Abstract:
A heat exchanger, including a core having a variable size or length and a support structure connected to the core, the support structure accommodating variations in the size of the core. This abstract is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. 37 C.F.R. 1.72(b).

Description:
BACKGROUND OF THE INVENTION  
         [0001]    To improve the overall efficiency of a gas turbine engine, a heat exchanger or recuperator can be used to provide heated air for the turbine intake. The heat exchanger operates to transfer heat from the hot exhaust of the turbine engine to the compressed air being drawn into the turbine. As such, the turbine saves fuel it would otherwise expend raising the temperature of the intake air to the combustion temperature.  
           [0002]    The heat of the exhaust is transferred by ducting the hot exhaust gases past the cooler intake air. Typically, the exhaust gas and the intake air ducting share multiple common walls, or other strictures, which allow the heat to transfer between the two gases (or fluids depending on the specific application). That is, as the exhaust gases pass through the ducts, they heat the common walls, which in turn heat the intake air passing on the other side of the walls. Generally, the greater the surface areas of the common walls, the more heat which will transfer between the exhaust and the intake air. Also, the more heat which transfers between the exhaust and the air, the greater the efficiency of the heat exchanger will be.  
           [0003]    As shown in the cross-sectional view of FIG. 1, one example of this type of device is a heat exchanger  5 , which uses a shell  10  to contain and direct the exhaust gases, and a core  20 , placed within the shell  10 , to contain and direct the intake air. As can be seen, the core  20  is constructed of a stack of thin plates  22  which alternatively channel the inlet air and the exhaust gases through the core  20 . That is, the layers  24  of the core  20  alternate between channeling the inlet air and channeling the exhaust gases. In so doing, the ducting keeps the air and exhaust gases from mixing with one another. Generally, to maximize the total heat transfer surface area of the core  20 , many closely spaced plates  22  are used to define a multitude of layers  24 . Further, each plate  22  is very thin and made of a material with good mechanical heat conducting properties. Keeping the plates  22  thin assists in the heat transfer between the hot exhaust gases and the colder inlet air.  
           [0004]    Typically, during construction of such a heat exchanger  5 , the plates  22  are positioned on top of one another and then compressed to form a stack  26 . Since the plates  22  are each separate elements, the compression of the plates  22  ensures that there are always positive compressive forces on the core  20 , so that the plates  22  do not separate. The separation of one or more plates  22  can lead to a performance reduction or a failure by an outward buckling of the stack  26 . As such, typically the heat exchanger  5  is constructed such that the stack  26  is under a compressive pre-load.  
           [0005]    Applying a high pre-load reduces the potential for separation of the plates  22 . However, this approach does have the significant drawback that all the components of the core  20  are placed under much greater stress than they would be without the pre-loading. In addition, the pre-loading requires that the structure supporting the stack  26  must be much stronger and thus thicker. This pre-load assembly or support structure  40  collectively includes strongbacks  28 , tie rods  30 , as well as the shell  10  structure. This support structure  40  adds to both the weight and the cost of the heat exchanger  5 .  
           [0006]    Because the support structure  40  supports the core  20  and is not a heat transfer medium, the components of the support structure  40  are typically made of much thicker materials than that of the core  20 . Unfortunately, these thicker materials cause the support structure  40  to thermally expand at a much slower rate than the quick responding core  20 , which has the thin plates  22 . The thickness (and thus the thermal response) of the support structure  40  will also be affected by the amount of the pre-load it must apply to the core  20 .  
           [0007]    Differential thermal expansion between elements of the heat exchanger  5  will cause a compression load to be applied to the quicker expanding sections (e.g. the core  20  and specifically the stack  26 ). As noted, a compression load is also applied to the stack  26  by the application of a pre-load. Compressive forces from pre-loading and differential thermal expansion can cause a variety of problems, such as buckling, fatigue failures and creep. Buckling is particularly  
           [0008]    problematic as it results in the stack  26  expanding outward (laterally) in one or more directions. This outward expansion causes the plates  22  to separate from one another, resulting in a nearly complete destruction of the heat exchanger. Fatigue and creep frequently occur when heat exchangers are repeatedly cycled between hot and cold stages. Depending on the particular application, a turbine (not shown) attached to a heat exchanger can be started, ran for a short period of time and then shutdown, over and over. One example of such cyclic use is a turbine and heat exchanger apparatus employed in the production of electric power. Typically, such devices are run only during recurring periods of peak power demand.  
           [0009]    An additional source of loading on the heat exchanger can be from the airflow in the core  20 . When the inlet air in the core  20  is pressurized, the core  20  will want to expand out against the support structure  40 . This increases the amount of support structure needed to contain the core  20 , which further reduces the thermal response of the supporting structure  40 .  
           [0010]    Prior approaches to providing for differential expansion between the core  20  and the shell  10 , have included providing a gap or space for the core to expand into. However, the use of such a gap greatly reduces the efficiency of the heat exchanger by allowing much of the exhaust gas to pass around the core and not through it. Because of the gas pressures typically involved, even a very small gap can allow a great deal of exhaust gas to bypass the core. When the exhaust gas bypasses the core, less heat transfers to the intake air, and as a result, the overall efficiency of the heat exchanger (and thus of the turbine) drops dramatically.  
           [0011]    Therefore, a need exists for a heat exchanger which allows for differential thermal expansion between the core and the supporting structure, thereby preventing core buckling, fatigue failures, creep or other similar problems. The heat exchanger must however apply, throughout the differential expansion, a force (e.g. pre-load) to the core, which is sufficient to keep the core plates from separating or otherwise deviating from their positions. In addition, the heat exchanger must maintain a seal between the core and the shell, so to prevent the gases from bypassing the core, which would otherwise reduce the efficiency of the heat exchanger. Further, such an apparatus should be relatively simple in construction and operation to minimize its cost, weight and complexity.  
         SUMMARY OF THE INVENTION  
         [0012]    In some embodiments, the present invention is a heat exchanger which includes a core having a variable size and a support structure connected to the core. The support structure has a deformable member for accommodating variations in the size of the core. The support structure also includes a biasing member for applying a biasing force to the core. In some embodiments, the deformable member and the biasing member share the same structure. The deformable member and/or the biasing member can include a tension spring, a compression spring, a bellows, or a piston assembly.  
           [0013]    In other embodiments the Applicant&#39;s invention is a heat exchanger which includes a core having a variable length and a support structure which receives the core. The support structure includes a fixed member and an attached biased deformable member. The biased deformable member accommodates variations in the length of the core while applying a biasing force to the core. The biased deformable member can include a tension spring, a compression spring, a bellows, or a piston assembly. The fixed member can include a first portion and a second portion which are positioned about and are in contact with the core with the biased deformable member being mounted between the first portion and the second portion.  
           [0014]    The biased deformable member can be a tie rod having a coiled spring section. The spring section allows the tie rod to deform to accommodate variations in the length of the core, while applying a biasing force to the first and second portions of the fixed member. In place of a coiled spring, the tie rod can have a shaped spring section, such as an ‘s-shape’. In other embodiments, the deformable member is a tie rod with a compression spring placed between the end of the tie rod and a portion of the fixed member. Examples of compression springs include a coiled spring or a Belleville washer.  
           [0015]    In other embodiments, the fixed member comprises a first end and a second end positioned about the core. The first end is in contact with the core and the biased deformable member is mounted between the core and the second end of the fixed member. The biased deformable member is positioned so that it can be deformed as the length of the core varies. In these embodiments the biased deformable member can be a compression spring (e.g. coil spring), a bellows or a piston assembly. The bellows includes a first plate, a second plate and an expandable sidewall mounted between the first plate and the second plate. The bellows can be narrower, the same width or wider than the core. The piston assembly includes a cylinder and a piston received by the cylinder. As with the bellows, the piston assembly can be narrower, the same width or wider than the core. 
       
    
    
     BRIEF SUMMARY OF THE DRAWINGS  
       [0016]    [0016]FIG. 1 is a side cut-away view of a portion of a heat exchanger.  
         [0017]    [0017]FIG. 2 is an isometric view of a turbine/heat exchanger system.  
         [0018]    [0018]FIG. 3 is an isometric view of a heat exchanger in accordance with the present invention  
         [0019]    FIG  4  is a side cut-away view of a portion of a heat exchanger in accordance with the present invention.  
         [0020]    [0020]FIG. 5 is an angled side cut-away view of a portion of a heat exchanger in accordance with the present invention.  
         [0021]    [0021]FIG. 6 is a side cut-away view of a portion of a heat exchanger in accordance with the present invention.  
         [0022]    [0022]FIGS. 7 a  and  b  are side cut-away views of a portion of a heat exchanger in accordance with the present invention.  
         [0023]    [0023]FIGS. 5 a  and  b  are side cut-away views of a portion of a heat exchanger in accordance with the present invention.  
         [0024]    [0024]FIGS. 9 a  and  b  are side cut-away views of a portion of a heat exchanger in accordance with the present invention.  
         [0025]    [0025]FIGS. 10 a  and  b  are side cut-away views of a portion of a heat exchanger in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0026]    The present invention allows differential thermal expansion to occur between the heat exchanger&#39;s core and the support structure, without damage resulting from buckling, fatigue failure, creep or any other similar cause. The Applicants&#39; invention provides for this differential expansion with a mechanically expandable support structure, which expands and contracts with the core, while applying a continuous biasing force to the core. The support structure uses a biased deformable member, which allows the support structure to accommodate variations in the core size. As described in detail herein, the present invention has several advantages over the prior art.  
         [0027]    Unlike prior devices, the Applicants&#39; invention allows for the differential thermal expansion of the core by allowing the support structure to expand not only thermally but also mechanically. Also, in at least some embodiments, the present invention employs a biasing means to maintain a compression force on the core. As such, an advantage is achieved with the present invention of allowing the core to thermally expand relatively freely while the core is kept under a compressive force (e.g. pre-load) to prevent the core from separating or otherwise displacing in an undesired manner.  
         [0028]    Another advantage of some embodiments of the Applicants&#39; invention is that the heat exchanger allows the core to thermally expand freely while maintaining contact between the core and the shell. This continuous core-to-shell contact prevents gaps from forming between the two structures, thus keeping exhaust gases from bypassing around the core. As a result, the efficiency of the heat exchanger is maximized by forcing the hot gases through the core, so that the maximum amount of heat can be transferred from the exhaust gases to the cooler intake air.  
         [0029]    Still another advantage of embodiments of the present invention is that by allowing the core to expand and contract relatively freely, the core is not placed under additional compressive loads caused by restraining the core&#39;s movement. As such, the problems of buckling, fatigue failure and creep typically associated with prior heat exchangers are avoided. Further since the core is not under these additional compressive loads, the pre-load placed on the core can be dramatically reduced. In at least some embodiments of the present invention, by carrying substantially less loads the shell requires less structure and can therefore thermally expand and contract much quicker. This also allows the shell to be simpler, lighter and less expensive to manufacture.  
         [0030]    Therefore, the present invention provides a heat exchanger, or similar apparatus, which reduces the potential for damage to the core (e.g. plate separation, buckling, fatigue failure, creep, etc.), which is more efficient, easier to manufacture, lighter, and less expensive.  
         [0031]    Heat exchanger apparatuses which provide for differential thermal expansion are set forth in U.S. patent application Ser. No. 09/652,949, filed oil Aug. 31, 2000, entitled HEAT EXCHANGER WITH BYPASS SEAL ALLOWING DIFFERENTIAL THERMAL EXPANSION, by Yuhung Edward Yeh, Steve Ayres and David Beddome, which is hereby incorporated by reference in its entirety, and U.S. patent application Ser. No. 09/864,581, filed on May 24, 2001, entitled HEAT EXCHANGER WITH MANIFOLD TUBES FOR STIFFENING AND LOAD BEARING, by David W. Beddome, Steve Ayres, Yuhung Edward Yeh, Ahmed Hammond, David Bridgnell and Brian Comiskey, which is hereby incorporated by reference in its entirety.  
         [0032]    As shown in FIG. 2, for some embodiments, the present invention is a heat exchanger  100  which can be used in conjunction with a gas turbine engine. The heat exchanger  100  functions to heat the inlet air prior to it entering the turbine and cool the turbine exhaust gases prior to exiting the heat exchanger  100 . This is achieved by directing the inlet air so that it passes adjacent to the exhaust gas, such that heat is transferred from the exhaust to the inlet air. Specifically, as set forth in FIG. 2, air enters at an air inlet and is directed through the heat exchanger  100  where it is heated by heat from the exhaust gases. Then, the heated air is directed from the heat exchanger  100  to the turbine. The turbine uses the air to operate and in so doing expels exhaust gas. The exhaust gas is directed into and through the heat exchanger  100  where it heats the inlet air. The cooled exhaust gas then exits from the heat exchanger  100 . A detailed description of the functioning and structure of the heat exchanger  100  is set forth herein. While FIG. 2 shows an example of a system that at least some embodiments of the present invention can be used, many other systems and uses are possible, including the use of engines other than a gas turbine.  
         [0033]    [0033]FIG. 3 shows an embodiment of the heat exchanger  100  with an air inlet  114  and an air outlet  118  to bring air into and out of a heat transfer core (not shown), and an exhaust gas inlet and an exhaust gas outlet to direct the exhaust gases through the heat exchanger  100 . The heat exchanger  100  also has a shell assembly  160  with an upper strongback  143  and a lower strongback  145  (not shown) on either end. Connecting the strongbacks is a set of tie rods  150 . FIG. 3 also sets forth the cross-sections of the heat exchanger  100  as shown in FIGS. 4 and 5.  
         [0034]    For some embodiments of the present invention, as shown in the cut-away views of FIGS. 4 and 5, the heat exchanger  100 , has a core  110  positioned within the shell assembly  160 . Outside the shell  160  are the upper strongback  143  and the lower strongback  145  connected by the tie rods  150 .  
         [0035]    The core  110  is positioned within the shell  160 . The core  110  functions to duct the inlet air pass the exhaust gas, so that the heat of the exhaust gas can be transferred to the cooler inlet air. The core  110  performs this function while keeping, the inlet air separated from the exhaust gas, such that there is no mixing of the air and the gas. By moving air near the gas without mixing the two, the heat exchanger  100  transfers heat at a high level of efficiency. Further, the heat exchanger  100  also maximizes engine performance by not allowing the exhaust gases to be introduced into the intake air of the turbine (or other engine).  
         [0036]    As shown in FIGS. 4 and 5, the core  110  has an exterior surface  112 . An air inlet  114  and an air outlet  118  to bring air into and out of the core  110 . The air inlet  114  receives relatively cool inlet air for passage through the core  110 . When the heat exchanger  100  is operating, the air exiting the air outlet  118 , having been heated in the core  110 , will have a much higher temperature than the inlet air. Between the air inlet  114  and the air outlet  118  are the inlet manifold  116 , a heat exchange region  122  and the outlet manifold  120 .  
         [0037]    While the heat exchanger  100  is operating the core  110  has a variable size (e.g. length) caused by thermal expansion or contraction. That is, as the core  110  is heated up by the exhaust gases passing through the shell, the core  110  will expand and as the heat exchanger  100  stops operating the core  110  will contract as it cools.  
         [0038]    The heat exchange region  122  can be any of a variety of configurations that allow heat to transfer from the exhaust gas to the inlet air, while keeping the gases separate. However, it is preferred that the heat exchange region  122  be a prime surface heat exchanger having a series of layered plates  128 , which form a stack  130 . The plates  128  are arranged to define heat exchange members or layers  132  and  136  which alternate from ducting air, in the air layers  132 , to ducting exhaust gases, in the exhaust layers  136 . These layers typically alternate in the core  110  (e.g. air layer  132 , gas layer  136 , air layer  132 , as layer  136 , etc.). Separating each layer  132  and  136  is a plate  128 .  
         [0039]    On either end of the stack  130  are a first end plate  142  and a second end plate  144 . The first end plate  142  is positioned against the upper portion of the shell assembly  160  and the second end plate  144  is positioned against the lower portion of the shell assembly  160 .  
         [0040]    Also shown in FIG. 4, are biased deformable members or tie rods  150   a . A series of tie rods  150   a  and an upper strongback or load bearing member  143  and a lower strongback or load-bearing member  145 , are used to hold the stack  130  together and carry loads. The tie rods  150   a  function to apply a compressive load to the strongbacks  143  and  145 . The tie rods  150   a  include a bar section  151   a  running between either end  152   a  and fasteners  153   a  at each end  152   a . The fasteners  153   a  function to hold the tie rods  150   a  to the strongbacks  143  and  145 .  
         [0041]    On the outside of the shell  160  and above and below the core  110 , are the upper strongback  143  and the lower strongback  145 . The tie rods  150   a  and the strongbacks  143  and  145  (as well as the shell  160 ) carry compressive loads applied to the stack  130 . These compressive loads can be from a variety of sources including pre-loading, differential thermal expansion, air pressure, and the like. The upper strongback  143 , the lower strongback  145 , the tie rods  150   a , as well as the shell  160 , collectively form a support structure  170   a  which functions to apply the compressive force to the stack  130  of the core  110 . In contrast to the tie rods  150   a , the upper strongback  143  and the lower strongback  145  (collectively a fixed member, with the upper strongback  143  a first portion of the fixed member and the lower strongback  145  a second portion of the fixed member) are generally not deformable.  
         [0042]    As can be seen, the plates  128  are generally aligned with the flow of the exhaust gas through the shell assembly  160 . The plates  128  can be made of any well-known suitable material, such as steel, stainless steel or aluminum, with the specific material dependent on the operating temperatures and conditions of the particular use. The plates  128  are stacked and connected (e.g. welded or brazed) together in an arrangement such that the air layers  132  are closed at their ends  134 . With the air layers  132  closed at ends  134 , the core  110  retains the air as it passes through the core  110 . The air layers  132  are, however, open at air layer intakes  124  and air layer outputs  126 . As shown in FIGS. 4 and 5, the air layer intakes  124  are in communication with the inlet manifold  116 , so that air can flow from the air inlet  114  through the inlet manifold  116  and into each air layer  132 . Likewise, the air layer outputs  126  are in communication with the outlet manifold  120 , to allow heated air to flow from the air layers  132  through the outlet manifold  120  and out the outlet  118 .  
         [0043]    In contrast to the air layers  132 , the gas layers  136  of the stack  130  are open on each end  138  to allow exhaust gases to flow through the core  110 . Further, the gas layers  136  have closed or sealed regions  140  located where the layers  136  meet both the inlet manifold  116  and the outlet manifold  120 . These closed regions  140  prevent air, from either the inlet manifold  116  or the outlet manifold  120 , from leaking out of the core  110  into the gas layers  136 . Also, the closed regions keep the exhaust gases from mixing, with the air.  
         [0044]    Therefore, as shown in FIGS. 4 and 5, the intake air is preferably brought into the core  110  via the inlet manifold  116  and distributed along the stack  130 , passed through the series of air layer intakes  124  into the air layers  132 , then sent through the air layers  132  (such that the air flows adjacent—separated by plates  128 —to the flow of the exhaust gas in the gas layers  136 ), exited out of the air layer  132  at the air layer outputs  126  into the outlet manifold  120 , and finally out of the core  110 . In so doing, as the air passes through the core  110  it receives heat from the exhaust gas.  
         [0045]    With the stack  130  arranged as shown in FIGS. 4 and 5, the hot exhaust gas passes through the core  110  at each of the gas layers  136 . The exhaust gas heats the plates  128  positioned at the top and bottom of each gas layer  136 . The heated plates  128  then, on their opposite sides, heat the air passing through the air layers  132 .  
         [0046]    As the plates  128  and the connected structure of the core  110  heat up, they expand. This results in an expansion of the entire stack  130  and thus of the core  110 . As noted, this expansion is typically faster than the thermal expansion of the supporting structure  170   a  (the shell  160 , strongbacks  143  and  145  and the tie rods  150   a ). The resulting differential expansion causes the core  110  to apply a force against the restraining support structure  170   a . As noted in detail below, the support structure  170   a  is biased and functions to mechanically expand with the thermal expansion of the core  110 . In this manner, support structure  170   a  allows the core  110  to thermally expand quicker, with minimal build-up of additional forces between the core  110  and the structure  170   a . This prevents the core  110  from being damaged by excess compressive forces which would otherwise be created if the support structure could not expand to accommodate the differential thermal expansion. In addition, in at least some embodiments, the support structure  170   a  continuously applies to the core  110  a compressive force which is at least sufficient to keep the plates  128  of the core  110  from being displaced.  
         [0047]    Although the core  110  can be arranged to allow the air to flow through it in any of a variety of ways, it is preferred that the air is channeled so that it generally flows in a direction opposite, or counter, to that of the flow of the exhaust gas in the gas layers  136  (as shown in the cross-section of FIG. 4). With the air flowing in an opposite direction to the direction of the flow of the exhaust gas, it has been found by the Applicants that the efficiency of the heat exchanger is significantly increased as compared to other flow configurations.  
         [0048]    The arrangement of the core  110  can be any of a variety of alternative configurations. For example, the air layers  132  and gas layers  136  do not have to be in alternating layers, instead they can be in any arrangement which allows for the exchange of heat between the two layers. For example, the air layers  132  can be defined by a series of tubes or ducts running between the inlet manifold  116  and the outlet manifold  120 . While the gas layers  136  are defined by the space outside of, or about, these tubes or ducts. Of course, the heating of such a configuration of the core most likely will still result in differential thermal expansion between the core and the support structure.  
         [0049]    To facilitate heat transfer, the core  110  can also include secondary surfaces such as fins or thin plates connected to the inlet air side of the plates  128  and/or to the exhaust gas side of the plates  128 .  
         [0050]    The core  110  and shell  160  can carry various gases, other than, or in addition to, those mentioned above. Also, the core  100  and shell  160  can carry any of a variety of fluids.  
         [0051]    As shown in FIGS. 4 and 5, the shell assembly includes side walls  162 , openings  164 , upper panel  166  and lower panel  168 . The shell assembly  160  functions to receive the hot exhaust gases, channel them through the core  110 , and eventually direct them out of the shell  160 . The shell  160  is relatively air tight to prevent the exhaust gases from leaking out of the shell  160 . The shell  160  is large enough to fully contain the core  110  and at least strong enough to withstand the pressure exerted on the shell  160  by the exhaust gas. Typically, the shell  160  is flexible and can be deformed to varying amounts depending on its specific construction.  
         [0052]    The openings  164  of shell  160  are positioned through the upper panel  166 . The shell assembly  160  can be made of any suitable well known material including, but not limited to, steel and aluminum. Preferably, the shell  160  is a stainless steel, when it is used in high temperature applications.  
         [0053]    The construction of the shell assembly  160  can vary depending on the particular embodiment of the present invention. In some embodiments the shell  160  is constructed to carry some of the compressive load generated by the support structure  170   a  and applied to the core  110 . The shell  160  can also be configured to carry other internally created loads (e.g. air pressure loads) and externally exerted loads (e.g. inertia loads or vibration loads). Because in some embodiments of the present invention, the walls  162 , upper panel  166  and lower panel  168  of the shell  160  are thick relative to the thin core plates  128 , the shell  160  will thermally expand at a slower rate than the core  110 . This can result in differential thermal expansion or contraction between the shell  160  and the core  110 , as the two are either heated or cooled, as the case may be. To avoid, or to minimize, gaps or spaces forming between the core  110  and the shell  160  during differential expansion, the shell  160  is flexible enough to be deformed by the forces applied by the strongbacks  143  and  145  and the tie rods  150   a.    
         [0054]    In other embodiments, the structure of the shell  160  is relatively thin. In such embodiments, the compressive loads created by the support structure  170   a  are primarily carried by the strongbacks  143  and  145  and the tie rods  150   a . In such embodiments, because the shell  160  is thinner than in other embodiments, the shell  160 , thermally expands and contracts much quicker. This allows any differential thermal expansion between the shell  160  and the core  110  to be minimized. Which, in turn, aids in preventing gaps from forming between the core  110  and the shell  160 . This thinner structure also increases the shell&#39;s flexibility and allows the shell  160  to be more easily deformed by the strongbacks  143  and  145  and the tie rods  150   a . As such, in these embodiments, the potential for exhaust gases being able to pass around the core  110 , through gaps between the core  110  and the shell  160 , is further reduced.  
         [0055]    The present invention, however, provides for differential thermal expansion between the structures of the heat exchanger  100  by employing a mechanically expandable support structure. As shown herein, a variety of embodiments of the support structure  170   a  exist.  
         [0056]    Coiled Tie Rod:  
         [0057]    One embodiment of the support structure  170   a  is shown in FIG. 4. As can be seen, the tie rods  150   a  of this embodiment include a coiled bar section  151   a  running between the ends  152   a . Fasteners  153   a  are attached to the bar section  151   a  at each end  152   a , and function to hold the tie rod  150   a  against the strongbacks  143  and  145 . The fasteners  153   a  are set at or near the ends  152   a  outboard of the strongbacks  143  and  145 . In this manner, the tie rods  150   a  are held in tension between the strongbacks  143  and  145 .  
         [0058]    In this embodiment, the tie rods  150   a  have the bar section  151   a  shaped to include a spring portion  154   a . A part of the bar section  151   a  of the tie rod  150   a  is shaped into a coil or spiral to form the spring portion  154   a . With the tie rods  150   a  stretched in tension, the strongbacks  143  and  145  exert a compressive force to the elements of the heat exchanger  100  set in between them, including the core  110 .  
         [0059]    In this embodiment, the length L tc  of the spring portion  154   a  is varied by the amount of the load placed on the tie rod  150   a . For example, an increase in the load in tension on the tie rod  150   a  will expand the spring portion  154   a , increasing the overall length L tc  of the tie rod  150   a . When deformed, the spring portion  154   a  applies a further biasing force in tension on the tie rod  150   a . The amount the spring portion  154   a  is deformed is related to the force it exerts on other portions of the heat exchanger  100 . In some embodiments a substantially linear relationship exists between the deformation of spring portion  154   a  and the force it exerts.  
         [0060]    The specific configuration of the spring portion  154   a  can vary depending on the requirements of the use. Namely, the spring portion  154   a  is shaped and/or has material properties which allow the spring portion  154   a  to supply a biasing force on the core  110 . The biasing force from the spring portion  154   a  is high enough to keep the core plates  128  together and in place, but low enough to allow the support structure  170   a  to mechanically expand in response to the differential thermal expansion of the core  110 , without damage to the core  110 . The specific configuration (e.g. size, coil shape, material, etc.) of the spring portion  154   a  for the particular application can be determined by one skilled in the design of such structures, using well known analytical and/or empirical methods.  
         [0061]    As such, the tie rods  150   a , as part of the support structure  170   a , function both to permit the support structure  170   a  to apply a continuous force onto the core  110  and to allow the support structure  170   a  to mechanically expand. In this manner, the heat exchanger  100  (1) keeps a sufficient pre-load on the core  110  to prevent the plates  128  from separating or otherwise displacing from their original positions, (2) keeps the shell  160  and the core  110  in contact to avoid gaps between them, and (3) allows the support structure  170   a  to mechanically expand to accommodate the differential thermal expansion of the core  10 , avoiding damage which could otherwise occur.  
         [0062]    Instead of shaping the bar portion of the tie rod into a coil shape, an another embodiment of the tie rod has a straight bar portion attached to a separate tension spring. In this manner the separate tension spring can be placed anywhere along the tie rod between the strongbacks.  
         [0063]    Shaped Tie Rod:  
         [0064]    As shown in FIG. 6, in some embodiments of a support structure  170   b , biased deformable members or shaped tie rods  150   b  are used. The shaped tie rods  150   b  function in a similar manner as the coiled tie rods  150   a  (not shown in FIG. 6), which are detailed above. That is, the tie rods  150   b  act as tension springs as their shape is deformed. As shown, the tie rods  150   b  are held in place at their ends  152   b  by fasteners  153   b . Preferably, the tie rods  150   b  are held in tension, such that a biasing force is exerted. With the tie rods  150   b  acting as tension springs, the strong backs  143  and  145  are biased against the shell  160  and the core  110 . In contrast to the tie rods  150   b , the upper strongback  143  and the lower strongback  145  (collectively a fixed member, with the tipper strongback  143  a first portion of the fixed member and the lower strongback  145  a second portion of the fixed member) are generally not deformable. As such, the core  110  can be kept under a constant compressive force (pre-load) which retains the plates  128  in place. Since the bar section  151   b  of the tie rods  150   b  can be deformed along the length L ts  of the shaped portion  154   b , the support structure  170   b  can mechanically expand in response to the differential thermal expansion of the core  110 .  
         [0065]    [0065]FIG. 6 shows an embodiment of the tie rods  150   b  with the shaped portion  154   b  in an ‘S-shape’ or ‘sine-wave’ pattern. In this configuration the tie rods  150   b  can be deformed along the length L ts  to allow the support structure  170   b  to mechanically expand. That is, as the core  110  differentially thermally expands against the support structure the tie rods  150   b  are pulled into a straighter shape. As the tie rods  150   b  are straightened out, they exert a further biasing force on the strongbacks  143  and  145 . Likewise, as the core  110  thermally contracts quicker than the support structure  170   b , the tie rods  150   b  will return to their original ‘S-shapes’, and in so doing they will mechanically contract the support structure  170   b  with the core  110 .  
         [0066]    In other embodiments, the tie rods  150   b  alternatively have any of a variety of other shapes which allow the tie rods  150   b  to be deformed along their lengths, such that they allow the support structure  170   b  to mechanical expand.  
         [0067]    Tie Bar with Compression Spring:  
         [0068]    In another embodiment of the present invention, a support structure  170   c , as shown in FIG. 7 a , employs biased deformable members or tie rods  150   c  which have springs positioned at their ends. Specifically, the tie rods  150   c  include a bar section  151   c  running between the ends  152   c , fasteners  153   c  attached to the bar section  151   c  at each end  152   c , and compression springs  154   c  positioned between the fasteners  153   c  and the strongbacks  143  and  145 . The compression springs  154   c  are compressed between the fasteners  153   c  and the strongbacks  143  and  145 . This results in a biasing force being applied by the compression springs  154   c  to the fasteners  153   c  and the strongbacks  143  and  145 . This biasing force causes the strongbacks  143  and  145  to, in turn, apply a compressive force to the core  110 . This compressive force allows the core  110  to be pre-loaded, preventing the plates  128  from separating or otherwise being displaced. In contrast to the tie rods  150   b , the upper strongback  143  and the lower strongback  145  (collectively a fixed member, with the upper strongback  143  a first portion of the fixed member and the lower strongback  145  a second portion of the fixed member) are generally not deformable.  
         [0069]    The compression springs  154   c  can further compress or alternatively expand to accommodate differential thermal expansion or contraction of the core  110 . That is, as the temperature of the heat exchanger  100  changes and the core  110  either thermally expands or contracts faster than the support structure  170   c , the compression springs  154   c  will allow the support structure  170   c  to mechanically expand so that the core  110  is not damaged. As such, the length of the springs  154   c  will change in response to the differential expansion or contraction of the core  110 .  
         [0070]    The specific configuration of the compression springs  154   c  and their force and displacement properties can vary depending on the requirements of the specific use in which they are employed. The necessary configuration and properties of the compressions springs  154   c  for the particular use can easily be determined by one skilled in the art of the design of such structures, using well known analytical and/or empirical methods.  
         [0071]    The compression springs  154   c  show in FIG. 7 a  are coil springs, however any of a variety of spring types can be used. For example, as shown in FIG. 7 b  a Belleville washer  154   c ′ is used. The Belleville washer  154   c ′ is curved so that it can deform to accommodate changes in the length of the core  110 .  
         [0072]    Compression Spring Apparatus:  
         [0073]    In some embodiments of the present invention, in place of a support structure utilizing the deformable tie rods  150   a - c  (as described in detail above), one or more biased deformable members or compression springs  180  are used. One embodiment of the present invention employing a compression spring  180  is shown in FIG. 8 a . Like the tie rods  150   a - c  (not shown FIG. 8 a ), the spring  180  allows a support structure  170   d , which includes the strongbacks  143  and  145 , tie rods  150   d  (the strong backs and ties rods collectively a fixed member with the strongback  143  at a first end and the strongback  145  at a second end of the fixed member), shell  160  and spring  180 , to expand and contract with the core  110 . The spring  180  also functions to apply a pre-load to the core  110 . The compression spring  180  is part of the support structure  170   d , and allows the support structure  170   d  to mechanically expand and contract, and to exert a biasing force.  
         [0074]    In the embodiment shown, the spring  180  is positioned between the lower panel  168  of the shell  160  and the core  110 . This allows the spring  180  to continuously apply a biasing force (pre-load) to the core  110 . Also, this prevents the core plates  128  from separating or moving, which might cause the core  110  to buckle. That is, the loading exerted by the spring  180  keeps the plates  128  in their original positions so that the structure of the heat exchanger  100  is not damaged or otherwise compromised.  
         [0075]    As the core  110  thermally expands or contracts independently from the support structure  170   d , the structure  170   d  will mechanically expand due to the compression or expansion of the spring  180 . That is, the spring  180  compresses as the core  110  expands, and it lengthens as the core  110  contracts. The overall length L s  of the spring  180  changes as the core differently expands and contracts. In the embodiment shown, the spring  180  is coil spring and includes a first mounting surface  182  and a second mounting surface  184 . The first surface  182  abuts the core  110  and the second surface  184  is in contact with the shell  160 .  
         [0076]    Depending on the amount of compressive force (pre-loading) that must be applied to the core  110 , the spring  180  can be compressed different amounts prior to being placed between the core  110  and the shell  160 .  
         [0077]    The specific aspects of the spring  180  (e.g. size, shape, spring constant, material used etc.) can vary depending on the requirements of the specific use. One skilled in the art of the design of such apparatuses can determine the specific characteristics of the spring  180  by well known analytical and/or empirical methods. While any of a variety of materials can be used, it is preferred that the spring  180  be constructed of a stainless steel.  
         [0078]    At least one embodiment of the present invention, as shown in FIG. 8 b , uses more than one compression spring. As shown, several springs  180 ′ can be used in place of the single spring  180  (as shown in FIG. 8 a ). Such an embodiment functions generally in the same manner as the single spring  180 . That is, the springs  180 ′ apply a biasing force on to the core  110  to prevent buckling, as shown in FIG. 8 b . Since the springs  180 ′ can expand and contract, the support structure  170   d ′ can also vary its size in response to differential movement of the core  110 .  
         [0079]    In other embodiments of the applicants invention, the spring  180  or springs  180 ′ are positioned in various other locations. For example, the springs can be positioned between the lower strongback  145  and the lower shell panel  168 . Likewise, the springs can be positioned above the core  110 , that is between the core  110  and the upper shell panel  166 . In still other embodiments of the present invention, the spring  180  or springs  180 ′ have shapes other than the coil shaped shown in FIGS. 8 a  and  b . In these embodiments the springs are any of a variety of shapes such as leaf, beam, curved or the like. One such embodiment uses a corrugated spring in place of the coil spring  180 . The corrugated spring can be made of sheet metal bent repeatedly into a corrugated shape.  
         [0080]    In some embodiments of the present invention, tie rods  150   d  are used in conjunction with the bellows  190  and  190 ′, as shown in FIGS. 8 a  and  b . However, in other embodiments, the tie rods can be positioned between the upper strongback  143  and the lower end of the core  110 . These embodiments allow at least some of the loading to not have to be carried by the springs  180  and  180 ′. This also allows lighter Springs to be used.  
         [0081]    Pressurized Bellows Apparatus:  
         [0082]    In other embodiments of the present invention the support structure employs a bellows mechanism to mechanically expand and contract while maintaining a compressive force on the core  110 . Embodiments of such support structures are shown in FIGS. 9 a  and  b.    
         [0083]    As shown in FIGS. 9 a  and  b , a support structure  170   e  includes the upper strongback  143 , the lower strongback  145 , tie rods  150   e  (the strong backs and ties rods collectively a fixed member with the strongback  143  at a first end and the strongback  145  at a second end of the fixed member), the shell  160  and a biased deformable member or sealed bellows  190 . The bellows  190  is a sealed structure which contains a pressurized gas or other fluid and which can expand or contract as necessary. Preferably pressurized air is used. The bellows  190  is mounted between components of the support structure  170   e  and the core  110 . In this position the bellows  190  can apply a force (e.g. pre-load) to the core  110 , to hold the core plates  128  together and/or prevent the plates  128  from being unacceptably displaced from their original positions (e.g. such that leaks in the core are created). When the pressure in the bellows  190  is raised, the force applied to the core  110  is likewise increased. The pressure in the bellows  190  is variable to be able to accommodate the requirements of the particular use in which it is employed.  
         [0084]    In at least some embodiments, the bellows  190  includes a first bellows plate  192 , a second bellows plate  194  and bellows sides  196 , as shown in FIGS. 9 a  and  b . The first bellows plate  192 , second bellows plate  194  and bellows sides  196  define a fluid space  197  for containing a pressurized fluid. The first bellows plate  192  is positioned against the lower portion of the core  110  so that a force generated by the bellows  190  is applied over the core  110 . The first bellows plate  192  can vary in size and can be larger or smaller than the core  110 , or it can be sized to match the core  110  as shown in FIGS. 9 a  and  b.    
         [0085]    The second bellows plate  194  is positioned against the lower shell panel  168 . Since the lower panel  168  abuts the lower strongback  145 , forces applied to the lower panel  168  by the second plate  194  are carried by the support structure  170   e.    
         [0086]    The bellow sides  196  contain the fluid (e.g. air) in the bellows  190 , and in so doing, carry loads generated by the fluid pressure. The sides  196  also function to allow the bellows  190  to expand and contract in a longitudinal direction (e.g. in a direction generally perpendicular to the plates  192  and  194 ). This expansion can be accommodated by any of variety of different bellows side structures. In some embodiments, as shown in FIGS. 9 a  and  b , a folding structure is employed for the sides  196 . This allows the bellows to freely expand and contract so that any differential expansion of the core  110  can be reacted to by the support structure  170   e . That is, the folding sides  196  allow the length L b  of the bellows  190  to vary. In this manner, the core  110  will not be damaged by buckling, creep and/or fatigue failures, which might otherwise result from support structure  170   e  not being able to expand and contract with the core  110 . As noted in detail below, other configurations for the sides  196  can be used as well.  
         [0087]    The fluid (gas, liquid, etc.) used in the bellows  190  is supplied via a port  198  which is connected to a supply source (not shown). The port  198 , supply source and the fluid space  197  are in fluid communication with one another. The supply source typically includes a control mechanism (not shown) for regulating flow and pressure of the fluid. Suitable supply sources and control mechanisms are commercially available. Preferably, a gas is used for the fluid in the bellows. In at least one embodiment, the supply source includes a high pressure bled from the turbine (not shown) which the heat exchanger  100  is attached to.  
         [0088]    Depending on the specific requirements of the use of the bellows  190 , the pressure can be kept at, or near, a constant value or the pressure can be varied. With a constant pressure the bellows  190  will exert a generally constant biasing force against the core  110 . Similarly, with variable pressure, the biasing force can be adjusted as necessary to accommodate the operation of the heat exchanger  100 . If the amount of fluid in the bellows  190  is kept substantially constant, then the pressure within the bellows  190  will change as the core  110  expands and contracts. In such an embodiment of the invention the biasing force exerted on the core  110  will increase as the core  110  expands, and decrease as it contracts.  
         [0089]    With the bellows  190  maintaining constant contact with the core  110 , the bellows  190  prevents, or at least greatly limits, any exhaust gas flow from bypassing the core  110 . By not allowing the exhaust gas to have an alternate route, all, or least substantially all, of the exhaust gas must pass through the core  110 . This maximizes the efficiency of the heat exchanger  100 .  
         [0090]    The specific configuration of the bellows  190  can vary depending on the requirements of the particular heat exchanger it is used with. That is, the particular size, shape, structure and material of the bellows  190  depend on a variety of factors including the amount of expansion and the force that the bellows  190  is required to provide. The specifics of the configuration of the bellows  190  for the particular use which it is employed can be determined by one skilled in the art of the design of such structures, using well known analytical and/or empirical methods.  
         [0091]    The material used to construct the bellows  190  can vary, but it is preferred if the bellows  190  is of a material which will not be damaged or unacceptably degraded when subjected to the typically high temperatures of the exhaust gases passing by the bellows  190 . Although a variety of suitable materials, including steel and aluminum, can be used for the bellows  190 , it is preferred that stainless steel is employed. Further, a high temperature resistant material such as a tightly woven ceramic cloth with a wire mesh can be used in conjunction with the other suitable materials.  
         [0092]    While the width of the bellows can vary, it is preferred that the bellows be wider than the core  110 . As shown in FIG. 9 b , in at least one embodiment of the present invention, a bellows  190 ′ is used which is larger across (wider) than the core  110 . In this manner the first bellows plate  192 ′ of the bellows  190 ′ provides a larger area for the pressure in the bellows  190 ′ to act upon. As such, the total amount of force applied to the core  110  by the bellows  190 ′ is increased as compared to a narrower bellows  190  (as shown in FIG. 9 a ). This embodiment also provides the benefit that the same force can be created with a lower fluid pressure. A lower fluid pressure in turn allows for a thinner and lighter structure for the bellows  190 ′.  
         [0093]    The bellows  190 ′ includes the first bellows plate  192 ′, a second bellows plate  194 ′, bellows sides  196 ′ and a port  198 ′. Preferably, the port  198 ′ is supplied air by a connected air supply port  199 ′ (connection not shown). As shown in FIG. 9 b , the port  199 ′ is tapped into the air inlet  114  of the core  110 . With the port  198 ′ in communication with the air inlet via the port  199 ′, the core  110  and the bellows  190 ′ have the same air pressure. However, because the bellows  190 ′ is wider than the core  110 , the air pressure in the bellows  190 ′ acts over a larger surface area than that of the core  110 . This results in a greater force being exerted by the bellows  190 ′ on to the core  110  than the force which is exerted by the core  110  on the bellows  190 ′. As such, by having the bellows  190 ′ pressurized by being connected to the air inlet  114 , a net compression force is applied by the bellows  190 ′ to the core  110 , preventing the core  110  from buckling or otherwise being displaced.  
         [0094]    The bellows  190 ′ is part of the support structure  170   e ′. The support structure  170   e ′ includes tie rods  150   e ′, strong backs  143  and  145  and the bellows  190 ′.  
         [0095]    Other embodiments of the present invention include using more than one bellows, in parallel (adjacent each other) or series (end-to-end). Also, the bellows  190  and/or  190 ′ are positioning in other locations than those shown in FIGS. 8 a  and  b  and  9   a  and  b . For example, the bellows  190  can be positioned in between the lower strongback  145  and the lower shell panel  168  or above the core  110  on either side of the upper shell panel  166 .  
         [0096]    In some embodiments of the present invention, tie rods  150   e  and  150   e ′ are used in conjunction with the bellows  190  and  190 ′, respectfully, as shown in FIGS. 9 a  and  b . However, in other embodiments, the tie rods can be positioned between the upper strongback  143  and the lower end of the core  110 . These embodiments allow at least some of the loading to not have to be carried by the bellows. This also allows the pressure in the bellows to be lowered without the core  110  excessively expanding.  
         [0097]    Pressurized Piston Apparatus:  
         [0098]    Other embodiments of the present invention allow for differential expansion and contraction, as well as application of a biasing force to the core  110 , by the use of a biased deformable member or pressurized piston assembly  200 . One such embodiment is a piston assembly  200  as shown in FIG. 10 a . As can be seen, the piston assembly  200  is part of a support structure  170   f  and is positioned between the core  110  and the other components of the support structure  170   f . The support structure  170   f  includes strongback  143 , strongback  145 , tie rods  150   f  (the strong backs and ties rods collectively a fixed member with the strongback  143  at a first end and the strongback  145  at a second end of the fixed member) and the shell  160 .  
         [0099]    The piston assembly  200  contains a fluid (a gas or a liquid) which is under pressure. Preferably pressurized air is used. The piston assembly  200  functions in a similar manner to that of the bellows  190 (not shown). The pressure causes the piston assembly  200  to exert a force onto the core  110 . This force is a biasing force which pre-loads the core  110 . Also, the length L p  of the piston  200  can be varied to allow for differential expansion between the core  110  and the support structure  170   f.    
         [0100]    The piston assembly  200  includes a cylinder  202  and a piston  206 . The cylinder  202  and piston  206  define a fluid space  209  for containing a pressurized fluid. The cylinder  202  in turn includes a first piston plate  203 , sides  204  and an fluid port  205 . The piston  206  includes a second piston plate  207  and a seal  208 .  
         [0101]    As shown in FIG. 10 a , the cylinder  202  abuts the core  110  at the first plate  203 , which allows the force generated by the piston assembly  200  to be applied to the core  110 . The cylinder  202  is sized and shaped to receive the piston  206 , preferably it is round to receive a cylindrical shaped piston. The piston  206  is held in the cylinder  202  by the cylinder sides  204 . The fluid port  205  allows the pressurized fluid to enter and leave the fluid space  209 . The fluid port  205  is attached to a fluid source (not shown) which supplies the pressurized fluid. In some embodiments this source is a high pressure bled from the turbine (not shown) attached to the heat exchanger  100 . The fluid port can include a valve (not shown) to control the flow of the fluid.  
         [0102]    The piston  206  can slide along the inside of the sides  204  of the cylinder  202 . In this manner the overall length L p  of the piston assembly  200  can be varied, allowing for the differential expansion and contraction of the core  110  relative to the support structure  170   f . FIG. 10 a  shows the second mounting surface  207  of the piston  206  abutting the lower shell panel  168  of the shell  160 . The piston  206  can also include the seal  208  to prevent fluid from escaping from the fluid space  209 . It is preferred that the piston is cylindrical in shape.  
         [0103]    As with the bellows  190  (not shown in FIG. 10 a ), the specific size and shape of the piston assembly  200  is dependent on the specific needs of the use and the available fluid pressure. The particular size, shape and extension of the piston assembly  200  to meet the needs of the use, can be determined by one skilled in the design of such structures using well known analytical and/or empirical methods.  
         [0104]    The material used to construct the piston assembly  200  can vary, but it is preferred if the piston assembly  200  is of a material which will not be damaged or unacceptably degraded when subjected to the typically high temperatures of the exhaust gases passing through the shell  160  and adjacent the piston assembly  200 . Although a variety of suitable materials, including steel and aluminum, can be used for the piston assembly  200 , it is preferred that a stainless steel is employed. Further, a high temperature resistant material such as a tightly woven ceramic cloth with a wire mesh can be used in conjunction with the other suitable materials.  
         [0105]    In some embodiments of the present invention, a piston assembly  200 ′ which is wider than the core  110  is used. One such embodiment is shown in FIG. 10 b . As with the similar embodiment of the bellows  190 ′ (not shown), the wider piston assembly  200 ′ provides increased forces for given fluid pressures, as compared to the narrower piston assembly  200  (as shown FIG. 10 a ). This is because the fluid pressure is applied over an increased surface area. For the same exerted force, the wider piston  200 ′, operates with lower fluid pressure and as such can be thinner and lighter in its constriction as compared with the piston assembly  200 .  
         [0106]    The piston assembly  200 ′ includes a cylinder  202 ′ and a piston  206 ′. The cylinder  202 ′ and piston  206 ′ define a fluid space  209 ′ for containing a pressurized fluid. The cylinder  202 ′ in turn includes a first piston plate  203 ′, sides  204 ′ and an fluid port  205 ′. The piston  206 ′ includes a second piston plate  207 ′ and a seal  208 ′.  
         [0107]    In some embodiments, the port  205 ′ is supplied air by a connected air supply port  210 ′ (connection not shown). As shown in FIG. 10 b , the port  210 ′ is tapped into the air inlet  114  of the core  110 . With the port  205 ′ in communication with the air inlet via the port  210 ′, the core  110  and the piston  200 ′ have the same air pressure. However, because the piston  200 ′ is wider than the core  110 , the air pressure in the piston  200 ′ acts over a larger surface area than that of the core  110 . This results in a greater force being exerted by the piston  200 ′ on to the core  10  than the force which is exerted by the core  110  on the piston  200 ′. As such, by having the piston  200 ′ pressurized by being connected to the air inlet  114 , a net compression force is applied by the piston  200 ′ to the core  110 , preventing the core  110  from buckling or otherwise being displaced.  
         [0108]    The piston  200 ′ is part of the support structure  170   f ′. The support structure  170   f ′ includes tie rods  150   f ′, strong backs  143  and  145  and the piston  200 ′.  
         [0109]    Many alternative embodiments of the piston assembly  200  exist. For example, in at least one embodiment the piston  206  is positioned against the core  110  and the cylinder  202  abuts the shell  160 . In another embodiment, the fluid port  205  is positioned in the piston  206 . Also, more than one fluid port can be used. In other embodiments of the present invention more than one piston assembly is used. In some embodiments of the present invention, tie rods  150   f  and  150   f ′ are used in conjunction with the pistons  200  and  200 ′, respectfully, as shown in FIGS. 10 a  and  b . However, in other embodiments, the tie rods can be positioned to attached between the upper strongback  143  and the lower end of the core  110 . These embodiments allow the pistons to carry less loads than they would otherwise carry.  
         [0110]    While the preferred embodiments of the present invention have been described in detail above, many changes to these embodiments may be made without departing from the true scope and teachings of the present invention. The present invention, therefore, is limited only as claimed below and the equivalents thereof.