Patent Publication Number: US-11649989-B2

Title: Heat station for cooling a circulating cryogen

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to improving the configuration of a heat station that transfers heat from a circulating cryogen cooling an external load to the reciprocating flow of gas internal to the cold end of a high capacity expander operating on the GM or Stirling cycle, producing refrigeration at cryogenic temperatures. 
     2. Background Information 
     GM and Stirling cycle refrigerators produce refrigeration at cryogenic temperatures in an expander by flowing gas at a high pressure through a regenerator type heat exchanger to the cold end of a piston reciprocating in a cylinder as the displaced volume is increasing, then lowering the pressure and flowing the gas back through the regenerator as the piston reduces the displaced volume. Refrigeration is made available to cool a load by conduction of heat through the walls of the cold end cap of the cylinder, that encloses the cold displaced volume. The cold end cap and means for transferring heat to the gas in the expander is referred to as the cold heat station. 
     Most cryogenic refrigerators that are used to cool cryopumps, superconducting MRI magnets, and laboratory research instruments use GM type refrigerators. Most of these applications require relatively small amounts of cooling, 1 to 50 W, at temperatures between 4 and 70 K that is transferred to the refrigerator heat station by conduction. There is now a growing need for refrigerators that can cool loads of 300 to 1,000 W at temperatures near 75 K, which can be cooled most practically by a circulating cryogen. The cryogen can be circulated as a gas by a cold fan or room temperature compressor, as a liquid by a pump, or as a gas or liquid by natural convection. The simplest form of natural convection is to condense a cryogen and have the liquid drain to a load where it evaporates, then returns to the condensing surface as a gas. 
     It is the object of this invention to provide a high capacity GM expander with a cold heat station that can cool or condense a cryogen, is compact, efficient, and easy to mount and connect to the circulating piping. This requires minimizing the temperature difference between the circulating cryogen and the gas in the expander while minimizing the pressure drop of the circulating cryogen that is flowing through the heat station. Minimizing the pressure drop is important because the power input to a cold fan or pump becomes part of the heat load on the refrigerator. Minimizing the temperature difference involves the design of the internal and external heat exchangers that transfers heat from the circulating gas, through the cold end cap to the internal heat exchanger, which transfers heat to the gas in the expander. 
     U.S. Pat. No. 4,277,949 to Longsworth shows a system that transfers heat from a remote load using helium that is circulated by a compressor at room temperature cooled by tubes wrapped around the expander heat stations. Loads at different temperatures are connected to the circulating helium by convective couplings which enable the load to be thermally disconnected from the refrigerator. An example of a system that cools a remote load by natural convection of a condensing cryogen is described in U.S. Pat. No. 8,375,742 to Wang.  FIG.  7    shows an expander with an extended surface on the cold end mounted in an insulated sleeve. Cryogen condenses on the cold end and drains down through an insulted tube to a dewar (where it could cool a load), and boil-off gas returns up the insulated tube to be recondensed. The option of bringing a small stream of gas to room temperature (to intercept heat leaks) then recondensing it, all by natural convection is also shown. 
     The heat station of this invention involves the novel combination of several components that enable an advantageous way to mount the expander. The advantageous way to mount the expander requires a compact heat station at the cold end of the expander so that the size of the hole in the mounting plate is minimized and the attachment of the circulating tubes is simplified. Heat exchangers that have been known to be used between the regenerator and expansion space in regenerative expanders include an annular gap, perforated plates, wire screens, corrugated sheet metal, and slots that are cut by wire electric discharge machining (EDM), milling or sawing. Narrow slots that create fins between the slots can be sized to have the best heat transfer relative to pressure drop and void volume. 
     It is advantageous to form closely spaced fins by using a folded copper ribbon. The ribbon can be formed to have a good balance between the three functional properties, heat transfer, pressure drop, and void volume, at a much lower cost than any of the machining methods. It can even be formed into narrower gaps than can be machined and can be stretched or compressed to change the relationships between the three functional properties. 
     Folded ribbons can be used to optimize heat transfer in the expander cold end, and more advantageously can be optimized for transferring heat from the circulating flow of cryogen that is bringing heat from a remote load to the outside of the expander cold end. An optimum geometry has been found to be to have an external folded ribbon, that is removing heat from the load, thermally bonded to the outside of a cylindrical cold heat station, and have fins, formed by machined slots or an internal folded ribbon, thermally bonded to the inside of the cold heat station. Heat is thus transferred radially directly from the external folded ribbon on the (copper) heat station shell to the internal fins with a minimal temperature difference. The reason why fins formed by a folded ribbon are more advantageous on the outside of the cold heat station than the inside is because there is no concern for void volume in the external fins thus the surface area and the flow area can be large and the cost advantage is much greater. The folded ribbon requires less material than machined fins and thus is more compact. This arrangement of internal and external heat exchangers enables the diameter of the cold end to be minimized and thus the mounting hole in the vacuum housing can be minimized. A small mounting hole is only possible however if there are no radial fittings on the cold heat station. A novel way of circulating cryogen within the outer housing enables having the tubes that connect to the circulating cryogen mounted on the bottom. 
     Heat is transferred most efficiently from a load if the circulating cryogen condenses in the external fins and evaporates at the load. Nitrogen can be used to condense and evaporate for loads in the temperature range of about 65 K to 85 K and neon can be used for loads in the temperature range of about 22 K to 35 K. Helium can be used at any temperature within the range of the refrigerators that use helium as a refrigerant. 
     SUMMARY OF THE INVENTION 
     The present invention comprises a heat station on a GM expander, for cooling a circulating cryogen, that is compact, efficient, and easy to mount and connect to the circulating piping. The heat station comprises a shell that has external and internal fins thermally connected to it that are aligned parallel to the axis of the shell, in a cylindrical housing that has inlet and outlet ports that connect to the circulating gas piping. The diameter of the housing is minimized by using folded ribbon on the external heat exchanger and locating the inlet and outlet ports on the bottom of the housing so that the diameter of the hole for mounting the expander on the warm flange of the cryostat is minimized. The fins in the external heat exchanger can be configured to allow different circulation patterns in the housing for different cryogens and orientations. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1 A  shows a schematic of a prior art pneumatically driven GM cycle expander which has an internal cold end heat exchanger like the one described in U.S. Pat. No. 6,256,997. The area that is circled is shown for the new designs shown in  FIGS.  3 - 5   . 
         FIG.  1 B  illustrates a plan view of a cold end having machined slots which form fins as the external heat exchanger and a partial section of an outer housing. 
         FIG.  2    shows a section of folded ribbon. 
         FIG.  3   a    shows a schematic of the cold end of GM expander  100  with a tube that brings gas from the regenerator to the bottom of the cylinder, then back up through an annular space with machined fins inside a circular shell, and into the expansion space. External to the shell is a folded ribbon in a housing designed to recondense a cryogen such as nitrogen. 
         FIG.  3   b    shows an enlarged view of a section of the cold end heat exchanger of GM expander  100  with machined fins internal to the circular shell and folded ribbon fins externally. 
         FIG.  4   a    shows a schematic of the cold end of GM expander  200  which has folded ribbon fins in both the internal and external heat exchangers and a housing with two ports. A break in the external folded ribbon allows gas to enter from the bottom then flow to the top where it is distributed to flow back down to the bottom through the fins. This configuration can be used to cool a circulating gas or a condensing cryogen. 
         FIG.  4   b    shows an enlarged view of a section of the annular gaps of GM expander  200  with the folded ribbons and the break in the outer folded ribbon where the return gas flows to the top. 
         FIG.  5   a    shows a schematic of the cold end of GM expander  300  that has the same inner and outer folded ribbon heat exchangers as GM expander  200  but an extension of the displacer and a seal forces gas from the regenerator to flow down through the inner annular space in the cold end to the expansion space. The housing has a partition across the bottom that causes the gas entering the bottom through a port on one side of the partition to flow up through about half of the external fins and down through the other half, then through the outlet port. 
         FIG.  5   b    shows an enlarged view of a section of the annular gaps of GM expander  300  with the folded ribbons and the sleeve inside the inner folded ribbon that the seal rides against. 
         FIG.  6    shows a schematic of the cold end of GM expander  400  with a single port in the end of the housing located such that a cryogen gas that flows into the housing can condense in the external fins and drain out through the port as a liquid when the expander is oriented between cold end down and horizontal. 
     
    
    
     DESCRIPTIONS OF THE PREFERRED EMBODIMENTS 
     The drawings use the same number to show the same part, and the words up and top refer towards the warm end while down and bottom refer towards the cold end. 
       FIG.  1    shows a schematic of a prior art pneumatically driven GM cycle expander which has the cold end heat exchanger design that is most widely used today. The present invention describes new designs for transferring heat from a load to the gas in the expander in the area that is circled at the cold end of the expander.  FIG.  1    shows a typical pneumatically driven GM expander in its entirety in order to describe the cycle and put the cold end in context. The system comprises or preferably consists of compressor  40  which supplies gas at a high pressure through line  31  to the expander which admits the gas through warm inlet valve  44  to warm displaced volume  30 , then into regenerator  3  in displacer  1 , through the regenerator and into expansion space  5  at the cold end of displacer extension  12   a . Displacer  1  moves up inside cylinder  2  filling displaced volume  5  with cold gas at high pressure. Inlet valve  44  is then closed and outlet valve  45  is opened causing the gas in displaced volume  5  to drop to a lower temperature as it drops to low pressure. The cold gas at low pressure is pushed out of cold displaced volume  5  as displacer  1  moves down. Heat from a load that is connected to cold end  37  is transferred to the cold gas as it flows through annular gap  7 , between displacer extension  12   a  and cold end  22 , and then through radial ports  15 , regenerator  3 , warm displaced volume  30 , outlet valve  45 , and low pressure line  32  to compressor  40 . Cylinder  2  has a warm cylinder flange  46  which mounts on cryostat flange  47 . Displacer  1  has drive stem  35  attached to the top which reciprocates in drive stem bore  36  in warm head  41 . Reciprocation of displacer  1  is caused by opening and closing valves  42  and  43  out of phase with valves  44  and  45  thus causing gas to alternate between high and low pressure as it flows through line  34  to drive stem volume  36 . 
       FIG.  1 A  includes a schematic of a system presently being built that circulates a cryogen to cool a device,  25 , in cryostat  26 . Cold end  37  has machined slots which form fins as the external heat exchanger on the outside of cold end cap  22 , (shown in  FIG.  1   b   ) and outer housing  16  which has inlet port  21   a  bringing circulating gas in radially above the fins and outlet port  21   b  below the fins on the bottom of outer housing  16 . Circulator  27 , which may be a fan or a pump, drives a cryogen through connecting tubes  28  and  29 , which are vacuum insulated. This cold end is very effective at transferring heat with a low pressure drop but the radial inlet port results in assembly complexity because it has to be added to cold end  37  after the rest of the expander has been inserted through the port in cryostat flange  47 . Also the machined fins add to the cost and size. The main advantage of this invention is to minimize the diameter of cold end  37  so that it fits through a port in cryostat flange  47  that is reasonably small and does not require additional assembly work before the piping that connects to the load, is connected to cold end  37 . 
       FIG.  2    shows a section of folded ribbon  13  which is usually formed from a sheet of copper. The shape of the folded ribbon is defined by the thickness T, the width W, the height H, and the gap G. Folded copper ribbons are presently being manufactured using sheets that are thinner and have narrower gaps than can be machined. Sheets with thicknesses in the range of 0.3 to 1.0 mm can be folded to a H/T ratio of about 15 and a G/(G+T) ratio&gt;0.6. The gaps can be further reduced after the sheet is folded by pushing the folds together. They can alternately be increased by stretching the folded ribbon. 
     The pressure boundary at the cold end of cylinder  2  of expander  100 , shown in  FIG.  3   a   , is comprised of cylindrical shell  4  and end plate  10 .  FIG.  3   b    shows details of internal heat exchanger  6 , formed by machined slots in core  9 , press fit into shell  4 , and external heat exchanger  14  comprising a folded ribbon which is thermally bonded to the outside of shell  4 . Core  9  has a close enough fit with tube  8  to bring most of the gas from regenerator  3  to the top of end plate  10 , then radially through flow channel  11 , then back up through internal heat exchanger  6  and into cold displaced volume  5 . Housing  16  encloses external folded ribbon  14 , has inlet port  21  and outlet port  22  on the bottom, and is mounted to cold flange  48  on cylinder  2 . These are arranged so that a cryogen gas, such as nitrogen, can flow through inlet port  21  into manifold  20  which distributes it to folded ribbon  14 , where it condenses, then drains as a liquid through outlet port  22  to a load that is being cooled. Manifold  19  above folded ribbon  14  plays a minor roll in distributing gas to the coldest surfaces. Heat flows from the condensing cryogen through external heat exchanger  14 , cylindrical shell  4 , internal heat exchanger  6 , and into gas that is flowing in and out of cold displaced volume  5 . The components that are conducting heat, internal and external heat exchangers  6  and  14 , and shell  4 , are made of materials having high thermal conductivity, copper being preferred, while housing  16  and ports  22  and  21  might preferably be made from SS. While the process of thermally bonding metals having high thermal conductivity usually involves soldering or brazing, it can be done by other means, such as a press fit, as long as the temperature difference across the joint is small relative to the temperature difference between the external and internal gas streams. Not shown is the option of wrapping a heater around housing  16  to facilitate warming the load. 
     Expander  200 , shown in  FIGS.  4   a  and  4   b   , shows folded ribbon as the internal heat exchanger  14  and is otherwise similar to expander  100  except the external components are designed to cool a circulating gaseous cryogen, rather than condensing a cryogen. This is done by having return port  21   a , which brings gas that has cooled a load, through the bottom of housing  16 , into flow passage  18  which connects to manifold  19  at the top of external folded ribbon  14 , and distributes the gas to flow back down through the folded ribbons. Cooled gas then flows out through outlet port  21   b . Flow passage  18  is separated from outlet manifold  20  by barrier  23 . 
     Another means of directing a circulating gaseous cryogen through external heat exchanger  14  is shown in  FIG.  5   a    for the cold end of expander  300 . Gas flowing through inlet port  21   a  is distributed in lower plenum  20   a  to flow up through the fins on one side of external heat exchanger  14  to the top plenum space  19  and return down through the fins on the other side, the bottom plenum space  20   b , and the outlet port. 
     Expander  300  has an extension  12   b  below regenerator  3  that has a close fit inside sleeve  17  which in turn has a close fit inside internal heat exchanger  6 . Extension  12   b  has a smaller diameter than displacer  1  and thus divides the cold displaced volume into an inner displaced volume,  5   a , and an outer displaced volume,  5   b . Seal  49  prevents gas from leaking between displaced volumes  5   a  and  5   b  and forces gas to flow through radial passages  15  into cold displaced volume  5   b , where some of it remains, and the balance flows through internal heat exchanger  6  into cold displaced volume  5   a . Volume  5   b  is approximately 15% of the total cold displaced volume, which means that only about 85% of the gas that would flow through internal heat exchanger  6  in expanders  100  and  200 , flows internal heat exchanger  6  in expander  300 . This might be thermodynamically advantageous because the last 15% of the gas that flows out of regenerator  3  is significantly warmer than the first 85% so even though less gas flows through internal heat exchanger  6  it is colder on average. 
       FIG.  6    shows a schematic of the cold end of expander  400  which has a single port,  21 , on the outer bottom of housing  16 . Expander  400  can be mounted horizontally such that liquid cryogen  39   b  can drain out through port  21  while gaseous cryogen  39   a  flows in. If the device being cooled is located below port  21  then a cryogen such as nitrogen can circulate by natural convection. 
     Table 1 has an example that compares an external heat exchanger made by machining fins on the outside of shell  4  with a folded ribbon. The design is based on transferring 400 W of cooling at 80 K by circulating 5 g/s of helium at 200 kPa in which both designs have the same temperature differences, in the gas and the fins, and the same pressure drop. The thickness of the machined fin is at its root and the weight of copper for the machined fin includes the material removed from the groove. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Comparison of Machined Fins with Folded Ribbon Fins 
               
            
           
           
               
               
               
            
               
                   
                 Machined 
                 Ribbon 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Outside Dia. of Shell 4 - mm 
                 115 
                 115 
               
               
                   
                 Inside Dia. of Housing 16 - mm 
                 140 
                 131 
               
               
                   
                 Width of Fin, W - mm 
                 100 
                 100 
               
               
                   
                 Gap, G - mm 
                 1.0 
                 0.8 
               
               
                   
                 Thickness, T - mm 
                 2.0 
                 0.5 
               
               
                   
                 Number of Gaps 
                 120 
                 310 
               
               
                   
                 Weight of Cu to form fins - kg 
                 4.0 
                 1.0 
               
               
                   
                   
               
            
           
         
       
     
     The folded ribbon is seen to provide a significant reduction in the diameter of housing  16  and the amount of material needed to make the fins. 
     In the claims top and bottom, and up and down, refer to the expander when the axis is vertical with the cold end down.