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

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

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

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