Salt water to fresh water conversion on ships


In the previous article we learnt as to how fresh water is generated on ship. Fresh water generators (evaporators) use heated water from the diesel engines to boil the sea water. Flash process can be used to increase the efficiency of the whole system. In the previous article we saw as to how water is generated in a single stage evaporator. Flash process can be used in a system having two stages of evaporation to obtain a better quality of fresh water. For this process 2-stage fresh water generators are used. A 2-stage FWG is similar to the single stage FWG, the only difference being that the whole single stage process is repeated twice in a 2-stage generator. Let’s see how a double stage evaporator works.

dual stage fwg


We know that a dual stage fresh water generator has the main chamber divided into two parts. Both the parts follow the same process. This to distillate the water to the finest quality by reducing the salt particles per million to the lowest number. There is also a pre-heater provided which pre-heats the salt water to facilitate the flash process. Rest of the arrangement is almost same as single stage FWG.

dual stage fwg

How does it work?

Sea water enters the evaporator chamber through a flow meter. This sea water is pre heated with the help of a pre heater to bring the temperature of the sea water as close to the jacket water temperature and also to enable the flash process from taking place. Pre heater uses steam for the process. Some of the sea water after entering the first stage evaporator flashes off due to the reduced pressure. The steam generated passes through the demister (steam water remover) where the water droplets are removed. The steam is then condensed into water droplets in the condenser.

The heated sea water now enters the second stage where it first passes through the evaporator. Due to the low pressure of the chamber, the hot water flashes off. The steam is again demisted and then condensed. The fresh water thus formed is collected with the distilled water from the first stage, with the help of a distiller pump.

The concentrated sea water, brine and the solid particles which remained at the end of the second process is collected with the help of a brine pump. Air ejector is used to maintain the air pressure inside the chamber and also to remove any access air in the chamber.


Due to high boiling temperature of sea water, scales are formed on the heating surface. The higher the boiling temperature required the more the formation of scales takes place. Other factors on which the formation of scale depends are flow rate and the density of brine formed.

Due to excessive scale deposits on heating surfaces, greater requirement of boiling temperature arises. Excessive scales will lead to low quality of fresh water and also reduction in the quantity of water produced. It will also prevent higher reduction of pressure inside the chamber from taking place.

Scale formation can be prevented by using a technique known as cold shocking. In this method the heating surface is alternatively heated and cooled, which reduces the formation of scales.

Also a routine maintenance of the generator should be carried out by shutting down the plant and removing the scale manually or by chemical treatments. The internal walls of the chamber or the shell should also be cleaned when the overall cleaning is done.

Air ejectors and educators should also be checked for holes or leakages, which can prevent the formation of desired vacuum.

The distiller, feed and brine pumps should also be properly maintained to prevent any interruption in the flow of fresh and sea water. The processes and the phenomena used in both plate and tube type FWGs are the same.

A constant check should be kept on the flow meter to prevent excessive or very less flow.

The salinometer alarm should be precisely set and given a constant watch. This is to prevent the degrading of the quality of fresh water produced.


Introduction to marine engineering(1996) by D.A Taylor


Introduction to marine engineering(1996) by D.A Taylor