The working principles of multistage roots vacuum pumps
Fore vacuum pumps are defined as those which exhaust to atmospheric pressure. They are also required to support secondary pumps or to attain the initial conditions for their operation. There are two types of fore vacuum pumps:
- Dry-running backing pumps such as scroll, screw and diaphragm pumps.
- Oil-sealed pumps such as rotary vane pumps. In this blog post, we’ll examine the key working principles of common multi-stage roots fore vacuum pumps.
MULTI-STAGE ROOTS PUMPS
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 use in combination with several types of fore pumps (such as rotary vane pumps, screw and liquid ring pumps) to improve (or ‘boost’) ultimate pressure and pumping speeds. When multistage roots pumps are employed, no fore pump is required and they can operate from atmospheric pressure. A multi-stage roots pump can consist of up to eight stages and employ several sets of rotors (on a shared shaft). 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, other industrial applications and for scientific instruments and research.
In its simplest form, roots pumps employ two counter-rotating interconnected ‘lobed’ rotor units rotating within a stator housing or casing. 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 stator. 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 — 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 25 to 200 m3/h — they create a very low noise of up to 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, rotation, speed of operation, ambient temperature, initial inlet pressure and the 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 25 and 200m3/hr
- Good performance when used to pump light gases
- Very low noise levels (even at high rotational speeds)
- Virtually no vibrations
Multistage roots pumps are predominantly employed where their dry, clean and high pumping 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 the laser industry. They’re also used 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 generated, or any need for oil which would otherwise contaminate the vacuum system and final product.
However, multistage roots pumps have several disadvantages. These include: relatively high service costs and lower pumping performance when operating near atmospheric pressure. They are also 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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Pompetravaini liquid ring pumps have a low noise level and water flow, a stainless-steel impeller, reliable suction valves, and a compact installation. We offer Atlas Copco LRP VSD+ intelligent liquid ring vacuum offers superior vacuum performance and connectivity. It provides optimum energy efficiency along with low operational and maintenance costs. Its ease of use, integration into existing processes and centralization capabilities make this a truly unique liquid ring vacuum pump.
Contact us today to find out more about our range of liquid ring pumps.
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We offer the Atlas Copco GVD series of 2-stage oil sealed rotary vane vacuum pumps deliver excellent ultimate vacuum pressure, high pumping speeds and superior vapor handling capabilities with quiet operation.
Also, the Atlas Copco GVS 16-630 A series operates according to the proven oil-sealed rotary vane principle that has been successfully used for many years in all general vacuum applications of industry packed with innovative features that ensure the highest possible performance at the lowest possible lifecycle cost.
At Ash Air we pride ourselves on supplying quality, reliable rotary vane vacuum pumps to our customers, with industry-leading servicing and spare parts for all brands. We offer 24/7 nationwide support with experienced engineers on hand to help when and where you need it.
We have a quality range of oil-sealed screw pumps for sale that are energy efficient and have low life-cycle costs.
The Atlas Copco GHS VSD⁺ series vacuum pumps are ideal for a range of applications in plastics, glass, bottling, canning, wood, packaging, printing and paper, meat packaging and many more. The GHS 350-900 VSD⁺ is the ideal solution when a dedicated pump is required in the production area. Atlas Copco GHS VSD⁺ is available in Turbo version for fast cycling and dedicated configuration and as well in Humid version with a greater water handling capability.
Contact the Ash Air team today for your screw vacuum pump needs.
Ash Air has been around in New Zealand since 1979, and we’ve grown into a nationwide company with international support and a
reputation for quality and reliability.We look after all things compressed air for your business!
Ash Air's range of Chicago Pneumatic, ALUP, Pneumatech, and Atlas Copco products are used extensively around the world in
industries ranging from oil and gas to food, automotive and farming, and we bring you these world-class compressors here in the land of
the long white cloud.Our technicians are compressed air equipment experts and are dedicated to addressing customer needs. Supported with
nationwide coverage, Ash Air offers one of the widest selections of compressed air, vacuum pump, and industrial gas equipment and parts
available today in New Zealand.
With Ash Air, you can count on reliability and high performance for even the most demanding applications. We focus our efforts
on the following:
Talk to the team today: There are two types of vacuum pumps: the ‘wet’ kind, which is essentially a hydraulic pump, and the pneumatic, ‘dry’ variety.
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Ash Air has been around in New Zealand since 1979, and we’ve grown into a nationwide company with international support and a reputation for quality and reliability.We look after all things compressed air for your business!
Ash Air's range of Chicago Pneumatic, ALUP, Pneumatech, and Atlas Copco products are used extensively around the world in industries ranging from oil and gas to food, automotive and farming, and we bring you these world-class compressors here in the land of the long white cloud.Our technicians are compressed air equipment experts and are dedicated to addressing customer needs. Supported with nationwide coverage, Ash Air offers one of the widest selections of compressed air, vacuum pump, and industrial gas equipment and parts available today in New Zealand.
With Ash Air, you can count on reliability and high performance for even the most demanding applications. We focus our efforts on the following:
Talk to the team today:
There are two types of vacuum pumps: the ‘wet’ kind, which is essentially a hydraulic pump, and the pneumatic, ‘dry’ variety. Read More…