453
Chapter
17
Engineering Fundamentals:
Part 2
Thermodynamics
17.1 Introduction
Thermodynamics is an aspect of physics which deals with the energy
characteristics of materials and with the behavior of systems under-
going changes in system energy levels. The field of thermodynamics
is quite broad as well as deep, and can vary in presentation and in
application from relatively simple to very complex. For the purposes
of this book, a relatively simple presentation is adequate. The concepts
of thermodynamics presented here are common to virtually all text-
books and reference books. For those who want greater detail, Chap.
1 of the ASHRAE Handbook Fundamentals is one presentation writ-
ten at a college upper-division or graduate-student level.
1,2
17.2 Thermodynamics Terms
One problem with understanding thermodynamics is that the basic
terms energy and entropy are defined in relatives rather than abso-
lutes.
Energy can be reduced to the concepts of heat and work and can be
found in various forms: potential energy, kinetic energy, thermal or
internal energy, chemical energy, and nuclear energy.
Potential energy is the energy of location or position of a mass in a
force field. A body or a volume of water at the top of a hill has potential
energy with respect to the bottom of the hill.
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freezing point and boiling point of water. The Celsius scale defined the
difference in terms of 100 units with 0 as the freezing point and 100
as the boiling point. The Fahrenheit scale uses the freezing point of a
salt solution as the 0 point with pure water freezing and boiling at 32
and 212ЊF, respectively. The lowest possible temperature, the condition
at which molecular motion ceases, is called absolute zero. The absolute
scale which uses the Celsius increment is called the Kelvin scale. It
places absolute zero at Ϫ273ЊC, or the ice melting point of water at
ϩ273K. The absolute scale which uses the Fahrenheit increment is
called the Rankine scale. It places absolute zero at Ϫ460ЊF, or the ice
melting point of water at ϩ492ЊR. There is no upper limit to a possible
temperature.
17.3 First Law of Thermodynamics
The first law of thermodynamics sounds like the law of the conser-
vation of mass, with different vocabulary. If the law of mass conser-
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Engineering Fundamentals: Part 2 455
vation asserts that matter can be neither created nor destroyed, then
the first law of thermodynamics states that energy cannot be created
or destroyed. This implies that the various forms of energy may be
converted, one to another. It means that we can account for all energy
conversions in a system with accuracy.
Energy in Ϫ Energy out ϭ Change in stored energy
Entropy is used to define the unavailable energy in a system. In
another sense, entropy defines the relative ability of one system to act
on another. As things move toward a lower energy level, where one is
less able to act upon the surroundings, the entropy level is said to

L
is the
temperature of the low energy region.
As the temperatures approach equilibrium (T
L
ϭ T
H
), the process
efficiency tends toward zero.
The second statement of the second law is credited to Clausius, who
said, ‘‘No machine whose working fluid undergoes a cycle can absorb
heat from one system, reject heat to another system and produce no
other effect.’’
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456 Chapter Seventeen
Both statements of the second law place constraints on the first law
by identifying that under natural conditions, things (including energy)
run downhill. We must expend energy to make it happen otherwise.
It takes energy to drive cold to hot, e.g., a refrigerator. It takes energy
to raise a weight against gravity. A corollary is that there is no such
thing as a perpetual-motion machine. We cannot get something for
nothing. This means that in all the energy balances of the first law,
we know that some things will not happen unless we expend or give
up something to make them happen.
Ⅲ
Example: A heat resource cannot be fully converted to work. Work
can be withdrawn from heat moving from a high-energy region to

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Engineering Fundamentals: Part 2 457
Q
evap
COP ϭ
cool
Q
in
In the classical sense, similar to the definition of efficiency, there is a
theoretical limit to the COP defined in terms of the temperature (low)
of the cold region T
L
related to the temperature (high) of the warm
region T
H
T
L
COP ϭ (17.1)
max
T Ϫ T
HL
For a building chiller evaporating at 40ЊF (500ЊR) and condensing at
100ЊF (560ЊR), the maximum COP would be
500
COP ϭϭ8.3 (17.2)
max
560 Ϫ 500

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