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

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

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Heat Transfer
Water always boils when its

Vapour pressure equals 76 cm of Hg
Temperature reaches 100° C
Saturated vapour pressure equals the external pressure on its surface
Saturated vapour pressure is less than the atmospheric pressure

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