Heat exchanger and method of constructing same

Information

  • Patent Grant
  • 6446336
  • Patent Number
    6,446,336
  • Date Filed
    Friday, September 3, 1999
    24 years ago
  • Date Issued
    Tuesday, September 10, 2002
    21 years ago
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'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.



Claims
  • 1. A method of forming a heat exchanger at a predetermined heat exchanger region on a wall having an interior surface and an exterior surface, the method comprising:(a) aligning a radially inwardly facing surface of an annular, outer ring coaxially with the exterior surface of the wall, the outer ring having a connected heat transfer means; (b) displacing the outer ring along an outer ring axis until the radially inwardly facing surface seats against the exterior surface of the wall at the predetermined heat exchanger region; (c) aligning a radially outwardly facing surface of an annular, inner ring coaxially with the interior surface of the wall, the inner ring having a connected heat transfer means; and (d) displacing the inner ring along an inner ring axis until the radially outwardly facing surface seats against the interior surface of the wall at the predetermined heat exchanger region and on an opposite side of the wall as the outer ring, thereby clampingly retaining the wall between the radially outwardly facing surface of the inner ring and the radially inwardly facing surface of the outer ring.
  • 2. A method in accordance with claim 1, wherein the inner ring is displaced in a first direction, and the outer ring is displaced in a second, opposite direction.
  • 3. A method in accordance with claim 1, further comprising creating a temperature difference between the rings.
US Referenced Citations (8)
Number Name Date Kind
2102581 McNeish Dec 1937 A
2643863 Buschow Jun 1953 A
2678808 Gier, Jr. May 1954 A
3002729 Welsh Oct 1961 A
3868754 Kawano Mar 1975 A
4348794 Kim et al. Sep 1982 A
4349959 Bowden Sep 1982 A
4419802 Riese Dec 1983 A