Process Equipment and Plant Design
Fouling factor must be included in the calculation of over all design heat transfer coefficient, when the liquid

Is highly viscous
Containing suspended solids flows at high velocity
Is of high specific gravity
Containing suspended solids flows at low velocity

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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

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
Same as that for fluid flow

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