A horizontal air stream entering this zone expands within a
chamber and converges towards the outlet (Fig. 1b). Particles
fed in the chamber with zero velocity accelerate horizontally
by the drag force Fd and also fall down. At the end of the
acceleration interval the horizontal velocity component vx of
all particles almost reaches the are flow velocity u and they
do not separate in this direction. Separation occurs due do
particles’ vertical motion across the air stream. Each particle
falls at its own terminal settling velocity. Accordingly, due
to the horizontal motion they land on the bottom at different
chamber locations: coarse-close to the inlet, smaller-close
to the outlet. Coarse fraction is gathered on the bottom of
the chamber, fines do not reach the bottom and are carried
away by the air flow through the outlet.
Thus, in the gravitational-crossflow zone particle separation
is a two-dimensional process, where their trajectories
are in fact ballistic tracks. Unlike the counterflow zone, the
cut size depends not only on the particle’s terminal velocity,
but mainly on the chamber length and height. These parameters
are chosen in such a way that particles with the cut
size land at the farthest point A of the bottom (Fig. 1b).