Patent Publication Number: US-6991026-B2

Title: Heat exchanger with header tubes

Description:
BACKGROUND OF THE INVENTION 
   The invention relates to a heat exchanger having header tubes. 
   BRIEF DESCRIPTION OF THE INVENTION 
   The invention provides a heat exchange cell for use in a recuperator. The cell includes top and bottom plates spaced apart to define therebetween an internal space, each of the top and bottom plates defining an inlet and outlet opening communicating with the internal space for the respective inflow and outflow of fluid with respect to the internal space. The cell also includes a plurality of internal matrix fins within the internal space and metallurgically bonded to the top and bottom plates. The cell also includes a plurality of inlet header tubes within the internal space and communicating between the inlet opening and the matrix fins, each inlet header tube being rigidly affixed to at least one adjacent inlet header tube and to the top and bottom plates. The cell also includes a plurality of outlet header tubes within the internal space and communicating between the matrix fins and the outlet opening, each outlet header tube being rigidly affixed to at least one adjacent outlet header tubes and to the top and bottom plates. 
   The inlet header tubes may include flat portions that are rigidly affixed to the top and bottom plates and to the adjacent inlet header tubes. The inlet header tube may, for example, have a substantially rectangular cross-section having four flat sides, wherein two of the flat sides are rigidly affixed to the respective top and bottom plates and the other two of the flat sides are rigidly affixed to adjacent inlet header tubes. The inlet header tubes may be metallurgically bonded to each other and to the top and bottom plates. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic representation of a microturbine engine including a recuperator according to the present invention. 
       FIG. 2  is a perspective view of the core of the recuperator of  FIG. 1 . 
       FIG. 3  is an exploded view of one cell of the recuperator of  FIG. 2 . 
       FIG. 4  is a perspective view of one of the header tubes of the recuperator of  FIG. 3 . 
       FIG. 5  is a cross-section view of a portion of a header of a recuperator cell. 
       FIG. 6  is an enlarged cross-section view of a known header fin. 
       FIG. 7  is an enlarged cross-section view of a portion of two adjacent header tubes according to the present invention. 
   

   DETAILED DESCRIPTION 
   Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limited. The use of “including,” “comprising” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. The terms “mounted,” “connected” and “coupled” are used broadly and encompass both direct and indirect mounting, connecting and coupling. Further, “connected” and “coupled” are not restricted to physical or mechanical connections or couplings, and can include electrical connections or couplings, whether direct or indirect. 
   Microturbine engines are relatively small and efficient sources of power. Microturbines can be used to generate electricity and/or to power auxiliary equipment such as pumps or compressors. When used to generate electricity, microturbines can be used independent of the utility grid or synchronized to the utility grid. In general, microturbine engines are limited to applications requiring 2 megawatts (MW) of power or less. However, some applications larger than 2 MW may utilize one or more-microturbine engines. 
     FIG. 1  illustrates a microturbine engine  10  that includes a compressor  15 , a recuperator  20 , a combustor  25 , a gassifier turbine  30 , a power turbine  35 , and a power generator  40 . Air is compressed in the compressor  15  and delivered to the recuperator  20 . With additional reference to  FIG. 2 , heat is exchanged in the recuperator  20  between a flow of hot gases  45  and the flow of compressed air  50 , such that the flow of compressed air  50  is preheated. The preheated air is mixed with fuel and the mixture is combusted in the combustor  25  to generate a flow of products of combustion or hot exhaust gases. The use of a recuperator  20  to preheat the air allows for the use of less fuel to reach the desired temperature within the flow of products of combustion, thereby improving engine efficiency. 
   The flow of hot exhaust gases drives the rotation of the gassifier turbine  30  and the power turbine  35 , which in turn drives the compressor  15  and power generator  40 , respectively. The power generator  40  generates electrical power in response to rotation of the power turbine  35 . After exiting the gassifier and power turbines  30 ,  35 , the flow of exhaust gases, which is still very hot, is directed to the recuperator  20 , where it is used as the aforementioned flow of hot gases  45  in preheating the flow of compressed air  50 . The exhaust gas then exits the recuperator  20  and is discharged to the atmosphere, processed, or used in other processes (e.g., cogeneration using a second heat exchanger). 
   The engine  10  shown is a multi-spool engine (more than one set of rotating elements). As an alternative to the construction illustrated in  FIG. 1  and described above, a single radial turbine may drive both the compressor  15  and the power generator  40  simultaneously. This arrangement has the advantage of reducing the number of turbine wheels. Also, the illustrated compressor  15  may be a centrifugal-type compressor having a rotary element that rotates in response to operation of the gassifier turbine  30 . The compressor  15  may be a single-stage compressor or a multi-stage compressor (when a higher pressure ratio is desired). Alternatively, compressors of different designs (e.g., axial-flow compressors, reciprocating compressors, scroll compressor) can be employed to supply air to the engine  10 . 
   The gassifier and power turbines  30 ,  35  may be radial inflow single-stage turbines each having a rotary element directly or indirectly coupled to the rotary element of the respective compressor  15  and power generator  40 . Alternatively, multi-stage turbines or axial flow turbines may be employed for either or both of the gassifier and power turbines  30 ,  35 . A gearbox or other speed reducer may be used to reduce the speed of the power turbine  35  (which may be on the order of 50,000 RPM, for example) to a speed usable by the power generator  40  (e.g., 3600 or 1800 RPM for a 60 Hz system, or 3000 or 1500 RPM for a 50 Hz system). Although the above-described power generator  40  is a synchronous-type generator, in other constructions, a permanent magnet, or other non-synchronous generator may be used in its place. 
     FIG. 2  illustrates the recuperator  20  constructed of a plurality of heat exchange cells  55 . The relatively hot and cool gases  45 ,  50 , respectively, flow generally parallel and opposite to each other through the center portion (hereinafter referred to as the matrix portion  60 ) of the recuperator  20 , with the hot gases  45  flowing between the cells and the relatively cool gases  50  flowing inside the cells  55 . Header portions  61  of the cells  55  direct the compressed air  50  into the matrix portion  60  along a flow path that is generally perpendicular to the flow path in the matrix portion  60 . In this regard, the illustrated recuperator  20  may be termed a counterflow recuperator with crossflow headers. 
   With reference to  FIG. 3 , the recuperator cells  55  include top and bottom plates  63  that are joined (e.g., by welding, fastening, or another means for substantially air-tightly joining the plates) together along their entire edges or peripheries. The generally flat central parts of the plates  63  are generally parallel to each other and define therebetween an internal space. The cell  55  includes inlet and outlet holes  65 ,  70  communicating with the internal space. 
   Internal matrix fins  75  are metallurgically bonded (e.g., by welding, brazing, or another joining process that facilitates heat transfer) to the inside surfaces of the top and bottom plates  63  and are thus within the internal space of the cell  55 . External matrix fins  80  are metallurgically bonded to the outer surfaces of the top and bottom plates  63  above and below the internal matrix fins  75 . The internal and external matrix fins  75 ,  80  are in the matrix portion  60  of the recuperator  20  and their corrugated fins are generally parallel to each other. Most of the heat exchange between the fluid  50  flowing through the cells  55  and the fluid  45  flowing between the cells  55  occurs in the matrix portion  60  and is aided by the internal and external matrix fins  75 ,  80 . It is therefore desirable to maximize the heat transfer capability of the recuperator  20  within the matrix portion  60 . 
   With reference to  FIGS. 3–5 , header tubes  90  are arranged in parallel fashion in the inlet and outlet header portions  61  of each cell  55 . The header tubes  90  are metallurgically bonded to the top and bottom plates  63  and are also metallurgically bonded to each other. The header tubes  90  have generally rectangular cross sections (e.g., they may be generally square or have another rectangular shape) with top, bottom, and side walls. The side walls of adjacent tubes  90  are generally parallel and in close proximity to each other, and are metallurgically bonded to each other. As seen in  FIG. 4 , the end  91  of each header tube  90  adjacent the inlet and outlet openings  65 ,  70  may be cut or formed to follow the curvature of the openings  65 ,  70  (as illustrated) or may be cut at right angles to the side and top walls of the tube  90 . The end  93  of each tube  90  adjacent the matrix fins  75  is cut at an angle so that each tube  90  communicates with a plurality of the matrix fins. 
   To construct the recuperator core (as in  FIG. 2 )  20 , each cell  55  is positioned with its inlet and outlet holes  65 ,  70  in alignment with the respective inlet and outlet holes  65 ,  70  of the other cells  55 . The top plate  63  of each cell  55  is joined to the bottom plate  63  of the cell  55  above it along the edge of the inlet and outlet holes  65 ,  70 . The resulting generally cylindrical spaces defined by the stacked inlet and outlet holes  65 ,  70  are referred to as the inlet and outlet manifolds  95 ,  100 , respectively, of the recuperator  20 . The inlet manifold  95  delivers the compressed air  50  to the internal space of the cells  55  and the outlet manifold  100  delivers preheated compressed air  50  to the combustor  25 . 
   The internal spaces of the cells  55  are pressurized by the compressed air flowing through them. The internal matrix fins  75  and the header tubes  90  must withstand the tensile load that results from the pressure forcing the top and bottom plates  63  away from each other. The purpose of the header regions  61  of the cells  55  is to deliver the compressed air to or from the matrix portion  60  with as little frictional loss (i.e., pressure drop) as possible while still maintaining the structural stability of the header portion  61 ; minimizing pressure drop is a more important design consideration in the header portion  61  than maximizing heat transfer. The purpose of the matrix portion  60  is to transfer as much heat as possible from the relatively hot gases  45  flowing between the cells  55  to the relatively cool gases  50  flowing within the cells  55 ; maximizing heat transfer is a more important design consideration in the matrix portion  60  than minimizing pressure drop. 
   The internal matrix fins  75  are constructed of a corrugated material (sometimes referred to as “folded fins”) having a relatively high fin density. The corrugated material is metallurgically bonded to the top and bottom plates  63  at each crest and trough. The high fin density provides more heat transfer and load bearing paths to enhance heat transfer and structural stability in the matrix portion  60 . 
     FIG. 6  illustrates the effect of high pressure in the header portion  61  of the cells  55  when corrugated header fins  105  are used. The fin density in the header portion  61  is typically kept as low as possible to reduce pressure drop across the header portion  61 . However, the lower fin density also reduces the number of tensile stress bearing fins in the header portion  61 . As the fin density in the header portion  61  is decreased, the degree to which the top and bottom plates  63  are separated as a result of the internal pressure increases. 
   Separation of the top and bottom plates  63  applies bending stresses to the fillets  110  connecting the corrugated fins  105  to the top and bottom plates  63 . As used herein, the term “fillet” means the deposit of metallurgically bonding material (e.g., welding flux, brazing material or the material used in any other metallurgically bonding process) connecting the top and bottom plates  63  and the illustrated corrugated header fins  105  or header tubes  90  (seen in  FIG. 6 ). More specifically, as seen in phantom in  FIG. 6 , as the top and bottom plates  63  move apart, the fins  105  stretch and achieve a steeper orientation as the angle θ decreases. This applies a bending stress on the fillet  110 . 
   One way to reduce the bending stress on the fillet  110  is to increase the size of the fillet  110  to cover the entire corner of the fin (e.g., a fillet bounded by the phantom line  115  in  FIG. 6 ). However, there is an upper limit to the practical size of a fillet  110  because larger fillets tend to result in voids, and metallurgical transformation in the fillet material that may weaken the fillet  110 . 
   Another way to reduce the bending stress on the fillet  110  is to increase the fin density to provide more tensile load bearing paths in the header portion  61 . This would reduce or eliminate the extent to which the top and bottom plates  63  can move apart, which would in turn reduce the deflection of the fin and the bending stress on the fillet  110 . However, there is a limit to the acceptable fin density in the header portion  61  of the cell  55  because of the resultant increase in pressure drop. 
     FIG. 7  illustrates the corners of adjacent rectangular header tubes  90 . Although the illustrated tubes  90  are metallurgically bonded to each other and to the top and bottom plates  63 , the tubes  90  may alternatively be joined to each other and to the top and bottom plates  63  in other suitable ways, especially because the heat transfer capability of the header portion  61  is not a driving design factor. The header tubes  90  may therefore, for example, be mechanically joined with fasteners, clips, or the like. The most economical means for joining the tubes  90  to the top and bottom plates  63  and to each other, however, is currently thought to be via metallurgical bonding via brazing, as illustrated. 
   Because the sides of the rectangular tubes  90  are fixed to each other, any deflection of one would have to be shared by the adjacent side of the adjacent tube  90 . Separation of the top and bottom plates  63  would require both angles θ and θ′ to decrease. The adjacent tubes  90  therefore offset each other and the tensile load is born by the tubes  90  without significant deflection of their sides and consequently without significant bending stresses on the fillets  110 . Thus, fillets  110  of optimal size may be used and the amount of structural material (e.g., fin density) may be kept relatively low to reduce pressure drop across the header portions  61 . A header fin constructed of a corrugated material  105  (as in  FIG. 6 ) is unable to take advantage of the structural superiority of the rectangular tubes  90  illustrated in  FIG. 7  because the fins of the corrugated material  105  do not have any adjacent fins to which they may be metallurgically bonded. 
   Although the illustrated header tubes  90  have rectangular cross-sections, other cross-sectional shapes are contemplated by the invention. For example, the tubes may be generally circular in shape with four flats that may be rigidly affixed to the top and bottom sheets and to the adjacent tubes. The header tubes could also have a polygonal cross-sectional shape, such as octagonal, which provides flat surfaces for rigidly affixing to the top and bottom sheets and to the adjacent tubes.