Abstract: An apparatus for conditioning a fluid stream or a flow of granular solids at high volume and low velocity comprising a substantially closed cylindrical body having its side walls defined by a permeable helical trough joined to a vertically inclined baffle; means to convey the fluid through the body and out through the permeable trough; in a fluid conditioning apparatus, the fluid being conditioned upon passing the fluid through the helical trough or through the helical trough and a layer of particulate supported thereon; and in a solids conditioning apparatus, the solids being supported on the helical trough, and being conditioned upon passing fluid through the solids and through the helical trough.