Heat Transfer
In the equation Q = UAΔt; Δt is

Geometric mean temperature difference
Logarithmic mean temperature difference
Arithmetic mean temperature difference
The difference of average bulk temperatures of hot and cold fluids

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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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