A History of the Growth of the Steam-Engine Part 34

Newcomen and Calley next separated the pump from the steam-engine proper, producing the modern steam-engine--the engine as a train of mechanism; and in their engine, as in Savery's, we noticed the use of surface condensation first, and subsequently that of the jet thrown into the midst of the steam to be condensed.

Watt finally effected the crowning improvements, and completed the movement o "differentiation" by separating the condenser from the steam-cylinder. Here this process of change ceased, the several important operations of the steam-engine now being conducted each in a separate vessel. The boiler furnished the steam, the cylinder derived from it mechanical power, and it 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 workmans.h.i.+p, thus making it possible to render available many of those improvements in detail upon which effectiveness is so greatly dependent and which are only useful when made by a skillful workman.

Watt and his contemporaries also commenced that movement toward higher pressures of steam and greater expansion which has been the most striking feature noticed in the progress of steam-engineering 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 pressure somewhat and increased the duty considerably. Watt started with a duty double that of Newcomen and raised it to 320,000 foot-pounds per pound of coal, with steam at 10 pounds pressure. To-day, Cornish engines of the same general plan as those of Watt, but worked with 40 to 60 pounds of steam and expanding three or four times, do a duty probably averaging, with the better cla.s.s of engines, 600,000 foot-pounds per pound of coal. The compound pumping-engine runs the figure up to above 1,000,000.

The increase in steam-pressure and in expansion since Watt's time has been accompanied by a very great improvement in workmans.h.i.+p--a consequence, very largely, of the rapid increase in perfection, and in the wide range of adaptation of machine-tools--by higher skill and intelligence in designing engines and boilers, by increased piston-speed, greater care in obtaining dry steam, and in keeping it dry until thrown out of the cylinder, either by steam-jacketing or by superheating, or both combined; it has further been accompanied by a greater attention to the important matter of providing carefully against losses by radiation and conduction of heat. We use, finally, the compound or double-cylinder engine for the purpose of saving some of the heat usually lost in internal condensation and reevaporation, and precipitation of condensed vapor from great expansion.

It is evident that, although there is a limit, tolerably well defined, in the scale of temperature, below which we cannot expect to pa.s.s, a degree gained in approaching this lower limit is more remunerative than a degree gained in the range of temperature available by increasing temperatures.[116]

[116] The fact here referred to is easily seen if it is supposed that an engine is supplied with steam at a temperature of 400 above absolute zero and works it, without waste, down to a temperature of 200. Suppose one inventor to adapt the engine to the use of steam of a range from 500 down to 200, while another works his engine, with equally effective provision against losses, between the limits of 400 and 100, an equal range with a lower mean. The first case gives an efficiency of one-half, the second three-fifths, and the third three-fourths, the last giving the highest effect.

Hence the attempt made by the French inventor, Du Trembly, about the year 1850, and by other inventors since, to utilize a larger proportion of heat by approaching more closely the lower limit, was in accordance with known scientific principles.

We may summarize the result of our examination of the growth of the steam-engine thus:

_First._ The process of improvement has been one, primarily, of "differentiation;"[117] 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.

[117] This term, though perhaps not familiar to engineers, expresses the idea perfectly.

_Secondly._ 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 portion of the machine. This is ill.u.s.trated by the two cylinders of the compound engine and by the duplication noticed in the binary engine.

_Thirdly._ The direction of improvement has been marked by a continual increase of steam-pressure, greater expansion, provision for obtaining dry steam, high piston-speed, careful protection against loss of heat by conduction or radiation, and, in marine engines, by surface condensation.

The direction which improvement seems now to be taking, and the proper direction, as indicated by an examination of the principles of science, as well as by our review of the steps already taken, would seem to be: working between the widest attainable limits of temperature.

Steam must enter the machine at the highest possible temperature, must be protected from waste, and must retain, at the moment before exhaust, the least possible amount of heat. He whose inventive genius, or mechanical skill, contributes to effect either the use of higher steam with safety and without waste, or the reduction of the temperature of discharge, confers a boon upon mankind.

In detail: In the engine, the tendency is, and may probably be expected to continue, in the near future at least, toward higher steam-pressure, greater expansion in more than one cylinder, steam-jacketing, superheating, a careful use of non-conducting protectors against waste, and the adoption of still higher piston-speeds.

In the boiler: more complete combustion without excess of air pa.s.sing through the furnace, and more thorough absorption of heat from the furnace-gases. The latter will probably be ultimately effected 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 antic.i.p.ate the use of better materials, and more careful workmans.h.i.+p, especially in the boiler, and much improvement in forms and proportions of details.

In management, there is a wide field for improvement, which improvement we may feel a.s.sured will rapidly take place, as it has now become well understood that great care, skill, and intelligence are important essentials to the economical management of the steam-engine, and that they repay, liberally, all of the expense in time and money that is requisite to secure them.

In attempting improvements in the directions indicated, it would be the height of folly to a.s.sume that we have reached a limit in any one of them, or even that we have approached a limit. If further progress seems checked by inadequate returns for efforts made, in any case, to advance beyond present practice, it becomes the duty of the engineer to detect the cause of such hinderance, and, having found it, to remove it.

A few years ago, 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 involved in such a use of steam, and of the precautions necessary to be taken to insure the gain which science taught us should follow. The hinderances are mechanical, and it is for the engineer to remove them.

The last remark is especially applicable to the work of the engineer who is attempting to advance in the direction in which, as already intimated, an unmistakable revolution is now progressing, the modification of the modern steam-engine to adapt it safely and successfully to run at the high piston-speed, and great velocity of rotation which have been already attained and which must undoubtedly be greatly exceeded in the future. As there is no known and definite limit to the economical increase of speed, and as the limit set by practical conditions is continually being set farther back as the builder acquires greater skill and attains greater accuracy of workmans.h.i.+p and the power to insure greater rigidity of parts and durability of wearing surfaces, we must antic.i.p.ate a continued and indefinite progress in this direction--a progress which must evidently be of advantage, whatever may be the direction that other changes may take.

It is evident that this adaptation of the steam-engine to great speed of piston is the work now to be done by the engineer. The requisites to success are obvious, and may be concisely stated as follows:

1. Extreme accuracy in proportions.

2. Perfect accuracy in fitting parts to each other.

3. Absolute symmetry of journals.

4. Ample area and maximum durability of rubbing surfaces.

5. Perfect certainty of an ample and continuous lubrication.

6. A nicely calculated and adjusted balance of reciprocating parts.

7. Security against injury by shock, whether due to the presence of water in the cylinder or to looseness of running parts.

8. A "positive-motion" cut-off gear.

9. A powerful but sensitive and accurately-working governor determining the degree of expansion.[118]

[118] The author is not absolutely confident on the latter point. It may be found more economical and satisfactory, ultimately, to determine the point of cut-off by an automatic apparatus adjusting the expansion-gear _by reference to the steam-pressure_, regulating the speed by attaching the governor elsewhere. The author has devised several forms of apparatus of the kind referred to.

10. Well-balanced valves and an easy-working valve-gear.

11. Small volume of "dead-s.p.a.ce," or "clearance," and properly adjusted "compression."

It would seem sufficiently evident that the engine with detachable ("drop") cut-off valve-gear must, sooner or later, become an obsolete type, although the subst.i.tution of springs or of steam-pressure for gravity in the closing of the detached valve may defer greatly this apparently inevitable change. The "engine of the future" will not probably be a "drop cut-off engine."

As regards the construction of the engine as a piece of mechanism, the principles and practice of good engineering are precisely the same, whether applied in the designing of the compound or of the ordinary type of steam-engine. The proportioning of the two machines to each other in such manner as to form an effective whole, by procuring approximately equal amounts of work from both, is the only essential peculiarity of compound-engine design which calls for especial care, and the method of securing success in practice may be stated to be, for both forms of engines, as follows:

1. A good design, by which is meant--

_a._ Correct proportions, both in general dimensions and in arrangement of parts, and proper forms and sizes of details to withstand safely the forces which may be expected to come upon them.

_b._ A general plan which embodies the recognized practice of good engineering.

_c._ Adaptation to the specific work which it is intended to perform, in size and in efficiency. It sometimes happens that good practice dictates the use of a comparatively uneconomical design.

2. Good construction, by which is meant--

_a._ The use of good material.

_b._ Accurate workmans.h.i.+p.

_c._ Skillful fitting and a proper "a.s.semblage" of parts.

3. Proper connection with its work, that it may do that work under the conditions a.s.sumed in its design.

4. Skillful management by those in whose hands it is placed.

_In general_, it may be stated that, to secure maximum economical efficiency, steam should be worked at as high a pressure as possible, and the expansion should be fixed as nearly as possible at the point of maximum economy for that pressure. In general, the number of times which the volume of steam may be expanded in the standard single-cylinder, high-pressure engine with maximum economy, is not far from 1/2 sqrt(P), where P is the pressure in pounds per square inch; it rarely exceeds 0.75 sqrt(P). This may be exceeded in double-cylinder engines. It is even more disadvantageous to cut off too short than to "'follow' too far." With considerable expansion, steam-jacketing and moderate superheating should be adopted, to prevent excessive losses by internal condensation and reevaporation; and expansion should take place in double cylinders, to avoid excessive weight of parts, irregularity of motion, and great loss by friction.

To secure this vitally important economy, it is advisable to seek some practicable method of lining the cylinder with a non-conducting material. This plan, as has been seen, was adopted by Smeaton, in constructing Newcomen engines a century ago. Smeaton used wood on his pistons, and Watt tried wood as a material for steam-cylinder linings.

That material is too perishable at temperatures now common, and no metal has yet been subst.i.tuted, or even discovered, which answers the same purpose. The loss will also be reduced by increasing the speed of rotation and velocity of piston. Where no effectual means can be found of preventing contact of the steam with a good absorbent and conductor of heat, it will be found best to sacrifice some of the efficiency due to the change of state of the vapor, by superheating it and sending it into the cylinder at a temperature considerably exceeding that of saturation. With low steam and slowly-moving pistons, it is better to pursue the latter course than to attempt to increase the efficiency of the engine by greater expansion.

External surfaces should be carefully covered by non-conductors and non-radiators, to prevent losses by conduction and radiation of heat.