A roots blower is a valve-less displacement compressor without internal compression. When the compression chamber comes into contact with the outlet port, compressed air flows back into the housing from the pressure side. Subsequently, further compression takes place when the volume of the compression chamber further decreases with continued rotation. Accordingly, compression takes place against full counter-pressure, which results in low efficiency and a high noise level. Two identical, usually symmetrical, counter-rotating rotors work in a housing, synchronized by means of a set of gear wheels. Blowers are usually air-cooled and oil-free. Their low efficiency limits these blowers to very low pressure applications and compression in a single stage, even if two- and three-stage versions are available. Roots blowers are frequently used as vacuum pumps and for pneumatic conveyance.
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In this article, learn more about the design and operation of a roots type blower and its application as a supercharger.
The animation below shows the operating principle of a Roots-type blower (Roots-type supercharger) with a pair of two-lobed rotary vanes (rotors), named after its inventor Francis Roots. The fluid is pumped from the suction side (inlet) to the pressure side (outlet) between the individual meshing lobes and the housing. Due to the counterpressure at the pressure side, the gaseous fluid is compressed.
Figure: Structure of a Roots blower (lobe pump) Animation: Operating principle of a two lobed Roots-type blower (lobe pump)The rotors are usually driven by a belt drive and a pair of gear wheels mounted on the rear. The rotors are therefore not driven by the contact of their meshing vanes! Rather, even a small clearance must remain between the rotary vanes, as otherwise the high speeds would result in inadmissible heat development and the rotors would wear out rapidly.
Figure: Driving pulley and gear drive of a Roots-type blowerInstead of two-lobe rotors, however, three-lobe or even four-lobe rotors are usually used. In order to achieve a more continuous compression and thus avoid pressure surges, the rotors are also twisted along their axis of rotation. The cross-sectional profile of the rotors consists of cycloids. Due to the contact-free meshing, however, it is not a cycloid gearing in the true sense!
Figure: Three lobed rotary vanes of a Roots-type blower (lobe pump) Animation: Operating principle of a three lobed Roots-type blower (lobe pump)Roots-type blowers are used, among other things, as compressors for supercharging combustion engines. They are also used for pumping liquids or foodstuffs such as rice or cereals.
Figure: Compressor for supercharging the air in combustion enginesThe animation below shows the structure and the functional principle of a Roots-type supercharger as it is used for charging the combustion air in automobiles.
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Animation: Operating principle of a Roots-type supercharger Figure: Roots type blower (supercharger)The air is sucked in through a slot at the front. The air enters the housing through this air inlet, which is equipped with cooling fins.
Figure: Air inlet of a Roots-type superchargerInside the Roots-type supercharger are the three-lobed rotary vanes (rotors), which pump the air between the rotors and the inner wall of the housing from the suction side (top) to the discharge side (bottom). Due to the accumulated air on the discharge side, the pressure there increases and the air is compressed accordingly.
Figure: Three lobed vanes of a Roots-type supercharger Animation: Operating principle of a Roots-type superchargerThe rotary vanes are driven by a belt drive, whereby the driving pulley drives only one of the two rotors directly. The second rotor is driven by two helical gears.
Figure: Driving pulley and helical gears of a Roots-type superchargerOnce the air has been pumped to the pressure side, the compressed air can be used for the combustion process. The unwanted increase in temperature during compression is counteracted by the cooling fins on the housing.
Figure: Air outlet of a Roots-type superchargerThe pressure ratio between the pressure side and suction side of a Roots-type supercharger is generally a maximum of 2. Depending on the rotational speed, the efficiency in these cases is around 50 %. With larger pressure ratios, the efficiency decreases rapidly! Basically, the smaller the pressure ratio, the more efficient Roots-type superchargers are.
The shape of the rotors in Roots-type blowers is made up of cycloids in the same way as cycloid gears. However, there is no power transmission between the rotary vanes as is the case with cycloid gears!
For the construction of two-lobed rotor, the rolling circles for the epicycloids and the hypocycloids are chosen to a quarter of the diameter of the base circle (see article Geometry of cycloidal gears).
Figure: Construction of the rotary vanes of a two lobed Roots blower (Roots-type supercharger) Animation: Construction of the rotary vanes of a two lobed Roots blower (Roots-type supercharger)For the construction of a three-lobed rotor, the rolling circles for creating the hypocycloids and epicycloids are to be chosen to a sixth of the diameter of the base circle.
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Figure: Construction of the rotary vanes of a three lobed Roots blower (Roots-type supercharger) Animation: Construction of the rotary vanes of a three lobed Roots blower (Roots-type supercharger)