Fundamentals of Vacuum Technology (1248463), страница 26
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Also,in this region the intermediate condenser must be able to reduce the vaporpartial pressure to at least 60 mbar. The stated arrangement is suitable Ðwhen cooling the condenser with water at 15 ¡C Ð for water vaporpressures between about 4 and 40 mbar.Region D: A roots pump and a gas ballast pump.In this region D the limits also depend essentially on the stages and ratiosof sizes of the pumps. In general, however, this combination can always beused between the previously discussed limits Ð therefore, between 10-2 and4 mbar.2.2.4 Drying processesOften a vacuum process covers several of the regions quoted here.
Inbatch drying the process can, for example (see Fig. 2.74), begin in region A(evacuation of the empty vessel) and then move through regions B, C, andD in steps. Then the course of the process would be as follows:A. Evacuating the vessel by a gas ballast pump and a Roots pumpwith a bypass line.B. Connecting the two condensers because of the increasing vaporpressure produced by heating the material.The choice of the pumping System is decided by the highest vapor partialpressure occurring and the lowest air partial pressure at the inlet.C. Bypassing the main condenserIt will now not have an effect. Instead it would only be pumped empty bythe pumping system with a further drop in vapor pressure.D. Bypassing the intermediate condenser.Roots pumps and gas ballast pumps alone can now continue pumping.With short-term drying, the separation of the condenser filled withcondensed water is particularly important, because the gas ballast pumpwould continue to pump from the condenser the previously condensedWater vapor partial pressure pvVacuum generation6=0.4/p ppvAirAir partialpartialpressurepressureppppFig.
2.74 Areas of application for Roots pumps and condensers pumping water vapor (o.G. =without gas ballast)water vapor at the saturation vapor pressure of water.With longer-term drying processes, it suffices to shut off the condensatecollector from the condenser. Then only the remaining condensate film onthe cooling tubes can reevaporate. Depending on the size of the gas ballastpump, this reevaporation ensues in 30 Ð 60 min.E. If the drying process should terminate at still lower pressuresWhen a pressure below 10-2 mbar is reached a previously bypassed oilvapor ejector pump should be switched on in addition.Drying of solid substancesAs previously indicated, the drying of solid substances brings about a seriesof further problems.
It no longer suffices that one simply pumps out avessel and then waits until the water vapor diffuses from the solidsubstance. This method is indeed technically possible, but it wouldintolerably increase the drying time.It is not a simple technical procedure to keep the drying time as short aspossible. Both the water content and the layer thickness of the dryingsubstance are important. Only the principles can be stated here.
In case ofspecial questions we advise you to contact our experts in our Colognefactory.The moisture content E of a material to be dried of which the diffusioncoefficient depends on the moisture content (e.g. with plastics) as afunction of the drying time t is given in close approximation by the followingequation:E=E0(1 + K · t )q%(2.31)E0 where E is the moisture content before dryingq is the temperature-dependent coefficient. Thus equation (2.31) servesonly for the temperature at which q was determinedK is a factor that depends on the temperature, the water vapor partialpressure in the vicinity of the material, the dimensions, and the64HomeVacuum generationproperties of the material.With the aid of this approximate equation, the drying characteristics ofmany substances can be assessed.
If K and q have been determined forvarious temperatures and water vapor partial pressures, the values forother temperatures are easily interpolated, so that the course of the dryingprocess can be calculated under all operating conditions. With the aid of asimilarity transformation, one can further compare the course of dryingprocess of a material with known properties with that of a material withdifferent properties.Fundamentally, in the drying of a material, a few rules are noteworthy:Experience has shown that shorter drying times are obtained if the watervapor partial pressure at the surface of the material is relatively high, thatis, if the surface of the material to be dried is not yet fully free of moisture.This is possible because the heat conduction between the source of heatand the material is greater at higher pressures and the resistance todiffusion in a moist surface layer is smaller than in a dry one.
To fulfill theconditions of a moist surface, the pressure in the drying chamber iscontrolled. If the necessary relatively high water vapor partial pressurecannot be maintained permanently, the operation of the condenser istemporarily discontinued. The pressure in the chamber then increases andthe surface of the material becomes moist again.
To reduce the water vaporpartial pressure in the vessel in a controlled way, it may be possible toregulate the refrigerant temperature in the condenser. In this way, thecondenser temperature attains preset values, and the water vapor partialpressure can be reduced in a controlled manner.2.2.5 Production of an oil-free(hydrocarbon-free) vacuumBackstreaming vapor pump fluids, vapors of oils, rotary pump lubricants,and their cracking products can significantly disturb various workingprocesses in vacuum.
Therefore, it is recommended that certainapplications use pumps and devices that reliably exclude the presence ofhydrocarbon vapors.a) Rough vacuum region (1013 to 1 mbar)Instead of rotary pumps, large water jet, steam ejector, or water ring pumpscan be used. For batch evacuation, and the production of hydrocarbon-freefore vacuum for sputter-ion pumps, adsorption pumps (see Section 2.1.8.1)are suitable. If the use of oil-sealed rotary vane pumps cannot be avoided,basically two-stage rotary vane pumps should be used. The small amountof oil vapor that backstreams out of the inlet ports of these pumps can bealmost completely removed by a sorption trap (see Section 2.1.4) insertedin the pumping line.b) Medium vacuum region (1 to 10-3 mbar)For the pumping of large quantities of gas in this pressure region, vaporejector pumps are by far the most suitable.
With mercury vapor ejectorpumps, completely oil-free vacua can be produced. As a precaution, theinsertion of a cold trap chilled with liquid nitrogen is recommended so thatthe harmful mercury vapor does not enter the vessel. With the mediumvacuum sorption traps described under a), it is possible with two-stagerotary vane pumps to produce almost oil-free vacua down to below 10-4mbar.Absolutely oil-free vacua may be produced in the medium vacuum regionwith adsorption pumps. Since the pumping action of these pumps for thelight noble gases is only small, vessels initially filled with air can only beevacuated by them to about 10-2 mbar.
Pressures of 10-3 mbar or lower canthen be produced with adsorption pumps only if neither neon nor helium ispresent in the gas mixture to be pumped. In such cases it can be useful toexpel the air in the vessel by first flooding with nitrogen and then pumping itaway.c) High- and ultrahigh vacuum region (< 10-3 mbar)When there is significant evolution of gas in the pressure regions that mustbe pumped, turbomolecular pumps, or cryopumps should be used. Asputter-ion pump is especially suitable for maintaining the lowest possiblepressure for long periods in a sealed system where the process does notrelease large quantities of gas.
Magnetically-suspended turbomolecularpumps also guarantee hydrocarbon-free vacua. However, while thesepumps are switched off, oil vapors can enter the vessel through the pump.By suitable means (e.g., using an isolating valve or venting the vessel withargon), contamination of the vessel walls can be impeded when the pumpis stationary.
If the emphasis is on generating a Òhydrocarbon-free vacuumÓwith turbomolecular pumps, then hybrid turbomolecular pumps withdiaphragm pumps or classic turbomolecular pumps combined with scrollpumps should be used as oil-free backing pumps.2.2.6 Ultrahigh vacuum workingTechniquesThe boundary between the high and ultrahigh vacuum region cannot beprecisely defined with regard to the working methods. In practice, a borderbetween the two regions is brought about because pressures in the highvacuum region may be obtained by the usual pumps, valves, seals, andother components, whereas for pressures in the UHV region, anothertechnology and differently constructed components are generally required.The ÒborderÓ lies at a few 10-8 mbar.
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