ft. at 100° C. ; what volume will it occupy at 0° C, the pressure on the gas remaining the same ? 6. What do you understand by the term absolute pressure ? 7. Explain the phenomenon of boiling. 8. What effect has pressure on the temperature at which water evaporates ? 9. Describe a simple process of obtaining pure water from muddy water. 10. What is the meaning of the word 'vacuum'? and explain why it is impossible in practice to obtain 2. perfect vacuum. 11. Describe an experiment illustrating the reality of atmospheric pressure. Questions 27 1 12. Sketch and describe the action of the pulsometer. 13. Sketch and describe the action of Newcomen's atmospheric engine. 1. Describe carefully the stages involved in the conversion of water into steam under a movable piston, noticing especially the heat quantities, and the changes in temperature and volume. 2. What vfork is done in raising a piston 2 sq. ft. in area through a height of 5 ft. against atmospheric pressure? and represent this by an area. 3. What is the total number of units of heat required to convert water at 32° F. into steam at 212° ? and explain how these units have been expended. 4. How is the efficiency of an engine expressed ? 5. Considering the work done per pound by steam during formation at varying pressures without expansion, what advantage is gained by using high pressures rather than low ? 5. Given that the volume per pound of steam at 120 lbs. pressure is 3 '65 cub. ft., find the external work done per pound during formation. 7. Find the weight of steam required per horse-power per hour in example 6. 8. What conditions affect the total quantity of heat rejected by steam to the condenser > 9. Define ' latent heat of steam ' and ' total heat of evaporation.' 10. Give formulfe for finding the total and latent heats of steam, and apply them, having given that the temperature of steam at 115 lbs. is 338° F. 11. A cylinder is 16 ins. diameter, and stroke of piston 2 ft. ; find the area of the piston and the volume displaced by the piston at each stroke, neglecting clearance. 12. If, in the previous question, the area and volume of the piston rod, 2j ins. diameter, be deducted, what will then be the effective area of the piston and volume of steam on the rod side ? 13. The cylinder of an engine is 36 ins. diameter, the initial pressure of steam 120 lbs. per sq. inch ; find the load on the piston in tons. 14. The area of a piston is 7o6'8 sq. ins. ; find the diameter of the air pump which is one-half that of the cylinder. 15. The cylinder of an engine is 74 ins. in diameter, and the stroke is 7i ft. ; what is the capacity of the cylinder ? How many pounds of water must be evaporated in order to fill such a cylinder with steam at an actual pressure of 15 lbs., it being given that steam at 15 lbs. pressure occupies a space equal to 1670 times that of the water from which it is generated ? (Sc. & A. 1871.) {Note. — I lb. of water = '016 cub. ft.) 2/2 Steam VI 1. What is saturated steam ? 2. What is the temperature of saturated steam at atmospheric pressure, also at pressures of 20, 60, 100, 150, 200 and 400 lbs. per sq. in. ? 3. Show the rel&tion between temperatures and pressures by a curve. 4. Draw a curve illustrating the relation between the volume and pressure of saturated steam. 5. If 2 lbs. of water at 200° F. be mixed with 2-5 lbs. of water at 212° F., iind the temperature of the mixture. 6. How much water at 60° F. must be mixed with t lb. of water at 212° F., so that the resulting temperature may be 120° F. ? 7. How much water at 60° F. will be necessary to condense I lb. of steam at 212°, so that the resulting temperature shall be 120° F. ? 8. Find the temperature of the mixture when 17 '63 lbs. of condensing water at 60° F. are used per lb. of steam at 212°. VII 1 . AMiat is Boyle's Law ? 2. Steam is admitted into the cylinder of an engine at the pressure of 45 lbs. per sq. in. absolute, and is cut off at one-third of the stroke ; find the pressure of the steam in pounds per sq. in. at half-stroke, and also at the end of the stroke. 3. Suppose the steam pressure in above example is 45 lbs. above the atmosphere, and the engine is non-condensing, what is the effective pressure on the piston at half-stroke and also at the end of the stroke ? 4. Find the steam pressure at the end of the stroke of the piston in an engine where the steam is admitted at a pressure of 30 lbs. above the atmosphere, and is cut off at two-fifths of the stroke. (Sc. & A. 1882.) 5. A steam cylinder is 4 ft. long ; the steam enters at 60 lbs. boiler pressure and is cut off at one-third of the stroke ; what is the steam pressure when the piston has travelled over 2 ft., 3 ft., and 4 ft. respec- tively ? Give your answer in pressures above the atmosphere. 6. Steam is admitted into a cylinder at a pressure of 25 lbs. on the square inch above the atmospheric pressure of 15 lbs. on the square inch, and is cut off at such a point that its pressure at the end of the stroke is 5 lbs. below that of the atmosphere. At what point of stroke was it cut off? Make a diagram, showing approximately the steam pressure on the piston throughout the stroke. (Sc. & A. E. 1885.) 7. Draw the theoretical indicator diagram when steam at 75 lbs. boiler pressure is cut off and expanded to three times its initial volume, first by calculation, and then by the graphical method. 8. In a cylinder having a piston with a 4-ft. stroke, steam at 75 lbs. absolute pressure is cut off at two fifths of the stroke ; find the pressure of the steam at the second, third, and fourth foot of the stroke (neglecting the effect of clearance). Questions 273 I, Explain the use of the hyperbolic logarithm in obtaining the area of the theoretical indicator diagram. Write down the expression for finding the total area of the figure. 2. Explain the reason of the advantage gained by using steam ex- pansively, and compare the effect in work done, and in steam and fuel consumption with steam at 60 lbs. absolute pressure — (a) when cut off at one-third of the stroke ; {b) when admitted throughout the whole stroke, omitting the effect of back pressure. 3. What is the effect of using a condenser on the total work done by the engine ? 4. Write down the operation of finding the mean effective pressure represented by an indicator figure. 5. Steam is admitted to a cylinder at 75 lbs. absolute and cut off at one-third of the stroke ; back pressure 15 lbs. Draw the theoretical indicator diagram, and find the mean effective pressure by measurement. 6. Write down the expression for finding the mean pressure by the use of a table of hyperbolic logarithms. 7. Given that the hyperbolic logarithm of 3 is I 'OgS ; find the mean pressure in question 5. (. 8. Write down the formula for finding the indicated horse-power of an engine. g. Find the indicated horse-power of an engine with a cylinder 1 6 ins. diameter, length of stroke 2 ft. , number of revolutions 70, mean effective pressure on piston 30 lbs. per sq. in. 10. A single-cylinder engine 24 ins. diameter, 3 ft. stroke, mean effective pressure of steam 40 lbs. , makes 60 revolutions per minute ; find its indicated horse-power. 11. The above 24-in. cylinder engine is required to indicate 250 horse-power ; what must be the mean effective pressure of steam « hen running at the same speed ? 12. Suppose that, in the previous case, instead of obtaining the 250 I. H. P. by increasing the mean pressure, it was done by increasing the number of revolutions ; how many revolutions per minute must the engine now make ? 13. Find the horse-power of a locomotive engine which can draw a train weighing 100 tons (including its own weight) along a level road at 30 miles per hour, the train resistance being taken at 10 lbs. per ton of load. (Sc. &A. 1884.) 14. What diameter of cylinder will develop 50 horse-power with a 4-ft. stroke 40 revolutions per minute, and a mean effective steam pressure of 30 lbs. above the atmosphere, the engine being non-condensing ? (Sc. & A. 1883.) T 274 Steam 15. In a beam engine the mean pressure of the steam on the piston is 20 tons, and the length of the crank is 2| ft. ; what is the horse-power when the crank shaft makes 30 revolutions per minute? (Sc. & A. 1883.) 16. An engine is required to indicate 50 horse-power, with a mean effective pressure on piston of 35 lbs. per sq. in. ; length of stroke 2 ft. ; number of revolutions 60 ; find the diameter of the cylinder. 17. Compare the economical effect of using steam at 80 lbs. absolute, and steam at 40 lbs. absolute in a single-cylinder condensing engine. Back pressure 3 lbs., and terminal pressure 10 lbs. in each case. 18. Suppose steam at 60 lbs. boiler pressure is used to drive two engines, one a non-condensing engine, and the other a condensing engine. In the non-condensing engine the steam is cut off at \ of the stroke, back pressure 18 lbs. absolute ; and in the condensing engine at ^ of the stroke, back pressure 3 lbs. absolute. The cylinders of both engines are the same size. Draw the theoretical indicator diagrams and compare the relative work done, and weight of steam used in the two cases. 19. Explain in what way the amount of back pressure limits the number of useful expansions of the steam in the cylinder. 20. What is ' clearance ' ? and explain its effect on the work done by the steam in the cylinder. 21. How may the loss by clearance be modified ? 22. Explain in what way ' cylinder condensation ' limits the useful range of expansion of the steam on the cylinder. 23. What are the remedies adopted to reduce the amount of conden- sation of the steam in the cylinder ? 1. Make a hand-sketch of the cylinder (fig. 44). 2. How is the piston-rod made steam-tight in passing through the cylinder cover ? 3. What are ' wire-drawing ' and ' cushioning ' ? 4. Make a sketch of a cylinder fitted with a liner (see fig. 126). 5. What is the object of the cylinder escape valve, and relief cocks ? 6. A cylinder is 36 ins. diameter, stroke of piston 3 ft. 6 ins. ; find the capacity of the cylinder, allowing 7 per cent, in addition for clearance space. 7. Find the weight of steam occupying 26'47 cub. ft. at a pressure of 12 lbs. per sq. inch absolute, given that steam at 12 lbs. absolute occupies 3l'g cub. ft. per pound. 8. The weight of steam passing through the engine per stroke is •83 lb. ; find the weight used per hour when the engine makes 85 revolu- tions per minute. 9. Sketch some form of steam-engine piston, and explain how it is made to work steam-tight in the cylinder. Questions 275 10. What is the 'packing ring,' the 'tongue piece,' and the 'junk ring,' and what is the purpose of each ? 11. What is the speed of the piston of a locomotive engine having 24 ins. stroke, with 7-ft. driving wheels when running at 40 miles per hour ? (Sc. & A. 1880.) 12. What is the piston displacement per minute in an engine with 20 ins. diameter cylinder, 2 ft. 6 in. stroke, running at 60 revolutions ? 13. What is the nature of the stress on the screwed end of the piston- rod during the to-and-fro motion of the piston ? 14. Sketch some form of engine crosshead. 15. Explain the nature of the thrust on the guides. 16. Of what use is the top guide when the thrust is usually carried on the bottom guide ? 17. Sketch some form of engine connecting-rod, 18. What are the ' dead centres ' ? 19. Draw a diagram showing the relative positions of piston and crank pin when the crank makes angles of 0°, 30°, 60°, 90°, 120°, 150°, 180°, v/hen the length of the connecting rod is i i times the stroke of the piston. Mark also upon the piston path, the piston positions for the same crank angles if the connecting rod were infinitely long. 20. In an engine with a cylinder 24 ins. diameter and 3 ft. stroke, the mean pressure of the steam on the piston is 45 lbs. per sq. in. ; find the mean pressure on the crank pin in the direction of its motion. 21. The crank ot an engine is 2 ft. long and the mean tangential force acting upon it is 17,000 lbs. What is the mean pressure of the steam upon the piston of the engine during each stroke ? (Sc. & A. 1S76.) 1. Sketch a sectional view of the steam and exhaust ports of an engine showing a valve, without lap, at the end of its stroke. Show by arrows the direction of the steam. 2. Define 'outside lap,' 'inside lap,' and 'lead' of a slide valve; make sketches illustrating your answer. 3. The width of a steam port is \\ in. ; the lap of the valve /j in., and the lead ^ in. Draw a diagram giving the travel of valve and angular advance of the eccentric. 4. What is the effect of outside and inside lap of the valve ? 5. Describe how you would set a slide valve. 6. Find the travel of a valve having J in. outside lap, and maximum port opening l| in. 7. Sketch a slide valve in mid position to the following dimensions : exhaust port 3 ins. wide, bars i in. wide, steam ports 2 ins. wide, outside lap I J in. Sketch also the same valve at the beginning of the pistoii stroke with \ in. lead. (Sc. & A. 1882.) T 2 276 Steam 8. Make a hand-sketch of a piston valve, describe its construction, and state what are its advantages. 9. Make a sketch of a double-ported slide valve, and explain how the pressure of steam at the back of the valve is largely removed. 10. Make a hand-sketch of an eccentric and describe its construction. 1 1. What is the object of the link motion ? and explain how that object is accomplished. 12. Make a sketch showing the approximately relative position of crank and eccentrics in a link motion. XI 1 . Make a sketch of a section of a Corliss cylinder, showing the steam and exhaust valves. 2. Make a sketch of the Reynolds-Corliss valve gear, and explain its action. 3. What advantages are claimed for the Corliss valve gear ? XII 1. Sketch a locomotive crank axle. 2. What is meant by ' the tangential pressure on the crank pin ' ? and how may it be determined geometrically, assuming the pressure is uniform throughout the stroke ? 3. What are the advantages of having two cranks at right angles rather than together or exactly opposite each other ? 4. Sketch a pedestal suitable to carry a shaft when the resultant load on the bearing is inclined to the vertical. 5. Why is it important that the bearings of shafts should be made sufficiently large ? XIII 1. What is the object of the condenser? 2. Make a sketch of a jet condenser, and sketch and explain the means adopted for removing the water from the condenser. 3. Sketch and describe the surface condenser. 4. Explain the circumstances which led to the abandonment of the jet condenser in favour of the surface condenser in steamships. 5. Make a sketch showing how the tubes are secured in the condenser tube plate. Questions 277 6. The index finger of a vacuum gauge points to 26, Explain the meaning of this. 7. Make a sketch of a feed pump, and explain its action. 8. The diameter of the plunger of a feed pump is 6 ins., length of stroke 10 ins. ; find the capacity of the pump. 9. Find the weight of water thrown per minute by a pump 1,000 cub. ins. capacity, and 25 deliveries per minute. 10. A pump is I ft. 9 ins. diameter ; length of stroke, 2 ft. 6 ins. ; the bucket is covered with water at each stroTce to a height of 2 ft. ; revolu- tions of engine 50 per minute ; find the weight of water lifted per hour. 11. A pump valve is made in the form of two rings, each i in. wide, and of internal diameter 5 and 10 ins. respectively ; what is the area of the openings in the seating ? 12. A pump valve is 3 ins. in diameter ; what should be its lift so that the opening for escape of water shall be the same as if there were no valve ? 13. A surface condenser has 1,725 tubes, each 13 ft. long, and J in. outside diameter ; what amount of condensing surface do they give ? Write down two numbers which express pretty nearly the relative conducting powers of copper and iron. (Sc. & A. 1876.) XIV 1. Sketch a Watt governor, and explain the object of the governor. 2. Say whether the governor fulfils its purpose perfectly, and give your reasons for your answer. 3. Describe the construction of the ' Porter ' governor, and sketch an arrangement showing how it may be made to act upon an expansion valve. 4. What is the object of the fly-wheel? 5. Make a sectional sketch of a locomotive showing the arrangement of the engine underneath the boiler. 1 . What are the purposes for which indicator diagrams are taken ? 2. Sketch a sectional view of an indicator, and describe its action. 3. Draw a good form of indicator-diagram for a non-condensing engine, and name and explain the different portions of the diagram. 4. Draw diagrams showing the probable effects of the slide-valve having moved out of position by the shortening or lengthening of the valve rod. 5. Compare by means of indicator diagrams the methods of regulating the power of the engine by ' throttling ' and by ' cut-off' respectively. 6. Draw and compare the diagrams from a condensing and a non- condensing engine respectively. 278 Steam XVI 1. What is the reason for the adoption of the compound engine, and in what respects is this engine superior to the single-cylinder engine ? 2. Make a hand-sketch of the cylinders of the compound engine in fig. 123, and explain how the steam is supplied to and exhausted from each cylinder. 3. How is the low-pressure cylinder of a compound engine proportioned ? 4. Find the number of expansions of the steam in a compound engine when the piston diameters are as i to 2, and the cut-off in the high-pressure cylinder is at half stroke. 5. Find the point of cut-off in the high-pressure cylinder of a two- cylinder compound condensing engine, when the volumes of the cylinders are as I to 3J ; initial pressure 90 lbs. by boiler gauge and terminal pressure 10 lbs. absolute ; allowing a loss of 5 lbs. between boiler pressure and initial pressure in the cylinder. XVII 1. Make a skeleton sketch of a compound tandem engine. 2. Draw a theoretical indicator diagram illustrating the distribution of the steam in the Woolf engine, assuming a cut-off at one-third of the stroke in the high-pressure cylinder, taking steam to end of stroke in low-pressure cylinder, and ratio of cylinder volumes i to 3. Combine the diagrams. 3. Compare, by an example, the range of temperatures in the separate cylinders of a compound engine, with the range on a single-cylinder, using the same number of expansions of the steam. 4. Make a sketch showing plan of the arrangement of cylinders and pipe connections for two-cylinder compound, three-cylinder triple, four- cylinder triple, and quadruple expansion engines. 5. Make a skeleton sketch of a two-cylinder compound receiver engine. 6. Explain the distribution of the steam in the two-cylinder compound receiver engine, and illustrate your .answer by drawing the theoretical dia- gram for a cut-off at half stroke in both cylinders. 7. Write what you know of the improvement in coal consumption which has taken place with the introduction of the various types of com- pound engines. 8. How do you account for this improvement ? 1. Compare the resistance of cylindrical vessels to internal pressure, longitudinally and transversely. 2. Make a sketch showing how Galloway tubes are fitted to boiler furnace flues. 3. Draw a longitudinal section of a Lancashire boiler, showing all the necessary fittings. 4. Make a sketch of an economizer, and explain its action. 5. Suppose the feed- water to enter the economizer at 98° F. and to Questions 279 leave it at 218° F., instead of entering the boiler direct at 98° F. : find the gain per cent, by using the economizer, supposing the total heat of the steam from 32° F. to be 1 190. 6. A steamship has two boilers, each with three furnaces, 3 ft. diameter by 6 ft. long ; find the fire-grate area. 7. Find the heating surface of a marine boiler of the following dimen- sions : — (a) 3 Furnaces, each 3 ft. diameter X 6 ft. long. {]/) 3 Combustion chambers : Top plates 2 (3 ft. 6 ins. x 2 ft. 3 ins.) I (3 ft. o in. X 2 ft. 3 ins.) Back plates 2 (3 ft. 6 ins. x 3 ft. 6 ins.) 1 (3 ft. o in. X 5 ft. o in.) Side plates 4 (2 ft. 3 ins. X 3 ft. 6 ins.) 2 (2 ft. 3 ins. X S ft. o in.) (<:) 3 Back tube plates : 2 (3 ft. 6 ins, X 3 ft. o in.) I (3 ft. o in. X 4 ft. 6 ins.) Less area of 200 holes, 3 ins. diameter. {d) 200 tubes, 3 ins. internal diameter, length between tube plates 6 ft. 3 ins. 8. Make a sketch of the longitudinal section of the locomotive boiler, showing how the flat crown of the surface is stayed. 9. Sketch and describe the Babcock and Wilcox boiler, and say what are the advantages and disadvantages of water-tube boilers. 10. Illustrate the advantage of small tubes over large ones, to provide large area of heating surface. 11. Make a sketch of a lever safety valve. 12. A valve, 3 ins. diameter, is held down by a lever and weight, length of the lever being 10 ins., and the valve spindle being 3 ins. from the fulcrum. You are to disregard the weight of the lever, and to find the pressure per square inch which will lift the valve when the weight hung at the end of the lever is 25 lbs. (Sc. & A. 1881.) 13. Sketch a Ramsbottom safety valve. 14. Find the dead weight required for a valve 3J ins. diameter required to blow off at 90 lbs. per sq. inch. 15. Sketch an equilibrium double beat valve. 16. Describe the construction and action of Bourdon's pressure gauge. XIX I. Make a sketch of a simple draught gauge, and explain how it acts ; what is the equivalent pressure per square inch of f in. of water-head in the gauge ? 28o Steam 2. Describe some of the causes of loss of efficiency in the management of the boiler furnace. 3. What should be the aim of the fireman who wishes to obtain the largest amount of steam from the boiler, and how can he secure and maintain it ? 4. What is generally the cause of the formation of smoke in boiler furnaces, and how would you manage a furnace so as to reduce the smoke to a minimum ? 5. Sketch a. Meldrum apparatus as fitted to a Lancashire boiler, and explain how it acts and why it is used. t. Sketch a section of the Serve tube, and explain the advantages that may be claimed for it. 2. What is meant by "equivalent evaporation from and at 212° F."? A boiler evaporates 7 '5 lbs. of water per pound of coal when working at 150 lbs. pressure, feed temperature 100° F. ; find the equivalent evaporation from and at 212° F. 3. Find the power of a marine boiler having three furnaces, each 3 ft. by 6 ft., and burning 20 lbs. of coal per square foot of grate per hour, allowing an evaporation of 10 lbs. of water per pound of coal. 1. Describe the chemical processes involved in the combustion of coal in a boiler furnace. 2. Name and describe the various classes of fuel used by engineers. 3. Describe the construction and use of a Thompson's calorimeter. 4. Write down the formula of Dulong for calculating the total heat of combustion, and find the heat value of a sample of coal having the following composition : — Carbon o'82, hydrogen 0'04, oxygen o'i2. 5. If 25 lbs. of air are supplied to a furnace per pound of coal burned, the temperature of chimney gases is 580° F., and the heat value of the coal 13>000 B.T.U. ; find approximately the percentage loss of heat at the chimney. Temperature of the atmosphere 50° F., specific heat of gases o'24. 6. Write down the formula showing the resulting gaseous products obtained when 20 lbs. of air are supplied to burn : lb. of carbon. 7. Given that the composition of the gases from a boiler-flue by volume is 10 per cent. CO^, i per cent. CO,' and 8'5 per cent, free oxygen, find the weight of air supplied per pound of coal consisting of 90 per cent, pure carbon. 8. In the above example find the percentage loss due to the presence of I per cent, of CO. Questions 28 1 XXII 1. What precautions would you take when' getting up steam? 2. Suppose the vacuum in the condenser was not satisfactory, what would you do ? 3. What points should be attended to by a man in charge of a boiler ? 4. How would you test {a) for a leaky slide valve ; {b) for a leaky piston ? 5. How would you adjust the brasses to their journal, after the journal had worked loose by wear ? ANSWERS I (3) -IS". -10°, 5°, 20°, 70°, 130° C,;and 33'8°, -22°, -13°, 194°, 248° F. (4) 25°. (5) 1° nearly. (6) 521°, 561°, 708°, and 273°, 373°. (7) 136° R., and 132° R. II (2) 157 units of heat ; or 121,204 units of work. (3) 40 units of heat ; 30,880 units of work. (4) 137J units of heat ; 106,150 units of work. (5) ll6o'6. IV (3) 7-5 cub. ft. (4) 36-47 lbs. (5) 11-34 cub. ft. V (2) 21,168 ft. lbs. (6) 63,072 ft. lbs. (7) 31-39. (10) 1183-4 and 878-4. (II) 201 sq. ins. ; 4824 cub. ins. (12) 197-024 sq. ins. ; 4728-576 cub. ins. (13) 54-53 tons. (14) 15 ins. (15) 224 cub. ft. ; 8-38 lbs. VI (5) 2o6-66°F. (6) 1-53 lbs. (7) 17-63 lbs. (8) 120° F. VII (2) 30 and 15. (3) 25 and 5. (4) 18 lbs. absolute. (5) 35, 18-33, 10. (6) \. (8) 60, 40, 30. 282 Steam VIII (5) 37-4- (7) 37-4- (9) Si'iS. (lo) 197-4- (II) 50-66. (12) 76. (13) 80. (14) Hi- (15) 407-27. (16) 15-74 ins. IX (6) 26-47 cub. ft. (7) -83 lb. (8) 8466 lbs. (11) 640 ft. per min. (12) 654-16 cub. ft. per min. (20) 12,960. (21) 26,703-6 lbs. (6) 4i ins. XIII (8) 282-7 cub. ins. (9)904-2^5. (10) 900,000 lbs. (Ii) 18-85 and 34-46 sq. ins. (12) finch. (13) 4403 sq. f.. ; 6 to I. XVI (4) 8. (5) jV XVIII (5) 10-67. (6) 108 sq. ft. (7) 1207-27 sq. ft. (12) 1 1 -9 lbs. per sq. inch. (14) 865-89 lbs. XX (2) 8-7 lbs. (3) 10,800 lbs. of steam per hour. XXI (4) 13,522 B.T.U. (5) 25-4 per cent. (6) 3-6 CO2 + 2 oxygen 4- I5'4 nitrogen. (7) 18 lbs. of air per pound of coal. (8) 6-3 per cent. Board of Education — Questions 283 BOARD OF EDUCATION STEAM EXAMINATION PAPERS 1896 First Stage, or Elementary Examination I Explain the difference between a simple non-condensing engine, a condensing engine, and a compound non-condensing engine. Give outline sketches of the general arrangement in a horizontal engine of each of the three classes. (15.) 2. Sketch and describe the escape valve as fitted to the cylinders of a marine engine. What is the use of such a valve ? Show, by a sketch, where it is fixed. (10.) 3. A steam-engine has a steam cylinder of 20 inches in diameter, the crank measures 18 inches from the centre of crank-shaft to centre of crank-pin, the engine runs at 85 revolutions per minute, and tlie mean effective pressure of steam on the piston is 28 lbs. per square inch ; find the indicated horse-power of the engine. (10.) 4. Make a sketch and describe the construction of one form of piston cross-head with which you are acquainted. Under what conditions may a slipper slide for the piston cross-head be employed in a hori- zontal engine ? (10.) 5- What would be the indicated horse-power of a locomotive when moving at a steady rate of 35 miles per hour on a level rail, the weight of the train being 130 tons, and the resistance to traction 10 lbs. per ton? (10.) 6. Make a sketch and describe the construction of an eccentric sheave and strap. Show the position of the crank-shaft through the eccentric, and indicate on your sketch the throw of the eccentric. Name the materials of which the several parts of the eccentric are made. (10.) 7. Describe and sketch the construction of a double-beat or equilibrium valve. When and for what purpose are such valves used ? In such a valve the two seats measure respectively 8 inches and 7J inches in diameter, and the weight of the valve is 70 lbs. What pressure per square inch would cause the valve to lift, the pressure between the valve-discs being disregarded ? (15) 8. Sketch the construction of a lever safety-valve with balance weight, and state under what circumstances such a construction could not be used. If the lever be 16 inches in length, and the centre of the valve seat is 4 inches from the fulcrum, while the diameter of the valve is 4 inches ; find the weight to be placed at the end of the lever so that steam may blow off at a pressure of 45 lbs. per square inch, the weight of the valve and of the lever being neglected. (15.) 284 Steam 9. Make a longitudinal and also a transverse section of a Lancashire boiler with its brickwork settings. Indicate the course of the gases through "the internal and external flues of the boiler to the chimney. Show also the construction of the fire-bridge and method of sup- porting the fire-bars. (20.) 10. Sketch and describe the construction and action of a non-return feed- water valve for either a land or a marine boiler. Where and at what level is such a valve placed on the boiler? (15-^ 11. What is meant by "sensible heat," "latent heat," and "total heat of evaporation " ? Calculate the total heat in British thermal units required to convert 30 lbs of water at 62° F. into steam at a tempera- ture of 212° F. If I lb. of coal develops 14,000 units of heat during its com- bustion, how many pounds of coal would be required to convert the 30 lbs. of water into steam under the above conditions, if there was no loss of heat in the operation ? (i5-) 12. Sketch and describe the construction of the air-pump of a condensing engine. What is the use of the air-pump ? If the temperature of the injection water supplied to a jet condenser be 62° F., and the water is pumped out of the hot well at a temperature of 106° F., and the steam to be condensed enters the condenser at a temperature of 212° F., what weight of injection water would be required per pound of steam condensed ? (20.) 1897 Describe clearly, with sketches, the working of any single-cylinder direct-acting non-condensing engine with slide-valve and eccentric. Do not give too much detail, but show that you understand how the piston and stuffing-box are made steam-tight ; how the piston is fastened to the rod ; how the ends of the connecting-rod are made ; the action of the governor and of the flywheel. The diameter of the cylinder of an engine is 30 inches, and the stroke of the piston is 4 feet. If steam is admitted at an absolute pressure of 70 lbs. per square inch, and is cut off when the piston has travelled I foot, what would be the total pressure on the piston at the point of cut-off, and also when the piston has travelled 2 feet, 3 feet, and 4 feet respectively ? Take the simplest law of expansion. Why is it economical to cut off steam before the end of the stroke? (10.) What are meant by temperature ; expansion by heat ; pressure of a fluid ; Fahrenheit scale ; Centigrade scale ; absolute scale ; latent heat ; Regnault's total heat in a pound of steam ; calorific value of a fuel ; combustion ; conduction of heat ; convection of heat ; radiation Board of Education — Questions 285 of heat ; indicated horse-power ; brake horse-power ? Very brief answers are expected. (15.) What information ought to be found written beside an indicator diagram? How would you proceed to find the indicated horse-power ? The average breadth of the two diagrams on one card is i '56 inches ; scale, -3'g ; piston, 12 inches diameter; crank, i foot; no revolu- tions per minute. Find the indicated horse-power. What is the actual horse-power given out by the crank-shaft likely to be? (ij.) Sketch and describe the construction of the air-pump bucket with its valves and packing, and show how it is worked in connection with a jet condenser. Of what materials are the body of the bucket and of the valves respectively made? (lo.) Why did Watt's invention of the condenser effect a great economy ? Why does condensation take place in the cylinders of modern engines, and how do we attempt to get rid of it ? (15.) Explain and show, with sketches, the construction and action of the force-pump employed for feeding the water into a boiler when an injector is not used. Sketch alio in section the "clack" or non- return valve attached to the boiler. How is the pump prevented from forcing water into the boiler when the engine is running, but a supply of water is not required ? The ram of such a pump is 2 inches in diameter, and has a stroke of 24 inches. How many gallons of water (neglecting leakages) would be forced into the boiler for each 1000 double strokes (one forward and one backward) of the pump ? I gallon of water = '16 cubic feet. (15.) Describe and show by a sketch the construction of Ramsbottom's safety valve for a locomotive engine. How are the lever and valves prevented from flying off in the event of the spring breaking ? If in a Ramsbottom valve the two valves each have a diameter of 2J inches, what would be the pull on the spring when the steam is just blowing off at a gauge-pressure of 140 lbs. to the square inch (neglect the weight of the valves and connections) ? (10.) What do you understand by the efficiency of an engine ? What would be the efficiency of a good marine engine and boiler which indicates one horse-power for every 2 lbs. of coal consumed in the furnace of the boiler per hour, supposing the coal to have a calorific power of 14,500 Fahrenheit thermal units per pound? (10.) Describe and sketch a Lancashire boiler and its seating. What precautions are taken in sloking to prevent smoke from the furnace of such a boiler ? How are the ends strengthened, and how are they fastened to the shell ? (15.) An engine gives 10 indicated horse-power and 7 '6 brake horse-power for a consumption of 230 cubic feet of coal-gas per hour. The calorific power of the gas is 530,000 foot-pounds per cubic foot. What is the efficiency ? ( 10. ) 286 Steam 1898 What heat must be given to I lb. of water at 80° F. to convert it into steam at 300° F. ? Regnault's formula for the total heat of a pound of steam from water at 32° F. being H = 1082 + 0'305 /, where i° F. is the temperature of the steam, how many pounds of this steam are equivalent in total heat to the calorific power (15,000 units of heat) of a jjound of coal ? Describe carefully how you would measure the pressure of steam at various temperatures. Show, roughly, what kind of curve ycu would obtain if you expressed your results on squared paper. ('S-) Define temperature, Fahrenheit scale, absolute temperature, unit of heat, Joule's equivalent, capacity for heat, total heat of a pound of steam, latent heat, intrinsic energy, entropy. (15.) Describe an indicator j how is it attached to a steam or gas or oil engine ? Choose some one of these and sketch the sort of diagram obtained, and state what information it gives us. Show how the horse-power is calculated. (15.) What is meant by " clearance " ? If a piston is 12 inches in diameter, and the crank I foot, what is the working volume in cubic inches ? If the clearance is such that 4 lbs. of water just fills it when the piston is at the end of its stroke, express it as a percentage of the working volume. If the working volume is represented to such a scale that a distance of three inches represents I cubic foot, what distance will represent the clearance ? (15.) What do we mean by " working steam expansively " ? Steam at 60 lb. per sq. in. absolute, is used in a cylinder whose stroke is 2 feet, and expansion begins when one-quarter of the stroke has been performed. What are the probable pressures at half, three-quarters, and the end of the stroke ? Show your answers in a diagram. Find the average pressure. (15.) Why is priming such a, great evil ? What is the cause of the con- densation of steam in a cylinder before cut-off? How do we try to diminish it? (15.) Describe and sketch a. slide-valve ; describe how it distributes the steam, and how it is worked. What are meant by the terms lap, half-travel, inside lap, and advance? (15.) Sketch and, describe briefly the construction of a piston, showing how it is made steam-tight. Sketch a gland and stuffing-box, and the crank-pin end of a connecting-rod. (15.) Describe, without too much detail, the working of any gas or oil engine with which you are acquainted. Board of Education— Qteestions 287 If 20 lb. of oil (calorific value 21,000 Fahrenheit units) are used per hour, the brake horse-power being 18, what is the efficiency ? (20.) Describe with sketches the bed or frame of any engine with which you are acquainted. If you choose either — (1) A large or small stationary engine, horizontal or vertical; (2) A locomotive engine ■ (3) A marine engine ; (4) A gas or oil engine ; sketch carefully how the cylinder is attached to the frame, and how the slide or slipper is guided in the engine you select ; show also the crank-shaft bearing, and how the frame is itself attached to, or supported from, the ground or to the frame of a ship. If you are better acquainted with the construction of a steam-turbine or an impulse wheel, describe and sketch one of these instead. (20.) Describe with sketches a boiler of any kind. You need not show any fittings. What are the most important things to remember in connection with the furnace and flue parts ? 1899 Answer only one of the following questions, A, B, or C : — A. Sketch and describe the staying of the top and sides of a loco- motive fire-box, and how the fire-bars are supported. (20.) B. Sketch and describe the construction of the front end plate of either a two-flued Lancashire boiler or a marine boiler (not a water-tube boiler), and show how it is connected with the shell plates, and how it is otherwise strengthened or stayed. (20.) C. Show by sketches how the piston-rod and connecting-rod are attached to the crosshead. With a crank of one foot and connecting-rod J feet, find by construction the distance of the piston from the near end of the stroke when the crank stands at 30° on either side of each dead point position. (20.) Answer only one of the following questions. A, B, or C : — A. Sketch in position in the frame and describe any construction of axle-box of a locomotive engine with which you are acquainted, and show the arrangement of the springs. (20.) 288 Steam B. Sketch and describe a tube igniter, and how the timing valve is worked, in, say, a 20 horse-power gas engine. (20.) C. Sketch the main casting of a large jacketed cylinder, and describe clearly how the cylinder liner and valve seat are attached. (20.) 3. What additional parts are required in order to convert a non-con- densing into a condensing engine ? Under what circumstances is it better to use a condensing engine ? When is it necessary to use a surface condenser ? How is a surface condenser constructed ? (15.) 4. Describe the construction, with the aid of sketches, showing the valve on its seating, of either a dead, weight, a lever, or a spring safety valve. Say to what class of boiler the valve you select is specially adapted, and whether it can be used alike on stationary, locomotive, or marine types of boiler. Give reasons for your answer. Suppose the steam pressure is just sufficient to lift a valve, is it sufficient to keep it well open ? (15.) 5. Describe the several parts, and show by sketches the construction of the eccentric of a steam engine. Sketch the eccentric in position on the shaft, and mark clearly the length of the throw of the eccentric which you sketch. What is meant by the angle of advance, and why do we have an angle of advance ? ( IS. ) 6. Describe, with sketches, either a gas or oil engine, and show by a diagram how it uses the Otto cycle of operations. Sketch the cylinder, showing piston, water-jacket, valves, shape of clearance space, and how the exhaust is provided for. (15.) 7. When steam pressure is acting on a piston, is the whole of it trans- mitted through the piston-rod to the crosshead ? If not, how is the difference employed ? And if the speed of the engine increased while the steam pressure remained about the same, would the force at the crosshead remain the same as before ? If not, why not ? (15) 8. How would you determine the mean pressure of steam in a stsam. engine cylinder, when the indicator diagram is given ? and besides the mean effective pressure, what other data are necessary to enable you to calculate the H.P. of the engine? The two cylinders of a locomotive are each 17" in diameter, the length of each crank is 12", the mean effective steam pressure is 80 lbs. per square inch, and the driving-wheel of the locomotive makes no revolutions per minute ; under these conditions, what is the H.P. of the engine? (i5-) 9. Steam enters a cylinder at any initial pressure you please, say 120 lbs. absolute, and is cut off at two-fifths of the stroke, it expands according to the law "pressure is inversely as volume." Find the average pressure absolute during the forward stroke ; what fraction is it of Board of Education — Questions 289 the initial pressure? Neglect clearance, and do the calculation by construction if you can. (15O 10. Answer only one of the following, either A ox B : — A. Change into horse-power the rates of conversion of chemical energy by combustion of the following : — i lb. of kerosene per hour ; I cubic foot of coal gas per hour ; i cubic foot of Dowson gas per hour ; I lb. of coal per hour. The calorific powers are, in Fahrenheit pound heat units, I lb. of kerosene, 22,000; I lb. coal, 15,000; I cubic foot of coal gas, 700 ; I cubic foot of Dowson gas, 160. (15.) B. Using the calorific powers given above, calculate the efficiencies of :— («) A large good condensing engine, using 2 lb. of coal per brake-horse-power-hour. (i) A gas engine using 26 cubic feet of coal gas per brake- horse-power-hour. {c) The Diesel oil engine which is said to use 0"56 lb. of kerosene per brake-horse-power-hour. i^S-) 11. Answer only one of the following questions, A or B : — A. One boiler produces 9 lb. of dry sleam at 402° F. from feed water at 62° F., and another 10 lb. of dry steam at 302° F. from feed water at 110° F. per pound of the same fuel; compare these performances. ('SO B. Reynolds found 1399 foot-pounds to be equivalent to the average heat to raise a pound of water one Cent, degree between 0° C. and 100° C. Regnault gives the total heat of a pound of water from 0° C. to 100° C, as loo'S ; what is the Joule's equivalent which suits Regnault's unit of heat? (150 12. State shortly why superheating, steam-jacketing, and successive expansion are now being used in steam engines. (I5-) 1900 Try only one of the following, A, B, or C : — A Describe with sketches the crank-pin end of any connecting rod. (I5-) B. Describe with sketches any form of locomotive regulator valve to admit steam from the boiler to the cylinder steam chests. (I5-) C. Sketch in section a gas-engine cylinder, showing the valves and piston. ('50 U 290 Steam 2. Try only one of the following, A, B, or C : — A. Describe with sketches a Parsons steam turbine. State why it is necessary to make the steam go in series through many elementary turbines. (IS-) B. Describe a mechanical stoker and how it acts. Under what circumstances is its use preferable to hand-firing? (15.) C. Sketch in section half the crank-shaft of an inside cylinder locomotive, describing the construction of the crank and driving wheel, and showing also two eccentric discs. ('SO 3. If a piston and rod weigh 290 lbs., and if at a certain instant when the resultant total force due to the steam pressure is 7 tons, the piston has an acceleration of 420 ft. per second in the same direction, what is the actual force acting on the cross-head ? (I5-) 4. Steam enters a cylinder at 180 lbs. pressure per square inch (absolute) ; is cut off at one-third of the stroke, and expands according to the law " p V constant." Find the average pressure (absolute) during the forward stroke, neglecting clearance. If the back pressure is 17 lbs. (absolute) per square inch, what is the average effective pressure ? ■ If the area of the cross-section of the cylinder is 112 sq. ins. and the stroke is 24 ins., what work is done in one stroke ? (15.) 5. Sketch a gas-engine indicator diagram. How is it used in finding the indicated horse-power ? State clearly what information is neces- sary. Why must we know the number of explosions per minute rather than the number of revolutions ? (I5') 6. Sketch a simple slide-valve showing cylinder ports, and the valve chest. What do we mean by outside lap of a valve, inside lap^ advance, lead ? What is the effect of each of these on the indicator diagram? (15.) 7. The printed table given to you will enable you to calculate Regnault's total heat of i lb. of steam. State exactly what this means. o'7 lb. of water at 145° C. is converted into steam at the same temperature ; how much heat is given ? How much entropy is given ? 8. When steam is admitted to the cylinder, why does much of it condense ? What becomes of the condensed water, (i) dunng expansion, (2) during exhaust ? How may we diminish this initial condensation ? (20.) 9. If steam is cut off both in the down and up strokes of a vertical engine (the crank being below the cylinder) when the crank makes an angle of 70° with a dead point, show that this means a later cut-off in the down stroke than in the up stroke. Is this a good or a bad result? ' (15.) 10. Why do we regulate an engine with both a fly-wheel and a governor ? Explain clearly how each effects the regulation. (15.) 1 1 . An electric light station, when making its maximum output of 6co kilowatts, uses 1920 lbs. of coal per hour. When its load factor is 30 per cent, (that is, when its output is 600 X 30 -i- 100), it uses 1026 lbs. of coal per hour. \^'hat will be the probable consumption of coal per hour when the load factor is 12 per cent. ? Use squared paper. (15.) Board of Education — Questions 291 1 2. Describe with sketches some one form of boiler with which you are acquainted. No fittings need be shown. What are the most impor- tant things to remember when designing the furnace and flues ? (15.) 1901. 1 . Describe, with sketches, only one of the following, A, B, C, ox D : — A. Any kind of cross-head, showing ends of piston rod and con- necting rod and guide. £. A gas or oil engine cylinder, showing valves and piston. C. A surface condenser, showing the stays and the attachment of the tubes. D. An air pump, showing foot, bucket, and delivery valves. (i6.) 2. Describe, with sketches, only one of the following, A, B, C, or D :^ A. I.ocomotive cylinders, and how they are fastened to the side frames. B. A steam or gas engine governor, and how it regulates. C. A spirit or oil engine for a motor car, showing how it drives the car and how it works. v. The frame of a marine engine, showing how the pumps are worked. (17.) 3. Answer only o?ie of the following. A, B, or C : — Describe how you would experimentally determine — A. How the pressure and temperature of steam depend upon one another. Why must there be no air present ? B. The calorific power of any kind of burning gas. C. The latent heat of steam. (16.) 4. State the following amounts of energy in foot-pounds : — A weight of I ton which may fall vertically 10 ft. 3 lbs. of water raised 20° C. One horse-power hour. 3 watts for 200 hours. ( 1 5 ■ ) 5. Draw the compression, ignition, and expansion part of a gas-engine diagram. If the volumes and pressures at four points on the diagram, to any scales whatsoever, are represented by — Points A. B. C. D. Volumes . 5 I '5 2 ♦ Pressures . I 4 10 3 and if at the point A we know that the temperature is 127° C, what are the temperatures at the other points ? Tabulate the results. (16.) A steam electric generator on three long trials, each with a different point of cut-off on steady load, is found to use the following amounts of steam per hour for the following amounts of power :— U 2 292 Steam Pounds of steam per hour . 4020 6650 10,800 Indicated horse-power . 210 480 706 Kilowatts produced 114 290 435 Find the indicated horse-power and the weight of steam used per hour when 330 kilowatts are being produced. (16.) Steam enters a cylinder at 150 lbs. (absolute) per square inch. It is cut off at one-fourth of the stroke and expands according to the law "/ V constant." Find the average pressure (absolute) in the forward stroke. If the back pressure is 17 lbs. (absolute) per square inch, what is the average effective pressure? If the area of the cross- section of the cylinder is 126 sq. ins., and the crank is II ins. long, what work is done in one stroke? Neglecting clearance and con- densation, what volume of steam enters the cylinder per stroke ? (17. ) Using the formula in the table of useful constants furnished you, find the volume of i lb. of the steam admitted to the cylinder of Ques- tion 7. What weight of steam is actually admitted to that cylinder per stroke ? If you do not care to use the formula, use the following information and squared paper : Steam at 150 lbs. pressure is at 181° C, and the following numbers are known : — Temperature .... 175° C. 180° C. 185° C. Volume in cubic feet of i lb.') of steam . . , . j 3'4i9 3-065 2-756 (16.) Sketch a simple slide valve placed symmetrically over the cylinder ports, and in dotted lines show it at the beginning of a stroke of the engine. What do we mean by outside lap, inside lap, lead of valvi, 3.nA advance of eccentric i (15.) If cut-off takes place on both sides of a piston when the crank makes an angle of 90° with the dead point, (i) assuming connecting rod infinitely long, (2) assuming connecting rod four times length of crank ; find in each case for each side of piston the fraction of stroke at which cut-off takes place. (16.) We endeavour to prevent condensation in the cylinder of a steam engine, {a) by a separator, (li) by superheating, (c) by drainage from the cylinder, {d) by steam jacketing, {e) by high speed. Explain how each of these methods tend to effect our object. (18.) Using the formula in the table of useful constants furnished you, find how much heat was given to each pound of feed water at 20° C. to convert it into the steam which is admitted to the cylinder of Ques- tion 8, if that admitted steam is at l8i° C. and is not wet. (15.) Board of Education — Questions 293 igo2 Describe, with sketches, only one of the following. A, B, C, or D : — A. A piston slide valve and its seat, showing packing and ports. B. Any engine, steam, spirit, or oil, used on motor cars. C. Any link motion or other reversing gear to work a slide valve with which you are acquainted. State exactly what is the effect of altering the gear. D. Either a Geipel or Sirius or Turnbull or Lancaster steam trap. Describe, with sketches, only one of the following. A, B, or C : — A. A double-ended cylindric marine boiler ; the usual positions of joints of plates and of stays to be indicated. Where and why is leakage probable under forced draught ? B. Any water -tube boiler ; the general construction to be clearly shown : some one part shown in good detail and more carefully described. C. A steam boiler for a traction engine or a motor car, the fuel being oil or spirit. Describe carefully any appliance necessary in this boiler which is not usually found on a stationary boiler. Answer only one of the following. A, B, or C. How would you experimentally determine : — A. Thelatentheatof steam at atmospheric pressure? Why is it more difficult to measure the latent heat at, say, two atmospheres? B. The total heat obtainable from the burning of one pound of kerosine ? C. How the rate of passage of heat from hot gas inside a tube to water outside the tube depends upon the velocity of gas along the tube ? State the following amounts of energy in foot-pounds : — A weight of I '6 tons may fall vertically 12 feet. The kinetic energy of a body of 100 lbs. moving at 1200 feet per second. 2'4 lbs. of water raised from 50° F. to 80° F. The latent heat of steam at atmospheric pressure. One horse-power hour. 2'3 kilowatts for 5 hours. The energy given to a mass of fluid at 150" C, increasing its entropy by the amount of 0'56 ranks, its temperature keeping constant. Answers — M,oo% \ 2,236,025; 56,016; 752,326; 1,980,000; 30,522,788; 117,012. Steam of 150 lbs. per square inch (absolute) is cut off at \ stroke, and expands according to the law/7/ constant. Find the average pressure in the forward stroke, using squared paper. The back pressure is 294 Steam 1 8 lbs. per square inch, what is Ihe effective pressure on the piston? The piston is 15 inches diameter ; crank I foot ; two strokes in the revolution ; 120 revolutions per minute ; find the work in one revolution and the horse-power. Ansiuers — 61,420 ft.-lbs. ; 223'4. 6. At an electric-power station, 4150 units of electric energy were sold in 24 hours, the coal consumed being j6,200 pounds. And on another occasion 2489 units were sold in the 24 hours, the coal consumption being 12,380 pounds. It is known that if units of electricity and weight of coal are plotted on squared paper, the points will lie fairly well in a straight line. The maximum output is 25,000 units. Find the coal consumed in the 24 hours, when there are the daily outputs of 8, 16, 24, and 50 per cent, of the maximum. In each case what is the coal per unit ? Tabulate your answers. Answers — 35,400520,900; 15,810; 11,210; 282; 3'48; 4'20; 561. 7. Sketch the compression, ignition, and expansion parts of a gas engine diagram. If the volumes and pressures at four points on the diagram, to any scales whatsoever, are represented by — Points A. B. C. D. Volumes 6 i'7 2 4-5 Pressures . z 5 13-8 3 '2 and if at the point A we know that the temperature is 140° C, what are the temperatures at the other points ? Tabulate your results. Anszuers—i/iO° C. ;-3i2° C. ; 1627° C. ; 718° C. Sketch the section of a simple slide valve placed symmetrically over the ports, and, in dotted lines, show it at the beginning of the stroke of the engine. What do we mean by outside lap, inside lap, lead of valve, and angular advance ? Draw another view of the valve, showing its face. A piston and rod and crosshead weigh 330 lbs. At a certain instant, when the resultant total forces due to steam pressure is 3 tons, the piston has an acceleration of 370 feet per second per second in the same direction. What is the actual force acting at the crosshead ? Answer — 2930 lbs. A vessel is filled by 100 tons of water at 210° C. How much steam must be taken away just dry at 175° C, through a reducing valve, for the temperature of the remainder to become 175° C. ? You are given that the latent heat of steam at 175° C. is 4827 centigrade units. Answer — 7-25 tons. There is a balance weight of 180 lbs. at a distance of 3 '4 feet from the centre, and another weight of 150 lbs. at a distance of 2 '56 feet from Board of Education — Questions 295 the centre, in a direction at right angles to the first, both on the same driving wheel of a locomotive. Find the amount and position of any single weight which would have the same balancing effect as these two. Answer — 241 lbs. at 3 feet ; angle 32°. 12. Describe, with sketches, a loaded Watt governor. Why is a load used ? 13. Describe, with sketches, how lubrication of the various parts of an engine (not encased) is now usually performed. Aiiswei' — See figs. 210 and 211. 1903 Describe, with good sketches, some one important detail of a modern steam or internal combustion engine with which you are well acquainted. If, for example, the crank pin and the end of a connecting rod be shown, it is of no use merely indicating the existence of a bolt and nut ; the bolt and nut, and the method of locking the nut, must be clearly shown. Again, it is no use making a sketch of so much of any engine that details cannot be clearly sketched. For example, a whole governor with its gear would be too much, but certain parts may be chosen. This question is to test your knowledge of details and your power to sketch. Describe, with good sketches, some one important part of any kind of boiler. For example : — a fitting like a safety valve ; the staying of the fire-box crown of a locomotive ; the arrangement of a furnace ; a feed-water heater, gauge glass and connections. The remarks in Question I apply here also. In connection with the steam or gas or oil or spirit engine work with which you are acquainted, there is testing of some sort to be done requiring careful measurement of work or heat. For example : — finding the calorific power of coal, gas, or oil ; finding the latent heat of steam ; or how its pressure depends upon temperature ; or finding the wetness of steam during an engine test ; comparing the power of an engine and the quantity of heat or of steam, gas, or oil used per hour. Describe, with sketches, some one such test. (Should you choose to answer also Question 10, there must be no repetition.) Steam enters a cylinder at 140 lbs. pressure (absolute) per sq. inch ; is cut off at 0'35 of the stroke, and expands according to the law "pv constant." Neglect clearance and cushioning, and draw the hypo- thetical diagram usually taken. Back pressure 17 lbs. per sq. inch. Find the effective pressure. Area of piston, i sq. foot ; stroke, 2 feet. What is the work done in one stroke? How many cubic feet of steam entered the cylinder ? What is the work done per cubic foot? Answer — 83'52 ; 24-053 ft.-lbs. ; 07 cub. ft. ; 34,362 ft. -lbs. 296 Steam 5. An engine whose speed and cut-off do not alter uses W lbs. steam per hour when its actual horse-power is P, and W and P have been carefully measured during three long tests. P. 152 no 56 w. 3190 2630 1850 What is the probable W when P is 125 horse-power ? In each of the four cases find the steam used per horse-power hour. Answers — 2830 lbs.; 2i'olbs. ; 23'9lbs. ; 33 '03 lbs. ; 227 lbs. What is Regnault's total heat of steam at 170° C. ? Use the formula on the outside page of the tables given you. State exactly what you mean by this total heat. How much of it is given to the water ? How much is called latent heat of steam ? Give these two answers for steam at 100° C. Ans'wers—d'^Z-l; 170; 488'3; loo; 537. Sketch a simple slide valve showing cylinder ports and no more of the cylinder ; show the valve in its mid position. Show in dotted lines the position of the valve when the piston has jast begun its stroke. What do we mean by outside lap of a valve, inside lap, advance, and half-travel ? How do these affect the distribution of steam ? A link motion or otlier gear for a slide valve will reverse an engine, but suppose we do not reverse the engine ; suppose we only change from say full to half gear, state clearly what it is that is really effected by the change. Sketch also the probable change in the indicator diagram. What is the cause of priming in boilers ? Even if the boiler does not prime, why may wet steam reach the cylinder ? What may be done to prevent it ? Even if only dry steam enters the cylinder, why may there be condensation on admission ? Why is this harmful ? What may be done to prevent it ? Describe the construction of an indicator, and how it is used. Give a sketch of a specimen indicator diagram from a steam, gas, or oil engine, and describe what each part of the diagram means. Why is an engine balanced? Describe generally any method of balancing the rotating parts that is known to you. Imagine a long railway truck containing an invisible caged lion on a level track ; axle bearings frictionless ; imagine the lion to walk backwards and forwards to the limits of its cage, what would an outsider observe ? Now suppose the wheels of the waggon blocked, what occurs ? State very clearly what are the conditions that must be satisfied for good combustion in a furnace, and for the efficient communication of heat from the hot gases to the water of a boiler. Board of Edtication— Questions 297 1904 Answer and illustrate by good sketches either A or B, but not both :— A. In reference to any modern example of a steam, gas, oil, or spirit engine you like to select, how is leakage prevented past the piston ? Also past the piston-rod if a steam engine is chosen ? B. Describe the construction of a steam-engine cylinder, showing the ports and the steam inlet and outlet, but omitting the covers of the cylinder and steam chest* Answer and illustrate by good sketches either A or B, but not both, in reference to the type of modern steam boiler with which you are best acquainted : — A. Show the arrangement for feeding the boiler with water under pressure. B. Explain how the steam is brought to the cylinder as dry as possible. Describe a valve for shutting off the supply of steam. Describe how you would test a boiler for strength, or an indicator for accuracy, or a feed-water meter. Choose only one of these. What heat is given to I lb. of water at 0° C. to convert it into dry saturated steam at 180° C. ? Use the formula on the outside page of the tables given you. How much of this is given to it as water to raise its temperature, and how much is latent heat? If instead of being at 0° C, it had been water at 30° C, how much heat would have been needed ? Anmoers — ii88; 324; 864'8 ; II34'8; (all in Fahrenheit units). The figure shows an indicator diagram with its atmospheric line, from a cylinder 15 inches diameter, 2 feet stroke. The scale of the diagram is known from the fact that the highest gauge pressure is 75 lbs. per sq. inch. Find the effective average pressure and the work done in one stroke. Answers — 277 lbs. sq. in. ; 9790 ft. -lbs. 298 Steam 6. Describe with slcetclies any method known lo you of admitting and exhausting steam to and from the two ends of a cylinder. You must show that you Itnow how the contrivance admits and releases before the ends of the stroke and allows expansion and cushioning. 7. If a locomotive of 1200 indicated horse-power uses 38 lbs. of feed- water per hour per indicated horse-power ; in a journey of 2\ hours what is the total amount of feed water ? If every pound of coal produces 9 lbs. of steam, what is the total weight of coal burnt on the journey ? If the mechanical efficiency of the engine is 0-85, what is the power actually spent in overcoming the resistance of the engine'and train? Answers— im,ooo lbs. per hour. Plot power and water on squared paper and assume a straight line law. What is the water per hour when the load factor is 10 per cent, (that is, the output is 600 kilowatts) ? Tabulate the numbers. State in each case the water per hour per kilowatt. Answers — 42,500 ; 22 ; 44' I ; 70'8. 9. How do we try to prevent condensation in a cylinder ? If any of the methods serves some other good object, state it . 10. One pound of a fuel contains o'S lb. of carbon and 0'I5 lb. of hydrogen and no free oxygen or nitrogen : what weight of oxygen is needed for complete combustion ? what weight of air ? Answers— y;i3 lbs. ; 14'48 lbs. 11. Choose any kind of boiler. Explain how, by its construction, ist, the combustion is made as complete as possible, 2nd, as much of the heat as possible is given to the water. You need not speak of careful tiring. 12. Explain why both the flywheel and governor are needed to regulate or govern the speed of an engine. 13. State in foot-pounds the following amounts of energy : — (a) A weight of 40 tons raised 30 feet ; {d) a projectile of 40 lbs. moving at 2000 feet per second ; {c) 3^4 horse-power-hours ; (d) 2'5 kilowatt- hours; {(?) the calorific energy of i lb. of average coal which is 8370 Centigrade heat units. Anszc/ers—2,6S8,ooo ; 2,484,000 ; 6,732,000 ; 6,636,000 ; 11,660,000. 1906 Describe, with good sketches, one and only one, of the following, A, B, C, or D .-— A . The crank shaft bearing of a horizontal or vertical engine. B. The crank axle of an inside cylinder locomotive. C. The piston of a gas or petrol engine, showing the packing, and the pin to which the connecting rod is attached. I). The rotating part of a Parsons or other steam turbine, showing how the vanes are fixed. Describe, with good sketches, one, and only one, of the following, A, B, C, D, or E :— A. A steam stop valve of the screw-down type. B. A locomotive regulator valve of any type. C. Two forms of boiler stays, stating the use of each. D. The front plate of a Lancashire, Cornish, or return tubQ marine boiler, showing how the boiler shell is attached. £, The carburettor of a petrol or oil engine. Board of Education — Questions 301 3. With a small experimental boiler you are finding the pressure of steam when its temperature is, say, ioo° C, iio° C, 120° C, etc. Show, with sketches, exactly how you would proceed. In what way does the presence of air with the steam spoil your results ? 4. State the following amounts of energy in foot-pounds : — (a) A weight of 35 tons may fall vertically 15 feet. (i5) The kinetic energy of a projectile of 60 lbs. moving at 2000 feet per second, (f) The calorific energy of I lb. of coal, 8500 Centigrade pound heat units. (d) 30 lbs. of water raised from 40° F. to 103° F. (e) One horse-power-hour. (/) One kilowatt-hour. Ansitiers — 1,176,000 ; 3,726,000 ; 11,840,000; 1,470,420; 1,980,000; 2,654,155- 5. It used to be thought that by cutting off earlier and earlier in the stroke, we got better and better results. Why is this untrue ? It used to be that the slide valve was never found on economical engines ; why is it now in use on many large and economical engines ? 6. The mean effective pressure on the piston, both in the forward and back strokes, is 62 lbs. per square inch ; cylinder 18 inches diameter ; crank, 18 inches long. What is the work done in one revolution ? Answer — 94,700 foot-pounds. 7. A pound of oil contains 0^85 lb. of carbon and o'i5 lb. of hydrogen. What weight of oxygen is sufficient to produce CO2 and H^O by com- bustion ? Take the atomic weights of C, 12 ; of O, 16 ; of H, I. If I lb. of oxygen is contained in 4^35 lbs. of air, how many pounds of air are needed for complete combustion ? Atiiiuers—yi,(>Vo%.; i5'o7 lbs. 8. A slide valve is worked directly from an eccentric. The advance is 30°- When the main crank has moved 20° from the line of centres, show the position of the eccentric crank. The half travel being 3 inches, mark off this radius and drop a perpendicular on the line of centres ; what have you thus found ? 9. A formula for Regnault's total heat H will be found on the tables supplied to you ; it is the total heat which must be given to i lb. of water at 0° C. to raise its temperature as water to 6° C, and then to convert it all into steam at 6° C. What is the heat which must be given to l lb. of water at 40° C. to convert it into steam at 170° C. ? Answer — 6i8'3. 10. A boiler furnace fire is about 12 inches thick. What do we know as to the way combustion is going on at various places in the coal and above it and in the space just on the furnace side of the flues ? Take any state you please, just before fresh coal is supplied or after, but you must say what the conditions are. 11. F lb. is the outward radial force on each ball of a governor required 302 Steam to keep it in equilibrium at the distance r feet from the axis when not revolving. The following are for the extreme cases ;^ r F. 07 lOO'l 144-6 The weight of each ball being 10 lbs., what is the centrifugal force of each at n revolutions per minute, the radius being ?'? What are the speeds for the above values of r when the governor is revolving? Answers— 2/ifl-b ; 246-5. 12. In a gas-engine cylinder where v = 2-2 and/ = 14-72, it was known that the temperature was 130° C. What is the temperature when p - 122 and » = 1-2? ^?my?/-— 1550° C. 13. The total heat, that is, the heat H required to convert a pound of water at 0° C. into a pound of wet steam at 6° C, having a dryness fraction x, is H = e + ^L where L is the latent heat of I lb. of dry saturated steam. If wet steam, 90 per cent, dry (that is, x = O'g) at 203-3 l^s. per square inch, is throttled by passing through a non-conducting reducing valve to loi -9 lbs. per square inch, what is its dryness at the lower pressure ? Remember that H is the same for the two kinds of steam ; it keeps constant when steam is throttled. p B L. 203-3 IOI-9 195 165 468-0 4S9-9 Answer — 0-92. 1907 Describe, with sketches, one, and only one, of the following A, B, C, D, or E .-— A. Any kind of governor. B. A large air pump for a steam engine. C. The crank axle of a locomotive showing the eccentric sheaves, the direction of the centre of each sheave being shown relatively to the directors of the cranks. Z>. An engine used on any kind of motor car. £. The rotating part of any steam turbine showing how the vanes are fixed. Board of Education — Questions 303 Describe, with sketches, one, and only one, of the following. A, B, C, or D .— A. The firebox of a locomotive, showing how it is stayed. Give larger sketches of a few details. B. Any important part of any water-tube boiler. C. Any form of safety valve now in common use. D. The carburettor of a petrol engine. Describe, with sketches, how you would experimentally determine any one and only one, of the following. A, B, C, or Z> : — A. The law connecting pressure, volume, and temperature of a quantity of air. B. The heat required to convert I lb. of water at 0° C. into dry saturated steam at, say, 100 pounds per sq. inch. C. The dryness of steam leaving a boiler. Z>. The calorific power of a gas or an oil or petrol. State the following amounts of energy in foot-pounds : — {a) The kinetic energy of the rim of a fly-wheel whose weight is 3 tons, average velocity 75 feet per second. (i) The calorific energy of one cubic foot of producer gas 95 Centi- grade heat units. ( Volume in cubic feet of i lb. of steam 4'35« Ansivers — 6'6 ; I'S3I ; i '242 ; 09482. 304 Steam 7. The area of a petrol engine diagram is (using the planimeter which subtracts and adds properly) 4' 12 sq. inches, and its length (parallel to the atmospheric line) is 3 '85 inches ; what is the average breadth of the figure ? If I inch represents 70 lbs. per square inch, what is the average effective pressure? The piston is 3'S inches in diameter with a stroke of 4 inches. What is the work done in one cycle ? If there are 800 cycles per minute, what is the horse-power ? Answers — I '07 ins. ; 74^9 ; 240'3 ; 5'82. 8. A formula for Regnault's total heat H will be found on the tables supplied to you ; it is the total heat which must be given to I lb. of water at 0° C. to raise its temperature as water to 6° C, and then to convert it all into steam at fl° C. What is the heat which must be given to I lb. of water at 0° C. to convert it into steam at 150° C. ? What amount of this was required to heat the water before any of it was converted into steam ? What name is given to the remainder, and how much is it? Answers — 652'2 ; 150 ; 5o2'2 lb. cent, units. 9. Why do we admit air by the fire-door as well as from the ashpit through the grate of a boiler furnace ? 10. Sketch and describe any form of steam or gas engine in-licator. ir. What is the formula for centrifugal force F pounds in terms of radius r feet, mass m or — — and n revolutions per minute. Given F, r, 32 2 and m, show how we find n. If F has the following values for the given values of r, and w is 9 '66 lbs., find n in each case. r feet. Fibs. 0-6 0-8 87 Answei' — 2io'i ; 2I3'9. Sketch the ports and a simple slide valve in its mid position. In dotted lines show the valve at the beginning of a stroke of the piston. What do we mean by laf, lead, and advance of a valve ? What methods are taken to prevent condensation of steam in the cylinder of an engine ? Why does such condensation tend to take place ? Describe, with good sketches, some one important detail of a modern steam, or gas, or oil, or petrol engine. Describe, with good sketches, that important part of any kind of boiler, or that boiler fitting with which you are best acquainted. Board of Education — Questions 305 3. Describe, with sketclies, any test in connection with heat engines in which you have talcen part, or even in which you have only been an onlooker. It is of no use attempting this question if you have merely read of such tests. 4. Steam enters a cylinder at 120 lbs. per square inch (absolute), and is cut off at 0"4 of the stroke. Draw the hypothetic diagram with an expansion law pv constant, no cushioning, no clearance. Back pressure 4 lbs. per sq. inch, what is the mean effective pressure? Area of piston 200 sq. inches, stroke 2'5 feet, what is the work done in one stroke ? What is the volume of the steam at cut-off ? You must not use a formula for the mean effective pressure unless you first prove it to be correct. Ansxuers — 87'9; 45,980; l'39. 5. One pound of feed water at 40° C. (or 104° F.) is converted into steam at 170° C. (or 338° V.), what is the total heat given to it? Use the formula on the outside page of your tables for Regnault's total heat, but recollect that he supposes the water to be heated from 0° C. (or 32° F.). Of the amount shown in your answer, how much heat is what we may call latent heat ? Answers — 6i8'3 ; 488'3. 6. On a certain steam-engine governor the centrifugal force on each ball must have the following values to keep the ball at the distance r feet from the axis. Each ball weighs 6^44 lbs. ; what speed (in revolutions per minute) at these values of r will produce these centrifugal forces ? - feet. 0-4 0-6 Fibs. 80 132 Answer — 302'2 ; 317. Describe, with sketches, any form of steain or gas-engine indicator, and how it is connected to the engine and used. Answer only one of the following, A ox B : — A. What are the conditions which must be satisfied to have good combustion in a boiler furnace ? B. How would you calculate the amount of air necessary for the complete combustion of a solid or liquid or gaseous fuel ? Why has an engine to be balanced ? Describe the balancing of any engine. The vane of the moving part of a turbine makes a certain angle with the surface at which the fluid enters ; that surface has a certain velocity ; the fluid has a certain velocity before it enters ; show by a figure what is the condition that the fluid shall enter without shock. A slide valve has a half travel 3 inches and an advance 30° ; how far is the valve from the middle of its stroke when the piston is at the end of its stroke ? Answer — \\ inches, X 3o6 Steam 12 Answer one but not both of the following, A at B : — A. Steam flows through an orifice from a place where it is at rest at 102 lbs. pressure per sq. inch to a place where it is at 98 lbs. per sq. inch ; its average density is 0'23 lbs. per cubic foot ; what is its speed ? Answer — 401 '6. B. A lb. of air has a volume 4 cubic feet, pressure 50 lbs. per sq. inch, temperature 127° C. ; it receives 250 (centigrade-lb.) units of heat, its volume remaining constant ; what is its new tempera- ture and its new pressure ? The specific heat of air at constant volume is 0T7. Answer — 1597'6° C. ; 233'9 lbs. per sq. inch. 13. Sketch a possible indicator diagram for an ordinary non -condensing engine, cut off about half stroke. How do we find the work done per stroke and the indicated horse-power ? 1909 Describe, with good sketches, one and only one of the following. A, B, or C :— A . The crank-pin end of any connecting rod. B. An important part of a steam pump. C. The cylinder of a petrol engine or of a gas engine, showing the valves. Describe, with good sketches, one and only one of the following, A, B, or C .•— A. Some important part of a water-tube boiler. B. A suction gas-producer for a gas engine. C. A spring loaded safety valve. You must answer two out of these three, A, B, and C, to get full marks. A. How would you show that to raise a pound of water one degree in temperature, almost exactly the same amount of heat is needed whatever the temperature of the water may be ? B. You will find a formula in front of the set of tables supplied you for the total heat of steam. What is the total heat of a pound of steam at 160° C. ? How much of this is latent heat ? Answers — 655'3 ; 495'3 lb. cent, units. C. How would you experimentally determine the pressure of steam corresponding to any particular temperature ? . Answer only one of the following, A ox B : — A. Sketch, in dotted ink lines or pencil, the hypothetic diagram of a steam engine : initial pressure, 100 lbs. per sq. inch ; back pressure, 1 7 ; cut off at half stroke. Draw in ink on the same part of your paper the real diagram such as might be expected from an engine with slide valve, going, say, at 150 revolu- tions per minute. Using your own diagram, find the average pressure, describing exactly how you have done it. Board of Education — Questions 307 B, A gas or petrol engine has a clearance space which is one-sixlh of the greatest volume behind the piston ; sketch, in dotted ink lines or pencil, the hypothetical diagram, stating what it means. Now draw in ink on the same part of your paper the real diagram which you would expect. Explain why it differ from the hypothetical diagram. Write out the formula for centrifugal force in terms of revolutions per minute. In a certain steam-engine governor the radial force F on each ball must have the following valves to keep the ball at the distance »• ft. from the axis. Each ball weighs 4'83 lbs. ; what speeds (in revolutions per minute) at the values of r will produce these centrifugal forces ? r feet. Fibs. 0-7 5fi 79 Answer — 220'7 ; 23 r2. 6. Explain why we cut off steam before the end of the stroke. Is there a limit to the usefulness of expansion, and, if so, why ? 7. Choose some particular boiler. About what proportion of the heat of the fuel in the furnace gets to the water {a) through the furnace itself, (b) through the flues? What is the usual cause of poor efficiency of a boiler ? 8. At the beginning of the compression part of the diagram of a gas- engine cylinder the pressure is represented by a distance 0'3I in. and the volume by 3 ins. ; the temperature is known to be 120° C. At a point on the ignition part of the diagram where the pressure is 7 ins. and the volume is 0'6 in., what is the temperature? Answer — 1502° C. 9. If a slide valve has no lap and no advance, state when the four events occur : admission, cut-off, release, and compression. What changes are produced by giving lap and advance to the valve ? 10. Convert the following amounts of energy into foot-pounds : — Three horse-power hours ; two Board of Trade electrical units ; the standard unit of evaporation, 536 Centigrade heat units ; the calorific energy of l lb. of average coal, 8570 Centigrade heat units ; the calorific energy of i cub. ft. of producer gas, 75 Centigrade heat units. Answers — 5,940,000: 5,309,000; 746,648 ; 11,940,000; 104,500. 11. Without sketching too many details, explain carefully the action of any steam turbine. 12. When steam is admitted to a cylinder, usually much of it condenses and is useless. Why does this occur ? How do we try to prevent it ? X 2 INDEX Absolute temperature, 6 — pressure, 19 Accelerated draught, 209 Air for combustion, 221 — per pound of coal, 223 — pump, 116 — vessel, 124 Answers to questions, 281 Anthracite, 218 Ash, 218 6abcock and Wilcox boilers, 189 Back pressure, 53 Bearings, 243 Bituminous coal, 218 Blades, 238, 242 Boiler, Babcock and Wilcox, 189 — Cornish, 170 — efficiency, 227 — Lancashire, 171 — marine, 181 — power of, 214 — Thorny croft, 192 — vertical, 180 — water-tube, 188 — Yarrow, 194 Boilers, 168 Boiling, 20 — point, 21, 39 Bourdon's gauge, 201 Boyle's law, 42 Brake horse-power, 254 Brasses, to adjust, 231 Calorimeter, Thompson's, 220 Capacity of pumps, 124 Carburettor, 261 Care of engines, 228 Centigrade thermometer, 3 Charles, law of, 2 1 Chimney gases, 221 Clearance, 63, 243 Clinker, 2i8 Coke, 218 Combustion, 215 — heat of, 217, 219 Comparison of turbines, 248 Compound engines, 141 — types of, 152, 162 Compression, 266 Condensation, 22 Condensers, jet, 115 — surface, 116 — tubes, 120 Conduction, 12 Connecting rod, 83 Convection, 13 Corliss valves, lOI Couplings, 113 Cranks, 107 Crossheads, 80 Curve, hyperbolic, 44 Cylinder, details of, 72 — condensation, 65 — escape valve, 74 — relief cocks, 75 Dead centres, 85 De Laval blades, 238 — losses, 239 — turbines, 234 Diesel oil engine, 257 Double beat valve, 201 — ported valve, 96 Draught gauge, 204 — forced, 210 — induced, 211 — natural, 204 309 3IO Steam ECC Eccentric, 97 Economizer, 175 Energy, 9 — intrinsic, 37 Engines, non-condensing, 69 — compound, 141 — locomotive, 131 — management of, 228 — petrol, 259, 265 — receiver, 152 — Woolf, 157 Equilibrium valve, 201 Evaporation, rate of, 213 — equivalent, 213 Evaporative power of fuel, 18 Expansion, coefficient of, 17 — economy of, 52 — limit of, 61 — of solids, 20 — of gases, 21 — of steam, 48 — joints, 18 Fahrenheit thermometer, 3 Fly-wheels, 130 Formation of steam, 28 Freezing point, 3 Fuel, combustion of, 215 — evaporative power of, 217 Fuels, 217 Furnace, the, 203 Galloway tubes, 171 Gas engine compression, 266 — crank positions, 251 — indicator diagram, 252 — Hornsby-Stockport, 257 — temperature, 267 Gauge glass, 174 Governors, 125 Green's economizer, 175 Guides, 80 Gusset stays, 174 Heat, i — latent, 37 — mechanical equivalent, to — sensible, 36 — total, 37 Heat transfer of, 12 — unit of, 7 Heating surface, 183, 188, 212 Hornsby-Stockport gas engine, 257 Horse-power, 9, 254 — indicated, 57 Hyperbolic curve, 44 — logarithms, 51 Indicated horse-{Jower, 57 Indicator, 135 — diagrams, 138 Intrinsic energy, 37 Jacket, steam, 67 Joule's experiment, 10 Journals, 114 Junk ring, 77 Lancashire boiler, 171 Lap, effect of, 94 — inside and outside, 92 Law of Boyle, 42 — Charles, 2i Lead, 92 Leaky piston, to test, 230 — valve, — , 230 Lever safety valve, 195 Link motion, 99 Locomotive, the, 131 — boiler, 186 Losses in turbines, 239, 248 Lubrication forced, 263 Marine boiler, 181 Mean pressure, 55 Mechanical efficiency, 255 — equivalent of heat, lo Meldrum furnace, 210 Mensuration, 269 Mixtures, temperature of, 40 Napier's formula, 237 Newcomen's atmospheric engine, 26 Nozzle, De Laval, 236 Index 311 Oil fuel, 219 — engine, 257 Otto cycle, 249 Parsons' turbine, 240 — blades, 242 — steam consumption, 247 Pedestals, 113 Petrol engine, 259 Piston displacement, 79 — leaky, 230 — rods, 80 — speed, 79 — valve, 95 Pistons, 76 Porter governor, 129 Power of boilers, 214 Pressure, absolute, 19 — gauge, 201 — mean, 55 — of the air, 19 Pulsometer, 25 Pumps, 122 — capacity of, 124 Quadruple 162, 166 Quantity of heat, 3 Questions, 269 expansion engines, Radiation, 12 Rate of combustion, 209 Reaumur thermometer, 3 Receiver engines, 157 Reversing gear, 100 Safety valve, 195 — dead weight, 199 — lever, 196 — spring, 198 Saturated steam, 38 Serve tube, 212 Shaft couplings, 113 Shafts, crank, 107 Shrinking on, 107 Slide valve, 89 — doub'e ported, 96 — to set, 94 Smoke, 208 Smoke prevention, 209 Specific heat, 5 Spring rings, 76 Steam consumption, 247 — expansion of, 48 — formation of, 28 — heat rejected by, 34 — properties of, 39 — regulating valve, 200 — saturated, 29, 38 — table, 39 — weight of, 39, 75 — work done by, 29 — volume of, 39, 75 Stop valve, 200 Strap, gib, and cotter, 84 Stuffing box, 73 Table of specific heats, 5 — heat of combustion, 217 — steam properties, 39 Tangential pressure, 108 Temperature, 3 — absolute, 6 — of mixtures, 40 Thermometers, 3 — compared, 4 Thompson's calorimeter, 220 Thornycroft boiler, 192 Triple-expansion engines, 164 Turbines, advantages and disadvan- tages, 246 — De Laval, 234 — exhaust steam, 248 — impulse and reaction, 233 Turning effort, 108 Unit of heat, 7 — of work, 8 Vacuum, 22 — gauge, 121 Valve, Corliss, loi — double-beat, 20I — double-ported, 96 — gridiron, 201 — slide, 89 — stop, 200 — lap and lead of, 92 312 Steam Valve, lift of, 124 — piston, 95 — regulating, 200 Velocity diagrams, 245 Vertical boiler, 180 Volume of steam, 39, 75 Water, condensing, 40 — gauge, 174 Water, weight of, 124 Water-tube boilers, 188 Watt governor, 125 Weight of steam, 39, 75 Wire-drawing, 73 Woolf engines, 152 Work, unit of, 8 Yarrow boiler, 194 PRINTED BY WILLIAM CLOWES AND SONS, LIMITED, LONDON AND BECCLES,