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Function of flash evaporator & fresh water generators for Ship service system

The evaporator , boils sea water at the saturation temperature corresponding to the uniform pressure through the evaporation and condensing chambers. With flash evaporators (Figure below) the water is heated in one compartment before being released into a second chamber in which the pressure is substantially lower.

The drop in pressure changes the saturation temperature below the actual temperature, so that some of the water instantly flashes off as vapour. Steam in the chamber at sub-atmospheric pressure is condensed by contact with tubes circulated with the salt feed and is removed by a distillate pump.

Suitably placed baffles and demisters, similar to those already described, prevent carry-over of saline droplets. The arrangements for continuous monitoring of distillate purity are similar to those described above. If two or more vessels in series are maintained at progressively lower absolute pressures, the process can be repeated. Incoming salt feed absorbs the latent heat of the steam in each stage, with a resultant gain in economy of heat and fuel. This is known as cascade evaporation, a term which is self-explanatory.

Figure below shows a two stage flash evaporator distiller.

The flash chambers are maintained at a very low absolute pressure by ejectors, steam or water operated; the salt feed is heated initially by the condensing vapour in the flash chambers, subsequently in its passage through the ejector condenser (when steam-operated ejectors are used) and is raised to its final temperature in a heater supplied with low pressure exhaust steam.

Brine density is maintained, as in the case of the evaporator-distillers described previously, by an excess of feed over evaporation and the removal of the excess by a pump. The re-circulation of brine may be provided for in plant. It should be noted that when distillate is used for drinking it may require subsequent treatment to make it potable.

Cascade evaporator
Figure : Flow diagrams - cascade evaporator

freshwater generator
Fig: Freshwater generator
Image credit :

Freshwater generator or freshwater distiller

The unit used for conversion of seawater into fresh water by vacuum distillation based on evaporation and condensation. Single-stage freshwater generator consists of a chamber with two titanium plate packs acting as an evaporator and a condenser respectively. A vacuum of 85-95% is maintained in the system by a brine/air ejector.

Seawater evaporates at a temperature of approximately
- The seawater pump is enabled through a control panel.
- After passing the condenser section the seawater will flow through the air-brine ejector.
- A small part of the seawater flow is used as feed water to the evaporation section, where it is then divided into each second channels.
- Through a suction process the air-brine ejector removes the air from inside the casing and the excesive feed water.
- After maximum of 5 minutes a vacuum of approx 90% is reached.
- Open the valves on the hot water inlet and outlet piping line.
- The evaporation of the feed water will start immediately.
- The vapour will flow through the demister and will be condensed in the condenser section.
- Please start the freshwater pump after a maximum of 2 minutes after opening for the hot water line.
40C due to the vacuum condition as it passes between the plates of evaporator heated by hot fresh water from the engine jacket cooling system, or by steam.

Generators can be equipped with disinfection units (Chlorination, UV-radiation and Silver ionization), pH-adjustment and rehardening filters.

Summarized below some of the basic procedure of machinery service systems and equipment :
  1. Ballast arrangements

  2. The ballasting of a vessel which is to proceed without cargo to the loading port is necessary for a safe voyage, sometimes in heavy weather conditions. On arrival at the port the large amount of ballast must be discharged rapidly in readiness for loading....

  3. Cargo ships bilge systems

  4. The essential purpose of a bilge system, is to clear water from the ship's 'dry' compartments, in emergency. The major uses of the system, are for clearing water and oil which accumulates in machinery space bilges as the result of leakage or draining, and when washing down dry cargo holds. The bilge main in the engine room, has connections from dry cargo holds, tunnel and machinery spaces.....

  5. Bilge system layout details

  6. All bilge suctions have screw down non-return valves with strainers or mud boxes at the bilge wells. Oily bilges and purifier sludge tanks have suitable connections for discharge to the oily water separator or ashore. The system is tailored to suit the particular ship......

  7. Domestic water system

  8. Systems using gravity tanks to provide a head for domestic fresh and sanitary water, have long been superseded by schemes where supply pressure is maintained by a cushion of compressed air in the service tanks....

  9. Reverse osmosis

  10. Osmosis is the term used to describe the natural migration of water from one side of a semi-permeable membrane into a solution on the other side. The phenomenon occurs when moisture from the soil passes through the membrane covering of the roots of plants,....
  11. Salinometer features

  12. The condensate or product, if of acceptable quality, is delivered to the appropriate tanks by the distilled water pump. Quality is continuously tested by the salinometer both at start up and during operation. If the device registers an excess of salinity it will dump the product and activate the alarm using its solenoid valves. The product is recirculated in some installations......

  13. Sewage systems

  14. The exact amount of sewage and waste water flow generated on board ship is difficult to quantify. European designers tend to work on the basis of 70 litres/person/day of toilet waste (including flushing water) and about 130-150 litres/person/day of washing water (including baths, laundries, etc.). US authorities suggest that the flow from toilet discharges is as high as 114 litres/person/day with twice this amount of washing water......

  15. Sewage zero discharge system

  16. A retention or holding tank is required where no discharge of treated or untreated sewage is allowed in a port area. The sewage is pumped out to shore reception facilities or overboard when the vessel is proceeding on passage at sea, usually beyond the 12 nautical mile limit. ...

  17. Biological sewage treatment

  18. A number of biological sewage treatment plant types are in use at sea but nearly all work on what is called the extended aeration process. Basically this consists of oxygenating by bubbling air through or by agitating the surface. ....

  19. Sterilization system

  20. Sterilization by the addition of chlorine, is recommended in Merchant Shipping Notice M1214. A later notice, M1401, states that the Electro-Katadyn process in use since the 1960s, has also been approved. Another problem with distilled water is that having none of the dissolved solids common in fresh water it tastes flat. It also tends to be slightly acidic due to its ready absorption of carbon dioxide (CO2). .....

  21. Treatment of water from shore

  22. There is a risk that water supplied from ashore may contain harmful organisms which can multiply and infect drinking or washing water storage tanks. All water from ashore, whether for drinking or washing purposes, is to be sterilized. When chlorine is used, the dose must be such as to give a concentration of 0.2 ppm....

  23. Water production low pressure evaporator

  24. A considerable amount of fresh water is consumed in a ship. The crew uses on average about 70 litre/person/day and in a passenger ship, consumption can be as high as 225 litre/person/day. Water used in the machinery spaces as make up for cooling system losses may be fresh or distilled but distilled water is essential for steam plant where there is a water tube boiler. Steamship consumption for the propulsion plant and hotel services can be as high as 50 tonnes/day.....

  25. Flash evaporator system

  26. The evaporator , boils sea water at the saturation temperature corresponding to the uniform pressure through the evaporation and condensing chambers. With flash evaporators the water is heated in one compartment before being released into a second chamber in which the pressure is substantially lower......

  27. Oil content monitor system

  28. In the past, an inspection glass, fitted in the overboard discharge pipe of the oil/water separator permitted sighting of the flow. The discharge was illuminated by a light bulb fitted on the outside of the glass port opposite the viewer......

  29. Oily water separator

  30. Oil/water separators are necessary aboard vessels to prevent the discharge of oil overboard mainly when pumping out bilges. They also find service when deballasting or when cleaning oil tanks. The requirement to fit such devices is the result of international legislation....

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