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

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Process Equipment and Plant Design
Generally, elliptical dished heads

All of these
Are manufactured on 2:1 ratio of major to minor axis and is recommended to be used for pressure vessels operating above a pressure of 1.5 MN/m²
Resist half the pressure rating compared to hemi spherical head provided on the cylindrical shell of the same thickness and diameter
Are approximately as strong as seamless cylindrical shell having the corresponding I.D and O.D

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