Evaporators
Combined supply
The first distillation plants, which 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 that used steam from the main boiler through coils within a steam drum or evaporator.[23] The distillate from this vessel was then passed to an adjacent vessel, the distillation condenser.[23] As these evaporators used a supply of "clean" seawater directly, rather than contaminated water from the boiler circuit, they could be used to supply feedwater and drinking water. These double stills appeared around 1884.[15] For safety against failure, ships, except the smallest, were equipped with two sets.[23].
Vacuum evaporators
Evaporators consume a large amount of steam, and therefore fuel, relative to the amount of fresh water produced. Its efficiency is improved by working with partial vacuum, supplied by the main engine condensers.[23][24][25] On modern diesel-powered ships, this vacuum can be produced by an ejector, usually operated by the outlet of the brine pump. Working under vacuum also reduces the temperature required to boil seawater and therefore allows the evaporators to be used with waste heat at lower temperatures from the diesel refrigeration system.
Tartar
One of the biggest operating problems with an evaporator is scale buildup. Its design is designed to reduce this and to make cleaning as effective as possible. The usual design, developed by Weir and Admiralty, is for a vertical cylindrical drum, heated by choked coils carrying steam at the bottom.[24] Since they are completely submerged, they avoid the most active region for scale deposition, around the waterline. Each coil consists of one or two spirals in one plane. Each coil is easily removed for cleaning, and is secured by individual pipe joints across the side of the evaporator. A large door is also provided, allowing the coils to be removed or replaced. Cleaning can be done mechanically, with a hand-held scale hammer.[25] This also has a risk of mechanical damage to the tubes, as smaller pits tend to act as a core for scaling or corrosion.[25] It is also common practice to release light scale by thermal shock, passing steam through the coils without cooling the water present[23][25] or by heating the coils, then introducing seawater. cold.[26] In 1957, the test ship HMS Cumberland&action=edit&redlink=1 "HMS Cumberland (57) (not yet drafted)"), an obsolete heavy cruiser, was used for the first tests of the "flexing element" still, where the non-rigid heating coils were flexed continuously during service and thus the scale was released as soon as a rigid layer was formed.
Despite the obvious salinity of seawater, salt is not a problem for deposition until it reaches saturation concentration.[20] As this is approximately seven times that of seawater and the evaporators are only operated at a concentration of two and a half times,[27] this is not a problem in service.
A major problem for scaling is the deposition of calcium sulfate.[24] The saturation point for this compound decreases with temperatures above 60 °C (140 °F), so from around 90 °C (194 °F) it forms a hard and tenacious deposit.
To further control scale formation, equipment can be provided to automatically inject a weak citric acid solution into the seawater feed. The ratio is 1:1350, with respect to seawater, by weight.[28].
Composite evaporators
The operation of an evaporator represents expensive consumption of steam from the main boiler, therefore of fuel. Evaporators for a warship must also be suitable to supply the boilers at full power when required, although this is rarely required. Varying the vacuum under which the evaporator operates, and therefore the boiling point of the feed water, can optimize production either for maximum throughput or increased efficiency, depending on what is needed at the time. The highest output is achieved when the evaporator operates at a pressure close to atmospheric and at a high temperature (for saturated steam, this will be limited to 100 °C), which can have an efficiency of 0.87 kg of feedwater produced for each kg of steam supplied.[24].
If the condenser vacuum is increased to maximum, the evaporator temperature can be reduced to around 72°C. Efficiency increases until the mass of feedwater produced is almost equal to that of steam supplied, although production is now restricted to 86% of the previous maximum.[24].
Evaporators are usually installed as a set, where two evaporators are coupled to a single still.[29] For greater reliability, large ships will have a pair of these sets.[29] It is possible to arrange these sets of evaporators in parallel or in series, for maximum or more efficient production.[24] This arranges the two evaporators so that the first operates at atmospheric pressure and high temperature (the maximum output box), but then uses the heat output resulting from the first evaporator. to drive a second, operating at maximum vacuum and low temperature (the maximum efficiency case).[29] The total feedwater production can exceed the weight of the steam supplied first, up to 160% of it. However, the capacity is reduced, up to 72% of the maximum.[24].
Evaporation pumps
Unevaporated seawater in an evaporator gradually becomes a concentrated brine and, like early seawater-fed steam boilers, this brine must be blown intermittently every six to eight hours and dumped overboard.[23] Early evaporators were simply mounted high and discharged their brine by gravity.[15] As the increasing complexity of surface condensers demanded better quality of the feedwater, a pump became part of the evaporator equipment. This pump had three combined functions as a seawater feed pump, a freshwater supply pump and a brine removal pump, each of progressively smaller capacity. The salinity of the brine was an important factor in the efficiency of the evaporator: too densely stimulated scaling, but too little represented a waste of hot seawater. The optimal operating salinity was thus set at three times that of seawater, so the brine pump had to remove at least one-third of the total feedwater delivery rate.[30] These pumps resembled the steam reciprocating feedwater pumps already in service. They were usually produced by well-known manufacturers, such as G & J Weir. Vertical and horizontal pumps were used, although horizontal pumps were favored as they encouraged deaeration of the feedwater. Electric rotary centrifugal pumps were adopted later, as more efficient and more reliable. There were initial doubts about whether these would be able to pump brine against the vacuum of the evaporator, so there was also a type of transition in which a worm-driven brine plunger pump was driven from the rotating shaft.[22].