Fundamentals of Vacuum Technology (1248463), страница 8
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Pumps where the pumping effect is based mainly on the diffusion ofgases into a gas-free high speed vapor jet (vapor pumps)4. Pumps which pump vapors by means of condensation (condensers) andpumps which pump permanent gases by way of condensation at verylow temperatures (cryopumps)5. Pumps which bond or incorporate gases by adsorption or absorption tosurfaces which are substantially free of gases (sorption pumps).A survey on these classes of vacuum pumps is given in the diagram ofTable 2.1.Vacuum pump(Operating principle)Gas transfervacuum pumpEntrapmentvacuum pumpPositive displacementvacuum pumpReciprocatingpositive displacementvacuum pumpDiaphragmvacuum pumpPistonvacuum pumpKineticvacuum pumpRotaryvacuum pumpDragvacuum pumpLiquid sealedvacuum pumpLiquid ringvacuum pumpRotary vanevacuum pumpMultiple vanevacuum pumpGaseous ringvacuum pumpTurbinevacuum pumpFluid entrainmentvacuum pumpIon transfervacuum pumpEjectorvacuum pumpAdsorptionpumpGetter pumpLiquid jetvacuum pumpBulk getter pumpAxial flowvacuum pumpGas jetvacuum pumpSublimationpumpRadial flowvacuum pumpVapor jetvacuum pumpGetter ion pumpRotary pistonvacuum pumpMolecular dragvacuum pumpDiffusion pumpEvaporation ion pumpRotary plungervacuum pumpTurbomolecular pumpSelf-purifyingdiffusion pumpSputter-ion pumpDry compressingvacuum pumpFractionatingdiffusion pumpCryopumpRootsvacuum pumpClawvacuum pumpDiffusion ejectorpumpCondenserScroll pumpTable 2.1 Classification of vacuum pumps19HomeVacuum generation2.1.1 Oscillation displacement vacuumpumps2.1.1.1 Diaphragm pumpsRecently, diaphragm pumps have becoming ever more important, mainlyfor environmental reasons.
They are alternatives to water jet vacuumpumps, since diaphragm pumps do not produce any waste water. Overall, adiaphragm vacuum pump can save up to 90 % of the operating costscompared to a water jet pump. Compared to rotary vane pumps, thepumping chamber of diaphragm pumps are entirely free of oil. By design,no oil immersed shaft seals are required.
Diaphragm vacuum pumps aresingle or multi-stage dry compressing vacuum pumps (diaphragm pumpshaving up to four stages are being manufactured). Here the circumferenceof a diaphragm is tensioned between a pump head and the casing wall(Fig. 2.1). It is moved in an oscillating way by means of a connecting rodand an eccentric. The pumping or compression chamber, the volume ofwhich increases and decreases periodically, effects the pumping action.The valves are arranged in such a way that during the phase where thevolume of the pumping chamber increases it is open to the intake line.During compression, the pumping chamber is linked to the exhaust line.The diaphragm provides a hermetic seal between the gear chamber andthe pumping chamber so that it remains free of oil and lubricants (drycompressing vacuum pump).
Diaphragm and valves are the onlycomponents in contact with the medium which is to be pumped. Whencoating the diaphragm with PTFE (Teflon) and when manufacturing the inletand exhaust valves of a highly fluorinated elastomer as in the case of theDIVAC from LEYBOLD, it is then possible to pump aggressive vapors andgases. It is thus well suited for vacuum applications in the chemistry lab.Due to the limited elastic deformability of the diaphragm only acomparatively low pumping speed is obtained. In the case of this pumpingprinciple a volume remains at the upper dead center Ð the so called ÒdeadspaceÓ Ð from where the gases can not be moved to the exhaust line.
Thequantity of gas which remains at the exhaust pressure expands into theexpanding pumping chamber during the subsequent suction stroke therebyfilling it, so that as the intake pressure reduces the quantity of inflowing newgas reduces more and more. Thus volumetric efficiency worsenscontinuously for this reason. Diaphragm vacuum pumps are not capable ofattaining a higher compression ratio than the ratio between Òdead spaceÓand maximum volume of the pumping chamber. In the case of single-stagediaphragm vacuum pumps the attainable ultimate pressure amounts toapproximately 80 mbar.
Two-stage pumps such as the DIVAC fromLEYBOLD can attain about 10 mbar (see Fig. 2.2), three-stage pumps canattain about 2 mbar and four-stage diaphragm pumps can reach about5·10-1 mbar.Diaphragm pumps offering such a low ultimate pressure are suited asbacking pumps for turbomolecular pumps with fully integrated Scroll stages(compound or wide range turbomolecular pumps, such as theTURBOVAC 55 from LEYBOLD). In this way a pump system is obtainedwhich is absolutely free of oil, this being of great importance tomeasurement arrangements involving mass spectrometer systems and leakdetectors. In contrast to rotary vane pumps this combination of pumps forleak detectors offers the advantage that naturally no helium is dissolved inthe diaphragm pump thereby entirely avoiding a possible build up of ahelium background.2.1.2 Liquid sealed rotary displacementpumps2.1.2.1 Liquid ring pumpsDue to the pumping principle and the simple design, liquid ring vacuumpumps are particularly suited to pumping gases and vapors which may alsocontain small amounts of liquid.
Air, saturated with water vapors or othergases containing condensable constituents, may be pumped withoutproblems. By design, liquid ring pumps are insensitive to any contaminationEXINa)b)c)d)(1)(2)(3)(4)Fig. 2.1Casing lidValvesLidDiaphragm disk(5)(6)(7)(8)1st stageDiaphragmDiaphragm support diskConnecting rodEccentric diskSchematic on the design of a diaphragm pump stage (Vacuubrand)2nd stageOpening and closing of the valves, path and pumping mechanism during foursubsequent phases of a turn of the connecting rod (a-d)Fig.
2.2Principle of operation for a two-stage diaphragm pump (Vacuubrand)20HomeVacuum generationConstant, minimum clearance a for the entire sealing passage b1 Rotor2 Rotor shaftFig. 2.33 Casing4 Intake channel5 Liquid ring6 Flex. discharge channelLiquid ring vacuum pump, schematic (Siemens)which may be present in the gas flow. The attainable intake pressures arein the region between atmospheric pressure and the vapor pressure of theoperating liquid used. For water at 15 ¡C it is possible to attain an operatingpressure of 33 mbar. A typical application of water ring vacuum pumps isventing of steam turbines in power plants.
Liquid ring vacuum pumps(Fig. 2.3) are rotary displacement pumps which require an operating liquidwhich rotates during operation to pump the gas. The blade wheel isarranged eccentrically in a cylindrical casing. When not in operation,approximately half of the pump is filled with the operating fluid.
In the axialdirection the cells formed by the blade wheel are limited and sealed off byÒcontrol discsÓ. These control discs are equipped with suction and ejectionslots which lead to the corresponding ports of the pump. After havingswitched on such a pump the blade wheel runs eccentrically within thecasing; thus a concentrically rotating liquid ring is created which at thenarrowest point fully fills the space between the casing and the blade wheeland which retracts from the chambers as the rotation continues.
The gas issucked in as the chambers empty and compression is obtained bysubsequent filling. The limits for the intake or discharge process are set bythe geometry of the openings in the control discs.Fig. 2.4Arrangement of the sealing passage in rotary vane pumpsalso known as Òduo sealÓAs shown in the classification Table 2.1, the oil sealed displacement pumpsinclude rotary vane and rotary plunger pumps of single and two-stage designas well as single-stage trochoid pumps which today are only of historicinterest. Such pumps are all equipped with a gas ballast facility which wasdescribed in detail (for details see 2.1.2.2.4) for the first time by Gaede(1935).
Within specified engineering limits, the gas ballast facility permitspumping of vapors (water vapor in particular) without condensation of thevapors in the pump.2.1.2.2.1Rotary vane pumps (TRIVAC A,TRIVAC B, TRIVAC E, SOGEVAC)Rotary vane pumps (see also Figs. 2.5 and 2.6) consist of a cylindricalhousing (pump-ing ring) (1) in which an eccentrically suspended and slottedrotor (2) turns in the direction of the arrow. The rotor has vanes (16) whichare forced outwards usually by centrifugal force but also by springs so thatIn addition to the task of compression, the operating fluid fulfills threefurther important tasks:1. Removal of the heat produced by the compression process.2. Uptake of liquids and vapors (condensate).3. Providing the seal between the blade wheel and the casing.2.1.2.2Oil sealed rotary displacement pumpsA displacement vacuum pump is generally a vacuum pump in which the gaswhich is to be pumped is sucked in with the aid of pistons, rotors, vanesand valves or similar, possibly compressed and then discharged.
Thepumping process is effected by the rotary motion of the piston inside thepump. Differentiation should be made between oiled and dry compressingdisplacement pumps. By the use of sealing oil it is possible to attain in asingle-stage high compression ratios of up to about 105. Without oil, ÒinnerleakinessÓ is considerably greater and the attainable compression ratio iscorrespondingly less, about 10.12345678Fig. 2.5Pump housingRotorOil-level sight glassSuction ductAnti-suckback valveDirt trapIntake portLid of gas ballast valve910111213141516Exhaust portAir inlet silencerOil filterExhaust valveExhaust ductGas ballast ductOil injectionVaneCross section of a single-stage rotary vane pump (TRIVAC A)21HomeVacuum generation1234567Fig. 2.6Intake portDirt trapAnti-suckback valveIntake ductVanePumping chamberRotor8 Orifice, connection forinert gas ballast9 Exhaust duct10 Exhaust valve11 Demister12 Spring13 Orifice; connection foroil filterCross section of a single-stage rotary vane pump (TRIVAC B)the vanes slide inside the housing.
Gas entering through the intake (4) ispushed along by the vanes and is finally ejected from the pump by the oilsealed exhaust valve (12).The older range of TRIVAC A pumps (Fig. 2.5) from LEYBOLD has threeradial vanes offset by 120¡. The TRIVAC B range (Fig. 2.6) has only twovanes offset by 180¡. In both cases the vanes are forced outwards by thecentrifugal forces without the use of springs.
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