read

Turbomolecular pumps: what you need to know.

By Dr Graham Rogers
turbovac family

If you have a high-vacuum application requiring a pumping speed between 50 l/s and 3000 l/s and minimum contamination from the pump, then a turbomolecular pump is an obvious choice.

The lowest pumping speeds of the other key high vacuum pump types, namely diffusion and cryo pumps, are around 3000 l/s. In addition, diffusion pumps can be a source of oil contamination, whilst cryo pumps require regeneration on a regular basis.

Turbopumps are classed as kinetic pumps and will require a forevacuum pump.

Turbos function by imparting momentum to the gas molecules and atoms by collision with the surfaces of the rapidly spinning rotor, thus controlling the flow of gases in such a manner that they are transported to the exhaust port of the pump.

EMEA-Blog-Turbo1-2020-11

 
Principle of a Turbomolecular Pump

A turbomolecular pump (TMP) is a molecular pump whose rotor is composed of discs with gas conveying channels. These discs rotate between the corresponding discs of the stator.

Disc with conveying channels = plane of the rotating blades

Disc of the stator = plane of stationary blades

 

 

 

 

The velocity of the tips the rotors approaches a value close to the mean free velocity of the gaseous species being pumped. The lighter the gas, the higher its thermal velocity. Thus helium has a velocity of 1245 m/s whilst air, with higher density, has a mean speed of 463 m/s, which results in TMPs having lower compression ratio for lighter gases.

Originally the 2 sets of bearings shown in the schematic were of a conventional mechanical design. These bearings required periodic replacement – typically every 2-3 years, depending on the application.  However this design is rare these days, and there are now two types of bearings used:

  • A hybrid version comprising one mechanical bearing and one permanent magnetic friction free bearing
  • 2 sets of frictionless active magnetic bearings

The hybrid version, with only one mechanical friction bearing requiring replacement, results in a reduction in maintenance costs.

The active magnetic variant generally offers the highest pumping speeds and is favoured in more demanding processes such as semiconductor etch processes, or where pumping speeds in excess of 1000l/s are demanded.

As noted, both variants pump light gases less well. For coating applications where gas throughput, rather than ultimate pressure, is key, the “classic” design as shown above is ideal.

The important analytical market demands high compression even for light gases. This is achieved by the addition of an additional compression stage close to the exhaust port of the pump. This greatly improves light gas pumping speed. Such a design is known as a compound turbopump. The diagram below illustrates the principle of a Holweck stage which achieves this increased compression: 

Molecular Pump - Holweck Design
EMEA-Blog-Turbo2-2020-11 EMEA-Blog-Turbo3-2020-11.png

 

Another advantage of this increased compression means higher backing pressures can be tolerated by the TMP, even up to a few mbar. Consequently, some diaphragm pumps are now capable of acting as a forevacuum pump for this design of pump. Use of a diaphragm pump is encountered in portable” dry” leak detectors and compact high vacuum pumping systems.

All turbopumps require a frequency controller to provide the high rotational speeds demanded by these pumps. Conventionally these were stand-alone units, but increasingly the controllers have been integrated onto the pump – so called onboard controllers. This innovation has reduced rack space requirements – often a key driver for OEMs.

It is worth noting that remote controllers are still preferred if the pump is subject to high radiation where the onboard electronics might be compromised. This is a requirement for high energy beam applications where such an environment is present.

The most recent development allows “smart” onboard controllers which monitor and control valves, pumps and gauges within an automated vacuum system.

Conclusions:
  • Turbomolecular pumps offer a reliable contamination-free high vacuum option.
  • Pumping speeds range from 50 l/s to 3000 l/s.
  • Pumps can be tailored to allow high throughput or high compression of light gases.
  • Smart onboard frequency convertors offer control functions for high vacuum systems.


Tags: R&D, Ultra High Vacuum, High Vacuum

About Dr Graham Rogers

Dr Graham Rogers
Dr Graham Rogers has extensive experience across the world of vacuum, having been involved in the detailed specification and technical selling of the complete range of products, principally for Leybold, over the past 30 years. He brings in-depth knowledge from the metallurgy, chemical, analytical, R&D and semiconductor sectors, and has a passion for helping customers in solving problems and developing solutions that will bring real process improvements and value. Graham is a science graduate from Oxford University, UK, where he gained his degree in Chemistry and a DPhil in Physical Chemistry. His journey into vacuum science began early on in his career when worked developing semiconductor processes particularly thin film coating, this led him into the exciting and inspiring world of vacuum and eventually Leybold. We are delighted to have Graham as a resident Consultant for Leybold, where he is able to share his knowledge and insight through our vacuum academy training, videos and blogs.

Read more articles by this author

Leak Detection 101

Download our e-Book "Fundamentals of Leak Detection" to discover leak detection essentials and techniques.

MockUp_BookAndPhone