Process Equipment and Plant Design
Liquid gradient over the tray results due to the

Large reflux ratio
Large plate spacing
Low gas velocity
Resistance offered to flow of liquids by caps & risers and the flow of gas

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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

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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