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Key working principles of rough vacuum pumps

February 11, 2021

Rough vacuum pumps (aka fore vacuum, primary vacuum) 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 rough 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 rough vacuum pumps. 

Diaphragm pumps

Diaphragm pumps operate in the low vacuum range and do not achieve high compression ratios with a single stage. They can produce a standard operating range from atmosphere down to the low mbar range.

These rough vacuum pumps use a diaphragm that is moved backwards and forwards by a rod. This oscillating motion compresses the pumped medium and activates the valves. The gas then moves in through an inlet valve, and when the diaphragm moves back, the valve is closed and the gas is pressurised before being expelled through the exitvalve.

The diaphragm and the valves are usually made of polytetrafluoroethylene (PTFE), which makes them resistant to corrosives and less vulnerable to vapour damage. Since diaphragm pumps are “dry” by design; they provide a hydrocarbon-free vacuum. Diaphragm pumps are simple to maintain and suitable for pumping many gases and laboratory chemicals. They also do not use oil, which means their operational and maintenance costs (diaphragm replacements) are comparatively low.

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. They combine a series of roots pump stages to overcome the pressure difference limitations of a single roots pump stage. 

They can compress atmospheric pressure down to the low 10-2 mbar range and are a dry pumping alternative to scroll pumps (where particle-free pumping is required).

A multi-stage roots pump can consist of up to eight stages and employ several sets of rotors (on a shared shaft). The geometry of the rotors creates compression, hence each stage produces progressively higher pressure. In this way, the product of a lower stage is the “feed gas” for the higher stage that follows (but without any interconnecting valves). They are compact and experience little to no wear because there are zero contacting parts. With that said, the assembly time is typically longer and suppliers may charge higher service costs.The ions created then bombard the titanium cathode plate, and pumping of molecular/gas ions can occur through implantation (physisorption). The bombardment causes the sputtering of Titanium atoms from the cathode lattice, resulting in deposits on surrounding surfaces of sputtered film. This film produces pumping via gettering (i.e. chemisorption of gas molecules).

Scroll pumps

Scroll pumps are one of the few pumps traditionally employed in low (i.e. 1000 mbar to 1 mbar) and medium (i.e. 1 mbar to 10-3 mbar) systems, and yet are now also frequently being employed as fore-pumps in high and ultra-high (i.e. 10-3 to 10-12 mbar) vacuum systems.

Scroll pumps consist of two co-wound spiral-shaped scrolls within a vacuum-housing, with an exit valve at the centre of the scroll assembly. One spiral is fixed whilst the other (the “orbiter”) moves eccentrically against the other, without rotating. Gas enters the (outside) open end of the spirals and, as one of the spirals orbits, becomes trapped between the scrolls. The gas is then moved into the centre as the void occupied by the gas is “squeezed and transported” between the two spirals.

When this finite “slug” of gas moves towards the centre, the volume it occupies decreases and the captive gas is continuously compressed until it is expelled at pressure via a non-return valve at the centre of the housing. Although none of the moving parts contained within the chamber requires lubrication, the PTFE tip seals are subject to wear and will need periodic changing.

Rotary vane pumps

Rotary vane pumps are wet positive displacement pumps, with the term “wet” meaning that the pump uses oil for sealing and lubrication.

As these pumps operate within a pressure range from atmosphere down to the approximately 10-4 mbar range, they are considered to be ideal backing pumps for any kind of medium and high vacuum pumps. Whereas the oil-sealed operation is a disadvantage for some applications, the use of oil facilitates higher compression ratios, a better internal cooling behaviour and ensures the pump is resistant to dirt, dust and condensate. 

In an oil-sealed rotary vane pump (or dry vane mechanism), an offset rotor fitted with vanes slides in and out of the housing (i.e in contact) within a stator chamber. The vanes rotate and trap a quantity of gas which enters through the pump’s inlet port, which then decreases the volume between the rotor and stator. The resulting compressed gas exits the outlet port to the atmosphere.

Screw pumps

Screw pumps are well-suited to a broad range of industrial applications that produce considerable quantities of dust and condensate. They use counter-rotating screw rotors to trap gas in the volume between the screws of their rotors. This void decreases as the screws rotate, compressing and moving the gas towards the exit port. They are also frequently used as fore vacuum pumps for roots pumps.

Screw pumps have numerous important facets: despite the micro-space between the two rotating screws, there are no contact parts nor is there any need for lubrication. As a result, there is no contamination of the medium being pumped. Furthermore, rotary wear is minimal, they have a high tolerance against particles, and employ high pumping speeds and low ultimate pressures. 

They are, however, less suitable for pumping light gasses, and cannot be scaled-down to small pumping speeds. Operational costs and maintenance requirements are also relatively low.

Choosing the correct rough vacuum pump for your application

In this article, we’ve explored some of the fundamental principles of common rough vacuum pumps. However, we’ve only just scratched the surface. There is a wealth of information that we simply could not cover here, such as selecting the right pump for your specific requirements.

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