How does a marine fresh water generator (evaporator) work?

<p>A marine fresh water generator, or evaporator, is a low-pressure distillation plant that boils seawater under about 90 to 95 percent vacuum using waste heat from the main engine jacket water, then condenses the vapour into fresh water.</p>A fresh water generator produces the ship's drinking, washing and technical water from seawater by low-pressure distillation, using heat the engine would otherwise throw away. Main engine jacket cooling water leaves the engine at roughly 70 to 85 degrees C and passes through the heating section of the generator, where it boils a feed of seawater. Water cannot boil at those temperatures at atmospheric pressure, so the shell is held under a vacuum of roughly 90 to 95 percent by an air ejector, bringing the boiling point down to around 40 to 60 degrees C. The vapour rises through a demister that knocks out entrained salt droplets, condenses on a seawater-cooled condenser section, and is pumped to the tanks by the distillate pump, while the concentrated brine is continuously extracted.

Quality control is automatic but must be understood. A salinometer measures the conductivity of the distillate and, above its setpoint, commonly from a few parts per million up to about ten, dumps the water back to the shell and raises an alarm so salty water never reaches the tanks. Water destined for drinking is additionally sterilised, by ultraviolet or chlorination, and usually rehardened through a mineraliser, since pure distillate is corrosive to pipework and flat to drink. Do not run the plant in harbours, estuaries or polluted coastal waters, where the feed can carry contamination that low-temperature distillation is not certain to deal with.

When output or quality falls, the usual suspects are few: - Falling production with normal vacuum: scale on the heating surfaces, low jacket water temperature or flow, or a throttled feed. - Poor vacuum: air leaks at gaskets and gauge connections, a worn or fouled ejector nozzle, or low pressure on the ejector pump. - High salinity: a damaged demister, condenser leakage, excessive brine level, or overboiling from trying to force output.

Operate it gently and it lasts: raise vacuum before applying heat, control the shell temperature because scale forms fast when it runs hot, dose antiscalant to the feed as recommended, and descale the heat exchange surfaces when performance trends down rather than waiting for output to collapse. Many ships also fit reverse osmosis plants, which force seawater through membranes at high pressure and suit ships with little waste heat, but the waste-heat evaporator remains standard on motor ships precisely because it turns jacket water heat into tonnes of good water per day at almost no fuel cost.