(7.7 Fresh Water Generator)

7.7 Fresh Water Generator

Fresh water is one of the main important items onboard. It is not only used for
drinking, cooking and bathing but also in boilers, and as cooling agent for diesel
engines and other machineries.
All large ships have “Fresh Water Generator” or being referred to as “Distilling Plant”,
referring to the process of distillation for generating fresh water onboard. It is either
operated by steam (either high or low pressure) or the heat coming from the diesel
engine which may vary in capacity and type with the size and the class of the ship.
In most merchant ship’s, fresh water generator design use the heat from main engine
jacket cooling fresh water system while the main plant is in operation and in some
cases an alternative supply of steam is provided so that the equipment can be used
even with the main plant is in idle condition.

7.7.1 Principle of Distillation
A ship is always surrounded by limitless quantities of water, except when it is in dry
dock. But this water is called the sea water which is unfit for human consumption and
in most case it cannot be used directly into diesel engine cooling system or boiler
system due to its corrosive effects. If the salt and other substances can be removed
there would be sufficient pure water at hand. Such substances can be removed by
distillation.
Distillation is a process simply by boiling the liquid and condensing the vapor back to
the liquid again. In boiling most or all impurities are left behind, so that the
condensed liquid is relatively pure.

7.7.2 Onboard Process of Distillation
When the pressure in the evaporating shell is reduced to vacuum condition, the
boiling point of sea water is reduced to a temperature lower than 100 oC (boiling pt.
of water at atmospheric pressure).

7.7.3 Types of Distilling Plants
There are various types of fresh water generator or distilling plant that are available
in the ships and we will only discuss the most common type of fresh water generator.
a. Flash Type Distilling Plants (For LNG Vessel)
The flash type evaporator, like all distilling plants, removes salts and other
impurities from raw seawater by the process of evaporation and condensation.
This is accomplished by boiling the water to convert it to steam, and condensing
this steam to formed distilled water. The flash evaporator is different from
other distilling plants, because evaporation takes place at temperatures well
below the normal boiling point of water and without the use of submerged
heat transfer surfaces. In the flash type distilling plant, the temperature of
the water is never raised beyond 80 °C, and is only raised to this temperature
within the last pass of tubes of the feed water heater. Flash evaporation takes
place at temperatures as low as 40°C. In addition, no boiling occurs on heat
transfer tube surfaces; as a result, the scale formation is greatly reduced and
operation at maximum efficiency is prolonged. The term “flash evaporation”
means that water is converted to steam as it enters an evaporating
chamber, without further addition of heat. Flashing at low temperatures is
possible only when a vacuum is maintained in the chamber, since the boiling
point of water decreases as the pressure in the chamber is reduced. As in other
methods of distillation, a portion of the water remains behind in the evaporating
chamber and is taken off as a concentrated waste (brine).

b. Low Pressure Submerge Tube Type Fresh Water Generator
Low pressure, submerged tube plants differ from ship to ship, but the operating
conditions and the maintenance procedures are basically the same.
This type of fresh water generator plant is employing tubes as heat exchangers.
The evaporator tube is submerged to sea water and heating is supplied using the
jacket cooling water or steam. The vapors that are generated in the evaporator is
condensed in the condenser using the cooling sea water. The system is operating
under vacuum state and boiling of sea water is taken place below the
atmospheric pressure. The plant requires proper heat balance to maintain
efficiency.

c. Plate Type Fresh Water Generator
Plate-type fresh water generator operating procedure is almost the same with the
low pressure submerge tube type fresh water generator. The main difference is
the structure as it is using plate type heat exchanger to generate fresh water.
Since the plate-type heat exchanger part is being used, it takes up less
installation space, and maintenance/inspection work is easier and less timeconsuming.
Efficiency is also improved depending on the operating condition.
Most of this Fresh Water Generator uses the waste heat from diesel engine
jacket cooling water to heat seawater and evaporation takes under vacuum to
produce fresh water.


d. Principle of Operation (Plate Type Fresh Water Generator)
The combine brine/air ejector driven by ejector pump creates vacuum in the
system in order to lower the evaporation temperature of the feed water.
The feed water is introduced into the evaporator section in a suitable pressure
and amount passing to an orifice through a connection to sea water line from
ejector pump or separate pump.
The hot water from diesel engine or a separate steam supply is added to the
evaporator, thus heat exchanging takes place that causes water to boil below
atmospheric pressure and the feed water goes to partial evaporation. The mixture
of vapour and brine enters to a separating chamber, where brine is separated
from the vapour and extracted by the combined brine/air ejector.
Having passed a demister the vapour enters the condenser section. The cooling
sea water cooling is supplied by combined cooling/ejector water pump into the
condenser, thus absorbing the heat being transferred from the condensing vapour.
From the outlet side of the condenser cooling, the sea water is being used as jet
water for the ejector and as feed water for evaporation.
The produced fresh water is extracted by the fresh water pump and led to the
fresh water tank.

e. Heat Balance
In order to maintain the generating efficiency and good operational condition of
the fresh water generator, heat balance requirements should be followed. Heat
balance requirements depend on the size and the generating capacity of the
plant. The Heat-Mass Balance diagram below shows the amount of heat require,
the temperature of the cooling and heating medium in the process, the mass of water (fresh water and sea water) passes through and displace in the heat exchanger and the pressure in the shell in order to generate distilled water. This diagram is very important to the user’s in operating fresh water generator to
maintain its efficiency
the amount of heat
required to generate 30 MT/day of distilled water is about 726,555 kcal/h(845.0
kW) with the following condition:
The heat required in the evaporator that should be adjusted by means of M/E
Jacket cooling water bypass valve is 49.8 T/h with an inlet temperature of 85 oC.
When the jacket cooling water temperature is to low, fresh water production is
decreases and if too high, will promote scale deposition in the tubes and
eventually reducing the over-all efficiency of the plant.
Vacuum/pressure in the shell that must be generated by the ejector is 637
mmHgV (-0.085 MPa) and with the shell evaporation temperature of 55.8 oC.
When evaporation temperature is too low, the water production quantity
increases, and the salinity of the production water turns high. When evaporation
temperature is too high, water production quantity decreases, and danger of the
scale generation increases in a heating tube.
The feed water supply required at a constant rate of 4200 kg/hr with the
temperature of 50.8oC and the brine that should be removed is 2950 kg/h. Brine
should be continuously removed as the rated amount maintain the evaporation
rate.
With efficient heat exchange in the evaporator and with the rated vacuum
condition, evaporation rate is 1250 kg/hr and the Jacket water outlet rated
temperature is 70.4 C.
The rated sea water cooling supply required is 70t/hr at an inlet temperature of 32 C, to condense the rated amount of vapor in the evaporator with the cooling
water outlet temperature of 42 C. The cooling sea water temperature difference
in the condenser inlet and outlet should be adjusted that it should not be more
than the rated temperature as shown in the diagram.
Always consult the maker’s instruction manual to clarify the heat balance
requirements of the fresh water generator before operating it.

7.7.4 Procedures for Operation
Operating procedure and cautions varies from each machine. It is better to consult
your Operating Manual to best understand these procedures. The operation
procedure discussed here is based on the following technical description of Fresh
Water Generator:
a. Starting Procedure
i. Open valves on the suction and discharge side of the ejector pump.
ii. Open overboard valve for combined brine/air ejector.
iii. Close air screw (“vacuum breaker valve”) at the separator.
iv. Start Ejector pump to create a vacuum of minimum 90% (about 10 mins.).
Notes:
a.) Sea Water pressure at combined brine/air ejector inlet minimum of 0.3 MPa.
b.) Back pressure at combined brine/air ejector outlet at maximum of 0.06 MPa.
v. When there is minimum of 90% vacuum (after maximum 10 mins.), open valve for
feed water treatment if any.
vi. Open the Jacket Cooling Fresh Water Valve inlet and outlet. Hot water is started
to supply by adjusting the bypass valve gradually in 100oC temperature
increments until the desired jacket water temperature is reached. The boiling
temperature now rises, while the obtained vacuum drops to approximately 85%.
This indicates that evaporation has started.

Note: Be careful when opening these valves as excessive heating of the evaporator
plates may promote scale and pressure fluctuation in the JCFW system line may
affect the operation condition of main engine or may gives trouble.

vii. After approximately 3 minutes, the boiling temperature will drop again and normal
vacuum is reestablished. Open valve to fresh water tank.
viii. Switch on the salinometer.
ix. Start fresh water pump. After starting the fresh water pump, the flow sight glass in
the air suction pipe must be empty.
Note: Fresh water pump discharge pressure must be between 0.12~0.16 MPa.

b. Stopping Procedure
i. Open the JCFW bypass valve gradually until fully open and close inlet and outlet
valve.
ii. Stop the fresh water pump and close the valve to fresh water tank.
iii. Switch off Salinometer.
iv. Continue to run sea water to the evaporator until it cools down then stop the
ejector pump and close all the valves.
v. Open air screw (“vacuum breaker”).
Note: All valves must be closed while the fresh water generator is not in used except for
the air screw (“vacuum breaker valve”).

7.7.5 Maintenance Procedures
Regular maintenance procedures will improve performance and availability of the
plant. Always refer to “Maker’s Maintenance Manual” for guidance and when the
plant is in operation, observe and analyze the plant performance efficiency and draw
out plan practically regarding the maintenance schedule.

a. Heat Exchanger Plates and Gaskets
i. Be careful not to scratch, deform or damage the plates. Scratches in the plate will
promote scales.
ii. Be careful with the arrangement of the plates as any incorrect arrangement will
damage the plates during tightening of securing nuts.
iii. When assembling the plate, the gasket must be fully secured to the plates and
kink gasket should not be allowed.


b. Cooling Water Ejector Pump and F.W. Pump
When overhauling the pump pay attention to the following:
i. The clearance between the impeller and the casing ring. If the clearance is more
than the wear limit the efficiency of the pump is affected.
ii. The condition of mechanical seal, metal bush and the shaft. Pitting is the most
common defects and breakage of mechanical seal sealing parts (carbon).


c. Ejector
An ejector in its simplest form is consists of actuating nozzle, suction chamber
and diffuser. In fresh water generator, the air/brine ejector works as follow:
The actuating fluid (sea water) enters the actuating nozzle is expanded from its
initial pressure to a pressure equal to that of the secondary fluid (air/brine) in the
suction chamber. In the process of being expanded, the actuating fluid is
accelerated from its initial entrance velocity, which is negligibly small, to a high
velocity.
In the suction chamber, the actuating fluid induces a region of low pressure-high
velocity flow which causes the secondary fluid to become entrained and mixed
with the actuating fluid. During the mixing process, the actuating fluid is retarded
and the secondary fluid is accelerated. As the mixture enters the diffuser, it is
compressed to the exit pressure by rapid deceleration. The purpose of the ejector
is to transport and compress a weight of induced fluid from the suction pressure
to the exit pressure
When overhauling and cleaning of air/brine ejector check the following;
i. The dimension of the ejector and compare to maker’s standard measurements. If
the wear is more than the limit renew if necessary.
ii. If the diffuser worn out is more than 20%, renew the diffuser.
iii. Check for corrosion and fittings, repair if necessary.

d. Salinometer
To keep an accurate salinity indication of the produce fresh water, clean the
salinometer probe cell regularly. Carefully wipe off dust and scale from its surface
with soft cloth. Do not use sand paper, a file or other material which may damage
the cell surface.

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