It is an auxiliary machinery used in ships to produce freshwater from seawater. The freshwater produced is used for domestic purposes such as cooking, drinking, heating, washing and for cooling of machinery parts. The major components of a freshwater generator are; vapour condenser, Demister, Evaporator,Orifice, Seawater pump,Brine ejector,Air ejector, Salinometer, Distillate pump,Swing valve.
How the freshwater is generated.
The seawater is boiled using the energy from a heating coil, and by reducing the pressure in the evaporator shell boiling can take place at about 60 degrees centigrade. The seawater pumped into the vessel is first circulated through the condenser and part of outlet is provided as a feed to evaporation chamber. The steam produced rises and salt and other solid is left in liquid. Hot diesel engine jacket water or steam is passed through the heater nest, because of the reduced pressure in the chamber the seawater boils. The steam produced rises and passes through a water separator or demister which prevents water droplets passing through. In the condensing section, the steam becomes pure water which is pumped out by a distillate pump or a discharge line. The seawater feed is regulated by a flow controller and about half the feed is evaporated. The rest are salty water or brine. A combined brine and air ejector draws out the air and brine from the evaporator.
Before starting the freshwater generator check that the ship is not in congested waters like canals and is at least 20 nautical miles from shore. This is to prevent chemicals and other effluents from getting into the machinery and damage it.
.Ensure that the engine is running above 50 rpm because an engine running at a lower rpm will produce jacket water at a lower temperature which will not be sufficient for evaporation of water.
Ensure that the drain valve located at the bottom of the generator is in the closed position.
.Open suction and discharge valves of the seawater pump which will provide water for evaporation,cooling and to the eductor to create vacuum.
.Open the seawater discharge valve from where the water is sent back into the sea after circulating inside the freshwater generator.
. Close the vacuum valve situated at the top of the generator.
Start the seawater pump and check the pressure of the pump. The pressure is usually 3-4 bars.
.Wait for the vacuum to build up which should be at least 90% as seen on the gauge. The time take to generate the vacuum is usually 10 minutes.
When the vacuum is fully generated open the feed water treatment valve to prevent scale formation inside the plates.
.Now open the jacket water inlet and outlet valves slowly to about half. The outlet valve should first be opened before the inlet valve. Slowly increase the opening of the valve till fully open.
.The evaporation temperature starts increasing as the vacuum begins to drop. The vacuum drops to 85% indicating that evaporation has began.
.Open the valve from freshwater pump to drain.
. Switch on the salinometer if it is not auto started.
.Start the freshwater pump and taste the water coming out of the drain.
. When freshwater starts producing the boiling temperature drops again slightly and vacuum returns back to normal value.
. Check to ensure that the water from the salinometer is not salty and that the salinometer is working properly. The salinometer value is kept below 10 ppm. If it is above that value then the salinometer is faulty and needs to be repaired or replaced.
. After checking that the salinometer is working properly open the valve for freshwater tank and close drain valve.
. Close the jacket water inlet valves firstand then the outlet valves.
. Close the valve for feed water treatment.
. Stop freshwater pump.
.Switch off the salinometer.
. Stop seawater pump (ejector pump)
. Open vacuum valve.
Close seawater suction valve and overboard valve especially if any leakage is detected.