The gravitational-counterflow zone exists in air stream rising
up with velocity u inside a vertical chamber with parallel
walls (Fig. 1a). Particles experience gravity G and drag Fd
forces acting in opposite directions. Coarse particles, having
terminal settling velocity VTS larger than air flow velocity,
move downwards, against the air stream, and fines
rise with the stream. The cut size d50 is calculated from the
equilibrium G = Fd combined with zero particle’s absolute
velocity, i.e. VTS = u. Theoretically, the cut size particles
are indefinitely suspended in the separation zone