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Production of distilled water for ships use - The evaporation process
Requirement of sea water distillation systems
Distillation is the production of pure water from sea water by
evaporation and re-condensing. Distilled water is produced as a result of
evaporating sea water either by a boiling or a flash process. This
evaporation enables the reduction of the 32000 parts per million of
dissolved solids in sea water down to the one or two present in distilled
water. The machine used is called an 'evaporator', although the word
'distiller' is also used.
Sea water is boiled using energy from a heating coil, and by reducing the
pressure in the evaporator shell, boiling can take place at about 60°C.
The sea water from the ship's services is first circulated through the
condenser and then part of the outlet is provided as feed to the
evaporation chamber .
Hot diesel engine jacket water or
steam is passed through the heater nest and, because of the reduced
pressure in the chamber, the sea water boils. The steam produced rises
and passes through a water separator or demister which prevents water
droplets passing through. In the condensing section the steam becomes
pure water, which is drawn off by a distillate pump. The sea water feed is
regulated by a flow controller and about half the feed is evaporated. The
remainder constantly overflows a weir and carries away the extra salty
water or brine. A combined brine and air ejector draws out the air and
brine from the evaporator.
Fig:Boiling process evaporator
Flash evaporation is the result of a liquid containing a reasonable
amount of sensible heat at a particular pressure being admitted to a
chamber at a lower pressure. The liquid immediately changes into
steam, i.e. it flashes, without boiling taking place. The sensible heat
content, water pressure and chamber pressure are designed to provide a
desired rate of evaporation. More than one stage of evaporation can take
place by admitting the liquid into chambers with progressively lower
Fig:Two stage flash evaporator
A two-stage flash evaporator is shown in Figure . The feed pump
circulates sea water through the vapour condensers and the preheater.
The heated sea water then passes to the first-stage flash chamber where
some of it flashes off. A demister removes any water droplets from the
steam as it rises and is then condensed in the first-stage condenser.
The heated sea water passes to the second-stage flash chamber, which
is at a lower pressure, and more water flashes off. This steam is demisted
and condensed and, together with the distilled water from the
first-stage, is drawn off by the distillate pump.
The concentrated sea water or brine remaining in the second-stage
flash chamber is drawn off by the brine pump. The preheater uses steam
to heat the sea water and most of the latent heat from the flash steam is
returned to the sea water passing through the condensers. An air ejector
is used to maintain the low pressure in the chambers and to remove any
gases released from the sea water.
During the operation of evaporating plants, scale will form on the
heating surfaces. The rate of scale formation will depend upon the
operating temperature, the flow rate and density of the brine.
Scale formation will result in greater requirements for heating to
produce the rated quantities of distilled water or a fall-off in production
for a fixed heating supply.
Cold shocking, the alternate rapid heating and cooling of the tube
surfaces, for a boiling process type, can reduce scale build-up.
Ultimately, however, the plant must be shut down and the scale removed
either by chemical treatment or manual cleaning.
The working density of evaporators should not be allowed to rise above the manufacturer’s recommendation. Feed water regulators and brine ejection equipment must be kept in good working order.
When the evaporator is in use it must be treated with the selected scale retarding chemical. The treatment is to be continuous rather than in "slug" form and is to conform to the supplier’s dosage rates.
- Compressed Air Systems for various shipboard operations
The main aim of a compressor, as the name suggests, is to compress air or any fluid in order to reduce its volume. Some of the main applications of compressors onboard ships are main air compressor, deck air compressor, AC compressor and refrigeration compressor. Failure to start or control air compressor can be inconvenient, costly and can carry risks, which need to be managed.....
- Marine air compressors working principles
Control or instrument air supplies have particular requirements with regard to being moisture and oil free and without impurities. A special type of oil-free compressor may be used to supply control air or it may be treated after delivery from an ordinary air compressor. This treatment results in the air being filtered and dried in order to remove virtually all traces of oil, moisture and any atmospheric impurities.....
- Coolers at sea- Shell and tube type coolers and plate type coolers Heat exchangers on board ship are mainly coolers where a hot liquid is cooled by sea water. There are some instances where liquid heating is required, such as heavy fuel oil heaters and sea water heaters for tank cleaning. Although being heat exchangers, the main condenser for a steam ship and the evaporator/distiller are dealt with separately .....
- Distillation system- Production of distilled water for ships use - The evaporation process
Distillation is the production of pure water from sea water by evaporation and re-condensing. Distilled water is produced as a result of evaporating sea water either by a boiling or a flash process. This evaporation enables the reduction of the 32000 parts per million of dissolved solids in sea water down to the one or two present in distilled water. The machine used is called an 'evaporator', although the word 'distiller' is also used.....
- Oily water separator and filter unit for 15 parts per million purity
Oil/water separators are used to ensure that ships do not discharge oil when pumping out bilges, oil tanks or any oil-contaminated space. International legislation relating to oil pollution is becoming more and more stringent in the limits set for oil discharge.....
Sewage treatment on board- biological and chemical sewage treatment plantThe discharge of untreated sewage in controlled or territorial waters is usually banned by legislation. International legislation is in force to cover any sewage discharges within specified distances from land. As a result, and in order to meet certain standards all new ships have sewage treatment plants installed.....
- Incinerator for ships Stricter legislation with regard to pollution of the sea, limits and, in some instances, completely bans the discharge of untreated waste water, sewage, waste oil and sludge. The ultimate situation of no discharge can be achieved by the use of a suitable incinerator. When used in conjunction with a sewage plant and with facilities for burning oil sludges, the incinerator forms a complete waste disposal package.....
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