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.
Fresh water Generator Starting Procedure
- 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.
Fresh water Generator Stopping Procedure
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.
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