Producing a Vacuum

It is often required in laboratory practice that the space in an apparatus should contain no air and other gases. Such a rarefied space is called a vacuum. Water-jet pumps produce a rarefaction of the 

Water-jet pumps can be used wherever there is an adequate pressure of the water in the mains 2 at or 196.2 kPa). The ultimate vacuum depends both on the design of the pump and the water temperature and cannot exceed the water vapour pressure at the given temperature (Appendix 1, Table 13). 

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. 

The initial pressure from which normal operation of a pump begins. 

The minimum pressure reached by the given pump, called the ultimate pressure. The ultimate vacuum is determined mainly by the vapour pressure of the working fluid. 

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The so-called hydro-vac pump, shown in Fig. 11, 22, 2 (the upper half of the mercury reservoir and the column above it are insulated by a layer of asbestos), is an inexpensive, all-glass, mercury diffusion pump, which can be used in series either with an oil pmnp or with a water Alter pmnp (compare Fig. 11,21, 1) capable of producing a vacuum of at least 2 mm. It is accordingly of particular value in the organic laboratory for vacuum distillations, fractionations, sublimations and pyrolyses as well as for molecular distillations (see Section 11,26). The hydro-vac... 

Example Assume the tank in Fig. 26-50 has a diameter of 4 ft and a capacity of 1000 gal, is filled with water, and discharges to the atmosphere. The shortcut calculation (tank is initially competely full) indicates that the internal pressure would be 10.65 in Hg. An initial fillage of 70 percent of the tank s volume would produce a vacuum of 6.93 in Hg, mich is 65 percent of the shortcut result. 

Other pieces may have to be elevated to enable the system to operate. A steam jet ejector with an intercondenser that is used to produce a vacuum must be located above a 34 ft (10 m) barometric leg. Condensate receivers and holding tanks frequently must be located high enough to provide an adequate net positive suction head (NPSH) for the pump below. For many pumps an NPSH of at least 14 ft (4.2 m) H2O is desirable. Others can operate when the NPSH is only 6 ft (2 m) H2O. See Chapter 8 for a method of calculating NPSH. 

This requires a heavy-walled flask. Ordinary lab vacuums are about 15 mm Hg. A simple water-forced suction vacuum requires only a water source to produce a vacuum of about 25 mm Hg, which is satisfactory for most purposes. Evaporation causes the temperature to drop which slows evaporation. Running a stream of warm water over the flask or putting it in a warm water bath avoids this. To avoid difficulties in getting residues out of the bottom of the flask, it is useful to do the evaporation in a vacuum exsiccator shaped as shown or in a flat dish in the exsiccator. Whenever a forced water vacuum is used, it is wise to place a water trap between the vacuum and the solvent being evaporated to prevent water from entering when the pressure fluctuates. 

A vacuum desiccator has a side arm with a cock by means of which it is connected via a protective wash bottle to a water-jet pump producing a vacuum in the desiccator. 

When a flame occurs there is started a current of air generated thereby. This draft serves to maintain and to increase the flame. The fire destroys without intermission the air which supports it and would produce a vacuum, if other air could not come to supply it. So soon as the air is no longer in condition to sustain a flame, no earthly crea ture can live in it any more than can the flame. 87... 

As an older contemporary of Newton, Boyle did not have the advantage of Newton s speculations that chemistry might be handled by a system of short-range forces operating on atoms. He believed in atoms moving in space. Some of his most widely reported experiments were those carried out with an air pump. He explained that it was possible in the jar of the pump to produce a vacuum, where that meant simply a space altogether or at least almost entirely devoid of air. Boyle, unlike Descartes, believed in the possibility of a vacuum or void, a space empty of matter. He thought that all material bodies were made up corpuscles, which were in turn made up of atoms combined in different ways, ft was from these different combinations of atoms that the qualities of bodies arose. 

Contact condensers employ liquid coolants, usually water, which come in direct contact with condensing vapors. These devices are relatively uncomplicated, with typical configurations illustrated in Figure 14. Some contact condensers are simple spray chambers, usually with baffles to ensure adequate contact. Others, incorporate high-velocity jets designed to produce a vacuum. 

Lavoisier came forward with an explanation of burning which completely rejected the old notion of phlogiston. That air was necessary for combustion and breathing was known. Leonardo da Vinci during the fifteenth century believed fire destroyed without intermission the air which supports it and would produce a vacuum if other air did not come to supply it." Paracelsus back in 1535 wrote that man dies like a fire when deprived of air." Robert Boyle, too, was prone to suspect that there may be dispersed through the rest of the atmosphere some odd substance on whose account the air is... 

For some tasks very cheap and simple fish tank-type diaphragm pumps are useful. These will produce a vacuum of about SOOmmHg and can also be used to provide compressed air for pressurizing chromatography columns (see Chapter 11). 

Occasionally a distillation will require a higher vacuum than that produced by a rotary pump. Higher vacuums are normally obtained by employing a mercury or oil vapour diffusion pump, in conjunction with a rotary pump. Oil vapour pumps are most commonly used today and various models are now commercially available which will produce a vacuum of about lO mmHg. These pumps are only used occasionally but it is very useful to have one shared between a large group or section. 

A vacuum source this can be a water pump (see p. 29) with an anti-suck-back trap producing a vacuum of 15-25 mm Hg, at best or a rotary vacuum pump (consult your instructor about its use since it is an expensive piece of equipment and easily contaminated or damaged), which will evacuate down to 0.1 mm Hg. Two-stage dry vacuum pumps produce a vacuum of 1-5 mm Hg, are resistant to organic and acid vapours and are easy to use. 

Check that all joints are sealed and that the water pump is producing a vacuum it will change note as the vacuum is produced, when it is working properly. If there is no vacuum, the solution may not boil and you will overheat it in trying to promote evaporation. The joints may suddenly seal and the solution will then boil vigorously under the reduced pressure and will bump into the condenser and receiving flask. 

FIG. 8 High-speed three-stage mercury diffusion pump capabie of producing a vacuum of 5 x 10 mm Hg. Mercury is boiied in the fiask the vapor rises in the center tube, is defiected downward in the inverted cups, and entrains gas moiecuies, which diffuse in from the space to be evacuated, A. The mercury condenses to a iiquid and is returned to the fiask the gas moiecuies are removed at B by an ordinary rotary vacuum pump. 

A modern autoclave is generally equipped with a water-ring pump that can produce a vacuum of approximately 70 residual mbar in the chamber. Accordingly, almost 7% of the air is not removed. The following two methods are essentially used for completing air removal. 

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