TMPs or Turbo Molecular Pumps are vacuum pumps that work using a very fast rotor (typically 24,000 to 90,000 rpm). Their typical operating pressures are in the high to an extreme pressure range of 10-3 and 10-11 mbar, with pump speeds of 10-4000 l/s.

The turbomolecular vacuum pump works on the principle: When gas molecules collide with a solid surface, they will move in a definite or intentional direction. In this case, the solid surface is a turbine-like rotor that rotates inside a chamber. The working parts are no different from a multi-blade turbine, with vanes running along the length of the shaft.

One of the main manufacturers of turbomolecular pumps in the world is AgilentTechnologies, Inc., an American analytical instrumentation development and manufacturing company (you can find a great variety on the Agilent Turbomolecular Pumps Section).

Types of Turbo pumps:

There are two main types of turbopumps: the “classic” version containing only one turbine molecular pump stage (made up of a series of rotor and stator stages) and the “wide range” version containing both turbine molecular pump stage and a mixed pump stage; The “wideband” TMP can achieve significantly increased exhaust pressures up to several mbar and is therefore used in conjunction with smaller primary vacuum pumps such as diaphragm pumps. The advantage of the “classic” rotor design is that these TMPs can handle higher airflow rates and can withstand more difficult handling conditions (e.g. particulate or dust). The complex rotors have three types of mechanisms.

• Holweck Mechanism:
  • Axial Pumping mechanism
  • Multiple spiral channels located on a stator tube
  • Spinning drum located close to the stator tube
• Siegbahn Mechanism:
  • Radial Pumping mechanism
  • Multiple spiral channels located on a stator disc
  • Spinning disc located close to the spiral channels
• Gaede Mechanism:
  • Circumferential pump mechanism
  • Single-channel with inlet and outlet ports on the stator disc
  • Spinning disc located close to the channel

Turbopump bearings:

Molecular vacuum pumps started using all mechanical rotor bearings (commonly known as cantilever designs with bearings at the lower end of the shaft and in the area of ​​the lower stages of the rotor). However, these require lubrication, and the life of the pump is dictated by the life of the bearings, and since bearing replacement is a specialized task, it must be done off-site. In addition, mechanical bearings cause less favorable rotor dynamics than other bearing types. On the plus side, however, turbopump mechanical bearings are highly resistant to external shocks or accidents and add to that a small footprint.

Five-axis active magnetic bearings (2x, 2y and z) (located at each end of the rotor shaft) with position sensing and variable magnetic field control are an alternative to mechanical bearings. These pumps do not require any lubrication compared to mechanical bearings, ensuring oil-free operation, and the pump life is not limited by bearing wear and transmission vibrations. Since there are no mechanical bearings, these TMPs are capable of injecting corrosive gases that would normally destroy lubricated bearings in a short period of time. Compared with TMP mechanical bearings, these bearings have a lower resistance to external shocks. In addition, they are large in size and more expensive than units using other types of bearings.

Hybrid bearings for TMP use a mechanical bearing located at the “lower” end of the rotor shaft with a pair of passive magnetic bearings located at the “upper” end of the shaft. Unlike the mechanical double bearing system, the life of the unit is mainly limited by the life of the single mechanical bearing, the transmitted vibrations are also less due to the absence of a second mechanical bearing. The hybrid bearing design benefits from favorable rotor dynamics that also allow field bearing exchange in the field. Similar to the fully magnetic flying TMP, the hybrid bearing TMP also has a lower tolerance to external shocks and a slightly larger footprint than mechanical bearing designs.

Applications of Turbo pumps:

Molecular pumps are extremely easy to use and require little maintenance. They provide hydrocarbon-free, low-vibration operation without regeneration. They provide continuous pumping speeds of up to 4000 l/s in the high, ultra-high, and ultra-high vacuum ranges. Molecular pumps are very compact and when combined with a dry primary pump, do not force oil into the vacuum system. Such conditions are necessary to produce the highest cleanliness conditions. These characteristics mean that molecular turbine pumps are suitable for a wide range of applications, from electron microscopy to semiconductor processing.

Turbine pumps are used in a wide range of high and ultra-high vacuum applications, including clean applications (e.g. in analytical instruments or R&D) and very difficult applications in an industry where the pump must handle corrosive gas or critical process conditions. The selection of the most appropriate TMP design should be based on the requirements of each application. It is important to keep in mind that each design (pump mechanism, different bearing design, etc.) will have certain pros and cons that will determine its suitability for a given application.