Fundamentals of Vacuum Technology (1248463), страница 12
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The maximumpermissible pressure difference Æpmax is influenced by the following factors:forevacuum or compression pressure pV, pumping speed of the backingpump SV, speed of the Roots pump n, gradation kth and the adiabaticexponent κ of the pumped gas. Æpmax increases when pV and SV increaseand decreases when n and kth increase. Thus the maximum differencebetween forevacuum pressure and intake pressure, pV Ð pa must Ð duringcontinuous operation Ð not exceed a certain value depending on the type ofpump. Such values are in the range between 130 and 50 mbar. However,the maximum permissible pressure difference for continuous operation maybe exceed for brief periods.
In the case of special designs, which use gascooling, for example, high pressure differences are also permissible duringcontinuous operation.Maintaining the allowed pressure differenceIn the case of standard Roots pumps, measures must be introduced toensure that the maximum permissible pressure difference between intakeand exhaust port due to design constraints is not exceeded. This is doneeither by a pressure switch, which cuts the Roots pump in and outdepending on the intake pressure, or by using a pressure difference oroverflow valve in the bypass of the Roots pumps (Fig.
2.20 and 2.21). Theuse of an overflow valve in the bypass of the Roots pump is the better andmore reliable solution. The weight and spring loaded valve is set to themaximum permissible pressure difference of the particular pump. Thisensures that the Roots pump is not overloaded and that it may be operatedin any pressure range. In practice this means that the Roots pump can beswitched on, together with the backing pump, at atmospheric pressure. Inthe process any pressure increases will not adversely affect combinedoperation, i.e. the Roots pump is not switched off in such circumstances.Types of motors used with roots pumpsStandard flange-mounted motors are used as the drive.
The shaftfeedthroughs are sealed by two oil sealed radial shaft seals running on awear resistant bushing in order to protect the drive shaft. Flange motors ofany protection class, voltage or frequency may be used.Integral leak tightness of this version is < 10-4 mbar · l · s-1.In the case of better leak tightness requirements of < 10-5 mbar · l · s-1 theRoots pump is equipped with a canned motor. The rotor is seated in thevacuum on the drive shaft of the pump and is separated from the stator bya vacuum-tight non-magnetic tube. The stator coils are cooled by a fanhaving its own drive motor.
Thus shaft seals which might be subject to wearare no longer required. The use of Roots pumps equipped with cannedmotors is especially recommended when pumping high purity-, toxic- orradioactive gases and vapors.Fig. 2.20 Cross section of a Roots pumps with bypass line30HomeVacuum generationflows into the pumping chamber via the pre-admission channel. Finally therotors eject the pumped medium via the discharge port. The cooled gas,which in the case of single-stage compression is taken from theatmosphere and admitted from the pre-admission cooler, and which in thecase of multi-stage pump systems is taken from downstream gas coolers,performs a pre-compression and removes by Òinner coolingÓ the heat ofcompression at the point of time it occurs.WAU 20012.1.3.2SOGEVAC SV 1200Fig.
2.21 Vacuum diagram Ð Roots pump with integrated bypass line and backing pumpPre-admission cooling (Fig. 2.22)In the case of Roots pumps with pre-admission cooling, the compressionprocess basically is the same as that of a normal Roots pump. Sincegreater pressure differences are allowed more installed power is needed,which at the given speed and the pressure difference between inlet anddischarge port is directly proportional and is composed of the theoreticalwork done on compression and various power losses.
The compressionprocess ends normally after opening of the pumping chamber in thedirection of the discharge port. At this moment warmed gas at higherpressure flows into the pumping chamber and compresses the transportedvolume of gas. This compression process is performed in advance in thecase of pre-admission cooling. Before the rotor opens the pumpingchamber in the direction of the discharge port, compressed and cooled gasClaw pumpsLike Roots pumps, claw pumps belong to the group of dry compressingrotary piston vacuum pumps (or rotary vacuum pumps).
These pumps mayhave several stages; their rotors have the shape of claws.The design principle of a claw pump is explained by first using anexample of a four-stage design. The cross section inside the pumpÕs casinghas the shape of two partly overlapping cylinders (Fig. 2.23). Within thesecylinders there are two freely rotating rotors in each pump stage: (1) withtheir claws and the matching recesses rotating in opposing directions abouttheir vertical axes. The rotors are synchronized by a gear just like a Rootspump. In order to attain an optimum seal, the clearance between the rotorat the center of the casing and the amount of clearance with respect to theinside casing wall is very small; both are in the order of magnitude of a few0.01 mm.
The rotors periodically open and close the intake and dischargeslots (5) and (4). At the beginning of the work cycle in position a, the rightrotor just opens the intake slot (5). Gas now flows into the continuallyincreasing intake space (3) in position b until the right rotor seals off theintake slot (5) in position c. After both claws have passed through thecenter position, the gas which has entered is then compressed in thea1b44c321 Intake port2 Discharge port3 Gas coolerFig.
2.22 Diagram of a Roots pump with pre-admission cooling4 Flow of cold gas1234RotorsCompression chamberIntake spaceExhaust slot5 Intake slot6 Intermediate stage purgegasFig. 2.23 Principle of operation31HomeVacuum generationINOUT1 mbarStage 13 mbarPIN3 mbarOUT 15 mbarStage 2ZIN15 mbarOUT 150Stage 3PZIN150OUTStage 41000PZPFig. 2.25 Pressures in pump stages 1 to 4Fig. 2.24 Arrangement of the pumps and guiding of the gas flow. P = Pump stage Z =Intermediate ringcompression chamber (2) (position a) so long until the left rotor releasesthe discharge slot (4) (position b) thereby discharging the gas. Immediatelyafter the compression process has started (position a) the intake slot (5) isopened simultaneously and gas again flows into the forming intake space(3) (position b). Influx and discharge of the gas is performed during two halfperiods.
Each rotor turns twice during a full work cycle. Located betweenthe pumping stages are intermediate discs with flow channels which runfrom the discharge side of the upper stage to the intake side of the nextstage, so that all inlet or exhaust sides are arranged vertically above eachother (Fig.
2.24). Whereas in a Roots pump the incoming gas is pumpedthrough the pump at a constant volume and compression is only performedin the forevacuum line (see Section 2.1.3.1), the claw pump compressesthe gas already within the pumping chamber until the rotor releases thedischarge slot. Shown in Fig. 2.25 are the average pressure conditions inthe individual pumping stages of a DRYVAC at an intake pressure of 1mbar. In order to meet widely differing requirements LEYBOLDmanufactures two different series of claw pumps, which chiefly differ in thetype of compression process used:1) Pumps with internal compression, multi-stage for the semiconductorindustry (DRYVAC Series) and2) Pumps without internal compression, two-stage for chemistryapplications (ÒALLáexÓ).Figs. 2.26 and 2.27 demonstrate the differences in design.
Shown is thecourse of the pressure as a function of the volume of the pumping chamberby way of a pV diagram.Fig. 2.26 shows the (polytropic) course of the compression for pumps withPP12W Compr.1W Compr.2334VFig. 2.26 Compression curve for a claw pump with internal compressionVFig. 2.27 Compression curve for a claw pump without internal compression (ÒisochoriccompressionÓ)32HomeVacuum generation1CasingsuctionIntakeline2Cooling water3Exhaust line1 Intake port2 Front panel / electronics3 Main switchFig.
2.28 DRYVAC pumpinternal compression. The pressure increases until the discharge slot isopened. If at that point the exhaust back pressure has not been reached,the compression space is suddenly vented with hot exhaust gas. As thevolume is reduced further, the gas at exhaust pressure is ejected. The workdone on compression is represented by the area under the p-v curve1-2-3-4. It is almost completely converted into heat. In the case of drycompressing vacuum pumps not much of this heat can be lost to the cooledcasing due to the low density of the gas.
This results in high gastemperatures within the pump. Experiences with claw pumps show that thehighest temperatures occur at the rotors.Shown in Fig. 2.27 is the principle of isochoric compression in a p-vdiagram. Here the compression is not performed by reducing the volume ofthe pumping chamber, but by venting with cold gas which is applied fromthe outside after completion of the intake process. This is similar to theadmission of a gas ballast when opening the gas ballast valve aftercompletion of the intake phase.
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