A cryocooler is a device used to cool the environment and anything inside it to extremely cold temperatures. Typically used in scientific and engineering applications, it is designed to achieve temperatures well below those reached by standard appliances. There is no officially defined temperature requirement for what is considered a cryocooler. A device that can cool to approximately -238 degrees Fahrenheit (or about –150 degrees Celsius) or colder, however, is usually referred to as a cryocooler.
While there are several types of cryocoolers, most operate on some variation of a common process. Gas is typically circulated through a closed cycle to absorb heat from the interior of the device and transfer it to the outside environment. This gas may be hydrogen, helium, or some other gas or mixture of gases. The ability of the device to cool its interior environment depends largely on the thermodynamic properties of the gas circulating through the system.
Cooling cycles in these devices typically begin with gas being compressed in a compressor. As the compressed gas passes through a heat exchanger, it absorbs heat from the inside of the cryocooler, thereby cooling anything inside it. When this gas absorbs heat at a constant volume in the heat exchanger, its pressure increases. It expands in volume and its pressure decreases in the next portion of the cycle. Finally, it returns to the compressor, which completes a closed loop through the cycle, and begins to circulate through the cycle again.
A cryocooler may sometimes mistakenly be referred to as a cryostat. There is a small but distinct difference, however, between the two. A cryostat is used to maintain cryogenic temperatures already in place, usually passively such as with insulation. On the other hand, a cryocooler actively operates to cool the environment to cryogenic temperatures rather than merely to maintain environmental conditions that already exist. This difference can be thought of as similar to the difference between a thermos and a refrigerator.
Many types of cryocoolers are available with different advantages and features suitable for a wide variety of applications. Common types of cryocoolers include the Joule–Thomson cooler, the Gifford–McMahon cooler, the Stirling cooler, the pulse tube refrigerator, and the adiabatic demagnetization refrigerator. While notably less efficient than many other devices, Joule-Thomson cryocoolers provide advantages in reliability and low electrical and mechanical noise levels. Gifford-McMahon coolers, on the other hand, generate some vibration due to a piston that pushes the gas through the system. They do, however, offer flexibility to the user as they can be operated in any orientation.
Special consideration is typically given to selecting a cryocooler for use in space. In such applications, power typically must be used efficiently and repair is extremely costly or even impossible — such as for missions to other planets. Noted for both reliability and efficiency, Stirling coolers are the first to have been used successfully in space. With even higher reliability than Stirling coolers, pulse tube coolers are often chosen for space even though typically they are slightly less efficient. An adiabatic demagnetization refrigerator may also be chosen for its excellent efficiency and ability to operate in zero gravity environments.
There are numerous fields in which cryocoolers play a vital role. These include medical, automotive, and aerospace applications, use in scientific research and military operations, and more. For example, cryogenic hardening of metal components can change their physical properties, increasing strength, hardness, and resistance to wear. Infrared sensors used in satellite-based surveillance and missile guidance, as well as atmospheric studies and more, typically require cryogenic cooling.