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. A simple fresh water generator diagram is shown below.
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.
A detailed line diagram of a tube type fresh water generator on board ship is shown below. Click on the diagram to enlarge.
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