industrial apparatus for converting liquid into vapour. The single-effect evaporator consists of a container or surface and a heating unit; the multiple-effect evaporator uses the vapour produced in one unit to heat a succeeding unit. Double-, triple-, or quadruple-effect evaporators may be required in industrial and steam heating plants.

Some evaporators are used to concentrate a solution by vapourizing and eliminating water, as, for example, in a concentration plant for sugar and syrup. In purification processes, such as the desalination of seawater, evaporators convert the water to vapour, leaving mineral residues in the evaporator. The vapour then is condensed into (desalinated) water. In a refrigeration system, the cooling effect is produced as the rapid evaporation of the liquid refrigerant absorbs heat.