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. The figure below shows a typical shell and tube freshwater generator used onboard ships.
Working Principle with Engine Jacket Cooling Water Heating
The freshwater generator 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.
The freshwater generator is based on two sets of shell and tube heat exchangers, one acting as evaporator or heater and other as condenser.
The water ejector creates evaporator chamber vacuum condition by driving sea water pass through water 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, feed water evaporates due to vacuum condition. 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 be sucked into vapour chamber 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.
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 simple line diagram shown below gives clearer picture about the plant. Click on the diagram to enlarge.
Working Principle with Saturated Steam Heating
In addition to engine jacket cooling water heating, saturated live steam can alternatively be used as heating medium for the shell and tube freshwater generator.
The saturated live steam is being delivered to the heat exchanger of the freshwater generator through a steam injector. This steam injector works as a circulating pump and heat up the circulated freshwater or steam condensate, which comes from the outlet branch of the heat exchanger.
The heat exchanger of the freshwater generator is filled with clean freshwater or steam condensate by starting the steam heating process. The produced steam condensate from supplied live saturated steam escapes from the socket in the vertical safety blow off pipe. This condensate is led back to the boiler feed water hot well or cascade tank.
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