takes place one or more throttle flaps one shown 7 may be provided to vary the cross section of elbow 4 at will if as illustrated such a flap is provided on the inner side of the elbow curvature there is no interference between it and the stream of coal particles which slide along the inner surface of the elbow at the outer side of the curvature thereof under the influence of centrifugal acceleration on the other hand however the flap 7 offers sufficient resistance to the flow of the carrier gas in stream s 39 so that a variation of this resistance can be used to determine the particle size of the coarser fractions which are separated by the device from the finer fractions in stream s that is to say that the stronger the flow of stream s 39 through elbow 4 is throttled the lower the proportion of small particle fractions which enter the bypass 2 with stream s it follows that the throttle 7 can be used to reduce the gas stream s 39 to a minimum if desired while mechanical strains on the particles e g abrasion and the like are largely avoided in the embodiment of.