| CPC F04C 18/084 (2013.01) [F04C 18/088 (2013.01); F04C 18/16 (2013.01); F04C 18/20 (2013.01); F04C 2240/20 (2013.01); F04C 2250/201 (2013.01)] | 10 Claims |

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1. A screw compressor or expander comprising:
a female rotor including a first section having a right-hand first groove and a second section having a left-hand second groove, wherein the first groove has a first variable helix, the second groove has a second variable helix, and the female rotor has a first variable profile and a first variable outer diameter; and
a male rotor including a third section having a left-hand first lobe and a fourth section having a right-hand second lobe, wherein the first lobe has a third variable helix, the second lobe has a fourth variable helix, and the male rotor has a second variable profile and a second variable outer diameter,
wherein the female rotor includes a first central section positioned between the first section and the second section and the male rotor includes a second central section positioned between the third section and the fourth section, and wherein the female rotor transitions to a substantially circular cross section at the first central section and the male rotor transitions to a substantially circular cross section at the second central section.
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