2.8.2 Application notes
Generating clean vacuum
Turbopumps are suitable for generating clean vacuums in the range of 10-3 to 10-10 mbar. Thanks to their high compression ratio, they reliably keep oil from the inlet area of rotary vane pumps away from the recipient. Models with stainless steel housings and CF flanges can be baked out. This makes these pumps ideally suited for research and development applications where contamination of the residual gas is undesirable. Turbopumps can be used for evacuating large vessels, with rotary vane pumps being employed as backing pumps. In the case of turbo drag pumps, even two-stage diaphragm pumps will suffice as backing pumps; however due to their lower pumping speed, it will take them a great deal of time to pump down larger vessels. The gas throughput of the combination will also be highly restricted by the diaphragm pump.
However this combination is an extremely cost-effective solution for a dry pumping station. It is often used in connection with differentially pumped mass spectrometers. Pumping stations consisting of a backing pump and a turbopump do not require valves. Both pumps are switched on at the same time. As soon as the backing pump has reached the necessary backing-vacuum, the turbopump quickly accelerates to its nominal speed and quickly evacuates the vessel to a pressure of p < 10-4 mbar with its high pumping speed. Brief power failures can be bridged by the high rotational speed of the rotor. In the case of longer power failures, both the pump and the recipient can be vented automatically if the RPMs decline below a minimum speed.
The effects that play a role in evacuating vessels are described in Chapter 7. Dimensioning issues as well as calculation of pump-down times are also presented in that chapter.
Evacuating load lock chambers
Evacuating load-lock chambers definitely requires clean handling when transferring the workpieces to be treated in a vacuum process. If these items are channeled in from atmospheric pressure, the chamber should first be pre-evacuated via a bypass line. The running turbopump is then connected between the backing pump and the chamber via valves.
In many cases, mass spectrometers are used in analysis devices today. Fluids are often injected and evaporated in the inlet chamber of the vacuum system. Pressure is reduced in several stages, and the individual chambers are isolated from one another by orifices. Since each chamber must be pumped, the objective is to combine the gas flows via taps on the turbopump through skillful combination of backing pumps and turbopumps. Specially modified turbopumps with taps are used for series applications.
Helium leak detectors, too, are equipped with turbopumps. In this case, the counter-flow principle is often used; i.e. a mass spectrometer is arranged on the high vacuum side of the pump. Due to the lower compression ratios of turbopumps for helium than for nitrogen or oxygen, the pump acts as a selective amplifier for the helium partial pressure.
Pumps with high gas loads in vacuum processes
Pumps with high gas loads in vacuum processes The turbopump offers two advantages when pumping high gas loads for vacuum processes: It generates clean vacuum at the beginning of each process step, and can then pump down process gas without any harmful backflow. In the second step, the primary objective is to maintain the given pressure at which the desired vacuum process should run. In this process, gas throughputs and working pressure will be determined by the application in question; i.e. a given volume flow rate will be pumped at a given gas throughput. Moreover, it should be possible to quickly achieve clean intermediate vacuum when changing workpieces. Since these are conflicting requirements, a turbopump of sufficient size for the required gas throughput and the required intermediate vacuum will be selected. The process pressure will be regulated via an inlet (butterfly) valve. An example of how to dimension this kind of pumping station is shown in Chapter 7. The maximum permissible gas loads specified in the technical data should be taken to mean permissible continuous loads. This applies subject to the assurance of sufficient cooling in accordance with the specification and a backing pressure that is less than 50 % of the critical backing pressure.
Pumping corrosive and abrasive substances
When pumping corrosive gases, measures must be taken to protect the motor / bearing areas and the rotor, in particular, against corrosion. To do this, all surfaces that come into contact with corrosive gas are either provided with a coating or made from materials that can withstand attacks by these gases. A defined inert gas flow is admitted into the motor / bearing area in the backing-vacuum via a special sealing gas valve. From there, the gas flows through labyrinth seals to the backing-vacuum area, mixes with the corrosive gas and is pumped down together with the corrosive gas.
The blades can wear mechanically should dust accumulate; this could necessitate repairs and replacement of the rotor. It should also be noted that deposits can be expected to form in the pump, which will necessitate shorter service intervals. In particular, it is necessary to ensure that deposits in the pump do not react with the moisture in the air to become aggressive substances. Consequently, the pumps should be vented with dry inert gases only, and should be fitted with sealed backing-vacuum and high vacuum flanges. Turbopumps for these applications are always classical turbopumps without a Holweck stage, as the narrow gaps and pump channels in the Holweck stage would quickly clog with dust deposits and the rotor would seize.
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