The twin screw is the most common type of screw compressor used today. It uses a double set of rotors (male and female) to compress the refrigerant gas. The male rotor usually has four lobes. The female rotor consists of six lobes. Normally, this is referred to as a 4 + 6 arrangement. However, some compressors, especially air conditioners are using other variations, such as 5 + 7.