VACUUM-PUMP. This has an opposite effect to an air compressor; that is, it withdraws air instead of forcing it in. The smaller pumps are employed for scientific purposes, and for the exhaustion of incandescent lamp bulbs and Röntgen tubes. Large units serve factories carrying on certain chemical processes, and making foodstuffs, sauces, varnishes, etc. A vacuum is largely used for boiling semi-liquids or liquids at a low tempera ture, the boiling point being determined by pressure. Condensed milk and milk powder are made by boiling off the water in a vacuum, while oil refining and sugar refining are also performed with the help of a vacuum.
The large pumps are built similarly to air-compressors, and driven by belt, steam, electric motor, or internal-combustion engine, and ranging in power requirement to more than 200 horse-power. Small types such as the well-known Sprengel mercury pump are composed of tubes and bulbs mounted against a vertical 'frame or board. But a mechanical pump, the Geryk, patented by Fleuss (and made by The Pulsometer Engineering Co. Ltd.) is extremely simple to use, and gives a very high
vacuum, whether on a small or a large scale. The single-cylinder type will give a vacuum of of a millimetre off perfect, while in an elaborated type, and under favourable conditions, a vacuum of 2, 1.0–.000 mm. can be obtained. The piston of the small pumps is reciprocated by a lever, or a lever worked from a handwheel, while larger units derive their power from an electric motor. Rotary pumps are also made on a modified principle.
In the diagram the top of the piston and the underface of the head valve are seen to be of the same form, so that when they are in contact no air space exists between them. As the piston reaches the top of its stroke it over-runs the cylinder bore slightly, and is therefore in close contact with the head valve, expelling all air. The piston in rising to this position comes into contact with the oil. The main valve is oil sealed with a special oil of low vapour tension. See also VACUUM. (F. H.) VACUUM TUBE : see THERMIONIC VALVE.