The operating vapor velocity is specified to minimize the entrainment of liquid drops in the exiting vapor.

The Souders-Brown equation was derived from a force balance on a droplet suspended in a gas flowing upwards. It was originally derived to size fractionating columns.

- ρ
_{L}is the liquid density (kg m^{-3}) - ρ
_{L}is the gas density (kg m^{-3}) - K is the vapor or gas load factor (m s
^{-1}) - V is the maximum allowable vapor velocity (m s
^{-1})

K has a value of 0.107 m s

Once you have the maximum allowable vapor velocity, you can then calculate the drum diameter by dividing the vapor flowrate by the allowable vapor velocity.

^{-1}for a separator with a mesh pad. However, the right value of K is largely dependent on the operating conditions and physical properties of the system. The GPSA Engineering Data Handbook tabulates values of K for various operating conditions.Once you have the maximum allowable vapor velocity, you can then calculate the drum diameter by dividing the vapor flowrate by the allowable vapor velocity.