Fresh water generator, one of the important machinery on board a ship, is something that cannot be done without. Fresh water produced from fresh water generator is used for drinking, cooking, washing and even running other important machinery which use fresh water as a cooling medium.
Fresh water is generally produced on board using the evaporation method. There are two things that are available in plenty on ship to produce fresh water –Seawater and heat. Thus fresh water is produced by evaporating sea water using heat from any of the heat source. The evaporated sea water is then again cooled by the sea water and the cycle repeats.
Generally the heat source available is taken from the main engine jacket water, which is used for cooling the main engine components such as cylinder head, liner etc. The temperature available from this jacket water is about 70 deg. centigrade. But at this temperature the evaporation of water is not possible as we all know that the evaporation of water takes place at 100 deg centigrade under atmospheric pressure.
Thus in order to produce fresh water at 70 degrees we need to reduce the atmospheric pressure, which is done by creating a vacuum inside the chamber where the evaporation is taking place. Also, as a result of the vacuum the cooling of the evaporated sea water will also take place at lower temperature. This cooled water is collected and transferred to the tank.
Nowadays, reverse osmosis is one of the methods which are used on board for generating fresh water. Generally this is used on passenger vessels wherein there is a large requirement of fresh water production. However, in merchant ships the evaporation method is used as reverse osmosis is costly and includes large maintenance cost for membrane.