VCD plants are provided on ships where the engine power are less and sufficient heat is not available in the engine cooling water system for producing fresh water in large quantities. These plants are completely independent of other systems and normally fitted on military crafts.
The working principle of the VD plant is as follows:
Initially, a small quantity of fresh water is heated in a heater and steam is raised. This steam is compressed in vapour compressor where the enthalpy of steam is increased. This heat is used for evaporating the sea water in the distiller. The sea water is fed to the plant by sea water pump which draws seawater from the sea.
The steam raised from sea water becomes the inlet steam to vapour compressor in the second cycle and there is no further requirement of a heater.
The compressed vapour from the compressor, after giving out its heat to the sea water, becomes condensate and comes out from the plant as fresh water.
This fresh water is also utilised to preheat the sea water to increase the overall efficiency.
The brine solution formed out of evaporation is also taken out continuously and the heat in brine solution is also utilised for preheating the sea water fed to the plant.