Early classifiers of such type [3] were constructed as vertical
chambers having within a rising air stream (Fig. 1a) with
particles fed from the top or from the side. Being inexpensive
and simple in construction and maintenance, these devices
have one important shortcoming. Namely, the fed particles
break air flow uniformity and create significant turbulence.
As such, coarse particles prevent fines from rising with the
air stream. This in combination with the single-stage operation
mode underlies low separation efficiency of counterflow
classifiers with rising flows. These classifiers are used
only for separation of coarse bulk materials, containing two
types of particles of well differing sizes, e.g. coarse sand or
pebbles of 1–10mm mixed with dust 0.1–0.2 mm.