Heat Transfer
Double pipe heat exchangers are used

when heat transfer area required is very low
because it is less costly
when heat transfer area required is very high
because it occupies less floor area

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Heat Transfer
Vacuum is generally maintained in the vapour space of an evaporator mainly to

Facilitate forward feeding in multiple effect evaporation
Get economical temperature difference by using moderate pressure steam
Achieve very high concentration of the final product
Concentrate heat sensitive materials

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