Fundamentals of Vacuum Technology (1248463), страница 13
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From the diagram it is apparent that in thecase of isochoric compression the work done on compression must beincreased, but cold gas instead of hot exhaust gas is used for venting. Thismethod of direct gas cooling results in considerably reduced rotortemperatures. Pumps of this kind are discussed as ÒALLáexÓ in Section2.1.3.2.2.2.1.3.2.1Fig. 2.29a Vacuum diagram for the DRYVAC Bwith perfluoropolyether (PFPE).
The gear box is virtually hermetically sealedfrom the pumping chamber by piston rings and a radial shaft seal. Thebearings in the upper end disk are lubricated with PFPE grease. In order toprotect the bearings and shaft seals against aggressive substances, abarrier gas facility is provided. A controlled water cooling system allows thecontrol of the casing temperature over a wide range as the pump issubjected to differing gas loads coming from the process. The four stagedesign is available in several pumping speed and equipment grades of 25,50 and 100 m3/h DRYVAC pumps:a) as the basic version for non-aggressive clean processes:DRYVAC 25 B, 50 B and 100 B (Fig.
2.29a)b) as a version for semiconductor processes: DRYVAC 25 P, 50 P and100 P (Fig. 2.29b)c) as a system version with integrated self monitoring: DRYVAC 50 S and100 SCasingsuctionIntakeline100 PCooling waterClaw pumps with internal compressionfor the semiconductor industry(“DRYVAC series”)Design of DRYVAC PumpsDue to the work done on compression in the individual pumping stages,multi-stage claw pumps require water cooling for the four stages to removethe compression heat.
Whereas the pumping chamber of the pump is freeof sealants and lubricants, the gear and the lower pump shaft are lubricatedExhaust lineInert gasFig. 2.29bVacuum diagram for the DRYVAC P33HomeVacuum generationTo be provided by the customerTSHTemperature switchPSLPressure switchPSHPressure switchFSLFlow switchMPSMotor protection switchPT 100 Temperature sensorFor DRYVAC with LIMSCSCurrent sensorEPSExhaust pressure sensorFig. 2.29c Vacuum diagram for the DRYVAC SFig. 2.30 Key to Figures 2.29a Ð 2.29cd) as a system version with integrated self monitoring offering anincreased pumping speed in the lower pressure range: DRYVAC 251 Sand 501 S (Fig. 2.29c)thus the formation of layers within the pump and reduces the risk that theclaw rotor may seize. Care must be taken to ensure that the velocity of thegas flow within the individual pumping stages is at all times greater than thesettling speed of the particles entrained in the gas flow.
As can be seen inFig. 2.31, the settling speed of the particles depends strongly on their size.The mean velocity (VGas) of the flowing gas during the compression phaseis given by the following equation:The ultimate pressure attainable with the DRYVAC 251 S or 501 S is Ðcompared to the versions without integrated Roots pump Ð byapproximately one order of magnitude lower (from 2 · 10-2 mbar to3 · 10-3 mbar) and the attainable throughput is also considerably increased.It is of course possible to directly flange mount LEYBOLD RUVAC pumpson to the DRYVAC models (in the case of semiconductor processes alsomostly with a PFPE oil filling for the bearing chambers).The pumps of the DRYVAC family are the classic dry compressing clawvacuum pumps that are preferably used in the semiconductor industry,whereby the pumps need to meet a variety of special requirements.
Insemiconductor processes, as in many other vacuum applications, theformation of particles and dusts during the process and/or in the course ofcompressing the pumped substances to atmospheric pressure within thepump, is unavoidable. In the case of vacuum pumps operating on the clawprinciple it is possible to convey particles through the pump by means of socalled Òpneumatic conveyingÓ. This prevents the deposition of particles andvGas=q pV mbar · `· s −1·=p · A mbar · cm2(2.22)10 · q pV m=·p·A sqpV = gas throughputp = pressureA = surface areaOne can see that with increasing pressure the velocity of the pumped gasflow slows down and attains the order of magnitude of the settling speed ofthe particles in the gas flow (Fig.
2.32). This means that the risk ofStage 22 500 mbar · `/sStage 3Stage 48 300 mbar · `/s 20 000 mbar · `/sGas velocityLimit speedParticle sizePressureThroughput in mbar · `/sThroughput cross section 6.5 cm2, constantPressureFig. 2.31 Settling speed as a function of pressure p. Parameter: particle sizeFig. 2.32 Mean gas velocity vg during compression without purge gas (left) and with purge gas(right) in stages 2, 3 and 434HomeVacuum generation• Losses in pumping speed and a reduction in ultimate pressure can bekept very small due to the special way in which the gas is made to passthrough the pump.Ultimate pressurembar2.1.3.2.2Purge gas flowClaw pumps without internalcompression for chemistry applications(“ALL·ex” )Fig. 2.33 Ultimate pressure of the DRYVAC 100S as a function of pure gas flow in stages 2 Ð 4depositing particles in the operating chamber of the pump and the resultingimpairment increases with increasing pressure.
In parallel to this thepotential for the formation of particles from the gaseous phase increases atincreasing compression levels. In order to keep the size of the formingparticles small and thus their settling speed low and to maintain a highvelocity for the gas, an additional quantity of gas is supplied into the pumpvia the individual intermediate discs (purge gas).
For this, the inflowingquantity of purge gas is matched to the pressure conditions prevailing in theindividual pumping stages (see top right part of Fig. 2.32). This keeps thevelocity of the gas flow high enough within the entire pump by so-calledpneumatic pumping. Through the way in which the gas is lead within thepump, i.e.
from the intake through the four pumping stages with the relatedintermediate discs to the exhaust, it is possible to reduce the influence ofthe purge gas on the ultimate pressure to a minimum. Test results (Fig.2.33) indicate that the influence of purge gas in the fourth stage is Ð as tobe expected Ð of the lowest level since there are located between thisstage and the intake side the three other pumping stages. The admission ofpurge gas via the second and third stages (Fig. 2.33) has a comparativelysmall influence on the ultimate pressure as can be seen from the pumpingspeed curve in Fig. 2.34.
Finally it can be said that the formation of particlesis to be expected in most CVD processes. When using dry compressingclaws vacuum pumps, the controlled admission of purge gas via theindividual intermediate discs is the best approach to avoid the formation oflayers. When applying this method several effects can be noted:• The admitted purge gas dilutes the pumped mixture of substances,particle-forming reactions will not occur, or are at least delayed.The chemical industry requires vacuum pumps which are highly reliable andwhich do not produce waste materials such as contaminated waste oil orwaste water.
If this can be done, the operating costs of such a vacuumpump are low in view of the measures otherwise required for protecting theenvironment (disposal of waste oil and water, for example). For operation ofthe simple and rugged ÒALLáexÓ pump from LEYBOLD there are norestrictions as to the vapor flow or the pressure range during continuousoperation. The ÒALLáexÓ may be operated within the entire pressure rangefrom 5 to 1000 mbar without restrictions.Design of the ÒALLáexÓ pumpThe design of the two-stage ALLáex is shown schematically in Fig.
2.35.The gas flows from top to bottom through the vertically arranged pumpingstages in order to facilitate the ejection of condensates and rinsing liquidswhich may have formed. The casing of the pump is water cooled andpermits cooling of the first stage. There is no sealed link between gaschamber and cooling channel so that the entry of cooling water into thepumping chamber can be excluded. The pressure-burst resistant design ofthe entire unit underlines the safety concept in view of protection againstinternal explosions, something which was also taken into account by directcooling with cold gas (see operating principle). A special feature of theÒALLáexÓ is that both shafts have their bearings exclusively in the gear. Onthe pumping side, the shafts are free (cantilevered).
This simple designallows the user to quickly disassemble the pump for cleaning and servicingwithout the need for special tools.In order to ensure a proper seal against the process medium in thepumping chamber the shaft seal is of the axial face seal type Ð a sealing• The risk of an explosion through self-igniting substances is significantlyreduced.• Particles which have formed are conveyed pneumatically through thepumpIntake portMotor1st stageClawsPumping speed2nd stageCouplingAxial facesealsmbar á l/sStage 2Stage 3Stage 4Gear, completewith shafts andbearingsmbar á l/smbar á l/sPressureFig. 2.34 Pumping speed with and without purge gasFig. 2.35 Simple arrangement of the dry compression ÒALLáexÓ pump35HomeVacuum generation13425671 Motor2 Pump3 Intake port4 Exhaust port5 Exhaust coolertakes in the quantity of cold process gas needed for venting the pumpingchamber back into the compression space on its own. This process,however, has no influence on the pumping speed of the ÒALLáexÓ becausethe intake process has already ended when the venting process starts.Designing the cooler as a condenser allows for simple solvent recovery.The method of direct gas cooling, i.e.
venting of the pumping chamber withcold gas supplied from outside (instead of hot exhaust gas) results in thecase of the ÒALLáexÓ in rotor temperatures which are so low that mixturesof substances rated as ExT3 can be pumped reliably under all operatingconditions. The ÒALLáexÓ thus fully meets the requirements of the chemicalindustry concerning the protection against internal explosions. A certaindegree of liquid compatibility makes the ÒALLáexÓ rinseable, thus avoidingthe formation of layers in the pump, for example, or the capability ofdissolving layers which may already have formed respectively. The rinsingliquids are usually applied to the pump after completion of the connectedprocess (batch operation) or while the process is in progress during briefblocking phases.
Even while the ÒALLáexÓ is at standstill and while thepumping chamber is completely filled with liquid it is possible to start thispump up. Shown in Fig. 2.39 is the pumping speed characteristic of anÒALLáexÓ 250. This pump has a nominal pumping speed of 250 m3/h andan ultimate pressure of < 10 mbar. At 10 mbar it still has a pumping speedof 100 m3/h.
The continuous operating pressure of the pump may be ashigh as 1000 mbar; it consumes 13.5 kW of electric power.6 Cooling water connection7 Cooling receiverFig. 2.36 ÒALLáexÓ pumpconcept well proven in chemistry applications. This type of seal is capableof sealing liquids against liquids, so that the pump becomes rinseable andinsensitive to forming condensate. Fig.
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