Abstract:
A method of forming a heat exchanger apparatus on a housing wall. The heat exchanger includes inner and outer annular rings. The rings have heat radiating, high surface area fins attached on oppositely facing surfaces. The inner ring has a radially outwardly facing surface that abuts the interior surface of the housing sidewall. The outer ring has a radially inwardly facing surface that abuts the exterior surface of the housing sidewall. When displaced longitudinally along the ring axes, which are coincidental, the sidewall is clampingly engaged therebetween, and an excellent thermal flow path is formed. Heat transferred into the inner fins from a working gas is conducted to the inner ring, through the sidewall, into the outer ring, then into the outer fins. Air impinging upon the outer fins absorbs the heat from the outer fins.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to heat exchangers, and more particularly to a heat exchanger for transferring heat between a fluid on the inside of a wall and a fluid at a different pressure on the outside of the wall, and a method of constructing such a heat exchanger. 
     2. Description of the Related Art 
     Heat exchangers transfer heat energy from one fluid to another. A common heat exchanger is an automobile radiator, in which heat is transferred from a warm water solution in the radiator to the cooler air. Heat is removed by passing the fluid, which can be a liquid or gas, through a thin-walled passage and directing air over the outside of the thin-walled passage. Gas molecules in the air impinge upon the walls of the passage, removing heat during contact. 
     In free piston Stirling cycle machines, there is a need to transfer heat from a gas on one side of a hermetically sealed housing to a fluid, such as environmental air, on the other. In free piston Stirling cycle cryocoolers in particular, a working gas, such as helium, within the housing is compressed, thereby raising its temperature. Heat is removed from the compression region of the housing as part of the process of absorbing heat in one region of the housing and rejecting it at another. 
     This heat pumping process requires the flow of heat energy through the housing wall. However, the most common housing wall material, stainless steel, is not a particularly good thermal conductor. A housing wall that is made thinner to transfer heat more rapidly cannot support the pressure within the housing. 
     Heat transfer in conventional Stirling cycle machines is assisted by attaching thin, highly thermally conductive fins to the inside and outside of the housing to promote heat transfer. The internal fins have high surface area upon which the working gas within the machine impinges, transferring heat energy to the fins. This heat energy flows through the housing wall to the cooler fins on the outside of the housing. Fluid coolant, such as ambient air, passes over the outer fins, removing heat. 
     Fins are conventionally attached by one of two methods. In one method, fins are brazed or soldered to the interior and exterior surfaces of the housing. In the second method, the housing is separated into two sections by cutting along a plane intersecting the housing. A fin structure is interposed between the two housing sections and brazed or soldered into place. 
     Two disadvantages to soldering or brazing fins to the housing are the high cost and the tendency brazing and soldering have to modify the metallurgical properties of both the housing and the fins. Disadvantages of interposing a fin structure include the high costs and metallurgical effects, and the possibility of leaks due to poor soldering or brazing. 
     Therefore, the need exists for an effective heat exchanger, and a method for forming the same, on a Stirling cycle machine in particular, and opposing sides of walls in general. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention is a heat exchanger for transferring heat energy from one side of a housing wall to the opposite side. The invention also contemplates a method of constructing the heat exchanger. In the preferred embodiment, the housing wall is the housing of a free piston Stirling cycle machine, such as a cryocooler. 
     The apparatus includes an inner ring that seats against the inner surface of the housing. An outer ring seats against an outer surface of the housing. The rings are positioned coaxially and aligned longitudinally on opposite sides of the housing wall, forming a thermal energy conduction path from ring to ring. The rings also support the housing wall under the stress created by the pressure within the housing. 
     Heat transfer means, preferably thin, highly thermally conductive fins, are mounted to the opposing sides of the rings. The inner fins promote conduction of heat from the working gas within the housing to the inner ring. The heat is conducted through the housing sidewall to the outer ring. The heat is then conducted to the outer fins and then removed by gas circulating through gaps between the outer fins. This gas is environmental air in the embodiment contemplated, but could alternatively be a fluid coolant. 
     A method of forming the apparatus comprises seating the inner ring against the interior surface of the housing and then displacing it longitudinally to a predetermined longitudinal position. The outer ring is seated against the exterior surface of the housing and displaced longitudinally to the predetermined longitudinal position, preferably aligned with the inner ring on the opposite side of the sidewall. The relative temperatures of the rings can also be changed if desired. 
     The heat exchanger constructed has an interference fit between the abutting surfaces of the rings and the housing sidewall, thereby preventing relative movement of the rings and the housing sidewall. Furthermore, the high-contact area between the rings and the housing provides an excellent path for thermal energy conduction. There is no weakening of the metallurgical properties of the housing due to soldering or brazing, and in fact the heat exchanger strengthens the housing. There is no need to re-seal the housing sidewall due to interposition of a structure. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     FIG. 1 is a side view in section illustrating the preferred embodiment of the present invention on the preferred free piston Stirling cycle cooler. 
     FIG. 2 is a side view in section of a schematic illustration of the preferred heat exchanger. 
     FIG. 3 is a side view in section illustrating the preferred heat exchanger and the relevant portion of the free piston Stirling cycle cryocooler of FIG.  1 . 
     FIG. 4 is an end view in partial section along the line  4 — 4  of FIG.  3 . 
     FIG. 5 is a magnified side view in section illustrating the preferred heat exchanger and the relevant portion of the free piston Stirling cycle cryocooler of FIG.  1 . 
     FIGS. 6 and 7 are end views in section illustrating alternative heat transfer means. 
    
    
     In describing the preferred embodiment of the invention which is illustrated in the drawings, specific terminology will be resorted to for the sake of clarity. However, it is not intended that the invention be limited to the specific terms so selected and it is to be understood that each specific term includes all technical equivalents which operate in a similar manner to accomplish a similar purpose. For example, the word connected or terms similar thereto are often used. They are not limited to direct connection but include connection through other elements, where such connection is recognized as being equivalent by those skilled in the art. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The heat exchanger  10  of the present invention is shown in FIG. 1 in a free piston Stirling cycle cryocooler  12 . However, as will become apparent to one of ordinary skill in the art from the description below, the invention can be used on any wall through which heat must be transferred, such as pipes, vessels and other structures. 
     The cryocooler  12  has a piston  14  that is slidably mounted in a cylindrical passage defined by the sidewall  18 . A displacer  16  is slidably mounted in a cylindrical passage defined by the sidewall  19 . The piston  14  is drivingly linked to an annular ring  22  to which magnets are mounted. The annular ring  22  is disposed within a gap in which a time-changing, alternating magnetic field is generated, driving the ring  22 , and therefore the linked piston  14 , in a reciprocating motion. 
     A working gas, such as helium, that is contained within the cryocooler  12  is compressed in the compression space  20  during a part of the reciprocation cycle of the piston  14 , thereby raising the working gas temperature in the compression space  20 . The heated working gas passes over the internal components of the heat exchanger  10  following the arrows  15  through apertures  17  in the housing  13 . Some of the heat that is absorbed by the internal components from the working gas is conducted to the external components of the heat exchanger  10 . Heat is removed by ambient air passing over the external components of the heat exchanger  10 . 
     The cryocooler  12  pumps heat according to a known thermodynamic cycle from the cold end  26  where the working gas expands, to the compression space  20  where the working gas is compressed. The cold end  26  of the cryocooler  12  can thereby cool, for example, gaseous oxygen to condense and liquefy the oxygen, electronic devices, superconductors and any other device requiring cryogenic (less than 150K) temperatures. 
     The preferred heat exchanger  10 , described briefly above and shown in more detail in FIGS. 3,  4  and  5 , is mounted at the warmer region  24  of the cryocooler  24  to remove heat energy from the working gas in the compression space in that region. 
     The cryocooler  12  has a sidewall  42  that is hermetically sealed to form a housing, only a portion of which is shown in FIGS. 3,  4  and  5 . The sidewall  42  has an interior surface  46  and an exterior surface  48 . The sidewall is very thin (approximately 0.3 mm), and around the compression space the housing diameter is large, increasing the stress in the sidewall  42  much more than an amount proportional to the increase in diameter. The heat exchanger supports this sidewall  42  where support is most needed. Next to the heat exchanger thicker sidewalls can be used as shown in FIG. 2, because heat transfer is not a substantial concern. 
     The heat exchanger  10  includes two main elements: an inner ring  32  and an outer ring  34 . The inner ring  32  is a thick, preferably copper annulus having a radially outwardly facing surface  36  that, when positioned as shown in the heat exchanger region  31 , seats against the interior surface  46  of the sidewall  42 . The heat exchanger region  31  is the region of the housing sidewall  42  at which the inner ring  32  and the outer ring  34  are mounted in their preferred operable position shown in FIGS. 3 and 5. 
     The inner ring  32  has a radially inwardly facing surface  35  to which a heat transfer means mounts. A heat transfer means is defined, for the purpose of the present invention, as a structure that facilitates the transfer of heat from a fluid to one of the rings or from one of the rings to a fluid. The preferred heat transfer means is a plurality of radially extending fins  37  shown in FIG.  4 . Alternative heat transfer means include a thermally conductive tube, such as a copper tube, mounted to the surface of the ring, or mounted within the ring, through which a fluid, such as water or another liquid or a gas, flows to transfer heat energy to or from the ring. Examples of such alternatives are shown in FIGS. 6 and 7. Another alternative heat transfer means includes a heat sink, such as a very large piece of thermally conductive material. 
     The fins  37  are preferably made from a thin copper strip that is pleated into a plurality of panels with corners joining adjacent panels at opposite edges. The inner corners are mounted to the inwardly facing surface  35  of the inner ring  32  by brazing or soldering. Alternatively, the fins  37  could be integral with the inner ring  32  by forming the ring and fins of one piece of material, or by forming a larger ring and cutting away material to leave the ring and the fins. 
     Referring again to FIG. 5, the outer ring  34  is a thick, preferably copper annulus having a radially inwardly facing surface  38  that, when positioned in the heat exchanger region  31 , seats against the exterior surface  48  of the sidewall  42 . The outer ring  34  has a radially outwardly facing surface  39  to which a plurality of radially extending fins  47  attach as shown in FIG.  4 . The fins  47  are preferably substantially similar in structure to the fins  37  formed on the inner ring  32 , and function as the preferred heat transfer means mounted to the outer ring  34 . The fins  47  are larger than the fins  37 . 
     In the schematic illustration of FIG. 2, the inner ring  32  and the outer ring  34  are shown prior to being displaced along their axes to their final positions in the heat exchanger region  31 . The rings  32  and  34  are first positioned as shown after being pre-assembled with the fins attached to the rings, and are subsequently forced into the positions shown in phantom. 
     The inner ring  32  is displaced to the left in FIG. 2 to the position shown in phantom, and the outer ring  34  is displaced to the right in FIG. 2 to the position shown in phantom. The order of ring displacement to the heat exchanger region  31  is not critical. It is critical, however, that the rings clampingly engage the sidewall  42  in a gap between them to provide a suitable thermal conduction path from the inner ring  32  to the outer ring  34 . Such a clamping engagement is assured when the rings and sidewall have the dimensions described below. The dimensions described ensure a tight interference fit that provides thermal conduction between the abutting surfaces of the sidewall  42  and the rings  32  and  34 . 
     There is a difference of approximately 0.504 mm in the diameter of the outwardly facing surface  36  of the inner ring  32  and the inwardly facing surface  38  of the outer ring  34 . This difference forms an annular gap with a thickness of 0.252 mm if the rings  32  and  34  are placed one inside the other. The preferred thickness of the sidewall  42 , which is positioned in that gap, is approximately 0.3 mm. 
     The difference in gap thickness and sidewall  42  thickness necessitates deformation of the inner ring  32 , the outer ring  34 , the sidewall  42  or a combination of some or all structures to position the structures as shown in FIG.  5 . The inner and outer rings are preferably made of a copper alloy and the sidewall is made of stainless steel. Because copper alloys are generally more prone to deformation than stainless steel, the deformation occurs primarily in the rings  32  and  34 , and most primarily in expansion of the inner diameter of the outer ring  34 . Alternatively, the rings  32  and  34  can be heated, cooled or a combination to create a temperature difference to form a gap closer to or larger than 0.3 mm. 
     During operation the inner ring  32  is maintained at a higher temperature than the outer ring  34 , which causes the inner ring  32  to expand more than the outer ring  34 . This outward thermal expansion by the inner ring  32  against the mechanical inwardly directed force of the outer ring  34  ensures a clamping engagement of the sidewall  42  under all contemplated conditions and supports the sidewall  42  against the outwardly directed gas compression forces against the housing. 
     The stainless steel wall  42  has the ability to conform to the shape of the gap between the rings  32  and  34 . Therefore, there can be a relatively loose fit between one ring and the wall&#39;s surface. However, because of the smaller gap between the facing surfaces of the rings, placing the second ring in place will cause the wall to conform essentially completely to the shape of the gap. This creates a substantial amount of ring to wall and wall to ring contact, providing excellent thermal conduction. 
     The sidewall  42  shown in FIG. 5 can be the preferred thickness of 0.3 mm because it is supported by the rings  32  and  34 . The pressure in the compression space  20  increases cyclically during operation of the cooler, creating significant stress in the sidewall  42  surrounding the compression space  20 . This stress could rupture a sidewall of the preferred thickness if it were not supported by the outer ring  34 . If the sidewall  42  were made substantially thicker to support the stress, it would not be as effective at conducting heat out of the compression space  20 . Therefore, the combination of the thin sidewall  42  supported by the heat exchanger  10  provides a desirable balance of rapid thermal conduction and strength. 
     As the cryocooler  12  utilizing the preferred heat exchanger operates, heat is pumped from the cold end  26  to the warmer region  24  by compression and expansion of the working gas. The heat must be transferred away from the working gas within the compression space  20  of the cryocooler through the heat exchanger to the environment. The fins  37  are positioned in the flow path of the working gas which is directed against the fins  37  by passing through apertures  17  formed all around the housing  13  just to the left of the leftward end of the sidewall  18  shown in FIG.  1 . When the warmer working gas in the cryocooler  12  flows through the gaps between the fins  37  shown in FIG. 4, the gas transfers heat to the fins  37  via convection, in which heated gas molecules impinge upon the fins  37 , conducting heat to the fins during the brief moment of contact. The working gas passes through the fins  37 , into a regenerator within the displacer  16  and toward the cold end  26  where it expands. 
     The heat exchanger  10  forms a thermal conduction path that flows “downhill” from, the internal fins  37  to the external fins  47 . The heat is conducted from the fins  37  to the cooler inner ring  32 . From the inner ring  32 , heat flows through the even cooler sidewall  22  toward the still cooler outer ring  34 . Finally, heat is conducted to the coolest part of the heat exchanger, the fins  47 . Atmospheric gas molecules impinging upon the fins  47  remove heat energy via convection, preferably to the atmosphere. 
     The heat exchanger could, alternatively, be used to transfer heat energy into a Stirling cycle cryocooler, for example at the cooler end  26 . Of course, the heat exchanger of the present invention could also be used on Stirling cycle engines, coolers and other non-Stirling cycle machines. 
     Alternative heat transfer means are shown in FIG. 6 and 7. The outer ring  134  and the inner ring  132  of the heat exchanger  110  of FIG. 6 form an interference fit with the sidewall  142  as in the preferred embodiment. The outer ring  134  has a fluid tube  140  that is mounted to the radially outwardly facing surface of the outer ring  134  by conventional mounting, such as soldering. The fluid tube  142  is mounted to the radially inwardly facing surface of the inner ring  132  by conventional mounting, such as soldering. 
     The fluid tube  142  transfers heat to the ring  132  from the fluid within the tube, and the ring  134  transfers heat to the fluid in the tube  140 . The tubes could, alternatively, be formed as passages within the rings, as in the heat exchanger  210  shown in FIG. 7 in which the rings  232  and  234  form an interference fit with the sidewall  252 . The fluid passages  240  and  242  are formed within the rings  234  and  232 , respectively, and fluid flows therethrough to transfer heat from the fluid to a ring or to the fluid from a ring. 
     While certain preferred embodiments of the present invention have been disclosed in detail, it is to be understood that various modifications may be adopted without departing from the spirit of the invention or scope of the following claims.