Introduction:
Fresh water generator, one of the important machinery on board a ship, is something that cannot be done without. Fresh water produced from fresh water generator is used for drinking, cooking, washing and even running other important machinery which use fresh water as a cooling medium.
Working principle of F.W generator:
1. In this system F.W is produced by boiling sea water by engine cooling out let water.
2. Inside the evaporation chamber vacuum is created by using air ejectors. So the relatively low temp jacket outlet water entering to the F.W.G at about 65 – 75 degree cel. will make s.w evaporate at lower temp. Than normally required temp (almost 45 deg cel).
3. Vapor produce from s.w is free from the salt, they accumulated in the demister where small and fine droplet become larger drops to fall back against the vapor flow.
4. These small particles of water are condensed in condenser to produce fresh water. Cooling is done by S.W in the condenser.
5. Partial evaporation of sea water leaves brine which is removed through the brine ejector.
6. Air & other gases released from the heating of sea water are removed by the air ejector.
Fresh Water Generator Arrangement
The main body of a fresh water generator on the ship consists of a large cylindrical body with two compartments. One of the compartments is the condenser and the other is the evaporator. The fresh water generator also consists of an educator which helps in generating the required vacuum. The fresh water pump and ejector pump helps in transfer of water to and from the fresh water generator.
Starting the Fresh Water Generator
1. Before starting the fresh water generator we have to check that the ship is not in congested water, canals and is 20 nautical miles away from the shore. This is done because near the shore the effluents from factories and sewage are discharged into the sea can get into the fresh water generator.
2. Check whether engine is running above 50 rpm, the reason for this is that at low rpm the temperature of jacket water which is around 60 degrees and not sufficient for evaporation of water.
3. Check the drain valve present at the bottom of the generator is in close position.
4. Now open suction and discharge valves of the sea water pump which will provide water for evaporation, cooling and to the eductor for creating vacuum.
5. Open the sea water discharge valve from where the water is sent back to the sea after circulating inside the fresh water generator.
6. Close the vacuum valve situated on top of the generator.
7. Now start the sea water pump and check the pressure of the pump. The pressure is generally 3-4 bars.
8. Wait for the vacuum to build up. Vacuum should be at least 90% which can be seen on the gauge present on the generator. Generally the time taken for the generation of vacuum is about 10 minutes.
9. When vacuum is achieved open the valve for feed water treatment, this is to prevent scale formation inside the plates.
10. Now open hot water (jacket water) inlet and outlet valves slowly to about half. Always open the outlet valve first and then inlet valve. Slowly start to increase the opening of the valves to full open.
11. Now we can see that the boiling temperature starts increasing and the vacuum starts dropping.
12. The vacuum drop to about 85% which is an indication that evaporation is started.
13. Open the valve from fresh water pump to drain.
14. Switch on the salinometer if it has to be started manually. Generally it is on auto start.
15. Now start fresh water pump and taste the water coming out of the drain.
16. When fresh water starts producing it is seen that the boiling temperature drops again slightly and vacuum comes back to the normal value.
17. Check the water coming out of the salinometer is not salty and also check the reading of the salinometer. This is done to see if the salinometer is working properly or not and to prevent the whole fresh water from getting contaminated with salt water. The value of salinometer is kept below 10ppm.
18. After checking the taste of the water coming out of the salinometer, open valve for tank from the pump and close drain valve.
Stopping the Fresh water Generator
1. Close the jacket water inlet valves. Generally inlet is closed first and then the outlet valve.
2. Close the valve for feed water treatment.
3. Stop fresh water pump.
4. Switch off the salinometer.
5. Stop sea water pump (also known as ejector pump).
6. Open vacuum valve.
7. Close sea water suction valve and overboard valve. This is generally not required as they are non- return valves. However, in case of valve leaking or damage, these valves are to be closed without fail.
Produced Fresh Water Should not be Used to drinking Directly:
1. The FW produce from generator may contain living diseases producing organisms.
2. In some case jacket cooling water can leak through to evaporator, it will contain chemical.(Chromate and dichromate) and this would be harmful for the human body.
What Will Happen While Vacuum Reach 100%?
1. Increase the salinity because of agitation. At that time boiling rate is very high.
2. To control this condition, open the vacuum breaker to maintain 95% vacuum. (back pressure can occur at distillate pump)
Causes of loss of vacuum effect in FWG?
1. Failure of ejector pump.(pump suction leak and pump efficiency)
2. Failure of ejector nozzle.(chocks, fouling, erosion)
3. Malfunction of check valve.(at each ejector nozzle)
4. Defective vacuum breaker valve.
5. Any air leakage into the system.
Watch this video for better understanding
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