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
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
Working
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.
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.
ReplyDeleteLotz 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.
DeleteIf the fresh water capacity is going down can have many reasons, f.i.
Delete1) not enough vacuum, owing to poor working vacuum unit or high seawater temperature in the condenser, or an air leakage in the flanges owing to dry packing or loose flange bolts.
2) Boiler scale on the heater unit or even on the condenser, the last has also a bad influence on the vacuum.
(If you clean the heater unit with acid, at same time clean also the condenser unit the same way)
So far my opinion
Regards
Alfons, retired C/E
Well said sir....
DeleteGood day! Just want to ask how can the three gas laws be applied in fresh water generator system? Please help me. Begging some ideas about this.
ReplyDeleteI keep getting this alarm " WMS system fault too much water separation stop " . Timer T22 goes into countdown mode ,right now the setting is 60s .. and from 60:s when it reached 0 s this alarm is generated . The water content in feed is not high as I run the standby purifier and it works perfect , also confirmed with a water in oil test . Filling water setting T02 is only 0.10 sec same as the standby purifier .I checked the solenoid valve isn't leaking . Any solution for this issue ?
ReplyDeleteMy salinometer is rising fast. Before, we clean FWG every 6 months. Now, it has been 2 months and salinity is now high. What could be the other factors affecting salinity aside from salt forming in plates?
ReplyDeletecheck salinometer for fault
Deletecheck if amount of water produced is more than desired capacity then quality of water effected,
check jacket temperature are in correct range not too high or too low
check demister, if it doesn't working properly, then there might be salt carry over
check and clean condenser with chemical, if chemical not cleaning properly use alternate chemical with appropriate dosage.
ejector pump pressure to be checked, if not then chances brine may settle
vacuum to be appropriate
Possible causes are:
ReplyDeleteBrine level inside shell too high.
Leaking condenser tubes or plates.
Operation of evaporator near shore with contaminated feed water.
Shell temperature and pressure too low.
Increased solubility of CO2 generated from the salt water due to reduced sea water temperature. This dissolved CO2 makes water acidic and conductivity of water increases. Hence salinometer shows increased salinity, which is a measure of conductivity ans not presence of salt
which one is more effective? the tube type or the plate type?
ReplyDeletePlate type...
DeleteHow priming is done in freshwater generator??
ReplyDelete