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
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

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Process Equipment and Plant Design
Which of the following is not a valid assumption in continuous binary distillation for calculating the number of equilibrium stages by Mc-Cabe-Thiele's method?

Heat of mixing of normal liquid is assumed to be zero
Molar latent heat of the two components are equal
Reflux is not a saturated liquid
Sensible heat changes for vapour & liquid are negligibly small

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