Explanation:
1) As shown in the sketch, sea water pipe takes clean sea water from the sea and discharges it to air ejector, brine ejector and at the bottom part of the evaporator shell.
2) In this part, a plate type heat exchanger is provided.
3) The sea water is around the heat exchanger and steam or jacket water from main engine is circulated from heat exchanger.
4) This causes the outer sea water to get heated up, boiled and produce vapors.
5) These vapors travel upwards through demister.
6) Demister trap/filters the salt particle in the water vapors and allow pure vapor to rise upward.
7) In the upper part of evaporator shell, one more plate type heat exchanger is arranged which is also called vapor condenser.
8) The vapor/steam is around the heat exchanger and cooling sea water is circulated from inner side of the heat exchanger.
9) This causes cooling and condensation of vapor and fresh water of distilled water is generated.
10) Which is then transferred to Fresh Water Tank.
Principle:
> Water generally boils at 100°C at atmospheric pressure.
> Boiling point of water reduces when subjected to vacuum.
> As shown in the sketch, air ejector maintains the evaporator at vacuum, thus the boiling point is reduced to 45°C
> Therefore the engine jacket water, which is normally at 70°C is able to boil sea water at 45°C - 50°C and vapors are generated.
> Brine ejector removes concentrated salt solution which is also called as Brine, from the bottom of the shell and throws it over board.
> Both air ejector and brine ejector works upon venturi principle.
Importance of FWG:
> Storage capacity of Fresh Water (FW) on board is limited.
> Day-to-day use of FW will reduce the stock of FW during voyage.
> In this case FW needs to be bunkered from different ports.
> This can be expensive and at the same time the quality of FW is inferior.
> Hence it is required to produce FW by making use of Salt Water.
> FW produced by FWG is of good quality and suitable for all domestic needs.
