Generating Freshwater on Ship: Fresh Water Generator Explained

Fresh water generator, one of the important machinery on board a ship, is something that cannot be done without. Fresh water produced from a freshwater generator is used for drinking, cooking, washing, and even running other important machinery which uses fresh water as a cooling medium.

Fresh water is generally produced on board using the evaporation method. Two things are available in plenty on the ship to produce fresh water –Seawater and heat. Thus, fresh water is produced by evaporating seawater using heat from any of the heat sources. The evaporated seawater is then again cooled by the seawater, and the cycle repeats.

Generally, the heat source available is taken from the main engine jacket water, which is used for cooling the main engine components, such as the cylinder head, liner, etc. The temperature available from this jacket water is about 70 degrees. centigrade. However, at this temperature, water evaporation is not possible, as we all know that it occurs at 100 degrees centigrade under atmospheric pressure.

Thus, to produce fresh water at 70 degrees, we need to reduce the atmospheric pressure, which is achieved by creating a vacuum inside the chamber where evaporation occurs. Also, as a result of the vacuum, the cooling of the evaporated seawater will take place at a lower temperature. This cooled water is collected and transferred to the tank.

Nowadays, reverse osmosis is one of the methods which are used on board for generating fresh water. Generally, this is used on passenger vessels, wherein there is a significant requirement for fresh water production. However, in merchant ships, the evaporation method is used as reverse osmosis is costly and includes considerable maintenance costs for the membrane.

Fresh Water Generator Arrangement

The main body of a freshwater 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 freshwater generator also consists of an educator, which helps in generating the required vacuum. The fresh water pump and ejector pump help in the transfer of water to and from the fresh water generator.

Starting the Fresh Water Generator

1. Before starting the freshwater generator, we must verify that the ship is not in congested waters, canals, or within 20 nautical miles of the shore. This is done because near the shore, effluents from factories and sewage are discharged into the sea, which can then contaminate the freshwater 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 that the drain valve present at the bottom of the generator is in a closed position.
4. Now open suction and discharge valves of the seawater pump, which will provide water for evaporation, cooling and to the eductor for creating a vacuum.
5. Open the seawater discharge valve from where the water is sent back to the sea after circulating inside the freshwater generator.
6. Close the vacuum valve situated on top of the generator.
7. Now start the seawater pump and check the pressure of the pump. The pressure is generally 3-4 bars.
8. Wait for the vacuum to build up. The vacuum should be at least 90%, as indicated by the gauge on the generator. Generally, the time taken for the generation of a vacuum is about 10 minutes.
9. Once the vacuum is achieved, open the valve for feed water treatment to prevent scale formation inside the plates.
10. Now open the hot water (jacket water) inlet and outlet valves slowly to about half. Always open the outlet valve first and then the inlet valve. Slowly increase the opening of the valves until they are entirely open.
11. Now we can see that the boiling temperature starts increasing and the vacuum starts dropping.
12. The vacuum drops to about 85% which is an indication that evaporation has started.
13. Open the valve from the freshwater pump to the drain.
14. Switch on the salinometer if it has to be started manually. Generally, it is on auto start.
15. Now start the 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 the vacuum returns to the normal value.
17. Check that the water coming out of the salinometer is not salty, and also check the reading of the salinometer. This is done to determine if the salinometer is functioning correctly and to prevent the entire freshwater supply from becoming contaminated with saltwater. The value of the salinometer is kept below 10ppm.
18. After checking the taste of the water coming out of the salinometer, open the valve for the tank from the pump and close the drain valve.

Stopping the Freshwater Generator

1. Close the jacket water inlet valves. Generally, the inlet is closed first, and then the outlet valve.
2. Close the valve for feed water treatment.
3. Stop the freshwater pump.
4. Switch off the salinometer.
5. Stop the seawater pump (also known as the ejector pump).
6. Open the vacuum valve.
7. Close the seawater suction valve and the 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.

Watch this video for a better understanding of how freshwater is generated:

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