Vacuum systems starting

If you spray a probe gas lighter than air (such as helium) on a vacuum system, start at the top of the system. Start at the bottom with gases heavier than air. Otherwise, the drifting of the respective gases may provide false or inconclusive leak identification. It may be necessary to close windows and doors and might even be necessary to set up baffles to minimize drifting gases to prevent false or inconclusive leak identification. 

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. 

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). 

Solution polymerizations of 1,3-butadiene were carried out in a high-pressure glass reactor (40 mL) connected with a vacuum system. In a typical procedure, 4 pmol of precatalyst (EAS/precatalyst =100 mol/mol) was dissolved in 20 mL of toluene. The polymerization started by adding 1.08 g of 1,3-butadiene and EAS to the solution in this order. The reaction mixture was stirred at a specific temperature (30 to 70 °C) for 40 min. The resulting solution was poured into acidified methanol (100 mL of a 5% v/v solution of HCl). The polymer was then isolated by filtration and washed with methanol before drying overnight at 40 °C. Polymer yield was determined by gravimetry. 

Another approach is to perform ex situ reactions and insert the sample into a high vacuum system without exposure to ambient conditions. Incorporating N2 glove boxes or reactor systems with X-ray photoelectron spectroscopy (XPS) sample handling can also provide information that is closer to operational conditions. In a similar manner ex situ reactions and sample handling are starting to be apphed to electron microscopy studies. Commercially available sample transfer systems will accelerate the application of this methodology. 

Caution. The disilathianes should be regarded as toxic and are vile smelling. Their exposure to air and/or moisture is likely to promote rapid oxidation. Manipulations should be carried out in a sound vacuum system in a well-ventilated area. All preparations may be scaled-up to use 10 mmole of starting material. 

Another point worth considering in detail before starting to build a vacuum system is the nature and quantities of solvents which will be required. Careful purification of solvents is a prerequisite for most h.v.t. and it is usually wasteful and tedious. Therefore, if large amounts of particular solvents will be required, then it is better to purify in larger batches and to incorporate correspondingly large solvent reservoirs in the system. 

To start a vacuum system such as that illustrated in Fig. 5.1, the clean main trap is fitted in place with an even coat of stopcock grease on the joint. A Dewar partially filled with liquid nitrogen is raised around this trap, and the fore pump is immediately turned on. CAUTION Oxygen from the atmosphere will condense in a trap heid at liquid nitrogen temperature therefore, it Is important never to leave a trap cooled to liquid nitrogen temperature exposed to the atmosphere for a significant length of time. When the line has pumped down to less than 1 torr, the... 

A stainless steel high vacuum system (V = 15L) has been pumped for 36 h and the pressure is 1 x 10 7 mbar. A pressure-rise test, starting at 10 5 mbar, shows a pressure of 1.5 x 10"3 mbar after 20 min. Calculate the amount of gas entering the chamber and comment on the source of the gas. 

Most residual gas analysers (RGAs) used routinely on vacuum systems are based on quadrupole mass spectrometers (see Figure 5.5). They have a mass range starting at 1 amu and ending at 100 amu. 

In all vacuum systems, the pressure obtained is determined by the gas load and the effective pumping speed. In UHV systems at equilibrium, the predominant gas load arises from the outgassing of the internal surfaces. Although gas sources in vacuum systems have been discussed in Chapter 4, no differentiation was made between gas adsorbed on a surface and that absorbed within its structure. Various applications of UHV technology involve the use of vacuum systems that cannot be baked in situ and consideration of the choice of material for the vacuum envelope and also its surface treatment is critical. An important starting point for this is an understanding of the interaction of gas with materials. 

The pump set on a vacuum system has to evacuate the system, starting from atmospheric pressure down to the required pressure, often in a given time. It must be able to maintain this pressure during operation of the vacuum process. Chapter 3 reviews the range of vacuum pumps available and the combinations that are used over the range from atmospheric pressure down to our current limits of measurement in the EHV range. 

Achieving a poor-quality vacuum when starting a vacuum system up for the first time (see Sec. 7.6.3)... 

If you are starting to use a vacuum system for the first time, Simply skim this entire chapter. Afterward, study your vacuum system and see what components you recognize from what you ve read. Finally, re-read the sections of this chapter that are pertinent to your system. This method of study may seem like a lot of work before you turn on a switch or twist a stopcock, but the vacuum system you save may be your own. 

When first starting up a vacuum system, let the pumps evacuate the system (if starting up the system for the first time) or the traps (if they have been vented to the atmosphere) for a few minutes before setting the traps into liquid nitrogen. Otherwise you are likely to condense oxygen in the traps and create a potentially dangerous situation when the pumps are turned off (see Sec. 7.4.3). 

Prolonged operation above 1CT4 torr without adequate trapping Review procedures for starting vacuum system and use of traps. 

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

Limit the Amount of Moisture Near the Top of Your Cold Trap. A common error when first starting up a vacuum system is pouring liquid nitrogen too high into a Dewar. People often overfill Dewars in the early startup process... 

When spraying probe gases on a vacuum system, be sure to start at the top with gases less dense than air and start at the bottom with gases denser than air. 

A number of ceramic materials meet these specifications fairly well. Thus, synthetic mullite having a melting point of about 1835°C. and an expansion coefficient of 45 X 10-7 cm./cm./°C. between 20° and 1320°C., and synthetic zircon having a melting point of 1775°C. and an expansion coefficient of 42 X 10 7 between 20° and 1550°C. are suitable materials. However, both ceramics start to decompose under vacuum conditions above 1300°C. and show a not negligible vapor pressure of SiO and Si02 at considerably lower temperatures. In spite of this, mullite and zircon are better than fused quartz in the temperature range up to 1200°C. Moreover, they are cheaper and easier to be sealed to the vacuum system. 

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