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

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Process Equipment and Plant Design
When one of the fluids is highly corrosive and has fouling tendency, it should

Preferably flow inside the tube for its easier internal cleaning
Flow at a very slow velocity
Flow outside the tube, when the flow is counter-current and inside the tube when the flow is co-current
Perferably flow outside the tube

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