As I understand it you want a formula for a dirty air pipe size in a flour or feed mill. With very little information this would take a wild guess.

The Industrial Ventilation Manual recommends the following for Grain Elevators, feed mills, and flour mills - Air volumes can vary considerably depending on degree of enclosure, flow rate of material and dustiness of the grain. Minimum duct velocity = 3,500 fpm.

Ventilation control is desirable for these operations to minimise the explosive characteristics of grain dusts and to preserve plant housekeeping standards.

Much of the information quoted is based on recommendations by the National Fire Protection Association. ■