Working Substance In Air Conditioning:

 A significant thing for the student of air conditioning is to value that the working substance under study, like, moist air, is a mixture of two of gases. One of these  gas is dry air that  itself is a mixture of a number of gases and the other one is water vapour that may exist in a saturated or superheated state.

One may ask whether moist air could be considered as a pure substance. However a pure substance is homogeneous & invariable in chemical composition. Therefore, a homogeneous mixture of gases is a pure substance till its components do not alter in phase. Dry air is a good instance of such a type of pure substance. Water vapour is surely a pure substance. However moist air is not a pure substance in any procedure in which condensation or evaporation of moisture take place. In that case, regular charts ought to be developed to explain the thermodynamic properties of the mixture under different conditions & compositions.

Thus it is seen that moist air consists of two portions: First, comprising dry air, referred as the fixed part, and the other one, solely of water vapour, referred as the variable part.

Dry air and water vapour both can be referred as perfect gases as both of exist in the atmosphere at low pressures. Therefore, perfect gas laws may be applied to them individually. Additionally, for non-reactive mixtures of gases Gibbs-Dalton laws may be applied to the dry air portion only to get its properties like a single pure substance, before set up the properties of moist air.