Operating method of cryogenic refrigerator and cryogenic refrigerator

Abstract

To shorten initial cooling time of a cryogenic refrigerator.SOLUTION: An operation method for a cryogenic refrigerator 10 comprises connecting a second compressor 80 in series with a first compressor 12 on a high-pressure line 63 or a low-pressure line 64, connecting a buffer volume 70 to the low-pressure line 64 via a supply valve 72, executing initial cooling for cooling an expander 14 from an initial temperature to a cryogenic temperature while the second compressor 80 and the buffer volume 70 are connected to the cryogenic refrigerator 10, and executing steady operation for keeping the expander 14 at the cryogenic temperature after the initial cooling. Executing the initial cooling comprises supplying working gas to the expander 14 by using the first compressor 12 and the second compressor 80, and controlling the supply valve 72 on the basis of a measured pressure in the high-pressure line 63 so as to keep the pressure in the high-pressure line 63 within a preset appropriate pressure range.SELECTED DRAWING: Figure 12

Description

Technical Field

[0001] The present invention relates to a method for operating a cryogenic refrigerator and a cryogenic refrigerator.

Background Art

[0002] Cryogenic refrigerators are used to cool various objects such as superconducting devices, measurement instruments, and samples that are used in cryogenic environments.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In order to cool an object with a cryogenic refrigerator, first, the cryogenic refrigerator must be started up to cool the cryogenic refrigerator from an initial temperature to a target cryogenic temperature. Such initial cooling of a cryogenic refrigerator is also referred to as cooldown. Since the initial cooling is merely preparation for starting the cooling of the object, it is desirable that the required time therefor be as short as possible.

[0005] One exemplary object of an aspect of the present invention is to shorten the initial cooling time of a cryogenic refrigerator.

Means for Solving the Problems

[0006] According to one aspect of the present invention, a method for operating a cryogenic refrigerator is provided. The cryogenic refrigerator comprises a first compressor, an expander, and a high-pressure line and a low-pressure line connecting the first compressor to the expander. The method comprises connecting a second compressor in series with the first compressor on the high-pressure line or on the low-pressure line, connecting a buffer volume to the low-pressure line via a supply valve, performing initial cooling to cool the expander from an initial temperature to a cryogenic temperature with the second compressor and buffer volume connected to the cryogenic refrigerator, and performing steady-state operation to maintain the expander at a cryogenic temperature following the initial cooling. Performing initial cooling comprises supplying working gas to the expander using the first and second compressors, and controlling the supply valve to maintain the pressure of the high-pressure line within a preset appropriate pressure range based on the measured pressure of the high-pressure line.

[0007] According to one aspect of the present invention, a method for operating a cryogenic refrigerator is provided. The cryogenic refrigerator comprises a first compressor, an expander, and a high-pressure line and a low-pressure line connecting the first compressor to the expander. The method comprises connecting a second compressor in series with the first compressor on the high-pressure line or the low-pressure line, performing initial cooling to cool the expander from an initial temperature to a cryogenic temperature with the second compressor connected to the cryogenic refrigerator, and performing steady-state operation to maintain the expander at a cryogenic temperature following the initial cooling. The first or second compressor has a compressor motor with a variable operating frequency and is driven by the compressor motor. Performing initial cooling comprises supplying working gas to the expander using the first and second compressors, and controlling the operating frequency of the compressor motor to maintain the pressure of the high-pressure line within a preset appropriate pressure range based on the measured pressure of the high-pressure line.

[0008] According to one aspect of the present invention, the cryogenic refrigerator includes an expander capable of performing initial cooling, which cools from an initial temperature to a cryogenic temperature, and steady-state operation, which maintains the cryogenic temperature following the initial cooling; a high-pressure line and a low-pressure line connected to the expander; a first pressure sensor for measuring the pressure in the high-pressure line; a second pressure sensor for measuring the pressure in the low-pressure line; a buffer volume for storing working gas; a supply valve connected to the buffer volume in the low-pressure line; and a controller configured to control the supply valve to maintain the pressure in the high-pressure line within a preset appropriate pressure range based on the pressure in the high-pressure line measured by the first pressure sensor during initial cooling. The controller is configured to interrupt control of the supply valve based on the pressure in the high-pressure line when the pressure in the low-pressure line measured by the second pressure sensor falls below a preset low-pressure threshold, and to control the supply valve to restore the pressure in the low-pressure line to the low-pressure threshold based on the pressure in the low-pressure line measured by the second pressure sensor.

[0009] Furthermore, any combination of the above components, or any substitution of components or expressions of the present invention between methods, apparatus, systems, etc., is also valid as an embodiment of the present invention. [Effects of the Invention]

[0010] According to the present invention, the initial cooling time of a cryogenic refrigerator can be shortened. [Brief explanation of the drawing]

[0011] [Figure 1] This is a schematic diagram showing a cryogenic refrigerator according to the first embodiment. [Figure 2] This is a schematic diagram showing a cryogenic refrigerator according to the first embodiment. [Figure 3] This is a flowchart illustrating the control method for a cryogenic refrigerator according to the first embodiment. [Figure 4] This is a flowchart illustrating the control method for a cryogenic refrigerator according to the first embodiment. [Figure 5]This is a graph showing an example of changes in temperature and pressure during operation of the cryogenic refrigerator according to the first embodiment. [Figure 6] Fig. 6(A) and Fig. 6(B) are graphs showing an example of changes in pressure during operation of the cryogenic refrigerator according to the first embodiment. [Figure 7] This is a graph showing an example of changes in temperature and pressure during operation of the cryogenic refrigerator according to the first embodiment. [Figure 8] This is a diagram schematically showing the cryogenic refrigerator according to the first embodiment. [Figure 9] This is a flowchart for explaining the control method of the cryogenic refrigerator according to the first embodiment. [Figure 10] This is a flowchart for explaining the control method of the cryogenic refrigerator according to the first embodiment. [Figure 11] This is a flowchart for explaining the control method of the cryogenic refrigerator according to the first embodiment. [Figure 12] This is a diagram schematically showing the cryogenic refrigerator according to the second embodiment. [Figure 13] This is a diagram schematically showing the cryogenic refrigerator according to the second embodiment. [Figure 14] This is a flowchart for explaining the operation method of the cryogenic refrigerator according to the second embodiment. [Figure 15] This is a diagram schematically showing the cryogenic refrigerator according to the third embodiment. [Figure 16] This is a flowchart for explaining the operation method of the cryogenic refrigerator according to the third embodiment. [Figure 17] This is a flowchart for explaining the operation method of the cryogenic refrigerator according to the third embodiment. [Figure 18] This is a diagram schematically showing the cryogenic refrigerator according to the third embodiment. [Figure 19] This is a diagram schematically showing the cryogenic refrigerator according to the third embodiment. [Modes for Carrying Out the Invention]

[0012] Hereinafter, embodiments for carrying out the present invention will be described in detail with reference to the drawings. In the description and the drawings, the same or equivalent components, members, and processes are denoted by the same reference numerals, and overlapping descriptions are omitted as appropriate. The scales and shapes of the respective parts shown in the drawings are set for convenience of explanation, and should not be interpreted in a limiting manner unless otherwise specified. The embodiments are illustrative 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 nature of the invention.

[0013] Figure 1 and Figure 2 are diagrams schematically showing a cryogenic refrigerator 10 according to the first embodiment. The cryogenic refrigerator 10 is, as an example, a two-stage Gifford-McMahon (GM) refrigerator. Figure 1 schematically shows a compressor 12 and an expander 14 that constitute the cryogenic refrigerator 10 together with a control device 100, and Figure 2 shows the internal structure of the expander 14 of the cryogenic refrigerator 10.

[0014] The compressor 12 is configured to recover the working gas of the cryogenic refrigerator 10 from the expander 14, pressurize the recovered working gas, and supply the working gas to the expander 14 again. The compressor 12 and the expander 14 constitute a refrigeration cycle of the cryogenic refrigerator 10, whereby the cryogenic refrigerator 10 can provide desired cryogenic cooling. The expander 14 is also referred to as a cold head. The working gas, also referred to as refrigerant gas, is usually helium gas, but other suitable gases may be used. For the sake of understanding, the flow direction of the working gas is indicated by arrows in Figure 1.

[0015] Note that in general, 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 considerably higher than the atmospheric pressure, and can be referred to as a first high pressure and a second high pressure, respectively. For convenience of explanation, the first high pressure and the second high pressure are also simply referred to as high pressure and 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 the sake of understanding, the flow direction of the working gas is indicated by arrows.

[0016] The expander 14 comprises a refrigerator cylinder 16 and a displacer assembly 18. The refrigerator cylinder 16 guides the linear reciprocating motion of the displacer assembly 18 and forms expansion chambers (32, 34) for the working gas between itself and the displacer assembly 18. The expander 14 also includes a pressure switching valve 40 that determines the timing for the start of intake of working gas into the expansion chamber and the timing for the start of exhaust of working gas from the expansion chamber.

[0017] In this book, for convenience, in order to explain the positional relationships between the components of the cryogenic refrigerator 10, the side of the displacer closer to the top dead center in its axial reciprocating motion will be referred to as "upper," and the side closer to the bottom dead center as "lower." 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 upper axial direction downwards, so the upper side can also be called the high-temperature side and the lower side the low-temperature side.

[0018] The refrigeration cylinder 16 has a first cylinder 16a and a second cylinder 16b. The first cylinder 16a and the second cylinder 16b are, for example, cylindrical members, with the second cylinder 16b having 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.

[0019] The displacer assembly 18 comprises a first displacer 18a and a second displacer 18b connected to each other, which move together as a single unit. The first displacer 18a and the second displacer 18b are, for example, cylindrical members, with the second displacer 18b having a smaller diameter than the first displacer 18a. The first displacer 18a and the second displacer 18b are arranged coaxially.

[0020] The first displacer 18a is housed in the first cylinder 16a, and the second displacer 18b is housed in the second cylinder 16b. The first displacer 18a is reciprocable in the axial direction along the first cylinder 16a, and the second displacer 18b is reciprocable in the axial direction along the second cylinder 16b.

[0021] As shown in Figure 2, the first displacer 18a houses the first cooler 26. The first cooler 26 is formed by filling the cylindrical body of the first displacer 18a with a wire mesh, such as copper, or other suitable first coolant. The upper and lower lids of the first displacer 18a may be provided as separate components from the body of the first displacer 18a, and the upper and lower lids of the first displacer 18a may be fixed to the body by appropriate means such as fastening or welding, thereby housing the first coolant in the first displacer 18a.

[0022] Similarly, the second displacer 18b houses the second regenerator 28. The second regenerator 28 is formed by filling the cylindrical body of the second displacer 18b with a non-magnetic regenerator such as bismuth, a magnetic regenerator such as HoCu2, or other suitable second regenerator. The second regenerator may be formed into granules. The upper and lower lids of the second displacer 18b may be provided as separate components from the body of the second displacer 18b, and the lower lid of the upper lid of the second displacer 18b may be fixed to the body by suitable means such as fastening or welding, thereby housing the second regenerator in the second displacer 18b.

[0023] The displacer assembly 18 forms a room temperature chamber 30, a first expansion chamber 32, and a second expansion chamber 34 inside the refrigerator cylinder 16. For heat exchange with the 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 room temperature chamber 30 is formed between the upper lid of the first displacer 18a and the upper part of the first cylinder 16a. The first expansion chamber 32 is formed between the lower lid of the first displacer 18a and the first cooling stage 33. The second expansion chamber 34 is formed between the lower lid 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.

[0024] The first regenerator 26 is connected to the room temperature chamber 30 through an operating gas passage 36a formed in the upper lid of the first displacer 18a, and to the first expansion chamber 32 through an operating 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 an operating 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 an operating gas passage 36d formed in the lower lid of the second displacer 18b.

[0025] A first seal 38a and a second seal 38b may be provided to ensure that the working gas flow between the first expansion chamber 32, the second expansion chamber 34 and the room temperature chamber 30 is directed to the first regenerator 26 and the second regenerator 28, rather than to the clearance between the chiller cylinder 16 and the displacer assembly 18. The first seal 38a may be mounted on the top cover of the first displacer 18a so as to be positioned between the first displacer 18a and the first cylinder 16a. The second seal 38b may be mounted on the top cover of the second displacer 18b so as to be positioned between the second displacer 18b and the second cylinder 16b.

[0026] As shown in Figure 1, the expander 14 includes a refrigeration housing 20 that houses a pressure switching valve 40. The refrigeration housing 20 is coupled to the refrigeration cylinder 16, thereby forming an airtight container that houses the pressure switching valve 40 and the displacer assembly 18.

[0027] The pressure switching valve 40, as shown in Figure 2, comprises a high-pressure valve 40a and a low-pressure valve 40b, and is configured to generate periodic pressure fluctuations within the refrigeration cylinder 16. The working gas outlet of the compressor 12 is connected to the room temperature chamber 30 via the high-pressure valve 40a, and the working gas inlet of the compressor 12 is connected to the room temperature chamber 30 via the low-pressure valve 40b. The high-pressure valve 40a and the low-pressure valve 40b are configured to open and close selectively and alternately (i.e., when one is open, the other is closed).

[0028] The pressure switching valve 40 may take the form of a rotary valve. That is, the pressure switching valve 40 may be configured such that the high-pressure valve 40a and the low-pressure valve 40b are alternately opened and closed by the rotational sliding of the valve disc relative to the stationary valve body. In this case, the expander motor 42 may be connected to the pressure switching valve 40 so as to rotate the valve disc of the pressure switching valve 40. For example, the pressure switching valve 40 may be positioned such that the valve rotation axis is coaxial with the rotation axis of the expander motor 42.

[0029] Alternatively, the high-pressure valve 40a and the low-pressure valve 40b may each be individually controllable valves, in which case the pressure switching valve 40 does not need to be connected to the expander motor 42.

[0030] The expander motor 42 is connected to the displacer drive shaft 44 via a motion conversion mechanism 43, such as a Scotch yoke mechanism. The expander motor 42 is mounted in the refrigerator housing 20. The motion conversion mechanism 43, like the pressure switching valve 40, is housed in the refrigerator housing 20. The motion conversion mechanism 43 converts the rotational motion output by the expander motor 42 into linear reciprocating motion of the displacer drive shaft 44. The displacer drive shaft 44 extends from the motion conversion mechanism 43 into the room temperature chamber 30 and is fixed to the top cover of the first displacer 18a. The rotation of the expander motor 42 is converted by the motion conversion mechanism 43 into axial reciprocating motion of the displacer drive shaft 44, causing the displacer assembly 18 to reciprocate linearly in the axial direction within the refrigerator cylinder 16.

[0031] The expander 14 may also be equipped with a temperature sensor 46 that measures the temperature of the second cooling stage 35 (and / or the first cooling stage 33) and outputs a measured temperature signal indicating the measured temperature.

[0032] The compressor 12 comprises a high-pressure gas outlet 50, a low-pressure gas inlet 51, a high-pressure passage 52, a low-pressure passage 53, a first pressure sensor 54, a second pressure sensor 55, a bypass line 56, a compressor body 57, and a compressor housing 58. The high-pressure gas outlet 50 is installed in the compressor housing 58 as the working gas discharge port of the compressor 12, and the low-pressure gas inlet 51 is installed in the compressor housing 58 as the working gas intake port of the compressor 12. The high-pressure passage 52 connects the discharge port of the compressor body 57 to the high-pressure gas outlet 50, and the low-pressure passage 53 connects the low-pressure gas inlet 51 to the intake port of the compressor body 57. The compressor housing 58 houses the high-pressure passage 52, the low-pressure passage 53, the first pressure sensor 54, the second pressure sensor 55, the bypass line 56, and the compressor body 57. The compressor 12 is also referred to as a compressor unit.

[0033] The compressor body 57 is configured to compress the working gas drawn in through its inlet and discharge it from its outlet. The compressor body 57 may be, for example, a scroll type, a rotary type, or another pump that pressurizes the working gas. In this embodiment, the compressor body 57 is configured to discharge a fixed, constant flow rate of working gas. Alternatively, the compressor body 57 may be configured to make the discharged flow rate of working gas variable. The compressor body 57 is sometimes referred to as a compression capsule.

[0034] The first pressure sensor 54 is positioned in the high-pressure channel 52 to measure the pressure of the working gas flowing through the high-pressure channel 52. The first pressure sensor 54 is configured to output a first measured pressure signal PH representing the measured pressure. The second pressure sensor 55 is positioned in the low-pressure channel 53 to measure the pressure of the working gas flowing through the low-pressure channel 53. The second pressure sensor 55 is configured to output a second measured pressure signal PL representing the measured pressure. Therefore, the first pressure sensor 54 and the second pressure sensor 55 can also be called the high-pressure sensor and the low-pressure sensor, respectively. In this document, the term "pressure sensor" may also be used to refer to either the first pressure sensor 54 or the second pressure sensor 55, or to refer to both collectively.

[0035] The bypass line 56 connects the high-pressure passage 52 to the low-pressure passage 53 so as to recirculate the working gas from the high-pressure passage 52 to the low-pressure passage 53, bypassing the expander 14. The bypass line 56 is provided with a relief valve 60 for opening and closing the bypass line 56 or controlling the flow rate of the working gas flowing through the bypass line 56. The relief valve 60 is configured to open when a pressure difference exceeding a set pressure acts between its inlet and outlet. The relief valve 60 may be an on / off valve or a flow control valve, and may be a solenoid valve, for example. The set pressure can be set appropriately based on the designer's empirical knowledge or experiments and simulations conducted by the designer. This prevents the pressure difference between the high-pressure line 63 and the low-pressure line 64 from exceeding this set pressure. It also prevents the pressure in the high-pressure line 63 from becoming excessive.

[0036] The relief valve 60 may be configured to act as a so-called safety valve, that is, it may be mechanically opened when a differential pressure greater than or equal to a set pressure acts between the inlet and outlet. Alternatively, the relief valve 60 may be opened and closed by control of the control device 100. The control device 100 may compare the differential pressure between the high-pressure line 63 and the low-pressure line 64 to be measured with a set pressure and control the relief valve 60 to open if the measured differential pressure is greater than or equal to the set pressure, and to close the relief valve 60 if the measured differential pressure is less than the set differential pressure. The control device 100 may acquire the measured differential pressure between the high-pressure line 63 and the low-pressure line 64 based on a first measured pressure signal PH from a first pressure sensor 54 and a second measured pressure signal PL from a second pressure sensor 55. As another example, the control device 100 may compare the measured pressure of the high-pressure line 63 with an upper limit pressure based on the first measured pressure signal PH and control the relief valve 60 to open if the measured pressure is greater than or equal to the upper limit pressure, and to close the relief valve 60 if the measured pressure is less than the upper limit pressure.

[0037] Furthermore, the compressor 12 may have various other components. For example, the high-pressure passage 52 may be provided with an oil separator, an adsorber, etc. The low-pressure passage 53 may be provided with a storage tank or other components. In addition, the compressor 12 may be provided with an oil circulation system for cooling the compressor body 57 with oil, and a cooling system for cooling the oil.

[0038] The cryogenic refrigerator 10 also includes a gas line 62 for circulating working gas between the compressor 12 and the expander 14. The gas line 62 includes a high-pressure line 63 connecting the compressor 12 to the expander 14 to supply working gas from the compressor 12 to the expander 14, and a low-pressure line 64 connecting the compressor 12 to the expander 14 to recover working gas from the expander 14 to the compressor 12. The refrigerator housing 20 of the expander 14 is provided with a high-pressure gas inlet 22 and a low-pressure gas outlet 24. The high-pressure gas inlet 22 is connected to the high-pressure gas outlet 50 by high-pressure piping 65, and the low-pressure gas outlet 24 is connected to the low-pressure gas inlet 51 by low-pressure piping 66. The high-pressure line 63 consists of high-pressure piping 65 and a high-pressure flow path 52, and the low-pressure line 64 consists of low-pressure piping 66 and a low-pressure flow path 53. The bypass line 56 may be considered part of the gas line 62. The bypass line 56 connects the high-pressure line 63 to the low-pressure line 64 so as to recirculate the working gas from the high-pressure line 63 to the low-pressure line 64, bypassing the expander 14.

[0039] Therefore, the working gas recovered from the expander 14 to the compressor 12 enters the low-pressure gas inlet 51 of the compressor 12 through the low-pressure piping 66 from the low-pressure gas outlet 24 of the expander 14, and then returns to the compressor body 57 via the low-pressure flow path 53, where it is compressed and pressurized by the compressor body 57. The working gas supplied from the compressor 12 to the expander 14 exits the high-pressure gas outlet 50 of the compressor 12 through the high-pressure flow path 52 from the compressor body 57, and is then supplied to the expander 14 via the high-pressure piping 65 and the high-pressure gas inlet 22 of the expander 14.

[0040] Furthermore, the cryogenic refrigerator 10 includes a buffer volume 70, a supply valve 72, and a recovery valve 74. The buffer volume 70 is a volume for storing working gas and may be, for example, a buffer tank. The supply valve 72 connects the buffer volume 70 to the low-pressure line 64, and the recovery valve 74 connects the buffer volume 70 to the high-pressure line 63. The supply valve 72 and the recovery valve 74 may be on / off valves or flow control valves, and may be, for example, solenoid valves.

[0041] The pressure in the buffer volume 70 is the pressure at which the working gas is supplied to the cryogenic refrigerator 10 when the cryogenic refrigerator 10 is stopped. When the cryogenic refrigerator 10 is in operation (for example, during initial cooling or steady-state operation), the pressure in the buffer volume 70 is an intermediate pressure between the pressure of the high-pressure line 63 and the pressure of the low-pressure line 64 (for example, the average pressure of the high and low pressures).

[0042] Therefore, during operation of the cryogenic refrigerator 10, when the supply valve 72 is open, working gas is supplied from the buffer volume 70 to the low-pressure line 64 through the supply valve 72. When the supply valve 72 is closed, the supply of working gas from the buffer volume 70 to the low-pressure line 64 is stopped. Also, when the recovery valve 74 is open, working gas is recovered from the high-pressure line 63 to the buffer volume 70 through the recovery valve 74. When the recovery valve 74 is closed, the recovery of working gas from the high-pressure line 63 to the buffer volume 70 is stopped. In this way, by opening and closing the supply valve 72 and the recovery valve 74, the amount of working gas circulating in the gas line 62 can be adjusted, and as a result, the pressures of the high-pressure line 63 and the low-pressure line 64 can also be controlled.

[0043] As shown in Figure 1, the control device 100 that controls the cryogenic refrigerator 10 includes a controller 110 that controls the supply valve 72 and the recovery valve 74. The controller 110 is electrically connected to the first pressure sensor 54 and the second pressure sensor 55 to acquire a first measured pressure signal PH and a second measured pressure signal PL. As will be described later, the controller 110 receives the first measured pressure signal PH from the first pressure sensor 54 and is configured to open and close the supply valve 72 and the recovery valve 74 based on the measured pressure of the high-pressure line 63 indicated by the first measured pressure signal PH. The controller 110 is also electrically connected to the temperature sensor 46 to acquire a measured temperature signal from the temperature sensor 46.

[0044] In the illustrated example, the control device 100 is provided separately from and connected to the compressor 12 and the expander 14, but this is not limited to the case. The control device 100 may be mounted on the compressor 12. The control device 100 may be provided on the expander 14, such as being mounted on the expander motor 42. The controller 110 may be provided on the supply valve 72, or on the recovery valve 74, or on the supply valve 72 and the recovery valve 74 respectively.

[0045] The control device 100 is implemented in hardware form by components and circuits such as the CPU and memory of a computer, and in software form by computer programs, etc., but in Figure 1, it is depicted as a functional block realized through the coordination of these components as appropriate. Those skilled in the art will understand that these functional blocks can be realized in various forms by combinations of hardware and software.

[0046] When the compressor 12 and expander motor 42 of the cryogenic refrigerator 10 are in operation, periodic volume fluctuations and synchronized pressure fluctuations of the working gas occur in the first expansion chamber 32 and the second expansion chamber 34. Typically, during the intake process, the low-pressure valve 40b closes and the high-pressure valve 40a opens, causing high-pressure working gas to flow from the compressor 12 into the room-temperature chamber 30 through the high-pressure valve 40a, and is supplied to the first expansion chamber 32 through the first regenerator 26 and to the second expansion chamber 34 through the second regenerator 28. In this way, the first expansion chamber 32 and the second expansion chamber 34 are pressurized from low pressure to high pressure. At this time, the displacer assembly 18 is moved upward from bottom dead center to top dead center, increasing the volumes of the first expansion chamber 32 and the second expansion chamber 34. The intake process ends when the high-pressure valve 40a closes.

[0047] In the exhaust process, the high-pressure valve 40a closes and the low-pressure valve 40b opens, opening the high-pressure first expansion chamber 32 and second expansion chamber 34 to the low-pressure working gas inlet of the compressor 12. As a result, the working gas expands in the first and second expansion chambers 32 and 34, and the resulting low-pressure working gas is discharged from the first and second expansion chambers 32 and 34 through the first and second regenerators 26 and 28 to the room temperature chamber 30. At this time, the displacer assembly 18 moves downward from top dead center to bottom dead center, reducing the volume of the first and second expansion chambers 32 and 34. The working gas is recovered from the expander 14 to the compressor 12 through the low-pressure valve 40b. The exhaust process ends when the low-pressure valve 40b closes.

[0048] In this way, a refrigeration cycle, such as a GM cycle, is configured, and the first cooling stage 33 and the second cooling stage 35 are cooled to a desired cryogenic temperature. The first cooling stage 33 can be cooled to a first cooling temperature, for example, in the range of about 20K to about 40K. The second cooling stage 35 can be cooled to a second cooling temperature lower than the first cooling temperature (for example, about 1K to about 4K).

[0049] The cryogenic refrigerator 10 is capable of initial cooling and steady-state operation following initial cooling. Initial cooling is the operating mode of the expander 14 that rapidly cools the cryogenic refrigerator 10 from an initial temperature to a cryogenic temperature when the cryogenic refrigerator 10 is started up, and steady-state operation is the operating mode of the expander 14 that maintains the cryogenic temperature achieved by initial cooling. The initial temperature may be the ambient temperature (e.g., room temperature). Initial cooling can also be performed when restarting a cryogenic device (e.g., a superconducting device such as an MRI (Magnetic Resonance Imaging) system) equipped with the cryogenic refrigerator 10 after maintenance. During maintenance, the objects to be cooled inside the cryogenic device may not be heated to the ambient temperature and may be kept at a relatively low temperature (e.g., 20-80K). In this case, the initial temperature may be such a low temperature.

[0050] The expander 14 is cooled to a standard cooling temperature by initial cooling, and during steady-state operation, it is maintained within the allowable cryogenic temperature range, including this standard cooling temperature. The standard cooling temperature varies depending on the application and settings of the cryogenic refrigerator 10, but is typically about 4.2K or lower for cooling applications such as superconducting devices. For other cooling applications, the standard cooling temperature may be, for example, about 10K to 20K, or even below 10K. Initial cooling can also be called cool-down, as described above.

[0051] Incidentally, during initial cooling, as the temperature drops from the initial temperature to the cryogenic temperature, the density of the working gas in the expander 14 increases. Consequently, the amount of working gas accumulating in the expander 14 increases, and in a sense, the working gas is absorbed from the gas line 62 into the expander 14. As a result, as the cooling of the expander 14 progresses, the pressure of the working gas circulating in the gas line 62 gradually decreases. This decrease in working gas pressure leads to a decrease in the cooling capacity of the cryogenic refrigerator 10, which is a concern as it may increase the time required for initial cooling. Since initial cooling is merely preparation for the cryogenic refrigerator to begin cooling the object, it is desirable that the time required for it be as short as possible.

[0052] To address these issues, in this embodiment, the controller 110 controls the supply valve 72 to maintain the pressure of the high-pressure line 63 within a preset appropriate pressure range, based on the pressure of the high-pressure line 63 measured by the first pressure sensor 54 during initial cooling. More specifically, the controller 110 may compare the measured pressure of the high-pressure line 63 with a lower limit Pc of the appropriate pressure range during initial cooling and operate the supply valve 72 to repeatedly open and close the supply valve 72 so that the pressure of the high-pressure line 63 does not fall below the lower limit Pc.

[0053] Furthermore, in this embodiment, the controller 110 controls the recovery valve 74 to maintain the pressure of the high-pressure line 63 within an appropriate pressure range based on the pressure of the high-pressure line 63 measured by the first pressure sensor 54 during initial cooling. More specifically, the controller 110 may compare the measured pressure of the high-pressure line 63 with the upper limit Pd of the appropriate pressure range during initial cooling and operate the recovery valve 74 to repeatedly open and close so that the pressure of the high-pressure line 63 does not exceed the upper limit Pd.

[0054] Figure 3 is a flowchart illustrating the control method for the cryogenic refrigerator 10 according to the first embodiment. This method is repeatedly executed at predetermined intervals by the controller 110 during the initial cooling of the cryogenic refrigerator 10. This method may be continuously executed not only during the initial cooling but also during the steady-state operation of the cryogenic refrigerator 10.

[0055] First, the pressure of the high-pressure line 63 is measured (S10). The first pressure sensor 54 measures the pressure of the high-pressure line 63 and outputs a first measured pressure signal PH representing the measured pressure of the high-pressure line 63. The controller 110 receives the first measured pressure signal PH and obtains the measured pressure of the high-pressure line 63.

[0056] Next, the measured pressure of the high-pressure line 63 is compared with the appropriate pressure range (S12). The lower limit Pc of the appropriate pressure range is set so that the cryogenic refrigerator 10 provides sufficient cooling capacity. The upper limit Pd of the appropriate pressure range is set so as not to generate excessive pressure in the high-pressure line 63. The upper limit Pd of the appropriate pressure range may be set to a pressure value smaller than the aforementioned set pressure at which the relief valve 60 opens. The appropriate pressure range can be set as appropriate based on the designer's empirical knowledge or experiments and simulations conducted by the designer. The appropriate pressure range may be pre-saved in the controller 110 as the initial setting of the cryogenic refrigerator 10, or it may be set in the controller 110 by the user before operation of the cryogenic refrigerator 10.

[0057] As an example, the upper limit value Pd and the lower limit value Pc of the appropriate pressure range may be selected, for example, from the range of 2 MPa to 3 MPa, or the range of 2.1 MPa to 2.7 MPa. The width of the appropriate pressure range, that is, the difference between the upper limit value Pd and the lower limit value Pc of the appropriate pressure range, may be set to a certain value within 0.5 MPa, or within 0.3 MPa, or within 0.1 MPa, for example. For example, the appropriate pressure range may be set to 2.45±0.05 MPa, in which case the width of the appropriate pressure range is 0.1 MPa, the upper limit value Pd is 2.5 MPa, and the lower limit value Pc is 2.4 MPa.

[0058] The controller 110 compares the measured pressure of the high-pressure line 63 with the lower limit value Pc of the appropriate pressure range, and opens the supply valve 72 when the measured pressure of the high-pressure line 63 falls below the lower limit value Pc (PH<Pc) (S14). As a result, working gas is supplied from the buffer volume 70 to the low-pressure line 64 through the supply valve 72. Since the amount of working gas circulating in the gas line 62 increases, the pressure in the high-pressure line 63 is restored.

[0059] The controller 110 closes the supply valve 72 when the measured pressure of the high-pressure line 63 is restored to the appropriate pressure range (S16). For example, the controller 110 compares the measured pressure of the high-pressure line 63 with the lower limit value Pc of the appropriate pressure range, and may close the supply valve 72 when the measured pressure of the high-pressure line 63 exceeds the lower limit value Pc (PH>Pc, or PH≧Pc). When the supply valve 72 is closed, the supply of working gas from the buffer volume 70 to the low-pressure line 64 is stopped. In this way, the present method ends and is executed again in the next control cycle.

[0060] The pressure threshold for closing the supply valve 72 may be different from the lower limit value Pc of the appropriate pressure range, and may be larger than the lower limit value Pc, for example. This pressure threshold may be set so as not to exceed the upper limit value Pd of the appropriate pressure range. For example, the pressure threshold may be a value obtained by adding a predetermined proportion of the width of the appropriate pressure range (upper limit value Pd - lower limit value Pc) to the lower limit value Pc. The predetermined proportion may be, for example, 50% or less, 30% or less, or 10% or less.

[0061] FIG. 4 is a flowchart illustrating a control method for the cryogenic refrigerator 10 according to the first embodiment. The present method is repeatedly executed at predetermined intervals by the controller 110 during initial cooling of the cryogenic refrigerator 10. The present method may be executed in parallel with the method shown in FIG. 3. Note that the present method may be continuously executed not only during initial cooling but also during steady operation of the cryogenic refrigerator 10.

[0062] First, the pressure in the high-pressure line 63 is measured using the first pressure sensor 54 (S20). The controller 110 receives the first measured pressure signal PH from the first pressure sensor 54, and acquires the measured pressure in the high-pressure line 63.

[0063] Next, the measured pressure in the high-pressure line 63 is compared with an appropriate pressure range (S22). The controller 110 compares the measured pressure in the high-pressure line 63 with the upper limit value Pd of the appropriate pressure range, and opens the recovery valve 74 when the measured pressure in the high-pressure line 63 exceeds the upper limit value Pd (PH>Pd) (S24). Thereby, the working gas is recovered from the high-pressure line 63 to the buffer volume 70 through the recovery valve 74, and the pressure in the high-pressure line 63 decreases.

[0064] The controller 110 closes the recovery valve 74 when the measured pressure in the high-pressure line 63 returns to the appropriate pressure range (S26). For example, the controller 110 may compare the measured pressure in the high-pressure line 63 with the upper limit value Pd of the appropriate pressure range, and close the recovery valve 74 when the measured pressure in the high-pressure line 63 falls below the upper limit value Pd (PH<Pd, or PH≦Pd). When the recovery valve 74 is closed, recovery of the working gas from the high-pressure line 63 to the buffer volume 70 is stopped. Thus, the present method ends and is executed again in the next control cycle.

[0065] Note that the pressure threshold for closing the recovery valve 74 may be different from the upper limit value Pd of the appropriate pressure range, and may be smaller than the upper limit value Pd, for example. This pressure threshold is selected from the appropriate pressure range, that is, it may be larger than the lower limit value Pc of the appropriate pressure range.

[0066] The appropriate pressure range may be changed during the operation of the cryogenic refrigerator 10. For example, the appropriate pressure range during initial cooling may differ from the appropriate pressure range during steady-state operation, and may be higher than the appropriate pressure range during steady-state operation. For example, the lower limit Pc during initial cooling may be higher than the lower limit Pc during steady-state operation, and / or the upper limit Pd during initial cooling may be higher than the upper limit Pd during steady-state operation.

[0067] In this case, the switching from initial cooling to steady-state operation and the change in the appropriate pressure range may be controlled by the control device 100. For example, the control device 100 may compare the measured temperature of the second cooling stage 35 (and / or the first cooling stage 33) with the standard cooling temperature based on the measured temperature signal from the temperature sensor 46, and perform initial cooling if the measured temperature is higher than the standard cooling temperature, and transition from initial cooling to steady-state operation if the measured temperature is lower than or equal to the standard cooling temperature. The controller 110 may change the appropriate pressure range in conjunction with the transition from initial cooling to steady-state operation.

[0068] Furthermore, as will be described later with reference to Figures 7 and 8, the switching from initial cooling to steady-state operation and the change in the appropriate pressure range may be performed based on the pressure of the buffer volume 70 or based on the differential pressure between the high-pressure line 63 and the low-pressure line 64. In this way, the control device 100 can complete the initial cooling of the cryogenic refrigerator 10 without relying on the temperature sensor 46.

[0069] Here, in order to ensure a reliable supply of working gas from the buffer volume 70, we consider the desired conditions for the buffer volume 70. From the ideal gas law, when the cryogenic refrigerator 10 is shut down (i.e., before initial cooling), PI(VH+VL+VB)=nRT (1) The following holds true. Here, PI (MPa) is the operating gas filling pressure of the cryogenic refrigerator 10 at temperature T (K), VH (L) is the volume of the high-pressure line 63, VL (L) is the volume of the low-pressure line 64, VB (L) is the volume of the buffer volume 70, n (mol) is the amount of operating gas in the cryogenic refrigerator 10, and R is the gas constant.

[0070] Similarly, during steady-state operation of the cryogenic refrigerator 10, PHVH + PLVL + PBVB = nRT (2) The following holds true. Here, PH (MPa) represents the pressure of the high-pressure line 63 during steady-state operation at temperature T, PL (MPa) represents the pressure of the low-pressure line 64 during steady-state operation at temperature T, and PB (MPa) represents the pressure of the buffer volume 70 during steady-state operation at temperature T.

[0071] From equations (1) and (2), PI(VH+VL+VB)=PHVH+PLVL+PBVB (3) This is the result.

[0072] To supply working gas from buffer volume 70 to low-pressure line 64 at any time during operation of cryogenic refrigerator 10, for any temperature T in the temperature range from the initial temperature to the cryogenic temperature of cryogenic refrigerator 10, PL ≤ PB (4) It should satisfy this condition.

[0073] Solving equation (3) for PB and substituting it into equation (4), we obtain the following relationship. VB≧VH(PH-PI) / (PI-PL)-VL (5)

[0074] Therefore, in order to ensure the supply of working gas from the buffer volume 70 to the low-pressure line 64, it is preferable that the buffer volume 70 satisfies equation (5) for any temperature in the temperature range from the initial temperature to cryogenic temperatures.

[0075] Similarly, to ensure gas recovery into the buffer volume 70, we consider the desired conditions for the buffer volume 70. In this case, in order to supply working gas from the buffer volume 70 to the high-pressure line 63 at any time during the operation of the cryogenic refrigerator 10, for any temperature T in the temperature range from the initial temperature to the cryogenic temperature of the cryogenic refrigerator 10, PB ≤ PH (6) It should satisfy this condition.

[0076] Solving equation (3) for PB and substituting it into equation (6), we obtain the following relationship. VB≧-VH+VL(PI-PL) / (PH-PI) (7)

[0077] Therefore, in order to ensure the recovery of working gas from the high-pressure line 63 to the buffer volume 70, it is preferable that the buffer volume 70 satisfies equation (7) for any temperature in the temperature range from the initial temperature to cryogenic temperatures.

[0078] Figure 5 is a graph showing an example of the time variation of temperature and pressure during operation of the cryogenic refrigerator 10 according to the first embodiment. The pressure changes shown were obtained experimentally. The upper part of Figure 5 shows the pressure PH of the high-pressure line 63 measured by the first pressure sensor 54 and the pressure PL of the low-pressure line 64 measured by the second pressure sensor 55. The lower part of Figure 5 shows the temperature T1 of the first cooling stage 33 and the temperature T2 of the second cooling stage 35. The horizontal axis represents time.

[0079] Before the cryogenic refrigerator 10 is started (time 0), the pressure PH of the high-pressure line 63 and the pressure PL of the low-pressure line 64 are both the sealing pressure PI, and the temperature T1 of the first cooling stage 33 and the temperature T2 of the second cooling stage 35 are both room temperature (approximately 300K). When the cryogenic refrigerator 10 is started and initial cooling begins, the compressor 12 and the expander 14 operate, the pressure PH of the high-pressure line 63 increases from the sealing pressure PI, and the pressure PL of the low-pressure line 64 decreases from the sealing pressure PI. Due to the initial cooling, the temperature T1 of the first cooling stage 33 and the temperature T2 of the second cooling stage 35 decrease. When the first cooling stage 33 and the second cooling stage 35 are cooled to the above-mentioned standard cooling temperatures (for example, T1 ≤ 30K, T2 ≤ 4K), initial cooling is completed and steady-state operation begins.

[0080] Figure 6(A) schematically shows an enlarged view of section A shown in Figure 5, and Figure 6(B) schematically shows an enlarged view of section B shown in Figure 5. Figure 6(A) shows the pressure PH of the high-pressure line 63 immediately after the start of initial cooling, along with the open / closed state of the recovery valve 74, and Figure 6(B) shows the pressure PH of the high-pressure line 63 after section A, along with the open / closed state of the supply valve 72.

[0081] As shown in Figure 6(A), when the pressure PH of the high-pressure line 63 exceeds the upper limit Pd of the appropriate pressure range, the recovery valve 74 opens. Since the working gas is recovered from the high-pressure line 63 into the buffer volume 70 through the recovery valve 74, the pressure PH of the high-pressure line 63 decreases. When the pressure PH of the high-pressure line 63 falls below the upper limit Pd, the recovery valve 74 closes. In this way, excessive pressure increase in the high-pressure line 63 can be avoided. The risk of emergency shutdown of the compressor 12 due to excessive pressure increase is reduced. In addition, since the buffer volume 70 is pressurized by the recovery of the working gas, it can be effectively used to supply working gas from the buffer volume 70 to the low-pressure line 64.

[0082] As shown in Figure 6(B), when the pressure PH of the high-pressure line 63 falls below the lower limit Pc of the appropriate pressure range, the supply valve 72 opens. Working gas is supplied from the buffer volume 70 to the low-pressure line 64 through the supply valve 72. As the amount of working gas circulating in the gas line 62 increases, the pressure in the high-pressure line 63 is restored. When the pressure PH of the high-pressure line 63 exceeds the lower limit Pc, the supply valve 72 closes.

[0083] As described above, the decrease in temperature of the expander 14 during initial cooling increases the density of the working gas within the expander 14, which has the effect of lowering the pressure PH of the high-pressure line 63. Therefore, even if the pressure PH of the high-pressure line 63 recovers once, it falls below the lower limit Pc again. The supply valve 72 opens again, the pressure in the high-pressure line 63 recovers, and the supply valve 72 closes. In this way, the supply valve 72 operates by repeatedly opening and closing to maintain the pressure PH of the high-pressure line 63 within the appropriate pressure range.

[0084] If the working gas were not supplied to the gas line 62 during the initial cooling, the pressure PH of the high-pressure line 63 could drop significantly due to the temperature decrease of the expander 14. Since the cooling capacity of the cryogenic refrigerator 10 is correlated with the pressure PH of the high-pressure line 63, the cooling capacity of the cryogenic refrigerator 10 may decrease as the initial cooling progresses. This could be a factor that prolongs the time required for initial cooling.

[0085] In contrast, according to this embodiment, the pressure PH of the high-pressure line 63 can be maintained within an appropriate pressure range by controlling the supply valve 72 during the initial cooling. Therefore, the cooling capacity of the cryogenic refrigerator 10 can be properly maintained, and the increase in the initial cooling time can be suppressed. Furthermore, by keeping the pressure PH of the high-pressure line 63 approximately constant, the cryogenic refrigerator 10 can provide a stable cooling capacity.

[0086] To maintain the pressure PH of the high-pressure line 63 within the appropriate pressure range, a method of controlling the supply valve 72 and the recovery valve 74 based on the pressure of the low-pressure line 64 is also conceivable. The pressure of the low-pressure line 64 is affected by the cooling temperature of the expander 14 (it fluctuates depending on the cooling temperature). Therefore, it is practically essential to set the appropriate pressure range of the low-pressure line 64, i.e., the pressure threshold of the low-pressure line 64 for opening and closing the supply valve 72 and the recovery valve 74, to different values ​​depending on the cooling temperature, which complicates the control design. Furthermore, even if the low-pressure line 64 is within the appropriate pressure range, depending on the cooling temperature, the pressure of the high-pressure line 63 may become excessively high. Therefore, the method based on the pressure of the high-pressure line 63, as in the embodiment, is advantageous in that it mitigates or prevents such problems.

[0087] Figure 7 is a graph showing an example of temperature and pressure changes during operation of the cryogenic refrigerator 10 according to the first embodiment. Figure 8 is a schematic diagram of the cryogenic refrigerator 10 according to the first embodiment.

[0088] Similar to the embodiment described above, the cryogenic refrigerator 10 includes a compressor 12, an expander 14, a buffer volume 70, and a control device 100. During initial cooling, the controller 110 controls the supply valve 72 to maintain the pressure of the high-pressure line 63 within a preset appropriate pressure range based on the pressure of the high-pressure line 63 measured by the first pressure sensor 54. The controller 110 also controls the recovery valve 74 to maintain the pressure of the high-pressure line 63 within an appropriate pressure range based on the pressure of the high-pressure line 63 measured by the first pressure sensor 54 during initial cooling.

[0089] The cryogenic refrigerator 10 includes a buffer pressure sensor 76 connected to the buffer volume 70 to measure the pressure in the buffer volume 70. The buffer pressure sensor 76 is electrically connected to the control device 100 and is configured to output a measured buffer pressure signal PB representing the measured pressure to the control device 100.

[0090] The upper part of Figure 7 shows the pressure PH of the high-pressure line 63 and the pressure PL of the low-pressure line 64, as shown in Figure 5, as well as the pressure PB of the buffer volume 70 measured by the buffer pressure sensor 76. The lower part of Figure 7 shows the temperature T1 of the first cooling stage 33 and the temperature T2 of the second cooling stage 35. As can be seen from Figure 7, once the cryogenic refrigerator 10 is sufficiently cooled upon completion of the initial cooling and the temperatures of the first cooling stage 33 and the second cooling stage 35 stabilize, the pressure PH of the high-pressure line 63 and the pressure PL of the low-pressure line 64 also stabilize. At this time, both the supply valve 72 and the recovery valve 74 are closed, and the buffer volume 70 is disconnected from the gas line 62. Therefore, the pressure PB of the buffer volume 70 also becomes constant (the final buffer pressure PF shown in Figure 7).

[0091] Therefore, the completion of initial cooling can be determined by detecting the stabilization of the pressure PB of the buffer volume 70. If the operating gas filling pressure PI and operating conditions (e.g., high pressure PH, low pressure PL, temperatures T1, T2, etc.) of the cryogenic refrigerator 10 are known, the final pressure of the buffer volume 70 at the completion of initial cooling can be predicted. In this case, the controller 110 may compare the predicted value of the final buffer pressure with the measured pressure PB of the buffer volume 70 and determine, based on the comparison result, whether the measured pressure PB of the buffer volume 70 is equal to the predicted value of the final buffer pressure. The controller 110 may complete initial cooling if the state in which the measured pressure PB of the buffer volume 70 is equal to the predicted value of the final buffer pressure continues for a predetermined time (e.g., several minutes).

[0092] Alternatively, the controller 110 may determine the completion of initial cooling by calculating the difference between the measured pressure PB of the buffer volume 70 and the reference pressure during initial cooling, and detecting the stabilization of the calculated pressure difference. The reference pressure may be the previously measured pressure of the buffer volume 70, or for example, the maximum value PM of the pressure of the buffer volume 70 measured during initial cooling. As can be seen from Figure 7, the pressure of the buffer volume 70 increases from the sealing pressure PI to the maximum value PM immediately after the start of initial cooling.

[0093] The controller 110 may compare the calculated pressure difference (i.e., the difference between the measured pressure PB of the buffer volume 70 and the reference pressure) with the target pressure difference value and, based on the comparison result, determine whether the calculated pressure difference is equal to the target pressure difference value. The controller 110 may complete the initial cooling if the state in which the calculated pressure difference is equal to the target pressure difference value continues for a predetermined time. The predetermined time may be selected from, for example, a range of 1 minute or more and 10 minutes or less. If the difference between the calculated pressure difference and the target pressure difference value is within a predetermined value (e.g., 0.05 MPa), the calculated pressure difference can be considered equal to the target pressure difference value. Since this target pressure difference value does not depend on the sealing pressure PI, the completion of initial cooling can be determined even if the sealing pressure PI is unknown.

[0094] As another example of a reference pressure, the pressure PH of the high-pressure line 63 (or the pressure PL of the low-pressure line 64), measured at the same time as the measured pressure PB of the buffer volume 70, may be used. The controller 110 may determine the completion of initial cooling by calculating the difference between the measured pressure PB of the buffer volume 70 and the measured pressure PH of the high-pressure line 63 (or the measured pressure PL of the low-pressure line 64) and detecting the stabilization of the calculated pressure difference. The controller 110 may also compare the calculated pressure difference with a target pressure difference value, as in the example above, and complete initial cooling if the calculated pressure difference is equal to the target pressure difference value for a predetermined period of time.

[0095] As a further alternative, the controller 110 may determine the completion of initial cooling by calculating the difference between the measured pressure PH of the high-pressure line 63 and the measured pressure PL of the low-pressure line 64, and detecting the stabilization of the calculated pressure difference.

[0096] In the above-described embodiment, during initial cooling, the supply valve 72 is controlled based on the measured pressure of the high-pressure line 63 (hereinafter also referred to as high-pressure priority control), and the pressure of the high-pressure line 63 is maintained within the appropriate pressure range. At this time, the pressure of the low-pressure line 64 is not controlled, so in some cases, undesirable phenomena such as the pressure of the low-pressure line 64 becoming too low may occur.

[0097] To address these issues, the controller 110 may be configured to interrupt the control of the supply valve 72 based on the pressure of the high-pressure line 63 (i.e., high-pressure priority control) when the pressure of the low-pressure line 64, as measured by the second pressure sensor 55, falls below a preset low-pressure threshold. The controller 110 may also be configured to control the supply valve 72 to restore the pressure of the low-pressure line 64 to the low-pressure threshold based on the pressure of the low-pressure line 64, as measured by the second pressure sensor 55. Control of the supply valve 72 based on the pressure of the low-pressure line 64 will hereafter be referred to as low-pressure priority control. An example of such a switching process from high-pressure priority control to low-pressure priority control will be described later with reference to Figure 9.

[0098] Figure 9 is a flowchart illustrating the control method for the cryogenic refrigerator 10 according to the first embodiment. This method is repeatedly executed by the controller 110 at a predetermined cycle during the initial cooling of the cryogenic refrigerator 10. This method may be continuously executed not only during the initial cooling but also during the steady-state operation of the cryogenic refrigerator 10.

[0099] First, the pressure of the low-pressure line 64 is measured (S30). The second pressure sensor 55 measures the pressure of the low-pressure line 64 and outputs a second measured pressure signal PL representing the measured pressure of the low-pressure line 64. The controller 110 receives the second measured pressure signal PL and obtains the measured pressure of the low-pressure line 64.

[0100] Next, the measured pressure PL of the low-pressure line 64 is compared with the low-pressure threshold Pe (S32). The low-pressure threshold Pe may be set as the lower limit of the pressure of the low-pressure line 64, for example, from the viewpoint of ensuring stable operation of the compressor 12. For example, the low-pressure threshold Pe may be selected from the range of 0.2 MPa to 0.4 MPa. The low-pressure threshold Pe can be set as appropriate based on the designer's empirical knowledge or experiments and simulations conducted by the designer. The low-pressure threshold Pe may be pre-saved in the controller 110 as an initial setting of the cryogenic chiller 10, or it may be set in the controller 110 by the user before operation of the cryogenic chiller 10.

[0101] The controller 110 compares the measured pressure of the low-pressure line 64 with the low-pressure threshold Pe, and if the measured pressure of the low-pressure line 64 exceeds this low-pressure threshold Pe (PL>Pe, or PL≧Pe), it selects high-pressure priority control (S34). In this case, as explained with reference to Figure 3, the controller 110 compares the measured pressure of the high-pressure line 63 with the lower limit Pc of the appropriate pressure range, and operates the supply valve 72 to repeatedly open and close the supply valve 72 so that the pressure of the high-pressure line 63 does not fall below the lower limit Pc. In this way, high-pressure priority control is continuously performed.

[0102] On the other hand, when the measured pressure of the low-pressure line 64 is below the low-pressure threshold value Pe (PL<Pe), the controller 110 selects low-pressure priority control (S36). The high-pressure priority control is interrupted, and the low-pressure priority control is started. An example of the low-pressure priority control will be described later with reference to FIG. 10. In this manner, the switching process from the high-pressure priority control to the low-pressure priority control is completed.

[0103] In the low-pressure priority control, first, as shown in FIG. 10, the pressure of the low-pressure line 64 is measured (S40). Next, the measured pressure of the low-pressure line 64 is compared with the low-pressure threshold value Pe (S42). When the measured pressure of the low-pressure line 64 is below the low-pressure threshold value Pe (PL<Pe), the controller 110 opens the supply valve 72 (S44). Thereby, working gas is supplied from the buffer volume 70 to the low-pressure line 64 through the supply valve 72, and the pressure of the low-pressure line 64 is recovered.

[0104] On the other hand, when the measured pressure of the low-pressure line 64 exceeds the low-pressure threshold value Pe (PL>Pe, or PL≧Pe), the controller 110 closes the supply valve 72 (S46). The supply of working gas from the buffer volume 70 to the low-pressure line 64 is stopped. Note that the pressure threshold for closing the supply valve 72 may be different from the low-pressure threshold value Pe, for example, may be somewhat larger than the low-pressure threshold value Pe. In this way, the present method ends and is executed again in the next control cycle.

[0105] According to the low-pressure priority control described above, the supply valve 72 is opened and closed based on the pressure of the low-pressure line 64, and the pressure of the low-pressure line 64 can be recovered to the low-pressure threshold value Pe. However, in the low-pressure priority control, the pressure of the high-pressure line 63 is not managed. Therefore, during the low-pressure priority control, an undesirable phenomenon that the pressure of the high-pressure line 63 becomes too low may occur this time.

[0106] Therefore, the controller 110 may be configured to interrupt the control of the supply valve 72 based on the pressure of the low-pressure line 64 when the pressure of the high-pressure line 63, as measured by the first pressure sensor 54, falls below the high-pressure threshold. The controller 110 may also be configured to control the supply valve 72 to restore the pressure of the high-pressure line 63 to the high-pressure threshold based on the pressure of the high-pressure line 63, as measured by the first pressure sensor 54. An example of such a process of returning from low-pressure priority control to high-pressure priority control will be described later with reference to Figure 11.

[0107] Figure 11 is a flowchart illustrating the control method for the cryogenic refrigerator 10 according to the first embodiment. This method is repeatedly executed at a predetermined cycle by the controller 110 while the low-pressure priority control described above is being performed.

[0108] First, the pressure of the high-pressure line 63 is measured (S50). Next, the measured pressure of the high-pressure line 63 is compared with the high-pressure threshold Pf (S52). The high-pressure threshold Pf may be, for example, the lower limit Pc of the appropriate pressure range. The high-pressure threshold Pf can be set appropriately based on the designer's empirical knowledge or experiments and simulations conducted by the designer. The high-pressure threshold Pf may be pre-stored in the controller 110 as an initial setting for the cryogenic refrigerator 10, or it may be set in the controller 110 by the user before operating the cryogenic refrigerator 10.

[0109] The controller 110 compares the measured pressure of the high-pressure line 63 with the high-pressure threshold Pf, and if the measured pressure of the high-pressure line 63 exceeds this high-pressure threshold Pf (PH>Pf, or PH≧Pf), it selects low-pressure priority control (S54). In this case, low-pressure priority control continues to be performed.

[0110] On the other hand, when the measured pressure of the high-pressure line 63 falls below the high-pressure threshold Pf (PH<Pf), the controller 110 selects high-pressure priority control (S56). In this way, the switching process from low-pressure priority control to high-pressure priority control ends. By switching back to the high-pressure priority control, the supply valve 72 is controlled based on the measured pressure of the high-pressure line 63, and the pressure of the high-pressure line 63 is maintained within an appropriate pressure range.

[0111] By the way, it is also possible to shorten the time required for initial cooling by increasing the operating differential pressure of the cryogenic refrigerator 10 (the pressure difference between the high-pressure line 63 and the low-pressure line 64), thereby increasing the refrigeration capacity of the cryogenic refrigerator 10. However, in the above-described embodiment, the relief valve 60 that bypasses the high-pressure line 63 and the low-pressure line 64 may become an obstacle. If the relief valve 60 is of a type that is mechanically opened when a differential pressure equal to or higher than a set pressure acts between its inlet and outlet, the operating differential pressure of the cryogenic refrigerator 10 may be limited to this set pressure. This is because when the pressure difference between the high-pressure line 63 and the low-pressure line 64 exceeds the set pressure of the relief valve 60, the relief valve 60 mechanically opens, the working gas flows out from the high-pressure line 63 to the low-pressure line 64 through the relief valve 60, and as a result, an increase in the operating differential pressure of the cryogenic refrigerator 10 may be hindered.

[0112] To address this, an additional compressor may be temporarily installed in the cryogenic refrigerator 10 for initial cooling, as described below. For convenience of explanation, hereinafter, the main compressor 12 of the cryogenic refrigerator 10 will be referred to as the first compressor 12, and the added sub compressor will be referred to as the second compressor 80.

[0113] Figure 12 and Figure 13 are diagrams schematically showing the cryogenic refrigerator 10 according to the second embodiment. Figure 12 shows the setting of the cryogenic refrigerator 10 during initial cooling, and Figure 13 shows the basic setting of the cryogenic refrigerator 10 in steady operation before initial cooling or after initial cooling. Figure 14 is a flowchart for explaining the operation method of the cryogenic refrigerator 10 according to the second embodiment.

[0114] The cryogenic refrigerator 10 and its operating method according to the second embodiment may be the same as the cryogenic refrigerator 10 and its operating method according to the first embodiment, except for the second compressor 80. Therefore, in Figures 12 and 13, the same reference numerals are used for components common to the first embodiment, and their detailed descriptions are omitted as appropriate to avoid redundancy.

[0115] Before initial cooling is performed, the cryogenic refrigerator 10 takes on a basic setup with a first compressor 12 and an expander 14, as shown in Figure 13. The first compressor 12 and the expander 14 are connected by a high-pressure line 63 and a low-pressure line 64. The second compressor 80 and the buffer volume 70 are not connected to the cryogenic refrigerator 10.

[0116] In the operation method of the cryogenic refrigerator 10 according to the second embodiment, as shown in Figure 14, the second compressor 80 and the buffer volume 70 are connected to the cryogenic refrigerator 10 as a pretreatment for initial cooling (S60, S61). The installation order of the second compressor 80 and the buffer volume 70 is not important.

[0117] The second compressor 80 is connected in series with the first compressor 12 on the high-pressure line 63. More specifically, as shown in Figure 12, the high-pressure gas outlet 50 of the first compressor 12 is connected to the inlet of the second compressor 80, and the discharge port of the second compressor 80 is connected to the high-pressure gas inlet 22 of the expander 14. Thus, the cryogenic refrigerator 10 has a two-stage compressor configuration consisting of the first compressor 12 and the second compressor 80.

[0118] The buffer volume 70 is connected to the low-pressure line 64 via a supply valve 72 and to the high-pressure line 63 via a recovery valve 74. The supply valve 72 and the recovery valve 74 may be housed together with the buffer volume 70 in a single housing to constitute a buffer volume unit. The buffer volume unit and the second compressor 80 may be brought to the site where the cryogenic refrigerator 10 is in operation, for example by a service technician, and connected to the cryogenic refrigerator 10.

[0119] Once the second compressor 80 and buffer volume 70 are connected to the cryogenic refrigerator 10, the cryogenic refrigerator 10 is started and initial cooling begins (S62). As described above, initial cooling is a process of cooling the cryogenic refrigerator 10 from its initial temperature to the desired cryogenic temperature in preparation for steady-state operation of the cryogenic refrigerator 10. The initial temperature may be the ambient temperature (e.g., room temperature), or it may be a temperature lower than the ambient temperature and higher than the desired cryogenic temperature (e.g., a temperature selected from the range of 20K to 80K). Through initial cooling, the first cooling stage 33 of the expander 14 is cooled to the first cooling temperature, and the second cooling stage 35 is cooled to the second cooling temperature.

[0120] Initial cooling is performed with the second compressor 80 and buffer volume 70 connected to the cryogenic refrigerator 10. Therefore, during initial cooling, the working gas is supplied to the expander 14 using the first compressor 12 and the second compressor 80. The first compressor 12 pressurizes the working gas of the cryogenic refrigerator 10, which is recovered from the expander 14 through the low-pressure line 64, and supplies the pressurized working gas to the second compressor 80. The second compressor 80 further pressurizes the working gas from the first compressor 12 and supplies it back to the expander 14.

[0121] Furthermore, during initial cooling, the buffer volume 70 is used to maintain the pressure of the high-pressure line 63 within an appropriate pressure range, similar to the first embodiment. That is, the supply valve 72 is controlled during initial cooling to maintain the pressure of the high-pressure line 63 within an appropriate pressure range based on the pressure of the high-pressure line 63 measured by the first pressure sensor 54.

[0122] Once the initial cooling is complete, the second compressor 80 and buffer volume 70 are removed from the cryogenic refrigerator 10 (S63, S64). The order of removal of the second compressor 80 and buffer volume 70 does not matter. The cryogenic refrigerator 10 is returned to the basic setting shown in Figure 14. Then, steady-state operation of the cryogenic refrigerator 10 is performed (S65).

[0123] According to the second embodiment, the operating differential pressure of the cryogenic refrigerator 10 can be increased by adding a second compressor 80. In particular, the operating differential pressure of the cryogenic refrigerator 10 can be increased beyond the limit caused by the set pressure of the relief valve 60 of the first compressor 12 described above. Increasing the operating differential pressure leads to an increase in the cooling capacity of the cryogenic refrigerator 10 and can shorten the time required for initial cooling.

[0124] Another way to increase the operating differential pressure of the cryogenic refrigerator 10 is to bring in a larger compressor with higher output than the first compressor 12 from an external source and replace the first compressor 12. However, such large compressors are generally large in both size and weight, making them unsuitable for transport. In contrast, the second compressor 80 is used in conjunction with the first compressor 12, so it can be relatively small and easy to transport.

[0125] In the above-described embodiment, the second compressor 80 is connected to the discharge side (outlet side) of the first compressor 12 in order to create a two-stage compressor configuration. However, other configurations are also possible. For example, in principle, the second compressor 80 may be connected to the suction side (inlet side) of the first compressor 12. That is, the second compressor 80 may be connected in series with the first compressor 12 on the low-pressure line 64.

[0126] It should be noted that the removal of the buffer volume 70 is not mandatory. In the second embodiment, as in the first embodiment, the cryogenic refrigerator 10 may be operated steadily with the buffer volume 70 connected to the cryogenic refrigerator 10, as shown in Figure 1. Alternatively, in the first embodiment, as in the second embodiment, the buffer volume 70 may be removed from the cryogenic refrigerator 10 after initial cooling.

[0127] Figure 15 is a schematic diagram showing the cryogenic refrigerator 10 according to the third embodiment. Figures 16 and 17 are flowcharts illustrating the operation method of the cryogenic refrigerator 10 according to the third embodiment.

[0128] The cryogenic refrigerator 10 and its operating method according to the third embodiment may be the same as the cryogenic refrigerator 10 and its operating method according to the first embodiment, except for the second compressor 80. Therefore, in Figure 15, the same reference numerals are used for components common to the first embodiment, and their detailed descriptions are omitted as appropriate to avoid redundancy.

[0129] In the third embodiment, as in the second embodiment, the cryogenic refrigerator 10 takes on a basic setting with a first compressor 12 and an expander 14, as shown in Figure 13, before initial cooling is performed. The first compressor 12 and the expander 14 are connected by a high-pressure line 63 and a low-pressure line 64. The second compressor 80 is not connected to the cryogenic refrigerator 10.

[0130] In the operation method of the cryogenic refrigerator 10 according to the third embodiment, as shown in Figure 16, a second compressor 80 is connected to the cryogenic refrigerator 10 as a pretreatment for initial cooling (S70). The second compressor 80 is connected in series with the first compressor 12 on the high-pressure line 63. As shown in Figure 15, the high-pressure gas outlet 50 of the first compressor 12 is connected to the inlet of the second compressor 80, and the discharge port of the second compressor 80 is connected to the high-pressure gas inlet 22 of the expander 14. Therefore, the cryogenic refrigerator 10 has a two-stage compressor configuration consisting of the first compressor 12 and the second compressor 80. The second compressor 80 may be brought to the site where the cryogenic refrigerator 10 is operated, for example by a service technician, and connected to the cryogenic refrigerator 10.

[0131] The second compressor 80, like the first compressor 12, includes a compressor body 57. The second compressor 80 also includes a compressor motor 82 with a variable operating frequency (i.e., rotational speed), and the compressor body 57 is driven by the compressor motor 82. The compressor motor 82 may be, for example, an electric motor or any other suitable type of motor. By increasing the operating frequency of the compressor motor 82, the discharge flow rate of the compressor body 57 is increased, and as a result, the pressure in the high-pressure line 63 may increase. Conversely, by decreasing the operating frequency of the compressor motor 82, the discharge flow rate of the compressor body 57 is decreased, and as a result, the pressure in the high-pressure line 63 may decrease.

[0132] The control device 100 includes an inverter 90 that controls the operating frequency of the compressor motor 82. The compressor motor 82 and the inverter 90 are powered by an external power source 92, such as a commercial power supply (three-phase AC power supply). The inverter 90 is configured to adjust the frequency of the power input from the external power source 92 under the control of the controller 110, as described later, and output it to the compressor motor 82 at an arbitrary frequency. The operating frequency of the compressor motor 82 corresponds to the output frequency of the inverter 90 and can be adjusted, for example, in the range of 30Hz to 100Hz or 40Hz to 70Hz.

[0133] With the second compressor 80 connected to the cryogenic refrigerator 10, the cryogenic refrigerator 10 is started and initial cooling is performed (S72). The working gas is supplied to the expander 14 using the first compressor 12 and the second compressor 80. The first compressor 12 pressurizes the working gas of the cryogenic refrigerator 10, which is recovered from the expander 14 through the low-pressure line 64, and supplies the pressurized working gas to the second compressor 80. The second compressor 80 further pressurizes the working gas from the first compressor 12 and supplies it back to the expander 14. As will be described later, the operating frequency of the compressor motor 82 of the second compressor 80 is controlled to maintain the pressure of the high-pressure line 63 within the appropriate pressure range, based on the measured pressure of the high-pressure line 63.

[0134] Once the initial cooling is complete, the second compressor 80 is removed from the cryogenic refrigerator 10 (S73). The cryogenic refrigerator 10 is returned to the basic settings shown in Figure 13. Then, steady-state operation of the cryogenic refrigerator 10 is performed (S75).

[0135] Referring to Figure 17, an example of the control process for the operating frequency of the compressor motor 82 based on the measured pressure of the high-pressure line 63 is described. This process is repeatedly performed by the controller 110 at a predetermined cycle during the initial cooling of the cryogenic refrigerator 10.

[0136] First, the pressure of the high-pressure line 63 is measured (S80). Next, the measured pressure of the high-pressure line 63 is compared with the high-pressure target value Pg (S82). The high-pressure target value Pg may be, for example, the lower limit Pc of the appropriate pressure range. The high-pressure target value Pg can be set appropriately based on the designer's empirical knowledge or experiments and simulations conducted by the designer. The high-pressure target value Pg may be pre-saved in the controller 110 as an initial setting for the cryogenic refrigerator 10, or it may be set in the controller 110 by the user before operating the cryogenic refrigerator 10.

[0137] The controller 110 compares the measured pressure PH of the high-pressure line 63 with the high-pressure target value Pg and outputs the relationship between the two as a comparison result. In other words, the comparison result from the controller 110 represents one of the following three states: (i) the measured pressure PH is less than the high-pressure target value Pg, (ii) the measured pressure PH is greater than the high-pressure target value Pg, or (iii) the measured pressure PH is equal to the high-pressure target value Pg.

[0138] Based on the comparison results from the controller 110, the inverter 90 is controlled, and the operating frequency of the compressor motor 82 is controlled according to the output frequency of the inverter 90. Specifically, (i) if the measured pressure PH is less than the high-pressure target value Pg, the controller 110 controls the inverter 90 to increase the operating frequency of the compressor motor 82 (S84). This may increase the pressure in the high-pressure line 63. (ii) if the measured pressure PH is greater than the high-pressure target value Pg, the controller 110 controls the inverter 90 to decrease the operating frequency of the compressor motor 82 (S18). This may decrease the pressure in the high-pressure line 63. (iii) if the measured pressure PH is equal to the high-pressure target value Pg, there is no need to increase or decrease the operating frequency of the compressor motor 82, so the controller 110 controls the inverter 90 to maintain the current operating frequency. Note that case (iii) may be included in either (i) or (ii).

[0139] When increasing or decreasing the operating frequency of the compressor motor 82, the controller 110 may increase or decrease the operating frequency of the compressor motor 82 by a predetermined amount from the current operating frequency value. However, if the current operating frequency value has already reached the upper limit when attempting to increase the operating frequency, the controller 110 may maintain the operating frequency at that upper limit without increasing it. For example, if the range of possible operating frequencies for the compressor motor 82 is 30Hz to 100Hz and the current value is already at the upper limit of 100Hz, the controller 110 will maintain the operating frequency at 100Hz rather than further increasing it from 100Hz. Similarly, if the current operating frequency value has already reached the lower limit when attempting to decrease the operating frequency, the controller 110 may maintain the operating frequency at that lower limit without decreasing it.

[0140] Alternatively, the controller 110 may control the inverter 90 to adjust the operating frequency of the compressor motor 82 (for example, by feedback control such as PID control) to minimize the deviation of the measured pressure PH from the high-pressure target value Pg. In this way, the controller 110 may control the inverter 90 to compare the pressure of the high-pressure line 63 with the target pressure, and to decrease the operating frequency of the compressor motor 82 when the pressure of the high-pressure line 63 exceeds the target pressure, and to increase the operating frequency of the compressor motor 82 when the pressure of the high-pressure line 63 falls below the target pressure.

[0141] According to the third embodiment, the operating differential pressure of the cryogenic refrigerator 10 can be increased by adding a second compressor 80. In particular, the operating differential pressure of the cryogenic refrigerator 10 can be increased beyond the limit caused by the set pressure of the relief valve 60 of the first compressor 12 described above. Increasing the operating differential pressure leads to an increase in the cooling capacity of the cryogenic refrigerator 10 and shortens the time required for initial cooling. Furthermore, since the second compressor 80 is used in combination with the first compressor 12, it can be relatively small and easily transported.

[0142] Similar to the second embodiment, in the third embodiment, as shown in Figure 18, the buffer volume 70 may be connected to the cryogenic refrigerator 10 for initial cooling together with the supply valve 72 and the recovery valve 74. As a pretreatment for initial cooling, the buffer volume 70 is connected to the low-pressure line 64 via the supply valve 72 and to the high-pressure line 63 via the recovery valve 74. During the execution of initial cooling, the buffer volume 70 is used to maintain the pressure of the high-pressure line 63 within an appropriate pressure range, similar to the first embodiment. That is, the supply valve 72 is controlled to maintain the pressure of the high-pressure line 63 within an appropriate pressure range during initial cooling, based on the pressure of the high-pressure line 63 measured by the first pressure sensor 54. After initial cooling, the buffer volume 70 may be removed from the cryogenic refrigerator 10 together with the supply valve 72 and the recovery valve 74. By using the buffer volume 70 for initial cooling in this way, the time required for initial cooling can be shortened, similar to the embodiments described above.

[0143] Furthermore, as shown in Figure 19, the second compressor 80 may be connected to the suction side (inlet side) of the first compressor 12 in order to form a two-stage compressor configuration. That is, the second compressor 80 may be connected in series with the first compressor 12 on the low-pressure line 64. In this case, the compressor motor 82 that drives the compressor body 57 of the first compressor 12 may have a variable operating frequency. The operating frequency of the compressor motor 82 may be controlled by the controller 110 and inverter 90 based on the measured pressure of the high-pressure line 63, in order to maintain the pressure of the high-pressure line 63 within an appropriate pressure range. Along with this, or instead, the operating frequency of the compressor motor 82 of the second compressor 80 may be controlled by the controller 110 and inverter 90, similar to the embodiment shown in Figure 15.

[0144] The present invention has been described above based on examples. Those skilled in the art will understand that the present invention is not limited to the above embodiments, that various design changes are possible, and that various modifications are possible, and that such modifications also fall within the scope of the present invention. Various features described in relation to one embodiment are applicable to other embodiments. New embodiments resulting from combinations will possess the combined effects of each of the embodiments combined.

[0145] For example, the control processes described in relation to the first embodiment (e.g., the process for completing initial cooling based on buffer pressure, the process for switching from high-pressure priority control to low-pressure priority control, and the process for switching from low-pressure priority control to high-pressure priority control) may also be applied to the second and third embodiments.

[0146] In the above-described embodiment, the case in which the expander motor 42 operates at a constant operating frequency (motor rotation speed) (i.e., the case in which the expander motor 42 operates at the same operating frequency during initial cooling and steady-state operation) is described as an example, but the present invention is not limited to this. The cryogenic refrigerator 10 may be equipped with an expander motor 42 with a variable operating frequency, and so-called accelerated cooling may be performed during initial cooling by operating the expander motor 42 at a higher operating frequency than that of steady-state operation. In this case, the switching from initial cooling (accelerated cooling) to steady-state operation and the change in operating frequency may be performed based on the pressure of the buffer volume 70 or based on the differential pressure between the high-pressure line 63 and the low-pressure line 64, as described above with reference to Figures 7 and 8. By performing accelerated cooling, the initial cooling time can be further shortened.

[0147] The pressure sensors, such as the first pressure sensor 54 and the second pressure sensor 55, are not necessarily required to be installed in the compressor 12, but may be installed in any location where pressure can be measured, such as the gas line 62 or the expander 14. For example, the first pressure sensor 54 may be installed in any location on the high-pressure line 63, and the second pressure sensor 55 may be installed in any location on the low-pressure line 64.

[0148] In the above-described embodiment, the supply valve 72 and the recovery valve 74 are provided as separate valves, each connected to the buffer volume 70, but the present invention is not limited thereto. For example, the supply valve 72 and the recovery valve 74 may be integrated, or they may be a three-way valve connected to the buffer volume 70. By switching the three-way valve, it may be possible to switch between a supply state in which the buffer volume 70 is connected to the low-pressure line 64 and a recovery state in which the buffer volume 70 is connected to the high-pressure line 63.

[0149] In the embodiments described above, the buffer volume 70 is a single buffer tank, but in some embodiments, the buffer volume 70 may consist of multiple buffer tanks. One buffer tank may be connected to the low-pressure line 64 by a supply valve 72, and another buffer tank may be connected to the high-pressure line 63 by a recovery valve 74. Also, in the embodiments described above, the buffer volume 70 is located outside the compressor 12 and the expander 14, but is not limited to this. For example, the buffer volume 70 may be located inside the compressor 12.

[0150] The above-described embodiment is explained using the case where the cryogenic refrigerator 10 is a two-stage GM refrigerator as an example, but is not limited to this. The cryogenic refrigerator 10 may be a single-stage or multi-stage GM refrigerator, or even another type of cryogenic refrigerator such as a pulse tube refrigerator.

[0151] Embodiments of the present invention can also be expressed as follows. 1. An expander capable of performing initial cooling, which cools the temperature from an initial temperature to an extremely low temperature, and steady-state operation, which follows the initial cooling and maintains the extremely low temperature. A high-pressure line connected to the aforementioned expander, through which the working gas drawn into the expander flows, A low-pressure line connected to the aforementioned expander, through which the working gas exhausted from the expander flows, A first pressure sensor for measuring the pressure of the high-pressure line, The buffer volume for storing the working gas, A supply valve connected to the buffer volume in the low-pressure line, A cryogenic refrigerator comprising: a controller that controls the supply valve to maintain the pressure of the high-pressure line within a preset appropriate pressure range based on the pressure of the high-pressure line measured by the first pressure sensor during the initial cooling; and a controller that controls the supply valve to maintain the pressure of the high-pressure line within a preset appropriate pressure range. 2. The cryogenic refrigerator according to item 1, characterized in that the controller compares the measured pressure of the high-pressure line with the lower limit of the appropriate pressure range during the initial cooling, and operates the supply valve to repeatedly open and close the supply valve so that the pressure of the high-pressure line does not fall below the lower limit. 3. The cryogenic refrigerator according to item 1 or 2, characterized in that, when the buffer volume is denoted as VB, the volume of the high-pressure line as VH, the volume of the low-pressure line as VL, the operating gas filling pressure at a certain temperature as PI, the pressure of the high-pressure line during steady-state operation at that temperature as PH, and the pressure of the low-pressure line during steady-state operation at that temperature as PL, the buffer volume satisfies VB ≥ VH(PH-PI) / (PI-PL)-VL for any temperature in the temperature range from the initial temperature to the cryogenic temperature. 4. The system further comprises a recovery valve that connects the buffer volume to the high-pressure line, The cryogenic refrigerator according to any one of claims 1 to 3, characterized in that the controller controls the recovery valve to maintain the pressure of the high-pressure line within the appropriate pressure range. 5. The cryogenic refrigerator according to item 4, characterized in that, when the buffer volume is denoted as VB, the volume of the high-pressure line as VH, the volume of the low-pressure line as VL, the operating gas filling pressure at a certain temperature as PI, the pressure of the high-pressure line during steady-state operation at that temperature as PH, and the pressure of the low-pressure line during steady-state operation at that temperature as PL, the buffer volume satisfies VB ≥ -VH + VL(PI - PL) / (PH - PI) for any temperature in the temperature range from the initial temperature to the cryogenic temperature. 6. Further comprising a buffer pressure sensor for measuring the pressure of the buffer volume, The cryogenic refrigerator according to any one of claims 1 to 5, characterized in that the controller completes the initial cooling based on the pressure of the buffer volume measured by the buffer pressure sensor. 7. Further comprising a second pressure sensor for measuring the pressure of the low-pressure line, The aforementioned controller, When the pressure of the low-pressure line measured by the second pressure sensor falls below a preset low-pressure threshold, the control of the supply valve based on the pressure of the high-pressure line is interrupted. The cryogenic refrigerator according to any one of claims 1 to 6, characterized in that it is configured to control the supply valve to restore the pressure of the low-pressure line to the low-pressure threshold based on the pressure of the low-pressure line measured by the second pressure sensor. 8. The controller is, When the pressure of the high-pressure line measured by the first pressure sensor falls below the first appropriate pressure range, the control of the operating frequency of the compressor motor based on the pressure of the low-pressure line is interrupted. The cryogenic refrigerator according to claim 7, characterized in that it is configured to control the operating frequency of the compressor motor so as to restore the pressure of the high-pressure line to a first appropriate pressure range based on the pressure of the high-pressure line measured by the first pressure sensor. 9. A first compressor connected to the expander through the high-pressure line and the low-pressure line, The cryogenic refrigerator according to any one of claims 1 to 8, further comprising a second compressor connected in series with the first compressor on the high-pressure line or the low-pressure line. 10. The first compressor or the second compressor has a compressor motor with a variable operating frequency and is driven by the compressor motor. The cryogenic refrigerator according to claim 9, characterized in that the controller is configured to control the operating frequency of the compressor motor to maintain the pressure in the high-pressure line within the appropriate pressure range, based on the pressure in the high-pressure line measured by the first pressure sensor during the initial cooling. 11. A method for operating a cryogenic refrigerator, wherein the cryogenic refrigerator comprises a first compressor, an expander, and a high-pressure line and a low-pressure line connecting the first compressor to the expander, and the method is: The second compressor is connected in series with the first compressor on the high-pressure line or the low-pressure line, The buffer volume is connected to the low-pressure line via a supply valve, The initial cooling of the expander from its initial temperature to an extremely low temperature is performed with the second compressor and the buffer volume connected to the cryogenic refrigerator. The system comprises performing a steady-state operation to maintain the expander at the cryogenic temperature, following the initial cooling. Performing the aforementioned initial cooling means The first compressor and the second compressor are used to supply the working gas to the expander, A method characterized by comprising: controlling the supply valve to maintain the pressure of the high-pressure line within a preset appropriate pressure range based on the measured pressure of the high-pressure line. 12. The method according to claim 11, further comprising removing the second compressor after the initial cooling. 13. The method according to claim 11 or 12, further comprising removing the buffer volume after the initial cooling. 14. Performing the initial cooling described above is When the measured pressure of the low-pressure line falls below a preset low-pressure threshold, the control of the supply valve based on the pressure of the high-pressure line is interrupted. The method according to any one of claims 11 to 13, further comprising controlling the supply valve to restore the pressure of the low-pressure line to the low-pressure threshold based on the measured pressure of the low-pressure line. 15. Performing the initial cooling described above is When the measured pressure of the high-pressure line falls below the high-pressure threshold, the control of the supply valve based on the pressure of the low-pressure line is interrupted. The method according to claim 14, further comprising controlling the supply valve to restore the pressure of the high-pressure line to the high-pressure threshold based on the measured pressure of the high-pressure line. 16. The first compressor or the second compressor has a compressor motor with a variable operating frequency and is driven by the compressor motor, The method according to any one of claims 11 to 15, characterized in that the initial cooling is performed by controlling the operating frequency of the compressor motor to maintain the pressure of the high-pressure line within a preset appropriate pressure range based on the measured pressure of the high-pressure line. 17. Performing the initial cooling described above is During the initial cooling process, the pressure of the buffer volume is measured, The method according to any one of claims 11 to 16, further comprising completing the initial cooling based on the pressure of the measured buffer volume. 18. A method for operating a cryogenic refrigerator, wherein the cryogenic refrigerator comprises a first compressor, an expander, and a high-pressure line and a low-pressure line connecting the first compressor to the expander, and the method is: The second compressor is connected in series with the first compressor on the high-pressure line or the low-pressure line, The initial cooling of the expander from its initial temperature to an extremely low temperature is performed while the second compressor is connected to the cryogenic refrigerator. The system comprises performing a steady-state operation to maintain the expander at the cryogenic temperature, following the initial cooling. The first compressor or the second compressor has a compressor motor with a variable operating frequency and is driven by the compressor motor. Performing the aforementioned initial cooling means The first compressor and the second compressor are used to supply the working gas to the expander, A method characterized by comprising: controlling the operating frequency of the compressor motor to maintain the pressure of the high-pressure line within a preset appropriate pressure range based on the measured pressure of the high-pressure line. 19. The method according to claim 18, further comprising removing the second compressor after the initial cooling. 20. Further comprising connecting the buffer volume to the low-pressure line via a supply valve, The method according to claim 18 or 19, characterized in that the initial cooling is performed by controlling the supply valve to maintain the pressure in the high-pressure line within a preset appropriate pressure range based on the measured pressure in the high-pressure line. 21. The method according to claim 20, further comprising removing the buffer volume after the initial cooling. 22. Performing the initial cooling described above is During the initial cooling process, the pressure of the buffer volume is measured, The method according to claim 20 or 21, further comprising completing the initial cooling based on the measured pressure of the buffer volume.

[0152] The present invention has been described above based on examples. Those skilled in the art will understand that the present invention is not limited to the above embodiments, that various design changes are possible, and that various modifications are possible, and that such modifications also fall within the scope of the present invention. [Explanation of symbols]

[0153] 10 Cryogenic refrigerator, 12 Compressor, 14 Expander, 54 First pressure sensor, 55 Second pressure sensor, 63 High pressure line, 64 Low pressure line, 70 Buffer volume, 72 Supply valve, 80 Second compressor, 82 Compressor motor, 110 Controller.

Claims

1. A method for operating a cryogenic refrigerator, wherein the cryogenic refrigerator comprises a first compressor, an expander, and a high-pressure line and a low-pressure line connecting the first compressor to the expander, and the method is as follows: The second compressor is connected in series with the first compressor on the high-pressure line or the low-pressure line, The buffer volume is connected to the low-pressure line via a supply valve, The initial cooling of the expander from its initial temperature to an extremely low temperature is performed with the second compressor and the buffer volume connected to the cryogenic refrigerator. The system comprises performing a steady-state operation to maintain the expander at the cryogenic temperature, following the initial cooling. Performing the aforementioned initial cooling means The first compressor and the second compressor are used to supply the working gas to the expander, A method characterized by comprising: controlling the supply valve to maintain the pressure of the high-pressure line within a preset pressure range based on the measured pressure of the high-pressure line.

2. The method according to claim 1, further comprising removing the second compressor after the initial cooling.

3. The method according to 1 or 2, further comprising removing the buffer volume after the initial cooling.

4. Performing the aforementioned initial cooling means When the measured pressure of the low-pressure line falls below a preset low-pressure threshold, the control of the supply valve based on the pressure of the high-pressure line is interrupted. The method according to 1 or 2, further comprising controlling the supply valve to restore the pressure of the low-pressure line to the low-pressure threshold based on the measured pressure of the low-pressure line.

5. Performing the aforementioned initial cooling means When the measured pressure of the high-pressure line falls below the high-pressure threshold, the control of the supply valve based on the pressure of the low-pressure line is interrupted. The method according to 4, further comprising controlling the supply valve to restore the pressure of the high-pressure line to the high-pressure threshold based on the measured pressure of the high-pressure line.

6. The cryogenic refrigerator further comprises a bypass line connecting the high-pressure line to the low-pressure line so as to recirculate the working gas from the high-pressure line to the low-pressure line, bypassing the expander, and a relief valve provided in the bypass line and configured to open when a differential pressure greater than or equal to a set pressure is acting upon it, The method according to 1 or 2, characterized in that the preset pressure range has an upper limit set to a pressure value lower than the set pressure at which the relief valve opens.

7. The method according to claim 1 or 2, characterized in that the preset pressure range has an upper limit and a lower limit selected from the range of 2 MPa to 3 MPa.

8. The method according to claim 7, characterized in that the difference between the upper limit and the lower limit is within 0.5 MPa.

9. An expander capable of performing initial cooling, which cools the temperature from an initial temperature to an extremely low temperature, and steady-state operation, which follows the initial cooling and maintains the extremely low temperature, A high-pressure line and a low-pressure line connected to the aforementioned expander, A first pressure sensor for measuring the pressure in the aforementioned high-pressure line, A second pressure sensor for measuring the pressure in the low-pressure line, The buffer volume for storing the working gas, A supply valve connected to the buffer volume in the low-pressure line, The system includes a controller configured to control the supply valve to maintain the pressure in the high-pressure line within a preset pressure range, based on the pressure in the high-pressure line measured by the first pressure sensor during the initial cooling process, The aforementioned controller, When the pressure of the low-pressure line measured by the second pressure sensor falls below a preset low-pressure threshold, the control of the supply valve based on the pressure of the high-pressure line is interrupted. A cryogenic refrigerator is characterized by being configured to control the supply valve to restore the pressure of the low-pressure line to the low-pressure threshold based on the pressure of the low-pressure line measured by the second pressure sensor.

10. The aforementioned controller, When the pressure of the high-pressure line measured by the first pressure sensor falls below the high-pressure threshold, the control of the supply valve based on the pressure of the low-pressure line is interrupted. The cryogenic refrigerator according to claim 9, characterized in that it is configured to control the supply valve to restore the pressure in the high-pressure line to the high-pressure threshold based on the pressure in the high-pressure line measured by the first pressure sensor.

11. A first compressor connected to the expander via the high-pressure line and the low-pressure line, The cryogenic refrigerator according to claim 9 or 10, further comprising a second compressor connected in series with the first compressor on the high-pressure line or the low-pressure line.

12. The cryogenic refrigerator further comprises a bypass line connecting the high-pressure line to the low-pressure line so as to recirculate the working gas from the high-pressure line to the low-pressure line, bypassing the expander, and a relief valve provided in the bypass line and configured to open when a differential pressure greater than or equal to a set pressure acts upon it, The cryogenic refrigerator according to claim 9 or 10, characterized in that the preset pressure range has an upper limit set to a pressure value lower than the set pressure at which the relief valve opens.

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