Vacuum engineering is the field of engineering that deals with the practical use of vacuum in industrial and scientific applications. Vacuum may improve the productivity and performance of processes otherwise carried out at normal air pressure, or may make possible processes that could not be done in the presence of air. Vacuum engineering techniques are widely applied in materials processing such as drying or filtering, chemical processing, application of metal coatings to objects, manufacture of electron devices and incandescent lamps, and in scientific research.
Vacuum techniques vary depending on the desired vacuum pressure to be achieved. For a "rough" vacuum, over 100 Pascals pressure, conventional methods of analysis, materials, pumps and measuring instruments can be used, whereas ultrahigh vacuum systems use specialized equipment to achieve pressures below one-millionth of one Pascal. At such low pressures, even metals may emit enough gas to cause serious contamination.
Design and mechanism
Vacuum systems usually consist of gauges, vapor jet and pumps, vapor traps and valves along with other extensional piping. A vessel that is operating under vacuum system may be any of these types such as processing tank, steam simulator, particle accelerator, or any other type of space that has an enclosed chamber to maintain the system in less than atmospheric gas pressure. Since a vacuum is created in an enclosed chamber, the consideration of being able to withstand external atmospheric pressure are the usual precaution for this type of design. Along with the effect of buckling or collapsing, the outer shell of vacuum chamber will be carefully evaluated and any sign of deterioration will be corrected by the increase of thickness of the shell itself. The main materials used for vacuum design are usually mild steel, stainless steel, and aluminum. Other sections such as glass are used for gauge glass, view ports, and sometimes electrical insulation. The interior of the vacuum chamber should always be smooth and free of rust and defects. High pressure solvents are usually used to remove any excess oil and contaminants that will negatively affect the vacuum. Because a vacuum chamber is in an enclosed space, only very specific detergents can be used to prevent any hazards or danger during cleaning. Any vacuum chamber should always have a certain number of access and viewing ports. These are usually in the form of a flange connection to the attachment of pumps, piping or any other parts required for system operation. Extremely important is the design of the vacuum chamber's sealing capability. The chamber itself must be airtight to maintain perfect vacuum. This is ensured through the process of leaking checking, generally using a mass spectrometer leak detector.[1] All openings and connections are also assembled with o-rings and gaskets to prevent any further possible leakage of air into the system.
Vacuum engineering uses techniques and equipment that vary 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, cryogenic 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.
Applications
Vacuum technology is a method used to evacuate air from a closed volume by creating a pressure differential from the closed volume to some vent, the ultimate vent being the open atmosphere.[2] When using an industrial vacuum system, a vacuum pump or generator creates this pressure differential. A variety of technical inventions were created based on the idea of vacuum discovered during the 17th century. These range from vacuum generation pumps to X-ray tubes, which were later introduced to the medical field for use as sources of X-ray radiation. The vacuum environment has come to play an important role in scientific research as new discoveries are being made by looking back to the fundamentals of pressure. The idea of “perfect vacuum” cannot be realized, but very nearly approximated by the technological discoveries of the early 20th century. Vacuum engineering today uses a range of different material, from aluminum to zirconium and just about everything in between. There may be the popular belief that vacuum technology deals only with valves, flanges, and other vacuum components, but novel scientific discoveries are often made with the assistance of these traditional vacuum technologies, especially in the realm of high-tech. Vacuum engineering is used for compound semiconductors, power devices, memory logic, and photovoltaics.
Another technical invention is the vacuum pump. Such invention is used to remove gas molecules from sealed volume, thus leaving behind a partial vacuum. More than one vacuum pump is used in a single application to create fluent flow. Fluent flow is used to allow a clear path made using vacuum to remove any air molecules in the way of the process. Vacuum will be used in this process to attempt to create a perfect vacuum. A type of vacuum such as partial vacuum can be caused by the usage of positive displacement type pumps. A positive displacement pump is able to transfer gas load from the entrance to the exit port, but due to its design limitation, it can only achieve a relatively low vacuum. In order to reach a higher vacuum, other techniques must be used. Using a series of pumps, such as following up a fast pump down with a positive displacement type pump, will create a much better vacuum than using a single pump. The combination of pumps used is usually determined by the need of vacuum in the system. Some applications in the chemical, pharmaceutical, oil and gas and other industries require complex process vacuum systems.
Materials
Materials for use in vacuum systems must be carefully evaluated.[3] Many materials have a degree of porosity, while unimportant at ordinary pressures, 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 are driven off. However, this requirement affects which materials can be used. For low pressure applications, it is possible to post process even 3-D printed plastic to make vacuum systems.[4]
Particle accelerators are the largest ultrahigh vacuum systems and can be up to kilometres in length.[5]
Vacuum systems have been studied for a long time so now the properties of basic materials used in vacuum tubes (carbon, ceramics, copper, glass, graphite, iron, mica, nickel, precious metals, refractory metals, steel, and all relevant alloys) and well understood, including their joining techniques and how to deal with common problems such as secondary emission and voltage breakdown.[6]
History
The word “Vacuum” is originated from the Latin word “vacua”, which is translated to the word “empty”. Physicists use vacuum to describe a partially empty space, where air or some other gases are being removed from one container. The idea of vacuum relating to the empty space has been speculated as early as 5th century from Greek philosophers, Aristotle (384-322 B.C.) was the one who came up with the relation of vacuum being an empty space in nature would be impossible to ever create.[7] This idea had stuck around for over centuries until the 17th century, when vacuum technology and physics was discovered. In the mid 17th century, Evangelista Torricelli studied the properties of a vacuum generated by a mercury column in a glass tube; this became the barometer, an instrument to observe variations in atmospheric air pressure. Otto von Guericke spectacularly demonstrated the effect of atmospheric pressure in 1654, when teams of horses could not separate two 20-inch diameter hemispheres, which had been placed together and evacuated. In 1698, Thomas Savery patented a steam pump that relied on condensation of steam to produce a low-grade vacuum, for pumping water out of mines. The apparatus was improved in the Newcomen atmospheric engine of 1712; while inefficient, it allowed coal mines to be exploited that otherwise would flood by ground water. During the years of 1564–1642, the famous scientist Galileo was one of the first physicist to conduct experiments to develop measured forces to develop vacuum using a piston in a cylinder. This was a big discovery for scientist and was shared among others. French scientist and philosopher Blaise Pascal used the idea that was discovered to look into further research of vacuum. Pascal discoveries were similar to Torricelli's research as Pascal used similar methods to pull vacuum using mercury. It was until the year 1661, when the mayor of the city of Magdeburg used this discovery to invent or retrofit new ideas. The mayor Otto von Guericke created the first air pump, modified the idea of water pumps, and also modified manometers. Vacuum engineering nowadays provides the solution for all thin film needs in the mechanical industry. This method of engineering is typically used for R&D needs or large scale material production.
Pump technology hit a plateau until Geissler and Sprengle in the mid 19th century, who finally gave access to the high-vacuum regime. This led to the study of electrical discharges in vacuum, discovery of cathode rays, discovery of X-rays and the discovery of the electron. The photoelectric effect was observed in high vacuum, which was a key discovery that lead to the formulation of quantum mechanics and much of modern physics.
Chemical vapor deposition (CVD) is a vacuum deposition method used to produce high-quality, and high-performance, solid materials. The process is often used in the semiconductor industry to produce thin films.
Distillation, or classical distillation, is the process of separating the components or substances from a liquid mixture by using selective boiling and condensation, usually inside an apparatus known as a still. Dry distillation is the heating of solid materials to produce gaseous products ; this may involve chemical changes such as destructive distillation or cracking. Distillation may result in essentially complete separation, or it may be a partial separation that increases the concentration of selected components; in either case, the process exploits differences in the relative volatility of the mixture's components. In industrial applications, distillation is a unit operation of practically universal importance, but is a physical separation process, not a chemical reaction. An installation used for distillation, especially of distilled beverages, is a distillery. Distillation includes the following applications:
Pressure measurement is the measurement of an applied force by a fluid on a surface. Pressure is typically measured in units of force per unit of surface area. Many techniques have been developed for the measurement of pressure and vacuum. Instruments used to measure and display pressure mechanically are called pressure gauges,vacuum gauges or compound gauges. The widely used Bourdon gauge is a mechanical device, which both measures and indicates and is probably the best known type of gauge.
A vacuum pump is a type of pump device that draws gas particles from a sealed volume in order to leave behind a partial vacuum. The first vacuum pump was invented in 1650 by Otto von Guericke, and was preceded by the suction pump, which dates to antiquity.
A vacuum is a space devoid of matter. The word is derived from the Latin adjective vacuus for "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often discuss ideal test results that would occur in a perfect vacuum, which they sometimes simply call "vacuum" or free space, and use the term partial vacuum to refer to an actual imperfect vacuum as one might have in a laboratory or in space. In engineering and applied physics on the other hand, vacuum refers to any space in which the pressure is considerably lower than atmospheric pressure. The Latin term in vacuo is used to describe an object that is surrounded by a vacuum.
A cryopump or a "cryogenic pump" is a vacuum pump that traps gases and vapours by condensing them on a cold surface, but are only effective on some gases. The effectiveness depends on the freezing and boiling points of the gas relative to the cryopump's temperature. They are sometimes used to block particular contaminants, for example in front of a diffusion pump to trap backstreaming oil, or in front of a McLeod gauge to keep out water. In this function, they are called a cryotrap, waterpump or cold trap, even though the physical mechanism is the same as for a cryopump.
Pneumatics is a branch of engineering that makes use of gas or pressurized air.
A getter is a deposit of reactive material that is placed inside a vacuum system to complete and maintain the vacuum. When gas molecules strike the getter material, they combine with it chemically or by absorption. Thus the getter removes small amounts of gas from the evacuated space. The getter is usually a coating applied to a surface within the evacuated chamber.
A vacuum chamber is a rigid enclosure from which air and other gases are removed by a vacuum pump. This results in a low-pressure environment within the chamber, commonly referred to as a vacuum. A vacuum environment allows researchers to conduct physical experiments or to test mechanical devices which must operate in outer space or for processes such as vacuum drying or vacuum coating. Chambers are typically made of metals which may or may not shield applied external magnetic fields depending on wall thickness, frequency, resistivity, and permeability of the material used. Only some materials are suitable for vacuum use.
A chiller is a machine that removes heat from a liquid coolant via a vapor-compression, adsorption refrigeration, or absorption refrigeration cycles. This liquid can then be circulated through a heat exchanger to cool equipment, or another process stream. As a necessary by-product, refrigeration creates waste heat that must be exhausted to ambience, or for greater efficiency, recovered for heating purposes. Vapor compression chillers may use any of a number of different types of compressors. Most common today are the hermetic scroll, semi-hermetic screw, or centrifugal compressors. The condensing side of the chiller can be either air or water cooled. Even when liquid cooled, the chiller is often cooled by an induced or forced draft cooling tower. Absorption and adsorption chillers require a heat source to function.
Ultra-high vacuum is the vacuum regime characterised by pressures lower than about 1×10−6 pascals. UHV conditions are created by pumping the gas out of a UHV chamber. At these low pressures the mean free path of a gas molecule is greater than approximately 40 km, so the gas is in free molecular flow, and gas molecules will collide with the chamber walls many times before colliding with each other. Almost all molecular interactions therefore take place on various surfaces in the chamber.
An injector is a system of ducting and nozzles used to direct the flow of a high-pressure fluid in such a way that a lower pressure fluid is entrained in the jet and carried through a duct to a region of higher pressure. It is a fluid-dynamic pump with no moving parts except a valve to control inlet flow.
A liquid-ring pump is a rotating positive-displacement gas pump, with liquid under centrifugal force acting as a seal.
A vacuum furnace is a type of furnace in which the product in the furnace is surrounded by a vacuum during processing. The absence of air or other gases prevents oxidation, heat loss from the product through convection, and removes a source of contamination. This enables the furnace to heat materials to temperatures as high as 3,000 °C (5,432 °F) with select materials. Maximum furnace temperatures and vacuum levels depend on melting points and vapor pressures of heated materials. Vacuum furnaces are used to carry out processes such as annealing, brazing, sintering and heat treatment with high consistency and low contamination.
Investment casting is an industrial process based on lost-wax casting, one of the oldest known metal-forming techniques. The term "lost-wax casting" can also refer to modern investment casting processes.
Thermal spraying techniques are coating processes in which melted materials are sprayed onto a surface. The "feedstock" is heated by electrical or chemical means.
An evaporator is a type of heat exchanger device that facilitates evaporation by utilizing conductive and convective heat transfer to provide the necessary thermal energy for phase transition from liquid to vapor. Within evaporators, a circulating liquid is exposed to an atmospheric or reduced pressure environment, causing it to boil at a lower temperature compared to normal atmospheric boiling.
Sputter deposition is a physical vapor deposition (PVD) method of thin film deposition by the phenomenon of sputtering. This involves ejecting material from a "target" that is a source onto a "substrate" such as a silicon wafer. Resputtering is re-emission of the deposited material during the deposition process by ion or atom bombardment. Sputtered atoms ejected from the target have a wide energy distribution, typically up to tens of eV. The sputtered ions can ballistically fly from the target in straight lines and impact energetically on the substrates or vacuum chamber. Alternatively, at higher gas pressures, the ions collide with the gas atoms that act as a moderator and move diffusively, reaching the substrates or vacuum chamber wall and condensing after undergoing a random walk. The entire range from high-energy ballistic impact to low-energy thermalized motion is accessible by changing the background gas pressure. The sputtering gas is often an inert gas such as argon. For efficient momentum transfer, the atomic weight of the sputtering gas should be close to the atomic weight of the target, so for sputtering light elements neon is preferable, while for heavy elements krypton or xenon are used. Reactive gases can also be used to sputter compounds. The compound can be formed on the target surface, in-flight or on the substrate depending on the process parameters. The availability of many parameters that control sputter deposition make it a complex process, but also allow experts a large degree of control over the growth and microstructure of the film.
Porosity sealing is done through the process of vacuum impregnation. Vacuum impregnation is a preferred OEM process that seals porosity and leak paths in metal castings, sintered metal parts and electrical castings that form during the casting or molding process. Vacuum impregnation stops casting porosity
Oxygen plants are industrial systems designed to generate oxygen. They typically use air as a feedstock and separate it from other components of air using pressure swing adsorption or membrane separation techniques. Such plants are distinct from cryogenic separation plants which separate and capture all the components of air.
References
↑ Atta, C.M. Van (1965). Vacuum Science and Engineering. New York: McGraw-Hill. p.303.
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