Multiple-effect distillation (MED) is a distillation process often used for sea water desalination. It consists of multiple stages or “effects”. In each stage the feed water is heated by steam in tubes. Some of the water evaporates, and this steam flows into the tubes of the next stage, heating and evaporating more water. Each stage essentially reuses the energy from the previous stage.

The tubes can be submerged in the feed water, but more typically the feed water is sprayed on the top of a bank of horizontal tubes, and then drips from tube to tube until it is collected at the bottom of the stage.