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

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

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Process Equipment and Plant Design
Vertical condenser is advantageous to the horizontal condenser from operation point of view, when

Hydrostatic head is required for refluxing the condensate
Only the function of condensation is to be carried out
Sub cooling of condensate is desired
Both the functions of condensation & sub cooling are carried out in a single unit

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