Turbomolecular pumps (TMPs) are kinetic vacuum pumps which operate using a very fast spinning rotor (usually rotating at between 24,000 and 90,000 RPM). Their typical operating pressures are in the high to ultra-high pressure range between 10-3 and 10-11 mbar, employing pumping speeds of between 10 and 4,000 l/s.
Turbomolecular vacuum pumps work on the principle that gas molecules, when struck by a solid surface, will move in a specific or deliberately biased direction. In this case the solid surface is a turbine-type blade rotating within a chamber. The working parts are not dissimilar to a multi-bladed turbine, with blades along the whole length of the shaft.
There are two major types of TMPs: the “classic” which contains only a turbomolecular pumping stage (consisting of a series of rotor and stator stages) and the “wide range” version which contains both the turbomolecular pumping stage and a compound pumping stage; “Wide range” TMP can obtain significantly increased exhaust pressures up to several mbar and thus be used in combination with smaller fore-vacuum pumps such as diaphragm pumps. Advantages of the “classic” rotor design are that those TMP can handle higher gas throughput and can tolerate harsher process conditions (e.g. particles or dust).
There are three different designs of compound stage mechanisms worth mentioning: the Holweck, the Siegbahn and the Gaede stage design.
Image source: Leybold UK
The graphic above describes how each mechanism works in relation their pumping performance, which primarily depends on the specific design parameters of each concept, such as number of stages, clearances etc. Overall, the Holweck Mechanism is the most efficient one from a pumping performance point of view, while the Gaede and Siegbahn design is more compact.
Turbomolecular vacuum pumps started off using all mechanical rotor bearings (typically in a so-called cantilevered design with bearings at the lower end of the shaft and in the area of the lower rotor stages). However, these require lubrication, and the pump’s lifetime is dictated by the bearing life, and since bearing exchange is a specialist task, this needs to be done off-site. In addition, mechanical bearings cause less favourable rotor dynamics than other bearing types. However, on the plus side, mechanical bearings TMP have a high resistance to external shocks or shock venting, and in addition possessing a small footprint.
An alternative to mechanical bearings is the five-axis (2x, 2y and z orientation) active magnetic bearings (located at either end of the rotor shaft) with position sensors and variable magnetic field control. These pumps require no lubrication compared to mechanical bearings, provide hydrocarbon-free operations, the lifetime of the pump is not limited to bearing wear, and there is very low transmission vibration. Due to the absence of mechanical bearings those TMP are able to pump corrosive gases which would typically destroy lubricated bearings in a short period of time. Compared with mechanical bearing TMP these units have a lower tolerance to external shocks., Furthermore, they have a large footprint and are more expensive than units that employ other types of bearings.
The hybrid bearing for TMPs employs 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. In contrast to the double-mechanical bearing system the lifetime of the unit is mainly limited by the life of one mechanical bearing only, also the transmitted vibrations are less due to the absence of the second mechanical bearing. The hybrid bearing designs benefits from favourable rotor dynamics which also allows on-site bearing exchange in the field. Similar to fully magnetic levitated TMP, hybrid bearing TMPs also show lower tolerance to external shocks and slightly bigger footprints versus the mechanical bearing designs.
Turbomolecular pumps are used in a wide range of high and ultra-high vacuum applications, covering both clean applications (e.g. in analytical instruments or R&D), and very harsh applications in Semiconductor industry where the pumps have to handle corrosive gases or critical process conditions. The selection of the most appropriate TMP design needs to based on the requirements of each application. It is important to keep in mind that each design (different pumping mechanisms, bearing designs etc.) will have certain pros and cons which will determine its suitability for a certain application.
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