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

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

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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

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

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