Cryogenic recondenser with remote cold box

Abstract

A recondenser cycles a working volume of cryogen gas through a remote cold box and a coaxial recondensing, heat exchanger transfer line which is inserted into a cryostat. The working volume of gas is compressed to a high pressure and cooled through cooling means which include a mechanical refrigerator of the regenerator-displacer type. The cooled gas is expanded through a first jt valve to a medium pressure and further cooled. The further cooled medium pressure gas is transferred in a closed coaxial transfer line to a cryostat in which boil-off is recondensed. A second jt valve in the cryostat end of an inner tube coaxially positioned in an outer tube forming the transfer line expands the gas to a lower pressure and forms a liquid-gas mixture. The liquid-gas mixture is passed in heat exchange relation with the boil-off from an inner tube to an outer tube of a coaxial recondensing heat exchanger. The outer surface of the outer tube at the cryostat end of the transfer line has burrs which provide the necessary surface area on which to recondense the boil-off. The gas is transferred back to the cooling means through intermediate channels formed between the outer tube and the coaxially positioned inner tube.

Description

BACKGROUND OF THE INVENTION

Several superconducting devices of today, such as superconducting computers and superconducting magnets of magnetic resonance imaging systems, use an inventory of liquid cryogen (i.e. helium) for continuous refrigeration. Usually a cryostat or vacuum jacketed reservoir of the liquid cryogen is used to cool the device to achieve superconductivity. As the device is used, heat is generated and the inventory of liquid cryogen boils off. In the case of mobile magnetic resonance imaging systems, it is necessary to demagnetize the device for each rod trip. The demagnetization process further causes several liters of cryogen to be boiled off. In order to maintain and replenish the inventory of liquid cryogen a continuous supply of gaseous cryogen must be provided, liquified and introduced into the liquid inventory; or a means of recondensing the boil off back into the liquid inventory must be provided.

One approach to recondensation has been to collect the venting gas and direct it to refrigeration apparatus outside of the cryostat which recondenses the cryogen. The liquid cryogen is reintroduced into the cryostat. However, problems arise in transferring the liquid cryogen back to the cryostat while maintaining the cold temperature.

Another approach has been to place a refrigerator directly in an access port or neck of the cryostat. Such refrigerators are disclosed in U.S. Pat. Nos. 4,223,540 and 4,484,458. Each discloses a displacer-expander refrigerator in conjunction with a Joule-Thomson heat exchanger. The refrigerator is disposed in at least one access port to cool heat shields of the cryostat and to recondense the cryogen boil-off. U.S. Pat. No. 4,223,540 minimizes heat transfer losses by matching the temperature gradient in the access port. U.S. Pat. No. 4,484,458 matches the thermal gradient in the heat exchanger with that of the refrigerator, to minimize heat loss in the cryostat when the refrigerator is in use.

Having the apparatus or a refrigerator disposed within the cryostat housing, it then becomes necessary to provide means to remove the refrigerator should it have to be serviced. With such removal, however, there is a danger of exposing the liquid cryogen inventory to ambient conditions and allowing heat infiltration which would in turn promote cryogen boil-off. One method to solve this problem of removal is to specially design the cryostat. However, the refrigerators for such cryostats typically have relatively high heat transfer losses, and the cryostats have large cross-sectional areas. U.S. Pat. No. 4,223,450 discloses a cryostat utilizing a closed-cycle refrigerator with several stages of refrigeration to intercept heat leak into the liquid cryogen and to recondense cryogen boil-off. The cryostat is adapted to removal, repair and replacement of the refrigerator while the superconducting device continues operation. However, designing such a cryostat for each different super conducting device is costly and impractical.

A further problem with cryostat refrigerators of prior art is the large access area to the cryostat necessitated by the refrigerator compared to the smaller access ports of todays devices. Smaller access ports are being made to decrease the amount of heat infiltration to the cryogen and therefore to prevent promotion of boil-off. More particularly, in the case of a magnetic resonance imaging system, the access port is about one inch in diameter which is much smaller in diameter than any refrigerator of prior art.

In another approach, it has been suggested to condense an outside source of helium gas to liquid form, transfer the liquid helium into a cryostat through a transfer line in heat exchange with the boil-off and thereby recondense the boil off to replenish the liquid cryogen contained in the cryostat.

SUMMARY OF THE INVENTION

The normnal boiling point of liquid helium is about 4.2 K. at about 1 atm pressure. In order to provide refrigeration below about 4.5 K. to condense boil-off of liquid helium contained in a cryostat, the present invention cools and expands a stream of helium gas to form a cold low pressure mixture of helium liquid and gas, and places the mixture in heat exchange relation with the boil-off. The stream of helium gas is precooled by means including a mechanical refrigerator. The precooled gas is then carried to the cryostat through a transfer line from the cooling means which are remote from the cryostat. The end of the transfer line in the cryostat has a Joule-Thomson (JT) valve through which the precooled gas is expanded to form the cold low pressure mixture of helium liquid and gas. The mixture is passed in heat exchange relation with the boil-off.

In a preferred embodiment, the mechanical refrigerator of the cooling means is of the regenerator-displacer type, such as the Gifford-McMahon refrigerator. In accordance with one aspect of the invention, the cooling means includes another JT valve positioned outside of the cryostat at an intermediate temperature. The JT valve expands the precooled helium gas to a medium pressure gas enabling greater thermodynamic efficiency in the expansion through the final JT valve at the end of the transfer line in the cryostat.

In accordance with another aspect of the invention, the end of the transfer line positioned in the cryostat comprises an outer tube having burrs on its outer surface and an inner tube positioned coaxially within the outer tube. The burrs are unitary with the outer tube and are formed by a series of radial and circumferential cuts into the outer surface to provide a large surface area per unit of projected area. Further, the finished outer diameter is less than about 1 inch to enable the transfer line to fit through the small access ports of an MRI cooling bath system and the like. With a small outer diameter of the transfer line which enables access to confined from form the vacuum. Charcoal adsorbent 17 is provided on the heat exchanger coils 47 and 49 to create a cryopumping surface which enables a high insulating vacuum. The mechanical refrigerator thus serves the added function of creating and maintaining an insulating vacuum.

As shown in FIG. 3, heat exchanger transfer line 61 is attached to cooling means 25 by connector piece 27. The outside surface of the connector piece 27 of transfer line 61 is about 300 degrees Kelvin. Tubing 81 extending from the piece 27 houses inner transfer tube 29 coaxially positioned in outer transfer tube 39. Inner transfer tube 29 serves as an extension of the line leading from adsorber 63 and is locked to the line by nut 97. Outer transfer tube 39 is the return line and is connected at a manifold 79 to line 37b. The coaxial transfer tubes provide for final counter flow heat exchange prior to expansion in the second JT valve 41. Inner transfer tube 29 has an outer diameter of about 3/16 inch and outer transfer tube 39 has an outer diameter of about 5/8 inch. Both tubes comprise stainless steel. A multilayer radiation shield 51 comprising aluminized mylar is packed around the outer transfer tube 39 to prevent heat leak from ambient.

Tubing 81 has an outer diameter of about 1.5 inches and houses inner and outer tube 29 and 39, respectively, in a vacuum. Nylon spacers 183 are positioned throughout tube 81 to support the transfer tubes. Bellows 93 allow for mechanical alignment when placing cold end 45 of the transfer line 61 into the subject cryostat 59. Elbow 83 provides about a 90 degree curve connecting housing tube 81 to tubing transition 85. Outer and inner tubes 39 and 29 have corresponding elbows within elbow 83. Transfer line 61 may be of other shapes for other cryostats in which case elbows of other degrees and bellows and the like are used to aid in mechanical alignment.

Around the bend of the "J" shape, tubing transition 85 extends into a thin poorly conducting stainless steel outer tubing 158 of about 15 inches in length. This enables the transition in outer surface temperature from 300 degrees Kelvin at the connector end to about 4.2 degrees Kelvin at the cold cryostat end 45. Tubing 158 provides a continuation of the vacuum housing for coaxial transfer tubes 29 and 39.

As shown in FIG. 4, the end of outer transfer tube 39 leading to JT valve 41 is adapted by tubing reducer 105 which is fitted into connecting tube 107. Within connecting tube 107 the end of inner transfer tube 29 is connected to JT valve 41.

JT valve 41 is positioned in the cryostat 59 at the cold end of tubing 158. This position minimizes the problems associated with transferring the liquid-gas mixture formed upon expansion through the JT valve at low pressure as in prior art systems. Further the thermodynamic efficiency of the system is enhanced by JT valve 41 expanding the cold working gas closer to the recondensing heat exchanger 50 such that the expanded gas is not effected by the returning gas of a warmer temperature or the pressure drop associated with flowing to the cold end 45.

Transfer line 61 itself serves as a coaxial heat exchanger. It provides the final precooling prior to the second JT valve 41 in cryostat 59 where final expansion of the working gas 41 results in a cold liquid-gas mixture in inner tube 55.

As shown in FIGS. 5 and 6, cold end 45 of the transfer line 61 comprises a recondensing heat exchanger structure 50 formed of inner tube 55 positioned coaxially within an outer tube 12. The inner walls of both tubes 55 and 12 comprise fins which protrude radially inward. The fins define flow channels and aid in heat transfer to the cryogen flowing through the tubes. In the preferred embodiment, outer tube 12 has about 14 fins 101 and tube 12 is pressed around inner tube 55 such that fins 101 are in mechanical contact with inner tube 55. This enhances the transfer of heat from outer tube 12 to inner tube 55 and helium flowing in channels 103.

End cap 80 plugs outer tube 12 at the cold end of tube 12. Hence, the working gas and liquid mixture is prevented from communicating with the cryostat cryogen and is transferred from inner tube 55 to channels 103 in outer tube 12. The working gas and liquid mixture in the coaxial tubes 55 and 12 absorbs heat from the cryogen boil-off in the cryostat through outer tube 12, fins 101 and end cap 80.

Between JT valve 41 and end cap 80, outer tube 12 comprises burrs 99 which are formed from the outer surface of outer tube 12. The outer surface of outer tube 12 is radially shaved to lift edges of material away from the surface of the tube. These shaved edges are then cut circumferentially into several burrs called spines. One type of such spining is performed by Heatron Inc. of York, Pa. In the preferred embodiment, outer tube 12 at cap end 80 has about 26 spines per turn with about 0.125 inch spacing between turns. The outer diameter of outer tube 12 around burrs 99 is less than about 0.9 inch which enables access in narrow ports of a cryostat.

The amount of heat absorbed from the cryogen boil-off is a function of the heat transfer coefficient of the working gas (i.e., helium) and the projected surface area of recondensing heat exchanger 50. Helium has a low heat transfer coefficient which necessitates large surface area in order to appreciably recondense the boil-off. The spined surface of outer tube 12 provides such an increase in surface area over other tubing used in prior art devices. The spined tubing provides a surface area per unit of projected area of about 5. The burrs 99 further provide many sites for condensate droplets to form and drip off the surface.

In the preferred embodiment, the working gas is transferred to end cap 80 through inner tube 55 qwhich which has an outer diameter of about 0.5 inch. Outer channels 103 formed between inner tube 55 and outer tube 12 carry the working gas in reverse direction back to line 91 through side "b" of heat exchangers 37, 35, 33 and 31. On the return, the working gas absorbs heat at each heat exchanger and exits through line 91 to form a closed loop system.

While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.

Claims

1. A cryogenic recondenser for recondensing cryogen retained in a storage vessel, the recondenser comprising:

cooling means comprising a mechanical refrigerator positioned outside of the storage vessel, said means precooling a volume of gaseous refrigerant;

a transfer line leading from the cooling means and removeably inserted into the storage vessel; and

a jt valve at an end of the transfer line in the storage vessel, the precooled refrigerant being transferred in the transfer line from the cooling means to the jt valve in heat exchange relation with returning refrigerant and being expanded through the jt valve to form a liquid-gas cryogen mixture within the end of the transfer line which is in heat exchange relation with boil-off from the cryogen retained in the storage vessel such that the boil-off is cooled and recondensed;

refrigerant being returned to the cooling means through the transfer line in a manner in which the returning refrigerant is in heat exchange relation with the refrigerant being transferred to the jt valve.

2. A cryogenic recondenser as claimed in claim 1 wherein said mechanical refrigerator is of the Gifford-McMahon regenerator displacer type system.

3. A cryogenic recondenser as claimed in claim 2 wherein said cooling means further include another jt valve for receiving the refrigerant precooled by the mechanical refrigerator and expanding the precooled refrigerant.

4. A cryogenic recondensor as claimed in claim 1 further comprising a recondensing heat exchanger connected to the jt valve for receiving the formed liquid-gas cryogen mixture and passing the mixture in heat exchange relation with the boil-off such that the boil-off is cooled and recondensed.

5. A cryogenic recondensor as claimed in claim 4 wherein the recondensing heat exchanger connected to the jt valve comprises an inner tube coaxially positioned within an outer tube, the formed liquid-gas cryogen mixture being transferred from the jt valve in one tube and passed to the other tube in heat exchange relation with the boil-off.

6. A cryogenic recondensor as claimed in claim 5 wherein the transfer line comprises an inner tube coaxially positioned within an outer tube, the precooled refrigerant to be expanded by the jt valve being transferred to the end of the transfer line in one tube and the precooled refrigerant expanded through the jt valve being transferred back to said cooling means through the other tube.

7. A cryogenic recondenser as claimed in claim 5 wherein the outer tube comprises an outer surface having a plurality of burrs on which cryogen condensate forms, and the outer tube has an outer diameter of less than about 1 inch.

8. A cryogenic recondenser as claimed in claim 4 wherein the recondensing heat exchanger has an outer diameter of less than about one inch.

9. A cryogenic recondensor as claimed in claim 1 wherein the transfer line comprises an inner tube coaxially positioned within an outer tube, the precooled refrigerant to be expanded by the jt valve being transferred to the end of the transfer line in one tube and the precooled refrigerant expanded through the jt valve being transferred back to said cooling means through the other tube in heat exchange relation with the precooled refrigerant in the one tube.

10. A cryogenic recondensor as claimed in claim 9 further comprising a coaxial recondensing heat exchanger connected to the jt valve for receiving the formed liquid-gas cryogen mixture and passing the mixture in heat exchange relation with the boil-off such that the boil-off is cooled and recondensed.

11. A cryogenic recondensor as claimed in claim 1 wherein the volume of gaseous refrigerant is helium.

12. A cryogenic recondensor as claimed in claim 1 wherein said cooling means further comprises a charcoal adsorbent for creating a vacuum about said mechanical refigerator.

13. Apparatus for cooling a bath of cryogen in a cryostat in which a magnetic coil of a magnetic resonance imaging system is cooled, the apparatus comprising:

a mechanical refrigerator positioned outside of the bath, said refrigerator precooling a volume of gaseous refrigerant;

a transfer line leading into the cryostat; and

a jt valve at an end of the transfer line in the cryostat, the transfer line transferring the precooled refrigerant from the mechanical refrigerator to the jt valve in heat exchange relation with returning refrigerant, the precooled refrigerant being expanded through the jt valve to form a liquid and gas cryogen mixture at the end of the transfer line in the cryostat, the formed liquid and gas mixture being in heat exchange relation with boil-off from the bath and thereby recondensing said boil-off;

the refrigerant being returned to the mechanical refrigerator through the transfer line in heat exchange relation with the precooled and expanded refrigerant being transferred to the jt valve.

14. Apparatus as claimed in claim 13 wherein the mechanical refrigerator is in conjunction with an external jt valve positioned outside of the bath, the external jt valve expanding the precooled refrigerant for a first time such that the transfer line transfers precooled and expanded refrigerant to the jt valve at the end of the transfer line in the bath, and the jt valve at the end of the transfer line further expanding the precooled and expanded refrigerant to form the liquid and gas cryogen mixture within the end of the transfer line leading into the bath.

15. Apparatus as claimed in claim 13 wherein the refrigerator is of the Gifford-McMahon regenerator-displacer type.

16. Apparatus as claimed in claim 13 wherein the transfer line comprises an inner tube coaxially positioned within an outer tube, the precooled and expanded refrigerant being transferred to the jt valve at the bath end of the transfer line in the inner tube and being transferred back to said mechanical refrigerant through the outer tube.

17. Apparatus as claimed in claim 16 further comprising a coaxial recondensing heat exchanger having an inner tube coaxially positioned within an outer tube, said coaxial recondensing heat exchanger positioned at the end of the jt valve for receiving the formed liquid and gas cryogen mixture, the cryogen mixture from the jt valve being received by the inner tube and passed to the outer tube in heat exchange relation with the boil-off.

18. Apparatus as claimed in claim 17 wherein the outer tube of the coaxial recondensing heat exchanger has an outer diameter of less than about 1 inch.

19. Apparatus as claimed in claim 13 wherein the volume of gaseous refrigerant is helium.

20. Apparatus as claimed in claim 13 wherein said mechanical refrigerator further comprises a charcoal adsorbent for creating and maintaining a vacuum about said mechanical refrigerator.

21. Apparatus for recondensing boil-off from a bath of cryogen retained in a cryostat comprising:

cooling and expansion means comprising a mechanical refrigerator positioned outside of the cryostat, said means precooling and expanding a volume of working gas;

a coaxial transfer line having an inner tube coaxially positioned within an outer tube leading into the cryostat;

a jt valve at an end of the coaxial transfer line in the cryostat, said precooled and expanded working gas being transferred through the inner tube of the coaxial transfer line to the jt valve in heat exchange relation with the working gas flowing in the outer tube of the coaxial transfer line and being expanded through the jt valve to form a liquid and gas cryogen mixture; and

a coaxial recondensing heat exchanger having an inner tubing coaxially positioned within an outer tubing and positioned at an end of the jt valve for receiving the formed liquid and gas cryogen mixture, the formed cryogen mixture being passed through the inner tubing and the outer tubing in heat exchange relation with the boil-off such that said boil-off is recondensed and said cryogen being returned to the cooling and expansion means through the outer tube of the coaxial transfer line.

22. Apparatus as claimed in claim 21 wherein the outer tubing of the coaxial recondensing heat exchanger has an outer diameter of less than about one inch and comprises an outer surface having a plurality of burrs on which the boil-off recondenses.

23. Apparatus as claimed in claim 21 wherein the outer tube of the coaxial transfer line and the outer tubing of the coaxial recondensing heat exchanger are less than about one inch in outer diameter.

24. A method of condensing cryogen gas comprising the steps of:

precooling a stream of compressed gas;

expanding the precooled gas through a first jt valve to form a stream of medium pressure gas;

cooling the stream of medium pressure gas; and expanding the cooled stream of medium pressure gas through a second jt valve in a cryostat which is remote from said first jt valve, expansion through the second jt valve forming a cold mixture of liquid and low pressure gas which is in heat exchange relation with cryogen boil-off from a volume of liquid cryogen contained in the cryostat and thereby recondenses the boil-off.

25. A method of condensing cryogen gas as claimed in claim 24 wherein the step of precooling is by means of a mechanical refrigerator of the regenerator-displacer type system.

26. A method of condensing cryogen as claimed in claim 24 wherein the stream of gas is helium.

27. A condenser comprising:

a mechanical refrigerator for precooling a stream of compressed gas;

a first jt valve for expanding the precooled stream of compressed gas to a medium pressure stream of precooled gas; and

a heat exchanging and transfer means for further cooling and transferring the medium pressure precooled gas between the first jt valve and a second jt valve, the second jt valve expanding the medium pressure stream of further cooled gas, said expansion by the second jt valve forming a cold mixture of liquid and gas at a pressure below the medium pressure, the first and second jt valves being remotely positioned from each other, the second being in a storage vessel and the first being outside of the storage vessel.

28. A heat exchange surface for condensing cryogen comprising a coaxial heat exchanger having an inner tube coaxially positioned within an outer tube, the outer tube having an end with a plurality of extensions from an outer surface of the end, condensate forming on said extensions, the extensions forming an outer diameter of the outer tube of less than about one inch; and

the outer tube having a plurality of radially inward protrusions along its inner walls, the protrusions bridging between the inner and outer tubes.

29. A heat exchange surface positioned at an end of a transfer line leading into a dewar of condensing cryogen in the dewar comprising a coaxial heat exchanger having an inner tube positioned within an outer tube, said outer tube having a plurality of extensions from an outer surface and an outer diameter of less than about one inch.

30. A heat exchange surface for condensing cryogen comprising:

an outer tube having a closed end and burrs on an outer surface; and

an inner tube coaxially positioned with the outer tube forming a central and intermediate channels, at least one channel for passing helium gas in one direction through one tube and the other channels for passing helium gas in an opposite direction through the other tube, the helium gas being transferred from one tube to the other in heat exchange relation with the cryogen to be condensed, the burrs being unitary with the outer tube and formed by a series of circumferential and radial cuts into the outer surface of the outer tube.

31. A heat exchange surface as claimed in claim 30 wherein said outer tube is less than about one inch in outer diameter.

32. A cryogenic recondenser for recondensing cryogen retained in a cryostat, the recondenser comprising:

cooling means for precooling a volume of gaseous refrigerant, the cooling means comprising a mechanical refrigerator positioned outside of the cryostat and a first jt valve for receiving the refrigerant precooled by the mechanical refrigerator and expanding the precooled gas;

a transfer line leading from the cooling means and removably inserted into the cryostat; and

a second jt valve coupled to the transfer line, the precooled and expanded refrigerant being transferred in the transfer line from the cooling means to the second jt valve in heat exchange relation with returning refrigerant and being expanded through the second jt valve to form a liquid-gas cryogen mixture for the end of the transfer line in the cryostat which is in heat exchange relation with boil-off from the cryogen retained in the cryostat such that the boil-off is cooled and recondensed;

refrigerant being returned to the cooling means through the transfer line in a manner in which the returning refrigerant is in heat exchange relation with the refrigerant being transferred to the second jt valve.

33. A cryogenic recondenser as claimed in claim 32 wherein said mechanical refrigerator is of the Gifford-McMahon regenerator displacer type system. 34. A cryogenic recondenser as claimed in claim 32 further comprising a recondensing heat exchanger connected to the end of the transfer line in the cryostat for receiving the formed liquid-gas cryogen mixture and passing the mixture in heat exchange relation with the boil-off such that the boil-off is cooled and recondensed. 35. A cryogenic recondenser as claimed in claim 34 wherein the recondensing heat exchanger comprises an inner tube coaxially positioned within an outer tube, the formed liquid-gas cryogen mixture being transferred from the second jt valve in one tube and passed to the other tube in heat exchange relation with the boil-off. 36. A cryogenic recondenser as claimed in claim 35 wherein the transfer line comprises an inner tube coaxially positioned with in an outer tube, the precooled refrigerant expanded by the second jt valve being transferred to the end of the transfer line in one tube and the refrigerant being transferred back to said cooling means through the other tube.

37. A cryogenic recondenser as claimed in claim 35 wherein the outer tube comprises an outer surface having a plurality of burrs on which cryogen condensate forms, and the outer tube has an outer diameter of less than about 1 inch. 38. A cryogenic recondenser as claimed in claim 34 wherein the recondensing heat exchanger has an outer diameter of less than about one inch. 39. A cryogenic recondenser as claimed in claim 32 wherein the transfer line comprises an inner tube coaxially positioned within an outer tube, the precooled refrigerant expanded by the second jt valve being transferred to the end of the transfer line in one tube and the refrigerant in the end of the transfer line being transferred back to said cooling means through the other tube in heat exchange relation with the precooled refrigerant in the one tube. 40. A cryogenic recondenser as claimed in claim 39 further comprising a coaxial recondensing heat exchanger connected to the end of the transfer line in the cryostat for receiving the formed liquid-gas cryogen mixture and passing the mixture in heat exchange relation with the boil-off such that the boil-off is cooled and recondensed.

1. A cryogenic recondenser as claimed in claim 32 wherein the volume of gaseous refrigerant is helium. 42. A cryogenic recondenser as claimed in claim 32 wherein said cooling means further comprises a charcoal adsorbent for creating a vacuum about said mechanical refrigerator. 43. Apparatus for cooling a bath of cryogen retained in a cryostat, the apparatus of the type comprising cooling means with a mechanical refrigerator positioned outside of the cryostat, a transfer line leading from the cooling means and removeably inserted into the cryostat, a first jt valve coupled to the transfer line for expanding precooled refrigerant transferred from the cooling means by the transfer line to form a liquid-gas cryogen mixture for the end of the transfer line in the cryostat which is in heat exchange relation with boil-off from the cryogen retained in the cryostat, the apparatus further characterized by:

a second jt valve for receiving the refrigerant precooled by the mechanical refrigerator and expanding for the first time the precooled refrigerant before the refrigerant is transferred through the transfer line to the first jt valve coupled thereto for additional expansion therethrough.

44. Apparatus as claimed in claim 43 wherein the refrigerator is of the Gifford-McMahon regenerator-displacer type. 45. Apparatus as claimed in claim 43 wherein the transfer line comprises an inner tube coaxially positioned within an outer tube, the precooled and first time expanded refrigerant being transferred to the first jt valve coupled to the transfer line in the inner tube and being transferred back to said mechanical refrigerator through the outer tube. 46. Apparatus as claimed in claim 45 further comprising a coaxial recondensing heat exchanger having an inner tube coaxially positioned with an outer tube, said coaxial recondensing heat exchanger positioned at the end of the transfer line for receiving the formed liquid and gas cryogen mixture, the cryogen mixture from the first jt valve coupled to the transfer line being received by the inner tube and passed to the outer tube in heat exchange relation with the boil-off. 47. Apparatus as claimed in claim 46 wherein the outer tube of the coaxial recondensing heat exchanger has an outer diameter of less than

about 1 inch. 48. Apparatus as claimed in claim 43 wherein the volume of gaseous refrigerant is helium. 49. Apparatus as claimed in claim 43 wherein said mechanical refrigerator further comprises a charcoal adsorbent for creating and maintaining a vacuum about said mechanical refrigerator. 50. A cryogenic recondenser for recondensing cryogen retained in a cryostat, the recondenser of the type comprising cooling means with a mechanical refrigerator positioned outside of the cryostat, a transfer line leading from the cooling means and removeably inserted into the cryostat, a jt valve coupled to the transfer line for expanding precooled refrigerant transferred from the cooling means by the transfer line to form a liquid-gas cryogen mixture for the end of the transfer line in the cryostat which is in heat exchange relation with boil-off from the cryogen retained in the cryostat, the cryogenic recondenser further characterized by:

a coaxial recondensing heat exchanger comprising an inner tube coaxially positioned within an outer tube, the coaxial recondensing heat exchanger connected to the end of the transfer line for receiving the formed liquid gas cryogen mixture and passing the mixture in heat exchange relation with the boil-off, such that the boil-off is cooled and recondensed.

1. A cryogenic recondenser as claimed in claim 50, wherein the transfer line comprises an inner tube coaxially positioned within an outer tube, the precooled refrigerant, expanded by the jt valve, being transferred to the end of the transfer line in one tube and the precooled and expanded refrigerant at the end of the transfer line being transferred back to said cooling means through the other tube. 52. A cryogenic recondenser as claimed in claim 50 wherein the outer-tube comprises an outer surface having a plurality of burrs on which cryogenic condensate forms and the outer-tube has an outer diameter of less than about 1 inch.