Heat Transfer
For condensation of pure vapors, if the heat transfer co-efficients in filmwise and drop-wise condensation are respectively hf and hd, then

hf < hd
hf > hd
hf = hd
hf could be greater or smaller than hd

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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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