In these devices classification occurs in the space over
a fluidized bed from which fine particles are carried away
with the air stream. The air velocity must essentially exceed
the maximal fluidization velocity of fine fraction. These
classifiers operate on the gravitational counterflow separation
principle with fluidized bed used to increase particles’
residence time in the separator. Coarse particles can move
horizontally across the bed to the exit, which is another advantage
of this device.