Miniature cryogenic heat exchanger

Abstract

A cryogenic heat exchanger has a pair of coaxial conduits. A pressurized refrigerant gas is passed through the internal conduit, out a thermal expansion valve and back through the annular space between the conduits. The closed end of the outer conduit is chilled by the expansion of the pressurized gas. The cryogenic effect is enhanced by precooling the pressurized warm gas with the depressurized cool return gas. A plurality of porous rings and disks bridging the lumen of the interior conduit and the annular space between the conduits assist in radial heat transfer to accomplish the desired precooling. The material chosen for the rings and disks is preferably highly porous, such as sintered metal, to increase the surface contact area and thereby increase heat transfer efficiency.

Description

CROSS REFERENCE TO RELATED APPLICATION

[0001] This is a §111(a) application relating to U.S. appln. Ser. No. 60/172,076 filed Dec. 23, 1999.

FIELD OF THE INVENTION

[0002] The present invention relates to cryogenic heat exchangers, and more particularly to miniature Joule-Thompson cryocoolers having an internal high pressure flow through an internal conduit to a thermal expansion valve and an external low pressure flow of refrigerant back to a compressor via a coaxial conduit.

BACKGROUND OF THE INVENTION

[0003] Joule-Thompson cryocoolers are known in the art of cryogenics and are used for numerous applications like sensor cooling and in miniature form for surgical instruments, such as in appliances for freezing diseased or precancerous tissue. The miniaturization of these apparatus is most critical for use in endoscopic surgery and for use with catheters for introduction into the circulatory system. A cryogenic cooler of this type is disclosed in U.S. Pat. No. 5,901,783 which utilizes a miniaturized heat exchanger having a plurality of laminated perforated plates and intervening spacers. The laminate structure provides an internal flow path for highly pressurized refrigerant that is conducted to a Joule-Thompson orifice through which the refrigerant is discharged. The expansion of the refrigerant at the orifice absorbs heat from the tip of the instrument, producing the desired cryogenic effect. A low pressure return conduit is defined by the laminate for conducting the expanded refrigerant away from the tip and back to a compressor. U.S. Pat. No. 5,901,783 recognizes that the low pressure return flow of refrigerant can be used to precool the pressurized refrigerant to enhance the cryogenic effect. The perforated plates in U.S. Pat. No. 5,901,783 are used as heat exchangers to increase the heat transfer from the warmer pressurized refrigerant to the cooler return flow, thus decreasing the temperature at the tip. U.S. Pat. No. 5,901,783 uses a dispersion welding process to laminate the perforated plates and spacers.

[0004] A similar cryogenic cooler was described in the Conference Proceeding Cryocoolers 10 in an article dated 1999 and entitled, “Experimental Comparison of Mixed-Refrigerant Joule-Thompson Cyrocoolers With Two Types of Counterflow Heat Exchangers” by Luo, et al., wherein a coaxial arrangement of inner and outer tubes define the pressurized and unpressurized refrigerant passageways and the perforated plates are stacked with interleaved spacers in the refrigerant passageways.

[0005] The perforated plates used in known Joule-Thompson cryocoolers are difficult and expensive to make, e.g., by a laser driven photo etching process, provide limited refrigerant-to-plate contact surface area, thereby limiting heat transfer, and constitute a significant flow restriction due to limited surface area per volume of material. Accordingly, it remains an objective to provide a cryogenic heat exchanger with more effective heat transfer and lower refrigerant flow restriction and further to provide for easier and cheaper methods for manufacturing a cryogenic cooler.

SUMMARY OF THE INVENTION

[0006] The limitations in the prior art are overcome by the present invention which includes a cryogenic heat exchanger having a first conduit for receiving pressurized refrigerant proximate a first end and discharging the refrigerant at a second end thereof. A thermal expansion valve is disposed at the second end of the first conduit. A second conduit, larger than the first conduit is disposed coaxially about the first conduit defining an annular space therebetween. The second conduit has a closed end proximate to the thermal expansion valve, such that refrigerant discharged from the first conduit through the thermal expansion valve impinges upon the closed end and is directed through the annular space.

[0007] A ring composed of porous material is disposed in the annular space proximate to the closed end for transferring heat from the refrigerant within the first conduit to the refrigerant within the annular space.

BRIEF DESCRIPTION OF THE FIGURES

[0008] For a better understanding of the present invention, reference is made to the following detailed description of an exemplary embodiment considered in conjunction with the accompanying drawings, in which:

[0009] FIG. 1 is a schematic diagram of a Joule-Thompson cryocooler cold head in accordance with the present invention;

[0010] FIG.2 is an exploded view of a pair of heat exchanger disks and an intervening spacer in accordance with an alternative embodiment of the present invention; and

[0011] FIG. 3 is a plan view of a heat exchanger ring in accordance with an alternative embodiment of the present invention.

DETAILED DESCRIPTION OF THE FIGURES

[0012] FIG. 1 shows a cryogenic cooler 5 having a thin-wall outer tube 10, a thin-wall inner tube 12, a tip 14 and a thermal expansion valve 16. Tubes 10 and 12 preferably have thin-walls to reduce conductive heat loss in the axial direction. The thermal expansion valve 16 is pressed, welded or threaded onto the end of the inner tube 12 and includes an upstanding nipple 18 with central inlet aperture 20. A discharge outlet 22 passes through the expansion valve 16 for conducting pressurized/fluidized refrigerant. Upon discharging from the discharge outlet 22, pressurized refrigerant expands and absorbs heat from the surroundings, including tip 14, which is used for contacting tissue to be excised or for application to some other work surface. The tip 14 is sealingly affixed to the outer tube 10 to prevent the leakage of refrigerant. Ports 24 are provided around the periphery of the expansion valve 16 to vent refrigerant that exits the expansion valve 16. The inner tube 12 defines an inlet passageway 25 for conducting pressurized refrigerant to the expansion valve 16. The outer tube 10 is substantially coaxial with the inner tube 12 and the annular space 27 therebetween defines a passageway for conducting expanded refrigerant back to a compressor or to a vent, depending on the application.

[0013] A plurality of ring-shaped heat exchange elements 26 are stacked in the annular space 27 between the inner tube 12 and outer tube 10 and are separated one from another by small and large ring spacers 28, 30, respectively, which are interposed therebetween. In a similar manner, the inner tube 12 contains a plurality of stacked heat exchanger disks 32 which are separated by internal ring spacers 34. The ring spacers 28, 30 and 34 are provided to interrupt heat transfer in the axial direction such that each heat exchange element 26 and heat exchanger disk 32, is preferably insulated from axially adjacent elements 26 and disks 32. In contrast, it is preferred to maximize the heat conduction between radially adjacent elements 26 and disks 32. In this manner, each radially adjacent set of heat exchanger 26 and heat exchanger disk 32 has an associated temperature range. The temperature ranges of the radially adjacent sets of heat exchanger 26 and heat exchanger disk 32 vary from one to the next in the axial direction in a stepwise temperature transition spanning the temperature differential between the tip 14 and the transfer tube portion 36 distal to the tip 14. The spacers 28, 30 and 34 can be formed in any cross-sectional shape so long as the open area thereof is sufficient to avoid diminishing the flow of refrigerant therethrough and causing a noticeable pressure drop.

[0014] The ring-shaped heat exchanger elements 26 and the heat exchanger disks 32 are dimensioned to promote heat transfer from the pressurized refrigerant in the inner tube 12 to the expanded refrigerant in the annular space 27. More particularly, the heat exchanger disks 32 preferably bridge the lumen of the inner tube 12 such that the refrigerant must pass through the disks 32. As the refrigerant passes through the disks 32, heat is transferred thereto, from the disks 32 to the inner tube 12, and from the tube 12 to the radially adjacent ring-shaped heat exchanger elements 26. The ring-shaped heat exchanger elements 26 preferably span the annular space 27 between the inner tube 12 and the outer tube 10 thereby forming a means for conducting heat from the inner tube to the refrigerant passing through the annular space 27 between the inner tube 12 and the outer tube 10. It can be appreciated that the flow of heat is radially outward.

[0015] A novel aspect of the present invention is the composition of the heat exchanger discs 32 and ring-shaped heat exchanger elements 26, which are formed from a porous material such as sintered metal. The use of sintered metal for the heat exchanger disks 32 and ring-shaped elements 26 increases the fluid-to-heat exchanger contact surface area while preserving a high flow rate of refrigerant through the sintered heat exchanger elements. The sintered elements therefore result in a net increase in heat transfer in the radial direction as described above which has the beneficial effect of pre-cooling the pressurized refrigerant. Pre-cooling allows the cryocooler to achieve lower temperatures and warms the expanded refrigerant preventing unwanted cryogenic effects from occurring at locations other than the tip, thereby allowing the surgeon to focus the cryogenic effect on the specific tissues needing treatment and avoiding unwanted, inadvertent destruction of healthy tissue through contact with an unnecessarily cold transfer tube 36 distal to the cold tip 14.

[0016] The porosity and heat conductivity along the transverse (radial) direction of the heat exchange elements 26, 32 is readily controlled by varying the thickness of the elements and the particle size of the metal used in sintering. In this manner, the requirements of fluid flow and heat exchange can be maximized for the particular refrigerant and dimensions of the cryocooler. More specifically, there are two different modes of heat transfer that are important to consider in this apparatus, viz., (1) conductive heat transfer through the heat exchanger elements 26, 32 in the radial direction or radial heat transfer Qr and (2) convective heat transfer (by convection between materials in two different phases) between the refrigerant and the heat exchanger elements 26, 32, Qf.

[0017] These two modes of heat transfer can be described by the following equations:

Qr=(kAs&Dgr;T F)/L  (1)

[0018] where

[0019] k is the thermal conductivity of the material;

[0020] As is the average-solid cross-sectional area of the conducting body;

[0021] &Dgr;T is the temperature differential between a first point on the body and a second point between which the conductivity is being measured;

[0022] F is a shape factor; and

[0023] L is the distance between the first and second points.

Qf=h AHT&Dgr;T  (2)

[0024] where

[0025] h is the heat transfer coefficient for the two materials/phases in question;

[0026] AHT is the heat transfer surface area (wetted area); and

[0027] &Dgr;T is the temperature differential between the materials/phases in question.

[0028] Comparing the porous heat exchanger of the present invention to the perforated plate heat exchanger of the prior art we can appreciate that Qr for the present invention might be slightly lower than Qr for a perforated plate, if all other conditions are the same. This is due to a difference in the shape factor F, which in the present invention is approximately in the range of 0.8 to 1.0, whereas in perforated plates, F approaches 1 as for a solid. Accordingly, the radial heat transfer Qr of the heat exchanger elements 26 and disks 32 of the present invention may be slightly less than perforated plate heat exchanger elements.

[0029] The foregoing loss of magnitude in Qr attributable to the shape factor F is more than compensated for in the present invention with respect to the heat transfer Qf in that the present invention has a much greater heat transfer contact area AHT (by an order of magnitude larger). Accordingly, the heat exchanger elements 26 and disks 32 of the present invention are about 10 to 100 times more efficient in transferring heat between the refrigerant and the heat exchange elements 26, 32. As a result, total heat transfer is far greater in the present invention than in heat exchangers using perforated plates for the same temperature difference.

[0030] In the present invention, the porosity can be varied between 5% to 90% for a given heat exchanger element 26, 32 volume. Porosity can be used to control flow volume and pressure. Moreover, for a given porosity, the hydraulic diameter, AH=Void Volume/(4) Surface Area, can be controlled, further contributing to control over fluid friction and pressure drop experienced as the refrigerant passes through the heat exchange elements 26, 32. Accordingly, the ratio of contact surface area to unit volume of material can exceed that of a perforated plate by a factor of 10 to 100.

[0031] Whereas sintered copper is described above, other sintered metals such as steel or any other material with high heat conductivity could be employed to form the heat exchanger elements. Further, other porous materials, such as metallized or ceramic foams, such as those which may be obtained from Ultramet of Pacoima, Calif. could be employed.

[0032] FIGS. 2 and 3 show alternative structures for the heat exchange elements. FIG. 2 shows disk heat exchangers 132 for placement into an inner tube 112. The disks 132 are formed from a wire mesh, such as copper screening. Wire mesh tends to have a greater open area ratio than either sintered metal or perforated plates. The heat conducting capacity of the wire mesh heat exchangers 132 can be increased by pressing down on the mesh with a suitable press which causes the individual wires to flatten at intersection points to increase the contact area between individual wires of the mesh, decreasing the porosity of the screen and increasing the radial heat transfer and the surface contact area per unit volume. As before, a spacer 130 is introduced between adjacent heat exchanger disks 132. Besides the spacer 130 configuration shown, the spacer could be any other porous shape that physically separates the heat exchanger disks 132 without impeding refrigerant flow. The spacer 130 may be formed from a thermal insulation material such as sintered polymer or nylon and may be in the form of a screen disk with greater open area than the heat exchanger disks 132.

[0033] FIG. 3 shows a ring-shaped heat exchange element 126 that is formed from a mesh material, such as copper or stainless steel screening, and that would be suitable for introduction into an annular space 27 (FIG. 1) for contacting a return flow of refrigerant. U.S. Pat. No. 5,901,783 is incorporated herein by reference for its teachings concerning the basic structure, assembly and operation of cryocoolers.

[0034] It should be understood that the embodiments described herein are merely exemplary and that a person skilled in the art may make many variations and modifications without departing from the spirit and scope of the invention as defined in the appended claims. All such variations and modifications are intended to be included within the scope of the present invention as defined in the appended claims.

Claims

1. A cryogenic heat exchanger, comprising:

a first conduit for receiving pressurized refrigerant proximate a first end and discharging the refrigerant at a second end thereof;

a thermal expansion valve disposed at said second end of said first conduit;

a second conduit, larger than said first conduit and disposed coaxially about said first conduit so as to define an annular space therebetween, said second conduit having a closed end proximate to said thermal expansion valve, whereby refrigerant discharged from said first conduit through said thermal expansion valve impinges upon said closed end and is directed through said annular space; and

at least one ring composed of porous material and disposed in said annular space proximate to said closed end, said at least one ring transferring heat from the refrigerant within said first conduit to the refrigerant within said annular space.

2. The heat exchanger of

claim 1, further including at least one disk composed of porous material and disposed within said first conduit for transferring heat from the refrigerant within said first conduit to the refrigerant within said annular space.

3. The heat exchanger of

claim 2, further including a plurality of rings composed of porous material disposed in said annular space and a plurality of disks composed of porous material disposed in said first conduit.

4. The heat exchanger of

claim 3, further including first spacers between said plurality of rings and second spacers between said plurality of disks, said first spacers and said second spacers slowing the rate of heat transfer in an axial direction relative to said first and second conduits.

5. The heat exchanger of

claim 4, wherein said first spacers and said second spacers have a greater open area than said rings and said disks, respectively.

6. The heat exchanger of

claim 5, wherein said first spacers and said second spacers have a material composition with a smaller heat conductivity than that of said rings and said disks, respectively.

7. The heat exchanger of

claim 6, wherein said first spacers and said second spacers are ring-shaped.

8. The heat exchanger of

claim 7, wherein said rings and said disks substantially bridge said annular space and a lumen of said first conduit, respectively.

9. The heat exchanger of

claim 2, wherein said porous ring material or said porous disk material is sintered metal.

10. The heat exchanger of

claim 9, wherein said sintered metal is copper.

11. The heat exchanger of

claim 9, wherein said sintered metal is stainless steel.

12. The heat exchanger of

claim 8, wherein said porous ring material and/or said porous disk material is metallized foam.

13. The heat exchanger of

claim 8, wherein said porous ring material and/or said porous disk material is metal screening.

14. The heat exchanger of

claim 8, wherein said porous ring material and/or said porous disk material is ceramic foam.

15. The heat exchanger of

claim 13, wherein said metal screening is compressed to flatten adjacent wires of said screening against each other to increase heat transfer through the junction made thereby.

16. The heat exchanger of

claim 3, wherein said ring material and said disk material are the same.

17. The heat exchanger of

claim 1, wherein said ring material has a porosity in the range of from about 5% to about 95% and a heat transfer contact area AHT that is about 10 to about 100 times greater than that of a comparably dimensioned perforated ring material.

18. A cryogenic heat exchanger, comprising:

a first conduit for receiving pressurized refrigerant proximate a first end and discharging the refrigerant at a second end thereof;

a thermal expansion valve disposed at said second end of said first conduit;

a second conduit, larger than said first conduit and disposed coaxially about said first conduit so as to define an annular space therebetween, said second conduit having a closed end proximate to said thermal expansion valve, whereby refrigerant discharged from said first conduit through said thermal expansion valve impinges upon said closed end and is directed through said annular space; and

means for transferring heat from the refrigerant within said first conduit to the refrigerant within said annular space.

19. A method for making a cryogenic heat exchanger, comprising the steps of:

(a) providing a first conduit for receiving pressurized refrigerant proximate a first end and discharging the refrigerant at a second end thereof;

(b) attaching a thermal expansion valve to said second end of said first conduit;

(c) positioning a second conduit, which is larger than said first conduit, coaxially about said first conduit to define an annular space therebetween;

(d) inserting a ring composed of porous material in said annular space proximate to said thermal expansion valve; and

(e) affixing a closure member over said second conduit proximate said thermal expansion valve such that refrigerant discharged from said first conduit through said thermal expansion valve impinges upon said closure member and is directed through said annular space.

20. The method of

claim 19, further comprising the steps of inserting a plurality of rings of porous material in said annular space with a plurality of intervening insulator spacers to form a stacked laminate of alternating spacers and rings, and further inserting a plurality of disks composed of porous material into a lumen of said first conduit, said disks inserted with a plurality of intervening spacers to form a stacked laminate of alternating disks and spacers.