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

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Process Equipment and Plant Design
In a bag filter, the pressure drop increases directly as the

None of these
Density of the gas and inversely as viscosity of the gas
Density of the gas, but is independent of the viscosity of the gas
Viscosity of the gas and inversely as density of the gas

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