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Fresh Water Generator or Evaporator (Alfa Laval Type)

Updated on April 12, 2010


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

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:

  1. Control over type of scale formed.
  2. Heating medium can be at relatively low temperature. (e.g. diesel engine jacket water or waste steam)
  3. 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.


Freshwater Generator (Alfa-Laval Type)
Freshwater Generator (Alfa-Laval Type)

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

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    • profile image

      Mohamed Fizal 4 years ago

      What will be the result when the shell temperature of fresh water generator increases

    • jabelufiroz profile image

      Firoz 4 years ago from India

      Yes, while at sea we produce up to 40 tonnes/day for both equipments and personnel on board ships.

    • tamarawilhite profile image

      Tamara Wilhite 4 years ago from Fort Worth, Texas

      Do you ever have to produce distilled water for equipment?

    • profile image

      nick0411 5 years ago

      what is the maximum vacuum press to produce fresh water ?

    • profile image

      mikatarefin 6 years ago

      jab what is the shell temperature of fresh water generator

    • profile image

      alpesh devghare 7 years ago

      nice dud

    • profile image

      jaypee alestre 7 years ago

      is verry excillent to post this basic diagram of f.w.g..we can helped to our marine student seacrhers

    • thevoice profile image

      thevoice 7 years ago from carthage ill

      terrific first rate hub work thanks

    • stars439 profile image

      stars439 7 years ago from Louisiana, The Magnolia and Pelican State.

      Very impressive hub. I enjoyed reading this. God Bless You.