Home General Engineering Knowledge Fresh water Generator on Ships
In a steam ship the consumption for the boiler can be as high as 30 tonnes/day. Sufficient potable water should be taken on in port to meet crew and passenger requirement. The stowage space that should 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. That said, the equipment used on board for the production of freshwater from seawater is known as fresh water generator. As a result, the quality of this water gonna be take only a minimum supply of potable water and make up the rest by distillation of sea water.
The shell and tube freshwater generator consist of heat exchanger, separator shell and condenser.
By maintaining a low pressure, water can be boiled at low temperatures say 50 degree Celsius. It’s a well the source of heat for the fresh water generator gonna be waste heat rejected by main engine jacket cooling water. Whenever boiling point of water can be reduced by reducing the pressure of the atmosphere surrounding it, Basic principle of all low pressure freshwater generator is that. Essentially, this water ejector, ejector pump, distillate pump, salinity indicator, demister or mesh separator, solenoid valve and water flow meter are also fitted as accessories.
While boiling can takes place at about 40 to 60 degree Celsius, using energy from a heating coil, and by reducing pressure in the evaporator shell. Here plate type heat exchangers are used for condenser and evaporator unit. During entire operation the feed rate to the evaporator is fixed by the orifice plate at the feed inlet to evaporator. This single type effect plant is designed to give better economy than obsolete Boiling Evaporators. Working principle of plate type fresh water generator is same as that of submerged tube type. Now please pay attention. Unevaporated water is discharged as brine. Besides, the evaporated water passes through the demister to the plate type vapour condenser. With that said, here, after condensation Undoubtedly it’s discharged to fresh water storage tank by fresh water distillate pump. Eventually, 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. Only difference is the heat type exchangers used.
In the event of salinity of fresh water exceeding a predetermined value the solenoid controlled dump valve diverts the flow back to the shell.
Excess salinity should be used by many factors include leakage of seawater at condenser or priming of evaporator or malfunctioning of demister, or many other reasons. This way the shell of fresh water generator is maintained at high vacuum, a must requirement to boil water at low temperatures. What can’t be condensed at the condenser are called ‘incondensable gases’ like air and these gases are continuously ejected out by air/brine ejector. This prevent contamination of the made water.
The shell is usually fabricated steel which is shot blasted so coated with some sort of protective. The important points about protective coatings are. For plate type, titanium plates are used for condenser and evaporator. Let me tell you something. Demister is created from layered knitted wire of monel metal. Heat exchangers use aluminium brass tubes and muntz netal tube plate in the case of tube type fresh water generator. One coating type is sheet rubber which is rolled and bonded to the plate hereafter hardened afterwards by heat treatment.
Extreme care must be taken in the course of the operation of fresh water generator onboard ships.
Capacity of a fresh water generator means the quantity of fresh water produced by it per day. Sudden opening and closing of valves may result in thermal shock to the main engine. Besides, the quantity to the evaporator to be increased a bit, when the temperature of jacket cooling water is comparatively low. Operate all the valves gradually. The capacity of fresh water generator can be varied by reducing or increasing the percentage of jacket cooling water to the evaporator. With all that said… Find out if distillate pump never runs dry. Anyways, at identical time cooling seawater pressure to the condenser also to be regulated accordingly. The quantity of jacket cooling water to the evaporator can be adjusted by adjusting the bypass valve provided.
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. Too high evaporation temperature causes scale formation in the heat exchanger. Just think for a moment. During high seawater temperatures, evaporation temperature can go up. In that case, increase the quantity of seawater to the condenser for reducing evaporation temperature. Shouldn’t run dry, The distillate pump discharge to be throttled.
This finally results in reduced evaporation quantity. Operation of freshwater going to be carried out in consultation with bridge watch keeper. This is as the seawater may contain harmful bacteria which can enter the freshwater produced. Amount of freshwater increases in the condenser and effective cooling area of the condenser reduces, when distillate pump isn’t able to extract the freshwater at the rate of production. Following procedure can be adopted for stopping fresh water generator. The rate of distillate pump discharge and rate fresh water produced in the condenser should match. Shallow water, and stuff it’s desirable to stop the fresh water generator, when ship approaches port.
Calcium carbonate and magnesium hydroxide scale formation mainly depends on the temperature of operation. Magnesium hydroxide scale is deposited, So if sea water is heated above80 degree Celsius. Calcium carbonate scale predominates, if the sea water in the fresh water generator is heated to a temperature below80 degree Celsius. Calcium sulphate scale formation depends mainly on the density of the evaporator contents or brine. The reaction takes place when sea water heated are.
Calcium sulphate scales are formed, So if the density of evaporator contents is in excess of 96000 ppm.
Scale formation due to calcium sulphate isn’t a real poser. Also, all these together will reduce the plant efficiency drastically. You see, Surely it’s recommended to operate fresh water generator at its rated capacity, not more. So, 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 gonna be difficult to remove. Generally, fresh water generator brine density is normally 80000 ppm and less.
Scale formation in fresh water generator can be controlled and minimized by continuous chemical treatment. Polysulphate compounds with anti foam is preferred by marine engineers and is extensively used on ships. Although, percentage of chemical to be dosed depends on the capacity of fresh water produced. Seriously. If my be continuously fed into the feed line using a metering pump or by gravity. Normally, the sea water temperature to be less than 90 degrees. Now let me tell you something. Important thing is that this chemical is effective only on low pressure fresh water generators. Fresh water generator chemical treatment to be religiously carried out to maintain its performance. Therefore, their trade name is different. These chemicals minimize calcium carbonate scale formation and possibility of foaming.
Faults in fresh water generator reduces performance of the system, thereby reducing quality and quantity of freshwater produced on ships.
Any fault in the ejector or brine extraction pump cause increase in the brine level. For the satisfactory operation of the freshwater generator, a constant degree of brine to be maintained in the shell. Brine is the concentrated sea water after liberation of water vapours.
These irregularities must be identified and rectified immediately to ensure that the optimum performance of the fresh water generator is achieved. So it’s achieved by the combined air brine ejector. Certainly, it extracts air as well as brine from the shell. For example, this brine is gradually extracted from the shell.