Encyclopaedia of cryogenic and related terms

Absorption - adsorption

From sorbere lat. to suck in. Absorption is the capability of materials to take in gases or liquids. Adsorption is the capability of solid materials to attract to their surface molecules of gases or solutions with which they are in contact.

Activated charcoal

Adsorber material. Made from e.g. coconut.Spec. surface area ca. 890m2/g, density ca. 490kg/m3.

 

adiabatic

see isenthalpic

Claude cycle

The Claude cycle is a widely used cooling process in refrigeration technology. It combines isenthalpic and isentropic expansion. A refrigerator based on the Claude cycle comprises a compressor at ambient temperature, a series of heat exchangers, an expansion turbine and a Joule-Thomson valve. The principle used is to expand a part flow of the process gas in the turbine and therewith cool it down ( isentropic expansion). The turbine part flow is then used to precool the main gas stream by the means of the heat exchangers. The precooled main gas stream is further cooled down by isenthalpic expansion via the Joule-Thomson valve.

 

Coaleszer

Droplet separator. Working principle: The droplets in the gas stream collide with the fibers of the coaleszer cartridge and stay adhered. More droplets lead to the formation of larger drops which flow down along the fibers of the coaleszer cartridge and fall into the sump.

 

Condensation

Transition from the vapour phase to the liquid or solid phase, e.g. by cooldown of water vapour on a cold window.

Cryopump

Vacuumpump for attaining high or ultrahigh vacua by cooling of parts of the vacuum chamber wall to extremely low temperatures by the means of liquefied gases at which the gas is then condensed as solid matter.

Cryostat

Cooling apparatus with temperature control for maintaining constant, usually very low temperatures. The cooling is in most cases achieved with liquefied gases (especially nitrogen, hydrogen and helium).

 

Dewar

Double walled and vacuum insulated storage and transport container for very cold liquefied gases named after its inventor James Dewar (*1842, +1923). [->Index]

 

Helium

Helium is a colourless, odourless and inert (chemically non reactive) gas from the eighth group of the periodic system. Helium liquefies at -268.9°C, it can, however, only solidify at elevated pressures (25bar).
Helium has been discovered in 1868 by Joseph Norman Lockeyer in the gaseous shell of the sun when he detected a spectral line that could not be attributed to any known element. Helium was named by Lockeyer after the Greek name for the sun - helios. On the earth helium was first found by Sir William Ramsay in rock containing uranium (Cleveit). The helium deposits on earth are the product of radioactive decay. Helium does not accumulate in the atmosphere as the gravitational force of the earth cannot stop the helium from diffusing into outer space.
Besides helium-4, small traces of helium-3 can be found on earth. Helium-3 is produced by the decay of the rare radioactive hydrogen isotope tritium (hydrogen-3). It makes up 1.3ppm (0.00013%) of the total helium deposits.
Helium is extracted from natural gas. Natural gas of some resources in the U.S. contains up to 7.6% helium.

    *
      atomic number 2

    *
      atomic weight 4.0026

    *
      boiling point at 1.013bar - 4.22K (-268.94°C)

    *
      density (273K, 1bar) - 176g/m³

    *
      valence 0

    *
      electron configuration 2 or 1s²

    *

 

HeI-HeII

Liquid helium (also known as HeI) does not solidify when cooled beyond temperatures below 2.17K, but it turns into another liquid phase which is called HeII.

HeII has characteristics like no other known liquid: It expands when cooled down and it shows an enormous heat conductivity (approx. 1000 times grater than copper). Furthermore the viscosity of HeII is much smaller than the viscosity of HeI. Neither the heat conductivity nor the viscosity of HeII do follow classical laws. The phase separation line in between

HeI and HeII is called λ line.

Supercritical helium

Helium at a pressure larger than the critical pressure (2.3bar). Supercritical helium is an one phase gas at any temperature, which is an advantage for the transport through long transferline systems. [->Index]

Inversion temperature

Characteristic temperature of real gases. By an adiabatic expansion the gas will heat up above the inversion temperature and cool down below the inversion temperature. [->Index]

isenthalpic

= adiabatic. Change of state with constant enthalpy i.e. no energy be in the form of work or heat is exchanged with the environment. Example: expansion of gas in an ideally insulated valve. [->Index]

 

isentropic

Change of state with constant entropy i.e. energy in the form of work is exchanged with the environment without friction losses. Example: ideal compression in a pump or ideal expansion in a turbine. (In the compression in a pump work is introduced by the pump motor and in the expansion in a turbine work is released via the turbine shaft e.g. to a generator). [->Index]

 

Joule-Thomson Effect

The temperature change effect in adiabatic expansions discovered by Joule and Thomson. [->Index]

 

Joule-Thomson valve

Valve in which gas can be cooled by isenthalpic expansion if the temperature of the gas is below the inversion temperature. Typically last valve in a refrigerator. [->Index]

 

LNG - Liquid Natural Gas

Natural gas (primarily methane) which has been liquefied by refrigeration below the boiling point (-161,5°C, 111,7K) for storage and transport. Liquid natural gas requires 600 times less storage space than gaseous natural gas (at 1bar). [->Index]

 

Molecular sieve

Adsorber material.It consists of synthetically made zeoliths i.e. cristallised alkaline-earth-alumino-silica with crystall water. The structure of the zeoliths is made up of SiO4- and AlO4-tetraeders which form a polyeder. Theses polyeders embody cavities in their centre which are interconnected by pores. If the crystall water is driven out from the cavities they are available for the adsorption of gases. [->Index]

 

Nitrogen

Nitrogen is a colourless, odourless and non combustible gas. Density (273K, 1bar) = 1234,2g/m3, boiling point at 1,013bar = 77,35K. It is produced by air separation. [->Index]

 

Phase

Physical state of a substance e.g. solid, liquid, gaseous but also crystalline, amorphous, glas, plasma etc. In cryogenics phase usually refers to the solid or the liquid phases of a substance. [->Index]

 

Quench

 
Random effect in superconducting material which leads locally for a short time to the loss of the superconductivity. If a larger number of local quenches occurs, the linked heat production can cause an avalanche effect and lead to the loss of superconductivity in the whole coil. If the coil is being cooled by gaseous or liquid helium, the quench will lead to a large change of density and therewith an increase in pressure. [->Index]

Refrigerator - Liquefier

 
The difference in between a refrigerator and a liquefier is the fact that the refrigerator permanently recirculates the cooling agent in a closed circuit. A liquefier in difference supplies cryogens for cooling in batch supply also called open circuit.

Closed circuit means that the cooled down cryogen is streaming in a permanent flow to the object to be cooled and back e.g. by the means of transferlines. The returned cold cryogen is directly used for cooling of heat exchangers in the refrigerator.
In a liquefier in contrary the cryogen is cooled, liquefied and stored. The object to be cooled is supplied with cold cryogen e.g. by transportable dewars. Evaporated cryogen from the object to be cooled is warmed up to ambient temperature and returned to the liquefier.
Due to the fact that cold gas is reused in a refrigerator the efficiency of the refrigerator process is higher than the efficiency of the liquefier process. [->Index]

 

Silicagel

SiO2*nH2O. Adsorber material. Produced by careful dehydration of acidified silicate solutions. [->Index]

Superinsulation

Very effective insulation material used in low temperature applications based on the principle of reducing the radiation by multiple radiation shields in high vacuum spaces. It consists of a reflective coated thin plastic foil and a spacer layer e.g. a fine plastic net. [->Index]

Thermoacoustic Oscillations  

Thermoacoustic oscillations are gas oscillations which can occur in gas filled tubes if one end of the tube is cold and the other end is warm. This is the case e.g. with siphons and safety pipes. The thermoacoustic oscillations do lead to a heat transport from the warm end to the cold end of the tube. [->Index]

Transferline  

System of pipes for the transport of cold gases. For reasons of insulation the cold pipes are located within an evacuated mantle pipe and frequently they are also insulated by the means of superinsulation foils or actively cooled shields. [->Index]