Marine Fresh Water Generators

Fresh Water Generators
(desalinator)
(distiller)
(evaporator)
Waste heat recovery
Derleyen : H.Nejat ÖZTEZCAN
Chief EngineerHüseyin Nejat ÖZTEZCAN Chief Engineer

Fresh water production from sea water for domestic and auxiliary
purposes is an essential requirement aboard ships.
A considerable amount of fresh water is consumed in a ship.
The crew consumes an average 100 liter/head/day. In a steam ship (a
ship whose main propulsion unit is steam turbine or a ship which is a
large tanker having steam turbine driven cargo oil pumps) the
consumption for the boiler can be as high as 30 tonnes/day.
Sufficient potable water may be taken on in port to meet crew and
passenger requirement. But the quality of this water will be too poor for
use in water tube boilers and for filling expansion tanks.
It is common practice to take on only a minimum supply of potable
water and make up the rest by distillation of sea water.
Hüseyin Nejat ÖZTEZCAN Chief Engineer

The stowage space that would have been used for fresh water can
hence be utilized for fuel or extra space made available for cargo when
fresh water generator is installed on a ship.
The equipment used on board for the production of freshwater from
seawater is known as fresh water generator.

There are two methods for generating fresh water:
• Distillation
• Reverse Osmosis (RO)
Distillation is a process in which impure water is boiled and the steam is
collected and condensed in a separate container, leaving many of the
solid contaminants behind.
Reverse osmosis (RO) desalination is a method of producing fresh water
from seawater by a process similar to filtration, rather than by
traditional evaporative distillation. A semipermeable membrane allows
water molecules to pass through while blocking the passage of most
other ions.

Distillation is cheaper and effective for less quantity but RO is costly and
for large quantity production. RO is used on Passenger ship where large
quantity of water is consumed.
Distillation = (Evaporation + Condensation)
Reverse Osmosis = (Semi permeable membrane - filter)
What ever type of plant is used, essential requirement of any fresh
water generator is that it should produce fresh water as economically as
possible.

Distillation process (method) is widely used on merchant ships.
Distillation is the combination of 2 process, evaporation and
condensation.
Evaporation can be done in 2 ways :
Evaporation by Boiling
Evaporation by Flash
Distillation can be done by 2 ways - Boiling or Flash.
Boiling and distillation process, on the basis of condenser's
structure it is divided into 2 following types :
(1) Tube type
(2) Plate type. Hüseyin Nejat ÖZTEZCAN Chief Engineer

FRESH WATER
PRODUCTION
DISTILATION
MEMBRANE
REVERSE OSMOSIS
FLASHBOILING

Treatment
Total
Hardness
Calcium
Hardness
Silica
Sodium
Chloride
TDS
Sea Water 250 200 14 15000 15000
Distillation <0.2 <0.2 <0.2 <20 <20
Reverse Osmosis 20 5 <1 <750 <750
Typical examples of water produced

What ever type of plant is used, essential requirement of any fresh
water generator is that it should produce fresh water as economically as
possible..
Even with a very efficient engine, only about 50% of the heat in the fuel
is converted into useful work at the crankshaft. The remainder
potentially wasted.
Main engine jacket cooling water also contains a considerable quantity
of heat this may be recovered in fresh water evaporators.

Shaft power
Output
49%
Lubricating oil
cooler
3%
Jacket water
cooler
5%
Air cooler
17%
Heat radiation
1%
Exhaust gas
26%
Standart Engine Fuel Consumption % 100 (167
gr/kW)

A desalination plant (also known as a fresh water generator)
onboard a floating structure is quite different from a land-based
desalination plant.
It operates in a very corrosive environment .
Rolling and pitching of the ship is also taken into consideration while
designing the desalination plant.
To avoid cavitation problems, an adequate quantity of water at
required pressure is always made available at a pump suction.
The vapours which are condensed on the condenser tubes are
collected in a product water sump. A sloping product water sump
may be provided depending on the extent of rolling to enable the
product water pump suction to be always full of liquid and avoid
cavitation problems or dry running of the pump.

Materials of Construction for Fresh Water Generator :
The shell is usually fabricated steel (or non-ferrous metal like
cupro-nickels) which has been shot blasted then coated with
some form of protective.
The important points about protective coatings are:
•They must be inert and prevent corrosion.
•They must resist the effect of acid cleaning and water
treatment chemicals
•They must have a good bond with the metal

Heat exchangers use aluminium brass tubes and muntz netal
tube plate (60/40 copper alloyed with zinc) in the case of tube
type fresh water generator.
For plate type, titanium plates are used for condenser and
evaporator. Demister is made of layered knitted wire of monel
metal.
A distilling plant be capable of operating for at least 90 days at
rated capacity without shutdown for cleaning.
Maritime Administration specifications require that "each
desalination unit be capable of unattended automatic operation
after being put on the line locally."

Boiling Process :
Sea Water is boiled in the evaporator at saturation temperature,
corresponding to the pressure in the evaporator.
Sea Water is kept at saturation temperature always.
It is of 2 types
If evaporator is plate type then it is called Plate type fresh water
generator
and If tubes are used for heating then it is called Tube type fresh
water generator. Also called submerged type, because heating
coils are submerged.

Conde
ns
Wapor
Condens
Sea water

Advantages and Disadvantages of Shell and Tube and Plate type Heat
Exchangers
A . Plate Type Heat Exchangers
Advantages
• Simple and Compact in size
• Heat transfer efficiency is more
• Can be easily cleaned
• No extra space is required for dismantling
• Capacity can be increased by introducing plates in pairs
• Leaking plates can be removed in pairs, if necessary without
replacement
• Maintenance is simple
• Turbulent flow help to reduce deposits which would interfere with
heat transfer

Disadvantages
• Initial cost is high since Titanium plates are expensive
• Finding leakage is difficult since pressure test is not as easy as tube
coolers
• Bonding material between plates limits operating temperature of
the cooler
• Pressure drop caused by plate cooler is higher than tube cooler
• Careful dismantling and assembling to be done
• Over tightening of the clamping bolts result in increased pressure
drop across the cooler
• Joints may be deteriorated according to the operating conditions
• Since Titanium is a noble metal, other parts of the cooling system
are susceptible to corrosion

B. Shell and Tube Heat Exchangers
Advantages
• Less expensive as compared to Plate type coolers
• Can be used in systems with higher operating temperatures and pressures
• Pressure drop across a tube cooler is less
• Tube leaks are easily located and plugged since pressure test is
comparatively easy
• Using sacrificial anodes protects the whole cooling system against corrosion
Disadvantages
• Heat transfer efficiency is less compared to plate type cooler
• Cleaning and maintenance is difficult since a tube cooler requires enough
clearance at one end to remove the tube nest
• Capacity of tube cooler cannot be increased.
• Requires more space in comparison to plate coolers

Fresh Water Generator Working Principle:
Water is generally produced on board using the distilation 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 80°C

•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 80°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.

Boiling
Point
Pressure Boiling Point Pressure
°C atm °C atm
5 0,0085 55 0,1553
10 0,0120 60 0,1965
15 0,0167 65 0,2468
20 0,0229 70 0,3075
25 0,0311 75 0,3804
30 0,0418 80 0,4674
35 0,0554 85 0,5704
40 0,0727 90 0,6919
45 0,0945 95 0,8342
50 0,1216 100 1,0000
WATER – PRESSURE AND BOILING POINTS

Flash Process

How we create a vacuum in fresh water generators ?
Vacuum is created using a device called “Air Ejector”
An air ejector is a device which uses the motion of moving fluid
(Motive Fluid) to transport another fluid (Suction fluid). It is has a
wide range of application in steam ejector in boiler condenser, fresh
water generator and in priming the centrifugal pump.
Ejector Pump
The ejector pump supplies seawater to the ejectors and also to the
heat exchanger.
The seawater goes to the ejectors in order to create a vacuum in the
boiling section.
This vacuum serves to lower the boiling point of the water, and to
allow the brine from the desalinated water to be returned to the
sea. Hüseyin Nejat ÖZTEZCAN Chief Engineer

An air ejector which uses the high pressure motive fluid such as sea
water to flow through the nozzle. The function of the nozzle is to
convert the pressure energy of the motive fluid into the velocity
energy.
P1-pressure of the fluid entering the nozzle.
V1- velocity of the fluid entering the nozzle.
P2- pressure of the fluid leaving the nozzle.
V2- velocity of the fluid leaving the nozzle.
By Bernoulli’s theorem:
P1 × V1 = P2 × V2.

This diagram shows the basic components of an Ejector used in the
Fresh Water Generators.
This Ejector was designed for use with sea water.
These are:
1) There are three connections. One for the high pressure sea water
(ejector pump discharge), one for the low pressure (LP) suction
entrained and one for the medium pressure discharge.
2) The suction (in this case air or brine) comes in at the side.
3) There is a nozzle for converting the pressure energy of the high
pressure motive into kinetic energy.

REGULATIONS REGARDING PRUDUCTION OF SEA
WATER ON BOARD SHIP
• Cannot be used in ports, anchorages and closer to shore than
12 nm because of domestic sewage and industrial effluents.
• Engine must be running at full ahead sea speed during start of
FWG
• Ensure main engine parameters are normal
• Shipo is not maneuvering
• There is no oil/chemical reported in the visinity of the ship
• Unfit as potable water because:
- Not sterilised
- Tasteless
- Slightly acidic in nature
- Devoid of any minerals requried for human body.

SHELL AND TUBE TYPES FRESH WATER
GENERATORS (SASAKURA)

PLATE TYPES FRESH WATER GENERATORS

ALFA LAVAL

OPERATION PRINCIPLES
• The Evaporator Chamber is kept under vacuum by a Water Ejector.
Sea water supplied by Ejector Pump drives Water Ejector, and
enters into the Tubes of Condenser as a cooling medium, then is
discharged overboard.
• Parts of the jacket cooling water (fresh water) circulates to the
outside of the heater tubes giving up some of its heat to the sea
water which flows inside the tubes.
• The heated sea water (feed water) evaporates as it enters the main
chamber due to the vacuum condition. Water droplets are removed
from the vapor by the deflector and mesh separator. The seperated
droplets fall back into the brine, which is extracted from the
chamber and discharged overboard.

• The vapor passes to the condenser tube bundle which is cooled by
sea water flowing inside the tubes. The condensed vapor is
collected and pumped to the fresh water storage tank by the
distillate pump.
• The sea water used in the condenser becomes warmed up as the
vapor gives up its heat of condensation. Part of this warm sea
water is used as the feed water to the FWG.
• The salinity of the distllate is monitored by a conductivity dedector.
If the salinity exceeds the specific level, the selenoid valve in the
discharge line of the distillate pump is automatically activated and
the faulty distillate is returned to the brine side of the evaporation
chamber.

FRESH WATER GENERATORS MAIN COMPONENTS AND FUNCTIONS
• 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.
• Condenser: It exchanges the latent heat ın the produced fresh
vapours to the cooling water so that the vapours are condensed
and accumulated ın the bottom of the condenser ‘s shell.
• Evaporator: It is used to boil off the seawater at lower temperature
with the help of vacuum created inside Fresh Water Generator
shell.

• An Air Ejector: To Create Vacuum In the main body.
• A Brine Ejector: The brine ejector removing brine and salt
deposits from the evaporator chamber.
• Combined brine and air ejector: The combined brine and air
ejector extracts brine and incondensable gases from the
separator vessel.
• A Sea Water Ejector Pump: For supplying necessary sea water
required for production of fresh water .
• A Fresh Water Distillate Pump: For pumping the f/w produced
from condensor chamber into f/w storage tank.

• A Salinometer: For measuring the ppm of Fresh Water produced
which is generally 1-10 ppm. If more than 10 ppm (as set by
operater), an alarm sounds and The Fresh Water produced is
bypassed back to the evaporater.
• Demister: The water vapour pass through the demister which will
remove the carried salt and only allow the water vapour to pass
through.
• Vacuum Breaker Valve: For releasing the vacuum at the time of
shutting down.
• Flow Meter: The flow meter ındicates the accumulated fresh water
produced.
• Relief Valve: For releasing the excess pressure.
• Control panel: Contains motor starters with thermal overload relays
and running lights for each pump, salinometer and alarm panel.

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.

CONTROL PANEL AND SALINOMETER

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 ejector
pump which will provide water for evaporation, cooling and to
the ejector 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 breaker 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. Hüseyin Nejat ÖZTEZCAN Chief Engineer

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.Hüseyin Nejat ÖZTEZCAN Chief Engineer

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.
Note : The distillate water shall be disposed out for min. 30 minutes at
the initial start up of the distillate pump.Hüseyin Nejat ÖZTEZCAN Chief Engineer

REGULATING THE CAPACITY
The capacity (quantity of produced water) of the Fresh Water
Generator is regulated by increasing or decreasing the quantities of
Jacket cooling water to the heat exchanger. The capacity of the plant
is measured by means of the water meter.
The quantity of the Jacket cooling water shall be regulated by the
by-pass valve to the fresh water cooIer until the plant produces its
normaI capacity.
In case that the temperature of the jacket cooling water is lower
than the prescribed one, the flow quantity passing throught the
heat exchanger shall be increased more.
The supply of cooling sea water to the condenser is regulated so
that the cooling sea water temperature rises about prescribed value
when passing through the cooling tubes of the condenser.

The evaporation temperature should be about 450C to 600 C.
Evaporation temp. may become much lower than suitable range
when ship sails in low sea water temp. area. In such case,
Evaporation temp. must be raised by means of either adjusting
"VACUUM ADJUST VALVE" on air extraction line, or reducing
condenser cooling sea water flow rate .
If the evaporation temperature is too high which may occur at high
cooling sea water temperature, the quantity of cooling sea water to
the condenser is increased which will make the evaporation
temperature drop.
Too high evaporation temperatures increase the risk of scale
formation in the tubes of the heat exchanger, and too low
evaporation temperature will owing to the resulting great vapour
volumes mean a risk that sea water drops air brought with to the
condenser resulting in fresh water with a too high salt content.

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 breaker valve.
7. Close sea water suction valve and overboard valve. This is
generally not required as they are non- return valves.

Precautions for Operation of Fresh water Generator :
1.Seawater pressure at the inlet of air ejector must be 3 bar or
more.
2.The pressure at ejector outlet should not exceed 0.8 bar.
3.Never start fresh water generator distillate pump in dry
condition.
4.Operate jacket cooling water valves to the fresh water
generator gradually to avoid thermal shock to the main engine.
5.Feed water to be supplied for a few minutes to cool down
the evaporator before stopping.
6.Never open the drain valve of evaporator before opening
vacuum breaker. Otherwise atmospheric pressure causesHüseyin Nejat ÖZTEZCAN Chief Engineer

MAİNTENANCE
Why you need to perform regular maintenance duties ?
Regular maintenance of the plant will improve performance and
availability.
The maintenance schedule will tell you how often service should be
performed on the main components. As the actual operating
conditions of the plant are of major influence on the life time, the
overhaul dates are not obligatory but only recommended intervals.
When the plant has been in operation for a longer period of time
and experience has been established as to the actual performance,
it will be possible to adapt the maintenance schedule.

During the operation of evaporating plants, scale will form on the
heating surfaces. The rate of scale formation will depend upon the
operating temperature, the flow rate and density of the brine.
Scale formation will result in greater requirements for heating to
produce the rated quantities of distilled water or a fall-off in
production for a fixed heating supply.
Cold shocking, the alternate rapid heating and cooling of the tube
surfaces, for a boiling process type, can reduce scale build-up.
Ultimately, however, the plant must be shut down and the scale
removed either by chemical treatment or manual cleaning.
Also a routine maintenance of the generator should be carried out
by shutting down the plant and removing the scale manually or by
chemical treatments. Hüseyin Nejat ÖZTEZCAN Chief Engineer

The internal walls of the chamber or the shell should also be
cleaned when the overall cleaning is done.
Air ejectors and educators should also be checked for holes or
leakages, which can prevent the formation of desired vacuum.
The distiller, feed and brine pumps should also be properly
maintained to prevent any interruption in the flow of fresh and sea
water. The processes and the phenomena used in both plate and
tube type FWGs are the same.
A constant check should be kept on the flow meter to prevent
excessive or very less flow.
The salinometer alarm should be precisely set and given a constant
watch. This is to prevent the degrading of the quality of fresh water
produced.

Maintenance of Plate surface :
Clean Plate surface as follows:
1. Remove tightening bolts
2. Open plate stack
3. Remove plate stack
4. Submerge plates completely in a hot, inhibited acid bath at
maximum 50ºC.
5. Scrub plates with a soft brush and plain hot water at maximum 50°C.
6. Examine plates and gaskets for possible damage, and remove
damaged plates and/ or replace damaged gaskets.
If a defective plate is found, remove the plate together with one of the
adjacent plates.
The end plate and start plate cannot be removed but must always be
replaced, with a corresponding plate.Hüseyin Nejat ÖZTEZCAN Chief Engineer

7. Reassemble the plate stack in accordance with attached assembly
scheme.
8. Tighten plate stack to measurements stated in technical specification.
9. Vacuum test the freshwater generator before start up.
10. The evaporator section is pressure tested by letting hot water
circulate through the section with bypass valve for hot water in normal
running position.
11. The condenser section is pressure tested by starting the ejector
pump and letting sea water circulate through the condenser section.
NOTE! Measure and note the tightening measure before removing
tightening bolts.
NOTE! Be careful not to damage the gasket during manual cleaning.Hüseyin Nejat ÖZTEZCAN Chief Engineer

• Whenever the sections are dismantled, inside the chambers;
isolated layers must be checked for defects. Repair any
damage according to the maintenance guide for coating. To
preserve this coating DO NOT scrape or scratch the inside
surface of the seperator vessel.
• Whenever the seperator vessel is opened check that the
anodes are functioning. If the anodes are not functioning
and/or worn replace them.

How Scale Formation Occurs in Fresh Water Generator:
The performance of fresh water generator reduces with the
formation of scales because of reduction in heat transfer
efficiency. Three scales which are normally found in fresh water
generators are:
•Calcium Carbonate, CaCO3
•Magnesium Hydroxide, Mg(OH)2
•Calcium Sulphate, CaSO4
Calcium carbonate and magnesium hydroxide scale formation
mainly depends on the temperature of operation. Calcium
sulphate scale formation depends mainly on the density of the
evaporator contents or brine.
It is recommended to operate fresh water generator at its rated
capacity, not more. More production of water than rated capacity
means higher concentration of brine and more scale formation.Hüseyin Nejat ÖZTEZCAN Chief Engineer

How to Minimize Scale Formation
Scale formation in fresh water generator can be controlled and
minimized by continuous chemical treatment. Their trade name
is different, like:
•Vaptreat (by “UNITOR”)
•Ameroyal (by “DREW CHEMICALS”)
These chemicals minimize calcium carbonate scale formation
and possibility of foaming.
The compound is non toxic, no-acidic, and can be used in fresh
water generator producing water for drinking purposes. It would
be continuously fed into the feed line using a metering pump or
by gravity.
Amount of chemical to be dosed depends on the capacity of
fresh water produced. Hüseyin Nejat ÖZTEZCAN Chief Engineer

CLEANING THE TUBES (DESCALJNG METHOD)
The fresh water generator is equipped with a heater, a condenser and a
preheater. Scale forms mainly in the heating tubes of the heater.
Chemical cleaning of the whole system can be made by fitting adapter
(option) to the corrosion plate connection of the condenser water
chamber.
Sea water boils and evaporates in the heating tubes, and consequently
sea water touching the heating tubes is considerably concentrated and
supersaturated. This is why scale is deposited in the heating tubes.
Cleaning (descaling) of the of the heating tubes should be made twice or
three times a year in general. However, the interval depends upon the
operating conditions and the properties of sea water.

CLEANING METHOD:
Scale may be either peeled off by physical methods or dissolved by
chemical methods. The former includes the use of brush and drill,
the rapid cooling method, injection of pressurized water, etc., but it
is rather difficult to completely remove scale by these methods.
• CHEMICAL METHODS
a) Submerged Cleaning
Pour chemical solution into the heater through the sight hole until
the upper tube plate is soaked and leave it as it is. The time required
for cleaning varies in the thickness of scale. When the solution
becomes saturation, it has no capacity for cleaning.
In this case interchange with·new solution a few times.

b) Circulated Cleaning
As the drawing shows, by fitting the adapter for the inlet of solution to
the connection of corrosion plate at cooling water inlet nozzle of
condenser water chamber, and using the socket of bottom cover for
outlet of solution, clean the whole system of heat exchangers.

FRESH WATER GENERATOR TYPE VSP-36-100/125 CC/SWC

MAINTENANCE

TROUBLE SHOOTING
A
C D B

B A C D

ALFA LAVAL AQUA-100-HW, AQUA-125-HWHüseyin Nejat ÖZTEZCAN Chief Engineer

•Half the seawater flow - compared to other freshwater generators
only half the seawater is needed, which means smaller seawater
pumps can be used. Optimized distribution prevents dry spots and
inhibits the natural scaling process.
•Lower costs and emissions - the reduction in seawater pumping
needs has a corresponding effect on the consumption of electrical
energy. Less fuel has to be burned, which reduces both operating
costs and CO2 emissions.
•3-in-1 plate technology - The AQUA Blue incorporates the
evaporation, separation and condensation processes into a single
type of titanium plate. Desalination is handled within a single plate
pack that also contains the process vacuum. No outer shell is
necessary.

The combined brine/air ejector driven by the cooling water creates
the necessary vacuum in order to lower the evaporation temperature
of the feed water.

The feed water is introduced into the evaporator section through an
orifice, and is distributed into every second plate channel (evaporation
channels).

The hot water is distributed into the remaining channels, thus
transferring its heat to the feed water in the evaporation channels.
Having reached boiling temperature – which is lower than atmospheric
pressure – the feed water undergoes a partial evaporation and
generates a mixture of vapour and brine. The brine is separated from
the vapour and extracted by the combined brine/air ejector.Hüseyin Nejat ÖZTEZCAN Chief Engineer

Having passed a separation zone the vapour enters every second plate
channel in the condenser section.
The cooling water supplied distributes itself into the remaining channels,
thus absorbing the heat being transferred from the condensing vapour.
The produced fresh water is extracted by the freshwater pump and
pumped to the freshwater storage tank.

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.

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

Water Disinfection Methods
• Chlorine sterilization
• Ultra violet light disinfection
• Ozone water disinfection

CHLORINE STERILIZATION AND CONDITIONING
The distillated water 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

CHLORINE STERILIZATION

Ultra violet light disinfection
The UV light is an effective and clean water disinfection method, it
inactivates bacterias and other harmful contaminants. UV light as a
disinfection method is non residual so is actually doesn’t leave any
disinfectant in the water.
Ozone water disinfection
Water disinfection methods also include the use of Ozone (O3), this is
a very unstable molecule which is a powerful oxidant that’s toxic for
organisms living in water. Ozone offers a very wide spectrum
disinfection ability. the Ozone must be produced on site using oxygen
and a UV light normally. Ozone disinfectant produces less hazardous by
products that Chlorine does.

Some of the important points that should be considered during
maintenance of drinking water systems on ships are:
• Check Salinity Alarm: The salinity alarm or salinity indicator needs
regular checks as it allows only pure fresh water to flow into the fresh
water tank.
• Stop Fresh Water Generator At Right Time: Whenever a vessel
approaches any port, land or estuary, the Fresh Water Generator
must be stopped as at such places the sea water is heavily infected
with bacteria ,which may be transferred to the fresh water stored
onboard. As per recommended in Safety Management System
Manual or Flag State Requirements, the Fresh Water Tanks are
generally cleaned once in six months or on yearly basis.
• Use High Pressure Spray While Cleaning Tanks: While cleaning the
fresh water tanks it is advisable to use high pressure spray of fresh
water.

• Be Careful While Using Chemicals and Scrubbing: Chemicals, if any,
are to be used should be biodegradable. Mostly fresh water tanks do
not get rusted and have a special coating inside. It should be kept in
mind not to scrub the tank surface too hard so that it results in
removal of coating from the tank walls.
• Take Proper Steps While Applying Paint: Paint if applied on the tank
surface must be of approved type, immiscible in water and suitable
to the surface.
• Follow Proper Enclosed Space Entry Procedures: If ship’s staff is
involved in cleaning fresh water tanks, enclosed space entry
checklist and procedures must be complied with.
• Open Separator Shell When Required: The separator shell and heat
exchanger covers can be opened up and inspected during scheduled
inspections for scale formation or if cooling tubes are fouled with
any sludge formation.

• Use Scale Inhibitors: Scale inhibitors are used to prevent scale
formation by dispersing scale deposits and delaying reaction.
Scale formation inside heat exchanger requires cleaning if specific
temperatures cannot be obtained for inlet and outlet of fresh
water.
• Remove Damaged Coating: In case coating inside fresh water
generator is damaged, the damaged covering is to be scraped off
and the surface should be then thoroughly dried. After putting
the undercoat on the steel surface, epoxy-resin or food coating
(as prescribed by FWG manufacturer) is to be applied.
• Clean Drinking Water Fountains: Various drinking water fountains
inside accommodation require scheduled cleaning and
replacement of filters as well.
• Cleaning of Fresh Water Tank: The fresh water tank must be
inspected and cleaned at regular intervals of time (normally 6
months).

SOME FWG PHOTOS TAKEN BY ME

FRESH WATER GENERATOR
QUESTIONS and ANSWERS

• Why fresh water generator is fitted on ships ?
To produce the high purity distilled water from sea water.
To provide make up water for boiler and portable water for
drinking and domestic use. So can save cost.
• What is temperature of Main Engine jacket cooling water
entering to fresh water generator?
It is about 80 C degree

• What are the causes of loss of vacuum in fresh water generator ?
•Failure of ejector pump
•Failure of ejector nozzle (fouling, erosion)
•Malfunction of check valve (at ejector nozzle)
•Defective vacuum breaker
•Any air leakage into the system (At joint)
• What will happen when vacuum reach 100% in fresh water
generator ?
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
93% vacuum.

• What are the reasons loss of facuum or over-pressure of shell?
The shell pressure of the fresh water generator rises and rate of
freshwater produced reduces. The reasons are:
1.Air leaks into the evaporator shell in large quantities and air
ejector cannot cope.
2.The cooling water flow through the condenser is reduced or
cooling water temperature is high. This cause saturation
temperature and hence saturation pressure within the condenser
to rise.
3.Malfunctioning of the air ejector.
4.Flow rate of the heating medium increased and excess water
vapour produced. Since this excess vapours cannot be condensed,
shell pressure increases or vacuum falls.

• REASON FOR FRESH WATER GENERATOR SALINITY ALARM?
1. Vaccum is too high,which is leading to rapid boiling of sea water in
fresh water generator.
2. Same goes with the low waccum but with less boiling temperature.
3. Jacket water from main engine is not properly set to flow in to the
generator.
4. Brine ejector is not working properly, hence too much brine
carryover in the condensation.
5. Demister mesh, is not working properly, leading to large carryovers.
6. Vaccum relief valve or the FWG space is leaking.
7. Alarm level hes been set too low, as compared to salinity maintained
by the FWG.

• What are the safeties in a FWG?
Safeties in a FWG are:
1. Vacuum breaker for releasing the vacuum at the time of shutting
down.
2. Relief Valve for releasing the excess pressure.
3. High Salinity Alarm: It is fitted to the salinometer as it measures
higher salt content in the water produced, it sounds the alarm.
4. Temperature Guage.

• Where does the ejector pump takes suction from?
• What if ejector pump fails and we have to run FWG?
Ejector pump has a separate sea water suction (a separate sea chest.)
In case the ejector pump fails and we need to run the FWG, there is a
separate line from fire and general service pump as the discharge
pressure of this pump is around 3-4 bar and ejector pump discharges at
pressure not less than 4 bar.
Main sea water cannot be used ın this case because main sea water
pump has discharge pressure around 1-2 bar.

• What are the reasons salt water carry over (Priming)?
Salt water may be carried over in large quantities during operation
of the freshwater generator. This is called priming. General reasons
of the priming are:
1.Level of salt water inside the shell is high. When water level is high
agitation due to boiling occurs and salt water may carry over along
with the vapours.
2.When the salt water brine density is too high, agitation of salt
water occurs which results in priming.
3.Increased evaporation rate.

• What are the reasons of the gradual increase in level of brine?
For the satisfactory operation of the freshwater generator, a
constant level of brine to be maintained in the shell.
Brine is the concentrated sea water after liberation of water
vapours.
This brine is gradually extracted from the shell. Usually this is
achieved by the combined air-brine ejector. It extracts air as well as
brine from the shell.
Any fault in the ejector or ejector pump cause increase in the brine
level.

• Reasons for increase in Salinity of Freshwater?
Possible causes are:
1.Brine level inside shell too high.
2.Leaking condenser tubes or plates.
3.Operation of evaporator near shore with contaminated feed water.
4.Shell temperature and pressure too low.
5.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.

Salinity of distilled water produced from fresh water generator onboard depends on
A. Amount of feed set in fresh water production
B. Amount of salt water leaking from condenser if any
C. Temperature of the sea water used
D. Efficiency of brine ejector from the evaporator shell
Answer-A, C & D
Scale formation in a fresh water generator evaporator can lead to _____________
A. Impaired heat transfer
B. Reduced capacity
C. Increased shell temperature
D. All of the above
Answer-D
Amount of distilled water produced in fresh water generator onboard decreases with
A. Increase in vacuum in the fresh water generator shell
B. Decrease in sea water temperature
C. Decrease in efficiency of heat exchanger
D. Increase in sea water temperature
Answer-C and D

. A high reading at a salinity cell located in the loop seal between
two stages of a flash type evaporator would indicate ______.
a) chill shocking is necessary to remove scale
b) leakage at the second-stage condenser
c) faulty operation of the brine overboard pump
d) carryover in the first-stage
• In which of the following Fresh Water Generators would an air
ejector be unnecessary?
a) Reverse Osmosis Unit
b) Submerged tube type FWG Unit
c) Plate type FWG Unit
d) Flash Type FWG Unit

REVERSE OSMOSIS

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.

Osmosis
To understand the purpose and process of Reverse Osmosis you must
first understand the naturally occurring process of Osmosis.
Osmosis is a naturally occurring phenomenon and one of the most
important processes in nature. It is a process where a weaker saline
solution will tend to migrate to a strong saline solution. Examples of
osmosis are when plant roots absorb water from the soil and our
kidneys absorb water from our blood.
What is meant by Osmosis ?
•When different concentration solutions are separated by a semi-
permeable membrane, water from less concentrated solution pass to
the other solution through the membrane to equalize the concentration
of the two solution.

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

What is meant by Reverse Osmosis ?
•The pressure greater than the osmotic pressure is applied to the
side of higher concentration solution, the osmosis process
is reversed.
•Water from the stronger solution is forced back through the semi-
permeable membrane to dilute the initially weak solution on the
other side and further increase the concentration of the
strong solution. The total pressure required for this process
consists of the osmotic pressure (up to 28 bar for sea water)
plus the system pressure losses and net driving pressures
(around 25 bar).

A semi-permeable membrane is a membrane that will allow some
atoms or molecules to pass but not others.
A simple example is a screen door. It allows air molecules to pass
through but not pests or anything larger than the holes in the
screen door.
Another example is Gore-tex clothing fabric that contains an
extremely thin plastic film into which billions of small pores have
been cut. The pores are big enough to let water vapor through, but
small enough to prevent liquid water from passing.

How does Reverse Osmosis work?
Reverse Osmosis works by using a high pressure pump to increase
the pressure on the salt side of the RO and force the water across
the semi-permeable RO membrane, leaving almost all (around 95%
to 99%) of dissolved salts behind in the reject stream.
The amount of pressure required depends on the salt concentration
of the feed water. The more concentrated the feed water, the more
pressure is required to overcome the osmotic pressure.
The desalinated water that is demineralized or deionized, is called
permeate (or product) water.
The water stream that carries the concentrated contaminants that
did not pass through the RO membrane is called the reject (or
concentrate) stream.

Salt Rejection %
This equation tells you how effective the RO membranes are
removing contaminants. It does not tell you how each individual
membrane is performing, but rather how the system overall on
average is performing.
A well-designed RO system with properly functioning RO membranes
will reject 95% to 99% of most feed water contaminants. You can
determine effective the RO membranes are removing contaminants
by using the following equation:
The higher the salt rejection, the better the system is performing.
A low salt rejection can mean that the membranes require cleaning or
replacement. Hüseyin Nejat ÖZTEZCAN Chief Engineer

Salt Passage %
This is simply the inverse of salt rejection described in the previous
equation.
This is the amount of salts expressed as a percentage that are
passing through the RO system.
The lower the salt passage, the better the system is performing.
A high salt passage can mean that the membranes require cleaning
or replacement.

Recovery %
Percent Recovery is the amount of water that is being 'recovered' as
good permeate water. Another way to think of Percent Recovery is the
amount of water that is not sent to drain as concentrate, but rather
collected as permeate or product water.
The higher the recovery % means that you are sending less water to
drain as concentrate and saving more permeate water.
However, if the recovery % is too high for the RO design then it can lead
to larger problems due to scaling and fouling.
The % Recovery for an RO system is established with the help of design
software taking into consideration numerous factors such as feed water
chemistry and RO pre-treatment before the RO system.
Therefore, the proper % Recovery at which an RO should operate at
depends on what it was designed for.

By calculating the % Recovery you can quickly determine if the
system is operating outside of the intended design.
For example, if the recovery rate is 75% then this means that for
every 100 gallons of feed water that enter the RO system, you are
recovering 75 gallons as usable permeate water and 25 gallons are
going to drain as concentrate.
Industrial RO systems typically run anywhere from 50% to 85%
recovery depending the feed water characteristics and other design
considerations.
The calculation for % Recovery is below:

RO Membrane Cleaning
RO membranes will inevitably require periodic cleaning, anywhere
from 1 to 4 times a year depending on the feed water quality.
As a general rule, if the normalized pressure drop or the normalized
salt passage has increased by 15%, then it is time to clean the RO
membranes.
If the normalized permeate flow has decreased by 15% then it is
also time to clean the RO membranes.
You can either clean the RO membranes in place or have them
removed from the RO system and cleaned off site by a service
company that specializes in this service. It has been proven that
offsite membrane cleaning is more effective at providing a better
cleaning than onsite cleaning skids.

One problem with any filtration system is that deposits accumulate
and gradually blocks the filter.
-The sea water is supplied at a pressure of about 60 bar, 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.

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.

Reverse Osmosis process flow chart.
SEA WATER
SAND FILTRATION
ANTISCALANT DOSING
CARTRIDGE FİLTRATION
HP FEED PUMP
MEMBRAN
FRESH WATERHüseyin Nejat ÖZTEZCAN Chief Engineer

Backwashing:
Backwashing of the filters is carried out to remove the
accumulated solid particulates from the filtering media layers; it
involves reversing the normal flow and discharging it to waste.
Backwashing is carried out on a set frequent depending upon the
feed quality or if the differential pressure increases by 1.0 bar
between the inlet to the outlet.
The backwash flow rate will vary depending upon the feed water
temperature. It is critical that the correct flow rate is used; a
satisfactory wash may not be achieved if it is too low or, on the
other hand, media may be washed away if the wash water flow
rate is too high.

2018

Marine Fresh Water Generators

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