Vacuum Technology

I

Introduction

Vacuum Technology, processes and equipment based on the principle that, when the quantity of gas such as air in a closed vessel is reduced, the remaining molecules, atoms, or any electrically charged particles that are derived from them, such as ions and electrons, can move about more freely. This freedom is proportional to the reduction in the gas pressure.

II

Development

Low and medium vacuums have been commonly used in such household equipment as vacuum bottles and vacuum cleaners since the late 19th century. The distillation of lubricating oils from petroleum residues and the removal of atmospheric oxygen from electric light bulbs also employ vacuum technology. Before World War II, however, high-vacuum techniques, achieving near-complete vacuum conditions, were mainly used in research laboratories, the one exception being vacuum-tube production. During the war, techniques for coating optical lenses with extremely thin films of magnesium fluoride, using high vacuum, became established. This process improved the optical quality of the lenses by reducing light reflection. High-vacuum techniques are also employed in the molecular distillation of fish oils to produce vitamin A concentrate, and in the electromagnetic separation of uranium-235 from the more abundant isotope of uranium with which it is associated in nature.

III

Uses

One of the more important recent applications of vacuum technology is in large-scale industrial refrigeration. The rate of evaporation of water is accelerated in vacuum conditions and the process is used for freeze-drying foods (see Food Processing and Preservation). The water in the food is removed by sublimation into ice, which at the same time freezes the food. Metal evaporation in high vacuum is used to coat plastics and other objects to give them a high, metallic lustre. This process was an outgrowth of the lens-coating process. The production rate of television tubes was greatly accelerated by the introduction of high-speed, high-vacuum pumps. High-vacuum treatment of melted, cast, or sintered metals improves their physical properties by removing gases and other impurities. Single-metal crystals used in transistors and similar electronic devices are “grown”, or prepared, in high-vacuum furnaces. Electrical transformers and high-voltage cables are vacuum-impregnated with high-dielectric material to improve the insulation. To obtain maximum insulation from heat for flasks and pipes that store and transport liquid oxygen, nitrogen, and helium, the container walls are maintained at high vacuum. Substrates, or bases, used in making electronic microcircuits are prepared by sputter-coating them with refractory materials such as tantalum and tungsten under high-vacuum conditions. See Integrated Circuit.

Vacuum technology is very important in scientific and technological research. Atomic particle accelerators depend on high and very high vacuum to provide a relatively gas-free unobstructed path for the particles. Large chambers, sometimes with capacities of thousands of cubic metres, requiring great pumping speeds for gas removal, are used to test aerospace equipment in simulated space conditions. In certain types of chemical analysis, in which the material to be analysed must be in a gaseous state or in the form of electrically charged ions, a vacuum must be used to satisfy these requirements. The mass spectrometer, electron microscope, and vacuum-fusion and nuclear magnetic resonance analysers are a few such instruments. New uses for vacuum operation are continually being discovered.

IV

Nature of Equipment

An operational vacuum system is made up, in general, of three parts: the chamber in which the work is done, the vacuum pumps, and the accessory equipment such as electrical controls and piping. A simple vacuum system is shown in Fig. 1. To make the equipment operable, the work chamber with its vacuum gauge is sealed vacuum-tight to the pumping port. When the high-vacuum and roughing valves have been closed and the forepressure valve opened, both the diffusion and mechanical pumps are started. When the diffusion pump is operating, it is isolated from the rest of the system by the closing of the forepressure valve. The work chamber is then pumped out, first by the mechanical pump. To do this, the air inlet valve is closed and the roughing valve opened. The pressure in the work chamber is lowered to around one ten-thousandth of a bar. (The bar is 100,000 pascals and equal to 29.53 inches of mercury, slightly less than standard atmospheric pressure.) Then the chamber is opened to the diffusion pump by first closing the roughing valve and then opening the forepressure and high-vacuum valves. The work chamber is then ready to carry out its high-vacuum operation. The work chamber is an airtight container with one or more places of access to the interior. A gasketed glass or steel bell jar is often used as the work chamber for simple operations. Leaktight connections to the chamber are provided for accessories such as sight glasses, devices that transfer mechanical motion into the vacuum, and electrical terminals.

An early type of vacuum pump resembled the reciprocating steam engine. This pump has been replaced in vacuum work today by the rotary oil-seal pump (Fig. 2) and the ejector pump (Fig. 3). In the rotary pump, an eccentric cylinder rotates within a hollow cylindrical casing. A reciprocating vane mounted in the casing and maintained in contact with the rotor provides a seal between inlet and outlet ports. The entire interior is flooded with a low vapour-pressure sealing oil. Ejection pumps operate on the principle that a liquid or gas under pressure, when released through a nozzle (sometimes called a jet) in a directed stream, will pick up gas molecules in a mixing chamber and eject them, thus producing a vacuum. If water is the moving force, the vacuum device is called an aspirator or barometric condenser; if steam is the mover, it is called a steam ejector. The diffusion pump operates on a similar principle but uses the boiled-off vapour of a very low vapour-pressure liquid, such as a specially chosen and prepared organic fluid, or mercury, as the moving medium. The boiled-off vapours are recycled by continually condensing and reboiling the returned condensate. A few of the many other types of vacuum pump include: ion pumps (used when a dry, vapour-free condition is essential), operating by ionizing the gas molecules and trapping them on electrically charged collector plates; chemical-ion pumps, which rely on the reaction of vapours of a metal such as titanium with the gas, followed by their condensation on the walls of the pump casing; and sorption pumps, which remove gases by adsorbing and absorbing them, using artificial zeolite, the “molecular sieve”. Cryogenic pumping is accomplished by condensing out the gases on surfaces maintained at extremely low temperatures.