When a gas is passed through a permeable support on which material rests which is able to flow, the packing of the bed of material will becomes less dense i.e. the bed will start to expand when a certain velocity is reached. At still greater velocities the particles in the bed will start to expand when a certain velocity is reached. At still greater velocities the particles in the bed will begin to be in turbulent motion. Than all particles of the product are thoroughly mixed in the turbulent layer and they dry therefore at uniform rate.

A further increase in the stream of gas finally leads to a velocity at which the particles float, which occurs when the upward force equals the force of gravity acting on the particles. Beyond this velocity it is impossible to  maintain a fluidized bed is therefore maintained at air velocities, which lie between those at which the bed starts to expand the those at which the particles float.