Page 27 - Entrophy Analysis in Thermal Engineering Systems

18 Entropy Analysis in Thermal Engineering Systems

wherever there exists a temperature difference. Carnot had also understood
that motion caused by heat can only be due to a change of volume. In
describing the characteristics of perfect engine that would produce a max-
imum power, he wrote ([13], p. 57):
The necessary condition of the maximum is, then, that in the bodies employed to
realize the motive power of heat there should not occur any change of tempera-
ture which may not be due a change of volume. Reciprocally, every time that this
condition is fulfilled the maximum will be attained.

Carnot viewed establishment of a thermal equilibrium without production
of power as an actual loss. He suggested that every temperature difference
that is not due to a volume change or chemical reaction like combustion
should be avoided to maintain the condition of maximum power. Contrary
to the commonly accepted notion among the present scholars according to
which combustion process is viewed as a large source of “loss” in a power
producing device, Carnot had assumed the absence of any chemical reaction
in his demonstration (Ref. [13], p. 56):
The chemical action which takes place in the furnace is, in some sort, a preliminary
action, -an operation destined not to produce immediately motive power, but to
destroy the equilibrium of the caloric, to produce a difference of temperature which
may finally give rise to motion.

Carnot then devised the operation of an engine that would operate in a cycle
while communicating with two thermal reservoirs of fixed but different tem-
peratures. Any change in the temperature of the working substance may occur
due to compression or expansion only, and the heat exchange processes take
place at the reservoirs’ temperatures. To prove that his proposed engine pro-
duces a maximum power, Carnot employs a rather philosophical reasoning.
He first argues that if the operation of the engine is reversed; that is, to operate
it as a refrigerator, the mechanical and thermal effects of the engine would be
identical to those if it were operated backward, i.e., as a refrigerator. He then
reasons that if there were any method (i.e., engine) to produce a greater power
than that of his proposed engine, it would be enough to spend a portion of this
power to transfer heat from the cold reservoir to the hot reservoir by execut-
ing the Carnot engine backward, whereby a combination of the two engines
would lead to a perpetual machine.
The above method of demonstration, despite founded erroneously on
caloric theory, has since been used by not only his successors like Clausius,