Fundamentals of Vacuum Technology (1248463), страница 62
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The products with considerably higher vapor pressures which canbe created through oxidation are removed again in a short period of time bythe fractionation and degassing equipment in the pump (see Section2.1.6.1). Even though the pump fluid which was originally light in color hasbeen turned brown by air ingress, this need not necessarily mean that themedium has become unusable. If, on the other hand, the pump fluid hasturned cloudy and has become more viscous, as well (which may be thecase following periods of air ingress lasting for several minutes) then themedium will have to be replaced.
It is possible that under certaincircumstances cracking products from the pump fluid may make the oil inthe forepump unserviceable so that here, too, an oil change will have to bemade.Mercury diffusion and vapor-jet pumps are less sensitive to air ingress thanoil diffusion pumps.
The oxidation of the hot mercury caused by the airingress is negligible in regard to the operating characteristics of the pumpwhen compared with the mercury loss in the forepump line.Changing the pump fluid: The interior section will be extracted from thepump and the contaminated pump fluid poured out. The interior section andthe pump body are then cleaned with pure petroleum ether (naphtha). Theinterior section and pump body of mercury pumps should have beencleaned beforehand with a clean brush; use a bottle brush for the nozzlebores. Ensure that all the nozzle orifices are properly cleaned.
It isadvantageous to evaporate all solvent residues in a drying kiln. Then theinside section is inserted once again and the fresh pump fluid is installedthrough the forevacuum port. It is necessary to ensure that the uppernozzle cover is not moistened with pump fluid! Do not install too muchpump fluid!8.3.4.2Operating errors with diffusion and vaporjet pumpsPotential sources of defects when the desired ultimate pressure is notreached• Coolant temperature is too high; inadequate water throughput. Thecoolant flow should always be monitored by a water flowmeter in order•••••to protect the pump from damage.
Remedy: Measure the exittemperature of the coolant water (it should not exceed 30 ¡C). Increasethe coolant flow-through rate. The cooling coils at the pump may have tobe decalcified.Forevacuum pressure too high: This is possible particularly wherevapors which are either evacuated from the vessel or are created ascracking products from the driving medium (e.g. following air ingress)get into the roughing pump. Check the forevacuum pressure with the oildiffusion pump disconnected. Remedy: Run the forevacuum pump for anextended period of time with gas ballast. If this is not sufficient, then theoil in the forepump will have to be changed.Pump fluid at the diffusion pump spent or unserviceable: Replace thedriving medium.Heating is incorrect: Check the heating output and check for goodthermal contact between the heating plate and the bottom of the boilersection. Replace the heating unit if necessary.Leaks, contamination:Remedy: if the pump has been contaminated by vapors it may help touse a metering valve to pass air through the apparatus for some periodof time; here the pressure should not exceed 10-2 mbar whenDIFFELEN is being used.Measurement system old or soiled (see Section 8.4.2).Potential sources of error where there is insufficient pumping speed:• Forevacuum pressure is too high: Check the forevacuum; allow the gasballast pump to run for a longer period of time with the gas ballast valveopen.
It may be necessary to change the oil in the forepump.• The pump fluid in the diffusion pump has become unserviceable: Drivingmedium will have to be replaced.• Nozzles at the diffusion pump are clogged: Clean the diffusion pump.• Heating is too weak: Check heating output; examine heating plate forgood thermal contact with the bottom of the boiler chamber.• Substances are present in the vacuum vessel which have a highervapor pressure than the driving medium being used: among these are,for example, mercury, which is particularly hazardous because themercury vapors will form amalgams with the nonferrous metals in the oildiffusion pump and thus make it impossible to achieve perfect vacuums.8.3.5 Adsorption pumps8.3.5.1Reduction of adsorption capacityA considerable reduction in pumping speed and failure to reach the ultimatepressure which is normally attainable in spite of thermal regenerationhaving been carried out indicates that the zeolite being used has becomecontaminated by outside substances.
It does not make good sense toattempt to rejuvenate the contaminated zeolite with special thermalprocesses. The zeolite should simply be replaced.8.3.5.2Changing the molecular sieveIt will be necessary to rinse the adsorption pump thoroughly with solventsbefore installing the new zeolite charge. Before putting the adsorption pumpwhich has been charged with fresh zeolite into service it is also necessaryto bake out the new zeolite charge, under vacuum and using the heating144HomeInstructions for equipment operationelement associated with the pump, for a period of several hours so thatcontaminants which might have collected during the storage period candissipate.8.3.6 Titanium sublimation pumpsEach of the turns in the titanium sublimation pump contains approximately1.2 g of useable titanium supply.
At a heating current of 50 A the surfacetemperature comes to about 1850 K, the sublimation rate approximately0.12 g/h, i.e. a turn can be operated continuously for about 10 hours. Sinceat pressures below 1 á 10-6 mbar sublimation is not continuous but ratheronly at intervals which Ð at low pressures (below 5 á 10-8 mbar) and low gasvolumes Ð are already more than ten times the actual sublimation period,one may assume a pumping period of almost one month at a workingpressure of 10-10 mbar per turn.The effective pumping speed of a titanium sublimation pump will depend onthe getter screen surface and the geometry of the suction opening.
Thepumping speed, referenced to the surface area of the getter surface, will bedependent on the type of gas and the getter screen temperature. Sinceinert gases, for example, cannot be pumped at all, titanium sublimationpumps should always be used only with an auxiliary pump (sputter-ionpump, turbomolecular pump) used to pump these gas components. Thesupplementary pump can be much smaller than the titanium sublimationpump.
Only in a few special cases can one do without the additional pump.The selection of the coolant is dictated by the working conditions and therequirements in terms of ultimate pressure. At high pressures, above1 á 10-6 mbar, more thermal energy is applied to the getter screen byfrequent sublimation cycles. It is for this reason that cooling with liquidnitrogen is more favorable.
At low pressures water cooling may besufficient. The getter screen should if at all possible be heated to roomtemperature before airing the pump as otherwise the humidity in the airwould condense out on the surface.8.3.7 Sputter-ion pumpsSputter-ion pumps use high-voltage current. Installation and connectionshould be carried out only under the supervision of a qualified specialist.The operating instructions shall be observed!The service life of sputter-ion pumps depends linearly on the pumpÕsoperating pressure. In the case of pumps manufactured by LEYBOLD, thefollowing applies:p á T = 45 á 10-3 mbar á h(p = operating pressure, T = service life)This means that for operating pressure of10-3 mbar the service life is 45hours10-6 mbar the service life is 45,000hours10-9 mbar the service life is 45,000,000 hoursIf a triode pump is not needed over an extended period of time it can eitherbe operated continuously at low pressure Ð with practically no influence onthe service life Ð or it can be aired, removed from the pump and packed ina dust-tight container.
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