1) When the drag impressed upon a bed of solid particles by a stream of fluid passing upward through it is sufficient to overcome its weight, the individual particles disengage and become mobile. The bed of particles is "fluidised". The pressure gradient required to maintain the slow flow of fluid through a fixed bed of spherical particles is given approximately by equation (1).
2) At the point of fluidisation, this pressure gradient is equal to the weight of the bed and the minimum fluidisation velocity is given by equation (2).