JP4354410B2 - Cryogenic operation method - Google Patents

Description

  The present invention relates to a technique of a cryogenic apparatus for cooling a superconducting magnet or the like.

  For example, in an apparatus equipped with a superconducting magnet (superconducting magnet), such as an MRI (magnetic resonance tomography apparatus) known as a medical device, in order to maintain the superconducting (superconducting) characteristics, Generally, a cryogenic device that keeps the magnet at an extremely low temperature close to absolute zero is provided.

  In the cryogenic device, there is one in which a superconducting magnet is cooled by immersing an object to be cooled such as the superconducting magnet in liquid helium as a refrigerant.

  This type of cryogenic device includes a storage container that stores a cooled object including the superconducting magnet in a state of being immersed in liquid helium, and a sleeve that protrudes from the storage container and communicates with the inside and outside of the storage container. A refrigerating machine inserted into the sleeve while closing the opening at the front end thereof, and the refrigerating helium vaporized in the containment vessel and the sleeve is recondensed by the refrigerating machine.

  Since the refrigerator operates for a relatively long time, regular maintenance is required. In this maintenance, the refrigerator is often extracted from the sleeve.

  In this extraction operation, the outside air (atmosphere, water vapor, etc.) of the volume occupied by the refrigerator in the volume in the sleeve enters the sleeve, and the outside air condenses and adheres inside the sleeve. There was a risk of malfunction.

Therefore, conventionally, for example, as disclosed in Patent Document 1, the opening of the sleeve is covered with a flexible maintenance bag, and the air in the maintenance bag is discharged, and then helium gas is introduced to be expanded. A method has been proposed in which the refrigerator is maintained in the maintenance bag (that is, in an atmosphere of helium gas).
Japanese Patent Laid-Open No. 5-223379 (page 3 to page 5, FIG. 4)

  However, in the method described in Patent Document 1, it is necessary to attach the maintenance bag to the sleeve during maintenance, and to prepare a suction blower or the like for sucking air from the maintenance bag. It was something.

  In addition, it is necessary for the worker to perform maintenance of the refrigerator by inserting his hand into the maintenance bag through the glove, which is complicated. Furthermore, the maintenance work may be interrupted due to a situation such as the maintenance bag breaking.

  The present invention has been made in view of the above-described problems, and relates to a cryogenic apparatus that can easily perform maintenance work of a refrigerator while suppressing the intrusion of air into a sleeve, and the operation of replacing the refrigerator The purpose is to provide a suitable driving method.

  In order to solve the above problems, the present applicant, when replacing a refrigerator, generates a purge gas flow in the sleeve from the position on the storage container side toward the opening side of the sleeve relative to the refrigerator. I came up with a way to operate the device.

  According to the operation method of the cryogenic apparatus, when the refrigerator is extracted from the sleeve, the outside air (atmosphere or water vapor) that tends to enter the sleeve can be blown off by the purge gas flow flowing in the sleeve. Can be surely prevented.

  However, in the operation method, when the refrigerator is pulled out, the refrigerator is stopped and a purge gas at room temperature is introduced.Therefore, heat intrusion into the liquid refrigerant by the purge gas occurs. The liquid helium was a direct cause of loss from the containment vessel.

  In view of the above, the present invention provides a storage container that stores the object to be cooled in a state of being immersed in a liquid refrigerant, a sleeve that extends from the storage container to above the liquid level of the liquid refrigerant, and an opening that closes the opening. An operation method suitable for replacing the refrigerator with respect to a cryogenic device provided with a refrigerator that is removably inserted into the sleeve and recondenses the liquid refrigerant vaporized in the sleeve. A subcooling step of cooling the liquid refrigerant by the refrigerator so that the liquid refrigerant has a supercooling temperature below the boiling point of the liquid refrigerant under atmospheric pressure conditions, and the liquid refrigerant is the supercooling temperature. In the cooled state, the opening and the liquid refrigerant liquid level are formed in the sleeve by gas flow generation means capable of introducing purge gas from a position closer to the storage container than the refrigerator to the sleeve. A gas flow generating step of generating a purge gas flow toward a method and a replacement step of replacing the refrigerator while performing said gas flow generating process.

  According to the operation method of the present invention, the introduction of the purge gas is started in a state where the liquid refrigerant is cooled to the supercooling temperature, and the refrigerator is replaced during that time, so that the liquid refrigerant evaporates by the introduction of the purge gas. It is possible to suppress the location to be mainly limited to the vicinity of the liquid level, and to reduce the evaporation amount of the liquid refrigerant.

  That is, in the operation method of the present invention, when the purge gas is introduced by the gas flow generating means, the liquid refrigerant is cooled to the supercooling temperature in advance, so even if the purge gas is sprayed on the liquid refrigerant surface, Although the liquid surface temperature rises relatively quickly and the liquid refrigerant near the liquid surface evaporates, the intrusion heat is difficult to be transmitted to the deep part of the liquid refrigerant, and the deep part can maintain a low temperature state for a while. By replacing the refrigerator during that time, the total evaporation amount of the liquid refrigerant can be effectively reduced.

  Here, although heat penetration to the liquid refrigerant liquid level occurs due to the purge gas, it is difficult to transfer heat to the deep part because the convection hardly occurs in the liquid refrigerant even when the purge gas is blown. It has become. That is, in the above operation method, although the liquid surface temperature of the liquid refrigerant rises relatively quickly due to the purge gas having a temperature higher than that of the liquid refrigerant, almost no convection occurs, so the heat that has penetrated by the purge gas moves to the deep side. However, since the temperature on the deep side does not rise immediately, the amount of evaporation can be effectively reduced if the refrigerator is replaced quickly during that time.

  The “replacement” of the refrigerator is not necessarily limited to the operation of replacing the refrigerator mounted on the sleeve with another refrigerator, and the maintenance of the refrigerator extracted from the sleeve is quickly performed. This includes the work of inserting the refrigerator after the maintenance into the sleeve again.

  In the operation method of the cryogenic device, the supercooling operation may always be performed, but during the normal operation, the liquid refrigerant is maintained at a temperature equal to or higher than the boiling point of the liquid refrigerant under atmospheric pressure conditions, More preferably, the supercooling step is performed when performing the replacement step.

  Thus, during normal operation, by maintaining the liquid refrigerant at a temperature equal to or higher than the boiling point under atmospheric pressure conditions, the inside of the containment vessel is set to a pressure environment above atmospheric pressure by the saturated vapor pressure of the refrigerant vaporized in the containment vessel. The above-mentioned effect can be achieved by avoiding the pressure situation in which outside air is easily sucked into the containment vessel from the place where the refrigerator is attached to the sleeve (airtight formation place), etc., while temporarily setting the supercooling temperature only when changing the refrigerator. Can be obtained.

  Such a transition from the normal operation to the supercooling operation can be performed by changing the cooling capacity of the refrigerator, but it is not easy to accurately adjust the cooling capacity. On the other hand, the cooling capacity of the refrigerator is set to the capacity capable of cooling the liquid refrigerant to the supercooling temperature, and during the normal operation, the refrigerator is operated while the refrigerator is being operated. The temperature of the liquid refrigerant is adjusted by adjusting the amount of heat by the heating means provided in the heating unit, and in the supercooling step, the heating means is stopped while the refrigerator is operated, so that the liquid refrigerant is cooled to a supercooling temperature. If it is cooled down to an appropriate temperature, it is easy to adjust the amount of heat by the heating means during normal operation, for example, by adjusting the power supply to the electric heater. It can shift to supercooling operation by operation.

  According to the present invention, it is possible to easily perform the maintenance work of the refrigerator while suppressing air from entering the sleeve, and to reduce the evaporation amount of the liquid refrigerant.

  Hereinafter, a specific configuration of a cryogenic apparatus in which an operation method suitable for the replacement operation of the refrigerator according to the present invention is employed will be described with reference to FIGS.

  FIG. 1 is a schematic front sectional view of an MRI equipped with a cryogenic apparatus according to an embodiment of the present invention, and FIG. 2 is an enlarged front sectional view showing a main part of the cryogenic apparatus of FIG. FIG. 3 is a graph showing the temperature of the liquid solvent in the cryogenic apparatus of FIG.

  The illustrated cryogenic device 1 includes a storage container 4 that stores a cooled object 2 (superconducting magnet 2 in the illustrated example) in a state of being immersed in liquid helium (liquid refrigerant) 3. The shield container 11 is accommodated in the decompression container 7. The superconducting magnet 2 and the containers 4, 7, 11 are each formed in a donut shape whose axis is directed in the horizontal direction. That is, in the MRI equipped with the cryogenic apparatus 1, an object to be inspected or a subject passes through the central hole 7 a of the decompression container 7. Further, the inside of the decompression vessel 7 is a vacuum chamber 6, whereby heat insulation between the shield vessel 11 and the outside is achieved.

  The containment vessel 4 is formed in a donut shape with a material having low thermal conductivity, and contains an amount of liquid helium 50 that is not completely filled with the superconducting magnet 2 immersed therein.

  A sleeve 5 is formed in the upper part of the storage container 4, and a refrigerator 9 is detachably inserted into the sleeve 5. The refrigerator 9 recondenses liquid helium (that is, helium gas) vaporized in the sleeve 5 as will be described in detail later. In the illustrated example, the refrigerator 9 is a two-stage refrigerator. Yes.

  The sleeve 5 is a tubular member made of a material having low thermal conductivity. The sleeve 5 extends upward from the storage container 4, and a middle part thereof penetrates the shield container 11 and the decompression container 7, and the opening 8 can open the inside of the storage container 4 to the outside air. ing.

  Specifically, the sleeve 5 has a configuration in which the connecting pipe 14, the cooling pipe 15, and the bellows pipe 16 are concentrically connected in order from the storage container 4.

  The connecting pipe 14 is a pipe having a lower end portion connected to the storage container 4, and an upper end portion of the connecting pipe 14 is a shoulder portion 14 a that is widened, and is connected to the cooling pipe 15 through the shoulder portion 14 a. Yes.

  The cooling pipe 15 has a flange portion 15a bulging inward and outward in the radial direction at its upper end, and this flange portion 15a is attached to the shield container 11 by a heat conductor 18 made of a copper braided wire or the like. It is connected thermally. The bellows pipe 16 is erected on the outer peripheral edge of the flange portion 15a.

  The bellows pipe 16 is flexible so as to suppress deformation in the vertical direction thereof, and has a flange portion 16a bulging outward in the radial direction at the upper end portion. The middle part of the bellows tube 16 is fixed to the decompression vessel 7 while forming an airtight state.

  Further, the sleeve 5 is provided with a gas flow generating means 10 for introducing a purge gas (helium gas in this embodiment) into the sleeve 5.

  The gas flow generation means 10 switches a gas introduction pipe 19 for introducing helium gas into the sleeve 5 and the supply and stop of helium gas from a gas supply source (not shown) to the gas introduction pipe 19. And a valve 20.

  The gas introduction pipe 19 has one end connected to the lower side surface of the cooling pipe 15 and the other end connected to a gas supply source (not shown) via the valve 20. Further, a middle portion of the gas introduction pipe 19 penetrates the decompression container 7 and the shield container 11 while forming an airtight state with the decompression container 7.

  That is, the gas introduction pipe 19 introduces helium gas into the cooling pipe 15 from the side, and the helium gas introduced into the cooling pipe 15 collides with the opposite side wall and is separated. As shown by an arrow Y1 in FIG. 2, a purge gas flow sprayed onto the liquid surface of the liquid helium 50 and a purge gas flow toward the opening 8 side of the sleeve 5 as shown by an arrow Y2 are constituted.

  On the other hand, the refrigerator 9 includes a substantially rod-shaped refrigerator main body 21 and a lid body 22 attached to the refrigerator main body 21, and the lid body 22 is hermetically sealed in the sleeve 5 via an O-ring 23. Thus, the refrigerator main body 21 can be detachably inserted into the sleeve 5 by being detachably attached to the flange 16a by a bolt 24.

  The refrigerator main body 21 is a two-stage refrigerator using a Gifford McMahon (GM) system.

  That is, the refrigerator main body 21 is inserted into the sleeve 5, the second-stage cooling stage 25 disposed above the gas introduction pipe 19, and the first-stage cooling stage in contact with the shield container 11. 26 in an integrated manner.

  The second cooling stage 25 is set to a cooling capacity capable of cooling the liquid helium 50 in the containment vessel 4 to a supercooling temperature TP (see FIG. 3), which will be described later, and a heater 27 provided at the tip thereof. The desired cooling capacity can be exhibited by adjusting the amount of heat.

  The first-stage cooling stage 26 is set to a temperature higher than that of the second-stage cooling stage 25, and the shield container 11 is cooled to a temperature that can shield the radiant heat to the storage container 4.

  The cover body 22 includes a discharge pipe 28 communicating with the sleeve 5 in a state of being fitted into the sleeve 5, and a check valve 29 capable of deriving gas only outside the sleeve 5 through the discharge pipe 28. When the differential pressure between the atmospheric pressure and the internal pressure of the sleeve 5 is equal to or higher than the operating pressure of the check valve 29, the gas in the storage container 4 is discharged through the check valve 29.

  In the illustrated cryogenic apparatus 1, safety means 13 is provided separately from the sleeve 5. The safety means 13 includes a safety discharge pipe 30 that penetrates the vacuum container 6 and the shield container 11 while forming an airtight state with the vacuum container 6, and a rupture provided at the tip of the safety discharge pipe 30. A valve 31 and a plurality of shielding plates 32 disposed in the safety discharge pipe 30 are provided.

  According to the safety means 13, when the amount of heat intrusion into the storage container 4 increases due to some cause such as deterioration of the vacuum degree of the vacuum container 6 and the pressure in the storage container 4 excessively increases, Since the rupture valve 31 is broken according to the pressure and the helium gas in the storage container 4 is discharged through the safety discharge pipe 30, damage to the storage container 4 due to the excessive pressure can be suppressed. Moreover, since each said shielding board 32 is arrange | positioned along with the axis line of the said safety discharge pipe 30, it can suppress that the radiant heat from the said rupture valve 31 side penetrate | invades into the storage container 4. FIG.

  Furthermore, the cryogenic apparatus 1 includes a pressure sensor (not shown) that can detect the pressure in the containment vessel 4 (that is, the vapor pressure of the liquid helium 50), and based on the detection result of the pressure sensor, The temperature of the liquid helium 50 in the storage container 4 is calculated, and the amount of heat of the heater 27 is adjusted based on the calculated temperature, so that the temperature of the liquid helium 50 can be maintained at a desired temperature. .

  The decompression container 7 can be decompressed inside (vacuum chamber 6) by a suction source (not shown).

  Hereinafter, the operation method of the cryogenic apparatus 1 will be described by dividing it into a normal operation time T1, a supercooling time T2, a boosting time T3 and a replacement time T4 shown in FIG.

1) Normal operation time T1 In the refrigerator 9, the amount of heat of the heater 27 is adjusted to maintain the temperature of the liquid helium 50 at 4.2K. This temperature is the boiling point of the liquid helium 50 under atmospheric pressure conditions. Therefore, in this normal operation time T1, the pressure by the liquid helium 50 vaporized in the storage container 4 becomes atmospheric pressure. In this way, during normal operation, avoiding the negative pressure inside the storage container 4 and eliminating the pressure difference between the inside and the outside of the storage container 4, the sleeve 5 and the refrigerator 9 are connected. It is possible to suppress a situation in which outside air is sucked from a portion (the O-ring 23) forming an airtight state.

2) Supercooling time T2
Prior to replacing the refrigerator 9, the heater 27 is turned off while continuing its operation, so that the temperature of the liquid helium 50 is changed from the boiling point of the liquid helium 50 under atmospheric pressure conditions from 4.2K. Until it reaches a lower supercooling target temperature TP (supercooling step).

  In this supercooling step, the liquid helium 50 is cooled from the liquid surface 3a side, so that convection occurs in the liquid helium 50, so that the heat from the refrigerator 9 moves from the liquid surface 3a to the deep side and the liquid helium 50 is cooled. Helium 50 can be effectively cooled.

3) Boosting time T3
As the supercooling operation proceeds, the temperature of the liquid helium 50 decreases, and the temperature is calculated by the detected pressure of the pressure sensor (not shown). The temperature of the liquid helium 50 is monitored, and when the temperature reaches the supercooling temperature TP, the refrigerator 9 is stopped and the gas flow generating means 10 causes the helium gas flow to flow into the sleeve 5. Generate (gas flow generation step).

  In this gas flow generation step, a helium gas flow toward the liquid surface 3a of the liquid helium 50 is introduced as indicated by an arrow Y1 in FIG. With the introduction of this helium gas flow, the pressure rapidly increases and the liquid surface temperature L1 (shown by a one-dot chain line in FIG. 3) of the liquid helium 50 cooled to the supercooling temperature TP rises relatively quickly. The deep temperature L2 of liquid helium 50 (indicated by a broken line in FIG. 3) does not rise immediately.

4) Exchange time T4
When the liquid surface temperature of the liquid helium 50 reaches 4.2K and the pressure sensor (not shown) detects that the pressure in the storage container 4 has reached atmospheric pressure, the operation of replacing the refrigerator 9 is performed. Perform (exchange process). Even during this exchange process, the introduction of helium gas by the gas flow generating means 10 continues. As a result, a helium gas flow toward the opening 8 (see arrow Y2) is continuously generated in the sleeve 5, so that outside air can enter the sleeve 5 from the opening 8 during the replacement process. Can be suppressed.

  That is, in this operation method, when the helium gas is introduced, the liquid helium 50 is cooled to the supercooling temperature TP in advance, so that even if helium gas is sprayed onto the liquid surface 3a of the liquid helium 50, Although the surface temperature L1 rises and the liquid helium 50 in the vicinity of the liquid surface 3a evaporates, the deep part of the liquid helium 50 can maintain a low temperature for a while. This is clear from the experimental results shown in FIG.

  FIG. 4 is a graph showing the temperature of the deep portion of the liquid helium 50 when the temperature of the liquid surface 3a of the liquid helium 50 is increased to 4.2K. In this figure, a solid line L3 is obtained by increasing the temperature of the liquid surface 3a of the liquid helium 50 by blowing the helium gas by the gas flow generating means 10, and a one-dot chain line L4 does not blow the helium gas, and the sleeve 5 Each is shown when the inside is not positively pressurized (hereinafter referred to as non-pressurized).

  When the helium gas is sprayed, the heat transfer accompanying the temperature of the helium gas reaches only about 3 cm in depth until the temperature of the liquid surface 3a of the liquid helium 50 is increased to 4.2K. At the time of non-pressurization, while the liquid level 3a of the liquid helium 50 rises to 4.2K, heat transfer accompanying the temperature of the outside air reaches a range of about 16 cm in depth. Therefore, at the time of non-pressurization, the temperature of the liquid helium 50 easily reaches 4.2K not only at the liquid level 3a but also at the deep part, and the liquid helium 50 evaporates from the deep part, whereas helium gas is blown. Sometimes, the temperature of the deep part can be maintained below 4.2K for a relatively long time.

  Here, heat transfer to the deep part occurs when no pressure is applied, while heat transfer to the deep part hardly occurs when helium gas is blown, because the time required for the liquid level 3a of the liquid helium 50 to reach 4.2K It is a big factor. In other words, since it takes a relatively long time for the temperature of the liquid surface 3a to rise to 4.2 K at the time of non-pressurization, heat transfer is gradually generated in the liquid helium 50 during this time, and the intruded heat reaches the deep part. On the other hand, when the helium gas is blown, the temperature of the liquid surface 3a rises relatively fast up to 4.2K, but almost no heat transfer occurs in the liquid helium 50. The temperature on the deep side does not rise immediately.

  In the replacement step, not only the operation of replacing the refrigerator 9 that has been mounted on the sleeve 5 with another refrigerator, but also the maintenance of the refrigerator 9 removed from the sleeve 5 is quickly performed, and the maintenance is performed. The operation of inserting the later refrigerator 9 into the sleeve 5 again can be performed. And after finishing the said replacement | exchange process, it will operate | move again on the conditions of the normal operation time T1 mentioned above.

  In the operation method described above, the gas flow generation step can be performed in a state where the temperature of the liquid helium 50 is cooled to the supercooling temperature TP. The surface 3a can be suppressed to only the surface 3a, and the amount of evaporation of the liquid helium 50 during the replacement process can be effectively reduced by performing the replacement process quickly.

  In the above embodiment, the liquid helium 50 is maintained at 4.2 K during the normal operation time T1 and is cooled to a temperature of less than 4.2 K during the supercooling time T2. However, the present invention is not limited to this. For example, as shown in FIG. 5, the liquid helium 50 is maintained at the supercooling temperature TP in advance during the normal operation time T1, that is, the supercooling process can be performed constantly during the normal operation time T1. .

  In this embodiment as well, at the time of pressurization time T3 of the refrigerator 9, the liquid surface temperature L5 rises relatively quickly, but the deep temperature L6 can be maintained at a temperature lower than 4.2K. The amount of evaporation of the liquid helium 50 can be effectively reduced.

  Further, according to this embodiment, it is not necessary to perform the supercooling process again before the replacement process of the refrigerator 9, and thus the time until the replacement process is performed is shortened (that is, the It is unnecessary to separately perform the supercooling time T2), and the maintenance workability can be improved.

  In the above-described embodiment, the cooling capacity of the second stage cooling stage 25 is adjusted by the heater 27 provided in the second stage cooling stage 25 of the refrigerator 9, and the pressure (helium in the containment vessel 4 is indirectly adjusted). 50), in place of or in addition to the heater 27, as shown in FIG. 2, a heater 33 (see FIG. 2) provided in the liquid refrigerant 3 (inside the storage container 4). The amount of heat applied to the helium 50 can be adjusted to adjust the pressure in the storage container 4.

1 is a schematic front sectional view of an MRI equipped with a cryogenic apparatus according to an embodiment of the present invention. It is front sectional drawing which expands and shows the principal part of the cryogenic apparatus of FIG. It is a graph which shows the temperature of the liquid solvent in the cryogenic apparatus of FIG. It is a graph which shows the temperature of the deep part of liquid helium when the temperature of the liquid level of liquid helium is raised to 4.2K. FIG. 4 is a view corresponding to FIG. 3 according to another embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Cryogenic apparatus 2 Superconducting magnet 3 Liquid helium 3a Liquid level 4 Storage container 5 Sleeve 8 Opening part 9 Refrigerator 10 Gas flow generation means 11 Shield container

Claims (3)

A storage container for storing the object to be cooled immersed in the liquid refrigerant, a sleeve that opens from the storage container to the upper side of the liquid refrigerant surface, and an opening that closes the opening while being detachable in the sleeve And a cryogenic device provided with a refrigerator that re-condenses the liquid refrigerant vaporized in the sleeve and is suitable for replacing the refrigerator,
A subcooling step of cooling the liquid refrigerant by the refrigerator so that the liquid refrigerant has a supercooling temperature below the boiling point of the liquid refrigerant under atmospheric pressure conditions;
In a state where the liquid refrigerant is cooled to the supercooling temperature, the opening and the liquid are formed in the sleeve by gas flow generation means capable of introducing a purge gas from a position closer to the storage container than the refrigerator with respect to the sleeve. A gas flow generating step for generating a purge gas flow toward the liquid level of the refrigerant;
And a replacement step of replacing the refrigerator while performing the gas flow generation step.   During normal operation of the cryogenic device, the liquid refrigerant is maintained at a temperature equal to or higher than the boiling point of the liquid refrigerant under atmospheric pressure conditions, and the subcooling operation from the normal operation to the subcooling operation is performed before the replacement step. The operation method of the cryogenic apparatus according to claim 1, wherein   The cooling capacity of the refrigerator is set to an ability capable of cooling the liquid refrigerant to the supercooling temperature, and is provided in the refrigerator or sleeve while operating the refrigerator during the normal operation. The temperature of the liquid refrigerant is adjusted by adjusting the amount of heat by the heating means, and the liquid refrigerant is cooled to the supercooling temperature by stopping the heating means while operating the refrigerator in the supercooling step. The operation method of the cryogenic apparatus according to claim 2.

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