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

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

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Process Equipment and Plant Design
Overall tray efficiency of a distillation column is the ratio of the number of

Real trays required to the number of ideal trays required
Overall gas transfer units to the number of ideal trays required
Ideal trays required to the number of real trays required
None of these

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