JP6529850B2 - Cryogenic refrigerator and operating method of cryogenic refrigerator - Google Patents

Description

  The present invention relates to a cryogenic refrigerator that generates refrigeration by expanding high pressure helium supplied from a compression device, and a method of operating the cryogenic refrigerator.

  As a cryogenic refrigerator, for example, there is one described in Patent Document 1. The displacer-type cryogenic refrigerator includes an expander configured to movably accommodate the displacer inside the cylinder. The displacer type cryogenic refrigerator generates refrigeration by expanding helium in the expander while reciprocating the displacer inside the cylinder. The cold of the helium generated in the expander is transferred to the cooling stage while being accumulated in the regenerator to reach a desired cryogenic temperature to cool the object to be cooled connected to the cooling stage.

  These cryogenic refrigerators usually generate about 4 [K] of refrigeration when used for producing liquid helium under atmospheric pressure, for example. If the ultimate temperature of the cold can be further lowered, it is possible to provide, for example, a helium superfluid transition temperature.

JP, 2006-242484, A

  An object of the present invention is to provide a technique for reducing the ultimate temperature of the refrigeration generated by a cryogenic refrigerator.

  In order to solve the above problems, a cryogenic refrigerator according to an embodiment of the present invention is a cryogenic refrigerator which generates refrigeration of 4 [K] or less by expanding helium, and expands helium under high pressure. An expander, and a compressor that compresses low pressure helium returned from the expander to generate high pressure helium and supplies the helium to the expander. When the temperature of helium in the expander is 2.17 [K] or less, the pressure of low pressure helium is the temperature of the horizontal axis and the pressure of the vertical axis of helium, the volumetric thermal expansion coefficient of helium is 0 The pressure of the curve is

  Another aspect of the present invention is a cryogenic temperature expander comprising: an expander for expanding high pressure helium; and a compressor for compressing low pressure helium returned from the expander to produce high pressure helium to supply the expander. It is an operation method of generating refrigeration of 4 [K] or less by expanding helium in a refrigerator. This method comprises the steps of detecting the temperature of helium in the expander and, when the detected temperature is 2.17 [K] or less, the temperature in the horizontal axis and the pressure in the helium with the vertical axis being the pressure of helium. Setting the pressure of the low pressure helium above the pressure of the curve where the volumetric thermal expansion coefficient is zero.

  According to the present invention, it is possible to provide a technique for lowering the ultimate temperature of the cold generated by the cryogenic refrigerator.

It is a schematic diagram which shows the cryogenic refrigerator which concerns on embodiment of this invention. It is a state diagram which shows the phase of helium 4 in cryogenic temperature. It is a schematic diagram which shows the cryogenic refrigerator which concerns on embodiment of this invention. It is a schematic diagram which shows the cryogenic refrigerator which concerns on embodiment of this invention.

  Embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a schematic view showing a cryogenic refrigerator 1 according to an embodiment of the present invention. The cryogenic refrigerator 1 according to the embodiment is a Gifford McMahon type refrigerator using helium helium 4 ( ⁴ He) as a refrigerant gas. The cryogenic refrigerator 1 has a cylinder 4 which forms an expansion space 3 for expanding high pressure helium between the displacer 2 and a bottomed cylindrical cooling stage adjacent to the expansion space 3 and positioned so as to be enveloped. 5 is provided. The cooling stage 5 functions as a heat exchanger that performs heat exchange between the object to be cooled and helium. Hereinafter, in the present specification, the entire configuration in which the displacer 2 is accommodated in the cylinder 4 and the helium is expanded is generically referred to as “expansion device 50”. The compressor 12 recovers the low pressure helium returned from the expander 50, compresses it, and then supplies the expander 50 with high pressure helium.

  The displacer 2 includes a main body 2a and a lid 2b provided at the low temperature end. The lid 2 b may be configured of the same member as the main body 2 a. Moreover, the cover part 2b may be comprised with the material whose heat conductivity is higher than the main-body part 2a. Then, the lid 2 b also functions as a heat conducting portion that exchanges heat with the helium flowing in the lid 2 b. For the lid 2b, for example, a material such as copper, aluminum, stainless steel, or the like having a thermal conductivity larger than at least the main body 2a is used. The cooling stage 5 is made of, for example, copper, aluminum, stainless steel or the like.

  The cylinder 4 accommodates the displacer 2 so as to be reciprocally movable in the longitudinal direction. For example, stainless steel is used for the cylinder 4 from the viewpoint of strength, thermal conductivity, helium blocking ability, and the like.

  At a high temperature end of the displacer 2, a scotch yoke mechanism (not shown) for reciprocatingly driving the displacer 2 is provided, and the displacer 2 reciprocates along the axial direction of the cylinder 4.

  The displacer 2 has a cylindrical outer peripheral surface, and the inside of the displacer 2 is filled with a cold storage material. The internal space of the displacer 2 constitutes a regenerator 7. An upper end side rectifier 9 and a lower end side rectifier 10 are provided on the upper end side and the lower end side of the regenerator 7, respectively, for rectifying the flow of helium.

  At the high temperature end of the displacer 2 is formed an upper opening 11 through which helium flows from the room temperature chamber 8 to the displacer 2. The room temperature chamber 8 is a space formed by the cylinder 4 and the high temperature end of the displacer 2, and the volume changes as the displacer 2 reciprocates.

  Of the pipes connecting the air supply and exhaust system including the compressor 12, the supply valve 13 and the return valve 14 to the room temperature chamber 8, a supply and discharge common pipe is connected. In addition, a seal 15 is mounted between a portion from the high temperature end of the displacer 2 and the cylinder 4.

  At the low temperature end of the displacer 2, a helium outlet 16 for introducing helium into the expansion space 3 is formed. Further, between the outer wall of the displacer 2 and the inner wall of the cylinder 4, a clearance C which is a flow path of helium connecting the inner space of the displacer 2 and the expansion space 3 is provided.

  The expansion space 3 is a space formed by the cylinder 4 and the displacer 2, and the volume changes as the displacer 2 reciprocates. A cooling stage 5 thermally connected to the object to be cooled is disposed at a position corresponding to the expansion space 3 on the outer periphery and bottom of the cylinder 4. Helium is supplied to the expansion space 3 by flowing into the expansion space 3 through the helium outlet 16 and the clearance C.

  Next, the operation of the cryogenic refrigerator 1 will be described.

  At a certain point in the helium supply process, the displacer 2 is located at the bottom dead center LP of the cylinder 4 as shown in FIG. At the same time or when the supply valve 13 is opened at a timing shifted slightly, high pressure helium is supplied from the supply / discharge common pipe into the cylinder 4 through the supply valve 13. As a result, high pressure helium flows into the regenerator 7 inside the displacer 2 from the upper opening 11 located at the top of the displacer 2. The high pressure helium flowing into the regenerator 7 is supplied to the expansion space 3 through the helium outlet 16 and the clearance C located at the lower part of the displacer 2 while being cooled by the regenerator material.

  When the expansion space 3 is filled with high pressure helium, the supply valve 13 is closed. At this time, the displacer 2 is located at the top dead center UP in the cylinder 4. The helium in the expansion space 3 is depressurized and expanded when the return valve 14 is opened at the same time or when the displacer 2 is located at the top dead center UP in the cylinder 4 or at a timing shifted slightly. The helium in the expansion space 3, which has become low temperature due to expansion, absorbs the heat of the cooling stage 5.

  The displacer 2 moves toward the bottom dead center LP, and the volume of the expansion space 3 decreases. The helium in the expansion space 3 is collected in the displacer 2 through the helium outlet 16 and the clearance C. Also at this time, helium absorbs the heat of the cooling stage 5. The helium returned from the expansion space 3 to the regenerator 7 also cools the regenerator material in the regenerator 7. The helium recovered by the displacer 2 is further returned to the suction side of the compressor 12 through the regenerator 7 and the upper opening 11. The above steps are one cycle, and the cryogenic refrigerator 1 cools the cooling stage 5 by repeating this cooling cycle.

  As described above, in the cryogenic refrigerator 1 according to the embodiment, the helium in the expansion space 3 expands due to the reciprocal movement of the displacer 2 in the cylinder 4 constituting the expander 50, and the cold is generated. Do.

  Here, in order to generate refrigeration of about 4.2 [K] which is the boiling point of helium under atmospheric pressure, the high pressure side of the operating pressure of the compressor 12 is 25 [bar] and the low pressure side is 8 [bar] ] And efficiency is good. That is, by repeating the cooling cycle of expanding the helium of 25 [bar] to 8 [bar] in the expander 50, the cryogenic refrigerator 1 converts the helium under atmospheric pressure to about 4 [K] Can be generated efficiently.

Subsequently, physical properties of helium 4 at a cryogenic temperature of 4 [K] or less will be described. Helium contains helium 4 ( ⁴ He) and helium 3 ( ³ He) as isotopes, but the two differ in physical properties at extremely low temperatures. The following description is based on the premise that helium is helium 4.

  FIG. 2 is a phase diagram showing the phase of helium 4 at cryogenic temperatures. FIG. 2 is a diagram generated using HePak (version 3.40) of Horizon Technologies, Inc., USA.

  FIG. 2 is a phase diagram of helium in which the horizontal axis represents temperature T [K] and the vertical axis represents pressure P [bar]. In FIG. 2, the temperature range of helium is 1.7 [K] to 2.4 [K], and the pressure range is 0 [bar] to 40 [bar]. The broken line indicated by m in FIG. 2 is a melting curve of helium. The broken line indicated by λ is a λ line (lambda line). When the temperature and pressure of the helium are below the λ line, the helium becomes superfluidity.

  In FIG. 2, the broken line indicated by α indicates a curve in which the volumetric thermal expansion coefficient α of helium is zero. In the following specification, in the phase diagram shown in FIG. 2, a curve in which the volumetric thermal expansion coefficient of helium is zero is referred to as “α curve” for convenience.

  In FIG. 2, an area above the α curve is an area where the volumetric thermal expansion coefficient α of helium takes a positive value. Further, a region below the α curve is a region where the volumetric thermal expansion coefficient α of helium takes a negative value. Adiabatic expansion of helium lowers the temperature of helium when the temperature and pressure of helium are above the alpha curve. On the other hand, when the temperature and pressure of helium are below the α curve, the adiabatic expansion of helium raises the temperature of helium.

  In FIG. 2, the solid lines shown with numbers indicate helium isentropic curves. Each number shows the entropy s [J / gK] per unit mass of helium. For example, the entropy s per unit mass of helium at a pressure of 24 [bar] and a temperature of 2.09 [K] is 1.407 [J / g K]. When helium adiabatically expands, the temperature and pressure of the helium change along the isentropic curve.

  The boiling point of helium is approximately 4.2 [K] at 1 atm (approximately 1 [bar]). When 1 bar of helium becomes 4.2 [K] or less, it becomes liquid helium. When 1 [bar] and 4.2 [K] of helium are depressurized and the vapor pressure is reduced to about 0.05 [bar], the temperature of helium becomes about 2.17 [K]. At this time, helium transfers to a superfluid state. That is, the superfluid transition temperature of helium is approximately 2.17 [K] at saturated vapor pressure.

  As shown in FIG. 2, the λ line of helium is a curve having a negative slope falling to the right in the state diagram. This means that as the pressure of helium increases, the superfluid transition temperature decreases. Therefore, in order to transfer helium to a superfluid state, a minimum of 2.17 [K] of refrigeration is required. Hereinafter, the “superfluid temperature range” means a temperature range of 2.17 [K] or less, which is the minimum temperature required to transfer helium to the superfluid state, unless otherwise specified in the present specification. I decided to.

  As apparent from FIG. 2, when helium is adiabatically expanded in the superfluidization temperature range, the temperature of helium does not fall below the temperature at the intersection of the isentropic curve and the α curve. That is, in the phase diagram of helium shown in FIG. 2, the temperature at the intersection of the isentropic curve and the α curve indicates the lower limit value of the temperature reached when helium adiabatically expands.

  As apparent from FIG. 2, the α curve is on the upper side of the λ curve and does not intersect with the λ curve. This means that when the helium is depressurized and adiabatically expanded in the superfluid temperature region, the helium reaches the minimum temperature before the λ transitions to the superfluid state. In other words, if the pressure is reduced to just before the λ transition of helium, it means that the temperature of the helium rises after reaching the minimum temperature. Therefore, in the case of adiabatically expanding helium in the superfluid temperature region, the pressure reduction is controlled so that the pressure of helium in the expansion space 3 is not less than the pressure at the intersection of the isentropic curve and the α curve. Thereby, the temperature rise of the helium by adiabatic expansion can be suppressed, and a cooling efficiency can be improved.

  Also, the α curve, like the λ curve, is a curve having a negative slope falling to the right in the phase diagram of helium shown in FIG. This means that the pressure at the intersection of the isentropic curve and the α curve increases as the entropy of helium decreases. Adiabatic expansion in the expansion space 3 lowers the temperature of helium and reduces the entropy per unit mass of helium. Therefore, as the helium repeats the cooling cycle in the superfluid temperature region, the entropy of the helium decreases and the pressure at the intersection of the isentropic curve and the α curve increases.

  Therefore, the entropy of helium at the temperature is calculated based on the lowest temperature reached by the cryogenic refrigerator 1. The temperature of helium in the expansion space 3 is detected, and at least when the detected temperature is 2.17 [K] or less, the pressure on the low pressure side of the operating pressure of the compressor 12 is calculated by the isentropic curve and α in the calculated entropy The pressure is set equal to or higher than the pressure at the intersection with the curve. As a result, the pressure of low pressure helium in the expansion space 3 fluctuates on the upper side of the helium α curve in the state diagram shown in FIG. Since the pressure of helium is equal to or higher than the pressure at the intersection of the isentropic curve and the α curve, the adiabatic expansion of helium can suppress the temperature rise of helium. As a result, the cooling efficiency in the superfluid temperature range of the cryogenic refrigerator 1 can be enhanced. If it is difficult to directly detect the temperature of helium in the expansion space 3, the temperature of the cooling stage 5 may be measured, and the measured temperature may be regarded as the temperature of helium in the expansion space 3.

  Alternatively, when the temperature of helium in the expansion space 3 is 2.17 [K] or less, the setting value of the pressure on the low pressure side of the operating pressure of the compressor 12 is adaptively changed according to the temperature of helium. Good. More specifically, in the state diagram shown in FIG. 2, the pressure at the intersection of the isentropic curve and the .alpha. Curve according to the entropy determined according to the temperature of helium is determined by the pressure on the low pressure side of the operating pressure of the compressor 12. It may be a set value. As a result, when the temperature of helium in the expansion space 3 is high, the set value of the pressure on the low pressure side of the operating pressure of the compressor 12 becomes low, and cold of lower temperature can be generated in the expansion space 3.

  As a non-limiting example, the pressure on the low temperature side of the compressor 12 is 15 [bar]. In this case, helium in the expansion space 3 is at least 15 [bar] or more. In the α curve shown in FIG. 2, when the pressure is 15 [bar], the temperature is approximately 2.06 [K]. That is, by setting the pressure on the low temperature side of the compressor 12 to 15 [bar], the lowest achieved temperature of the cold generated by the cryogenic refrigerator 1 becomes 2.06 [K]. This temperature is 0.1 [K] or more lower than 2.17 [K] which is the minimum temperature required to transfer helium to the superfluid state. Therefore, the cryogenic refrigerator 1 can be stably used as a refrigerator for transferring helium to a superfluid state.

  The cryogenic refrigerator 1 is often used for liquefying helium. As described above, when the high pressure side of the operating pressure of the compressor 12 is 25 [bar], refrigeration of about 4.2 [K] which is the boiling point of helium under atmospheric pressure can be efficiently generated. For this reason, in many cases the high pressure side of the operating pressure of the existing compressor is set to about 25 [bar], and even when the cryogenic refrigerator 1 as a whole is designed to withstand about 25 [bar] There are many.

  In general, when the differential pressure between the pressure on the low pressure side of the compressor 12 and the pressure on the high pressure side of the cryogenic refrigerator 1 is small, the operating efficiency of the cryogenic refrigerator 1 is reduced. When using the existing cryogenic refrigerator 1 in which the high pressure side of the operating pressure of the compressor 12 is set to about 25 [bar], even if the pressure on the low pressure side of the compressor 12 is 15 [bar], the differential pressure is There are 10 [bar]. For this reason, the operating efficiency of the cryogenic refrigerator 1 is also considered to be in the practical range. Therefore, by setting the pressure on the low pressure side of the compressor 12 to 15 [bar], sufficient refrigeration is generated to transfer helium to the superfluid state without changing the pressure design of the cryogenic refrigerator 1 can do.

  As another non-limiting example, the pressure on the low pressure side of the compressor 12 may be 25 [bar]. In this case, helium in the expansion space 3 is at least 25 [bar] or more. In the α curve shown in FIG. 2, when the pressure is 25 [bar], the temperature is approximately 1.93 [K]. In this case, the cryogenic refrigerator 1 can generate refrigeration below 2 [K], and can more stably provide the superfluid transition temperature of helium.

  When the pressure on the low pressure side of the compressor 12 is set to 25 [bar], the pressure on the high pressure side is set to 25 [bar] or more. In order to enhance the operation efficiency of the cryogenic refrigerator 1, it is preferable that the pressure on the high pressure side of the compressor 12 be sufficiently higher than the pressure on the low pressure side. However, if the pressure on the high pressure side of the compressor 12 is too high, the pressure of the helium also increases, and the helium becomes solid regardless of the temperature.

  As described above, in the phase diagram shown in FIG. 2, the broken line indicated by m is the melting curve of helium. If the temperature and pressure of the helium are above the melting curve in the phase diagram shown in FIG. 2, then the helium becomes solid. Therefore, in order to operate the cryogenic refrigerator 1, the pressure on the high pressure side of the compressor 12 is set so that the pressure of helium is less than or equal to the melting curve of helium in the phase diagram.

  As a non-limiting example, the pressure on the high pressure side of the compressor 12 is 35 [bar]. In this case, helium in the expansion space 3 is at most 35 [bar] or less. In the melting curve of helium shown in FIG. 2, when the pressure is 35 [bar], the temperature is approximately 1.91 [K]. Entropy s per unit mass of helium at a pressure of 35 [bar] and a temperature of 1.91 [K] is about 1.25 [J / g K]. In the phase diagram shown in FIG. 2, the isentropic curve whose entropy s per unit mass is 1.25 [J / gK] intersects the α curve at a point of approximately 1.82 [K] and 28 [bar]. . Therefore, by setting the pressure on the low pressure side of the compressor 12 to 28 [bar], the cryogenic refrigerator 1 can generate refrigeration of 1.9 [K] or less. In addition, it can also be suppressed that helium becomes solid.

  Next, an equation representing an α curve in the phase diagram of helium shown in FIG. 2 will be described.

When helium is adiabatically expanded, i.e., depressurized while keeping the entropy of helium constant, the temperature of helium changes depending on the pressure. As shown in FIG. 2, in the superfluidization temperature range, the temperature of helium has a local minimum value with respect to pressure. This is ∂T / ∂P = 0 in the superfluid temperature region, where T [K] is the temperature of helium, P [bar] is the pressure, and s [J / gK] is the entropy per unit mass. It means that the pressure P ₀ is present. Also, let the temperature of helium at this time be T ₀ .

The pressure P _{0 at} which ∂T / ∂P = 0 in the superfluidization temperature region changes in accordance with the entropy s per unit mass of helium. Thus, the pressure P ₀ can be expressed as P ₀ (s) as a function of the entropy s per unit mass of helium. Similarly, the temperature T ₀ of helium when ∂T / ∂P = 0 is also expressed as T ₀ (s) as a function of the entropy s per unit mass of helium. From the above, the α curve can be represented as a point (T ₀ (s), P ₀ (s)) in the helium phase diagram shown in FIG. 2 with the entropy s per unit mass of helium as a parameter. That is, the α curve is expressed as a locus drawn by points (T ₀ (s), P ₀ (s)) when the entropy s per unit mass of helium is changed.

図２より、ヘリウムの単位質量あたりのエントロピーｓは、およそ１．２［Ｊ／ｇＫ］＜ｓ＜１．６［Ｊ／ｇＫ］の間を変化する。 The α curve is expressed by the following equation using partial differentiation.

From FIG. 2, the entropy s per unit mass of helium changes between approximately 1.2 [J / gK] <s <1.6 [J / gK].

  In the above equation, in the phase diagram of helium shown in FIG. 2, the point at which the temperature gradient with respect to pressure change of helium gas in the superfluidization temperature region is 0 is drawn while changing the entropy s per unit mass of helium gas Represents a locus. The α curve is a curve that gives the lowest temperature that helium gas can reach when adiabatically expanding helium gas in the superfluidization temperature range.

  As explained above, the cryogenic refrigerator 1 according to the embodiment can lower the ultimate temperature of the cold generated by the expansion of helium.

  In particular, the cryogenic refrigerator 1 according to the embodiment can stably generate cold of 2.17 [K] or less, which is the superfluid transition temperature of helium 4. Therefore, the cryogenic refrigerator according to the embodiment can also be used as a refrigerator for performing superfluid transfer of helium 4. There are refrigerators that use helium 3 to generate refrigeration in this temperature range, but helium 4 is much less expensive than helium 3. Therefore, the cryogenic refrigerator 1 according to the embodiment can provide the superfluid transition temperature of helium 4 at low cost.

  FIG. 3 is a schematic view showing a cryogenic refrigerator 60 according to the embodiment of the present invention. The cryogenic refrigerator 60 includes an expander 62, a compressor 64, a helium gas line 66, a helium tank unit 68, and a helium tank control unit 70. The cryogenic refrigerator 60 is a two-stage refrigerator, so the expander 62 has a single-stage cooler 72 and a two-stage cooler 74. The two-stage cooling unit 74 includes a two-stage helium expansion chamber 76 and a two-stage heat exchanger 78 or a two-stage cooling stage that encloses the two-stage helium expansion chamber 76.

  The helium gas line 66 couples the expander 62 to the compressor 64 so as to recover low pressure helium from the expander 62 to the compressor 64 and to supply high pressure helium from the compressor 64 to the expander 62. The low pressure side pressure of the compressor 64 is hereinafter also referred to as the operating low pressure of the compressor 64. The helium gas line 66 includes a valve unit 84 including a supply valve 80 and a return valve 82. The helium gas line 66 further includes a low pressure pipe 86, a high pressure pipe 88, and a common supply / discharge pipe 90. Low pressure piping 86 connects return valve 82 to the low pressure port of compressor 64. A high pressure line 88 connects the supply valve 80 to the high pressure port of the compressor 64. The common supply and discharge piping 90 connects the valve unit 84 to the room temperature chamber of the one-stage cooling unit 72.

  The helium tank portion 68 is connected to the cryogenic refrigerator 60 so as to supply helium to the cryogenic refrigerator 60. The helium tank portion 68 includes a helium tank 92, a connecting pipe 94 connecting the helium tank 92 to the helium gas line 66 of the cryogenic refrigerator 60, and a valve 96 installed in the connecting pipe 94.

  The helium tank 92 is a pressure vessel configured to store helium gas having a predetermined pressure. The pressure and volume of the helium tank 92 are designed such that the operating low pressure of the compressor 64 is increased to the target pressure by the helium supply from the helium tank 92 to the helium gas line 66. The target pressure is equal to or higher than the pressure value determined from the α curve at or near the above-mentioned superfluidization temperature range. For example, the helium tank 92 is designed to increase the operating low pressure of the compressor 64 from the initial operating low pressure (e.g., 8 [bar]) to 15 [bar] or higher at or near the superfluid temperature range There is.

  The valve 96 is configured to control the flow of helium gas in the connection pipe 94. The valve 96 is controlled in accordance with a valve control signal V input from the helium tank control unit 70. That is, the valve 96 is opened and closed or the opening degree is adjusted in accordance with the valve control signal V. The valve 96 is communicably connected to the helium tank control unit 70 to receive a valve control signal V.

  When the valve 96 is opened, the helium tank 92 is in communication with the helium gas line 66 through the connection pipe 94, and helium gas flow between the helium tank 92 and the helium gas line 66 is permitted. When the valve 96 is closed, the helium tank 92 is disconnected from the helium gas line 66 and the helium gas flow between the helium tank 92 and the helium gas line 66 is shut off.

  The helium tank portion 68 is connected to the low pressure side of the compressor 64. The connecting pipe 94 connects the helium tank 92 to the low pressure pipe 86. If the helium tank pressure is higher than the operating low pressure of the compressor 64, helium is supplied from the helium tank 92 to the cryogenic refrigerator 60 when the valve 96 is open. If the helium tank pressure is less than the operating low pressure of the compressor 64, helium is recovered from the cryogenic refrigerator 60 to the helium tank 92 when the valve 96 is open. Therefore, by connecting the helium tank portion 68 to the low pressure side of the compressor 64, the helium tank pressure can be made relatively low. This helps to simplify the structure of the helium tank 92 and reduce its weight.

  The helium tank portion 68 may be connected to the high pressure side of the compressor 64. In this case, in order to supply helium from the helium tank 92 to the cryogenic refrigerator 60, the helium tank pressure must be higher than the high pressure side pressure of the compressor 64.

  The cryogenic refrigerator 60 includes a two-stage temperature sensor 98 that measures the temperature of the two-stage helium expansion chamber 76 and / or the two-stage heat exchanger 78. The two-stage temperature sensor 98 is attached to the two-stage heat exchanger 78 of the expander 62. The two-stage temperature sensor 98 is communicably connected to the helium tank control unit 70 so as to output the measured temperature T 2 to the helium tank control unit 70.

  The helium tank control unit 70 starts the supply of helium from the helium tank unit 68 to the cryogenic refrigerator 60 based on the temperature of the two-stage helium expansion chamber 76 and / or the two-stage heat exchanger 78. Are configured to control the

  The helium tank control unit 70 includes a temperature comparison unit 100 and a valve control unit 102. The temperature comparison unit 100 is configured to compare the measured temperature T2 with the temperature threshold T0. The temperature comparison unit 100 is configured to output the result of the temperature comparison to the valve control unit 102. The valve control unit 102 is configured to generate a valve control signal V in accordance with the input from the temperature comparison unit 100. The valve control unit 102 closes the valve 96 when the measured temperature T2 is higher than the temperature threshold T0, and opens the valve 96 when the measured temperature T2 is lower than the temperature threshold T0. The temperature threshold value T0 is predetermined from a temperature range higher than 2.17 [K] and 5 [K] or lower. The temperature threshold T0 may be, for example, 4 [K]. The helium tank control unit 70 may include a storage unit 104 that stores the temperature threshold value T0.

  With such a configuration, the cooling temperature of the two-stage cooling unit 74 is monitored in the process of cooling from room temperature to cryogenic temperature. Since the measurement temperature T2 is higher than the temperature threshold T0 at the beginning of operation of the cryogenic refrigerator 60, the valve 96 is closed and the helium tank 92 does not supply helium to the helium gas line 66. At this time, the pressure of the helium tank 92 is maintained at the design initial pressure. The cryogenic refrigerator 60 is operated at the initial operating pressure of the compressor 64. When the cooling process proceeds and the measured temperature T2 drops to the temperature threshold T0, the valve 96 is opened to start supply of helium from the helium tank 92 to the low pressure pipe 86 of the helium gas line 66. Thus, the helium tank portion 68 increases the amount of helium gas of the cryogenic refrigerator 60. As a result, the operating low pressure of the compressor 64 is increased above the pressure value determined from the α curve at or near the superfluidization temperature range.

  Therefore, the cryogenic refrigerator 60 can generate refrigeration of 2.17 [K] or less as described above. In addition, the cryogenic refrigerator 60 can be operated at a low helium pressure suitable for temperature ranges higher than 4 [K].

  The cooling temperature may increase somewhat soon after the valve 96 is opened. This is a transient phenomenon accompanying an increase in the amount of helium gas of the cryogenic refrigerator 60. Therefore, the helium tank control unit 70 may be configured to temporarily ignore the measurement temperature T2 immediately after the valve 96 is opened. For example, the valve control unit 102 may be configured to keep the valve 96 open for a predetermined time regardless of the input from the temperature comparison unit 100 once the valve 96 is opened. In this way, it is possible to avoid closing the valve 96 and stopping the supply of helium due to a transient temperature rise.

  Also, in order to reduce or prevent such a transient temperature rise, the helium tank control unit 70 controls the helium tank unit 68 to supply helium from the helium tank unit 68 to the cryogenic refrigerator 60 stepwise. It may be configured. To that end, the valve control unit 102 may repeat the opening and closing of the valve 96. Thus, the temperature rise can be suppressed by gradually supplying helium.

  The helium tank control unit 70 controls the helium tank unit 68 to stop the supply of helium from the helium tank unit 68 to the cryogenic refrigerator 60 based on the operating low pressure of the compressor 64 and / or the pressure of the helium tank 92. It may be configured as follows. The operating low pressure of the compressor 64 may be measured by a compressor pressure sensor built into the compressor 64. The pressure of the helium tank 92 may be measured by a tank pressure sensor attached to the helium tank 92. The pressure sensor is communicably connected to the helium tank control unit 70 so as to output the measured pressure to the helium tank control unit 70.

  The helium tank control unit 70 may include a pressure comparison unit configured to compare the measurement pressure with a predetermined pressure threshold and output the result of the comparison to the valve control unit 102. The pressure threshold is, for example, the target pressure described above. The valve control unit 102 may be configured to generate the valve control signal V according to the input from the pressure comparison unit. The valve control unit 102 may close the valve 96 when the measured pressure is equal to or higher than the pressure threshold, and may continue opening the valve 96 when the measured pressure is lower than the pressure threshold. The pressure threshold may be stored in the storage unit 104.

  The initial pressure of the helium tank 92 may be the average pressure of the high pressure and low pressure of the compressor 64. In this way, the helium tank 92 can be restored to the initial pressure for the next operation by opening the valve 96 while the cryogenic refrigerator 60 is shut down. Alternatively, the helium tank 92 may be connected to the high pressure side of the compressor 64 for restoration to the initial pressure.

  FIG. 4 is a schematic view showing a cryogenic refrigerator 110 according to the embodiment of the present invention. The cryogenic refrigerator 110 includes a first cooling unit 112 providing a pre-cooling function, and a second cooling unit 114 providing a cooling function to a superfluid temperature region. The second cooling unit 114 is precooled by the first cooling unit 112. Thus, the cryogenic refrigerator 110 has the high-temperature stage precooling refrigerator and the low-temperature refrigerator separately.

  The first cooling unit 112 includes a first expander 116, a first compressor 118, and a first helium gas line 120. The first expander 116 has a helium expansion chamber 122 at its low temperature side. The first helium gas line 120 is configured to recover the first low pressure PL1 helium from the first expander 116 and to supply the first high pressure PH1 helium from the first compressor 118 to the first compressor 118. Connect to The illustrated first cooling unit 112 is a single-stage refrigerator, but the first cooling unit 112 may be a two-stage refrigerator (for example, a 4K-GM refrigerator).

  The second cooling unit 114 includes a second expander 124, a second compressor 126, and a second helium gas line 128. The second expander 124 has a helium receiving chamber 130 on its high temperature side. The helium receiving chamber 130 is thermally connected to the helium expansion chamber 122 of the first cooling unit 112 by the heat transfer member 132. A portion of the heat transfer member 132 is attached to the helium expansion chamber 122 of the first cooling unit 112, and another portion is attached to the helium receiving chamber 130 of the second cooling unit 114. The first cooling unit 112 precools the second cooling unit 114 by conduction cooling from the helium expansion chamber 122 to the helium receiving chamber 130.

  The second helium gas line 128 is configured to recover the second low pressure PL2 helium from the second expander 124 and to supply the second high pressure PH2 helium from the second compressor 126 to the first compressor 118. Connect to The second helium gas line 128 is separated from the first helium gas line 120. Thus, the helium circulation circuit of the second cooling unit 114 is isolated from the helium circulation circuit of the first cooling unit 112.

  The second cooling unit 114 is operated at a helium pressure different from that of the first cooling unit 112. The second low pressure PL2 is higher than the first low pressure PL1. The second low pressure PL2 may be 15 [bar] or more. The first low pressure PL1 may be 8 [bar] or less. Also, the second high pressure PH2 may be higher than the first high pressure PH1.

  Therefore, the cryogenic refrigerator 110 can be operated at a helium pressure suitable for each of the first cooling unit 112 and the second cooling unit 114. That is, the first cooling unit 112 can be operated at a low helium pressure suitable for precooling, and the second cooling unit 114 can be operated at a high helium pressure suitable for cooling of 2.17 [K] or less.

  Although the preferred embodiments of the present invention have been described in detail, the present invention is not limited to the above-described embodiments, and various modifications and substitutions may be added to the above-described embodiments without departing from the scope of the present invention. be able to.

  In the above, the cryogenic refrigerator 1 has been described on the premise of the GM refrigerator. The cryogenic refrigerator 1 may also be a displacer type Stirling refrigerator that uses helium 4 as a working fluid. Also in this case, the pressure on the low pressure side of the compressor may be set with reference to the α curve shown in FIG. 2 based on the target temperature of the Stirling refrigerator. Also, the pressure on the high pressure side of the compressor may be set so that the pressure of helium does not exceed the melting curve. Thereby, it is possible to suppress the rise of the temperature of the helium gas due to the adiabatic expansion of the helium while lowering the lowest reachable temperature of the Stirling refrigerator.

  In the above, the cryogenic refrigerator 1 has been described on the premise of a single-stage GM refrigerator. The cryogenic refrigerator 1 may be a multistage GM refrigerator of two or more stages. Also in this case, the pressure on the low pressure side of the compressor may be set with reference to the α curve shown in FIG. 2 based on the target temperature of the refrigerator. Also, the pressure on the high pressure side of the compressor may be set so that the pressure of helium does not exceed the melting curve.

  1 cryogenic refrigerator, 2 displacer, 2a main body, 2b lid, 3 expansion space, 4 cylinders, 5 cooling stages, 7 regenerators, 8 room temperature room, 9 upper end side rectifier, 10 lower end side rectifier, 11 upper opening, 12 compressor, 13 supply valve, 14 return valve, 15 seal, 16 outlet, 50 expander, 60 cryogenic refrigerator, 62 expander, 64 compressor, 68 helium tank part, 70 helium tank control part, 76 two Stage helium expansion chamber, 78 two-stage heat exchanger, 92 helium tank, 94 connection piping, 96 valve, 98 two-stage temperature sensor, 100 temperature comparison unit, 102 valve control unit, 110 cryogenic refrigerator, 112 first cooling unit , 114 second cooling unit, 116 first expander, 118 First compressor, 120 first helium gas line, 122 helium expansion chamber, 124 second expander, 126 second compressor, 128 second helium gas line, 130 helium receiving chamber.

Claims (12)

A cryogenic refrigerator that generates refrigeration of 4 [K] or less by expanding helium,
An expander for expanding high pressure helium;
A compressor for compressing low pressure helium returned from the expander to generate high pressure helium and supplying the helium to the expander;
When the temperature of helium in the expander is 2.17 [K] or less, the pressure of the low pressure helium is the temperature of the horizontal axis and the pressure of the vertical axis is the pressure in helium. The cryogenic refrigerator according to the present invention is characterized in that the pressure of the curve is equal to or more than 0.   The cryogenic refrigerator according to claim 1, wherein the pressure of the low pressure helium is 15 bar or more.   The cryogenic refrigerator according to claim 1, wherein the pressure of the low pressure helium is 25 bar or more.   The cryogenic refrigerator according to any one of claims 1 to 3, wherein a pressure of the high pressure helium is equal to or less than a melting curve of helium in the phase diagram.   The cryogenic refrigerator according to claim 4, wherein the pressure of the high pressure helium is 35 [bar] or less. Assuming that the temperature of helium in the expander is T [K], the pressure is P [bar], and the entropy per unit mass is s [J / g K], the curve whose volumetric thermal expansion coefficient is 0 is

The cryogenic refrigerator according to any one of claims 1 to 5, which is a curve represented by   The cryogenic refrigerator according to any one of claims 1 to 6, wherein the helium is helium 4. The expander includes a helium expansion chamber, and a heat exchanger enclosing the helium expansion chamber.
The cryogenic refrigerator is
A helium tank portion coupled to the cryogenic refrigerator to supply helium to the cryogenic refrigerator;
A helium tank control unit for controlling the helium tank unit to start supply of helium from the helium tank unit to the cryogenic refrigerator based on the temperature of the helium expansion chamber and / or the heat exchanger The cryogenic refrigerator according to any one of claims 1 to 7, characterized in that it comprises. The cryogenic refrigerator is attached to the expander to measure the temperature of the helium expansion chamber and / or the heat exchanger, and communicates with the helium tank control unit to output the measured temperature to the helium tank control unit. Further comprising a temperature sensor, which is connected to the
The helium tank unit includes a helium tank, a connecting pipe that connects the helium tank to the cryogenic refrigerator, and a valve installed in the connecting pipe.
The helium tank control unit is a temperature comparison unit configured to compare the measured temperature with a temperature threshold, and the valve is closed when the measured temperature is higher than the temperature threshold, and the measured temperature is the temperature. A valve control unit configured to control the valve according to an input from the temperature comparison unit so as to open the valve when the temperature threshold is less than or equal to 2.17 [The temperature threshold is 2.17 [ The cryogenic refrigerator according to claim 8, characterized in that K] is higher than 5 [K] in advance.   The cryogenic refrigerator according to claim 8 or 9, wherein the helium tank unit is connected to the low pressure side of the compressor. A first expander having a helium expansion chamber, a first compressor, and a first expansion for recovering a first low pressure helium from the first expander and supplying a first high pressure helium from the first compressor. A first helium gas line connecting the first compressor to the first compressor;
A second expander having a helium receiving chamber thermally connected to the helium expansion chamber, a second compressor, and the first helium gas line, and a second low pressure helium from the second expander. A second cooling unit comprising: a second helium gas line connecting the second expander to the second compressor so as to recover and supply a second high pressure helium from the second compressor; The cryogenic refrigerator according to any one of claims 1 to 7, wherein the second low pressure is higher than the first low pressure. Helium is expanded in a cryogenic refrigerator comprising: an expander for expanding high pressure helium; and a compressor for compressing low pressure helium returned from the expander to generate high pressure helium and supplying the high pressure helium to the expander. Driving method to generate cold of 4 [K] or less by
Detecting the temperature of the helium in the expander;
When the detected temperature is 2.17 [K] or less, the low-pressure helium has a pressure lower than the pressure of a curve at which the volumetric thermal expansion coefficient of helium becomes 0 in the phase diagram of helium with the horizontal axis as temperature and the vertical axis as pressure And b. Setting the pressure of the cryogenic refrigeration system.

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