Generation of Fresh Water Onboard

Fresh water is required for domestic purpose, cooling intricate spaces. Although costly, freshwater cooling doesn’t lead to corrosion. The generation of fresh water onboard uses two things which are available in abundance onboard- Heat and Seawater.

The generation of fresh water onboard takes place in two ways:-

1. Distillation
2. Osmotic de-salinators

Freshwater generator (also called evaporator or distiller) is a device used for the production of freshwater from seawater by application of the evaporation process (distillation).

The working principle of the freshwater distiller is based on evaporation of seawater and then condensation of generated vapour.

The following media can be used in the evaporation process: steam and water.

In a case of water heated evaporators, fitted on ships propelled by reciprocating internal combustion engines, freshwater that cools down the main propulsion engine is used as a heating medium. This type of distillers is installed on the majority of vessels.

The usage of hot water from cooling systems lowers the ship’s operating costs.

Evaporators are primarily used for the production of distilled water for cooling systems, feeding steam boilers, or electrolyte refilling. However, the resulting freshwater surplus can be used for sanitary purposes. The salinity of freshwater, depending on its purpose, varies from 2 to 200 ppm, while the salinity of seawater is about 35 000 ppm.

It can be assumed that vacuum freshwater evaporators are exclusively installed in today’s engine rooms. Reducing the pressure lowers the temperature of the water evaporation to 41-44oC, while the absolute pressure in vacuum evaporators is 0.007-0.009 MPa. Besides obvious energetic reasons, such low temperatures prevent intensive salt deposits on heat transfer surfaces. Thus, vacuum evaporators can be cleaned in long intervals (2-3 times a year). On the other hand, such a low boiling point cannot guarantee appropriate bacteriological water properties with a maximum production of 6.3 tons of fresh water a day.

All construction elements such as evaporation chamber with tubular heater, tubular condenser, demister, and other auxiliaries, piping, and fittings are mounted in frame construction.

Starting the Fresh Water Generator

Before starting the freshwater generator we have to check that the ship is
– Not in congested waters.
– Not transiting through canals
– 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 freshwater generator.

– Check whether the 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.

– Check the drain valve present at the bottom of the generator is in close position.

Open suction and discharge valves of the seawater pump which will provide water for evaporation, cooling, and to the eductor for creating a vacuum.
Open the seawater discharge valve from where the water is sent back to the sea after circulating inside the freshwater generator.

Close the vacuum valve situated on top of the generator.

Now start the seawater pump and check the pressure of the pump. The pressure is generally 3-4 bars.
Wait for the vacuum to build up. The 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.

When the vacuum is achieved open the valve for feed water treatment, this is to prevent scale formation inside the plates.

Now open Hot Water (Jacket Water) inlet and outlet valves slowly to about half. Always open the outlet valve first and then the inlet valve. Slowly start to increase the opening of the valves to full open.
Now we can see that the boiling temperature starts increasing and the vacuum starts dropping.

The vacuum will drop to about 84%-85% which is an indication that evaporation is started.
Open the valve from the freshwater pump to drain.
Switch on the salinometer if it has to be started manually. Generally, it is on auto start.
Now start the freshwater pump.

When freshwater starts producing it is seen that the boiling temperature drops again slightly and vacuum comes back to the normal value.
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 freshwater from getting contaminated with saltwater. The value of the salinometer is kept below 10ppm.

After checking the taste of the water coming out of the salinometer, open valve for the tank from the pump and close drain valve.

Stopping the Fresh water Generator

Close the jacket water inlet valves. Generally inlet is closed first and then the outlet valve.
Close the valve for feed water treatment.
Stop fresh water pump.
Switch off the salinometer.
Stop sea water pump (also known as ejector pump).
Open vacuum valve.
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.

Osmotic desalinators
Nowadays, reverse osmosis is one of the methods which are used onboard for generating fresh water. Usually employed on passenger vessels where there is a large requirement of freshwater production. However, in merchant ships, the evaporation method is used as reverse osmosis is costly and includes large maintenance costs for membrane.

Osmosis is a natural process of diffusive (spontaneous) movement of solvent molecules through a semipermeable membrane into a region of higher solute concentration. The process stops when the hydrostatic pressure of liquid column (h) will offset osmotic pressure differences.

Mechanism of osmosis phenomenon can be seen from the above figure.

In the beginning of the process (On Left)
1 – Fresh water and 2 – sea water are present.

Once additional pressure is added on the sea water sided such that it reaches the Osmotic Pressure (On Right)
3 – Newly-generated Fresh Water and 4 – Sea Water with higher salt concentration are present.

So, this is the basics of generation of fresh water onboard.

About the Author:- Cadet Nikhil Kumar Choudhary is a first-year student at the Tolani Maritime Institute, Pune and currently taking up Marine Engineering, where he developed the gift of declamatory and is said to have eloquent writing skills. His chief incentive of writing is to convey about Marine Technology and Machines.