The Vacuum System

One might expect the bigger the vacuum pump the better. But oversizing may render the system unreliable Let s consider a medium sized plant to be operated at 0.02 mbar (Ptoti)- measured a leak rate of 0.05 mbar-l-s for the empty 

The stream contains 50% of condensable vapour, that is also 1.05 mbar xls (vap loadj). 

At this vacuum level the conductance ofthe here used cold trap between still and pump amounts to approximately250 l-s . On its way through the cold trap, due to chilling and further condensation, the partial pressure of the condensable vapour decreases from 0.01 to 0.0005 mbar (Pvj,p2)- pressure loss (Ap) is approximately 

Iteration Gas load step vapl Vapour load, Ap Ptot2 vap2 Vapour loadj 

The required effective pumping speed amounts to (1.05 mbar X1 s t) 

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Clearly, the lower the ionization energy with respect to the work function, the greater is the proportion of ions to neutrals produced and the more sensitive the method. For this reason, the filaments used in analyses are those whose work functions provide the best yields of ions. The evaporated neutrals are lost to the vacuum system. With continued evaporation of ions and neutrals, eventually no more material remains on the filament and the ion current falls to zero. 

Ions and neutral molecules headed for skimmer orifice, drawn mainly by the vacuum system... 

Direct-inlet probe. A shaft or tube having a sample holder at one end that is inserted into the vacuum system of a mass spectrometer through a vacuum lock to place the sample near to, at the entrance of, or within the ion source. The sample is vaporized by heat from the ion source, by heat applied from an external source, or by exposure to ion or atom bombardment. Direct-inlet probe, direct-introduction probe, and direct-insertion probe are synonymous terms. The use of DIP as an abbreviation for these terms is not recommended. 

The helium leak detector is a common laboratory device for locating minute leaks in vacuum systems and other gas-tight devices. It is attached to the vacuum system under test a helium stream is played on the suspected leak and any leakage gas is passed into a mass spectrometer focused for the helium-4 peak. The lack of nearby mass peaks simplifies the spectrometer design the low atmospheric background of helium yields high sensitivity helium s inertness ensures safety and its high diffusivity and low adsorption make for fast response. 

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 ... 

Ultrasound frequencies can be introduced into the walls of the vacuum system. If a source of ultrasound is placed on the wall of an ultrahigh vacuum system, a large hydrogen peak is observed. Related phenomena, presumably from frictional effects, are observed if the side of a vacuum system is tapped with a hammer a desorption peak can be seen. Mechanical scraping of one part on another also produces desorption. 

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. 

For materials of moderate to low porosity, a good starting vacuum level is 0.6 to 0.7 bar (18 to 21 in Hg), as the capacity of most vacuum pumps starts to fall off rapidly at vacuum levels higher than 0.67 bar (20 in Hg). Unless there is a critical moisture content which requires the use of higher vacuums, or unless the deposited cake is so impervious that the air rate is extremely low, process economics will favor operation at vacuums below this level. When test work is carried out at an elevation above sea level different than that of the plant, the elevation at the plant should be taken into account when determining the vacuum system capacity for high vacuum levels (>0.5 bar). 

For example, in rotary vacuum dryers it is possible to prevent the formation of explosible dust-air mixtures by setting and monitoring a certain partial vacuum (negative pressure). This pressure value must be determined by experiment for each type of dust. With pressures of less than O.I bar, in general, hazardous effects of dust explosions need not be anticipated. If the vacuum system malfunctions, the partial vacuum must be released by inert gas and the instaUation shut down. 

Outlet Temperature and Pressure. It is important to have proper subcooling in the vent end of the unit to prevent large amounts of process vapors from going to the vacuum system along with the inerts. 

The inerts will blanket a portion of the tubes. The blanketed portion has very poor heat transfer. The column pressure is controlled by varying the percentage of the tube surface blanketed. When the desired pressure is exceeded, the vacuum system will suck out more inerts, and lower the percentage of surface blanketed. This will increase cooling and bring the pressure back down to the desired level. The reverse happens if the pressure falls below that desired. This is simply a matter of adjusting the heat transfer coefficient to heat balance the system. 

Another example of pressure control by variable heat transfer coefficient is a vacuum condenser. The vacuum system pulls the inerts out through a vent. The control valve between the condenser and vacuum system varies the amount of inerts leaving the condenser. If the pressure gets too high, the control valve opens to pull out more inerts and produce a smaller tube area blanketed by inerts. Since relatively stagnant inerts have poorer heat transfer than condensing vapors, additional inerts... 

First, one must estimate air or other gas leakage into the vacuum system. Of course every effort is made to keep it as tight as possible. The author is aware of possible leak points being sealed with polystyrene, which produces an excellent seal. When tests cannot be made, one must use rules of thumb. Many such rough estimating techniques exist. 

Put the vacuum system control valves at the highest point of a horizontal run and the control valve bypass in the same horizontal plane. This is in compliance with item 8. 

Electron spectroscopic techniques require vacuums of the order of 10 Pa for their operation. This requirement arises from the extreme surface-specificity of these techniques, mentioned above. With sampling depths of only a few atomic layers, and elemental sensitivities down to 10 atom layers (i. e., one atom of a particular element in 10 other atoms in an atomic layer), the techniques are clearly very sensitive to surface contamination, most of which comes from the residual gases in the vacuum system. According to gas kinetic theory, to have enough time to make a surface-analytical measurement on a surface that has just been prepared or exposed, before contamination from the gas phase interferes, the base pressure should be 10 Pa or lower, that is, in the region of ultrahigh vacuum (UHV). 

In two stage units, it is often economical to distill more gas oil in the vacuum stage and less in the atmospheric stage than the maximum attainable. Gas formed in the atmospheric tower bottoms piping at high temperatures tends to overload the vacuum system and thereby to reduce the capacity of the vacuum tower. The volume of crude vaporized at the flash zone is approximately equal to the total volume of distillate products. Of course, the vapor at this point contains some undesirable heavy material and the liquid still contains some valuable distillate products. The concentration of heavy ends in the vapor is reduced by contact with liquid on the trays as the vapor passes up the tower. This liquid reflux is induced by removing heat farther up in the tower. 

Gas formed in the atmospheric tower bottoms piping at high temperatures tends to overload the vacuum system and thereby to reduce the capacity of the vacuum tower. 

A vacuum pump seal drum design which provides a liquid seal (hydraulic flame arrester) to mitigate flame propagation backward into the vacuum system. The seal liquid is an organic stream (mostly Cg aromatics) that comes from the vacuum pump discharge drum overflow. 

A detonation flame arrester with an integral thermocouple at the inlet to the process heater firebox to prevent backflash into the vacuum system. 

If pressure is expressed as inches of mercury vacuum, the reading of the local barometer (or a reference barometer) is necessary to establish the absolute sucdon pressure, or pressure in the vacuum system. 

To prevent/reduce the undesirable condensation in the pump, a small hole is drilled in the pump head to admit air or other process non-condensable gas (gas ballast) into the latter portion of the compression stroke. This occurs while the vapor being compressed is sealed off from the intake port by the piston. By reducing the partial pressure of the vapor s condensables, the condensation is avoided. Obviously, this can reduce the capacity of the pump, as the leakage past the seals allows the gas ballast to dilute the intake volume of ba,se suction gas. For most process applications, the effect of this leakage is negligible, unless the vacuum system suction is below 1 torr [22]. 

CVD does not usually require the low pressures which are necessary with sputtering, MBE, and other PVD processes. Consequently the vacuum system is simpler and less costly. Mechanical pumps are adequate for many operations. Vane pumps, built with corrosion resistant material, are preferred. If properly maintained, these pumps will operate for long periods of time. 

Figure 1. Apparatus for the preparation of radical anions (11). On connection of the entire vessel to the vacuum system, traces of water and oxygen on the wall are removed by heating and discharging with a tesla coil. When the apparatus is filled with purified nitrogen through A, the weighed sample of the hydrocarbon is put into B through C, a piece of sodium is put into D, and dimethoxyethane is distilled into E, where a small amount of an Na-K alloy is added. After the system is again evacuated the solvent is distilled from E into B, the bulb E is,sealed off at F, and the sodium is sublimed to form a mirror on the wall of the bulb G. After tubes at C and H are sealed off, the apparatus is pumped to high vacuum for 1 hr and then sealed off at J. Then the solution of the hydrocarbon is poured from B into G. After a time varying from several minutes to several hours, a color is observed, and the sample is ready for optical and esr measurements.

When constructing high vacuum apparatus, waxed tools should only be used when the apparatus under construction can be thoroughly cleaned afterwards. Otherwise wax may be present in the vacuum system, and will be removed by pumping only with extreme slowness. 

Different options are available for LC-MS instruments. The vacuum system of a mass spectrometer typically will accept liquid flows in the range of 10-20 p,L min-1. For higher flow-rates it is necessary to modify the vacuum system (TSP interface), to remove the solvent before entry into the ion source (MB interface) or to split the effluent of the column (DLI interface). In the latter case only a small fraction (10-20 iLrnin ) of the total effluent is introduced into the ion source, where the mobile phase provides for chemical ionisation of the sample. The currently available commercial LC-MS systems (Table 7.48) differ widely in characteristics mass spectrometer (QMS, QQQ, QITMS, ToF-MS, B, B-QITMS, QToF-MS), mass range m/z 25000), resolution (up to 5000), mass accuracy (at best <5ppm), scan speed (up to 13000Das-1), interface (usually ESP/ISP and APCI, nanospray, PB, CF-FAB). There is no single LC-MS interface and ionisation mode that is readily suitable for all compounds... 

Recently, a revival of electron ionisation LC-MS has been proposed by development of LC-SMB-MS [529]. In this system, the LC output (50-250 p,Lmin 1) is vaporised at atmospheric pressure and expanded from a supersonic nozzle into the vacuum system... 

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