The Evolving Turbomolecular Pump
Already the leader in high-reliability high-vacuum pumping, design improvements continue to enhance their capabilities.
Just as in the aerospace arena where jet turbine technologies continue to see dramatic improvements in thrust and reliability, the turbomolecular vacuum pump (TMP) continues to see new designs that improve their effectiveness and performance. Most TMPs in the past had mean time between failure (MTBF) values in excess of 100,000 hr (11.4 yr), but current products now have MTBFs in excess of 200,000 hr due to improved balancing, bearings, and rotor designs. Similarly, most previous TMPs were mostly fixed designs with limited capabilities. Many current products, however, can be customized for specific applications.
TMPs are a category of momentum transfer pumps and are simply a series of rotating turbine-like blades. Each blade is termed a ‘stage’ and many newer pump rotors have all the TMP stages machined from one piece of aluminum to minimize weight and design complexity. In most pumps, there is a number of alternating rotor and stator stages arranged along a drive shaft rotating at speeds of up to 90,000 rpm. The rotating blades impact the incoming gas molecules and impart momentum to the gas particles that exit the pump through gas transfer holes.
A molecular drag pump varies from the TMP in that the momentum imparted to the incoming gas molecules is not by impact with a rotating blade, but by impact with a solid rotating surface—the molecules are ‘dragged’ along the surface. Hybrids are combinations of TMPs and molecular drag pumps in the same device.
One of the latest offerings is the multi-inlet TMP from Leybold Vacuum GmbH, Cologne, Germany (www.leybold.com). Up to three analysis chambers can be pumped down simultaneously by a single multi-inlet pump. The benefit offered with this pump is that you can replace two-TMPs in a system with just one TMP, thereby improving the overall system reliability, maintenance, and cost. This particular design also includes a cartridge capability, which, should a failure or maintenance need arise, allows the user to simply and quickly remove the central core of the TMP as a cartridge without involved assembly and leak detection work.
In this design, the first section of the TMP rotor includes two TMP stages, while the second represents a Holweck molecular drag-type stage. The Holweck pumping stage increases the permissible fore vacuum pressure level markedly when compared with the classical TMP. The dual-inlet pump has inlets directly into the turbo stages. Triple-inlet variations have a third gas inlet that feeds directly into the Holweck stage.
A number of TMP manufacturers now offer what they refer to as hybrid TMPs—turbo pumps with integrated Holweck molecular drag pump stages. Alcatel Vacuum Technology, Annecy, France (www.adixen.com), offers what they refer to as the smallest pump in the market designed for analytical instruments, the Adixen ATH 30. This pump fits into a 10 cm cube, and with its 10 x 10 cm controller is the most compact solution for an analytical instrument. Operating at 42,000 rpm, it provides an ultimate pressure of 10-8 bar (10-6 Pa) with a dry fore pump. Variations on this pump include a three-stage Holweck pump for increased compression ratio.
Other variations on TMPs and molecular drag vacuum pumps are available from BOC Edwards, Wilmington, Mass. (www.bocedwards.com), Pfeiffer Vacuum, Asslar, Germany (www.pfeiffer-vacuum.net), Varian Vacuum Technologies, Lexington, Mass. (www.varianinc.com), Shimadzu, Columbia, Md. (www.shimadzu.com), and Ebara, Sacramento, Calif. (www.ebaratech.com).