Page 50 - Curriculum Visions Dynamic Book

Condenser tanks are kept at low pressure
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New supply of seawater pumped in
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Distilled “fresh” water pumped out
Seawater is chemically treated on entry to the condenser tanks.
Some seawater is discharged since it was only used for cooling.
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Desalination
Distillation of water is used on a very large scale in countries where water is scarce, for example, in desert countries. In this case it is known as desalination.
Water is distilled from salt water by spraying the seawater into heated tanks in which the air pressure is low. This causes water to boil and steam to form at a temperature below its normal boiling point. This process is called “flashing.” The steam is then condensed onto cooled collector tubes. The process is called flash distillation. It uses
a very large amount of energy and can only work where energy is plentiful, such as in some Middle Eastern states. The largest desalination plants are on the Arabian Peninsula, and here a series of flash chambers is used to improve the efficiency of the process.
Large plants can produce millions of cubic metres of water a day.
Some brine leaving
the condenser tanks is discharged, and some is
5 recirculated along with the fresh seawater.
We can say that in general, the more
that a solid is dissolved in water, the more concentrated the solution and the lower the freezing point will be. This property is used, for example, when salt is put on roads to reduce icing. The freezing point of a high concentration of salt in water is several degrees below zero (the exact value depending on the concentration of the salt). We also find that salty water boils above 100°C. In fact, water used for boiling vegetables with salt added for flavoring has the unintended effect of raising the boiling point of the water and so slightly speeding up the cooking.
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The rest of the seawater joins the brine (concentrated seawater).
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Water condenses on pipes that carry cooler seawater and brine solution (condensers). This “fresh” distilled water is then collected in trays and piped off for use.
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