Abstract:
A plate fin heat exchanger is disclosed. The heat exchanger includes a plurality of plates defining a set of hot fluid passages between adjacent plates of the plurality of plates and a set of cold fluid passages between adjacent plates of the plurality of plates. A hot fluid inlet and outlet are located at a first face of the heat exchanger. A barrier is located between adjacent plates defining the hot fluid passages. The barrier extends between the adjacent plates and extends from the first face of the heat exchanger at a location between the hot fluid inlet and the hot fluid outlet in a direction perpendicular to the first face, and defines a first pass of hot fluid passages on a first side of the barrier and a second pass of hot fluid passages on a second side of the barrier.

Description:
BACKGROUND 
       [0001]    This disclosure relates to heat exchangers, and in particular to a multi-pass plate fin heat exchanger. 
         [0002]    Heat exchangers are often used to transfer heat between two fluids. For example, in aircraft environmental control systems, heat exchangers may be used to transfer heat between a relatively hot air source (e.g., bleed air from a gas turbine engine) and a relatively cool air source (e.g., outside air). Some heat exchangers, often referred to as plate fin heat exchangers, include a plate fin core having multiple heat transfer sheets arranged in layers to define air passages there between. Closure bars seal alternating inlets of hot air and cool air inlet sides of the core. Accordingly, hot air and cool air are directed through alternating passages to form alternating layers of hot and cool air within the core. Heat is transferred between the hot and cool air via the heat transfer sheets that separate the layers. In addition, to facilitate heat transfer between the layers, each of the passages can include heat transfer fins, often formed of corrugated material (e.g., steel), that are oriented in a direction of the flow within the passage. The heat transfer fins increase turbulence and surface area that is exposed to the airflow, thereby enhancing heat transfer between the layers. 
         [0003]    Bleed air from a gas turbine engine such as on an aircraft is at a high temperature, often in excess of 1000° F. This bleed air is too hot for an ECS air cycle machine to effectively utilize for producing conditioned cabin air, and is also too hot for easy transport from the aircraft engine to ECS air cycle machines typically located in the aircraft wing stem. Accordingly, the bleed air is usually passed through a heat exchanger in a location proximate to the engine. This heat exchanger, often referred to as a precooler, is typically tasked with cooling the bleed air from a temperature in excess of 1000° F. to a temperature below 400° F. Such temperature differences between inlet and outlet temperatures on the heat rejection side of the heat exchanger can subject the heat exchanger to undesirable levels of thermal stress. Additionally, if the heat rejection side of the heat exchanger has multiple passes, the overall large temperature difference will necessitate large temperature differences between adjacent passes in the heat exchanger, which can contribute to reduced heat transfer efficiency from heat being conductively transferred through the pass barrier from a hotter pass closer to the hot side inlet to an adjacent cooler pass closer to the hot side outlet. 
       BRIEF DESCRIPTION 
       [0004]    In some aspects of this disclosure, a plate fin heat exchanger is disclosed, comprising a plurality of plates defining a set of hot fluid passages between adjacent plates of the plurality of plates and a set of cold fluid passages between adjacent plates of the plurality of plates. A hot fluid inlet and a hot fluid outlet are located at a first face of the heat exchanger. A barrier is located between adjacent plates defining the hot fluid passages. This barrier extends between the adjacent plates and extends from the first face of the heat exchanger at a location between the hot fluid inlet and the hot fluid outlet in a direction perpendicular to the first face. The barrier defines a first pass of hot fluid passages on a first side of the barrier and a second pass of hot fluid passages on a second side of the barrier. The barrier also comprises a void space isolated from the first and second passes of hot fluid passages. 
         [0005]    In some aspects of this disclosure, an environmental conditioning system comprises a gas turbine engine, a precooler that receives and cools a bleed flow of compressed air from the gas turbine engine, and an air cycle machine that conditions air received from the precooler. The precooler comprises a plurality of plates defining a set of hot air passages between adjacent plates of the plurality of plates and a set of cold air passages between adjacent plates of the plurality of plates. A hot air inlet is in fluid communication with the bleed flow from engine, and a hot air outlet is in fluid communication with the air cycle machine, with the hot air inlet and hot air outlet located at a first face of the heat exchanger. A barrier is located in the space between adjacent plates defining the hot air passages. This barrier extends between the adjacent plates and extends from the first face of the heat exchanger at a location between the hot air inlet and the hot air outlet in a direction perpendicular to the first face. The barrier defines a first pass of hot air passages on a first side of the barrier and a second pass of hot air passages on a second side of the barrier. The barrier also comprises a void space isolated from the first and second passes of hot air passages. 
         [0006]    In some aspects of this disclosure, a method of cooling a fluid, comprises passing the fluid through a heat rejection side of a plate fin heat exchanger comprising a plurality of plates defining a set of hot fluid passages between adjacent plates of the plurality of plates and a set of cold fluid passages between adjacent plates of the plurality of plates. A hot fluid inlet and a hot fluid outlet are located at a first face of the heat exchanger. A barrier is located between adjacent plates defining the hot fluid passages. This barrier extends between the adjacent plates and extends from the first face of the heat exchanger at a location between the hot fluid inlet and the hot fluid outlet in a direction perpendicular to the first face. The barrier defines a first pass of hot fluid passages on a first side of the barrier and a second pass of hot fluid passages on a second side of the barrier. The barrier also comprises a void space isolated from the first and second passes of hot fluid passages. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The subject matter of the disclosure is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the present disclosure are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
           [0008]      FIG. 1  is a schematic depiction of an exemplary heat exchanger in an isometric view; 
           [0009]      FIG. 2  is a schematic depiction of cross-section of a heat rejection side of an exemplary heat exchanger; 
           [0010]      FIG. 3  is a schematic depiction of cross-section of a heat absorption side of an exemplary heat exchanger; and 
           [0011]      FIG. 4  is a schematic depiction of an exemplary environmental control system utilizing a heat exchanger as described herein. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    Referring now to the Figures, an isometric view of exemplary heat exchanger  100  is shown in  FIG. 1 . As shown in  FIG. 1 , heat exchanger  100  includes stacked cold side fin structures  102  and stacked hot side fin structures  104 . Cold fin structures  102  are typically configured in an accordion pattern that act to remove heat from adjacent hot fin structures  104  by thermal conduction. Hot fin structures  104  typically comprise stacked layers of metal foil fin elements in a wavy pattern to maximize contact between the wavy fins and hot bleed air passing through the hot fin structures  104 . Cold air, such as aircraft ram air enters the stacked cold fin structures  102  in the direction of arrows C and cools the cold fin structures  102 . Hot air such as bleed air from an engine bleed system enters stacked hot fin structures  104  flows in the direction of arrows H, entering through hot air inlet header  112  and exiting through hot air outlet header  114 , and is cooled from air flow in stacked cold fin structures  102 . Hot and cold air flow in heat exchanger  100  is configured for cross-flow. 
         [0013]    Stacked cold fin structures  102  contain metal foil heat exchanger elements in an accordion pattern that act to remove heat from adjacent hot fin structures  104  by conduction. Stacked hot fin structures  104  typically contain stacked layers of metal foil fin elements in a wavy pattern to maximize contact between the wavy fins and hot air passing through the hot fin structure. The stacked hot fin structures  104  and cold fin structures  102  are separated by parting sheets  110 . Parting sheets  110  can also be formed from metal alloys and act to support the foil elements in the hot and cold fin structures  104  and  102 . Closure bars  118  form the outside barriers of hot fin structures  104 , and a similar set of closure bars  119  form the outside barriers of cold fin structures  102  when viewed from the face of the heat exchanger having the hot air inlet and outlet headers  112  and  114 , respectively. Mounting brackets  120  are disposed located on a face of the heat exchanger  100  corresponding to the internal location of the centrally-located barrier  130  ( FIG. 2 ), which can promote structural integrity and stress management. End caps  34  isolates barrier  130  ( FIG. 2 ) from hot side fluid flow, as described in more detail below. Additional structural elements (not shown) include core bands which act to support the overall stack of hot and cold fin structures of heat exchanger  100 . 
         [0014]    The metal components of heat exchanger  100  may be any metal known in the art of heat exchanger design. In some embodiments, the metal components are made from a steel alloy. 
         [0015]    Turning now to  FIG. 2 , a cross-sectional view of hot fin structures  104  is shown. As shown in  FIG. 2 , hot fluid enters through hot fluid inlet header  112  and is guided by the hot side fin structures  104  through the heat exchanger hot side first pass  122  and the second pass  124 . The flow of hot fluid is redirected at junctions  126  and  128  using a mitered fin to turn the first pass  122  flow 90° counter to the cold inlet and then again turn the flow another 90° into the second pass  124 . 
         [0016]    As further shown in  FIG. 2 , a barrier  130  separates the first pass  122  and the second pass  124 . The barrier provides a physical separation between the first and second hot fluid passes, and contains a void space isolated from the first and second passes of hot fluid passages. By “void space” is meant a space that is void of structure, e.g., an air space. In some embodiments, the physical separation provided by the barrier  130  between the first pass  122  and the second pass  124  can range from 0.2 to 3.0 inches, and more specifically 0.9 to 2.6 inches. Being isolated, the void space provides a space of non-moving or ‘dead’ fluid or air, thus providing a thermal barrier between the first and second passes. Such a thermal barrier reduces heat transferred by conduction directly from the first pass to the second pass, thereby promoting the ability for the heat exchanger to accommodate larger temperature differentials between the hot fluid inlet header  112  and the hot fluid outlet header  114 , while managing thermal stresses on the heat exchanger structure. The barrier can be configured in a variety of ways. For example, the barrier can be provided by a wall or walls extending vertically between the adjacent parting sheets  110  ( FIG. 1 ) and horizontally from the face of the heat exchanger with hot inlet and outlet headers  112 ,  114 , along the dashed line depicted in  FIG. 2 . However, a separate wall structure is not necessary, as the barrier can be provided by fin structures  104 , e.g., by a fin that is sealed to end cap  134  so that the fin provides a barrier between the hot fluid flow area and the barrier  130  isolated from fluid flow. It should be noted here that the dashed line representing the border of barrier  130  in  FIG. 2  is of course conceptual, and the interruption of the fin structure adjacent to the dashed line is for ease of illustration with regard to the dashed line. Embodiments are also included where the fin pattern is not interrupted, such as illustrated in  FIG. 1 , although fin spacing and thickness can be different inside the barrier  130  compared to the fin spacing and thickness in the fluid flow area outside the barrier  130 , as the structural or thermal characteristics of the barrier  130  can be different than that of the fluid flow area. The fin structures can be sealed where they intersect with the parting sheets  110  ( FIG. 1 ). A void space isolated from the hot fluid flow in first and second passes  122 ,  124  can be provided by sealing the ends of the fin structures  104  with an end cap  134  to isolate the spaces between the fin structures  104  from the hot fluid flow at the headers. The void space can be left unsealed at the end of the barrier  130  opposite the headers  112 ,  114 , as hot fluid will tend not flow into the void space since it has no place to exit. Alternatively, the void space can be sealed at the end of the barrier  130  opposite the headers  112 ,  114  to provide a fully sealed void space in the gaps between the fin structures  104 . 
         [0017]    Any of the fin structures  102  or  104  can be equipped with slots such as those described in US published patent application US 2015/0053380 A1 of Army, Jr. et al., the disclosure of which is incorporated herein by reference in its entirety. As described in this publication, the slots can provide a technical effect of arresting crack propagation. Additionally, with respect to the fin structures  104 , the slots can provide additional compliance for thermal stress management. Fin dimensions and spacing can vary depending on system requirements and specifications. Fin height of course depends on the distance separating adjacent parting sheets, and can range from 0.04 to 0.5 inches. Fin thickness can range from 0.002 to 0.012, and fin spacing can range from 10 fpi (fins per inch) to 35 fpi. In some embodiments, the fin dimensions (e.g., thickness, spacing) for the barrier fin structures  104  than for the fin structures  102  to provide desired compliance and other physical characteristics to the barrier  130 . 
         [0018]    The cold side of the heat exchanger  100  is depicted in  FIG. 3 , where cold air such as aircraft ram air flows across the cold fin structures  102  in the direction of arrows C to cool the cold fin structures  102 . Headers  112  and  114  for the hot side of heat exchanger  100  are also depicted in  FIG. 3 . 
         [0019]    The heat exchanger embodiments described herein can be used in operating conditions where high temperatures, large temperature differences, or both, are encountered. In some aspects, the heat exchanger is operated under conditions where there is a temperature difference of at least 200° F. between the hot side inlet and hot side outlet. In some aspects, fluid temperatures at the hot side inlet temperatures can range from 400° F. to 1400° F. In some aspects, fluid temperatures at the hot side outlet temperatures can range from 100° F. to 400° F. 
         [0020]    In some aspects, the heat exchanger  100  can be used as a precooler or other heat exchanger in an environmental conditioning system (ECS) with a gas turbine engine. Such a system is schematically depicted in  FIG. 4 . As shown in  FIG. 4 , ECS system  200  comprises a gas turbine engine  202 , from which bleed air  203  is directed to a precooler  204  (i.e., heat exchanger  100 ), and then the cooled bleed air  205  is directed to an air cycle machine  206 . The air cycle machine  206  can be any of a number of known variations of such air cycle machines. An exemplary air cycle machine is described in U.S. Pat. No. 7,188,488 to Army, Jr. et al., the disclosure of which is incorporated herein by reference in its entirety. Further details regarding the ECS are known in the art, and do not require additional detailed description herein. 
         [0021]    While the present disclosure has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the present disclosure is not limited to such disclosed embodiments. Rather, the present disclosure can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the present disclosure. Additionally, while various embodiments of the present disclosure have been described, it is to be understood that aspects of the present disclosure may include only some of the described embodiments. Accordingly, the present disclosure is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.