A hollow shaft of a screw conveyor for transporting powder material receives pressurized air in its interior. A plurality of openings through the wall of the hollow shaft permits the pressurized air to exit energetically to prevent build-up of the powder material on the inside wall of the casing of the screw conveyor. In one embodiment, the openings are slits angled with respect to the axis of the screw conveyor at about the same angle as the helical blade. This produces a swirling flow of air which, in addition to preventing build-up of materials, enhances the advance of the powder material and, when the powder material is a mixture of different materials, helps prevent separation of the materials during transport.