3. Fresh Water Generator

Ocean going vessels do not have the space on board for storing large quantities of fresh water, therefore, much of what is required must be made on board. The majority of fresh water generators are of the flash type. These operate on the principle that the boiling temperature of water varies with pressure. By creating a vacuum water will boil at a lower temperature and the engine jacket cooling water can be used as the heat source.

In a flash type fresh water generator the engine jacket cooling water is fed to the heat exchanger where it circulates outside the tubes. Sea water feed passes through the inside of the tubes.

The pressure inside the evaporator is maintained at a vacuum by the air ejector and the reduction in pressure results in some of the sea water flashing off into vapour. The remaining brine is taken overboard via the ejector.

The vapour passes upwards through a demister and is condensed by a cooler. The condensate is collected in a tray and fed to the distillate pump. The salinity of the distillate must be monitored and once it falls to a preset value, usually 2ppm, the distillate pump will start to pump the water to the fresh water storage tanks.

Instruction Concerning Operation

• Open up ejector pump system and start the pump.

• Open the inlet and outlet valves for the condenser.

• Open the feed valve and adjust the flow until the flow indicator is at the correct position.

• Allow a vacuum to build up inside the evaporator. This should fall to 700mm Hg.

• When a vacuum is achieved, the heating inlet and outlet valves should be opened to admit jacket cooling water to the heater. This should be done slowly because the heater acts as another jacket water cooler. The jacket water temperature controller may not respond quickly enough if the valves are opened suddenly, resulting in a fall in jacket water temperature.

• Switch on the salinometer. This will be high initially, but should gradually fall as fresh water is produced.

• Set the distillate pump to automatic and open the valves to the fresh water storage tank.

• When the salinity falls to the preset level the distillate pump will start.

• Open up the chemical feed system and set the flowmeter for the correct dosage rate.

• The capacity of the fresh water generator is regulated by increasing or decreasing the amount of jacket water to the heater. The evaporation temperature should be between 45 – 60°C. This will vary depending upon the sea water temperature. When the sea water temperature is low the evaporation temperature will be lower. It can be raised by reducing the sea water flow to the condenser or adjusting the vacuum if possible. The temperature rise across the condenser should be about 6°C. When the sea water temperature is high, the sea water to the condenser should be opened up more if possible.

• The fresh water generator should not be used within 50 miles of land or in estuarial water due to harmful bacteria found in the water. It is important to disinfect the water produced before use. A disinfecting unit is fitted to the domestic fresh water system and may be either ultra violet or an electrolytic salt cell which produces sodium hypochlorite.