Evaporator (marine)

Two evaporators for the Olympic liners, 1910

The first distilling plants that boiled a separate water supply from that of the main boiler, appeared around 1867.[15] 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.[23] The distillate from this vessel then passed to an adjacent vessel, the distilling condenser.[23] 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.[15] For security against failure, ships except the smallest were fitted with two sets.[23]

Evaporators consume a great deal of steam, and thus fuel, in relation to the quantity of fresh water produced. Their efficiency is improved by working them at a partial vacuum, supplied by the main engine condensers.[23][24][25] On modern diesel-powered ships, this vacuum can instead be produced by an ejector, usually worked by the output from the brine pump. Working under vacuum also reduces the temperature required to boil seawater and thus permits evaporators to be used with lower-temperature waste heat from the diesel cooling system.

One of the greatest operational problems with an evaporator is the build-up of scale. Its design is tailored to reduce this, and to make its cleaning as effective as possible. The usual design, as developed by Weir and the Admiralty, is for a vertical cylindrical drum, heated by steam-carrying drowned coils in the lower portion.[24] As they are entirely submerged, they avoid the most active region for the deposition of scale, around the waterline. Each coil consists of one or two spirals in a flat plane. Each coil is easily removed for cleaning, being fastened by individual pipe unions through the side of the evaporator. A large door is also provided, allowing the coils to be removed or replaced. Cleaning may be carried out mechanically, with a manual scaling hammer.[25] This also has a risk of mechanical damage to the tubes, as the slightest pitting tends to act as a nucleus for scale or corrosion.[25] It is also common practice to break light scaling free by thermal shock, by passing steam through the coils without cooling water present[23][25] or by heating the coils, then introducing cold seawater.[26] In 1957, the trials ship HMS Cumberland, an obsolete heavy cruiser, was used for the first tests of the 'flexing element' distiller, where non-rigid heating coils flexed continually in service and so broke the scale free as soon as it formed a stiff layer.

Despite the obvious salinity of seawater, salt is not a problem for deposition until it reaches the saturation concentration.[20] As this is around seven times that of seawater and evaporators are only operated to a concentration of two and a half times,[27] this is not a problem in service.

A greater problem for scaling is the deposition of calcium sulphate.[24] The saturation point for this compound decreases with temperature above 60 °C (140 °F), so that beginning from around 90 °C (194 °F) a hard and tenacious deposit is formed.

To further control scale formation, equipment may be provided to automatically inject a weak citric acid solution into the seawater feed. The ratio is 1:1350, by weight of seawater.[28]

Operation of an evaporator represents a costly consumption of main boiler steam, thus fuel. Evaporators for a warship must also be adequate to supply the boilers at continuous full power when required, even though this is rarely required. Varying the vacuum under which the evaporator works, and thus the boiling point of the feedwater, may optimise production for either maximum output, or better efficiency, depending on which is needed at the time. Greatest output is achieved when the evaporator operates at near atmospheric pressure and a high temperature (for saturated steam this will be at a limit of 100 °C), which may then have an efficiency of 0.87 kg of feedwater produced for each kg of steam supplied.[24]

If condenser vacuum is increased to its maximum, evaporator temperature may be reduced to around 72 °C. Efficiency increases until the mass of feedwater produced almost equals that of the supplied steam, although production is now restricted to 86% of the previous maximum.[24]

Evaporators are generally installed as a set, where two evaporators are coupled to a single distiller.[29] For reliability, large ships will then have a pair of these sets.[29] It is possible to arrange these sets of evaporators in either parallel or in series, for either maximum or most efficient production.[24] This arranges the two evaporators so that the first operates at atmospheric pressure and high temperature (the maximum output case), but then uses the resultant hot output from the first evaporator to drive a second, running at maximum vacuum and low temperature (the maximum efficiency case).[29] The overall output of feedwater may exceed the weight of steam first supplied, as up to 160% of it. Capacity is however reduced, to 72% of the maximum.[24]

The unevaporated seawater in an evaporator gradually becomes a concentrated brine and, like the early steam boilers with seawater feed, this brine must be intermittently blown down every six to eight hours and dumped overboard.[23] Early evaporators were simply mounted high-up and dumped their brine by gravity.[15] As the increasing complexity of surface condensers demanded better feedwater quality, a pump became part of the evaporator equipment.[23] This pump had three combined functions as a seawater feed pump, a fresh water delivery pump and a brine extraction pump, each of progressively smaller capacity.[22] The brine salinity was an important factor in evaporator efficiency: too dense encouraged scale formation, but too little represented a waste of heated seawater. The optimum operating salinity was thus fixed at three times that of seawater, and so the brine pump had to remove at least one third of the total feedwater supply rate.[30] These pumps resembled the steam-powered reciprocating feedwater pumps already in service. They were usually produced by the well-known makers, such as G & J Weir. Vertical and horizontal pumps were used, although horizontal pumps were favoured as they encouraged the de-aeration of feedwater. Electrically powered rotary centrifugal pumps were later adopted, as more efficient and more reliable. There were initial concerns whether these would be capable of pumping brine against the vacuum of the evaporator and so there was also a transitional type where a worm gear-driven plunger pump for brine was driven from the rotary shaft.[22]

Vapour-compression distillers were installed on US submarines shortly before World War 2.[34] Early attempts had been made with evaporators running from diesel engine exhaust heat, but these could only be used when the submarine was running at speed on the surface. A further difficulty with submarines was the need to produce high-quality water for topping up their large storage batteries. Typical consumption on a war patrol was around 500 US gallons (1,900 litres) per day for hotel services, drinking, cooking, washing[lower-roman 2] etc. and also for replenishing the diesel engine cooling system. A further 500 gallons per week was required for the batteries.[34] The standard Badger model X-1 for diesel submarines could produce 1,000 gallons per day. Tank capacity of 5,600 gallons (1,200 of which was battery water) was provided, around 10 days reserve.[34] With the appearance of nuclear submarines and their plentiful electricity supply, even larger plants could be installed. The X-1 plant was designed so that it could be operated when snorkelling, or even when completely submerged. As the ambient pressure increased when submerged, and thus the boiling point, additional heat was required in these submarine distillers, and so they were designed to run with electric heat continuously.[34]