Vacuum systems Evacuation time

Ten grams (0.03 mole) of antimony(III) sulfide is poured into the ampul with the aid of a suitable funnel. After the inside of the constriction has been cleaned with a cotton plug, the ampul is connected to a vacuum system, evacuated, and sealed off. During this operation the level of the liquid nitrogen should be corrected from time to time to avoid thawing of the acid, which would immediately react with the antimony(III) sulfide. 

Significant errors will arise if gas purity is not accounted for. It should be noted that the code lays down no conditions for this, and a figure of 99 / or better should be targeted. In order to obtain a good purity at the start, all pipe joints should be taped and the system evacuated to a low vacuum several times with intermediated purging with the test gas to remove the residual contaminants. 

Booster vacuum pumps are used to shorten the pump-down on evacuation (time) of a vacuum system before switching to the smaller vacuum pump to maintain the system opening vacuum and to handle the air inleakage to the system. 

Mechanical separations, 224 Mechanical vacuum systems, 342 Applications, 352, 353 Barometric iniercondenser, 349 Evacuation times, 387 Operating range, 355 Performance curves, 386 Pump down, 380... 

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... 

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.

Measurements with a vacuum system of equilibrium oxygen partial pressures as a function of temperature indicate desorption energies. There is some difficulty in choosing a representative state of comparison. Generally, investigators evacuate at increased temperature for a long time. 

Then 10 to 15 g. of the anhydrous disodium monoamido-phosphate is weighed in a 50-ml. flask with a glass joint. The flask is attached to a high-vacuum system, via a stopcock and a trap filled with solid sodium hydroxide, and evacuated. The flask is then heated in an oil bath to 80° and this temperature maintained for 6 hours. The temperature is then raised, and pressure builds up from the ammonia liberated. This gas is pumped off at intervals and the temperature increased to 210°. The formation of ammonia slows down after some time, and the reaction comes to an end after about 7 days. 

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. 

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). 

The gas oil injector system included a l(X)0 tL glass syringe connected to the injection needle and to a m gas oil reservoir by means of a two-way valve (sample/inject). It was also equipped with electrically actuated switches which controlled the timer/actuator assembly on the four-port valve as well as the data acquisition system. The data acquisition system allowed for collecting the pressure proriles in the reactor and vacuum system as a function of time during the reaction and post-reaction evacuation periods. 

The flow of gas in a vacuum system is of all three types successively when the pressure is being reduced from atmospheric to 1 tort or less. Turbulent flow occurs for a short time at the beginning of the evacuation when the gas velocity is high. Laminar flow takes place when the mean free path is small compared with the tube diameter under these conditions the gases near the tube wall are almost stationary, and those near the centre of the tube have the maximum velocity. As the pressure is reduced to about 1 toiT, molecular flow is first established (Fig. 10.1). 

The successful operation of any vacuum system is the result of experience and familiarity, rather than of instruction. The best results are usually obtained by an operator who has been using the system for some time who knows the pressure conditions in any part of the vacuum line because he knows what has been done who can estimate, from the sound of a rotary oil pump, when it is time to bring the diffusion pump into operation, or whether there is a leak in the system or not. For example, an experienced neon mbe processor can evacuate and degas the glassware and electrodes of such a tube, fill it with some 20 mmHg pressure of spectrally pure neon gas and seal the tube from the vacuum manifold, all in about 10 min. The only measurement he will make will be the pressure of the filling gas. 

Into a dry tube fitted with a Youngs tap and a Quickfit joint (ofthe type used to polymerize butyl acrylate in Protocol 1) is placed a small piece of sodium (about the size of a match-head). The tube is connected to the manifold or a vacuum line, evacuated and a sodium mirror formed (as for potassium above.) The tube is then cooled ( 78°C, dry ice/acetone) and ethylene oxide (ca. 8 mL) is added from the cylinder. The tube is then held at -10°C for 24 h. After this time the liquid is once again connected to the gas-line, frozen (liquid nitrogen), and the tube evacuated. An identical tube containing a sodium mirror is also prepared on the same line and cooled to 78°C. The first tube is allowed to return to room temperature and ethylene oxide is then transferred by virtue of its low vapour pressure. The second tube is then isolated from the system and once again allowed to stand under vacuum for 24 h. 

---