Structural/mechanical measures are used so that it is possible either continuously or in increments over the length of a wire cage (2) of a screen to maintain a differential velocity of at least 5 m/s between the peripheral velocity of the screening vanes (3) of a rotor and the fibrous suspension. As a result the throughput is considerably greater than with conventional wire cages. This may be done by having a wire cage and rotor whose diameters increase steplessly, or in a stepped manner, from the input to the output end of the apparatus. Alternatively or additionally baffles may be provided at discontinuities along the internal wall of the cage. <IMAGE>
