In a centrifugal fan or the like including an impeller housed in a casing, recirculation means are provided to recycle a portion of the gas discharge back to the blades of the impeller. Specifically these means comprise a radial passage between an impeller shroud and a wall of the casing, and aperture means in the shroud. 15% to approximately 30% of the gas discharge may be recycled. This arrangement has the advantage of increasing the fan pressure at low discharge rates in comparison with previous fans with no gas recirculation, and this is advantageous in some fan installations where operation is at reduced rates for considerable periods.
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