JP2008215783A - Cryogenic refrigerating machine and cryogenic refrigerating method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a cryogenic refrigerating machine having a pre-cooling means and capable of enhancing the refrigerating efficiency by setting the expanding pressure to be equal to or lower than the critical pressure of the refrigerant. <P>SOLUTION: The cryogenic refrigerating machine is a GM refrigerating machine 16 which includes a high-compression compressor 10 for compressing the refrigerant, and a cold storage container 6 filled with a two-stage cold storage material 2a and a one-stage cold storage material 2b with the compressed refrigerant passing therethrough and reciprocating in a cylinder 5. The cryogenic refrigerating machine comprises at least one pre-cooling GM refrigerating machine 20 provided on a side of the cold storage container 6 for performing the pre-cooling, a pressure adjusting means for adjusting the expanding pressure of the refrigerant in the high-pressure compressor 10 to be equal to or lower than the critical pressure of the refrigerant, and a heat exchange means 19 provided on a bottom in a cooling stage 1 of the final stage of the GM refrigerating machine 16, performing the heat exchange with the refrigerant including a liquid generated when the refrigerant is expanded. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a regenerative cryogenic refrigerator and a cryogenic refrigeration method realized by using a Gifford McMahon refrigerator.

  A cryogenic environment is indispensable for realizing superconducting technology. There are various types of cryogenic refrigerators to realize this cryogenic environment. Among these, a regenerative cryogenic refrigerator called a Gifford McMahon refrigerator (hereinafter referred to as a GM refrigerator) is known (for example, see Patent Document 1).

  Here, the two-stage expansion GM refrigerator 16 will be described with reference to FIG. The GM refrigerator 16 includes a high-compression compressor 10 that compresses refrigerant, a supply / exhaust switching valve 7 connected to the high-compression compressor 10 via a pipe, and a cold storage in which the supply / exhaust switching valve 7 is installed. And a container 6. The regenerator container 6 and the high-compression compressor 10 are disposed with a vacuum container wall 18 therebetween. The regenerator container 6 functions as a displacer and reciprocates in the cylinder 5. A cooling stage 1 is provided at the lower end of the cylinder 5. The high compression compressor 10 is normally operated at a high pressure (discharge) side of about 2 MPa and a low pressure (intake) side of about 0.8 MPa. In order to perform 4K level cooling by this GM refrigerator, the regenerator vessel 6 is filled with a two-stage regenerator material 2a and a first-stage regenerator material 2b such as a rare earth metal or a ceramic having a high specific heat at a very low temperature.

  As a pressure adjusting means for adjusting the refrigerant intake pressure and the refrigerant exhaust pressure of the refrigerator head of the GM refrigerator, a configuration in which a flow rate adjusting valve is arranged between the intake valve, the exhaust valve and the compressor is also known ( For example, see Patent Document 2).

Also, a cryogenic refrigerator that combines a regenerative refrigerator called a GM / JT refrigerator and a Joule-Thomson refrigeration loop is known (see, for example, Patent Document 3). The GM / JT refrigerator, which is a kind of this cryogenic refrigerator, is accompanied by a GM refrigerator for pre-cooling, but the JT refrigerator is a refrigerant by reducing the exhaust pressure to about 0.1 MPa of atmospheric pressure. Helium is liquefied and cooled by latent heat. The amount of cold generated per unit flow rate of the refrigerant is larger than the GM refrigeration capacity, and the GM / JT refrigerator is superior in the refrigeration efficiency in the 4K level cooling.
JP-A-4-52468 Japanese Patent Laid-Open No. 10-132405 JP-A-4-52467

  In order to perform 4K level cooling by a regenerative cryogenic refrigerator using the GM refrigerator 16, it is necessary to use a regenerator material such as rare earth metal or ceramic having a high specific heat at a very low temperature.

  However, when the helium gas, which is a refrigerant, is cooled at a 4K level, for example, at an operating pressure of an ordinary GM refrigerator or the like (supplying air 2 MPa, exhausting about 0.8 MPa), in principle, the cooling capacity is only about 20% of the Carnot efficiency. There was a problem that could not be obtained.

  In addition, according to the GM / JT refrigerator, by reducing the exhaust pressure to about 0.1 MPa of the atmospheric pressure on the JT refrigerator side, the refrigeration efficiency in the 4K level cooling is superior to that of GM. Yes.

  However, since the GM / JT refrigerator is a counter-current refrigerator that uses cold generation due to expansion through a JT valve (throttle valve), the refrigeration performance is degraded due to clogging of the throttle valve portion due to contamination with impurities. It is easy to generate. Further, in general, there is a problem that the countercurrent type heat exchanger is expensive because it requires three stages.

  Moreover, in the conventional regenerative refrigerator described above, the refrigerant gas reciprocates instead of a one-way flow. In addition, since there is no throttle portion corresponding to a JT valve, performance deterioration due to impurities hardly occurs. In addition, a regenerator corresponding to a countercurrent heat exchanger of a JT refrigerator can be manufactured at a relatively low cost because of its simple configuration. For this reason, assuming the realization of a refrigerator that combines the advantages of the reliability and low cost of a regenerative refrigerator represented by the GM refrigerator and the high efficiency of the GM / JT refrigerator, the expansion pressure is assumed. A regenerative refrigerator that is reduced to the atmospheric pressure level can be considered.

  However, in principle, the GM refrigerator has a reduced refrigeration efficiency due to an increase in the compression ratio. Therefore, there is a problem that the refrigeration efficiency cannot be improved simply by reducing the operating pressure of the multistage GM refrigerator as it is.

  Furthermore, a technique using a pulse tube refrigerator as a refrigerator that performs cooling at a 4K level is known. However, when a pulse tube refrigerator is used, the volume of the expansion space on the low temperature side of the pulse tube that contributes to 4K level cooling is required in the pulse tube, so the required flow rate of the refrigerant increases due to an increase in the invalid volume, There was a problem where efficiency was reduced.

  The present invention has been made to solve the above-described problems. In a GM refrigerator, a cryocooling system capable of improving the refrigeration efficiency by providing precooling means and setting the expansion pressure below the critical pressure of the refrigerant. It is an object to provide a machine and a cryogenic refrigeration method.

  In order to achieve the above object, the cryogenic refrigerator of the present invention includes a compressor for compressing a refrigerant and a regenerator container that is filled with a regenerator material through which the compressed refrigerant passes and reciprocates in a cylinder. A cryogenic refrigerator having a Gifford-McMahon refrigerator for performing Simon expansion on the side of the regenerator vessel, the at least one precooling means provided on the side of the regenerator container, and the expansion pressure of the refrigerant in the compressor. Pressure adjusting means for adjusting the pressure below the critical pressure, and heat exchanging means for exchanging heat with a refrigerant that is provided at the bottom of the cooling stage of the final stage of the Gifford McMahon refrigerator and that contains a liquid generated when the refrigerant expands It is characterized by having.

  In order to achieve the above object, the present invention provides a Gifford McMahon refrigerator including a compressor step for compressing a refrigerant and a regenerator container that is filled with a regenerator material through which the compressed refrigerant passes and reciprocates in a cylinder. In the cryogenic refrigeration method using the expansion of Simon during the refrigeration cycle, the step of pre-cooling by at least one pre-cooling means provided on the side surface of the regenerator container, and the expansion pressure of the refrigerant in the compressor Adjusting to a critical pressure or less, and exchanging heat with a refrigerant containing a liquid generated when the refrigerant expands by heat exchanging means provided at the bottom of the cooling stage of the final stage of the Gifford McMahon refrigerator It is characterized by having.

  According to the cryogenic refrigerator and the cryogenic refrigeration method of the present invention, it is possible to improve the refrigeration efficiency by providing precooling means in the refrigerator and setting the expansion pressure to be equal to or lower than the critical pressure of the refrigerant.

  Embodiments of a cryogenic refrigerator and a method thereof according to the present invention will be described below with reference to the drawings. Here, the same or similar parts are denoted by common reference numerals, and redundant description is omitted.

  FIG. 1 is a configuration diagram showing a regenerative cryogenic refrigerator according to a first embodiment of the present invention.

  As shown in this figure, the cryogenic refrigerator includes the regenerator-type GM refrigerator 16 of the present embodiment that performs Simon expansion in the refrigeration cycle. Here, first, the two-stage expansion GM refrigerator 16 will be described.

The GM refrigerator 16 includes a high compression compressor 10 that compresses the refrigerant. In order to adjust the expansion pressure of the refrigerant in the high compression compressor 10 below the critical pressure of the refrigerant, a pressure adjusting means is provided. By this pressure adjusting means, the expansion pressure of the refrigerant of the high compression compressor 10 is set to be equal to or lower than the critical pressure of the refrigerant. Further, when 4K level cooling is performed using the high compression compressor 10, helium is used as the refrigerant. This helium includes helium 4 (He ⁴ ) and helium 3 (He ³ ) which is an isotope. When helium 3 is used, the boiling point is 3.19K, and when helium 4 is used, the boiling point is 4.215K.

  The high compression compressor 10 is connected to a regenerator container 6 constituting a regenerator of the GM refrigerator 16 via a high pressure side pipe 23 and a low pressure side pipe 24. An air supply / exhaust switching valve 7 that functions as an intake valve and an exhaust valve is interposed in the high pressure side pipe 23 and the low pressure side pipe 24. A scotch yoke 8 is installed on the upper part of the main body of the GM refrigerator 16. The scotch yoke 8 is a mechanism that converts the circular motion of the displacer and the shaft of the valve drive motor 9 into a linear vertical reciprocating motion by using an eccentric crank. The other end of the output shaft 25 that reciprocates up and down of the scotch yoke 8 is connected to the regenerator container 6 that reciprocates within the cylinder 5.

  The GM refrigerator 16 includes a regenerator container 6 that reciprocates in the cylinder 5 and functions also as a displacer. An output shaft 25 of the scotch yoke 8 is connected to the upper part of the regenerator container 6, and the regenerator container 6 reciprocates in the cylinder 5. A first-stage seal 3b and a second-stage seal 3a are provided at the upper part and the middle part of the regenerator container 6, respectively, to restrict the movement of the refrigerant.

  A first-stage regenerator material 2b is built in the upper part of the regenerator container 6 constituting this regenerator, and a two-stage regenerator material 2a is built in the lower part. In addition, a one-stage precooling means cooling stage 4 is installed on the side surface of the regenerator container 6. A cooling stage 1 is installed below the cylinder 5. In order to perform 4K level cooling with this regenerator, it is necessary to use a regenerator material made of a rare earth metal or ceramic having a large specific heat at a very low temperature.

  At least one stage of pre-cooling means is pre-cooled on the side surface of the regenerator container 6 constituting the regenerator. As this pre-cooling means, a GM refrigerator, a pulse tube refrigerator, a Stirling refrigerator, or the like can be used. Here, an example in which the first-stage precooling GM refrigerator 20 is installed on the side surface of the regenerator container 6 via the precooling means cooling stage 4 is illustrated. The precooling GM refrigerator 20 includes a precooling refrigerator cold head 12 and includes a precooling refrigerator compressor 13.

  Further, a heat exchanging means 19 is provided which is provided inside the cooling stage 1 at the lowest stage of the GM refrigerator 16 and exchanges heat with a refrigerant containing a liquid generated when the refrigerant expands. The heat exchange means 19 will be described later.

  In the present embodiment configured as described above, the high-pressure refrigerant compressed by the high-compression compressor 10 is introduced into the regenerator container 6 via the supply / exhaust switching valve 7. The regenerator container 6 reciprocates in the cylinder 5 by the output shaft 25 of the scotch yoke 8, so that the high-pressure refrigerant comes into contact with the two-stage regenerator material 2a and the first-stage regenerator material 2b built in the regenerator container 6. Replaced and released. At this time, the temperature of the gas decreases due to Simon expansion, and a low temperature is generated.

  When performing 4K level cooling, in order to efficiently cool in consideration of the cold storage loss from room temperature, etc., the GM refrigerator is usually cooled by making the regenerator of the two-stage expansion type or the three-stage expansion type multi-stage. is doing. Here, the two-stage expansion type GM refrigerator 16 is illustrated.

  In the two-stage expansion type GM refrigerator 16, the temperature at the inlet of the first stage is at a 300K level, and at the outlet of the last stage is at a 4K level. During this period, precooling means, which is an intermediate cooling means of one stage at 50K level, is installed. When installing a three-stage expansion type GM refrigerator, two-stage precooling means of 80K level and 20K level are installed between the first stage and the last stage.

  Next, the refrigeration capacity of the GM refrigerator will be described.

  In the 4K level refrigeration, the cooling efficiency is higher when the operating pressure is reduced to the atmospheric pressure, the refrigerant is liquefied, and cooling is performed using the latent heat. On the other hand, in the 80K, 50K, and 20K level cooling, which is a precooling means, if the expansion pressure is reduced to atmospheric pressure, a certain high supply pressure (usually about 2 MPa) is required to obtain a predetermined refrigeration capacity. Thus, the compression ratio becomes high. Since the efficiency of the GM refrigerator becomes lower as the compression ratio becomes higher in principle, there is no merit to lower the expansion pressure to the atmospheric pressure in the cooling by the pre-cooling means.

  Therefore, in the present embodiment, the GM refrigerator 16 that performs 4K level cooling and the precooling GM refrigerator 20 that performs precooling are divided into separate independent systems, and the operating pressure is set to an optimum condition for each. Drive under. That is, the precooling GM refrigerator 20 has a supply pressure of about 2 MPa, which is an operating pressure of a normal GM refrigerator, and an exhaust pressure of about 0.8 MPa. On the other hand, the GM refrigerator 16, which is a regenerative refrigerator that performs 4K level cooling, liquefies the refrigerant by reducing the exhaust pressure (= expansion pressure) to about atmospheric pressure or below the critical pressure of the refrigerant. By efficiently performing the 4K level cooling with the latent heat associated with this liquefaction, it is possible to achieve a highly efficient 4K level refrigeration as a whole.

  As shown in FIG. 5, a conventional GM refrigerator that performs cooling at a 4K level has conventionally used a gap-type heat exchanger because helium gas exceeding the critical pressure is used as a refrigerant. It was. That is, the gap 26 for performing forced convection heat transfer of the refrigerant exists in the cooling stage that performs cooling at the 4K level. When the refrigerant passed through the gap 26 between the inner wall of the cooling stage 1 and the outer wall 6a of the regenerator container, the cooling stage 1 was cooled by forced convection.

  On the other hand, in the GM refrigerator 16 of the present embodiment, the expansion pressure is reduced to about the atmospheric pressure and the expansion is performed. Then, the refrigerant becomes liquid when the refrigerant expands in the refrigeration cycle of about 1 Hz and is stored at the bottom of the cooling stage. When a heat load is applied to the cooling stage, heat transfer is performed efficiently by boiling heat transfer. For this reason, heat exchange in the gap portion is unnecessary, and the refrigerant inlet / outlet port from the regenerator container 6 can be directly provided on the cooling stage 1 side. In this way, the pressure loss during expansion can be reduced. Further, the heat transfer area at the bottom of the cooling stage can be increased, the temperature difference between the wall and the liquid due to the heat flux can be reduced, and the refrigeration efficiency can be improved and the cooling temperature can be lowered.

  According to the present embodiment, in the Gifford McMahon refrigerator, by providing one or more pre-cooling stages and setting the expansion pressure below the critical pressure of the refrigerant, the inner wall of the cooling stage and the outer wall of the regenerator The refrigeration efficiency of the cryogenic refrigerator can be further improved.

  FIG. 2 is a block diagram showing a regenerative cryogenic refrigerator according to the second embodiment of the present invention. This figure shows the lower part of the regenerator container 6 of the cryogenic refrigerator shown in FIG. 1 by cutting, and the same or similar parts as in FIG. .

  As shown in this figure, the regenerator container 6 is built in the cylinder 5 and reciprocates. The regenerator container 6 contains a two-stage regenerator material 2a through which the refrigerant passes. A cooling stage 1 is installed below the cylinder 5. In the lower part of the cylinder 5, a porous heat transfer body 14 is provided as a heat exchange means 19. The porous heat transfer body 14 is made of a spherical metal joined body, a porous metal, a honeycomb metal, or the like. The spherical metal-like joined body, porous metal, honeycomb-like metal, and the like are made from a rare earth metal having the properties of a good heat conductor or a high specific heat body.

  In this way, a metal ball such as copper having high thermal conductivity is spread on the bottom, and is brought into thermal contact with the cooling stage by means such as plating or diffusion bonding. Alternatively, it is possible to reduce the temperature amplitude of the cooling stage and improve the refrigeration efficiency by using a rare earth metal having a high specific heat at a very low temperature as the metal sphere.

  According to the present embodiment, in the GM refrigerator, one or more pre-cooling stages are provided, and the expansion pressure is made equal to or lower than the critical pressure of the refrigerant. By using 14, it is possible to further improve the refrigeration efficiency of the cryogenic refrigerator.

  FIG. 3 is a block diagram showing a regenerative cryogenic refrigerator according to a third embodiment of the present invention. This figure shows the lower part of the regenerator container 6 of the cryogenic refrigerator shown in FIG. 1 by cutting, and the same or similar parts as in FIG. .

  As shown in this figure, the heat transfer area can be increased by disposing a heat conductor 15 provided with a large number of upward recesses 15a not penetrating as the heat exchanging means 19 at the bottom of the cooling stage 1. Alternatively, although not shown in the figure, it is also effective to provide unevenness on the cooling stage 1 to increase the surface area. At that time, by providing a concavity and convexity on the bottom of the tip of the regenerator container 6 as well, the ineffective volume with respect to the refrigeration cycle can be reduced, and the refrigeration efficiency can be improved.

  That is, as shown in FIG. 3, the heat transfer area can be increased by providing a large number of upward recesses 15 a that do not penetrate the bottom of the cooling stage 1. Alternatively, although not shown in the figure, it is also effective to provide unevenness on the cooling stage 1 to increase the surface area. At this time, by processing the concaves and convexes facing the bottom of the tip of the regenerator container 6 as well, it is possible to reduce the ineffective volume with respect to the refrigeration cycle and to improve the refrigeration efficiency.

  According to the present embodiment, in the GM refrigerator, one or more precooling stages are provided, and the expansion pressure is set to be equal to or lower than the critical pressure of the refrigerant. By arranging the heat conductor 15 provided with a large number of 15a, the refrigeration efficiency of the cryogenic refrigerator can be further improved.

  FIG. 4 is a block diagram showing a regenerative cryogenic refrigerator of the fourth embodiment of the present invention. In this figure, a heat conductor 15 is provided in the lower part of the regenerator container 6 of the cryogenic refrigerator shown in FIG. 1, and the same or similar parts as in FIG. The duplicated explanation is omitted.

  As shown in this figure, the regenerator container 6 is built in the cylinder 5 and reciprocates. The regenerator container 6 contains a two-stage regenerator material 2a through which the refrigerant passes. A cooling stage 1 is installed below the cylinder 5. A heat conductor 15 processed as a heat exchanging means is provided in the lower part of the cylinder 5. The processed heat conductor 15 is processed with fine through holes 15b and further with a porous downward recess 15c. The processed heat conductor 15 is made of a rare earth metal having the properties of a good heat conductor or a high specific heat body. Further, a tip seal 3c as tip seal means is provided at the tip of the regenerator container 6b of the regenerator container 6 at the final stage. The tip seal 3 c can seal between the regenerator vessel tip 6 b and the inner wall of the cylinder 5.

  In the present embodiment configured as described above, the processed heat conductor 15 has the through hole 15b and the downward recess 15c, so that even when the refrigerator cold head is tilted, a predetermined temperature is obtained. Refrigerating capacity can be obtained. In addition, a tip seal 3 c is provided between the regenerator vessel tip 6 b of the regenerator vessel 6 and the inner wall of the cylinder 5. By this tip seal 3c, it is possible to suppress heat intrusion due to convection caused by a high-density low-temperature refrigerant flowing between the cylinder 5 and the regenerator container 6. At this time, the tip seal 3 c is installed at a position away from the cooling stage 1 so that frictional heat generation is not directly transmitted to the cooling stage 1.

  According to the present embodiment, in the GM refrigerator, by providing one or more precooling stages and setting the expansion pressure to be equal to or lower than the critical pressure of the refrigerant, the through hole 15b and the downward direction as the heat exchanging means 19 are provided. By providing the heat conductor 15 in which the recess 15c is processed, the refrigeration efficiency of the cryogenic refrigerator can be further improved.

  Also, a processed heat conductor 15 in which a through hole 15b is processed and a downward recess 15c is processed is disposed at the bottom of the cylinder 5. For this reason, even if the refrigerator cold head is tilted, the condensed liquid can be stored because of the downward recess 15c. Thus, it is possible to suppress the deterioration of the refrigeration performance related to the error in the mounting direction of the refrigerator.

  Furthermore, the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the present invention by combining the embodiments of the present invention. it can.

The block diagram which shows the cool storage type cryogenic refrigerator of the 1st Embodiment of this invention. The block diagram which shows the cool storage type cryogenic refrigerator of the 2nd Embodiment of this invention. The block diagram which shows the cool storage type cryogenic refrigerator of the 3rd Embodiment of this invention. The block diagram which shows the cool storage type cryogenic refrigerator of the 4th Embodiment of this invention. The block diagram which shows the conventional cool storage type cryogenic refrigerator.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Cooling stage, 2a ... Two-stage cool storage material, 2b ... One-stage cool storage material, 3a ... Two-stage seal, 3b ... One-stage seal, 3c ... Tip seal, 4 ... Pre-cooling means cooling stage, 5 ... Cylinder, 6 ... Cold storage Vessel container, 6a ... outer wall of regenerator vessel, 6b ... tip of regenerator vessel, 7 ... air supply / exhaust switching valve, 8 ... scotch yoke, 9 ... displacer and valve drive motor, 10 ... high compression compressor, 10a ... air supply (High pressure) side, 10b ... exhaust (low pressure) side, 11 ... heat transfer plate, 12 ... precooling refrigerator cold head, 13 ... precooling refrigerator compressor, 14 ... porous heat transfer body, 15 ... heat conductor, DESCRIPTION OF SYMBOLS 15a ... Upward dent, 15b ... Through-hole, 15c ... Downward dent, 16 ... GM refrigerator, 18 ... Vacuum vessel wall, 19 ... Heat exchange means, 20 ... Precooling GM refrigerator, 23 ... High pressure side piping, 24 ... Low pressure side piping, 25 ... Output , 26 ... gap.

Claims (9)

A cryogenic refrigerator having a compressor for compressing a refrigerant and a Gifford-McMahon refrigerator that performs a Simon expansion in a refrigeration cycle including a regenerator container that is filled with a regenerator material through which the compressed refrigerant passes and reciprocates in a cylinder. There,
At least one stage of precooling means provided on the side surface of the regenerator container for precooling;
Pressure adjusting means for adjusting the expansion pressure of the refrigerant in the compressor below the critical pressure of the refrigerant;
A heat exchanging means for exchanging heat with a refrigerant including a liquid generated when the refrigerant expands, provided at the bottom in the cooling stage of the final stage of the Gifford McMahon refrigerator,
A cryogenic refrigerator characterized by comprising:   The cryogenic refrigerator according to claim 1, wherein the refrigerant is helium.   The cryogenic refrigerator according to claim 1 or 2, wherein the precooling means is at least one selected from a Gifford McMahon refrigerator, a pulse tube refrigerator, and a Stirling refrigerator.   The cryogenic temperature according to any one of claims 1 to 3, wherein the heat exchange means is made of at least one selected from a spherical metal joined body, a porous metal, and a honeycomb metal. refrigerator.   The cryogenic refrigerator according to any one of claims 1 to 4, wherein the heat exchange means is made of a rare earth metal.   The passage through which the refrigerant that has passed through the regenerator material in the final stage of the Gifford McMahon refrigerator is introduced into the expansion space in the cooling stage is provided at the bottom of the regenerator container. The cryogenic refrigerator according to any one of 5.   The cryogenic refrigerator according to any one of claims 1 to 6, wherein the heat exchanging means includes a heat transfer body in which a through hole is processed and a downward recess is processed.   The cryogenic refrigerator according to claim 7, wherein the heat exchanging means includes tip sealing means provided at the tip of the regenerator container at the final stage and sealing between the inner wall of the cylinder. . A cryogenic refrigeration that performs Simon expansion during a refrigeration cycle using a Gifford McMahon refrigerator that includes a compressor step that compresses the refrigerant and a regenerator container that is refilled with a regenerator material through which the compressed refrigerant passes. In the method
Precooling by at least one stage of precooling means provided on a side surface of the regenerator container;
Adjusting the expansion pressure of the refrigerant in the compressor below the critical pressure of the refrigerant;
Heat exchanging with a refrigerant containing a liquid generated when the refrigerant expands by heat exchanging means provided at the bottom in the cooling stage of the final stage of the Gifford McMahon refrigerator,
A cryogenic refrigeration method comprising:

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