The principle of the molecular pump - well known since 1913 - is that the gas particles to be pumped receive, through impact with the rapidly moving surfaces of a rotor, an impulse in a required flow direction. The surfaces of the rotor - usually disk-shaped - form, with the stationary surfaces of a stator, intervening spaces in which the gas is transported to the backing port. In the original Gaede molecular pump and its modifications, the intervening spaces (transport channels) were very narrow, which led to constructional difficulties and a high degree of susceptibility to mechanical contamination.
At the end of the Fifties, it became possible - through a turbine-like design and by modification of the ideas of Gaede - to produce a technically viable pump called the “turbomolecular pump”. The spaces between the stator and the rotor disks were made in the order of millimeters, so that essentially larger tolerances could be obtained. Thereby, greater security in operation was achieved. However, a pumping effect of any significance is only attained when the circumferential velocity (at the outside rim) of the rotor blades reaches the order of magnitude of the average thermal velocity of the molecules which are to be pumped. Kinetic gas theory supplies for c- o the equation 1.17:
in which the dependency on the type of gas as a function of molar mass M is contained. The calculation involving cgs-units (where R = 83.14 · 106 mbar · cm3 / mol · K) results in the following Table:
Whereas the dependence of the pumping speed on the type of gas is fairly low
the dependence of the compression k0 at zero throughput and thus also the compression k, because of
is greater as shown by the experimentally determined relationship in Fig. 2.55.
from theory it follows that
This agrees well, as expected (order of magnitude), with the experimentally determined value for k0 (N2) = 2.0 · 108 from Fig. 2.55. In view of the optimizations for the individual rotor stages common today, this consideration is no longer correct for the entire pump. Shown in Fig. 2.56 are the values as measured for a modern TURBOVAC 340 M.
In order to meet the condition, a circumferential velocity for the rotor of the same order of magnitude as c high rotor speeds are required for turbomolecular pumps. They range from about 36,000 rpm for pumps having a large diameter rotor (TURBOVAC 1000) to 72,000 rpm in the case of smaller rotor diameters (TURBOVAC 35 / 55). Such high speeds naturally raise questions as to a reliable bearing concept. Leybold offers three concepts, the advantages and disadvantages of which are detailed in the following:
+ Good compatibility with particles by circulating oil lubricant
- Can only be installed vertically
+ Low maintenance
+ Installation in any orientation
+ Suited for mobile systems
± Air cooling will do for many applications
+ Lubricated for life (of the bearings)
+ No wear
+ No maintenance
+ Absolutely free of hydrocarbons
+ Low noise and vibration levels
+ Installation in any orientation
Even a brief tear in the thin lubricating film between the balls and the races can - if the same type of material is used - result in microwelding at the points of contact. This severely reduces the service life of the bearings. By using dissimilar materials in so called hybrid bearings (races: steel, balls: ceramics) the effect of microwelding is avoided.
The most elegant bearing concept is that of the magnetic suspension. As early as 1976 Leybold delivered magnetically suspended turbomolecular pumps - the legendary series 550M and 560M. At that time a purely active magnetic suspension (i.e. with electromagnets) was used. Advances in electronics and the use of permanent magnets (passive magnetic suspension) based on the “System KFA Jülich” permitted the magnetic suspension concept to spread widely. In this system the rotor is maintained in a stable position without contact during operation, by magnetic forces. Absolutely no lubricants are required. So-called touch down bearings are integrated for shutdown.
Fig. 2.52 shows a sectional drawing of a typical turbomolecular pump. The pump is an axial flow compressor of vertical design, the active or pumping part of which consists of a rotor (6) and a stator (2). Turbine blades are located around the circumferences of the stator and the rotor. Each rotor - stator pair of circular blade rows forms one stage, so that the assembly is composed of a multitude of stages mounted in series. The gas to be pumped arrives directly through the aperture of the inlet flange (1), that is, without any loss of conductance, at the active pumping area of the top blades of the rotor – stator assembly. This is equipped with blades of especially large radial span to allow a large annular inlet area. The gas captured by these stages is transferred to the lower compression stages, whose blades have shorter radial spans, where the gas is compressed to backing pressure or rough vacuum pressure. The turbine rotor (6) is mounted on the drive shaft, which is supported by two precision ball bearings (8 and 11), accommodated in the motor housing. The rotor shaft is directly driven by a medium-frequency motor housed in the forevacuum space within the rotor, so that no rotary shaft lead-through to the outside atmosphere is necessary. This motor is powered and automatically controlled by an external frequency converter, normally a solid-state frequency converter that ensures a very low noise level. For special applications, for example, in areas exposed to radiation, motor generator frequency converters are used.
The vertical rotor - stator configuration provides optimum flow conditions of the gas at the inlet. To ensure vibration-free running at high rotational speeds, the turbine is dynamically balanced at two levels during its assembly.
The pumping speed (volume flow rate) characteristics of turbomolecular pumps are shown in Fig. 2.53. The pumping speed remains constant over the entire working pressure range. It decreases at intake pressures above 10-3 mbar, as this threshold value marks the transition from the region of molecular flow to the region of laminar viscous flow of gases. Fig. 2.54 shows also that the pumping speed depends on the type of gas.
The compression ratio (often also simply termed compression) of turbomolecular pumps is the ratio between the partial pressure of one gas component at the forevacuum flange of the pump and that at the high vacuum flange: maximum compression k0 is to be found at zero throughput. For physical reasons, the compression ratio of turbomolecular pumps is very 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 340 M for N2, He and H2 as a function of the backing pressure. Because of the high compression ratio for heavy hydrocarbon molecules, turbomolecular pumps can be directly connected to a vacuum chamber without the aid of one or more cooled baffles or traps and without the risk of a measurable partial pressure for hydrocarbons in the vacuum chamber (hydrocarbon-free vacuum! – see also Fig. 2.57: residual gas spectrum above a TURBOVAC 361). As the hydrogen partial pressure attained by the rotary backing pump is very low, the turbomolecular pump is capable of attaining ultimate pressures in the 10-11 mbar range in spite of its rather moderate compression for H2. To produce such extremely low pressures, it will, of course, be necessary to strictly observe the general rules of UHV technology: the vacuum chamber and the upper part of the turbomolecular pump must be baked out, and metal seals must be used. At very low pressures the residual gas is composed mainly of H2 originating from the metal walls of the chamber. The spectrum in Fig. 2.57 shows the residual gas composition in front of the inlet of a turbomolecular pump at an ultimate pressure of 7 · 10-10 mbar nitrogen equivalent. It appears that the portion of H2 in the total quantity of gas amounts to approximately 90 to 95%. The fraction of “heavier” molecules is considerably reduced and masses greater than 44 were not detected. An important criterion in the assessment of the quality of a residual gas spectrum are the measurable hydrocarbons from the lubricants used in the vacuum pump system. Of course, an “absolutely hydrocarbon-free vacuum” can only be produced with pump systems which are free of lubricants, for example with magnetically-suspended turbomolecular pumps and dry compressing backing pumps. When operated correctly (venting at any kind of standstill) no hydrocarbons are detectable also in the spectrum of normal turbomolecular pumps.
M = Mass number = Relative molar mass at an ionization 1
I = Ion current
A further development of the turbomolecular pump is the hybrid or compound turbomolecular pump. This is actually two pumps on a common shaft in a single casing. The high vacuum stage for the molecular flow region is a classic turbomolecular pump, the second pump for the viscous flow range is a molecular drag or friction pump.
Leybold manufactures pumps such as the TURBOVAC 55 with an integrated Holweck stage (screw-type compressor) and, for example, the HY. CONE 60 or HY. CONE 200 with an integrated Siegbahn stage (spiral compressor). The required backing pressure then amounts to a few mbar so that the backing pump is only required to compress from about 5 to 10 mbar to atmospheric pressure. A sectional view of a HY. CONE is shown in Fig. 2.52a.
As a rule turbomolecular pumps should generally be started together with the backing pump in order to reduce any backstreaming of oil from the backing pump into the vacuum chamber. A delayed start of the turbomolecular pump, makes sense in the case of rather small backing pump sets and large vacuum chambers. At a known pumping speed for the backing 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:
Simultaneous start when
and delayed start when
at a cut-in pressure of:
Simultaneous start when
and delayed start when
at a cut-in pressure of:
When evacuating larger volumes the cut-in pressure for turbomolecular pumps may also be determined with the aid of the diagram of Fig. 2.58.
After switching off or in the event of a power failure, turbomolecular pumps should always be vented in order to prevent any backdiffusion of hydrocarbons from the forevacuum side into the vacuum chamber. After switching off the pump the cooling water supply should also be switched off to prevent the possible condensation of water vapor. In order to protect the rotor, it is recommended to comply with the (minimum) venting times stated in the operating instructions. The pump should be vented (except in the case of operation with a barrier gas) via the venting flange which already contains a sintered metal throttle, so that venting may be performed using a normal valve or a power failure venting valve.
In the case of pumps equipped with a barrier gas facility, inert gas – such as dry nitrogen – may be applied through a special flange so as to protect the motor space and the bearings against aggressive media. A special barrier gas and venting valve meters the necessary quantity of barrier gas and may also serve as a venting valve.
TURBOVAC pumps are precisely balanced and may generally be connected directly to the apparatus. Only in the case of highly sensitive instruments, such as electron microscopes, is it recommended to install vibration absorbers which reduce the present vibrations to a minimum. For magnetically suspended pumps a direct connection to the vacuum apparatus will usually do because of the extremely low vibrations produced by such pumps.
For special applications such as operation in strong magnetic fields, radiation hazard areas or in a tritium atmosphere, please contact our Sales Department which has the necessary experience and is available to you at any time.
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