^ T p /i^ R IPP F M I t S0EI1LL UNIVIESITY LUKSEY. I i s O00KJS ^i@t to be taKen '© B from tlia;?ftea-dii].g Room. © *5s ..#- X.,,. c i i ^^"S. © WHE!)|rf>^t5NE #'8TH, HSfbKi-l'^T Os^Ct;! TO © I fe.#^ sHbL' ,.2^. :£:',_^. .© v« ^jj^0_y;0i'@/e?i"yf7^: ffij' -•^■^ c^f^Vvr- .'^^i^^g^5-;>^ i Lon^aaas' Eleiuentaw Science :Maimais. ■ ^1^ Cornell mnibersitB HiftracB. THK GIFT OF LONGMANS, GREEN & CO., arV10529 Steam / Cornell University Library 3 1924 031 243 615 olin.anx Cornell University Library The original of tliis book is in tine Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31 924031 24361 5 STEAM PRINTED BY SPOTTISWOODE AND CO., NEW-STREET SQUARI LONDON S T e.,A>KM BY WILLIAM RIPPER MEMBER OF THE INSTITUTION OF MECHANICAL ENGINEERS PROFESSOR OF MECHANICAL ENGINEERING IN THE SHEFFIELD TECHNICAL SCHOOL I AUTHOR OF 'MACHINE DRAWING AND design' 'practical CHEMISTRY* ETC. LONDON i.ONGMANS, GREEN, AND CO. AND NEW YORK : 15 EAST i6"> STREET 1889 ® All rights reserved ORiNEL UNiVERSPTYf ,. 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 work on the Steam Engine. W. R. Sheffield : October 1889. CONTENTS CHAPTER I I'AGE Introduction — Heat, its nature and effects — Temperature — Ther- mometers — Specific heat — Absolute temperatures . . . i 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 1 5 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 . . 2& Contents vii CHAPTER VII PAGE Saturated steam— Tatle of properties — Water heated in a closed vessel — Temperature of mixtures— Condensing water . . 38 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 speeds 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 loi CHAPTER XIII Condensers — The jet condenser — The air-pump — The surface con- denser — The vacuum gauge — Pumps 109 viii Steam CHAPTER XIV PAGE Governors— The Watt governor— The Porter governor with auto- matic expansion gear — Fly wheels— The locomotive engine, arrangement and construction of 119 CHAPTER XV Compound engines — compared with single cylinder engine — The two-cylinder compound engine illustrated .... 129 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 139 CHAPTER XVII Boilers — Resistance of cylindrical vessel"- — 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 158 CHAPTER XVIII Practical notes on the care and management of engines and boilers — Annual inspection of engines and boilers . . . .181 APPENDIX: — Questions and Exercises . . . .186 INDEX . . . , 200 ST EA 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 Steatn 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 |(g) 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 H| temperature of the water is the same throughout, but the quantity of heat varies as the weight of water in each vessel. Thermometers are used to indicate temperature, and they do so by the rise or fall of a little column of mer- cury enclosed in a tube of very fine bore, and having a small bulb at the bottom containing a store of mercury. If the thermometer be warmed, the mercury ex- pands or tends to occupy a larger volume, and the column therefore rises in the stem of the tube ; or, if the thermometer be cooled, the mercury will con- tract or diminish in volume, and the column will shorten or fall. A graduated numbered scale is affixed, and the smallest change of temperature, shown by the movement of the surface of the column, is thus easily detected. To graduate the scale the thermometer is placed in melting ice, and the point to which the mercury in the stem has fallen is marked, and called the freezing point. It is then placed in boiling water at the pressure of the atmosphere, and the level of the column of mercury is j!^j_ again marked, and called the boiling point. The distance between these two marks is divided on the Fahrenheit thermometer into 180 equal parts or degrees ; on the Centigrade thermometer the distance between the two marks is divided into 100 equal parts or degrees ; and on the Reaumur scale the same distance is divided into 80 equal parts or degrees. English engineers mostly use the Steam Fahrenheit scale. The following sketch will show that the difference between the various thermometers is not in the height of the mercury, but in the scales of degrees by which the height is expressed. It will be seen from the fig. that the scales are marked as follows : p^^^^i^g B„;,i„g point. point. Fahrenheit . . 32 212 Centigrade . . o 100 Reaumur . . o 80 It will also be clear that 212° — 32"= 180° F. occupy the same space as 100° C, or 80° R. Now 100° C.= 180° F. .or^^i8o°^ 82* (i) Q*t) (*> Boiling point . Free2ing point * ■ 100 . _9° F. S also 180° F. ■= 100° C. 1° F. ^ 100° C. 180 Fig ^. = 5_ C 9 From which we obtain the Rules. — To convert degrees Fahrenheit into degrees Centigrade : Subtract 32, multiply the remainder by 5, and divide by 9. Thus, convert 158° F. to degrees C. Then(is8 — 32) 5 = 70° C. Or, to convert degrees Centigrade into degrees Fahrenheit: Multiply by 9, divide by 5, and add 32. Thus, convert 70° C. into degrees F. Then (70 x^) + 32 = 158° F. The relation between degrees Fahrenheit and Centigrade may also be expressed thus : or, conversely, Specific Heat C. = (F.-32) 5; 9 F. = |C. +32. Similarly, the student will see from fig. 2 that 180° F. occupy the same space as 80° R.; hence 1° F. = i_R. = 2r. 80 4 Also, since 100 C. = 80 R. .•. i C. = - R. 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-ooo 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 o'ii3, or about \ ; that is to say, the quantity of heat which would raise i lb. of wrought iron through i ° 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, /, of the mixture by cal- culation : Note. — The total heat per lb. in any given substance is found by multiplying its temperature by its specific heat ('see table, p. 5). Then Heat lost by iron = Heat gained by water ; (212— if) xsp. ht. xweight=(/— 50) xsp. ht. x weight ; (212—/) X-II3 xi=(/— 5o)xi X2 ; 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 -I- 46 1=67 3° 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 in 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 1 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 \ 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 s w^. ItjeJb 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 mean weight = 3 X 7+o_ 3 X3^=io|, or one-half that in the pre- vious case. It should be noticed that the unit of work has no reference Horse-power 9 to the time taken, for the same amount of work is done in lifting the weight, whether it be done in one second or one hour. Ihs. power of an agent is measured by the rate at which it can do work, and depends upon the amount of work done in the unit of time. 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 : lime m mmutes *^ ' ov,^ Work done , ^ , ana -, ^ ., — =horse-power exerted. 33,000 X tmie 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 different 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. 10 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 vers&, 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§=i390 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 =3^=4-75. 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 ways : 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 having 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 n 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, loo Steel, ii-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 j 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 mpthod 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.,^ 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 (H2O). 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 j 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 ; thirdly, the remaining solid residue of the coal is burnt. Considering the gases distilled from the coal, which consist principally of marsh gas (CH4) and defiant gas (C2H4) : 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. When 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 17 II. Table of Heat of Combustion Combustible. Total units of heat of combustion per lb. lbs. of water evaporated from and at 212°. Hydrogen Carbon burned to carbonic 62,032 64-2 oxide Carbon burned to carbonic 4,400 4-55 acid Anthracite • 14,500 14,700 15-0 15-2 Newport coal Durham coke • 14,000 13,640 14-5 14-1 Wigan cannel coal . Petroleum • 14,000 20,360 14-5 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 (CO3). 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 sufKcient 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)94° thermal units. One pound of good coal yields (when the combustion is c 1 8 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 v capable of exertmg an energy of — ? '- = 5 horse-power ^ DO/ 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 i^ 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 evaporative power of i lb. of pure carbon. Total heat evolved per lb. 14, wo , ,, , e— : ^-T lur — 7- = VV = I S lbs. of water. Heat required per lb. of water 966 Example 2. — Check the accuracy of the results given in last column of 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 Power 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. 62 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 e:;3 Fig. 8. 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 cjrlinder under a movable pistoii 22 Steam occupies 10 cub. ft. at 60° K. ; 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 60° x ?-" = iox7ii 521 = 13 '65 cub. ft. Example 2. — A volume of air at 212° F. is confined in a rigid cylin- drical vessel, and exerts 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 ,, 761 673 = i6'96 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 15° C, what volume will it occupy if its temperature is raised to 100° C, the pressure on the gas remaining constant ? 1 5° C. = 1 5 + 273 = 288 absolute 100° C. = 100 + 273 = 373 ., then 20 X 373 ^ 2 J .Q ^^ £j_ 200 Pressure of the Air— Absolute Pressure On the surface of the earth we Hve, as it virere, at the bottom of an aerial sea, vi^hich 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, when its finger points to rolbs., 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+ 15=25 lbs, pressure absolute. Application of Heat to Water Water is a compound substance, c6nsisting 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 ™ ^mC-~), \ 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, p^^ 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 the 24 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 atmosJ>here,\ihe.n 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 boiUng 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 boiUng 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 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 : Fig. Condensation Under a pressure of 5 lbs. the boiling temp. is 162° F. » 10 193° » i) „ I atmosphere „ 212° „ » „ 2 atmospheres „ 249° „ I) 3 273° „ )j » 4 291° M » 5 306° „ » 10 357° » 25 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- turns to the liquid state and becomes water. Thus, let a flask A contain a known weight of pure water. Fit a cork and glass tube to it as shown, and connect with a spiral tube surrounded by flowing cold water ; let the lower end of the tube pass into a vessel B. Boil the water in A. It will pass off' as steam by the tube C to the spiral ; and if the spiral be sur- rounded by a stream of cold water, the steam will be condensed to water, which will drop from the end of the tube. At the end of the operation the loss of weight by A is equal Ovtrflm: Fig. zi 26 Steam to the gain by B. This illustrates the process of distillation, and by this method pure water may be obtained from water containing impurities. Advantage was taken by the early engineers of the property possessed by steam of being easily condensed. They valued steam not so much for its own sake, but because by condensa- tion they were able to call to their aid the pressure of the atmosphere in the performance of work. A vacuum is literally an empty space — that is, a space abso- lutely free from air or vapour of any kind capable of exerting pressure. Vapour arises from water at all temperatures, and exerts an appreciable pressure. And the lowness to which the pressure can be reduced in condensers depends on the temperature of , the condensed steam, and this temperature in practice cannot economically be reduced below about 102° F., at which tempe- rature the vapour of water exerts Fig. 12. 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— Tak& 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 t 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 yg g-jr °f 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. 28 Steam CHAPTER VI ACTION OF HEAT JN THE FORMATION OF STEAAf 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 Bm Stage I. Stage 2. Stage 3. Stage 4. Fig. 15. 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 andthe piston commences toascend in the cylinder (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 i lb. of water at 32° F. be contained at the bottom of a cylinder i sq. ft., or 144 sq. ins., in sectional area. Then, first to find the height of the water in the cylinder ; since the area of the vessel is i sq. ft, and the weight of I cubic foot of water is 62-5 lbs., 62-5 lbs. of water will stand 1 ft. high, lib. „ „ -i-ft. 62 5 =•016 ft. 30 Steam 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=21 16'8 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 I 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, =180-1-966 = 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 : c5 irrnn Ml f 1 -■'. 1 1 ■■; ^ ;• ^^ 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 from which it was generated ; and, secondly, the piston has been raised from the surface of the water in stage i to that of the steam in stage 3. The heat, usually called latent heat, has been expended, then, in two ways : firstly in overcoming the internal molecular resistances of the water in changing its condition from water to steam ; and, secondly, in overcoming the external resistance of the piston to its increasing volume during formation. The first of these effects of ' latent ' heat is called internal work, because the changes have been wrought within the body itself ; and the second is called external yiork, because the work has teen 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,n6'8 lbs. through a height of 26-36 ft.= 2,ii6-8X26-36 = 55,799 foot Ibs^ ; or, S5>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 jnay 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 32 Steam per square foot multiplied by the area of the piston in square feet gives the load on the piston, and this multiplied by the height / through which the piston moves in feet gives the work done ; or External work = P x a x /. But axl =v = the volume occu- pied by the i lb. of steam ; there- fore External work = P x z;. 11 SSYqq ft. a%. CO •J I I 1 I -r -¥- Jnfevnal Work m ConvtrCin^ steam. V-=Zi,-%(uJ,.ff Fig. 17. If, then, a rectangle be constructed, as in fig. 1 7, having one side=P, and an adjacent side=z', to any convenient scale, J 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. 1 8), making A B ^ pressure and B ^ = volume to any scale to represent the external work done by the steam. To the base B(5 add the rectangle BCiTiJ = 1 2 -36 times the rectangle A ^. This is done by making B C = 12-36 times AB. Make also €0 = 2-48 times AB and complete the rectangle. Then the total heat required to heat 1 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 Hfftt 'fe raiie weiCtt from, .32* fc 2/2* Fig. 18. Work done by Steam 33 by the remarkably small area given by the rectangle K'&ba. 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= ■= . or about — . areaADiia i5'84 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 -jJg-x 100 = 6-25 per cent. And this is better than would be the case in practice under the same circumstaijces, 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 xs = 14,400 X4-33 = 62,352 ft. lbs. Comparing this with the external work done by i lb. of steam at atmospheric pressure, we have extetnal work in ft, lbs. 1 lb. steam at 100 lbs. pressure = 62,352 I lb. „ 147 „ = 55,799 and these numbers do not differ very greatly. From this we see that, when steam is not used expansively' — that is, when it is supplied at full pressure throughout the stroke — i lb. of high-pressure steam is not capable of doing much more useful work than the same weight of low-pressure steam. D 34 Steam In comparing the work done by high- and low-pressure steam, it will be noticed we have taken the work done by equal weights and no expansion. The same would not be true of equal volumes, for evidently if the cylinder were sup- plied with high-pressure steam, it would do more work on the piston than the same volume of steam at a lower pressure ; but then there would be a proportionally greater weight of steam used, and, therefore, a greater quantity of fuel consumed ; and the object of the engineer is to get the greatest amount of work from the least consumption of fuel. Thus, if a cylinder is filled at each stroke with steam at loo lbs. pressure per square inch throughout, then, assuming there is no back pressure, this steam would do twice as much work as steam at 50 lbs. ; but the weight of each cylinder full at 100 lbs. pressure is approximately twice that of the cylinder full at 50 lbs. Hence, though we have done twice the work, we have used twice the weight of steam, and, therefore, weight for weight, the work done in both cases is equal. To find the work done per lb. of steam during formation, without expansion, at any given absolute pressure per square inch p : — Find by Table III. (p. 39) the given pressure, the volume V per lb. in cubic feet ; then/ x 144 X w = 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 'Oi = 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 ,, =-^;— i — = 317 lbs. 62,352 Heat rejected to Condenser 35 Heat rejected by Steam to Condenser When steam is condensed, the heat rejected by it to the condensing water is not always the same, but depends upon the conditions under which it is condensed. If it is con- densed under the same constant pressure at which it was formed, the heat given out will be the same as the total heat supplied ; in other words, the heat rejected is the same as its total heat of formation ; but if it be condensed under any other conditions, the heat rejected by the steam to the con- densing water will be different. This statement may be. illus- trated by taking three cases : \st 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 = 180-I-8937 -1-72-3 = 1,146 units. = total heat supplied. ind 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- jr,^_ ^^^ 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, Da 36 Steam Heat rejected^ total heat— external work = 1,146-72-3 ^1^0737 units for this particular case. This corresponds to the amount of heat rejected when the steam is exhausted to a condenser without back pressure. 2,rd case. SvL^^iOse. 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 212" 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 J of that done on the piston by the steam ; hence Heat rejected=heat of water from 212° to 32°=i8o -1- internal heat of steam =8937 Fig. 20. -1-^ external work = I of 72-3 = 24-1 i,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 instead of 32°, then the number of thermal units required to raise water at 50° to water at 2i2°=2i2— 50=163. The latent heat of steam is defined as the amount of heat required to convert i lb. of water at a given temperature into steam at the same temperature. The total heat of evaporation is the sum of the latent and sensible heats and is defined as the quantity of heat required to raise i lb. of water from 32° to the temperature of evaporation, and to convert it into steam at that temperature. The total heat of evaporation for steam at any particular temperature, t, may be obtained approximately from the follow- ing 'formula : Total heat= 1,082 + "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 t. Example. — Find 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 = 875 -a- 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 formulse given -above for the total and latent heats of steam, it will be evident that the total heat increases as the temperature of the steam increases, while the latent heat 'decreases as the temperature increases. 38 Steam CHAPTER VII 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. Exatnple. — A 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, io'28 cub. ft. of steam at 40 lbs. pressure weigh i lb. I >j ') . i> _ lbs. IO'28 'S !> »> „ I5x-i_-lbs. 10-28 = I -46 lb?. Properties of Saturated Steam 39 m own m ■«:}• i^ mco vo vD Ov fo os t^\D fO ^ r^oo p r>. y^ fO iH oncw vo c*^ »-< on t^^o ^ m n 00 10 ro •-< io^^^^rorororOPOW « N N « M m w w m Total heat of evapora- tion from water at 32° F. rt-\p op g\ ^ p yi p a\vp Nop pi^^^ ji-w « J**^ !>. 1>*00 00000000000000 0\0\0\0\OQ\0 >H i-t M p M r^ pvp p hH M ONPO^cop inop 00 w w p ^ b Tt^ t^ ►"" Vdb ^ t^ N 00 codb fn r>. w b r^ b in ■-H N « N fOr^fO^i-rj-iJ-) Lnvo vO i^^ i^oo •-< rr> •* Absolute pressure in lbs. per sq. in. 00&ONOO>-fi-.NrOri- invo i>.00 On O ^n O m O -^ mvD Ti- 00 vo r>. \o inoo 00 « m m on t^oo m Total heat of evapora- tion from water at 32° F. fO^ pNvp P p ^ N op p p 7d-Np p N tN. p t^ hK w ro ro ^ iovo NO r^ t^cx) w cnint^b m m V^j^^^ II *^ pop M IT) f^Cp 0ptN.iopp TfO w\p pCpu-iw b fTl iocJo b « Tl-'i> 00 b ONr^^^t^WWjNt^N c^f^f^rO'^-'^-^'^'^iJ-) invO l->.CO 00 ON ON '-' Absolute pressure in lbs. per sq. in. w N ro Th imo r^oo onO ^no ^no mo "^o »no 1J| >ooo««\Ow«:i-00'^poO rN.vo vn N vo 00 « r^ lO^O NO M ON r>.00 00 NO ON in fOOO ro On ts. t^ r^ en t^ WOO t-N^o in';j--*mcr)cnwNWNNW«M Total heat of evapora- tion from water at 32° F. in M NO :^cp ON t^ ^ p\ p y-) ^N.^p On on pNcp vp y^ ro b u^oo *-< fnmi->.ONO w fn ■^vo "O ^N00 On >-« w M W M r^^^om^rJrOT^•■^■^'!^■^■*t'>;l-■«t'^lnli^ Ttvp f f^ r" P^ P :* P°P P S^P ^* py^v^pp Absolute pressure in lbs. per sq. in. M N fn -^ invo i>.oo ON M « m ■<* mvo *^oo o» 40 Steam Water heated in a Closed Vessel Let water at 32° be heated in a closed vessel, such as an ordinary steam boiler, containing space for the accumulation of steam, and let heat be gradually applied. Then the temperature of the water will gradually rise, and steam will be formed at once, and not only when some definite temperature is reached, as was the case with the movable piston. As the heat is increased, the temperature, pressure, and density, or weight per cubic foot, of the steam increase indefi- nitely, so long as the strength of the boiler is not exceeded ; and the relation between the temperature, pressure, and density always bears a certain fixed relation, as given by Regnault's Tables, p. 39. If heat is applied so as to maintain the temperature con- stant, the pressure and density remain constant also, and evaporation ceases. If a communication be opened between the boiler and engine, on escape of steam from the boiler the pressure is momentarily reduced and re-evaporation commences rapidly. So long as the temperature is maintained, no sensible variation of pressure is noticeable in a boiler supplying steam to an engine. Temperature of Mixtures — Condensing Water Example i. — If i lb. of water at 212° F. be mixed with 5 lbs. of water at 50° F. , find the temperature of the mixture. Note. — In order to avoid confusion in problems of this kind, it is necessary to remember that the total heat in water or steam is always reckoned from 32° F. or 0° C. Hence it is necessary to subtract 32 from the temperature given in Fahrenheit degrees. Let t = temperature required. Then Total heat in i lb. of Total heat in 5 lbs. of^ Total heat in 6 lbs. of water at 2 1 2° water at 50° water at /°. 1(212-32)+ 5(50-32) =6(^-32) 180+ 90 =6^-192 6/=462 Condensing Water 41 Example 2. — How much water at SS° F. must be mixed with i lb. of water a.i 212° F. so that the resulting temperature of the mixture may be 105° F. ? Let W c weight of water required ; then Total heat in i lb. Total heat in W lbs. ^ Total heat in (W+ i) lbs of water at 212° of water at 55° of the mixture at 105° 1(212-32)+ W(5S-32) =(W+i)(io5-32) 180+ 23W =73W + 73 5oW=io7 ■W = 2-I4 lbs. In this connection it is interesting and important to compare the difference in the weight of water required to cool a given weight of water, with that required to cool the same weight of steam at the same temperature. In the following example it is shown that it takes ten times as much water to cool i lb. of steam at 212° as it takes to cool the same weight of water at 212° to the same final tempera- ture of 105°- Example 3. — How much water at 55° F. will be necessary to condense I lb. oi steam at 212° so that the resulting temperature in the vessel shall be 105° F., assuming condensation takes place at the pressure due to the temperature of the steam ? Let W = weight of water required ; then Total heat of i lb. Total heat in W lbs. ^ Total heat in (W + I ) lbs. of steam at 212° of water at 55° of water at 105° 1146+ W(5S-32) =(W+ 1) (105-32) 1 146+ 23W =73W+73 SOW =1073 W = 2i -46 lbs. Compare this answer with that in Ex. 2 above. Example 4.^ — Find the temperature of the mixture when 21 -J lbs. of con- densing water at SS°F. are used per lb. of steam at atmospheric pressure. Let t = the temperature required ; then Total heat in i lb. Total heat in 21-5 lbs. of _ Total heat in 22-5 lbs. of steam at 212° condensing water at 55° of mixture. 1 146 + 21-5(55-32) =22-5(if-32) 1 146 + 494-5 =22-5^-720 22'5; = 2360'5 ^= 104-9° F, 42 Steam CHAPTER VIII RELATION BETWEEN PRESSURE AND VOLUME OF GASES Let a portion of gas be introduced into a cylinder which is closed at one end and fitted with a movable piston. Then the gas will fill every part of the space beneath the piston, and exert a uniform pressure on each square inch of surface with which it is in contact. If the internal volume of the cylinder be in- creased, by lifting the piston, the gas will still completely fill the space, but it will be less dense— that is, it will weigh less per cubic foot — and it will exert less pressure per square inch of surface with which it is in contact. If the gas be compressed into a smaller space, it will become more dense, and it will exert a greater pressure per square inch. The relation between the volume and pressure of a perfect gas at constant temperature is expressed in the following terms, known as Boyle's Law : ' The volume of a given portion of gas varies inversely as the pressure, the temperature remaining constant.' This may be illustrated as follows : Let a cylinder (fig. 21) be closed at one end and contain a movable piston, and let the piston, when in position a, enclose one cubic foot of gas under atmospheric pressure, or, say, 1 5 lbs. per square inch. Suppose, now, that weights be added to the piston till the pressure on the enclosed gas is equal to 30 lbs. per square inch, or two atmospheres. Then, by the law just stated, the pressure Boyle's Law 43 ,r.. tsrm on th^gas being doubled, the volume will be reduced one-half. Hence the piston now occupies position b, so that e b=\ e a. Again, apply to the piston a pressure equal to 60 lbs. per sq. in., or four atmospheres. The pressure on the gas being now four times the original pressure, its volume is one-fourth of its original volume, and the piston now falls to c, so that e c-=-\ e a. Again, apply to the piston a pressure equal to 120 lbs. per sq. in., or eight atmospheres. The pressure on the gas being eight times the original pressure, the volume is now one-eighth of the original volume, and the piston tiOf^- 30> ■6o;l d Fig. falls to d, so that e d=^ e a. If now horizontals be drawn from the respective piston positions the length of which is equal to the pressure at these positions to any scale, and a curve be drawn through the extremities of the lines, the student will recognise the curve as being similar to that of an engine indicator diagram if the book be held so that the cylinder is horizontal. This curve is called a rectangu- lar hyperbola. Boyle's Law may also be expressed thus.: If Vis the volume at pressure P then 1 V jj )j 2P iv )> )? 3P iv j> 39 4P and so on ; or. 2 V 39 )> 4P 3V }) J) iP 4V )j » iP From which it is evident that in each case, if the pressure be multiplied by the volume, the result is a constant number, 44 Steam The Hyperbolic Curve • Boyle's Law and the properties of the hyperbolic curve may be further illustrated as follows : Suppose that one cubic foot of gas at 120 lbs. pressure is enclosed in a cylinder and expanded to eight times its original volume. Then the successive changes of volume and pressure may be represented by lines, thus : Draw two lines O M, O N from a common point O at right angles to one another. Let the vertical line O M be called the line of pressures, and the horizontal line O N the line of volumes. - 'OS- a 5 '?'^ a g ys- i bo- in g US- u) a 30- IS- * VOL M 120* , "^ j,. VOL.AT60'--fl \t. VOL At30- Ij— * Volumes. Fig. 22. Mark off on the line of pressures to a scale of, say, ^V inch= 15 lbs., a series of divisions, and on this scale place the figures 15, 30, 60, 120, &c., opposite the points representing these pressures. On the line of volumes take, say, | inch=i cub. ft., and mark i, 2, 4, 8, &c., opposite their respective positions. From these points raise verticals to represent to scale the pressure of the gas at the various volumes. Thus i a = 120 = the initial pressure of the i cub. ft. of gas. The gas is now expanded to 2 cub ft., the temperature meanwhile being supposed to be kept constant ; and the pressure 2 b will now have fallen to 4 of 120 = 60 ; at 4 cub. ft. the pressure 4c = i of 120 = 30 : The Hyperbolic Curve 4S and at 8 cub. ft. the pressure 8^= A of 120 = 15. If the free ends a, b, c, d (fig. 22) of the verticals are now joined, the curve formed is called a rectangular hyperbola, fig. 23. Then the four rectangles Qa, Q)b,Oc, Od represent the conditions of the gas as to volume and pressure for the respec- tive piston positions, and these rectangles are all equal in area : for, 1 20 XI =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 UlNE or VOCUMES Fig. 33. same throughout, hence it is called an isothermal curve, mean- ing the curve formed when the gas expands at equal or uniform temperature. The curve also describes fairly accurately the relation between the varying pressures and volumes of expand- ing steam in an engine cylinder. It is not, however, an ' iso- thermal ' for steam, because the temperature of saturated steam varies with the pressure (see Table III., p. 39), unless it be superheated, which is not usually the condition of steam in a steam-engine cylinder. Example. — Steam at 85 lbs. boiler pressure, or 100 lbs. pressure per square inch absolute, is admitted to a cylinder 5 ft. long, and cut off at \ 46 Steam of the stroke. Draw the theoretical indicator diagram, assuming that the hyberbolic curve is sufficiently accurate. Note. — In drawing theoretical indicator diagrams, always use absolute pressures. Let O M = line of pressures, and on it mark a scale of pressures, say ^ inch = 5 lbs. Let O N = line of volumes to scale of, say, | inch = i ft. of stroke of piston, and divide this line into ten equal parts. Com- plete the rectangle O M a A. Then O A = the volume of the steam and A a the pressure, at the point where the steam is cut off. To find the pressures B i5, C c, &c., at B, C, D, &c., corresponding to the successive volumes O B, O C, O D, &c., advancing by distances along O N of 0-5 ft. Since the pressure at any point is inversely as the volume : O A 2 Pressure at B = ^^^ x initial pressure = - x 100 = 66 -66 C = D = OB OA OC OA OD ^ = OE F = G H N OA OF OA OG OA OH OA ON -- X I00=S0'00 4 2 = - X 100 = 40-00 = gX 100 = 33-33 2 = -x ioo = 28-i;7 2 = 5X 100 = 25-00 o 2 = - X 100=22-23 9 = — X 100 = 20-00 10 The Hyperbolic Curve 47 The above method of finding the pressure at any point during the expansion when the initial pressure is given may be ex- pressed as follows : Multiply the initial pressure in lbs. per sq. in. by the length of stroke to point of cut-off, and divide by the distance of the given point from the beginning of the stroke. The hyperbolic curve may be described without any calcu- lation by the following simple geometrical method. Draw the lines O M and O N as before. (Note. — The point O in the line O M is the zero of pressure, and not the point through which the line of atmospheric pressure passes.) Complete the parallelogram O M a A as in fig. 25. Produce M a parallel to O N. To find the pressure at any point B Fig. 25. corresponding to the volume O B, draw the vertical B b and join Ob, cutting A a in b' . Then the horizontal through b' to cut the vertical B b gives a point / in the curve. Any number of other points may be obtained in the same way, and the curve drawn through the points may be completed. This curve describes the relation between the varying pressures and volumes of a gas, whether the gas is expanding or being compressed, provided the temperature remains constant. Boyle's Law, however, is not absolutely obeyed by any known gas, and less so by steam ; but the knowledge of the law is of great value in enabling us to obtain results which, for ordinary purposes, correspond with sufficient accuracy to the behaviour of the steam expanding in the cylinder. 48 Steam CHAPTER IX EXPANSIVE WORKING When engines are required to exert their full power for a short period — as happens, for example, with the locomotive in mounting an incline — steam is admitted to the cylinder at full pressure through the greater part of the stroke, without regard to economy in the consumption of steam or fuel. But this is not the way in which steam is used for any length of time in well-constructed and well-managed engines ; and although extra work is obtained from the engine by neglecting to use the steam expansively, it is being very dearly paid for in the excessive proportion of steam and fuel consumed compared with the extra work done, as we shall now proceed to show. Work done by Steam used expansively We have seen that the work done per lb. of steam without expansion at high pressures only slightly exceeds that done by the same weight of steam at low pressures. We will now call attention to the increased work which may be ob- '■ — tained from high- pressure steam when advantage is taken of its expan- sive properties. Let I lb. of steam at loo lbs. per sq. in. absolute be admitted to a cylinder (fig. 26), when the piston P is at the end I I ■ ^v^^^^x^^^xvsl.l.^.kM■'.^!.W.'■^.'■l■m'.k'.^l,^'.T.Tr;. Expansion of Steam 49 a of the cylinder, and let the supply of steam be continued for one-fourth of the stroke, namely, till the piston reaches b, when we will suppose it just contains i lb. of steam. The supply is now cut off and the piston is driven for the remainder of the stroke by the expansive force of the steam thus enclosed. At the end of the stroke , the steam occupies " four times its original volume, and its pres- sure is now one-fourth its original or initial pressure, and the work done by the i lb. of steam will be clearly shown by the aid of a diagram. Thus let ap \ ! i i I T * V 2 3 H- 2 Fig. 27. (fig. 27) be drawn to (^ any convenient scale of pressures to equal 100 lbs.; and make aw, equal to 4-33 to any other convenient scale. iJSfote : i lb. of steam at 100 lbs. pressure absolute occupies 4"33 cub. ft., and if we assume the area of the piston = 1 sq. ft, then length a Z'i=4'33 ft.) Produce the line to a V4, making a 2^4=4 times avi. Complete the figure by the graphical method (p. 47). Now the whole work done by the steam is equal to the area of the figure p a v^fr, and this whole area is made up of two parts, namely : (i) area/ a v^ r=work done during admission ; (2) area r v^ &4_/=work done during expansion. Hence, by making use of the expansive properties of steam, we obtain the additional work out of it represented by the latter area. To find the area of the figure would be a simple process if the line /"/(fig. 27) had been a straight line instead of a curve, for then the area of the admission portion=ffl pxavi ; and the area of the expansion portion=»ie'4 multiplied by the mean height m m. But the curve falls below this line, hence the E so Steam area thus obtained is too large, and the greater the expansion the greater the error. Much greater accuracy, however, is secured by this method if several divisions are taken, as shown by the dotted lines re, en, nf, in fig. 27, and the greater the number of divisions taken the greater the accuracy of the result. This is practically the method used by engineers in finding the area of the indicator diagram, the figure being divided into ten equal portions, as explained on p. 55. The exaet value of the expansion portion of a theoretical diagram may be readily obtained by referring to a table of hj'perbolic logarithms; for, when the curve rf is hyperbolic, the hyperbolic logarithm expresses the relation between the area during expansion and the area during admission. Thus, if the steam is cut off at half-stroke, it is expanded to twice its original volume ; and if the area during admission=i, then the area during expansion=the hyperbolic logarithm of 2 ; hence total area=i+hyp. log. 2. Now, on turning to a table (given in most engineers' pocket- books), the hyp. log. of 2 is "696 ; then total area= i + -696 = i'696 ; and for a general case, if R=the ratio of expansion, or the volume of the steam at end of stroke divided by the volume at point of cut-off, then area during expansion =hyp. log. R, and total area=i +hyp. log. R. Thus, in the example (fig. 27), the steam was cutoff at one- fourth of the stroke, therefore at the end of the stroke the volume occupied by the steam was four times its volume at the point of cut-off; in which case R=4, and the whole area = i-t-hyp. log. 4, but by table (p. 51), hyp. log. of 4=1-386 ; therefore whole area=i -f 1-386 =2 -386. That is to say, if the area /aw, r=.\, then the area rv^v^f = 1-386, and the whole area=2-386, Expansion of Steam 51 To express the work done in foot lbs. : Work done during admission =/ v = 100x144x4-33 =62,352 ft. lbs. Total work done during admission and expansion to four volumes=62,352 x 2-386 = 148,771-872 ft. lbs. Example. — Find the weight of steam required per indicated horse- power per hour, working at a pressure of 100 lbs. per sq. in. absolute, with a cut-off at one-fourth of the stroke, assuming there is no back pressure or loss from other causes. Then ^^''^ P^"^ I. H. P. per hour Work per lb. of steam _ 1, 980, 000 _ ~ 148,772 ~ Compare this with result on p. 34. 13-3 lbs. The following table gives the proportional values of the work done for various degrees of expansion. The work done by the steam during admission is taken as I, and corresponds with the area aprvi (fig. 27). Ratio of Work dene Work done Steam in cylinder expansion during during expansion Total work done R admission ' =hyp. log. R Cut off at f stroke li 0-262 1-262 „ i ,i 2 0-693 1-693 »» 3 3 I -098 2-098 »» 5" 4 1-386 2-386 X s 1-609 2-609 .. * 8, 2-079 3-079 " ? 9 2-197 3-197 " 10 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 efificiency of steam with in- creased pressures and increased degrees of expansion may be further shown by the aid of the following diagram. Lime of Volumes Fig. 2S. On the diagram (fig. 28) let oV„ oVj, &c., represent the volume occupied by i lb. of steam at pressures varying from 15 lbs. to 100 lbs. per sq. in. absolute. Now, suppose in each case the steam is used non-expan- sively, that is, is supplied at full pressure throughout the stroke, and then allowed to escape into the air or condenser. Then, neglecting back pressure, the effect of which will be considered presently, the work done by the i lb. of steam under each of the several conditions is represented by an area as follows : Expansion of Steam 53 (i) Work done by steam at 100 lbs.=area o P, ; (2) „ „ 6olbs.=area 0P2 ; (3) )i ), 3olbs.=area 0P3 ; (4) !, „ 15 lbs.=area o P4. But, assuming that steam obeys the law of Boyle, which is sufiSciently 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 Pi the further area P, P4 V4 Vj, which shows a very considerable in- crease in the work done. If the area oP, = i, then the area PiP^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 = 1, then by using the same weight of steam at 100 lbs. pressure and expanding down to 15 lbs. without back pressure, nearly thrSe times the amount of work is done per pound of steam used, and practically also per pound of fuel consumed, for, as has been already shown, the consumption of fuel depends upon the weight of steam used, and is nearly independent of the pressure of the steam, owing to the fact that the total heat in steam at high pressures is only a very little greater than the total heat in steam of lower pressures (see table, p. 39). Back Pressure Back pressure has a considerable influence on the total work done by a given weight of steam. Suppose the piston of a steam engine to be acted upon on one side by steam of 45 lbs. pressure absolute, and, if it be possible, let there be no pressure at all acting on the other side. Then, if the pressure of the steam were maintained uniform throughout the stroke, the diagram of pressures and volumes, 54 Steam or, in other words, the diagram of worK, would be a simple rectangle, thus (fig. 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 efi"ective pressure is therefore 45 — 15 = 30 lbs. per sq. in. ; and the effect on the diagram is to remove all the lower part from zero to 15 lbs., and thus reduce the area of the figure, and therefore also the effective work done. In practice there is an additional back pressure of 2 to 4 lbs., due to incompleteness of exhaust, making a 'total back pressure of 17 lbs. to 19 lbs. per sq. in. It may be much more than this with high-piston speeds. If, however^ the cylinder were put, during exhaust, into communication with a condenser, then a large portion of the atmospheric pressure is removed, and a back pressure of not more than about 3 lbs. absolute will now oppose the motion of the piston. In this case the area of the figure representing the effective work done will be extended down to within about 3 lbs. of the zero line (fig. 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. Me.\n Pressure To find the mean effective pressure of steam per square inch on the piston, by measurement from the indicator diagram : Mean Pressure 55 (i) Divide the line of volumes into ten equal parts. (2) Measure the 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.— Y\\\^ 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. X \ ^ 1 W J ^ h 0- 20 Otlmo ■Kfi Vl\£. Sj^ T^ ">^ II 1 >- r- £ : fc "J 5 •^ ^ 5 5 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 cyUnder. R = range of expansion, or ratio of volume at end of stroke to volume at point of cut-off. Then/ = P X i±hyP^gL^_ back pressure. 56 Steam Thus, for steam at 80 lbs. per sq. in. absolute, cut off at one-fourth of the stroke, 1 + 1-386 / = 80 X -^ — ^ — — 3 4 = 4772-3 = 4472 lbs. per sq. in. The following is useful for reference in obtaining the theoretical mean pressure : IV. Table of Mean Pressures Number of times steam is expanded final volume Mean pressure throughout stroke. Initial Number of times steam is expanded final volume Mean pressure throughout stroke. Initial initial volume I* 2 3 pressure = 1 ■964 ■937 •846 •699 initial volume 6 7 8 9 pressure = 1 ■465 •421 •385 •355 4 •596 10 •330 5 ■522 For back pressure, subtract from result obtained by above table, 3 lbs. for condensing engines and 17 lbs. for non- condensing engines, and the remainder will give the mean effective pressure. Example.— Sifixa at 100 lbs. absolute is expanded down to 20 lbs., back pressure 17 lbs. ; find the mean effective pressure. Here — = 5 = number of expansions mean pressure (by table) = -522 100 X •522 = 52-2 lbs. mean absolute pressure, or, 52-2— 17 = 35-2 lbs. per sq. in. mean effective pressure. Indicated Horse-power The diagram representing the work done on the piston has been called an indicator diagram. From this diagram, having obtained the mean pressure, the work done per stroke may be found. The work done per minute = the work done per Indicated Horse-power 57 stroke x number of strokes per minute. The result may be expressed in horse-power by dividing the work done per minute by 33,000. The horse-power obtained from the indi- cator diagram in this way is called the Indicated Horse-power. It represents the effective work done on the piston .by the steam. The formula for Indicated Horse-power (I.H.P.) may be written, so as to be easily remembered, as follows : J TT p units of work done per minute _ P L A N 33,000 33.°°° Where P = mean effective pressure in lbs. per sq. in. on piston. A = area of piston in sq. ins. ■=■ (diameter of cylinder in inches)^ x 7854 L — length of stroke in feet, or distance travelled by the piston from end to end of cylinder. N = number of strokes per minute ; or ■=■ number of revolutions of engine x 2. For work is always estimated by a force of so many pounds acting through so many feet, and (P x A) pounds pressure on piston, acting through (L x N) feet per minute passed through = work done on piston per minute in foot lbs., which, divided by 33,000 = work done expressed in horse- power. This formula will repay for careful study. It shows that a given indicated horse-power can be obtained by a variety of conditions, providing that the product P x L x A x N remains constant. Thus P, the mean pressure, may be made up by high-pressure steam cut off at an early point in the stroke, or by low-pressure steam acting through the greater part of the stroke. If P is increased by substituting high-pressure steam for low pressure, then A, the area of the piston, may be less, which means that the engine may be made smaller. If N, the number of revolutions, be increased, then L, the length of the stroke, may be decreased. As a matter of fact, this is what has taken place in the development of the steam engine, namely, increased steam 58 Steam pressures and higher piston speeds, which has resulted in a smaller, and therefore cheaper, type of engine. The early engines using low-pressure steam and running at a comparatively small number of revolutions assumed the type of the massive beam engine. The modern engine develops the same power with high pressures and high-piston speeds, and its dimensions are therefore proportionally decreased. PLAN In the equation I. H. P. = ■ we have five indefinite 33,000 terms, any one of which may be found when values are substi- tuted for the remaining terms. Example I. — Find the indicated horse-power of an engine with a cylinder 12 ins. diameter, length of stroke 18 ins., number of revolutions per minute 90, mean effective pressure per square inch on piston 40 lbs. Then I. H. P. = —^-^?' 33,000 _ (P X A) lbs. X (L X N) ft. per min. 33.000 _ (40x I2X I2X 7854) lbs. x(i'5 X90X2) ft. per min. 33,000 _ 4, 520 lbs. X 270 ft. per min. 33,000 = 37 nearly. Example 2. — An engine is required to indicate 37 horse-power with a mean effective pressure on piston of 40 lbs. per sq. in., length of stroke 18 ins., number of revolutions per minute 90; find the diameter of the cylinder. First find the area from the formula : i.H.p.=LiiA? 33,000 . _ 33,oooI.H.P. PxLxN ^ 3 3,000 X 37 40 X I -5 X 90 X 2 A, or area of piston = 1 1 3 sq. ins. From which the diameter may be obtained thus : Diameter = a/ ^IH5. = A / Jil. ^ Vr^= 12 inches. V 7854 V 7854 The horse-power of a compound engine is obtained in practice by finding the horse-power exerted in each cylinder separately from the indicator diagrams by the method above Expansion of Steam 59 explained, and adding the results together ; the sum then gives the total indicated horse-power. Or, its theoretical value may be obtained from an ideal diagram, by considering that the whole of the work is done in the low-pressure cylinder only, working with steam at the initial pressure- of the high-pressure cylinder, expanding down to the terminal pressure of the low- pressure cylinder. Examples illustrating Economy of Expansive Working Example I. — A condensing engine works with steam at 30 lbs. boiler pressure, cut off in the cylinder at half-stroke. It is proposed to increase the boiler pressure to 60 lbs. and to cut off at one-fourth of the stroke. Compare the relative work done and weight of steam used in the two cases. Steam at 60 lbs. Steam at 30 lbs. boiler pressure boiler pressure cut off at cut off at \ stroke ^ stroke Absolute initial pressure . . 75 lbs. 45 lbs. Theoretical mean pressure . . 447 lbs. 38 lbs. Effective mean pressure, allow- ing 3 lbs. back pressure . . 417 lbs. 35 lbs. Relative density or weight of steam per cub. ft 75 45 Relative volumes used per stroke I vol. 2 vols. Relative weight used per stroke 75 >< 1=75 45x2=90 From which we gather that by using the higher pressure of steam with earlier cut-off there is a gain in effective mean pres- sure of 417— 35=67 lbs. per sq. in.= -^x 100=19 per cent, and a reduced consumption of steam =90 — 75 = 15 lbs., saving on each 90 lbs. formerly used, or a saving of — x ioo=i6'6 90 per cent. As formerly explained, there will be a saving in fuel con- sumption corresponding with the saving in the weight of steam. 1st case 2nd case 60 lbs 45 lbs. 75 lbs. 60 lbs. 10 bs. 10 lbs. 30 lbs. is 27-9 lbs. 26 lbs. 23-9 lbs. 60 Steam In practice it would be necessary to set against the above result — (i) Probable increased initial condensation of steam in the cylinder, with the higher pressure and greater expansion. (2) Increased initial stresses on the engine. Example 2. — Two engines, single cylinder condensing, have cylinders of equal dimensions, each works with steam having a terminal pressure of 10 lbs. absolute, back pressure 4 lbs. The boiler pressure by gauge for the first engine is 60 lbs. and for the second 45 lbs. Compare the result in the two cases. Boiler pressure .... Absolute initial pressure . Terminal pressure Cut-off (neglecting clearance) Theoretical mean pressure Effective mean pressure, allow- ing 4 lbs. back pressure . The weight of steam used per stroke is the same in each case, for the cylinders are the same size, and the terminal pressures are equal. They, therefore, hold at the end of the stroke equal volumes of steam at the same pressure, and, there- fore, of the same weight. It is true that the total heat required to generate steam at 7 5 lbs. absolute is greater than that required for steam at 60 lbs, absolute, but this difference is so small that it may be neglected, and the consumption of fuel per lb. of steam generated may be assumed to be the same. There is, however, as seen by the table, a gain in mean effective pressure of 26 — 23'9^2'i lbs., or a gain of — x 100=9 P^'' <^6nt. in power exerted with the 23-9 higher pressure. To set against this we have greater initial stresses on the parts of the engine working with the higher pressure, in the proportion of '^^l^—'-^, or an increase of 60—4 56 27 per cent., requiring an engine in this case 27 per cent. stronger. To further illustrate the advantage of expansive working, Expansion of Steam 6i we will take another case from actual practice. A steamer of i,ooo I.H.P., having a pair of two-cylinder compound oscillat- ing paddle-wheel engines, made by Messrs. Laird of Birkenhead, runs at the following speeds and coal consumption for varying degrees of cut-off in each cylinder : Point of Knots Coal Consumption Coal Consumption cut-off per hour per hour in cwts. per mile in cwts. 3-ioths 8 6 075 4-ioths 9 9 i-oo 5-ioths lO 12 I'20 6-ioths 12 20 1-66 From this table it will be seen how the weight of steam sup- plied to the cylinder affects the speed and coal consumption. Comparing the effects of the two extreme points of cut-off, when cutting-ofifat 6-ioths instead of 3-ioths, twice the volume and weight of steam is used, 3-3 times the weight- of coal is consumed per hour, and 2-2 times the weight of coal is con- sumed per mile, for i -5 times the number of revolutions. The effect of expansive working on the possible distance which a vessel can run with a given weight of fuel will also be evident ; for in the case we are considering the vessel would run 2"2 times the distance when cutting-off at 3-ioths that she would run when cutting-off at 6-ioths. The influence of this increased economy of expansive working on the power to run longer voyages where coal is not easily obtained has had im- mense influence on British commerce with distant parts of the globe. Limit of useful Expansion of Steam The amount of back pressure in a cylinder determines the lifnit of useful expansion of the steam in the cylinder, for it would evidently be useless to expand the steam to a pressure below that of the pressure at the back of the piston. Thus, let steam at 45 lbs. per sq. in. boiler pressure, or 60 lbs. per sq. in. absolute, be admitted to a cylinder, and cut off at one-fourth of the stroke of the piston. Let the engine be non-condensing with a back pressure of 15 lbs. per sq. in. 62 Steam Then, referring to fig. 33, the useful work done by the expanding steam is represented by the shaded portion from the commencement of the stroke to the point d on the hne of volumes. Here the pressure of the steam (d t) and the back pressure are equal, and it will be evident that any further ex- tension of the diagram would be useless. If the expansion be continued beyond four, to five or six expansions, the work done by the atmosphere against the piston in the later stages is greater than that done on it by the steam, to the extent repre- sented by the area of the shaded part marked negative work. Hence, in an engine cylinder the steam should never be cut off so early as to cause it to expand to a pressure below that of the back pressure acting against the piston. In practice the expansion cannot be carried with advantage so far as this. Theoretical limit of number of expansions := '™tial pressure . back pressure Thus, in above case —, initial pressure _6o_ = __=4 expansions. In back pressure practice the maximum number of expansions should not exceed three-fourths of the theoretical limit. In condensing engines the steam is expanded down to a final pressure of about 10 lbs. per sq. in. absolute. And by dividing the known initial abso- lute pressure by the known terminal pressure we determine the number of expansions required. Thus, for a condensing engine Clearance 63 working with steam at an initial pressure of 150 lbs. absolute and expanding to a terminal pressure of 10 lbs. absolute, initial pressure iso_ number of expansions: = 15- terminal pressure 10 Such a large number of expansions could not be economically carried out in one cylinder, but in practice would require three successive cylinders. Clearance in the Cylinder ^Vhen the piston in a cylinder is at the end of its stroke it does not touch the end or cover of the cyhnder, but there is always a certain space left between them to prevent the danger of their coming into actual contact. In addition to this is the passage between the face of the slide valve and the cylinder by which the steam is conducted to the cylinder. These two spaces (marked c c, fig. 34), which make up the whole space between the face of the valve and the face of the piston when the piston is at the end of its stroke, are called the clearance. ,^//777!rr Let the volume displaced by ^^^ the piston during its stroke = 9 ^^ '^ cub. ft ; and the volume of the clearance = i cub. ft. Then, when steam is admitted, i cub. ft. is used to fill the clearance space before the piston moves ; and if steam is used at full pressure throughout the stroke, 9 cub. ft. more is required to displace the piston. Thus i cub. ft. in every 10 cub. ft. of steam passes through the engine with- out doing any work, representing a loss of 10 per cent. But suppose the steam is cut off at ^th of the stroke. Then, during admission there is first i cub. ft. of steam to fill the clearance, and which so far does no work on the piston, and then I cub. ft. to displace the piston, when the steam is cut off. There are now 2 cub. ft. of steam at initial pressure Fig. 34. 64 Steam enclosed in the cylinder. Expansion commences, and at the end of the stroke the volume occupied by the steam will evi- dently be 10 cub. ft. Hence, pressure of steam at end of stroke =-/ijths, or -Jth of initial pressure. If there had been no clearance, then we should have had I cub. ft. of steam in the cylinder at point of cut-off, which would expand to 9 cub. ft. with a terminal pressure of ^th, the initial pressure. Suppose the initial pressure had been 180 lbs. absolute. Then, neglecting the effect of clearance, the terminal pressure =i-?=2o lbs. per sq. in. absolute. 9 Including the effect of clearance, the terminal pressure = —=36 lbs. per sq. in. absolute, or nearly twice the terminal pressure obtained neglecting clearance. ISOr- FlG. 35. Although the steam required to fill the clearance space does no work on the piston during admission, yet when cut-off takes place the piston receives the advantage of the expansive force of this steam, and its effect in increasing the total work done is shown by the shaded part of the diagram (fig. 35). To draw the diagram, set off a f =the stroke of the piston, to any scale and divide it into nine equal parts, construct the curve /, 20, by the graphical method from the point a, representing the expansion of steam of volume a b and pressure b p. To the Priming 65 left of a draw a d, making a ^=^th a c, that being the proportion of the voluine of the cyHnder occupied by the clearance space. Draw the curve /, 36, from the point d, representing the ex- pansion of steam of volume db, and pressure bp. The loss by clearance may be much reduced by closing the exhaust passage in the cylinder before the end of the stroke, so that the steam so enclosed may be compressed and fill the clearance space at a pressure and temperature approaching that of the newly entering steam. Priming A boiler is said to ' prime ' when the steam supplied by it to the cylinder is not dry, but is charged with more or less moisture. Priming is frequently due to rapid evaporation, too small steam space, defective circulation, or the presence of certain impurities in the water. Of the total water used by a boiler, from 5 to 10 per cent, may be accounted for by priming. Cylinder Condensation When steam is admitted to a cylinder which is colder than the entering steam, the steam parts with some of its heat to the cylinder walls, a portion of the steam is condensed and deposited on the metallic surface, and more steam from the boiler enters the cylinder to take its place, while the temperature of the cylinder rises to that of the steam in contact with it. If the steam be supplied to the cylinder at the initial pres- sure and temperature throughout the whole stroke, and the exhaust port be then opened, the steam will escape into the air, and the pressure in the cylinder will fall to that of the atmosphere, or nearly so. But the water (which exists probably as a film), being in contact with the metallic walls of the cylinder at the temperature of the initial steam, will evaporate immediately the pressure is reduced by the opening of the exhaust, and become reconverted into steam at the expense of the heat in the walls of the F 66 Steam cylinder, thereDy cooling them to the temperature of the steam during exhaust. The steam thus re-evaporated during exhaust not only absorbs heat, which will have to be made up again from the entering steam during the next stroke, but it passes away to the air without doing any useful work ; in fact, it acts rather as back pressure against the piston. When the piston reaches the end of its stroke, the boiler steam is readmitted into the cylinder and comes in contact with the cooled surface of the cylinder cover, piston, and steam passages, which have been exposed to the temperature of the exhaust steam, and the same process of condensation and re- evaporation will be repeated. If the cylinder had been in communication with a condenser instead of with the air, the temperature of the cylinder during exhaust would have fallen still lower, namely, to that due to the decreased pressure in the condenser, and the condensation of the initial steam during admission would have been still greater. If the steam is cut off at an early point in the stroke, con- densation occurs, as before, during admission, while the steam is hotter than the cylinder ; but as the expansion proceeds, the pressure of the steam is reduced below that at which water at the increased temperature of the cylinder evaporates, and a portion of the condensed steam is consequently re -evaporated, the re-evaporation increasing as the pressure decreases towards the end of the stroke. On the opening of the port to exhaust, the pressure is still further reduced, and re-evaporation is com- pleted. Condensation, then, takes place during the early part of the stroke, while re-evaporation occurs partly towards the end of the stroke and partly during exhaust. The re-evaporation during expansion behind the piston helps the piston, and in- creases the total work done ; but the steam re-evaporated during exhaust in a single cylinder engine passes away to waste. The loss due to condensation of steam in the cylinders of all engines varies from lo to 50 per cent., or more, of the whole steam consumed, the loss becoming greater as the degree of Cylinder Condensation 67 expansion increases, or, in other words, as the variation of tem- perature in the walls of the cylinder increases. The economical advantage of using high-pressure steam is due to the power it possesses of doing work by expanding behind the piston after the supply is cut off from the boiler. But the temperature of saturated steam varies with the pressure, and, therefore, if, in a single cylinder, steam at high pressure and ternperature be admitted and expanded to a low pressure and temperature, the greater the degree of expansion the greater the difference in temperature between the steam on entering and leaving the cylinder. Thus, suppose in each of the following cases the tempera- ture of the exhaust to be that at atmospheric pressure, namely, 2 12° F. If the initial pressure of the steam — that is, the pressure on admission to the cylinder — be 45 lbs. absolute, temperature 274° R, we have a difference of temperature in the cylinder of 274—212=62° F., and on the admission of steam for the new stroke we shall have steam at 274° coming in contact with cylinder walls at 212". But suppose the initial pressure of the steam is raised to 90 lbs. absolute, temperature 320° F., which is expanded in the cylinder and exhausted into the atmosphere, then the difference of temperature in the cylinder is 320 — 212^108° F., and steam on admission at 320° comes into contact with cylinder walls at 212°. But the loss due to initial condensation of steam in the cylinder increases as the variation in temperature in the walls of the cylinder increases ; hence there is a limit to the useful ex- pansion of steam in a single cylinder, owing to the excessive condensation in the cylinder, with high degrees of expansion, resulting in increased consumption of fuel instead of a saving, and giving rise to the expression ' Expansive working is expen- sive working.' The secret of economy is in the carrying out of the principle laid down by Watt, namely, that the cylinder should be kept as hot as the steam which enters it, and engineers from Watt's day to the present have striven to accomplish this result. The laws which govern the condensation of steam in the 68 Steam cylinder are not at present well understood. The remedies so far adopted, which are, however, only partially successful, are : (i) Jacketing the cylinder with hot steam (an example of jacketed cylinders is given in figs. 105 and 106). The addition of the steam jacket has a considerable influence in reducing the amount of condensation in the cylinder. The jacket is the more necessary the greater the degree of expansion in one cylinder, and the slower the piston speed. (2) Cushioning, that is, compressing a portion of the exhaust steam by closing the exhaust port before the end of the stroke, and allowing the piston to compress the steam and thereby raise its pressure and temperature, and therefore als'o the temperature of the cylinder cover, steam passage, and piston, before the new steam is admitted. (3) Compounding the cylinders, that is, adding one or more separate cylinders into which the steam may be expanded, and thereby reducing the variation of temperature in each cylinder. 69 CHAPTER X THE STEAM ENGINE Non-condensing Engines Engines which exhaust their steam into the air after it has done work in the cylinder are called non-condensing engines. The locomotive and most factory and mill engines belong to this type. Non-condensing engines are known by the pufiSng of the escaping steam up the chimney, a phenomenon which is familiar to every reader. The puff of the exhaust steam occurs as the piston arrives at the end of its siroke, the escaping steam having previously driven the piston from one end of the cylinder to the other. In condensing engines there is no puffing of exhaust steam into the air, the steam being passed instead into a box or con- denser, where it is cooled and condensed by actual contact with a jet of cold water, or by contact with cold pipes through which cold water is flowing. The essential parts of all ordinary non-condensing engines, whether the engine be an horizontal or vertical one, are practi- cally the same, the difference in appearance among engines by different makers being due for the most part to a difference in shape or arrangement of the essential details. The following diagrams (figs. 36 and 37) give a front view and side view of a small vertical non-condensing steam engine, as in use for various kinds of factory and mill work. The pressure of steam used in such engines is about 60 lbs. per sq, in. above the atmosphere. The action of the parts is as follows : The steam is con- 70 Steam ducted from the boiler by the steam pipe to the slide jacket or chamber in which the slide valve S V works. Here, by a sUding motion of the slide valve on the face of the ports, the Fig. 36. C, cylinder ; P, piston ; P R, piston rod ; G, guides ; C R, connecting rod ; C P, crank pin ; E, eccentric ; E R, eccentric rod ; S V, slide valve. Non-Condensing Engines 71 steam passages or ports are alternately opened, admitting steam to one side of the piston, and allowing it to escape from the .other side into the air ; or, if a condensing engine, into a con- FlG. 37. C H cross-head ; C R connecting rod ; B, bearings ; £ P, exhaust pipe ; S P, steam pipe. 72 Steam denser. (The exact action of the slide valve will be explained more fully presently.) The piston is thus made to move from end to end of the cylinder against the resistance due to the load which is communicated through the piston rod. Attached to the outer end of the piston rod is the crosshead, having a flat base called a slipper, which sHdes to and fro between guides, and compels the piston rod to move parallel to the axis of the cylinder, thus preventing the angular action of the connecting rod from bending the piston rod. The con- necting rod is attached at one end to the crosshead by a pin, sometimes called a gudgeon, which passes completely through the block and the fork end of the rod as shown, and at the other end to the crank pin. The reciprocating motion of the piston is by this means converted into the circular motion of the crank pin and shaft, and from the shaft by means of a pulley and belt, or by wheel gearing, the power of the engine is transmitted as required. See also figs. 104 and 105. Engine Details The Cylinder. — The cylinder, which is made of cast-iron, consists of the cylindrical chamber, bored out perfectly true, and of the slide jacket or valve box. The cylindrical chamber is connected at each end with the slide jacket by passages called steam ports, S, through which steam passes to or from the cylinder. The pas- sage between the two steam ports leads to the air, or to a con- denser, and is called the exhaust port, E. This passage is put in SP, steam pipe . _, SV, slide valve G, gland. Fig. 38. steam port ; E, exhaust port ; ; P, piston, P R, piston rod ; The Cylinder 73 communication with either end of the cylinder as required by means of the slide valve. The ends of the cylinder are closed by covers bolted to the flanged ends. In the example (fig. 38) the bottom end is cast solid with the body of the cylinder. In order to make the hole in the cover through which the piston rod passes steam-tight, a stuffing box is used, the con- struction of which will be understood from the figure. The casting is so formed as to leave a small space around the rod, which is filled with packing, or stuffing, consisting of tallowed hemp or other substitute, and the packing is pressed down on the rod by means of a cover or gland fitted with two screwed bolts. A similar arrangement of stuffing box and gland is fitted to the slide valve rod ; it is also used for pump rods and other similar purposes. The steam passages should be made as short as possible, because at each stroke the passage must be filled with its own volume of steam before the steam acts upon the piston. The effect of this has been described under the heading of Cleat ance, on p. 63. The steam ports must be made large enough to admit sufficient steam to the cylinder during the instant the port is open, otherwise the steam will be wiredrawn. Wiredrawing is the gradual fall of pressure of the steam behind the piston, as it pro- ceeds on its stroke, owing to small and re- stricted steam passages. Its effect may be illus- trated by the diagram (fig. 39). If the pres- sure of the steam on ad- mission to the cylinder = O A, then the pres- sure, instead of being maintained at a pressure NE to the point of cut-off, E, gradually falls from A to B. The stroke of the piston from end to end of the cylinder Fig. 39. 74 Steam (which is equal to the diameter of the crank-pin path) deter- mines the internal length of the cylinder from cover to cover, which must evidently be equal to the stroke of the piston, plus the thickness of the piston, plus twice the clearance allowed between the piston and cylinder cover, when the piston is at the end of its stroke. This clearance, which is kept as small as possible, varies from ^ in. to \ in., according to the size of the cylinder. It will be noticed that the shape of the cylinder cover must be made to conform to that of the piston, otherwise a con- siderable volume of steam might be wasted at each stroke, in filling unnecessarily large clearance spaces. Cylinder liner. Steamjacket. — Cylinders are sometimes fitted with a separate internal barrel, called a cylinder liner, as shown in the sectional view of the compound engines (fig. 105), made of hard cast-iron or of steel. Between the liner and the body of the cylinder is a space called the steam jacket, which is filled with steam direct from the boiler. The depth of the jacket is about the same as the thickness of metal in the cylinder. Sometimes the cylinder covers are jacketed, as well as the body of the cylinder. Cylinder escape valves. — To avoid the danger of the piston bursting the cylinder cover as it approaches the end of its stroke, owing to the occa- sional presence of water through prim- ing or condensation, cylinder escape valves are often fitted on the cylinder covers. The diagram (fig. 40) will explain the construction of these valves. The valve is of the ordinary conical kind, kept in position by a spring loaded a little above the pressure in the boiler. Cylinder relief cocks (fig. 41) are also fitted to all cylinders to drain off the water, or to blow through the cylinder with the steam, and thus clear it of water, especially on starting the engine. Fig. 40. The Cylinder 7S Example l.— A cylinder is 15 ins. diameter, stroke of piston 25 ins. ; find the capacity of the cylinder, allowing an addition of 7 per cent, for clearance space. Ans. 47267 cub. ins. Note. — This represents the volume of steam in the cylinder at end of stroke ; the following example shows how to find the weight of this volume. Example 2. — Find the weight of 47267 cub. ins. of steam at 20 lbs. pressure per sq. in. absolute. By Table III. I lb. of steam at 20 lbs. pressure absolute occupies 197 cub. ft. Then 197 cub. ft. of steam at 20 lbs. pressure weigh i lb. 47267 1728 -^Ib. 197 47^. ' lbs. 1728 197 = -1388 lb. The above two examples give the volume and weight of steam used per stroke in a cylinder of the above dimensions, CYLINDER Fig. 41. working with steam at 20 lbs. terminal pressure. To find the volume or weight of steam passing through the engine as steam vapour in a given time, multiply the above results by the number of times the cylinder is filled ; in other words, multiply by the number of strokes made by the piston in the given time. Example 3. — The engine in the above case runs at 100 revolutions per minute ; find the weight of steam used per hour. In I stroke the weight of steam used = '1388 lb. „ I revolution „ „ =(-i388 x 2) lb. ,, I minute ,, „ =(-1388 x 2 x 100) lbs. „ I hour „ „ =(-1388 X 2x 100x60) lbs. = 1665-6 lbs. 76 Steam Exampk 4. — Suppose it is known that the horse-power of the above engine, when working at 100 revolutions, is 90 ; find the number of lbs. of steam used per horse-power per hour. Ans. i8'5 Fig. 42. "^1 Pistons The piston is the movable plug which moves from end to end of the cylinder, under the pressure of the steam, and through which the energy of the steam is converted into the motion of the mechanism. The piston must form a steam-tight division between the two ends of the ii. ylinder. If it were possible to turn up a olid piston, which should so exactly fit .he bore of the cylinder that it would be steam-tight, and at the same time move freely without friction, this would be a perfect piston. In the early days of the steam engine, when steam pressures were very low, pistons were made steam-tight by coiling rope or junk in a groove on the rim of the piston, and this method is still adopted for pump buckets which only require to be water- tight. But for the pistons of steam cylinders a more perfect ar- rangement was soon found necessary. As at present made, the body of the piston is turned to an easy fit in the cylinder, and it is then made steam-tight by means of spring rings. A common and simple arrangement is that of Ramsbottom's spring rings, which are simple steel or gun-metal rings of \ in. to f in. square section (fig. 43). They are turned at first to a diameter a little larger than that of the cylinder they are required to fit ; and a small piece is then taken out to enable them to close up to the bore of the cylinder when in their place. They are then sprung over the piston and fitted into grooves turned in the piston rim (fig. 43). Pistons 77 Figs. 43 and 44 are types of locomotive pistons; fig. 44 is fitted with two cast-iron packing rings about \ in. thick by /j5''^ s I in. wide, turned, cut and sprung into position as before. The rings are some- times placed iri the same groove and sometimes in separate grooves. For marine work, pis tons of the type shown in fig. 45 are much used. The packing ring consists of one large cast-iron ring, PR, which is pressed outwards against the cylinder by means of a series of (E X rp-^ ti-. 45. J R, junk ring ; P R, packing ring ; T P. tongue piece ; S, spring. Springs, S, placed behind the packing ring. For horizontal cylinders, the bottom spring is removed and a cast-iron block is substituted, which takes the weight of the piston. Instead of 78 Stcavi V... o 3Q3) o the small separate springs, various patent coiled springs are used in vertical engines. The packing ring is turned a little larger {I: in. per foot diameter) than the bore of the cylinder ; it is then cut through by an oblique slit and tends to spring open as wear takes place. The steam is pre- vented from leaking through this opening by a brass tongue piece, T P, which is fitted in another groove cut across the slit as shown. The tongue piece is secured to a plate fastened to the back of the ring, and on one side of the slit (fig. 46). The packing ring is held in its place between two flanges, one of which is cast solid with the piston, the other being formed by a loose flat ring called the junk ring, J R. The junk ring is secured to the piston by screwed bolts which screw into brass nuts in- serted in a cavity left for the purpose in the body of the pis- ton. These bolts are prevented from slacking back by a guard ring or pieces of a ring fitted be- tween the heads, as Fk- ^7■ shown. Fig. 47 is a section of Buckley's Patent Piston. The packing consists of two separate rings with a continuous coiled spring behind it. The action of the coiled spring is to keep the rings steam-tight, not only against the cylinder, but against the junk ring and flange of the piston. Pistons 79 The friction between the packing ring and the cylinder should be as little as possible consistent with steam-tightness, and the piston should be as light as possible consistent with strength. Steel pistons are now becoming common, and by using this material the weight of the piston can be considerably reduced. A fruitful and all too common source of loss of efficiency in steam engines is the presence of leaky pistons, the steam passing from one side of the piston to the other. Such steam is worse than wasted, as it not only does no work on the piston but acts as back pressure against it. The pistons of locomotives are usually kept in good condition, and the short sharp exhaust of the locomotive is in striking contrast with the asthmatical exhaust of too many factory and mill engines. Piston speed. — The mean speed of the piston in feet per minute = length of stroke x number of revolutions per minute X 2. Example. — An engine with a 3-ft. stroke makes 80 revolutions per minute ; find the mean speed of the piston. 3 ft. X (80 X 2) strokes = 480 ft. per minute. The mean speed of the piston in practice varies from about 250 ft. per minute for small stationary engines, to from 500 to 750 ft. per minute for marine engines, and in some cases it exceeds 1,000 ft. per minute in the locomotive. There has been, and is, an increasing tendency towards high piston speeds and light moving parts. Fiston displacement per minute is the space swept through by the piston at each stroke, multiplied by the number of strokes per minute ; = the area of the piston in square feet, multiplied by the speed of the piston in feet per minute. Example. — Find the displacement of the piston per minute in an engine, diameter of cylinder 18 ins. and length of stroke 2 ft. ; revolutions per minute, 70. Then (i "S x i -5 x 7854) sq- ft. x 2 ft. x (70 x 2) strokes = 49476 cub. ft. per minutei Piston rods are subjected to alternate pushing and pulling stresses which occur in rapid succession, and which must severely test the material of the rod, and they are now invari- 8o Steam ably made of steel. The weakest part of the rod is at the screwed end which takes the nut. This part, however, is only subject to tension, and not to alternate tension and compres- sion, for when the steam enters the cylinder underneath the piston (fig. 43) the whole load is carried by the screwed part of the piston rod ; but on the return stroke, when the piston is descending, the stress is removed from the screwed part and comes on the tapered part of the rod and the collar. The load to be carried by the piston rod equals the differ- ence between the pressure on the two sides of the piston. Thus, in a condensing engine the effective pressure per sq. in. on the piston equals the boiler pressure by gauge, plus 15 lbs. pressure of atmosphere, minus loss of pressure between boiler and cylinder, minus back pressure due to imperfect vacuum in condenser. Crossheads and Guide Blocks The crosshead forms a head at the outer end of the piston rod, to which the connecting rod is attached by a pin passing through the crosshead. It varies very con- siderably in design. Guide blocks are sometimes attached to each end of the pin, on either side of the cross- head, as in fig. 48. Another arrange- ment is to make a foot solid with the crosshead, which acts as a guide block, and works between guides, as shown in fig. 49. The blocks and guides prevent the oblique thrust or pull of the connecting rod from bending the piston rod. This can be seen by reference to fig. 50. When the piston P is being im- pelled forward, so that the rotation of the crank pin is in the direction of the arrow, the resistance at the crank pin c causes a downward thrust through the connecting rod eg, which may be resolved into two forces, one tending to compress the piston rod and the other to bend it in the direction T, causing a down- FlG. 48. Crossheads and Guides ward thrust upon the guides. Again, when the piston is being driven back by the steam, the resistance of the crank pin at d causes a downward pull at the point g of the piston rod, the tendency again being to cause a downward thrust upon the guides. If the engines were reversed the whole of the condi- FlG. 49. tions would be reversed, and the thrust ^T would be upwards instead of downwards. Hence the prevailing direction in which horizontal engines should run is that shown by the arrow in the figure, so that the pressure on the guides should be upon t I Fig. so. , the lower rather than upon the upper guide bar ; this is especially important for the sake of efficient lubrication. It should be noticed that when the crank pin drags the piston, as it does, for example, when steam is shut off while the engine continues to rotate, the direction of the thrust on the guides is 82 Steam reversed ; hence the necessity for a top and bottom guide bar under all circumstances. The amount of the thrust on the guides varies according to the angularity of the connecting rod, being greatest when the crank is at right angles to the axis of the piston rod, and being reduced to nothing at each end of the stroke ; hence the guides wear hollow in the middle, and arrange- ments should exist for removing the guides and truing them up. The amount of the thrust on the guides in the middle of the stroke may be found from the following simple formula : ,, . r, . Pressure on piston X radius of crank in ins. Maximum thrust = = r^t-? ■. 3-^ — -. Length of connectmg rod m ms. Example. — Find the maximum thrust on the guides when pressure on piston at half-stroke = 20,000 lbs. ; radius of crank = 15 ins. ; length of connecting rod = 5 ft. Maximum thrust = '^°'°°° "'^ ^ S.ooolbs. 60 Fig. si. (Dotted circle = crank-pin path.) When engines are required to rotate in either direction equally, as the locomotive, the surfaces in contact between the block and the guide are made equally large, as is the case in iig. 52, with the top and bottom guide bar ; but when the engine is intended to rotate always in one direction, or nearly so, as in the marine engine and in factory engines, the surface on which the thrust comes is made sufficiently large, while the opposite surface may be much reduced, as is the case with the slipper, or shoe guide (fig. 49), the prevailing direction of the thrust being taken on the largest surface of the block. The Connecting Rod The connecting rod connects the crosshead with the crank pin, and by its means the reciprocating or to-and-fro motion The Connecting Rod 83 of the piston is transformed into the rotatory or circular motion of the crank pin. The length of the connecting rod, which is measured from the Fig. 521 centre of tlie crank pin to the centre of the crosshead pin, varies from two to three times the length of stroke of the engine. By the stroke is meant the distance travelled by the piston from one £ Fig. 53. end of the cylinder to tjie other, which is equal to the diameter of the crank-pin path, or to twice the length of the crank arm. Fig. 53 is an illustration of the marine type of connecting rod. 84 . Steam Fig. 54 shows a ' strap, gib, and cotter ' arrangement for a connecting rod end. ^^ L Fig. s4- S, strap ; G, gib ; C, cotter. Relative positions of piston and crank pin. — When the piston P is at either end of the stroke, the centre Hne of the A — IcS) ' m^//////Ammj^ Fig. s5. connecting rod B C and of the crank o C he on the axis of the cyhnder produced (see figs. 55 and 56), and the crank is then said to be on its dead centre ; for if the enguie come to rest in this position, it will remain at rest, even when steam is admitted to The Connecting Rod 85 the cylinder, because the pressure of the piston is felt merely as a thrust on the crank shaft main bearing, and it has no tendency to cause the crank to rotate. In such a case it is necessary to ' bar ' the engine round by the fly wheel till the crank has ( moved off the dead centre, before admitting steam against the piston. There are two ' dead centres ' in a revolution. ^////////MMM^ Fig. 56. Let the dotted circle, fig. 57, represent the path of the crank pin "about the centre of the crank shaft. If the connect- ing rod were infi- nitely long, or if we neglect the obliquity of the connecting rod, then, when the crank pin is at any .. position between 0° and 180°, the cor- responding position of the piston is found by dropping a perpendicular up- on the diameter as shown, which diameter may be taken to represent the stroke of the piston. In such a case, when the crank pin is at 90° or one-fourth of a revolution from 0°, the piston would be in the middle of its stroke ; but this is not the case in practice, because of the obliquity of the connecting rod, as will now be shown. Since the position of the crosshead corresponds exactly with the'position of the piston, we may for the present purpose Fig. 57- 86 Steani suppose the connecting rod to take hold of the piston direct, without the intervention of the crosshead and piston rod. First, to obtain the position of the piston for a given position of the crank pin. In fig. 58 let C] C2 be the diameter of the crank-pin path, and let the length of the connecting rod be i^ times the stroke, namely, li times Ci C2. From Cj, with radius equal to length of connecting rod, mark P,, the position of the piston when crank is at Ci ; also from C2 with the same radius mark Pj, the position of piston when crank is at Cj. From any intermediate position on the circular crahk-pin path, and with radius equal to length of connecting rod, cut the line of stroke P i Po, then the intersection will give the corresponding position of the piston. Thus, when \ I \'^ I 1 -m-—"'^ Fig. 58. the crank pin is at C with the crank at right angles to the line of stroke, the piston position is not at half stroke, but at some position P^ beyond half stroke ; and the shorter the connect- ing rod. the greater the distance travelled by the piston beyond the centre of the stroke ; or, the longer the connecting rod the more nearly P„ would coincide with the middle of the stroke. From the figure it will be clearly seen that while the crank pin rotates at a uniform velocity through the first quarter of a revolution, the piston travels at the same time from rest at P, a distance Pi P^, greater than half the stroke, when its velocity is equal to that of the crank pin ; and, during the uniform rota- tion of the crank pin through the second quarter, the piston travels a distance P^ P2, or less than half the stroke, again coming to rest at Pj. The Connecting Rod 87 Conversely, to obtain the position of the crank pin for a given position of the piston. When the crank pin is at Ci (fig. 59), the piston is at P, ; and when the crank pin is at C2 the piston is at Pj ; and if the connecting rod were loose from the crank pin, and held at the centre of the crank shaft Co the piston would be at P^, namely, at the middle of the stroke. Now let the piston end of the rod remain in this middle position and move the other end of the rod from C^ in an arc of a circle from, centre P^ till it cuts the crank-pin path at C. Then C is the position, of the crank pin when the piston is in the middle, orthe.stroke. Any other position of the crank pin. for a given position of the piston may be similarly obtained. ? ''i ^ 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 : mi_j or as I : 1-5708. Thus, if the mean piston speed is 1,000 ft. per minute, the mean speed of the crank pin is 1000 x i'57o8 = i57o-8 ft. per minute. By the principle of work, since the work done on the piston is the same as that done on the crank pin, and that the mean speed oi\k\& crank pin is i'57o8 times that of the piston, there- 88 Steam fore the mean pressure on the crank pin in the direction of its motion is of the mean pressure on the piston. Example. — In a direct acting engine the diameter of the cylinder is 17 ins., and the mean pressure 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 -■ 17 x 17 x 7854 x 60 = 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. But, 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 X2=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 be helpful to the student to have a clear idea of the actual shape of the cylinder face. This is shown in the following diagram, fig. 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 cylinder face to the outside of the cylinder, one end of the passage being the rectangular opening, called the exhaust port, and the other end the circular opening with a flange, shown in the figure, to which the exhaust pipe may be bolted. The two other ports are the steam ports — one leading to one end of the cylinder, and the other to the other end. The slide valve is shaped somewhat like a hollow, rectangu- lar, inverted dish ; the edges of the dish, constituting the face of the valve, are planed and scraped to a perfectly true plane surface, and this works on a similarly prepared surface on the cylinder face. The following diagram will explain the action of the slide valve. 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. I ^ --\- -4e- -- Fig. 6z. The valve then uncovers the right-hand port and the distribu- tion of steam is reversed. The valve which we have so fqr described has two impor- tant disadvantages : (i) It admits steam to the cylinder throughout the whole length of the stroke of the piston. The waste of steam involved in not cutting off the supply at an early point of the stroke, and using it expansively, has been already pointed out. (2) It opens the ports to steam and exhaust just after the 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 kad—Xhui 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 / i, by increasing the width of the face from the width of the port S to the width ci. The amount of opening of the port for the admission of '°' *■*■ steam when the piston 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 OE (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- The Slide Valve 93 senting the eccentricity of the eccentric or the half trave! ol the valve = width of port + lap. Let the piston be situated at the beginning of 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 r-' V o "/ ■)T3 y Fig. 66. angle. To find this position : From the centre O on the centre line C D set off O « equal to a f— that is, equal to the lap plus the lead — and from n raise a perpendicular n E' to cut the circular path of the eccentric centre. Then E is the position required, and E' produced is the centre line of the eccentric (see also fig. 67). But the piston is assumed at the end of its stroke, therefore O C is the position of the crank, and we now have the relative positions of the crank and eccentric centre hnes. 94 Steam The angle E O E' which the centre Une of the eccentric is moved through beyond 90° ahead of the crank is called the angular advance of the eccentric. (See also fig. 67.) Example.— The width of a steam port is 1 in., the lap of the valve J in., and lead ^ in. Find the eccentricity of the eccentric, and the angular advance of the eccentric. Eccentricity of eccentric = half travel of valve Half travel of valve = lap + port opening. = i + l in- = ij in. Therefore, from centre 0, with radius O E= ij in., draw a circle represent- ing the path of the centre of the eccentric. (Fig. 67, half size. ) Let C be the position of the crank, and _C draw cE at right angles to Co. On C « produced make a = J in. and ab = ^ in., and from b draw iE' perpendicular to Co to cut the path of the eccentric centre in E'. Join O E'. E O E' is the angular advance of the eccentric. The action of the valve on the face of the ports may be easily followed by drawing the ports, and marking off the valve on the edge of a piece of paper, and moving the valve on the ports as required. The effect of outside lap is : (r) To increase the travel of the valve ; (2) To cut off the steam at some point before the end of the stroke ; (3) To cause the eccentric to be moved forward on the shaft, which results in an early opening of the exhaust port. The effect of inside lap is : (i) To close the exhaust port at an earlier point in the stroke, producing compression of the steam at the back of the piston ; (2) To delay the opening to exhaust. To set a slide valve. — Put the crank alternately on its two dead centres. Measure the opening of the port to steam allowed by the valve at each end of the stroke. When these Fig. 67. Piston Valve 95 are equal to the lead allowed in each case, the valve is correctly set. The travel of a slide valve from end to end of its stroke is equal to twice the distance moved by it on each side of its middle positian= 2 x (outside lap + maximum opening of steam port). Example. — Find the travel of a valve having \ in. outside lap, maximum opening of steam port i^ in. 2x(f+l^) = 3}ins. Piston valves. — Fig. 68 illustrates the type of slide valve known as the piston valve, so called because it consists of two pistons, each work- ing in a short barrel, in which an opening extending right round the barrel acts as the steam port. The chief advantage of the piston valve is that it is in a — equilibrium, there being no pressure of the valve against the cylinder face, as with the common flat or locomotive type of valve. It is therefore much used for the high- pressure cylinder of triple expansion engines. The pistons are each fitted with spring rings. In the example given in fig. 68, which is taken from a drawing kindly sup- plied to the author by Messrs. Bow, MacLachlan & Co., of Paisley, the piston rings are of phosphor bronze. There are two rings in each groove made eccentric, and one inside the other, as shown in the section A A. The rings are prevented from catching in the ports by diagonal bars across the ports, as SecTioN atAA. Fig. 68. 96 Steam shown also in the section. The face of each piston is the same as the length of the face of the common valve, the inside and outside lap being also the same. The steam is admitted at the two ends of the valve, and exhausts into the space between the two pistons, and thence to the next cylinder. Double-ported slide valve. — For large cylinders the travel of the valve, in order to open the port to supply sufficient steam, would necessarily be large. To reduce the travel and thereby also to reduce the work to be done by the eccentric in moving the valve, the double-ported sUde valve is used as shown in fig. 69. The steam passage C of the cyUnder terminates in two ports instead of one, and the steam ports are each made one-half the width of a single port, and therefore the travel of the double-ported valve is only half that of the common valve. Fig. 69. The valve is so constructed that, when in the middle of its stroke, each of the four steam ports is covered by it, the inside and outside lap in each case being the same as with the simple valve having the same travel, and hence its action in the dis- tribution of the steam is exactly the same as with the simple valve. The arrangement is equivalent to two separate slide valves, the steam being supplied to the inner portion of the valve by steam passages in the sides of the valve. B B are the exhaust passages. In large engines with a single flat slide valve, the pressure of the steam on the back of the valve would be so excessive, unless reduced by some means, that the load thrown on the eccentrics and working parts of the valve gear, owing to the friction between the valve and cylinder faces, would be enormous. To prevent this a packing ring is often fitted to Eccentrics 97 the back of the valve, as shown in the drawing (fig. 69). The ring is fitted in a circular groove in the slide jacket cover E, and it is tightened against a planed surface on the back of the valves by set screws a a pressing against a spring or packing. The set screws permit of a careful adjustment of the ring, so as to work steam tight without being excessively tight. By this arrangement the steam pressure is removed from a large portion of the back of the valve, and the enclosed space is connected instead by a pipe with the condenser. Eccentrics Eccentrics are used when a very small to-and-fro motion is required to be derived from a revolving shaft. They are applied mostly to drive steam-engine slide valves, or pumps having a short stroke. The simplest form of eccentric is a circular solid disc called a sheave, secured to and revolving with the shaft, the centre of the disc being ' out of centre ' or ' eccentric ' with the centre of the shaft. This arrangement is equivalent to a small crank (fig 70), the length of whose arm r is the same as the distance c e between the centre of sheave and centre of shaft This length ceis called the eccen- tricity of the eccen- tric. The travel of the valve is equal to twice the ■ _qj , ■ _ eccentricity of the I 1 / T. eccentric. The \ /^^^^ / [ / sheave is sur- ^ JimioO»m. / — 1 rounded by a thin metal hoop, or bands (fig. 71), called the strap, to which the eccentric rod is attached. The ro- tation of the sheave H about the centre of the shaft is trans- mitted through the strap and rod, and results in the to-and-fro motion of the valve. The sheave may be considered as a very large crank pin, and the eccentric rod and strap as an ordinary Fig. 70. 98 Steam o connecting rod. The sheave rotates within the strap just as the crank pin rotates within the head of the connecting rod. In order to get the eccentric in its place on the shaft it is mostly necessary to make the sheave in halves. The halves are secured together by two bolts, not shown, which are passed through holes drilled in the sheave and se- cured by spUt cotters. The strap is also made in halves, each half having lugs to take the bolts which secure them together. A small oil cup L is cast solid with the strap. Fig. 71. The sheave is secured to the shaft by a key fitting in a key- way cut in the shaft and in the sheave. Reversing Gear — The Link Motion ^- ^_^ Not the least important quality possessed by the steam engine is the ease with which it lends itself to the most perfect control. For by the movement of a handle the massive engines of a steamship running at a high speed may be instantly stopped ^ ^ and as quickly reversed. The fol- ~ " lowing simple diagram will explain the principle of reversing gears. It has been shown that when the crank is in some position The Link Motion 99 OC (fig. 72), the centre line of the eccentric will be in a direc- tion O E ahead of the crank, the direction of rotation being shown by the arrow. But suppose we wished to re- verse the engine — in other words, to change the direction of rota- tion, as in fig. 73 — then, unless we have some means of shifting the eccentric from E to E', the engine will not reverse, but will only rotate one way. Fig. This difficulty is easily overcome by the link motion, which is one of the most common methods of reversing, and it is done in the fol- lowing way : Two ec- centrics are used, one having its centre at E, and the other at E', and by means of the link we have the power to use which eccentric we please, and to throw the other out of gear ; hence the engine can be made to rotate in either direc- tion with the greatest ease. Each eccentric is attached by a rod to one end of the slotted bar or link shown in fig. 74, and the link is moved transversely by the levers so as to bring the slide valve under the influence .A. I i SVR sv Fig. 74. S V, slide valve : SVR, slide valve rod ; L, link R L, reversing lever ; S H, starting handle : W S, weigh shaft ; E, eccentric ; E R, eccentric rod. H 2 lOO Steam of either eccentric as required. The slide valve is attached to a little block which fits in the slot of the link, so that any move- ment of the link in the direction of the axis of valve rod affects the position of the valve. When the block is in the middle of the link, the valve is influenced equally by both eccentrics, virith the result that the engine will not run in either direction. The nearer the block is to its mid position in the slot, the less is the travel of the valve and the earlier the steam is cut off in the cylinder. The link motion is therefore useful, not only for reversing but as an arrangement for working the steam expansively in the cylinder by varying the point of cut-off. lOI CHAPTER XII CRANKS AND CRANK SHAFTS Cranks are used to convert the reciprocating motion of the piston into circular motion. Fig. 75 shows two views of a simple overhanging crank. This crank consists of an arm with a boss at each end — one to take the main shaft, and the other the crank pin. The crank is secured firmly in its place on the shaft, either by keying alone, or by ' shrinking ' and key- ing. The shrinking is done by boring out the hole a shade smaller than the shaft, then heating the crank round the hole, and thus causing the material to expand and the hole to become larger. The crank is then slipped in its place on the shaft, and on cooling it contracts and grips the shaft tightly. Forc- ing on by hydraulic pressure is now frequently adopted in preference to shrinking on. The crank pin is shrunk in position or forced in by hydraulic pressure, and riveted over the end as shown. The radius of the crank arm is measured from the centre of the shaft to the centre of the crank pin. The throw of the crank is equal to the diameter of the crank-pin path, and to the stroke of the piston. The following is a crank axle for a locomotive with inside Fig. 73. A=crank shaft ; C=crankpin; B = web; D, D'=bosses; E=key. I02 Steam cylinders, showing the cranks at right angles. The webs are here shown strengthened by wrought-iron straps shrunk on. Fig. 76. With such a shaft the engines will start in any position ; for, if one crank is on its ' dead centre,' the other is in the best possible position for starting. Examples of crank shafts are also given in figs. 36, 105, &c, E«-SN- Tangential Pressure on the Crank Pin The tangential pressure on the crank pin is that share of the total pressure on the pin which tends to turn it about the centre of the shaft. To present this subject in the simplest form we will suppose the pressure of the steam on the piston uniform throughout the stroke, the connecting rod to be of infinite length, or, in other words, to act always parallel to the centre line of the engine, and the moving parts to be without weight. In fig. 77, ABC represents the path of the crank pin. Let P =the uniform pressure on the piston, and let O A the radius of the circle be chosen equal to P to any scale. When the Fig. 77. Pressure on Crank Pin 103 crank is in the position O A, the pressure P acts towards the centre only, and there is no tendency to turn the crank about the centre, but only to press the shaft against the bearing ; hence in this position the tangential pressure is nothing. The same is true of the position O C, and O A and O C are termed the ' dead centres.' At the position O B of the crank at right angles to the direction of the force, the whole of the force is expended in turning the pin about O, and the tangential pres- sure on the pin is therefore equal to P. Between these two points A and B the tangential pressure varies from nothing at A to a maximum zX, B, and again falls to nothing at C. To find the tangential pressure at an intermediate point, as E, the force P may be resolved into two forces, one acting towards the centre of the shaft, and one perpendicular to it, or tangential to the crank-pin path at E. Thus at E draw the line E E' parallel to A C and equal to the force P to scale, and by the parallelogram offerees resolve it into two forces, T E tangentially and R E radially. Then the line T E measures to scale the share of the force P acting at E, tending to turn the pin about the centre of the shaft, and R E measures also the share of force pressing the crank on the bearing. But when OE=EE', the perpendicular E F is equal to T E for any position of E. Therefore for any point on the crank-pin path, under the condi- tions above described, a perpendicular Jet fall from it upon the diameter A C represents the tangential pressure on the pin at that point. The diagram (fig. 78) furdier illustrates the variable nature of the turning effort on the crank pin, the amount of the turn- ing effort at points i, 2, &c, being proportional to the perpendiculars i a}, 2 cfi, &c., and varying from nothing at A, I a' at I and so on, to a maximum at B, and again gradually falling to nothing at C. Further, it will be seen that this variation of twisting stress in the crank shaft occurs twice in every revolution of the crank I04 Steam also that by increasing the pressure on the crank pin, either by increasing the area of the piston or the pressure of steam, the variation in the twisting stress is also increased in the same proportion. If a pair of engines of equal power work on to one crank shaft, and the cranks are placed at angles of o° or 180° with each other — that is, with the cranks together or exactly opposite — the twisting stresses on the shaft will be double those pro- duced by the single engine alone, also the maximum and mini- mum twisting stresses on each crank will occur atlhe same time. But if the cranks be placed at right angles with each other, then, with the same engines, the maximum stress due to one engine will occur at the same time as the minimum stress due to the other engine, so that the total maximum stress will be reduced, and there will also be a much more uniform distribution of the stresses in the shaft. This will be more clearly seen by re- ferring to the following figures. Fig. 79 is a continuous diagram of the turning effort on the crank pin for a single engine, the value of which at any point fl', a^, &c., is given by the vertical ordinate a' c', a^c^, and so on, varying from nothing at c° to a maximum a' c^ at a^. Fig. 80 shows the effect of the addition of another engine of equal power to the same shaft, when the cranks are at 0° or 180° apart. In this case the stresses are doubled throughout, varying from nothing to a' maximum a' b^, which is twice a^ ^ in fig. 79. Fig. 81 shows the effect of placing the cranks at right angles to one another, the maximum turning effort as b° (f for one engine occurring when the turning effort of the other engine is nothing. The maximum stress is therefore never so Pressure on Crank Pin lOS great as twice that due to the single engine, and the minimum stress never falls below the maximum due to one engine alone. There is, therefore, a much more regular distribution of the stress. The uniformity or otherwise of the turning effort can be more clearly seen by setting up the ordinates from a horizontal Fig. 8o. base, as in fig. 82. Thus, draw a horizontal line A C, and mark from A divisions A c, &c., ' equal and corresponding to the divisions on the semicircumferences, fig. 81. Then from these divisions (fig. 82) set up a' €=■ a' d (fig. 81), and cV (fig. 82) Fig. 81. ■=■ V n' (fig. 81), and so on, and join the free ends of the lines. Then the total breadth of the figure gives the combined turn- ing eifort on the shaft. The variation in the stress may be still more conveniently represented by constructing the whole figure above the line A C as shown by the dotted parts ; thus io6 Steam c b' is set off above a' = a' d, and so on ; and the tops of the ordinates are joined by a free curve. The nearer this curve becomes to a horizontal line, the less the variation in the twist- ing stresses. For the treatment of this subject, when account is taken of the varying pressures of the steam throughout the -<>- - Fig. 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 1 20° 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 crossheadj which start from rest. Fig. 83. Shaft Couplings 107 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 fl^g/a (Tl I I ~ J.-i-i-LI 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 Fig. 86. 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 suflSciently 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. 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 1 ' ' 1 1 r JP EP, exhaust pipe from cylinder ; C, condenser ; AP, air pump ; HW, 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 j 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, theyV^f condenser and the surface condenser — the one, as its name implies, con- X densing the steam by meaiis of z. jet of cold water, and the other by bringing the steam into contact with a cold metallic 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. The condensed steam and hw hot well; b, air-pump bucket :HV, head injection water must now be ™'™= ^^' '■°°' ^^^''^■ removed, and a pump A P is provided for the purpose. This pump is called the air puvip 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 III from the water. It is the air and vapour in the condenser which are the cause of whatever pressure exists therein. The condensed steam, injection water, air and vapour, are pumped into the hot-well H W, and thence to waste ; and from the hot-well the water is taken to feed the boiler. The suction valve of the air pump is called the foot valve, and the delivery valve is called the head valve. 1 HW =o' m ^^ Wa 1 HV rv '^QT Fig. ga Fig. 89 is a more com- plete drawing of an air pump as applied to ver- s 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 delivery 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. JBut 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 "3 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 7b Jiir R^-mfi Vt/attr from Ctj'tuUiting ^mfi Fig. 91, vertical engines ; it is also frequently made cylindrical with fiat 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 114 Steam space surrounding the tubes. The tubes are made of brass, | or f inch outside diameter, and ^V inch 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. CT 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, W F" W^^^^^^^^^ "^^^ stuffing box is packed with tape or cord packing. Fig. 93 is a wood ferrule W F made to fit the tube exactly, but a little too large for the hole. It is driven in between the tube and the hole in the tube 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 IIS 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 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 begins to indicate pressure when the pressure of the steam rises above the pres- sure of the atmosphere, this being the starting point or zero. The vacuum gauge also starts from the same zero, namely, the pressure of the atmosphere, and reckons backwards. But, further, the figures on the vacuum gauge are doubled, and to understand the reason of this it should be remembered that they represent not pounds pressure but inches of mercury by the barometer, every two inches of mercury being equivalent , lo- «) ts - t'^i 10- 5 i; a.^ S ^(0 5 - CU*HosfiA£yu: . n 9r\ ins. 4° 4° 14° 14° ih ins. 3i ins. 3« ins- si ins. I in. I in. Iixl4 ins. \\ X 17 ins. 40 per cent. 50 per cent. 129 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, ahd sometimes a third or fourth cylinder, before escaping to the condenser. The compound engine enables the fullest advantage to be taken of the expansive power of high-pressure steam ; (i) By reducing the range of temperature in any one cylinder, and thereby reducing initial condensation of the steam in the cylinder. (2) By taking advantage of the re-evaporation which accom- panies cylinder condensation. For, since the bulk of the re.- evaporation in a cylinder takes place during exhaust, it is obvious that in a single-cylinder engine the steam formed by re-evaporation during exhaust passes away to the air or the con- denser to waste without serving any useful purpose. But when the steam is exhausted into a second or third cylinder, the steam formed by re-evaporation in one cylinder is utilised in doing useful work on the pistons of the succeeding cylinders. (3) By the ease with which it may be adapted to work on to two or more cranks, thereby reducing the excessive variation of stress which occurs in a single-cylinder engine when working with steam at a high initial pressure expanded to a greatly reduced terminal pressure. 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. I30 Steam Suppose I lb. of steam at 150 lbs. pressure absolute admitted to a single cylinder and expanded down to 1 2 lbs. pressure absolute and exhausted into a condenser, when the pressure averages 3 lbs. absolute. Then the action of the steam in the single cylinder is represented by the whole figure shown cross- lined. In such a case the temperature in the cylinder would vary from 358° F., the temperature of the steam at 150 lbs. pressure down to 142° F., the temperature of the steam at 3 lbs. pres- sure ; or a difference of 358—142=216° F. between the initial ) zqr 60 ^ •.{ 2Zt' "m^^^TTA 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 1 50 lbs. and 60 lbs. pressure, there is a variation of 65° F. ; in the intermediate cylinder, working between 66 lbs. and 20 lbs. pressure, there is again a variation of 65° F. ; in the low-pressure Compound Engines 131 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 HZ=i6-3 times the terminal stress. This would be a most 9 objectionable variation of stress on the working parts, and as the engine must be made strong enough to bear the maximum stresses due to the high initial pressure acting on a large piston area, a much heavier engine would be required than if the stresses were more judiciously distributed. If now the expan- sion of the steam, the range of temperature, the initial stresses, and the total work are distributed among three cylinders con- nected with three cranks, a much more economical and mechanically perfect engine is the result. The shaded parts marked high, intermediate, low, represent the distribution of the work among three separate cylinders. The diagram further illustrates the historical growth of the steam engine, for'the bottom part of the figure represents the condition of the early engines working up to 20 lbs. pressure with a single cyhnder ; then came higher pressures, higher rates of expansion, and two-cylinder compound engines, and later, with the introduction of steel for boilers, and surface condensa- tion, we have had a rapidly increased boiler pressure and rate of expansion, and the introduction of the three-cylinder or triple expansion compound engine. Pressures are still increasing, while the terminal pressure remains constant, and a fourth cylinder is in many instances now being added, forming a quadruple expansion engine. Figs. 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 K 2 132 Steam Q. M a " o ■ • u s". EW Is ..a ^1 ^1 - S'S .Sfa Compound Engines 133 134 Steam high-pressure into the low-pressure cylinder, 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. io6. 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. 107. 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 13S 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 greater 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 Fig. 108. as for a single-cyhnder engine, to exert the required power with the given initial pressure of steam of the high-pressure cyUnder, 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. 136 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 : 8^=9 : 25 : 64=1 : 278 : 7-11. The number of expansions of the steam in any engine, whether simple or compound, = . "^ '^^""'"^ ■ and this is ap- mitial volume proximately equal to '"^ !^ pressure ^j^^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 cylinder to point of cut-off. Yox 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 137 But if the steam is cut off in the high-pressure cylinder at one third of the stroke, the number of expansions = final vol. vol. of L. P. cylinder _ 4 _ initial vol. ^(vol.ofH.P.cylinder)~^ of i~" Or, again, if the initial pressure of the steam in the high-pres- sure cylinder is 90 lbs. absolute, and the terminal pressure in the low-pressure cylinder is 10 lbs. absolute, then the number c ■ initial pressure 90 of expansions = -. — ^ =^=9. terminal 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 : ■J. Tj _vol. of L. P. cylinder_ vol. of H. P. cylinder Then the point of cut-off in the high-pressure cylinder = R _4 number of expansions 9 of the stroke. Example. — ^The ratio of the cylinder volumes of a two-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 =55=:5-^j 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 105 lbs. absolute, the number of expansions = — 5=io"5,or 138 Steam allowing for losses = 10, in which case a two-cylinder com- pound would be necessary. Suppose the engine required is to be non-condensing, then with a terminal pressure of, say, 5 lbs. above the atmosphere, or 20 lbs. absolute, and a boiler pressure at command of 80 lbs., or 95 lbs. absolute, the number of expansions = ^^=475, or 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. 139 CHAPTER XVI TYPES OF COMPOUND ENGINES Ffom SoUdf HP c\ LP To Cffndensei' mm r^ m m r^ Compound engines may be roughly divided into two classes (i) those in which the pistons of each cylinder commence the stroke simultaneously. In such engines the cranks are either at o° or i8o° apart. These engines are known as the 'Woolf type. (2) Those in which the cranks are set at various angles other than 0° or 180°, and exhaust from one cylinder before the next cyUnder 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. TAe Tandem Compound En- gine with cylinders, 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. log, H P is the high-pres- sure cylinder and L P the low-pressure. n I I I I I I I I c— Fig. Steam is conducted 140 Steam from the boiler direct to the high-pressure cylinder H. P., where it is admitted alternately at either end of the stroke, cut oifat about one-half or one-third of the stroke, expanded nearly to the end of the stroke and then exhausted into the low-pressure cylinder L P, where it further expands, acting as back pressure on the high-pressure piston, and forward pressure on the low-pres- sure piston, and is finally exhausted into the air or a condenser. The distribution of the steam in the cylinders of the tandem engine at various points in the stroke may be clearly followed by the aid of an ideal diagram. In order to simplify the diagram we will neglect the effect of clearance at the end of the cylinders, the connecting passages between the cylinders, the friction of the steam in the passages, compression, &c., and suppose the vacuum perfect. In fig. no, let the relative volumes of the high- and low- pressure cylinders be as i : 4, then make « ^ = i = volume of high -pressure cylinder, and ar= 4= volume of low-pressure cylinder. From a set off a ^ = the initial absolute pressure of steam in the high-pressure cylinder, the horizontal through a being the zero of pressures. Then, if the steam be admitted to the high-pressure cylinder for one-third of the stroke, de-=^\ 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 141 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 ^w^ represents the gradual fall of pressure as the volume of the low-pressure cylinder increases, and the curve /« h represents the decreasing back pressure on the high-pressure piston during the same period ; bf=ag ; p n=:rm ; and ck-=a k. Then defh is the theoretical indicator diagram for the high-pressure cylinder, and ag kc for 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 i'. The varying pressures and volumes throughout "the stroke in compound 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. 142 Steam Initial steam pressure=ioo lbs, per sq. in. absolute. Then, the volume of steam admitted to high-pressure cylinder =j volume of cylinder + clearance =3 of 5 + '35=2 02 cub. ft. The final volume of the steam is that contained by volume of low-pressure cylinder -f 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-|- i'2 + '35=2i'5s cub. ft. final volume Then total ratio of expansion = initial volume =££55^10.67, and the terminal pressure of steam in the low-pressure cylinder 2'02 ,, = ioo X =9"4 lbs. per sq. m. 21-55 ^^ ^ ^ We may now trace the varying pressures of the steam in passing from the high-pressure cylinder, through the receiver, to the end of the stroke of the low-pressure cylinder. Pressure at end of stroke of high-pressure cylinder 2'02 „ = ioo X - — =37-75 lbs. per sq. m. The steam is now exhausted at this pressure into the receiver. If there were no intermediate chamber between the two cylinders — the steam passing from one immediately to the other — and no clearance, then the terminal pressure of the high-pressure cylinder, namely, 3775 lbs., would be the initial pressure of the low-pressure cylinder (as in fig. 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 143 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 (5-35 X 37-75) + (2-3 X 9-4 )^ .,,6 lbs. 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-(-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. To find the pressure at any intermediate point in the stroke, say 5th ; then the volume occupied by the steam will be : | volume of high-pressure cylinder -H clearance in high-pressure cylinder + volume of receiver -I- clearance in low-pressure cylinder ■\-\ volume of low-pressure cylinder, =1 of 5 -F '35 + 2 '3 -I- 1 '2 -f J of 2o=i2-6 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 - — 1=1774 lbs. per sq. in. 1 2 '6 The pressure at the end of the stroke may also be found from the same data ; for volume of steam at end of stroke= volume of low-pressure cylinder -|- clearance of low-pressure cylinder -f volume of receiver -f- clearance of high-pressifre cylinder =2o-H-2-l-2-3-|- •35=23-85 cub. ft., and its terminal pressure =29-226 X ' ^ =9'4 lbs. per sq in., and this is the same result as we obtained before. 144 Steam 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. Fig. - Forward Stroke Exhaust Stroke Total Range 170° 119° 108° Single-Cylinder Engine Woolf Engine H. P. Cylinder Woolf Engine L.P. Cylinder 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 cylinder and meets with an internal metallic surface including cylinder cover, piston face, and steam port at a temperature of 142°. Compound Engines 145 (This assumes that there is no compression at the end of the stroke, the effect of which is to increase the temperature of the cylinder before admission.) But, further, during admission and expansion of steam in the forward stroke of the piston, the cyUnder 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 cylinder 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 tKe 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 cylinder. 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) -I- (^ X 3) is less than P X 4. Again, at the end of the stroke of the tandem engine, L 146 Stemn the pressure on the crank pin is equal to the sum of the effective pressures on the two pistons ; but in a single- cylinder 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 mean 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-cylinder 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 fig. 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. J12. Compound Engines U7 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 EC m H P Hi. J !I=0 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. - RECEIVER. a. y 11!! iillll / t Ir ^ r RECi ^^ a. ■ lull: MllUill A b- a. f III' . :lllll 0- T Fig. 114. Fig. IIS. pressure cyhnders at half stroke. Then, at the moment of ex- haust from the high-pressure cylinder, the low-pressure piston 148 Steam is only at half stroke (see fig. 1 14), and the low-pressure cylinder is therefore not yet ready to receive the steam. The slide valve of the low-pressure cylinder, fig. 1 14, covers the port for admission of steam from the receiver to the side a of the low-pressure cylinder, because it is at present connected with the condenser ; and as the cut-off in the low-pressure cylinder occurs at half stroke, the port will also be closed for admission of steam to the side b. The position of the pistons is now as shown in fig. 114. The low-pressure piston proceeds to the end of its stroke, and the high-pressure piston (fig. 115) also returns from the bottom of the cylinder against the back pressure of its exhaust steam which fills the high-pressure cylinder and receiver, thereby reducing its volume and increasing its pressure. This proceeds till the high-pressure piston reaches its half stroke, by which time the low-pressure piston has reached the end of its stroke, and its steam port opens for admission of steam from the receiver to the end a, fig. 115. The confined steam in the receiver and high-pressure cylinder now expands, driving the low-pressure piston forward, and acting as back pressure on the high-pressure piston, and forward pressure on the low-pressure piston. The initial pressure in the low-pressure cylinder (neglecting loss by friction in the passages) is equal to the pressure in the receiver when the high-pressure piston has reached the middle of its return stroke. This steam expands in the low-pressure cylinder till its piston reaches, say, half stroke, when its steam port closes. The supply of steam from the receiver being now cut off, it expands to the terminal pressure, and is exhausted into the condenser. These operations may also be readily traced from an ideal indicator diagram. Suppose the steam cut off in both the high- and low-pressure cylinders at half stroke. In fig. 116 let the relative volumes of the high- and low-pressure cylinders be as I : 3. Make ab =-\-=. volume of high-pressure cylinder, and ac =3 = volume of low-pressure cylinder. From a set o&ad = the initial absolute pressure of steam in the high-pressure cylinder, the horizontal through a being the zero of pressures. Compound Engines 149 Then, if the steam be admitted to the high-pressure cylinder for one-half the stroke, de—-\ab\s, the line of admission, 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 ^/= \ad. Communication is now opened with the receiver, and the pressure falls to g, the pressure bg depending on the volume of the receiver and on the pressure of the steam in it. But there is as yet no admission to the low-pressure cylinder till another half stroke has been made (as shown by fig. 1 14). The diagram of work done by the high-pressure piston will therefore show an increasing back-pressure curve gf as that piston returns, till it reaches half stroke, when the low- FlG. 116. pressure steam port opens and admits steam at the initial pressure ah = st. The pressure now falls by expansion of the steam behind the low-pressure piston, the terminal pressure a ;^ in the high-pressure cylinder being equal to the pressure rvi in the low-pressure cylinder at half stroke. Cut-off now takes place in the low-pressure cylinder, and the steam expands behind the piston \.ock=^\ad—^bf,2X which point it escapes to the condenser, when the pressure falls to the hue 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- I so 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 gtn, 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=a d, cut-off at half stroke=(f e, and expanded to end of stroke=e/ where the terminal pressure bf-=6o lbs. r,,, . . , . r ■ final volume ac \k , The total rate of expansion=.-^^-i — = =_=__3. = 5 • mitial volume «j 2-5 and the terminal pressure ck m low-pressure cylinder= — 6 =20 lbs. per sq. in. The steam in the low-pressure cylinder is expanded twice in that cylinder, therefore the pressure r m at half stroke=i: kxi =20 X 2 -f 40 lbs. per sq. in. ; and this is also the pressure a n in the receiver at the point of cut-off. The high-pressure cylinder exhaust now opens to the receiver, and we have two volumes of steam at different pressures, com- bining to fill the space, namely : volume of high-pressure cylinder at pressure bf, and volume of receiver at pressure a n, making a total volume of5-|-8'5 = i3'5 cub. ft. , at a resultant pressure bg ='^3 °''"'"^_ ^ — 4_;=:4^-4 lbs. per sq. in., showing a 'drop' or fall in pressure /^=6o—47'4=i2-6 lbs. per sq. in. This steam, however, cannot yet be admitted to the low- pressure cylinder because the steam port of that cylinder remains closed for another half stroke : hence the enclosed steatn is Compound Engines 151 compressed behind the high-pressure piston during one half of its exhaust stroke, as shown by the line g t. The volume of the steam has by this time been reduced to 13-5— 2-5=:ii cub. ft, and its pressures / has been increased to 47-4 x^^=s8'2 lbs. II The steam port of the low-pressure cylinder is now Opened, and steam at 58-2 lbs. initial pressure {a h) acts against the low- pressure piston. Here it is driven before the high-pressure piston and drives the low-pressure piston before it until the latter reaches half stroke, when the volume of the steam is now equal to ^ volume of low-pressure cylinder + volume of receiver =7-5 + 8 ■5= 16 cub. ft., and its pressure has fallen to 47-4 X ^^=^0 lbs. per sq. in.=r»z. The supply of steam to the low-pressure cylinder is now cut off from the receiver, and we have in that cylinder a volume of steam equal to 7-5 cub. ft., which expands to end of stroke occupying 15 cub. ft.> and having a terminal pressure of 40 X Z^=20 lbs. per sq. 'm.-=ck as obtained before. IS Communication is now opened with the condenser, and the pressure ck falls to the line of back pressure. The range of temperature in the high-pressure cylinder of the receiver compound engine with the cranks at right angles is less than in the Woolf engine. Here the total range in the high-pressure cylinder varies theoretically from the initial tem- perature of the steam to the temperature in the receiver when the low-pressure piston is in the middle of its stroke. In practice the range is not greater than that given by the difference between the initial temperatures in the two cylinders. The advantage of the distribution of the stresses between two cranks at right angles has been explained at p. 104. Triple and quadruple expansion engines — namely, those in which the steam is expanded in three or four cylinders re- spectively — are the necessary outcome of increased pressures of steam ; for, since the terminal pressure is about constant, in- creased pressures involve an increased number of expansions. And in order to prevent undue range of stress and temperature, 152 Stemn three and even four cylinders are now employed. Thus the same reasons which led to the rejection of the single-cylinder engine in favour of the two-cylinder compound, have now led to the rejection of the two-cylinder engine (at least, in marine work), and the adoption of the triple compound, and in some cases the quadruple compound, in its stead. The economy of fuel which resulted from the introduction of high-pressure steam, and the compound engine with surface condensation for steamships, was very remarkable, as may be seen from the following table : — Year. Pressure of steam by boiler gauge per sq. in. Consumption of coal per I.H.P. per hour. 1830 2 to 3 lbs. 9-0 lbs. 1840 8 » 5-5 .. 1850 14 ,, 4"o ., i860 30 „ 3'o .. 1870 40 to 50 „ 2-6 „ 1880 70 to 80 „ 2-2 „ 1886 150 to 160 „ I "5 >. 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'4lbs. 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 \ lb. 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=^]b. of coal. „ „ per day =-\ x 24lbs, of coal. Compound Engines 153 Sh'ailt Fig. ii8. Quadruple Expansion Engines 155 -!.->_- "^ Fig. 119- 1 56 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 —^ 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. 1 1 7, 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 lo^ 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 157 and the junk ring pins felt without disturbing the upper cylinders. 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. „ Feed pump . . 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. l5^ Steam 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 tofxdxl. Boilers 159 The area of the material to resist this tendency to burst along the lines a^ and bd^= (ac+bd) t= 2/x/. Hence the stress (s) per square inch on the plate may be expressed thus : load pdl _pd I ^ area 2 1 1 2t' ' ' ' 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 oif. Area of end — d^x 7854. Total pressure on end = {tPx 7854)/. The area of the material to resist this tendency (neglecting deductions for rivet holes) = circumference of shell x thickness of plate = ^x 3"i4i6 x /. Hence the stress (s) per square inch on the plate may be expressed thus : ^_ load ^^^x^78s4X/_/£ ... (2) area ^X3'i4i6x/ 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. i6o Steam Stationary Boilers The Cornish boiler. — This form of boiler was first adopted by Trevithick, the Cornish engineer, at the time of the intro- "''^^=^^^^=^. B n, ^^^^ikmmm mmmmmm^mmmmmsi:^^mim:is^^m 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 of their heat by the time they reach the bottom flue, are less liable Fig. 122. Boilers i6i Fig. 123. 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 furnace tube, and the joints made good by riveting the flanges of the water tube round the hole. Water 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, M t62 Steam Boilers 163 164 Steam Fig. 127. 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. 1 39), 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 i6s 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 water. Vertical Boilers The illustration, fig. 129, shows the construction of a vertical boiler. These boilers are used for small powers, and where ' From Tie Marine Engine, by Mr. R. Sennett (Longmans). 1 66 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 liner. 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. sfl 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 167 00000 /ooooooo JOOOOOOO looooooo 5000000 lOOOOOOo. I ioooooot fooboop \ Soooooo , loooooo '■ .-a .0 I* ofi u.. o ss •a >» 'u i3 ,13 (Q li Sh4 (J ,„ o l68 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=54sq. 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 5 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 1 69 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 X — ^—z. area of triangle a be. 360 To give the front and back plates of shell the necessary stiffness, large circular plate washers, i o 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 \\ in. diameter, and g,ooo 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 FB, 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 I/O Steam Boilers 171 ^;^ o. .0, 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 tubes. — 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. For the smaller the diameter of the tubes used to fill a given sec- tional area, the greater the area of heating surface obtained. 1/2 Steam Thus, take a circle i in. in diameter, fig. 135, then its cir- cumference=i x 3"i4i6 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 -^ in. diameter, be ranged along the same dia- meter, the sum of their diameters being i in., the sum of their circumferences is ( ^gXyi^iG) 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^ to 3I ins. outside diameter. The student will note that the heating surface is measured from the outside diameter of the tube. Safety Valves The safety valve provides for the safety of boilers 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 will not permit the pressure Safety Valves 173 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 Ak — ^B L 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 = pressure of steam per sq. in. a = area of valve. V = weight of valve. 1 74 Steam (i) 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 A^ .^ Pff AC AC W = ^'^^AB (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. ^><^ + -^i^ + V == P. Example. — Let it be required to find weight W at end of lever when AB = 36 ins., AC = 4i ins., zy = 5 lbs., V = 2 lbs., P = ioo lbs., and a = S sq. ins. From (i) omitting weight of valve and lever we have ■W = (looxS)x4i.--36 W = 62ilbs. From (2) including weight of valve and lever 8 W + 20 + 2 = 500 W = 593 lbs. These results show that, if a weight of 62^ lbs. was placed on the lever instead of the proper weight, 59J lbs., the valve would not blow off at 100 lbs. as required. A suitable length of lever A B for a given weight W may be obtained from the same equations. At least two valves are fitted to each boiler. The valve seating may be either flat or coned to 45°. A bearing surface on the edge of the seating, -^ in. in width, is found to be quite sufficient, and to answer better than a wider surface. Safety Valves 175 Spring-loaded Safety Valve for Locomotive The following diagram, fig. 137, illustrates what is known as Ramsbottom's safety valve. It consists of two sepa- rate valves and seatings A A, having one lever, B, bearing on the two valves, and loaded by a spring D, the spring being placed between the valves. The tension on the spring can be adjusted by the nut E. By pulling or raising the lever B, the driver can re- lieve the pressure from either F'°- '37- valve separately, and ascertain that it is not sticking on the seating. The Dead-weight Safety Valve Fig. 138 illustrates a dead- weight safety valve as used for stationary boilers. The valve a rests on the seating b, which is fixed on the top of a long pipe, as shown. The valve is secured to a large casting A, which fits down over the pipe like a cap. This casting is provided with a ledge on which circular rings of metal, which act as weights, may rest. To find the dead weight re- quired (including casting and weights) for a valve of given area : Multiply the area of the valve by the pressure per square inch at which the valve is required to lift. Thus a valve 3 ins. diameter 176 Steam to blow off at 100 lbs. pressure requires the following dead weights : area X pressure per sq. in. = 3 x 3 x 7854 x 100 = 706-86 lbs. dead weight. Steam Regulating Valves The stop valve is used to open or close the communication between the boiler and engine. A common form of valve is Fig. 139. Fig. 140. shown in fig. 139. It consists of a ralve which may be opened or closed by means of a screwed spindle which is turned by a hand wheel. Steam Regulating Valves 177 The gridiron valve (fig. 140) is an arrangement for giving a large opening to the passage of steam with a comparatively small travel of the valve. It consists of a flat valve composed of a number of bars which move on a seating, having a number of ports or openings which are covered by the valve, as shown in the figure. The valve V is opened or closed by the screw turned by a handle at H. The equilibrium double-beat valve (fig. 141) consists of two disc-valves, A and B, on one spindle, each of which has its own seating. The arrows show the direc- tion of the steam on entering the valve box from the passage E. The valve B is made a little larger than A, to enable the valve A to be put in its place from the ^'°' '"*'■ top. The pressure acts on the top of one valve and on the bottom of the other, hence the two valves are nearly in equi- librium and may be easily lifted from their seating when under pressure. This arrangement provides a large opening to steam with valves of comparatively small diameter. Bourdon's Pressure Gauge The pressure gauge registers the pressure of steam ^ri the boiler above the pressure of the atmosphere. The following figure (fig. 142) illustrates the construction of Bourdon's gauge, which is the one commonly used. The gauge consists of a curved tube, B B, of a flattened or elliptical cross- section, as shown enlarged at C. The tube is closed at one end and open at the other, by which the interior of the tube is put in communication with the boiler pressure through the cock A. The closed end of the tube is attached (as shown in the figure) to a sector D, provided with teeth which gear with those of a small pinion on the same axis as the finger E F on the face. The effect of steam pressure in the N 178 Steam curved tube is that the tube tends to straighten itself, and thus, as the pressure increases, the closed end moves the sector, which acts on the finger and indicates the pressure. These gauges are carefully graduated by comparing their indications with those of a mercurial gauge. The efficiency of the boiler is a fraction, and is estimated thus : ^~. . _ water evaporated per lb. of fuel theoretical evaporative power of fuel Thus a good stationary boiler evaporates lo lbs. of water per lb. Fig. 142. of fuel ; but the theoretical evaporative power of the fuel is estimated at 14 lbs. Therefore efficiency of boiler = — = 714, or 71 "4 per cent. 14 The pounds of water evaporated per pound of fuel varies with different types of boilers, as will be seen from the following approximate results .- lbs. of water evaporated per lb. of coal. Lancashire boilers, with water tubes . 1 1 Locomotive boilers .... 10 Marine boilers 8 Boiler Efficiency 179 lbs. of water evaporated per lb. of coal. Cornish boilers 8 Torpedo-boat locomotive boiler . . 7-5 From the above we see that the evaporative efficiency of the Lancashire water-tube bailer is high, while that of the tor- pedo-boat locomotive type of boiler is low. But it will be noticed that nothing is here said as to the time taken— which, in practice, is a highly important point. It is more important, for example, that a torpedo-boat boiler should "be small, and yet capable of rapidly generating large quantities of steam, than that it should be economical. It is largely so also with the loco- motive boiler. Such boilers, therefore, require (i) to be strong enough to carry steam at the highest pressures ; (2) to have large and efficient heating surface ; (3) to maintain a vigorous combustion. The strongest form of boiler is the cylindrical form of small diameter. The requisite heating surface of the locomotive type of "boiler is provided by the large number of tubes of small diameter ; and the vigorous combustion is obtained by the ex- haust steam (or ' blast ') passing into the chimney, which acts as a pump, creating a vacuum in the chimney and drawing the air through the fire-bars, thus providing a very strong draught, causing intense combustion in the furnace, and at the same time drawing the flame and hot gases through the tubes, and bringing them into contact with the large heating surface here provided. The same effect is produced in torpedo-boat boilers, and in modern man-of-war boilers, by means of forced draught sup- plied by a fan, worked by the main machinery or by a separate small engine. In this way the combustion is much more per- fect, the temperature of the furnace is greatly increased, and the escape of unburnt gas and smoke prevented ; the wear and tear, however, is greatly increased The efficiency of boilers with respect to the rate of evapora- tion may be seen from the following table, taken from Hutton's ' Practical Engineer's Handbook ' : N 2 i8o Steam Description of boiler. lbs. of water evaporated per sq. ft. of heating surface per hour. Vertical boiler, with cross tubes . . 2-20 lbs. Vertical tubular boiler . • 2-25 „ Cornish boiler • 2-30 „ Lancashire boiler • 2-50 „ Marine boiler . . . . . S'o° >. ,, with forced draught . 13 to 15 lbs Locomotive boilers . 8 to 9 lbs. Torpedo-boat boilers, locomotive type (forced draught) . . 18 lbs. I8l CHAPTER XVIII PRACTICAL NOTES ON THE 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 cyhnders should be opened to allow the steam to flow through, and the condensed steam to pass away. This will prevent the possi- bility 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 cylinder 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 tie slide-valve oif 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 82 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 sUce-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 183 (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 shde 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 valve. 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 are all clean and bright ; remove fatty substances which have accumulated from lubrication. Turn engines round and test lead of valve. {2) Take off" cylinder 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. 1 84 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 the 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 185 (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 I. Area of rectangle 2. Area of triangle 3. Diameter of circle 4. Circumference of circle 5. Area of circle . 6. Area of sector of circle = length X breadth. = base X \ perpendicular height. = radius x 2. = diameter X 3'i4i6 = diam. x diam. x '7854 _ area of circle x No. of degrees in arc. 7. Area of surface of cylinder = circumference x length + area of two ends. 8. To find diameter of circle having given area : Divide the area by •7854, and extract the square root. 9. To find the volume of a cylinder : Multiply the area of the section in square inches by the length in inches = the volume in cubic inches. Cubic inches divided by 1728 = volume in cubic feet. QUESTIONS AND EXERCISES 1. Explain the nature of the phenomenon which we call ' heat.' 2. Distinguish between ' temperature' and 'quantity of heat.' 3. Convert s", 14°, 41°, 68°, 158°, 266° Fahrenheit to Centigrade ; and 1°, - 30°, - 25°, 90°, 120° Centigrade to Fahrenheit. 4. A Fahrenheit thermometer rises through 45° ; how many degrees would this rise indicate on the Centigrade thermometer ? 5. What is meant by the ' specific heat ' of a substance ? One ounce Questions 1 87 of copper at 212° F. is immersed in i lb. of water 55° F. ; find the increase of temperature of the water. 6. Express the following temperatures in degrees absolute : 60° F., 100° F., 247° F., and 0° C, 100° C. 7. Convert 338° F. and 165° C. into degrees Reaumur. 1. Define the ' unit of heat,' the ' unit of work,' ' horse-power,' and the 'mechanical equivalent of heat.' 2. Find the units of heat required to raise 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 oxygen 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 carbon burnt to carbonic oxide, and carbon burnt to carbonic acid gas ? 6. What horse-power should be obtained by burning 1 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 per 24 hours. I. Give what practical illustrations you can of the expansion of metals by heat. 1 88 Steam 2. What is • the law of Charles' ? 3. A gas occupies <,-ty 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 pistoii 2 sq. ft. in area through a height of 5 ft. against atmospheric pressure ? and represent this by an area. 3. What is the total number of units of heat required to convert water at 32° F. into steam at 212° ? and explain how these units have been expended. 4. In what way is the ' latent ' heat expended during the formation of steam ? 5. How is *he 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° 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 ? !0. 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 115 lbs. is 338° F- 12. A cylinder is 16 ins. diameter, and stroke of piston 2 ft. ; find the Questions 189 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^ ft. ; what is the_ capacity of the cylinder ? How many lbs. of water must be evaporated in order to fill such a cylinder with steam at an actual pressure of 15 lbs., it being given that steam at 15 lbs. pressure occupies a space equal to 1,670 times that of the water from which it is generated ? (Sc. &A. 1871.) [Note. — I lb. of water ='0i6 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° F. be mixed with 2'S 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 2{2° F., so that the resulting temperature may be 120° F. ? 7. How much water at 60° F. will be necessary to condense i lb. of steam at 212°, so that the resulting temperature shall be 120° F. ? 8. Find the temperature of the mixture when 17 '63 lbs. of condensing water at 60° F. are used per lb. of steam at 212°. VIII r. 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 I90 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 fl. respec- tively ? Give your answer in pressures above the atmosphere. 6. Steam is admitted into a cylinder at a pressure of i$ 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 S lbs. below that of the atmosphere. At what point of stroke was it cut off? Make a diagram, showing approximately the steam pressure on the piston throughout the stroke. . (Sc. & A. E. 1885.) 7. Draw the theoretical indicator diagram when steam at 75 lbs. boiler pressure is cut off and expanded to three times its initial volume, first by calculation, and then by the graphical method. 8. In a cylinder having a piston with a 4-ft. stroke, steam at 75 lbs. absolute pressure is cut off at two-fifths of the stroke ; find the pressure of the steam at the second, third, and fourth foot of the stroke (neglecting the effect of clearance). 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 ; (i) 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-opS; 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 Cylinder 16 ins. Questions igi 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 2J ft. ; what is the horse-power when the crank shaft makes 30 revolutions per minute? (Sc. & A. 1883.) 16. An engine is required to indicate 50 horse-power, with a mean effective pressure on piston of 35 lbs. per sq. in. ; length of stroke 2 ft. ; number of revtjlutions 60 ; find the diameter of the cylinder. 17. Compare the economical effect of using steam at 80 lbs. absolute, and steam at 40 lbs. absolute in a single-cylinder condensing engine. Back pressure 3 lbs., and terminal pressure 10 lbs. in each case. 18. Suppose steam at 60 lbs. boiler pressure is used to drive two engines, one a non-condensing engine, and the other a condensing engine. In the non-condensing engine the steam is cut off at 5 of the stroke, back pressure 18 lbs. absolute ; and in the condensing engine at ^ of the stroke, back pressure 3 lbs. absolute. The cylinders of both engines are the same size. Draw the theoretical indicator diagrams and compare the relative work done, and weight of steam used in the two cases. 19. Explain in what way the amount of back pressure limits the number of useful expansions of the steam in the cylinder. 20. What is ' clearance ' ? and explain its effect on the work done by the steam in the cylinder. 21. How may the loss by clearance be modified ? 22. What is ' priming ' ? 23. Explain in what way 'cylinder condensation' limits the useful range of expansion of the steam on the Cylinder. 192 Steam 24. What are the remedies adopted to reduce the amount of condensa- tion of the steam in the cylinder ? 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 3 1 "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 having 24 ins. stroke, with 7-ft. driving wheels when running at 40 miles per hour ? (Sc. &A. 1880.) 12. What is the piston displacement per minute in an engine with 20 ins. diameter cylinder, 2 ft. 6 in. stroke, running at 60 revolutions ? 13. What is the nature of the stress on the screwed end of the piston- rod during the to-and-fro motion of the piston ? 14. Sketch some form of engine crosshead. 1 5. 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°, when the length of the connecting rod is i^ times the stroke of the piston. Mark also upon the piston path, the piston positions for the same crank angles if the connecting rod were infinitely long. 20. In an engine with a cylinder 24 ins. diameter and 3 ft. stroke, the mean pressure of the steam on the piston is 45 lbs. per sq. in. ; find the mean pressure on the crank pin in the direction of its motion. Questions ig? 21. The crank of 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.) XI J. 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 I J in. ; the lap of the valve j^ in., and the lead ^ in. Draw a diagram giving the travel of valve and angular advance of the eccentric. 4. What is the effect of outside and inside lap of the valve ? 5. Describe how you would set a slide valve. 6. Find the travel of a valve having' | 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 ^ in. Sketch also the same valve at the beginning of the piston stroke with \ 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. 1 1. What is the object of the link motion ? and explain how that object is accomplished. 12. Make a sketch showing the approximately relative position of •crank and eccentrics in a link motion. 1. Sketch a locomotive crank axle. 2. What is meant by ' the tangential pressure on the crank pin ' ? and Tiow 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 ? O 1 94 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 strolie 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 fin. 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. 1 876. ) XIV 1. Sketch a Watt governor, and explain the object of the governor. 2. Say whether the governor fulfils its purpose perfectly, and give your reasons for your answer. 3. Describe the construction of the ' Porter ' governor, and sketch an arrangement showing how it may be made to act upon an expansion valve. 4. What is the object of the fly-wheel ? 5. Make a sectional sketch of a locomotive showing the arrangement of the engine underneath the boiler. I. 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 igc, 2. Make a hand-sketch of the cylinders of the compound engine in figs. I OS 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 cylinder of a two- cylinder compound condensing engine, when the volumes of the cylinders are as I to 3^; initial pressure 96 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. Make a skeleton sketch of a compound tandem engine. 2. Draw a theoretical indicator diagram illustrating the distribution of the steam in the Woolf engine, assuming a cut-off at ^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 _eceiver engine, and illustrate your answer by drawing the theoretical dia- gram for a cut-off at half stroke in both cylinders. 7. Write what you know of the improvement in coal consumption which has taken place with the introduction of the various types of com- pound engines. 8. How do you account for this improvement ? 1. Compare the resistance of cylindrical vessels to internal pressure, longitudinally and transversely. 2. Make a sketch showing how 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 1 marine boiler, and explain how the boiler is stayed. ig6 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. {b) 3 Combustion chambers : Top plates 2 (3 ft. 6 ins. x 2 ft. 3 ins.) I (3 ft. o in. X 2 ft 3 ins.) Back plates 2 (3 ft. 6 ins. x 3 ft. 6 ins. ) 1 (3 ft. o in. X 5 ft. o in.) Side plates 4 (2 ft. 3 ins. x 3 ft. 6 ins. ) 2 (2 ft. 3 ins. X S ft. o in.) {c) 3 Back tube plates : 2 (3 ft. 6 ins. X 3 ft. o in.) 1 (3 ft. o in. X 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. 9. 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. 1 1. A valve, 3 ins. diameter, is held down by a lever and weight, length of the lever being 10 ins., and the valve spindle being 3 ins. from the fulcrum. You are to disregard the weight of the lever, and to find the pressure per square inch, which will lift the valve when the weight hung at the end of the lever is 25 lbs. (Sc. & A. 1881.) 12. Sketch a Ramsbottom safety valve. 13. Find the dead weight required for a valve 3| ins. diameter required to blow off' at 90 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. XVIII 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 197 3. what points should be attended to by a man in charge of a boiler ? 4. How would you test (a) for a leaky slide valve ; (i) for a leaky piston ? 5. How would you adjust the brasses to their jotimal, after the journal had worked loose by wear ? ANSWERS I (3) -15°. -10°. S°. 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 Oi heat ; or 121,204 units of work. (3) 40 units of heat ; 30,880 units of work. (4) 137^ units of heat ; 106,150 units of work. {5) ll6o-6. IV (4) 14 -5 lbs. (5) 10,100 units. (6) 5 H.P. (7) ii2'S tons. V (3) 7-S cub. ft. (4) 36-47 lbs. (5) n-34 cub. ft. VI (2) 21,168 ft. lbs. (7) 63,072 ft. lbs. (8) 31 •39- (II) 1183-4 and 878-4. (12) 201 sq. ins. ; 4,824 cub. ins. (13) 197-024 sq. ins. ; 4728-376 cub. ins. (14) 54-53 tons. (15) 15 ins. (16) 224 cub. ft. ; 8-38 lbs. VII (5) 206-66° F. (6) I -S3 lbs. (7) 17-63 lbs. (8)i20°F. VIII (2) 30 and 15. (3) 25 and 5. (4) 18 lbs. absolute. (5) 35. 18-33. 10. (6) \. (8) 60, 40, 30. IX (5) 37-4- (7) 37-4- (9) 5i-i6- (1°) J97-4. (II) 50-66. (12) 76. (13) 80. (14) 14a. (15) 407-27. (16) 16 ins. 1 98 Steam (6) 26-47 cut), ft. (7) -83 lbs. (8) 8466 lbs. (II) 640 ft. per min. (12) 654'l6 cub. ft. per min. (20) 26703-6 lbs. (6) 4I ins. XI XIII (8) 282-7 cub ins. (9) 904-2 lbs. (10) 900,000 lbs. (II) 18-85 and 34-46 sq. ins. (12) J inch. (13) 4,403 sq. ft. ; 6 to I. XV (4) 8. (S) ^. XVII (5) 108 sq. ft. (6) 1207-27 sq. ft. (11) 11-9 lbs. per sq. in. (13) 865-89 lbs. (16) -785. SCIENCE AND ART DEPARTMENT EXAMINATION PAPER— \i&^ 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. (15.) 2. Describe, with sketches, the alterations made by Watt in order to convert a single-acting into a double-acting engine. (15.) 3. What 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 off ? The pressure of the atmosphere may be taken at 15 lbs. (15.) Questions igg 5. Describe, with sketches, the construction of a horizontal direct acting engine, working with high-pressure steam and withput condensation, showing how the steam is admitted into the cylinder and let out again as required. (20.) 6. 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. (I5-) 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. (i5-) 8. Marine engines are fitted vith 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 that 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. ( 1 5- ) 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 axis of rotation? (15.) 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. (15.) 14. Sketch a longitudinal sectional elevation through the cylinder and condenser of a trunk engine, showing the air-pump and valves. How are the piston and trunk kept steam-tight ? (20. ) 200 INDEX ABS Absolute temperature, 6 Air pump, I lo — vessel, Ii8 Back pressure, S3 Boilers, 158 — stationary, 160 — Lancashire, 161 — vertical, 165 — marine, 166 — locomotive, 169 — efficiency of, 176 Boiling, 23 — point, 3 Bourdon's gauge, 177 Boyle's Law, 42 Brasses, to adjust, 184 Capacity of pumps, 118 Centigrade thermometer, 3 Charles, law of, 31 Clearance, 63 Combustion, 15 — heat of, 1 7 — chamber, 166 Compound engines, 129 — types of, 139 Condensation, 25 Condensers, jet, 109 — surface, 112 — tubes, 114 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, 161 Double-beat valve, 177 — ported valve, 96 Eccentric, 97 Energy, 9 Engines, non-condensing, 69 — ' compound, 129 — locomotive, 125 — receiver, 146 — Woolf, 139 — management of, 181 Equilibrium valve, 177 Evaporation, rate of, 179 Evaporative power of fuel, 18 Expansion, economy of, 52 -limit of, 61 - of solids, 20 - of gases, 21 - of steam, 48 Index 20 1 FAH Fahrenheit thermometer, 3 Formation of steam, 28 Freezing point, 3 Fuel, evaporative powrer of, 18 STE Mean pressure, 55 Mechanical equivalent of heat, 10 Mensuration, 186 Mixtures, temperature of, 40 Gauge glass, 164 Gland, 73 Governors, 119 Grate area, 168 Guides, 80 Gusset stays, 164 Heat, i — latent, 36 — mechanical equivalent, 10 — sensible, 36 — total, 36 — transfer of, 12 — unit of, 7 Heating surface, 168, 171 Horse-power, 9 — indicated, 57 Hyperbolic curve, 44 Indicated horse-power, 57 Jacket, steam, 68, 74 Joule's experiment, 10 Journals, 108 Junk ring, 77 Pedestals, 108 Pistons, 76 Piston displacement, 79 — rods, 80 — speed, 79 — leaky, 187 -— valve, 95 Porter governor, 123 Pressure, absolute, 22 — of the air, 22 — gauge, 177 — mean, 55 Priming, 65 Pumps, 116 — capacity of, 118 Quantity of heat, 3 Quadruple expansion engines, 1 54 Questions, 186 Radiation, 12 Range of temperature, 130, 144 Reaumur thermometer, 3 Receiver engines, 146 Reversing gear, 98 Rotary engines, 89 Lancashire boiler, 161 Lap, effect of, 94 — inside and outside, 92 Law of Boyle, 42 — Charles, 21 Lead, 92 Leaky piston, to test, 183 — valve, — 183 Lever safety valve, 173 Link motion, 98 Locomotive, the, 125 — boiler, 169 Marine boiler, 166 Safety valve, 172 — lever, 173 — spring, 17s — dead weight, 175 Saturated steam, 38 Shafts, crank, loi Shaft couplings, 107 Shrinking on, loi Slide valve, 89 — - to set, 94 — double ported, 96 Specific heat, 5 Spring rings, 76 Steam, expansion of, 48 — formation of, 28 202 Steam STE Steam, heat rejected by, 34 — properties of, 39 — regulating valve, 176 — saturated, 38 — weight of, 39, 75 — work done by, 29 — volume of, 39, 75 Stop valve, 176 Strap, gib, and cotter, 84 Stuffing box, 73 Table of specific heats, 5 — heat of combustion, 17 — steam properties, 39 Tangential pressure, 102 Temperature, 3 — absolute, 6 — of mixtures, 40 — range of, 130, 144 Thermometers, 3 — compared, 4 Triple expansion engines, 1 53 Tubes, boiler, 168 — condenser, 114 Turning effort, 102 WOR Unit of heat, 7 — of work, 8 Vacuum, 25 — gauge, IIS Valve, double-beat, 177 ■ — double ported, 96 — gridiron, 177 — slide, 89 — stop, 176 — lap and lead of, 92 — lift of, 117 — piston, 95 — regulating, 1 76 Vertical boiler, 165 Volume of steam, 39, 75 Water, condensing, 40 — weight of, 118 • — tubes, 161 Watt governor, 119 Weight of steam, 39, 75 Wire-drawing, 73 Woolf engines, 139 Work, unit of, S PRINTED BY SPOTTISWOODE AMD CO., NEW-STREET SQUAiX LONDON BY THE SAME AUTHOR. Published in One Volume complete, royal quarto, text and plates, bound in cloth, gilt lettered, 25^. ; or in Five Parts, paper covers, at 4J. bd. per Part ; or in sets, mounted and varnished, letter-press bound separately, £\. Y]S. bd. MACHINE DRAWING & DESIGN ENGINEERING STUDENTS AND PRACTICAL ENGINEERS: BEING A COMPLETE COURSE OF INSTRUCTIOiST IN MECHANICAL DRAWING, WITH EXERCISES ON THE APPLICATION OF PRINCIPLES TO ENGINE AND MACHINE DESIGN. ILLUSTRATED BY 55 PLATES AND NUMEROUS EXPLANATORY ENGRAVINGS. jfrom 'ITbe Engineer,' Bugust 2, 1889. ■ NuMEROtJS as are the books for instruction in machine drawing and design, we do not know of one which, preceding this, has anticipated it. The system upon which the author proceeds is to imitate the sequence of operations which would be followed by a competent draughtsman in his work in the drawing office. With a given subject to be dealt with, the type of design for the article being chosen, the calculations necessary to determine dimen- sions and strength are made, and the design finished accordingly. The examples chosen are all of the actual working drawing type representing modern practice, so that the student learns, not only the application of usua. calculations in designing, but a knowledge of the proper form of parts and complete engines, boilers, and lathes, as made in practice. He learns nothing that will have to be unlearned. The set of exercises on the steam engine, for instance, is arranged so as to encourage the student to make working drawings of details to as large a scale as possible, and afterwards, from his own draw- ings, to build up and complete the general drawing. This system Mr. Ripper who is the professor of mechanical engineering in the Technical School, Sheffield, found to work very successfully, the students becoming really in- terested in what is very like making parts and putting together a real thing. The same system is followed out with the lathe. The student is led to make those calculations which are necessary to enable him to do the thing in hand ; he learns to make a calculation for its immediate practical application, and to find out the reasons for things instead of merely drawing things from a copy. Drawing is thus made a really valuable training. The book is illustrated with a large number of excellent plates, but besides these a large number of explanatory sketches are given in the text. Besides the subjects we have men- tioned, there are many others, including instructions in drawing, drawing . instruments, sketching, &c., calculations of stress and strain, strength of materials, as far as necessary to the proper understanding of the work referred to. Examples are given of the methods of setting out spur, bevel, helical, and other kinds of gearing, propellers, &c. Ihe plates are well drawn, some are coloured, and the whole of the work of the book does credit to author and printer. We can strongly recommend it.' Post free from the Author, W. RIPPER, Sheffield.