An MED evaporator consists of several consecutive cells (or effects) maintained at decreasing levels of pressure (and temperature) from the first (hot) cell to the last one (cold). Each cell mainly consists of a horizontal tubes bundle. The top of the bundle is sprayed with the sea water make-up that then 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. At the same time sea water warms up and partly evaporates by recovering the condensation heat (latent heat). 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 heating media for the next effect where the process is repeated. The decreasing pressure from one cell to the next one allows brine and distillate to be drawn to the next cell where they will flash and release additional amounts of vapour at the lower pressure. This additional vapour will condense into distillate inside the next cell.
This process is repeated in a series of effect (Multiple Effect Distillation) (see sketch with 3 effects). In the last cell, the produced steam condenses on a conventional shell and tubes heat exchanger. This exchanger, called "distillate condenser" is cooled by sea water. At the outlet of this 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 from where they are extracted by centrifugal pumps.
The thermal efficiency of such evaporator can be quantified as the number of kilos of distillate produced per one kilo of steam introduced in the system. Such number is called the Gain Output Ratio (GOR).