Fres water generators are provided on board the ships for producing fresh water from sea water by distillation process:
There are two types of fresh water generators:
A) Vacuum Distillation Plants
B) Vapour Compression Distillation Plants (VCDs)
VACCUM DISTILLATION DISTILLATION PLANTS:
In this type of plants the heat available in the engine, the cooling water system is utilised for heating the sea water and for raising the steam. Normally, the engine cooling water will have a temperature of about 85-degree celsius and since steam cannot be produced from sea water at this temperature at atmospheric pressure, a vacuum is created inside the fresh water generators for reducing the boiling point. Two ejectors, (converging-diverging nozzles) one for brine solution and one for air, are provided on the fresh water generators for removal of brine and air and vacuum is created inside the fresh water generators.
The sea water feed for the plant is provided by the ejector pump which also serves the ejectors.
The sea water is heated in the heat exchanger by engine cooling water under vacuum and the steam produced is condensed in a condenser, utilising sea water as the cooling medium. The condensate (fresh water) produced is taken out by a freshwater pump, which discharges the fresh water to a storage tank.
A salinometer is provided on the freshwater generator which senses the salinity of fresh water produced, and in case the salinity is higher than 15p.p.m the fresh water produced is returned back to feed.
Multi-stage fresh water generators (vacuum type):
Freshwater generators are produced in multi-stages to produce more amount of freshwater, utilising the same heat available in an engine cooling water system.
In 2 stage distillation plants, the feed water (sea water) is heated in the first stage utilising the heat available in the engine fresh water cooling system and steam is raised under vacuum and condensed in the condenser.
In the second stage, the brine solution from 1st stage heat exchanger is utilised as a heating medium and steam is raised under vacuum.
In this process, more amount of fresh water can be produced utilising the same heat available in an engine cooling water system.
We shall talk about Vapour Compression Distillation Plants (VCDs) in the next part.