Dew Point Temperature:

The normal thermodynamic state 1 as illustrate in the given Figure (a) of moist air is referred as unsaturated air. The water vapour presenting at temperature T of the mixture and partial pressure p_v of the vapour in the mixture is in a superheated state normally.

If a sample of unsaturated moist air having superheated water vapour is cooled up (at constant pressure), the mixture shall eventually attain the saturation temperature t_d of water vapour correspondent to its partial pressure p_v, at that point the primary drop of dew will be created, that means the water vapour in the mixture shall begin condensing. This temperature t_d is known as the dew point temperature (DPT).

           (a) Thermodynamic State of Water Vapour in Moist Air                               

Moisture may be eliminate from humid air through bringing the air in contact along a cold surface or cooling coil whose temperature is below its dew point temperature. Throughout the procedure of cooling, the partial pressure p_v of water vapour and specific humidity ω remain constant till the vapour begin condensing.