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
Mc Cable Thiele method used for finding theoretical stages in a distillation column assumes that the

Liquid/vapor loading across the column remains constant
All of these
Sensible heat differences are small, because the temperature changes from tray to tray is small
Trouton's rule is applicable

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