Question

Prove that the enthalpy of the humid air remains constant along a wet bulb temperature
line on the psychrometric chart.​

Answer 1

In order to perform air conditioning calculations, it is essential first to estimate

various properties of air. It is difficult to estimate the exact property values of moist

air as it is a mixture of several permanent gases and water vapour. However, moist

air upto 3 atm. pressure is found to obey perfect gas law with accuracy sufficient for

engineering calculations. For higher accuracy Goff and Gratch tables can be used

for estimating moist air properties. These tables are obtained using mixture models

based on fundamental principles of statistical mechanics that take into account the

real gas behaviour of dry air and water vapour. However, these tables are valid for a

barometric pressure of 1 atm. only. Even though the calculation procedure is quite

complex, using the mixture models it is possible to estimate moist air properties at ther pressures also. However, since in most cases the pressures involved are low,

one can apply the perfect gas model to estimate psychrometric properties