Abstract: A rotary radial flow machine such as a centrifugal compressor for compressing gas, and having an annular shroud encompassing the rotor blades thereof and forming with the surrounding portion of the machine an enclosed restricted access gas containment zone is provided with a gas supply to provide a low-drag gas to the enclosed zone to substantially exclude working gas passing through the compressor from entering the enclosed zone, so as to reduce the drag on the rotor. A shroud seal between the shroud and the surrounding portion of the machine limits the passage of the gas relative to the zone.