REFRIGERATION REFRIGERANTS
to its then present state, the underlying theory of steam engines was not well-understood. Carnot asks the question, much discussed at the time, whether there is an upper bound to the amount of ‘motive power’ (work) that can be derived from heat. His arguments are somewhat diffi cult to follow because he develops them using the 18th
century theory of ‘caloric’.
Mendoza (see foot note) discusses this in his introduction to the Dover edition of Carnot’s monograph suggesting caloric can equated to ‘entropy’, although Carnot sometimes used heat and caloric interchangeably.
Carnot states that the availability of a high temperature heat/caloric source is not suffi cient in itself to produce motive power, a low temperature sink must also be available. His second key point is that all substances capable of undergoing a volume change when their temperatures are changed are capable of producing work from heat, including metal bars and liquids. He then poses a second question. “It is natural to ask here this curious and important question: Is the motive power of heat invariable in quantity, or does it vary with the agent employed to realize it as the intermediary substance, selected as the subject of action of the heat?”
Carnot describes the concept of the steam passing through a cycle: steam generation, expansion producing work and condensation, although the description at this point in the paper is incomplete since the water injection into the boiler is absent.
He then describes what is essentially a heat pump, a device acting as the opposite of a steam engine. (The concept of the vapour- compression refrigeration cycle was fi rst mooted by the American inventor, Oliver Evans, in 1805. But since Carnot generously acknowledges
Carnot describes the concept of the steam passing through a cycle: steam
generation, expansion producing work and condensation, although the description at this point in the paper is incomplete since the water injection into the boiler is absent.
earlier published work, I suspect he was unaware of Evans’ suggestion.) Carnot states: “The operations which we have just described [referring to the steam engine] might have been performed in an inverse direction and order. There is nothing to prevent forming vapor with the caloric of the body B, and at the [low] temperature of that body, compressing it [i.e. doing work on the steam] in such a way as to make it acquire the [high] temperature of the body A, fi nally condensing it by contact with this latter body, and continuing the compression to complete liquefaction.” This is equivalent to Clausius’ statement of the Second Law of Thermodynamics: “It is impossible to construct a device that produces no other eff ect than transfer of heat from lower temperature body to higher temperature body.”
Carnot then writes that if the work delivered Download the ACR News app today
by a steam engine drives a heat pump that raises more heat to the high temperature than the quantity of heat used by the engine, then overall the system would be a perpetual motion machine, generally accepted to be impossible, concluding: “… that the maximum of motive power resulting from the employment of steam is also the maximum of motive power realizable by any means whatever.” A more robust argument is given a few pages later where a complete cycle is described in detail using expanding and contracting with the absorption and rejection of heat with air as the working fl uid could apply to any substance, concluding that by this cyclic sequence “…we are led to establish this general proposition: The motive power of heat is independent of the agents employed to realise it; its quantity is fi xed solely by the temperatures of the bodies between which is eff ected, fi nally, the transfer of the caloric… The necessary condition of the maximum is… that in the bodies employed to realize the motive power of heat there should not occur any change of temperature which may not be due to a change of volume.” Carnot’s genius lay in his ability to see through the complexities of steam and other ‘heat engines’ to the fundamental principles, without being distracted by various ineffi ciencies caused by losses such as friction and adventitious heat loss. As the father of Thermodynamics, in particular by laying the foundations for the Second Law of Thermodynamics, Carnot perhaps received his greatest accolade from the 20th Century astrophysicist Sir Arthur Eddington, who wrote: “The law that entropy always increases holds, I think, the supreme position among the laws of Nature. If someone points out to you that your pet theory of the universe is in disagreement with Maxwell’s equations — then so much the worse for Maxwell’s equations. If it is found to be contradicted by observation — well, these experimentalists do bungle things sometimes. But if your theory is found to be against the second law of thermodynamics I can give you no hope; there is nothing for it but to collapse in deepest humiliation”. (New Pathways in Science, 1935). Albert Einstein clearly concurred: ‘[classical thermodynamics] is the only physical theory of universal content which I am convinced will never be overthrown, within the framework of applicability of its basic concepts’. In 2024, those of us working in the modern
refrigeration and HVAC industries should raise a glass of wine (French!) to toast Carnot, and, adapting Newton’s words, acknowledge that if we, “…have seen further than others it is by standing on the shoulders of giants.”
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