Fundamentals of Vacuum Technology (1248463), страница 30
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3Hence the Roots pump RUVAC WA 1001 would be the suitable pump. Thepermissible remaining moisture in the product determines the attainableultimate pressure. The relationship between the ultimate pressure and theremaining moisture is fixed for every product but different from product toproduct. LEYBOLD has many years of experience to its record regardingapplications in this area. Assume that a 0.1 % residual moisture content isrequired, for which the necessary ultimate pressure is 6 · 10-2 mbar. Duringthe last 5 h the remaining 6 % of the moisture content, or 5 kg of water, isremoved. At a mean pressure of about 0.65 mbar, 2000 m3/h of vapor isevolved. Two possibilities are offered:a) One continues working with the above-mentioned Roots pumpWA 1001.
The ultimate total pressure settles at a value according to thewater vapor quantity evolving. One waits until a pressure of about6.5 · 10-2 mbar is reached, which naturally takes a longer time.b From the beginning, a somewhat larger Roots pump is chosen (e.g., theRUVAC WA 2001 with a pumping speed of 2000 m3/h is suitable).
Forlarger quantities of paper (5000 kg, for example) such a pumpingsystem will be suitable which at a pumping speed for water vapor of upto 20,000 m3/h automatically lowers the pressure from 27 to 10-2 mbar.The entire time need for drying is significantly reduced when using suchpumps.72HomeVacuum generation2.3.6 Flanges and their sealsIn general, demountable joints in metallic vacuum components, pumps,valves, tubulations, and so on are provided with flanges. Vacuumcomponents for rough, medium, and high vacuum from LEYBOLD areequipped with the following standardized flange systems:• Small flanges (KF) (quick-action connections to DIN 28 403) of nominalwidths 10, 16, 20, 25, 32, 40 and 50 mm.
The values 10, 16, 25 and 40are preferred widths according to the PNEUROP recommendations andthe ISO recommendations of the technical committee ISO/TC 112 (seealso Section 11). For a complete connection of two identical flanges oneclamping ring and one centering ring are required.• Clamp flanges (ISO-K) of nominal widths 65, 100, 160, 250, 320, 400,500 and 630 mm. Also, these flanges correspond to the nominal widthsand construction of the PNEUROP and ISO/TC 112 recommendations.Clamp flanges are joined together by clamps or collar rings. Centeringrings or gaskets are needed for sealing.• Bolted flanges (ISO-F) for the same nominal widths as above(according to PNEUROP and ISO/TC 112). In special cases boltedflanges having a smaller nominal width are used. Clamp flanges andbolted flanges are in accordance with DIN 28 404.The nominal width is approximately equal to the free inner diameter of theflange in millimeters; greater deviations are exceptions, so the clamp flangeDN 63 has an inner diameter of 70 mm.
See also Table XI in Section 9).High vacuum components are made of aluminum or stainless steel.Stainless steel is slightly more expensive but offers a variety of advantages:lower degassing rate, corrosion resistant, can be degassed at temperaturesup to 200 ¡C, metal seals are possible and stainless steel is much moreresistant to scratching compared to aluminum.Generating pressures below 10-8 mbar requires higher bake-outtemperatures. As explained above (see Section 2.2.6) work in the UHVrange requires a basically different approach and the use of CF flangesfitted with metallic sealing rings.2.3.7 Choice of suitable valvesVacuum technology puts great demands on the functioning and reliability ofthe valves, which are often needed in large numbers in a plant. Thedemands are fulfilled only if correct shut-off devices are installed for eachapplication, depending on the method of construction, method of operation,and size.
Moreover, in the construction and operation of vacuum plants,factors such as the flow conductance and leak-tightness of valves are ofgreat importance.Valves are constructed so that they will not throttle pumping speed. Hence,when opened fully, their conductance in the rough and medium vacuumregions equals that of corresponding tube components. For example, theconductance of a right-angle valve will equal the conductance of a benttube of the same nominal bore and angle. Similarly, the conductance of thevalve for molecular flow (i.
e., in the high and ultrahigh vacuum regions), isso high that no significant throttling occurs. Actual values for theconductance of various components are given in the catalog.To meet stringent leak-tightness demands, high-quality vacuum valves aredesigned so that gas molecules adhering to the surface of the valve shaftare not transferred from the outer atmosphere into the vacuum duringoperation. Such valves are therefore equipped with metal bellows forisolating the valve shaft from the atmosphere, or alternatively, they are fullyencapsulated, that is, only static seals exist between atmosphere andvacuum.
This group is comprised of all medium and high vacuum valvesfrom LEYBOLD that are operated either manually or electropneumaticallyUltrahigh vacuum components are made of stainless steel and have CFflanges bakeable to a high temperature. These components, including theflanges, are manufactured in a series production, starting with a nominalwidth of 16 up to 250 mm. CF flanges are available as fixed flanges or alsowith rotatable collar flanges. They may be linked with CONFLAT flangesfrom almost all manufacturers. Copper gaskets are used for sealingpurposes.Basically, the flanges should not be smaller than the connecting tubes andthe components that are joined to them.
When no aggressive gases andvapors are pumped and the vacuum system is not exposed to atemperature above 80 ¡C, sealing with NBR (Perbunan) or CR (Neoprene)flange O-rings is satisfactory for work in the rough-, medium-, and highvacuum regions. This is often the case when testing the operation ofvacuum systems before they are finally assembled.All stainless steel flanges may be degassed at temperatures up to 200 ¡Cwithout impairment. However, then Perbunan sealing material is notsuitable as a flange sealant. Rather, VITILAN¨ (a special FPM) sealingrings and also aluminum seals, which allow heating processes up to150 ¡C and 200 ¡C respectively, should be used.
After such degassing,pressures down to 10-8 mbar, i.e. down to the UHV range, can be attainedin vacuum systems.1 Casing2 Valve disk3 Compression spring4 Solenoid coilFig. 2.79 Right angle vacuum valve with solenoid actuator73HomeVacuum generationThe various applications require valves with different drives, that is, valvesthat are manually operated, electropneumatically- or magnetically-operated,and motor driven, such as variable-leak valves. The variety is even moreenhanced by the various housing designs.
In addition to the variousmaterials used, right-angle and straight-through valves are required.Depending on their nominal width and intended application, flanges fitted tovalves may be small (KF), clamp (ISO-K), bolted (ISO-F), or UHV (CF).1 Casing2 Valve disk3 Bellows4 Compressed air supply5 PistonFig. 2.80 Right angle vacuum valve with electropneumatic actuator(Fig.
2.80) and (Fig. 2.79). The leak rate of these valves is less than10-9 mbar á l/s.In addition to the vacuum valves, which perform solely an isolation function(fully open Ð fully closed position), special valves are needed for specialfunctions. Typical are variable leak valves, which cover the leakage rangefrom 10-10 cm3/s (NTP) up to 1.6 · 103 cm3/s (NTP). These valves areusually motor driven and suitable for remote control and when they areconnected to a pressure gauge, the process pressures can be set andmaintained.
Other special valves fulfill safety functions, such as rapid,automatic cut-off of diffusion pumps or vacuum systems in the event of apower failure. For example, SECUVAC valves belong to this group. In theevent of a power failure, they cut off the vacuum system from the pumpingsystem and vent the forevacuum system.
The vacuum system is enabledonly after a certain minimum pressure (about 200 mbar) has been attainedonce the power has been restored.When aggressive gases or vapors have to be pumped, valves made ofstainless steel and sealed with VITILAN¨ sealant are usually used. Fornuclear technology, valves have been developed that are sealed withspecial elastomer or metal gaskets. We will be pleased to provide furtherdesign information for your area of application upon request.Valves sealed with oil or grease can be used for highly stringent demands.Their leakage rate is also about 10-9 mbar á l/s.
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