A tube for moving gas between an entry end into which gas is introduced and an exit end through which gas exits the tube, the tube comprising a plurality of adjoining adjacent□ guides, each guide comprising an outer half of a smoothly-curved, modified torus, and an outer rigid tube wall. Each guide forms an internal cavity with a cavity mouth opening into an inner portion of the tube, the cavities shaped such that a vortex forms within each of the cavities as gas passes through the tube, and the flow of fluid in the tube is unidirectional and axial from the entry end to the exit end.