Fundamentals of Vacuum Technology (1248463), страница 18
Текст из файла (страница 18)
The temperature and thevapor molecules adsorbed at the surface of the vessel determine exactlythe pressure in the vessel. If, the surfaces are not fully covered withadsorbed pump-fluid molecules after a bake-out process, their vaporpressure contributes only insignificantly to the pressure in the vessel.After a certain time, the Òstay-down timeÓ, a continuous layer of oilmolecules builds up, and the ultimate pressure is practically determined bythe vapor pressure of the pump fluid at the temperature of the vessel walls.This Òstay-downÓ time can even amount to several hours, indeed even todays, with the use of low-temperature baffles.Oil can reach the vessel not only as vapor, but also as a liquid film,because oil wets readily and thus creeps up the wall.By installation of an anticreep barrier (see Fig.
2.50) made of Teflonpolymer, a material that is not wetted by oil and can stand a bake-outtemperature up to 200 ¡C, further creeping of the oil can be effectivelyprevented. It is most appropriate to arrange the anticreep barrier above theupper baffle (see Fig. 2.50).1 Diffusion pumpwith cold cap baffle (cooledby contact),2 Shell or chevron baffle3 Anticreep barrier4567Sealing gasketBearing ringLN2 cold trap,Vacuum chamberFig.
2.50 Schematic arrangement of baffle, anticreep barrier and cold trap above a diffusionpumpcondensed, so that the inevitable loss of pump fluid from the condensationof the pump fluid on the low-temperature surface is kept at a minimum.With longer-term operation it is always advisable to install, in place of thecold cap, a water-cooled shell or chevron baffle between the diffusion pumpand the low-temperature baffle or cold trap (see Fig. 2.50).LEYBOLD manufactures cold traps made of metal so called LN2 coldtraps. These cold traps are to be used in those cases where a cold trap isto be operated for prolonged periods of time without requiring a filling facilityfor liquid nitrogen. The temperature increase at the vessel containing therefrigerant is so slight over the operating time that Ð as the liquid leveldrops Ð no significant desorption of the condensate takes place.
Located onthe pumping side is an impact panel made of copper. The low temperatureof this panel ensures that the greater part of the condensed pump fluidremains in the liquid state and may drip back into the pump. Today the oilsused to operate diffusion pumps have a very low vapor pressure at roomtemperature (for example DIFFELEN light, 2 · 10-8 mbar;DC 705, 4 · 10-10 mbar). The specified provisions with a liquid-nitrogencooled baffle or cold trap would enable an absolutely oil-free vacuum to beproduced.In practice, however, complete suppression of oil-backstreaming is neverattained.
There are always a few pump-fluid molecules that, as a result ofcollisions with one another, reach the vessel without having hit one of thecooled surfaces of the baffle or the cold trap. Moreover, there are always aNote:It must be noted that data on backstreaming as specified in catalogs applyonly to continuously-operated oil diffusion pumps.
Shortly after starting apump the uppermost nozzle will not eject a well directed vapor jet. Insteadoil vapor spreads in all directions for several seconds and thebackstreaming effect is strong. When switching a diffusion pump on and offfrequently the degree of oil brackstreaming will be greater.2.1.6.5Water jet pumps and steam ejectorsIncluded in the class of fluid-entrainment pumps are not only pumps thatuse a fast-streaming vapor as the pump fluid, but also liquid jet pumps. Thesimplest and cheapest vacuum pumps are water jet pumps.
As in a vaporpump (see Fig. 2.46 or 2.51), the liquid stream is first released from anozzle and then, because of turbulence, mixes with the pumped gas in themixing chamber. Finally, the movement of the water Ð gas mixture is sloweddown in a Venturi tube. The ultimate total pressure in a container that ispumped by a water jet pump is determined by the vapor pressure of thewater and, for example, at a water temperature of 15 ¡C amounts to about17 mbar.Essentially higher pumping speeds and lower ultimate pressures areproduced by steam ejector pumps. The section through one stage isshown in Fig.
2.51. The markings correspond with those shown in Fig. 2.46.In practice, several pumping stages are usually mounted in cascade. Forlaboratory work, two-stage pump combinations are suitable and consist of asteam ejector stage and a water jet (backing) stage, both made of glass.The water jet backing stage enables operation without other backingpumps. With the help of a vapor stream at overpressure, the vacuumchamber can be evacuated to an ultimate pressure of about 3 mbar. Thecondensate from the steam is led off through the drain attachment. Thewater jet stage of this pump is cooled with water to increase its efficiency.Steam ejector pumps are especially suitable for work in laboratories,particularly if very aggressive vapors are to be pumped.
Steam ejectorpumps, which will operate at a pressure of a few millibars, are especiallyrecommended for pumping laboratory distillation apparatus and similar45HomeVacuum generationrotor blades reaches the order of magnitude of the average thermal velocityof the molecules which are to be pumped. Kinetic gas theory supplies for -cothe equation 1.17:c=8· R ·Tπ ·Min which the dependency on the type of gas as a function of molar mass Mis contained. The calculation involving cgs-units (where R = 83.14 á 106mbar · cm3 / mol · K) results in the following Table:GasH2HeH2ONeCON2AirO2ArCO2CC13F (F11)1 Steam inlet2 Jet nozzle3 Diffuser4 Mixing region5 Connection to thevacuum chamberFig.
2.51 Schematic representation of the operation of a steam ejector pumpplants when the pressure from a simple water jet pump is insufficient. Inthis instance, the use of rotary pumps would not be economical.Even in spite of their low investment costs water jet pumps and steamejectors are being replaced in the laboratories more and more bydiaphragm pumps because of the environmental problems of using wateras the pump fluid. Solvent entering the water can only be removed againthrough complex cleaning methods (distillation).Table 2.4MolarMass M241820282828.96324044134.78Mean thermalvelocity (m/s)1761124558755747147146344039437568-c as a function of molar mass MWhereas _the dependence__ of the pumping speed on the type of gas is fairlylow (S ~ c ~ 1 / E M ), the dependence of the compression k0 at zerothroughput and thus also__the compression k, becauseof k0 ~ eEM log k0 ~ E M, is greater as shown by the experimentallydetermined relationship in Fig.
2.55.Example:from theory it follows thatlog k0(He)411===287 2.65log k0(N2)⇒ log k0(N 2) = 2.65 · log k0(He)2.1.7 Turbomolecular pumpsthis with k0 (He) = 3 · 103 from Fig. 2.55 results in:The principle of the molecular pump Ð well known since 1913 Ð is that thegas particles to be pumped receive, through impact with the rapidly movingsurfaces of a rotor, an impulse in a required flow direction. The surfaces ofthe rotor Ð usually disk-shaped Ð form, with the stationary surfaces of astator, intervening spaces in which the gas is transported to the backingport. In the original Gaede molecular pump and its modifications, theintervening spaces (transport channels) were very narrow, which led toconstructional difficulties and a high degree of susceptibility to mechanicalcontamination.This agrees Ð as expected Ð well (order of magnitude) with theexperimentally determined value for k0 (N2) = 2.0 · 108 from Fig.
2.55. Inview of the optimizations for the individual rotor stages common today, thisconsideration is no longer correct for the entire pump. Shown in Fig. 2.56are the values as measured for a modern TURBOVAC 340 M.At the end of the Fifties, it became possible Ð through a turbine-like designand by modification of the ideas of Gaede Ð to produce a technically viablepump the socalled Òturbomolecular pumpÓ. The spaces between thestator and the rotor disks were made in the order of millimeters, so thatessentially larger tolerances could be obtained. Thereby, greater security inoperation was achieved. However, a pumping effect of any significance isonly attained when the circumferential velocity (at the outside rim) of theIn order to meet the condition,_ a circumferential velocity for the rotor of thesame order of magnitude as c high rotor speeds are required forturbomolecular pumps.
They range from about 36,000 rpm for pumpshaving a large diameter rotor (TURBOVAC 1000) to 72,000 rpm in the caseof smaller rotor diameters (TURBOVAC 35 / 55). Such high speedsnaturally raise questions as to a reliable bearing concept. LEYBOLD offersthree concepts, the advantages and disadvantages of which are detailed inlog k0 (N2) = 2.65 · log (3 á 103) = 9.21or k0 (N2) = 1.6 · 109.46HomeVacuum generationthe following:• Oil lubrication / steel ball bearings+ Good compatibility with particles by circulating oil lubricant- Can only be installed vertically+ Low maintenance• Grease lubrication / hybrid bearings+ Installation in any orientation+ Suited for mobile systems± Air cooling will do for many applications+ Lubricated for life (of the bearings)• Free of lubricants / magnetic suspension+ No wear+ No maintenance+ Absolutely free of hydrocarbons+ Low noise and vibration levels+ Installation in any orientationSteel ball bearings / hybrid ball bearings (ceramic ball bearings): Evena brief tear in the thin lubricating film between the balls and the races can Ðif the same type of material is used Ð result in microwelding at the points ofcontact.
This severely reduces the service life of the bearings. By usingdissimilar 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. Asearly as 1976 LEYBOLD delivered magnetically suspended turbomolecularpumps Ð the legendary series 550M and 560M. At that time a purely activemagnetic suspension (i.e. with electromagnets) was used. Advances in -electronics and the use of permanent magnets (passive magneticsuspension) based on the ÒSystem KFA JŸlichÓ permitted the magneticsuspension concept to spread widely.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
