Environmental Engineering Reference
In-Depth Information
As a consequence, in a process for which
d
0, which is called an
adiabatic
process, the
entropy may remain the same or increase but may never decrease. An adiabatic process for which
the entropy increases is an irreversible process because the reverse of this process, for which the
entropy would decrease (
dS
Q
=
0), violates the second law as expressed in Clausius' inequality.
There are many important consequences of Clausius' inequality that we will not examine in
detail in this chapter, but will be identified as such at the appropriate occasion of use. Among them
are the conditions for thermodynamic equilibrium, including thermochemical equilibrium, and the
limits on the production of useful work in cycles or processes.
3
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3.6
THERMODYNAMIC PROPERTIES
The most common methods for utilizing the energy of fossil or nuclear fuels require the use of
fluids as the means to generate mechanical power or to transport energy to a desired location. The
thermodynamic properties of fluids thereby assume a great importance in the systems that transform
energy.
We know that in a steam power plant the working fluid, water, undergoes large changes in
temperature and pressure as it moves through the boiler, turbine, and condenser. The thermodynamic
properties pressure
p
and temperature
T
are called
intensive
properties because their values are not
proportionate to the mass of a fluid sample but are the same at all points within the sample. On the
other hand, the energy
E
, volume
V
, and entropy
S
are
extensive
properties in that their values are
directly proportionate to the mass of a fluid sample.
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But if we divide an extensive property, such
as
E
, by the mass
M
of fluid whose energy is
E
, then the ratio
E
M
, called the
specific energy,
is
independent of the amount
M
of fluid. Denoting specific extensive properties by a lowercase letter,
we have for the specific energy
e
, volume
/
v
, and entropy
s
E
M
,
V
M
,
S
M
e
≡
v
≡
s
≡
(3.12)
The use of specific extensive properties simplifies the analysis of thermodynamic systems utilizing
fluids and other materials to produce work or transform energy. By following the changes expe-
rienced by a unit mass of material as it undergoes a change within the system, the work and heat
quantities per unit mass may be determined. The total work and heat amounts for the system may
then be calculated by multiplying the unit quantities by the total mass utilized in the process.
The first and second law properties, energy and entropy, are sometimes not convenient to
use in analyzing the behavior of thermodynamic systems. Rather, particular combinations of the
properties
p
,
T
,v,
e
, and
s
turn out to be more helpful. One of these useful properties is called the
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The third law of thermodynamics is an additional principle that is closely related to the second law. It states
that the entropy of all thermodynamic systems is zero at the absolute zero of temperature. Among other things,
it is important to the determination of the free energy change in combustion reactions.
4
In the following discussion, we disregard the kinetic and potential energy components of the total energy as
expressed in equation (3.1) as we are considering the properties of a system that is stationary in the earth's
gravity field. When it is necessary to take this motion into account, as in a flow through a turbine, we will
explicitly add these additional energy components at the appropriate point.
