Turbomolecular pumps what you need to know
November 11, 2020
6 MIN READ
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 the obvious choice.
Turbomolecular pumps vs. other pumps
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.
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 of 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. This results in TMPs having lower compression ratio for lighter gases.
Originally the 2 sets of bearings shown in the schematic below 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 of one mechanical bearing and one permanent, magnetic, friction free bearing
2 sets of frictionless, active magnetic bearings
The hybrid version results in reduced maintenance costs, with only one mechanical friction bearing requiring replacement.
The active magnetic variant generally offers the highest pumping speeds and is favored in more demanding processes such as semiconductor etch processes, or where pumping speeds in excess of 1000l/s are demanded.
As noted, both variants are not as effective at pumping light gases. 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
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. Diaphragm pumps are often used 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 as 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.
