The rotatable impeller of a single-stage centrifugal blower is housed in a casing one side of which comprises a spiral plenum having an opening which extends along a predetermined limited portion of the perimeter of the impeller and the other side of which includes a port located near the periphery of the impeller but displaced from the opening of the plenum so that it is out of register with that opening. The spiral plenum includes an exit section that provides a first fluid outlet from the casing, and the port serves as a second, independent fluid outlet.
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