Fundamentals of Vacuum Technology (1248463), страница 24
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Deviations also result when the cryopanels aresurrounded by a cooled baffle at which the velocity of the penetratingmolecules is already reduced by cooling.2.2Service life / duration of operation top (s): The duration of operation ofthe cryopump for a particular gas depends on the equation:Vacuum technology has undergone rapid development since the Fifties. Inresearch and in most branches of industry today, it is indispensable.t op, GCG =∫ QG (t)dtwith0CG = Capacity of the cryopump for the gas GQG(t) = Throughput of the cryopump for the gas at the point of time t__If the constant mean over time for the throughput QG is known, thefollowing applies:t op, G =CGC= GpQGG · SG(2.30)After the period of operation top,G has elapsed the cryopump must beregenerated with respect to the type of gas G.Starting pressure po: Basically it is possible to start a cryopump atatmospheric pressure.
However, this is not desirable for several reasons.As long as the mean free path of the gas molecules is smaller than thedimensions of the vacuum chamber (p > 10-3 mbar), thermal conductivity ofthe gas is so high that an unacceptably large amount of heat is transferredto the cryopanels. Further, a relatively thick layer of condensate would formon the cryopanel during starting. This would markedly reduce the capacityof the cryopump available to the actual operating phase. Gas (usually air)would be bonded to the adsorbent, since the bonding energy for this islower than that for the condensation surfaces. This would further reducethe already limited capacity for hydrogen.
It is recommended thatcryopumps in the high vacuum or ultrahigh vacuum range are started withthe aid of a backing pump at pressures of p < 5 · 10-2 mbar. As soon as thestarting pressure has been attained the backing pump may be switched off.Choice of pumping process2.2.1 Survey of the most usual pumpingprocessesCorresponding to the many areas of application, the number of technicalprocedures in vacuum processes is extraordinarily large.
These cannot bedescribed within the scope of this section, because the basic calculations inthis section cover mainly the pumping process, not the process taking placein the vessel. A survey of the most important processes in vacuumtechnology and the pressure regions in which these processes are chieflycarried out is given in the diagrams (Figures 2.71 and 2.72).Generally, the pumping operation for these processes can be divided intotwo categories Ð dry- and wet Ð vacuum procedures, that is, into processesin which no significant amounts of vapor have to be pumped and those inwhich vapors (mostly water or organic) arise.Distinctions between the two categories are described briefly:Dry processes work primarily in a narrow and limited pressure region.The system is usually evacuated to a suitable characteristic pressurebefore the actual working process begins.
This happens, for example, inplants for evaporative coating, electron-beam welding, and crystal pulling; inparticle accelerators, mass spectrometers, electron microscopes; andothers.Further, there are dry processes in which degassing in vacuum is the actualtechnical process. These include work in induction- and arc furnaces, steeldegassing plants, and plants for the manufacture of pure metals andelectron tubes.Wet processes are undertaken primarily in a prescribed workingoperation that covers a wider pressure region. This is especiallyimportant in the drying of solid materials. If, for instance, work is undertakenprematurely at too low a pressure, the outer surfaces dry out too quickly.
Asa result, the thermal contact to the moisture to be evaporated is impairedand the drying time is considerably increased. Predominantly processesthat are carried out in drying, impregnating, and freeze-drying plants belongin this category.In the removal of water vapor from liquids or in their distillation, particularlyin degassing columns, vacuum filling, and resin-casting plants, as well as inmolecular distillation, the production of as large a liquid surface as possibleis important.
In all wet processes the provision of the necessary heatfor evaporation of the moisture is of great importance.Basic pumping procedures are given in the following paragraphs.If you have specific questions, you should get in touch with a specialistdepartment in LEYBOLD where experts are available to you who can drawon many years of experience.60HomeVacuum generationUltrahigh vacuumHigh vacuumMedium vacuumRough vacuumMass spectrometersMolecular beam apparatusIon sourcesParticle acceleratorsElectron microscopesElectron diffraction apparatusVacuum spectographsLow-temperature researchProduction of thin filmsSurface physicsPlasma researchNuclear fusion apparatusSpace simulationMaterial researchPreparations forelectron microscopy10–1310–1010–710–3100103Pressure [mbar]Fig. 2.71 Pressure ranges (p < 1000 mbar) of physical and chemical analytical methodsClassifications of typical vacuum processes and plants according tothe pressure regionsRough vacuum 1013 mbar Ð 1 mbar• Drying, distillation, and steel degassing.Medium vacuum 1 -10-3 mbar• Molecular distillation, freeze-drying, impregnation, melting and castingfurnaces, and arc furnaces.High vacuum 10-3 Ð 10-7 mbarUltrahigh vacuum• Evaporative coating, crystal pulling, mass spectrometers, tubeproduction, electron microscopes, electron beam plants, and particleaccelerators.Ultrahigh vacuum: < 10-7 mbar• Nuclear fusion, storage rings for accelerators, space research, andsurface physics.High vacuumMedium vacuumRough vacuumAnnealing of metalsMelting of metalsDegassing of molten metalsSteel degassingElectron-beam meltingElectron-beam weldingEvaporation coatingSputtering of metalsZone melting and crystal growing in high-vacuumMolecular distillationDegassing of liquidsSublimationCasting of resins and lacquersDrying of plasticsDrying of insulating papersFreeze-drying of mass materialsFreeze-drying of pharmaceutical productsProduction of incandescent lampsProduction of electron tubesProduction of gas-discharge tubes10–1010–710–3100103Pressure [mbar]Fig.
2.72 Pressure ranges of industrial vacuum processes61HomeVacuum generation2.2.2 Pumping of gases (dry processes)For dry processes in which a non condensable gas mixture (e.g., air) is tobe pumped, the pump to be used is clearly characterized by the requiredworking pressure and the quantity of gas to be pumped away.
The choiceof the required working pressure is considered in this section. The choice ofthe required pump is dealt with in Section 2.3.Each of the various pumps has a characteristic working range in which ithas a particularly high efficiency.
Therefore, the most suitable pumps foruse in the following individual pressure regions are described. For everydry-vacuum process, the vessel must first be evacuated. It is quite possiblethat the pumps used for this may be different from those that are theoptimum choices for a process that is undertaken at definite workingpressures. In every case the choice should be made with particularconsideration for the pressure region in which the working processpredominantly occurs.a) Rough vacuum (1013 Ð 1 mbar)The usual working region of the rotary pumps described in Section 2 liesbelow 80 mbar.
At higher pressures these pumps have a very high powerconsumption (see Fig. 2.11) and a high oil consumption (see Section8.3.1.1). Therefore, if gases are to be pumped above 80 mbar over longperiods, one should use, particularly on economic grounds, jet pumps,water ring pumps or dry running, multi-vane pumps. Rotary vane and rotarypiston pumps are especially suitable for pumping down vessels fromatmospheric pressure to pressures below 80 mbar, so that they can workcontinuously at low pressures. If large quantities of gas arise at inletpressures below 40 mbar, the connection in series of a Roots pump isrecommended.
Then, for the backing pump speed required for the processconcerned, a much smaller rotary vane or piston pump can be used.b) Medium vacuum (1 Ð 10-3 mbar)If a vacuum vessel is merely to be evacuated to pressures in the mediumvacuum region, perhaps to that of the required backing pressure fordiffusion or sputter-ion pumps, single- and two-stage rotary pumps areadequate for pressures down to 10-1 and 10-3 mbar, respectively. It isessentially more difficult to select the suitable type of pump if mediumvacuum processes are concerned in which gases or vapors are evolvedcontinuously and must be pumped away.
An important hint may be given atthis point. Close to the attainable ultimate pressure, the pumping speed ofall rotary pumps falls off rapidly. Therefore, the lowest limit for the normalworking pressure region of these pumps should be that at which thepumping speed still amounts to about 50 % of the nominal pumping speed.Between 1 and 10-2 mbar at the onset of large quantities of gas, Rootspumps with rotary pumps as backing pumps have optimum pumpingproperties (see Section 2.1.3.1).
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