Process Equipment and Plant Design
In a double pipe concentric heat exchanger, the equivalent diameter of annulus for heat transfer would be (where, D₁ and D₂ are I.D. of inner and outer pipes)

(D₂² - D₁²)/D₁²
D₂ - D₁
None of these
(D₂² - D₁²)/D₂²

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Process Equipment and Plant Design
Smoker's equation for the calculation of number of equilibrium stages in a continuous binary distillation column is used, when the

Number of equilibrium stages required is likely to be very small
Feed is not at its bubble point
Number of equilibrium stages in only stripping section is to be calculated
Relative volatility is close to one (e.g., separation of close boiling isomers)

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