Source: Minutes of the Proceedings of the Institution of Civil Engineers, Vol 12, N° 886
Date: Feb 15, 1853
Title: On the Use of Heated Air as a Motive Power - Minutes of Discussion
Author: Benjamin Cheverton
Mr. Lefroy then proceeded to state, that he had devoted much attention to the question of the practicability of conducting the combustion of the coal, in such a manner as should render the application of the elastic forces of the gaseous products of combustion, really convenient and economical.
It appeared to him, that these bodies, having nearly the same specific heats as atmospheric air, and the caloric being developed in combination with their molecules, there must be an economy in using the elasticities of these bodies, in preference to the elastic force, either of atmospheric air, or of steam, due to that portion of the caloric of these bodies which it might be possible to abstract from them, in their rapid passage through the flues, and acting through the metal plates of the vessel, containing the air, or water.
The practical difficulties which had hitherto prevented the economical use of the elastic forces of these gases, resulted from their high temperature, and the grit, or incombustible parts of the fuel, which they carried with them from the furnace ; but Mr. Lefroy thought, that in the form of apparatus which he would proceed to describe, these difficulties would be entirely overcome.
He proposed to place within the same circular boiler, but near its circumference, a number, probably four, of closed furnaces, each completely immersed in the water ; they would be separately stoked, by orifices in the boiler, corresponding to each furnace, and which would be closed by plates screwed tightly down after the stoking.
Each furnace would also be separately supplied with air, and the gases generated in each, would be poured out, near the bottom of the water in the boiler, by three, or more pipes, springing from the upper part of each furnace, and each terminating in a small perforated box, through which the gases would be poured in five columns, into the water, whence it was supposed they would rise purified from all grit and with their temperature reduced.
If it were found by experiment, that this arrangement did not insure a sufficient purification of the gases, a number of wire gratings, each covered with a stratum of gravel, or sand of different degrees of fineness, could be horizontally extended through the boiler, and the gases being compelled to pass through these as they rose through the water, could be purified to any degree required.
A hollow metal ball would be placed in each of the perforated boxes, which would be forced against and completely close the extremity of each pipe, when by opening the orifices to stoke the furnace, to which it was attached, the pressure in that furnace should be reduced to that of the atmosphere.
The ash, grit, etc., which would be poured down the gas pipes into the boiler, would sink to the lower part of the spherical bottom of the boiler, whence they would be ejected through the brine pipe.
On account of the intense heat, it would probably be found advantageous to use hollow fire-bars, through which it might be advantageous to pass the supply water into the lower part of the boiler. If the system were worked under high-pressure, say of 30 or 40 lbs. per inch, it was supposed, that the combustion of the coal, would be so rapid and perfect, that an apparatus of comparatively small dimensions would furnish very considerable power.
Should it be found, that under the conditions of very high temperature and pressure, gases of a much more elastic nature than those he had assumed, were generated in the furnaces, Mr. Lefroy expected no inconvenience, but rather increased economy, in many respects from it, for the form of the apparatus which was proposed, appeared so simple and to be so susceptible of indefinite strength, both from diminution of dimensions, and increased thickness of the parts, that he could conceive no elasticity too great to be safely handled, in a properly proportioned apparatus of the form proposed.
Finally he would enumerate, as the principal advantages of the apparatus and power he proposed to use, over the steam furnaces at present constructed, a suppression of the funnel, or chimney, a considerable saving of fuel, and a great economy in prime cost, space occupied, and labour of attendance.
Mr. W. G. Armstrong observed, that the first question to be considered was, whether a given quantity of heat applied to vaporize water, would produce a greater, or a less expansive effect, than when it was applied to the heating of air. If an estimate was made of the quantity of heat, by degrees of temperature, the comparison would appear to be in favour of water, but when it was taken into account, that the "specific" heat of water was nearly four times greater, than that of air, and consequently, that one degree of temperature in water was equivalent to nearly four degrees of temperature in air, it was easy to show, that the actual quantity of heat consumed, in expanding air to a given volume, was less, than was necessary to produce a corresponding effect with water.
On the other hand, the feed-pump of an air-engine would absorb an enormously larger proportion of power, than the feed-pump of a steam-engine ; but if it were practicable, as he believed it was, to recover and use over again a large proportion of the heat applied, he conceived the balance of economy, so far as theory was concerned, would be found in favour of air.
What the practical difficulties might prove to be, was another question ; but he thought the system was too much in its infancy, to enable any one to speak confidently on that point. The extraordinary size of the cylinders and pistons required in Captain Ericsson's engine, appeared to be a great difficulty, but time might develop some means of obviating that objection.
Mr. Rennie said he was present at the trials of Sir G. Cayley's engine, which had to a certain extent performed satisfactorily. It must, however, be borne in mind, that to place air on an equality with water the air must be heated to 360°, in order to double its bulk, and the chief practical difficulties arose from the destruction of the material of the heating chambers, from the direct action of the fire ; this was the cause of the cessation of employment of Stirling's engine, which in its form and arrangement was a decided improvement on any of its predecessors, and in fact on any of its successors.
The mentionned 360° is an error and air must be heated at 480° F to double its volume.
In 1806 a somewhat similar system had been tried, in France, by Niepce, by the ignition of an explosive powder in the cylinder. The machine had, however, never proceeded beyond a mere model.
Mr. C. W. Siemens, after sketching a diagram explanatory of the action of Ericsson's engine, stated that he had not seen the machine, but he believed the description which had been given of the arrangement was substantially correct.
He had followed its progress with considerable interest, having himself been engaged, for a number of years, in maturing an engine, in which steam was employed in a highly heated state.
The pistons being on their bottom stroke, the air from the reservoirs was admitted below, urging the working piston upward. Being heated, in its passage through the regenerator to 400° Fahrenheit, the volume of the air was increased in the proportion of 2 to 3, and hence the reservoirs were deprived of only two-thirds the contents of a working cylinder of compressed air. An additional means of economising the supply of compressed air was, by shutting off the admission, before the upward stroke was completed ; allowing it to act expansively.
The pumping cylinder had, in the mean time, discharged its contents of fresh atmospheric air into the reservoir, to make up for the supply of the working cylinder, so that the pressure of 10 lbs. per square inch was always maintained. The position of the slide-valve being then reversed, the air from beneath the working piston was free to escape into the atmosphere, but having to pass through the regenerator, the free and sensible heat contained in it was restored to the metallic wire gauze, in the inverse order to that in which it was taken up, issuing finally at little above the temperature at which it entered from the reservoir.
The descending stroke was effected by the mere weight of the pistons. When completed, the position of the slide-valve was again altered, and the air from the reservoir entered and forced the piston upward. In its passage through the regenerator it absorbed the heat which had been deposited there, and with an additional supply from the fire, its volume was again doubled, in filling the working cylinder.
Supposing the action of the regenerator could be made perfect, so that the air left the regenerator at precisely the same temperature at which it had entered, it might seem, at first sight, that the engine would work without any expenditure of heat, beyond the mere losses from radiation, etc. This view had indeed been maintained by Captain Ericsson and others ; but upon consideration it became apparent, that there was a theoretical consumption of heat, which might be very accurately calculated, from the fact that the air entered the regenerator in a compressed state and returned through it, after expansion to atmospheric pressure had taken place.
This expansion was accompanied by a diminution of temperature of some 70° or 80° Fahrenheit, which became latent and had to be replaced by the fire. The theoretical consumption of a perfect caloric engine, amounted to only one-fourteenth part of the theoretical consumption of a Boulton and Watt condensing engine. The practical arrangement of Ericsson's engine, however, rendered the attainment of such a result impossible, for the following reasons:
Fully two-thirds of the power of the engine must be expended in working the air-pump, independent of the resisting pressure of the atmosphere, which was equal to 3/5 lb. the total working pressure. The consequence was, that to produce the effective displacement of the piston, for one single volume of air at its full pressure, from 7 to 8 volumes had to be cooled and heated alternately.
The working piston of Ericsson's engine had, moreover, to work air-tight in a heated cylinder, which Mr. Siemens had practically found to be a matter of great difficulty. The lubricating material would become rapidly carbonised and would fill up the meshes of the regenerator.
The extent of heating surface provided, also appeared to be too small for the quantity of heat required to be transmitted. It was understood, from good authority, that the present working cylinders of 14 feet diameter, were now being replaced by others of 16 feet, which was the greatest size the breadth of beam of the vessel would permit.
Mr. Armstrong's views required correction, owing to the evolution of heat, in compressing the air in the pump, which would produce expansion and increased resistance. A corresponding cooling effect was, moreover, produced through its expansion in the working cylinder, which would diminish the power shown by him to be obtainable. These objections would, he expected, mark the anticipated results of this interesting experiment.
Mr. Bidder thought so little was known, accurately, either of the dimensions of Ericsson's engine, or of the results obtained, that the discussion could hardly be conclusive. Still, reasoning from the data before the meeting, it appeared, that unless the regenerator enabled any given portion of heat to be utilized over and over again, it was obvious, that no theoretical advantage was obtained, in using heated air, instead of vaporized water, as a motive power, and it was incapable of being applied practically with as much convenience.
This was evidenced by the enormous size of the cylinders on board the " Ericsson," - 12 feet and 14 feet diameter - by which, however, a speed of only 6 to 7 miles per hour had been attained. As the power was as the cube of the velocity, it followed, that to obtain a velocity of 14 miles per hour, with the same vessel, there would be required cylinders of between 30 feet and 40 feet diameter.
He had carefully investigated the operation of the regenerator, supposing it to be theoretically perfect, and had arrived at the conclusion, that no mechanical advantage could be attained by its use.
Mr. Spiller had also, from the data, reported by the public prints to have been furnished by Captain Ericsson himself, made a calculation of the power of the engines, and, assuming those data to be correct, he computed the total motive force produced by the expansion of the air, to be 208 H.P., taking 33,000 lbs. raised 1 foot per minute, as the standard of the horse-power ; that whether in a steam, or an air-engine, friction and waste were inevitable, and if only a sufficient amount was deducted, for these causes of loss, it would be found, that many of the marine-engines were producing power more economically, than the heated air engines in question.
He also remarked, that although any given amount of heat, acting on air, might produce more motive force, than the same amount employed to produce motive force by steam, still he believed, that in consequence of the very low pressure of air, except at so high a temperature as to be destructive of the necessary perfection of the working parts of every engine, steam would be found more economical than air, as an agent for the production of motive power, and hence however laudable the attempt might be, he could see but little prospect of success in this trial for substituting air for steam.
As to the regenerator, which had excited so much interest, that some economy might be produced by such an apparatus, there could be no reasonable doubt, notwithstanding what had been said of the impossibility of employing heat a second time, or "over again." The metallic wire acted as a reservoir, to embody, or take up a portion of the heat in the outgoing air ; which heat, so embodied, was given out to the incoming air, for the succeeding stroke, a principle carried into effect in every non-condensing engine, where the eduction-steam was employed to heat the feed-water, before it entered the boiler, and also by Messrs. Maudslay and Field, in ocean steamers, where the brine-pumps, etc., were so arranged, as to communicate a very large portion of the heat of the outgoing water, to that drawn in to feed the boilers.
Dr. Faraday said, that twenty years ago he had directed his attention to this question, and from theoretical views, he had been induced to hope for the successful employment of heated air as a motive power ; but even then he saw enough to discourage his sanguine expectation, and he had, with some diffidence, ventured to express his conviction of the almost unconquerable practical difficulties surrounding the case, and of the fallacy of the presumed advantages of the regenerator. He still retained his doubts as to the success of the innovation and feared the eventual results, even of Captain Ericsson's spirited and ingenious efforts.
Mr. Brunei, V.P., agreed in considering the regenerator to be a mystification, and the difficulty of the matter arose from its plausibility. It was extremely difficult to disprove that which did not exist at all. He believed the stated gain of power, from the action of the regenerator, to be mere assumption, and he was inclined to regard it just as he would any attempt to produce perpetual motion ; still he admitted the difficulty of exposing the fallacy, as he contended it to be, when it was asserted, that the power derived from the expansion of air by heat, could be used effectively, and then be recovered and used again.
He could not gather, from any of the statements, how power was actually obtained ; and he could only arrive at the conviction, that if there was any development of power, in lending heat to the metallic webs and borrowing it again, the natural consequence must be, that after a time there might be such an accumulation of borrowed heat, as would enable the machine to work, without any fire under the heating vessels. It was scarcely worth while to expend time in the refutation of such a fallacy.
The same kind of error had been fallen into, with respect to the hot water from the condenser of a steam-engine ; but in that case it was clear, that the advantage arose, not from recovering heat which had been previously used, but by obtaining some that had not been used before, and which would otherwise have been wasted.
He was of opinion, that Stirling's engine was not only a prior introduction, but that it was a better machine than Ericsson's ; still he thought that neither of them could be advantageously compared with a steam-engine.
Mr. Hawksley believed, that the machine involved a mechanical fallacy, as the regenerator produced no mechanical effect whatever. It might be granted, that the regenerator of Ericsson's engine received and redelivered the heat in the manner described, and that when the working piston was descending, the heat was deposited, and when ascending, the heat was restored ; but that operation could only result as a consequence of the motion of the piston and not as a cause of its motion - hence no mechanical effort was made.
This result was easily shown by assuming the contents of the pump to be 1, and the contents of the working cylinder to be 2. If the working piston was at the bottom of the cylinder, and in equilibrio with the external atmosphere, as regarded the pressure on a unit of surface, and then began to move and the air to be heated, in its passage through the regenerator, from 32° to a temperature of 512°, so as to double its volume, the lower piston would constantly produce a vacuity, so to speak, of 2, to be constantly fed by a supply of 1, from the pump, expanded into 2 by the increase of temperature, consequently the piston, at every instant of its motion, remained in cquilibrio with the external atmosphere, and no mechanical effect could result.
Still in Ericsson's engine a mechanical effect had been produced; but then this mechanical effect was no greater than would be produced without the aid of the regenerator, by the simple action of the furnace itself, and not so economically as by the use of steam.
Captain Fitzroy, R.N., said it appeared, that after imparting motion to the piston, the heated air, in passing through the regenerator, gave out to the metallic web 520 parts out of 570 parts of heat it contained,' and that a part of that heat was taken up again by the next supply of cold air on entering ; therefore, the mystery consisted in the action of the regenerator, by which the same caloric, husbanded and given out again, could, by a small addition of heat, continue to impart motion to the pistons.
The chief argument against the imputed fallacy, was the fact of the ship "Ericsson " having actually been propelled at a given speed through the water. The relative economy of the system, compared with steam, was entirely another question.
Mr. Bidder begged it might be understood, that in his remarks he had assumed the regenerator to have taken nearly all the heat, but he contended that when so taken up, it was not of any practical utility ; in fact, this was, to some extent, shown by the result obtained in the "Ericsson " ship ; for, with so fine a model, even a small engine and a much less consumption of fuel would have produced a greater speed.
Mr. Brunel, V.P., had also assumed the regenerator to be perfect. He believed that such a mass of wire-gauze would take up the heat perfectly, but he could not admit the soundness of the reasoning, as to the assumed gain of power from the regenerator.
Mr. F. Braithwaite recalled the circumstances attending the first trials of Ericsson's caloric engine, in England : there was not any regenerator, but there was a separate vessel for heating the air, and a refrigerator for cooling it. There was not any difficulty in obtaining power from the expanded air, but there was great difficulty in finding any lubricator, that would enable the pistons to work for any length of time continuously, as the high temperature carbonized all the fatty matters that were tried.
The caloric-engine was tested by pumping water, but the results were not equal to those of the steam-engine, in a commercial point of view. He did not consider the regenerator to be necessary for the engine ; it certainly was not a source of power, and it was problematical whether it was a means of economy.
Mr. W. G. Armstrong said his view of the action of the regenerator was, that it retained a portion of the heat, which would otherwise have been lost ; whether that heat was practically as beneficial, in the subsequent working of the engine, as had been assumed by the advocates of the system, did not yet appear to be fully proved. He thought, that for the credit of the Institution, the question should be more carefully considered.
Mr. Rendel, President, would not have the meeting arrive at a hasty, or erroneous conclusion, on the question of this engine ; and he, therefore, suggested, that Mr. Siemens should draw up a paper on the subject, and that the Members should collect, for a future meeting, all the information within their reach, in order to the calm and deliberate discussion of the question.