In this article, I am going to discuss about an equipment called Fresh water Generator which is used onboard to produce fresh water on ships. We will discuss it’s working principle, types, how it operates and it’s troubleshooting.
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If you any doubt or any problems related this Topic.You are at right place. After reading this 7 minute article on this topic, I am damn sure you will not have any Doubts remaining.
In this Article we will learn :-
- What is Fresh Water Generator ?
- HOW PURE water produced on ship ?
- Distillation system
- Working principle of Fresh Water Generator ? Fresh Water Generator Working Principle ?
- How it works ?
- Plate type
- Tube type
- How vacuum is created in Fresh Water generator ?
- Why losses of vacuum of Vaccum in Fresh Water Generator ?
- Fresh Water Generator starting procedure
And so many questions.
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What is Fresh Water Generator ?
Fresh water generator is Device on ship for production of fresh water from ocean water for domestic and auxiliary functions ,which is a vital demand aboard ships.
A considerable quantity of H2O 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 a turbine or a ship which could be a giant tanker with turbine-driven oil pumps) the boiler consumption may be as high as thirty tons per day.
The equipment used on board to generate freshwater from seawater is known as a freshwater generator.
It is used to produce fresh water onboard for drinking,cooking, washing etc.
How pure Water produced on ship ?
Pure water produced on ship ships generally using two principles or Method ; either
- Distillation or
- Reverse Osmosis.
Reverse osmosis is normally used in passenger ships where large quantities of water is consumed .
Here I am trying to illustrate the working principle of the freshwater generator that works on the basis of the distillation principle that is very common in cargo ships.
Fresh water from sea on ship is produced mainly by Distillation process.
What is Distillation ?
- Distillation is the method of production of pure water from sea water by evaporating and re condensing .
Distilled water is made as a results of evaporating ocean water either by a boiling or a flash process.
This evaporation enables the reduction of the 3200 parts per million of dissolved solids in sea water down-to the one or two percent in distilled water.
This type of evaporator boils sea water at a saturation temperature corresponding to the evaporator pressure and is known as a boiling evaporator.
In a boiling Evaporator, water is maintained continuously at its saturation temperature-in other words,Latent heat is added.
While in the flash evaporator,sensible heat is supplied.
Submerged tube type- Boiling Evaporator or tube type
Boiling Process Evaporator ( low pressure evaporator ) Alfa laval or plate type :-
This both type of generator discuss below in details.
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Flash Process (Flash Evaporator )
- This type of evaporator heats the water in one compartment before it is released into a second compartment in which the pressure is substantially lower, causing some of the water to flash into vapour .
- This type of evaporator is known as flash evaporator .
- In flash evaporator, sensible heat is supplied.
On the basis of of Working Principle ,it is classified into
- Distillate Type
- Reverse Osmosis
- Distillation is cheaper and efficient for less quantity, but RO is expensive and used for production in a large quantity.
- RO is used on a passenger ship, where a large amount of water is consumed.
- Distillation = (Evaporation + Condensation)
- Reverse Osmosis = (Semi permeable membrane – filter)
Distillate type is again classified based on the evaporator and condenser structure, i.e
- Plate type and
- Tube type fresh water generator
- Tube type ,also known as the submerged type, because the steam coils are submerged.
- Sometimes it is known as Boiling FWG.
Main or Various type of fresh water generator which are mainly used on ships are :
- Submerged Tube Type
- Plate Type
- Reverse Osmosis Plant
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The main body of a fresh water generator on the ship consists of
- Heat Exchanger,
- Distillate pump
- ejector pump,
- air brine eductor
- demisters or mesh separator,
- Evaporator :- It is used to boil off the sea water at lower temperature with the help of vacuum created inside the fresh water generator shell.
- Condenser: It use s the sea water to cool down; and condense the steam to achieve distilled water
2.Fresh Water Pump / Distillate pump
- It is used to supply the generated fresh water to ship’s fresh water tank by taking the suction from fresh water generator.
Normal rated capacity -3m^3/hr
- It is used to supply pressurised water to the eductor for creating vacuum.It also supplies cooling water to condenser(to cool the fresh water vapours)
Pressure- 3-6 bar
4.Air brine eductor:
- It is used to to remove accumulated brine and salts deposits from the generator and create necessary vacuum.
- It is connected to the distillate output just before the solenoid operated three way valve. It is used for measuring the ppm of fresh water produced which is generally (1-2ppm)
The salinometer works on the simple principle that pure water does not conduct electricity; and its conductivity increases with increased dissolved impurities and salts.
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What is demisters in Fresh Water generator ?
- This is used to separate sea water droplet from the steam vapour.
- A demisters is a thickened layer of mesh structure; fitted in between the evaporator and the condenser element.
- A demisters can be made of nickel, monel metals, copper, stainless steel and synthetic fibers; such as Polypropylene and PVC.
- Typically; demisters made of monel metal are used for the generation of fresh water.
- when the water evaporates it carry over some fine little molecules of water along with the rising steam.
- When the source of such water is sea; it can considerably increase the salinity of output water.
So to maintain salinity as low as 5 to 15 ppm; we use demisters which restrict the passage of mist and pass dry steam.
How Fresh Water Generator Works ?
- The basic principle of all low-pressure freshwater generators is that the boiling point of the water can be reduced by reducing the pressure of the surrounding atmosphere.
- Water can be boiled at low temperatures by maintaining a low pressure, say 50 degrees Celsius.The heat source for the freshwater generator could be waste heat rejected by main engine jacket cooling water.
- Hence,boiling can take place at about 40 to 60 degrees Celsius by using energy from a heating coil and by reducing pressure in the evaporator shell.
- This type of single-effect plant is designed to provide a better economy than obsolete boiling evaporators.
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Plate Type Fresh water generator ( Alfa – level Type ) Working
- If the condenser and evaporator Heat exchangers of a fresh water generator is composed of plates then that type of freshwater generator is called Plate type freshwater generator.
- The main components are condenser and evaporator heat exchangers, brine air ejectors, seawater pumps, distillate pumps, salinometer, demister, water flow meters, etc.
Below You can see the line diagram.
Fig :- Plate Type ( Alfa – Lavel Type )
- Fresh water generator uses heat from main engine jacket cooling system which often cooling the engine passes through evaporator to evaporate the sea water feed into it.
- But the jacket cooling water temperatures available is about 70-80 degree celcius,whereas boiling of water is 100 degree Celsius at 1 atm.
- so in order to evaporate sea water at 70 degree Celsius we need to reduce pressure.
- This is done by creating vacuum inside chamber si that sea water get evaporated below 100 degree celcius and also vacuum helps to evaporate easily.
- This vaccum is created by air or brine ejector.
- Now,the evaporated sea water passes through demisters which scrubs off sea water droplets from water vapour.
- Unevaporated water/ particles is discharged as brine (by means of a combined air / brine ejector).
- This vapour passes through the condenser which condense the vapour and get collected at the bottom which is transferred to fresh water tank ,where it is passed through salinometer and controlled by three way solenoid valve.
- The feed rate to the evaporator is fixed at the feed inlet to the evaporator by the orifice plate throughout the entire process.
- If the salt content of the produced water is high, the solenoid valve diverts the freshwater to the shell side of the freshwater generator and emits an alarm signal.
This prevent contamination of the made water.Excess salinity caused by so many factors including leakage of seawater at condenser or priming of evaporator or malfunctioning of demister, or many other reasons.In FWG,What cannot be condensed at the condenser Is called ‘incondensable gasses’ such as air and these gases are continuously ejected out by air/brineejector.This way, the fresh water generator shell is kept at high vacuum, which is a must to boil water at low temperatures.Suggested Read:
Tube Type Fresh Water Generator
- Tube type FWG also, known as the submerged type, because the steam coils are submerged.
- Sometimes it is known as Boiling FWG.
- The working and principle of the freshwater type tube generator is the same as the plate type fwg.
- Only difference in instead of plates, condenser and evaporators are tubes.
A typical freshwater generator tube-type line diagram is given below..
- The submerged tube type fresh water generator uses heat from the main engine jacket cooling water to produce water drinkable,by evaporating seawater due to high vacuum, which allows the feed water to evaporate at a comparatively low temperature.Steam can also be used as a source of heat instead of the main engine jacket cooling water.
- This type of freshwater generator is based on two sets of shell and tube heat exchangers, one act as an evaporator or heater and the another act as a condenser.
- The combined air / brine ejector creates vacuum condition in the evaporator chamber by driving sea water through the air / brine ejector and sea water supplied by the ejector pump to be delivered to the ejector for taking out the brine (concentrated seawater) and air.
- The temperature of the feed water in the evaporator chamber is about 50 degrees Celsius. The rate of supply of water to the evaporator is fixed by an orifice fitted at the feed inlet.
- Due to the vacuum condition inside the evaporator, the feed water evaporates at this temperature.The water spray and the droplets are partly removed from the vapor by the deflector mounted on the top of the evaporator and partly by the demister.
- The water droplets, which are separated, fall back into the brine, which is extracted by the water ejector.
- The desalted vapor, which passes through the demister, will come into contact with the condenser, where it will be condensed by incoming cold sea water.
- The distilled water is then removed by an integral freshwater pump (distillate pump) and controlled by a salinometer and a solenoid valve.
- If the salt content of the water generated is high, the solenoid valve transfers the freshwater to the freshwater generator shell side and gives an alarm signal.
- To get a better suction head, the distillate pump is placed in the freshwater generator plant at the lowest possible location.This is because the shell of the freshwater generator is at a lower pressure.
With the height of liquid column in the suction line, the distillate pump gets maximum net positive suction head.
Thermometers are installed to control the seawater to the condenser and the cooling cooling water to the evaporator .These thermometers did the work of controlling of both heating and cooling of these units.
The salinometer or salinity indicator is connected to the remote alarm, so that at the ship’s engine control room, very high salinity is immediately registered.
What are Safety device fitted on Fresh Water generator ?
- relief valve
- vaccum gauge
- vent cock
What are faults in fresh water generator
What happens when there is fault in FWG ?
- Faults in the freshwater generator reduce the performance of the system, reducing the quality and quantity of freshwater produced on ships.
- These irregularities must be identified and corrected immediately to ensure that the optimum performance of the freshwater generator is achieved.
Faults in FWG are :-
1.Loss of Vacuum or Over-pressure of Shell
What reasons may there be for vaccum losses in a fresh water generator?
The shell pressure of the freshwater generator increases and the rate of freshwater produced decreases.
The reasons are:
a) Air leaks into the evaporator shell in large quantities and air ejector cannot cope.
b) 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.
c) Malfunctioning of the air ejector.
d) Flow rate of the heating medium increased and excess water vapour produced.
Since this excess vapour can not be condensed, the pressure of the shell increases or the vacuum drops.
2.Salt Water Carry Over
During the operation of the freshwater generator salt water can be carried over in large quantities.
This is called priming.
General reasons of the priming are:
a) 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.
b) When there is a high level of water agitation due to boiling and salt water may be carried along with the vapours.
c) Rate of evaporation increased.
3.Gradual Increase in Level of Brine
A constant level of brine must be maintained in the shell for the satisfactory operation of the freshwater generator.
Brine is the concentrated water of the sea after the release of water vapours.
This brine is gradually extracted from the shell. Typically, this is obtained by the combined air-brine ejector.
It extracts air as well as brine from the shell of fwg.
Any fault with the ejector or the brine extraction pump (in some models) causes the brine level to increase.
4.Increase in Salinity of Freshwater
Possible causes are:
a) Brine level inside shell too high.
b) Leaking condenser tubes or plates.
c) Operation of evaporator near shore with contaminated feed water.
d) Shell temperature and pressure too low.
e) 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.
Salinometer therefore shows increased salinity, which is a measure of conductivity and non-salt presence.
How Scale Formation Occurs in Fresh Water Generator
Fresh water generator performance reduces with scale formation because it causes reduction in heat transfer efficiency.
In FWG three scales are normally found these are :-
Calcium Carbonate, CaCO3
Magnesium Hydroxide, Mg(OH)2
Calcium Sulphate, CaSO4
The formation of calcium carbonate and magnesium hydroxide depends mainly on the operating temperature.And,the formation of calcium sulphate depends mainly on the density of the contents of the evaporator or the brine.The reaction takes place when the sea water is heated:
Ca(HCO3)2 ————> Ca + 2HCO3
2HCO3 ————> CO3 + H2O + CO2
If it is heated up to approx. 80 degrees Celsius
CO3 + Ca ————> CaCO3
If it is heated above 80 degrees Celsius
CO3 + H2O ————> HCO3 + OH
Mg + 2OH ————> Mg(OH)2
Hence, if the sea water is heated to a temperature below 80 degrees Celsius in the freshwater generator, the calcium carbonate scale will predominate.
The magnesium hydroxide scale is deposited when sea water is heated above 80 degrees Celsius.
If the evaporator content density is greater than 96000 ppm, the calcium sulphate scales are formed.But, brine density of FWG is normally 80000 ppm and less.Hence, the formation of scales due to calcium sulphate is not a problem.
That’s why It is recommended that the freshwater generator be operated at its rated capacity, not more.More water production than the rated capacity means a higher concentration of brine and a more formation of scale.Similarly higher shell temperatures result in hard scale formation that will be hard to remove.All of these together will dramatically reduce efficiency of plant.
How to minimize scale formation
The formation of a scale in a freshwater generator can be controlled and minimized by continuous treatment of chemical.
Marine engineers prefer polysulphate compounds (such as sodium polysulphate) with anti-foam, which are commonly used on ships.
These chemicals reduce the scale formation of calcium carbonate and possibility of foaming.
The compound is
- non toxic,
and can be used in fresh water generator producing water for drinking purposes.
It will be continuously feed via a metering pump or by gravity to the feed line.
The quantity of chemical to be dosed depends on the capacity of the fresh water produced.
The main thing is that,this chemical doses is effective only on low pressure fresh water generators.
The temperature of the sea-water is less than 90 degrees.
In order to maintain performance of Fresh water generator chemical treatment to be religiously carried out.
What are the causes for low production of Fresh Water ?
Reasons of Low Production are following :-
- Ships draft is less.
- level of brine is too high.
- Filter before ejector pump is dirty .
- Faulty ejector pump- not developing enough pressure
- Faulty Ejector nozzle/ nozzle chocked
- Incorrect feed
- scale formation in evaporator
- shell temperature is too high
- scale formation in condenser
- condenser cooling water flow is reduced
- Condenser cooling water temp. too high
- Incorrect assembly of plates
- Leakage in plant like from pressure gauge, vent, distillate pump seal etc.
- Distillate pump faulty
- Faulty flow meter
- Faulty solenoid valve
How Do I Start a fresh water generator ?
Starting of fresh water generator
Starting the Fresh Water Generator,few important point to be noted:-
We need to check before starting the fresh water generator that the ship is not in congested water, canals and is 20 nautical miles away from the shore.This is done because the effluents from factories and sewage are discharged into the sea near the shore which can get into the FWG.
Check whether the engine runs above 50 rpm, which is because the temperature of the jacket water at low rpm is around 60 degrees and is not sufficient for water evaporation.
- Check the drain valve is in close position,Which is present at the bottom of the generator.
- Now open the sea water pump’s suction and discharge valves which provide water for evaporation, cooling, and to the eductor for vacuum formation.
- Open the seawater discharge valve through which the water is sent back to the sea after , circulating inside the freshwater generator.
- Close the vacuum valve, which is situated on top of the generator.
- Now, we should Start the sea-water pump and check the pump pressure. In general, the pressure is 3-4 bar.
- Wait until the vacuum builds up. Vacuum should be at least 90 percent, which can be clearly seen on the generator gauge ,situated on FWG . The time taken for vacuum generation is usually around 10 minutes.
- When vacuum is achieved, open the valve for the treatment of feed water, this is designed to prevent the formation of a scale within the plates.
- Now open the inlet and outlet valves of hot water (jacket water), slowly to about half.Always,First open the outlet valve and then the inlet valve. Slowly start increasing the opening of the valves to full opening.
- Now we can see that the boiling temperature starts to rise and the vacuum starts to fall.
- The vacuum drops to about 85 percent, which is an indication that evaporation begins.
- Open the valve for drain from the fresh water pump.
- Switch ON to the salinometer if it has to be started manually. Generally, it’s on the auto start mode.
- Now start the fresh water pump and test the water that comes out of the drain.
- When fresh water starts generating, it can be seen that the boiling temperature drops slightly again and that the vacuum goes back to normal value.
- Check that the water, which coming out of the salinometer is not salty and also check the reading of the salinometer.This is done to see whether the salinometer is working properly or not, and to avoid contamination of the entire fresh water with salt water.Salinometer values are kept below 10ppm.
- Open valve for tank from the pump and close drain valve after testing the taste of the water coming out of the salinometer.
Fresh water Generator Stopping Procedure
It is desirable to stop the fresh-water generator as ship approaches port, shallow water, etc.This is because the seawater may contain harmful bacteria that can enter into the produced freshwater.The operation of freshwater should be carried out in consultation with the bridge watchkeeper.
Following, procedure to stop the fresh water generator can be adopted.
- Open the bypass valve , slowly for main engine jacket cooling water.
- Ensure that the cooling water temperature of the main engine jacket is within normal limits.
- Close inlet and outlet valves of jacket cooling water for the freshwater generator respectively.
- Close the feed water chemical dosing valve.
- Stop the distillate pump and shut down the discharge valve.
- Switch off the salino meter.
- Close the filling valve of the freshwater tanks.
- Wait for the temperature of the evaporator shell to drop below 50 deg celcius.
- Close the evaporator feed-water valve.
- Stop the pump ejector. Shut down overboard valve of fresh water generator.
- Open the vacuum breaker valve so that the side pressure of the shell is equal to the atmospheric pressure.
- Open the evaporator drain valve to drain all the seawater from the freshwater generator.
Precautions for Operation of Fresh water Generator
- The pressure of seawater at the inlet of air ejector must be 3 bar or more.
- The ejector outlet pressure should not exceed 0.8 bar.
- The distillate pump of fresh water generator never start in dry condition.
- To prevent thermal shock to the main engine, operate the jacket cooling water valves slowly to the fresh water generator.
- Feed water to be supplied to cool down the evaporator for a few minute before stopping.
- Never open the evaporator drain valve before the vacuum breaker is opened. Otherwise, the atmospheric pressure causes seawater to hit the deflector insiders.
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