Refrigeration – Gas compression – heat regeneration and expansion – e.g.,...
Reexamination Certificate
2002-01-08
2003-10-07
Capossela, Ronald (Department: 3744)
Refrigeration
Gas compression, heat regeneration and expansion, e.g.,...
C060S520000
Reexamination Certificate
active
06629418
ABSTRACT:
BACKGROUND OF THE INVENTION
The pulse tube refrigerator is a cryocooler, similar to Stirling and Gifford-McMahon refrigerators, that derives cooling from the compression and expansion of gas. However, unlike the Stirling and Gifford-McMahon (G-M) systems, in which the gas expansion work is transferred out of the expansion space by a solid expansion piston or displacer, pulse tube refrigerators have no moving parts in their cold end, but rather an oscillating gas column within the pulse tube (called a gas piston) that functions as a compressible displacer. The elimination of moving parts in the cold end of pulse tube refrigerators allows a significant reduction of vibration, as well as greater reliability and lifetime, and is thus potentially very useful in many applications, both military and commercial.
Cryogenic temperatures such as those achievable using two stage pulse tube refrigerators, are highly desirable in such commercial applications as cooling the superconducting magnets used in magnetic resonance imiaging (MRI) systems to 4 K or for cooling cryopumps, which are often used to purge gases from semiconductor fabrication vacuum chambers, to 10 K.
Smaller cryocoolers are desirable in the most common applications to which pulse tube refrigerators lend themselves, such as semiconductor fabrication chambers, where continual efforts are made to reduce component size. Conventional two-stage pulse tube refrigerators, while capable of achieving two-stage refrigeration (e.g. 4 K and 10 K), require a relatively large buffer volume(s) for the two stages and are potentially less compact than Stirling or G-M refrigerators, in which the two stages require no buffer volume. Thus, any size reduction in pulse tube refrigerators is highly desirable, especially in two-stage designs that utilize one or more buffer volumes. What is needed is a way to design a more compact two-stage pulse tube refrigerator.
Conventional cryocoolers, such as Stirling and G-M refrigerators, include a moving displacer, which necessitates the inclusion of elements such as seals in the expansion space; this presents reliability problems and necessitates maintenance of such systems at regular intervals. The typical interval of 12,000 to 15,000 hours between maintenance is not a long time considering that many applications require the cryocoolers to operate indefinitely. It is desirable in such applications to strive for maintenance-free cryocooler designs. What is needed is a way to increase the maintenance interval and the reliability of a cryogenic refrigerator.
The exclusion of moving parts in the cold end of pulse tube refrigerators results in a great reduction in the level of vibration when compared to systems that are cooled by more conventional refrigerators, such as G-M and Stirling systems. The quality and uniformity of the chips produced in semiconductor fabrication vacuum chambers, in which pulse tube refrigerators may be used in cryopumps to “freeze out” or purge gases, may be greatly affected by the vibration of components within the chamber, which is likely to stir up dust and other particulate matter. Likewise, pulse tube refrigerators lend themselves nicely to MRI applications, in which a large superconducting magnet must remain cooled to as low as 4 K. Even the slightest vibration of any metal component in the magnetic field produced by the superconducting magnet results in interference and degrades the quality of the produced image. What is needed is a way to minimize vibration in applications requiring two-stage cryogenic refrigeration.
Conventional pulse tubes with single or double orifice control use large buffer volumes to get good efficiency, or “four valve” control to eliminate or minimize the size of the buffer volume but at the expense of efficiency. What is needed is away to design a compact pulse tube with good efficiency.
Gao et al., U.S. Pat. No. 5,974,807, entitled “Pulse tube refrigerator,” describes a pulse tube refrigerator capable of generating cryogenic temperatures of below 10 K that includes first and second refrigeration stages. Each stage includes a pulse tube and an associated regenerator provided at the low temperature side of the pulse tube. A pressure fluctuation generator having a compressor and a first to a fourth valve is provided at the high temperature side of each regenerator. The high temperature sides of each pulse tube are connected by a continuous channel, while the high temperature sides of each pulse tube and the high temperature sides of each regenerator are connected by a by-pass channel. A magnetic material having a rare-earth element and a transition metal is used as a regenerative material for the regenerator.
When pressure fluctuation is generated in each pulse tube at the phase difference angle of 180 degrees, respectively, a working gas is transferred between the high temperature sides of each pulse tube by an active valve, thereby optimizing the phase angle between the pressure fluctuation in each pulse tube and the displacement of the working gas. The flow amount of the operating gas sent to each pulse tube from the regenerator is limited using a fixed orifice in the by-pass channel.
This patent describes active and passive inter-phase control with fixed restrictors for the second orifices. No buffer volume is included. This is possible because there two identical two-stage pulse tubes that are interconnected so the volumes and temperatures match.
Matsui et al., U.S. Pat. No. 5,845,498, entitled “Pulse tube refrigerator,” describes a pulse tube refrigerator where the cryostat includes regenerators and pulse tubes. Each regenerator has a cold stage at an upper end thereof. Each pulse tube has a low-temperature end portion at a lower end thereof and a high-temperature end portion thereof, the low-temperature end portion being located lower than the cold stage. The cold stage and the low-temperature end portion are connected to each other through a line whose cubic volume is substantially negligible in comparison with that of the pulse tube. Since the pulse tube has working gas of relatively high density in an upper portion thereof and working gas of relatively low density in a lower portion thereof, there is no convection of working gas induced by the gravity.
This patent exemplifies the problems of applying prior art concepts to creating a configuration that is preferred for cooling cryopumps, namely having the valve mechanism below the cryopump housing. The hot end of a pulse tube has to be above the cold end in order to avoid serious convection losses in the pulse tube. This patent describes several different conventional control mechanisms for single warm regenerator designs (no inter-phase control).
FIG. 2
illustrates the problems of having large dead volumes in connect tubes
36
,
37
, and
38
, which are needed to keep the warm end of the pulse tube above the cold end with the valve mechanism below the pulse tube. The conventional construction shown as prior art in
FIG. 1
is suitable for cooling a cryopump if there is room for the valve mechanism above the cryopump housing.
Matsui et al., U.S. Pat. No. 5,711,156, entitled “Multistage type pulse tube refrigerator,” describes a multistage G-M type pulse tube refrigerator comprising a regenerator-side pressure oscillation generator, first regenerator connected to the regenerator-side pressure oscillation generator, first cold head connected to the low temperature side of the first regenerator, a first pulse tube having one end connected to the first cold head and the other end connected by way of a first flow regulating mechanism to a first pulse tube-side phase shifter, second regenerator having one end connected to the first cold head and the other end connected to the second cold head, a second pulse having one end connected to the second cold head and the other end connected to second pulse tube-side phase shifter by way of second flow regulating mechanism, in which the first pulse tube-side phase shifter and the second pulse tube-side phase shifter are controlled independently of each
Gao Jin Lin
Longsworth Ralph C.
Capossela Ronald
Katten Muchin Zavis & Rosenman
SHI-APD Cryogenics, Inc.
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