Fundamentals of Vacuum Technology (1248463), страница 22
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On a surface cooled to Å 10 K all gases except Heand Ne may be pumped by way of condensation. A surface cooled withliquid helium (T Å 4.2 K) is capable of condensing all gases except helium.132In continuous flow cryopumps the cold surface is designed to operate asa heat exchanger. Liquid helium in sufficient quantity is pumped by anauxiliary pump from a reservoir into the evaporator in order to attain asufficiently low temperature at the cold surface (cryopanel).The liquid helium evaporates in the heat exchanger and thus cools downthe cryopanel.
The waste gas which is generated (He) is used in a secondheat exchanger to cool the baffle of a thermal radiation shield whichprotects the system from thermal radiation coming from the outside. Thecold helium exhaust gas ejected by the helium pump is supplied to a heliumrecovery unit. The temperature at the cryopanels can be controlled bycontrolling the helium flow.Today refrigerator cryopumps are being used almost exclusively (coldupon demand).
These pumps operate basically much in the same way as acommon household refrigerator, whereby the following thermodynamiccycles using helium as the refrigerant may be employed:1 Compressor unit2 Flexible pressure lines3 Cold head (withoutcondensation surfaces)Fig. 2.65 All items of a refrigerator cryopump54HomeVacuum generation2.1.9.2The cold head and its operating principle(Fig. 2.66)Within the cold head, a cylinder is divided into two working spaces V1 andV2 by a displacer.
During operation the right space V1 is warm and the leftspace V2 is cold. At a displacer frequency f the refrigerating power W of therefrigerator is:W = (V2,max Ð V2,min) á (pH Ð pN) á f(2.26)The displacer is moved to and fro pneumatically so that the gas is forcedthrough the displacer and thus through the regenerator located inside theV2 (cold)RegeneratorV2 (cold)RegeneratorV2 (cold)displacer.
The regenerator is a heat accumulator having a large heatexchanging surface and capacity, which operates as a heat exchangerwithin the cycle. Outlined in Fig. 2.66 are the four phases of refrigeration ina single-stage refrigerator cold head operating according to the GiffordMcMahon principle.The two-stage cold headThe series-manufactured refrigerator cryopumps from LEYBOLD use a twostage cold head operating according to the Gifford-McMahon principle (seeFig. 2.67). In two series connected stages the temperature of the helium isreduced to about 30 K in the first stage and further to about 10 K in theV1 (warm)DisplacerV1 (warm)DisplacerPhase 1:The displacer is at the left dead center; V2where the cold is produced has its minimumsize. Valve N remains closed, H is opened. Gasat the pressure pH flows through theregenerator into V2.
There the gas warms up bythe pressure increase in V1.Phase 2:Valve H remains open, valve N closed: thedisplacer moves to the right and ejects the gasfrom V1 through the regenerator to V2 where itcools down at the cold regenerator.; V2 has itsmaximum volume.V1 (warm)Phase 3:RegeneratorV2 (cold)DisplacerValve H is closed and the valve N to the lowpressure reservoir is opened. The gas expandsfrom pH to pN and thereby cools down.
Thisremoves heat from the vicinity and it istransported with the expanding gas to thecompressor.V1 (warm)Phase 4:RegeneratorDisplacerWith valve N open the displacer moves to theleft; the gas from V2,max flows through theregenerator, cooling it down and then flows intothe volume V1 and into the low pressurereservoir. This completes the cycle.Fig. 2.66 Refrigerating phases using a single-stage cold head operating according to the Gifford-McMahon process55HomeVacuum generation1 Electric connections and currentfeedthrough for the motor in the coldhead2 He high pressure connection3 He low pressure connection4 Cylinder, 1st stage5 Displacer, 1st stage6 Regenerator, 1st stage7 Expansion volume, 1st stage8 1st (cooling) stage(copper flange)9 Cylinder, 2nd stage10 Displacer, 2nd stage11 Regenerator, 2nd stage12 Expansion volume, 2nd stage13 2nd (cooling) stage(copper flange)14 Measurement chamber for thevapor pressure15 Control piston16 Control volume17 Control disk18 Control valve19 Gauge for the hydrogen vaporpressure thermometer20 Motor in the cold headFig.
2.67 Two-stage cold headsecond stage. The attainable low temperatures depend among other thingson the type of regenerator. Commonly copperbronze is used in theregenerator of the first stage and lead in the second stage. Other materialsare available as regenerators for special applications like cryostats forextremely low temperatures (T < 10 K). The design of a two-stage coldhead is shown schematically in Fig. 2.67. By means of a controlmechanism with a motor driven control valve (18) with control disk (17) andcontrol holes first the pressure in the control volume (16) is changed whichcauses the displacers (6) of the first stage and the second stage (11) tomove; immediately thereafter the pressure in the entire volume of thecylinder is equalized by the control mechanism.
The cold head is linked viaflexible pressure lines to the compressor.12345678High vacuum flangePump casingForevacuum flangeSafety valve for gas dischargeThermal radiation shieldBaffle2nd stage of the cold head (≈10 K);CryopanelsFig.
2.68 Design of a refrigerator cryopump (schematic)inside and polished as well as nickel plated on the outside. Under no-loadconditions the baffle and the thermal radiation shield (first stage) attain atemperature ranging between 50 to 80 K at the cryopanels and about 10 Kat the second stage. The surface temperatures of these cryopanels aredecisive to the actual pumping process. These surface temperaturesdepend on the refrigerating power supplied by the cold head, and thethermal conduction properties in the direction of the pumpÕs casing.
Duringoperation of the cryopump, loading caused by the gas and the heat ofcondensation results in further warming of the cryopanels. The surfacetemperature does not only depend on the temperature of the cryopanel, butalso on the temperature of the gas which has already been frozen on to thecryopanel. The cryopanels (8) attached to the second stage (7) of the coldhead are coated with activated charcoal on the inside in order to be able topump gases which do not easily condense and which can only be pumpedby cryosorption (see 2.1.9.4).2.1.9.42.1.9.3The refrigerator cryopumpFig.
2.68 shows the design of a cryopump. It is cooled by a two-stage coldhead. The thermal radiation shield (5) with the baffle (6) is closely linkedthermally to the first stage (9) of the cold head. For pressures below10-3 mbar the thermal load is caused mostly by thermal radiation. For thisreason the second stage (7) with the condensation and cryosorption panels(8) is surrounded by the thermal radiation shield (5) which is black on the9 1st stage of the cold head(≈ 50Ð80 K)10 Gauge for the hydrogen vaporpressure thermometer11 Helium gas connections12 Motor of the cold head with casingand electric connectionsBonding of gases to cold surfacesThe thermal conductivity of the condensed (solid) gases depends verymuch on their structure and thus on the way in which the condensate isproduced.
Variations in thermal conductivity over several orders ofmagnitude are possible! As the condensate increases in thickness, thermalresistance and thus the surface temperature increase subsequentlyreducing the pumping speed. The maximum pumping speed of a newlyregenerated pump is stated as its nominal pumping speed. The bondingprocess for the various gases in the cryopump is performed in three steps:first the mixture of different gases and vapors meets the baffle which is at a56HomeVacuum generationtemperature of about 80 K. Here mostly H2O and CO2 are condensed. Theremaining gases penetrate the baffle and impinge in the outside of thecryopanel of the second stage which is cooled to about 10 K. Here gaseslike N2, O2 or Ar will condense.
Only H2, He and Ne will remain. Thesegases can not be pumped by the cryopanels and these pass after severalimpacts with the thermal radiation shield to the inside of these panels whichare coated with an adsorbent (cryosorption panels) where they are bondedby cryosorption. Thus for the purpose of considering a cryopump the gasesare divided into three groups depending at which temperatures within thecryopump their partial pressure drops below 10-9 mbar:1st group:ps < 10-9 mbar at T Å 77K (LN2): H2O, CO22nd group:ps < 10-9 mbar at T Å 20K: N2, O2, Ar3rd group:ps < 10-9 mbar at T < 4.2K: H2, He, NeA difference is made between the different bonding mechanisms as follows:Cryocondensation is the physical and reversible bonding of gas moleculesthrough Van der Waals forces on sufficiently cold surfaces of the samematerial.
The bond energy is equal to the energy of vaporization of the solidgas bonded to the surface and thus decreases as the thickness of thecondensate increases as does the vapor pressure. Cryosorption is thephysical and reversible bonding of gas molecules through Van der Waalsforces on sufficiently cold surfaces of other materials. The bond energy isequal to the heat of adsorption which is greater than the heat ofvaporization. As soon as a monolayer has been formed, the followingmolecules impinge on a surface of the same kind (sorbent) and the processtransforms into cryocondensation. The higher bond energy forcryocondensation prevents the further growth of the condensate layerthereby restricting the capacity for the adsorbed gases. However, theadsorbents used, like activated charcoal, silica gel, alumina gel andmolecular sieve, have a porous structure with very large specific surfaceareas of about 106 m2/kg.
Cryotrapping is understood as the inclusion of alow boiling point gas which is difficult to pump such as hydrogen, in thematrix of a gas having a higher boiling point and which can be pumpedeasily such as Ar, CH4 or CO2. At the same temperature the condensatemixture has a saturation vapor pressure which is by several orders ofmagnitude lower than the pure condensate of the gas with the lower boilingpoint.2.1.9.5Pumping speed and position of thecryopanelsConsidering the position of the cryopanels in the cryopump, theconductance from the vacuum flange to this surface and also thesubtractive pumping sequence (what has already condensed at the bafflecan not arrive at the second stage and consume capacity there), thesituation arises as shown in Fig.
2.69.The gas molecules entering the pump produce the area related theoreticalpumping speed according the equation 2.29a with T = 293 K. The differentpumping speeds have been combined for three representative gases H2, N2and H20 taken from each of the aforementioned groups. Since water vaporis pumped on the entire entry area of the cryopump, the pumping speedHydrogenWater vaporNitrogenArea related conductance of the intake flange in l / s · cm2:43.914.711.8Area related pumping speed of the cryopump in l / s · cm2:13.214.67.1Ratio between pumping speed and conductance:30 %99 %60 %Fig. 2.69 Cryopanels Ð temperature and position define the efficiency in the cryopumpmeasured for water vapor corresponds almost exactly to the theoreticalpumping speed calculated for the intake flange of the cryopump. N2 on theother hand must first overcome the baffle before it can be bonded on to thecryocondensation panel.
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