Psychrometric Properties:
The properties of moist air are known as psychrometric properties and the subject that deals with the behaviour of moist air is called psychrometry.
Moist air is a combination of dry air & water vapour. They make a binary combination. A combination of two substances needs three properties to fully define its thermodynamic state, unlike a pure substance that needs only two. One of the three properties may be the composition. Water vapour is existed in the atmosphere on a very low partial pressure. At this low pressure & atmospheric temperature, the water vapour acts as a perfect gas. The partial pressure of dry air is also below one atmosphere that can also be refereed to act very much like a perfect gas. The Gibbs-Dalton laws of perfect gas combination can be applied to the moist air.
As the water vapour part is constantly variable, all calculations in air- conditioning practice are depending on the dry air part.
For calculating & defining the psychrometric properties, we can consider a definite volume V of moist air at pressure p & temperature T, having ma kg of dry air and mv kg of water vapour as illustrated in given Figure. The actual temperature t of moist air is known as the dry bulb temperature (DBT). The net pressure p that is equal to the barometric pressure is constant.
