Gas replacement method for expander of cryogenic refrigerator, cryogenic refrigerator, and gas replacement pipe

Abstract

To provide a gas replacement technique for an expander of a cryogenic refrigerator for avoiding contact between an electric device in the expander and combustible gas.SOLUTION: A gas replacement method for an expander 14 of a cryogenic refrigerator 10 is provided. The expander 14 comprises an expander cylinder 16, a pressure switching valve 42 for switching a pressure in the expander cylinder 16, a connection flow passage 20b from the pressure switching valve 42 to the expander cylinder 16, and an expander motor 40 for driving the pressure switching valve 42. The method comprises the steps of: connecting a noncombustible gas source to the connection flow passage 20b or the expander cylinder 16; and purging accumulated gas in the expander cylinder 16 by noncombustible gas from the noncombustible gas source when the expander motor 40 is stopped.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a gas replacement method for an expander of a cryogenic refrigerator, and to a cryogenic refrigerator. The present invention also relates to a gas replacement pipe that can be used in such a gas replacement method. [Background technology]

[0002] One application of cryogenic refrigerators, such as Gifford-McMahon (GM) refrigerators, is to install them in storage vessels of liquefied gases and use them to recondense the gas. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2001-241796 A Summary of the Invention [Problem to be solved by the invention]

[0004] In the case where the gas recondensed by the cryogenic refrigerator contains a flammable gas, such as hydrogen, the atmosphere surrounding the cryogenic refrigerator may contain the flammable gas. The inventors have noticed that flammable gas may be drawn into the expander of the cryogenic refrigerator during maintenance work performed by suspending the operation of the cryogenic refrigerator. The maintenance work involves disassembly and reassembly of the expander for inspection and replacement of internal parts. Therefore, when maintenance work is performed at the site where the expander is installed, the surrounding gas, which may contain flammable gas, may be drawn into the expander. As a result, the flammable gas may come into contact with electrical equipment in the expander, such as a motor. Operating electrical equipment in contact with flammable gas may pose a risk of fire.

[0005] An exemplary object of an embodiment of the present invention is to provide a gas replacement technique for an expander of a cryogenic refrigerator to prevent contact between an electric device in the expander and a flammable gas. [Means for solving the problem]

[0006] According to one aspect of the present invention, there is provided a gas replacement method for an expander of a cryogenic refrigerator. The expander includes an expander cylinder, a pressure switching valve for switching the pressure in the expander cylinder, a connecting flow path from the pressure switching valve to the expander cylinder, and an expander motor for driving the pressure switching valve. The method includes connecting a non-combustible gas source to the connecting flow path or the expander cylinder, and purging the stagnant gas in the expander cylinder with non-combustible gas from the non-combustible gas source while the expander motor is stopped.

[0007] According to one aspect of the present invention, a cryogenic refrigerator comprises an expander cylinder, a pressure switching valve that switches the pressure in the expander cylinder, a high-pressure port and a low-pressure port connected to the expander cylinder via the pressure switching valve, a connection flow path from the pressure switching valve to the expander cylinder, and a gas receiving port different from the high-pressure port and the low-pressure port, which is connected to the connection flow path so as to enable gas to flow into the connection flow path.

[0008] According to one embodiment of the present invention, there is provided a gas replacement piping used for gas replacement in an expander of a cryogenic refrigerator. The gas replacement piping includes a main pipe connectable to a non-flammable gas source, a first branch pipe branching from the main pipe and connectable to a gas receiving port of the expander, a second branch pipe branching from the main pipe and connectable to a high-pressure port of the expander, and a third branch pipe branching from the main pipe and connectable to a low-pressure port of the expander. Effect of the Invention

[0009] According to the present invention, it is possible to provide a gas replacement technique for the expander of a cryogenic refrigerator, which prevents contact between an electric device in the expander and a flammable gas. [Brief description of the drawings]

[0010] [Figure 1] 1 is a diagram illustrating a schematic diagram of a cryogenic refrigerator according to an embodiment; [Diagram 2] 1 is a diagram illustrating a schematic diagram of a cryogenic refrigerator according to an embodiment; [Diagram 3] 1 is a diagram illustrating a schematic diagram of a cryogenic refrigerator according to an embodiment; [Figure 4] FIG. 2 is an exploded perspective view illustrating a main part of a motion conversion mechanism of a cold head according to an embodiment of the present invention. [Diagram 5] 4 is a flowchart showing a gas replacement method for an expander of a cryogenic refrigerator according to an embodiment. [Figure 6] 1 is a schematic diagram showing an expander of a cryogenic refrigerator in a gas replacement method according to an embodiment. FIG. [Figure 7] 1 is a schematic diagram showing an expander of a cryogenic refrigerator in a gas replacement method according to an embodiment. FIG. [Figure 8] 1 is a diagram illustrating a schematic diagram of a cryogenic refrigerator according to an embodiment; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Hereinafter, the embodiments for carrying out the present invention will be described in detail with reference to the drawings. In the description and drawings, the same or equivalent components, members, and processes are given the same reference numerals, and duplicated descriptions are omitted as appropriate. The scale and shape of each part shown in the drawings are set for convenience in order to facilitate the description, and are not to be interpreted as being limiting unless otherwise specified. The embodiments are merely examples, and do not limit the scope of the present invention in any way. All features and combinations thereof described in the embodiments are not necessarily essential to the invention.

[0012] 1 to 3 are schematic diagrams showing a cryogenic refrigerator 10 according to an embodiment. FIG. 1 shows the external appearance of the cold head of the cryogenic refrigerator 10, and FIG. 2 shows the internal structure of the low-temperature part of the cold head. FIG. 3 shows the internal structure of the drive part of the cold head. The cryogenic refrigerator 10 is usually installed in a vacuum vessel (not shown) such that the low-temperature part is disposed within the vacuum vessel and the drive part is disposed in the ambient environment (e.g., room temperature and atmospheric pressure environment) outside the vacuum vessel. The cryogenic refrigerator 10 is, for example, a two-stage Gifford-McMahon (GM) refrigerator.

[0013] As an example of an application, the cryogenic refrigerator 10 may be mounted on a cryogenic liquid reservoir that stores a cryogenic liquid. The cryogenic refrigerator 10 may be used to recondense vaporized cryogenic liquid. In this embodiment, the cryogenic liquid may be a refrigerant having a boiling point lower than the boiling point of nitrogen (approximately 77 K), such as liquid hydrogen, liquid helium, or liquid neon.

[0014] The cryogenic refrigerator 10 includes a compressor 12 and an expander 14. The compressor 12 is configured to recover the working gas of the cryogenic refrigerator 10 from the expander 14, increase the pressure of the working gas, and supply the working gas to the expander 14 again. The compressor 12 and the expander 14 form a refrigeration cycle of the cryogenic refrigerator 10, thereby enabling the cryogenic refrigerator 10 to provide the desired cryogenic cooling. The expander 14 is often referred to as a cold head. The working gas, also referred to as a refrigerant gas, is typically helium gas, although other suitable gases may be used. For ease of understanding, the flow direction of the working gas is indicated by arrows in FIG. 1.

[0015] Generally, the pressure of the working gas supplied from the compressor 12 to the expander 14 and the pressure of the working gas recovered from the expander 14 to the compressor 12 are both significantly higher than atmospheric pressure, and can be referred to as the first high pressure and the second high pressure, respectively. For convenience of explanation, the first high pressure and the second high pressure are also simply referred to as the high pressure and the low pressure, respectively. Typically, the high pressure is, for example, 2 to 3 MPa. The low pressure is, for example, 0.5 to 1.5 MPa, for example, about 0.8 MPa. For ease of understanding, the flow direction of the working gas is indicated by an arrow.

[0016] The expander 14 includes an expander cylinder 16, a displacer assembly (hereinafter sometimes simply referred to as a displacer) 18, and an expander housing (hereinafter sometimes simply referred to as a housing) 20. The expander cylinder 16 guides the linear reciprocating motion of the displacer 18, and forms an expansion chamber (32, 34) between the expander cylinder 16 and the displacer 18 as an expansion space for the working gas. The expander cylinder 16 is fixed to the expander housing 20, thereby forming a housing for the expander 14, and an airtight space for accommodating the displacer 18 is formed in the expander cylinder 16.

[0017] The expander 14 also includes a high-pressure port 13a and a low-pressure port 13b. The high-pressure port 13a is provided in the expander housing 20 as an inlet for the working gas to the expander 14, and the low-pressure port 13b is provided in the expander housing 20 as an outlet for the working gas from the expander 14. Thus, the high-pressure port 13a is connected to the high-pressure side of the compressor 12 by a high-pressure side pipe, and the low-pressure port 13b is connected to the low-pressure side of the compressor 12 by a low-pressure side pipe. Thus, the high-pressure working gas is supplied from the high-pressure side of the compressor 12 into the expander 14 through the high-pressure port 13a. Furthermore, the low-pressure working gas is recovered from the expander 14 to the low-pressure side of the compressor 12 through the low-pressure port 13b.

[0018] In this document, in order to explain the positional relationship between the components of the cryogenic refrigerator 10, for convenience, the side closer to the top dead center of the axial reciprocating motion of the displacer will be referred to as "top" and the side closer to the bottom dead center as "bottom." The top dead center is the position of the displacer where the volume of the expansion space is maximum, and the bottom dead center is the position of the displacer where the volume of the expansion space is minimum. During operation of the cryogenic refrigerator 10, a temperature gradient occurs in which the temperature decreases from the top to the bottom in the axial direction, so the top side can also be called the high temperature side and the bottom side the low temperature side.

[0019] The expander cylinder 16 has a first cylinder 16a and a second cylinder 16b. The first cylinder 16a and the second cylinder 16b are, for example, members having a cylindrical shape, and the second cylinder 16b has a smaller diameter than the first cylinder 16a. The first cylinder 16a and the second cylinder 16b are arranged coaxially, and the lower end of the first cylinder 16a is rigidly connected to the upper end of the second cylinder 16b.

[0020] The displacer assembly 18 includes a first displacer 18a and a second displacer 18b that are connected to each other and move together. The first displacer 18a and the second displacer 18b are, for example, cylindrical members, and the second displacer 18b has a smaller diameter than the first displacer 18a. The first displacer 18a and the second displacer 18b are arranged coaxially.

[0021] The first displacer 18a is accommodated in the first cylinder 16a, and the second displacer 18b is accommodated in the second cylinder 16b. The first displacer 18a can reciprocate in the axial direction along the first cylinder 16a, and the second displacer 18b can reciprocate in the axial direction along the second cylinder 16b.

[0022] As shown in Fig. 2, the first displacer 18a accommodates the first regenerator 26. The first regenerator 26 is formed by filling a tubular main body of the first displacer 18a with a wire mesh such as copper or other suitable first regenerator material. The upper and lower lids of the first displacer 18a may be provided as separate members from the main body of the first displacer 18a, and the upper and lower lids of the first displacer 18a may be fixed to the main body by suitable means such as fastening or welding, thereby accommodating the first regenerator material in the first displacer 18a.

[0023] Similarly, the second displacer 18b accommodates the second regenerator 28. The second regenerator 28 is formed by filling the cylindrical main body of the second displacer 18b with a non-magnetic regenerator material such as bismuth, a magnetic regenerator material such as HoCu2, or other suitable second regenerator material. The second regenerator material may be formed in a granular shape. The upper and lower lids of the second displacer 18b may be provided as separate members from the main body of the second displacer 18b, and the lower lid of the upper lid of the second displacer 18b may be fixed to the main body by suitable means such as fastening or welding, thereby accommodating the second regenerator material in the second displacer 18b.

[0024] The displacer 18 forms an upper chamber 30, a first expansion chamber 32, and a second expansion chamber 34 inside the expander cylinder 16. For heat exchange with a desired object or medium to be cooled by the cryogenic refrigerator 10, the expander 14 includes a first cooling stage 33 and a second cooling stage 35. The upper chamber 30 is formed between the upper cover of the first displacer 18a and the upper part of the first cylinder 16a. The first expansion chamber 32 is formed between the lower cover of the first displacer 18a and the first cooling stage 33. The second expansion chamber 34 is formed between the lower cover of the second displacer 18b and the second cooling stage 35. The first cooling stage 33 is fixed to the lower part of the first cylinder 16a so as to surround the first expansion chamber 32, and the second cooling stage 35 is fixed to the lower part of the second cylinder 16b so as to surround the second expansion chamber 34.

[0025] The first regenerator 26 is connected to the upper chamber 30 through a working gas passage 36a formed in the upper lid of the first displacer 18a, and is connected to the first expansion chamber 32 through a working gas passage 36b formed in the lower lid of the first displacer 18a. The second regenerator 28 is connected to the first regenerator 26 through a working gas passage 36c formed from the lower lid of the first displacer 18a to the upper lid of the second displacer 18b. The second regenerator 28 is also connected to the second expansion chamber 34 through a working gas passage 36d formed in the lower lid of the second displacer 18b.

[0026] A first seal 38a and a second seal 38b may be provided so that the working gas flow between the first expansion chamber 32, the second expansion chamber 34 and the upper chamber 30 is guided to the first regenerator 26 and the second regenerator 28, instead of through the clearance between the expander cylinder 16 and the displacer 18. The first seal 38a may be attached to the top cover of the first displacer 18a so as to be disposed between the first displacer 18a and the first cylinder 16a. The second seal 38b may be attached to the top cover of the second displacer 18b so as to be disposed between the second displacer 18b and the second cylinder 16b.

[0027] 3, the expander housing 20 includes a housing body 22 having a lower opening 21, and a lower cover 24 that closes the lower opening 21. The lower opening 21 is formed on the lower surface of the housing body 22. A housing internal volume 20a formed by the housing body 22 and the lower cover 24 may be connected to the low pressure side of the compressor 12 as shown in the figure, and maintained at low pressure.

[0028] The lower cover 24 separates the housing internal volume 20a from the displacer accommodating space (upper chamber 30) in the expander cylinder 16. The lower cover 24 has a disk-like shape as a whole, and more specifically, has an upper large diameter portion and a lower small diameter portion. A first seal member 25a is provided between the lower cover 24 and the expander cylinder 16 to maintain the airtightness of the internal volume of the expander cylinder 16, and a second seal member 25b is provided between the lower cover 24 and the housing body 22 to maintain the airtightness of the housing internal volume 20a. As shown in the figure, the first seal member 25a may be attached to the small diameter portion of the lower cover 24, and the second seal member 25b may be attached to the large diameter portion of the lower cover 24.

[0029] The lower cover 24 is removably fitted into the lower opening 21, and the upper flange portion of the expander cylinder 16 is fastened to the housing main body 22 with fastening members such as bolts. In this way, the lower cover 24 is sandwiched between the housing main body 22 and the upper flange portion of the expander cylinder 16. The lower cover 24 is not fixed to the housing main body 22 by fastening. However, a structure in which the housing main body 22 and the lower cover 24 are fastened with fastening members such as bolts may be employed.

[0030] The expander 14 also includes an expander motor 40, a pressure switching valve 42, and a motion conversion mechanism 43. The expander motor 40 is attached to the expander housing 20, more specifically, to a side surface of the housing main body 22. In order to maintain the airtightness of the housing internal volume 20a, a seal member (not shown) may be provided on the attachment surface between the expander motor 40 and the housing main body 22. The pressure switching valve 42 and the motion conversion mechanism 43 are accommodated in the expander housing 20.

[0031] The expander motor 40 is provided in the expander 14 as a drive source for the displacer 18 and the pressure switching valve 42. The expander motor 40 may be an appropriate electric motor and may be configured to rotate the motor rotating shaft 40a at a constant rotational speed, or may be configured so that the rotational speed of the motor rotating shaft 40a can be variably controlled.

[0032] The pressure switching valve 42 connects the high pressure port 13a and the low pressure port 13b to the expander cylinder 16 (i.e., the upper chamber 30, the first expansion chamber 32, and the second expansion chamber 34). The pressure switching valve 42 alternately connects the high pressure port 13a and the low pressure port 13b to the expander cylinder 16, and is configured to periodically switch between intake and exhaust of the expander cylinder 16.

[0033] Furthermore, when switching the connection between the high pressure port 13a and the low pressure port 13b, the pressure switching valve 42 may be configured to temporarily cut off the connection between both the high pressure port 13a and the low pressure port 13b and the expander cylinder 16. In this case, the pressure switching valve 42 does not instantly switch the connection between the high pressure port 13a and the low pressure port 13b, but has a period during which the expander cylinder 16 is not connected to either the high pressure port 13a or the low pressure port 13b.

[0034] In this embodiment, the pressure switching valve 42 takes the form of a rotary valve. Thus, the pressure switching valve 42 includes a valve rotor 42a and a valve stator 42b, and the valve rotor 42a is in contact with the valve stator 42b so as to rotate while sliding relative to the valve stator 42b. The valve rotor 42a is rotatably supported by the housing body 22, and the valve stator 42b is non-rotatably supported by the housing body 22. An elastic body such as a spring may be interposed between the valve stator 42b and the housing body 22 to press the valve stator 42b toward the valve rotor 42a in the direction of the rotation axis of the valve rotor 42a.

[0035] The expander housing 20 is formed with a connection passage 20b from the pressure switching valve 42 to the expander cylinder 16. The connection passage 20b connects the pressure switching valve 42 to the upper chamber 30. A valve internal passage is formed in the valve rotor 42a and the valve stator 42b of the pressure switching valve 42 so as to alternately connect the connection passage 20b to the high pressure port 13a and the housing internal volume 20a (i.e., the low pressure port 13b). The valve internal passage can adopt various known forms and will not be described in detail here.

[0036] The motion conversion mechanism 43 is configured to connect the expander motor 40 to the pressure switching valve 42 and the displacer 18 so as to transmit the rotation of the motor rotating shaft 40a to the pressure switching valve 42 and convert it into a linear reciprocating motion of the displacer 18. An example of the motion conversion mechanism 43 will be described later. One rotation of the motor rotating shaft 40a causes one reciprocating motion of the displacer 18 via the motion conversion mechanism 43, thereby periodically changing the volume of the expansion space of the working gas. At the same time, one rotation of the motor rotating shaft 40a causes one rotation of the pressure switching valve 42 via the motion conversion mechanism 43, thereby alternately connecting the high pressure port 13a and the low pressure port 13b to the expander cylinder 16 via the pressure switching valve 42, and periodically changing the pressure of the expansion space of the working gas.

[0037] In this way, synchronized volume fluctuations and pressure fluctuations are brought about in the expansion space, forming a refrigeration cycle of the cryogenic refrigerator 10, thereby allowing the cryogenic refrigerator 10 to provide the desired cryogenic cooling. The first cooling stage 33 can be cooled to a first cooling temperature, and the second cooling stage 35 can be cooled to a second cooling temperature lower than the first cooling temperature. The first cooling temperature may be, for example, in the range of about 10K to about 100K, or in the range of about 20K to about 40K. The second cooling temperature may be, for example, about 20K or less, or about 10K or less, or in the range of about 1K to about 4K. When the cryogenic refrigerator 10 is used for recondensing the vaporized cryogenic liquid, the cooling temperature of the cryogenic refrigerator 10 (for example, the second cooling temperature) may be a temperature equal to or lower than the boiling point of the cryogenic liquid.

[0038] The expander 14 of the cryogenic refrigerator 10 may be installed in an airtight container 11 as illustrated in FIG. 1. The airtight container 11 may be a vacuum container such as a cryostat. When the cryogenic refrigerator 10 is used to recondense vaporized cryogenic liquid, the airtight container 11 may constitute a cryogenic liquid storage tank for storing the cryogenic liquid. The expander 14 may be inserted into the airtight container 11 through an opening in the wall of the airtight container 11, and the expander housing 20 may be fixed to the airtight container 11 at this opening. The expander housing 20 is exposed to the ambient environment of the airtight container 11 (e.g., room temperature and atmospheric pressure environment), and the first cooling stage 33 and the second cooling stage 35 are disposed within the airtight container 11. The expander 14 may be installed in the airtight container 11 vertically, with the expander housing 20 facing upward and the first cooling stage 33 and the second cooling stage 35 facing downward.

[0039] 4 is a schematic exploded perspective view of a main part of the cold head motion conversion mechanism 43 according to the embodiment. An exemplary embodiment of the motion conversion mechanism 43 will be described with reference to FIGS.

[0040] The motion conversion mechanism 43 is a Scotch yoke in this embodiment, and includes a crank 44 having a crank pin 44a, a Scotch yoke shaft 45, and a crank pin bearing 46. The Scotch yoke shaft 45 includes a Scotch yoke plate 45a, an upper rod 45b, and a lower rod 45c. The Scotch yoke shaft 45 may be made of a metal material such as stainless steel.

[0041] The crank 44 is fixed to the motor shaft 40a. The crank pin 44a extends parallel to the motor shaft 40a at a position eccentric to the motor shaft 40a. The crank pin 44a extends from the crank 44 toward the opposite side of the motor shaft 40a.

[0042] The Scotch yoke plate 45a is a rectangular plate-like member having a horizontally long window 47. The horizontally long window 47 extends in the axial direction and in a direction perpendicular to the motor rotating shaft 40a. A crankpin bearing 46 is rotatably disposed in the horizontally long window 47. The crankpin bearing 46 may be, for example, a roller bearing. An engagement hole 46a that engages with the crankpin 44a is formed in the center of the crankpin bearing 46, and the crankpin 44a passes through the engagement hole 46a.

[0043] On the opposite side of the Scotch yoke plate 45a from the crank 44, the valve rotor 42a of the pressure switching valve 42 is arranged with its central axis aligned with the motor rotating shaft 40a, and the tip of the crank pin 44a, which passes through the engagement hole 46a, is fixed to the valve rotor 42a.

[0044] The upper rod 45b extends upward from the center of the upper frame of the Scotch yoke plate 45a, and the lower rod 45c extends downward from the center of the lower frame of the Scotch yoke plate 45a, and these rods are arranged coaxially. The Scotch yoke plate 45a and the upper rod 45b are accommodated in the expander housing 20, and the lower rod 45c extends outside the expander housing 20 through the lower cover 24. The tip of the lower rod 45c is connected to the displacer 18 inside the expander cylinder 16.

[0045] The first sliding bearing 48a is provided between the upper rod 45b and the housing body 22, and the second sliding bearing 48b is provided between the lower rod 45c and the lower cover 24. The housing body 22 has a recess in its upper part for receiving the upper rod 45b, and the first sliding bearing 48a is disposed in this recess and supports the upper rod 45b slidably in the axial direction. The lower cover 24 has a through hole in the center, and the second sliding bearing 48b is disposed in this through hole and supports the lower rod 45c slidably in the axial direction. The second sliding bearing 48b is provided with a seal such as a slipper seal or a clearance seal and is configured to be airtight, so that the housing internal volume 20a is isolated from the upper chamber 30. There is no direct gas communication between the housing internal volume 20a and the upper chamber 30.

[0046] A collar portion 50 is fixed to the tip of the lower rod 45c connected to the displacer 18 by a fixing pin 49. The collar portion 50 is a short cylindrical member into which the tip of the displacer assembly 18 is inserted. A through hole is formed in the tip of the lower rod 45c and the collar portion 50 in a direction perpendicular to the axial direction, and the fixing pin 49 is fitted into this through hole to fix the collar portion 50 to the lower rod 45c.

[0047] The first displacer 18a has a cover portion 52a and a main body portion 52b. The cover portion 52a is an upper cover of the first displacer 18a and has a disk-like shape. The cover portion 52a is formed of a metal material or other material, such as an aluminum alloy that has been anodized. The main body portion 52b has a cylindrical shape and has a regenerator therein. The main body portion 52b is formed of a synthetic resin material or other material, and may be formed of a phenolic resin such as Bakelite. The working gas flow path 36a described above is formed by penetrating the cover portion 52a and the upper end portion of the main body portion 52b in the axial direction. The first seal 38a described above may be sandwiched between the cover portion 52a and the main body portion 52b at their respective outermost peripheries. The cover portion 52a and the main body portion 52b are fixed to each other using a fastening member such as a bolt, or by another method such as adhesion.

[0048] A through hole is formed in the center of the lid portion 52a to receive the tip of the lower rod 45c and the collar portion 50. The collar portion 50 has a flange portion at its lower end portion that extends radially outward, and the lower rod 45c and the collar portion 50 are connected to the first displacer 18a by sandwiching this flange portion between the lid portion 52a and the main body portion 52b of the first displacer 18a. In this manner, the displacer 18 is attached to the Scotch yoke shaft 45.

[0049] Therefore, when the expander motor 40 is driven and the motor shaft 40a rotates, the crank pin bearing 46 engaged with the crank pin 44a rotates in a circular motion. At this time, the crank pin bearing 46 reciprocates through the horizontally elongated window 47 of the Scotch yoke plate 45a, and together with this, the Scotch yoke shaft 45 and the displacer 18 reciprocate in the axial direction. In this way, the expander motor 40 drives the axial movement of the Scotch yoke shaft 45.

[0050] 1 and 3, the expander 14 includes a gas receiving port 54 different from the high pressure port 13a and the low pressure port 13b. The gas receiving port 54 is provided in the expander housing 20 and connected to the connecting passage 20b so as to allow gas to flow into the connecting passage 20b. Thus, the gas receiving port 54 is provided on the expander cylinder 16 side of the pressure switching valve 42, not on the compressor 12 side (i.e., on the high pressure port 13a and low pressure port 13b side).

[0051] The gas receiving port 54 is not used and is closed in normal operation of the cryogenic refrigerator 10 for cryogenic cooling. As described below, the gas receiving port 54 is opened and used in a gas replacement method of the expander 14 for returning the cryogenic refrigerator 10 to normal operation after maintenance work.

[0052] The gas receiving port 54 is provided in the expander housing 20, and is therefore disposed outside the airtight container 11. Therefore, with the expander 14 attached to the airtight container 11, an operator can access the gas receiving port 54 from outside the airtight container 11 and easily open and close the gas receiving port 54.

[0053] Like the high pressure port 13a and the low pressure port 13b, the gas receiving port 54 may be a detachable fitting such as a self-sealing coupling, so that piping can be easily attached and detached to the gas receiving port 54 for gas replacement operations in the expander 14.

[0054] The gas receiving port 54 may have a different size than the high pressure port 13a and the low pressure port 13b, for example, a smaller size than the high pressure port 13a and the low pressure port 13b. For example, the gas receiving port 54 may be a fitting having a smaller size (for example, a nominal diameter) than the fitting constituting the high pressure port 13a (or the low pressure port 13b). In this way, by making the size of the gas receiving port 54 different from the other ports, it becomes easy to identify the gas receiving port 54. This helps to prevent piping from being connected incorrectly between the gas receiving port 54 and the other ports.

[0055] The flow passage cross-sectional area of ​​the gas receiving port 54 may be smaller than that of the connecting flow passage 20b. Here, the flow passage cross-sectional area means the flow passage cross-sectional area in a direction perpendicular to the flow direction of the working gas. The gas receiving port 54 (and the flow passage volume from the gas receiving port 54 to the connecting flow passage 20b) is not used in normal operation of the cryogenic refrigerator 10, and therefore may become a dead volume that may reduce the refrigeration capacity of the cryogenic refrigerator 10. Therefore, by making the flow passage cross-sectional area of ​​the gas receiving port 54 smaller than that of the connecting flow passage 20b, it is possible to reduce such dead volume and reduce adverse effects on the refrigeration capacity of the cryogenic refrigerator 10.

[0056] Fig. 5 is a flowchart showing a gas replacement method for the expander 14 of the cryogenic refrigerator 10 according to the embodiment. Fig. 6 is a schematic diagram showing the expander 14 of the cryogenic refrigerator 10 in the gas replacement method according to the embodiment.

[0057] This gas replacement method may be performed during shutdown of the cryogenic refrigerator 10, for example, after performing maintenance on the cryogenic refrigerator 10 and before resuming normal operation of the cryogenic refrigerator 10. The maintenance may involve disassembly and reassembly of the expander 14 for inspection and replacement of internal components of the expander 14, such as the displacer assembly 18. If the cryogenic refrigerator 10 is mounted on a storage vessel for cryogenic liquid and used to recondense vaporized cryogenic liquid, the ambient atmosphere surrounding the cryogenic refrigerator 10 may contain vaporized cryogenic liquid if maintenance is performed at the site. This gas may be drawn into the expander 14 during the maintenance. If the cryogenic liquid is liquid hydrogen, the trapped gas drawn into the expander 14 may contain flammable gas, since hydrogen is a flammable gas. Resuming operation of the cryogenic refrigerator 10 in this state (i.e., starting to drive the expander motor 40) would not be sufficient to prevent contact between the electrical equipment inside the expander 14 (i.e., the expander motor 40) and flammable gas, since there is no guarantee that the internal volume of the expander 14 is filled with only non-flammable gas.

[0058] The gas replacement method for the expander 14 according to the embodiment includes connecting the non-flammable gas source 60 to the connection flow path 20b (S10), and purging the accumulated gas in the expander cylinder 16 with the non-flammable gas from the non-flammable gas source 60 while the expander motor 40 is stopped (i.e., with the expansion motor 40 deenergized) (S20). In addition, in this method, when the purging of the accumulated gas is completed, the connection between the non-flammable gas source 60 and the connection flow path 20b is released, and the operation of the cryogenic refrigerator 10 is resumed.

[0059] Here, it is convenient to use the non-flammable gas as the working gas of the cryogenic refrigerator 10, typically helium gas, for example. In this way, after the purging of the retained gas is completed, the internal volume of the expander cylinder 16 is filled with the working gas of the cryogenic refrigerator 10, and the operation of the cryogenic refrigerator 10 can be immediately resumed.

[0060] In S10, the non-flammable gas source 60 is connected to the gas receiving port 54 of the connection flow path 20b by the gas replacement piping 56. An example of the connection between the expander 14 of the cryogenic refrigerator 10 and the non-flammable gas source 60 is shown in FIG.

[0061] The gas replacement pipe 56 includes a gas supply valve 57. The gas supply valve 57 is provided on a main pipe 56a of the gas replacement pipe 56. A non-flammable gas source 60 is connected to one end of the main pipe 56a, and a joint that can be detached from the gas receiving port 54 of the expander 14 is provided on the other end of the main pipe 56a, and the main pipe 56a is connected to the gas receiving port 54 by this joint. By opening the gas supply valve 57, non-flammable gas is supplied from the non-flammable gas source 60 to the gas receiving port 54, and by closing the gas supply valve 57, the supply of non-flammable gas is stopped. The non-flammable gas source 60 may be, for example, a gas cylinder filled with a non-flammable gas (e.g., an inert gas such as helium gas) at high pressure.

[0062] The gas replacement pipe 56 also includes a gas outlet port 58 branched off from the main pipe 56a. The gas outlet port 58 may be, for example, an on-off valve. By opening the gas outlet port 58, gas can be discharged from the gas replacement pipe 56 to the outside, and by closing the gas outlet port 58, gas discharge from the gas replacement pipe 56 is stopped.

[0063] In S20, non-combustible gas from the non-combustible gas source 60 is supplied from the gas replacement piping 56 through the gas receiving port 54 to the expander cylinder 16. In addition, gas remaining in the expander cylinder 16 is discharged from the gas outlet port 58 of the gas replacement piping 56. In other words, the supply of non-combustible gas dilutes the gas remaining in the expander cylinder 16, and this diluted gas is discharged from the expander cylinder 16. The supply of non-combustible gas and the discharge of the gas remaining are performed while the expander motor 40 is stopped.

[0064] The supply of non-flammable gas and the discharge of retained gas may be performed simultaneously. In this case, the gas supply valve 57 and the gas outlet port 58 are opened simultaneously. Alternatively, the supply of non-flammable gas and the discharge of retained gas may be performed alternately. In this case, a supply state in which the gas supply valve 57 is opened and the gas outlet port 58 is closed and a discharge state in which the gas supply valve 57 is closed and the gas outlet port 58 is open may be alternately repeated.

[0065] When the cryogenic refrigerator 10 is stopped, the pressure switching valve 42 can be in a first state in which the high-pressure port 13a is connected to the expander cylinder 16, a second state in which the low-pressure port 13b is connected to the expander cylinder 16, or a third state in which the expander cylinder 16 is not connected to either the high-pressure port 13a or the low-pressure port 13b. Therefore, in an existing cryogenic refrigerator not provided with a gas receiving port 54, even if a non-flammable gas source is connected to the high-pressure port 13a or the low-pressure port 13b and non-flammable gas is supplied from there, it is not guaranteed that the purging of the gas remaining in the expander cylinder 16 will be realized. For example, in the third state, both the high-pressure port 13a and the low-pressure port 13b are blocked from the expander cylinder 16 by the pressure switching valve 42, so that the non-flammable gas is not supplied into the expander cylinder 16.

[0066] In contrast, according to the embodiment, since the gas receiving port 54 is connected to the connection flow path 20b from the pressure switching valve 42 to the expander cylinder 16, the non-flammable gas source 60 can be connected to the expander cylinder 16 by the gas replacement piping 56 so as to be in parallel with the pressure switching valve 42. Therefore, regardless of the state of the pressure switching valve 42, the stagnant gas in the expander cylinder 16 can be purged with the non-flammable gas from the non-flammable gas source 60, and the internal volume of the expander cylinder 16 can be reliably replaced with the non-flammable gas (e.g., helium gas). Thereafter, when the operation of the cryogenic refrigerator 10 is resumed, it is guaranteed that the stagnant gas that may contain flammable gas does not exist in the expander 14, and the expander 14 is filled with the non-flammable gas. Therefore, the cryogenic refrigerator 10 can be returned to normal operation from the maintenance work while avoiding contact between the electrical equipment in the expander 14 and the flammable gas.

[0067] In addition, in S20, non-combustible gas may be supplied to the expander 14 from the high pressure port 13a and the low pressure port 13b. This may be performed simultaneously with the supply of non-combustible gas from the gas receiving port 54 to the expander cylinder 16. Alternatively, the supply from the gas receiving port 54, the supply from the high pressure port 13a, and the supply from the low pressure port 13b may be performed in any order. In order to supply non-combustible gas from the high pressure port 13a to the expander 14, the non-combustible gas source 60 may be connected to the high pressure port 13a using the above-mentioned gas replacement piping 56. Similarly, in order to supply non-combustible gas from the low pressure port 13b to the expander 14, the non-combustible gas source 60 may be connected to the low pressure port 13b using the above-mentioned gas replacement piping 56. In this way, the stagnant gas can also be replaced with non-combustible gas in the internal volume of the expander 14 connected to the high pressure port 13a and the low pressure port 13b.

[0068] FIG. 7 is a schematic diagram showing the expander 14 of the cryogenic refrigerator 10 in the gas replacement method according to the embodiment. As shown in the figure, a three-way gas replacement pipe 56 may be used. The gas replacement pipe 56 may include a main pipe 56a connectable to the non-flammable gas source 60, a first branch pipe 56b branching from the main pipe 56a and connectable to the gas receiving port 54 of the expander 14, a second branch pipe 56c branching from the main pipe 56a and connectable to the high-pressure port 13a of the expander 14, and a third branch pipe 56d branching from the main pipe 56a and connectable to the low-pressure port 13b of the expander 14. By using such a gas replacement pipe 56, non-flammable gas can be supplied to the expander 14 from three ports (gas receiving port 54, high-pressure port 13a, and low-pressure port 13b) of the expander 14 at once, and stagnant gas can be purged, which is convenient.

[0069] As in the above-described embodiment, the gas replacement piping 56 may include a gas supply valve 57 in the main piping 56a. The gas replacement piping 56 may include a gas outlet port 58 branched off from the main piping 56a. Instead of or in addition to the gas outlet port 58 branched off from the main piping 56a, a gas outlet port 59 may be provided in the expander 14 (e.g., the expander housing 20). The gas replacement piping 56 may also include a pressure sensor 62 that measures the pressure of the gas supplied from the non-flammable gas source 60.

[0070] FIG. 8 is a schematic diagram of a cryogenic refrigerator 10 according to an embodiment. FIG. 8 shows the internal structure of the cold head drive unit. As shown in the figure, a so-called gas-assisted cryogenic refrigerator 10 is known in which a gas assist chamber 64, which is an airtight space, is formed between the upper rod 45b of the Scotch yoke shaft 45 and the expander housing 20. The gas assist chamber 64 is connected to the high pressure port 13a and the low pressure port 13b via the pressure switching valve 42, similar to the expander cylinder 16. Thus, the expander housing 20 is formed with a second connection flow path 20c that connects the pressure switching valve 42 to the gas assist chamber 64. The pressure switching valve 42 is configured to alternately connect the high pressure port 13a and the low pressure port 13b to the gas assist chamber 64. As a result, the differential pressure acting periodically between the gas assist chamber 64 and the internal volume of the expander housing 20 can assist the axial reciprocating motion of the Scotch yoke shaft 45, i.e., the displacer 18.

[0071] The expander 14 having such a gas assist chamber 64 may include a second gas receiving port 55 connected to the second connection passage 20c in addition to the first gas receiving port 54 connected to the first connection passage 20b. The gas replacement method according to the embodiment may be applied to the gas assist chamber 64 through the second gas receiving port 55. In this way, the gas remaining in the gas assist chamber 64 can be purged with a non-flammable gas. For gas replacement in the expander 14 having the gas assist chamber 64, a four-way gas replacement pipe may be used in which a fourth branch pipe that branches off from the main pipe 56a and can be connected to the gas assist chamber 64 of the expander 14 is added to the gas replacement pipe 56 shown in FIG. 7.

[0072] The present invention has been described above based on examples. It will be understood by those skilled in the art that the present invention is not limited to the above-described embodiments, that various design changes are possible, that various modifications are possible, and that such modifications are also within the scope of the present invention. Various features described in relation to one embodiment can also be applied to other embodiments. A new embodiment resulting from a combination has the combined effects of each of the combined embodiments.

[0073] In the above embodiment, the non-flammable gas source 60 is connected to the connection flow passage 20b through the gas receiving port 54 using the gas replacement piping 56, but other connection configurations are possible. In one embodiment, the non-flammable gas source 60 may be connected to the expander cylinder 16 instead of the connection flow passage 20b. In this case, the gas receiving port 54 may be provided in the expander cylinder 16. The gas replacement piping 56 may be provided through the wall of the airtight container 11 to connect the gas receiving port 54 located inside the airtight container 11 to the non-flammable gas source 60 located outside the airtight container 11.

[0074] In the above embodiment, the cryogenic refrigerator 10 is a two-stage GM refrigerator, but the cryogenic refrigerator 10 according to the embodiment may be, for example, a single-stage GM refrigerator (providing cryogenic cooling of, for example, 20 K to 80 K) or another cryogenic refrigerator having a pressure switching valve in the expander. In such cases, the gas replacement method according to the embodiment can be similarly applied.

[0075] The present invention has been described above based on examples. It will be understood by those skilled in the art that the present invention is not limited to the above-mentioned embodiments, that various design changes are possible, that various modifications are possible, and that such modifications are also within the scope of the present invention. [Explanation of symbols]

[0076] 10 cryogenic refrigerator, 13a high pressure port, 13b low pressure port, 14 expander, 16 expander cylinder, 20 expander housing, 20b connecting flow path, 40 expander motor, 42 pressure switching valve, 54 gas receiving port, 56 gas replacement piping, 58 gas outlet port, 60 non-flammable gas source.

Claims

1. A gas replacement method for an expander of a cryogenic refrigerator, the expander including an expander cylinder, a pressure switching valve that switches a pressure in the expander cylinder, a connection flow path from the pressure switching valve to the expander cylinder, and an expander motor that drives the pressure switching valve, the method comprising the steps of: Connecting a non-flammable gas source to the connecting passage or the expander cylinder; purging trapped gas in the expander cylinder with non-flammable gas from the non-flammable gas source while the expander motor is stopped.

2. The connecting step includes connecting the non-flammable gas source to a gas receiving port of the connection flow passage by a gas replacement pipe; 2. The method of claim 1, wherein the purging comprises: supplying non-flammable gas from the non-flammable gas source from the gas replacement piping through the gas receiving port to the expander cylinder; and discharging the stagnant gas from the gas replacement piping or a gas outlet port of the expander.

3. The expander includes a high pressure port and a low pressure port connected to the expander cylinder via the pressure switching valve, 3. The method of claim 2, wherein said purging comprises supplying a non-combustible gas to said expander from said high pressure port and said low pressure port.

4. 4. The method according to claim 1, wherein the non-flammable gas is a working gas for the cryogenic refrigerator.

5. An expander cylinder; A pressure switching valve for switching the pressure in the expander cylinder; A high pressure port and a low pressure port connected to the expander cylinder via the pressure switching valve; A connection flow path from the pressure switching valve to the expander cylinder; a gas receiving port different from the high pressure port and the low pressure port, the gas receiving port being connected to the connecting flow path so as to enable gas to flow into the connecting flow path.

6. The pressure change valve may be provided in a housing connected to the expander cylinder.

6. The cryogenic refrigerator according to claim 5, wherein the gas receiving port is provided in the expander housing.

7. 7. The cryogenic refrigerator according to claim 6, wherein the expander housing is provided with a gas outlet port in addition to the high pressure port, the low pressure port, and the gas inlet port.

8. 6. The cryogenic refrigerator of claim 5, wherein the gas receiving port has a different size than the high pressure port and the low pressure port.

9. 9. The cryogenic refrigerator according to claim 5, wherein a flow passage cross-sectional area of ​​the gas receiving port is smaller than a flow passage cross-sectional area of ​​the connecting flow passage.

10. A gas replacement pipe used for gas replacement of an expander of a cryogenic refrigerator, a main pipe connectable to a source of non-flammable gas; A first branch pipe that branches off from the main pipe and is connectable to a gas receiving port of the expander; A second branch pipe that branches off from the main pipe and is connectable to a high-pressure port of the expander; a third branch pipe branched off from the main pipe and connectable to a low-pressure port of the expander.

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