CRYOGENIC REFRIGERATOR

Abstract: A cryogenic refrigerator comprising a closed cylinder (111) provided with an inlet (134) and an outlet (135) for introducing and discharging a coolant gas into and out of the cylinder (111), a displacer (119) slidably housed in the closed cylinder (111) and housing a cooling member (124) therein and having a passage (141) through which the coolant gas flows, a device (142) coaxially arranged in and along the passage (141) of the displacer (119) in which the cooling member (124) is housed to divide the passage (141) into outer and inner ones, a device for reciprocating the displacer (119) in the cylinder (111), and a device for repeating the process of introducing the high pressure coolant gas into the cylinder (111) through the inlet (134) and discharging it out of the cylinder (111), synchronizing with the reciprocating displacer (119).

Description: The present invention relates to a cryogenic refrigerator and, more particularly, a refrigerator of the refrigerant-accumulating type. Various kinds of cryogenic refrigerators are now on the market. One of them is of the Gifford-McMahon type. This refrigerator is usually arranged as shown in Fig. 1. The refrigerator comprises generally a cold head 1 and a coolant gas introducing and discharging system 2. The cold head 1 includes a closed cylinder 11, a displacer 12 freely reciprocating in the cylinder 11, and a motor 13 for driving the displacer 11. The cylinder 11 includes a first large-diameter cylinder 14 and a second small-diameter cylinder 15 coaxially connected to the first cylinder 14. The border wall between the first 14 and the second cylinder 15 forms a first stage 16 as a cooling face and the front wall of the cylinder 15 forms a second stage 17 which is lower in temperature than the first stage 16. The displacer 12 includes a first displacer 18 reciprocating in the first cylinder 14 and a second displacer 19 reciprocating in the second cylinder 15. The first and second displacers 18 and 19 are connected to each other in the axial direction of the cylinder 11 by a connector 20. A fluid passage 21 is formed in the first displacer 18, extending in the axial direction of the displacer 18, and a cooling member 22 formed of copper meshes or the like is housed in the fluid passage 21. Similarly, a fluid passage 23 is formed in the second displacer 19, extending in the axial direction of the displacer 19, and a cooling member 24 formed of lead balls or the like is formed in the fluid passage 23. Seal systems 25 and 26 are located between the outer circumference of the first displacer 18 and the inner circumference of the first cylinder 14 and between the outer circumference of the second displacer 19 and the inner circumference of the second cylinder 15, respectively. The top of the first displacer 18 is connected to the rotating shaft of the motor 13 through a connector rod 31 and a Scotch yoke or crankshaft 32. When the shaft of the motor 13 is rotated, therefore, the displacer 12 reciprocates, as shown by an arrow in Fig. 1, synchronizing with the rotating shaft of the motor 13. An inlet 34 and an outlet 35 for introducing and discharging coolant gas extend from the upper portion of one side of the first cylinder 14 and they are connected to the coolant gas introducing and discharging system 2. The coolant gas introducing and discharging system 2 serves as a helium gas circulating system, comprising connecting the outlet 35 to the inlet 34 through a low-pressure valve 36, a compressor 37 and a high-pressure valve 38. Namely, this system 2 is intended to compress low-pressure (about 5 atm) helium to high-pressure (about 18 atm) one by the compressor 37 and send it into the cylinder 11. The low- and high-pressure valves 36 and 38 are opened and closed, as will be described later, in a relation to the reciprocation of the displacer 12. That portions in the refrigerator where cooling is effected or which act as cooling faces are the first and second stages 16 and 17, which are cooled or refrigerated to about 30 K and 10 K, respectively, when no thermal load is present. Therefore, a temperature gradient ranging from a normal temperature (300 K) to 30 K exists between the top and bottom of the first displacer 18 and a temperature gradient ranging from 30 K to 10 K exists between the top and bottom of the second displacer 19. These temperature gradients, however, are changed by thermal loads at the step stages and it usually ranges from 30 K to 80 K at the first stage 16 while it ranges from 10 K to 20 K at the second stage 17. When the motor 13 starts its rotation, the displacer 12 reciprocates between top and bottom dead centers. When the displacer 12 is at the bottom dead center, the high-pressure valve 38 is opened, allowing high-pressure helium gas to flow into the cold head 1. The displacer 12 then moves to the top dead center. As described above, the seal systems 25 and 26 are arranged between the outer circumference of the first displacer 18 and the inner circumference of the first cylinder 14 and between the outer circumference of the second displacer 19 and the inner circumference of the second cylinder 15, respectively. When the displacer 12 moves to the top dead center, therefore, high-pressure helium gas flows into a first stage expansion chamber 39 formed between the first 18 and the second displacer 19 and then into a second stage expansion chamber 40 formed between the second displacer 19 and the front wall of the second cylinder, passing through the fluid passage 21 in the first displacer 18 and the fluid passage 23 in the second displacer 19. While flowing in this manner, high-pressure helium gas is cooled or refrigerated by the cooling members 22 and 24, so that high-pressure helium gas flowing into the first stage expansion chamber 39 can be cooled to about 30 K and high-pressure helium gas flowing into the second stage expansion chamber 40 can be cooled to about 8 K. Here, the high-pressure valve 38 is closed and the low-pressure valve 36 is opened. When the low-pressure valve 36 is opened, high-pressure helium gas in the first stage expansion chamber 39 and the second stage expansion chamber 40 is expanded and cooling is effected. The first stage 16 and the second stage 17 are cooled by this cooling phenomenon. Then, the displacer 12 moves to the bottom dead center again and helium gas in the first stage expansion chamber 39 and the second stage expansion chamber 40 is removed as the movement of the displacer 12. The expanded helium gas is warmed by the cooling members 22 and 24 while passing through the fluid passages 21 and 22, and is an ordinary temperature and discharged. Thereafter, the above-mentioned cycle is repeated and the refrigerating operation is performed. This type of the refrigerator is used for cooling a superconducting magnet or an infrared sensor, or as a cooling source of a cryopump. However, the above-structured conventional cryogenic refrigerators had the following problems. Specifically, the cylindrical fluid passage 23 is formed in the second displacer 19 and the inside of the passage is filled with the ball or grain-like cooling member 24. Speed distribution in helium gas flowing through the passages which were filled with balls or grains was measured and it was found that velocity of flow was the lowest in the center of the flow of helium gas and that it became higher and higher as coming remoter from the center of the flow of helium gas outward in the radial direction thereof. This means that a larger amount of helium gas flows only into some area of the cooling member 24 and that the cooling member 24 must exchange heat with excessive helium gas at this area thereof when heat exchange is to be done between helium gas and the cooling member 24. This teaches us that the cooling member 24 is not efficiently used. Therefore, cooling efficiency (or heat exchanging efficiency achieved by a cooling means) is reduced at the area of the cooling member, thereby resulting in reducing refrigerating capacity at a certain temperature. US-A-4366676 describes a Malone-type final stage in a Stirling Cycle Cryogenic Refrigerator. The displacer of the final stage has an annular cross-section passage which is divided axially into two halves. The coolant flows down through one half channel, and up through the other half channel, with check valves being provided to ensure unidirectional flow. A static thermodynamic medium is housed in the central cylindrical chamber which is sealed. An object of the present invention is to provide a cryogenic refrigerator capable of causing coolant gas to uniformly flow through a cooling member to increase the refrigerating capacity of the refrigerator. According to a first aspect of the present invention, there is provided a cryogenic refrigerator comprising a closed cylinder provided with an inlet and an outlet for introducing and discharging a coolant gas into and out of the cylinder; a displacer slidably housed in the closed cylinder and housing a cooling member therein and having a passage through which the coolant gas flows; a means coaxially arranged in and along the passage of the displacer in which the cooling member is housed for dividing the passage into outer and inner ones; a means for reciprocating the displacer in the cylinder; and a means for repeating the process of introducing the high pressure coolant gas into the cylinder through the inlet and discharging it out the cylinder, synchronizing with the reciprocating displacer. According to a second aspect of the present invention, there is provided a cryogenic refrigerator comprising a closed cylinder provided with an inlet and an outlet for introducing and discharging a coolant gas into and out of the cylinder; a displacer slidably arranged in the closed cylinder and housing a particulate cooling member therein and having a passage through which the coolant gas flows; plural gas permeable diaphragms arranged in the passage in which the cooling member is housed and separated from one another in a direction perpendicular to the direction in which the passage is directed, the particulate cooling member being housed between the gas permeable diaphragms; a means for reciprocating the displacer in the cylinder; and a means for repeating the process of introducing the coolant gas into the cylinder through the inlet and discharging it out of the cylinder through the outlet in a relation to the reciprocating displacer. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a cross sectional view showing a conventional Gifford-McMahon type cryogenic refrigerator; Fig. 2 is a cross sectional view showing a second displacer of the refrigerator of Fig. 1; Fig. 3 is a graph showing the relationship between a rate of leakage and temperature of stages in a sealing mechanism of the refrigerator of Fig. 1; Figs. 4 to 6 are cross sectional views showing the sealing mechanism of the refrigerator of Fig. 1; Fig. 7 is a graph showing the relationship of a rate of leakage and temperature of stages between a second displacer and a cooling member of the refrigerator of Fig. 1; Fig. 8 is a cross sectional view showing a Gifford-McMahon type cryogenic refrigerator relating to one embodiment of the present invention; Fig. 9 is a cross sectional view showing a second displacer of the refrigerator of Fig. 8; Fig. 10 is a graph showing the comparison between the speed distribution in helium gas in the cooling member of the second displacer of the refrigerator of Fig. 1 and that of the second displacer of the refrigerator of Fig. 8; Fig. 11 is a graph showing the comparison between the cooling curve of the refrigerator of Fig. 1 and that of the refrigerator of Fig. 8; Fig. 12 is a cross sectional view showing a Gifford-McMahon type cryogenic refrigerator relating to a second embodiment of the present invention; Fig. 13 is a cross sectional view showing a second displacer of the refrigerator of Fig. 12; Fig. 14 is a graph showing the comparison between the speed distribution in helium gas in the cooling member of the second displacer of the refrigerator of Fig. 1 and that of the second displacer of the refrigerator of Fig. 12; Fig. 15 is a cross sectional view showing a Gifford-McMahon type cryogenic refrigerator relating to a third embodiment of the present invention; Fig. 16 is a cross sectional view showing a second displacer of the refrigerator of Fig. 15; Fig. 17 is a graph showing the comparison between the speed distribution in helium gas in the cooling member of the second displacer of the refrigerator of Fig. 1 and that of the second displacer of the refrigerator of Fig. 15; Some preferred embodiments of the present invention will be described in detail. Fig. 8 is a sectional view showing an example of the Gifford-McMahon type refrigerator, which is same in arrangement as the one shown in Fig. 1 except a fluid path or passage 123. The refrigerator includes generally a cold head 101 and a coolant gas introducing and discharging system 102. The cold head 101 comprises a closed cylinder 111, a displacer 112 housed in the cylinder 111 and freely reciprocating therein, and a motor 113 for driving the displacer 112 to reciprocate in the cylinder 111. The cylinder 111 includes a first large-diameter cylinder 114 and a second small-diameter cylinder 115 coaxially connected to the cylinder 114. The border wall between the first cylinder 114 and the second cylinder 115 forms a first stage 116 which serves as a cooling face, and the front wall of the cylinder 115 forms a second stage 117 which is lower in temperature than the first stage 116. The displacer 112 includes a first displacer 118 reciprocating in the first cylinder 114 and a second displacer 119 reciprocating in the second cylinder 115. The first and second displacers 118 and 119 are connected to each other by a connector member 120 in the axial direction of the cylinder 112. A fluid passage 121 is formed in the first displacer 118, extending in the axial direction of the displacer 118, and a cooling member 122 made by copper meshes or the like is contained in the fluid passage 121. Similarly, a fluid passage 123 is also formed in the second displacer 119, extending in the axial direction of the displacer 119, and a cooling member 124 made by copper balls or the like is contained in the fluid passage 123. Seal systems 125 and 126 are located between the outer circumference of the first displacer 118 and the inner circumference of the first cylinder 114 and between the outer circumference of the second displacer 119 and the inner circumference of the second cylinder 115, respectively. The top of the first displacer 118 is connected to the rotating shaft of the motor 113 through a connector rod 131 and a Scotch yoke or crankshaft 132. When the shaft of the motor 113 is rotated, therefore, the displacer 112 is reciprocated as shown by an arrow in Fig. 8, synchronizing with the rotating shaft of the motor 113. An inlet 134 and an outlet 135 for coolant gas extend outwards from the upper portion of one side of the first cylinder 114 and they are connected to the coolant gas introducing and discharging system 102. This system 102 serves to circulate helium gas flowing through the cylinder 111 and comprises connecting the outlet 135 to the inlet 134 through a low-pressure valve 136, a compressor 137 and a high-pressure valve 138. The system 102 also serves to compress low pressure helium gas (about 5 atm) to high pressure one (about 18 atm) through the compressor 137 and send it into the cylinder 111. The low- and high-pressure valves 136 and 138 are opened and closed in a relation to the reciprocating displacer 112. As shown in Fig. 9, a pipe 142 is coaxially housed in the fluid passage 123 and allows helium gas to flow inside and outside the pipe 142. A fluid passage 143 inside the pipe 142 is filled with a cooling member 145 shaped like balls each having a diameter of 0.4 mm and another fluid passage 144 outside the pipe 142 is filled with a cooling member 146 shaped like balls each having a diameter of 0.2 mm. The passage of helium gas is divided into two in the same direction as helium gas flows, and the large-diameter cooling balls 145 are housed in the inner fluid passage 143. This reduces the pressure loss of helium gas flowing through the inner fluid passage 143 and the amount of helium gas flowing through the passage 143 is increased accordingly. The partial flow of helium gas can be thus reduced to a greater extent. This enables the cooling efficiencies of the cooling balls 145 and 146 to be increased so as to enhance the refrigerating capacity of the refrigerator. Fig. 10 shows results obtained by measuring the flow speed distributions of helium gas flowing through the cooling members in the fluid passages shown in Figs. 2 and 9. These results were obtained under normal temperature and with the refrigerators kept static, providing that the outer diameters of the fluid passages, the amounts of the cooling members contained in the fluid passages and the materials by which the cooling members are made are same. These conditions are different from those (cryogenic temperature and reciprocating motion) under which the refrigerators are practically operated, but it is understood that the flow speed distribution of helium gas flowing through the cooling member in the fluid passage shown in Fig. 9 is more uniform. It is supposed that this trend can be kept under the practical conditions. Fig. 11 shows refrigerating curves achieved by the conventional cryogenic refrigerator in which the fluid passage 23 shown in Fig. 2 is incorporated and by the cryogenic refrigerator of the present invention in which the fluid passage 123 shown in Fig. 9 is incorporated. The horizontal axis of the coordinate shown in Fig. 11 represents temperatures (K) of the second stage 117 and the vertical axis thereof heat loads (W) added to the second stage 117. As apparent from Fig. 11, refrigerating capacity under same temperature is higher in the case of the cryogenic refrigerator according to the present invention. It is therefore understood that refrigerating capacity can be increased when the fluid passage 123 which has the above-described arrangement is employed. Although the fluid passage in this example is divided into two concentric ones, it may be divided into three or more ones. The diameter of the ball is not limited to 0.4 mm or 0.2 mm. Figs. 12 and 13 show a second example of the cryogenic refrigerator according to the present invention, in which the pipe 142 is coaxially housed in the fluid passage 141, the passage of helium gas is divided to flow inside and outside the pipe 142, and a cooling member 124 contained in the inner and outer passages 143 and 144 is shaped like balls each having same size. The passage of helium gas is divided into two in same direction as helium gas flows, so that the partial flow of helium gas can be reduced to a greater extent, as compared with that in the conventional case. Therefore, cooling efficiency achieved by the cooling member 124 can be increased to thereby enhance the refrigerating capacity of the refrigerator. Fig. 14 shows results obtained by measuring the flow speed distributions of helium gas flowing through the cooling members contained in the fluid passages shown in Figs. 2 and 13. These results were obtained under normal temperature and with the refrigerators kept static, providing that the outer diameters of the fluid passages, the amounts, shapes and sizes of the cooling members contained in the fluid passages, and the materials by which the cooling members are made are same. These conditions are different from those (cryogenic temperature and reciprocating motion) under which the refrigerators are practically operated but it is understood that the flow speed distribution of helium gas flowing through the cooling member in the fluid passage shown in Fig. 13 is more uniform. It is supposed that this trend can be kept under the practical conditions. Although the fluid passage in this example is divided into two concentric ones, it may be divided into three or more ones. They may be neither concentric nor cylindrical. Fig. 15 shows a third example of the cryogenic refrigerator according to the present invention. This third example is different from the first example in the arrangement of a fluid passage 141 which is formed in the second displacer 119 and in which the cooling member 124 is contained. As shown in Fig. 16, the cooling member 124 shaped like balls, and sheets of meshes 147 are contained in the fluid passage 141 in such a way that they are alternately piled in the fluid passage 141 in direction perpendicular to the flow of helium gas. When the fluid passage 141 is arranged in this manner, helium gas flowing through the passage 141 can be made uniform by the sheets of meshes. The partial flow of helium gas can be thus reduced to a greater extent, as compared with that in the conventional case. Therefore, cooling efficiency achieved by the cooling member 124 can be increased so as to enhance the refrigerating capacity of the refrigerator. Fig. 17 shows results obtained by measuring the flow speed distributions of helium gas flowing through the cooling members in the fluid passages shown in Figs. 2 and 16. These results were measured under normal temperature and with the refrigerators kept static, providing that the outer diameters of the fluid passages, the amounts, shapes and sizes of the cooling members and the materials by which the cooling members are made are same. These conditions are different from those (cryogenic temperature and reciprocating motion) under which the refrigerators are practically operated, but it is understood that the flow speed distribution of helium gas flowing through the fluid passage shown in Fig. 16 is more uniform. It is supposed that this trend can be kept under the practical conditions. Glass wool or the like may be used as spacers instead of the sheets of meshes. Although the fluid passage in the second displacer has been arranged as shown in Figs. 9, 13 and 16 in the case of the above-described three examples, the fluid passage in the first displacer may be arranged as shown in Figs. 9, 13 and 16. These arrangements of the fluid passage can be applied to the cryogenic refrigerator which includes third and fourth displaces. The fluid passage in which the cooling member is housed may be arranged as shown in Figs. 9, 13 and 16 even in the case of those cryogenic refrigerators in which the displacers and the cooling accumulator are not combined as a unit. Although description has been made about those refrigerators in which the displacer and the cooling accumulator are combined with each other as a unit, the present invention can be applied to the other refrigerators in which the displacer and the cooling accumulator are not combined as a unit. Further, description has been made about the refrigerator of the Gifford-McMahon type which is typical of the cryogenic refrigerators, but the present invention can be applied to the other cryogenic refrigerators of the improved Solvay, Stirling and cycle types.

Claims: A cryogenic refrigerator comprising: a closed cylinder (111) provided with an inlet (134) and an outlet (135) for introducing and discharging a coolant gas into and out of the cylinder (111); a displacer (119) slidably housed in the closed cylinder (111) and housing a cooling member (124) therein and having a passage (123) through which the coolant gas flows; a means (142) coaxially arranged in the passage (123) of the displacer (119) in which the cooling member (124) is housed, for dividing the passage (123) into outer and inner ones; a means for reciprocating the displacer (119) in the cylinder (111); and a means for repeating the process of introducing the high pressure coolant gas into the cylinder (111) through the inlet and discharging it out of the cylinder (111), synchronizing with the reciprocating displacer (119). The cryogenic refrigerator according to claim 1, characterised in that said passage-dividing means (142) is a cylindrical member (142). The cryogenic refrigerator according to claim 1, characterised in that said cooling member (124) is particulate. The cryogenic refrigerator according to claim 3, characterised in that grains of the cooling member (124) housed in the outer passage (144) of the displacer (119) each have a size smaller than those of the cooling member (124) housed in the inner passage (143). A cryogenic refrigerator comprising: a closed cylinder (111) provided with an inlet (134) and an outlet (135) for introducing and discharging a coolant gas into and out of the cylinder (111); a displacer (119) slidably arranged in the closed cylinder (111) and housing a particulate cooling member (124) therein and having a passage (123) through which the coolant gas flows; plural gas permeable diaphragms (147) arranged in the passage (141) in which the cooling member (124) is housed and separated from one another in a direction perpendicular to the direction in which the passage (141) is directed, the particulate cooling member being housed between the gas permeable diaphragms; a means for reciprocating the displacer (119) in the cylinder (111); and a means for repeating the process of introducing the coolant gas into the cylinder (111) through the inlet and discharging it out of the cylinder (111) through the outlet (135) in a relation to the reciprocating displacer. The cryogenic refrigerator according to claim 5, characterised in that said gas permeable diaphragms (147) are mesh members.

Inventors: KURIYAMA, TORU, NAKAGOME, HIDEKI, TOKAI, YOICHI, KURIYAMA, TORU, C/O INTELLECTUAL PROPERTY DIVISION, NAKAGOME, HIDEKI, C/O INTELLECTUAL PROPERTY DIV., TOKAI, YOICHI, C/O INTELLECTUAL PROPERTY DIV.
Applicants: KABUSHIKI KAISHA TOSHIBA
Application year: 1990
Main classification: F25B9
IPC codes: F25B9, F02G1