Heat Transfer
The advantage of backward feed multiple effect evaporators over forward feed units is that

Heat sensitive material can be handled
Most concentrated liquid is at highest temperature
Equal heat transfer co-efficients exist in various effects
There is no additional cost of pumping

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

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

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