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

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Process Equipment and Plant Design
In a shell and tube heat exchanger for a given heat transfer surface area, smaller diameter tubes are favoured as compared to larger diameter ones ; because the smaller diameter tubes

None of these
Are less prone to fouling
Are easier to clean
Can be fitted into a smaller shell diameter hence the cost of the heat exchanger would be less

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