Complex screw rotors having multiple helical profiles joined by a centeral portion with a pocket

A screw compressor or expander comprising:

• a housing having an inlet port, a discharge port, and a body at least partially defining a compression chamber having a first portion and a second portion;
• a female rotor rotatably positioned in the first portion of the compression chamber, the female rotor including a first section having a first groove with a right-hand helical profile, a second section having a second groove with a left-hand helical profile, and a first central section having a first curved transition connecting the first and second groove;
• and
• a male rotor rotatably positioned in the first portion of the compression chamber, the male rotor including a third section having a first lobe with a right-hand helical profile, a fourth section having a second lobe with a left-hand helical profile, and a second central section having a second curved transition connecting the first and second lobes, wherein at least one of the first and second curved transitions includes a pocket formed in a surface of the at least one of the first and second curved transitions, the pocket configured to relieve air trapped by the at least one of the first and second curved transitions.