in foreshortened form in the graph of fig4 in order to ensure for the transport of fibre and thread materials also at the branch off or diversion locations the air velocity v a must not be less than 6 m sec at the region of the branch off or diversion channels i e the connecting channels 9 the cross sectional area of the second duct or channel 8 is chosen such that notwithstanding each discharge from a connecting channel 9 and the increase in the air volume which is associated therewith the average air velocity v k as shown in fig3 attains a value of preferably about 8 5 m sec higher values of v k for instance 17 m sec are still acceptable as concerns pressure losses inlet velocities at the fans amount to as a rule up to 20 m sec the sawtooth shaped velocity characteristic v k is formed with continuous widening or enlargement of the cross sectional area of the second duct or channel 8 as has been represented by the full or solid lines in fig2 the air volumes or quantities of air in the suction duct 2 and the suction duct 8 arrive at the common suction chamber or space 10 and by the action of the not particularly illustrated but conventional suction fan connected after the drum filter 11 are further conveyed through such drum filter and downstream thereof depending upon the prevailing requirements the second duct 8 can be provided either as shown in fig2 with a continuous cross sectional area increase or enlargement or as has been depicted with broken lines with a stepwise increasing cross section area in the last mentioned case the regular air velocity v k essentially corresponds to the mean air.