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

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Process Equipment and Plant Design
A stuffing box is used for

Prevention of fluid leakage around moving parts
Reducing the resistance of fluid flow
Absorbing the contraction/expansion of pipeline due to temperature changes
Facilitating smooth opening and closing of a valve

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