In physics and engineering, cryogenics is the study of the production of very low temperatures (below –150 °C, –238 °F or 123 K) and the behavior of materials at those temperatures. Rather than the familiar temperature scales of Fahrenheit and Celsius, cryogenicists use the Kelvin (and formerly Rankine) scales.
Contents |
The terms cryogenics, cryobiology and cryonics are frequently confused. Other new terms with the prefix cryo- have also been introduced.
The word cryogenics means "the production of freezing cold"; however the term is used today as a synonym for the low-temperature state. It is not well-defined at what point on the temperature scale refrigeration ends and cryogenics begins. The workers at the National Institute of Standards and Technology at Boulder, Colorado have chosen to consider the field of cryogenics as that involving temperatures below –180 °C (93.15 K). This is a logical dividing line, since the normal boiling points of the so-called permanent gases (such as helium, hydrogen, neon, nitrogen, oxygen, and normal air) lie below -180 °C while the Freon refrigerants, hydrogen sulfide, and other common refrigerants have boiling points above -180 °C.
Liquefied gases, such as liquid nitrogen and liquid helium, are used in many cryogenic applications. Liquid nitrogen is the most commonly used element in cryogenics and is legally purchasable around the world. Liquid helium is also commonly used and allows for the lowest attainable temperatures to be reached.
These liquids are held in either special containers known as Dewar flasks, which are generally about six feet tall (1.8 m) and three feet (91.5 cm) in diameter, or giant tanks in larger commercial operations. Dewar flasks are named after their inventor, James Dewar, the man who first liquefied hydrogen. Museums typically display smaller vacuum flasks fitted in a protective casing.
Cryogenic transfer pumps are the pumps used on LNG piers to transfer Liquefied Natural Gas from LNG Carriers to LNG storage tanks.
The field of cryogenics advanced during World War II when scientists found that metals frozen to low temperatures showed more resistance to wear. Based on this theory of cryogenic hardening, the commercial cryogenic processing industry was founded in 1966 by Ed Busch. With a background in the heat treating industry, Busch founded a company in Detroit called CryoTech in 1966. Though CryoTech later merged with 300 Below to create the largest and oldest commercial cryogenics company in the world, they originally experimented with the possibility of increasing the life of metal tools to anywhere between 200%-400% of the original life expectancy using cryogenic tempering instead of heat treating. This evolved in the late 1990s into the treatment of other parts (that did more than just increase the life of a product) such as amplifier valves (improved sound quality), baseball bats (greater sweet spot), golf clubs (greater sweet spot), racing engines (greater performance under stress), firearms (less warping after continuous shooting), knives, razor blades, brake rotors and even pantyhose. The theory was based on how heat-treating metal works (the temperatures are lowered to room temperature from a high degree causing certain strength increases in the molecular structure to occur) and supposed that continuing the descent would allow for further strength increases. Using liquid nitrogen, CryoTech formulated the first early version of the cryogenic processor. Unfortunately for the newly-born industry, the results were unstable, as components sometimes experienced thermal shock when they were cooled too fast. Some components in early tests even shattered because of the ultra-low temperatures. In the late twentieth century, the field improved significantly with the rise of applied research, which coupled microprocessor based industrial controls to the cryogenic processor in order to create more stable results.
Cryogens, like liquid nitrogen, are further used for specialty chilling and freezing applications. Nonetheless these cryogens have many applications in the control and tampering with human beings. Some chemical reactions, like those used to produce the active ingredients for the popular statin drugs, must occur at low temperatures of approximately -100 °C. Special cryogenic chemical reactors are used to remove reaction heat and provide a low temperature environment. The freezing of foods and biotechnology products, like vaccines, requires nitrogen in blast freezing or immersion freezing systems. Certain soft or elastic materials become hard and brittle at very low temperatures, which makes cryogenic milling (grinding) an option for some materials that cannot easily be milled at higher temperatures.
Another use of cryogenics is cryogenic fuels. Cryogenic fuels, mainly liquid hydrogen, have been used as rocket fuels. (Oxygen is used as an oxidizer of hydrogen, but oxygen is not, strictly speaking, a fuel.) For example, NASA's workhorse space shuttle uses cryogenic hydrogen fuel as its primary means of getting into orbit, as did all of the rockets built for the Soviet space program by Sergei Korolev. (This was a bone of contention between him and rival engine designer Valentin Glushko, who felt that cryogenic fuels were impractical for large-scale rockets such as the ill-fated N-1 rocket spacecraft.)
Russian aircraft manufacturer Tupolev is currently researching a version of its popular design Tu-154 with a cryogenic fuel system, known as the Tu-155. The plane uses a fuel referred to as liquefied natural gas or LNG, and made its first flight in 1989.
Cryogenic cooling of devices and material is usually achieved via the use of liquid nitrogen, liquid helium, or a cryocompressor (which uses high pressure helium lines).
Cryogenic temperatures, usually well below 77 K (-196 °C) are required to operate cryogenic detectors.
|
||||||||||||||||||||||||||