For any ocean going vessel, dependable supply of fresh water is essential. Even though a few small ships can have their fresh water tanks charged from the docks, the majority of them choose to have a fresh water generator installed aboard. When the berthing is not frequent, a fresh water tank assumes prohibitive proportions in a ship where space is a major constraint. This is particularly true of naval vessels where, apart from high consumption like in passenger ships, the sailing time is much longer.
A fresh water generator delivers the required potable water by conversion of the sea water. The various shipping companies in the country at present engage a large number of vessels and a considerable number are still on order. So far, all the ships including those of the Navy had been fitted with imported fresh water generators. If the country manufactures the units indigenously and provides for the future requirement, considerable foreign exchange can be saved. There are indications that these units can also be exported.
Various commercially acceptable processes are available for obtaining potable water from sea water. Amongst these, those based on evaporation and vapour compression cycle find favor. The design of fresh water generators should satisfy stringent conditions of weight and size and be capable of trouble free operations during the movements of ships especially on roughs seas. The unit developed by BARC essentially utilizes a low temperature evaporation process. The heating medium could be either low pressure steam (suitable for steam ships) or main engine cooling water (for diesel ships). In the latter applications, the engine cooling water gets cooled to the required extent in the heater circuit of the fresh water generators, which is an advantage. The fresh water generator essentially consists of three portions i.e. heater, separator and condenser. In the heater shell, vertical tubes are used. Feed sea water enters the unit at the bottom of the tubes and partly evaporates by the time it comes out at the top. After water and vapour mixture come out of the tubes, the vapour rises through the vertical shell, enters the horizontal tube bundle kept at the top of the vertical shell and gets condensed around the tubes (which are cooled by sea water flowing inside) forming fresh water. The product water is pumped out.