Vacuum system efficiency

Starting a vacuum system should include the following trap use procedures for safe and efficient vacuum system operation ... 

Metallization layers are generally deposited either by CVD or by physical vapor deposition methods such as evaporation (qv) or sputtering. In recent years sputter deposition has become the predominant technique for aluminum metallization. Energetic ions are used to bombard a target such as soHd aluminum to release atoms that subsequentiy condense on the desired substrate surface. The quaUty of the deposited layers depends on the cleanliness and efficiency of the vacuum systems used in the process. The mass deposited per unit area can be calculated using the cosine law of deposition ... 

Space needs to be provided for the auxiliaries, including the lube oil and seal systems, lube oil cooler, intercoolers, and pulsation dampeners. A control panel or console is usually provided as part of the local console. This panel contains instmments that provide the necessary information for start-up and shutdown, and should also include warning and trouble lights. Access must be provided for motor repair and ultimate replacement needs to be considered. If a steam turbine is used, a surface condenser is probably required with a vacuum system to increase the efficiency. AH these additional systems need to be considered in the layout and spacing. In addition, room for pulsation dampeners required between stages has to be included. Aftercoolers may also be required with knockout dmms. Reference 8 describes the requirements of compressor layouts and provides many useful piping hints. 

Because of the low efficiency of steam-ejector vacuum systems, there is a range of vacuum above 13 kPa (100 mm Hg) where mechanical vacuum pumps are usually more economical. The capital cost of the vacuum pump goes up roughly as (suction volume) or (l/P). This means that as pressure falls, the capital cost of the vacuum pump rises more swiftly than the energy cost of the steam ejector, which iacreases as (1 /P). Usually below 1.3 kPa (10 mm Hg), the steam ejector is more cost-effective. 

Pressure/vacuum, 435, 466 Vacuum systems, 343 Absolute pressure conversions, 363 Air inleakage, 366 Calculations, 366-375 Dissolved gases release, 368 Estimated air inleakage, table, 366 Evacuation time, 371 Maximum air leakage, chart, 367 Specific air inleakage rates, 368 Temperature approach, 375 Classifications, 343 Diagrams, 380 Pressure drop, 353 Pressure levels, 343, 352 Pressure terminology, 348 Pump down example, 381 Pump down time, 380 Thermal efficiency, 384 Valve codes, 26... 

The burner and associated equipment must be wrapped up or otherwise protected against dirt and soot. The front doors are opened and plastic sheeting is attached to minimize the spread of soot. The tubes are brushed, and the soot and dirt are vacuumed away while brushing takes place. Some contractors have truck-mounted vacuum systems to increase efficiency and minimize post-boiler cleaning work. 

Air streams from the digestion system, vacuum cooler, concentrator, and other areas where fluorine is evolved are connected to a highly efficient absorption system, providing extremely high volumes of water relative to the stream. The effluent from this absorption system forms part of the recycled water and is eventually discharged as part of the product used for fertilizer manufacture. The Minnesota plant requires a constant recirculating water load in excess of 3000 gpm (11.4 m /min), but multiple use and recycle reduce makeup requirements to less than 400 gpm (1.5 m /min) or a mere 13% of total water use. 

The LC column can operate for several weeks without any loss of separation efficiency. The flow rate of solvent into the mass spectrometer is 2-3 pL/min under normal conditions. The pressure in the ion source housing is 10 Pa and 10 Pa in the analyzer. No deterioration of the vacuum system during two years of operation has been observed. 

Two models can explain the events that take place as the droplets dry. One was proposed by Dole and coworkers and elaborated by Rollgen and coworkers [7] and it is described as the charge residue mechanism (CRM). According to this theory, the ions detected in the MS are the charged species that remain after the complete evaporation of the solvent from the droplet. The ion evaporation model affirms that, as the droplet radius gets lower than approximately 10 nm, the emission of the solvated ions in the gas phase occurs directly from the droplet [8,9]. Neither of the two is fully accepted by the scientific community. It is likely that both mechanisms contribute to the generation of ions in the gas phase. They both take place at atmospheric pressure and room temperature, and this avoids thermal decomposition of the analytes and allows a more efficient desolvation of the droplets, compared to that under vacuum systems. In Figure 8.1, a schematic of the ionization process is described. 

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]. 

The conductance of any system depends on the nature of the gas flow at higher pressures a viscous flow regime prevails and at lower pressures (p < 10 Torr) a molecular flow regime. Turbulent flow, seldom considered when assessing the efficiency of a vacuum system, is encountered only when the pressure in the system is close to atmospheric pressure. As a general rule, if the efficiency of the system is adequate for the viscous flow regime then it will also be suitable for turbulent flow. 

After this period, the dropping funnel and the vacuum takeoff are replaced by the short-path distillation assembly shown in Figure 2. The system is protected by a Drierite tube and the benzene is distilled under reduced pressure (water aspirator). After the benzene is removed, the benzene-containing receiver is replaced with a clean, dry flask, and the system is connected to an efficient vacuum pump. The pressure in the system is reduced to 0.02 mm., and the flask is immersed deeply in an oil bath (Figure 2) heated to about 200°. After about 1 ml. of fluid forerun is collected, the diethylaluminum cyanide distils at 162° (0.02 mm.) (Note 7) and is collected in atared 200-ml. receiver by heating the side arm and the adaptor with a stream of hot air or an infrared lamp (Note 8). After all the distillate is collected in the receiver (Note 9), dry nitrogen is admitted to the evacuated apparatus and the receiver is stoppered and weighed. Diethylaluminum cyanide is obtained usually as a pale yellow syrup (Note 10) in 60-80% yield (26.7-35.6 g.) (Note 11). 

A 37.5-g. (0.266-mole) sample of coumalic add (Note 1) is placed in a 30 x 10 cm. cylindrical flask attached horizontally to a 55 x 3 cm oven-heated Vycor tube (Note 2) loosely packed with 20 g. of fine copper turnings (Note 3). Following the Vycor tube successively are two ice-cooled 50-ml. receivers and a dry ice trap. The latter is connected to an efficient vacuum pump (Note 4). The system is evacuated, and the Vycor tube is heated to 650— 670° Then the flask containing the coumalic acid is heated with a nichrome wound heating jacket to 180°, and the temperature is allowed to rise slowly to 215°. During this time coumalic acid sublimes into the Vycor tube and a-pyrone distills into the ice-cooled receivers. The pressure is held below 5 mm (Note 5) The yield of pale yellow crude material is 18-19 3 g. (70-75%). 

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