Page 85 - Industrial Ventilation Design Guidebook
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50 CHAPTER 4 PHYSICAL FUNDAMENTALS
state is adiabatic and reversible. Such an adiabatic reversible process is called
an isentropic state change: one in which the entropy remains constant.
The thermodynamic equations to be considered at this stage are
where
1
s is the entropy, kj kg" K" 1
l
b is the enthalpy, kj kg~
For isentropic process we can write
The specific enthalpy change is defined as
The state equation gives
When dT is eliminated from this equation, the following differential equation
results:
Solving,
When c p and R can be treated as constants, the equation is usually written as
For a gas of one-atom molecules K =5/3 = 1.67. For a gas of two-atom mole-
cules K = 7/5 = 1.4. For gas of molecules containing three or more atoms
K = 9/7 = 1.3.
For air (mostly a mixture of N 2 and O 2) the following is valid:
Water vapor is considered as an ideal gas and is defined by
where a and b are constants. Converting,
and as
giving
