Heat Transfer
The advantage of using a 1 - 2 shell and tube heat exchanger over a 1 - 1 shell and tube heat exchanger is

Higher tube side heat transfer co-efficient
Lower shell side pressure drop
Lower tube side pressure drop
Higher shell side heat transfer co-efficient

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

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

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