Vacuum pumps, mechanical evacuation

Friction losses, air steam, 131 Pressure losses chart, 134 Vacuum pumps, mechanical, 382 Liquid ring pumps, 383-385 Liquid ring volume displaced/ evacuation, 387... 

Efficient high-vacuum pumps generally do not operate well near atmospheric pressure. Thus the vacuum system must have a mechanical vacuum pump to evacuate the system to a pressure where the high-vacuum pumps are effective. Mechanical pumps require routine maintenance, such as ballasting and replacing the pump oil. The diffusion pump is the least expensive and most reliable high-vacuum pump. Turbomolecular pumps and cryopumps are also used on mass analyzers. The high-vacuum pumps also require... 

Reduced pressures may be obtained by connecting a water aspirator pump or a mechanical vacuum pump to a vacuum adapter that is fitted between the condenser and the receiving flask. The vacuum produced by a water aspirator is limited by the vapor pressure of the water at the ambient temperature and the condition of the aspirator pump. Pressures as low as 8-10 torr may be obtained from a water aspirator with cold water, but pressures in the range of 15-25 torr are more common. A good mechanical vacuum pump can evacuate the apparatus to less than 0.01 torr it is important to clean the oil periodically and maintain tight coimections in the distillation apparatus to achieve the lowest possible pressures for a particular pump. Some... 

A solution of 30 g (0.1 mole) of 17j5-hydroxy-5a-androstan-3-one (androstano-lone), 20 ml of pyrrolidine and 200 ml of benzene is heated at reflux temperature for 2.5 hr under a Bidwell-Sterling water trap. The benzene solution is evaporated to dryness in a rotating evaporator connected to a water aspirator. The white cake which remains is broken up and dried further by immersing the flask in a water bath at 60-75° and evacuating the flask with a mechanical vacuum pump. After 0.5 hr the solid cake is broken up again and dried for another 0.5 hr at 60-75°. The enamine (9) obtained should smell only faintly of pyrrolidine. 

Mechanical Pumps. Perhaps the most common form of vacuum pump is a mechanical pump that operates with some sort of rotary action, with moving parts immersed in oil to seal them against back-streaming of exhaust as well as to provide lubrication. These pumps are used as forepumps for diffusion pumps. Other common laboratory applications are the evacuation of desiccators and transfer lines and distillation under reduced pressure. These pumps have ultimate pressures ranging from 10 to 0.05 Torr, and pumping speeds from 0.16 to 150 L s or more, depending on type and intended application. 

Figure 10-8. Equipment needed for lyophilization or freeze drying small samples, (a) The unit is cooled and evacnated by means of a self-contained refrigeration system and vacuum pump. Sample vessels are inserted into the rubber nozzles and the valve (little white knobs) opened for each nozzle that contains a vessel, (b, c) The units are evacuated by connecting them to a mechanical vacuum pump (connection indicated by arrows) and... 

Impregnation Process. In both processes the first step in the impregnation of wood is carried out by evacuating the air from the wood vessels and cell lumens (27). Any type of mechanical vacuum pump is adequate if it can reduce the pressure in the apparatus to 750 mm of mercury or less. Some industrial producers only reduce the pressure to about 711 mm of mercury. Experience has shown that the air in the cellular structure of most woods is removed as fast as the pressure in the evacuation vessel is reduced. Pumping for 30 min at 1 mm pressure is sufficient to remove the air. The vacuum pump is isolated from the system at this point. 

Commercial freeze-dryers have a vacuum chamber, with product shelves that can be heated or cooled and a cooled condenser. In a typical operation the frozen salt is introduced in trays onto cooled shelves to prevent melting. The chamber is evacuated by means of a mechanical vacuum pump, usually to a pressure of less than 100 Pa. The condenser coils are cooled to about —50 to — 60°C, and the product shelf is then gently heated. The condenser coils trap water as ice rather than inefficiently pumping the large volumes of water vapor generated. The heated product shelves provide the needed heat of sublimation to the frozen material. 

After calcining is complete the frequency response of the system can be measured. The system can be studied either in its actual state, or after an impurity or contaminant (such as oxygen) of known composition and amount is allowed to enter the system. The system is evacuated either after the calcining operation or after the addition operation by opening all valves except 6c and 8a and activating the mechanical vacuum pump 13. 

The experimental results which lead to these conclusions are summarized In Table I. Elemental analyses, conducted by a commercial laboratory, are presented In the form of a ratio of the relative numbers of atoms of C, H, N, and 0 normalized to 3 carbon atoms per monomer unit. IR spectra were obtained In-house using a Perkin Elmer 298 IR spectrophotometer. Solid probe magic angle spinning NMR experiments were performed at the National Bureau of Standards. All of these analyses were performed on bulk (l.e. pyrolyzed In batches of lg) samples of 485,000 average molecular weight ultrapure PAN prepared In a tube furnace evacuated by a mechanical vacuum pump, and are reported on In detail elsewhere.(13) Table I also summarizes electrical conductivity and optical absorption experimental results obtained using thin films of PAN solution cast In dlmethylformamlde. 

A pumping system is available, capable of evacuating the retort to a pressure of 0 05 fi (mercury equivalent). This consists of an oil diffusion pump, backed by a mechanical pump. In order to reduce the maintenance rate of the main pumping system, a small portable vacuum pump is used to achieve a pressure of a few millimetres, before the main pump is started. The furnace... 

Useful life for mechanical vacuum pumps is considerably shorter than for the vessel which it evacuates. This is simply because it has parts moving at a high frequency and making contact with one another. When evaluating the purchase of an enclosed vapor degreaser with a vacuum pump, one ought to insist on a warranty life of the vacuum pump of at least 2 years, and consider premium value as one which has a 3-year warranty. 

In GC-MS systems, analyte ionization, mass analysis, and ion detection take place in a high-vacuum system. In most benchtop GC-MS systems, the vacuum system consists of one pumped chamber, evacuated by means of a small turbomolecular pump, backed by a mechanical fore pump. Large systems as well as modern research-grade mass spectrometers generally contain two differentially pumped vacuum chambers, separated by means of a baffle containing a slit, i.e., the ion source housing and the analyzer region. 

The first attempt to determine whether dU/dV)x vanishes for real gases was made by Joule around 1843. His apparatus is schematically depicted in Figure 2.5. A sample of a gas (the system) was placed in one side of the apparatus and the other side of the apparatus was evacuated. Mechanical vacuum pumps were not yet invented, so a water aspirator was used. The system was allowed to equilibrate and the initial temperature of the apparatus was measured. The stopcock was then opened and the gas expanded irreversibly into the vacuum. 

The synthesis equipment consists of a stainless-steel cylinder with an inside diameter of 100 mm and a height of 265 mm and is shown schematically in Fig. 1. The wall of the cylinder is 3 mm and the top plate 10 mm in thickness. The cylinder is mounted with 12 MX 10 stainless-steel screws on a baseplate made of stainless steel. The baseplate is furnished with a rabbet that contains the O-ring Teflon seal between the cylinder flange and base. In addition, three 6-mm-o.d. stainless-steel lines are welded to the base. One line leads to a vacuum pump, another line to the gas bottle, and the third to a mechanical safety valve that opens for pressure higher than 75 atm. The inlet line has connections to a manometer and a second electrochemical safety valve that can be controlled by the manometer. A Pt/Pt 10% Rh, stainless-steel shielded thermocouple allows the temperature to be recorded during the procedure. The rubber stopper of the crucible is removed after the crucible has been placed on the pedestal of the synthesis equipment. The equipment is then evacuated to a pressure of about 10" torr, flushed two times with N2 at about 5 atm pressure, and loaded with 10 atm of N2. 

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