Steve W
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For low-salinity water, reverse osmosis can be used.
For high-salinity water, the water is boiled and recondensed.
However, the process of boiling the water does not necessarily mean that it is brought to a temperature of 100 C (212 F). It is common to apply a vacuum to the process, to get the water to boil at lower temperatures. This reduces the amount of input energy required to boil the water.
Where possible, the engineers try to design the boiling process desalination plants to use low-grade waste heat, like what comes out the exhaust of a power plant. This is where the use of vacuum is very helpful.
?
This is as simple an answer that I can give without losing too much technical detail. (I worked on a De-sal. plant of 8 units like that described below on the North African Coast of the Sahara desert.
A de-salination unit is operated under high vacuum in order to minimise heat input for economical reasons.
A single unit consists of a large, domed vessel (or drum), through which sea water is constantly flowing.
A huge amount of heat is added to the water by a large heat exchanger placed inside the vessel with Superheated steam at about 8 atm and 350°C passing through it.
The flowing water is heated to boiling point and the pure steam evolved passes out of the top dome into a Surface Condenser cooled by the incoming sea water feed to the unit thereby preheating the water to the process.
The surface condenser totally condenses the steam which, due to the large decrease in volume, (by about 1500 times from steam to water), causes a vacuum within the distillation drum down to about 5"Hg absolute at which pressure the sea water inside the vessel will be boiling at about 65°C.
The water leaving the condenser is cooled further, sanitised and sent to storage for domestic and industrial facilities.
Some water goes to the boilers for the production of the steam used in the process.
The Surface condenser also has 'Ejector' systems which are used to 'pull out' uncondensed gases like air, nitrogen ..etc and return them to the atmosphere. The build up of uncondensed gases in the unit will destroy the vacuum and decrease the unit performance.
The sea water leaving the unit after processing is higher in salt content and is returned to the sea well away from the area it originally came from.
eric l
Here is the fundamentals;
Heat gets water to boil.The boiling water does not take salt or other minerals. Only the water evaporates. The steam goes to another chamber and cools down and turns back in to a liquid.
Anonymous
The mechanism is reverse osmosis. Basically you have a membrane that lets water through but not salt, and you force water through it.
http://www.howstuffworks.com/question29.htm