We have learnt a lot about fresh water generators used on board ships and also talked about vapour compression type water generator. The fresh water is generated by Basket Type Steam Evaporators which operate on principle of brisk boiling of seawater surrounding a Steam Basket. This causes the formed vapor to carry along some salt water particles which are separated in two stages before the vapor goes to distiller cooler. They are very efficient evaporators and the water produced by this method has purity of less than 4 ppm of salt content.
They are available in two variants namely;
Seawater Cooled in which the vapor is cooled by seawater and the latent heat in the vapor is lost to sea and,
Distillate Cooled in which the vapor is cooled by distillate from main condenser and the latent heat in the vapor goes back to the boiler through feed water.
Construction and Operation
The evaporator consists of vertical basket type heater surrounded by seawater and located at bottom of a cylindrical shell. A brisk and violent boiling takes place inside the shell resulting in vapor with a lot of salt water particles carried along with it in suspension.
The suspended seawater particles are separated from vapor in two stages prior to reaching the condenser. In the first stage, most of the suspended seawater particles are separated by Centrifuging and, any remaining small amount of suspended seawater particles are removed in the second stage which is a Demister.
The dry vapor is then condensed to water in condenser cooled by seawater in the Seawater Cooled Type Evaporator. A vacuum is maintained in the condenser and the evaporator with the help of a steam ejector system.
The seawater feed to the evaporator is preheated in several stages commencing from distillate cooler to steam outlet from basket heater via condenser and vacuum ejector cooler. Only about 15% of the preheated seawater is used as feed and remaining is discarded back to sea.
In Condensate Cooled Type Evaporators, the vapor after the two stage separation goes to condenser which is cooled by condensate water from main condenser of the boiler system. In this way, the condensate from the main condenser gets preheated prior to being fed back to the boiler saving some energy.
Both the variants can be designed and operated in multistage versions with two or more stages for optimization of efficiency.