for changing the cross sectional passage area of the conveyor duct 6 for this purpose the latter may have for example a plurality of wall elements 17a 17b and 17c which may be shifted towards or away from the opppositely located wall portions so that the cross section of the conveyor duct 6 may be reduced or enlarged the wall elements 17a 17c may be made of an elastic material for example rubber or the like to thus ensure an advantageous seal according to the embodiment illustrated in fig3 the cross sectional area of the conveyor duct 6 is changed in such a manner as a function of the air quantities that the value of the desired flow speed is maintained turning now to fig4 the control apparatus 9 as opposed to the showing in fig3 is connected with the shutoff gates 11a 11c at the head of the reserve chutes 7 39 in this manner the cross sectional area of the conveyor duct 6 is preselected as a function of the position of the shutoff gates according to fig5 a lateral wall of the reserve chute 7 39 is provided with air outlet openings 7a which may be closed by a gate 20 rotatably supported at one end at the chute wall the position of the gate 20 may be varied by a power cylinder 21 which is connected with the control apparatus 9 not shown in fig5 turning now to fig6 to the conveyor duct 6 there are connected a pressure measuring device 18 and a flow rate measuring device 14 the.