An evaporative cooler is a device that cools air by saturating it with water vapor.
A packaged indirect evaporative cooler cools the air stream by evaporating water indirectly and without adding moisture into the air stream. On one side of the heat exchanger, the secondary air stream is cooled by evaporation, while on the other side of heat exchanger, the primary air stream (conditioned air to be supplied to the room) is sensibly cooled by the heat exchanger surfaces.