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A desalination device requires energy to produce fresh water from sea water. In thermal desalination device, water is heated to the boiling point to produce the maximum amount of water vapor.

To do this economically in a desalination plant, the applied pressure of the water being boiled is adjusted to control the boiling point, because of the reduced atmospheric pressure on the water, the temperature required to boil water decreases as one moves from sea level to a higher elevation.

To boil, water needs two important conditions: the proper temperature relative to its ambient pressure and enough energy for vaporization. When water is heated to its boiling point and then the heat is turned off, the water will continue to boil only for a short time because the water needs additional energy (the heat of vaporization) to permit boiling. Once the water stops boiling, boiling can be renewed by either adding more heat or by reducing the ambient pressure above the water. If the ambient pressure were reduced, the water would be at a temperature above its boiling point (because of the reduced pressure) and would flash to produce vapor (steam), the temperature of the water will fall to the new boiling point. If more vapors can be produced and then condensed into fresh water with the same amount of heat, the process tends to be more efficient.

To significantly reduce the amount of energy needed for vaporization, the distillation desalting process usually uses multiple boiling in successive vessels, each operating at a lower temperature and pressure.

Typically 8 tons of distillate can be produced from 1 ton of steam. This process of reducing the ambient pressure to promote additional boiling can continue downward and, if carried to the extreme with the pressure reduced enough, the point at which water would be boiling and freezing at the same time would be reached.

Nowadays desalinated water is more expensive than oil, and therefore the growth of desalination technology should be considered as a strategic industry

The most popular desalination methods in this kind of technology are:
MED(Multi Effect desalination) Method
MSF (Multi Stage Flash) system