Handbook of air conditioning and refrigeration / Shan K

2.2 CHAPTER TWO
The amount of water vapor present in moist air at a temperature range of 0 to 100°F (�17.8 to
37.8°C) varies from 0.05 to 3 percent by mass. It has a significant influence on the characteristics of
moist air.
Water vapor is lighter than air. A cloud in the sky is composed of microscopic beads of liquid
water that are surrounded by a thin layer of water vapor. These layers give the cloud the needed
buoyancy to float in the air.
Equation of State of an Ideal Gas
The equation of state of an ideal gas indicates the relationship between its thermodynamic properties,
or
pv � RTR where p � pressure of gas, psf (Pa)
v � specific volume of gas, ft3 /lb (m3 /kg)
R � gas constant, ft�lbf/lbm�°R (J/kg�K)
TR � absolute temperature of gas, °R (K)
Since v � V/m, then Eq. (2.1) becomes
where V � total volume of gas, ft 3 (m 3 )
m � mass of gas, lb (kg)
pv � mRT R
Using the relationship m � nM, and R � R o/M, we can write Eq. (2.2) as
pv � nRoTR where n � number of moles, mol
M � molecular weight
Ro � universal gas constant, ft�lbf/lbm�°R (J/mol�K)
Equation of State of a Real Gas
A modified form of the equation of state for a real gas can be expressed as
where A, B, C, ����virial coefficients and Z � compressibility factor. The compressibility factor
Z illustrates the degree of deviation of the behavior of the real gas, moist air, from the ideal gas
due to the following:
1. Effect of air dissolved in water
Nitrogen 78.08
Oxygen 20.95
Argon 0.93
Carbon dioxide 0.03
Other gases such as neon, sulfur dioxide, etc. 0.01
pv
RT R
� 1 � Ap � Bp 2 � Cp 3 �� � ��Z
(2.1)
(2.2)
(2.3)
(2.4)