Medium vacuum systems

Feed Slurry Temperature Temperature can be both an aid and a limitation. As temperature of the feed slurry is increased, the viscosity of the hquid phase is decreased, causing an increase in filtration rate and a decrease in cake moisture content. The limit to the benefits of increased temperature occurs when the vapor pressure of the hquid phase starts to materially reduce the allowable vacuum. If the hquid phase is permitted to flash within the filter internals, various undesired resiilts may ensue disruption in cake formation adjacent to the medium, scale deposit on the filter internals, a sharp rise in pressure drop within the filter drainage passages due to increased vapor flow, or decreased vacuum pump capacity. In most cases, the vacuum system should be designed so that the liquid phase does not boil. 

Unlike other mediums, the dielectric strength of a vacuum increases with a gap. but only mtirginally, which is the limiting factor in producing such breakers beyond 36 kV. These breakers are therefore used only for medium-voltage systems (2.4-36 kV). Some manufacturers have attempted to produce them up to 66 kV but they have not shown the desired results so far. The application of these breakers therefore continues to be up to 36 kV only. 

In order to estimate an acceptable air inleakage rate for sizing a vacuum pump for use in the medium to high vacuum system, consider ... 

Fig. 12. A flash photolysis apparatus. 1, high-voltage power supply 2, 10 M12 resistor 3. high-voltage capacitor 4, coaxial cable 5, flash tube 6, vacuum system 7, reflector 8, pulsed spectroscopic light source 9, measuring cell 10, Hilger medium quartz spectrograph. (From Vallotton and Wild, Ref. ))...

Initial evacuation of a vacuum chamber is influenced in the medium-, high-, and ultrahigh vacuum regions by continually evolving quantities of gas, because in these regions the escape of gases and vapors from the walls of the vessel is so significant that they alone determine the dimensions and layout of the vacuum system. 

Production of strand breaks by very low energy electrons (5-25 eV) in thin solid DNA films using ultrahigh vacuum systems have been reported in a number of studies [107-109]. Such studies have demonstrated the efficiencies of low energy electrons and photons to induce DNA damage. In the vacuum ultraviolet (UV) region, examination of experimental data [86,110,111] shows that the induction of strand breaks depends on the absorption spectrum of the components in the medium and the sensitivity spectrum of DNA [112]. Introduction of a variable with the wavelength for the induction of SSB by OH radicals, in conjunction with a fixed value for the quantum efficiency for the production of OH radical (sensitivity spectrum for induction of SSB in aqueous system [112]. 

Two widely used vacuum pumps are the mechanical rotary oil-seeded pump and the vapor pump. The former provides a medium vacuum and works relative to the atmosphere. The vapor pump, on the other hand, provides a high or very high vacuum and operates relative to a medium vacuum provided by a rotary pump, referred to as a backing pump in this connection. Thus, the most widely used high-vacuum system able to establish an ultimate pressure of about 10-< torr or below consists of a vapor pump backed by a rotary pump. 

Medium-resolution absorption spectrometer emission spectrometer with red-sensitive photomultiplier or CCD detector laser excitation source such as listed in Table 1 (or medium pressure mercury arc such as described in earlier editions of this text) neon calibration lamp and power supply (available from, e.g.. Oriel Corp., Stratford, CT) reagent-grade iodine 100-mm glass cell with Teflon stoppers for absorption studies heating tape with controlling Variac 50-mm cell for emission studies vacuum system, preferably with a diffusion pump and cold trap, for pumping down emission cell. 

Medium vacuum 0.1 x absolute pressure 1 to 2 bar 0.5 x system gauge pressure... 

Stopcock leading to the vacuum system is dosed, and the flask (still attached to the distillation apparatus) is placed in a constant temperature bath at 60°. The flask is occasionally swirled to aid in dissolution of AIBN and to recover AIBN, which condenses on the upper walls of the flask. The transition to a very viscous medium occurs in approximately 2 hr. At this point, the mixture is dissolved in 50 mL of dichloromethane and the resulting solution is dropped slowly down the wall of a 400-mL beaker containing 200 mL of stirred methanol. The polymer is allowed to settle, and most of the solvent is decanted. Approximately 200 mL of additional methanol is added and then decanted after the polymer has settled. The polymer is isolated by filtration through coarse filter paper, dried in an oven at 50°, and then placed in vacuo (0.02 torr) overnight. A yield of 0.27 g (12% conversion) is obtained. Conversions of 10 to 20% are typically achieved with higher conversions being possible with extended reaction times. 

Preliminary filtration tests may be made with a Buchner funnel or a small filter leaf, covered with canvas or other appropriate medium and connected to a vacuum system. Usually the suspension is poured carefully into the vacuum-connected funnel, whereas the leaf is immersed... 

Most CVD systems operate in the low or medium vacuum regimes. To ensure a clean chamber, however, CVD systems are often pumped into the high vacuum regime before introducing the reactant precursor gases. 

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