Ergun equation

${\displaystyle f_{p}={\frac {150}{Gr_{p}}}+1.75}$

where ${\displaystyle f_{p}}$ and ${\displaystyle Gr_{p}}$ are defined as

${\displaystyle f_{p}={\frac {\Delta p}{L}}{\frac {D_{p}}{\rho v_{s}^{2}}}\left({\frac {\epsilon ^{3}}{1-\epsilon }}\right)}$ and ${\displaystyle Gr_{p}={\frac {\rho v_{s}D_{p}}{(1-\epsilon )\mu }};}$

where:

${\displaystyle Gr_{p}}$ is the modified Reynolds number,
${\displaystyle f_{p}}$ is the packed bed friction factor
${\displaystyle \Delta p}$ is the pressure drop across the bed,
${\displaystyle L}$ is the length of the bed (not the column),
${\displaystyle D_{p}}$ is the equivalent spherical diameter of the packing,
${\displaystyle \rho }$ is the density of fluid,
${\displaystyle \mu }$ is the dynamic viscosity of the fluid,
${\displaystyle v_{s}}$ is the superficial velocity (i.e. the velocity that the fluid would have through the empty tube at the same volumetric flow rate), and
${\displaystyle \epsilon }$ is the void fraction (porosity) of the bed.

To calculate the pressure drop in a given reactor, the following equation may be deduced

${\displaystyle \Delta p={\frac {150\mu ~L}{D_{p}^{2}}}~{\frac {(1-\epsilon )^{2}}{\epsilon ^{3}}}v_{s}+{\frac {1.75~L~\rho }{D_{p}}}~{\frac {(1-\epsilon )}{\epsilon ^{3}}}v_{s}|v_{s}|.}$

This arrangement of the Ergun equation makes clear its close relationship to the simpler Kozeny-Carman equation which describes laminar flow of fluids across packed beds via the first term on the right hand side. On the continuum level, the second order velocity term demonstrates that the Ergun equation also includes the pressure drop due to inertia, as described by the Darcy–Forchheimer equation.

The extension of the Ergun equation to fluidized beds, where the solid particles flow with the fluid, is discussed by Akgiray and Saatçı (2001).

• Kozeny–Carman equation

• Ergun, Sabri. "Fluid flow through packed columns." Chem. Eng. Prog. 48 (1952).
• Ö. Akgiray and A. M. Saatçı, Water Science and Technology: Water Supply, Vol:1, Issue:2, pp. 65–72, 2001.