Vacuum pumps, mechanical Operating range

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... 

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. 

A liquid ring vacuum pump or a mechanical dry pump is used. Operating pressures are in the range of 1 to several hundred Torr over the time inten/al of one cycle. Operating pressures and temperatures are just below the saturation temperature for the operating fluid. 

Speciai working range of vacuum pumps I I Normal working range of vacuum pumps Figure 5.2 Common operating pressure ranges of mechanical vacuum pumps. 

If the pump is a filter pump off a high-pressure water supply, its performance will be limited by the temperature of the water because the vapour pressure of water at 10°, 15°, 20° and 25° is 9.2, 12.8, 17.5 and 23.8 mm Hg respectively. The pressure can be measured with an ordinary manometer. For vacuums in the range lO" mm Hg to 10 mm Hg, rotary mechanical pumps (oil pumps) are used and the pressure can be measured with a Vacustat McLeod type gauge. If still higher vacuums are required, for example for high vacuum sublimations, a mercury diffusion pump is suitable. Such a pump can provide a vacuum up to 10" mm Hg. For better efficiencies, the pump can be backed up by a mechanical pump. In all cases, the mercury pump is connected to the distillation apparatus through several traps to remove mercury vapours. These traps may operate by chemical action, for example the use of sodium hydroxide pellets to react with acids, or by condensation, in which case empty tubes cooled in solid carbon dioxide-ethanol or liquid nitrogen (contained in wide-mouthed Dewar flasks) are used. 

A vacuum up to 10 mmHg can be produced by a rotary (or mechanical) pump. Steam-jet pumps operate within the range from 10- ... 

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