This separation zone is characterized by a flat air vortex
prevailing in a cylindrical chamber with tangential inlet and
central outlet (Fig. 1c). In this vortex air rotates and flows
radially towards the chamber centre. The radial air movement
(radial sink flow type) serves as the particles’ separation
track.
Particles fed tangentially with air stream are involved in
rotation. Due to the centrifugal (inertial) force Fc their trajectories
gradually deviate from the air streamlines. Separation
is governed by the balance between Fc and the drag
force component Fdr induced by the radial air movement.
Coarse particles drift towards the chamber walls, where they
fall down to the outlet. Fines move towards the chamber
axis where they are eventually carried out by the air stream.
Particles with cut size theoretically move with zero radial
velocity, but practically go to one of the products. As the
centrifugal force depends on the radial position, so does the
cut size d50. This distinguishes the separation process in the
centrifugal-counterflow zone from the separation occurring
in the gravitational-counterflow zone. To keep the cut size
constant along the radial direction, the classifier should be
designed to provide constant tangential air velocity. This
feature is characteristic of the Archimedean spiral or free
vortex flow [2].