The MED evaporator consists of several consecutive cells maintained at a decreasing level of pressure (and temperature) from the first (hot) to the last (cold). Each cell (also called effect) contains a horizontal tube bundle. The top of the bundle is sprayed with sea water make-up that flows down from tube to tube by gravity. Heating steam is introduced inside the tubes. Since tubes are cooled externally by make-up flow, steam condenses into distillate (fresh water) inside the tubes. The heat released by the condensation (latent heat)warms up the sea-water outside the tubes and partly evaporates it. Due to evaporation, sea water slightly concentrates when flowing down the bundle and gives brine at the bottom of the cell. The vapour raised by sea-water evaporation is at a lower temperature than heating steam. However it can still be used as a heating medium for the next effect where the process repeats.In the last cell, the produced steam condenses in a conventional shell and tubes heat exchanger. This exchanger, called"distillate condenser" or "final condenser" is cooled by sea-water. At the outlet of the final condenser, part of the warmed sea-water is used as make-up of the unit, the other part is rejected to the sea.
Brine and distillate are collected from cell to cell till the last one, where from they are extracted by centrifugal pumps.
The heating steam of the first effect is generally low pressure condensing steam (as low as 0.3 bar abs). Other heating media (such as hot water) may be used.