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

Thermodynamics: Vapour Pressure and Boiling Temperature

Every liquid has its tendency to vaporize. Some liquid are more volatile and some are less, for example Hg has a very less tendency to vaporize so, we use Hg in the manometer for measuring pressure. 
Vapour Pressure is the pressure exerted by the vapours in equilibrium with the liquid in pure form at a constant temperature. For an ideal mixture, we can calculate the partial pressure exerted by the vapour, by multiplying its vapour pressure and mole fraction in liquid form. Vapour pressure is a function of temperature, as temperature increases vapour pressure increases and when the vapour pressure becomes equal to surrounding pressure, the corresponding temperature is called Boiling Temperature. So, boiling temperature is also a function of pressure. As pressure increases, boiling temperature increases and with decrease in pressure boiling point decreases.
Pressure cooker is used to cook vegetables, because it increases the temperature the temperature of steam by pressing it and the vegetables cook at high temperature and takes less time.