FIELD OF THE INVENTION

[0001] The present disclosure relates generally to cryocoolers and, in particular, to a generally vaporless, sealed rotary drive for a cryocooler and cryocoolers featuring such a rotary drive.

BACKGROUND

[0002] Cryocoolers are refrigeration devices capable of pumping heat from very low temperatures, typically described as below 150K (-123C). Most cryogenic applications are focused at the normal temperature of liquid nitrogen, LN2, at 77K, but others exist (e.g., liquefied natural gas, or LNG, at about 130K). LN2 is relatively inexpensive and widely available in developed countries, so it has become the coolant default.

[0003] In some applications, however, even lower temperatures are required. For example, for magnetic resonance imaging (MRI) systems used in medicine, liquid helium (4K) is used as a coolant. Due to the expense of liquid helium, it is recovered with on-site refrigeration in the form of very expensive cryocoolers, typically of the well- known Gifford-McMahon type or variants thereof. Examples of such devices are available from Cryomech, Inc. of Syracuse New York. Such devices require

disassembly and a rebuild on an annual basis to maintain performance.

[0004] For imaging applications at slightly less severe temperatures, and with low sensitivity to cost (e.g., spacecraft and military optical systems), fully-sealed Stirling- cycle devices are preferred. These are essentially maintenance-free, but are much more expensive than Gifford-McMahon type cryocoolers. Examples of such cryocoolers include the CRYOTEL free-piston units available from Sunpower, Inc. of Athens, Ohio and the QDRIVE cryocoolers available from Qdrive of Troy, New York (a Chart

Industries, Inc. company). Larger, non-hermetic offerings also exist for this temperature range, examples of which include units from Stirling Cryogenics of Eindhoven,

Netherlands, but these are not maintenance-fee and require annual rebuilds.

[0005] In view of the above, prior art cryocoolers are generally either unsealed and in need frequent maintenance (e.g., the Gifford-McMahon products of companies like Cryomech, Inc. of Syracuse, New York or the cryocoolers of Stirling Cryogenics) or are sealed, with specially built internal motors and costs too expensive for many

applications.

[0006] The underlying reason for one or the other of these two requirements: (1 ) sealed, expensive construction, or (2) repairable, open construction but with frequent rebuilds, is the need for excluding all contaminants from the internal cycle or cycle space of a cryocooler where there exists, by definition, a very low temperature region and a warm or hot region (where heat lifted from the cold space is rejected). Anything which tends to condense or freeze at the extremely low temperatures present within the cycle space will, in fact, do so, even if such a contaminant is nominally located in the warmer regions. Such condensation is driven by the difference in vapor pressures between the regions at different temperature. Higher vapor pressure at the warm conditions produces outgassing from materials with significant vapor pressure there. The resulting vapor then travels to, and then re-condenses in, the colder region. This process continues as long as the temperature difference and evaporable materials are present. In general, liquids and gels have such vapor pressure differences, whereas solids like metals do not. [0007] Helium, a common working fluid in cryocoolers, does not condense except at 4K and below. On the other hand, nitrogen, oxygen, carbon dioxide, and water - all common atmospheric gases that can easily be present in the cycle space of a cryocooler if the device is not cleaned extremely well, all condense at temperatures from 273K (water) to 77K (nitrogen). Such condensation tends to block heat exchange passages in the cryocooler, leading to flow obstructions and loss of performance.

[0008] The presence of hydrocarbons (such as methane, ethane, and higher paraffins, including oil and grease components) within the cycle space of the cryocooler are also detrimental to cooler performance. Hydrocarbons partially vaporize at normal ambient temperatures. If present in a cooler cycle space, such vapors are transported to the cold region and condense or freeze to the same bad effect as atmospheric gases.

[0009] For this reason, the existing sealed units are completely oil -free and depend instead on costly gas bearings, flexure supports or, in a few cases, solid-state lubricant surfaces. All of these arrangements enable movement of the internal parts without oil.

[0010] The open systems depend instead on dynamic seals to separate oil- lubricated mechanisms from dry extension of those mechanisms reaching into and wholly within the cycle space. This minimizes the presence of hydrocarbons and other contaminants in the cycle space even without perfect seals, but not for long. Hence, as noted previously, such devices must be rebuilt and cleaned regularly to remove the accumulated contamination that gets by the dynamic seals.

[0011] A robust cryocooler with the low cost of open units and the low maintenance needs and durability of the sealed units, suitable in scale for applications from a few watts to a few kilowatts of capacity at temperatures from 50-150K, is desirable. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Fig. 1 is a schematic of an embodiment of the rotary drive of the disclosure as used in an acoustic-Stirling cryocooler;

[0013] Fig. 2 is a schematic of an embodiment of the rotary drive of the disclosure as used in a Stirling cryocooler;

[0014] Fig. 3 is a schematic of an embodiment of the rotary drive of the disclosure as used in a Gifford-McMahon cryocooler;

[0015] Fig. 4 is a schematic of an embodiment of the rotary drive of the disclosure as used in a Joule-Thompson cryocooler.

SUMMARY OF THE DISCLOSURE

[0016] There are several aspects of the present subject matter which may be embodied separately or together in the devices and systems described and claimed below. These aspects may be employed alone or in combination with other aspects of the subject matter described herein, and the description of these aspects together is not intended to preclude the use of these aspects separately or the claiming of such aspects separately or in different combinations as set forth in the claims appended hereto.

[0017] In one aspect, a rotary drive device for generating a gas pressure wave for a cryocooler is provided, where the rotary drive includes a crankcase that is hermetically or semi-hermetically sealed with respect to the ambient environment. A crankshaft is rotatably mounted within the crankcase, and a cylinder is positioned within the crankcase and adapted to be in fluid communication with a coldhead of the cryocooler. A piston is slidably positioned within the cylinder and a connecting rod or other kinematic link joins the piston to the crankshaft. A motor is mounted external to the crankcase and there is a non-contact coupling between the motor and the crankshaft so that the crankshaft is rotated when the motor is activated. The crankshaft may be rotatably mounted within the crankcase on bearings that are lubricated solely with a grease having a vapor pressure at the bearings' operating temperature that is not substantially higher than the grease's vapor pressure at a cryogenic temperature of the cryocooler, or the crankshaft may be rotatably mounted within the crankcase on bearings that are lubricated solely with a grease having a vapor pressure at the bearings' operating temperature that is less than approximately 1 E-5 Torr.

[0018] In another aspect, a cryocooler is provided where a rotary drive includes a crankcase that is hermetically or semi-hermetically sealed with respect to the ambient environment. A crankshaft is rotatably mounted within the crankcase, and a cylinder is positioned within the crankcase. A piston is slidably positioned within the cylinder and a connecting rod or other kinematic link joins the piston to the crankshaft. A motor is mounted external to the crankcase and there is a non-contact coupling between the motor and the crankshaft so that the crankshaft is rotated when the motor is activated. The crankshaft may be rotatably mounted within the crankcase on bearings that are lubricated solely with a grease having a vapor pressure at the bearings' operating temperature that is not substantially higher than the grease's vapor pressure at a cryogenic temperature of the cryocooler, or the crankshaft may be rotatably mounted within the crankcase on bearings that are lubricated solely with a grease having a vapor pressure at the bearings' operating temperature that is less than approximately 1 E-5 Torr. A coldhead is in fluid communication with the cylinder so that the cold head receives a gas pressure wave when the piston slides within the cylinder.

[0019] In another aspect, a process of providing a gas pressure wave to a coldhead of a cryocooler includes the steps of rotating a crankshaft positioned within a

hermetically or semi-hermetically sealed crankcase using a motor positioned external to the crankcase, reciprocating a piston within a cylinder using the rotating crankshaft so that a gas pressure wave is produced and directing the gas pressure wave to the coldhead.

DETAILED DESCRIPTION OF EMBODIMENTS

[0020] In accordance with the disclosure, a rotary electric motor (or other prime mover) outside the cycle space of a cryocooler drives elements, including a shaft, inside the cycle space through a rotary magnetic coupling, or other non-contact coupling, that transfers power across a hermetic or a semi-hermetic seal that may bear internal pressure within the cycle space.

[0021] The driven elements within the cycle space are rotary (and substantially coaxial) with the driving element outside the space. Such internal rotary elements, specifically a crankshaft and a connecting rod, or other kinematic link, with a piston, either require lubricants such as grease or oil in their bearings, or must be limited to unacceptably low speeds and forces for dry-lubricated bearing materials. As noted previously, the presence of hydrocarbon lubricants in the cycle space is undesirable as they may contaminate the cycle space.

[0022] In the illustrated embodiments, rolling element bearings are used in the cycle space, but are provided with a lubricating grease that possesses extremely low vapor pressure at the elevated operating temperatures of the bearings. As an example only, the grease preferably has a vapor pressure of 1 E-5 Torr or below when bearing temperatures are approximately 50°C. Notably, a low vapor pressure at elevated temperature means that there is negligible difference between that and the vapor pressure at cryogenic temperature. This equates to no significant tendency for vapor to condense in the cold region of the cryocooler cycle space.

[0023] In addition, the grease also preferably exhibits a load capacity comparable to conventional bearing greases as indicated, for instance, by a comparably high weld point (where the lubrication film would fail, causing local metal -metal bonding).

[0024] As an example only, a fluorocarbon grease may be used. An example of a suitable fluorocarbon grease is KRYTOX GPL226, available from The Chemours Company of Wilmington, Delaware, which has a vapor pressure estimated at around 1 E-10 Torr at 20C temperature (approaching that of solid metals).

[0025] The illustrated embodiments thus apply an effectively 'dry' grease to rolling- element bearings and dry lubricated sliding seals on a crank-driven piston-crankshaft assembly, all within the cryocooler cycle space, driven through a magnetic coupling, to create the oscillatory pressure wave that drives a Stirling, an acoustic-Stirling, or other type of refrigeration cycle. Indeed, as illustrated below, by inclusion of appropriate valves, the illustrated rotary drive may also be used drive a Gifford-McMahon cycle as a vapor-free compressor.

[0026] An acoustic Stirling cryocooler including an embodiment of the rotary drive of the disclosure is indicated in general at 10 in Fig. 1. The cryocooler includes an embodiment of the drive of the disclosure, indicated in general at 12, and a coldhead, indicated in general at 14.

[0027] The drive 12 includes a rotary electric motor 16 mounted to a hermetically or semi-hermetically sealed crankcase 18. While an electric motor is shown and described in the illustrated embodiments, alternative types of rotary or non-rotary motors may be used including, but not limited to, combustion motors.

[0028] The motor 16 spins a shaft 22 to which is attached a ring of driving magnets 24 via a bracket 26. A sealed non-contact coupling, indicated in general at 28, takes the form of a magnetic coupling and includes the driving magnets 24, corresponding driven magnets 32, and an interposed non-ferromagnetic sealing vessel 34 that closes or seals the crankcase 18 with respect to the ambient environment in a hermetic or semi-hermetic fashion. As an example only, the sealing vessel may be constructed of austenitic stainless steel or high-strength borosilicate glass and secured to the crankcase 18 using bolts (or other fasteners) with gasket material at the junction of the vessel and the crankcase. This enables retention of working fluid (which may be pressurized) within the crankcase and prevents contaminants from entering, yet also passes work into the driven components within the crankcase through the interaction of the magnetic field flux that passes freely through the sealing vessel 34.

[0029] Alternative types of non-contact couplings may be used in place of the illustrated magnetic coupling.

[0030] The driven magnets 32 are mounted on crankshaft 36, which is mounted within the crankcase in a rotating fashion through two rolling-element bearings 38 and 42, each lubricated with a one-time charge of low vapor pressure grease that has a vapor pressure at the bearings' operating temperature that is not substantially higher than its vapor pressure at cryogenic temperature. If the vapor pressure at the bearing operating temperature (for example, approximately 50°C) is sufficiently low (for example, below 10-5 Torr), than a vapor pressure of zero at the cryogenic temperature (for example, approximately -200°C) would even be acceptable. As an example only, as noted above, the grease may be a fluorocarbon grease. Crankshaft 36 preferably is shaped generally as shown (in a cross sectional, side elevational view), with stepped diameters mating to inner diameters of bearings 38 and 42 and an eccentric reduced diameter mating to the inner diameter of piston bearing 44. Piston bearing 44 is similarly lubricated with a one-time charge of low vapor pressure grease.

[0031] Piston bearing 44 is mounted within the proximal end portion of a rod 46, the distal end of which is provided with a piston head 48. While a rocker-type piston (without a separate wrist pin) is illustrated, alternative types of pistons may be used. Piston head 48 reciprocates in a cylinder defined by cylinder wall 52, riding on dry- lubricated seal 54, and, together with the interior surface of cylinder wall 52 and portions of crankcase 18, forms a variable-volume compression chamber 56. As an example only, seal 54 may be a cup seal including a polytetrafluoroethylene-based compound, such as RULON, available from Saint-Gobain S.A. of Paris, France.

[0032] Chamber 56 is connected by duct 62 to an acoustic Stirling (pulse tube) coldhead 14 which includes a coldfinger, indicated in general at 64, heat rejectors 66a and 66b and a main body, indicated in general at 68, that includes an inertance tube 72 and 74. A near end of the tube 72 communicates with the heat exchangers in the cycle space of the cold finger 64. A distal end of the inertance tube 74 is an open end that communicates with a compliance tank, which in this figure is the volume enclosing the tube 74. Together, the inertance tube and compliance tank form a phasing network which is tuned to impose a pressure wave at the heat exchangers which lags the pressure wave imposed at the opposite end of the heat exchangers by the wave in duct 62 created by the drive portion 12. The drive portion 12 serves as a pressure wave generator (PWG) for the coldhead that cyclically compresses and expands the helium gas working fluid relative to the mean pressure (charge pressure) of the coldhead 14. While the coldhead of the present embodiment and the embodiments described below are described as using helium gas as the working fluid or coolant, alternative cryogenic fluids may be used as the working fluid or coolant.

[0033] Briefly, with each forward stroke of the piston, during which the piston head 48 moves towards the position illustrated in solid lines in Fig. 1 , a pressurized wave or pulse of compressed helium gas travels through duct 62 and into an annular passage 76 of the coldhead. The working fluid wave then flows through a warm heat exchanger 78, where heat is removed via heat rejectors 66a and 66b. The working fluid wave is further cooled by passage through a regenerator 82. The cooled helium gas wave then flows through a cold heat exchanger 84.

[0034] As the gas wave moves towards the cold heat exchanger, gas in the buffer tube 92 moves towards the inertance tube 72. Even as the driven gas stops advancing (i.e. when the piston 48 of Fig. 1 reaches its right-most position shown in solid lines in Fig. 1 ), the gas in the pulse tube and inertance tube, driven by its own inertia, keeps moving away from the cold heat exchanger 84 of the cold finger. The gas in the buffer and inertance tube therefore serves as a virtual piston to expand the gas in the area of the cold heat exchanger 84 of cold finger 64.

[0035] As a result, the tip of cold finger 64, which is in thermal communication with the space or process to be refrigerated - indicated in phantom at 86, absorbs heat and thus provides refrigeration.

[0036] As the piston 48 begins withdrawing, that is,moving to the left of Fig. 1 towards the position illustrated in phantom at 48', the helium gas slows and reverses its flow, drawn back through inertance tube 74 and 72 in the main body 68, and the regenerator 82. The above cycle then repeats and approximates a two-piston Stirling cycle in the region of the heat exchangers.

[0037] Heat exchanger 78 may alternatively be liquid cooled or air cooled as shown, to reject the heat lifted from cold exchanger 84.

[0038] Additional details regarding the acoustic-Stirling coldhead 14 are disclosed in commonly owned U.S. Patent Application No. 14/450, 142 to Corey et al., the contents of which are hereby incorporated by reference.

[0039] The drive 12 therefore provides an inexpensive and readily connected and serviced rotary electric motor 16 that provides the pressure wave required to drive the coldhead 14 with inexpensive intermediate components, but there is no contamination of the working fluid inside of the crankcase 18 or the cycle space of the coldhead 14.

[0040] A cryocooler that uses the drive described above with a conventional Stirling coldhead is indicated in general at 100 in Fig. 2.

[0041] The drive 1 12 features the same construction as the drive 12 of Fig. 1 , and thus the same references numbers are used. As with the drive 12 of Fig. 1 , the drive 1 12 uses an external rotary electric motor 16, that is positioned outside of crankcase 18, to drive elements within the interior cycle space of the crankcase, including a shaft 36, through a rotary magnetic coupling that transfers power across a semi-hermetic seal. As with the drive of Fig. 1 , the shaft bearings 38 and 42 and piston bearing 44 are lubricated with a one-time charge of low vapor pressure grease. As with the drive 12 of Fig. 1 , the motor 16 therefore drives the piston head 48 in a reciprocating fashion so that the drive serves as a pressure wave generator for the Stirling coldhead 1 14.

[0042] As the cycle begins, the displacer 116 is in the bottom-most position in its cylinder, illustrated in solid lines in Fig. 2, to provide maximum space for upper chamber 1 18. The piston head 48, initially at the top-most position (illustrated in phantom at 48' in Fig. 2), moves downwards (towards the position illustrated in solid lines for piston 48 in Fig. 2) so that a pressure wave of helium gas working fluid flows through heat exchanger 122 and is thus compressed isothermally (with heat rejected through heat exchanger 122). As in all heat exchangers disclosed herein, heat exchanger 122 may either be a liquid-cooled or air-cooled heat exchanger.

[0043] The compressed helium gas wave travels from heat exchanger 122 to space 1 18 of the displacer cylinder. The displacer 1 16 then moves upwards towards the position illustrated in phantom at 1 16' in Fig. 2 so that the gas from the upper chamber 1 18 is forced through regenerator 124 and into the lower chamber space 126. As a result, the gas is further cooled at constant volume, with the heat energy stored in the regenerator matrix. The lower chamber 126 is provided with maximum space when the displacer is in the position illustrated in phantom at 1 16' in Fig. 2. [0044] The piston 48 next travels upwards (towards the position illustrated in phantom at 48' in Fig. 2) which provides isothermal expansion of the gas at cold heat exchanger 128, which is in thermal communication with the space or process to be refrigerated. As a result, heat is absorbed by the cold heat exchanger 128 so as to provide the required refrigeration.

[0045] The displacer 1 16 next moves downwards so that gas is forced back through the regenerator 124, where it is heated with the energy stored previously. The cycle may then be repeated.

[0046] The displacer 1 16 may be reciprocated (via rod 130) off of the crankshaft 36 of the drive 1 12, or may use a dedicated drive which may or may not be of the type illustrated for drive 1 12.

[0047] A cryocooler may also use the drive described above with a Vuilleumier coldhead.

[0048] A cryocooler that uses the drive described above with a Gifford-McMahon coldhead is indicated in general at 300 in Fig. 3. In this embodiment, however, inlet check valve 304 and outlet check valve 306 have been added so that an inlet and outlet to drive 312 is formed. This converts the drive 312 from an oscillatory compressor to a continuous compressor to provide pressurized helium gas to the coldhead 314.

[0049] The drive 312 features the same construction as the drive 12 of Fig. 1 , and thus the same references numbers are used. As with the drive 12 of Fig. 1 , the drive 312 uses an external rotary electric motor 16, that is positioned outside of crankcase 18, to drive elements within the interior cycle space of the crankcase, including a shaft 36, through a rotary magnetic coupling that transfers power across a semi-hermetic seal. As with the drive of Fig. 1 , the shaft bearings 38 and 42 and piston bearing 44 are lubricated with a one-time charge of low vapor pressure grease. As with the drive 12 of Fig. 1 , the motor 16 therefore drives the piston head 48 in a reciprocating fashion.

[0050] Due to check valves 304 and 306, the drive 312 serves as a continuous compressor for the Gifford- c ahon coldhead 314. In operation, the drive 312 operates as a compressor to build a supply of pressurized (to a pressure of P ₂ ) helium gas within high pressure surge volume 314 and a lower pressure P1 in low pressure surge volume 336.

[0051] A reciprocating displacer 316 is positioned within a cylinder 318 so that upper and lower expansion spaces 320 and 322 are defined. The volume of these spaces depends on the position of the displacer 316.

[0052] With the displacer at the bottom of the cylinder, that is, when the displacer 316 is in the position illustrated in solid lines in Fig. 3, an intake valve 324 is opened and the pressure within the upper expansion space 320 is increased from a low pressure Pi to a higher pressure P ₂ . An exhaust valve 326 remains closed during this step, and the volume of the lower expansion space 322 is minimized as the displacer is at its lowest position.

[0053] While the intake valve 324 remains open and the exhaust valve 326 remains closed, the displacer 316 is moved to the top of the cylinder 318, to the position illustrated in phantom at 316'. As a result, the gas in the upper expansion space 320 is pushed out and flows down through a regenerator 328 and then through line 332 to the lower expansion space 322. Lower expansion space 322 reaches its maximum volume when the displacer is at the top of the cylinder (316'). The gas is cooled as it passes through the regenerator 328 and thus decreases in volume. As a result, gas is drawn in through the intake valve 324 to maintain a constant pressure within the system.

[0054] When the displacer is at the top of the cylinder, the intake valve 324 is closed and the exhaust valve 326 is opened. This allows the gas within the lower expansion space 322 to expand to the lower pressure Pi, which causes the gas in the lower expansion space to drop to a still lower temperature.

[0055] The displacer 316 is next moved downward to the bottom of the cylinder 318, into the position illustrated in solid lines in Fig. 3, to force the low temperature gas out of the lower expansion space 322. The cold gas flows through a heat exchanger 334, which is in thermal communication with the process or space that is refrigerated. As a result, the gas is warmed in heat exchanger 334, providing a refrigeration effect.

[0056] After passing through heat exchanger 334, the helium gas flows back through the regenerator 328 whereby the gas is further warmed back to near ambient temperature by the heat energy stored in the regenerator. The gas then passes through the exhaust valve to low pressure surge volume 336.

[0057] The displacer 316 may be reciprocated (via rod 338) off of the crankshaft 36 of the drive 312, or may use a dedicated drive which may or may not be of the type illustrated for drive 312.

[0058] A cryocooler that uses the drive described above with a Joule-Thompson coldhead is indicated in general at 400 in Fig. 4. In this embodiment, however, check valves 404 and 406 have been added so that an inlet and outlet to drive 412 are formed. This converts the drive 412 from an oscillatory compressor to a continuous compressor to provide pressurized helium to the coldhead 414. [0059] The drive 412 features the same construction as the drive 12 of Fig. 1 , and thus the same references numbers are used. As with the drive 12 of Fig. 1 , the drive 412 uses an external rotary electric motor 16, that is positioned outside of crankcase 18, to drive elements within the interior cycle space of the crankcase, including a shaft 36, through a rotary magnetic coupling that transfers power across a semi-hermetic seal. As with the drive of Fig. 1 , the shaft bearings 38 and 42 and piston bearing 44 are lubricated with a one-time charge of low vapor pressure grease. As with the drive 12 of Fig. 1 , the motor 16 therefore drives the piston head 48 in a reciprocating fashion.

[0060] As illustrated in Fig. 4, the coldhead 414 includes a heat exchanger 416, a Joule-Thompson expansion valve 418 and an evaporator 420. In operation, the compressed helium gas passes through valve 406 and then through the passage 422 of heat exchanger 416 where it is cooled to a low temperature by heat exchange with a cold outgoing gas stream passing through heat exchanger passage 424. The cooled gas stream is then expanded through the Joule-Thompson expansion valve 418 and directed into the evaporator 420, which is in thermal communication with the space or process being refrigerated. In the evaporator, the liquid helium 426 formed from a portion of the helium gas after the expansion process is evaporated (at constant temperature) by absorbing heat from the space or process being refrigerated. The resulting vapor then returns to the compressor/drive 412 through passage 424 of the heat exchanger and valve 404.

[0061] It will be clear to one skilled in the art that the rotary drive of the disclosure can serve coldheads in addition to those described above. All can benefit from the low- cost (both in terms of equipment cost and operation) and generally vapor-free operation offered by the drive of the disclosure.

[0062] In addition, many applications could be better served with the affordable and reliable low-maintenance cry cooler of the disclosure (combined with a gas source and storage dewar), especially given the practicality of such a cryocooler for installation at the point of use. Examples include, but are not limited to, home liquefaction of concentrated oxygen for ambulatory medical treatment of lung disease; in-office production of LN2 for dermatology use; in-tank recovery of LNG boil -off; and even primary production of some liquefied gases at the point of use for commercial and agricultural needs.

[0063] While the preferred embodiments of the disclosure have been shown and described, it will be apparent to those skilled in the art that changes and modifications may be made therein without departing from the spirit of the disclosure, the scope of which is defined by the following claims.