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

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

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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)
Less than that for fluid flow
More than that for fluid flow

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