How to apply cryogenics. (Vacuum Tech).
Cryogenics applies to the study of materials at or -150[degrees] C (-238[degrees]F or 123 K). The word cryogenic itself is a derivative of the Greek kyros, meaning "ice cold," Cryogenic temperatures are typically obtained by rapidly evaporating volatile liquids or by expanding gases confined at approximately 10 to 20 MPa.
The study of cryogenics is more than a century old. Toward the end of the 19th century (1877), French physicist Louis Paul Callietet and Swiss scientist Raoul Pictet produced droplets of liquid oxygen via the cascade principle. The inversion temperatures--the temperature at which the Joule-Thomson effect of a gas changes sign--of two premier cryogenic gases, hydrogen and helium, are quite low. In order to achieve a temperature reduction of these gases through expansion, they must be precooled below their inversion temperature--hydrogen by liquid air and helium by liquid hydrogen. By cascading these effects, the liquefaction temperatures can be obtained.
The evaporation of liquid helium at reduced temperatures is used to produce temperatures down to 0.7 K. Even lower temperatures can be obtained through the application of adiabatic demagnetization.
Cryogenic temperatures can be obtained with the use of a specially designed and constructed refrigerator (cryocooler). The gases most typically used to achieve cryogenic temperatures are nitrogen (77 K) and helium (4 K).
The temperature range for most cryogenic applications is about -50[degrees] to -160[degrees] C. In addition to both propulsion and cooling, cryogenic applications are specialized and include high-speed turbines and compressors using gas and magnetic bearing systems.
One of the most important applications of cryogenic liquids is the production of liquid nitrogen and liquid oxygen from ambient air. Typical applications can be in rocket engines, cutting and welding torches, blast furnace systems, and life support systems in marine and space vehicles.
Other applications include cryopumps, coolers, and helium compressors found in the semiconductor manufacturing and coating industries. Medical treatment (including Alzheimer's Disease) is finding increased use of cryogenic liquids by freezing tissues and then removing the dead tissue.
Web Resources for Cryogenics:
Cryogenics: The study and use of materials at very low temperatures.
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|Author:||Studt, Tim; Poliski, Iris|
|Publication:||R & D|
|Date:||Jul 1, 2003|
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