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.The first water supply by distillation of boiler steam appeared on early paddle steamers and used a simple iron box in the paddle boxes, cooled by water splash. A steam supply direct from the boiler, avoiding the engine and its lubricants, was led to them. With the development of steam heating jackets around the cylinders of engines such as the trunk engine, the exhaust from this source, again unlubricated, could be condensed.The first distilling plants that boiled a separate water supply from that of the main boiler, appeared around 1867. These were not directly heated by a flame, but had a primary steam circuit using main boiler steam through coils within a steam drum or evaporator. The distillate from this vessel then passed to an adjacent vessel, the distilling condenser. As these evaporators used a 'clean' seawater supply directly, rather than contaminated water from the boiler circuit, they could be used to supply both feedwater and drinking water. These double distillers appeared around 1884. For security against failure, ships except the smallest were fitted with two sets.A later form of marine evaporator is the flash distiller. Heated seawater is pumped into a vacuum chamber, where it 'flashes' into pure water vapour. This is then condensed for further use.Diesel-powered motorships do not use steam boilers as part of their main propulsion system and so may not have steam supplies available to drive evaporators. Some do, as they use auxiliary boilers for non-propulsion tasks such as this. Such boilers may even be heat-recovery boilers that are heated by the engine exhaust.