Fundamentals of Vacuum Technology (1248463), страница 16
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The pump fluidvapor, which constitutes the vapor jet, is condensed at the cooled wall ofthe pump housing, whereas the transported gas is further compressed,usually in one or more succeeding stages, before it is removed by thebacking pump. The compression ratios, which can be obtained with fluidentrainment pumps, are very high: if there is a pressure of 10-9 mbar at theinlet port of the fluid-entrainment pump and a backing pressure of10-2 mbar, the pumped gas is compressed by a factor of 107!Basically the ultimate pressure of fluid entrainment pumps is restricted bythe value for the partial pressure of the fluid used at the operatingtemperature of the pump. In practice one tries to improve this by introducingbaffles or cold traps.
These are ÒcondensersÓ between fluid entrainmentpump and vacuum chamber, so that the ultimate pressure which can beattained in the vacuum chamber is now only limited by the partial pressureof the fluid at the temperature of the baffle.The various types of fluid entrainment pumps are essentially distinguishedby the density of the pump fluid at the exit of the top nozzle facing the highvacuum side of the pump:1. Low vapor density:Diffusion pumps2.1.6.1 (Oil) Diffusion pumpsThese pumps consist basically (see Fig. 2.44) of a pump body (3) with acooled wall (4) and a three- or four-stage nozzle system (A Ð D).
The oilserving as pump fluid is in the boiler (2) and is vaporized from here byelectrical heating (1). The pump fluid vapor streams through the riser tubesand emerges with supersonic speed from the ring-shaped nozzles (A Ð D).Thereafter the jet so-formed widens like an umbrella and reaches the wallwhere condensation of the pump fluid occurs. The liquid condensate flowsdownward as a thin film along the wall and finally returns into the boiler.Because of this spreading of the jet, the vapor density is relatively low. Thediffusion of air or any pumped gases (or vapors) into the jet is so rapid thatdespite its high velocity the jet becomes virtually completely saturated withthe pumped medium. Therefore, over a wide pressure range diffusionpumps have a high pumping speed.
This is practically constant over theentire working region of the diffusion pump (² 10-3 mbar) because the air atthese low pressures cannot influence the jet, so its course remainsundisturbed. At higher inlet pressures, the course of the jet is altered. As aresult, the pumping speed decreases until, at about 10-1 mbar, it becomesimmeasurably small.The forevacuum pressure also influences the vapor jet and becomesdetrimental if its value exceeds a certain critical limit. This limit is calledmaximum backing pressure or critical forepressure.
The capacity of theOil diffusion pumps(Series: LEYBODIFF, DI and DIP)Mercury diffusion pumps2. High vapor density:Vapor jet pumpsWater vapor pumpsOil vapor jet pumpsMercury vapor jet pumps3. Combinedoil diffusion/ vapor jet pumps4. Water jet pumpsCooling of fluid entrainment pumpsThe heater power that is continuously supplied for vaporizing the pump fluidin fluid-entrainment pumps must be dissipated by efficient cooling. Theenergy required for pumping the gases and vapors is minimal.
The outsidewalls of the casing of diffusion pumps are cooled, generally with water.Smaller oil diffusion pumps can, however, also be cooled with an air streambecause a low wall temperature is not so decisive to the efficiency as formercury diffusion pumps. Oil diffusion pumps can operate well with walltemperatures of 30 ¡C, whereas the walls of mercury diffusion pumps mustbe cooled to 15 ¡C. To protect the pumps from the danger of failure of thecooling water Ð insofar as the cooling-water coil is not controlled bythermally operated protective switching Ð a water circulation monitor shouldbe installed in the cooling water circuit; hence, evaporation of the pumpfluid from the pump walls is avoided.12345HeaterBoilerPump bodyCooling coilHigh vacuum flange6 Gas molecules7 Vapor jet8 Backing vacuumconnectionABCDHNozzlesFig.
2.44 Mode of operation of a diffusion pump41HomeVacuum generationchosen backing pump must be such (see 2.3.2) that the amount of gasdischarged from the diffusion pump is pumped off without building up abacking pressure that is near the maximum backing pressure or evenexceeding it.The attainable ultimate pressure depends on the construction of the pump,the vapor pressure of the pump fluid used, the maximum possiblecondensation of the pump fluid, and the cleanliness of the vessel.Moreover, backstreaming of the pump fluid into the vessel should bereduced as far as possible by suitable baffles or cold traps (see Section2.1.6.4).Degassing of the pump oilIn oil diffusion pumps it is necessary for the pump fluid to be degassedbefore it is returned to the boiler.
On heating of the pump oil, decompositionproducts can arise in the pump. Contamination from the vessel can get intothe pump or be contained in the pump in the first place. These constituentsof the pump fluid can significantly worsen the ultimate pressure attainableby a diffusion pump, if they are not kept away from the vessel. Therefore,the pump fluid must be freed of these impurities and from absorbed gases.This is the function of the degassing section, through which the circulatingoil passes shortly before re-entry into the boiler.
In the degassing section,the most volatile impurities escape. Degassing is obtained by the carefullycontrolled temperature distribution in the pump. The condensed pump fluid,which runs down the cooled walls as a thin film, is raised to a temperatureof about 130 ¡C below the lowest diffusion stage, to allow the volatilecomponents to evaporate and be removed by the backing pump. Therefore,the re-evaporating pump fluid consists of only the less volatile componentsof the pump oil.Pumping speedThe magnitude of the specific pumping speed S of a diffusion pump Ð thatis, the pumping speed per unit of area of the actual inlet surface Ð dependson several parameters, including the position and dimensions of the highvacuum stage, the velocity of the pump fluid vapor, and the meanmolecular velocity -c of the gas being pumped (see equation 1.17 in Section1.1).
With the aid of the kinetic theory of gases, the maximum attainablespecific pumping speed at room temperature on pumping air is calculatedto Smax = 11.6 l · s-1 · cm-2. This is the specific (molecular) flowconductance of the intake area of the pump, resembling an aperture of thesame surface area (see equation 1.30 in Section 1.5.3). Quite generally,diffusion pumps have a higher pumping speed for lighter gases comparedto heavier gases.To characterize the effectiveness of a diffusion pump, the so called HOfactor is defined.
This is the ratio of the actually obtained specific pumpingspeed to the theoretical maximum possible specific pumping speed. In thecase of diffusion pumps from LEYBOLD optimum values are attained (of0.3 for the smallest and up to 0.55 for the larger pumps).The various oil diffusion pumps manufactured by LEYBOLD differ in thefollowing design features (see Fig. 2.45 a and b).12a) LEYBODIFF-Pump with fractionatingfacility1 Center2 Middle section3 Outer part of the boiler (fractionation)b) DI pump; side view on to the internalheater1 Thermostat sensor2 Heating cartridgeFig. 2.45 Diagram showing the basic differences in LEYBOLD oil diffusion pumpsa) LEYBODIFF seriesThis series of pumps is equipped with a fractionating device.
The variousconstituents of the pump fluid are selected so that the high vacuum nozzleis supplied only by the fraction of the pump fluid that has the lowest vaporpressure. This assures a particularly low ultimate pressure. Fractionatingoccurs because the degassed oil first enters the outer part of the boiler,which serves the nozzle on the backing vacuum side. Here a part of themore volatile constituents evaporates. Hence the already purified pumpfluid reaches the intermediate part of the boiler, which serves theintermediate nozzle.
Here the lighter constituents are evaporated in greaterquantities than the heavier constituents. When the oil enters the centralregion of the boiler, which serves the high vacuum nozzle, it has alreadybeen freed of the light volatile constituents.b) DI seriesIn these pumps an evaporation process for the pump fluid which isessentially free of bursts is attained by the exceptional heater designresulting in a highly constant pumping speed over time. The heater is of theinternal type and consists of heating cartridges into which tubes withsoldered on thermal conductivity panels are introduced. The tubes made ofstainless steel are welded horizontally into the pumpÕs body and are locatedabove the oil level.
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