Rotary-Lobe Compressors

At the heart of each of our custom packages lies an oil-free rotary-lobe compressor. The rotary-lobe compressor is a variant of rotary-type positive displacement compressor.

Rotary-lobe compressors are typically known as ‘roots-type’ blowers or vacuum boosters, according to the situation in which they are used:

• Blower – Discharge pressure above atmospheric pressure
• Vacuum Booster – Suction and discharge pressure below atmospheric pressure

Since these machines are of the positive-displacement type, changes in gas properties do not greatly affect the inlet volumetric flow. Rotary-lobe compressors are simple to operate without the need for surge control, pressurised lube oil systems, speed-reducing gearboxes, etc. Control of these machines is generally via transmitters which are wired up to a junction box at skid edge and communicate with the DCS. There is no need for a standalone PLC-based unit control panel.

Principles of Operation

The rotary-lobe compressor incorporates two intermeshing rotors mounted on parallel shafts. In a twin-lobe compressor, each rotor has two lobes (four lobes per compressor). In a tri-lobe machine each rotor has three lobes (six lobes per compressor). Twin-lobe (bi-lobe) machines are normally used for process gas applications.  

• The two rotors rotate in opposite directions.
• As each rotor passes the blower inlet, it traps a definite volume of gas (the ‘displaced volume’) and carries it around the rotor housing to the blower outlet. With constant speed operation, the displaced volume remains approximately the same at different inlet temperatures, inlet pressures and discharge pressures.
• As each rotor passes the blower outlet the gas is compressed to the system pressure there and expelled.
• Small but definite clearances allow operation without lubrication being required inside the air casing.
• Timing gears control the position of the rotors relative to each other.

The rotary-lobe compressor was invented by two brothers, Philander Higley Roots and Frances Marion Roots, who patented their design in 1860.