Fundamentals of Vacuum Technology (1248463), страница 14
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2.36 shows the componentssupplied with the ALL.ex, together with a gas cooler and a receiver.Operating principleIsochoric compression, which also serves the purpose of limiting thetemperature ultimately attained during compression, especially in the stageon the side of the atmosphere, and which ensures protection againstinternal explosions, is performed by venting the pumping chamber with coldgas from a closed refrigerating gas cycle (Fig.
2.37). Fig. 2.38-1 indicatesthe start of the intake process by opening the intake slot through the controledge of the right rotor. The process gas then flows into the intake chamberwhich increases in size. The intake process is caused by the pressuregradient produced by increasing the volume of the pumping chamber. Themaximum volume is attained after 3/4 of a revolution of the rotors (Fig.2.38-2).
After the end of the intake process, the control edge of the leftrotor opens the cold gas inlet and at the same time the control edge of theright rotor opens the intake slot (Fig. 2.38-3) once more. In Fig. 2.38-4 thecontrol edge of the left rotor terminates the discharge of the gas which hasbeen compressed to 1000 mbar with the cold gas; at the same time thecontrol edge of the right rotor completes an intake process again.The total emissions from the system are not increased by the largequantities of cold gas, since a closed refrigerating cycle is maintained byway of an externally arranged gas cooler and condenser (Fig.
2.37). Thehot exhaust gas is made to pass through the cooler and is partly returnedin the form of cold gas for pre-admission cooling into the pump. The pump36HomeVacuum generation1000mSaugvermˆgenPumping speedCooling gasExhaust gas3. h-110010864211246810100AnsaugdruckIntakepressureFig. 2.37 Circulation of the cold gas in the ÒALLáexÓ with cooler / condensermbar1000Fig.
2.39 Pumping speed characteristic of an ALLáex 250VmaxExhaust slot1Volume of the pumpchamber starts to increaseIntakeslotSuctionCold gas inletVmin100(10)1000 Pmbar(100)100(10)1000 Pmbar(100)Vmax2Volume of the pumpchamber at maximumEnd of suctionVminCold gas inletVmax3Volume of the pump chamberstars to decrease (withoutcompression). Pressureincrease to 1000 mbaronly by admitting cold gas.VminBeginning of admitting cold gas100(10)1000 Pmbar(100)100(10)1000 Pmbar(100)Vmax4Ejection of the mixturecomposed of suckedin gas and cold gas.Exhaust slotCold gas inletVminFig. 2.38 Diagrams illustrating the pumping principle of the ALLáex pump (claw pump without inner compression)37HomeVacuum generation2.1.4 Accessories for oil-sealedrotary displacement pumpsDuring a vacuum process, substances harmful to rotary pumps can bepresent in a vacuum chamber.Elimination of water vaporWater vapor arises in wet vacuum processes.
This can cause water to bedeposited in the inlet line. If this condensate reaches the inlet port of thepump, contamination of the pump oil can result. The pumping performanceof oil-sealed pumps can be significantly impaired in this way. Moreover,water vapor discharged through the outlet valve of the pump can condensein the discharge outlet line. The condensate can, if the outlet line is notcorrectly arranged, run down and reach the interior of the pump through thedischarge outlet valve. Therefore, in the presence of water vapor and othervapors, the use of condensate traps is strongly recommended. If nodischarge outlet line is connected to the gas ballast pump (e.g., withsmaller rotary vane pumps), the use of discharge filters is recommended.These catch the oil mist discharged from the pump.Some pumps have easily exchangeable filter cartridges that not only holdback oil mist, but clean the circulating pump oil.
Whenever the amount ofwater vapor present is greater than the water vapor tolerance of the pump,a condenser should always be installed between the vessel and the pump.(For further details, see Section 2.1.5)Elimination of dustSolid impurities, such as dust and grit, significantly increase the wear onthe pistons and the surfaces in the interior of the pump housing.
If there isa danger that such impurities can enter the pump, a dust separator or adust filter should be installed in the inlet line of the pump. Today not onlyconventional filters having fairly large casings and matching filter insertsare available, but also fine mesh filters which are mounted in the centeringring of the small flange. If required, it is recommended to widen the crosssection with KF adaptors.Elimination of oil vaporThe attainable ultimate pressure with oil-sealed rotary pumps is stronglyinfluenced by water vapor and hydrocarbons from the pump oil.
Even withtwo-stage rotary vane pumps, a small amount of back-streaming of thesemolecules from the pump interior into the vacuum chamber cannot beavoided. For the production of hydrocarbon-free high and ultrahigh vacuum,for example, with sputter-ion or turbomolecular pumps, a vacuum as freeas possible of oil is also necessary on the forevacuum side of thesepumps. To obtain this, medium vacuum adsorption traps (see Fig. 2.40)filled with a suitable adsorption material (e.g., LINDE molecular sieve 13X)are installed in the inlet line of such oil-sealed forepumps. The mode ofaction of a sorption trap is similar to that of an adsorption pump. For furtherdetails, see Section 2.1.8.
If foreline adsorption traps are installed in theinlet line of oil-sealed rotary vane pumps in continuous operation, twoadsorption traps in parallel are recommended, each separated by valves.Experience shows that the zeolite used as the adsorption material losesmuch of its adsorption capacity after about 10 Ð 14 days of running time,after which the other, now-regenerated, adsorption trap can be utilized;hence the process can continue uninterrupted. By heating the adsorptiontrap, which is now not connected in the pumping line, the vapors escaping12345HousingBasket holding the sieveMolecular sieve (filling)Sealing flangesIntake port with small flange6 Upper section7 Vessel for the heater orrefrigerant8 Connection on the side of thepump with small flangeFig.
2.40 Cross section of a medium vacuum adsorption trapfrom the surface of the zeolite can be most conveniently pumped away withan auxiliary pump. In operation, pumping by the gas ballast pump generallyleads to a covering of the zeolite in the other, unheated adsorption trap andthus to a premature reduction of the adsorption capacity of this trap.Reduction of the effective pumping speedAll filters, separators, condensers, and valves in the inlet line reduce theeffective pumping speed of the pump. On the basis of the values of theconductances or resistances normally supplied by manufacturers, theactual pumping speed of the pump can be calculated. For further details,see Section 1.5.2.2.1.5 CondensersFor pumping larger quantities of water vapor, the condenser is the mosteconomical pump.
As a rule, the condenser is cooled with water of suchtemperature that the condenser temperature lies sufficiently below the dewpoint of the water vapor and an economical condensation or pumpingaction is guaranteed. For cooling, however, media such as brine andrefrigerants (NH3, Freon ) can also be used.When pumping water vapor in a large industrial plant, a certain quantity of38HomeVacuum generationFor a mathematical evaluation of the combination of condenser and gasballast pump, it can be assumed that no loss of pressure occurs in thecondenser, that the total pressure at the condenser entrance ptot 1, is equalto the total pressure at the condenser exit, ptot 2:ptot 1 = ptot 2(2.23)The total pressure consists of the sum of the partial pressure portions of theair pp and the water vapor pv:Fig.
2.41 Condenser (I) with downstream gas ballast pump (II) for pumping of large quantitiesof water vapor in the rough vacuum range (III) Ð adjustable throttleair is always involved, which is either contained in the vapor or originatesfrom leaks in the plant (the following considerations for air and water vaporobviously apply also in general for vapors other than water vapor).Therefore, the condenser must be backed by a gas ballast pump (see Fig.2.41) and hence always works Ð like the Roots pump Ð in a combination.The gas ballast pump has the function of pumping the fraction of air, whichis often only a small part of the water-vapor mixture concerned, withoutsimultaneously pumping much water vapor.
It is, therefore, understandablethat, within the combination of condenser and gas ballast pump in thestationary condition, the ratios of flow, which occur in the region of roughvacuum, are not easily assessed without further consideration. The simpleapplication of the continuity equation is not adequate because one is nolonger concerned with a source or sink-free field of flow (the condenser is,on the basis of condensation processes, a sink). This is emphasizedespecially at this point. In a practical case of Ònon-functioningÓ of thecondenser Ð gas ballast pump combination, it might be unjustifiable to blamethe condenser for the failure.In sizing the combination of condenser and gas ballast pump, the followingpoints must be considered:a) the fraction of permanent gases (air) pumped simultaneously with thewater vapor should not be too great.
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