Air in the inlet port is trapped between the rotor lobes and the casing (left rotor in fig. 1) and carried round to the discharge port without being compressed on the way. There is a minimal clearance between the rotors and the casing so oil is not needed as a lubricant or to form a seal. The more precise the machining of the rotors and casing the smaller the gap between them, the higher the volumetric efficiency and the lower the discharge temperature of the air. Both of these factors contribute to extended blower life. The casing bore toward the discharge port is slightly eccentric so that as the lobe approaches the port the gap between it and the casing begins to widen. This allows gradual equalisation of pressure between the air in the discharge port and that in the chamber behind the advancing lobe (left rotor in fig. 2 and 3). This is the main reason why three-lobe blocks generate significantly less pulsation than two-lobe blocks.
Pressure equalisation in two-lobe blocks occurs abruptly as the advancing lobe crosses the lip of the discharge port. Figure 4 shows the final phase where the entrapped air is pushed out against the resistance in the pipework.