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Fresh Water Generator or Evaporator (Alfa Laval Type)
A considerable amount of fresh water is consumed in a ship. The crew consumes an average 100 liter/head/day. In a steam ship (a ship whose main propulsion unit is steam turbine or a ship which is a large tanker having steam turbine driven cargo oil pumps) the consumption for the boiler can be as high as 30 tonnes/day.
Sufficient potable water may be taken on in port to meet crew and passenger requirement but the quality of this water will be too poor for use in water tube boilers. It is common practice to take on only a minimum supply of potable water and make up the rest by distillation of seawater. It is statutory requirement to have a distillation plant for emergency use if otherwise ship has carried sufficient potable water.
Working principle of the low pressure evaporator is that, boiling point of water can be reduced by reducing the pressure of the atmosphere surrounding it. By maintaining a low pressure, water can be boiled at low temperatures say 50 degree Celsius. The source of heat could be waste heat rejected by main engine jacket cooling water.
Hence using energy from a heating coil, and by reducing pressure in the evaporator shell, boiling can takes place at about 40 to 60 degree Celsius. This type of single effect plant is designed to give better economy than obsolete Boiling Evaporators.
Such plants are widely used because of:
- Control over type of scale formed.
- Heating medium can be at relatively low temperature. (e.g. diesel engine jacket water or waste steam)
- Improved heat transfer across the heating element. This is due to higher temperature difference for lower pressures.
Operation of the Plant
Heat from the diesel engine cooling water is used to evaporate a small fraction of the seawater feed in the plate type evaporator. Unevaporated water is discharged as brine (by brine ejector) and that which is evaporated passes through the demister to the plate type vapour condenser, where, after condensation it is discharged to fresh water storage tank by fresh water distillate pump. During entire operation the feed rate to the evaporator is fixed by the orifice plate.
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In the event of salinity of freshwater density exceeding a predetermined value (maximum usually 10 ppm) the solenoid controlled dump valve diverts the flow to the bilge, preventing contamination of the made water. Excess salinity could be used by many factors include leakage of seawater at condenser or priming of evaporator or malfunctioning of demister, or many other reasons.
What cannot be condensed at the condenser are called 'uncondensable gases' like air and these gases are continuously ejected out by air ejector. This way the shell is maintained at high vacuum, a must requirement to boil water at low temperatures.
"Basic Marine Engineering" by J.K.DHAR