G r p {\displaystyle Gr_{p}} is the modified Reynolds number, f p {\displaystyle f_{p}} is the packed bed friction factor Δ p {\displaystyle \Delta p} is the pressure drop across the bed, L {\displaystyle L} is the length of the bed (not the column), D p {\displaystyle D_{p}} is the equivalent spherical diameter of the packing, ρ {\displaystyle \rho } is the density of fluid, μ {\displaystyle \mu } is the dynamic viscosity of the fluid, v s {\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) ϵ {\displaystyle \epsilon } is the void fraction (porosity) of the bed. R e {\displaystyle Re} is the particle Reynolds Number.