Vacuum engineering deals with technological processes and equipment that use vacuum to achieve better results than those run under atmospheric pressure. The most widespread applications of vacuum technology are:
Pyrolytic chromium carbide coating
Vacuum coaters are capable of applying various types of coatings on metal, glass, plastic or ceramic surfaces, providing high quality and uniform thickness and color. Vacuum dryers can be used for delicate materials and save significant quantities of energy due to lower drying temperatures.
Vacuum engineering uses techniques and equipment that vary greatly depending on the level of vacuum used. Pressure slightly reduced from atmospheric pressure may be used to control airflow in ventilation systems, or in material handling systems. Lower-pressure vacuums may be used in vacuum evaporation in processing of food stuffs without excessive heating. Higher grades of vacuum are used for degassing, vacuum metallurgy, and in the production of light bulbs and cathode ray tubes. So-called "ultrahigh" vacuums are required for certain semiconductor processing; the "hardest" vacuums with the lowest pressure are produced for experiments in physics, where even a few stray atoms of air would interfere with the experiment in progress.
Apparatus used varies with decreasing pressure. Blowers give way to various kinds of reciprocating and rotary pumps. For some important applications, a steam ejector can quickly evacuate a large process vessel to a rough vacuum, sufficient for some processes or as a preliminary to more complete pumping processes. The invention of the Sprengel pump was a critical step in the development of the incandescent light bulb as it allowed creation of a vacuum that was higher than previously available, which extended the life of the bulbs. At higher vacuum levels (lower pressures), diffusion pumps, absorption, cyrogenic pumps are used. Pumps are more like "compressors" since they gather the rarefied gases in the vacuum vessel and push them into a much higher pressure, smaller volume, exhaust. A chain of two or more different kinds of vacuum pumps may be used in a vacuum system, with one "roughing" pump removing most of the mass of air from the system, and the additional stages handling relatively smaller amounts of air at lower and lower pressures. In some applications, a chemical element is used to combine with the air remaining in an enclosure after pumping. For example, in electronic vacuum tubes, a metallic "getter" was heated by induction to remove the air left after initial pump down and closure of the tubes. The "getter" would also slowly remove any gas evolved within the tube during its remaining life, maintaining sufficiently good vacuum.
Materials for use in vacuum systems must be carefully evaluated. Many materials have a degree of porosity, unimportant at ordinary pressures, but which would continually admit minute amounts of air into a vacuum system if incorrectly used. Some items, such as rubber and plastic, give off gases into a vacuum that can contaminate the system. At high and ultrahigh vacuum levels, even metals must be carefully selected - air molecules and moisture can cling to the surface of metals, and any trapped gas within the metal may percolate to the surface under vacuum. In some vacuum systems, a simple coating of low-volatile grease is sufficient to seal gaps in joints, but at ultrahigh vacuum, fittings must be carefully machined and polished to minimize trapped gas. It is usual practice to bake components of a high-vacuum system; at high temperatures, any gases or moisture adhering to the surface is driven off. However, this requirement affects which materials can be used.
Particle accelerators are the largest ultrahigh vacuum systems and can be up to kilometres in length.