r/ppHR Cornell University Library The original of tliis bool< 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/cu31924003956251 Cornell University Library TJ 275.R59 1897 Steam. 3 1924 003 956 251 STEAM STEAM BY WILLIAM RIPPER ME.MBER OF THh: INSTITL:TI0.\ OF CI\IL ENGINEERS MEMBER OF THE INSTITUTION OF MECHANICAL ENGINEERS AUTHOR OF 'machine DRAWING AND DESIGN' ' I'KACTTCAU CHI:MISTRY' FTC. NEJr JJ\:PIiESSION LONGMANS, GREEN, AND CO. 39 PATERNOSTER ROW, LONDON NEW YORK AND BOMBAY 1899 Ai/ ris-hts j-cscrved PREFACE The following pages are based on the notes of lectures recently given by the author to an evening class of young mechanical engineers on steam, steam engines, and boilers. The rapid progress made in engineering science during recent years, and the limited space at disposal, have necessitated the omission of descriptions of obsolete types of steam engines, and the substitution of other matters of more importance to the present generation of engineering students. Special prominence has been given to the principles involved in the economical use of steam, and it is hoped that the book may be found of value, not only to the student, but to the practical engineer whose time and opportunities for the study of principles are limited. Most of the diagrams have been prepared specially for this work. I am, however, indebted to Mr. Geo. C. V. Holmes for the use of a few diagrams from his vrork on the Steam Engine. W. R. Sheffield : Uctohr 1889. PAGE CONTENTS CHAPTER I Introduction— Heat, its nature and effects — Temperature — Ther- mometers — Specific heat — Absolute temperatures CHAPTER II Unit of heat and unit of work — Horse-power — Mechanical equivalent of heat . . . ...... 7 CHAPTER III Transfer of heat — Radiation— Conduction — Convection . . .12 CHAPTER IV Combustion of fuel— Heat of combustion — Evaporative power of fuel 15 CHAPTER V Application of heat to solids — Application of heat to gases — Pressure of the air — Absolute pressure — Application of heat to water — Boiling — Condensation of steam — Vacuum . . . 20 CHAPTER VI Action of heat in the formation of steam — Work done by steam during formation at low- and high-pressures respectively — Efficiency of the steam — Heat rejected by steam to condenser — Sensible heat — Latent heat — Total heat of evaporation . 28 Contents vii CHAPTER VII PAGE Saturated steam — Table of properties — Water heated in a closed vessel — ^Temperature of mixtures— Condensing water . . 3 CHAPTER VIII Relation between the pressure and volume of gases — The hyperbolic curve . . ..... 42 CHAPTER IX 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 X 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 XI 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 ...... 89 CHAPTER XII Cranks and crank shafts —Tangential pressure on crank pin— Shaft couplings -Journals— Bearings 101 CHAPTER XIII Condensers — The jet condenser — The air-pump— The surface con- denser — The vacuum gauge — Pumps ..... 109 viii Steam CHAPTER XIV FACE Governors— The Watt governor— The Porter governor with auto- matic expansion gear— Fly wheels— The locomotive engine, arrangement and construction of . ... 119 CHAPTER XIVa The Indicator . . 129 CHAPTER XV Compound engines— compared with single cylinder engine — The two-cylinder compound engine illustrated . . . -135 CHAPTER XVI 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 145 CHAPTER XVII Boilers — Resistance of cylindrical vesseh — Descriptions of boilers — The Cornish boiler — Water tubes — The Lancashire boiler — The vertical boilers — Marine boilers — Steam room — The loco- motive boiler — Roof stays — Bar stays — Heating surface of tubes — Safety valves — To graduate the lever— Spring loaded . valves — The dead-weight safety valve — Bourdon's pressure gauge — Boiler performance and efficiency . . .164 CHAPTER XVIII Practical notes on the care and management of engines and boilers — Annual inspection of engines and boilers . . . 1S7 APPENDIX :— Questions and Exercises . . , .192 INDEX . , , .-,-»,, 221 ST E A M CHAPTER I INTRODUCTION The object of the study of steam and its applications is to obtain from the steam engine the greatest possible amount of 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 composed of minute particles called mole- cules, held together by mutual attraction or cohesion, and these molecules are in a state of continual agitation or vibra- tion. The hotter the body the more vigorous the vibrations of its constituent particles. In solid bodies the vibrations are limited in extent. If this limit is exceeded, owing to addition 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 afiSxed, 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 Sft'ain 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 : 32* (*)(*)(*) SO* Boiling point ■"reezii g Boiling point. point. 3- 212 o lOO o So , Freezing point I'ahrenhcit Centigrade Reaumur It will also lie clear that 212' — 32° = 180° F. occuity the same space as 100^ C, or So" R. Now 100° C.= iSo° F. jOQ __iSo° p 100 = 9! I, 5 also iSo° F.= ioo' C. J 80 C. Fk; 2. =^ Q 9 From which we obtain the Rules. — To lonvcyt degrees Falirciiheit into degrees Centigrade : Subtract 32, multiply the remainder b)- 5, and divide by i; Thus, convert 158° F. to degrees C. Then (15S-32) 5=1.70-- C. 9 Or, to convert degrees Centigrade into degr-ees Fahrenheit : Multiply by 9, divide by 5, and add 32. Thus, convert 70° C. into degrees F. Then (70 x +32^ :I5S°F. The relation between degrees Fahrenheit and Centigrade may also be expressed thus : specific Heat or, conversely, C. = (F. -32) 5 9 F. = 9 c. + 32. 5 Similarly, the student will see from fig. 2 that 180° F. occupy the same space as 80° R. j hence 1° F. Also, since 100 C. = 80 R. 80 „ 4„ —- R. = -R. 180 9 iC. =4r. 5 Temperatures are reckoned from zero both above and below that point. Temperatures below zero are marked with the negative sign ; thus — 25° reads minus 25 degrees, and indicates 25 degrees below zero. Specific Heat The ratio of the amount of heat required to raise the tem- perature of a substance one degree to the amount of heat required to raise an equal weight of water one degree is called the specific heat of the substance. The specific heat of bodies varies very considerably, as will be seen from the following table ; I. Table of Specific Heats Water = I"000 Cast Iron . =0-130 Steel . =o-ii8 Wrought Iron =0-113 Copper =0-100 Bismuth =0-031 Lead . =0-031 Mercury =0-033 Coal . ^0-241 Water has the highest specific heat of any substance (except hydrogen), and the metals have the lowest. In other words, it takes more heat to raise the temperature of a given weight of 6 Steam water one degree than to raise the same weight of any other substance one degree. The specific heat of water is i. The specific heat of wrought iron by the table is 0-113, or about \-, that is to say, the quantity of heat which would raise i lb. of wrought iron through 1° F. would only raise the temperature of I lb. of water through about ^° F. The following experiment may be easily performed : A mass of iron weighing i lb. is immersed in boiling water ; the iron is raised to the temperature of the water, namely, 212° F., and is then immersed in 2 lbs. of water at 50° F. The temperature of the water can now be taken by a thermometer. To find the resulting temperature, t, of the mixture by cal- culation : Heat lost by iron = Heat gained by water ; (212— /)xsp. ht. xweight=(/'— 50) xsp. ht. x weight ; (212— /)x -113 xi=(^— 5o)x 1x2; 23'956 — •113 -^=2/— 100; /'=58-66° F. ; that is, the temperature of the mixture is 58'66° F. Absolute Temperature The zero of temperature on the Centigrade and Fahrenheit scales has been chosen arbitrarily, on one the zero being the freezing point of water, and on the other a point 32° F. below it. For scientific purposes it is necessary to have a uniform zero, and such a point, called the zero of absolute temperature, has been chosen (from considerations explained in the Advanced Series), the position of which is 461° F. below the zero Fahren- heit, or 273° C. below the zero Centigrade. Hence, to express degrees Fahrenheit in degrees of absolute temperature, add 461. Thus the boiling point of water at atmospheric pressure=2i2° F. = 2i2 -1-461=673° absolute tem- perature. CHAPTER II UNIT OF HEAT AND UNIT OF WORK Before quantities of heat can be measured, we must have a unit of heat, just as we require a unit of length, namely, the inch or foot, in order to measure distance ; or the pound or ton, in order to measure weight. The unit of heat is the amount of heat necessary to raise the temperature of \ lb. of water i° F., when the water is at its greatest density, namely, from 39° to 40° F. But the all-important point with the engineer is the conver- sion of heat into work. We will therefore now consider what is understood by work, how it is measured, and what the relation is which exists between the two. By the term work in mechanics is understood ' the over- coming of a resistance through a space,' and the amount of work done is measured by the resistance overcome, multiplied by the distance through which it is overcome, the resistance being measured in pounds, and the distance in feet. Thus, if a body weighing 7 lbs. be lifted through a height of 3 feet, then the resistance, namely, 7 lbs., multiplied by the distance through which it is overcome, namely, 3 feet, is equal to 7x3=21 foot-pounds of work. Hence, work is measured neither by the pound nor by the foot, but by the product of the two. Thus the unit of work is the work done in raising one pound through a vertical height of one foot, and is called the foot-pound. Or, since action and reaction are equal and opposite, we may consider the force which overcomes the resistance. The work done by a force is measured by the intensity of the force- 8 Steam multiplied by the distance through which it acts, measured m the direction of the force. Thus, as before, in the above ex- ample, a force of 7 lbs. overcame the resistance due to the weight, and acted through a space of 3 feet, doing thereby 7 X3=2i foot-pounds of work. Since the unit of work is a product of two numbers, it may be represented by an area, and this is important, for we intend by-and-by to estimate the work done by an engine from the area of an indicator figure. Thus, if I inch be taken to repre- sent pounds on one line, and \ inch to represent feet on a line at right angles to it, then the unit of work is given by the small cross-lined rectangle, and the 21 foot-pounds in the above example by the whole rectangle (fig. 3). 7 G s WM Jitt Fig. Fig. Again, suppose the weight lifted in the previous case to be a vessel containing 7 lbs. of water, and that the water should escape by a tap at a uniform rate, all the while it is being lifted, until, when a height of 3 feet is reached, there is no water left. This result also may be well shown by a diagram (fig. 4), where the weight, varying from 7 lbs. to nothing, is given by a diagonal falling from 7 to the zero line of weight. The total work done is again given by the area of the figure, and is evidently equal to the distance 3 multiplied by the 7)iect7i weight = 3 X LJ^ = T,X2,\=io\, or one half that in the pre- vious case. It should be noticed that the unit of work has no reference Horse-power g to the /tme taken, for the same amount of work is done in lifting the weight, whether it be done in one second or one hour. The power of an agent is measured by the raU at which it can do work, and depends upon the amount of work done in the unit of ^I'me. The unit of power adopted by engineers is the horse-power. A horse-power represents the performance of 33,000 foot- pounds of work per minute. The addition of the words per minute should be particularly noticed. Work done -^ r 1 j „. — . . =units of work done per mmute ; 1 mie m mmutes '^ and . . — =horse-power exerted. 33,000 X time m mms. Energy is defined as ' the power of doing work.' When heat is applied to water, it confers upon the steam which is produced the power of doing work, such as driving the piston from one end of the cylinder to the other against a resistance ; and if we take the case of the locomotive, for example, the heat energy in the boiler furnace is capable of being transformed into the energy of motion of the moving train. If the brakes are put on the moving train, then the energy of motion of the train is retransformed into heat, sparks fly from the wheels and rails, and the train is brought to a stand- still. It is a fundamental principle in nature that, just as matter can neither be created nor destroyed, though it may be made to assume diiferent forms, visible or invisible, so energy, whether heat energy or any other, cannot be destroyed. It may take a variety of different forms, but the sum total of the energy remains the same. This principle is called the principle of the conservation of energy. Hence the heat which is carried to the engine in the steam is either transformed into useful work, or it passes away to waste in various ways, and the sum of the heat usefully em- ployed plus the heat which is wasted always equals exactly the heat which was applied. lO Steam Mechanical Equivalent of Heat We may now consider the important question of the relation between the unit of heat and the unit of work. The following diagram (fig. 5) will give an approximate idea of the apparatus used by Joule to determine this relation. Fig. 5. The weight W is attached to a string which is wound round the barrel B. A spindle S passes through the barrel, having thin pieces of sheet metal P P forming paddles or vanes attached to and radiating from it. The paddles are immersed in a vessel of water. When the weight W falls, the paddles rotate in the water, the water itself being prevented from rotating by fixed pieces not shown. When the weight descends one foot, 772 foot-pounds of work have been done, for the weight could have lifted an equal weight at the other end of the string. This work, which cannot be lost, now appears as heat in the water, the agitation of the paddles having increased the temperature of the water by an amount which can be measured by the thermometer. Unit of Heat 1 1 By this method (here only roughly indicated) Dr. Joule de- termined with the utmost care that i lb. of water was increased in temperature one degree by the work done upon it during the descent of 772 lbs. through i ft. Hence I unit of heat=772 units of work. To convert units of heat into units of work : Multiply the units of heat by 772. And vice versd, to convert units of work into units of heat : Divide the units of work by 772. If the unit of heat be measured by the Centigrade scale, namely, the heat necessary to raise the temperature of i lb. of water 1° C., then substitute the number 1390 for 772 ; for 772 x"=i39o nearly. The above experiment establishes the relation between heat and work by converting work into heat. The business of the mechanical engineer consists of con- structing machines by means of which the converse process, namely, the conversion of heat into work, may be carried out ; and the result of a large number of engine trials goes to prove conclusively the truth of the relation between heat and work as established by Joule, namely, that one unit of heat=772 units of work. A horse-power expressed in thermal or heat units =33^=4275- 772 12 Steam CHAPTER III TRANSFER OF HEAT When bodies of unequal temperature are placed near each other, the hot body tends to part with its heat to the colder body until the temperature in each is equal ; and when there is no tendency to a transfer of heat between them they are said to be of equal temperature. The transfer of heat from one to the other may take place in any of the following wa}s : namely, by radiation, conduction, or convection. Radiation Heat is given off from hot bodies in rays which radiate in all directions in straight lines. The heat from the burning coal in a furnace is transferred to the crown and sides of the furnace by radiation ; it passes through the furnace plates by conduction, and the water is heated by convection. Conduction The process by which heat passes from hotter to colder parts of the same body, or from a hot body to a colder body in contact with it, is called conduction. A bar of iron havino- one end placed in the fire soon becomes hot at the other ex- tremity, the heat being conducted from particle to particle throughout its entire length. A piece of burning wood can be held with the hand close to the burning part. Evidently, therefore, some bodies conduct heat much more readily than others. Conduction 13 If a piece of clean paper be pasted on the bottom of a copper kettle containing water, and the kettle be placed on a bright fire or over a strong gas flame, the water will soon be warmed, but the paper will not be charred in the least • the reason of this being that the heat is so rapidly conducted by the copper to the water. Bodies which conduct heat readily are called good conductors ; those which conduct heat slowly are called bad conductors. Bad conductors are used by engineers to prevent loss of heat by radiation ; hence boilers, steam pipes, and cylinders are covered with some non-conducting material, such as hair felt, or asbestos. Bodies of a finely fibrous texture are the worst conductors of heat. The following table gives the relative conducting power of metals : Silver, 100 Steel, 11 -6 Copper, 74 Lead, 8-5 Brass, 23 Bismuth, i'8 Iron, 1 1 "9 Liquids and gases are bad conductors, and it is impossible to heat them by conduction ; but they may be very readily and quickly heated by convection. Convection Convected or carried heat is that which is transmitted from one place to another by currents. The following experiment will clearly show that water is a bad conductor, and the necessity there- fore of heating it by some other method than conduction. Take a test tube nearly full of cold water, and hold the tube with the upper surface of the water against a flame, as shown in fig. 6. The water will soon boil at its upper surface, while j.,c. g, the temperature of the water in the bottom of the tube is not appreciably changed ; for, if a piece 14 Steam of ice be placed in the bottom of the tube, it will remain un- melted. If, however, the heat be applied at the bottom of the vessel, fig. 7, the heated lower layers, becoming less dense, rise towards the surface, while the cold upper and denser layers Fig. 7. fall, and thus circulating currents are set up which can be very plainly seen by dropping a little bran into the water, and which soon result in the water being heated throughout. Steam boilers should be so constructed as to secure the ready circulation of the water. 15 CHAPTER IV COMBUSTION OF FUEL The principal fuel used by engineers is coal, and the chief constituents of coal are carbon and hydrogen. Atmospheric air consists of two invisible gases, oxygen and nitrogen, in the proportion of 23 parts of oxygen and 77 parts of nitrogen in every 100 parts of air by weight. These gases are not united in any way, they are merely mixed together. The oxygen is the active element in air, and it is ready to unite with anything for which it has affinity, providing the surround- ing temperature is raised sufficiently high to enable it to do so. All fuels contain elements which readily unite with oxygen. The nitrogen of the air takes no part whatever in the process of combustion, and merely serves to dilute the oxygen. The process of combustion may be easily understood by considering the case of the common gas flame in the house. When we wish to ' light the gas ' — that is, to set in operation the process of combustion, or chemical union between the oxygen of the air and the carbon and hydrogen of the gas — we have first of all to apply heat with a match ; otherwise, if the tap is turned on, the gas will escape, but it will not burn. Once started, however, the burning proceeds vigorously and uniformly, and results in the evolution of heat. Before the escaping gas was lighted we could -detect the strongly charac- teristic odour of unburnt coal gas, but no such odour can be detected from the burning gas flame. The reason of this is that the carbon and hydrogen of the coal gas have united with the oxygen of the air to form two odourless and invisible com- pounds, namely, carbonic acid gas (CO2) and steam (HjO). In such a gas flame, however, the combustion is not per- 1 6 Steam feet, owing to the incomplete mixture of" the coal gas with the oxygen of the air ; hence the ceiling of the room is eventually blackened by the deposit of unburnt carbon. The combustion of coal differs, however, from the case just considered ; for, when coal is thrown on a furnace, there are three distinct stages in its combustion : first, the gases contained in the coal are distilled off as in the ordinary process of gas making ; secondly, these gases are either consumed or pass up the chimney unconsumed ; thirdl}', the remaining solid residue of the coal is burnt. Considering the gases distilled from the coal, which consist principally of marsh gas (CH,) and olefiant gas (CjHj) : in order that they may be completely burnt, (i) they must be thoroughly mixed with a sufficient supply of oxygen ; hence the necessity of admitting air above the coal, not, however, in excess, otherwise our object would be defeated by the cooling of the furnace. (2) The temperature of the mixed gases must be sufficiently high to allow of chemical combina- tion taking place. AVhen the distilled gases from the coal are not mixed with a sufficient supply of oxygen, or the temperature is not suffi- ciently high, then large quantities of carbon are disengaged from the gas, and pass up the chimney in the form of smoke, part of which is deposited in the flues as soot. But if the disengaged carbon is supplied with sufficient oxygen, and the temperature is sufficiently high for ignition or combination to take place, it burns with a bright flame. Considering the solid fuel which remains as coke or carbon, it should be explained that carbon is capable of forming two different compounds with oxygen, namely, carbonic oxide (chemical symbol, CO) and carbonic acid gas (chemical symbol, CO 2), depending on the abundance of the supply of oxygen to the carbon during the process of combustion. When the supply of oxygen is sufficient, and is intimately mixed with the fuel, the carbon is completely burnt to carbonic acid (CO2); but when there is an insufficient supply of oxygen, or the oxygen is not intimately mixed with the fuel, then carbonic oxide (CO) is formed. The effect of this on the production of heat may be seen by the following table : Combustion of Fuel II. Table of Heat of Combustion Total units of heat lbs. of water Combustible. of combustion per lb. evaporated from and at 212°. Hydrogen 62,032 64-2 Carbon burned to carbonic oxide 4,400 4'SS Carbon burned to carbonic acid 14,500 i5'° Anthracite 14,700 IS"2 Newport coal 14,000 14-5 Durham coke 13,640 14-1 Wigan cannel coal . 14,000 14-5 Petroleum 20,360 21'0 Coal gas 20,800 21-5 Oak wood (dried) . 7,700 8 When the air for combustion in a boiler furnace passes between the fire-bars under the fuel, combination takes place between the oxygen and the under layers of glowing carbon, forming carbonic acid (CO2). This gas, in passing on through the upper layers of carbon, here loses part of its oxygen, and the carbonic acid gas (CO2) is now reduced to carbonic oxide (CO) ; the remainder of its oxygen having united with more carbon to form carbonic oxide also. If now sufficient air is supplied at the surface of the fuel, this carbonic oxide will burn with a blue flame, with further evolution of heat ; but if it is not so supplied, it will pass up the chimney unconsumed, and the difference between the heat of complete and incomplete combustion of carbon, namely, 10,100 units of heat per lb. of carbon, will be lost. Example. — At a recent engine trial the coal burnt in the boiler furnace had a calorific value of 14,200 thermal units per lb. ; the weight of coal used per hour was 407 lbs. : find the total number of thermal units per hour to be accounted for. Ans. 577,940 thermal units. One pound of good coal yields (when the combustion is ' c 1 S Steam perfect) 14,000 units of heat; or, 14,000x772 = 10,808,000 units of work, or approximately 10,000,000 units of work. A horse-power is equal to 33,000 units of work done per minute; or, 33,000x60=1,980,000 units of work per hour; or, roughly, 2,000,000 units of work per hour. Hence the heat from i lb. of coal, if all utilised, would be , , - . r 10,000,000 1 capable of exertmg an energy of — '- = 5 horse-power '^ 00/ 2,000,000 per hour (approximately), or, in other words, i lb. of coal might be expected to yield one horse-power per hour. But our best stationary engines consume 2 lbs. of coal per horse-power per hour, or ten times as much as would be necessary if the whole of the heat of the coal could be converted into work. This, how- ever, can be shown to be impossible, and the ideal performance beyond which the steam engine is never likely to go is a consump- tion of about I lb. of coal per indicated horse-power per hour. Triple expansion marine engines now consume about li lb. of coal per indicated horse-power per hour. Evaporative Power of Fuel It will be shown in a future chapter that the amount of heat required to convert i lb. of water at 212" F. into steam at 212" is equal to 966 units. Hence, if we know the number of units of heat obtained by the complete combustion of I lb. of fuel, we can find the number of pounds of water at 212'' which this heat will convert into steam at the same temperature, or, in other words, the evaporative power of the fuel. Example I.— Find the ev.tporative power of i lb. of pure carbon. Total heat evolved per lb. 14, coo tr — I '■ — 3 11 r = -^^ = 15 lbs. of water. Heat required per lb. of water 966 Example 2. — Check the accuracy of the results given in last column cf Table II. The values given in the table of the evaporative power of fuels assume that the combustion is perfect, that the water is at 212° F., and that the whole of the heat goes to evaporate Evaporative Poiver of Fuel 19 water. This, however, is not the case in actual practice, for the heat in the boiler furnace is expended in (i) heating the water in the boiler and converting it into steam ; (2) heating the furnace gases, which escape by the chimney, carrying their heat with them to waste ; (3) heating surrounding objects by radiation ; (4) evaporating the moisture in the coal. The evaporative power of the best types of steam boilers at the present time is about 9 to 10 lbs. of water per lb. of coal. At a recent engine trial a compound portable boiler evaporated 18 -6 lbs. of water for an expenditure of i'86 lbs. of coal per indicated horse-power per hour. This is equivalent to 10 lbs. of water evaporated per lb. of fuel consumed. 20 Steam CHAPTER V APPLICATION OF HEAT TO SOLIDS All bodies expand by the action of heat. Numerous examples of the application of this law of expansion of metals will occur to students of engineering. Thus the bars of boiler furnaces are left free at the ends to enable them to expand. Boiler plates are riveted with red-hot rivets which cool and contract and draw the plates together at the joint with great force. In laying railways, a small space is left between successive lengths of rail ; and the bolt-holes by which they are secured to the fish-plates are elongated. Tires of wheels are fitted on when red hot, and as they cool they contract and grip the wheel with great firmness. Cranks are ' shrunk on ' crank shafts in a similar way. The walls of buildings which bulge out in the centre have been drawn back into position by passing iron bars through the walls from side to side of the building. They are screwed at the end with nuts and have large plate washers. The bars are heated inside the building and the nuts are tightened up. On cooling, the bars contract and draw the bulged walls together. Steam pipes, which are rigidly secured between two cylinders, should be fitted with an expan- sion joint or connection. A flanged copper connecting pipe, which admits of being extended or compressed, as in fig. 8, is sometimes used. Engine cylinders, which are heated to the temperature of the steam, instead of being rigidly bolted down on a horizontal Fig. S. Application of Heat to Solids 21 bed-plate, are frequently, especially for high-pressure steam, secured by the front face, the rest of the cylinder overhanging the bed of the engine. A small space is allowed between the crank bosses and main bearings of engines having cast-iron bed- plates to allow of expansion of the crank shaft in case of hot bear- ings, &c. If glass is heated or cooled suddenly, it is very liable to crack, because glass conducts heat slowly, and the two sides of the glass are unequally heated, and therefore unequally ex- panded : hence the fracture. The same thing is liable to occur in steam cylinders, which should always be carefully warmed by opening the stop valve a little while the steam is being generated, and blowing gently through with steam for some time before starting the engines, and thus bringing the cylinders and jackets gradually up to the working temperature. Steam boilers also require great care for similar reasons. They should not be hurriedly heated or cooled, and all sudden changes of temperature should be avoided ; otherwise, unequal contraction will take place, resulting in leakages. Less harm is done to a boiler by steaming steadily for a length of time than by repeatedly getting up steam and draw- ing the fires, which brings about repeated expansions and contractions of the boiler. The force exerted by heat in expanding a bar of metal is the same as would be required to stretch it to the same extent by mechanical means. Application of Heat to Gases Gases, such as air, expand on the action of heat much more freely than liquids or solids. The law which expresses the behaviour of gases under the influence of heat is known as the Law of Charles, and it may be stated thus : — The volume of a gas under constant pressure, or the pressure of a gas at constant volume, varies as the absolute temperature. The meaning of this law will be clear on considering the following applications. Example I. — A quantity of air in a cylinder under a movable piston 2 2 Steam occupies 10 cub. ft. at 60° F. ; what volume will it occupy if heated to 250° F. under the same constant pressure ? Here, the volume occupied by the air will evidently be greater, and in proportion to the absolute temperature, thus : 60° F. = 60 + 461 = 521 absolute temperature 250° F. = 250 + 461 = 711 Then, vol, at 250° F. =vol. at (i& x 7" ^ 521 711 = 10 X -~- 521 = 13-65 cub. ft. Example 2. — A volume of air at 212° F. is confined in a rigid cylin- drical vessel, and e.\erts a pressure of 15 lbs. per square inch ; find the pressure exerted by the air when the temperature is increased to 300° F., the volume, of course, remaining the same. Here, by the above law, the pressure exerted by the air will be greater, and in proportion to the absolute temperature ; then, 212° F. = 212 + 461 = 673 absolute 300° F. =300 + 461 = 761 Then pressure at 300° F. = pressure at 212 x ' 673 ^ 673 = 1 6 '96 lbs. per sq. in. If the temperatures are given in degrees Centigrade instead of Fahrenheit, then to find the absolute temperature add 273. (see p. 6) thus : Example 3.— A certain quantity of gas occupies 20 cubic feet at I5°C., what volume will it occupy if its temperature is raised to 100° C, the pressure on the gas remaining constant ? I5°C. = 15 +273 = 288 absolute 100° C. = 100 + 273 = 373 „ then 20 X 1Z| = 25 -9 cub. ft. 2 00 Pressure of the Air— Absolute Pressure On the surface of the earth we live, as it were, at the bottom of an aerial sea, which we call the atmosphere, and its weight causes a pressure in every direction of 147 lbs., or, roughly, 15 lbs. per square inch. Pressures are usually reckoned from the pressure of the Boiling 23 atmosphere. Thus the boiler pressure gauge, \Yhen its finger points to 10 lbs., indicates a pressure of 10 lbs. above the atmo- spheric pressure. To express this in absolute pressure add the pressure of the atmosphere to the gauge pressure. Thus, 10 lbs. pressure by boiler gauge=io + T5=.25 lbs. pressure absolute. Application of Heat to Water Water is a compound substance, consisting of hydrogen and oxygen chemically combined in the proportion of two volumes of hydrogen to one volume of oxygen, written in chemical symbols HjO. When water is subjected to the action of heat it is converted into steam, whch is water in the gaseous state. Though a change thus takes place in the physical condition of the substance, the chemical composition of the steam is a -'f^^^ in no way different from that ™ O-.i^-i. \ of the water from which it is generated. Boiling If heat be applied to the bottom of a vessel, as in fig. 9, the air contained in the water will first appear as little bub- bles which rise to the surface. Then the water immediately in contact with the source of heat will be converted into steam. The steam will form as bubbles on the bottom, and these will rise through the liquid ; but at the commencement of the operation they will at once be condensed by the cold upper layers of water. The condensation of the bubbles of steam is the cause of thp Fig. 9. H Steam ' singing ' of the water before boiling. Finally, the water be- comes heated throughout until it reaches a temperature of 212° F. under the pressure of the atmosphere, ^yhen the bubbles rise to the surface and boiling begins. It should be particularly noted that the temperature at which boiling takes place depends upon the pressure on the liquid, and that for every different pressure there is a fixed temperature at which boiling takes place, so that water has an indefinite number of boiling points. An experiment illustrat- ing boiling at a low tempera- ture will be understood by reference to fig. lo. Water is boiled in a glass flask as in fig. 9. When the water has been boiling a little time, and all the air is expelled, the heat is removed, and the flask is closed by a cork, turned upside down, and placed on the stand as shown. Meantime the water has, of course, ceased to boil. If now cold water be poured gently on the flask, the steam J.JJ, j^ which occupies the space above the water will be con- densed, the pressure on the water will therefore be reduced, and the water will again boil vigorously, although the temperature of the water has by this time fallen considerably below 212° F. Similarly, owing to the reduced pressure of the atmosphere on the tops of high mountains, boiling water is not sufficiently hot to cook food. On the other hand, the temperature of boiling water at the bottom of deep mines is higher than at the surface. The boiling temperatures for water under varying pressures are given in Table III., p. 39. The following are a few important pressures and temperatures : Condensation Under a pressure of 5 lbs. the boili ng temp. is 162° F. 10 193° „ „ „ I atmosphere 212" „ „ „ 2 atmospheres 249° „ 3 273° ,. 4 291" „ 5 306° „ 10 357° „ 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 — Vacuum Steam is water in the gaseous condition, and when the steam is cooled, it again re- j— , turns to the liquid ^:>, state and becomes U water. '^7 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 26 Steam Si 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 tp call to their aid the pressure of the atmosphere in the performance of work. A vacuum is literally an empty space^thzt is, a space abso- lutely free from air or vapour of any kind capable of exerting pressure. Vapour arises from water at att 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- 12. Now, when the plunger or pump bucket P is lifted, the valve V will lift by virtue of the difference 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.— Take a thin tin cylinder closed at both ends, having a tap, t, 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 driven most of the air out. Now the vessel contains steam and very little air. Condensation 27 Fig. 13. Close the tap and pour cold water on the vessel. The steam is immediately condensed to water ; and since water occupies only about ygVo "of the 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 had 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. Fig. 14. ?.8 Steam CHAPTER VI 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. 15) contain i lb. of water at 32° F., and let the pressure of the atmosphere be t in i ' illl l XI Stage 1. Stage 2. Stage 3. Stage 4. Fig. is. 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 expansion 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 cyhnder (stage 2), Work done by Steam 29 rising higher 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. 16, let 1 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. „ „ -^ft. 62 5 = ■016 ft 30 Steam c5 m 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 144=2116-8 lbs. (i) 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. 15. (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 212° 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, ^180 + 966 = 1,146 units. Now, in stage i, fig. 15, 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. The term 'latent,' however, is not well chosen for the following reasons : S3 I Fig. 16. 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 firom 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 ^ork, 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. = 2,116-8x26-36 = 55,799 foot lbs. ; or, 55,799-^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, (i) 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 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 ; II si S5-(qq ft. a%. External work = P x a x /. But ay.l=-v = the volume occu- pied by the i lb. of steam ; there- fore External work = P x w. . 4 ■3 ■Si Jntimol Work In converting Water Jnio- steam. Fig. 17. If, then, a rectangle be constructed, as in fig. 17, having one side=P, and an adjacent side=z', to any convenient scale, the area of the rectangle will equal the work done. Similarly, the proportion which 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 7 2 '3, we have 2'48 : I2-36 : i. Draw the rectangle A B (5 a (fig. 18), making A B = pressure and Bd = volume to any scale to represent the external work done by the steam. To the base B i> add the rectangle BCci = 1 2 -36 times the rectangle A i5. This is done by making B C =; t2'36 times AB. Make also CD = 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 AD da, and the share of this which goes to perform useful work is represented I -4- naXcr frtrm, 32' to 212° Fig. JVoj'k done by Steam 33 by the remarkably small area given by the rectangle h.'&h a. But the ratio which the useful work done bears to the total heat expended is called the efficiency of the steam. Hence, in this case, the efficiency=E5?LAMf =.^_ or about i^. areaAD^rt iS"84 16 In other words, in such an engine as this, taking steam at full pressure throughout the whole stroke, only ^'^ 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 -jJj^ 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 z* = 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 I lb. . „ 147 „ =S5>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 loo 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 ,, =-^| — - — = 317 lbs, 62,352 Heat rejected to Condenser 35 Heat rejected by Steam to Condenser Wlien 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. 15, 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 = 72-3 units; and, therefore, Heat rejected = i8o-f8937 + 72-3 = 1,146 units. = total heat supplied. 2nd case. — Suppose, in fig. 19, that, when the cooling commenced, the piston had been secured so that it could not fall as the volume of the steam de- p^^ ^^ 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, 36 Steam Heat rejected= total heat— external work = i,i46-72'3 = 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(/ira^f.— 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. 20). Then, when the steam has been cooled till it only exerts a pressure of 5 lbs. ^ per square inch, the piston will begin to ^ fall, and, on continuing the cooling ope- 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 -f internal heat of steam =8937 Fig. 20. + 1" external work = I of 72-3 = 24-1 1,097-8 We 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=2 12 — 32=180. If the temperature of the water to begin with is, say, 50'' F. Heat rejected to Condenser 37 inst'fead of 32'^; then the number of thermal units required to ..raise water at 50° to water at 2i2°=2i2-— 50=162. The latent heat of steam is defined as the amount of heat required to convert i lb. of water at a given tempieratqre into isteam at the same temperature. The total heat of evaporation t the susi of the latent and sensible heats and is defined as the qfiantity of lieat' 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, /, 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. — Y\aA the latent heat of steam at 120 lbs. pressure absolute, given that the temperature of steam at this pressure is 341° F. Then, latent heat = 1,114— 7' ' = 1,114-7x341 = 87s -s- The data required as to temperature, total heat, &c., of saturated steam are best obtained by reference to tables which have been prepared on the basis of the exhaustive experiments of Regnault. (See Table III.) 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. CHAPTER Vn 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. ' J' >> >i ^, lbs. 10-28 15 " » .. I5x_i-lbs. 10-28 = I -46 lbs. Properties of Saturated Steam 39 Volume per lb. in cubic feet. p r-» u-i CD w o^op \p po ^ a» r>.\p ^ rn n go y^ p t^ inrhV^^COCOP*^n^r^« « N « M W ►- ^ w M Total heat of evapora- tion from water at 32° F. 7^-^ op p^ Ti- p \n p 0^^p «oppi-^^-i ;^N N Mr-^ 00 C\ b >-' N V ^vb t^ b> ^ W ^ io i^OD ^ o> N r>. t^ t^cx) 00000000000000 ONONO\0\0\a«o o *-" ^ MI-IM«-lt-(l-l>-HMI-Hl-.l-l»-.t-ll-II-.l-HC(N«n la 11 ^ rn w t^ pvp p ^ « P'Pr^pp v^9P °p r* t^ 9 \b b V i^ ►"" Vob ^ t^ w 00 cooo CO i>^ I-" b t-^ o ij-i Absolute pressure in lbs. per £q. in. li-iOmO^^O^J^OOOOOOOOOOOOO' 00O^C^O O'-''-' « roT^ u->vo t^OO CT» O "^ O "^ Q MMI-Ht-thHt-HMI-HI-lt-ll-lt-HC^dCOCO^ Volume per lb. in cubic feet. 1 '^d- f^-o -^ o 00 vo i^ o ys 1^00 00 M w w a\ r^oo w) CO p W vp p pop fO pN ^^ M ^ CD\p p 7^ P ^ P Total heat of evapora- tion from water at 32° F. rn Mt p\vp p p ^ « 03 p p Tl-'P ^ p M r-^ p t^ w N CO fO 'd- u-j'O yD r^ t-^oo t-" r<^Lot^O '-' N ^^or^ II • 11 r>. pooo N p t^cp op f^ppp ^t-p ^-O pop p^ b fO ^00 b M ^^ «) O ONt-^'^t-' t^MOO Mr;;.W mc^po^D■«:^■<^■^■^■^u~) "J^vo r^oo 00 c^ ON o O '-' ««NMNW««(M««W«MCSW«COPO(ri Absolute pressure in lbs. per sq. in. HH N r^ T^ \r^\Q t^fXi OnO iJ^O ^^^0 ^^O "^0 i^^O yDOO NM\0 1- -^ O O ri-irjO I>.v5 un - -VOL at60--)( Volumes. Fig. 22. Mark off on the line of pressures to a scale of, say, y''u inch= IS 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 Une 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 1 20 = 60 ; at 4 cub. ft. the pressure ^c — \ of 120 = 30 ; TJie Hyperbolic Curve 45 and at 8 cub. ft. the pressure 8 rf = |- of 1 20 = 15. If the free ends a, h, c, d (fig. 22) of the verticals are now joined, the curve formed is called a rectangular hyperbola, fig. 23. Then the four rectangles O «, Ob, Oc, O d 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, 120x1^60x2 = 30x4=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 s oc tn > S 1-609 2-609 .. i „ 8 2-079 3 -079 ' , !, J » 9 2-197 3-197 J, 10 I> 10 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 i : 2-098.) Now, if the steam had been admitted at initial pressure throughout the whole stroke, then three times 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. Lime or Volumes Fig. 2S. On the diagram (fig. 28) let oV,, oV,, &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) .1 „ 3olbs.=area 0P3 ; (4) >, „ 15 lbs.=area o P4. But, assuming that steam obeys the law of Boyle, which is sufficiently 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 Pj the further area Pj P^ V4 V„ which shows a very considerable in- crease in the work done. If the area oPi = i, then the area P.P^V^ ¥, = 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 15 lbs. pressure :=!, 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 wonc, would be a simple rectangle, thus (fig. 29) : Fig. 29. Fig. 30. Fig. 31. ■ 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. 30). 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, howe^-er, 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. 31) ; 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 = Difference between pressures on each side of piston. Mean 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 width 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. Fig. 32. 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 I+^yP- ^°?- ^- backpressure. 56 Steam Thus, for steam at 80 lbs. per sq. in. absolute, cut off at one-fourth of the stroke, ^ = 8oxi±i:386_ 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 •964 initial volume 6 pressure = i •465 2 3 4 ■937 ■846 •699 ■596 7 8 9 10 ■421 •385 •355 ■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. Stj. T. / 2 < Fig. 59. 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 1-5708 = 1570-8 ft. per minute. By the principle of work, since the work done on the pistoh is the same as that done on the crank pin, and that the mean speed ol^G. crank pin is 1-5708 times that of the piston, there- 88 Steam fore the mean pressure on the crank pin in the direction of its motion is of the mean pressure on the piston. 1-5708 Example. — In a direct acting engine the diameter of the cyhnder is 17 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 --17X 17X 7854x60 = 13614 and mean pressure on crank pin = 13614 x = 8670 lbs. Rotary engines. — Endless time and money have been spent on the invention of rotary engines, consisting for the most part of a piston which revolves within the cylinder, instead of one which moves from end to end of the cylinder, the object being to prevent the supposed loss of power due to the frequent changes of the direction of motion in the moving parts. Butj as a matter of fact, there is no such loss, for the energy exerted during the first half of the stroke in accelerating the piston from rest to its maximum velocity at the middle of the stroke, is restored to the crank pin during the latter half of the stroke- in again bringing the piston to rest. And, further, although the mean tangential pressure on the crank pin is less than the mean pressure on the piston, yet the distance through which the tangential pressure acts is greater than that through which the pressure on the piston acts in the proportion of 2 : 3'i4i6, namely, two strokes of the piston to one revolution of the crank pin, and by the principle of work (neglecting friction). Pressure on piston x 2 =mean tangential pressure on crank pin 3-1416. Hence there is no loss by the arrangement. 89 CHAPTER XI THE SLIDE VALVE Before explaining the action of the valve, it will l)e 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. 60, Fig. 60. 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 cyhnder 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 bplted. 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. We 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. 6 1 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. 6i. 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. 62. 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. Fig. 61. FROM QOILER TO AIR k- -{^E- -I Fig. 62. 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 oif 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 lap to the valve- that is, by extending the width of its face— and (2) by giving it /ra^f— 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. 63, the lightly shaded part shows the valve with no lap. The darker parts show the addition of outside lap c c and inside lap ii, by increasing the width of the face from the width of the port S to the width ci. , The amount of opening of W/A' v/A im v////^^ the port for the admission of ^"^' ^''' steam when the pistoti is at the beginning of its stroke is called the leadoi the valve (pronounced leed). Thus the opening b (fig. 64) 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 / 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. 6i)=width of port. But, when lap is added to the valve (fig. 65), 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. 65) repre- K I '1 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 ot the stroke (fig. 66); 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 Fig. 65. I ID Fig. 66. angle. To find this position : From the centre O on the centre line C D set off O « equal to a - FlG. 82. stroke, the obliquity of the connecting rod, and the weight of the moving parts, the student is referred to the work on the same subject in the Advanced Series. Crank shafts having three cranks usually placed at 120'' still further distribute the stresses, and cause a still more regular and uniform motion of the shaft. The variable character of the tan- gential or turning pressure on the crank pin is due to three causes : (i) The communication of the pres- ~ ~ ~ " sure to the crank pin from a reciprocating piston through the connecting rod, the effect of which is that the tangential pressure varies from zero at the ' dead centres ' to a maximum in the middle of the stroke. (2) The expansive working of the steam by which the pres- sure falls from the beginning to the end of the stroke. (3) The influence of the weight and velocity of the recipro- cating piston, piston rod, and crosshead, which start from rest, Shaft Couplings I07 and are accelerated till they acquire a maximum velocity at the middle of the stroke, in accomplishing which a large portion of the steam pressure is absorbed, and is therefore not transmitted to the crank pin ; while, during the latter part of the stroke, they are again brought to rest, the effect of which is to cause a Fig. 84. greatly increased pressure on the crank pin in addition to that due to the steam pressure on the piston. It will be seen that the influence of the weight and of the velocity of the reciprocating parts tend to modify the variable nature of the stresses due to expansive working, and this is especially so at high speeds. Shaft couplings. — The above diagram, fig. 84, illustrates Q.0/Q jTl rr n I I p: ■* — ^■""■"' ■■"■ ' ^ Fig. 85. the method of joining lengths of shafting together at the ends. The ends of the shafts have flanges forged on them which are turned with the shaft and butt together end to end. Holes are drilled through the flanges, and they are firmly secured by bolts as shown. io8 Steam Journals. — The journal of a shaft is that part of it which fits in the bearing (figs. 85, 86). It is of the greatest importance that the bearing sur- faces of working parts should be sufficiently large. The length of the journal and of the bearing is proportioned so that the pressure per square inch on the bearing shall not be so great as to squeeze the oil out of the bearing, and so prevent proper lubrication. The length of the journal and bearing are increased for high speeds, and the bearing nearest the work is made longer than one further away from it. Pedestals, or Plummer Blocks (figs. 85, 86), consist of a body which holds the brasses, and a cap which is bolted down on the brasses to keep the bearing rigid. When the resultant of the forces acting on the shaft is not vertical, but inclined at some angle with the vertical, the pedestal is constructed as shown in fig. 86. For large engines the bearings are fitted with ' white metal,' as in fig. 87, on which shafts run more smoothly and with less friction and tendency to heat. The ' white metal ' is run into grooves left for it in the brass. Fig. 87. I09 CHAPTER XIII CONDENSERS The condenser is a box or chamber into which the steam is passed and condensed after doing its work in the cyhnder, in- stead of being exhausted into the air. The object of the condenser is two-fold, being first to re- E P, exhaust pipe from cylinder ; C, condenser ; A P, Mr pump ; H W, hot well. move as far as possible the effect of atmospheric pressure from the back of the piston by receiving the exhaust steam and con- densing it to water, thus creating a partial vacuum ; and secondly to enable the steam which acts on the piston to be no Steam expanded down to a lower pressure before leaving the cylinder than can profitably be done when the steam exhausts into the air, There are two kinds of condensers, namely, they>^ condenser and the sitrface condenser— the one, as its name implies, con- ;tx densing the steam by means of a. Jet of cold water, and the Other by bringing the steam into contact with a cold 7netallic surface. The jet condenser is il- lustrated by fig. 88 on pre- vious page. The steam, on being ex- hausted from the cylinder, passes into the chamber C, called the condenser. Here it meets with a jet of cold water in the form of spray, which condenses the steam. „ „ Fig. Sg. The condensed steam and HW hot well; B,ak-pump bucket : HV, head injection water must now be ™'^'' ■ ^ ^' f°°' ™'"=- removed, and a pump A P is provided for the purpose. This pump is called the air pump because it removes, not only the water, but also the air which passes into the condenser mixed with the injection water, as well as the vapour which arises Condensers 1 1 1 from the water. It is the air and vapour in the condenser winch are the cause of \)\&\.tyQx pressure exists therein. The condensed steam, injection water, air and vapour, are pumped into the hot-well H \\\ 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 valvt, and the delivery valve is called the head valve. ^sr ~w Fig. 90. Fig. 89 is a more com- plete drawing of an air pump as applied to ver- — I tical 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. 90, 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 2 Steam condenser for marine engines as the 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 -^^ 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 into contact with cold metallic surfaces. The general arrangement of a surface condenser is shown in fig. 91. The cold metallic surface required, by which to condense Condensers 113 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 Toju-Rim/, VlaXtr from. Fig. 91. vertical engines ; it is also frequently nriade cylindrical with flat ends, as in fig. 91. 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 I 114 Steam The tubes are made of brass, | space surrounding the tubes, or I inch outside diameter, and ^V i"*^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. 92. C T P=condenser tube plate. The diagrams figs. 92 and 93 show two methods of connect- ing the tubes to the tube plates so as to make them tight. Fig. 92 shows a little stuffing box and screwed gland, which is very generally used. WF W^M^^^^^^ ^^^ stuffing box is packed with tape or cord packing. Fig. 93 is a wood ferrule WF 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 : Fig. 93. plate. When in its place it absorbs moisture and swells, forming a perfectly tight connection. Fig. 94 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 115 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 S o 20- 15- 10- S - 9r(issu.-K 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 (}Jcmosf{ki*lc 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 reckcns 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 ^1 ^ 10 S-- /o 20 . JS-I30 Fig. 95 Ii6 Steam approximately to i lb. pressure, 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 2S-;-2 = i2| 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^ = 2 1 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 iu horsf power by using the condenser may be calculated by the usual formula, H P:= — ^ — where P is the gam of pressure by 33,000 using a condenser, namely, in the present case, 12^ lbs. Pumps The feed pump is used to feed the boiler, 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 2I times this quantity. The feed pump is sometimes worked from the engme direct, or from the shaft b^r an eccentric attached to the plu/iger (see fig. 104). When it is worked independently of the main engine it is called a 'donkey pump.' The following diagram, fig. 96, 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 whole interior of the pump to be full of air. A\'hen the plunger rises the pressure on the suction valve S will be reduced, and the air in the supply pipe will lift the valve and flow into the barrel. The pressure of the air in the supply pipe is Pumps 117 now less than before, and accordingly the pressure on the external surface of the water forces water 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, tvhen the plunger re- turns, it forces water in- stead 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. in. In practice, however, the supply can never be drawn from a depth greater than about 25 ft. The valves are pre- vented from rising above a certain heigiit by stops shown 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 unnecessary. 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 3-1416 xlift = dia.'^ x 7854 ; iift = ^-ii-. Fig. 96. 1 1 8 Steam Large valves are prevented by the stop 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. 96. The air in the water collects in this vessel and forms a cushion or spring which enables the water to be delivered in a continuous stream instead of inter- mittently. 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 1,000 ounces. The weight of a cubic foot of salt water .= 64 lbs. I lb. of water occupies '016 cub. ft. I gallon of water=io lbs. 119 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 xe.%vXx. 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 th£ 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. 97- I20 SteaiH The central spindle S of the governor, fig. 97, 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. It 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 Fig. 98. 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 weight 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 121 sides A C, A B, and B C (fig. 98), which are respectively parallel to the forces. The vertical distance C A is called the height ol the governor or the height of the cone of revolution, and this height is constant for a given number of revolutions per 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 cl /'\T'-/'\ Fig. 99. height of the governor to take place, as from C A to C a, fig. 98. It is the raising of the sleeve A to a 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 CA of the cone of revolution. But a perfect governor would permit of no increase either in the number of 122 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. 98. When, however, the arms are suspended from points E and F (fig. 99), 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 of engine building has long been towards higher speeds, and for quick-running engines a Watt governor geared so as to run slower than the engine is not sufficiently sensitive. This governor is, therefore, now largely superseded by various forms of high-speed governors, of which the 'Porter' governor, illustrated by fig. 100, 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 123 flG. 1 24 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. 100 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. 141. In fig. 100 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-wheels 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 125 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. 126 and 127 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 126 Steam The Locomotive 127 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 the next chapter. 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 at the ends and secured to the piston by a gun-metal nut with a taper steel pin through the nut. I2t 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 best Yorkshire 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 : — High pressure. Low pressure. Cylinder, diameter 1 6 ins. 23 ins. Ratio of piston areas . I 2-1 Length of stroke 24 ins. 24 ins. Length of connecting rod . 6 ft. 6 ft. Throw of eccentrics .... . (>\ ins. 6} ins. Angle of advance, forward gear 4"" 4" ,, ,, back gear 14" 14- Travel of valve, full forward gear • 3^ ins. 3l ins. „ full back gear 3t§ ins. 3j ins. Lap of valve I in. I in. Steam ports .... . Iixi4 ins. i| X 17 in.^. Cut-off, ordinary running . 40 per cent. 50 per cent. Pressure of steam in boiler, 175 lbs. per sq. in. above the atmosphere. Ejiercise 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. Alls. 7_5t_^_ _ g.j ]ij5_ of water per lb. of coal. 79 X 112 '■ 129 CHAPTER XI Va 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 Bteam 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 Indicator.— 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 five K 1 30 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. I02A 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 131 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. VJIfJfJtJJJJJffJ. /^\ >>>ii>ll>lliirm Fig. 102A. The Indicator Diagram .—Y\g. 102B is an example of a common form of diagram from a single cylinder non-con- densing engine running under good working conditions. I3S 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 XY to represent the pressure in the boiler. There is always a certain fall of pressure between the cylinder and the boiler in Fig. 102B — X Y = atmospheric line ; A B = admission line ; B C = steam line ; CD = 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, : insufficient lead ; t = late exhaust ; X = late compression. '. = excess of lead ; / = wiredrawing ; J = early release ; r= early compres- sion. Fig. I0 2C 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 133 Fig. I02D. 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. 102D, 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. T\\Q point of cut-off C, fig. 102B, 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. 102E and 102F); 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. 1020). 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. 102E for non-condensing engines, and in fig. 102F Fig. 102E. Fig. I02F. in condensing engines with a trip gear, the cut-off being fairly sharp. Fig. I02G 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. io2e) 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 uressure of the steam during expansion 134 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. 102G. The expansioti 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. 102B) 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 exhmist line DE (fig. io2b) represents the fall of pressure which occurs in the cylinder when the exhaust port opens. Fig. 102c shows early opening to exhaust at s, and late opening to exhaust at t. A late opening to exhaust, as shown at t, is a very grave defect in a diagram. The back pressure line E F (fig. io2b) 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. io2b) 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. 135 CHAPTER XV 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, and 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 gready reduced terminal pressure. The following diagram (fig. 103) illustrates the difference between the action of the steam in a simple engine and in a triple expansion compound engine. 136 Steam Suppose I lb. of steam at 150 lbs. pressure absolute admitted to a single cylinder and expanded down to 12 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° R, the temperature of the steam at 3 lbs. pres- sure ; or a difference of 358—142 = 216° F. between the initial ^S8° ZqZ .22g' ItfZ' Fig. 103. 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 150 lbs. and 60 lbs. pressure, there is a variation of 65" F. ; in the intermediate cylinder, working between 60 lbs. and 20 lbs. pressur.e, there is again a variation of 65° F. ; in the low-pressure Compound Engines 137 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 -^=16-3 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 qnd 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. 104, 105, and 106 illustrate a two-cylinder compound mill engine, HP being the high-pressure and LP 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 cylinder 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. 106, from the ■^3^ Steam > ■- o >- a tj o > .. a ?« ..S ^'^^ o ■-" I- (0 h ^^ - d .CO b oj ft) -^ 2 - Compound Engines 139 140 Steam high-pressure into the low-pressure cyhnder, 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. 106. 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. T07. 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 unequal area, as Compound Engines j^j in fig. 1 08, 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 ; 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 greaiter the pressure between the two pistons, and the less Fig. 107. 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 V 12* h Fig. 108. 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. 142 Steam 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 cylinder 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 32 : 52 : 82=9 : 25 : 64=1 : 278 : yu. The number of expansions of the steam in any engine, whether simple or compound, = __navourne ^^^ ^^^ .^ mitial volume proximately equal to '"^ !^ pressure ^j^^^.^ ^^ 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 cyhnder 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. _vol. of L. P. cylinder_ initial vol. "vol. of H. P. cylinder ~'^' Compound Engines 143 But if the steam is cut off in the high-pressure cyhnder at one third of the stroke, the number of expansions = final vol. _ vol. of L. P. cylinder _ 4 _ initial vol.~i(vol. of H.P. cylinder)~rori~" ^^" 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 „r,^„„ „■ initial pressure qo or expansions = ^ ■=y—=.Ci. 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 : Let R ^ vol- of L-P- cylinder ^ vol. of H. P. cylinder Then the point of cut-off in the high-pressure cylinder = i , — =- of the stroke. number of expansions 9 Example. — The ratio of the cylinder volumes of a two-cylinder 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. Ans. ^ 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 = " = 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 90 lbs. per sq. in. by boiler gauge, or loq lbs. absolute, the number of expansions = — 3 = io-5,or •> 10 144 Steam allowing for losses = i o, 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 20 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. ^45 CHAPTER XVI TYPES OF COMPOUND ENGINES ffom SoiUr HP c\ LP J To condensei' WEL [-^ HB \J 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 iSo" 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 cyHnders, as shown in fig. 109, 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. 109, H P is the high-pres- H I 1^- Fig. 109. sure cylinder and L P the low-pressure. Steam is conducted r46 Steam 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 foUoAved 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. no, let the relative volumes of the high- and low- pressure cylinders be as i : 4, then make a b ^ 1 = volume of high -pressure cylinder, and a <: = 4 = volume of low-pressure cylinder. From a seX ofi a d =■ 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 Compound Engines l\y 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 ck^\ 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 gmk represents the gradual fall of pressure as the volume of the low-pressure cylinder increases, and the curve /« A represents the decreasing back pressure on the high-pressure piston during the same period ; bfr=ag ; p n=^rm ; and ck^a h. Then defh is the theoretical indicator diagram for the high-pressure cylinder, and agkcior 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 ccfmpound engines, as in fig. 109, 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 . =1-2 „ 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. L 2 148 Steam Initial steam pressure=ioo lbs per sq. in. absolute. Then, the volume of steam admitted to high-pressure cylinder =-^ volume of cylinder + clearance =:g- of 5 + ■35 = 202 cub. ft. The final volume of the steam is that contained by volume of low-pressure cylinder -l- clearance of low-pressure cylinder + clearance of high-pressure cylinder (omitting the steam in the intermediate chamber, which is a constant volume at the end of each stroke)=2o-F i'2 + -35 = 2i'55 cub. ft. final volume Then total ratio of expansion: initial volume - 21-55 - 2 '02 :IO-67, and the terminal pressure of steam in the low-pressure cylinder = 100 X =g'4 lbs. per sq. in. 21-55 ^\'e 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—; — =37-75 lbs. per sq. \n. 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, namel)', 3775 lbs., would be the initial pressure of the low-pressure cylinder (as in fig. no) ; 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- Compound Engines ^49 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)^3 .3,6 1^3. 5'3S + 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 7-6s-f i-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. 8'85 To find the pressure at any intermediate point in the stroke, say :jth ; 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 + ^ volume of low-pressure cylinder, :=f of 5 + '35 + 2 "3 -I- 1 "2-1-:^ 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 '—^=iT';4 lbs. per sq. in. 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 cylinder-!- volume of receiver -f clearance of high-pressure cylinder =2o-H-2 + 2-3-1- •35=23-85 cub. ft, and its terminal pressure = 29-226 X ^--^=9-4 lbs. per sq in., 23-85 and this is the same result as we obtained before. ISO Steam S0Jbs--3l2° Communication is now opened with the condenser, and the pressure falls to that in the condenser. The range of temperature in the cylinders may be followed for a special case by referring to a similar diagram to the pre- vious one, having the pressures and tem- peratures marked upon it. Thus, suppose steam at 80 lbs. absolute is expanded to 30 lbs. in the for- ward stroke of the ^^^» high-pressure cylin- der, and then to 10 lbs. in the low- pressure cylinder, after which it is exhausted into the condenser at 3 lbs. pressure. The temperatures due to these pressures are marked on the diagram, from which we may prepare the follow- ing table, showing the range of temperature in the cylinders of a compound Woolf engine as compared with a single-cylinder engine. ,m Fig. Single Cylinder Engine Woolf Engine H.P. Cylinder Woolf Engine L. P. Cylinder Forward Stroke Exhaust Stroke Total Range 170° 119° 108° 312- 142= 170° 312-250=62° 250- 142= 108° Open to condenser at 142° 250-193 = 57° Open to condenser at 142° From the table it will be noticed that the range of tempera- ture for the single-cylinder varies from the initial temperature of the steam to the temperature of the condenser — the two extremes — during the single forward stroke. The hot steam at the initial temperature of 312" enters the cyhnder and meets with an internal metallic surface including cylinder cover, piston face, and steam port at a temperature of 142". Compound Engines 151 (This assumes that there is no compression at the end of the stroke, the effect of which is to increase the temperature of the cyhnder before admission.) But, further, during admission and expansion of steam in the forward stroke of the piston, the cylinder barrel on the other side of the piston is in com- munication with the condenser at 142° ; and as the piston moves forward it brings more and more of the cold barrel into communication with the hot steam. It will not be surprising, therefore, that condensation of steam occurs in the cylinder under such conditions. In the high-pressure cyHnder of the Woolf engine, how- ever, it will be seen that the range is very much less than in the single-cylinder engine. The steam at the initial tempera- ture of 312° is admitted, when the temperature of the cylinder walls is 193°, or 51° warmer than with the single-cylinder engine. But, further, the cylinder barrel on the other side of the piston, the surface of which the piston in its forward movement brings more and more into communication with the initial steam, instead of being in communication with a condenser at 142°, contains steam at a temperature varying from 250° to 193°. There is, therefore, an evident gain in this arrangement over that of the single-cylinder engine. The variation of stress oti the mechanism of the engine is reduced by adopting the compound system. In the tandem arrangement, fig. 109, it will be evident that the maximum effec- tive pressures of the steam upon the two pistons, the whole of which are transmitted through the working parts to the crank pin, occur at the same time. The sum of these pressures, how- ever, is less than if the steam at the same initial pressure had been admitted direct from the boiler to a single cylinder of the diameter of the low-pressure cyhnder. For if P = initial pressure per sq. ft. on high-pressure piston, and /= initial pressure per sq. ft. on low-pressure piston ; and if the areas of the pistons are I sq. ft. and 4 sq ft. respectively, then P x i=pressure on high- pressure piston, and /x(4— i)=effective pressure on low- pressure piston, and since/ is less than P, then evidently (P x i) + (/ X 3) is less than P x 4. Again, at the end of the stroke of the tandem engine, 152 Steam the pressure on the crank pin is equal to the sum of the effective pressures on the two pistons ; but in a single- cyhnder engine working down to the same terminal pressure, the pressure would be that due to the pressure on the large piston only. We have, therefore, in the compound tandem engine the initial pressures less and the terminal pressures greater than ia a single-cylinder engine of the same power, working through the same range of pressures. The effect may be represented by a diagram, fig. 112, which is approximately the figure that would be ob- tained if the pres- sures on the crank pin in the two cases were plotted by verti- cal ordinates measured from the zero line of pressure. From which we see that, though the mea7i pressures in the two cases may be the same, the range of stress is less in the compound engine. This limited range of stress, however, is not altogether an advantage, especially at high speeds. A further advantage in point of strength is gained in this engine over the single-cyhnder engine, by using the steam of high initial pressure in a cylinder of small diameter. II. The Compound Engine, with the cylinders placed side by side, and with the cranks at right angles, as shown at fio-. 113. In this engine the steam enters the high-pressure cylinder 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. Fig. 112. Compound Engines 153 In practice it is usually found unnecessary to have a separate special chamber for a receiver, as the exhaust pipe of the high-- L P m H P E rt^ Fig. 113. 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- ReceivER. a. y mil iiiiii / t Ir f,2it which point it escapes to the condenser, when the pressure falls to the line of back pressure. If the cut-off in the low-pressure cylinder occurs later than half stroke, which it frequently does, there will be a momentary rise of pressure in the middle of the low-pressure diagram, due to the augmented pressure in the receiver from the high- ic6 Steam pressure exhaust ; there will also be a corresponding fall of back pressure on the high-pressure piston. In practice, the changes indicated^do not occur so as to produce sharp corners as shown on the ideal diagram. All the corners would be rounded, and the line g t n, for example, would be a gentle curve. We may further illustrate this case by a numerical example. Take an engine as in fig. 113, with cranks at right angles, cut- off at half stroke in each cylinder. Volume of high- pressure cylinder . . =5 cub. ft. Volume of low-pressure cylinder . . =15 ,, Volume of receiver .... =8'5 ,, Initial pressure of steam:=i2o lbs. per sq in. absolute. Then, omitting the effect of clearance, steam is admitted to the high-pressure cylinder at an initial pressure of 120 lbs=« d, cut-off at half stroke=(/^, and expanded to end of stroke3=«_/^ where the terminal pressure bf=^6o lbs. rr,, , . 1 ^ /• ■ final volume ac iK , I he total rate of expansion=. =:_=:_^ =: 6 • mitial volume aj- 2 '5 and the terminal pressure ck in low-pressure cylinders ?^ 6 = 20 lbs. per sq. in. The steam in the low-pressure cylinder is expanded twice in that cylinder, therefore the pressure r m a. 1870 1880 40 to 50 „ 70 to 80 „ 2-6 „ 2-2 „ 1886 150 to 160 „ I'S » 1889 - 1-4 „ Between i860 and 1870, when the pressure of steam used for marine engines was about 30 lbs. by boiler gauge, and the steam expanded in a single cylinder, the amount of coal con- sumed by the best engines was about 4 lbs. per I.H.P. per hour. On the introduction of the compound engine, the con- sumption fell to a little over 2 lbs. per I.H.P. per hour. The triple expansion engine has reduced this to as low as i'4 lbs. per I.H.P. per hour, and the quadruple expansion engine still further reduces the consumption by about 10 per cent. To appreciate the significance of so apparently small a gain as \ ib. of coal per I.H.P. per hour, we will take an example : Suppose a vessel of 6,000 I.H.P. steams from London to Melbourne and back in eighty-four days, find the saving on such a trip. Gain per I.H.P. per hour=;^lb. of coal. „ ,j per day =\ x 24lbs. of coal Compound Engines 159 i6o Steam Quadruple Expansiou Eiigiues i6i i'lG. iiy- 1 62 Steam Gain per I.H.P. per 84 days =;^ x 24 x 84 lbs. of coal. „ per 6,000 I.H.P. per 84 days =i x 24 x 84 x 6,000 lbs. = 3,024,000 lbs. = 1,350 tons. The principles which govern the construction of triple and quadruple expansion engines are merely an extension of those already considered. The diagram, fig. 117, is a section through the cylinders of a set of triple expansion paddle engines made by Messrs. Bow, McLachlan and Co., of Paisley. Thecylinders of these engines are : High-pressure cylinder, 16 in.; intermediate, 25 in. ; low-pressure, 39 in. diameter, respec- tively, having a stroke of 36 in. The cranks are set at an angle of 120° relative to each other. The high-pressure cylinder is fitted with a piston valve, the exhaust steam being led round by a passage to the casing of intermediate cylinder, which cylinder is fitted with an ordinary flat-faced slide valve ; from this cylinder the steam is led round by a passage formed on inter- mediate cylinder to low-pressure cylinder, which is fitted with a flat-faced slide valve. The exhaust steam is led from thence to the condenser, which is arranged under the crank shaft. An enlarged drawing of the piston valve is shown in fig. 68. Figs. 118 and 119 show a sectional view of a set of quadruple expansion engines made by Messrs. Fleming and Ferguson, of Paisley. Fig. 119 gives a section through all four cylinders, show- ing the slide valves, steam ports, and the construction of the pistons and stuffing boxes. The steam enters the smallest or high-pressure cylinder only, direct from the boiler ; and it is then successively expanded to the second, third, and fourth cylinders (in the order of their diameters) by means of the pipes shown on the other view; and finally into the condenser, shown as a rectangular box, forming part of the engine framing. The diameters of the cylinders are 10^ ins., 14 ins., 20 ins., and 28 ins. respectively ; the stroke is 20 ins. ; and the indi- cated horse- power 360. The whole of the low-pressure cylinder, and the bottom of the third cylinder, are jacketed. The upper cylinders form the covers for the lower. The lower cylinders have hand holes in front, to allow of their pistons being sighted, Compound Engines 163 and the junk ring pins felt without disturbing the upper cyUnders. The packing between the cylinders is of the self-adjusting spring metallic type, and will last for years without attention. The crank shaft is 5^ ins. diameter, with cranks at right angles ; it is forged from the best wrought-iron scrap, and has three bearings, 10 ins. in length. The condenser has 390 sq. ft. of cooling surface. Air pump . .12 ins. diameter, 12 ins. stroke. Circulating pump . 7 ins. „ 12 ins. „ Feedpump . . 2| ins. ,, 12 ins. ,, Bilge pump . . 2| ins. ,, 12 ins. ,, The heating surface in the boiler is 752 sq. ft., and the grate surface 27 sq. ft. Working pressure of steam in boiler, 200 lbs. per sq. in. M 2 164 . Sfca77t CHAPTER XVII boilers: The vessel in which the steam is generated is called the boiler. In the early days of boiler construction, the pressures used were not higher than 3 or 4 lbs. on the square inch ; and boilers were then constructed without regard to suitability of form to resist internal pressure. But, as steam pressures began to in- crease, increased attention to this point became necessary. To- day 150 lbs. on the square inch is not uncommon, and to carry this safely the strongest possible form of boiler must be adopted. The sphere is the strongest form of vessel to resist internal pressure, but there are many practical reasons which prevent its being used for the purpose. Next to the sphere the cylin- drical form is the simplest and strongest, and it is now universally adopted. Resistance of cylindrical vessels when subjected to internal pres- sure. — Let fig. 120 represent a thin cylindrical vessel subjected to internal pressure. Let/ = internal pres- sure per square inch ; d = diameter of cylinder ; t = thickness of plate ; and / = length of cylinder. It is evident that the internal pressure/ is acting radially from the centre on every part of the internal circumference of the shell ; but if these forces be resolved into components parallel and perpendicular to a given plane, the resultant forces, tending to separate the cylinder into two parts through a plane abed, can be shown to be equal to pxdx I. Boilers 165 The area of the material to resist this tendency to burst along the lines ac &nA bd^^ {ac+bd) t= 2 Ixt. Hence the stress {s) per square inch on the plate may be expressed thus : load pdl _pd , ■. area 2 / / 2 / ' From which we learn that the stress on the material increases as the pressure or the diameter increases ; and the stress per sq. in. on the material decreases as the thickness of the material is increased. In practice the section would be taken through the weakest part, which, in a new boiler, is through the rivet holes. The strength of a single riveted joint is taken as 56 per cent, that of the solid plate, and of a double riveted joint 70 per cent. Again, to find the pressure tending to tear the boiler in two in a plane perpendicular to the axis ; in other words, tending to blow the end off. Area of end = d'^ x 7854. Total pressure on end = (^^ x 7854)/. The area of the hiaterial to resist this tendency (neglecting deductions for rivet holes) = circumference of shell x thickness of plate = ^x 3-1416 xt. Hence the stress {s) per square inch on the plate may be expressed thus : s = ^°^^ = ^X7854X/ _ld _ _ , (2) area ^X3'i4i6 v/ 4/ Comparing this result with that in (i) above, we see that the stress is only half as great in the latter case ; in other words, theoretically the plate is twice as likely to give way in the direction of the length of the boiler as circumferentially. For this reason the longitudinal joints are made stronger than the circumferential by the addition of an extra row of rivets. Descriptions of Boilers Boilers may be divided into three classes : stationary, locomotive, and marine. 1 66 Steam Stationary Boilers The Cornish boiler. — This form of boiler was first adopted by Trevithick, the Cornish engineer, at the time of the intro- Fig. 121. duction of high-pressure steam to the early Cornish engine, and it is still much used. It consists of a cylindrical shell A, with flat ends, through which passes a smaller tube B containing the furnace, as shown in fig. 121. The products of combustion pass from the fire-grate forward over the brickwork bridge to the end of the furnace tube ; they then return by the two side flues m m' to the front end of the boiler, and again pass to the back end by a flue n n along the bottom of the boiler to the chimney. Fig. 122 shows a trans- verse section of the boiler and flues. One advantage possessed by this type of boiler is that the sediment contained in the water falls to the bottom, where the plates are not brought into contact with the hottest portion of the furnace gases. The reason for carrying the products of combustion first through the side flues, and lastly through the bottom flue, will now be evident ; for the gases, having parted with much ol their heat by the time they reach the bottom flue, are less liable Fig. 123. Boilers j ^j to unduly heat the plates in the bottom of the boiler, where sediment may have collected. Water tubes are often fitted to Cornish and Lancashire boilers. Their shape and position will be understood from the diagram, fig. 123. Holes are _ __ _ cut opposite each other in the ^5 = =^3_-Jv:^:^S^f3"V^r^"Yi furnace tube, and the joints made good by riveting the flanges of the water tube round the hole. AVater can thus flow freely through the tube. They pass right across the furnace beyond the furnace bars, so that the flame and hot gases have a considerably increased surface to act upon. Besides increas- ing the heating surface, these tubes improve the circulation of the water, and act as a stay to the furnace tube. They are not an unmixed good, however, for they cool the furnace gases and retard combustion. The Lancashire boiler differs from the Cornish boiler in having two internal furnace tubes instead of one. The separate furnaces are intended to be fired alternately, so that the mixture of smoke and unburnt gases from the newly-fired furnace may be consumed in the flues by the aid of the high temperature of the gases from the bright fire of the other furnace. The following figures (figs. 124, 125 and 126) illustrate the construction of a Lancashire boiler. Fig. 124 shows a longi- tudinal section. The furnace door, P, opens to the furnace where the fuel is supported on two or three successive lengths of fire-bars, under- neath which is the ash-pit. At the back end of the furnace is a low brickwork bridge. Besides limiting the length of the fire- grate, the bridge causes the flame to rise against the upper surface of the tube. The fire-bars are supported on bearers. The front bearer, which is a cast-iron plate, is called the dead plate. Beyond the furnace are shown the Galloway tubes a a a, 1 68 Steatii noih; 169 170 Steam Fig. seen also in fig. 126. In the Lancashire boiler the furnace gases pass to the end of the furnace tube, and then by the flue under- neath the boiler to the front, where it divides and again passes by side flues (see fig. 126) to the back end of the boiler and up the chimney. The flat ends of the boilers are prevented from bulging by the furnace tubes and by longi- tudinal stays, A A, also by gusset stays e e, shown in fig. 124 and enlarged in fig. 127. The level of the water is shown, and the space above this is occupied by the steam. The steam is collected in the pipe S, which is perforated with holes all along the top so as to admit the steam, and at the same time pre- vent water spray from passing to the engine with the steam. On opening the stop valve T (see also fig. 139), the steam passes by the steam pipe to the engines. Two safety valves are shown, one a dead-weight safety valve B (see also fig. 138), and the other a lever safety valve C. The float F is balanced so as to float on the surface of the water. Should the water fall .below a safe level, the float F, which falls with the water, causes the valve to open by means of levers, and allows steam to escape, giving warning of shortness of water. A manhole M is shown, by which access is obtained to the interior of the boiler for cleaning and inspection or repairs. Boilers 171 A mudhole H is also required for cleaning out the boiler, and removing the sediment which accumulates. A blow-off cock and pipe is shown in the bottom of the boiler at the front end. On the front of the boiler, fig. 125, is shown a pressure gauge with a finger indicating the pressure of the steam in the boiler above the atmosphere ; two water-gauge glasses showing the height of the level of the water in the Fig. 129.* boiler (see enlarged view, fig. 128) ; the furnace doors P ; the feed-pipe K, which is shown extending some distance into the boiler in fig. 124 ; and the scum cock R for blowing off the scum which accumulates on the surface of the v;ater. Vertical Boilers The illustration, fig. 129, shows the construction of a vertical boiler. These boilers are used for small powers, and where ■ From The Marine Engine, by Mr. R. Sennett (Longmans). 172 Steam space is limited. The internal fire-box is frequently made slightly tapering towards the top to allow of the ready passage of the steam to the surface. The bottom of the fire-box is attached to the bottom of the outer shell by being flanged out as shown, or by means of a solid wrought-iron ring, as shown in the locomotive boiler, fig. 132, the rivets passing right through the plates and solid ring. The water tubes pass across the internal fire-box, and increase the heating surface as well as improve the circulation, though they cool the furnace gases. The plate forming the passage leading from the top of the fire-box to the chimney — called the uptake — is frequently protected either with fireclay or with a cast-iron hner. The Marine Boiler Marine boilers, which are now usually constructed to carry steam at pressures up to 150 or 160 lbs. per square inch, are cylindrical tubular boilers, short as compared with their dia- meter, as illustrated by the accompanying diagram (fig. 130). Description of the figure. — The boiler is of the cylindrical, multitubular type, fired from one end, with three furnaces. The products of combustion in the furnaces are carried forward by the draught into the combustion chambers C C, and thence through the tubes in the direction of the arrow to the front of the boiler, whence they pass up the funnel. The outside shell is 12 ft. if in. extreme diameter, and 9 ft. 5fa ins. extreme length. The plates are of steel, |-} in. thick, in three rings united together circumferentially by double-riveted lap joints. The longitudinal seams are treble- riveted. The end plates are made in three pieces, and are joined together by double-riveted lap joints, and flanged to meet the shell and the furnace flues. The furnaces are 3 ft. inside diameter, constructed of Fox's corrugated steel plates \ in. thick. They are flanged at the back end, and riveted to the combustion chambers. The combustion chambers are flat on the top, and are sup- ported by wrought-iron girder stays. The back and sides of these chambers are stayed with if-in. screwed stays, fitted with nuts on both ends. Boilers 173 ' 74 Steam The boiler contains 200 tubes, 3 ins. diameter outside, of which 42 are stay tubes. The stay tubes are of wrought iron, -^ in. thick, and screwed into the plates with nuts on the front ends. The remainder of the tubes are of brass. Longitudinal stays, i| in. diameter, steel, pass through the steam space from end to end, and support the front and back plates of shell. Fire-grate area=:a.rea. of grate X number of furnaces= (3x6)x3=S4sq. ft. The heating surface. — The effective heating surface of a marine boiler is obtained by finding the sum of the following areas : 1. Area of furnace above level of fire-bars. 2. Area of sides and crown of combustion chamber above level of bridge. 3. Area of back tube plate, less area of holes for tubes. 4. Area of surface of tubes, namely, the area obtained by multiplying the external circumference by the length between the tube plates. The area of the front tube plate is omitted. The length of furnace should not exceed 6 ft,, otherwise it becomes difficult to stoke. The fire-doors are made of three pieces of plate placed about 2\ ins. apart, the two inner ones being perforated. It will be noticed that the back of the combustion chamber slopes a little inwards towards the top. This enables the steam to rise more freely. The space allowed between the tubes is i in., and the tubes are arranged in vertical rows to allow of the boiler being properly cleaned internally. Manholes are placed on the top and front of the boiler, to get at the upper and lower parts of the furnaces for cleaning and repairing. The furnace bars are of wrought iron, and in three lengths, sloping towards the bridge \ in. per foot. Dis- tance between bars \ in., maintained by widened ends of bars. Steam room. — It is important to have as large a reservoir of steam as possible above the level of the water in the boiler, to prevent too great fluctuations of pressure. The water level should be at least 7 ins. above the top row of tubes. To find the cubic contents of the steam space : Find the Boilers I7S area of the segment of the circle occupied by the steam, and multiply by the internal length of the boiler ; and from this subtract the contents of the stays which occupy part of the steam space. To find the area of the segment of a circle (fig. 131) : Area of whole circle angle acb . . , , X — 5_^ — area of triangle abc. 360 To give the front and back plates of shell the necessary stiffness, large circular plate washers, 10 ins. diameter, are riveted on to outside of plates. ^"^' '^'' The maximum stress allowed on these stays is 8,000 lbs. per square inch for stays under i^ in. diameter, and 9,000 lbs. for stays over •i\ in. The Locomotive Boiler The following diagram (fig. 132) is a longitudinal section of the locomotive boiler. The fire-box F B, or furnace, is of rectangular section, and is made of copper, stayed by means of screwed and riveted copper stays, \ in. in diameter and 4 ins. apart, to the outer shell of the boiler. The crown plate of the fire-box being flat requires to be very efficiently stayed, and for this purpose girder stays called fire-box roof stays are mostly used, as shown in the figure. These stays are now being made of cast steel for locomotives. They rest at the two ends on the vertical plates of the fire-box, and sustain the pressure on the fire-box crown by a series of bolts passing through the plate and girder stay, secured by nuts and washers. Fig. 133 is a plan and elevation of a wrought-iron roof stay. Another method adopted in locomotive types of marine boilers for staying the flat crown of the fire-box to the circular shell plate is shown in fig. 134 — namely, by wrought-iron vertical bar stays secured by nuts and washers to the fire-box and with a fork end and pin to angle-iron pieces riveted to the outer shell. 176 Steam Boilers 177 e® o. o. o. Fig. 133. The barrel of the boiler contains the tubes through which the products of combustion pass. The advantage of the tubes is the large amount of heating surface they expose to the heated gases. If the tubes are placed too closely together the steam generated round the tubes can- not freely escape ; and as steam cannot absorb the heat so readily as water, the surface of the tube is liable to be overheated and to rapidly deteriorate. The part of the tube nearest the fire-box is the most effective heating surface ; and the value of the heating sur- face of the tube rapidly decreases towards the smoke-box end. The upper surface of the tube is also far more effective than the lower, even when the tube is clean ; but when soot is deposited in the lower portion of the tube, that part of it is valueless as heating surface. The chamber beyond the tubes and below the chimney is called the smoke-box, SB. A dome, S D, is usually provided, Fig. 134. from which the steam is taken to supply the engines ; and a safety valve, S V, is placed as shown. Heating surface of iuies.— The student will be aware that, in order to obtain large heating surface in a boiler, a number of small tubes are used in preference to a few large ones. Foi the smaller the diameter of the tubes used to fill a given sec- tional area, the greater the area of heating surface obtained. 178 Steam Thus, take a circle i in. in diameter, fig. 135, then its cir- cumference=i x 3-1416 ins. But if two circles, \ in. in dia- meter, be placed on the diameter of the i-in. circle, touching each other and the large circle, then their circumferences = (5X3'i4i6) 2=3'i4i6 ins., the same as before; or, if 10 small circles, each -^-j^ in. diameter, be ranged along the same dia- meter, the sum of their diameters being i in., the sum of their circumferences is ( -jJ„ X3'i4i6) 10 = 3'i4i6 ins. as before. But, the smaller the circles used, the more room remains for the insertion of other circles within the area of the large circle ; and, therefore, the smaller the diameter of the tubes, the greater the number possible in a given area, and the greater the heating surface obtained. Fig. 135. The practical limit to the diameter of the tube depends upon the possibility of keeping them from being choked up with soot and dirt. The tubes used in locomotive boilers are about 2I ins., and in marine boilers from 2 i to 3^ ins. outside diameter. Safety A'alves The safety valve provides for the safety of hollers by allowing the steam to escape when its pressure exceeds a certain limit. The safety valve is kept in its place on its seating either by a weight at the end of a lever, by a strong spring, or by a heavy weight, placed directly over the valve, and these three forms will here be described. A good safety valve is one which wiU not permit the pressure Safety Valves 179 in the boiler to rise above a fixed point, and, having reached that point, will allow all excess of steam to escape as fast as it is generated by the boiler. Mr. Webb, of the London and North- Western Railway, in an experiment on a locomotive boiler fired hard, found that a pipe i\ in. diameter was sufficient to allow all the steam to escape as fast as generated without the pressure increasing beyond the initial pressure. The Lever safety valve. — This valve (fig. 136) rests on a circular brass seating, and is prevented from rising by the steam pressure underneath the valve, by the weight at the end Fig. 136. of the lever. The disadvantage of this valve is that it admits of being tampered with, and the effect of a small addition to the weight is magnified considerably in its action on the valve. To find the weight W, or length of lever A B, for a given pressure of steam : Let A B = length of lever from fulcrum A to centre of weight W, A C = distance between centre of valve and fulcrum. W = weight at end of lever. ■w = weight of lever acting at centre of gravity of lever, assumed at centre of lever. P r= pressure of steam per sq. in. a = area of valve. V = weight of valve. N 2 1 80 Steam (1) If the effect of the weights of valve and lever be omitted, we have, when valve is just about to lift, — Downward pressures = upward pressures. Pressure on valve Upward steam due to W. pressure. W X ^J - ^" w = P.x;^ (2) Taking the effect of weights of valve and lever into account (which should always be done where accuracy is re- quired) we have, when valve is about to lift, — Downward pressures = upward pressures. Pressure on Pressure due Weight Total upward valve due to weight of pressure on to W. of lever. valve. valve. ^^^4c + ->' ., with forced draught . 13 to 15 lbs Locomotive boilers . 8 to 9 lbs. Torpedo-boat boilers, locomotive (forced draught) type . 18 lbs. 187 CHAPTEE XVHI PRACTICAL NOTES ON THK CARE AND MANAGEMENT OF ENGINES AND BOILERS (i) Before getting up steam the boiler water-gauge cocks should be tried to see that the water is in the boiler. (2) The stop-valve should be opened a little, before the fire is lighted, so that, while the steam is being generated in the boiler, it may pass through the cyhnders and jackets and warm them gradually, the temperature rising as the pressure rises. Meanwhile all drain-cocks from the slide jackets and cylinders should be opened to allow the steam to flow through, and the condensed steam to pass away. This will prevent the possi- bihty of the cylinder cracking owing to sudden admission of hot steam against the cold metallic walls of the cylinder. This is especially important in cold weather. (3) The drain-cocks should remain open for a few revolu- tions till all water has been blown out of the cylinder, and then closed. (4) Should these precautions not have been attended to, then, since the exhaust port closes before the end of the stroke, the water in the cyHnder would be compressed, and a difficulty found in starting the engines. Any attempt to force the engine by ' barring round ' would tend to burst the cylinder cover, or to push the slide-valve off the face of the ports. (5) See that all the lubricators are in good condition, the holes clear, and the worsteds clean, and that the lubricators are well supplied with oil. (6) Should there be any tendency to heating of the bearings, the cap nuts should be eased and the lubricator examined to 1 88 Steam see whether it is working properly. Should the bearing be very hot, the engine must be stopped, the cap removed, and the brass taken out and examined to see the cause. (7) If a condensing engine, the vacuum gauge should be watched ; and, if the vacuum is not maintained, the injection, or circulating water, should be regulated. If this does not pro- duce the desired effect, there is probably an air leak through the piston-rod gland, or the air-pump-rod gland, which should be screwed up ; and, if the vacuum is still defective, the cause must be looked for in the foot and head valves or the air-pump bucket valve (if any), or in leaky condenser tubes. (8) See that the water in the boiler-gauge glass is kept at the proper height, namely, about half-way up the glass, and that the fires are kept in proper condition, and that the steam pressure is kept uniform. The feed water supply should be as uniform as possible, and not be shut off at one time, and wide open at another. (9) When feeding the furnace the coals should be laid on in thin layers, and in small quantities at a time, care being taken to fill up all hollow places, and to keep the fire level. The fire-door should not be kept open a moment longer than is necessary. (10) The damper regulating the draught should be kept only sufficiently open to generate the quantity of steam required. (11) The ashes should not be allowed to accumulate in the ashpit, because the heat from them may cause the fire-bars to bend under the weight of the fuel in the furnace. (12) To clean the fire, which should be done when it is dirty from the presence of clinker, scrape the fire from one side of the furnace to the other with the slice-bar, then break up the clinkers from the fire-bars with the slice-bar and draw them out with the rake with as much speed as possible. As soon as this half of the furnace is clear of clinker, turn the fire over from the other side on to the clean side, and throw a little round coals on the fire before cleaning the second half ; then clear of clinker as before. Now level the fire over the bars with the slice-bar, and throw on a thin layer of round coals, and close the fire-door. Care of Machinery 189 (13) All cocks and valves connected with the boiler should be moved daily, especially the safety valve. In the Navy the safety valves are lifted at least once every watch, to see that they are in working order. (14) Should the engines stand idle for any length of time they should be turned partly round each day. To test for a leaky slide valve. — Block the fly-wheel when the slide valve is in the middle of its stroke (seen by the position of the eccentric, which is in mid position a little before the piston reaches the end of the stroke) and open the indicator taps, or the relief cocks, or look at the exhaust pipe. A steady escape of steam indicates a leaky vahe. To test for a leaky piston. — Block the fly-wheel when the piston is situated at a short distance beyond the beginning of the stroke. Admit steam to the piston and open the indicator tap, or relief cock, on the exhaust side of the piston. An escape of steam will indicate a leaky piston. The leak may be caused by a leaky slide valve, so this should be tested first. Annual Inspection of Engines and Boilers Engines. — (i) Take off slide-valve cover and examine valve faces and fastenings of valves to rods ; see that surfaces arc all clean and bright ; remove fatty substances which have accumulated from lubrication. Turn engines round and test lead of valve. (2) Take off c\linder cover, examine cylinder for cracks or other defects. (3) Examine condition of piston whether steam-tight, and its attachment to . piston rod ; take off junk ring, remove springs and spring rings, and see that they are in good condi- tion. (4) Air-pump and condenser to be opened out and cleaned, foot and head valves, bucket valves, and bucket packing to be examined, and defects made good. Also see that the injection valve and orifice is in good condition, and that air-pump rod is properly secured to bucket. If surface condenser, tube packings renewed where necessary. 190 Steam (5) Caps to be taken off main bearings, cap brasses taken out and adjusted, and oil-ways cleaned. (6) Connecting-rod brasses to be examined and adjusted if necessary. To adjust connecting-rod brasses. — 'When fitted with liners or distance pieces between the two half brasses, remove the liners, screw down the brass on the journal and measure the distance between the two brasses with a pair of internal cal- lipers. Take a gauge from this with external callipers and fit th£ liner tight between the brasses. Then slack the nuts off, put the liners in their places, and screw up the nuts. The brasses will then fit properly on the journal. When the brasses are not fitted with liners, place a piece of thin lead wire on the journal and tighten up the brasses upon it. When the brasses are right up, the lead wire will be flat- tened, and the thickness of the flattened wire will indicate the amount the brasses require to be set up. The proper freedom of the journal in the brass may be tested by disconnecting the other end of the rod, and swinging the rod on the journal ; when tightened up, the rod should move . freely on its bearing without any tendency to grip the journal. (7) All stuffing boxes to be repacked. Boilers. — (i) Stationary boilers, even when using clean feed water, should be opened and thoroughly cleaned out at least once a year, and all parts of the boiler and the boiler fittings carefully examined. The frequency of cleaning out the boiler will depend upon the kind of service and the character of the feed water : for example, locomotive boilers are cleaned out two or three times a week. (2) Find the water-line inside the boiler and trace with a paying hammer for defective parts. Examine carefully for indications of pitting &c. Where decay has commenced, it should be carefully watched and steps taken to prevent further corrosion by scraping off all rust, and coating with a thin wash of Portland cement, or other substitute. Test the thickness of the plate where suspiciously thin by drilling a hole ; tap it, and put in a screwed plug. Test the boiler by hydraulic pressure to twice its working pressure. Care of Machinery ^9^ (3) Safety valves. — Remove the weights or springs from the valve, and take the valve out to clean and examine it ; see that the seating is not pitted and that the valve works freely. Weigh the weights on the valve (or test the springs), and divide the sum by the area of the valve. This will give the pressure per sq. in. at which the valve will lift. Check this result by the pressure gauge. (4) Stop-valve. — Remove the cover and take out and ex- amine the valve and its seating. (5) Water-gauge cocks. — Take all plugs out and see that all the passages are clear. (6) Feed-valve. — Take out and examine the condition of valve and its seating. (7) Blow-off and scum cocks. — Take out, clean, and examine condition of plugs and bearing surfaces. Also examine the gland bolts of these cocks ; if they break, the plug is blown out. (8) Fire-bars. — See that the fire-bars are not too far apart. To fit fire-bars, fill the furnace tight with bars, then remove one bar ; this will allow for expansion. APPENDIX MENSURATION OF SURFACES AND VOLUMES 1. Area of rectangle . . = length x breadth. 2. Area of triangle . = base x \ perpendicular height. 3. Diameter of circle . = radius x 2. 4. Circumference of circle . =diameter x 3'l4i6 5. Area of circle . . . =diam. x diam. x 7854 , , r , / • I area of circle x No. of degrees in arc. 5. Area of sector of circle . = . - ^ -- 360 7. Area of surface of cylinder = circumference x length -h area of two ends. 8. To find diameter of circle liaving given area : Divide the area by ■7854, and extract the square root. 9. To find the volume of a cylinder : JNIultiply 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. E.xplain 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 \vould this rise indicate on the Centigrade thermometer ? 5- What is meant by the ' specific heat ' of a substance ? One ounce Questions 193 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. 11 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° F". 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 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.' 1. Explain the process of the combustion of coal. 2. What conditions are necessary for the complete combustion of the gases distilled from the coal ? 3. What is the effect of an incomplete supply of o.xygen during the combustion of solid fuel ? 4. Given that 966 units of heat are required to evaporate i lb. of water from and at 212°, how many lbs. of water should i lb. of coal evaporate whose total heat of combustion is 14,000 units per lb. ? 5. What is the difference in the heat of combustion between carljon burnt to carbonic oxide, and carbon burnt to carbonic acid gas ? 6. What horse-power should be obtained by burning i lb. of coal if there were no waste ? (Sc. & A. 1883.) 7. An engine of 6,000 H.P. burns if lb. of coal per I.H.P. per hour; find the consumption of coal psr 24 hours. V I. Give what practical illustrations you can of the expansion of metals by heat. 194 Steam 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 15-5 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 evapo. rates ? 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 a perfect vacuum. 11. Describe an experiment illustrating the reality of atmospheric pressure. 1. Describe carefully the stages involved in the conversion of water Into steam under a movable piston, noticing especially the changes in tempera- ture and volume. 2. What work 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. Whjit 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. In what way is the ' latent ' heat expended during the formation of steam ? 5. How is the efficiency of an engine expressed ? 6. Considering the work done per lb. by steam during formation at varying pressures without expansion, what advantage is gained by using high pressures rather than low ? 7. Given that the volume per lb. of steam at 120 lbs. pressure is 3'65 cub. ft. ; find the external work done per lb. during formation. 8. Find the weight of steam required per horse-power per hour in example 7. 9. What conditions affect the total quantity of heat rejected by steam to the condenser ? 10. Define ' latent heat of steam ' and ' total heat of evaporation.' 11. Give formulae for finding the total and latent heats of steam, and apply them, having given that the temperature of steam at IIJ lbs. is .338° F- 12. A cylinaer is 16 ins. diameter, and stroke of piston 2 ft. ; find the Questions 1 95 area of the piston and the volume displaced by the piston at each stroke neglecting clearance. 13. If, in the previous question, the area and volume of the piston rod, 2\ ins. diameter, be deducted, what will then be the effective area of the piston and volume of steam on the rod side ! 14. The high-pressure cylinder of an engine is 36 ins. diameter, the initial pressure of steam 120 lbs. per sq. in. ; find the load on the piston in tons. 15. The area of a piston is 706-8 sq. ins. ; find the diameter of the air pump which is one-half that of the cylinder. 16. The cylinder of an engine is 74 ins. in diameter and the stroke is 7-3 ft. ; what is the capacity of the cylinder ? How many lbs. of wafer 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 1,670 times that of the water from which it is generated ? (Sc. & A. 1871.) {Xote. — I lb. of water="0l6 cub. ft.) VII 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 relation 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° K. be mixed with 2-5 lbs. of water at 212° F., find the temperature of the mixture. 6. How much water at 60^ F. must be mixed with I 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 1 7 '63 lbs. of condensing water at 60° F. are used per lb. of steam at 212°. VIII 1. What 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 O 2 1 96 Steam 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 ii 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 oi the steam at the second, third, and fourth foot of the stroke (neglecting the effect of clearance). 1 . 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 ; (/') 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 -098 ; find the mean pressure in question 5. 8. Write down the formula for finding the indicated horse-power of an engine. 9. Find the indicated horse-power of an engine with a cyhnder 16 ins. Questions \ 97 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 when 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.) 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.) l5. 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 to 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 tlie steam in the cylinder. 21. How may the loss by clearance be modified ? 22. What is ' priming ' ? 23. Explain in what way ' cylinder condensation ' limits the useful range of expansion of the steam on the cylinder. iqS Steam 26^.. What are the remedies adopted to reduce the amount of condensa- tion of the steam in the cyhnder? 1. Make a hand-sketch of the cylinder (fig. 38). 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. 105). 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. in. absolute ; given that steam at 12 lbs. absolute occupies 31 '9 cub. ft. per lb. 8. The weight of steam passing through the engine per stroke is ■83 lbs. ; 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. 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 havinc 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 \\ 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. Questions 199 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. 1876.) 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 1 j in. ; the lap of the valve ~^ 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 i in. Sketch also the same valve at the beginning of the pistol! stroke with J in. lead. (Sc. & A. 1882.) 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. 11. 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. 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 ? 200 Steam 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. 6. Tb"; 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 stroke 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 f 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.) 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 is the reason for the adoption of the compound engine, and in what respects is this engine superior to the single-cylinder engine ? Questions 201 2. Make a hand-sketch of the cylinders of the compound engine in figs. 105 and io6, and explain how the steam is supplied to and exhausted from each cylinder. 3. How is the low-pressure cylinder of a compound engine propor- tioned ? 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 cyhnder 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. 1. ifake 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 ^rd 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. Show that the variation of stress on the mechanism of the engine is less in the compound engine than in the single-cylinder engine, working with the same weight of steam through the same range of pressures. 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. S. 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 water tubes are fitted to boiler furnace flues. 3. Draw a longitudinal section of a Lancashire boiler, showing all the necessary fittings. 4. Sketch a longitudinal section of a marine boiler, and explain how the boiler is stayed. 202 Steam 5. A steamship has two boilers, each with three furnaces, 3 ft. diameter by 6 ft. long ; find the fire-grate area. 6. Find the heating surface of a marine boiler of the following dimen- sions : {a) 3 Furnaces, each 3 ft. diameter x 6 ft. long. ij}] 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 5 ft' o in.) (c) 3 Back tube plates : 2 (3 ft. 6 ins. X 3 ft. o in.) 1 (3 ft. o in. » 4 ft. 6 ins.) Less area of 200 holes, 3 ins. diameter. (d) 200 tubes, 3 ins. external diameter, length between tube plates 6 ft. 3 ins. 7. Make a sketch of the longitudinal section of the locomotive boiler, showing how the flat crown of the surface is stayed. 8. Sketch a method of staying the flat crowns of furnaces by bar stays and angle-iron. g. Illustrate the advantage of small tubes over large ones, to provide large area of heating surface. 10. Make a sketch of a lever safety valve. 11. A valve, 3 ins. diameter, is held down by a lever and weight, length of the lever being 10 ins., aijd 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.) 12. Sketch a Ramsbottom safety valve. 13. Find the dead weight required for a valve 3j ins. diameter required to blow off at'go lbs. per sq. in. 14. Sketch an equilibrium double beat valve. 15. Describe the construction and action of Bourdon's pressure gauge. 16. Find the efficiency of a boiler which evaporates 11 lbs. of water per lb. of fuel. 1. What precautions would you take when getting up steam? 2. Suppose the vacuum in the condenser was- not satisfactory, what would you do ? Questions 203 3. ^A^hat 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) -15°. -10°, 5°. 20°, 70°, 130° C. ; and 33-8°, -22°, -13°, 194°, 248° F. (4) 25°. (5) 1° nearly. (6) 520°, 560°, 707°, and 273°, 373°. (7) 136° R., and 132° R. II (2) 157 units o; heat ; or 121,204 ""i's of work. (3) 40 units of heat ; 30,880 units of work. (4) 1 37i units of heat ; 106,150 units of work. (5) ii6o-5. IV (4) 14-5 lbs. (5) 10,100 units. (6) 5 H.P. (7) II2-S tons. V (3) 7-5 cub. ft (4) 36-47 lbs. (5) 11-34 cub. ft. VI (2) 21,168 ft. lbs. (7) 63,072 ft. lbs. (8) 3I-39. (II) 1183-4 and 878-4. (12) 201 sq. ins. ; 4,824 cub. ins. (13) 197-024 sq. ins. ; 4728-576 cub. ins. {14) 54-53 tons. (15) 15 ins. (16) 224 cub. ft. ; 8-38 lbs. VII (5) 206'66°F. (6) 1-53 lbs. (7) 17-63 lbs. (8) 120= F. VIII (2) 30 and 15. (3) 25 and 5. (4) 18 lbs. absolute. (5) 35. 18-33. 10- (6) i- (S) 6°' 40. 30- IX (5) 37-4- (7) 37-4- (9) Si'iS. (10) I97-4- (II) 50-66. (12) 76. (13) 80. (14) 14=5. (15) 407-27. {16) 16 in? 204 StcaiK (6) 26-47 cub. ft. (7) -83 lbs. (8) 8466 lbs. (n) 640 ft. per min. (12) 654'i6 cub. ft. per min. (20) 12960. (21) 26703 6 lbs. (6) 4lins. XI XIII (8) 2827 cub ins. (9) 904-2 lbs. (10) 900,000 lbs. (II) 18-85 and 34-46 sq. ins. (12) finch. (13) 4,403 sq. ft. ; 6 to I. XV (4) 8. (5) J,. XVII (5) loS sq. ft. (6) 1207-27 sq. ft. (ll) 11-9 lbs. per sq. in. (13) 865-89 lbs. (16) -785. SCIENCE AND ART DEPARTMENT EXAMINATION PAPER— i%%c, SUBJECT XXII. STEAM First Stage or Elementary Examination Instructions. You are not permitted to attempt more than six questions. The value attached to each question is shown in brackets after the question. 1. In an atmospheric pumping engine how is the injection water and condensed steam got rid of? Sketch and explain some of the princi- pal improvements made by Watt in engines of this kind. (I5.) 2. Describe, with sketches, the alterations made by Watt in order to convert a single-acting into a double-acting engine. (I5-) 3. \Miat is the latent heat of steam at 212° F. expressed in foot-pounds '? If I lb. of steam at 212° F. is mixed with 10 lbs. of water at 60° F., find the resulting temperature. {15.) 4. Steam expands in the cylinder of an engine from a pressure of 30 lbs. above the atmosphere to 5 lbs. below the atmosphere, at what part of the stroke was the steam cut cff ? The pressure of the atmosphere may be taken at 15 lbs ('SO Questions 205 5. Describe, with sketches, the construction of a horizontal direct acting engine, working with high-pressure steam and without condensation, showing how the steam is admitted into the cyhnder and let out again as required. (20.) 5. Define the lap of a slide valve, and explain your answer by reference to a sketch. Account for the difference in the working of two engines, one of which has lap on the steam side of this valve and the other has not. (15.) 7. Sketch the end of the connecting rod of a locomotive engine which embraces the crank pin. Show clearly the method of tightening the brasses on the crank pin by a gib and cotter. (iSO S. Marine engines are fitted yith a so-called air pump, circulating pump, feed pump, and bilge pump. What are the respective uses of these pumps ? Show, with a sketch, the construction of any one of them, and explain how it acts. (15 ) 9. The stroke of a direct acting engine is 5 feet, and the crank shaft makes 30 revolutions per minute, find the mean speed of the piston in feet per minute. State your reasons for concluding *hat there is no loss of work from the oblique action of the connecting rod during successive portions of the stroke. Friction is neglected. (15.) 10. Why are the longitudinal joints in cylindrical boilers usually double riveted, while the transverse joints are only single riveted ? Sketch in longitudinal section a marine high -pressure boiler. (iS-) 11. Sketch and describe the following safety valves : — (1) An ordinary lever valve. (2) The Ramsbottom spring loaded valve. (3) An ordinary dead weight valve. (20. ) 12. Explain the principle of Watt's pendulum governor for a steam engine, and sketch the apparatus. Why is it important that the points of suspension of the arms should be near to the vertical axi.s of rotation? (IS-) 13. Compare the crank with the eccentric. Show that they both produce the same motion. State reasons for employing one or the other in particular cases. ('S-) 14. Sketch a longitudinal sectional elevation through the cylinder and coridenser of a trunk engine, showing the air-pump and valves. How are the piston and trunk kept steam-tight? (zo.) 1890 First Stage, or Elementary Examination 1. Define the ierra-^ latent heat, fool-potiiid, thermal unif. Writedown the number which expresses the latent heat of steam at 212° F., and explain how that number is arrived at. (15.) 2o6 Steam 2. Describe Hoinblower's compound or double cylinder engine, and explain by sketches the manner in which the stearn passes through the cylinders into the condenser. You can take either the single or double-acting engine. (I5-) 3. Explain the principle of Watts' invention of a parallel motion for connecting the ends of the piston and air-pump rods with the working beam of a steam-engine. Make such sketches as you may think necessary for explaining your answer. (15.) 4. The piston rod of a steam cylinder and the piston rod of a blowing cylinder are each connected to the opposite ends of a working beam. The steam cylinder is 55 inches in diameter, with a stroke of 13 feet, and the blowing cylinder is 144 inches in diameter, with a stroke of 12 feet : sketch the arrangement. What pressure in the steam cylinder will balance an air pressure of 3 lbs. on the square inch above the atmosphere in the blowing cylinder ? ('SO 5. Sketch in section the ports of a steam cylinder, and show by means of three separate sketches any form of slide-valve which you prefer to illustrate, wherein (i) The valve has no lap. (2) The valve has lap on the steam side. (3) The valve has lap on both the steam and exhaust sides. In each drawing the valve must be shown in its mid-position. (20.) 6. Sketch in section the piston of a small engine working with high- pressure steam, such as a locomotive engine, together with the piston- rod and cylinder cover, showing the metallic packing for the piston and the stuffing-box and gland for making the piston-rod steam-tight. Explain the reasons for the construction which you adopt. (15.) 7. What is the use of a fly-wheel as applied to a steam-engine ? How does its action upon the working of an engine differ from that of the governor? Sketch in elevation the governor of an engine. (15.) 8. Steam is admitted into the cylinder of an engine at 9 lbs. above the atmosphere (taken at 15 lbs. per square inch), and is expanded down to 7 lbs. below the atmosphere : find the pressure of the steam at half-stroke, and the point of cut-off. (l5-) 9. Sketch a section through a vertical air-pump together with the foot and delivery valves. The bucket plunger has a valve in it : describe the construction of the valve and the packing of the plunger ; and show by your drawing the method of attaching the pump-rod to the bucket. Which valves are open during the descent of the bucket? (ij.) 10. The diameter of a steam cylinder is 24 inches, the number of revolu- tions = 30, and the mean effective pressure of the steam = 35 lbs. What should be the length of stroke in order that the horse-power may be 50? (l^,) Questions ^^7 11. Draw a vertical section through the fire-box and tubes of a low- pressure marine boiler. Describe the method adopted for strengthen- ing the weak portions. Mark in your drawing the combustion chamber, and state the object which it fulfils. (15.) 12. The diameter of a steam pipe is 12J inches, and the upper and lower discs of an equilibrium valve which closes it are 12 and loj inches respectively. Sketch the arrangement, and find the lift of the valve when the opening of the valve is equal to the area of the steam pipe. (15.) 13. Taking steam at 45 lbs. pressure above that of the atmosphere (which is taken at 15 lbs.), sketch three diagrams showing the amounts of work obtained from a given weight of steam — (i) When used in an engine without expansion or condensation, (z) When the steam is cut ofif at half-stroke but not condensed . (3) When the steam is cut off at half-stroke with condensation. (15.) 14. Describe, with such sketches as you think necessary, the construction and arrangement of parts in either a marine, or stationary, or loco- motive engine, whichever you are most familiar with. (20.) 1891 First Stage, or Elementary Examination 1. When does heat become latent? What do you understand by the expression latent heat of steam ? What unit is adopted for measuring and comparing quantities of heat ? Write down the number express- ing the latent heat of steam at 212° F. (IS-) 2. Sketch the Newcomen engine in sectional elevation. During what portion of each stroke, and in what manner, was heat unnecessarily wasted by Newcomen's arrangement? How did Watt propose to lessen the waste of heat, and in what way did he carry out his idea ? . ('5-) 3 What do you understand by the expansion of air or gas accordmg to Boyle's law? Assuming that steam expands in this way, find the pressure of steam on its admission into a steam cylinder under the following conditions :— Length of stroke of piston is 5 feet, steam is cut off after the piston has described 2 feet, and expands down to 3 lbs. below the atmospheric pressure (taken at IJ lbs.). (15.) 4. Give a longitudinal section through the cylinder and steam-chest of an engine, showing the steam and exhaust ports, the steam slide-valve, the valve-rod, and stufhng-box for the latter. Put the valve in its mid -position. *'5-) 5. Sketch a vertical section of a dead-weight safety-valve. (No marks will be given for the ordinary lever safety-valve.) If the valve is 2o8 Steam 2\ inches in diameter, with a dead weight of 300 lbs. , at what steam pressure per square inch will the valve lift ? (Take ir = 3^.) (15.) 6. What is meant by the terms clearance and cushioning ? At what part of the stroke does cushioning occur? Show, by a diagram, the manner in which the slide-valve produces cushioning. (20.) 7' Sketch an eccentric and rod for connecting it with the slide-valve of an engine. Show, by a diagram, the positions of the crank and eccentric (l) when steam is admitted into the cylinder; (2) when steam is cut off, the valve having a fixed amount of lap and lead, which you are to allow for in the diagram. (I5-) 8. The cylinder of a steam-engine is 20 inches in diameter, the crank-arm is I foot long, and the connecting-rod is 4 feet long. Find the turning effort on the crank-shaft at the instant when the crank-arm makes a right angle with the connecting-rod, the pressure of the steam being 60 lbs. per square inch. (15.) 9. Give (i) a longitudinal, (2) a transverse section through a Lancashire doubly-flued boiler, showing the probable water-line. Where is the feed- water admitted, and for what reason? What is done to strengthen the shell of the boiler at the parts where it is most liable to give way under pressure ? How are the flues secured against collapse ? (20. ) 10. Slcetch an equilibrium or double-beat valve. Where is such a valve introduced ? If the upper and lower discs of a double-beat valve are 12 and II inches in diameter respectively, and the pressure of the steam is 50 lbs. per square inch, while there is a pressure of 3 lbs. per square inch in the space between the upper and lower valves, what pressure would be necessary to open the valve (ir = 3^) ? (15.) 11. When is steam said to be ja/ara/^i/.? Distinguish between saturated steam and superheated zK.c2.xa. When air is compressed without change of temperature, its pressure is increased according to Boyle's law : state what happens when saturated steam is compressed in like manner without change of temperature. (15.) 12. Sketch a sectional elevation of a double-acting air-pump with foot and delivery valves, such as is suitable for a direct-acting engine. Describe clearly the construction of an indiarubber disc valve. (15.) 1892 First Stage, or Elementary Hxamination What is the thermal unit employed in this country? State its measure in foot-pounds. How many thermal units are expended in converting I lb. of water at 60° F. into i lb. of steam at 212° F. ? (15.) State concisely the chief improvements which Watt made in the Questions 209 steam-engine, so as to give some idea of the mode in which they were carried out. Sketch in section Watt's single-acting engine, and explain its working. (15.) Sketch the diagram of work as applied by Watt to explain the advantage of expanding steam in the cylinder of an engine. Steam enters the cylinder at 60 lbs. pressure (absolute), and is cut off at Jth of the stroke. Find pressure at }, J, J, and end of stroke. (15.) Draw in section the cylinder of any steam-engine, showing the piston-rod and mode of securing it to the piston. Show also the slide-valve just on the point of cutting off steam, and pface the piston in the proper position, marking the directions in which the piston and slide-valves will be respectively moving. (15.) Describe and sketch the construction of a glass water-gauge and its mounting, as adapted for use in a steam boiler. Where is such a. o-auge placed, and at what height ? How is its working tested ? (10.) What is meant by the lead of a valve ? What is the object of lead ? How is lead obtained when an eccentric actuates the valve ? (15.) Explain the manner in which a marine engine is reversed when fitted with a single eccentric sheave which can be shifted in position upon the shaft. (15.) Show, by a sketch, the manner in which the crosshead in a direct- acting horizontal engine is kept in its true path by guide-bars. Why are rectangular notches usually cut across the ends of the sliding surfaces of guide-bars ? ( 10. ) Draw a vertical longitudinal section of a single-ended return tube marine boiler of circular section. Show the method of staying the tube plate and combustion chamber. Show also the fire bridge, and state what end it serves. (15.) Which joints of a Lancashire boiler are single riveted and which are double riveted ? Explain why all the joints are not made of the same strength, and give some reason for such construction. Show, by a sketch, a double-riveted butt-joint with cover plates. (:5.) Sketch and describe any form of balanced slide-valve. What is the purpose of the arrangement ? What is the advantage of a double- ported slide-valve over one with single ports ? and when would such a valve be used ? (20-) Explain the action of an ordinary governor of a steam-engine. How is the throttle-valve connected with the governor? Sketch the arrangement. Explain also the action of the fly-wheel, and why it is so necessary in stationary engines. (20.) Steam 1893 First Stage, or Elementary Examination 1. Distinguish between the sensible and latent heat of steam. How many thermal units must be added to I lb. of water at 32° F. to raise it to 212° F. and evaporate it into steam ? How many of these units go to sensible and how many to latent heat ? (15.) 2. What is the difference between a simple non-condensing engine, a condensing engine, and a compound non-condensing engine ? Describe, with a sectional sketch, the arrangement and action of Hornblower's compound engine. You may take either the single or double-acting engine. {25.) 3. What are the functions of a slide-valve in a steam-engine? Sketch a single-ported D slide-valve, and show it opening the port by the amount of lead. (15.) 4. Define the angle of advance of an eccentric. Show by a diagram the position of the crank and the angle of advance in an ordinary direct-acting engine. Sketch separately one usual form of eccentric and eccentric strap as made in parts, showing how the separate halves of the eccentric and strap are respectively connected together. (I5-) 5. What diameter of cylinder will be required to develop 50 horse-power in a non-condensing engine which has a stroke of 4 feet, and makes 45 revolutions per minute when working with a mean effective pressure of 30 lbs. above the atmosphere ? (Take ir = ','.) (10.) 6. The pressure of steam is 30 lbs. above the atmosphere, and the cut- off takes place when the piston has moved 5 inches. The mean resistance of the load = 18 lbs., and the steam is supposed to expand according to Boyle's law : how much farther will the piston have moved when the actual pressure of the steam just balances the resistance? (i^.) 7. What is a Galloway tube, and what is its object? Sketch the transverse and longitudinal sectional elevations of a boiler in which such tubes are used. How are the ends of the boiler stayed ? (15.) 8. What is the object of a cylinder escape valve, and where is it placed ? Sketch a sectional elevation of such a valve. To what extent is it usually loaded ? (15.) 9. Sketch in section the high-pressure cylinder, with slide and expansion valve, as forming part of a compound cylinder marine engine. Describe briefly the arrangement of the engine, and how the con- densation of steam is effected. (20.) 10. Write a short description of the " Rocket " locomotive, as invented Questions 2 1 1 by Stephenson, and point out its chief features, giving any reasons you can for its success over rival inventions. Sketch a longitudinal section through a modern locomotive boiler, showing the smoke-box and the method of staying the fire-box, and point out briefly the chief improvements now introduced. (20.) Describe a form of safety-valve such as is frequently used on locomo- tive boilers where two valves connected by a lever are held down by a spring applied between the valves, showing by sketches the construction of the appliance. (15.) 1894 First Stags, or Elementary Examination I. Using Fahrenheit's thermometer, define the British standard unit of heat. To what amount of energy, expressed in British units of work, is this heat unit equivalent ? How many units of work must be converted into heat in order to raise the temperature of 3 lbs. of water from 50° F. to 120° F. ? (10.) z. What is a slide-valve, and for what purpose is it used in a steam- engine ? Show clearly by sketches the position of both ends of such a valve in relation to the ports when the piston is at the beginning, middle, and end of its stroke (cut-off being at half-stroke). (15.) 3. Explain the difference between — (i) A simple non-condensing engine ; (2) A compound double-cylinder condensing engine ; (3) A triple expansion engine ; and mention some of the advantages of each type of engine. (1.5.) 4. Describe and sketch a dead-weight safety-valve. Such a valve is 2 inches in diameter ; the weight of the valve and spindle is 20 lbs. What dead weight would require to be added so that steam should blow off when the pressure reaches 80 lbs. per square inch? (15.) 5. What do you understand by the statement that there is a vacuum of 10 lbs. registered on the vacuum gauge of a condenser ? Sketch and describe fully some form of gauge for testing the pressure in a con- denser. ('S') 6. Describe, with sketches, an oscillating marine engine. How is the steam conveyed to the steam-chest, and afterwards to the condenser ? In what class of steamers are oscillating engines commonly employed, and why? (20-) 7. Show that by the use of a double valve a steam passage may be opened with small exertion of force. Sketch the contrivance, show- ing how the steam passes. Make a sectional sketch of a Cornish P 2 2 1 2 Steam crown valve, and explain the principle of its construction for the above purpose. ('S-) 8. Explain the difference between the functions of a fly-wheel and governor, giving a clear idea of the influence of each upon the running of an engine. Sketch any form of governor, showing what are the attachments by which it controls the engine. (15.) 9. How is the piston-rod of a steam-engine made to work steam-tight through the cylinder cover? Sketch the arrangement in plan and sectional elevation, and state the material employed for each part of the combination. How is the piston made steam-tight ? (15.) 10, What were the chief improvements made by Watt in the atmospheric pumping engine ? Draw an approximate indicator diagram of such an engine, before the improvements to which you refer were intro- duced. (15.) 11, Assuming, as was done by Watt, that the actual expansion curve of steam is the same as that of air when expanding at a constant temperature, set out an approximate expansion curve when steam of a volume 10 cubic feet, and pressure 65 lbs. per square inch above the atmosphere, is expanded to a vohime of 40 cubic feet. (15.) 12, Describe, with such sketches as you think necessary, the general construction of a locomotive engine. (20.) 1895 First Stage, or Elementary Ezamiuatian Distinguish between heat and temperature. What are the units by which each is measured ? How many units of heat are required for raising i lb. of water from 32° F. to 212° F., and then for evaporat- ing it into steam ? How much mechanical work would be done in each operation ? (15.) Describe and slcetch the general arrangement of the cylinder, the piston and its rod, with the connecting-rod, and the method of coupling it to either the crank or beam, and the method of keeping the end of the piston-rod in a straight line, in one of the following types of engine : — (1) An inverted vertical high-pressure engine. (2) A beam engine. (3) A horizontal stationary engine. State some fundamental differences of construction between (i) and (2)- (15) Define the " lap " and " lead " of a slide-valve, and state the purpose for which each is employed. Sketch in section the cylinder and its Questions 2 1 3 ports with th.e slide-valve in the middle of its stroke, and show from your sketch the amount of lap both on the steam and exhaust sides which you have put upon the valve. (15.) Describe, with the necessary sketches, the changes introduced by Watt in order to convert an engine in which the steam was employed for lifting a pump-rod, into the modern form of engine as employed for driving the machinery in factories. (iSO Describe, with the necessary sketches, the construction of a locomotive piston, piston-rod, and cross-head ; and state clearly how the several parts are secured together. (10.) A safety-valve of 3 inches diameter is held down by a lever and weight. The lever is 30 inches long, and the valve-centre is 4 inches from the fulcrum, the suspended weight on the lever being 56 lbs. At what pressure of steam would the valve be lifted, the weight of the lever being neglected in the computation ? Sketch the valve, its seating, and the general arrangement. (15.) The crank of a direct-acting engine is 2 feet in length, and the con- necting-rod 5 feet. Find the positions of the piston when the crank has described an angle of 60° from the dead centre in both the forward and backward strokes. (10.) Assuming that the steam in a steam cylinder expands according to Boyle's law, show how to find the terminal pressure for any given boiler pressure and cut-ofif, and how to find the mean pressure during the strolie ; then, assuming that the boiler pressure is 100 lbs. absolute per square inch, and that the cut-off talces place at J of the stroke, determine what would be the terminal pressure, and also the mean pressure in the cylinder for the whole stroke. (15.) Describe, with a sketch, an oscillating marine engine. How is the steam conveyed to and from the slide-valve casing? In what class of steamers are oscillating engines generally employed, and why? (20.) Sketch and describe fully the construction of an air-pump bucket with its valve or valves ; show also the packing of the bucket; and explain the use and mode of action of the air-pump. (I5-) What do you understand by jet and surface condensation respec- tively? Give a sketch of each arrangement. There are in a surface condenser 1000 brass tubes, each of 6 feet in length, and 1 inch outside diameter. What amount of cooling surface would such a condenser provide ? ' (IS') What are the chief causes of the collection of scale within a boiler ? Of what does boiler incrustation usually consist, and in what parts ot the boiler is the collection most objectionable, and why? Give a section through the furnace of either a land or marine boiler, showing its general construction, and mark where the scale may be expected to collect. (20-) 2^4 Steam 1896 First Stage, or Elementary Examination 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. (iSO 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.) 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 the mean effective pressure of steam on the piston is 28 lbs. per square inch : find the indicated horse-power of the engine. (10.) 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 ) 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? (lo.) 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.) 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 7| inches in diameter, and the weight of the valve is 70 lbs. What jjressure per square inch would cause the valve to lift, the pressure between the valve-discs being disregarded? ('5 ) 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.) Make a longitudinal and also a transverse section of a Lancashire boiler with its brickwork settings. Indicate the course of the gases Questions 2 1 S 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.) 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.) 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 ? (15.) 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 1 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 j Centigrade scale ; absolute scale ; latent heat ; Regnault's total heat in a pound of steam ; calorific value of a fuel ; combustion j conduction of heat ; convection of heat j radiation 2i6 Steam of heat; indicated horse-power; bralie horse-power? f^r)/ brief answers are expected. (i5') 4. 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, -Jg- ; 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? (15.) 5. 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.) 6. 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.) 7. 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 also 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.) 8. 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 2\ 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.) 9. 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.) 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.) ri. 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 fool. What is the efficiency? (10.) Questiojis 2 1 7 1898 What heat must be given to i lb. of water at 80° F. to convert it into steam at 3CX3° F. ? Regnault's formula for the total heat of a pound of steam from water at 32° F. being H = 1082 + 0-305 /, where t° 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 pound of coal ? Describe carefully how you would measure the pressure of steam at various temperatures. Show, roughly, what kind of curve you 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 ; 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 1 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. (IS-) 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? (iS-) 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 ? ( ' 5- ) 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. 2 1 8 Steam If 20 lb. of oil (calorific value 2r,ooo Fahrenheit units) are used per hour, the brake horse-power being i8, what is the efficiency ? (20.) II. 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.) 12 Dw'scribe 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 fiue parts ? 1899. Answer only oue 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 5 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, ox 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.) Questions 219 B. Sketch and describe a tube igniter, and how the timing valve is worked, in, say, a 20 horse-power gas engine. (20.) C. Slcetch 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 w.ight, a. Itver, or a spring safety valve. Say to what class of boiler the valve you select is specially adapted, and whether it can be us3d 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 pjrts, 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 c!o we have an angle of advance ? (i5') 6. Describe, with sketches, cither 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 sleam pressure remained about the same, would the force at the crosshead remain the same as before ? If not, why not ? (IS-) 8. How would you determine the mean pressure of steam in a steam- 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 1 10 revolutions per minute ; under these conditions, what is the H.P. of the engine? ('S-) 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 pressuie absolute during the forward stroke ; what fraction is it of 220 Steam the initial pressure ? Neglect clearance, and do the calculation by construction if you can. ('SO 10. Answer only otie 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-hovse-power-hour. {//) 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. (IS-) 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 1 10° F. per pound of the same fuel ; compare these performances. (iSO 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 vi-ater from 0° C. to 100° C, as I00'5 ; what is the Joule's equivalent which suits Regnault's unit of heat? (I5-) 12. State shortly why superheating, steam-jacketing, and successive expansion are now being used in steam engines. (150 INDEX ABS Absolute temperature, 6 Air pump, no — vessel, Ii8 Back pressure, 53 Boilers, 164 — efficiency of, 182 — Lancashire, 167 — locomotive, 175 — marine, 172 — stationary, 166 — vertical, 171 Boiling, 23 — point, 3 Bourdon's gauge, 183 Boyle's Law, 42 Brasses, to adjust, 190 Capacity of pumps, 118 Centigrade thermometer, 3 Charles, law o"^, 21 Clearance, 63 Combustion, 15 — chamber, 172 — heat of, 17 Compound engines, 1 35 — types of, 145 Condensation, 25 Condensers, jet, 109 — surface, 1 tz — tubes, H4 Condensing water, 40 Conduction, 12 EXP Connecting rod, 83 Convection, 13 Couplings, 107 Cranks, loi Crossheads, 80 Curve, hyperbolic, 44 Cushioning, 68 Cylinder, details of, 72 — condensation, 65 — escape valve, 74 — relief cocks, 75 Dead centres, 85 — plate, 167 Double-beat valve, 183 — ported valve, 96 Eccentric, 97 Energy, 9 Engines, non-condensing; 69 — compound, 135 — locomotive, 125 — management of, 187 — receiver, 152 — Woolf, 145 Equilibrium valve, 183 Evaporation, rate of, 185 Evaporative power of fuel, iS Expansion, economy of, 52 — limit of, 61 — of solids, 20 — of gases, 21 — of steam, 48 222 Steam FAH Fahrenheit thermometer, 3 Formation of steam, 28 Freezing point, 3 Fuel, evaporative power of, 18 STE Mean pressure, 55 Mechanical equivalent of heat, 10 Mensuration, 192 Mixtures, temperature of, 40 Gauge glass, 170 Gland, 73 Governors, 119 Grate area, 174 Guides, 80 Gusset stays, 170 Heat, i — latent, 36 — mechanical equivalent, — sensible, 36 — total, 36 — transfer of, 12 — unit of, 7 Heating surface, 174, 177 Horse-power, 9 — ■ indicated, 57 Hyperbolic curve, 44 Indicated horse-power, 57 Indicator, 129 Indicator diagrams, 131 Jacket, steam, 68, 74 Joule's experiment, 10 Journals, 108 Junk ring, 77 Pedestals, 108 Piston displacement, 79 — leaky, 193 — rods, 80 — speed, 79 — valve, 95 Pistons, 76 Porter governor, 123 Pressure, absolute, 22 — gauge, 183 — mean, 55 — of the air, 22 Priming, 65 Pumps, 116 — capacity of, 118 Quadruple expansion engines, 160 Quantity of heat, 3 Questions, 192 Radiation, 12 Range of temperature, 136, 150 Reaumur thermometer, 3 Receiver engines, 152 Reversing gear, 98 Rotary engines, 89 Lancashire boiler, 167 Lap, effect of, 94 — inside and outside, 92 Law of Boyle, 42 — Charles, 21 Lead, 92 Leaky piston, to te^t, 185 — valve, — , i8g Lever safety valve, 1 79 Link motion, 98 Locomotive, the, 125 — boiler, 175 Marine boiler, 172 Safety valve, 178 — dead weight, 181 — lever, 1 79 — spring, 181 Saturated steam, 38 Shaft couplings, 107 Shafts, crank, loi Shrinking on, loi Slide valve, 89 — double ported, 96 — to set, 94 Specific heat, 5 Spring rings, 76 Steam, expansion of, 48 — formation of, 2S Index 223 STE Steam, heat rejected by, 34 — properties of, 39 — regulating valve, 182 — saturated, 38 — weight of, 39, 75 — work done by, 29 — volume of, 39, 75 Stop valve, 182 Strap, gib, and cotter, 84 Stuffing box, 73 Table of specific heats, 5 — heat of combustion, 1 7 — steam properties, 39 Tangential pressure, 102 Temperature, 3 — absolute, 6 — of mixtures, 40 — range of, 136, 150 Thermometers, 3 — compared, 4 Triple expansion engines, 159 Tubes, boiler, 174 — condenser, 1 14 Turning eftbrt, 102 WOR Unit of heat, 7 — of work, 8 Vacuum, 25 — gauge, 115 Valve, double-beat, 183 — double ported, 96 — gridiron, 183 — slide, 89 — stop, 182 — lap and lead of, 92 — lift of, 117 — piston, 95 — regulating, 182 X'ertical boiler, 1 7 1 Volume of steam, 39, 75 Water, condensing, 40 — weight of, 118 — tubes, 167 Watt governor, 119 Weight of steam, 39, 75 Wire-drawing, 73 Woolf engines, 145 Work, unit of, 8 PRINTED BY WILLIAM CLOWES AND SONS, LIMITEDi LONDON AND BECCLES. / 4