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
Seamless pipe is made

By forging a solid round, piercing it by simultaneously rotating & forcing it over a piercer point and further reducing it by rolling gas drawing
From rolled strips formed into cylinders and seam-welded
By extrusion & casting into static and centrifugal molds
Both B and C

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