An evaporative cooler is a device that cools air by saturating it with water vapor.
A wet coil indirect evaporative cooler cools the air stream by evaporating water indirectly and without adding moisture into the air stream. Water is sprayed directly on the tubes of the heat exchanger where latent cooling takes place and the vaporization of the water on the outside of the heat exchanger tubes allows the simultaneous heat and mass transfer which removes heat from the supply air on the tube side.