Fenske uses approximate shortcut methods to calculate the number of theoretical trays required for a particular separation of two key components at various reflux ratios.

For the specified separation of the key components, the program calculates the minimum number of theoretical stages (at total reflux) and the minimum reflux ratio (for an infinite number of stages). At these points, and a number of operating points in between, the program calculates a reflux ratio, a number of theoretical stages, and the distribution of all components in the top and bottom products.