Cryocooler

A cryocooler is a refrigerator designed to reach cryogenic temperatures (below 120 K, -153 °C, -243.7 °F). 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. Their input powers can be as high as 1 MW. In most cases cryocoolers use a cryogenic fluid as the working substance and employ moving parts to cycle the fluid around a thermodynamic cycle. The fluid is typically compressed at room temperature, precooled in a heat exchanger, then expanded at some low temperature. The returning low-pressure fluid passes through the heat exchanger to precool the high-pressure fluid before entering the compressor intake. The cycle is then repeated.

A cryocooler is a refrigerator designed to reach cryogenic temperatures (below 120 K, -153 °C, -243.7 °F). 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. Their input powers can be as high as 1 MW. In most cases cryocoolers use a cryogenic fluid as the working substance and employ moving parts to cycle the fluid around a thermodynamic cycle. The fluid is typically compressed at room temperature, precooled in a heat exchanger, then expanded at some low temperature. The returning low-pressure fluid passes through the heat exchanger to precool the high-pressure fluid before entering the compressor intake. The cycle is then repeated.

Components

The basic type of Stirling-type cooler is depicted in Fig.1. It consists of (from left to right): a piston a compression space and heat exchanger (all at ambient temperature T a ) a regenerator a heat exchanger an expansion space a piston (all at the low temperature T L ). Left and right the thermal contact with the surroundings at the temperatures T a and T L is supposed to be perfect so that the compression and expansion are isothermal. The work, performed during the expansion, is used to reduce the total input power. Usually helium is the working fluid.

Cooling cycle

The cooling cycle is split in 4 steps as depicted in Fig.2. The cycle starts when the two pistons are in their most left positions: From a to b, the warm piston moves to the right while the cold piston is fixed. The temperature of compressed gas at the hot end is isothermal (by definition), so heat Q a is given off to the surroundings at ambient temperature T a . From b to c, the two pistons move to the right. The volume between the two pistons is kept constant. The hot gas enters the regenerator with temperature T a and leaves it with temperature T L . The gas gives off heat to the regenerator material. From c to d, the cold piston moves to the right while the warm piston is fixed. The expansion is isothermal and heat Q L is taken up. This is the useful cooling power. From d to a, the two pistons move to the left while the total volume remains constant. The gas enters the regenerator with low temperature T L and leaves it with high temperature T a so heat is taken up from the regenerator material. At the end of this step the state of the cooler is the same as in the beginning. In the pV diagram (Fig.3) the corresponding cycle consists of two isotherms and two isochores. The volume V is the volume between the two pistons. In practice the cycle is not divided in discrete steps as described above. Usually the motions of both pistons are driven by a common rotary axes which makes the motions harmonic. The phase difference between the motions of the two pistons is about 90°. In the ideal case the cycle is reversible so the COP (the ratio of the cooling power and the input power) is equal to the Carnot COP given by T L /(T a − T L ). It is not so practical to have a cold piston, as described above, so, in many cases, a displacer is used instead of the cold piston. A displacer is a solid body which moves back and forth in the cold head driving the gas back and forth between the warm and the cold end of the cold head via the regenerator. No work is required to move the displacer since, ideally there is no pressure drop over it. Typically its motion is 90 degrees out of phase with the piston. In the ideal case the COP also equals to the Carnot COP. Another type of Stirling cooler is the split-pair type (Fig.4), consisting of a compressor, a split pipe, and a cold finger. Usually there are two pistons moving in opposite directions driven by AC magnetic fields (as in loudspeakers). The pistons can be suspended by so-called flexure bearings. They provide stiffness in the radial direction and flexibility in the axial direction. The pistons and the compressor casing do not touch so no lubricants are needed and there is no wear. The regenerator in the cold finger is suspended by a spring. The cooler operates at a frequency near the resonance frequency of the mass-spring system of the cold finger.

Cooling cycle

The cycle can be divided in four steps, with Fig.6, as follows: The cycle starts with the low-pressure (LP) valve closed, the high-pressure (HP) valve open, and the displacer all the way to the right (so in the cold region). All the gas is at room temperature. From a to b. The displacer moves to the left while the cold head is connected to the HP side of the compressor. The gas passes the regenerator entering the regenerator at ambient temperature T a and leaving it with temperature T L . Heat is released by the gas to the regenerator material. From b to c. The HP valve is closed and the LP valve opened with fixed position of the displacer. Part of the gas flows through the regenerator to the LP side of the compressor. The gas expands. The expansion is isothermal so heat is taken up from the application. This is where the useful cooling power is produced. From c to d. The displacer moves to the right with the cold head connected to the LP side of the compressor forcing the cold gas to pass the regenerator, while taking up heat from the regenerator. From d to a. The LP valve is closed and the HP valve opened with fixed position of the displacer. The gas, now in the hot end of the cold head, is compressed and heat is released to the surroundings. In the end of this step we are back in position a.

Cooling cycle

In Fig.8 the pressures and temperatures refer to the case of a nitrogen liquefier. At the inlet of the compressor the gas is at room temperature (300 K) and a pressure of 1 bar (point a). The compression heat is removed by cooling water. After compression the gas temperature is ambient temperature (300 K) and the pressure is (point b). Next it enters the warm (high-pressure) side of the counterflow heat exchanger where it is precooled. It leaves the exchanger at point c. After the JT expansion, point d, it has a temperature of and a pressure of 1 bar. The liquid fraction is x. The liquid leaves the system at the bottom of the reservoir (point e) and the gas (fraction 1 − x) flows into the cold (low-pressure) side of the counterflow heat exchanger (point f). It leaves the heat exchanger at room temperature (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. When used as a cryocooler it is preferable to use gas mixtures instead of pure nitrogen. In this way the efficiency is improved and the high pressure is much lower than 200 bar, normally between 20 and 40 bar. Mixed refrigerants are designed according to different requirements: cooling temperature, flammability, environmental aspects, and compressor's compatibility. A more detailed description of Joule-Thomson coolers and Joule-Thomson refrigerators can be found in.

Applications

Cryocoolers are a key enabling technology for applications infrared detection and applied superconductivity. Applications include superconducting electronics and quantum computing. Compact cryocoolers have been developed for superconducting photon detectors.

See also

Cryogenic processor Adiabatic demagnetization refrigerator Dilution refrigerator Hampson-Linde cycle Pulse tube refrigerator Stirling engine (Stirling cryocooler) Entropy production

References

Text derived from https://en.wikipedia.org/w/index.php?oldid=1363111304 under CC BY-SA 4.0; modifications have been made.