Fundamentals of Vacuum Technology (1248463), страница 17
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The thermal conductivity panels made of copper areonly in part immersed in the pump fluid. Those parts of the thermalconductivity panels are so rated that the pump fluid can evaporateintensively but without any retardation of boiling. Those parts of the thermalconductivity panels above the oil level supply additional energy to thevapor. Owing to the special design of the heating system, the heatercartridges may be exchanged also while the pump is still hot.42HomeVacuum generation1 Nozzle (Laval)2 Diffuser nozzle (Venturi)3 Mixing chamber4 Connection to the vacuum chamberFig. 2.46 Operation of a vapor jet pump2.1.6.2 Oil vapor ejector pumps1 High vacuum portFig. 2.472 Diffusion stages3 Ejector stagesDiagram of an oil jet (booster) pumpnot remain constant toward low inlet pressures. As a consequence of thehigh vapor stream velocity and density, oil vapor ejector pumps cantransport gases against a relatively high backing pressure. Their criticalbacking pressure lies at a few millibars.
The oil vapor ejector pumps used inpresent-day vacuum technology have, in general, one or more diffusionstages and several subsequent ejector stages. The nozzle system of thebooster is constructed from two diffusion stages and two ejector stages incascade (see Fig. 2.47). The diffusion stages provide the high pumpingspeed between 10-4 and 10-3 mbar (see Fig. 2.48), the ejector stages, thehigh gas throughput at high pressures (see Fig. 2.49) and the high criticalbacking pressure.
Insensitivity to dust and vapors dissolved in the pumpfluid is obtained by a spacious boiler and a large pump fluid reservoir. Largequantities of impurities can be contained in the boiler without deteriorationof the pumping characteristics.Intake pressure paFig.
2.48 Pumping speed of various vapor pumps as a function of intake pressure related to anominal pumping speed of 1000 l/s.End of the working range of oil vapor ejector pumps (A) and diffusion pumps (B)Pumping speed [mbar ál á sÐ1]Pumping speed [l áPumping speed [Torr á l á sÐ1]The pumping action of a vapor ejector stage is explained with the aid of Fig.2.46. The pump fluid enters under high pressure p1 the nozzle (1),constructed as a Laval nozzle. There it is expanded to the inlet pressurep2. On this expansion, the sudden change of energy is accompanied by anincrease of the velocity.
The consequently accelerated pump fluid vapor jetstreams through the mixer region (3), which is connected to the vessel (4)being evacuated. Here the gas molecules emerging from the vessel aredragged along with the vapor jet. The mixture, pump fluid vapor Ð gas, nowenters the diffuser nozzle constructed as a Venturi nozzle (2). Here thevapor Ð gas mixture is compressed to the backing pressure p3 withsimultaneous diminution of the velocity. The pump fluid vapor is thencondensed at the pump walls, whereas the entrained gas is removed by thebacking pump. Oil vapor ejector pumps are ideally suited for the pumping oflarger quantities of gas or vapor in the pressure region between 1 and10-3 mbar.
The higher density of the vapor stream in the nozzles ensuresthat the diffusion of the pumped gas in the vapor stream takes place muchmore slowly than in diffusion pumps, so that only the outer layers of thevapor stream are permeated by gas. Moreover, the surface through whichthe diffusion occurs is much smaller because of the special construction ofthe nozzles. The specific pumping speed of the vapor ejector pumps is,therefore, smaller than that of the diffusion pumps. As the pumped gas inthe neighborhood of the jet under the essentially higher inlet pressuredecisively influences the course of the flow lines, optimum conditions areobtained only at certain inlet pressures.
Therefore, the pumping speed doesIntake pressure paFig. 2.49 Pumping speed of various vapor pumps (derived from Fig. 2.48)43HomeVacuum generation2.1.6.3Pump fluidspossible, various measures must be undertaken simultaneously:a) OilsThe suitable pump fluids for oil diffusion pumps are mineral oils, siliconeoils, and oils based on the polyphenyl ethers.
Severe demands are placedon such oils which are met only by special fluids. The properties of these,such as vapor pressure, thermal and chemical resistance, particularlyagainst air, determine the choice of oil to be used in a given type of pumpor to attain a given ultimate vacuum. The vapor pressure of the oils used invapor pumps is lower than that of mercury. Organic pump fluids are moresensitive in operation than mercury, because the oils can be decomposedby long-term admission of air.
Silicone oils, however, withstand longerlasting frequent admissions of air into the operational pump.a) the high vacuum-side nozzle and the shape of the part of the pumpbody surrounding this nozzle must be constructed so that as few aspossible vapor molecules emerge sideways in the path of the vaporstream from the nozzle exit to the cooled pump wall.Typical mineral oils are DIFFELEN light, normal and ultra.
The differenttypes of DIFFELEN are close tolerance fractions of a high quality baseproduct (see our catalog).Two chief requirements must be met in the construction of baffles or coldtraps for oil diffusion pumps. First, as far as possible, all backstreamingpump-fluid vapor molecules should remain attached to (condensed at) theinner cooled surfaces of these devices.
Second, the condensation surfacesmust be so constructed and geometrically arranged that the flowconductance of the baffles or cold traps is as large as possible for thepumped gas. These two requirements are summarized by the termÒoptically opaqueÓ. This means that the particles cannot enter the bafflewithout hitting the wall, although the baffle has a high conductance. Theimplementation of this idea has resulted in a variety of designs that takeinto account one or the other requirement.Silicone oils (DC 704, DC 705, for example) are uniform chemicalcompounds (organic polymers).
They are highly resistant to oxidation in thecase of air inrushes and offer special thermal stability characteristics.DC 705 has an extremely low vapor pressure and is thus suited for use indiffusion pumps which are used to attain extremely low ultimate pressuresof < 10-10 mbar.ULTRALEN is a polyphenylether. This fluid is recommended in all thosecases where a particularly oxidation-resistant pump fluid must be used andwhere silicone oils would interfere with the process.APIEZON AP 201 is an oil of exceptional thermal and chemical resistancecapable of delivering the required high pumping speed in connection withoil vapor ejector pumps operating in the medium vacuum range. Theattainable ultimate total pressure amounts to about 10-4 mbar.b) MercuryMercury is a very suitable pump fluid. It is a chemical element that duringvaporization neither decomposes nor becomes strongly oxidized when airis admitted.
However, at room temperature it has a comparatively highvapor pressure of 10-3 mbar. If lower ultimate total pressures are to bereached, cold traps with liquid nitrogen are needed. With their aid, ultimatetotal pressures of 10-10 mbar can be obtained with mercury diffusionpumps. Because mercury is toxic, as already mentioned, and because itpresents a hazard to the environment, it is nowadays hardly ever used as apump fluid.
LEYBOLD supplies pumps with mercury as the pump fluid onlyon request. The vapor pressure curves of pump fluids are given in Fig.9.12, Section 9.2.1.6.4Pump fluid backstreaming and itssuppression (vapor barriers, baffles)In the vapor stream from the topmost nozzle of a diffusion pump, pumpfluid molecules not only travel in the direction of streaming to the cooledpump wall, but also have backward components of velocity because ofintermolecular collisions. They can thus stream in the direction of thevessel.
In the case of LEYBODIFF and DI pumps, the oil-backstreamingamounts to a few micrograms per minute for each square centimeter ofinlet cross-sectional area. To reduce this backstreaming as much asb) the method for cooling the pump wall must allow as complete aspossible condensation of the pump fluid vapor and, after condensation,the fluid must be able to drain away readily.c) one or more pump-fluid traps, baffles, or cold traps must be insertedbetween the pump and the vessel, depending on the ultimate pressurethat is required.A cold cap baffle is constructed so that it can be mounted immediatelyabove the high vacuum nozzle. The cold cap baffle is made of metal of highthermal conductivity in good thermal contact with the cooled pump wall, sothat in practice it is maintained at the cooling-water temperature or, with aircooled diffusion pumps, at ambient temperature.
In larger types of pumps,the cold cap baffle is water cooled and permanently attached to the pumpbody. The effective pumping speed of a diffusion pump is reduced by about10 % on installation of the cold cap baffle, but the oil backstreaming isreduced by about 90 to 95 %.Shell baffles consist of concentrically arranged shells and a central baffleplate. With appropriate cooling by water or refrigeration, almost entirely oilvapor-free vacua can be produced by this means. The effective pumpingspeed of the diffusion pump remains at least at 50 %, although shell bafflesare optically opaque. This type of baffle has been developed by LEYBOLDin two different forms: with a stainless-steel cooling coil or Ð in the so-calledAstrotorus baffles Ð with cooling inserts of copper. The casing of the formertype is made entirely of stainless steel.For the smaller air-cooled, oil diffusion pumps, plate baffles are used.
Theair-cooled arrangement consists of a copper plate with copper webs to thehousing wall. The temperature of the plate baffle remains nearly ambientduring the operation of the diffusion pump.Hydrocarbon-free vacuumIf extreme demands are made on freedom from oil vapor with vacuumproduced by oil diffusion pumps, cold traps should be used that are cooledwith liquid nitrogen so that they are maintained at a temperatureof - 196 ¡C.Low-temperature baffles or cold traps should always be used with a coldcap in place. On this the greatest part of the backstreaming oil is44HomeVacuum generationfew highly volatile components of the pump fluid that do not remainattached to the very low temperature surfaces.
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