The ship traveling on long voyages must be able to generate their own fresh water for the boilers, machinery and for washing. Fresh Water Generators or Desalinators are installed to convert the seawater from the sea to freshwater. 

Fresh water generator using vacuum to boil off the sea water at low temperatures

Fresh water generator using vacuum to boil off the sea water at low temperatures

The most common type makes use of the distillation process. Using the heat from the engines, the seawater is evaporated into vapor. This vapor is then led into a cooling section and it condenses into water again – distilled water. The cooling section consist of tubes or plates which uses sea water as the cooling agent.

The air inside the evaporation chamber is evacuated to a near vacuum, so that the boiling point of water becomes lower. It then becomes possible to evaporate the seawater at a temperature of 60 degree C. The diesel engine jacket cooling water is sufficiently hot to boil the seawater, and it is commonly used on Motorships.