Fresh Water Generator

Mohd. Hanif Dewan, Chief Engineer and
Maritime Lecturer & Trainer, Bangladesh.
FRESH WATER GENERATOR
6/23/2014 1

Fresh Water Generator Working Principle:
- Water is generally produced on board using the
evaporation method.
- Fresh water is produced by evaporating sea water using
heat from any of the heat source.
- The evaporated sea water is then again cooled by the sea
water 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 cylinder head, liner etc.
-The temperature available from this jacket water is about
70°C.
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- But at this temperature the evaporation of water is not
possible as we all know that the evaporation of water takes
place at 100°C under atmospheric pressure.
- Thus in order to produce fresh water at 70°C, we need to
reduce the atmospheric pressure, which is done by
creating a vacuum inside the chamber where the
evaporation is taking place.
- Also, as a result of the vacuum the cooling of the
evaporated sea water will also take place at lower
temperature.
- This cooled water is collected and transferred to the tank.
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Function of All Parts of Shell & Tube
Type FWG
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Vent Cock:
It is used to break the vacuum after stopping. In idle condition
and maintenance period, it is used for venting purpose.
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Relief Valve:
It is used as a pressure-relief device .
Vacuum Gauge:
It is used to show the percentage of vacuum.
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Operation of a Shell & Tube Type FWG
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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.
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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.
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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.
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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.
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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 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.
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SALINOMETER
• Pure distilled water may be considered a non-conductor
of electricity. The addition of impurities such as salts in
solution increases the conductivity of the water, and this
can be measured. Since the conductivity of the water is,
for low concentrations, related to the impurity content, a
conductivity meter can be used to monitor the salinity of
the water.
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SALINOMETER ALARM UNIT
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DRINKING WATER TREATMENT
• Filtration – to remove any solid particulate matter –
using carbon filter, membrane filter etc.
• Sterilisation – to remove bacteria – through
chlorination, UV treatment, ozonisation etc.
• Neutralisation – to neutralise acidic nature – add
calcium or magnesium carbonate
• Mineralisation – to add minerals required for human
body by dosing calcium or magnesium carbonate
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Drinking water treatment
• The low operating temperature of the evaporator
is not sufficient to sterilize. Harmful organisms
may enter with the sea water and pass through
to the domestic water tank. There is a likelihood
that while in the domestic tank, water may
become infested with bacteria. Sterilization by
the addition of chlorine, is recommended.
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CHLORINE STERILIZATION AND CONDITIONING
•The distillate is passed through a neutralite unit containing
magnesium and calcium carbonate. Some absorption of CO2 from
the water and the neutralizing effect of these compounds, removes
acidity. The addition of hardness salts also gives the water a better
taste. The sterilizing agent chlorine, being a gas, is carried into the
water as a constituent of sodium hypochlorite (a liquid) or in
granules of calcium chloride dissolved in water. The addition is set
to bring chlorine content to 0.2 ppm. The passage of water from
storage tanks to the domestic system, is by way a carbon filter
which removes the chlorine taste.
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REVERSE OSMOSIS
Reverse Osmosis (RO) 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 large requirement of
fresh water production. However, in merchant
ships the evaporation method is used as reverse
osmosis is costly and includes large
maintenance cost for membrane.

Working Principle Of RO:
Osmosis describes the process whereby a fluid will
pass from a more dense to a less dense solution
through a semi-permeable membrane. It is very
important to the water absorbtion processes of
plants. RO is a process which uses a semi-
permeable membrane which retains both salt and
impurities from sea water while allowing water
molecules to pass. Filtration of up to 90% is
possible thus making the produced water
unsuitable for boiler feed without further
conditioning. Improved quality is possible using a
two or more pass system.
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An experiment to determine this osmotic head is shown.6/23/2014 39

The parchment paper acts as the semi-permeable
membrane and allows the water molecules to pass but not
the larger salt molecules.6/23/2014 40

Operation of Reverse Osmosis Plant:
- Reverse osmosis is the process whereby a
pressure greater than the osmotic head pressure
is applied to a solution of high density.
- Fluid is forced from the high density side to the
less dense side.
- For desalination plants the pressure is applied to
sea water and the water is forced through the
semi-permeable membrane.
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Operation of Reverse Osmosis Plant (cont’d)
- Design of the cartridges is therefore such that the sea
water feed passes over the membrane sheets so that the
washing action keeps the surfaces clear of deposits.
- A dosing chemical is also injected to assist the action.
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- The two membranes sealed on the outer three edges,
enclose porous under-layer through which the permeate
spirals to central collecting tube.
- Pressurized feed water passes lengthways through the
tubular spiral wound membrane element.
- Freshwater permeate travels through the membrane
layers as directed along a spiral bath in-out a central
perforated tube, while brine is discharged out the end of the
membrane element.
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- The fluid could be water and the solutions sea water.
Under normal conditions the water would pass from the
less saline solution to the more saline solution until the
salinity was the same.
- This process will cease however if the level in the more
saline side raises to give a difference greater than the
Osmotic height.
- For practical use to allow the generation of large
quantities of water. It is necessary to have a large surface
area of membrane which has sufficient mechanical strength
to resist the pressurized sea water.
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- One problem with any filtration system is that deposits
accumulate and gradually blocks the filter.
- The sea water is supplied at a pressure of 60bar, a relief
valve is fitted to the system.
- The Osmosis production plant is best suited to the
production of large quantities of water rather than smaller
quantities of steam plant feed quality.
SEMI PERMEABLE MEMBRANE:
The semi permeable membrane which is typically made of
polyamide membrane sheets wrapped in a spiral form
around a perforated tube resembling a loosely wound like a
toilet paper roll.
The material used for sea water purification is spirally
wound polyamide or polysulphonate sheets.
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47
REVERSE OSMOSIS
SEAWATER TO POTABLE WATER
SEAWATER
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Pretreatment and post treatment
Sea water feed for reverse osmosis plant is pretreated
before being passed through. The chemical sodium hexa-
phosphate is added to assist wash through of salt deposits
on the surface of the elements and the sea water is
steriliazed to remove bacteria which could otherwise
become resident in the filter. Chlorine is reduced by
compressed carbon filter while solids are removed by other
filters. Treatment is also necessary to make the water
drinkable.
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49
REVERSE OSMOSIS
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REVERSE OSMOSIS
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RO FILTER MODULE
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RO FILTER MODULE
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ANY QUESTION?
THANK YOU!
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Fresh Water Generator

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