The Stirling Regenerator

Source: Minutes of the Proceedings of the Institution of Civil Engineers, Vol 12, N° 892, p.563
Date: May17, 1853
Title: On the Principle of the Caloric, or Heated Air Engine
Author: James Leslie, M. Inst. C.E.

The following observations are intended to show, that the "regenerator," as it has been called, but which ought more properly to be termed the "economizer," is based on true principles, and is attended, in practice, with real economy of heat, and consequently of fuel — that Messrs. Stirling's Air Engine, with its economizer, has precedence, in point of date, of Capt. Ericsson's Engine — and that the former is free from several of the objections that have been urged against the latter, and is, on the whole, decidedly superior to it.

It was admitted, in the recent discussion at the Institution, that the air is cooled, in passing one way, and is heated in returning the other way through the "regenerator;" but it was at the same time denied, that this alternate heating and cooling was attended with any useful, or economical result.

The admission and the inference would appear to be quite inconsistent, unless it be maintained, and can be shown, that the mechanical power required to drive the air through the passages of the regenerator, is greater than any advantage to be derived from the alternate heating and cooling ; for that some advantage is to be gained by the heating being effected, in a great measure, by means independent of the heat of the fire, is surely not to be denied, unless it be attended with some countervailing loss, or disadvantage.

It has been found in practice, that the force required to drive the air through the passages of the regenerator, is very small indeed, and that it does not increase in proportion as the density of the air is increased. It could also be easily shown, by an apparatus available for the purpose of making the experiment, that by the removal of the regenerator, the Air Engine would become almost entirely deprived of its power, and that its economy would be entirely gone.

The principle of the regenerator can be best explained by a quotation from a description by the Rev. Dr. Stirling, with whom the first idea of it originated.

To demonstrate the efficiency of the "regenerator," or as it might more properly be called the "economizer," we only require to assume as an axiom, what is universally known and believed; that if two bodies of unequal temperatures be brought into contact their temperatures will speedily be equalized. We also require, as in other physical demonstrations, to lay out of consideration all circumstances that are not essential, such as the gradual dissipation of caloric by radiation etc.
It will also simplify the demonstration if we confine our attention to the change produced upon the temperature of the air, and suppose that of the parts of the apparatus to remain invariable. This approaches very near to the truth, even in practice, since the capacity for caloric in the apparatus must be vastly greater than that of the mass of air to be heated and cooled.

Now let A be a minute portion, or particle of air, at a temperature of 50° F, and let B C D E F G H I K L be points, or portions of the economizer having following temperatures respectively attached to them.

  • A: 50°
  • B: 60°
  • C: 70°
  • D: 80°
  • E: 90°
  • F: 100°
  • G: 110°
  • H: 120°
  • I: 130°
  • K: 140°
  • L: 150°
  • A': 160°

Let A be brought into contact with B and it will immediately acquire the temperature of 60°, let it touch C and it will be raised to 70°, and so on, till at last, by contact with L, it acquires the temperature of 150°, having been heated in all 100°, and having taken only 10° from each point or body in the series. The same thing will also hold good in all the successive particles of air which compose the whole mass to be heated.

Suppose farther, that by any means the temperature (of A') has been raised to 160° — has attained its maximum — and requires to be cooled. For this purpose let it first be applied to L and it falls to 15O°, then to K and it becomes
140°, and so on, till at last meeting with B, it is cooled to 60°.

It has thus lost 100° imparting only 10° to each of the points to which it was applied. If now by any means 10° be abstracted from it, it will be reduced to its original temperature of 50°. In the process of being heated it has imbibed 10-11ths of its maximum temperature from the bodies represented by B C D, etc. and in process of being cooled it has returned the very same quantity to these bodies, distributing its caloric equally, and giving 10° to each member of the series.

Let this process be repeated and it is evident that at every successive heating the air requires only 10° to be added to the temperature acquired from the series of bodies B C D etc., to raise it to its supposed maximum of 160°, and at every successive cooling it requires 10° to be abstracted to bring it to the original temperature of 50°. And thus it appears, that by applying air successively to a series of bodies regularly increasing in temperature, and moving it alternately from one end of the series to the other, it may be heated and cooled ten times, with an expenditure of caloric which would barely have heated it once, if it had been applied at once, to the hottest body (i.e. beyond the series).

It is evident also that if the series had been composed of twenty points, or bodies, having a difference of temperature of 5°, the air might be heated and cooled twenty times at no greater expense of caloric. Nay it is evident, that by multiplying the members of the series indefinitely, air could be heated and expanded and made to do work at no appreciable expense.

But let no mathematician be alarmed with the idea of a perpetual motion, or the creation of power. There are many enemies to contend with in the air-engine besides friction, which alone prevents perpetuity in some mechanical motions. We have no means, without consuming a part of our power, of applying the air so closely to the apparatus as to make it absolutely assume the temperature of the bodies to which it is applied.

There is therefore a loss in the very act of heating and cooling. The change of temperature which takes place in each of the economizing bodies, during the passage of the air, though small, yet prevents the absolutely uniform heating of the whole mass, and thus causes waste.

But the greatest enemy of the economizing principle, is the continual passage of caloric from the hot to the cold parts of the engine, by radiation, conducting, etc., which requires a continual supply of caloric, to maintain the proper temperature of each. This defect, however, is not peculiar to the air engine, and by multiplying the steps by which caloric must make its way in escaping, and by opposiDg various obstacles to its progress, we can so detain it as to make it frequently perform the duty of expansion, before it altogether escapes.

Dr. Stirling might have added, as a more convincing proof, and more easy of comprehension than any abstract reasoning, that the machine really did act for a number of years most efficiently and economically, which it could not have done, had there been a fallacy in the regenerator. Air does not possess the advantage of steam, in being converted by heat, per saltum, from a liquid into a gaseous fluid, occupying about 1800 times its former bulk, and of being reconverted, by condensation, into water; but it is simply increased in volume by degrees, in proportion to the heat applied.
Therefore, an air-engine, without a regenerator, would be a much less effective and economical application of heat than the steam-engine.

If the correctness of the principle of the regenerator is to be denied, the same objection may with equal truth, be applied to that beautiful apparatus "Jeffrey's respirator," which is an adaptation, although a more recent one, of the same principle ; and it might then be asserted that it is of no use to the lungs of invalids, and has no effect in keeping the body warm. The air in passing from the lungs to the atmosphere, gives out a certain portion, but not the whole of its heat, to the metal plates of the respirator, and in being inhaled again takes back the greater part of its heat, and reaches the lungs considerably warmer than the atmosphere, though not so warm as the lungs.

Calling the temperature of the lungs A that of the atmosphere B, that of the inhaled air when it reaches the lungs C, and that of the exhaled air, when it reaches the atmosphere, D, the difference of temperature, between that of the lungs, and that of the air exhaled, when it reaches the atmosphere, viz., A—D, or conversely between that of the atmosphere and that of the air inhaled when it reaches the lungs, or C—B, in the case of the respirator, represents what is gained by the regenerator, or economizer of the air-engine.

The difference between the temperature of the air exhaled, when it reaches the atmosphere, and that of the atmosphere viz., D—B, or conversely between that of the air inhaled, when it reaches the lungs, and that of the lungs, or A—C, in the case of the respirator, represents what is not saved by the regenerator of the air-engine, and which must be made up by the application of heat to the one end of the air-vessel, and of a cooling process at the other, say by cold water led through pipes, as in Stirling's engine, or by the admission of cold air from the atmosphere, as in Ericsson's ; the force required to drive the air through the passages, being a farther deduction from the economical process.

The originality of the invention can be best shown by reference to the following extracts from Messrs. Stirling's specifications, and to their dates. It may be here mentioned, that while Dr. Stirling has the merit of the invention of the economizing process, on which the principle of the air-engine is based, to his brother Mr. James Stirling, C.E., is due, the idea of reducing materially the bulk of the engine, by using compressed air, instead of air at the ordinary density of the atmosphere.
This improvement was first patented in 1827, and afterwards with farther improvements in 1840.

Extract from Dr. Stirling's specification of an air-engine, patented in November 1816.

All my improvements for diminishing the consumption of fuel consist of different forms, or modifications of a new method, contrivance, or mechanical arrangement, for heating and cooling liquids, airs, or gases, and other bodies, by the use of which, heat is abstracted from one portion of such liquids, airs, &c., and communicated to another portion with very little loss ; so that, in all cases, where a constant succes sion of heated liquids, airs, &c., is required, the quantity of fuel necessary to maintain, "or supply it, is, by this arrangement, greatly diminished.
The Stirling regenerator
The first modification of the said contrivance, is described as follows : A B is a pipe, tube, channel, or passage, formed of metal, stone, bricks, or other suitable materials. The hot liquid, gas, or body to be cooled is by mechanical, or other means, made to enter the passage at A, and to pass along to its other extremity B. In its progress, it gives out its heat to the sides of the tube, or passage, or to any bodies contained in it, and issues at B at nearly the original temperature of the passage. In this manner, the extremity at A, and a considerable portion of the passage, is heated to nearly the same temperature of the hot liquid, air, &c., while the extremity B still retains nearly its original temperature.

When the temperature of the passage, at B, has been raised a few degrees, the motion of the liquid, air, etc., from A to B is stopped, and a portion of liquid, air, etc., which is required to be heated, and which is supposed to be a few degrees colder, than the extremity of the passage, at B, is made to traverse the same passage, in a contrary direction, i. e., from B to A, by which means it receives heat from the sides of the passage, or other bodies contained in it, and issues at A of nearly the same temperature with the liquid, air, &c., to be cooled. When the heat of the passage at A has thus been lowered a few degrees, the process is again stopped, and a portion of the liquid, air, &c., to be cooled is made to pass from A to B, and so on alternately.

After describing other modifications of the same contrivance the specification proceeds.

The form and construction of the tubes and plates, etc., may be varied, according to circumstances, but the benefit to be derived from this arrangement arises from the fluids, airs, etc., to be heated, and those to be cooled, being made to move in opposite directions, etc.

As to the respective merits of Stirling's and Ericsson's engine, it must be admitted, that the former is much more compact in all its parts, and occupies much less space than the latter, owing to the use of compressed air, as already mentioned, which increases the power of the engine almost directly in the ratio of the density of the air, and of course very much reduces the friction of the working parts. The compressing of the air, which is generally from about seven to ten atmospheres, is, in the first instance, necessarily effected by hand, by pumping it into a receiver, or magazine, in the same way as water requires, at the outset, to be pumped into the boiler of a steam-engine.

After this, the air is pumped in by the engine itself, and the power required for that purpose is inconsiderable, as it is only the small portion of air which is wanted to make up for leakage, that is required to be supplied, the same air being used over and over again.

In Stirling's engine, the working cylinder being a separate apparatus from the air vessels, and being connected with the cold end of them, is kept quite cool, and thus is free from the objection urged against a former engine of Ericsson's, that it carbonized the oil by its excessive heat.

In an engine of 45 horse-power, which was actually constructed, and was in use for upwards of three years, driving all the lathes and other machinery of the Dundee Foundry, the economizer was composed of slips of the thinnest sheet iron that could be got, 38 inches long, and 11 inch broad, placed in the direction of the radii. The passages for the air, between the slips, were about one-fiftieth of an inch in width, and the slips being about the same thickness (one-fiftieth) there would be twenty-five slips in each inch of the circumference of the air vessels. These would expose an aggregate surface of about 460,000 square inches, or 3,200 square feet nearly, to heat and cool the air alternately in each air vessel.

The quantity of compressed air transferred through the economizer, or regenerator, at each stroke of the plunger, was 9 cubic feet nearly, so that for each cubic foot of air, there was an economising surface of 3200/9 = 355 square feet ; or, the compressed air being reduced to air of the ordinary density of the atmosphere, would give 9 x 10 (viz., 10 atmospheres) = 90 cubic feet, and 3200/90 = 35.5 square feet of surface, for each cubic foot of air of the ordinary density.

Ericsson's wire gauze is stated at 15,000 square feet, and as the supply cylinder sends in nearly 620 cubic feet of air at each stroke, 15000/620 = 24 square feet only of surface, for each cubic foot of air.

As it has been asserted, that Stirling's engine did work effectively and economically for a number of years, it will naturally be asked, for what reason was it abandoned at the Dundee Foundry? From information furnished by Mr. David Mudie, now one of the lessees of the Foundry, it appears, that the motion of the engine was not perfectly smooth and uniform, which was the only mechanical objection, and that not an important one ; but that the real cause of the engine having been set aside, or re-converted into a steam-engine, was, that the bottoms of the air vessels could not be made to withstand the heat to which they were exposed.

The engine of 45 horse-power was started in March 1843. In December 1845, (viz., two years and nine months after starting,) one air vessel gave way ; in May 1846, another failed, and in January 1847, a third, when the parties carrying on the Foundry, Mr. Stirling having left the management and gone to Edinburgh, got discouraged by these repeated failures, and removed the engine.

Mr. Stirling has since turned his attention to the best mode of remedying the defects, and there can be little doubt that he could satisfactorily accomplish his object, if he were to meet with support and encouragement, in carrying out and completing his experiments. At present, however, the only active movement being made towards turning this engine to account, seems to be in New York, where certain parties having collected all the information they can procure, as to the patents, and as to the engines constructed at Dundee, are endeavouring to improve Stirling's air-eugine, with the intention of bringing it out in preference to Ericsson's.

It remains only to offer a protest, against the use of the name "caloric engine," as being improper and as leading to misapprehension. Caloric may be a correct enough, although a somewhat pedantic, word for heat, but it by no means signifies heated air, any more than it does heated water, or steam, and a steam-engine is therefore quite as much entitled to the name as an air-engine. That the use of this name does engender a confusion of ideas is certain, and may be shown by reference to paragraphs and advertisements, in American newspapers, in which caloric, as a motive power, is freely spoken of in contradistinction to steam.