以下の論文は1851年にケルビン（Ｗ．トムソン）が、空気ポンプによって建物を暖房あるいは冷房することの可能性を論じたものです。今日の熱ポンプの本質を、当時明らかになった熱力学原理により、あますとこなく見事に説明している。
[グラスゴー哲学協会紀要 Vol.Ⅲ 1852年12月]
“On the Economy of the Heating or Cooling of Buildings by means of Currents of Air”; From the Glasgow Phil, Soc. Proc. Vol. Ⅲ. Dec. 1852年
On the Economy of the Heating or Cooling of Buildings by means of Currents of Air.
If it be required to introduce a certain quantity of air at a stated temperature higher than that of the atmosphere into a building, it might at first sight appear that the utmost economy would be attained if all the heat produced by the combustion of the coals used were communicated to the air ; and in fact the greatest economy that has yet been aimed at in heating air or any other substance, for any purpose whatever, has had this for its limit.
If an engine be employed to pump in air for heating and ventilating a building (as is done in Queen's College, Belfast), all the waste heat of the engine, along with the heat of the fire not used in the engine, may be applied by suitable arrangements to warm the entering current of air; and even the heat actually converted into mechanical effect by the engine, will be reconverted into heat by the friction of the air in the passages, since the overcoming of resistance depending on this friction is the sole work done by the engine.
It appears therefore that whether the engine be economical as a converter of heat into mechanical work, or not, there would be perfect economy of the heat of the fire if all the heat escaping in any way from the engine, as well as all the residue from the fire, were applied to heating the air pumped in, and if none of this heat were allowed to escape by conduction through the air passages.
It is not my present object to determine how nearly in practice this degree of economy may be approximated to ; but to point out how the limit which has hitherto appeared absolute, may be surpassed, and a current of warm air at such a temperature as is convenient for heating and ventilating a building may be obtained mechanically, either by water power without any consumption of coals, or, by means of a steam engine, driven by a fire burning actually less coals than are capable of generating by their combustion the required heat; and secondly, to show how, with similar mechanical means, currents of cold air, such as might undoubtedly be used with great advantage to health and comfort for cooling houses in tropical countries*, may be produced by motive power requiring (if derived from heat by means of steam engines) the consumption of less coals perhaps than are used constantly for warming houses in this country.
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The mode of action and apparatus proposed for this purpose differs tram that proposed originally by Professor Piazzi Smyth for the same purpose, only in the use of an egress cylinder, by which the air is made to do work by its extra pressure and by expansion in passing from the reservoir to the locality where it is wanted, which not only saves a great proportion of the motive power that would be required were the air allowed simply to escape through a passage, regulated by a stop-cook or otherwise, but is absolutely- essential to the success of the project, as it has been demonstrated by Mr. Joule and the author of this communication, that the cold of expansion would be so nearly compensated by the heat generated by friction, when the air is allowed to rush out without doing work, as to give not two-tenths of a degree of cooling effect in apparatus planned for 30 degrees.
The use of an egress cylinder has (as the meeting was informed by Mr. Macquorn Bankine), recently been introduced into plans adopted by a committee of the British Association appointed to consider the practicability of Professor Piazzi Smyth's suggestion, with a view to recommending it to government for public buildings in India.
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In the mathematical investigation communicated with this paper, it is shown in the first place, according to the general principles of the dynamical theory of heat, that any substance may be heated thirty degrees above the atmospheric temperature by means of a properly contrived machine, driven by an agent spending not more than about 1/35 of energy of the heat thus communicated;
and that a corresponding machine, or the same machine worked backwards, may be employed to produce cooling effects, requiring about the same expenditure of energy in working it to cool the same substance through a similar range of temperature.
When a body is heated by such means, about 34/35 of the heat is drawn from surrounding objects, and 1/35 is created by the action of the agent; and when a body is cooled by the corresponding process, the whole heat abstracted from it, together with a quantity created by the agent, equal to about 1/35 of this amount, is given out to the surrounding objects.
A very good steam engine converts about 1/10 of the heat generated in its furnace into mechanical effect; and consequently, if employed to work a machine of the kind described, might raise a substance thirty degrees above the atmospheric temperature by the expenditure of only 10/35, or 2/7, that is, less than one-third of the coal that would be required to produce the same elevation of temperature with perfect economy in a direct process.
If a water-wheel were employed, it would produce by means of the proposed machine the stated elevation of temperature, with the expenditure of 1/35 of the work, which it would have to spend to produce the same heating effect by friction.
The machine by which such effects are to be produced must have the properties of a "perfect thermo-dynamic engine," and in practice would be either like a steam engine, founded on the evaporation and re-condensation of a liquid (perhaps some liquid of which the boiling point is lower than that of water), or an air engine of some kind.
If the substance to be heated or cooled be air, it will be convenient to choose this itself as the medium operated on in the machine.
For carrying out the proposed object, including the discharge of the air into the locality where it is wanted, the following general plan was given as likely to be found practicable.
Two cylinders, each provided with a piston, ports, valves, and expansion gearing, like a high-pressure double-acting steam engine, are used, one of them to pass air from the atmosphere into a large receiver, and the other to remove air from this receiver and discharge into the locality where it is wanted.
The first, or ingress cylinder and the receiver, should be kept with their contents as nearly as possible at the atmospheric temperature, and for this purpose ought to be of good conducting material, as thin as is consistent with the requisite strength, and formed so as to expose as much external surface as possible to the atmosphere, or still better, to a stream of water.
The egress cylinder ought to be protected as much as possible from thermal communication with the atmosphere or surrounding objects.
According as the air is to be heated, or cooled, the pistons and valve gearing must be worked so as to keep the pressure in the receiver below, or above, that of the atmosphere.
If the cylinders be of equal dimensions, the arrangement when the air is to be heated, would be as follows: The two pistons working at the same rate, air is to be admitted freely from the atmosphere into the ingress
cylinder, until a certain fraction of the stroke, depending on the heating effect required, is performed, then the entrance port is to be shut, so that during the remainder of the stroke the air may expand down to the pressure of the receiver, into which, by the opening of another valve, it is to be admitted in the reverse stroke ; while the egress cylinder(*) is to draw air freely from the receiver through the whole of each stroke on one side or the other of its piston, and in the reverse stroke first to compress this air to the atmospheric pressure (and so heat it as required), and then discharge it into a pipe leading to the locality where it is to be used.
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In this case the egress cylinder acts merely as an air pump, to draw air from the receiver and discharge it into the locality where it is wanted, and the valves required for this purpose might be ordinary self-acting pomp-valves.
A similar remark applies to the action of the ingress cylinder in the use of the apparatus for producing a cooling effect on the air transmitted, which will then be that of a compressing air-pump to force air from the atmosphere into the receiver.
But in order that the same apparatus may be used for the double purpose of heating or cooling, as may be required at different seasons, it will be convenient to have the valves of each cylinder worked mechanically, like those of a steam engine.
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If it be required to heat the air from 50° to 80° Fahr., the ratio of expansion to the whole stroke in the ingress cylinder would be 18/100, the pressure of the air in the receiver would be 82/100 of that of the atmosphere (about 27 lbs. on the square inch below the atmospheric pressure), and the ratio of compression to the whole stroke in the egress cylinder would be 13/100.
If 1 lb. of air (or about 13(1/2) cubic feet, at the stated temperature of 80°, and the mean atmospheric pressure,) be to be delivered per second, the motive power required for working the machine would be 283 of a horse power, were the action perfect, with no loss of effect, by friction, by loss of expansive power due to cooling in the ingress cylinder, or otherwise.
If each cylinder be four feet in stroke, and 26.3 inches diameter, the pistons would have to be worked at 26.1 double strokes per minute.
On the other hand, if it be desired to cool air, either the ingress piston must be worked faster than the other, or the stroke of the other must he diminished, or the ingress cylinder must be larger, or an auxiliary ingress cylinder must he added.
The last plan appears to he undoubtedly the best, as it will allow the two principal pistons to be worked stroke for stroke together, and consequently to be carried by one piston rod, or by a simple lever, without the necessity of any variable connecting gearing, whether the machine be used for heating or for cooling air; all that is necessary to adapt it to the latter purpose, besides altering the valve gearing, being to connect a small auxiliary piston to work beside the principal ingress cylinder, with which it is to have free communication at each end.
If it were required to cool air from 80° to 50° Fahr., the auxiliary cylinder would be required to have its volume 1/17 of that of each of the principal cylinders; and, if its stroke be the same, its diameter would therefore be a little less than a quarter of theirs.
The valves would have to be altered to give compression in the ingress cylinder during the same fraction of the stroke as is required for expansion when the air is heated through the same range of temperature, and the valves of the egress cylinder would have to give the same proportion of expansion as is given of compression in the other case; and the pressure kept up in the receiver, by the action of the pistons thus arranged, would be 1(18/100) atmos., or about 3.2 lbs. on the square inch above the atmospheric pressure.
The principal cylinders being of the same dimensions as those assumed above, and the quantity of air required being the same (1 lb. per second), the pistons would have to be worked at only 21.4 double strokes per minute instead of 26.1, and the horse power required would be 288, instead of as formerly 283, when the same machine was used for giving a supply of heated air.
[Note added June 26, 1881.
The method of cooling air in unlimited quantities described in this article has been realized by Mr. Coleman, first in refrigerators used for the distillation of paraffin, and after that in the Bell-Coleman refrigerator, for carrying supplies of fresh meat from North America to Europe; in a great refrigerator recently sent out for the Abattoir at Brisbane, Queensland; and other large practical applications of a similar kind. The Bell-Coleman machine sends large quantities of air cooled 10° or 20℃ below freezing point into the chamber to be kept cool; and the general temperature of this chamber is thus maintained at the desired point, which, for the case of carrying fresh meat from America to this country is about 35° F.
The method of heating air described in the article remains unrealized to this day.
When Niagara is set to work for the benefit of North America through electric conductors, it will no doubt be largely employed for the warming of houses over a considerable part of Canada and the United States.
But it is probable that it will also have applications though less large in other cold countries, to multiply the heat of coal and other fuel, and to utilize wind and water power (with aid of electric accumulators) for warming houses.]