The term is most often used for smaller systems, typically table-top size, with input powers less than about 20 kW.
Some can have input powers as low as 2–3 W. Large systems, such as those used for cooling the superconducting magnets in particle accelerators are more often called cryogenic refrigerators.
Note that even a perfect heat exchanger will not affect the entrance temperature Ti of the gas.
A regenerator consists of a matrix of a solid porous material, such as granular particles or metal sieves, through which gas flows back and forth.
The thermal contact with the gas must be good and the flow resistance of the matrix must be low.
In its most extreme form an ideal regenerator has the following properties: Progress in the cryocooler field in recent decades is in large part due to development of new materials having high heat capacity below 10 K.[1] The basic type of Stirling-type cooler is depicted in Fig.1.
Usually there are two pistons moving in opposite directions driven by AC magnetic fields (as in loudspeakers).
The pistons and the compressor casing don't touch so no lubricants are needed and there is no wear.
Gifford-McMahon (GM) coolers[2] have found widespread application in many low-temperature systems e.g. in MRI and cryopumps.
The pressure variations in the cold head are obtained by connecting it periodically to the high- and low-pressure sides of a compressor by a rotating valve.
During the opening and closing of the valves irreversible processes take place, so GM-coolers have intrinsic losses.
It is a simple type of cooler which is widely applied as cryocooler or as the (final stage) of coolants.
At the inlet of the compressor the gas is at room temperature (300 K) and a pressure of 1 bar (point a).
In order to keep the system in the steady state, gas is supplied to compensate for the liquid fraction x that has been removed.
[3] Cryocoolers are a key enabling technology for applications infrared detection and applied superconductivity.
[5] This article incorporates public domain material from the National Institute of Standards and Technology