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

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

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Process Equipment and Plant Design
In a shell and tube heat exchanger, the clearance of the tube is generally

More in case of triangular pitch as compared to the square pitch tube layout
Equal to the tube diameter
More than the tube diameter
Not less than one-fourth of the tube diameter or 3/16"

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