Multistage roots pumps are dry vacuum pumps used in low, medium, high and ultra-high vacuum systems to produce “dry” conditions.
The simple (single-stage) roots pump is most commonly employed as a booster pump for several types of fore-pumps (such as rotary vane pumps, screw and liquid ring pumps) to improve ultimate pressure and pumping speeds. When multistage roots pumps are employed, no fore pump is required and they can operate from atmospheric pressure. Roots pumps are suitable where a dry and clean atmosphere is important or more likely essential. Consequently, they are frequently used in the manufacture of semiconductors and solar panels, as well as for coatings and other industrial applications.
In its simplest form, roots pumps employ two counter-rotating interconnected ‘lobed’ rotor units rotating within a housing or chamber. Gas enters through an inlet flange located perpendicular to the rotating units, and is then “isolated” between the rapidly rotating rotors (which are spinning in opposite directions) and the chamber wall. The compressed gas is then expelled via the exhaust port.
Building upon this simplistic model of a roots pump, a multi-stage roots pump (which can consist of up to eight stages) employs several sets of rotors (on a shared shaft). The geometry of the rotors creates compression and hence each stage produces a progressively higher pressure. In this way, the product of a lower stage is the “feed gas” for the next higher stage (but without any interconnecting valves).
As with single-stage (i.e. roots blower) pumps, in multistage units there is no contact between rotors and the stator housing, there is no mechanical loading of seals in the pump housing/chamber, the units are hermetically sealed, they are robust and they can operate for long periods between service. Additionally, the new generation of multistage roots pumps, have been specifically designed to be used in quiet and clean environments, such as in analytical instruments and research laboratories where, operating with pumping speeds of between 40 and 50 m3/h they create very low noise 52 dB(A).
Performance characteristics of Roots pumps
The performance characteristics of roots pumps vary depending on a number of factors, including the number of stages, the unit size, speed of operation, ambient temperature, initial inlet pressure and characteristics of the gases being pumped.
The following listings provide some of the more obvious and significant performance characteristics:
- operating pressure range between atmosphere down to the low 10-2 range;
- impressively high pumping speed of between 40 and 200m3/hr;
- good performance when used to pump light gases;
- very low noise levels (even at high rotational speeds);
- and virtually no vibrations
Multistage roots pumps are predominantly employed where their dry, clean and high speed characteristics can be used to the best advantage. As a result their applications include: analytical instruments, R&D and space, the semiconductor and solar industries, and in the laser industry etc., as well as in furnaces, metallurgy and a variety of coatings applications (where their very dry nature is highly advantageous).
Advantages and DIFFICULTIES
Multistage roots pumps have many advantages over other pumps, and surprisingly few disadvantages or limitations.
The main advantages of multistage roots pumps is that they are very compact and quiet, enjoy a long service life, have no contact between moving parts (and thus no wear, no particles, nor any need for “contact” oil which would otherwise contaminate the vacuum system and final product). However, multistage roots pumps have several disadvantages: relatively high service costs and lower pumping performance when operating near atmospheric pressure. Furthermore, they are only capable of achieving small pressure differences, which is why they are employed in a multistage format where each incremental stage is able to build upon the pressure increase achieved from the compression of the previous stage.
Advantages of Multistage Roots Pumps
- Compact design
- Frequency controlled motors
- Robust, with long service intervals
- Lower noise levels than human conversation
- No contact between moving parts (and therefore no wear)
- Clean pumping – no particle contamination and no oil
- Hermetically sealed units
Disadvantages of Multistage Roots Pumps
- Relatively higher service costs
- When pumping near atmospheric pressure, they display lower pumping performances than other pump types
- Unsuitable for pumping liquids
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