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Cryocooler
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===Cooling cycle=== {{See also|Stirling cycle}} <div class="skin-invert-image"> {{multiple image | align = right | total_width = 450 | image1 = Stirling Cycle Cryocooler.jpg | caption1 = Fig.2 Four states in the [[Stirling cycle]]. | image2 = Stirling cycle pV diagram.jpg | caption2 = Fig.3 pV-diagram of the ideal [[Stirling cycle]]. }}</div> [[File:Split Stirling Cooler02.jpg|class=skin-invert-image|300px|thumb|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.]] 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''<sub>a</sub> is given off to the surroundings at ambient temperature ''T''<sub>a</sub>. * 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''<sub>a</sub> and leaves it with temperature ''T''<sub>L</sub>. 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''<sub>L</sub> 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''<sub>L</sub> and leaves it with high temperature ''T''<sub>a</sub> 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 90Β°. 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 cycle|Carnot]] COP given by ''T''<sub>L</sub>/(''T''<sub>a</sub> β ''T''<sub>L</sub>). 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 don't 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. {{clear}}
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