Popular Science Monthly/Volume 13/May 1878/The Growth of the Steam-Engine VI
|THE GROWTH OF THE STEAM-ENGINE.|
By Professor R. H. THURSTON,
OF THE STEVENS INSTITUTE OF TECHNOLOGY.
THE STEAM-ENGINE OF THE FUTURE, AND ITS BUILDER.
HAVING thus rapidly outlined the history of the steam-engine, and of some of its most important applications, we may now take up the question—
What is the problem, stated precisely and in its most general form, that engineers have been attempting here to solve?
After stating the problem, we will examine the record with a view to determine what direction the path of improvement has taken hither-to; and, so far as we may judge the future by the past, by inference, to ascertain what appears likely to be its course in the present and in the immediate future. Still further, we will inquire what are the conditions, physical and intellectual, which best aid our progress in perfecting the steam-engine.
This important problem may be stated in its most general form thus:
To construct a machine which shall, in the most perfect manner possible, convert the kinetic energy of heat-motion, as derived from the combustion of fuel, into mechanical power, using steam as the receiver and conveyer of that heat.
The problem embodies two distinct and equally important inquiries: The first, What are the scientific principles involved in the problem, as stated? The second, How shall we construct a machine that shall most efficiently embody and accord with not only known scientific principles, but also with all well-settled principles of engineering practice?
The one question is addressed to the man of science; the other to the engineer. They can only be satisfactorily answered, even so far as our knowledge at present permits, after studying with care the scientific principles involved in the theory of the steam-engine, under the best light that science can afford us, and by a careful study of the various steps of improvement that have already taken place, and of accompanying variations of structure, analyzing the effect of each change and tracing the reasons therefor. The theory of the steam-engine is too important and too extensive a subject to be treated in even the space available for a complete course of college lectures; and we can only here attempt an exceedingly concise statement of the principles, pointed out by science, as those applicable in the endeavor to increase the economic efficiency of the steam-engine.
The teachings of science indicate that, in the modern steam-engine: Success in economically deriving mechanical power from the energy of heat-motion will be the greater as we work between more widely-separated limits of temperature, and as we more perfectly provide against losses by dissipation of heat in directions in which it is unavailable for the production of power.
Scientific research has proved that, in all varieties of heat-engines, a very great loss of effect is unavoidable from the fact that we cannot reduce the lower limit of temperature, in working, below a point that is far above the absolute zero of temperature: the point corresponding to the mean temperature of the surface of the earth in our latitude is now practically our lower mean limit of temperature. The higher the temperature of the steam, however, when it enters the engine, and the lower the temperature at which it leaves the cylinder, and the more thoroughly we provide against waste of heat by conduction and radiation, and of power by friction, the greater will be our success.
Now, looking back over the history of the steam-engine, we may rapidly note the prominent points of improvement and the most striking changes of form; and we may thus obtain some idea of the general direction in which we are to look for further advance.
Beginning with the machine of De Caus, at which point we may first take up an unbroken thread, it will be remembered that we there found a single vessel performing the functions of all the parts of a modern pumping-engine; it was at once boiler, steam-cylinder, and condenser, as well as both a lifting and a forcing pump.
The Marquis of Worcester, and, still earlier, Da Porta, divided the engine into two parts; using one part as a steam-boiler, and the other as a separate water-vessel.
Savery duplicated those parts of the earlier engine which acted the several parts of pump, steam-cylinder, and condenser, and added the use of the jet of water to effect rapid condensation.
Newcomen and Cawley next introduced the modern type of engine, and separated the pump from the steam-engine proper: in their engine, as in Savery's, we notice the use of surface-condensation first; and, subsequently, that of a jet of water thrown into the midst of the steam to be condensed.
Watt finally effected the crowning improvement of the single cylinder-engine, and completed this movement of differentiation by separating the condenser from the steam-cylinder, thus perfecting the general structure of the engine.
Here this movement ceased, the several important processes of the steam-engine now being conducted each in a separate vessel. The boiler furnished the steam; the cylinder derived from it mechanical power; the vapor was finally condensed in a separate vessel; while the power, which had been obtained from it in the steam-cylinder, was transmitted through still other parts to the pumps, or wherever work was to be done.
Watt also took the initiative in another direction: He continually increased the efficiency of the machine by improving the proportions of its parts and the character of its workmanship; and thus made it possible to render available many of those improvements in detail which are only useful when the parts can be skillfully made.
Watt and his contemporaries also commenced that movement toward higher pressures of steam, used with greater expansion, which has been the most striking feature noticed in the progress of the steam-engine since his time. Newcomen used steam of barely more than atmospheric pressure, and raised 105,000 pounds of water one foot high, with a pound of coal consumed. Smeaton raised the steam-pressure to eight pounds, and increased the duty to 120,000. Watt started with a duty of double that of Newcomen, and raised it 320,000 foot-pounds per pound of coal, with steam at ten pounds. To-day, Cornish engines of the same general plan as those of Watt, but worked with forty to sixty pounds of steam, and expanding three to six times, do a duty that will probably average, with good ordinary engines, 600,000 foot-pounds per pound of coal.
The increase of steam-pressure and expansion which has been seen since Watt's time has been accompanied by a very great improvement in workmanship, a consequence of rapid increase in the perfection and the wide range of adaptation of machine-tools, of higher skill and intelligence in designing engines and boilers, increased piston-speed, greater care in obtaining dry steam, and in keeping it dry until thrown out of the cylinder—either by superheating, or by steam-jacketing, or by both means combined; and it has been further accompanied by greater attention to the important matter of providing carefully against losses by conduction and radiation, and by internal wasteful transfer of heat. The use, finally, of the "compound" or double-cylinder engine for the purpose of reducing friction, as well as of saving some of that heat which is usually lost in consequence of internal condensation and reëvaporation due to great expansion, has already been considered when treating of the marine engine.
It is evident that, although there is a limit, which is tolerably well defined, in the scale of temperature, below which we cannot expect to pass, using the now standard type of engine, a degree gained in approaching this lower limit is more remunerative than a degree gained in the range of available temperature, by increasing the maximum temperature. Hence, the attempt made by the French inventor, Du Trembly, a quarter of a century ago, and by other inventors since, to utilize a larger proportion of heat by approaching more closely the lower limit, was in accordance with what are now well-known scientific principles.
The form of engine here referred to is known among engineers as the Binary Vapor-Engine. In it the heat usually carried away by the water delivered from the condenser of the steam-engine is made to evaporate some very volatile liquid, as ether or carbon bisulphide, which, in turn, by the expansion of its vapor, develops additional mechanical power. Mechanical difficulties have hitherto prevented the success of this form of engine; but it cannot be pronounced impossible that coming inventors may make the system commercially valuable.
An important consequence of the still unchecked rise of piston-speed in the modern steam-engine is the approach to a limit beyond which the now standard form of "drop cut-off," or "detachable" valve-gear, cannot be used. For the piston would, at that limit of speed, reach the end of its stroke before the dropped valve could reach its seat, and the point of cut-off and degree of expansion could no longer be determined accurately and invariably by the governor. This limit has probably already been attained in some engines; and the engineer adopting such piston-speeds as 1,000 feet per minute or more is driven back to the use of the older types of "positive-motion" valve-gearing, and is compelled to devise special forms of governor which shall have sensitiveness, and yet power sufficient to control these less tractable kinds of mechanism, and to invent reliable and durable forms of balanced valves, and to practise every practicable expedient for making the movement of the valve, and its adjustment by the regulator, perfectly easy. Positive motion and ease of adjustment by the governor are, therefore, evidently the requisites of a successful valve-gear for the engine which will probably succeed the standard engine of to-day.
We may now summarize the results of our examination of the growth of the steam-engine thus:
1. The process of improvement has been one, primarily, of "differentiation";" the number of parts has been continually increased, while the work of each part has been simplified, a separate organ being appropriated to each process in the cycle of operations.
2. A kind of secondary process of "differentiation" has, to some extent, followed the completion of the primary one, in which secondary process one operation is conducted partly in one and partly in another part of the machine. This is illustrated by the two cylinders of the compound engine, and by the duplication noticed in the binary vapor-engine.
3. The direction of improvement has been marked by a continual increase of steam-pressure, greater expansion, special provision for obtaining dry steam, higher piston-speed, careful protection against loss of heat by conduction or radiation, and, in marine engines, by surface condensation.
The direction of improvement, as indicated by science as well as by our own review of the actual steps already taken, would seem to be: En résumé, working between the widest attainable limits of temperature, and the saving of heat previously wasted in the apparatus or rejected from it.
Steam must enter the machine at the highest possible temperature, must be protected from waste or loss of heat, and must retain, at the moment before exhaust, the least possible proportion of originally available heat. He whose inventive genius, or mechanical skill, contributes to effect either of these objects—to secure either the use of higher steam with safety, or the more effective conversion of heat into mechanical power without waste, or the reduction, by transformation into work, of the temperature of the rejected working-fluid—confers an inestimable boon upon mankind.
In detail, in the engine proper the tendency is, and may be expected to continue, in the near future at least, toward higher steam, greater expansion in more than one cylinder, steam-jacketing, superheating, a careful use of non-conducting protectors against waste, and higher piston-speed with rapid rotation, and to the adoption of special proportions and of forms of valve-gear adapted to such high-speed engines.
In the boiler, more complete combustion, without excess of air passing through the furnace, is sought, and a more thorough absorption of heat from the furnace-gases. The latter may be ultimately found most satisfactorily attainable by the use of a mechanically-produced draught, in place of the far more wasteful method of obtaining it by the expenditure of heat in the chimney.
In construction, we may anticipate the use of better materials, as already seen in the substitution of "mild steels" for the cruder material, iron, and more careful workmanship, especially in the boiler, and still further improvement in forms and proportions of details.
In management, there is an immense field for improvement, which improvement we may feel assured will rapidly take place, as it is now becoming well understood that care, skill, and intelligence, are absolutely essential to economical management, as well as to safety, and that they repay liberally all the expenditure of time and money that is requisite to secure them. It is truer of labor than of anything else in the market that "the best is the cheapest."
In attempting improvement in the directions that I have indicated, it would be the height of folly to assume that we have reached a limit in any one of them, or that we have even approached an impassable limit. If further progress seems checked by inadequate returns, when efforts are made to advance, in any promising direction, beyond present practice, it becomes the duty of the engineer to detect the cause of such hinderance, and, having found it, to find a way to remove it, if such removal is not physically impossible.
A few years since the movement toward the expansive working of high steam was checked by experiments seeming to prove positive disadvantage to follow advance beyond a certain point. A careful revision of results, however, showed that this was true only with engines built, as was then common, in utter disregard of all the principles which should have been observed in such use of steam, and of the precautions necessary to be taken to insure the gain which science has taught us should follow the intelligent use of higher pressures of steam. The obstructions are purely physical and mechanical, and it is for the engineer to remove them.
An analysis of the methods of waste of heat, in the operation of the modern steam-engine, would show that a very large proportion—nearly all, in fact—is due to the rejection of unutilized heat with the exhaust steam. In the best engines in general use this loss amounts to from eight-tenths to nine-tenths of the total amount of heat derived from the fuel. Modern steam-engines lose nearly all wasted heat in this way; the losses by conduction and radiation are comparatively small. It is at once evident that the only way in which any very great additional economy can be secured is to reduce to a minimum the quantity of heat remaining at the opening of the exhaust-valve, and then to retain this rejected heat within the system, so far as is possible, and to thus prevent its waste by escape from the system. The reduction of the great quantity of heat left for rejection at the end of the stroke of the piston can only be effected, to any important degree, by expedients which check that internal condensation and reëvaporation which, with great expansion, transfer to the condenser, unutilized, an immense amount, often, of the heat supplied. As already stated, these expedients are the use of dry steam, the adoption of the steam-jacket and of high engine-speed, and the use of a material for the interior lining of the cylinder which has the least possible conductivity.
The retention of the heat actually rejected from the cylinder, and its complete utilization by reworking, is practically a matter of difficulty, although not certainly impossible; and the author has proposed a new type of steam-engine, in which the water of condensation and the steam rejected from the engine shall be separated and returned, by pumps of proper proportion and construction, to the boiler. The return of the water demands the expenditure of an insignificant amount of power. To return the rejected steam with its charge of heat—which usually forms so large a proportion of the total heat generated by the combustion of the fuel, assuming all transfer of heat to the exhaust by the operation of internal condensation and reëvaporation to have been prevented—demands the expenditure of precisely the amount of power which has been developed by its expansion. In an ideal engine of this type, therefore, the efficiency is perfect, and all heat-energy is utilized by transformation into mechanical energy; but the engine cannot develop as much power as an engine of the common type of the same size. The size of engine will be nearly inversely proportional to the "efficiency of the fluid" under similar conditions in this and the ordinary type of engines. The heat rejected from the cylinder has been degraded so low on the scale of temperature as to be no longer available for the production of power; nevertheless, restored to the boiler, it serves with perfect efficiency as a basis upon which to "pile up a new stock of utilizable energy" in the form of heat derived from the furnace, and at a higher temperature.
The obstacles to the realization of this theoretically perfect type of engine are those which make it so difficult to reduce internal condensation and reëvaporation, and those conditions of practice which make the engine of this type exceptionally bulky and mechanically inefficient.
Whether this type of heat-engine can ever be made of practical value will be determined by the rate of condensation of steam expanding against a resisting piston; the extent to which high pressures and great expansion can be practically carried; the extent to which internal transfer of heat, without doing work, can be reduced; the practical limit of engine-speed; and the perfection attainable in the engine considered as a piece of mechanism. All these conditions remain to be experimentally determined, and it is only by their determination that it can be known whether the "Steam-Engine of the Future" will greatly exceed the engine of to-day in efficiency, and whether this newly-proposed type may ultimately succeed.
That the changes in practice already indicated may go on almost indefinitely seems unquestionable. That this latter modification of the steam-engine will ever actually take place, and become generally adopted, cannot be as positively asserted. We may, at least, hope that it may.
We have seen that the most important problem offered the engineer for solution is a double one, and that it requires the aid of both the scientist and the mechanist in its solution.
But it is sufficiently evident that, before the engineer can determine what form of machine will best yield to him full control of these forces of Nature, he must have sufficient knowledge of science to be able to understand what scientific principles are to be rendered available, and what phenomena of Nature are operating in the production of the power which he is to seize upon and usefully to apply. Otherwise, he will grope in the dark, and will only learn, by the bitter experience of costly failures, to make slow progress toward perfection.
We have seen that the larger proportion of the principal improvements which have yet been effected in the steam-engine were due to the united engineering skill and experience and scientific attainments of James Watt. We have seen that his improvements followed a long course of intelligent and truly scientific research; and that, directed by the results of this investigation, the engineering talent and the mechanical knowledge of the great inventor accomplished more in a single lifetime than had been previously accomplished in the whole period embraced in the history of civilization.
This great example confirms what we should infer from the nature of the problem itself, that—
He who would accomplish most in the profession of the mechanical engineer must best combine scientific attainments—and especially experimental knowledge—with mechanical taste and ability and a good judgment refined by engineering experience.
As one of our oldest engineers tells him, he must "cultivate a knowledge of physical laws, without which eminence in the profession can never be securely attained." He must become familiar not only with science and the arts, but he must train himself to make the one assist the other; he must learn just how to make use of scientific principles in planning his work, and how to do his work most thoroughly, efficiently, and economically, when he has determined his general design. He must be able to determine how far standard designs are in accordance with correct scientific and mechanical principles, to detect their defects and the causes of those defects, and to provide a remedy correct in principle and mechanically efficient. Science and Art must always work hand-in-hand.
But how are the rising generation of engineers to acquire this proficiency in both branches of knowledge? How are they to be made mentally and manually accomplished; how fitted for the great work which is laid out for them?
The time has gone by when, in any art, the ignorant and merely dexterous workman can compete with even a less skillful shopmate, who possesses and uses brains as well as hands, and knows how to make the one direct and aid the other. We to-day find him occupying a decided vantage-ground who is at the same time familiar with the schools and at home in the workshop. For whatever department in the arts a youth may be designed, he must, to insure success in the future, be taught not "in either the school or the workshop," the alternative formerly offered him, but in the school and the workshop.
Here, then, arises the necessity for Technical and Trade Schools, in which, if properly conducted, knowledge is imparted so as not only to train the mind to habits of thought and study, to give it capacity for logical deduction and the rapid acquirement of information, but in such manner as shall at the same time make the student familiar with the principles of the art which he is to practise, and shall prepare him to learn the lessons taught, in the workshop and in the manufactory, rapidly and well.
It is the tardy recognition of these facts, of this vital necessity, that has placed a great nation, formerly far in advance of all others in manufactures and the useful arts, in a position relatively to her neighbors that is causing the greatest uneasiness to the more intelligent of her people and to all her statesmen. They see other nations, who were formerly far behind, now rapidly overtaking her, if not already taking the lead, in consequence of their earlier adoption of a system of technical instruction for their people.
Two hundred years ago, Edward Somerset, the second Marquis of Worcester, the inventor, whose work has become familiar to us, admonished his fellow-countrymen of the growing necessity of such a system of education for the people, and urged the establishment of technical schools. For this he deserves higher honor than for his improvements in the steam-engine. But the system first took a definite shape, a century ago, upon the Continent of Europe; and, during the past half-century, it has grown with the growth and strengthened with the strength of the western European nations, until, to-day, it has become a most important element of their national power.
In our own country, this great need has long been recognized; but the policy of our Government has not permitted it to institute systems of teaching at the expense of the nation, as has been done in European countries, and it has remained to a great degree unprovided for. It is to our sad deficiency in this respect, and to the tardy and unconcerted action of our educators and our legislators—few of whom seem to have the calibre of the real statesman—that we are to-day so seriously behind Continental nations in the industrial education of youth, and are threatened with serious evils in the future. Without general and systematic technical and trade education, the most enterprising people on the globe, brought into competition in the markets of the world with better-educated people and with nations of trained artisans, must inevitably become a great nation of paupers.
Such education cannot be provided at the small cost that the working-man can afford to pay; and, even if that were possible, it is doubtful whether the vital necessity of such education, to the people rather than to the individual, and to the coming rather than to the present generation, would be sufficiently well understood by the average citizen to induce the payment of its actual cost, far below its full value as it may be.
It becomes, therefore, the privilege and the duty of the wealthy among our citizens to provide this great want of our country, and to aid thus most effectively in giving her that preëminence among nations that every patriotic citizen desires her to attain.
The Stevens Institute of Technology.
- An abstract of "A History of the Growth of the Steam-Engine," to be published by D. Appleton & Co.
- "On a New Type of Steam-Engine," etc., by R. H. Thurston, Journal of the Franklin Institute, October, November, December, 1877. "Proceedings of the American Association for the Advancement of Science," 1877.
- Charles Haswell.