A dewar is a type of cryostat named after Sir James Dewar, the researcher who first developed the concept of a vacuum insulated container with silvered walls to reflect thermal radiation. Dewar was the first to liquefy hydrogen, and he created the device to store his discovery. The thermos bottle is a rudimentary form of … Continue reading Dewar

Stirling and Gifford-McMahon Cryocoolers

Stirling and Gifford-McMahon (GM) cryocoolers are two of the most commonly used cryocoolers in cryogenics. Both devices have a significant industrial base and operate at a wide range of temperatures and capacities. The thermodynamic cycles for both of these cryocoolers are quite similar. The Stirling cycle consists of a compressor, regenerator and a cold displacer. … Continue reading Stirling and Gifford-McMahon Cryocoolers


Stability refers to the ability of a device employing superconductors to remain in its superconducting state after part of the superconductor transitions back to its normal conducting state due to a disturbance. While the concept can apply to many superconducting devices—transmission lines, generators, motors, etc.—it is most commonly considered in the context of superconducting magnets. … Continue reading Stability

Bayonet Coupling

A bayonet coupling is a demountable joint that allows for quick and easy connection and disconnection of cryogenic components, including transfer lines, cryostats, liquefiers and refrigerators. Bayonets provide a number of advantages, chief among them the ability to connect and disconnect the bayonets while components they connect are still at cryogenic temperatures. The ability provided … Continue reading Bayonet Coupling

Space Cryogenics

Space cryogenics is, somewhat obviously, the application of cryogenics to space exploration and science. The use of cryogenics in space optimizes the launching of vehicles, provides power and life support to spacecraft and is critical for many scientific observations.Space cryogenics has a number of challenges and has driven both pure and applied research and development … Continue reading Space Cryogenics


The science of thermodynamics plays a major role in cryogenics. It underlies the various refrigeration cycles, from Carnot to Brayton (Cold Facts Vol. 32 No. 2) and Joule-Thomson, is a fundamental part of the definition of terms such as coefficient of performance (Cold Facts Vol. 31 No. 1) and even explains why it is more … Continue reading Exergy

Cryogenic Treatment

Cryogenic treatment is the process of cooling materials to cryogenic temperatures temporarily to improve their material properties at room temperature. This is distinct from cooling materials down to cryogenic temperatures to take advantage of phenomena such as superconductivity that only occur at cryogenic temperatures. Cryogenic treatment, sometimes also referred to as deep cryogenic treatment, is … Continue reading Cryogenic Treatment

Two-Phase Flows

Two-phase flows are those flows in which there is a mixture of two physical states (solid, liquid or vapor). In cryogenic applications, such flows are almost always a mixture of a cryogenic liquid along with its corresponding vapor. A mixture of liquid helium and helium vapor would be a typical example. While the complexity and … Continue reading Two-Phase Flows

Brayton Cycle

The Brayton cycle is one of the many thermodynamic cycles used to generate cooling at cryogenic temperatures. Strictly speaking, when referring to cooling we should call this the reverse Brayton cycle as the original Brayton cycle describes the process of power generation or propulsion via a gas turbine. In many publications in cryogenics, however, the … Continue reading Brayton Cycle

Fountain Pumps and He II Phase Separators

Helium II (He II), the second liquid phase of the 4He isotope described in this column in Cold Facts Spring 2010 (, can be modeled as consisting of two interpenetrating fluids. One, the superfluid component, has zero viscosity and entropy and the other, the normal fluid component, has nonzero viscosity and entropy. Research has shown … Continue reading Fountain Pumps and He II Phase Separators


A vital technology in the refrigerators and liquefiers described in Cold Facts Volume 31 Number 3 is that of turboexpanders. These devices are rotating machines in which the process fluid (e.g., helium) does work against the turboexpander while moving from high pressure to a lower pressure and thus is cooled. Such a process approximates an … Continue reading Turboexpanders

Air Separation

Air separation is one of the largest, as well as earliest, industrial applications of cryogenics. In this process, cryogenic temperatures are used to separate air into its constituent gases: nitrogen (78.08%), oxygen (20.95%), argon (0.93%) and carbon dioxide (0.3%). Trace gases such as krypton, neon, xenon and helium total far less than 1%. Water vapor … Continue reading Air Separation

Coefficient of Performance and Figure of Merit

The coefficient of performance (COP) is used to describe the effectiveness of refrigerators, including those operating at cryogenic temperatures. The COP is defined as the amount of heat removed at the cryogenic operating temperature of the refrigerator divided by the amount of work that must be applied to remove the heat. If two refrigerators remove … Continue reading Coefficient of Performance and Figure of Merit


Regenerators or regenerative heat exchangers are a key component of cryocoolers such as pulse tube cryocoolers (Cold Facts, August 2014). Regenerator performance greatly affects the coefficient of performance of cryocoolers. Improvements in regenerator design and, in particular, regenerator materials have been an important factor in the improvement of the performance of cryocoolers below 10K. A … Continue reading Regenerators