The fresh water generator is an equipment installed onboard vessels which is used to produce fresh water from sea water. There are two major ways of producing fresh water onboard vessels: vacuum evaporation and reverse osmosis, of which the vacuum evaporation comprises of two types: tube type and plate type. Each system has its own pro and cons. The reverse osmosis type has the advantage that it can produce the designated amount of water even at anchorage or while vessel is drifting, but the equipment and its parts are very expensive. This system is mainly common for offshore vessels. Vacuum evaporation type is the most used onboard vessels, especially the plate type, as is reliable, efficient, easy to maintain and makes use of the energy generated by the main engine jacket cooling water system. The fresh water generator can be used for extended periods even at anchor by heating it with the main engine jacket water heater (of course the water production will be low and vessel must use more fuel in the boiler).
The combined brine/air ejector, which is driven by the ejector pump, creates a vacuum in the system, lowering the feed water’s boiling temperature.
A spring-loaded regulating valve is used to introduce feed water into the fresh water generator.
Each second plate channel of the evaporator portion receives feed water.
The remaining channels are filled with hot water from the jacket cooling system, which transfers its heat to the supply water in the evaporation channels.
Once the feed water reaches its boiling point (which is lower than the pressure at atmospheric pressure), it undergoes partial evaporation. The vapors and brine combination enters the separator vessel, where it is separated from the vapors and extracted using the combined brine/air ejector. After passing through a demister the vapors enter every second plate channel of the condenser section.
The remaining channels are filled with sea water supplied by the combination cooling/ejector water pump, which absorbs the heat from the vapour and condenses it into fresh water.
The distillate pump extracts the produced fresh water and discharges it through a salinometer, which checks the water’s salinity. A flow meter is located at the distillate pump outflow.
The distillate from the FW generator is routed through the re-hardening filter/neutralizing unit and the silver ion type electric sterilizer before being released into the fresh water storage tanks.
To continuously check the quality of the produced fresh water, a salinometer is provided, together with an electrode unit fitted on the fresh water pump delivery side. If the salinity of the produced fresh water exceeds the chosen maximum value, the dump valve and alarm are activated to automatically dump the produced water to the bilge tank.
In conclusion, the fresh water generator consists of the following components:
- Evaporator section which consists of a plate heat exchanger and is enclosed within the separator vessel.
- Separator vessel which separates the brine from the vapour.
- Condenser section which like the evaporator section consists of a plate heat exchanger which is enclosed within the separator vessel.
- The ejector extracts brine and uncondensed gases from the separator vessel.
- The sea water supply pump is a single-stage centrifugal pump. This pump supplies the condenser with sea water, the brine/air ejector with jet water, and feed water for evaporation.
- The distillate pump is a single-stage centrifugal pump. The distillate pump extracts the produced fresh water from the condenser and pumps the water to the fresh water tank.
- The salinometer continuously checks the salinity of the produced water. The alarm set point is adjustable. The salinometer control panel is located at the fresh water generator side with LCD indicators ranging from 0.5 – 20ppm. The panel also contains a 10ppm test function and control buttons to set the alarm point.
The operating procedures for fresh water generator is described in the video below for a better understanding:
It is very important that you do not operate the plant in polluted water. Fresh water must not be
produced from polluted water, as the produced water will be unsuitable for human consumption. It is advisable and safe to start fresh water generator when the vessel is at least 20 nm from the shoreline.
Chemical treatment is added to the sea water feed in order to minimize foaming and restrict the formation of salt scale in the FW generator. It is essential that the correct dosage of chemical is used and frequent checks must be made on the dosing unit to ensure that the correct treatment is being applied.
The condensate produced in the FW generator is pumped to the storage tanks by the distillate pump. The distilled water may be pumped to the drain tank directly (for boiler feed water use), or it may be pumped to the fresh water tanks for domestic use throughout the ship.
Alfa Laval developed the new AQUA Blue S-type which maximizes energy efficiency and capacity-to-footprint ratio by making use of the vessel’s existing seawater cooling system pumps. This cuts electrical power needs by 70% compared to conventional freshwater generators, and it shrinks the already small AQUA Blue footprint by up to 15%. Because it makes use of the vessel’s seawater cooling system pumps, it employs a smaller ejector and a smaller, separately installed ejector pump. Likewise, the pipework can be both shorter and smaller in diameter.
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