Mass Transfer: Evaporative cooling

When hot dry air comes in contact with water, water will evaporate because the partial pressure of water in liquid phase {vapor pressure X mole fraction (1)} is greater than partial pressure of water in air (total pressure X mole fraction of water in air). Due to difference between partial pressure of water in liquid and gas phase, the water will evaporate till thr partial pressure becomes equal in both phase. The heat required for evaporation of water (540 kcal/kg) comes from the specific heat of water (1 kcal/kg K) and air (0.2404 kcal/kg K), so the temperature of air and water decreases. This principle is used in the following:

1.         Desert/air cooler: The objective is to get cold air

2.         Cooling tower:  The objective is to get cold water in chemical industries

3.         Earthen pots: Cold water

4.         Evaporation of sweat from the body makes the temperature of body cooler