Fundamentals of Vacuum Technology (1248463), страница 27
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Therefore, pressures below 10-7 mbarshould generally be associated with the UHV region.The gas density is very small in the UHV region and is significantlyinfluenced by outgasing rate of the vessel walls and by the tiniest leakagesat joints. Moreover, in connection with a series of important technicalapplications to characterize the UHV region, generally the monolayer time(see also equation 1.21) has become important. This is understood as thetime τ that elapses before a monomolecular or monatomic layer forms onan initially ideally cleaned surface that is exposed to the gas particles.Assuming that every gas particle that arrives at the surface finds a freeplace and remains there, a convenient formula for τ isτ=3.2 – 6· 10 sp(p in mbar)Therefore, in UHV (p < 10-7 mbar) the monolayer formation time is of theorder of minutes to hours or longer and thus of the same length of time asthat needed for experiments and processes in vacuum.
The practicalrequirements that arise have become particularly significant in solid-statephysics, such as for the study of thin films or electron tube technology. AUHV system is different from the usual high vacuum system for thefollowing reasons:65HomeVacuum generationa) the leak rate is extremely small (use of metallic seals),b) the gas evolution of the inner surfaces of the vacuum vessel and of theattached components (e.g., connecting tubulation; valves, seals) can bemade extremely small,c) suitable means (cold traps, baffles) are provided to prevent gases orvapors or their reaction products that have originated from the pumpsused from reaching the vacuum vessel (no backstreaming).To fulfill these conditions, the individual components used in UHVapparatus must be bakeable and extremely leaktight. Stainless steel is thepreferred material for UHV components.The construction, start-up, and operation of an UHV system also demandsspecial care, cleanliness, and, above all, time.
The assembly must beappropriate; that is, the individual components must not be in the leastdamaged (i.e. by scratches on precision-worked sealing surfaces).Fundamentally, every newly-assembled UHV apparatus must be tested forleaks with a helium leak detector before it is operated. Especially importanthere is the testing of demountable joints (flange connections), glass seals,and welded or brazed joints. After testing, the UHV apparatus must bebaked out.
This is necessary for glass as well as for metal apparatus. Thebake-out extends not only over the vacuum vessel, but frequently also tothe attached parts, particularly the vacuum gauges. The individual stages ofthe bake-out, which can last many hours for a larger system, and the bakeout temperature are arranged according to the kind of plant and theultimate pressure required. If, after the apparatus has been cooled and theother necessary measures undertaken (e.g., cooling down cold traps orbaffles), the ultimate pressure is apparently not obtained, a repeated leaktest with a helium leak detector is recommended. Details on thecomponents, sealing methods and vacuum gauges are provided in ourcatalog.2.3.
Evacuation of a vacuumchamber and determination ofpump sizesBasically, two independent questions arise concerning the size of avacuum system:1. What effective pumping speed must the pump arrangement maintain toreduce the pressure in a given vessel over a given time to a desiredvalue?2. What effective pumping speed must the pump arrangement reach duringa vacuum process so that gases and vapors released into the vesselcan be quickly pumped away while a given pressure (the operatingpressure) in the vessel, is maintained and not exceeded?During the pumping-out procedure of certain processes (e.g., drying andheating), vapors are produced that were not originally present in thevacuum chamber, so that a third question arises:3. What effective pumping speed must the pump arrangement reach sothat the process can be completed within a certain time?The effective pumping speed of a pump arrangement is understood asthe actual pumping speed of the entire pump arrangement that prevailsat the vessel.
The nominal pumping speed of the pump can then bedetermined from the effective pumping speed if the flow resistance(conductances) of the baffles, cold traps, filters, valves, and tubulationsinstalled between the pump and the vessel are known (see Sections 1.5.2to 1.5.4). In the determination of the required nominal pumping speed it isfurther assumed that the vacuum system is leaktight; therefore, the leakrate must be so small that gases flowing in from outside are immediatelyremoved by the connected pump arrangement and the pressure in thevessel does not alter (for further details, see Section 5).
The questionslisted above under 1., 2. and 3. are characteristic for the three mostessential exercises of vacuum technology1. Evacuation of the vessel to reach a specified pressure.2. Pumping of continuously evolving quantities of gas and vapor at acertain pressure.3. Pumping of the gases and vapors produced during a process byvariation of temperature and pressure.Initial evacuation of a vacuum chamber is influenced in the medium-, high-,and ultrahigh vacuum regions by continually evolving quantities of gas,because in these regions the escape of gases and vapors from the walls ofthe vessel is so significant that they alone determine the dimensions andlayout of the vacuum system.2.3.1 Evacuation of a vacuum chamber(without additional sources of gas orvapor)Because of the factors described above, an assessment of the pump-downtime must be basically different for the evacuation of a container in therough vacuum region from evacuation in the medium- and high vacuumregions.66HomeVacuum generation2.3.1.1Evacuation of a chamber in the roughvacuum regionIn this case the required effective pumping speed Seff, of a vacuum pumpassembly is dependent only on the required pressure p, the volume V ofthe container, and the pump-down time t.With constant pumping speed Seff and assuming that the ultimate pressurepend attainable with the pump arrangement is such that pend << p, thedecrease with time of the pressure p(t) in a chamber is given by theequation:−dp Seff=·pVdtExample: A vacuum chamber having a volume of 500 l shall be pumpeddown to 1 mbar within 10 minutes.
What effective pumping speed isrequired?500 l = 0.5 m3; 10 min = 1/6 hAccording to equation (2.34) it follows that:0.51013· 2.3 · log1/ 61= 3 · 2.3 · 3.01 = 20.8 m3/hSeff =(2.32)Beginning at 1013 mbar at time t = 0, the effective pumping speed iscalculated depending on the pump-down time t from equation (2.32) asfollows:p dpSeff·t∫ p =−V1013`norders of magnitude lower than the desired pressure.Sp= − eff · t1013VV1013 V1013Seff = · `n= · 2.3 · logtppt(2.33a)(2.33b)(2.34)For the example given above one reads off the value of 7 from the straightline in Fig. 2.75.
However, from the broken line a value of 8 is read off.According to equation (2.35) the following is obtained:0.5· 7 = 21 m 3/ h or160.5·8 = 24 m 3/ hSeff =16Seff =under consideration of the fact that the pumping speed reduces below10 mbar. The required effective pumping speed thus amounts to about24 m3/h.Introducing the dimensionless factor10131013σ = `n p = 2.3 · log p(2.34a)into equation (2.34), the relationship between the effective pumping speedSeff, and the pump-down time t is given bySeff =V·σt(2.35)The ratio V/Seff is generally designated as a time constant τ. Thus thepump-down time of a vacuum chamber from atmospheric pressure to apressure p is given by:t=τ⋅σPressure →(2.36)withτ= Vandσ = `n 1013SeffpThe dependence of the factor from the desired pressure is shown in Fig.2.75. It should be noted that the pumping speed of single-stage rotary vaneand rotary piston pumps decreases below 10 mbar with gas ballast andbelow 1 mbar without gas ballast.
This fundamental behavior is different forpumps of various sizes and types but should not be ignored in thedetermination of the dependence of the pump-down time on pump size. Itmust be pointed out that the equations (2.32 to 2.36) as well Fig. 2.75 onlyapply when the ultimate pressure attained with the pump used is by severalDimensionless factor σFig.
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