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

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Process Equipment and Plant Design
Overall distillation column efficiency for most of the refinery columns can be given by (for μ = 0.07 - 1.4 cp and relative volatility < 4) (where η = overall column efficiency, % μ = molal average viscosity of feed at average column temperature and pressure, cp Δp = pressure drop per tray, psi)

Η = 24.6 (Δp/μ)
Η = 17 - 61.1 log μ
Η = 0.25 log μ - 1.67
Η = 1.66 - 0.25 log μ

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Process Equipment and Plant Design
Number of evaporators which can be used in service in a multiple effect evaporation system is limited by the

Both A & B
Practical limit reached, due to low heat transfer rate because of inadequate temperature difference
Neither A nor B
Economic limit reached, when the plant cost exceeds the saving of steam

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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
It eliminates the foaming and entrain-ment tendency during operation
Of its flexibility and nearly constant efficiency under varying conditions of operation
None of these

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