Evaporator (marine)

An evaporator, distiller or distilling apparatus is a piece of ship's equipment used to produce fresh drinking water from sea water by distillation. As fresh water is bulky, may spoil in storage, and is an essential supply for any long voyage, the ability to produce more in mid-ocean is important for any ship.

Although distillers are often associated with steam ships, their use pre-dates this. Cook's Pacific exploration ship HMS Resolution of 1771 carried a distiller and Nelson's HMS Victory of 1805 was fitted with distilling apparatus in her galley. Distilling apparatus was only fitted to larger warships and some exploratory ships at this time: a warship's large crew needed more water and they could ill afford the space to carry enough. Cargo ships, and their smaller crews, merely carried their supplies with them.

With the development of the marine steam engine, their boilers also required a continual supply of feedwater.

Early boilers used seawater directly, but this gave problems with the build-up of brine and scale. For efficiency, as well as conserving feedwater, marine engines have usually been condensing engines. By 1865, the use of an improved surface condenser permitted the use of fresh water feed, as the additional feedwater now required was only the small amount required to make up for losses, rather than the total passed through the boiler. Despite this, a large warship could still require up to 100 tons of fresh water makeup to the feedwater system, when under full power. Attention was also paid to de-aereating feedwater, to further reduce boiler corrosion.

The distillation system for boiler feedwater at this time was usually termed an evaporator, partly to distinguish it from a separate system or distiller used for drinking water. Separate systems were often used, especially in early systems, owing to the problem of contamination from oily lubricants in the feedwater system and because of the greatly different capacities required in larger ships. In time, the two functions became combined and the two terms were applied to the separate components of the system.

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