(57) An improved supercharger or blower (10) of the Roots-type with reduced airborne noise.
The blower includes a housing (12) defining generally cylindrical chambers (32, 34)
containing meshed lobed rotors (14, 16) having the lobes (14a, 14b, 14c, 16a, 16b,
16c) thereon formed with an end-to-end helical twist according to the relation 360012n, where n equals the number of lobes per rotor. Blower housing (12) also defines
inlet and outlet ports (36, 38). The inlet port includes a longitudinal extent defined
by housing wall surfaces (20f, 20h) and a transverse extent defined by housing wall
surfaces 20g, 20i. Transverse wall surfaces (20g, 20i) are disposed substantially
parallel to the associated rotor lobes. The outlet port includes a longitudinal extent
defined by housing surfaces (20m, 20r) and a transverse extent defined by housing
surfaces (20p, 20s). Spaces (32a, 34a) between adjacent lobes of each rotor transfer
volumes of low-pressure inlet port air to relatively high-pressure outlet port air.
Associated with the outlet port are first and second expanding orifices (42, 44) disposed
on transversely opposite sides of the outlet port for controlling the rate of backflow
into the transfer volumes and operative at predetermined rotor speed and pressure
differential relationships to maintain a substantially constant backflow rate into
each of the transfer volumes. In an alternative embodiment an outlet port 138 has
a somewhat hourglass shape.
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