Heat Transfer
Air is to be heated by condensing steam. Two heat exchangers are available (i) a shell and tube heat exchanger and (ii) a finned tube heat exchanger. Tube side heat transfer area are equal in both the cases. The recommended arrangement is

Finned tube heat exchanger with air inside and steam outside
Shell and tube heat exchanger with air on shell side and steam inside tubes
Shell and tube heat exchanger with air inside tubes and steam on shell side
Finned tube heat exchanger with air outside and steam inside

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

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

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

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

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