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:

In this blog post, we'll examine the key working principles of common multi-stage roots fore vacuum 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).





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:


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).​



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

Disadvantages of Multistage Roots Pumps

Browse our full range of vacuum pumps

Claw Pumps

The Ash Air range of rotary claw vacuum pumps is the best alternative to rotary vane pumps. A vacuum claw pump is virtually zero maintenance and compared to rotary vane pumps can work at vacuum levels of 400 to 1000 mbar without overheating. Rotary claw vacuum pumps are the best choice for CNC guided wood routers.

Sub Categories

Liquid Ring Pumps

The Ash Air range of humble liquid ring pumps is still the most reliable form of large volume vacuum generation available today. These liquid ring pumps are simple to use and very reliable, and the best option when sucked air contains a large amount of saturated gas.

Sub Categories

Rotary Vane Pumps

Dry and oil-sealed vane vacuum pumps are robust, affordable and easy in maintenance. You can use DVP vane pumps for wide range of application, for skin packaging, holding, lifting, thermoforming.

Sub Categories

Screw Vacuum Pumps

We have a quality range of oil-sealed screw pumps for sale that are energy efficient and have low life-cycle costs.

Sub Categories

Do you have an idea for our #expertcornerLet us know!

Who we are and how Ash Air can help your business!

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!

  • Rotary Screw and Piston Air Compressors
  • Vacuum Pumps and Blowers
  • Nitrogen Generation
  • An extensive line of Air Treatment components
  • AIRnet Piping System and  Filtration
  • Mobile Compressors, Generators, and Light Towers

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.

Reliability and Efficiency

With Ash Air, you can count on reliability and high performance for even the most demanding applications. We focus our efforts on the following:

  • Increasing uptime
  • Reducing unexpected repairs
  • Reducing energy consumption and meeting our sustainability goals
  • Improving the cleanliness of compressed air

Talk to the team today:  CONTACT US CONTACT US

Read more from our #expertcorner

When it comes to laser cutting and other industrial processes, the choice of assist gas is crucial in achieving optimal results. Nitrogen and oxygen are commonly utilized as assist gases, each with its own unique properties and applications. Understanding the characteristics of both gases, will help you to make an informed decision to ensure precision, efficiency, and cost-effectiveness in your operations.