within the dirt outlet opening 12 in the area adjacent the bordering wall 14 of the housing 2 located at the rearward side of the dirt outlet opening 12 as viewed in the direction of rotation of the opening roller 3 as shown in fig1 this air guidance system 15 preferably defines a dirt removal conduit 16 extending parallel with the bordering wall 14 and an auxiliary air duct 17 defined by a deflecting wall 18 to open into the dirt removal conduit 16 preferably in the upper third of the lengthwise extent of the dirt removal conduit 16 as viewed in the direction of dirt flow therealong a somewhat modified embodiment of the air guidance system 15 is represented in fig2 in this embodiment a further air connector duct 19 is formed through the bordering wall 14 to extend from an air entrance location directly connected with the atmosphere and to open into the dirt removal conduit 16 in addition to the auxiliary air duct 17 a third variant of the air guidance system is represented in fig3 an auxiliary air duct 17 such as is provided in fig1 and 2 is omitted in this embodiment and only an air connector duct.