Refrigeration – Gas compression – heat regeneration and expansion – e.g.,...
Reexamination Certificate
2000-02-15
2001-07-10
Capossela, Ronald (Department: 3744)
Refrigeration
Gas compression, heat regeneration and expansion, e.g.,...
C060S520000
Reexamination Certificate
active
06256997
ABSTRACT:
BACKGROUND OF THE INVENTION:
This application relates to vibration reduction in a GM displacer/regenerator, and more particularly, relates to vibration reduction in a pneumatically-driven GM displacer/regenerator. Cryogenic refrigerators of the GM type frequently include a multi-stage displacer/regenerator as a key element in expanding high pressure gaseous refrigerant to achieve extremely low temperatures.
There is an abundance of prior art that describes various pneumatically-driven and mechanically-driven displacers and their operations in cryogenic systems and in achieving cryogenic temperatures. For example, basic principals of operation are described in the original Gifford-McMahon (GM) U.S. Pat. No. 2,906,101, issued Sep. 29, 1959. In that patent, which is incorporated herein by reference, the displacer is reciprocatingly driven in a cylinder by a conventional crank mechanism. Thus, low temperature refrigeration is effected with auxiliary equipment, such as connecting rods, crank shafts, or the like, to cycle the displacer. These mechanical parts produce mechanical vibrations that in many instances are undesirable and shorten the time between necessary maintenance or repairs.
U.S. Pat. No. 3,620,029, issued Nov. 16, 1971 by the present inventor, and incorporated herein by reference, replaces mechanical drive of the displacer with a pneumatic drive. The mechanical problems associated with the crank type drive, or cam type drive, as in other designs, are substantially eliminated and the operating life of the systems has been enhanced by such pneumatic drives. However, other mechanical problems, noise and vibration producing problems arise through the use of the pneumatically-driven displacer. These problems have roots also in the thermodynamics of the refrigeration cycle.
In a mechanically-driven or pneumatically-driven displacer/expander, the displacer includes a piston that reciprocates within a cylinder. When the piston moves to what is known as the “bottom” of the cylinder, it is most desirable thermodynamically that the clearance volume be zero, or as near to that volume as possible. Thus, unless careful control is provided for the motion of the displacer, collisions can occur between the displacer piston and the closed end of the cylinder. These collisions create noise and vibration. Also, when the displacer moves in the opposite direction, unless careful control is provided, there can be an impact when the displacer is at the “top” of its stroke. Further noise and vibration are produced. (The use of the words “top”, “bottom”, “up”, “down”, and the like does not necessarily indicate a physical orientation. No orientation is excluded from use.)
The original GM U.S. Pat. No. 2,906,101, describes a rectangular pressure-volume (P-V) diagram but actually it is best from a thermodynamic standpoint to close the inlet valve before the displacer reaches the top. This causes the gas pressure in the expander to drop before the displacer reaches the top. Similarly it is best to close the exhaust valve before the displacer reaches the bottom. This causes an increase in pressure before the displacer reaches the bottom. In a pneumatically driven expander this causes the displacer to decelerate before it reaches the end of the stroke.
Many vibration isolation systems have been developed to improve cycle efficiency and to prevent collisions between the displacer and its surroundings, or where collisions occur, to reduce vibrations caused by the impact. These include both electrical and mechanical concepts.
For example, repelling magnets have been used to constrain the motion of the displacer at the top and bottom ends of its motion. Elastomer vibration absorbers have been used with some success. However, these devices are only effective at the warm end of the displacer motion, but are not able to operate effectively at the cryogenic temperatures. Therefore, impact forces at the cold end have been absorbed, for example, using delrin plastic pads, which can take the low temperatures. However, there is still a considerable impact and vibration problem when using delrin absorbers. Such impacts and vibrations have been known to affect the quality and resolution of images obtained in MRI apparatuses that use cryogenically cooled magnets.
What is needed is an improved expander that has the advantages of a simplified pneumatic drive, long operating life, low vibration in operation and an efficient thermodynamic cycle.
SUMMARY OF THE INVENTION
In accordance with the invention, a displacer in a GM expander has a pneumatic drive that reduces the speed of the displacer before it hits at the top and bottom of the stroke. This velocity control is accomplished by closing the inlet and exhaust valves after the displacer has traveled about two-thirds of its stroke. Thereby driving pressure difference is reduced and the displacer slows down before hitting the top (warm end) and bottom (cold end) of the cylinder.
Historically, bumpers machined from delrin have been installed at the top and bottom of the cylinder to absorb some of the impact energy of the reciprocating displacer. Within the past few years, manufacturers have started to use “O” rings or an equivalent elastomer material to absorb the impact energy at the top end where the temperature is near room ambient. Unfortunately, elastomer materials become every rigid at the cold end temperatures so that machined bumpers of delrin continue to be used at the cold end.
In the present invention, an elastomer “O” ring, or other elastomer shape is used at the warm end to absorb the impact energy when the displacer reaches the bottom (cold end) of the stroke, before it hits the cylinder end cap. Also, when the displacer reaches the top of its stroke, before the displacer hits the internal mechanisms of the expander, another elastomer “O” ring absorbs the kinetic energy of the displacer. It has been reported that the resultant reduction in vibration by using two resilient “O”rings, reduces the electrical noise imparted to an MRI signal by more than fifty percent.
Accordingly, it is an object of the present invention to provide an improved expander/displacer unit that is a low producer of mechanical vibration and noise.
Yet another object of the invention is to provide an improved expander/displacer that is pneumatically driven and thereby has extended operating life and simplified construction.
Yet another object of the invention is to provide an improved expander/displacer that provides a refrigeration cycle of relatively high efficiency.
Still other objects and advantages of the invention will be apparent from the specification.
The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts, which will be exemplified in the constructions hereinafter set forth, and the scope of the invention will be indicated in the claims.
REFERENCES:
patent: 2906101 (1959-09-01), McMahon
patent: 3119237 (1964-01-01), Gifford
patent: 3620029 (1971-11-01), Longsworth
patent: 4389850 (1983-06-01), Sarcia
patent: 4490974 (1985-01-01), Colgate
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patent: 5735128 (1998-04-01), Zhang et al.
patent: 5737925 (1998-04-01), Sekiya et al.
Capossela Ronald
Helfgott & Karas P.C.
Intermagnetics General Corporation
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