Patent Publication Number: US-9410725-B2

Title: Refrigerator installing structure

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application is based upon and claims the benefit of priority of Japanese Patent Application No. 2012-020017 filed on Feb. 1, 2012, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to a refrigerator installing structure. More specifically, the present invention relates to a refrigerator installing structure of accommodating a refrigerator inside a sleeve provided inside a vacuum vessel. 
     2. Description of the Related Art 
     For example, a Gifford-McMahon refrigerator (hereinafter, referred to as a “GM refrigerator” has been widely used as a means for cooling a cryostat (a cryo temperature vacuum vessel) such as a superconducting magnet apparatus. When such a GM refrigerator is used for a long time, maintenance is required. 
     In a method of maintaining the superconducting magnet apparatus or the like, the entire superconducting magnet apparatus is required to have an ordinary temperature. It takes from one day to six days to cause the temperature of the superconducting magnet apparatus to reach an ordinary temperature. In the meantime, because the superconducting magnet apparatus is continuously stopped under an idle condition, running efficiency of the superconducting magnet apparatus is greatly reduced. 
     Meanwhile, there is proposed a method of extracting a displacer while a cylinder of the GM refrigerator is fixed to the vacuum vessel. Within the method, the cylinder is continuously exposed to the air and the cylinder is continuously cooled by the vacuum vessel. Therefore, moisture in the air changes to be an ice film and adheres to the inner surface of the cylinder. Therefore, it is impossible to insert the displacer again inside the cylinder. Consequently, the maintenance becomes impossible. 
     Then, as a maintenance method by which the superconducting magnet apparatus or the like is maintained while preventing the ice film from adhering to the inner surface of the cylinder, there is proposed a method of forming a space separated from a vacuum zone of a vacuum vessel and installing a cylinder of the GM refrigerator in the space (Patent Document 1). 
     In this structure, when the GM refrigerator is installed in the sleeve, a cooling stage of the GM refrigerator is connected to an object to be cooled located inside the vacuum vessel via the sleeve. Further, the space is formed between the sleeve and a flange of the GM refrigerator. The space is vacuated. Further a sealing member (an O-ring) is provided between the sleeve and the cylinder so as to maintain a degree of vacuum in the space. 
     In the above structure, when the superconducting magnet apparatus or the like is maintained, a cylinder is separated from the sleeve by a small amount, e.g., several mm, thereby releasing a thermal connection between the cylinder and the sleeve is released. However, even if the cylinder is moved relative to the sleeve, sealing by the O-ring is ensured to thereby maintain a vacuum in the space formed between the sleeve and the cylinder. 
     As described, because the vacuum space exists between the sleeve and the cylinder and the sleeve and the cylinder are thermally separated, a low temperature of the vacuum vessel does not thermally conduct from the cylinder to the inside of the vacuum vessel. Therefore, the ice film, which causes a problem in the maintenance, does not adhere to the inside of the cylinder. The displacer can be easily replaced within a short time. 
     [Patent Document 1] Japanese Laid-open Patent Publication No. 2004-053068 
     SUMMARY OF THE INVENTION 
     According to an aspect of the present invention, there is provided a refrigerator installing structure that enables a refrigerator including a cylinder and a displacer to be installed in a vacuum vessel in which an object to be cooled is accommodated, the displacer being removed from the cylinder during maintenance, the cylinder being movable inside a sleeve between a position at which the cylinder thermally contacts the sleeve and another position at which the cylinder does not thermally contact the sleeve includes a discharge mechanism configured to discharge a gas inside a space formed between the sleeve and the cylinder if a pressure inside the space becomes greater than or equal to a predetermined pressure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view for illustrating a refrigerator installing structure as an embodiment of the present invention where a GM refrigerator is installed in a sleeve; 
         FIG. 2  is a cross-sectional view of the refrigerator installing structure of the embodiment of the present invention where displacers are removed from the cylinder; 
         FIG. 3  is a plan view of a sleeve and a GM refrigerator installed in the sleeve; 
         FIG. 4  is an enlarged view of a safety valve in a closed state; and 
         FIG. 5  is an enlarged view of a safety valve in an opened state. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     According to the structure of installing a refrigerator of Patent Document 1, the temperature of the cylinder increases when the cylinder is separated from the cryo temperature vacuum vessel during the maintenance. In this case, if air or moisture is leaked into the space formed between the sleeve and the cylinder, the air or moisture frozen at a time of cooling is rapidly vaporized and expanded by the temperature increment of the cylinder. Therefore, there is a problem that the pressure inside the space formed between the sleeve and the cylinder rapidly increases to thereby damage the sleeve or the cylinder. 
     The present invention is provided in consideration of the above. The object of the present invention is to provide a refrigerator installing structure which can suppress a sudden increment of pressure inside the space formed between the sleeve and the cylinder. 
     A description is given below, with reference to the  FIG. 1  through  FIG. 5 . 
       FIGS. 1 to 3  illustrate a refrigerator installing structure of a first embodiment of the present invention. Within the embodiment, a Gifford-McMahon refrigerator R (hereinafter, referred to as a “GM refrigerator”) is explained as an example. However, the present invention can be widely applied to a refrigerator having a structure where an internal portion is removed from a cylinder at a time of maintaining the refrigerator. 
     The GM refrigerator R cools an object to be cooled while the GM refrigerator R is inserted in a vacuum vessel (not illustrated), in which the object to be cooled is accommodated. Said differently, the GM refrigerator R is inserted into the vacuum vessel and the object to be cooled is accommodated in the vacuum vessel. The GM refrigerator R includes a motor driving unit M, displacers engaged with the motor driving unit M so as to be driven by the motor driving unit M, and cylinders accommodating the displacers so that the displacers can reciprocate inside the cylinders. Within the embodiment, the GM refrigerator is a two stage type. Therefore, a first stage cylinder C 1 , a second stage cylinder C 2 , a displacers D 1 , and a displacer D 2  are included in the GM refrigerator R. 
     The present invention is not limited to the GM refrigerator R of the two stage type, and is applicable to a GM refrigerator R of a single stage type or a GM refrigerator R of a three or greater stage type. 
     A first stage cold head H 1  is formed at the lower end of the first stage cylinder C 1 . A second stage cold head H 2  is formed at the lower end of the second stage cylinder C 2 . 
     A flange  41  is formed in an upper opening periphery of the first stage cylinder C 1 . The flange  41  is provided to attach a motor driving unit M and the vacuum vessel to the upper opening periphery of the first stage cylinder C 1 . Specifically, the flange  41  lies between the first stage cylinder C 1  and the vacuum vessel in order to attach the first stage cylinder C 1  to the vacuum vessel via the flange  21 . The displacers D 1  and D 2  are inserted into the first and second stage cylinders C 1  and C 2  via the opening of the flange  41 . 
     The sleeve  2  includes a first stage sleeve  2   a  having a first stage cooling flange F 1  at an lower end of the first stage sleeve  2   a  and a second stage sleeve  2   b  connected to the first stage cooling flange F 1  at an upper end of the second stage sleeve  2   b  and having a second stage cooling flange F 2  at an lower end of the second stage sleeve  2   a . The flange  21  is provided at an opening periphery of the first stage sleeve  2   a  for attaching the first stage sleeve  2   a  to a top panel of the vacuum vessel. 
     Indium sheets  3   a  and  3   b  having thicknesses of about 0.5 mm to 1 mm are provided on a thermally contacting interface between a first stage cold head H 1  of a GM refrigerator R and the first stage cooling flange F 1  and a thermally contacting interface between a second stage cold head H 2  and the second stage cooling flange F 2 , respectively, in order to further ensure thermal contacts. 
     The GM refrigerator R can achieve a cryo temperature of 40 K (degrees kelvin) to 70 K using the first stage cold head H 1  and a cryo temperature of 4 K to 20 K using the second stage cold head H 2 . Therefore, the object to be cooled can be cooled to have a predetermined temperature by the first and second stage cold heads H 1  and H 2 . 
     The first and second stage cylinders C 1  and C 2  forming the GM refrigerator R are installed inside the sleeve  2 . In this installed state, a space  60  is formed between the inner surface of the sleeve  2  (the first and second stage sleeves  2   a  and  2   b ) and outer surfaces of the first and second stage cylinders C 1  and C 2  except for the thermally contacting interfaces. The thermally contacting interfaces are perpendicular to the extending direction of the first and second stage cylinders C 1  and C 2 . 
     The flange  41  formed in an upper opening periphery of the first stage cylinder C 1  faces the flange  21  of the sleeve  2 . The flange  41  includes a plate member  41 - 1  in an annular shape and a cylindrical part  41 - 2 . The plate member  41 - 1  is provided to attach the motor driving unit M to the first stage cylinder C 1  while the displacers D 1  and D 2  are inserted into the first and second stage cylinders C 1  and C 2 . The cylindrical part  41 - 2  is inserted into an upper portion of the sleeve  2  to seal a space inside the sleeve together with the plate member  41 - 1 . 
     The plate member  41 - 1  and the cylindrical part  41 - 2  are integrated by a bolt (not illustrated) or the like. An O-ring  41 - 3  is provided at a joining portion between the plate member  41 - 1  and the cylindrical part  41 - 2 . As described, the first and second stage cylinders C 1  and C 2  are accommodated in the sleeve  2  while the first and second stage cylinders C 1  and C 2  are separated from a vacuum zone inside the vacuum vessel. 
     An O-ring  42  made of rubber is provided on an inner peripheral surface of the flange  21  and a peripheral surface of the cylindrical part  41 - 2 . The inner peripheral surface of the flange  21  and the peripheral surface of the cylindrical part  41 - 2  mutually face. The-ring  42  seals an interface between the cylindrical part  41 - 2  and an inner wall of the sleeve  2  (specifically, the flange  21 ) facing the inner wall of the sleeve  2 . 
     As described below, the cylindrical part  41 - 2  can move upward and downward relative to the sleeve  2 . Therefore, there is a small gap between the inner surface of the sleeve  2  (the flange  21 ) and the outer surface of the cylindrical part  41 - 2 . The O-ring  42  is provided to prevent air, moisture or the like from intruding into the space  60  from the outside via the gap. 
     The flange  41  and the flange  21  are fastened by plural bolts  43  arranged at equal angular intervals. The bolts  43  are inserted on the lower side of the flange  21  to fasten the flange  21  and the flange  41 . By a reason to be described below, the bolt  43  is inserted into the flange  21  with some looseness. 
     In addition, at least one guide pin  44  is provided to regulate inclinations of the first and second stage cylinders C 1  and C 2  when the cylindrical part  41 - 2  is inserted into the sleeve  2 . In this embodiment, the number of the guide pins  44  is four. The four guide pins  44  are arranged at equal angular intervals of 90°. The guide pins  44  are arranged on the flange  21  in a standing state. The cylindrical part  41 - 2  and the plate member  41 - 1  have through holes at positions corresponding to the guide pins  44 . 
     Spring washers  45  are interposed between the heads of some bolts  43  among the plural bolts  43  and the flange  21  facing the heads. The spring washers  45  generate a biasing force of pulling the flange  41  onto the lower side of the refrigerator R (as illustrated in  FIG. 1 ) via the bolts  43 . 
     Said differently, when the cooling is initiated, the first stage cylinder C 1  begins to be cooled. Then, the first stage cylinder C 1  contracts. With this, the first stage cold head H 1  starts to move away from the first stage cooling flange F 1 . However, as stated, the first stage cylinder C 1  is pushed down by the spring washers  45 . Therefore, a surface contact between the first stage cold head H 1  and the first stage cooling flange F 1  (a thermal connection) is maintained. 
     The flange  21  includes a connector  46  and a vacuating port  47 . One end of the vacuating port  47  communicates with the space  60  formed between the sleeve  2  and the first and second stage cylinders C 1  and C 2 . The connector  46  is provided on the other end of the vacuating port  47 . A depressurizing means such as a vacuum pump is connected to the connector  46 . The depressurizing means performs vacuuming of the space  60 . 
     A measurement port  52  is provided in the flange  41  (specifically, the plate member  41 - 1 ). An end of the measurement port  52  communicates with the space  60 , and the other end of the measurement port  52  is extracted outside the plate member  41 - 1 . 
     A first stage temperature sensor S 1  is provided at a portion of the first stage cylinder C 1  thermally contacting the first stage cooling flange F 1 . A second stage temperature sensor S 2  is provided at a portion of the second stage cylinder C 2  thermally contacting the second stage cooling flange F 2 . The first and second stage temperature sensors S 1  and S 2  are provided to detect cooling temperatures of the first and second stage cooling flanges F 1  and F 2 , respectively. 
     Wirings  65  and  66  respectively connected to the first and second stage temperature sensors S 1  and S 2  are helically wound around the outer peripheries of the first and second stage temperature sensors S 1  and S 2 , respectively. Further, the wirings  65  and  66  are outwardly pulled via the measurement port  52 . 
     Within the embodiment, the safety valve  50  (a discharge mechanism in the claims) is provided in the measurement port  52 .  FIGS. 4 and 5  are enlarged view of the safety valve arranged in the measurement port  52 . 
     The safety valve  50  includes a fixed flange  53 , a movable flange  54 , bolts  55 , springs  57  and so on. The fixed flange  53  is shaped like a disk and fixed by the measurement port  52  by welding or the like. Through holes (not illustrated) are formed at positions of the fixed flange  53  corresponding to the bolts  55 . Further, a surface of the fixed flange  53  directing along an arrow A 1  in  FIG. 5  is a sealing surface  53   a.    
     The movable flange  54  is shaped like a disk having the same diameter as that of the fixed flange  53 . The movable flange  54  is movable in the directions of the arrows A 1  and A 2  relative to the fixed flange  53 . The bolts  55  are screwed into the movable flange  54 . Further, a surface of the movable flange  54  directing along the arrow A 2  in  FIG. 5  is a sealing surface  54   a.    
     The heads of the bolts  55  are positioned on the side directed by the arrow A 2  in  FIG. 5 . Columnar portions  56  of the bolts  55  extend in the direction along the arrow A 1  from the heads. Threaded portions are formed in ends of the bolts  55  over the columnar portions  56 . The threaded portions are screwed in internal thread portions formed in the movable flange  54 . With this, the bolts  55  and the movable flange  54  are structured so as to be integrated. 
     Further, the columnar portions  56  are inserted into the through holes formed in the fixed flange  53 . The diameter of the columnar portions  56  are smaller than the diameters of the through holes formed in the fixed flange  53 . Therefore, the movable flange  54  is guided by the bolts  55  (specifically, the columnar portion  56 ) relative to the fixed flange  53  so as to be movable in the directions along the arrows A 1  and A 2 . 
     The springs  57  are arranged between the fixed flange  53  and the heads of the bolts  55 . The springs  57  are coil springs for biasing elastic force in directions of stretching the coil springs. Therefore, the movable flange  54  is placed in a position where the movable flange  54  contacts the fixed flange  53  by being pressed against the fixed flange  53 . 
     As described, the fixed flange  53  has the sealing surface  53   a , and the movable flange  54  has the sealing surface  54   a . Therefore, since the movable flange  54  contacts the fixed flange  53  by being pressed against the fixed flange  53  by the elastic force of the springs  57 , airtightness is ensured by the contact between the sealing surfaces  53   a  and  53   b  to properly close the fixed flange  53 . Then, even if the measurement port  52  communicates with the space  60 , air or moisture does not leak inside the space  60  from the measurement port  52 . 
     Meanwhile, as described in detail later, if the pressure inside the space  60  becomes greater than or equal to a predetermined pressure, the movable flange  54  moves in the direction along the arrow A 1  relative to the fixed flange  53 . Said differently, if the pressure inside the space  60  increases, the pressure directly effects the sealing surface  54   a  of the movable flange  54 . Along with the pressure increment inside the space  60 , the force effecting on the sealing surface  54   a  exceeds the biasing force of the springs  57 . At this time, movable flange  54  moves in the direction along the arrow A 1  relative to the fixed flange  53 . 
     Referring to  FIG. 5 , the movable flange  54  moves and is positioned apart from the fixed flange  53 . By the movement of the movable flange  54 , a gap  59  is formed between the sealing surface  53   a  and the sealing surface  54   a . With this, the space  60  is opened to the air via the measurement port  52 . A gas or the like inside the space  60  is discharged via the measurement port  52  and the safety valve  50 . With this, the pressure inside the space  60  can be reduced. 
     Next, operations carried out in maintaining the refrigerator installing structure as configured above are described. 
     When the GM refrigerator R is maintained, the first and second stage cylinders C 1  and C 2  are maintained, and the motor driving unit M and the displacers D 1  and D 2  are replaced by a new motor driving unit M and new displacers D 1  and D 2 . 
     During replacement operations, the fastening between the flanges  21  and  41  are released, and the GM refrigerator R is pulled up in a range of ensuring sealing by the O-ring  42  without totally extracting the GM refrigerator R from the sleeve  2  (said differently, without exposing the inside of the sleeve  2  to the air). The amount of pulling up the GM refrigerator R is, for example, about 2 mm to 3 mm indicated by arrows ΔW 1  and ΔW 2  in  FIG. 2 . 
     By this operation, the GM refrigerator R moves away from the sleeve  2  to cancel surface contacts (thermal connections) between the sleeve  2  and the first and second stage cold head H 1  and H 2 . Thus, heat does not transfer on the thermally contacting interfaces. 
     Next, under the cryo temperature, the displacers D 1  and D 2  are extracted together with the motor driving unit M while the first and second stage cylinders C 1  and C 2  are fixed as is. Thereafter, the new displacers D 1  and D 2  and the new motor driving unit M are installed. 
     Referring to  FIG. 2 , the motor driving unit M is extracted together with the displacers D 1  and D 2 , and the surface contacts (the thermal connections) between the sleeve  2  and the first and second stage cylinders C 1  and C 2  are released. 
     While the surface contacts are released, the first stage sleeve  2   a  and the first stage cold head H 1  are thermally separated, and the second stage sleeve  2   b  and the second stage cold head H 2  are thermally separated. Further, the space  60  is depressurized by the depressurizing means via the connector  46 . Thus, the space  60  maintains a vacuum. 
     Next, the new motor driving unit M and the new displacers D 1  and D 2  are installed inside the first and second stage cylinders C 1  and C 2 . However, when the old motor driving unit M and the old displacers D 1  and D 2  are removed, the first and second stage cylinders C 1  and C 2  are exposed to the air. Because the temperature of the inside of the first and second stage cylinders C 1  and C 2  is low, an ice film or frost is formed on the inner surfaces of the first and second stage cylinders C 1  and C 2 . 
     Therefore, it is difficult to install the motor driving unit M and the displacers D 1  and D 2  in the first and second stage cylinders C 1  and C 2 . Because of this, the inside of the first and second stage cylinders C 1  and C 2  is heated. 
     In heating the inside of the first and second stage cylinders C 1  and C 2 , a heating device (e.g., a dryer) is inserted into the cylinders C 1  and C 2  to increase the temperature. Thus, the ice film or frost is removed and cleaned. The increased temperature is about 20° C. to 40° C. After heating the inside of the first and second stage cylinders C 1  and C 2 , the indium sheets  3   a  and  3   b  attached to lower end surfaces of the first and second cold heads H 1  and H 2  are softened. 
     After completing the heating, the new motor driving unit M and the new displacers D 1  and D 2  are inserted inside the first and second stage cylinders C 1  and C 2 . The pulled-up first and second stage cylinders C 1  and C 2  are pulled down to be in an original state as illustrated in  FIG. 1 . Thus, a performance of heat transfer in the first and second stage cold heads H 1  and H 2  can be similar to that in the original first and second stage cold heads H 1  and H 2  in the brand-new state before the replacement. 
     As described, in the refrigerator installing structure of the embodiment, the space  60  being vacuum exists between the sleeve  2  and the first and second stage cylinders C 1  and C 2 . Further, the surface contacts (the thermal connections) between the sleeve  2  and the first and second stage cylinders C 1  and C 2  are canceled. 
     Therefore, the first and second stage cylinders C 1  and C 2  are not directly cooled by the low temperature on the side of the vacuum vessel. Further, the first and second stage cylinders C 1  and C 2  are exposed to the air by removing the displacers D 1  and D 2 . However, the existence of the space  60  prevents heat from intruding toward the vacuum vessel via the first and second stage cylinders C 1  and C 2 . Thus, the temperature increment of the vacuum vessel can also be prevented. 
     Because the space  60  formed between the sleeve  2  and the first and second stage cylinders C 1  and C 2  is vacuum, air, moisture or the like likely intrude thereinto from the outside. Further, vibration occurs by driving the motor driving unit M. Therefore, the sealing property may degrade over a period of time. Because of these reasons, air, moisture or the like may leak (intrude) into the space  60 . If the air, moisture or the like leaks into the space  60 , since the sleeve  2  and the cylinders C 1  and C 2  are in a cryo temperature state, the inner wall of the sleeve  2  and the outer walls of the first and second stage cylinders C 1  and C 2  catches ice so that the ice is accumulated on the inner wall and the outer walls. 
     When the maintenance is done while the leaked air, moisture or the like freezes and are caught by the inner wall of the sleeve  2  and the outer walls of the first and second stage cylinders C 1  and C 2 , the first and second stage cylinders C 1  and C 2  are thermally separated from the sleeve  2  maintained to be cooled, and are exposed to the air. Therefore, the temperature of the first and second stage cylinders C 1  and C 2  increases. Along with the temperature increment, the frozen air, moisture or the like is vaporized and expands to thereby rapidly increase the pressure inside the space  60 . Specifically, when the air, moisture or the like is vaporized and expands, the pressure inside the space  60  increases greater than or equal to, for example, 400 times in comparison with the pressure before increasing the temperature. 
     However, in the refrigerator installing structure of the embodiment, the safety valve  50  is provided in the measurement port  52  communicating with the space  60 . Therefore, if the pressure inside the space  60  increases, the increased pressure is applied to the movable flange  54  forming the safety valve  50 . Then, the increased pressure biases to move the movable flange  54  in the direction along the arrow A 1 . 
     When the pressure inside the space  60  becomes a predetermined pressure or greater, the movable flange  54  moves against the elastic force of the springs  57 , as described with reference to  FIG. 5 . Thus, the safety valve  50  is opened to enable the vaporized and expanding gas inside the space  60  to be discharged from the gap  59  to the outside. 
     In the refrigerator installing structure, the predetermined pressure is set in a pressure range without causing the first and second stage cylinders C 1  and C 2  to move relative to the sleeve  2  even if the pressure increases and stays within a pressure range that will not cause damages in the sleeve  2 , the first and second stage cylinders C 1  and C 2 , the O-ring  42  and so on due to the pressure increment. The predetermined pressure in the safety valve  50  can be changed by adjusting the spring constant of the springs  57 , the areas of the sealing surfaces  53   a  and  53   b , or the like. 
     As described, in the refrigerator installing structure of the embodiment, even if air, moisture or the like leaks into the space  60 , it is possible to prevent the damages from occurring in the sleeve  2 , the first and second stage cylinders C 1  and C 2 , the O-ring  42 , or the like during the maintenance. Further, along with the pressure increment inside the space  60 , it is possible to prevent the first and second stage cylinders C 1  and C 2  from being separated from the sleeve  2 . Thus, safety of the maintenance operations can be enhanced. 
     Within the embodiment, the measurement port  52  conventionally used is utilized, and the safety valve  50  is provided in the measurement port  52 . Meanwhile, it is also possible to form a port communicating with the space  60  in addition to or instead of the measurement port  52  and provide a safety valve in the port. 
     However, in a case where the structure is employed, the number of the parts increases and the refrigerator installing structure becomes complicated. Therefore, as described in the embodiment, when the measurement port  52  is utilized and the safety valve  50  is provided in the measurement port  52 , it is possible to simplify the refrigerator installing structure and lower the cost of the refrigerator installing structure. 
     Further, as the port communicating with the space  60 , a vacuating port  47  used to vacuate the space  60  is provided in addition to the measurement port  52 . However, in the embodiment, the safety valve  50  is provided in the measurement port  52  without providing the safety valve  50  in the vacuating port  47 . 
     According to the structure, in a case where the pressure inside the space  60  increases, the gas inside the space  60  can be discharged via the safety valve and the measurement port  52 . Further, the gas inside the space  60  can be discharged via the connector  46  and the vacuating port  47 . Thus, the gas inside the space  60  can be efficiently discharged. 
     The refrigerator installing structure of the embodiment of the present invention can be used for a superconducting magnet apparatus provided for a monocrystal puller, a superconducting magnet apparatus for other usage, or the like. 
     According to the embodiment of the present invention, when the pressure inside the space formed between the sleeve and the first and second stage cylinders increases greater than or equal to the predetermined pressure, the gas inside the space is discharged by a discharge mechanism. Therefore, it is possible to prevent the sleeve and the first and second stage cylinders from being damaged. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the embodiments and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of superiority or inferiority of the embodiments. Although the refrigerator installing structure has been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.