F-factor (process engineering)

The F-factor (F = fluid) is a variable used to describe the gas load (amount of gas) in a thermal separation apparatus, such as a column during rectification , stripping or absorption . It is calculated from the gas velocity in m / s related to the empty cross-section multiplied by the root of the gas density in kg / m³.

${\displaystyle F=v_{G}\cdot {\sqrt {\rho _{G}}}}$

The unit of the F-factor is , but is often left out in practice. ${\displaystyle \mathrm {\frac {m}{s}} \cdot {\sqrt {\mathrm {\frac {kg}{m^{3}}} }}={\sqrt {\mathrm {Pa} }}}$

It follows from this that the F-factor and thus the gas load is higher, the higher the amount of gas passed through the apparatus, since for a given column diameter the gas velocity increases with the amount of gas.

In the English-language literature, the capacity factor C _G is usually used instead of the F-factor , which is directly related to the F-factor:

${\displaystyle C_{G}={\frac {F}{\sqrt {\rho _{L}-\rho _{G}}}}}$

The F-factor plays a role above all in the fluid dynamic design of columns.

Above a certain F-factor, liquid droplets are entrained by the gas in countercurrent operation (liquid trickling down, gas flowing up). Due to the back-mixing caused by this, the performance of the separating device is reduced. In the extreme case, all of the liquid is carried away and discharged from above. One then speaks of "floods".

Common F-factors in rectification or absorption:

Pressure area	F factor ${\displaystyle \left[{\sqrt {\mathrm {Pa} }}\right]}$
vacuum	3.0
Standard pressure	2.0
Overpressure	1.5

The F-factor is not only used for two-phase countercurrents, but also for single-phase flows. Velocity and density then relate to the phase under consideration, which is not necessarily the gas phase. The F-factor is therefore a well-suited variable for describing a flow condition, because it is proportional to the velocity on the one hand, so with a constant density proportional to the amount transported, and at the same time it is a measure of the kinetic energy. It applies

${\displaystyle {\dot {m}}=F\cdot {\sqrt {\rho }}\cdot A}$

${\displaystyle E_{\mathrm {kin} }={\frac {1}{2}}\cdot \rho \cdot v^{2}={\frac {1}{2}}\cdot F^{2}}$

where is the cross-sectional area flowed through. ${\displaystyle A}$

Individual evidence

1. ↑ ^{a b} R. Goedecke: Fluid process engineering: history, methodology, practice, technology. P. 758, Wiley-VCH, 2006, ISBN 9783527311989