Heat Transfer
Fouling factor for a heat exchanger is given by (where, U₁ = heat transfer co-efficient of dirty surface U₂ = heat transfer co-efficient of clean surface)

1/U₁ - 1/U₂
1/U₂ - 1/U₁
U₂ - U₁
U₁ - U₂

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

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

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