Special Vacuum Systems

1) forepump 2) diffusion pump 3) forevacuum flask (3-5 liters) 4) drying vessel (PaOg) 5) fore-vacuum manometer 6) Moser manometer or McLeod gauge 7) condensation trap for I 8) two-way stopcock 9) Toepler pump 10) gas burette 11) level vessel. 

Low-temperature distillation columns have been devised for improving fractionation. These greatly shorten the process and in very many cases are absolutely necessary for fractionation of mixtures of substances with very close boiling points. Clusius and Wolf [Z. 

Naturforsch. 495 (1947)] have described such an apparatus, which is shown in Fig, 45. This microcolumn is similar to those described by Clusius and Schanzer pZ, I iys. Chem. (A) 192, 273 (1943)], but has a capacity of 6-10 ml. 

While the Stock system, sometimes modified for special purposes, is the standard apparatus for manipulation of readily volatile 

For many purposes one manages with the much simpler dry bags. A bag of thin elastic transparent plastic (e.g., polethylene). 

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To make these measurements, a special vacuum system is required. A complete derivation of the formulas and procedures can be found in the book The Design of High Vacuum Systems and the Application of Kinney High Vacuum Pumps by C.M. Van Atta, 1955 by Kinney Manufacturing Division, New York Air Brake Company. 

Smith undertook an exhaustive study of the nature of these films. To do so, he had to build his own specialized vacuum system because he had to deposit his films on flat plates in order to examine them by electron diffraction. He also had to be able to transfer them into the electron diffraction camera without exposure to atmospheric contamination. For pressure measurements he, too, used a McLeod gauge because the ionization gauges one could build at that time were prone to outgassing, which would contaminate the films. He made films of a great many transition metals, in addition to nickel. 

Few of the naturally occurring elements have significant amounts of radioactive isotopes, but there are many artificially produced radioactive species. Mass spectrometry can measure both radioactive and nonradioactive isotope ratios, but there are health and safety issues for the radioactive ones. However, modem isotope instmments are becoming so sensitive that only very small amounts of sample are needed. Where radioactive isotopes are a serious issue, the radioactive hazards can be minimized by using special inlet systems and ion pumps in place of rotary pumps for maintaining a vacuum. For example, mass spectrometry is now used in the analysis of Pu/ Pu ratios. 

The radioactive isotopes available for use as precursors for radioactive tracer manufacturing include barium [ C]-carbonate [1882-53-7], tritium gas, p2p] phosphoric acid or pP]-phosphoric acid [15364-02-0], p S]-sulfuric acid [13770-01 -9], and sodium [ I]-iodide [24359-64-6]. It is from these chemical forms that the corresponding radioactive tracer chemicals are synthesized. [ C]-Carbon dioxide, [ C]-benzene, and [ C]-methyl iodide require vacuum-line handling in weU-ventilated fume hoods. Tritium gas, pH]-methyl iodide, sodium borotritide, and [ I]-iodine, which are the most difficult forms of these isotopes to contain, must be handled in specialized closed systems. Sodium p S]-sulfate and sodium [ I]-iodide must be handled similarly in closed systems to avoid the Uberation of volatile p S]-sulfur oxides and [ I]-iodine. Adequate shielding must be provided when handling P P]-phosphoric acid to minimize exposure to external radiation. 

Vacuum systems, largely for the semiconductor industry, are the main source of sales (see Semiconductors). The sales of all vacuum equipment, pumps (qv), valves, sensors (qv), etc, in the United States, including apphcations not in vacuum systems, generally exceed 500 X 10 /yr. A reasonably comprehensive hst of high vacuum manufacturers is supphed by the American Vacuum Society s exhibitor s hst. In Europe, a special issue of the journal A acuum serves similady. 

Vacuum systems are typically used when flows do not exceed 6800 kg/h (15,000 Ib/h), the equivalent conveyor length is less than 305 m (1000 ft), and several points are to be supplied from one source. They are widely used for finely divided materi s. Of special interest are vacuum systems designed for flows under 7.6 kg/min (1000 Ib/h), used to transfer materials short distances from storage bins or bulk containers to process units. This type of conveyor is widely used in plastics and other processing operations where the variety of conditions requires flexibility in choosing pickup devices, power sources, and receivers. Capital investment can be kept low, often in the range of 2000 to 7000. 

As a rule, steam systems do not require special vacuum protection, since they are normally capable of withstanding vacuum developed if steam generation should fail and residual steam condense. However, low-pressure steam systems should be examined. 

Two-stage and multiple-stage vented extruders are commonly used to remove volatile components from molten resin streams prior to downstream processing. The vent ports can be open to the atmosphere, or they can be attached to elaborate vacuum systems. For very specialized systems, stripping agents such as water, nitrogen, and carbon dioxide can be added upstream of the vent, mixed into the resin stream, and then devolatilized in the vent area. This technique can be employed to remove difficult components or components at a higher rate from the resin. 

In addition to the vacuum valves, which perform solely an isolation function (fully open - fully closed position), special valves are needed for special functions. Typical are variable leak valves, which cover the leakage range from 10" ° cm /s (NTP) up to 1.6 10 cm /s (NTP). These valves are usually motor driven and suitable for remote control and when they are connected to a pressure gauge, the process pressures can be set and maintained. Other special valves fulfill safety functions, such as rapid, automatic cut-off of diffusion pumps or vacuum systems in the event of a power failure. For example, SECUVAC valves belong to this group. In the event of a power failure, they cut off the vacuum system from the pumping system and vent the forevacuum system. The vacuum system is enabled only after a certain minimum pressure (about 200 mbar) has been attained once the power has been restored. 

Components must be stable at boiling point of mixture (probably mn under vacuum) vacuum lower than 7—13 kPa (50—100 torr) may require specialized (scraped-film) crystallizer, more complicated vacuum system. 

All the monomers and solvents were purified with special care for the manipulation under high vacuum system. The synthesis of the blockcopolymer was carried out by the following scheme. 

At present, there are two basic techniques of producing a vacuum, namely, by means of vacuum pumps and by using special gas absorbers. A third auxiliary technique uses freezing out traps that lower the pressure of the vapour in a vacuum system. 

Now that we have determined what processes the facility will be used for, we can finalize utility requirements. The following utilities are required for our solid-dose facility heating, ventilation, and air conditioning (HVAC), hot and cold water, steam, electrical service, compressed air, vacuum systems, dust collection, chillers, effluent stream, and purified water. For the more specialized processes or special material handling, we may need specialized gases and breathing air. Purified water is one of the more difficult utilities to maintain the quality of. From a source of potable water, a series of treatments must be performed to control microbiological quality. Typical treatment options include carbon filters, reverse osmosis, and UV radiation. 

The subject of vacuum-line technique is approached here from the perspective of the novice user who may wish to construct a system in collaboration with a glassblower. The literature on vacuum technology [3-11] and glassblowing [12-17] is vast and there are many commercial firms that specialize in it. Within the past decade, newer components such as Teflon-glass needle valves, O-ring seals, and oil diffusion pumps have been introduced into vacuum systems. 

Several methods may be used to attach an NMR tube to a vacuum system. The O-ring union, illustrated in Fig. 8.5, permits attachment of standard NMR tubes to a vacuum outlet. Once the NMR tube has been filled with an air-sensitive sample on the vacuum system, it is necessary to bring the tube up to atmospheric pressure with inert gas, remove the tube, and quickly stopper it. Recently, a small in-line valve has been introduced which has a provision for an O-ring seal with a specially constructed NMR tube.22 This valve allows the NMR tube to be sealed before it is disconnected from the vacuum line, and the valve s left on the tube during data collection. Owing to its symmetric design, the valve does not interfere with the spinner. 

A specialized method for sample preparation is to fracture or cleave the sample inside the vacuum system, thus creating a fresh surface for immediate analysis. Some crystalline materials (semiconductors, anisotropic structures such as graphite, etc.) have preferred cleavage planes that can be sectioned inside the chamber using a blade or chisel (operated through bellows... 

In concluding this section, it is pertinent to take note of a special kind of isotopic fractionation ubiquitous, often quite severe, and arguably the most important source of fractionation that must be taken into consideration in noble gas geochemistry. This fractionation arises in mass spectrometric analysis contributory effects can and do arise in gas extraction and transport through the vacuum system, in the ion source (especially when a source magnet is used), in beam transmission, and in ion collection and detection (especially when an electron multiplier is used). As noted in Section 1.3, sample data are corrected for instrumental (and procedural) discrimination, which is calibrated by analysis of some standard gas (usually air). This is a roundabout and imperfect near-equivalent to the 8 value convention, which is the norm in stable isotope geochemistry (O, C, H, S, N, etc.). The reproducibility of instrumental discrimination inferred from repeated calibration analysis is usually quite satisfactory, but seldom is any care taken to try to match operating conditions in samples and calibration analyses. It is thus a matter of faith - undoubtedly quite... 

Ball-and-socket joints should not be used for high-vacuum systems because they are not intended for vacuum work. It is possible to obtain specially made ball joints with O-rings that are acceptable for some vacuum work. Some manufacturers supply sockets that have not been ground, and when used in tandem with ball joints with O-rings, they can achieve satisfactory vacuum performance. 

On the other hand, gases require special, cumbersome handling and often require you to enclose a given section of a vacuum system in some sort of bag. Bagging can facilitate localizing the area of a leak, but cannot help in locating the exact location of the leak. 

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