Process Equipment and Plant Design
Bubble cap tray is the most commonly used tray in large distillation column, because

It incurs less pressure drop as compared to any other try designed for the same duty
Of its flexibility and nearly constant efficiency under varying conditions of operation
It eliminates the foaming and entrain-ment tendency during operation
None of these

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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

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
More than that for fluid flow

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