The Superiority of Air

Source: The Engineer, July 26th, 1867.
Date: July 22nd, 1867 - Paris
Title: The Second Law of Thermodynamics
Author: Philander Shaw

SIR,— In my letter published in THE ENGINEER of July 12th, I endeavoured to show that the superior economic value of atmospheric air as a fluid for the transmission of the power of heat is a real substantial thing, which may be made available in practice.

I proved that the second law of thermodynamics - which informs us that in order that the whole heat expended in a heat engine may be converted into external work it is necessary that the temperature of the condenser or refrigerator should be the absolute zero, a temperature unattainable by human means - has no force as applied to the air engine, and that, theoretically, the whole heat expended in a heat engine may be converted into external work within a range of temperature which is perfectly practical.

In THE ENGINEER of July 19th, in referring to the subject, Professor Rankine observes, that "when a heat engine is driven by a working substance that is not perfectly gaseous – when the substance, for example, exists successively in the conditions of liquid and of vapour – the mutual attractions of the particles produce a diminution of the work obtained through expansion with a given expenditure of heat, and an exactly equal diminution of the work expended in compression, so that the final result as to the maximum possible efficiency between given limits of temperature is exactly the same as in the case of a perfect gas".

If I understand the above statement rightly it is just this — theoretically there is no difference in the economic value of the fluids, air and water, when used for the transmission of the power of heat, and, as in the steam engine, the nature of the fluid used requires that three-fourths of all the heat imparted to it shall be “consumed in interior work, pulling the liquid molecules asunder,” exactly the same amount of heat is expended in an air engine in compressing the air to the density required to work the engine.

"Heat and mechanical energy are mutually convertible. Mechanical energy expended in compressing bodies is a source of heat. Heat by expanding bodies is a source of mechanical energy". If then, to work the air engine a certain amount of mechanical energy is exerted to compress the air, that mechanical energy is not lost but is converted into heat and stored up in the volume of compressed air, which in my air engine is immediately converted into mechanical energy, and used to compress another volume of air.

Here, then, the issue is fairly made between the possible economy of the steam and air engines, and if I succeed in showing that no part of the heat generated by the fuel consumed in an air engine is expended or mechanical energy lost by compressing the air, the great theoretical superiority of air over water as a fluid for the transmission of the power of heat must be admitted.

"The total energy of substance or system cannot be altered by the mutual actions of its parts"; and as no exterior work results from the mechanical energy used to compress the air, the heat produced must be the exact mechanical equivalent of the energy expended. The compressed air is a spring capable of exerting the same amount of mechanical energy by expansion that has been expended in compressing it.

The heat produced in the compressed air is no part of the heat produced by combustion in the engine, and consequently, it follows that instead of three-fourths of the heat expended in an air engine being consumed in compressing the air, there is theoretically none at all, and the superior economic value of air, as a fluid for the transmission of the power of heat, remains as shown in my letter of July 9th.