The Evolution of the Stationary Steam Engine

~apers.1 ROBERTSON ON THE STATIONARY STEAM-ENGINE. 353

(Paper No. 3188.) ‘‘ The Evolution of the Stationary Steam-Engine.” By ANDREWROBERT ROBERTSON. INthe time of Newcomen the steam-engine was used only as a means of driving a pump, as the natural outcome of the older idea of elevating water by displacing it with steam. Savery’s engine, or pump,worked on the latter principle, but the limits of the conditions under which it would act were so quickly reached, that Newcomen’s engine soon after its advent almost entirely replaced it ; but although at that time unable to do the work demanded of it, modern methods and knowledge have enabled Savery’s engine to reappear in the many forms of Pulsometer pumps. Early engines were called “ fire-engines,” and are now classed as“atmospheric,” the steam having been used not as a direct means of driving, but only to displace t,he air in the cylinder, which, in condensing, produced avacuum on one side of the piston and allowed the pressure of the atmosphere to act on the other side. Since the daysof Newcomen the name most intimately connected with the steam-engine is that of James Watt, and in his hands it assumed a shape and embodied principles which later engineers have not alteredbut only added toand improved. To show, therefore, that the main features and principle of the engine as laid down by Watt and his contemporaries differ only in detail from those employed by successive experimentersand designers will be the mainobject of this Paper. Watt began his study of the principles of Newcomen’s engine by investigating the quantity of water necessary to condense a givenamount of steam, withthe object of determiningthe temperature corresponding to varioussteam pressures. These experiments soon showed him that the Newcomen engine required more water to effect condensation than the volume of steam in the cylinder warranted. This apparent anomaly gave Watt his clue. Why should the engine inpractice demand more condensing water than was necessary to condense the same volume of steam experi- [THIE INST. C.E. VOL. CXXXVIII.] 2A Downloaded by [ Columbia University] on [15/09/16]. Copyright © ICE Publishing, all rights reserved. 354 ROBERTSON ON THE STATIONARY STEAM-ENUINE. [Selected mentally?The only answer that reasoning gave was, thatin some way more steam than the content of the cylinder was being used at everystroke. Further consideration pointed to the fact that when thesteam was admitted into the cylindera considerable amount was condensed by the walls cooled by the condensation of the previousstroke. The separate condenser was thus suggested as a means of preventing this excessive cooling and reheating of the cylinder, and Watt wasled to announce his leading principle, viz., that at allcosts the temperature of the cylinder mustbe kept as high as that of the entering steam. The success attendingthe first experimentsgave Watt such encouragement that he seriously took up the improvement of the engine, andthis led to the following alterations and improve- mentsthat made him famous. Theseparate condenser; the use of air-pumps;the steam-jacket ; the expansive use of steam; governing the speed of the engine ; guiding the piston-rod ; improvement of cylinder details ; the, mannerof converting reciprocating into rotarymotion ; the admission of steam to both sides of piston ; the steam-engine indicator ; the engine-counter and the pressure-gauge. The separate condenser led to the use of the air-pump, as otherwise the vacuumformed by thecondensation of the steam gradually began to fail owing to the accumulation within the condenser of the gases: brought in with the steam, and although Watt knew how to make a Torricellian condenser the height necessary was not available, and the air-pump was the natural alternative. The steam-jacket was part of the first principlementioned ; the expansive use of steam, however, was the outcome of a separate set of observations though brought about by noticing the power given up by steam expanding into the separatecondenser. The governing of the speed came as a necessary adjunct of the automatic working of the engine which now did not require that every stroke should be watched. The parallel motion replaced Newcomen’s chain and sector, and must beregarded as a very beautiful means of overcoming the difficulties caused at that time by imperfect workmanship which rendered theslipper guides now usedalmost impossible. The cylinderdetails wereimproved inworkmanship and design, stuffig-glandsand properly madepistons were used, and ultimately correctly bored cylinders became a possibility. Theconverting of the reciprocatorymovement of thepump into a rotary one required for driving a fly-wheel was most simply performed bythe crank and connecting-rod ; butthis ancient

device was patentable at the time, and Watt wasforestalled, hence his invention of the sun-and-planet gear. The admission of steam to both sides of the piston completed the making of the steam-engine type, and the recording instruments, theindicator, the pressure-gauge, andthe revolution- counter, enabled Watt to determine if his designs developed the results anticipated. Watt’s work was essentially practical in the true sense of the word, in that he combined a thorough study of what theory he could master with his experimenting-in fact, his philosophy was absolutelyexperimental. In all work, however, theresults obtainedare apt tomake theoriginator pay more attentionto engineering details rather than to pure philosophy, and as a result the precise study of abstract principle is generally leftto theorists who make a life-work of such subjects. In Watt’s case there was a commercial end in view; and while, therefore, the engine owes most of its details to Watt, it owes much of its modern efficacy to those who, making thermo-dynamics their special study, put Watt’s discoveries into scientific form, laying down certainrules and principles which all heat-engines should follow. Watt himself enjoyed the friendship and help of Dr. Black, of Glasgow ; but it is to Sadi Carnot that the honour must be given of originating the study now called Thermo-dynamics, which has been inseparable ever since from the proper understanding of the steam-engine, all problems of the modern heat-engine of whatever form being discussed withdue consideration of bothThermo- dynamic theory and practice ; the abstract theory being limited bythe possibilities obtainablein practice, andif the word is properly used including these limitations as part of the theory. Since Carnot’s time the theory has been more clearly elucidated, and the limitations of practice not only more clearly defined, but diminished, and the two brought into better relation, and therefore the theory is now placed in due and proper relation to the expediency of manufacture and the use to which the engine is to be put. Havingstated the principle of theseparate condenser and worked in accordance with it, Watt obtained a result considered satisfactory ; but his suocessors, aided by the theorist, havebecome more familiar with the principles and apply them further. The effect of the cold cylinder is now calleci initial condensation, and after Watt’s time it came to be recognized that lin removing the condenser from the cylinder all thecauses of initial condensation did notdisappear, and means of decreasing them were sought. 2A2

Downloaded by [ Columbia University] on [15/09/16]. Copyright © ICE Publishing, all rights reserved. 356 ROBERTSON ON THE STATIONARY STEAM-ENGINE. [Selected The steam’enters the cylinder ata pressure above that at whichit leaves it, and therefore the incoming steam meets a cylinder cooled bythe exhaust. Consequently modern engineers, intrying to make the difference of temperatures of entrance and exit as small as possible, compound the engine. Thisaction may fairly be described as a furthercarrying out of theprinciple of Watt’s separate condenser, and it may be accurately said that en,‘Vlneers have simply moved the condenser further from the engine. Com- pounding is aiso intimately connected with theuse of high-pressure steam, as the long ranges of expansion which would otherwise be necessary render it especiallyadvantageous. Thatthe efficiency of the engine is raised by the use of high-pressure steam is clear when Carnot’s statement for the efficiency of a heat-engine is considered. This shows that the efficiency of a perfect heat-engine dependswholly on the difference between thetemperature of entrance and exit of the working fluid, andthough the steam- engine is not a perfect “ heat-engine,’’ it belongs to that class and is governed by the same thermo-dynamic laws. The value of Carnot’s statement of efficiency, Temperature of entrance - temperature of exit __ ~~~~ ~ ~ Temperature of entrance increases with the temperature of entrance, so that the higher the pressure the greater the efficiency, and modern practice therefore tendstowaras the highest pressuresconsistent with safetyand durability. High-pressure engines (so called) are mentioned by Watt in his patents, but he did not employ them. It was for Trevithick, his competitor, to be the pioneer of high-pressure steam, and next to Watt he must be considered as having exercised, the strongest influence on the later history of the engine. From a study of his life it would appear that he lived before his time with greatideals which,hampered by circumstances,he had perforce to leaveto others to perfect. Watt, content to make his engine as perfect as circumstances would allow him,outlined possibilities he considered beyond the power of his generation. Trevithick, on the contrary, went boldly at apparently insurmountableobstacles, and produced enginesthat would now be considered criminally unsafe. He drove his engines with high-pressuresteam generated in thecrude, leaky boilers of that day, at what must then have been considered a high speed. Nevertheless, the combination of Watt’s engine withTrevithick’s use of high pressurecompleted thetypical engine. Watt and Trevithick found it an 6‘ atmospheric engine,”

Downloaded by [ Columbia University] on [15/09/16]. Copyright © ICE Publishing, all rights reserved. Papers.] ROBEBTSON ON THE STATIONARY STEAN-ENGINE. 357 made it into a singleacting steam-pressure engine, and finally left it in the form of a double-acting high-pressure steam-engine. The steam-jacketwas a natural outcome of Watt’s statement that the cylinder should be kept as hot as possible ; but though one of his inventions, he made less use of it than did Trevithick, who seems tohave used it as few engineers do now. In his characteristic style he did not employ steam for his jacket, but passed the flue gases round his cylinder, and thus obtained a far more efficient jacket than the lowLpressure steam-jackets which 80 many of his successors have thought of so highly. The value of the jacket is or was much discussed, but it has come to be recog- nised that the steam used for this purpose should be high-pressure boiler-steam and not steam below the temperatureof that admitted to the cylinder,because the jacket is to impart heat to the working fluid to overcome ‘‘ initial condensation ” and assist ‘‘ re-evapora- tion,” andwill therefore be the more efficient as a jacketthe higherthe temperature of thesteam it contains andthe more perfectly it is drained. Therefore Trevithick’s use of the hot flue gases was strictly correct and the increased economy thus obtained is readily understood. To the use of superheated steam, still periodically brought up for discussion and experiment, Trevithickalso paid much attention. It also is a means of attacking the initial condensation, and the advisability of its use is now limited to that temperature that can be used withoutinjury by burning, first thesuperheater and second the oil lubricating thevalves and piston-rings of the engine. This question of high temperatures also applies to the use of high- pressure steam, but not to the same extent, as the very water of initial condensation assists lubrication. Theexpansive use of steam andthe economy to be gained thewby, were clearly understood by Watt, but were not employed by him as often as hewould have liked, as there was no means of preventing users of the engine from altering and disarranging the setting of the valves. The only question now to be considered in engine design in this relation is the extent to which expansion is to be carried, as with too much, other matters are brought into consideration, viz., in compound enginesthe increased friction caused by the complication of moving parts, and in simple engines the extra range of temperature in the cylinder and consequent troubles caused by initial condensation ; while in both, the extra surface necessitated by the larger cylinder givesmore play to loss by condensation and radiation. Although Watt originally intendedin introducing steam to.both ..I

Downloaded by [ Columbia University] on [15/09/16]. Copyright © ICE Publishing, all rights reserved. 358 ROBERTSON ON THE STATIONARY STEAM-ENGINE. [Selected sides of the piston to reduce initial condensation, it resulted in enablinghim to obtain more work from hisengine without increasing its size, andthe double-acting engine became the general type, till of late years the use of very high speeds has led to a return of the single-acting form in order to obviate theshock and noise caused by the rapidly moving reciprocating parts, the designer of high-speed engines finding that the single-actingform allows of otherwise impossible piston-speeds bykeeping the pressure on his pins, etc., always in one direction. The perfection of valve-gears is intimately connected with that of the engine, and they have a history as wonderful as its own. The first valve-gear has been termed the “ crude artifice of a lazy boy whodesired toshirk his work.” This is a decidedly opprobrious termto apply to one of theearliest labour-saving devices, and to-day, when the greatest incentive to the inventive faculty is the ‘‘ saving of labour ” this jibe should be forgotten, or selve rather asa sign of want of appreciation on the part of those who use it of the merit of Humphrey Potter’s crude idea which only required the touch of Smeaton’s hand to become a veritable valve-gear. Valve-gears, in spite of their great variety, may be divided into two distinct classes, namely, those working independent valves where each valve has only one function, and those working slide-valves which have many. The first was the form of gear employed in theearliest engines, and,although modified tosuit modern requirements, it is still used. It is, however, now divided into two great classes, namely, valves worked positively and those which at certain moments lose their direct connection with the engine and complete their cycle underthe influence of some other mechanism. The use of the latter may be considered to mark an epoch in the growth of the largestationary mill engine, as they came into use whenthe economical production of large powers was being more carefully considered, and they were accompanied by a better application of first principles; in themselves, however, they do not necessarily suit all conditions, and the misuse and over-complication of drop or Corliss cut-off gearhas often proved a stumbling block to successful design. The second great class represented by the slide-valve had also an early origin, being generally credited to Murdoch, andthough it has passed through a multiplicity of shapes, it may be briefly classified thus :- First, the plain D slide-valve actuated by a simple eccentrio which in all treatiseson valves serves as an exposition of the first principles of valve-gear. Second, the slide-valve actuated by a

Downloaded by [ Columbia University] on [15/09/16]. Copyright © ICE Publishing, all rights reserved. Papers.] ROBERTSON ON THE STATIONARY STEAM-ENGINE. 359 link and two eccentrics; this is the most ingenious arrangement ever applied to anengine, as it combines in a very simple manner more possibilities and applications than almost any other part of the engine. We do not know that Stephenson really understood allthe possibilities of thelink, but in the hands of modern engineers it has, in its latest form, its place in many types of engines. Third, the shifting eccentrics, which enable changes and reversals to bemade such as fixed eccentrics do notallow of. Fourth, thedouble valves, which give variations in thecut-off and allow of governing by the amount of expansion. Fifth, the so- calledradial class, which,actuated by an eccentric, havetheir motion modified by some arrangement of radius rods worked by some other part of the engine. Watt and his contemporaries having established the principle of the expansive use of steam, subsequent inventors seem to have devoted most of their energies to the design of valve-gears, most of whichare now unknown.The modern engine shouldbe as simple in all its details as good working will render possible, and all the wonderful valve gears belong to a sort of “ middle age ” in the history of the engine. The plain slide-valves with, some of its better developments, the piston-valve, the independent positive valve-gears, and the better forms of drop cuboff or Corliss gear itre now theprincipal forms employed, butwhatever is used the greatest simplicitypossible is always sought. Valve-gears naturally suggest governors, which in the development of theengine have passed throughmany stages ofcom- plication only at length to be cnt down in modern practice to their simplest elements. Watt’s cataract governor has now become thedash-pot; his centrifugal governor as a means of regulating the throttle-valve remains much as he left it, bat its uses have been extended and its theory mastered in such a way as toleave it now onlyan outward semblance to Watt’s original form. After its application to the throttle came its application to the cut-off, and the greater number of modern governors alter by means of various mechanisms theamount of expansion of the steam inthe cylinder. These arethe principal methods of governing, thoughthe intermediate mechanisms differ with the valve-gear, and in many cases show a complete departure from Watt’s ideas. Apart from consideration of details, their particular application merits attention, and it is the modernengineer’s knowledge of how to make his engine best fit the conditions it has to work under, which is one of the chief characteristics of modern engineer-

ing, and in this connection the much abused word “ economy” must be considered, and its meaning coupled tothat of

‘L expediency,” before any engine is passed over as uneconomical or unsuited to its work. For example, the large powered multiple expansioncondensing-engine, fitted with all possibilities in the way of steam-saving appliances, carefully housed and looked after, never allowed to get ont of order, receiving its steam dry and at unvarying pressure from a battery of economical boilers, fed by mechanical stokers, etc., and whose load is nearly constant, is a very differentmachine from theengine which goes out to the Colonies, getting little or no attention, whose load is far from constant, receiving steamof all pressures and all degrees of dry- ness, which is started and stopped at irregular intervals, and must lie idle for months, instead of starting regularly every day; yet both may be best examples of their respective types. The engineer hasalways to consider the localcircumstances, and make his engine so that it is in the true sense the most economical. Thus in the spinning mill he has an engine of regular habits, which during its long life has a daily consumption of expensive fuel, a small saving of which soon repays the engine’s first cost, but in the engine which uses cheap fuel he has an example of a possible saving so small as not to repay its cost. Again, the engine carefully looked after may have a design whichwould be impracticable in the case of the one constantly abused, and in many instances hundreds, orit may be thousands of miles away from the possibility of obtainingspare parts. The nmltiple expansion and condensing engine also have their special application, and in the sugar factory or refinery, or in the dye-house, where steam is used for evaporating purposes, not only is the condenser not used in connection withthe engine, butthe engine actually exhausts against a considerable back pressure, theexhaust steam passing to the evaporating vessels and thus being utilised as regards the factory in a much more economical manner than would have been otherwise possible. These latter examplesmust be as engines uneconomical, because of the abuse they receive. Theymust possess great simplicity of design, and the fewer refinements the better. They often work only one hundred days in the year, and then must of necessity come to pieces, to avoid destruction fro111 corrosion. The repairs they receive depend on circumstances and are often very crude. The questions of weight and space bring in other considerations, and the high-speed engine, withits short stroke and large number of revolutions, belongs to another type, which, while embodying all the principles used on other types,

Downloaded by [ Columbia University] on [15/09/16]. Copyright © ICE Publishing, all rights reserved. P8pers.J ROBERTSON ON THE STATIONARY STEAM-ENGINE. 361 calls for qualities and differences in detail which are elsewhere out of place. There are also engines which are started andstopped at short intervals, say every15 minutes or so, whose duty is to do a large amountof work suddenly, and which arestopped almost as soon as theyreach full speed. Suchengines are those used to drive hydro-extractors, or rolling mills, and would be out of place where longcontinuous work is desired. Another example is offered by the comparison of fire-extinguishing pumps andthe waterworks pumping-engine. The one has to throw a maximum amount of water for a short time, utterly regardless of expense as to fueland steam-consumption, butthe other has towork continuously, and its fuel bill has beto paid by theever inquisitive ratepayer, and must therefore be calculated in proportion to the water pumped. The foregoing examples will be sufficient to show how any improvement on the engine of the earlier days must be considered as an improvement only when properly applied, and an engine which is perfect in one place is by no means so in another. Theprinciples concerned inmaking the enginewere mostly outlined by Watt, but their better understanding belongs to the later engineers. Thethermo-dynamics of theengine come first, and it is entirely due to themodern perfection of this study, both abstractly and experimentally, that the modern designer is able to take full advantage of the power stored in steam without losing efficiency in one directionwhile seeking it in another. For instance, expansion can be carried to a degree which renders its use uneconomical, as also compounding and the proper extent to which these can be with advantage pushed depends on a perfect understanding of the properties of steam not only as steam, but also as steam in its relation to the engine. This again introduces the commercial consideration, and it is often hard for the abstract theorist to understand that the price of fuel, the nature of the work expected, etc., must all find a symbol in his formulas. This principle must, for want of a better expression, be called “Theory inrelation to expediency.” Thepurist will consider thattrue theorytakes due account of expediency. Butin the ordinary acceptance of the word “ theory ” it does not, and, as a result, engineering papers are full of so-called conflicts between theory and practice. The one principlewhich is entirely modern andwhich has made the noiseless high-speed engine apossibility is that of dynamic balance. It is true that early engines were to a certain extent balanced, but these ear1-j- attempts onlyexemplify static balance,which being used on engines of low speed wasquite

Downloaded by [ Columbia University] on [15/09/16]. Copyright © ICE Publishing, all rights reserved. 362 ROBERTSON ON THE STATIONARY STEAN-ENGINE. [Selected sufficient. It was not untilthe introduction of higher speeds showed a necessity for better balance of moving parts that a study of theprinciples involvedwas necessitated. This subject has been of late given a great amount of attention and is yet subject to continual research and discussion. All effects are due to causes known or unknown, and any treat- ment of the development of the enginewould be incompletewithout reference to those at work in the evolution of the engine as used to-day. First the pressing need of some means of applying greater power to a pump than that exercised by human or animal labour brought Savery’s engineinto being, which in its turn was replaced bythe more powerful Newcomen engine;the latter

was, however, such a 6‘ steam-eater ” asto almost nullifythe profit to be gained by the greater depth at which it allowed the Cornish mines to be worked. Watt was, therefore, met by a great demandfor an economical pumping-engine, whichhe supplied; hadthe engine, however, merelyremained as a motor for pumpingplants, it would probablynot have developed tothe extent it has. But if a demand at times calls forth a supply, a qupply may in its turn create a demand. The engine being in existence, there was no reason why it should not be used to drive other than pumping machinery, and this extended use caused its ultimate development. At first the process was slow, andthe pump was regarded as such an integral part of the engine that water was actually pumped on to a water-wheel in order to do work. Gradually, however, theengine was separated from the pump and put to do work alone, from which point its evolution may be said to begin. First the increased power given by the still imperfectly made engine allowed of improvements in manufacture; improved methodsof manufacture led improvementsto in the engine itself; finer and heavier machinery maketo it evolved the more skil- ful workman, the designer, and the theorist; and so each advance on one side or the other leading to advances on all the others, till,increasing commercial demands aiding,the many types of heavy-powered engines of modern days grewby a process of evolution from the seeds sown by Newcomen, Watt,and Trevithick. The development of the engine, whileextending over considerable time and attended by such surprising results, may be briefly stated as follows : An amplification, better understanding, and extended use of theprinciples laid down byWatt and Trevithick. The dynamic balancing of the engine, the perfection of old details, and the introductionof a few new ones, all modified

Downloaded by [ Columbia University] on [15/09/16]. Copyright © ICE Publishing, all rights reserved. Papers.] ROBERTSON ON THE STATIONARY STEAM-ENGINE. 363 by properknowledge of first principles. The comprehension of thefact that an enginemust bedesigned forits own specific work, and, above all, that the first principle in engineering work is that good designconsists in the greatestsimplicity possible compatible with the object in view. When to this is added the increasedperfection attainablein materials,workmanship, and instruments of precision, all is said except that stress should once more be laid on that which has really developed the engine and made it what it is,namely, theever-varying and increasing demand for its use.