Fundamentals of Vacuum Technology (1248463), страница 9
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At low ambient temperaturesthis possibly requires the use of a thinner oil. The A-Series is lubricatedthrough the arising pressure difference whereas the B-Series pumps havea geared oil pump for pressure lubrication. The TRIVAC B-Series isequipped with a particularly reliable anti-suckback valve; a horizontal orvertical arrangement for the intake and exhaust ports. The oil level sightglass and the gas ballast actuator are all on the same side of the oil box(user friendly design).
In combination with the TRIVAC BCS system it maybe equipped with a very comprehensive range of accessories, designedchiefly for semiconductor applications. The oil reservoir of the rotary vanepump and also that of the other oil sealed displacement pumps serves thepurpose of lubrication and sealing, and also to fill dead spaces and slots.
Itremoves the heat of gas compression, i.e. for cooling purposes. The oilprovides a seal between rotor and pump ring. These parts are ÒalmostÓ incontact along a straight line (cylinder jacket line). In order to increase theoil sealed surface area a so-called sealing passage is integrated into thepumping ring (see Fig. 2.4). This provides a better seal and allows a highercompression ratio or a lower ultimate pressure. LEYBOLD manufacturesthree different ranges of rotary vane pumps which are specially adapted todifferent applications such as high intake pressure, low ultimate pressure orapplications in the semiconductor industry.
A summary of the moreimportant characteristics of these ranges is given in Table 2.2. The TRIVACrotary vane pumps are produced as single-stage (TRIVAC S) and twostage (TRIVAC D) pumps (see Fig. 2.7). With the two-stage oil sealedpumps it is possible to attain lower operating and ultimate pressuresTRIVAC ATRIVAC BTRIVAC BCSTRIVAC ESOGEVACVanes per stage32223 (tangential)Pumping speed[m3/h]1 Ð 1.52Ð48 Ð 1630 Ð 601.64Ð816 Ð 2540 Ð 65Ð16 Ð 2540 Ð 65ÐÐ2.5ÐÐ16 Ð 2540 Ð 100180 Ð 280585 Ð 1200Sealing passageyesyesyesyesnoUltimate pressure,single-stage [mbar]< 2 á 10Ð2< 2 á 10Ð2< 2 á 10Ð2Ð< 5 á 10Ð1Ultimate pressuretwo-stage [mbar]< 2.5 á 10Ð4< 1 á 10Ð4< 1 á 10Ð4< 1 á 10Ð4ÐOil supplyPressure differenceGear pumpGear pumpEccentric pumpPressure differenceSlotsComparable for all types: about 0.01 to 0.05 mmBearing/lubricationAxial face / oilAxial face / oilAxial face / oilBall / greaseBall / oilSpecialcharacteristicsÐHydropneumaticanti-suckback valveCoated parts incontact with mediumManyaccessoriesCost-effectiveMediaNo ammoniaClean tolight particlesAggressive andcorrosiveClean tolight particlesCleanMain areas ofapplicationMultipurposeMultipurposeSemiconductorindustryMultipurposePackagingindustryTable 2.2 Rotary vacuum pump ranges22HomeVacuum generationValve stopLeafspringof thevalveI High vacuum stageII Second forevacuum stageFig.
2.7Cross section of a two-stage rotary vane pump, schematiccompared to the corresponding single-stage pumps. The reason for this isthat in the case of single-stage pumps, oil is unavoidably in contact with theatmosphere outside, from where gas is taken up which partially escapes tothe vacuum side thereby restricting the attainable ultimate pressure. In theoil sealed two-stage displacement pumps manufactured by LEYBOLD, oilwhich has already been degassed is supplied to the stage on the side ofthe vacuum (stage 1 in Fig. 2.7): the ultimate pressure lies almost in thehigh vacuum range, the lowest operating pressures lie in the rangebetween medium vacuum / high vacuum.
Note: operating the so called highvacuum stage (stage 1) with only very little oil or no oil at all will Ð in spiteof the very low ultimate pressure Ð in practice lead to considerabledifficulties and will significantly impair operation of the pump.2.1.2.2.2Fig. 2.8a Cross section of a two-stage rotary vane pump (TRIVAC E)through the intake channel of the slide valve (12) into the pumping chamber(14). The slide valve forms a unit with the piston and slides to and frobetween the rotatable valve guide in the casing (hinge bar 13). The gasdrawn into the pump finally enters the compression chamber (4). Whileturning, the piston compresses this quantity of gas until it is ejected throughthe oil sealed valve (5).
As in the case of rotary vane pumps, the oil reservoiris used for lubrication, sealing, filling of dead spaces and cooling. Since thepumping chamber is divided by the piston into two spaces, each turncompletes an operating cycle (see Fig. 2.10). Rotary plunger pumps aremanufactured as single and two-stage pumps. In many vacuum processescombining a Roots pump with a single-stage rotary plunger pump may offerRotary plunger pumps (E-Pumps)Shown in Fig. 2.9 is a sectional view of a rotary plunger pump of the singleblock type. Here a piston (2) which is moved along by an eccentric (3) turningin the direction of the arrow moves along the chamber wall. The gas which isto be pumped flows into the pump through the intake port (11), passes12345678Fig.
2.8b SOGEVAC pump SV 300 with three tangential vanesFig. 2.9CasingCylindrical pistonEccentricCompression chamberOil sealed pressure valveOil-level sight glassGas ballast channelExhaust pot91011121314Gas ballast valveDirt trapIntake portSlide valveHinge barPumping chamber(air is flowing in).Cross section of a single-stage rotary plunger pump (monoblock design)23HomeVacuum generation2.1.2.2.31 Upper dead point2 Slot in suction channel of slide valve isfreed Ð beginning of suction period3 Lower dead point Ð slot in suctionchannel is quite free, and pumped-in gas(arrow) enters freely into the pumpingchamber (shown shaded)4 Slot in suction channel is closed againby swivelling hinge bar Ð end of suctionperiod5 Upper dead point Ð maximum spacebetween rotating piston and stator6 Shortly before beginning ofcompression period, the front surface of therotating plunger frees gas ballast opening Ðcommencement of gas ballast inlet7 Gas ballast opening is quite free8 End of gas ballast inlet9 End of pumping period.Trochoid pumpsTrochoid pumps belong to the class of so called rotary piston pumps, which(see overview of Table 2.1) in turn belong to the group of rotary pumps.With rotary piston pumps, the pistonÕs center of gravity runs along a circularpath about the rotational axis (hence rotary piston machines).
A rotarypiston pump can Ð in contrast to the rotary plunger pump Ð be completelybalanced dynamically. This offers the advantage that larger pumps canoperate without vibration so that they can be installed without needingfoundations. Moreover, such pumps may be operated at higher speed,compared to rotary plunger pumps (see below).
The volume of the pumpingchamber with respect to the volume of the entire pump Ð the so calledspecific volume Ð is, in the case of trochoid pumps, approximately twice ofthat of rotary plunger pumps. Larger rotary plunger pumps run at speeds of500 rpm. The trochoid pump may run at 1000 rpm and this applies also tolarger designs. It is thus about four times smaller compared to a rotaryplunger pump having the same pumping speed and runs without producingany vibrations.
Unfortunately the advantages in the area of engineering arecombined with great disadvantages in the area of manufacturing, so thattoday LEYBOLD does not produce trochoid pumps any more. Operation ofsuch a pump is shown in the sectional diagram of Fig.
2.12.Fig. 2.10 Operating cycle of a rotary plunger pump (for positions 1 to 9 of the plunger)more advantages than a two-stage rotary plunger pump alone. If such acombination or a two-stage pump is inadequate, the use of a Roots pump inconnection with a two-stage pump is recommended. This does not apply tocombinations involving rotary vane pumps and Roots pumps.2.1.2.2.4The gas ballastMotor powerThe motors supplied with the rotary vane and rotary plunger pumps deliverenough power at ambient temperatures of 12 ¡C and when using ourspecial oils to cover the maximum power requirement (at about 400 mbar).Within the actual operating range of the pump, the drive system of thewarmed up pump needs to supply only about one third of the installedmotor power (see Fig.
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