ASHRAE Journal - June 2021 - 34

REFRIGERATION APPLICATIONS
COLUMN
Andy Pearson
Celebrating Sadi Carnot
BY ANDY PEARSON, PH.D., C.ENG., FELLOW ASHRAE
June 1, 2021, is the 225th birthday of Sadi Carnot, an original thinker who was not
afraid to ask the questions that nobody else had thought. His attempts to answer them
were hampered by deficiencies in the scientific theory of the day, and sadly he died in
a cholera outbreak at the age of 36 before he had managed to untangle himself from
the contemporary understanding of the nature of heat that was holding him back.
Carnot is often portrayed as a maverick loner, working
outside the establishment, whose ideas about heat
popped out of nowhere and failed to make an impact at
the time because of his isolation from mainstream scientific
thought. Like most of history this is partly true,
but misses a lot of the detail that helps to put his work in
context.
Carnot trained for three years from the
age of 16 at the École Polytechnique in Paris,
where the staff at that time included Ampère,
Gay-Lussac, Arago and Poisson and their
international contacts included Biot, von
Humboldt, Foucault and Faraday. However,
he had the misfortune to graduate in 1814 as
a second lieutenant in the Corps of Engineers
in Napoleon's army the year before the Battle
of Waterloo.
Carnot's father, who had been Minister of the Interior
in Napoleon's government, was exiled and shortly afterwards
Carnot left active military service and transferred
to the General Staff in Paris. This gave him the opportunity
to reconnect with scientific thinking and he furthered
his education by attending lectures in industrial
chemistry by Professor Clément, striking up a friendship
with the Professor in 1819 and working with him on
the improvement of steam engine efficiency. Clément
was also responsible for defining the calorie as a unit of
heat-the amount of heat required to heat one kilogram
of water by one degree Celsius.
Rather than popping out of nowhere, Carnot's now
famous publication " Réflexions sur la Puissance Motrice du
Feu " ( " Reflections on the Motive Power of Fire " ) was the result
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At last, a picture of the Carnot
cycle that I understand.
of five years of intense study with Clément and his colleagues,
probing the shortcomings of existing theory.
Carnot's major significant breakthrough was the observation
that " The motive power of a waterfall depends
on its height and on the quantity of liquid; the motive
power of heat depends also on the quantity of caloric
used, and on what may be termed, on what
in fact we will call, the height of its fall. (The
matter here dealt with being entirely new, we
are obliged to employ expressions not in use
as yet, and which are perhaps less clear than
is desirable.) " His insight that the " height
of the fall " of heat set limits on the process
took another quarter-century to travel from
the fringes of scientific thought to the mainstream.
His big idea was almost lost to posterity
because most of his papers were burned
after his death due to fear of cholera infection and it was
a descriptive interpretation of Réflexions by Clapeyron,
published two years after Carnot's death, that provided
a picture signpost to the next generation of heat investigators
by plotting Carnot's ideal cycle on a graph of pressure
against volume.
I am indebted to ASHRAE Journal reader Bob Hanlon
who has produced an excellent (and very accessible)
textbook called Block by Block: The Historical and Theoretical
Foundations of Thermodynamics. The extract from Réflexions
quoted above and much of the historical context comes
from Bob's book, which he says took 20 years to write. It
should be on every refrigeration engineer's bookshelf.
Andy Pearson, Ph.D., C.Eng., is group managing director at Star Refrigeration in
Glasgow, UK.
https://www.ashrae.org/
ASHRAE Journal - June 2021

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