relation to the raw air chamber 14 some of the transport air continuously penetrates through the filter 4 on the other hand the fibres from which there is separated the transport air upon passage through the filter slide along a helical shaped path on the filter surface the flow velocity of the transport air upon entry into the raw air chamber 14 is chosen such that there is effectively prevented any adherence of fibres to the surface of the filter 4 under the action of the centrifugal force the heavier fibre material for instance in the form of balls or entanglement in any case is propelled along the inner wall of the housing and does not come into contact with the filter but also as to the material deposited at the filter surface the force acting in the tangential direction is sufficient in order to displace the fibres along such surface the fibres which have been practically completely separated from the transport air collect at the neighborhood of the floor 10 due to the spin flow there prevails internally of the housing 2 a pressure which drops from the outside towards the inside the pressure prevailing at the center of the housing.