Process Equipment and Plant Design
In a double pipe (concentric) heat exchanger, the hydraulic radius for heat transfer (for a fluid flowing through the annulus) would be

Same as that for fluid flow
D₂ - D₁ (D₁ and D₂ are I.D. of inner and outer pipes respectively)
More than that for fluid flow
Less than that for fluid flow

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Process Equipment and Plant Design
The function of manholes provided in the shell of a distillation column is to

Keep a check on the liquid gradient over the plate by direct visual observation
Give access to the individual trays for cleaning, maintenance and installation
All of these
Guard against foaming & entrainment by dumping antifoaming agent through it

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