Cryogenic expander with collar bumper for reduced noise and vibration characteristics

A cryogenic expander maximizes the energy absorbing capacity of bumpers that prevent the displacer or piston in a pneumatically driven expander from hitting the cold or warm end of a cylinder. A collar at the warm end of the piston which has the same outside diameter as the piston and a lip at the warm end that engages an “O” ring before the piston hits the cold end or bottom of the cylinder. The warm end of the collar also engages an “O” ring before the pistons hits the warm end or top of the cylinder. 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.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a cryogenic expander having reduced noise and vibration characteristics. More specifically, the invention relates to a high capacity expander having a pneumatically driven reciprocating piston producing refrigeration at cryogenic temperatures and which includes a collar bumper with reduced noise and vibration characteristics.

2. Background Information

Most cryogenic refrigerators that are used to cool cryopumps, superconducting MRI magnets, and laboratory research instruments use 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 pistons 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 piston to hit the top or bottom at the end of the stroke. The pneumatic drives are simpler but can produce significant noise if the piston hits the top or bottom of the cylinder at the end of the stroke. The same is true for expanders that operate on the Brayton cycle.

U.S. Pat. No. 3,045,436, 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 piston 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 piston, and a second pair of valves that cycles gas to the warm end of the piston out of phase with the gas flow to the regenerator. U.S. Pat. No. 3,119,237, by W. E. Gifford shows an improvement of the concept in the '436 patent in the form of a drive stem at the warm end of the piston which reduces the amount of gas used to drive the piston up and down The expander configuration and valve cycling are shown in FIGS. 2-9 in the '237 patent.

The typical GM type expander being built today has the regenerator located inside the piston. The piston/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 is nearly the same, the force required to cause the displacer to reciprocate is small, and can be provided by either a mechanical or pneumatic mechanism. In the descriptions that follow the term piston is used when it may also refer to a displacer.

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 piston/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.

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

U.S. Pat. No. 6,256,997 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. While the '997 patent describes the general principal and its application to relatively small and light displacers, the present invention describes a means of applying the principal to larger displacers and pistons in expanders that are producing more refrigeration and have larger and heavier pistons. This is accomplished by adding a collar extending from the top (warm end) of the piston that can have the same outside diameter as the piston and a lip at the top of the collar that engages an “O” ring before the piston hits the bottom (cold end) of the cylinder. The top end of the collar also engages an “O” ring before the piston hits the top (warm end) of the cylinder. Since 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 maximize the amount of energy they can absorb. “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.

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. Having the collar be the same diameter as the piston means that the clearances and machining tolerances that make them different are small.

SUMMARY OF THE INVENTION

The present invention provides a means of maximizing the energy absorbing capacity of bumpers that prevent the displacer or piston in a pneumatically driven cryogenic expander from hitting the cold or warm end of a cylinder. A collar is added to the warm end of the piston which can have the same outside diameter as the piston and a lip at the top end that engages an “O” ring before the piston hits the cold end or bottom of the cylinder. The top end of the collar also engages an “O” ring before the piston hits the warm end or top of the cylinder. 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 collar can also be used to drive the piston up and down in place of the typical drive stem. This design is referred to as a “collar bumper”

The options of having the bottom bumper be external to the collar for the Brayton expanders are not shown. Components that are equivalent in the drawings have the same identifying number.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1shows a schematic of a prior art pneumatically driven GM cycle expander that differs from the one shown in U.S. Pat. No. 3,119,237 only in having the regenerator internal to the piston rather than external to the cylinder All of the systems illustrated inFIGS. 1 through 7show the same compressor30, supply line31at high pressure, and return line32at 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 will allow pneumatically actuated expanders with higher cooling capacities to run quietly. These will require larger compressors which may be screw type compressors.

The expander has four main subassemblies. The cylinder subassembly comprises cylinder6a, cold end cap9, and warm flange7. The piston subassembly that reciprocates in the cylinder assembly comprises piston body1, regenerator19, drive stem2, and piston seal26near the warm end of piston body1. The cylinder head subassembly comprises cylinder head8a, stem cylinder18, and stem seal27. The valve subassembly, which is usually in a housing attached to the cylinder head subassembly, comprises valves12,13,14, and15. These valves are typically contained in a ported rotary valve driven by a motor. When piston1reciprocates it displaces gas in cold displaced volume3, warm displaced volume4, and drive stem displaced volume5. While most of these volumes are displaced as piston1reciprocates they also include void volumes in the form of clearances and gas ports. Valves14and15cycle gas to warm displaced volume4through line33then through ports21, regenerator19, and port20to cold displaced volume3. Valves12and13cycle gas to drive stem displaced volume5through line34. Seal17seals cylinder head8ato warm flange7.

A GM refrigeration cycle starts with the piston at the cold end, (cold displaced volume3minimized), the pressure in the cylinder and on the drive stem is high (valves12and14open, valves13and15closed). Valve12is then closed and13opened. Low pressure on the drive stem causes piston1to move up and draw high pressure gas into cold displaced volume3. Before the piston reaches the top valve14is closed and the pressure in the cylinder drops to a first pressure intermediate to the high and low pressures as the piston moves to the top. This pressure decrease results from warm gas being transferred from the warm displaced volume to the cold displaced volume. Valve15is then opened and the pressure in the cylinder drops to low pressure. Valve13is closed and12opened putting high pressure gas on the drive stem and pushing the piston down. Before reaching the bottom valve15is closed and the pressure in the cylinder increases to a second intermediate pressure as the piston moves to the bottom. This pressure increase results from cold gas being transferred from the cold displaced volume to the warm displaced volume. Valve14is then opened and the pressure increases to high pressure and the beginning of the next cycle. The P-V work done in cold displaced volume3is equal to the refrigeration produced per cycle.

FIG. 2shows GM expander100which differs from the prior art design ofFIG. 1, by the addition of collar22to piston1, and bumper “O” rings24and25. Collar22has an outside diameter that is about the same as piston1and does not rub the inside diameter of cylinder6ain the length that reciprocates in the cylinder. Cylinder head8bhas a neck that extends inside collar22and supports “O” ring bumper25in a lip at the bottom end that is near the inner diameter of collar22. Collar22has an internal lip at the top that engages “O” ring25when piston1reaches the cold end but before it hits cold end9. When piston1reaches the warm end the top of collar22engages “O” ring24before it hits cylinder head8b. The piston stroke is thus the distance piston1travels between compressed “O” rings24and25, and the length of the collar has to be longer than the stroke by the length of the lips on collar22and cylinder head8b. The space that is swept by drive collar22,11, is void volume that is connected to and adds to the void volume of displaced volume4. Pressurizing and depressurizing volume11may use 2 to 5% of the compressor flow. The refrigeration cycle of GM expander100is the same as that of the GM expander ofFIG. 1.

FIG. 3shows GM expander200which differs from GM expander100by having collar23have a lip on the top end of the collar that is external to the outside diameter of piston1. Cold bumper25is trapped in a section of the inside diameter of cylinder6babove the area where piston seal26slides. The external lip at the top of collar23engages “O” ring25when piston1reaches the cold end but before it hits cold end9. When piston1reaches the warm end the top of collar23engages “O” ring24before it hits cylinder head8c.

FIG. 4shows GM expander300which differs from GM expander200by replacing drive stem2, as the means to cause the piston to reciprocate, with collar23. This alternate means of driving the piston simplifies the design by eliminating the need for drive stem2, and drive stem cylinder18, and replaces stem seal27with inside collar seal28in cylinder head8d. The annular area between piston seal26and inner collar seal28is about the same as the area within stem seal27. An area that is about 15% of the cross section area of the piston is usually sufficient to overcome friction, pressure drop, and inertial forces needed to drive the piston. The line between valves12and13and volume10is designated as line35. GM expander300is more efficient than GM expanders100and200because volume10of GM expander300now includes the gas flow to drive the piston up and down that had been going to stem volume5and the void volume associated with the collar bumper is reduced. This is a preferred embodiment of this invention because cylinder head8dis simpler and the assembly is simpler than other embodiments. This drive mechanism is referred to as a “collar drive” which is analogous to the conventional “stem drive”.

FIG. 5shows GM expander400which differs from GM expander300by replacing drive collar23, which has the same outside diameter as the piston, with collar23bwhich has a smaller outside diameter. Cylinder head8ehas a smaller diameter neck and inside collar seal,28. Cylinder head8ealso has an outer section that holds bottom bumper25and also outer collar seal29. The cross section area of collar23b(between seals28and29) is also about 15% (<20%) of the cross section area of the piston. Gas ports37in the base of collar23bare needed to connect the inner and outer volumes of warm displaced volume4. GM expander400has the same advantages of efficiency as GM expander300relative to GM expanders100and200. Bumper “O” rings24and25are smaller than those that are about the same diameter as the piston but can be used with lighter pistons that do not need the maximum energy absorption of the larger bumper “O” rings. This is not a preferred embodiment of the collar bumper because it requires an additional seal,29.

FIG. 6shows Brayton expander500which has a stem drive and collar22with an internal lip, the same as GM expander100but the regenerator in the piston is replaced with external heat exchanger41and gas flow to cold displaced volume3is controlled by cold inlet valve43at high pressure and cold outlet valve44at low pressure, through line36. Brayton piston40separates cold displaced volume3from warm displaced volume4. A Brayton cycle expander has a big advantage over a GM expander in many applications because it makes the refrigeration available in remote heat exchanger42rather than only end cap9. It is easier to scale to larger sizes but it also has the disadvantage of being larger and more mechanically complex. The timing of opening and closing the valves to effect the same cycle as described for the GM cycle is shown in FIG. 7 of U.S. Pat. No. 9,080,794 in connection withFIG. 1option B.

FIG. 7shows Brayton expander600which has a collar drive. Collar22has an internal lip at the top that engages bottom bumper25before piston40hits cold end9. Cylinder head8fhas a neck that holds bottom bumper25and inner collar seal28. The operation of Brayton expander400is the same as Brayton expander300.

The object of this invention is to allow a cryogenic expander with a pneumatically driven piston to operate quietly in higher capacity refrigerators. The size of an “O” ring bumper is maximized by having it be about the same diameter as a piston and having a collar on the warm end of a piston with a lip at the top of the collar that engages the “O” ring bumper before it hits the cold end, and a similar “O” ring bumper that prevents it from hitting the warm end. Prior art “O” ring bumpers which have had smaller diameters have been adequate for pistons producing small amounts of refrigeration.

The rate at which refrigeration is produced is proportional to the high to low pressure difference and the rate of displacement, dV/dt, in the expansion space of a reciprocating expander. Given the same pressures the refrigeration rate is thus proportional to the square of the diameter of the piston, D, the stroke, S, and the cycle rate, N, eg. dV/dt=(SπD2N)/4. The kinetic energy of a piston is proportional to its mass, M, and velocity squared, (SN)2. If the displacement rate (refrigeration rate) is doubled by doubling the stroke or speed then the energy that has to be absorbed by the “O” ring bumpers is increased by a factor of four but the capacity of the bumper to absorb the additional energy has not changed. If the displacement rate is increased by doubling the area of the piston, and its length, stroke, and speed are kept the same then the kinetic energy is doubled, but an “O” ring bumper that is the diameter of the piston only increases in length by D √2. That is, if the displacement rate is increased by doubling the area of the piston, and its length, stroke, and speed are kept the same, then the kinetic energy is doubled and therein an “O” ring bumper that is the diameter of the piston only increases in length by D times 2.super.0.5. Regardless of what strategy is used to make larger displacement pistons lighter, a bumper “O” ring that is about the same diameter as the piston will maximize the refrigeration rate that can be produced by a pneumatically driven piston that runs quietly. A piston with a collar bumper enables this to be accomplished.