Based on the physical model of classification in vaned rotors, the fundamental difference between the exclusive use of a forced vortex flow (i.e. classification at the outer edge of the vanes) and the combined use of forced vortex/free vortex flow (i.e. classification in the interior vane free area) is derived. The classifier used for the comparison is a CFS-HDS model. Results received from this classifier equipped with a Convor-wheel with constant radial velocity of the flow in the interior vane free area are compared with results from a classifier wheel designed for classification at the outer edge of the vanes. Topics of special interest: range of fineness, sharpness of cut, pressure drop. (C) 2004 Elsevier B.V. All rights reserved.