Multiple-effect distillation (MED) is the low temperature thermal process of obtaining fresh water by recovering the vapour of boiling sea water in a sequence of vessels, (called effects) each maintained at a lower temperature than the last. Because the boiling point of water decreases as pressure decreases, the vapour boiled off in one vessel can be used to heat the next one, and only the first one (at the highest pressure) requires an external source of heat.
Water boiling point table
|Pressure||1bar||0.47 bar||0.32 bar (Top of Everest)||0.25 bar||0.1 bar|
By maintaining the effects at low pressure the sea water remains at temperatures below 65°C, thus avoiding unnecessary heating and allowing a good control of scaling.