Fundamentals of Vacuum Technology (1248463), страница 25
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For this pressure range, a single-stagerotary pump is sufficient, if the chief working region lies above 10-1 mbar. Ifit lies between 10-1 and 10-2 mbar, a two-stage backing pump isrecommended. Below 10-2 mbar the pumping speed of single-stage Rootspumps in combination with two-stage rotary pumps as backing pumpsdecreases. However, between 10-2 and 10-4 mbar, two-stage Roots pumps(or two single-stage Roots pumps in series) with two-stage rotary pumps asbacking pumps still have a very high pumping speed.
Conversely, thispressure region is the usual working region for vapor ejector pumps. Forwork in this pressure region, they are the most economical pumps topurchase. As backing pumps, single-stage rotary positive displacementpumps are suitable. If very little maintenance and valveless operation areconvenient (i.e., small vessels in short operation cycles are to be pumpedto about 10-4 mbar or large vessels are to be maintained at this pressureunattended for weeks), the previously mentioned two-stage Roots pumpswith two-stage rotary pumps as backing pumps are the suitablecombinations. Allthough, such a combination does not work aseconomically as the corresponding vapor ejector pump, it can operate for amuch longer time without maintenance.c) High vacuum (10-3 to 10-7 mbar)Diffusion, sputter-ion, and turbomolecular pumps typically operate in thepressure region below 10-3 mbar.
If the working region varies during aprocess, different pumping systems must be fitted to the vessel. There arealso special diffusion pumps that combine the typical properties of adiffusion pump (low ultimate pressure, high pumping speed in the highvacuum region) with the outstanding properties of a vapor ejector pump(high throughput in the medium vacuum region, high critical backingpressure).
If the working region lies between 10-2 and 10-6 mbar, thesediffusion pumps are, in general, specially recommended.d) Ultrahigh vacuum (< 10-7 mbar)For the production of pressures in the ultrahigh vacuum region, sputter-ion,and sublimation pumps, as well as turbomolecular pumps and cryopumps,are used in combination with suitable forepumps. The pump best suited to aparticular UHV process depends on various conditions (for further details,see Section 2.5).2.2.3 Pumping of gases and vapors (wetprocesses)When vapors must be pumped, in addition to the factors working pressureand pumping speed, a third determining factor is added namely the vaporpartial pressure Ð which may vary considerably in the course of a process.This factor is decisive in determining the pumping arrangement to beinstalled.
In this regard, the condensers described in Section 2.15 are veryimportant adjuncts to rotary displacement pumps. They have a particularlyhigh pumping speed when pumping vapors. The next section coverspumping of water vapor (the most frequent case). The considerations applysimilarly to other non-aggressive vapors.Pumping of Water VaporWater vapor is frequently removed by pumps that operate with water orsteam as a pump fluid, for example, water ring pumps or steam ejectorpumps.
This depends considerably on circumstances, however, becausethe economy of steam ejector pumps at low pressures is generally farinferior to that of rotary pumps. For pumping a vapor Ð gas mixture in whichthe vapor portion is large but the air portion is small, the vapor can bepumped by condensers and the permanent gases, by relatively small gasballast pumps (see Section 2.1.5).Comparatively, then, a pump set consisting of a Roots pump, condenser,and backing pump, which can transport 100 kg/h of vapor and 18 kg/h of airat an inlet pressure of 50 mbar, has a power requirement of 4 Ð 10 kW(depending on the quantity of air involved).
A steam ejector pump of thesame performance requires about 60 kW without altering the quantity of airinvolved.62HomeVacuum generationSection 2.1.5. At a water vapor tolerance of 60 mbar, the lower limit of thisregion isWater vapor partial pressure pvpv > 6O + 0.46 pp mbar6p=0.4/ppvAir partial pressure ppFig. 2.73 Areas of application for gas ballast pumps and condensers pumping water vapor(o.G. = without gas ballast)For the pumping of water vapor, gas ballast pumps and combinations ofgas ballast pumps, Roots pumps, and condensers are especially suitable.Pumping of water vapor with gas ballast pumpsThe ratio of vapor partial pressure pv to air partial pressure pp is decisive inthe evaluation of the correct arrangement of gas ballast pumps, as shownpreviously by equations 2.2 and 2.3.
Therefore, if the water vapor toleranceof the gas ballast pump is known, graphs may be obtained that clearly givethe correct use of gas ballast pumps for pumping water vapor (see Fig.2.73). Large single-stage rotary plunger pumps have, in general, anoperating temperature of about 77 ¡C and hence a water vapor tolerance ofabout 60 mbar. This value is used to determine the different operatingregions in Fig. 2.73. In addition it is assumed that the pressure at thedischarge outlet port of the gas ballast pump can increase to a maximum of1330 mbar until the discharge outlet valve opens.Region A: Single-stage, rotary plunger pumps without gas ballastinlet.At a saturation vapor pressure pS of 419 mbar at 77 ¡C, according toequation 2.2, the requirement is given that pv < 0.46 pp, wherepv is the water vapor partial pressurepp is the partial pressure of airpv + pp = ptot total pressureThis requirement is valid in the whole working region of the single-stagerotary plunger pump Ð hence, at total pressures between 10-1 and 1013mbar.Region B: Single-stage rotary plunger pumps with gas ballast and aninlet condenser.In this region the water vapor pressure exceeds the admissible partialpressure at the inlet.
The gas ballast pump must, therefore, have acondenser inserted at the inlet, which is so rated that the water vaporpartial pressure at the inlet port of the rotary pump does not exceed theadmissible value. The correct dimensions of the condenser are selecteddepending on the quantity of water vapor involved. For further details, seeRegion C: Single-stage rotary plunger pumps with a gas ballast.The lower limit of region C is characterized by the lower limit of the workingregion of this pump.
It lies, therefore, at about ptot = 1 mbar. If largequantities of vapor arise in this region, it is often more economical to inserta condenser: 20 kg of vapor at 28 mbar results in a volume of about1000 m3. It is not sensible to pump this volume with a rotary pump. As arule of thumb:A condenser should always be inserted at the pumpÕs inlet if saturatedwater vapor arises for a considerable time.As a precaution, therefore, a Roots pump should always be inserted in frontof the condenser at low inlet pressures so that the condensation capacity isessentially enhanced.
The condensation capacity does not depend only onthe vapor pressure, but also on the refrigerant temperature. At low vaporpressures, therefore, effective condensation can be obtained only if therefrigerant temperature is correspondingly low. At vapor pressures below6.5 mbar, for example, the insertion of a condenser is sensible only if therefrigerant temperature is less than 0 ¡C. Often at low pressures a gas Ðvapor mixture with unsaturated water vapor is pumped (for further details,see Section 2.1.5).
In general, then, one can dispense with the condenser.Region D: Two-stage gas ballast pumps, Roots pumps, and vaporejector pumps, always according to the total pressure concerned inthe process.It must again be noted that the water vapor tolerance of two-stage gasballast pumps is frequently lower than that of corresponding single-stagepumps.Pumping of water vapor with roots pumpsNormally, Roots pumps are not as economical as gas ballast pumps forcontinuous operation at pressures above 40 mbar.
With very large pumpsets, which work with very specialized gear ratios and are provided withbypass lines, however, the specific energy consumption is indeed morefavorable. If Roots pumps are installed to pump vapors, as in the case ofgas ballast pumps, a chart can be given that includes all possible cases(see Fig. 2.74).Region A: A Roots pump with a single-stage rotary plunger pumpwithout gas ballast.As there is merely a compression between the Roots pump and the rotaryplunger pump, the following applies here too:pv < 0.46 ppThe requirement is valid over the entire working region of the pumpcombination and, therefore, for total pressures between 10-2 and 40 mbar(or 1013 mbar for Roots pumps with a bypass line).Region B: A main condenser, a Roots pump with a bypass line, anintermediate condenser, and a gas ballast pump.This combination is economical only if large water vapor quantities are tobe pumped continuously at inlet pressures above about 40 mbar.
The size63Homeof the main condenser depends on the quantity of vapor involved. Theintermediate condenser must decrease the vapor partial pressure below 60mbar. Hence, the gas ballast pump should be large enough only to preventthe air partial pressure behind the intermediate condenser from exceedinga certain value; for example, if the total pressure behind the Roots pump(which is always equal to the total pressure behind the intermediatecondenser) is 133 mbar, the gas ballast pump must pump at least at apartial air pressure of 73 mbar, the quantity of air transported to it by theRoots pump. Otherwise, it must take in more water vapor than it cantolerate.
This is a basic requirement: the use of gas ballast pumps iswise only if air is also pumped!With an ideally leak-free vessel, the gas ballast pump should be isolatedafter the required operating pressure is reached and pumping continuedwith the condenser only. Section 2.1.5 explains the best possiblecombination of pumps and condensers.Region C: A Roots pump, an intermediate condenser, and a gasballast pump.The lower limit of the water vapor partial pressure is determined throughthe compression ratio of the Roots pump at the backing pressure, which isdetermined by the saturation vapor pressure of the condensed water.
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