Fresh Water Generator Principle, Working, Line Diagram, Parts and Functions

        As you all are aware of the fact that freshwater having a lot of uses on board ; it is used on board for drinking, cooking ,washing ,cooling medium, boilers,  tank cleaning etc .And because of all these reasons there is no wonder why freshwater consumption is  very high on board ship. Generally 100 litres are considered  as per day consumption of a man on ship.
It is not recommended to use portable water for boilers from port due to poor quality, so distilled water generation is unavoidable in ship. The equipment used to generate freshwater on board ship is called fresh water generator also known as evaporator.

Principle of fresh water generator
    In ships generally two principles are used to generate freshwater ; either Distillation or Reverse Osmosis. Reverse osmosis is normally used in passenger ships where large quantities of water is consumed .Here I am trying to explain about the working principle of  fresh water generator that works under the principle of distillation which is very common in cargo ships.

     Distillation is the process of separating freshwater from sea water by boiling sea water in evaporator and the evaporated vapor gets condensated in the condenser. In ship, boiling of sea water is done by the use of jacket water which will increase the overall efficiency. But as you know that  jacket water  with 80 degree cannot boil the sea water at atmospheric pressure; in order to boil sea water at 80 degrees we need to reduce the atmospheric pressure, which is done by creating a vacuum inside the chamber where the evaporation is taking place. 

Types Of Fresh Water Generators
   As we have discussed above fresh water generators are classified based on working principles ie, reverse osmosis type and Distillate type. Distillate type is again classified based on the evaporator and condenser structure, ie Plate type and Tube type fresh water generator.

Plate Type  Fresh Water Generator
 If the condenser and evaporator Heatexchangers of a fresh water generator is composed of plates then that type of freshwater generator is called Plate type freshwater generator. Main components are condenser and evaporator heatexchangers, brain air ejectors, sea water pump, Distillate pump, salinometer, Demister, Water flow meter etc. Below You can see the line diagram.

>Heat Exchangers: Condenser and evaporators are corrugated titanium plates. The plates are corrugated with horizontal or chevron pattern corrugations which will help to increase the heat exchange surface .

>Brain Air Ejectors: It is used to remove air and brain from the chamber which helps in maintaining the vacuum.

>Sea water and distillate pump: Both are centrifugal pumps. Distillate pump help to pump out the freshwater from condenser and seawater pump supply sea water to condenser and evaporator.

> Demister:  Demister help to remove the carry over of sea water droplets and water spray. It is made of layered knitted wire of monel metal.

> Salinometer: Check the salinity of the water. Three way valve is controlled by the salinometer as shown in fig. If the salinity is high, water goes back to the evaporator inlet.(Salinity usually max 10ppm)
Plate Type Fresh Water Generator Line Diagram  

 Main Engine jacket water passes through the evaporator  at 80 degree. Feed sea water enters the evaporator through the orifice at the feed inlet. Due to low pressure in the chamber, sea water boils and get converted to steam. The carry over water droplet and  water spray is removed by Demister. The separated water droplets gets collected as brain which is extracted by brain ejector. The steam then enters into the condenser, where it gets cooled down to form fresh water. Distillate pump help to pump out this fresh water from the condenser.

Tube Type Fresh Water Generator
Working and principle of tube type fresh water generator is same as plate type. Only difference in instead of plates, condenser and evaporators are tubes. A typical line diagram of tube type freshwater generator is given below.

You Can Answer Following Questions
> Principle, working of fresh water generator
> Fresh water generator line diagram
> Types of fresh water generators onboard ship
> Why corrugated plates are used in fresh water generator
> How vacuum achieved in fresh water generators
> Materials of Construction for Fresh Water Generator.

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  1. Recently, I have problem with our fresh water generator, unable to produce fresh water significanly only 20% from capacity 10000ltr/day, vacum only 85%, please help me to solve my problem. Maker alfa laval.

    1. Lotz of things affect vaccum.. even though common problems are leaking air and low sea water pressure these two things badly affect vacuum. Hope you didn't alter the feed.


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