Method and apparatus for the cooling of an object
United States Patent [I 1 Asztalos Apr. 22, 1975 METHOD AND APPARATUS FOR THE Primary Examiner-Meyer Perlin COOLING OF AN OBJECT Assistant E.\&#39;aminerR0na1d C Capossela [75] Inventor: Stefan Asztalos, Munich, Germany Agent or Firm-Karl Ross; Herbert Assignee: Linde Akiengesellschaft,  
 Wiesbaden, Germany [22] Filed: Feb. 13., 1974 [21] Appl. No.: 441,955  
 [30] Foreign Application Priority Data Feb. 20. 1973 Germany 2308301 [52] US. Cl. 62/62; 62/50; 62/467; 62/514 [51] Int. Cl. F2 5d 31/00 [58] Field of Search 62/45, 50-55, 62/467, 512. 514, 62, 64  
 [56] References Cited UNITED STATES PATENTS 3,159 008 12/1964 Nebgcn 62/512 X 3,364,687 1/1968 Kolm 62/467 X 3.415.077 10/1968 Collins... 62/467 3.611,740 10/1971 Gigcr .7 62/514 X R4014 new 3 [57] ABSTRACT A method of and an apparatus for the cooling of an object. such as a superconductive magnet. superconductive cable system or the like wherein the liquid cryogen (e.g. a liquified gas such as helium) is passed continuously through or in heat-exchanging relationship with the object from a supply reservoir. After traversing the object or a cryostat therefor. the liquid cryogen is partially expanded to form a gas/liquid phase mixture which is passed in heat-exchanging relationship with previously warmed and compressed coolant and is fully vaporized thereby. The vaporized cryogen is then used to pick up heat from the system. is compressed and is cooled by the aforementioned heat exchange before being expanded to bring about liquefaction with the liquid being collected in the reservoir.  
 17 Claims, 1 Drawing Figure CROSS REFERENCE TO RELATED APPLICATION The present Application is related to the commonly assigned copcnding application Ser. No. 435,856 filed Jan. 23, 1974 of myself and other and entitled METHOD OF AND APPARATUS FOR THE COOL- ING OF AN OBJECT FIELD OF THE INVENTION My present invention relates to a method of and an apparatus for the cooling of an object such as a superconductive magnet or a superconductive cable in a housing or cryostat with a liquid coolant or cryogen, e.g. liquid helium.  
 BACKGROUND OF THE INVENTION The deep cooling of objects has been found to be especially advantageous in recent years for the cooling of conductors in electrical systems, the conductivity of a conductor increasing as its temperature is reduced in the great majority of cases. The development of superconductive materials has caused increasing interest in cooling systems adapted to reach superconductive temperatures, i.e. temperatures of I4K or below, and in the handling of cryogenic liquids, i.e. liquefied gases capable of reaching these low temperatures.  
  Superconductors are used, for example, in magnets of particle accelerators, spectrometers and other systerns in which high magnetic field strengths must be developed with limited cross-sections of the conductor.  
 Moreover, superconductors are used in cables for the transmission of large currents over both small and large distances.  
  A typical cryogen-liquid-cooled cable may comprise a plurality of coaxial ducts in which the superconductor is received in an inner duct and an outer space is evacuated and/or provided with so-called superinsulation composed of alternating layers of filamentary material and heat-reflective foil. The cryogen liquid or cryogen is caused to flow through the innermost duct in intimate (direct) contact or heat-exchanging relation with the conductor. Superconductor magnets are often enclosed in highly insulated housings or cryostats to which the conductive liquid is admitted or are provided with channels, e.g. the magnet windings themselves, to which the cryogenic liquid is admitted.  
  it has been the practice prior to the system described in the aforementioned copcnding application, to supply liquid helium to the object from a first storage vessel and to conduct the liquid after it has traversed the object into a second storage vessel. A pressure differential, produced by some pressure-buildup means, is maintained across the vessel to obtain the driving pressure necessary to displace the liquid from the first vessel through the object and into the second vessel.  
  When the first vessel is emptied, the pressure differential is reversed and the liquid, now collected in the second vessel, is displaced in the opposite direction by an opposite pressure differential through the housing and into the first vessel.  
  The disadvantage of that system is that the housing cannot be supplied for prolonged periods continuously with the cryogenic liquid from one vessel and hence there are periods in which the flow of the cryogen must be interrupted. This of course has the concomittant disadvantage that uniform flow and cooling cannot be guaranteed and that even brief interruptions in the con- I tinuity of coolant flow may cause detrimental results when the cooled object is a superconductive magnet or superconductive cable.  
  To overcome these disadvantages, the system described in the aforementioned copcnding application provides that the cryogenic liquid is collected after traversing the object from a first vessel and after being expanded to transform it into a gas/liquid phase mixture which is separated in the second or receiving vessel with the liquid phase being transferred to a third vessel and intermittently discharged by an appropriate pressure differential into the first vessel.  
  More particularly, the prior application discloses a method of cooling an object in a housing, especially a superconductive magnet or a superconductive cable, which comprises feeding a cryogenic liquid from a first or supply vessel to the object (e.g. a housing, duct, or cryostat), collecting the cryogenic liquid from the housing in a second vessel, expanding the liquid in the second vessel to cool the liquid and produce by partial vaporization thereof a vapor/phase mixed with a liquid/phase, separating the vapor phase from the liquid phase of the second vessel, feeding the liquid phase to a third vessel and at least intermittently returning liquid from the third or storage vessel to the first or supply vessel. Thus that application provided for expansion of the liquid cryogen or coolant after it had been used to cool the object, thereby lowering the temperature of the liquid phase and abstracting heat therefrom equivalent to the latent heat of vaporization of the cryogen. Thereafter, a phase separation was carried out whereby the liquid component was collected in the third or storage vessel and was then resupplied to the first or supply vessel. That system had the advantage that it was able to achieve cooling of an object, e.g. a superconductive system, with a liquid coolant or cryogen, e.g. liquid helium, with one-way, continuous and long duration (noninterrupted) flow of the cryogen from a single supply vessel to the object.  
  The liquid coolant was displaced through the system with appropriate driving pressures and thus the pressure differential between the first and second vessel was maintained at the level necessary to drive the cryogen from the first or supply vessel through the cryostathousing or object and into the second or phaseseparation vessel. During the accumulation of the liquid in the storage vessel the latter was maintained at the same pressure as the second vessel, i.e. at a pressure lower than that in the first vessel. Even the expansion step within the second vessel took place to a pressure below that in the first or supply vessel.  
  While the system of the aforementioned copending application represents a major advance over the art of cryogenic cooling of objects, especially superconductive systems, it has some characteristics which are not always desirable. For example, the continuous cooling system required substantially periodic pressure buildup and pressure reduction in the third or storage vessel and consequently heat losses are unavoidable, furthermore, the pressure control system necessary to provide these periodic pressure variations may be more complex than is desirable.  
 OBJECT OF THE INVENTION It is, therefore, an important object of the present invention to provide a method of and an apparatus for the cooling of a body, eg a superconductive system, with a liquid cryogen, which enlarges the principles originally set forth in the aforementioned copending and commonly assigned application.  
  It is another object of the invention to provide a process for the cooling of an object in housing, e.g. a superconductive magnet in a cryostat, a conductive cable system or a superconductive magnet directly traversed by a cryogen, whereby the aforementioned disadvantages are obviated.  
  Another object of the invention is to provide an apparatus or system for the cooling of objects with a liquid cryogen whereby the continuity of flow to the cooled object from a supply vessel can be maintained.  
  Still another object of the invention is to provide a method of and an apparatus for the continuous supply of a cryogen to, and effective cooling of, an object to be cooled especially a superconductive system for long periods and with a single supply vessel serving as a source of the liquid cryogen to the object to be cooled.  
 SUMMARY OF THE INVENTION These objects and others which will become apparent hereinafter are attained, in accordance with the present invention, in that the partly vaporized coolant (i.e. the mixture of gas and liquid phases of the cryogen) is passed in heat exchanging relationship with previously warmed compressed coolant and is fully evaporated and heated thereby before being compressed anew; the compressed coolant or cryogen passes after this heat exchange and cooling thereby into the storage vessel into which it is expanded.  
  This process, according to the present invention, has been found to constitute a simple and energyconserving solution to the problems of continuously cooling a body with cryogenic liquids.  
  The cooling of the object according to the present invention is thus effected with a deep-cooled liquid coolant or cryogen from a storage vessel, preferably after some supcrcooling (i.e. cooling to a temperature below its boiling point) while the heat input from the exterior to the cryostat in which the object is maintained and the heat input to the connecting ducts between the storage vessel and the cryostat is compensated by the heat pickup by the expanded coolant from the object so that a part of the liquid coolant is thereby vaporized. A further portion of the vapor phase of the coolant is used for the supercooling of the cryogen supplied from the storage vessel to the object.  
  The sensible&#39;cold and the cooling capacity of the partly expanded cryogen is first transferred to the compressed cryogen by heat exchange so that the compressed cryogen is cooled and is thereafter expanded into the storage vessel. All of the aforementioned steps of the present invention can be carried out continuously so that periodically effective pressurizations and depressurizations are avoided and the complex control systems characterizing earlier techniques, can be minimized, or eliminated.  
  The cold loss of the process is not only compensated by vaporization of the liquid coolant stored in the vessel but also by utilization of the gas enthalpy of the vaporized cryogen so that the evaporation rate of the liquid coolant is significantly reduced.  
  The process and apparatus of the present invention is most advantageously utilized for the continuous cooling of superconductive systems as, for example, superconductive magnets, utilizing supercooled liquid helium as the cryogen. From the point of view of advantageous control of the process, the compressed and cooled cryogen is directly expanded into the storage vessel to the displacement pressure, i.e. the pressure whereby the cryogen is forced from the storage vessel to the cooled object.  
  According to a further feature of the invention, radiation shields, eg as described in the aforementioned copending application, which constitute further heat barriers within the cryostat in which the object to be cooled is disposed and within the ductwork between the storage vessel and the cryostat, are cooled by a part of the gaseous coolant generated in the system and which has a temperature greater than about 10K.  
  Of course, the process may be used for the cooling of a single object or for the cooling of a plurality of such objects traversed by the coolant in series and/or in parallel.  
  According to another aspect of the invention, an apparatus for carrying out the process and for cooling an object, such as a superconductive device in a cryostat, comprises a storage vessel for the deep-cooled liquid cryogen, feed-duct means for delivering the liquid cryogen from the storage vessel to the cryostat, return duct means for carrying the liquid cryogen away from the cryostat or the object to be cooled, a throttle valve permitting partial expansion of the cryogen after it has traversed the object, a heat exchanger for passing the resulting gas/liquid phase mixture in heat-exchanging relationship with the compressed cryogen, a compressor to which the fully expanded cryogen is supplied and which delivers the compressed and warmed cryogen to the heat exchanger. The heat exchanger is advantageously coupled with an evaporator having a flow cross section for the compressed cryogen while the apparatus may comprise a plurality of heat exchangers connected in series and to the evaporator. An expansion valve may be disposed at the inlet to the storage vessel.  
 BRIEF DESCRIPTION OF THE DRAWING The above and other objects, features and advantages of the present invention will become more readily apparent from the following description, reference being made to the accompanying drawing, the sole FIGURE of which is a flow diagram illustrating the invention.  
 SPECIFIC DESCRIPTION AND EXAMPLES In the drawing, I have shown a system for the cooling of a superconductive magnet 6, preferably in a cryostat not illustrated in detail but of the type described in the aforementioned copending application, with liquid helium. Liquid helium at a displacement pressure of about 2 bars and a temperature of about 5K from a storage vessel 1 through a heat exchanger 2 in which the liquid helium is supercooled to a temperature of about 4.6K.  
  The supercooled liquid helium is passed through the central duct 3 of a conduit system 4 (consisting of coaxially nested ducts as described in the aforementioned copending application) to the superconductive magnet 6 disposed within the cryostat represented by the dotdash line 5. The superconductive magnet 6&#39;and the interior of the cryostat 5 is held by the cooling process to a constant temperature of about 4.6K.  
  The liquid cryogen flows from the superconductive magnet 6 through an expansion valve 8 in which it is expanded against a pressure of 1.2 bars to form a gas/liquid phase mixture traversing the tube coil 9 which may represent a heatshield surrounding the superconductive magnet so as to prevent entry .of external heat into the cooling. zone. A portion of the liquid coolant is transformed to:vapor at the expansion valve 8 and a further portion is vaporized in the tube coil 9 by pickup of such leaked heat. From the tube coil 9 the gas/liquid phase mixture passes through the innermost annular duct 10 of the conduit system 4 and another portion is evaporated. in this passage 10, the cryogen mixture acts as a heat shield absorbing heat leaking from the exterior inwardly.  
  The gas/liquid phase mixture then enters the heat exchanger 2 in which it is passed in heat-exchanging relationship with the liquid helium from the storage vessel 1 to supercool the latter, the heat abstracted by the mixture resulting in a further evaporation of liquid in the latter stream. The remainder of the liquid of this stream is evaporated in the evaporator 11 connected to the warm side of the heat exchanger 2 and in the heat exchangers 13, 14, 15 and 16 connected in series with one another and the evaporator 11. The gaseous coolant is then compressed in compressor 17 to a pressure of about 15 bars and is at a temperature of 290K.  
  compressing said fully gaseous fluid to form a compressed fluid and abstracting heat from said compressed fluid by passing same in heat-exchanging relationship with said mixture to supply thereto at least part of the heat required for the complete vaporization of the liquid phase thereof, thereby &#34;forming a cooled compressed gas; and expanding said cooled gas into said vessel to supply liquefied fluid thereto. I v  
  2. The method defined in claim 1 wherein said compressed cooled gas expanded to said displacement pressure in said vessel. 7  
  3. The method defined in claim 1, further comprising derivingva warm partial stream of gas from the mixture during the evaporation thereof and, forming therefrom a heat-absorbingshield for a colder portion of the cryo- The compressed helium flows through the heat exchangers 16, l5, 14, 13 in succession and through the evaporator 11 in sections separated from those transverscd by the expanded helium and is cooled to a temperature of about 5.2K.  
  The fluid is then permitted to expand to a pressure of 2 bars, the displacement pressure of the liquid helium, at the throttle valve 18 into the storage vessel 1. This expansion reduces the temperature to about 5K and introduces a mixture of 42% by weight liquid and 58% by weight vapor into the storage vessel 1.  
  To cool the annular passage 19 serving as a radiation shield, the passages 20 and 21 of the duct system are evacuated and a portion of the low pressure helium at a temperature of about llK is drawn off via line 23 and fed to the radiation shield 19. This portion of the low pressure helium can be fed also to the tube coil 22 which may be disposed in an outer radiation shield of the cryostat (see the aforementioned copending application). After heating to a temperature of 100K, this partial stream is returned via line 24 to the compressor 17. A pressure controller 25 controls the throttle valve 18 to maintain the displacement pressure in the vessel 1.  
  I claim: 1. A method of cooling an object with a lowtemperature cryogenic fluid, comprising the steps of:  
 passing the liquefied fluid at a cryogenic temperature continuously from a storage vessel into heatexchanging relationship with said object and under a displacement pressure; partly expanding the liquefied fluid to form a gas/liquid phase mixture; fully vaporzing said mixture by heat exchange to form a completely gaseous fluid;  
 genie fluid.  
  4. The method defined in claim 1, further comprising the step of supercooling the liquefied fluid between said vessel and said object in heat exchange with said mixture.  
  5. The method defined in claim 4 wherein said liquefied fluid between said vessel and said object is passed through a duct, said method further comprising the step of shielding said duct with a sheath of said mixture.  
  6. The method defined in claim 5 wherein said object is enclosed in a cryostat provided with a radiation shield, said method further comprising the step of cooling said radiation shield with said mixture.  
  7. The method defined in claim 6, further comprising the step of deriving a partial gas stream from said mixture during the evaporation thereof and enclosing said duct in a sheath of said partial gas stream.  
  8. The method defined in claim 7 wherein said cryostat is provided with another radiation shield outwardly of the first-mentioned radiation shield, said method further comprising the step of cooling said further radiation shield with said partial gas stream.  
  9. An apparatus for cooling an object contained in a cryostat, said apparatus comprising:  
 a storage vessel for a liquid cryogen;  
 a duct connecting said storage vessel with said cryostat for passing said liquid cryogen into heatexchanging relationship with said object;  
 means for leading cryogenic fluid from said object;  
 heat-exchanger means having a first flow section traversed by said cryogenic fluid for converting same to a fully gaseous state;  
 a compressor connected to said heat-exchanger means for compressing the fully gaseous cryogenic fluid and forming a heated compressed fluid, said heat-exchanger means having a second flow crosssection traversed by said heated compressed fluid to cool the latter; and  
 an expansion valve between said second flow crosssection and said vessel for expanding the compressed fluid cooled in said heat-exchanger means into said vessel and liquefying the fluid expanded therein.  
  10. The apparatus defined in claim 9 wherein said heat-exchanger means includes an evaporator and a plurality of heat exchangers connected in series and to said evaporator, said apparatus further comprising another heat exchanger traversed by the fluid derived from said object and the liquid cryogen for passing same in heat-exchanging relationship to supercool said liquid cryogen and partial evaporate the fluid led from said object.  
 11. The apparatus defined in claim 10 wherein said means for leading fluid from said object comprises a duct traversed by the latter fluid, said apparatus further pansion valve for transforming liquid cryogcn into a gas/liquid phase mixture.  
  15. The apparatus defined in claim 14 wherein said cryostat is formed with a radiation shield disposed between the exterior and said object, said apparatus further comprising means for cooling said radiation shield with said mixture.  
  16. The apparatus defined in claim 15 wherein said cryostat further comprises another radiation shield outwardly of the first-mentioned radiation shield, and  
  means for passing said partial stream through said other radiation shield.  
 17. The apparatus defined in claim 16 wherein said object is a superconductive magnet.