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

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

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Process Equipment and Plant Design
For a cylindrical shell, (subject to the thickness of uppermost course being more than the minimum for diaofthe tank in question), the thickness of the courses of shell

Decreases upwards
May decrease or increase upwards depending upon whether vacuum or positive pressure would be maintained inside the shell
Remains same throughout
Increases upwards

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