Evaporator (marine)

Obtaining fresh water from seawater is a theoretically simple system that, in practice, presented many difficulties.

While there are numerous effective methods today, early desalination efforts had low yields and often could not produce potable water.

A selection of documented systems is as follows: With the development of the marine steam engine, their boilers also required a continual supply of feedwater.

Despite this, fresh water makeup to the feedwater system of a large warship under full power could still require up to 100 tons per day.

[26] Evaporators consume a great deal of steam, and thus fuel, in relation to the quantity of fresh water produced.

[26][27][28] 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.

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.

[28] 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.

[29] 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.

To further control scale formation, equipment may be provided to automatically inject a weak citric acid solution into the seawater feed.

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.

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.

[17] As the increasing complexity of surface condensers demanded better feedwater quality, a pump became part of the evaporator equipment.

[25] 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.

The cold seawater passes through a condenser coil in the upper part of each chamber before being heated by steam in an external feedwater heater.

[34] The advantage of the flash distiller over the compound evaporator is its greater operating efficiency, in terms of heat supplied.

This is due to working under vacuum, thus low temperature, and also the regenerative use of the condenser coils to pre-heat the seawater feed.

[34] A limitation of the flash distiller is its sensitivity to seawater inlet temperature, as this affects the efficiency of the condenser coils.

To avoid the risk of priming and a carry over of saltwater into the vapour, the evaporator is divided by a bubble cap separator.

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[ii] etc.