A Roots-type blower comprises a housing defining a rotor chamber and an inletand outlet to the rotor chamber. First and second rotors are mounted in the rotorchamber, each rotor defining a plurality of lobes, adjacent lobes and the housingcooperating to define gas transport chambers. The blower is configured so thata net flow rate of gas into a gas transport chamber is generally or approximatelyconstant, whereby a change in gas pressure in the gas transport chamber is generallyor approximately linear, as the gas transport chamber approaches the outlet.In one embodiment, this is accomplished by providing flow channels extendingfrom the outlet towards the inlet, and from the inlet towards the outlet, correspondingto each rotor. The flow channels permit gas to flow from the high pressure outletto a gas transport chamber and from the gas transport chamber to the low pressureinlet. The resulting amelioration of pressure spikes associated with flow backsubstantially reduces the operational noise level of the blower.
