Therefore,

If

(Blake – Plummer equation)

It is pointed out that the Ergun's equation is applied to calculate pressure- drop across packed bed consisting of small size particles

Also, note that is to be interpreted as energy loss due to drag or friction per unit mass of the fluid , so that the term can be substituted in the general mechanical energy balance equation, consisting of KE, gravitational and shaft-work heads.

where frictional–pressure drop