Evaporator - Marine Use

Marine Use

For more details on this topic, see Evaporator (marine).

Large ships usually carry evaporating plants to produce fresh water, thus reducing their reliance on shore-based supplies. Steam ships must be able to produce high-quality distillate in order to maintain boiler-water levels. Diesel-engined ships often utilise waste heat as an energy source for producing fresh water. In this system, the engine-cooling water is passed through a heat exchanger, where it is cooled by concentrated seawater (brine). Because the cooling water (which is chemically treated fresh water) is at a temperature of 70–80 °C (158–176 °F), it would not be possible to flash off any water vapour unless the pressure in the heat exhanger vessel was dropped. To alleviate this problem, a brine-air ejector venturi pump is used to create a vacuum inside the vessel. Partial evaporation is achieved, and the vapour passes through a demister before reaching the condenser section. Seawater is pumped through the condenser section to cool the vapour sufficiently to precipitate it. The distillate gathers in a tray, from where it is pumped to the storage tanks. A salinometer monitors salt content and diverts the flow of distillate from the storage tanks if the salt content exceeds the alarm limit. Sterilisation is carried out after the evaporator.

Evaporators are usually of the shell-and-tube type (known as an Atlas Plant) or of the plate type (such as the type designed by Alfa Laval). Temperature, production and vacuum are controlled by regulating the system valves. Seawater temperature can interfere with production, as can fluctuations in engine load. For this reason, the evaporator is adjusted as seawater temperature changes, and shut down altogether when the ship is manoeuvring. An alternative in some vessels, such as naval ships and passenger ships, is the use of the reverse osmosis principle for fresh-water production, instead of using evaporators.