Fundamentals of Vacuum Technology (1248463), страница 19
Текст из файла (страница 19)
In this system the rotor is maintainedin a stable position without contact during operation, by magnetic forces.Absolutely no lubricants are required. So-called touch down bearings areintegrated for shutdown.Fig. 2.52 shows a sectional drawing of a typical turbomolecular pump. Thepump is an axial flow compressor of vertical design, the active or pumpingpart of which consists of a rotor (6) and a stator (2). Turbine blades arelocated around the circumferences of the stator and the rotor.
Each rotor Ðstator pair of circular blade rows forms one stage, so that the assembly iscomposed of a multitude of stages mounted in series. The gas to bepumped arrives directly through the aperture of the inlet flange (1), that is,without any loss of conductance, at the active pumping area of the topblades of the rotor Ð stator assembly. This is equipped with blades ofespecially large radial span to allow a large annular inlet area. The gascaptured by these stages is transferred to the lower compression stages,whose blades have shorter radial spans, where the gas is compressed tobacking pressure or rough vacuum pressure.
The turbine rotor (6) ismounted on the drive shaft, which is supported by two precision ballbearings (8 and 11), accommodated in the motor housing. The rotor shaft isdirectly driven by a medium-frequency motor housed in the forevacuumspace within the rotor, so that no rotary shaft lead-through to the outsideatmosphere is necessary. This motor is powered and automaticallycontrolled by an external frequency converter, normally a solid-statefrequency converter that ensures a very low noise level. For specialapplications, for example, in areas exposed to radiation, motor generatorfrequency converters are used.2Turbomolecularpump stage34Siegbahn stage567811234High vacuum inlet flangeStator packVenting flangeForevacuum flangeFig.
2.525678Splinter guardRotorPump casingBall bearings9 Cooling water connection10 3-phase motor11 Ball bearingsSchematic diagram of a grease lubricated TURBOVAC 151 turbomolecular pump1 Vacuum port2 High vacuum flange3 Rotor4 Stator5 Bearing6 Motor7 Fan8 BearingFig. 2.52a Cross section of a HY.CONE turbomolecular pump47HomeVacuum generation10000l á sÐ1` á sÐ11000/1000 MC6001000500S340M20036115110050/5510ñ621046 8ñ510ñ410pñ3ñ21010mbarñ110Fig.
2.53 Pumping speed for air of different turbomolecular pumpsFig. 2.54 Pumping speed curves of a TURBOVAC 600 for H2, He, N2 and ArThe vertical rotor Ð stator configuration provides optimum flow conditions ofthe gas at the inlet.component at the forevacuum flange of the pump and that at the highvacuum flange: maximum compression k0 is to be found at zero throughput.For physical reasons, the compression ratio of turbomolecular pumps isvery high for heavy molecules but considerably lower for light molecules.The relationship between compression and molecular mass is shown in Fig.2.55. Shown in Fig.
2.56 are the compression curves of a TURBOVAC 340M for N2, He and H2 as a function of the backing pressure. Because of thehigh compression ratio for heavy hydrocarbon molecules, turbomolecularpumps can be directly connected to a vacuum chamber without the aid ofone or more cooled baffles or traps and without the risk of a measurablepartial pressure for hydrocarbons in the vacuum chamber (hydrocarbon-freevacuum! Ð see also Fig. 2.57: residual gas spectrum above a TURBOVAC361). As the hydrogen partial pressure attained by the rotary backing pumpTo ensure vibration-free running at high rotational speeds, the turbine isdynamically balanced at two levels during its assembly.The pumping speed (volume flow rate) characteristics of turbomolecularpumps are shown in Fig.
2.53. The pumping speed remains constant overthe entire working pressure range. It decreases at intake pressures above10-3 mbar, as this threshold value marks the transition from the region ofmolecular flow to the region of laminar viscous flow of gases. Fig. 2.54shows also that the pumping speed depends on the type of gas.The compression ratio (often also simply termed compression) ofturbomolecular pumps is the ratio between the partial pressure of one gask0MFig. 2.55 TURBOVAC 450 Ð Maximum compression k0 as a function of molar mass MFig. 2.56 Maximum compression k0 of a turbomolecular pump TURBOVAC 340 M for H2, Heand N2 as a function of backing pressure48HomeVacuum generationM = Mass number = Relative molar mass at an ionization 1I = Ion currentFig.
2.57 Spectrum above a TURBOVAC 361Fig. 2.58 Determination of the cut-in pressure for turbomolecular pumps when evacuating largeis very low, the turbomolecular pump is capable of attaining ultimatepressures in the 10-11 mbar range in spite of its rather moderatecompression for H2. To produce such extremely low pressures, it will, ofcourse, be necessary to strictly observe the general rules of UHVtechnology: the vacuum chamber and the upper part of the turbomolecularpump must be baked out, and metal seals must be used. At very lowpressures the residual gas is composed mainly of H2 originating from themetal walls of the chamber. The spectrum in Fig.
2.57 shows the residualgas composition in front of the inlet of a turbomolecular pump at anultimate pressure of 7 · 10-10 mbar nitrogen equivalent. It appears that theportion of H2 in the total quantity of gas amounts to approximately 90 to 95% . The fraction of ÒheavierÓ molecules is considerably reduced and massesgreater than 44 were not detected. An important criterion in the assessmentof the quality of a residual gas spectrum are the measurable hydrocarbonsfrom the lubricants used in the vacuum pump system.
Of course anÒabsolutely hydrocarbon-free vacuumÓ can only be produced with pumpsystems which are free of lubricants, i.e. for example with magneticallysuspended turbomolecular pumps and dry compressing backing pumps.When operated correctly (venting at any kind of standstill) no hydrocarbonsare detectable also in the spectrum of normal turbomolecular pumps.Information on the operation of turbomolecular pumpsStartingAs a rule turbomolecular pumps should generally be started together withthe backing pump in order to reduce any backstreaming of oil from thebacking pump into the vacuum chamber.
A delayed start of theturbomolecular pump, makes sense in the case of rather small backingpump sets and large vacuum chambers. At a known pumping speed for thebacking pump SV (m3/h) and a known volume for the vacuum chamber (m3)it is possible to estimate the cut-in pressure for the turbomolecular pump:A further development of the turbomolecular pump is the hybrid orcompound turbomolecular pump. This is actually two pumps on a commonshaft in a single casing. The high vacuum stage for the molecular flowregion is a classic turbomolecular pump, the second pump for the viscousflow range is a molecular drag or friction pump.LEYBOLD manufactures pumps such as the TURBOVAC 55 with anintegrated Holweck stage (screw-type compressor) and, for example, theHY.CONE 60 or HY.CONE 200 with an integrated Siegbahn stage (spiralcompressor). The required backing pressure then amounts to a few mbarso that the backing pump is only required to compress from about 5 to 10mbar to atmospheric pressure.
A sectional view of a HY.CONE is shown inFig. 2.52a.Simultaneous start whenSv> 40 h−1Vand delayed start whenSv< 40 h−1Vat a cut-in pressure of: SV pV, Start = e 6 · V mbar(2.24)When evacuating larger volumes the cut-in pressure for turbomolecularpumps may also be determined with the aid of the diagram of Fig. 2.58.VentingAfter switching off or in the event of a power failure, turbomolecularpumps should always be vented in order to prevent any backdiffusion ofhydrocarbons from the forevacuum side into the vacuum chamber. Afterswitching off the pump the cooling water supply should also be switched offto prevent the possible condensation of water vapor.
In order to protect therotor it is recommended to comply with the (minimum) venting times statedin the operating instructions. The pump should be vented (except in thecase of operation with a barrier gas) via the venting flange which alreadycontains a sintered metal throttle, so that venting may be performed using anormal valve or a power failure venting valve.49HomeVacuum generationBarrier gas operationIn the case of pumps equipped with a barrier gas facility, inert gas Ð suchas dry nitrogen Ð may be applied through a special flange so as to protectthe motor space and the bearings against aggressive media.
A specialbarrier gas and venting valve meters the necessary quantity of barrier gasand may also serve as a venting valve.Decoupling of vibrationsTURBOVAC pumps are precisely balanced and may generally beconnected directly to the apparatus. Only in the case of highly sensitiveinstruments, such as electron microscopes, is it recommended to installvibration absorbers which reduce the present vibrations to a minimum. Formagnetically suspended pumps a direct connection to the vacuumapparatus will usually do because of the extremely low vibrations producedby such pumps.For special applications such as operation in strong magnetic fields,radiation hazard areas or in a tritium atmosphere, please contact ourTechnical Sales Department which has the necessary experience andwhich is available to you at any time.1234Inlet portDegassing portSupportPump body5 Thermal conductingvanes6 Adsorption material(e.g. Zeolith)Fig.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
