Cryocooler

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

Fig.1 Schematic diagram of a Stirling cooler. The system has one piston at ambient temperature T a and one piston at low temperature T L .
Fig.4 Schematic diagram of a split-pair Stirling refrigerator. The cooling power is supplied to the heat exchanger of the cold finger. Usually the heat flows are so small that there is no need for physical heat exchangers around the split pipe.
Fig.5 Schematic diagram of a GM-cooler. V l and V h are buffer volumes of the compressor. The compression heat is removed by the cooling water of the compressor via a heat exchanger. The rotary valves alternatingly connect the cooler to the high- and the low-pressure sides of the compressor and runs synchronous with the displacer.
Fig. 6 The four stages in the cooling cycle of the GM cooler.
Fig.7 Schematic diagram of a Stirling-type single-orifice PTR.
Fig. 8 Schematic diagram of a JT liquefier. A fraction x of the compressed gas is removed as liquid. At room temperature it is supplied as gas at 1 bar, so that the system is in the steady state.