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

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

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Heat Transfer
LMTD for counterflow and prallel flow heat exchanger will be the same, when the

None of these
Outlent temperature of both the hot and cold fluid are same
Outlet temperature of hot fluid is less than the outlet temperature of the cold fluid
Cold fluid is heated to a certain temperature by condensing steam (isothermal fluid)

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