From the equation for enthalpy of moist air, the humid specific heat of moist air can be written as:
where
cpm = humid specific heat, kJ/kg.K
cp = specific heat of dry air, kJ/kg.K
cpw = specific heat of water vapor, kJ/kg
W = humidity ratio, kg of water vapor/kg of dry air.
Since the second term in the above equation (w.cpw) is very small compared to the first term, for all practical purposes, the humid specific heat of moist air, cpm can be taken as 1.0216 kJ/kg dry air.K
Specific volume: The specific volume is defined as the number of cubic meters of moist air per kilogram of dry air. From perfect gas equation since the volumes occupied by the individual substances are the same, the specific volume is also equal to the number of cubic meters of dry air per kilogram of dry air, i.e.,
