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.

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 A HT  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.