a) Field of the Invention
The present invention relates to a refrigerator, particularly a refrigerator using gas coolant such as helium and having a regenerator accommodating regenerating material.
b) Description of the Related Art
As a refrigerator using gas coolant such as helium and having a regenerator accommodating regenerating material, there are known a Gifford-McMahon (GM) cycle refrigerator, a (reverse) Stirling cycle refrigerator, and the like. A refrigerator will be described taking a Gifford-McMahon (GM) refrigerator as an example which is intended not to be limitative. A GM refrigerator cools helium gas supplied from a helium gas compressor via a gas passage controlled by a valve, by expanding the gas in an expansion space. An extremely low temperature is generally obtained by using a plurality of cooling stages. A Joule-Thomson (JT) valve mechanism may be used with the GM refrigerator.
A cryopump is used for obtaining clean vacuum in a sputtering system for manufacturing semiconductor devices. Recently, a GM refrigerator has been used as a cryopump type refrigerator. Not only as a cryopump, a GM refrigerator can be used for various purposes.
FIG. 2 is a schematic diagram showing an example of the structure of a GM refrigerator. This structure is made of two stages suitable for obtaining an extremely low temperature of about several K. to 20 K.
A helium gas compressor 10 compresses helium gas to about 20 Kgf/cm.sup.2 and supplies high pressure helium gas. This high pressure helium gas is supplied yo the inside of a first stage cylinder 11 via an intake valve V1 and a gas passage 16. The first stage cylinder 11 is coupled to a second stage cylinder 12.
First and second displacers 13 and 14 integrally formed are housed in the first and second stage cylinders 11 and 12. A shaft S extends upward from the first displacer 13, and is coupled to a crank mechanism 15 which is coupled to a driver motor M.
The first and second displacers 13 and 14 each have a hollow space for accommodating regenerating material. The first and second displacers 13 arid 14 are formed with gas passages 23 and 24 which communicate with the outside spaces.
Expansion spaces 21 and 22 are defined by, and formed between, the first displacer 13 and the first stage cylinder 11, and between the second displacer 14 and the second cylinder 12.
The first and second stage cylinders 11 and 12 are made of, for example, stainless steel (e.g., SUS 304) having a sufficient strength, a low heat conductivity, arid a sufficient shielding ability of helium gas.
The first and second displacers 13 and 14 are made of, for example, phenol (bakelite) containing cloth having a small specific gravity, a sufficient abrasion proof, a relatively high strength, and a low heat conductivity.
The high pressure helium gas supplied from the helium gas compressor 10 via the intake valve V1 is supplied to the inside of the first stage cylinder 11 via the gas passage 16, and to the first stage expansion space 21 via a gas passage 23a, a first stage regenerating material 17 such as a copper wire screen, and a gas passage 23b.
The compressed helium gas in the first stage expansion space 21 is supplied to the second stage expansion space 22 via a gas passage 24a, a second stage regenerating material 18 such as lead balls, and a gas passage 24b. The gas passages 23 and 24 are functionally shown in FIG. 2, and the real structure thereof is different.
When the intake valve V1 is closed and an exhaust valve V2 is opened, the high pressure helium gas in the second and first state cylinders 12 and 11 is recovered back to the helium gas compressor 10 via the flow route opposite to the intake passages, and via the gas passage 16 and the exhaust valve V2.
In operation of the GM refrigerator, the driver motor M rotates so that the first and second displacers 13 and 14 are reciprocally moved up and down as indicated by a double-headed arrow in FIG. 2. While the first and second displacers are driven downward, the intake valve V1 is opened so that the high pressure helium gas is supplied to the inside of the first and second cylinders 11 and 12.
While the first and second displacers 13 and 14 are driven upward by the driver motor M, the intake valve V1 is closed and the exhaust valve V2 is opened so that the helium gas is recovered into the helium gas compressor 10 and the expansion spaces in the first and second stage cylinders 11 and 12 lower their pressures.
At this time, the helium gas in the expansion spaces 21 and 22 are expanded and cooled. The cooled helium gas cools the regenerating materials 18 and 17.
At the next intake cycle, the supplied high pressure helium gas is cooled while it passes through the regenerating materials 17 and 18. The cooled helium gas is expanded and cooled further. At the steady state, the expansion space 21 in the first stage cylinder 11 is maintained at a temperature of, for example, 40 K. to 70 K., and the expansion space 22 in the second stage cylinder 12 is maintained at a temperature of several K. to 20 K.
A first stage heat station 19 surrounds the lower portion of the first stage cylinder 11 and thermally couples thereto, whereas a second stage heat station 20 surrounds the lower portion of the second stage cylinder 12 and thermally couples thereto.
The first heat station 19 is coupled, for example, to the panel of a cryopump to adsorb gas molecules. The second heat station 20 is coupled, for example, to an adsorption panel accommodating adsorbent such as activated carbon, to adsorb residual gas molecules. A cryopump having such a structure is used when a sputtering system or the like requires generation of a clean vacuum.
In the GM refrigerator constructed as above, it is designed to supply the gas in the upper portion of a cylinder to the lower portion of the cylinder. In order to prevent helium gas from passing through a gap between a displacer and a cylinder, a seal mechanism is provided between a displacer and a cylinder.
Although not shown in FIG. 2, a seal ring is inserted between the first stage displacer 13 and first stage cylinder 11 to provide the first stage cylinder 11 with a seal mechanism. Similarly, a seal ring is inserted between the second stage displacer 14 and second stage cylinder 12 to provide the second stage cylinder 12 with a seal mechanism.
FIGS. 9A and 9B show an example of the, second stage displacer. As shown in FIG. 9A, a tubular member 80 of a circular shape in section is made of phenol resin containing cloth, and formed with a groove 81 at its outer periphery in a circumferential direction. A seal ring is inserted in the groove 81. Openings 82 forming a gas passage are formed in the lower wall of the tubular member 80.
A lid 83 made of phenol resin containing cloth is inserted in the tubular member 80 at, its bottom, and bonded thereto. The lid 83 is a blank lid, and hermetically seals the bottom opening of the tubular member 80. The lid 83 may be made of material other than the phenol resin containing cloth. It is preferable to use material having a small specific gravity in view of easy motion of the displacer.
The upper surface of the lid 83 is slightly lower than the gas passage 82 to dispose a wire screen 84 on the upper surface of the lid 83. The height of the wire screen 84 is flush with the openings 82. The outer diameter of the tubular member 80 at the position lower than the openings 82 is slightly smaller than the outer diameter at the position higher than the openings 82. Therefore, a gap is formed between the outer circumference of the tubular member 80 and the inner circumference of the cylinder. This gap is a gas passage communicating the inside of the tubular member 80 with the expansion space 22 shown in FIG. 2.
A felt plug 85 is disposed on the wire screen 84, and the regenerating material 18 such as lead balls is filled in the inner space of the tubular member 80. Another felt plug 86 is disposed on the regenerating material 18, and a punched metal plate 87 is disposed on the felt plug 86.
A coupling mechanism 88 for coupling the tubular member 80 to the first stage displacer is inserted into the tubular member 80 and mounted above the punched metal 87. The coupling mechanism 88 is made of Al or Al alloy.
FIG. 9B shows the structure of a seal ring disposed between the tubular member 80 and cylinder 12. An expander ring 89 is inserted into the groove 81 of the tubular member 80 and a piston ring 90 is inserted into the groove 81 over the expander ring 89.
FIGS. 10A and 10B show an example of the structure of the first stage displacer. As shown in FIG. 10A, a tubular member 100 of a circular shape in section made of phenol resin containing cloth has an upper lid. An opening 101 forming a gas passage is formed in the upper lid of the tubular member 100. A circumferential step 102 for accommodating a seal ring is formed at the periphery of the upper plane of the tubular member 100.
As shown in FIG. 10B, an O ring 103 and a slipper seal 104 are filled in the circumferential step 102. The O ring 103 and slipper seal 104 are fixed by a flange 105 mounted on the upper plane of the tubular member 100 by a bolt. The outer periphery of the slipper seal 104 slightly projects from the outer periphery of the tubular member 100, and contacts the inner surface of the first stage cylinder 11.
As shown in FIG. 10A, a drive shaft S for moving the tubular member 100 up and down in the direction indicated by a double-headed arrow is formed on the upper plane of the flange 105.
A wire screen 106 is provided contacting the top of the inner space. Regenerating material 17 such as a copper wire screen is filled in the inner space the tubular member 100 under the wire screen 106. Another wire screen 107 is disposed under the regenerating material 17. Openings 108 forming a gas passage are formed in the side wall of the tubular member 100 at the height of the wire screen 107.
A lid 109 made of phenol, resin containing cloth is inserted into the tubular member 100 under the wire screen 107, and bonded to the tubular member 100. The lid 109 is a blank lid, and hermetically seals the bottom opening of the tubular member 100. A recess is formed on the bottom surface of the lid 109 to mount the coupling mechanism 88 shown in FIG. 9A.
The outer diameter of the tubular member 100 at the position lower than the openings 108 is set slightly smaller than the inner diameter of the cylinder. Therefore, a gap is formed between the inner circumference of the first stage cylinder 11 and the outer circumference of the tubular member 100 at the position lower than the openings 108. This gap forms a gas passage communicating the inside of the tubular member 100 with the expansion space 21 shown in FIG. 2.
In the refrigerator with a regenerator described above, a cooled temperature becomes higher than a designed temperature in some cases, or a temperature change becomes large in other cases.
A predetermined cooling performance is obtained in some cases by disassembling the refrigerator arid replacing the seal ring (e.g., a combination of the expander ring 89 and piston ring 90 shown in FIG. 9B disposed between the second stage displacer 14 and second stage cylinder 12 of the structure shown in FIG. 2) between the displacer and cylinder disposed at a low temperature area by a new seal ring. It can be presumed from such experiences that a cooling performance is greatly influenced by a seal mechanism between the displacer and cylinder.