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. 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. It is statutory requirement to have a distillation plant for emergency use if otherwise ship has carried sufficient potable water. The equipment used on board for the production of freshwater from seawater is known as fresh water generator.
Various types of fresh water generators used on board ships are mainly:
- Submerged tube type fresh water generator
- Plate type fresh water generator, and
- Reverse osmosis plant
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.
Submerged Tube Type Fresh Water Generator
The shell and tube freshwater generator consist of heat exchanger, separator shell and condenser. In addition to this water ejector, ejector pump, distillate pump, salinity indicator, demister or mesh separator, solenoid valve and water flow meter are also fitted as accessories.
Basic principle of all low pressure freshwater generator is that, boiling point of water can be reduced by reducing the pressure of the atmosphere surrounding it. By maintaining a low pressure, water can be boiled at low temperatures say 50 degree Celsius. The source of heat for the fresh water generator could be waste heat rejected by main engine jacket cooling water.
Hence using energy from a heating coil, and by reducing pressure in the evaporator shell, boiling can takes place at about 40 to 60 degree Celsius. This type of single effect plant is designed to give better economy than obsolete Boiling Evaporators.
The submerged tube type fresh water generator explained below uses the heat from main engine jacket cooling water to produce drinkable water by evaporating seawater due to the high vacuum, which enables the feed water to evaporate at a comparative low temperature. Steam can also be used as a heat source instead of main engine jacket cooling water.
This type of fresh water generator is based on two sets of shell and tube heat exchangers, one acting as evaporator or heater and other as condenser.
The combined air/brine ejector creates evaporator chamber vacuum condition by driving sea water pass through air/brine ejector, and sea water supplied by the ejector pump to be delivered to ejector for taking out the brine (concentrated seawater) and air.
While entering to the evaporator chamber temperature of feed water will be around 50 degree Celsius. Feed water supply rate to the evaporator is fixed by an orifice fitted at the feed inlet. Because of the vacuum condition inside evaporator feed water evaporates at this temperature. The water spray and droplets are partly removed from the vapour by the deflector mounted on top of the evaporator and partly by a build in demister. The separated water droplets fall back into the brine, which is extracted by the water ejector.
The desalted vapour, which passes through the demister, will come in contact with the condenser, where it will be condensed by means of incoming cold seawater.
The distilled water is then taken out by integral freshwater pump (distillate pump) and controlled by salinometer and solenoid valve. If the salt content of produced water is high, solenoid valve diverts the freshwater to the shell side of freshwater generator, and issues an alarm signal. In order to get better suction head, distillate pump is placed at the lowest possible location in the fresh water generator plant. This is because the fresh water generator shell is at a lower pressure. Distillate pump get maximum net positive suction head with the height of liquid column in the suction line.
Thermometers are installed for control of seawater to the condenser and jacket cooling water to the evaporator. These thermometers permit control of both heating and cooling of these units. The salinometer or salinity indicator is connected to remote alarm so that very high salinity is immediately registered at the engine control room of the ship.
Plate Type Fresh Water Generator
Working principle of plate type fresh water generator is same as that of submerged tube type. Only difference is the type of heat exchangers used. Here plate type heat exchangers are used for condenser and evaporator unit. Heat from the diesel engine cooling water is used to evaporate a small fraction of the seawater feed in the plate type freshwater generator or evaporator. Unevaporated water is discharged as brine (by combined air /brine ejector). The evaporated water passes through the demister to the plate type vapour condenser. Here, after condensation it is discharged to fresh water storage tank by fresh water distillate pump. During entire operation the feed rate to the evaporator is fixed by the orifice plate at the feed inlet to evaporator. A typical plate type freshwater generator line diagram is shown below.
In the event of salinity of fresh water exceeding a predetermined value (maximum usually 10 ppm) the solenoid controlled dump valve diverts the flow back to the shell. This prevent contamination of the made water. Excess salinity could be used by many factors include leakage of seawater at condenser or priming of evaporator or malfunctioning of demister, or many other reasons.
What cannot be condensed at the condenser are called ‘incondensable gases’ like air and these gases are continuously ejected out by air/brine ejector. This way the shell of fresh water generator is maintained at high vacuum, a must requirement to boil water at low temperatures.
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. One type of coating is sheet rubber which is rolled and bonded to the plate then hardened afterwards by heat treatment. 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 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.
Extreme care must be taken during the operation of fresh water generator onboard ships. Operate all the valves gradually. Sudden opening and closing of valves may result in thermal shock to the main engine. Also make sure that distillate pump never runs dry.
Starting of Fresh water Generator
- Make sure seawater ejector pump suction, discharge and overboard valves are open. Start the ejector pump. Seawater pressure at the air ejector must be 3 bar or more.
- Wait for vacuum to build up inside fresh water generator shell. (About 92 % vacuum).
- Open the feed water valve to feed seawater to the evaporator. Adjust the feed water pressure. Normally marking is provided on the pressure gauge for desired feed water pressure.
- Open main engine jacket cooling water inlet and outlet to the evaporator gradually.
- Open the air vent clock at the top of the evaporator to make sure the evaporator is filled with jacket cooling water. Air must be purged out if any.
- Switch on the salinity alarm panel for measuring purity of the freshwater produced.
- There will be a sight glass provided at the suction line for the distillate pump. Make sure condensed water is coming to the suction line. Now start the distillate pump and open discharge valve to lead generated water to specified storage tanks.
- Do checks While Running Fresh water Generator
- Through the sight glass provided in the evaporator shell, observe flashing of water.
- Also check for the brine level inside. It should not be too high or too low.
- Shell temperature must be around 50 deg cel.
- Make sure shell vacuum is more than 90% from the vacuum gauge.
- Check seawater inlet and outlet temperature to the condenser.
- Ensure seawater pressure at air ejector inlet more than 3 bars.
- Check for distillate pump pressure and water flow meter.
- Check salinity of fresh water produced.
- Check level and flow of dosing chemical.
- Check ampere of ejector pump and distillate pump motor.
Regulating the Capacity of Fresh water Generator
Capacity of a fresh water generator means the quantity of fresh water produced by it per day. The capacity of fresh water generator can be varied by reducing or increasing the amount of jacket cooling water to the evaporator. The quantity of jacket cooling water to the evaporator can be adjusted by adjusting the bypass valve provided. When the temperature of jacket cooling water is comparatively low, the quantity to the evaporator to be increased a bit. At the same time cooling seawater pressure to the condenser also to be regulated accordingly.
During very low seawater temperatures, evaporation temperature can falls to a lower value. In that case, adjust vacuum adjusting valve to control vacuum inside the shell. Cooling seawater quantity to the condenser also can be reduced to increase the evaporator temperature. During high seawater temperatures, evaporation temperature can go up. In that case, increase the quantity of seawater to the condenser for reducing evaporation temperature.
Too high evaporation temperature causes scale formation in the heat exchanger. On the other hand, too low evaporation temperature results in seawater carry over which increases salinity of fresh water produced.
The distillate pump discharge to be throttled so that pump should not run dry. The rate of distillate pump discharge and rate fresh water produced in the condenser should match. When distillate pump is not able to extract the freshwater at the rate of production, level of freshwater increases in the condenser and effective cooling area of the condenser reduces. This finally results in reduced evaporation quantity.
Stopping of Fresh water Generator
When ship approaches port, shallow water, etc. it is desirable to stop the fresh water generator. This is because the seawater may contain harmful bacteria which can enter the freshwater produced. Operation of freshwater should be carried out in consultation with bridge watch keeper. Following procedure may be adopted for stopping fresh water generator.
- Slowly open bypass valve for main engine jacket cooling water.
- Ensure that main engine jacket cooling water temperature is within normal limits.
- Close jacket cooling water inlet and outlet valves for the freshwater generator respectively.
- Close the feed water chemical dosing valve.
- Stop the distillate pump and shut discharge valve.
- Switch off salinity meter.
- Close filling valve to freshwater tanks.
- Wait for evaporator shell temperature to drop below 50 deg cel.
- Close the feed water valve to evaporator.
- Stop ejector pump. Shut fresh water generator overboard valve.
- Open the vacuum breaker valve to make shell side pressure equal to atmospheric pressure.
- Open the drain valve of the evaporator to drain all the seawater from the fresh water generator.
Precautions for Operation of Fresh water Generator
- Seawater pressure at the inlet of air ejector must be 3 bar or more.
- The pressure at ejector outlet should not exceed 0.8 bar.
- Never start fresh water generator distillate pump in dry condition.
- Operate jacket cooling water valves to the fresh water generator gradually to avoid thermal shock to the main engine.
- Feed water to be supplied for a few minutes to cool down the evaporator before stopping.
- Never open the drain valve of evaporator before opening vacuum breaker. Otherwise atmospheric pressure causes seawater inside to hit the deflector.
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. The reaction takes place when sea water heated are:
Ca(HCO3)2 ————> Ca + 2HCO3
2HCO3 ————> CO3 + H2O + CO2
If heated up to approx. 80 degree Celsius
- CO3 + Ca ————> CaCO3
If heated above 80 degree Celsius
- CO3 + H2O ————> HCO3 + OH
- Mg + 2OH ————> Mg(OH)2
Hence if the sea water in the fresh water generator is heated to a temperature below 80 degree Celsius, calcium carbonate scale predominates. If sea water is heated above 80 degree Celsius, magnesium hydroxide scale is deposited.
If the density of evaporator contents is in excess of 96000 ppm, calcium sulphate scales are formed. But fresh water generator brine density is normally 80000 ppm and less. Hence scale formation due to calcium sulphate is not a problem.
Hence 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. Similarly higher shell temperatures result in formation of hard scales which will be difficult to remove. All these together will reduce the plant efficiency drastically.
How to Minimize Scale Formation
Scale formation in fresh water generator can be controlled and minimized by continuous chemical treatment. Polysulphate compounds (like sodium polysulphate) with anti foam is preferred by marine engineers and is extensively used on ships. 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. If 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. Important thing is that this chemical is effective only on low pressure fresh water generators. The sea water temperature to be less than 90 degrees.
DongHwa Entec Heat Exchanger Solutions
“Basic Marine Engineering” by J.K.DHAR