Bumper retainer for cryogenic expander systems

Abstract

A bumper retainer assembly prevents the reciprocating displacer in a pneumatically driven cryogenic expander from hitting the cold and warm ends of a cylinder, while providing means of reducing the void volume relative to previous designs. A collar at the top of the displacer reciprocates in an annular space in the cylinder head. An annular recessed length, between lips at top and bottom, is located either on the collar or in the cylinder head. One or two bumper “O” rings are retained by ring shaped discs to form an assembly that is held in a groove opposite the annular recessed length. The lips at the top and bottom ends of the annular recessed length engage the bumper retainer at the ends of the stroke to absorb the inertia of the displacer, and prevent it from hitting the warm and cold ends of the cylinder assembly.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Patent Application No. 63 / 757,453,filed on Feb. 12, 2025, the entire contents of which are incorporated herein by reference.BACKGROUND1. Field of the Invention

[0002] This invention relates to reducing the noise and vibration of a high capacity expander having a pneumatically driven reciprocating displacer or piston producing refrigeration at cryogenic temperatures.2. Description of Related Art

[0003] Most cryogenic refrigerators that are used to cool cryopumps, superconducting MRI magnets, and laboratory research instruments use Gifford-McMahon (GM) type refrigerators. These typically use air conditioning compressors that have been modified to compress helium and draw less than 12 kW of input power. The expanders have reciprocating displacers that are either mechanically or pneumatically driven. The mechanical drive is relatively quiet because it provides a nearly sinusoidal motion that does not cause the displacer to hit the top or bottom at the end of the stroke. The pneumatic drives are simpler but can produce significant noise if the displacer hits the top or bottom of the cylinder at the end of the stroke. The same is true for pistons in expanders that operate on the Brayton cycle.

[0004] U.S. Pat. No. 3,045,436 (“the '436 patent”), by W. E. Gifford and H. O. McMahon describes the basic GM cycle. This refrigerator system consists of a compressor that supplies gas at a high pressure to an expander which admits the gas through a warm inlet valve to the warm end of a regenerator heat exchanger, through the regenerator, and then into an expansion space at the cold end of a displacer from whence it returns back through the regenerator and a warm outlet valve to the compressor at a low pressure. The '436 patent shows the regenerator external to the cylinder with the displacer, and a second pair of valves that cycles gas to the warm end of the displacer out of phase with the gas flow to the regenerator. U.S. Pat. No. 3,119,237 (“the '237 patent”), by W. E. Gifford shows an improvement of the concept of the '436 patent in the form of a drive stem at the warm end of a displacer which reduces the amount of gas used to drive the displacer up and down. The expander configuration and valve cycling are shown in FIGS. 2-9 in the '237 patent.

[0005] The typical GM type expander being built today has the regenerator located inside the displacer. The displacer / regenerator becomes a displacer that moves from the cold end to the warm end with the gas at high pressure, then from the warm end to the cold end with the gas at low pressure. Since the pressure above and below the displacer / regenerator is nearly the same, the force required to cause the displacer / regenerator to reciprocate is small and can be provided by either a mechanical or pneumatic mechanism. In the descriptions that follow, the term displacer is used for the displacer / regenerator in the GM type expander in which the pressure difference between the warm and cold ends is small, and piston is used when the pressure difference may be large.

[0006] A pneumatically driven expander operating on the Brayton cycle is described in U.S. Pat. No. 9,080,794 by Longsworth. The Brayton cycle differs from the GM cycle in using a counterflow heat exchanger instead of a regenerator heat exchange to precool the high pressure gas before it is expanded. This requires an additional pair of valves at the cold end of the expander that have to be synchronized with the valves at the warm end. The counterflow heat exchanger has to be external to the displacer / cylinder and is substantially larger than an equivalent regenerator. An important advantage that a Brayton cycle refrigerator has relative to a GM cycle expander is its ability to distribute cold gas to a remote load, while the cold expanded gas in a GM expander is contained within the expansion space.

[0007] A compressor system that can be used to supply gas to either a GM cycle expander or a Brayton cycle engine is described in U.S. Pat. No. 7,674,099 titled “Compressor with Oil Bypass” by S. Dunn. High and low pressures are typically 2.2 and 0.8 MPa.

[0008] U.S. Pat. No. 6,256,997 (“the '997 patent”) to Longsworth describes the use of elastomer “O” rings at the warm end of a GM type displacer as “impact absorbers” to absorb the impact energy of the displacer when it is at the ends of the stroke to avoid the noise and vibration associated with having the displacer hit the warm and cold ends of the cylinder. It accomplishes this by locating “O” rings around the central drive mechanism. U.S. Pat. No. 10,677,498 to Longsworth describes a means of increasing the diameter of the bumper “O” rings by locating them on a disc, located in the warm end housing, and having the shoulders of a recessed length on the drive stem of a Brayton engine engage the inner edge of a hole in the center of the disc. The disc has the disadvantage of having an increasing inertia force as it gets larger that increases the contact stresses with the drive stem.

[0009] While the '997 patent describes the general principle and its application to relatively small and light displacers, U.S. Pat. No. 10,634,393 (“the '393 patent”) to Longsworth describes a means of applying the principal to larger displacers and pistons in expanders that produce more refrigeration and have larger and heavier displacers. This is accomplished in a preferred design by having a collar extending from the top (warm end) of the displacer that has the same outside diameter as the displacer and a lip at the top of the collar that engages an “O” ring before the displacer hits the bottom (cold end) of the cylinder. The top end of the collar also engages an “O” ring before the displacer hits the top (warm end) of the cylinder. As described in the '393 patent the energy that an “O” ring can absorb is proportional to its volume.

[0010] The present application improves upon the '393 patent by reducing the void volume associated with the bumpers; the volume swept by the lip on the top of the collar as the displacer reciprocates. Gas from the compressor flows into the void volume at high pressure and out at low pressure without doing any work. The lip of the collar in the '393 patent must be at least as wide as the bumper plus the width of the wall of the groove that limits the compression of the bumper to be within its fatigue limit. This is shown and described in the '997 patent. The '393 patent states the void volume uses 2 to 5% of the compressor flow. The present invention places the bumper in a retainer and provides the option of reducing the width of the lip that engages the retainer to less than the width of a bumper “O” ring, such that the void volume is reduced.SUMMARY

[0011] The present invention provides a means or structure of maximizing the energy absorbing capacity of bumpers that prevent the reciprocating displacer or piston in a pneumatically driven cryogenic expander from hitting the cold and warm ends of a cylinder while providing the means of reducing the void volume relative to previous designs. A collar at the warm end of the displacer has an annular recessed length on the outside of the collar with lips at the top and bottom ends that engage a ring-shaped retainer with one or two “O” rings that absorb the inertia of the displacer and prevent it from hitting the warm and cold ends of the cylinder assembly. The collar can be a separate piece integrated with the displacer or an extension of the displacer. The single or two “O” ring bumpers held by the retainer is / are trapped in a groove in the cylinder head, preferably between the cylinder head and the cylinder warm flange. The outside diameter of the collar is slightly less than the ID of the cylinder so that it does not contact the cylinder wall as it reciprocates.

[0012] A first variation or embodiment of the present invention places the bumper retainer in a groove on the outside of the collar, and the annular recess, with lips at either end, in the cylinder head.

[0013] A second variation or embodiment of the present invention places the bumper retainer in a groove in the neck of the cylinder head, and the annular recess, with lips on either end, inside the collar.

[0014] A third variation or embodiment of the present invention places the bumper retainer in a groove on the inside of the collar, and the annular recess, with lips at either end, in the neck of the cylinder head.

[0015] The pneumatic force that activates the displacer can act on a drive stem or the collar. The energy that an “O” ring can absorb is proportional to its volume. Having “O” rings that are near the maximum diameter of the cylinder maximizes the amount of energy they can absorb, and thus permits quiet operation of larger size expanders than prior designs. The “O” rings that are used for the purpose of absorbing energy are referred to herein as bumpers or impact absorbers and are not necessarily round. While the elastomer Buna N is a preferred material, other materials can also be used.

[0016] While top and bottom are used to refer to the warm and cold ends respectively, and up refers to moving from the cold end to the warm end, and down refers to moving from the warm end to the cold end, the expanders can all be operated in any orientation.BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The drawing figures depict one or more implementations in accord with the present invention, by way of example only, not by way of limitations. In the figures, like reference numerals refer to the same or similar elements.

[0018] FIG. 1 shows a schematic of a conventional GM cycle system with a pneumatically driven expander having a collar and bumpers, as described in U.S. Pat. No. 10,634,393.

[0019] FIG. 2 shows a schematic of a GM cycle system of the present invention with a pneumatically driven expander having a collar, with a recessed length between two lips on the collar, that engage a bumper assembly that comprises a single piece retainer with two “O” ring bumpers that are held in a groove in the cylinder head.

[0020] FIG. 2a shows a more detailed view of the bumper retainer assembly of FIG. 2

[0021] FIG. 3 shows another embodiment of a GM cycle system that has similar features of the GM cycle system shown in FIG. 2, but has a different bumper retainer assembly that comprises two retainers with a single bumper “O” ring that are held in a groove in the cylinder head.

[0022] FIG. 3a shows a more detailed view of the bumper retainer assembly of FIG. 3

[0023] FIGS. 4a-4b show schematics of two variations of the warm end of a pneumatically driven GM expander. FIG. 4a shows the recessed length between two lips in the wall of the cylinder head and the groove that holds the bumper retainer assembly in the collar. FIG. 4b shows the recessed length between two lips in the neck of the cylinder head and the groove that holds the bumper retainer assembly in the collar.

[0024] FIGS. 5a-5b show schematics of two more variations of the location of the recessed length between two lips, and the location of the groove that holds the bumper retainer assembly. FIG. 5a shows the recessed length between two lips on the inside of the collar, and the groove that holds the bumper retainer assembly on the neck of the cylinder head. FIG. 5b shows the recessed length between two lips on the outside of the collar, and the groove that holds the bumper retainer assembly in the wall of the cylinder head.

[0025] FIG. 6 shows a schematic of a GM system that is configured to apply the drive force to the collar instead of the stem.DETAILED DESCRIPTION

[0026] FIG. 1 shows a schematic of a prior art pneumatically driven GM cycle expander having a collar and bumpers as described in U.S. Pat. No. 10,634,393. All of the systems illustrated in FIGS. 1 through 6 have the same compressor 30, supply line 31 at high pressure, and return line 32 at low pressure. These gas lines can be several meters long thus providing flexibility in mounting the expander. Compressors in use today are typically oil lubricated scroll type compressors that are manufactured for air conditioning applications and are adapted to compress helium, the working fluid in most cryogenic refrigerators. Operating pressures are typically about 2.2 / 0.8 MPa and input power is in the range of about 2 to 12 kW. The present invention allows pneumatically actuated expanders with relatively high cooling capacities to run quietly. These may include larger compressors which may be screw type compressors.

[0027] The expander has four main subassemblies. The cylinder subassembly comprises cylinder 6, cold end cap 9, and warm flange 7. The displacer subassembly that reciprocates in the cylinder assembly comprises displacer body 1, regenerator 19, drive stem 2, collar 23, and displacer seal 26 near the warm end of displacer body 1. Collar 23 extends above displacer body 1 and has a lip 23′ at the top, the underside of which engages bumper “O” ring 24b when the displacer reaches the bottom of the stroke. All of the collars of the GM expanders have means to have the same pressure around them, either by clearances around them or by “breathing” holes through the collar. The cylinder head subassembly comprises cylinder head 8, stem cylinder 10, and stem seal 27. The top of collar 23 engages bumper “O” ring 24a when displacer 1 reaches the top of the stroke. The valve subassembly, which is usually in a housing attached to the cylinder head subassembly, comprises valves 12, 13, 14, and 15. These valves are typically contained in a ported rotary valve driven by a motor. When displacer 1 reciprocates it displaces gas in cold displaced volume 3, warm displaced volume 4, and drive stem displaced volume 5. While most of these volumes are displaced as displacer 1 reciprocates, they also include void volumes in the form of clearances and gas ports. Valves 14 and 15 cycle gas to warm displaced volume 4 through line 33 then through ports 18, regenerator 19, and port 20 to cold displaced volume 3. Valves 12 and 13 cycle gas to drive stem displaced volume 5 through line 34. Seal 28 seals cylinder head 8 to warm flange 7.

[0028] A GM refrigeration cycle starts with the displacer 1 at the cold end, (cold displaced volume 3 minimized), the pressure in the cylinder and on the drive stem 2 is high (valves 12 and 14 open, valves 13 and 15 closed). Valve 12 is then closed and 13 opened. Low pressure on the drive stem 2 causes displacer 1 to move up and draw high pressure gas into cold displaced volume 3. Before the displacer reaches the top, valve 14 is closed, and the pressure in the cylinder drops to a first pressure intermediate to the high and low pressures as the displacer moves to the top. This pressure decrease results from warm gas being transferred from the warm displaced volume 4 to the cold displaced volume 3. Valve 15 is then opened and the pressure in the cylinder drops to low pressure. Valve 13 is closed and 12 opened putting high pressure gas on the drive stem 2 and pushing the displacer 1 down. Before reaching the bottom, valve 15 is closed and the pressure in the cylinder increases to a second intermediate pressure as the displacer 1 moves to the bottom. This pressure increase results from cold gas being transferred from the cold displaced volume to the warm displaced volume. Valve 14 is then opened, and the pressure increases to high pressure and the beginning of the next cycle. The P-V work done in cold displaced volume 3 is equal to the refrigeration produced per cycle.

[0029] FIG. 2 shows GM system 100 of the present invention, which differs from the prior art design of FIG. 1, by having bumper retainer assembly 22 that includes bumper retainer 25 and bumper “O” rings (or bumper rings) 24a and 24b mounted in the bumper retainer 25, which is shown in more detail in FIG. 2a. The bumper retainer 25 may have a ring shape or a disc shape that can be placed to surround the collar 23a. The bumper retainer 25 may include base 25′ and protrusion 25″ projecting from the base 25′. As shown in FIG. 2a, the bumper “O” rings 24a, 24b are placed on the upper and lower sides of the protrusion 25″, respectively.

[0030] Collar 23a is integrated to the warm end of the displacer 1, and extends upward from the warm end of the displacer 1. The collar 23a is configured to slide in the internal space 11 formed in the cylinder head 8a between the neck 8a′ and the wall 8a″. The cylinder head 8a includes neck 8a′ and wall 8a″ that is connected to and surrounds the neck 8a′. The internal space 11 is located between the neck 8a′ and the wall 8a″, and is configured to receive the collar 23a.

[0031] The collar 23a has first lip 17a at the top and second lip 17b at the bottom. A recessed space is provided between the lips 17a and 17b and between the collar 23a and the wall 8a″ / cylinder warm flange 7, defined by annular recessed length 16 between the lips 17a and 17b. Groove 29a is formed in the wall 8a″ on the side facing the neck 8a′. Bumper retainer assembly 22 is held in the groove 29a and is supported by cylinder warm flange 7. An end portion of the bumper retainer 25 (an end portion of the base 25′) projects into the recessed space defined by the annular recessed length 16 between lips 17a and 17b. The height of groove 29a, and its equivalents, is such that bumper “O” rings 24a and 24b can be compressed less than their fatigue limit, about 30% of compression, when lips 17a and 17b hit bumper retainer 25 at either end of the stroke; thus preventing the displacer subassembly from hitting cylinder cold end 9 or cylinder head 8a. This is true for all applications of the bumper retainer assemblies. Void volume due to the recessed length 16 is minimized if its depth is less than the diameter of bumper “O” rings 24a and 24b. Assembly of the expander is facilitated by cutting bumper retainer 25 radially so that it can be pulled apart enough to fit over the lip on top of collar 23a; followed by insertion of displacer 1 in cylinder 6 and attaching cylinder head 8 to flange 7, while the collar 23a is inserted into the internal space 11. The radial tolerance between the recessed length of collar 23a and retainer 25 allows collar 23a to slide while maintaining concentricity.

[0032] FIG. 3 shows GM system 200 of the present invention, which has similar features of the GM system 100 but has a different bumper retainer assembly 21 that fits in groove 29b. Bumper retainer assembly 21, as shown in detail in FIG. 3a, comprises upper and lower bumper retainers 25a, 25b and a single bumper “O” ring 24 between the upper and lower bumper retainers 25a and 25b. Specifically, the bumper retainer 25a includes base 25a′ and protrusion 25a″ projecting from the base 25a′, and the bumper retainer 25b includes base 25b′ and protrusion 25b″ projecting from the base 25b′. The bumper “O” ring 24 is placed between the protrusions 25a″ and 25b″. Bumper retainer assembly 21 projects into the recessed space between the lips 17a and 17b and between the collar 23a and the wall 8a″ cylinder warm flange 7, defined by annular recessed length 16a. Specifically, as shown in FIG. 3, end portions of the protrusions 25a″, 25b″ project into the recessed space.

[0033] FIGS. 4a-4b show GM systems 300 and 400 of the present invention, respectively, which have variations of the warm ends of GM systems 100 and 200. For description purpose, FIGS. 4a-4b show only left or right side of warm end portion of the GM systems 300 and 400.

[0034] The GM system 300 shown in FIG. 4a has groove 29c, formed in the collar 23b on the side facing the wall of the cylinder head 8b. A recessed space is formed on the wall 8b″ of the cylinder head 8b, on the side facing the neck 8b′, defined by the first lip 17a″ on the top portion of the wall 8b″. In this case, a portion of the upper surface of the cylinder warm flange 7, which is exposed to the recessed space, becomes the second lip 17b″. Therefore, the recessed space is provided on the wall of the cylinder head 8b between the lips 17a and 17b, defined by annular recessed length 16b on the wall 8b′ of the cylinder head 8b. The groove 29c holds the bumper retainer assembly 22a near the top of collar 23b, An end portion of the bumper retainer of the bumper retainer assembly 22a projects into the recessed space. FIG. 4a exemplarily shows the bumper retainer assembly 22a that has the structure of the bumper retainer assembly 22 shown in FIG. 2a, but the bumper retainer assembly 21 shown in FIG. 3a may be employed in the GM system 300.

[0035] The GM system 400 shown in FIG. 4b has groove 29d formed in the collar 23e on the side facing the neck 8e′ of the cylinder head 8e. A recessed space is formed on the neck 8e′ of the cylinder head 8e, on the side facing the wall 8e″, defined by the first lip 17a′ on the top portion of the neck 8e′ and the second lip 17b′ on the bottom portion of the neck 8e′. Therefore, the recessed space is provided between the lips 17a and 17b defined by annular recessed length 16c on the neck 8e′ of the cylinder head 8e. The groove 29d holds the bumper retainer assembly 22b near the top of collar 23e, An end portion of the bumper retainer of the bumper retainer assembly 22b projects into the recessed space. FIG. 4b exemplarily shows the bumper retainer assembly 22b that has the structure of the bumper retainer assembly 22 shown in FIG. 2a, but the bumper retainer assembly 21 shown in FIG. 3a may be employed in the GM system 400.

[0036] FIGS. 5a-5b show GM systems 500 and 600 of the present invention, respectively, which have variations of the warm ends of GM systems 100 and 200. For description purpose, FIGS. 5a-5b show only left or right side of warm end portion of the GM systems 500 and 600.

[0037] The GM system 500 shown in FIG. 5a has groove 29e formed in the neck 8d′ of the cylinder head 8d on the side facing the wall 8d″. The collar 23d has lips 17a and 17b on the side of the collar 23d facing the neck 8d′. A recessed space is provided between the lips 17a and 17b, defined by annular recessed length 16d on the collar 23d. The groove 29e holds the bumper retainer assembly 21a near the bottom of the neck of the cylinder head 8d. An end portion of the bumper retainer of the bumper retainer assembly 21a projects into the recessed space. FIG. 5a exemplarily shows the bumper retainer assembly 21a that has the structure of the bumper retainer assembly 21 shown in FIG. 3a, but the bumper retainer assembly 22 shown in FIG. 2a may be employed in the GM system 500.

[0038] The GM system 600 shown in FIG. 5b has groove 29f formed in the wall 8c″ of the cylinder head 8c. The collar 23c has lips 17a and 17b on the side of the collar 23c facing the wall 8c″. A recessed space is provided between the lips 17a and 17b defined by annular recessed length 16e on the collar 23c. The groove 29f holds the bumper retainer assembly 22c near the bottom of the wall 8c″ of the cylinder head 8c. An end portion of the bumper retainer of the bumper retainer assembly 22c projects into the recessed space. FIG. 5b exemplarily shows the bumper retainer assembly 22c that has the structure of the bumper retainer assembly 22 shown in FIG. 2a, but the bumper retainer assembly 21 shown in FIG. 3a may be employed in the GM system 600.

[0039] The GM system 600 has a collar that has a smaller diameter than the displacer and might be used with a smaller, lighter, displacer as an alternative to the bumper configuration of U.S. Pat. No. 6,256,997. The GM systems 400, 500 and 600 require two-piece construction of the cylinder head or collar.

[0040] FIG. 6 shows GM system 700 of the present invention, which has a variation of the warm ends of the GM systems 100 and 200. For description purpose, FIG. 6 shows only warm end portion of the GM system 700. The GM system 700 is configured to apply the drive force to collar 23f instead of the stem 2. The collar 23f may be referred to as a drive collar. In the GM system 100 and 200, the line 34 from valves 12 and 13 is connected to the space 5 to drive the stem 2. In the GM systems 700, the line 34 from the valves 12 and 13 is connected to the internal space 5a to d rive the collar 23f. Seal 27 on drive stem 2 in systems 100 and 200 is replaced with seal 27a on the outer upper end of collar 23f and seal 27b on the outside of the neck of warm cylinder head 8f. The gas in warm volume 4a is connected to the gas in the annular volume of recessed length 16 through holes 35 in collar 23f. Seal 27b is located between the top of collar 23f and holes 35 as displacer 1 reciprocates.

[0041] FIG. 6 exemplarily shows the GM system 700 having the groove 29b and the bumper retainer assembly 21 that are employed for the GM system 200 of the present invention. However, any of the structures of the grooves 29a, 29c, 29d, 29e, and 29f and the corresponding structures of the bumper retainer assembly 22, 22a, 22b, 21a, and 22c can be employed in the GM system 700.

[0042] FIGS. 2-6 show the bumper retainer assemblies 21, 21a, 22, 22a, 22b, and 22c based on the GM system, and these bumper retainer assemblies are not shown based on pneumatically driven Brayton expanders. However, the bumper assemblies 21, 21a, 22, 22a, 22b, and 22c as shown in FIGS. 2 through 6 can be employed in pneumatically driven Brayton cycle expanders that have a drive stem and a collar. The Brayton cycle expanders having the drive stem and collar are shown in, for example, FIGS. 6 and 7 of U.S. Pat. No. 10,634,393.

[0043] The terms and descriptions used in the following claims are not meant as limitations. Those skilled in the art will recognize that variations are possible within the spirit and scope of the invention and the embodiments described herein.

Examples

Embodiment Construction

[0026]FIG. 1 shows a schematic of a prior art pneumatically driven GM cycle expander having a collar and bumpers as described in U.S. Pat. No. 10,634,393. All of the systems illustrated in FIGS. 1 through 6 have the same compressor 30, supply line 31 at high pressure, and return line 32 at low pressure. These gas lines can be several meters long thus providing flexibility in mounting the expander. Compressors in use today are typically oil lubricated scroll type compressors that are manufactured for air conditioning applications and are adapted to compress helium, the working fluid in most cryogenic refrigerators. Operating pressures are typically about 2.2 / 0.8 MPa and input power is in the range of about 2 to 12 kW. The present invention allows pneumatically actuated expanders with relatively high cooling capacities to run quietly. These may include larger compressors which may be screw type compressors.

[0027]The expander has four main subassemblies. The cylinder subassembly compri...

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Patent Application No. 63 / 757,453,filed on Feb. 12, 2025, the entire contents of which are incorporated herein by reference.BACKGROUND1. Field of the Invention

[0002] This invention relates to reducing the noise and vibration of a high capacity expander having a pneumatically driven reciprocating displacer or piston producing refrigeration at cryogenic temperatures.2. Description of Related Art

[0003] Most cryogenic refrigerators that are used to cool cryopumps, superconducting MRI magnets, and laboratory research instruments use Gifford-McMahon (GM) type refrigerators. These typically use air conditioning compressors that have been modified to compress helium and draw less than 12 kW of input power. The expanders have reciprocating displacers that are either mechanically or pneumatically driven. The mechanical drive is relatively quiet because it provides a nearly sinusoidal motion that does not cause the displacer to hit the top or bottom at the end of the stroke. The pneumatic drives are simpler but can produce significant noise if the displacer hits the top or bottom of the cylinder at the end of the stroke. The same is true for pistons in expanders that operate on the Brayton cycle.

[0004] U.S. Pat. No. 3,045,436 (“the '436 patent”), by W. E. Gifford and H. O. McMahon describes the basic GM cycle. This refrigerator system consists of a compressor that supplies gas at a high pressure to an expander which admits the gas through a warm inlet valve to the warm end of a regenerator heat exchanger, through the regenerator, and then into an expansion space at the cold end of a displacer from whence it returns back through the regenerator and a warm outlet valve to the compressor at a low pressure. The '436 patent shows the regenerator external to the cylinder with the displacer, and a second pair of valves that cycles gas to the warm end of the displacer out of phase with the gas flow to the regenerator. U.S. Pat. No. 3,119,237 (“the '237 patent”), by W. E. Gifford shows an improvement of the concept of the '436 patent in the form of a drive stem at the warm end of a displacer which reduces the amount of gas used to drive the displacer up and down. The expander configuration and valve cycling are shown in FIGS. 2-9 in the '237 patent.

[0005] The typical GM type expander being built today has the regenerator located inside the displacer. The displacer / regenerator becomes a displacer that moves from the cold end to the warm end with the gas at high pressure, then from the warm end to the cold end with the gas at low pressure. Since the pressure above and below the displacer / regenerator is nearly the same, the force required to cause the displacer / regenerator to reciprocate is small and can be provided by either a mechanical or pneumatic mechanism. In the descriptions that follow, the term displacer is used for the displacer / regenerator in the GM type expander in which the pressure difference between the warm and cold ends is small, and piston is used when the pressure difference may be large.

[0006] A pneumatically driven expander operating on the Brayton cycle is described in U.S. Pat. No. 9,080,794 by Longsworth. The Brayton cycle differs from the GM cycle in using a counterflow heat exchanger instead of a regenerator heat exchange to precool the high pressure gas before it is expanded. This requires an additional pair of valves at the cold end of the expander that have to be synchronized with the valves at the warm end. The counterflow heat exchanger has to be external to the displacer / cylinder and is substantially larger than an equivalent regenerator. An important advantage that a Brayton cycle refrigerator has relative to a GM cycle expander is its ability to distribute cold gas to a remote load, while the cold expanded gas in a GM expander is contained within the expansion space.

[0007] A compressor system that can be used to supply gas to either a GM cycle expander or a Brayton cycle engine is described in U.S. Pat. No. 7,674,099 titled “Compressor with Oil Bypass” by S. Dunn. High and low pressures are typically 2.2 and 0.8 MPa.

[0008] U.S. Pat. No. 6,256,997 (“the '997 patent”) to Longsworth describes the use of elastomer “O” rings at the warm end of a GM type displacer as “impact absorbers” to absorb the impact energy of the displacer when it is at the ends of the stroke to avoid the noise and vibration associated with having the displacer hit the warm and cold ends of the cylinder. It accomplishes this by locating “O” rings around the central drive mechanism. U.S. Pat. No. 10,677,498 to Longsworth describes a means of increasing the diameter of the bumper “O” rings by locating them on a disc, located in the warm end housing, and having the shoulders of a recessed length on the drive stem of a Brayton engine engage the inner edge of a hole in the center of the disc. The disc has the disadvantage of having an increasing inertia force as it gets larger that increases the contact stresses with the drive stem.

[0009] While the '997 patent describes the general principle and its application to relatively small and light displacers, U.S. Pat. No. 10,634,393 (“the '393 patent”) to Longsworth describes a means of applying the principal to larger displacers and pistons in expanders that produce more refrigeration and have larger and heavier displacers. This is accomplished in a preferred design by having a collar extending from the top (warm end) of the displacer that has the same outside diameter as the displacer and a lip at the top of the collar that engages an “O” ring before the displacer hits the bottom (cold end) of the cylinder. The top end of the collar also engages an “O” ring before the displacer hits the top (warm end) of the cylinder. As described in the '393 patent the energy that an “O” ring can absorb is proportional to its volume.

[0010] The present application improves upon the '393 patent by reducing the void volume associated with the bumpers; the volume swept by the lip on the top of the collar as the displacer reciprocates. Gas from the compressor flows into the void volume at high pressure and out at low pressure without doing any work. The lip of the collar in the '393 patent must be at least as wide as the bumper plus the width of the wall of the groove that limits the compression of the bumper to be within its fatigue limit. This is shown and described in the '997 patent. The '393 patent states the void volume uses 2 to 5% of the compressor flow. The present invention places the bumper in a retainer and provides the option of reducing the width of the lip that engages the retainer to less than the width of a bumper “O” ring, such that the void volume is reduced.SUMMARY

[0011] The present invention provides a means or structure of maximizing the energy absorbing capacity of bumpers that prevent the reciprocating displacer or piston in a pneumatically driven cryogenic expander from hitting the cold and warm ends of a cylinder while providing the means of reducing the void volume relative to previous designs. A collar at the warm end of the displacer has an annular recessed length on the outside of the collar with lips at the top and bottom ends that engage a ring-shaped retainer with one or two “O” rings that absorb the inertia of the displacer and prevent it from hitting the warm and cold ends of the cylinder assembly. The collar can be a separate piece integrated with the displacer or an extension of the displacer. The single or two “O” ring bumpers held by the retainer is / are trapped in a groove in the cylinder head, preferably between the cylinder head and the cylinder warm flange. The outside diameter of the collar is slightly less than the ID of the cylinder so that it does not contact the cylinder wall as it reciprocates.

[0012] A first variation or embodiment of the present invention places the bumper retainer in a groove on the outside of the collar, and the annular recess, with lips at either end, in the cylinder head.

[0013] A second variation or embodiment of the present invention places the bumper retainer in a groove in the neck of the cylinder head, and the annular recess, with lips on either end, inside the collar.

[0014] A third variation or embodiment of the present invention places the bumper retainer in a groove on the inside of the collar, and the annular recess, with lips at either end, in the neck of the cylinder head.

[0015] The pneumatic force that activates the displacer can act on a drive stem or the collar. The energy that an “O” ring can absorb is proportional to its volume. Having “O” rings that are near the maximum diameter of the cylinder maximizes the amount of energy they can absorb, and thus permits quiet operation of larger size expanders than prior designs. The “O” rings that are used for the purpose of absorbing energy are referred to herein as bumpers or impact absorbers and are not necessarily round. While the elastomer Buna N is a preferred material, other materials can also be used.

[0016] While top and bottom are used to refer to the warm and cold ends respectively, and up refers to moving from the cold end to the warm end, and down refers to moving from the warm end to the cold end, the expanders can all be operated in any orientation.BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The drawing figures depict one or more implementations in accord with the present invention, by way of example only, not by way of limitations. In the figures, like reference numerals refer to the same or similar elements.

[0018] FIG. 1 shows a schematic of a conventional GM cycle system with a pneumatically driven expander having a collar and bumpers, as described in U.S. Pat. No. 10,634,393.

[0019] FIG. 2 shows a schematic of a GM cycle system of the present invention with a pneumatically driven expander having a collar, with a recessed length between two lips on the collar, that engage a bumper assembly that comprises a single piece retainer with two “O” ring bumpers that are held in a groove in the cylinder head.

[0020] FIG. 2a shows a more detailed view of the bumper retainer assembly of FIG. 2

[0021] FIG. 3 shows another embodiment of a GM cycle system that has similar features of the GM cycle system shown in FIG. 2, but has a different bumper retainer assembly that comprises two retainers with a single bumper “O” ring that are held in a groove in the cylinder head.

[0022] FIG. 3a shows a more detailed view of the bumper retainer assembly of FIG. 3

[0023] FIGS. 4a-4b show schematics of two variations of the warm end of a pneumatically driven GM expander. FIG. 4a shows the recessed length between two lips in the wall of the cylinder head and the groove that holds the bumper retainer assembly in the collar. FIG. 4b shows the recessed length between two lips in the neck of the cylinder head and the groove that holds the bumper retainer assembly in the collar.

[0024] FIGS. 5a-5b show schematics of two more variations of the location of the recessed length between two lips, and the location of the groove that holds the bumper retainer assembly. FIG. 5a shows the recessed length between two lips on the inside of the collar, and the groove that holds the bumper retainer assembly on the neck of the cylinder head. FIG. 5b shows the recessed length between two lips on the outside of the collar, and the groove that holds the bumper retainer assembly in the wall of the cylinder head.

[0025] FIG. 6 shows a schematic of a GM system that is configured to apply the drive force to the collar instead of the stem.DETAILED DESCRIPTION

[0026] FIG. 1 shows a schematic of a prior art pneumatically driven GM cycle expander having a collar and bumpers as described in U.S. Pat. No. 10,634,393. All of the systems illustrated in FIGS. 1 through 6 have the same compressor 30, supply line 31 at high pressure, and return line 32 at low pressure. These gas lines can be several meters long thus providing flexibility in mounting the expander. Compressors in use today are typically oil lubricated scroll type compressors that are manufactured for air conditioning applications and are adapted to compress helium, the working fluid in most cryogenic refrigerators. Operating pressures are typically about 2.2 / 0.8 MPa and input power is in the range of about 2 to 12 kW. The present invention allows pneumatically actuated expanders with relatively high cooling capacities to run quietly. These may include larger compressors which may be screw type compressors.

[0027] The expander has four main subassemblies. The cylinder subassembly comprises cylinder 6, cold end cap 9, and warm flange 7. The displacer subassembly that reciprocates in the cylinder assembly comprises displacer body 1, regenerator 19, drive stem 2, collar 23, and displacer seal 26 near the warm end of displacer body 1. Collar 23 extends above displacer body 1 and has a lip 23′ at the top, the underside of which engages bumper “O” ring 24b when the displacer reaches the bottom of the stroke. All of the collars of the GM expanders have means to have the same pressure around them, either by clearances around them or by “breathing” holes through the collar. The cylinder head subassembly comprises cylinder head 8, stem cylinder 10, and stem seal 27. The top of collar 23 engages bumper “O” ring 24a when displacer 1 reaches the top of the stroke. The valve subassembly, which is usually in a housing attached to the cylinder head subassembly, comprises valves 12, 13, 14, and 15. These valves are typically contained in a ported rotary valve driven by a motor. When displacer 1 reciprocates it displaces gas in cold displaced volume 3, warm displaced volume 4, and drive stem displaced volume 5. While most of these volumes are displaced as displacer 1 reciprocates, they also include void volumes in the form of clearances and gas ports. Valves 14 and 15 cycle gas to warm displaced volume 4 through line 33 then through ports 18, regenerator 19, and port 20 to cold displaced volume 3. Valves 12 and 13 cycle gas to drive stem displaced volume 5 through line 34. Seal 28 seals cylinder head 8 to warm flange 7.

[0028] A GM refrigeration cycle starts with the displacer 1 at the cold end, (cold displaced volume 3 minimized), the pressure in the cylinder and on the drive stem 2 is high (valves 12 and 14 open, valves 13 and 15 closed). Valve 12 is then closed and 13 opened. Low pressure on the drive stem 2 causes displacer 1 to move up and draw high pressure gas into cold displaced volume 3. Before the displacer reaches the top, valve 14 is closed, and the pressure in the cylinder drops to a first pressure intermediate to the high and low pressures as the displacer moves to the top. This pressure decrease results from warm gas being transferred from the warm displaced volume 4 to the cold displaced volume 3. Valve 15 is then opened and the pressure in the cylinder drops to low pressure. Valve 13 is closed and 12 opened putting high pressure gas on the drive stem 2 and pushing the displacer 1 down. Before reaching the bottom, valve 15 is closed and the pressure in the cylinder increases to a second intermediate pressure as the displacer 1 moves to the bottom. This pressure increase results from cold gas being transferred from the cold displaced volume to the warm displaced volume. Valve 14 is then opened, and the pressure increases to high pressure and the beginning of the next cycle. The P-V work done in cold displaced volume 3 is equal to the refrigeration produced per cycle.

[0029] FIG. 2 shows GM system 100 of the present invention, which differs from the prior art design of FIG. 1, by having bumper retainer assembly 22 that includes bumper retainer 25 and bumper “O” rings (or bumper rings) 24a and 24b mounted in the bumper retainer 25, which is shown in more detail in FIG. 2a. The bumper retainer 25 may have a ring shape or a disc shape that can be placed to surround the collar 23a. The bumper retainer 25 may include base 25′ and protrusion 25″ projecting from the base 25′. As shown in FIG. 2a, the bumper “O” rings 24a, 24b are placed on the upper and lower sides of the protrusion 25″, respectively.

[0030] Collar 23a is integrated to the warm end of the displacer 1, and extends upward from the warm end of the displacer 1. The collar 23a is configured to slide in the internal space 11 formed in the cylinder head 8a between the neck 8a′ and the wall 8a″. The cylinder head 8a includes neck 8a′ and wall 8a″ that is connected to and surrounds the neck 8a′. The internal space 11 is located between the neck 8a′ and the wall 8a″, and is configured to receive the collar 23a.

[0031] The collar 23a has first lip 17a at the top and second lip 17b at the bottom. A recessed space is provided between the lips 17a and 17b and between the collar 23a and the wall 8a″ / cylinder warm flange 7, defined by annular recessed length 16 between the lips 17a and 17b. Groove 29a is formed in the wall 8a″ on the side facing the neck 8a′. Bumper retainer assembly 22 is held in the groove 29a and is supported by cylinder warm flange 7. An end portion of the bumper retainer 25 (an end portion of the base 25′) projects into the recessed space defined by the annular recessed length 16 between lips 17a and 17b. The height of groove 29a, and its equivalents, is such that bumper “O” rings 24a and 24b can be compressed less than their fatigue limit, about 30% of compression, when lips 17a and 17b hit bumper retainer 25 at either end of the stroke; thus preventing the displacer subassembly from hitting cylinder cold end 9 or cylinder head 8a. This is true for all applications of the bumper retainer assemblies. Void volume due to the recessed length 16 is minimized if its depth is less than the diameter of bumper “O” rings 24a and 24b. Assembly of the expander is facilitated by cutting bumper retainer 25 radially so that it can be pulled apart enough to fit over the lip on top of collar 23a; followed by insertion of displacer 1 in cylinder 6 and attaching cylinder head 8 to flange 7, while the collar 23a is inserted into the internal space 11. The radial tolerance between the recessed length of collar 23a and retainer 25 allows collar 23a to slide while maintaining concentricity.

[0032] FIG. 3 shows GM system 200 of the present invention, which has similar features of the GM system 100 but has a different bumper retainer assembly 21 that fits in groove 29b. Bumper retainer assembly 21, as shown in detail in FIG. 3a, comprises upper and lower bumper retainers 25a, 25b and a single bumper “O” ring 24 between the upper and lower bumper retainers 25a and 25b. Specifically, the bumper retainer 25a includes base 25a′ and protrusion 25a″ projecting from the base 25a′, and the bumper retainer 25b includes base 25b′ and protrusion 25b″ projecting from the base 25b′. The bumper “O” ring 24 is placed between the protrusions 25a″ and 25b″. Bumper retainer assembly 21 projects into the recessed space between the lips 17a and 17b and between the collar 23a and the wall 8a″ cylinder warm flange 7, defined by annular recessed length 16a. Specifically, as shown in FIG. 3, end portions of the protrusions 25a″, 25b″ project into the recessed space.

[0033] FIGS. 4a-4b show GM systems 300 and 400 of the present invention, respectively, which have variations of the warm ends of GM systems 100 and 200. For description purpose, FIGS. 4a-4b show only left or right side of warm end portion of the GM systems 300 and 400.

[0034] The GM system 300 shown in FIG. 4a has groove 29c, formed in the collar 23b on the side facing the wall of the cylinder head 8b. A recessed space is formed on the wall 8b″ of the cylinder head 8b, on the side facing the neck 8b′, defined by the first lip 17a″ on the top portion of the wall 8b″. In this case, a portion of the upper surface of the cylinder warm flange 7, which is exposed to the recessed space, becomes the second lip 17b″. Therefore, the recessed space is provided on the wall of the cylinder head 8b between the lips 17a and 17b, defined by annular recessed length 16b on the wall 8b′ of the cylinder head 8b. The groove 29c holds the bumper retainer assembly 22a near the top of collar 23b, An end portion of the bumper retainer of the bumper retainer assembly 22a projects into the recessed space. FIG. 4a exemplarily shows the bumper retainer assembly 22a that has the structure of the bumper retainer assembly 22 shown in FIG. 2a, but the bumper retainer assembly 21 shown in FIG. 3a may be employed in the GM system 300.

[0035] The GM system 400 shown in FIG. 4b has groove 29d formed in the collar 23e on the side facing the neck 8e′ of the cylinder head 8e. A recessed space is formed on the neck 8e′ of the cylinder head 8e, on the side facing the wall 8e″, defined by the first lip 17a′ on the top portion of the neck 8e′ and the second lip 17b′ on the bottom portion of the neck 8e′. Therefore, the recessed space is provided between the lips 17a and 17b defined by annular recessed length 16c on the neck 8e′ of the cylinder head 8e. The groove 29d holds the bumper retainer assembly 22b near the top of collar 23e, An end portion of the bumper retainer of the bumper retainer assembly 22b projects into the recessed space. FIG. 4b exemplarily shows the bumper retainer assembly 22b that has the structure of the bumper retainer assembly 22 shown in FIG. 2a, but the bumper retainer assembly 21 shown in FIG. 3a may be employed in the GM system 400.

[0036] FIGS. 5a-5b show GM systems 500 and 600 of the present invention, respectively, which have variations of the warm ends of GM systems 100 and 200. For description purpose, FIGS. 5a-5b show only left or right side of warm end portion of the GM systems 500 and 600.

[0037] The GM system 500 shown in FIG. 5a has groove 29e formed in the neck 8d′ of the cylinder head 8d on the side facing the wall 8d″. The collar 23d has lips 17a and 17b on the side of the collar 23d facing the neck 8d′. A recessed space is provided between the lips 17a and 17b, defined by annular recessed length 16d on the collar 23d. The groove 29e holds the bumper retainer assembly 21a near the bottom of the neck of the cylinder head 8d. An end portion of the bumper retainer of the bumper retainer assembly 21a projects into the recessed space. FIG. 5a exemplarily shows the bumper retainer assembly 21a that has the structure of the bumper retainer assembly 21 shown in FIG. 3a, but the bumper retainer assembly 22 shown in FIG. 2a may be employed in the GM system 500.

[0038] The GM system 600 shown in FIG. 5b has groove 29f formed in the wall 8c″ of the cylinder head 8c. The collar 23c has lips 17a and 17b on the side of the collar 23c facing the wall 8c″. A recessed space is provided between the lips 17a and 17b defined by annular recessed length 16e on the collar 23c. The groove 29f holds the bumper retainer assembly 22c near the bottom of the wall 8c″ of the cylinder head 8c. An end portion of the bumper retainer of the bumper retainer assembly 22c projects into the recessed space. FIG. 5b exemplarily shows the bumper retainer assembly 22c that has the structure of the bumper retainer assembly 22 shown in FIG. 2a, but the bumper retainer assembly 21 shown in FIG. 3a may be employed in the GM system 600.

[0039] The GM system 600 has a collar that has a smaller diameter than the displacer and might be used with a smaller, lighter, displacer as an alternative to the bumper configuration of U.S. Pat. No. 6,256,997. The GM systems 400, 500 and 600 require two-piece construction of the cylinder head or collar.

[0040] FIG. 6 shows GM system 700 of the present invention, which has a variation of the warm ends of the GM systems 100 and 200. For description purpose, FIG. 6 shows only warm end portion of the GM system 700. The GM system 700 is configured to apply the drive force to collar 23f instead of the stem 2. The collar 23f may be referred to as a drive collar. In the GM system 100 and 200, the line 34 from valves 12 and 13 is connected to the space 5 to drive the stem 2. In the GM systems 700, the line 34 from the valves 12 and 13 is connected to the internal space 5a to d rive the collar 23f. Seal 27 on drive stem 2 in systems 100 and 200 is replaced with seal 27a on the outer upper end of collar 23f and seal 27b on the outside of the neck of warm cylinder head 8f. The gas in warm volume 4a is connected to the gas in the annular volume of recessed length 16 through holes 35 in collar 23f. Seal 27b is located between the top of collar 23f and holes 35 as displacer 1 reciprocates.

[0041] FIG. 6 exemplarily shows the GM system 700 having the groove 29b and the bumper retainer assembly 21 that are employed for the GM system 200 of the present invention. However, any of the structures of the grooves 29a, 29c, 29d, 29e, and 29f and the corresponding structures of the bumper retainer assembly 22, 22a, 22b, 21a, and 22c can be employed in the GM system 700.

[0042] FIGS. 2-6 show the bumper retainer assemblies 21, 21a, 22, 22a, 22b, and 22c based on the GM system, and these bumper retainer assemblies are not shown based on pneumatically driven Brayton expanders. However, the bumper assemblies 21, 21a, 22, 22a, 22b, and 22c as shown in FIGS. 2 through 6 can be employed in pneumatically driven Brayton cycle expanders that have a drive stem and a collar. The Brayton cycle expanders having the drive stem and collar are shown in, for example, FIGS. 6 and 7 of U.S. Pat. No. 10,634,393.

[0043] The terms and descriptions used in the following claims are not meant as limitations. Those skilled in the art will recognize that variations are possible within the spirit and scope of the invention and the embodiments described herein.

Examples

Embodiment Construction

[0026]FIG. 1 shows a schematic of a prior art pneumatically driven GM cycle expander having a collar and bumpers as described in U.S. Pat. No. 10,634,393. All of the systems illustrated in FIGS. 1 through 6 have the same compressor 30, supply line 31 at high pressure, and return line 32 at low pressure. These gas lines can be several meters long thus providing flexibility in mounting the expander. Compressors in use today are typically oil lubricated scroll type compressors that are manufactured for air conditioning applications and are adapted to compress helium, the working fluid in most cryogenic refrigerators. Operating pressures are typically about 2.2 / 0.8 MPa and input power is in the range of about 2 to 12 kW. The present invention allows pneumatically actuated expanders with relatively high cooling capacities to run quietly. These may include larger compressors which may be screw type compressors.

[0027]The expander has four main subassemblies. The cylinder subassembly compri...

Claims

1. A cryogenic expander system for providing reduced noises and vibrations, the cryogenic expander comprising:a cylinder having a warm cylinder end and a cold cylinder end;a displacer disposed in the cylinder, the displacer having a warm displacer end and a cold displacer end, the displacer reciprocating between the warm cylinder end and the cold cylinder end;a cylinder head disposed on the cylinder, wherein the cylinder head comprises a neck and a wall surrounding the neck, providing an internal space between the neck and the wall, wherein the neck and the warm displacer end provide a warm displaced volume in the cylinder between the neck and the warm displacer end;a collar integrated to the warm displacer end and extending upward from the warm displace end, wherein the collar is disposed in the internal space between the neck and the wall, wherein either:(i) the collar includes a first lip at a top portion of the collar protruding toward the wall and a second lip at a bottom portion of the collar protruding toward the wall, providing a recessed space between the first and second lips, and the wall has a groove on a side of the wall facing the neck,(ii) the wall has a recessed space on a side of the wall facing the neck defined by a first lip and second lip of the wall, and the collar has a groove on a side of the collar facing the wall,(iii) the neck has a recessed space on a side of the neck facing the wall defined by a first lip and second lip of the neck, and the collar has a groove on a side of the collar facing the neck, or (iv) the collar includes a first lip at a top portion of the collar protruding toward the neck and a second lip at a bottom portion of the collar protruding toward the neck, providing a recessed space between the first and second lips, and the neck has a groove on a side of the neck facing the wall;a bumper retainer assembly disposed in the groove and comprising at least one bumper ring and at least one bumper retainer to place the at least one bumper ring, wherein an end portion of the at least one bumper retainer projects into the recessed space.

2. The cryogenic expander system of claim 1, wherein the bumper retainer comprises a base and a protrusion projecting from the base, and the at least one bumper ring comprises a first bumper ring and a second bumper ring, and wherein one of the first and second bumper rings is disposed on a side of the protrusion and the other of the first and second bumper rings is disposed on an opposite side of the protrusion.

3. The cryogenic expander system of claim 1, wherein the bumper retainer comprises a first bumper retainer and a second bumper retainer, and each of the first and second bumper retainers comprises a base and a protrusion projecting from the base, and wherein the at least one bumper ring is disposed between the protrusions of the first and second bumper retainer.

4. The cryogenic expander system of claim 1, wherein the outside diameter of the collar is either (i) the same as the outside diameter of the displacer, or (ii) the outside diameter of the collar is less than the outside diameter of the displacer.

5. The cryogenic expander system of claim 1, further comprising a drive stem integrated at the warm displacer end, wherein the cylinder head has a stem displaced volume to receive the drive stem, wherein the displacer is activated by pneumatic force applied to the drive stem through the stem displaced volume.

6. The cryogenic expander system of claim 1, wherein the displacer is activated by pneumatic force applied to the collar through the internal space.

7. The cryogenic expander system of claim 1, wherein the displacer comprises a regenerator.

8. The cryogenic expander system of claim 1, wherein the cryogenic expander system operates on a Gifford-McMahon (GM) cycle or a Brayton cycle.

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