Patent Publication Number: US-2023147928-A1

Title: Apparatus and method for making internally finned pressure vessel

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 16/694,860, filed Nov. 25, 2019 and entitled “APPARATUS AND METHOD FOR MAKING INTERNALLY FINNED PRESSURE VESSEL,” which relates to and claims the benefit of U.S. Provisional Application No. 62/872,059, filed Jul. 9, 2019 and entitled “INTERNALLY FINNED HIGH-PRESSURE VESSEL AND METHOD OF FORMING THE SAME,” the entire disclosure of each of which is expressly incorporated herein by reference. 
    
    
     STATEMENT RE: FEDERALLY SPONSORED RESEARCH/DEVELOPMENT 
     Not Applicable 
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates generally to thermal energy conversion and, more particularly, to pressure vessels for storing or transporting fluids. 
     2. Related Art 
     In a thermal energy conversion engine, energy in the form of heat must be removed or added to a fluid stored at high pressure. The typical shape of a container to store or transport fluids at high pressure is spherical or cylindrical. Since liquids and gases generally have much lower thermal conductivity than solid materials, the distance that heat must travel in the fluid must be reduced to maximize heat transfer efficiency. The structural integrity of the vessel containing the fluids at high pressure must not be impacted by the heat transfer enhancement. 
     BRIEF SUMMARY 
     The present disclosure contemplates various apparatuses and methods for overcoming the above challenges accompanying the related art. One aspect of the embodiments of the disclosure is an apparatus for fabricating an internally finned pressure vessel. The apparatus may include a plurality of positioning discs, each of the positioning discs defining a plurality of circumferentially spaced slots extending radially into the positioning disc from a perimeter thereof, and one or more rods extending through the plurality of positioning discs, the plurality of positioning discs being held in axial alignment by the one or more rods. 
     The positioning discs may be made of stainless steel or a nickel-chromium based alloy. 
     The apparatus may include a pair of biasing discs, one on either side of the plurality of positioning discs and held in axial alignment therewith by the one or more rods, each of the biasing discs defining a plurality of circumferentially spaced slots extending radially into the biasing disc from a perimeter thereof, the slots of the biasing discs terminating farther than the slots of the positioning discs from an axis defined by the axial alignment of the plurality of positioning discs. The biasing discs may be made of rubber. 
     The one or more rods may comprise a center rod at an axis defined by the axial alignment of the plurality of positioning discs. The apparatus may include a pair of leveling discs, one on either side of the plurality of positioning discs and held in axial alignment therewith by the center rod, each of the leveling discs defining a surface that overlaps the slots of the positioning discs. The leveling discs may be made of stainless steel. The one or more rods may comprise at least one off-axis rod parallel to the center rod. The positioning discs may be fixed to the at least one off-axis rod with the center rod being movable relative thereto along the axis. The positioning discs may be fixed to the at least one off-axis rod by a plurality of nuts threadably connected to the at least one off-axis rod. The at least one off-axis rod may be made of stainless steel or a nickel-chromium based alloy. 
     The apparatus may comprise a pair of retaining rings installable respectively on either side of the plurality of positioning discs. Each of the retaining rings may have a relaxed diameter that is greater than a diameter defined by interior ends of the slots of the positioning discs. The retaining rings may be made of stainless steel. 
     The one or more rods may comprise a center rod at an axis defined by the axial alignment of the plurality of positioning discs. The apparatus may comprise a leveling disc on one side of the plurality of positioning discs and held in axial alignment therewith by the center rod, the leveling disc defining a surface that overlaps the slots of the positioning discs. The leveling disc may be made of stainless steel. The apparatus may comprise an adjustment assembly including a fixture connected to the leveling disc by the center rod and one or more adjustment screws arranged to protrude from the fixture in a longitudinal direction defined by the center rod. The fixture may be made of stainless steel. The fixture may comprise a bar that extends transversely to the center rod. 
     The one or more rods may include one or more grouping rods that connect a first subset of the plurality of positioning discs to define a first group of positioning discs, one or more grouping rods that connect a second subset of the plurality of positioning discs to define a second group of positioning discs, and one or more rods that connect the first group of positioning discs to the second group of positioning discs. The apparatus may comprise one or more spacer tubes through which the one or more rods that connect the first group of positioning discs to the second group of positioning discs extend, the one or more spacer tubes being disposed between the first and second groups of positioning discs. 
     Another aspect of the embodiments of the disclosure is a brazing jig for fabricating an internally finned pressure vessel. The brazing jig may comprise a plurality of axially aligned positioning discs, each of the positioning discs defining a plurality of circumferentially spaced slots extending radially into the positioning disc from a perimeter thereof, and a pair of retaining rings installable respectively on either side of the plurality of positioning discs, each of the retaining rings having a relaxed diameter that is greater than a diameter defined by interior ends of the slots of the positioning discs. 
     Another aspect of the embodiments of the present disclosure is a brazing jig for fabricating an internally finned pressure vessel. The brazing jig may comprise a plurality of axially aligned positioning discs, each of the positioning discs defining a plurality of circumferentially spaced slots extending radially into the positioning disc from a perimeter thereof, a rod extending through the plurality of positioning discs, and a leveling disc on one side of the plurality of positioning discs and held in axial alignment therewith by the rod, the leveling disc defining a surface that overlaps the slots of the positioning discs. 
     Another aspect of the embodiments of the disclosure is a method of fabricating an internally finned pressure vessel. The method may include providing an apparatus including a plurality of positioning discs, each of the positioning discs defining a plurality of circumferentially spaced slots extending radially into the positioning disc from a perimeter thereof, the apparatus further including one or more rods extending through the plurality of positioning discs, the plurality of positioning discs being held in axial alignment by the one or more rods. The method may further include loading a plurality of fins into the slots of the positioning discs, inserting the apparatus containing the plurality of fins into a pressure vessel, attaching the plurality of fins to the pressure vessel by a brazing process, and removing the apparatus from the pressure vessel. 
     The positioning discs may be made of stainless steel or a nickel-chromium based alloy. 
     The fins may be made of aluminum. 
     The apparatus may include a pair of biasing discs, one on either side of the plurality of positioning discs and held in axial alignment therewith by the one or more rods, each of the biasing discs defining a plurality of circumferentially spaced slots extending radially into the biasing disc from a perimeter thereof, the slots of the biasing discs terminating farther than the slots of the positioning discs from an axis defined by the axial alignment of the plurality of positioning discs. The method may include tack welding the plurality of fins to the pressure vessel after the inserting of the apparatus containing the plurality of fins into the pressure vessel and, thereafter, removing the pair of biasing discs prior to the attaching of the plurality of fins to the pressure vessel by the brazing process. The biasing discs may be made of rubber. 
     The one or more rods may comprise a center rod at an axis defined by the axial alignment of the plurality of positioning discs. The apparatus may include a pair of leveling discs, one on either side of the plurality of positioning discs and held in axial alignment therewith by the center rod, each of the leveling discs defining a surface that overlaps the slots of the positioning discs. The method may include abutting the plurality of fins against the surface of a first leveling disc of the pair of leveling discs and, while the fins are abutted against the surface of the first leveling disc, abutting the plurality of fins against the surface of a second leveling disc of the pair of leveling discs. The abutting of the plurality of fins against the surface of the second leveling disc may include moving the second leveling disc along the axis relative to the center rod. The leveling discs may be made of stainless steel. 
     The method may include welding a pair of end caps to the pressure vessel, one on either side of the plurality of fins. The end caps may be made of aluminum. 
     Another aspect of the embodiments of the disclosure is a method of fabricating a plurality of internally finned pressure vessels. The method may include providing an apparatus including a plurality of positioning discs, each of the positioning discs defining a plurality of circumferentially spaced slots extending radially into the positioning disc from a perimeter thereof, the apparatus further including one or more rods extending through the plurality of positioning discs, the plurality of positioning discs being held in axial alignment by the one or more rods. The method may further include loading a plurality of fins into the slots of the positioning discs, inserting the apparatus containing the plurality of fins into a pressure vessel, attaching the plurality of fins to the pressure vessel by a brazing process, removing the apparatus from the pressure vessel, loading a new plurality of fins into the slots of the positioning discs, inserting the apparatus containing the new plurality of fins into a new pressure vessel, and attaching the new plurality of fins to the new pressure vessel by a brazing process. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features and advantages of the various embodiments disclosed herein will be better understood with respect to the following description and drawings, in which like numbers refer to like parts throughout, and in which: 
         FIG.  1    is a partially exploded perspective view of an apparatus according to an embodiment of the present disclosure; 
         FIG.  2    is a side view of a brazing jig of the apparatus; 
         FIG.  3    is a perspective view of the brazing jig together with a plurality of fins; 
         FIG.  4    is a cross-sectional view taken along line  4 - 4  in  FIG.  3   ; 
         FIG.  5    is the same view as  FIG.  4    but with the plurality of fins having been inserted into the brazing jig; 
         FIG.  6    is a perspective view of the brazing jig and fins together with a pressure vessel; 
         FIG.  7    is a perspective view of the pressure vessel with the brazing jig and fins inserted therein and a tack welding jig of the apparatus shown in exploded view; 
         FIG.  8    is a perspective view of the pressure vessel with the apparatus including the brazing jig and the tack welding jig inserted therein together with the fins; 
         FIG.  9    is a cross-sectional view taken along line  9 - 9  in  FIG.  8   ; 
         FIG.  10    is a perspective view of an internally finned pressure vessel made according to an embodiment of the present disclosure; 
         FIG.  11    is a cross-sectional view taken along line  11 - 11  in  FIG.  10   ; 
         FIG.  12    is a cross-sectional view taken along line  12 - 12  in  FIG.  11   ; 
         FIG.  13    is a partially exploded perspective view of an apparatus according to another embodiment of the present disclosure; and 
         FIG.  14    is a perspective view of the pressure vessel (shown in phantom) with the apparatus inserted therein together with the fins. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure encompasses various apparatuses and methods for fabricating an internally finned pressure vessel. The detailed description set forth below in connection with the appended drawings is intended as a description of several currently contemplated embodiments. It is not intended to represent the only form in which the disclosed subject matter may be developed or utilized. The description sets forth the functions and features in connection with the illustrated embodiments. It is to be understood, however, that the same or equivalent functions may be accomplished by different embodiments that are also intended to be encompassed within the scope of the present disclosure. It is further understood that the use of relational terms such as first and second and the like are used solely to distinguish one from another entity without necessarily requiring or implying any actual such relationship or order between such entities. 
       FIG.  1    is a partially exploded perspective view of an apparatus  100  according to an embodiment of the present disclosure. The apparatus  100  may be used for fabricating an internally finned pressure vessel (see  FIGS.  10 - 12   ), which may be used to store or transport a fluid at high pressure such as a working fluid of a thermal energy conversion system. Unlike more typical constructions in which the fins of the pressure vessel are located externally, an internally finned pressure vessel may improve the heat transfer rate from the exterior of a thermally conductive pressure vessel to a fluid of low thermal conductivity contained within. When fabricating such an internally finned pressure vessel, attachment of the fins to the interior of a tube by drilling or welding through the tube from the outside may compromise the integrity of the vessel and is impractical, especially considering that the fins must be as thin as possible in order to add minimum weight and to maximize void space in the vessel for the working fluid. By positioning the fins on the interior of the vessel for brazing, the apparatus  100  may avoid such difficulties, allowing for the production of an internally finned pressure vessel having high structural integrity with minimal weight and obstruction to fluid flow in radial and/or axial directions. 
     As shown in  FIG.  1   , the apparatus  100  may include a plurality of positioning discs  110 , each of the positioning discs  110  defining a plurality of circumferentially spaced slots  112  extending radially into the positioning disc  110  from a perimeter thereof. In the example of  FIG.  1   , five positioning discs  110  are shown, but fewer or more positioning discs  110  may be used, depending on the length of the internally finned pressure vessel to be produced. The positioning discs  110  may be made of stainless steel or a nickel-chromium based alloy (e.g. Inconel) in order to withstand brazing temperatures. The apparatus  100  may additionally include one or more rods  120  extending through the plurality of positioning discs  110  and holding them in axial alignment, a pair of biasing discs  130 , and a pair of leveling discs  140 . In general, the positioning discs  110  and associated rod(s)  120  may be thought of as constituting a brazing jig used in the brazing process, while the biasing discs  130 , leveling discs  140 , and associated rod(s)  120  may be thought of as constituting a separate tack welding jig used prior to the brazing process as described below. In this respect, the one or more rods  120  may include a center rod  122  that is associated with the tack welding jig and one or more off-axis rods  124  that are associated with the brazing jig and extend parallel to the center rod  122 . 
       FIG.  2    is a side view of the brazing jig. As shown, the positioning discs  110  may be fixed to the rod(s)  124  by a plurality of nuts  160  threadably connected to the rod(s)  124 . For example, the rod(s)  124  may be threaded to allow the nuts  160  to be traversed up and down their length, and each of the positioning discs  110  may be fixed to the rod(s)  124  by positioning the nuts  160  on either side of the positioning disc  110 . To change the longitudinal position of the positioning discs  110  along the rod(s)  124  (e.g. to change the spacing between positioning discs  110 ), one may loosen the nuts  160 , freely slide the positioning disc  110  along the rod(s)  124 , and re-tighten the nuts  160  around the positioning disc  110  at the desired longitudinal position. In this regard, the positioning discs  110  may each define one or more holes  114  through which the rod(s)  120  extend (see  FIG.  1   , in which central holes  114  are visible and off-axis holes  114  are obscured by nuts  160 ). 
       FIG.  3    is a perspective view of the brazing jig together with a plurality of fins  200 .  FIG.  4    is a cross-sectional view taken along line  4 - 4  in  FIG.  3   . As shown in  FIGS.  3  and  4   , a method of fabricating an internally finned pressure vessel may include loading a plurality of fins  200  into the slots  112  of the positioning discs  110 . Twenty fins  200  are shown, with four of the fins  200  omitted in  FIG.  3    in order to more clearly show the brazing jig. Correspondingly, each of the positioning discs  110  has twenty slots  112 . However, any number of fins  200  and slots  112  is contemplated, depending on the heat transfer needs of the internally finned pressure vessel to be fabricated. Each positioning disc  110  may be arranged such that its slots  112  are aligned with corresponding slots  112  of the other positioning discs  110  as shown. As such, each fin  200  may be held by a set of corresponding slots  112 , one per positioning disc  110 . The fins  200  may be made of aluminum or another material that can be attached to the interior of the pressure vessel by brazing. In the example shown, the fins  200  are all the same length but alternate between two widths, and the slots  112  correspondingly alternate between two radial depths into the positioning discs  110 . In the case of an internally finned pressure vessel having a circular cross-section as shown, the use of alternating fin widths and slot depths may allow for an increased number of fins  200  while maintaining a desired minimum distance between the fins  200 . However, other constructions are possible as well, including those with only a single fin width and single slot depth. 
       FIG.  5    is the same view as  FIG.  4    but with the plurality of fins  200  having been inserted into the brazing jig.  FIG.  6    is a perspective view of the brazing jig and fins  200  together with a pressure vessel  300  (e.g. a tube). Prior to insertion into the pressure vessel  300 , the fins  200  may be prevented from falling out of the slots  112  by various means including, for example, a press fit between the fins  200  and the slots  112  (i.e. friction between the fins  200  and the walls of the slots  112 ) or a temporary wrapping or tie circumferentially surrounding the brazing jig with the fins  200  loaded therein. Alternatively, the brazing jig may be arranged vertically so that the fins  200  may be stood upright as shown in  FIG.  6    or slightly inclined toward the brazing jig to be held in the slots  112  by the force of gravity, with the pressure vessel  300  subsequently brought down over the top of the brazing jig and fins  200 . In this way, the pressure vessel  300  itself may serve to hold the fins  200  within the slots  112 . The brazing jig containing the fins  200  may thus be inserted into the pressure vessel  300 . 
       FIG.  7    is a perspective view of the pressure vessel  300  with the brazing jig and fins  200  inserted therein and a tack welding jig of the apparatus  100  shown in exploded view. As explained above, the tack welding jig may include the biasing discs  130 , leveling discs  140 , and associated rod(s)  120  (e.g. the center rod  122 ) of the apparatus  100 . Similar to the positioning discs  110  of the apparatus  100 , each of the biasing discs  130  may define a plurality of circumferentially spaced slots  132  extending radially into the biasing disc  130  from a perimeter thereof. Each biasing disc  130  may be arranged such that its slots  132  are aligned with corresponding slots  112  of the positioning discs  110 . In a case where the slots  112  of the positioning discs  110  have alternating slot depths as described above and shown, the corresponding slots  132  of the biasing discs  130  may likewise have alternating slot depths corresponding thereto. 
     The slots  132  of the biasing discs  130  may terminate farther than the corresponding slots  112  of the positioning discs  110  from the axis  170  defined by the axial alignment of the plurality of positioning discs  110 . For example, a distance d 2  (see  FIG.  1   ) between a given slot  132  of a biasing disc  132  and the center of the biasing disc  132  may be greater than a distance d 1  between a corresponding slot  112  of a positioning disc  110  and the center of the positioning disc  110 . Due to the slots  132  of the biasing discs  130  terminating at a greater distance d 1  from the axis  170  than the corresponding slots  112  of the positioning discs  110 , the biasing discs  130  may apply a biasing force on the fins  200  tending to push the fins  200  outward against the interior wall of the pressure vessel  300 . In this way, a close contact between the fins  200  and the pressure vessel  300  can be achieved. The biasing discs  130  may be made of rubber or another material with sufficient elasticity to allow the fins  200  to be fitted in the slots  132  within the pressure vessel  300  against such biasing force. 
     With the brazing jig and fins  200  inserted in the pressure vessel  300  as shown in  FIG.  7   , the fabrication of the internally finned pressure vessel may continue with placing the pair of biasing discs  130  one on either side of the plurality of positioning discs  110  of the brazing jig. To this end, similar to the positioning discs  110 , the biasing discs  130  may each define one or more holes  134  through which the rod(s)  120  extend. Because the biasing discs  130  will not be used to position the fins  200  during brazing, the biasing discs  130  may simply lie flat against the outermost positioning discs  110 . The off-axis hole(s)  134  of the biasing discs  130  may be large enough to receive the outermost nuts  160  in addition to the off-axis rod(s)  124  as they protrude from the outer-most positioning discs  110 . The off-axis rod(s)  124  need not extend beyond the biasing discs  130  and may in some cases terminate just after the outermost positioning discs  110  upon entering the biasing discs  130 . As noted above, the elasticity of the biasing discs  130  may make it possible to fit the slots  132  of the biasing discs  130  around the fins  200  even though the slots  132  may terminate farther from the axis  170  than the slots of  112  of the positioning discs  110 . In this way, the biasing discs  130  may be used to tightly hold the fins  200  against the interior wall of the pressure vessel  300  for a tack welding process as described below and may thereafter be removed for the subsequent brazing process. 
     The center rod  122  of the tack welding jig may be passed all the way through the brazing jig through the center holes  114  of the positioning discs  110  and, if biasing discs  130  are used, through the center holes  134  of the biasing discs  130  on either end thereof, allowing free movement of the center rod  122  relative to the positioning discs  110 , biasing discs  130 , and off-axis rod(s)  124 . The fabrication of the internally finned pressure vessel may continue with placing the pair of leveling discs  140  on the ends of the center rod  122 . The leveling discs  140 , which may be made of stainless steel or aluminum for example, may each define a surface that overlaps the slots  112  of the positioning discs  110  and, if the biasing discs  130  are used, overlaps the slots  132  of the biasing discs  130  as well. For example, a radius d 3  (see  FIG.  1   ) of the leveling discs  140  may be greater than the distance from center to slot of any of the slots  112 ,  132  of the positioning discs  110  and biasing discs  130 . In particular, the surface of each of the leveling discs  140  may prevent longitudinal movement of the fins  200  past the leveling disc  140 . The leveling discs  140  may thus be used to ensure alignment of the fins  200  in the longitudinal direction of the pressure vessel  300  prior to brazing (or prior to tack welding). For example, the plurality of fins  200  may first be abutted against the surface of one of the leveling discs  140  (e.g. the bottom leveling disc  140  in  FIG.  7   ), which may be fixed to the center rod  122  by a nut  150 . While the fins  200  are abutted against the surface of the one leveling disc  140 , the other leveling disc  140  (e.g. the top leveling disc) may be moved along the axis  170  relative to the center rod  122 , for example, by sliding the leveling disc  140  down until it abuts the fins  200  and fixing the leveling disc  140  at the new position with another nut  150 . To this end, the leveling discs  140  may each define a center hole  144  through which the rod  122  extends and the ends of the center rod  122  may be threaded as shown in  FIG.  7    (or alternatively the entire center rod  122  may be threaded). 
       FIG.  8    is a perspective view of the pressure vessel  300  with the entire apparatus  100  including the brazing jig and tack welding jig inserted therein together with the fins  200 .  FIG.  9    is a cross-sectional view taken along line  9 - 9  in  FIG.  8   . With the fins  200  sandwiched by the leveling discs  140  of the tack welding jig as described above, it can be ensured that the fins  200  are longitudinally aligned relative to each other in the brazing jig. Meanwhile, a close contact between the fins  200  and the pressure vessel  300  can be ensured by outward force applied by the biasing discs  130  (partially obscured in  FIG.  8   ). The method of fabricating the internally finned pressure vessel may thus proceed with tack welding the plurality of fins  200  to the pressure vessel  300 . 
     For example, on both ends of the pressure vessel  300 , a tack weld of, e.g., less than one inch may be made to hold each of the fins  200  to the interior wall of the pressure vessel  300 . The tack weld will serve to temporarily hold each of the fins  200  in the pressure vessel  300  in the desired position as established by the positioning discs  110  and as fine-tuned by the biasing discs  130  and leveling discs  140 . 
     Once each of the fins  200  has been tack welded on both ends, the tack welding jig may be removed from the pressure vessel  300 . In particular, the nuts  150 , leveling discs  140 , biasing discs  130 , and center rod  122  may be removed. The brazing jig containing the fins  200  that are now tack welded to the pressure vessel  300  may remain in the pressure vessel  300 . Thereafter, the plurality of fins  200  may be attached to the pressure vessel  300  by the brazing process. For example, a filler metal (e.g. 88% aluminum and 12% silicon) may be applied to the root of the fins  200  either in the form of a paste or a wire. The entire pressure vessel  300  with fins  200  and brazing jig (e.g. positioning discs  110 , rods  124 , and nuts  160 ) may be preheated in an air furnace and subsequently immersed in a bath of molten salt to melt the filler metal and bond the fins  200  to the vessel  300 . As noted above, the positioning discs  110 , rods  124 , and nuts  160  may be made of stainless steel or a nickel-chromium based alloy (e.g. Inconel) so that brazing of the fins  200  to the brazing jig may be avoided. However, various other combinations of materials may be used with appropriate control of the brazing temperature. By leaving the brazing jig in the pressure vessel  300  during the brazing process, excessive warpage of the fins  200  may be prevented. After the brazing process is complete, the brazing jig may be removed from the pressure vessel  300 . 
       FIG.  10    is a perspective view of an internally finned pressure vessel  300  made according to an embodiment of the present disclosure.  FIG.  11    is a cross-sectional view taken along line  11 - 11  in  FIG.  10   .  FIG.  12    is a cross-sectional view taken along line  12 - 12  in  FIG.  11   . The internally finned pressure vessel  300 , including the plurality of fins  200 , may be made using the apparatus  100  having the brazing jig and tack welding jig described above, for example, by loading the plurality of fins  200  into slots  112  of the positioning discs  110 , inserting the brazing jig into the pressure vessel  300 , biasing the fins  200  against the interior wall of the pressure vessel  300  using the biasing discs  130  of the tack welding jig, leveling the fins  200  using the leveling disc  140  of the tack welding jig, tack welding the fins  200  to the pressure vessel  300 , removing the tack welding jig, brazing the fins  200  to the pressure vessel  300 , and removing the brazing jig. The internally finned pressure vessel  300  may be completed by closing the ends with a pair of end caps. For example, a pair of aluminum end caps may be welded to the pressure vessel  300 , one on either side of the plurality of fins  200 , resulting in a vessel  300  capable of sustaining high internal and external pressure while maximizing radial heat transport from the external environment to the contained working fluid. The apparatus  100  including the brazing jig and tack welding jig may then be reused in the fabrication of additional internally finned pressure vessels  300 . In this regard, a new plurality of fins  200  may be loaded into the slots  112  of the positioning discs  110  and the remainder of the process may be repeated to attach the new plurality of fins  200  to a new pressure vessel  300 . 
     In general, in a thermal energy conversion engine, heat transfer enhancement may be needed for both moving and stationary fluids, making a simple increase of the convective heat transfer coefficient by increasing the fluid velocity insufficient. Without convection, the heat transfer rate is controlled solely by the thermal resistance to conduction in the fluid and the solid material of the pressure vessel. The thermal resistance may be decreased by reducing the distance that energy must travel in the medium of low thermal conductivity and by increasing the interface area between fluid and solid material. Both measures can be achieved using extended surfaces such as fins. Heat transfer from such extended surfaces may be used to increase the rate of heat transfer without increasing the convection coefficient or the temperature differential between solid material and fluid. Unlike more typical constructions in which the fins of the pressure vessel are located externally and intended to increase the heat transfer rate from a solid with high thermal conductivity to air with very low thermal conductivity, the internally finned pressure vessel  300  made according to the present disclosure may improve the heat transfer rate from the exterior of a thermally conductive pressure vessel to a fluid of low thermal conductivity contained within. By using the disclosed apparatus  100 , the difficulties associated with attaching fins to the interior of a pressure vessel may be overcome, allowing for the production of an internally finned pressure vessel  300  having high structural integrity with minimal weight and obstruction to fluid flow. Advantageously, the internally finned pressure vessel  300  produced by the disclosed embodiments and shown in  FIGS.  10 - 12    does not create separate compartments within the pressure vessel  300  and therefore allows unobstructed radial flow in the cross-sectional plane of the pressure vessel  300 . This may be important as certain working substances may exhibit substantial volumetric change when undergoing thermal expansion and/or phase change. 
     In U.S. Pat. No. 7,987,674, the disclosure of which is expressly incorporated herein by reference, a power generation system is described in which a Phase Change Material (PCM) working substance is melted (expanded) at warm temperatures, for example 15° C., and frozen (contracted) at cooler temperatures, for example 8° C. Tubes containing the PCM, which could be a paraffin such as pentadecane, would be equipped with a flexible center tube containing hydraulic oil. When the PCM is melted (expanded), it pushes the oil through a hydraulic motor to generate rotational mechanical energy which is then converted to electricity using common methods. The elapsed time during which the unit must remain at PCM freezing and melting temperatures is critical as it defines the amount of energy that can be generated in a specific timeframe. To keep the freeze and melt time low, previous units have been built with many small diameter tubes to minimize the distance energy has to be transferred through the PCM with low thermal conductivity. The disadvantage of this concept is the increased complexity of the system and the high overall system mass per unit of PCM volume. The disclosed apparatus  100  makes it possible to fabricate one large diameter tube  300  instead of many small ones while maintaining fast freeze and melt times. By the above-described processes, radially arranged thin aluminum fins  200  may be mechanically and thermally attached to the inside wall of the tube  300  to transport heat into the PCM. 
     In the illustrated examples described above, the pressure vessel  300  is assumed to be cylindrical. However, the disclosed subject matter is not intended to be so limited. For example, in the case of a spherical pressure vessel  300 , the fins  200  may be thicker or curved outward in the middle and the positioning discs  110  may be larger toward the middle of the brazing jig. In such case, the relationship between the slot distances d 1 , d 2  of the positioning discs  110  and biasing discs  120  (see  FIG.  1   ) may be modified as needed to create the desired outward biasing while accounting for differences in the sizes of the positioning discs  110  and/or differences in the thicknesses of the fins  200 . 
       FIG.  13    is a partially exploded perspective view of an apparatus  1100  according to another embodiment of the present disclosure. Like the apparatus  100 , the apparatus  1100  may be used for fabricating the internally finned pressure vessel  300  (see  FIGS.  10 - 12   ). The apparatus  1100  may be the same as the apparatus  100  except as shown and described herein and may, for example, include a plurality of positioning discs  1110  that may be the same as the positioning discs  110  except as follows, with each of the positioning discs  1110  similarly defining a plurality of circumferentially spaced slots  1112  extending radially into the positioning disc  1110  from a perimeter thereof.  FIG.  13    illustrates, for example, that each slot  1112  may widen between its two ends  1113   a,    1113   b,  with only the ends  1113   a,    1113   b  of each slot  1112  contacting the fins  200 . This structure may help to provide some flexibility in the resulting “finger” of the positioning disc  1110  defined between each two adjacent slots  1112 , allowing the clearance at the ends  1113   a,    1113   b  of each slot  1112  to be small enough for pressure fitting the fins  200  in the slots  1112 . It is also contemplated that the widening of each slot  1112  between its ends  1113   a,    1113   b  may promote free passage of fluid in the longitudinal direction of the apparatus  100  during the brazing process (as the fins  200  are only gripped at the ends  1113   a,    1113   b ) and may also reduce the amount of material (and thus weight) of the positioning discs  1110 . 
     The apparatus  1100  may additionally include one or more rods  1120  that may be the same as the rods  120  except as described herein and may similarly include a center rod  1122  that is the same as the center rod  122  as well as one or more off-axis rods  1124  that are the same as the off-axis rod(s)  124 , with the off-axis rods  1124  similarly extending all the way through the plurality of positioning discs  1110 . The positioning discs  1110  may similarly be fixed to the rod(s)  1124  by a plurality of nuts threadably connected to the rod(s)  1124  as described in relation to the apparatus  100  and may similarly each define one or more holes  1114  through which the rod(s)  1120  extend. Advantageously, the one or more rods  1120  of the apparatus  1100  may additionally include grouping rods  1126  that may be shorter than the rods  1124  and used to connect a subset of the positioning discs  1110  to define a group thereof. In the example of  FIG.  13   , two such groups are defined, with three grouping rods  1126  (the fatter rods) connecting the upper two positioning discs  1110  to define a first group and with three more grouping rods  1126  connecting the lower two positioning discs  1110  to define a second group. Spacing between the two groups of positioning discs  1110  may be achieved using spacer tubes  1180 , with the longer rods  1124  that span both groups extending through respective spacer tubes  1180 . Grouping the positioning discs  1110  in this way may allow for easier removal of the components of the apparatus  1100  from each end after brazing. 
     Instead of the slotted biasing discs  130  described above in relation to the apparatus  100  of  FIG.  1   , the apparatus  1100  may include one or more pairs of retaining rings  1130  as shown in  FIG.  13    (only the retaining rings  1130  at the top of the apparatus  1100  are clearly visible). A pair of retaining rings  1130  may include one retaining ring  1130  at the top of the apparatus  1100  and another retaining ring  1130  at the bottom of the apparatus  1100 . Like the biasing discs  130 , the retaining rings  1130  (which may be tapered section retaining rings, for example) may apply a biasing force on the fins  200  tending to push the fins  200  outward against the interior wall of the pressure vessel  300 . To this end, each retaining ring  1130  may have a relaxed diameter that is greater than a diameter defined by the interior ends  1113   b  of the slots  1112  (and, in particular, a relaxed radius greater than the difference between the radius of the pressure vessel  300  and the radial length of the fins  200 ). In this way, a close contact between the fins  200  and the pressure vessel  300  can be achieved as the retaining rings  1130  are first compressed to be installed in the bore defined by the collective interiors of the fins  200  and then allowed to expand against the fins  200  to firmly press the fins  200  outward against the interior wall of the pressure vessel  300 . The retaining rings  1130  may be made of a material that can withstand brazing temperatures, such as stainless steel. Each individual retaining ring  1130  may have a gap where it compresses. Depending on the gap size of the retaining rings  1130  (and, in particular, whether it is greater than the circumferential pitch of the fins  200 ), it is contemplated that a second pair of retaining rings  1130  may be employed (as visible at the top of the apparatus  1100  in  FIG.  13   ), with the gap of one ring  1130  offset from the gap of the other  1130  so as to effectively eliminate the gaps and apply force on all of the fins  200 . 
     Instead of two leveling discs  140  as described above in relation to the apparatus  100  of  FIG.  1   , the apparatus  1100  may advantageously make use of only a single leveling disc  1140  (which may otherwise be the same as either of the leveling discs  140 ), which may be provided on one end of the center rod  1122 . The leveling disc  1140  may define a surface that overlaps the slots  1112  of the positioning discs  1110  and may prevent longitudinal movement of the fins  200  past the leveling disc  1140 . By orienting the apparatus  1100  with the leveling disc  1140  at the bottom, longitudinal movement of the fins  200  away from the leveling disc  1140  (i.e., upward) may be prevented or at least limited by gravity. The leveling disc  1140  may thus be used to ensure alignment of the fins  200  in the longitudinal direction of the pressure vessel  300  prior to and during brazing. The leveling disc  1140  may prevent the fins  200  from falling out of the pressure vessel  300  during brazing in the event that the frictional forces applied by the retaining rings  1130  are not sufficient. The plurality of fins  200  may be abutted against the surface of the leveling disc  1140 , which may be fixed to the center rod  1122  by a nut  1150 . To this end, the leveling disc  1140  may define a center hole  1144  through which the rod  1122  extends and the ends of the center rod  1122  may be threaded as shown in  FIG.  13    (or alternatively the entire center rod  1122  may be threaded). The use of only a single leveling disc  1140  may allow the fins  200  to expand freely when heat is applied during the brazing process, thereby preventing excessive warpage that might otherwise occur when the fins  200  are confined between the two leveling discs  140  of the apparatus  100 . 
     In order to adjust the longitudinal position of the fins  200  within the pressure vessel  300  prior to brazing, the apparatus  1100  may further include an adjustment assembly  1190  comprising a fixture  1192  and one or more adjustment screws  1194 . The fixture  1192  (which may be a bar made of stainless steel, for example) may be connected to the leveling disc  1140  by the center rod  1122  (secured by another nut  1150 ). The adjustment screw(s)  1194  may be arranged to protrude from the fixture  1192  in a longitudinal direction defined by the center rod  1122  and may extend through the fixture  1192  as shown, for example. 
       FIG.  14    shows the apparatus  1100  disposed within the pressure vessel  300  with the fins  200  inserted into the slots  1112  of the positioning discs  1110  (and biased outward against the inner wall of the pressure vessel  300  by the retaining rings  1130  as described above). As shown, the fixture  1192  of the adjustment assembly  1190  may be positioned by the center rod  1122  above the positioning discs  1110  and fins  200  and may be shaped so as to extend transversely to the center rod  1122  so as to span the interior of the pressure vessel  300  when the apparatus  1100  is disposed therein (e.g., extending from one side of the pressure vessel  300  to the other in the case of the illustrated bar). The adjustment screws  1194  may be arranged so as to abut the pressure vessel  300  on either side of the apparatus  1100  to allow the height of the fixture  1192  to be adjusted relative to the pressure vessel  300 . In the illustrated embodiment, for example, the fixture  1192  is disposed above the pressure vessel  300  and the adjustment screws  1194  extend from the fixture  1192  to meet the longitudinal edge of the pressure vessel  300  at different positions along the perimeter thereof. In this way, the adjustment screws  1194  may be turned to adjust the height of the fixture  1192  relative to the pressure vessel  300 . Since the fixture  1192  is connected to the leveling disc  1140  that holds the fins  200  (with the positioning discs  1110  held at a fixed position along the center rod  1122  by nuts  1123 , for example), the adjustment of the height of the fixture  1192  in this way causes the entire apparatus  1100 , and in particular the fins  200  held by the leveling disc  1140 , to be moved longitudinally upward or downward relative to the pressure vessel  300 . (The pressure vessel  300  may rest on a stand or any surface during this adjustment, as well as during the subsequent brazing.) Once the fins  200  are at the desired longitudinal position within the pressure vessel  300 , they may be brazed to the inner wall of the pressure vessel  300  to produce the internally finned pressure vessel  300 . 
     The illustrated bar represents one relatively simple example of the fixture  1192 , which may be arranged to support adjustment screws  1194  that abut with the pressure vessel  300  at two points (e.g., on opposite sides). However, other types of fixtures  1192  are contemplated as well, such as a fixture  1192  having three or more arms (e.g., star or spider-shaped fixture  1192 ) to support adjustment screws  1194  that abut the pressure vessel  300  at three or more points (e.g., equally spaced points about the perimeter) or a disc-shaped fixture  1192  supporting any number of adjustment screws  1194  about its perimeter. 
     The process of fabricating an internally finned pressure vessel  300  using the apparatus  1100  may be the same as when using the apparatus  100  except in the particulars described above. One exemplary method may include grouping the positioning discs  1110  as desired using the rods  1124 ,  1126  and spacers  1180 , loading a plurality of fins  200  into the slots  1112  of the positioning discs  1110 , inserting the arranged positioning discs  1110  containing the plurality of fins  200  into a pressure vessel  300 , installing the retaining rings  1130  to bias the fins  200  against the interior surface of the pressure vessel  300 , inserting the center rod  1122  and attaching the leveling disc  1140  and adjustment assembly  1190  on either end thereof, aligning the fins  200  against the leveling disc  1140 , adjusting the longitudinal position of the fins  200  relative to the pressure vessel  300  using the adjustment assembly  1190 , attaching the plurality of fins  200  to the pressure vessel  300  by a brazing process, and, after brazing, removing the apparatus  1100  from the pressure vessel  300  (after which a new plurality of fins  200  may be loaded for use in fabricating another internally finned pressure vessel  300  using the same apparatus  1100 ). Advantageously, the components of the apparatus  1100  may be made out of materials such as stainless steel that can withstand high brazing temperatures, such that the entire apparatus  110  may remain in place while brazing. As a result, a preliminary tack welding step may be unnecessary and can be omitted, simplifying and shortening the fabrication process. 
     The above description is given by way of example, and not limitation. Given the above disclosure, one skilled in the art could devise variations that are within the scope and spirit of the invention disclosed herein. Further, the various features of the embodiments disclosed herein can be used alone, or in varying combinations with each other and are not intended to be limited to the specific combination described herein. Thus, the scope of the claims is not to be limited by the illustrated embodiments.