Abstract:
A heat exchanger is provided that includes a shell defining a first fluid space and one or more tubes within the first fluid space having interiors fluidly isolated therefrom. The tubes define a second fluid space and are configured to permit thermal energy transfer between the first fluid space and the second fluid space. One or more heat pipes are disposed within one of the first fluid space and the second fluid space and are configured to transfer thermal energy within the respective fluid space.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The embodiments herein generally relate to heat exchangers and more particularly to shell and tube heat exchangers. 
         [0002]    Numerous heat exchangers have been devised for transferring heat stored in a first medium or fluid to a second medium or fluid. One example of a heat exchanger for high temperature/high pressure applications is a shell and tube heat exchanger. Several features are essential for efficient heat transfer in shell and tube type heat exchangers. 
         [0003]    A large tube surface area is necessary for effective heat transfer, wherein the surface area increases with tube length and tube diameter. However, the advantage gained from a larger tube diameter is offset by a decreased thermal energy exchange which results from the medium inside of the large tubes tending to flow through the middle area of the tube where thermal energy transfer is lowest rather than adjacent the peripheral tube wall where thermal energy exchange is greatest. Further, a long tube length poses a problem with longitudinal expansion. When a high temperature shell fluid is employed, the tube temperature increases resulting in thermal expansion of the tubes, which can lead to damage and/or leaks between the mediums. Thus, there are size constraints that impact the efficiency of tube and shell heat exchangers, resulting in smaller heat exchangers. 
         [0004]    Another factor affecting the thermal energy transfer between mediums is the flow of the fluids in relation to each other. Optimum thermal energy transfer is achieved when the shell fluid and tube fluid are in a contraflow, or counter-flow, configuration allowing for small heat exchangers that are efficient. However, in extreme temperature conditions, a counter-flow configuration may not be sufficient to warm a cold fluid at the point where the cold fluid enters the heat exchanger. If the cold fluid is not warmed sufficiently, icing or other impacts on fluid flow may occur. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0005]    According to one embodiment, a heat exchanger is provided that includes a shell defining a first fluid space and one or more tubes within the first fluid space having interiors fluidly isolated therefrom. The tubes define a second fluid space and are configured to permit thermal energy transfer between the first fluid space and the second fluid space. One or more heat pipes are disposed within one of the first fluid space and the second fluid space and are configured to transfer thermal energy within the respective fluid space. 
         [0006]    According to another embodiment, a method of transferring thermal energy between two mediums is provided. The method includes providing a heat exchanger defining a first fluid space and a second fluid space that is fluidly isolated from the first fluid space, the heat exchanger configured to allow thermal energy transfer between the first fluid space and the second fluid space, and providing one or more heat pipes within one of the first fluid space and the second fluid space, the heat pipes configured to transfer thermal energy within the respective first fluid space or second fluid space. 
         [0007]    Technical effects of embodiments of the invention include providing an improved heat exchanger that enables efficient thermal energy transfer between mediums, or fluids, in a shell and tube heat exchanger that is configured for high pressure applications. Further, thermal energy transfer for a given heat exchanger size can be optimized in accordance with embodiments disclosed herein. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The subject matter which is regarded as the invention 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 invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
           [0009]      FIG. 1  is a cross-sectional illustration of an exemplary shell and tube heat exchanger; 
           [0010]      FIG. 2A  is a schematic view of a heat exchanger showing a parallel-flow configuration; 
           [0011]      FIG. 2B  is a relative temperature plot of the temperatures of the mediums within the parallel-flow heat exchanger of  FIG. 2A  as they flow therethrough; 
           [0012]      FIG. 3A  is a schematic view of a heat exchanger showing a counter-flow configuration; 
           [0013]      FIG. 3B  is a relative temperature plot of the temperatures of the fluids within the counter-flow heat exchanger of  FIG. 3A  as they flow therethrough; 
           [0014]      FIG. 4  is a cross-sectional illustration of a heat exchanger in accordance with an exemplary embodiment of the invention; 
           [0015]      FIG. 5  is a relative temperature plot of the temperatures of the fluids within the heat exchanger of  FIG. 4  as they flow therethrough. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0016]    Referring to  FIG. 1 , a cross-sectional illustration of an exemplary shell and tube heat exchanger  100  is shown. The heat exchanger  100  includes a shell  102  and one or more tubes  104  located within the shell  102 . Shell  102  defines a domed pressure vessel having a cylindrical body  106 , a domed first end  108 , and a domed second end  110 . Of course, the first and second domed ends  108 ,  110  could take on other shapes and/or geometries. 
         [0017]    The cylindrical body  106  defines a first fluid space, labeled as interior shell space  112 , located in the center of the shell  102  and bounded at a first end by a first tube sheet  114  and at a second end by a second tube sheet  116 . The first end tube sheet  114  and the second end tube sheet  116  fluidly isolate the shell space  112  from a first end cavity  128  and a second end cavity  130 . The first end cavity  128  and the second end cavity  130  are fluidly connected by the interior(s) of the one or more tubes  104 . A second fluid space may be defined as the volume within the tubes  104 , and may further include the first and second end cavities  128 ,  130 . It shall be understood that in order for the first and second end cavities  128 ,  130  to fluid connect to the tubes  104 , at least one tube  104  may pass completely through each tube sheet  114 ,  116 . 
         [0018]    A first medium  101 , such as a fluid, flows through the shell space  112  by entering the shell space  112  at a point  103  through first port  118  and exiting the shell space  112  at a point  105  through second port  120 . The first medium in the shell space  112  is in contact with the exterior surfaces of the tubes  104 . This allows for thermal energy transfer between a medium within the shell space  112  (first medium  101 ) and a medium within the tubes  104  (second medium  107 ), without mixing of the two mediums. The flow path of the first fluid within the shell space  112  can be controlled or directed by the inclusion of one or more baffles  122 ,  124 . As shown in  FIG. 1 , the first medium enters the first port  118  and flows downward, around the first baffle  122 , upward and around the second baffle  124 , and then downward and out the second port  120 , as indicated by the arrows within the shell space  112 . The first medium generally flows from left to right in  FIG. 1 , and defines a first fluid path. 
         [0019]    A second medium  107  flows through the heat exchanger  100  along a second fluid path. The second medium  107  enters the heat exchanger  100  at point  109  through a third port  126  and enters the first end cavity  128 . The second medium  107  then flows through the tubes  104  and into the second end cavity  130 . The second medium  107  will then exit the heat exchanger  100  at point  111  by way of a fourth port  132 . Similar to the first medium  101 , the second medium  107  also flows generally from left to right through heat exchanger  100  in  FIG. 1 . 
         [0020]    As noted, the first tube sheet  114 , the second tube sheet  116 , and the tubes  104  fluidly isolate the first medium  101  and the second medium  107  from each other to prevent mixing. This allows for the first medium  101  and the second medium  107  to be of different compositions and, more importantly, of different temperatures. The tubes  104  are formed from thermally conductive material(s) in order to transfer thermal energy from the first medium  101  to the second medium  107 , or vice versa. For example, thermal energy from a relatively warm or hot medium can be transferred to a relatively cool or cold medium when passing through the heat exchanger  100 . 
         [0021]    In order to facilitate heating of a cold medium (or cooling of a hot medium), the cold medium is passed through the heat exchanger  100  in one of the shell space  112  and the tubes  104 , such as shown in  FIG. 1 . At the same time a hot medium is passed through the heat exchanger  100  in the other of the shell space  112  and the tubes  104 . For example, the cold medium may be a fuel for an aircraft and the hot medium may be oil of an aircraft. Due to the low temperatures and other conditions of flight, the fuel may chill to temperatures that are sufficient to cause icing. The icing results from water that is in the fuel freezing and forming ice crystals that may clog lines through which the fuel flows and either reduces the fuel flow or, in extreme cases, may prevent fuel flow entirely. To heat the cold fuel and prevent icing, the cold fuel is passed through the tubes  104  and the hot medium, e.g., hot oil, is passed through the shell space  112 . The hot medium surrounds the tubes  104  and transfers heat through the surfaces of the tubes  104 , thus heating the fuel. 
         [0022]    As shown in  FIG. 1 , the first fluid path and the second fluid path flow generally in the same direction, i.e., generally from left to right. This fluid flow configuration is a parallel-flow configuration (see  FIG. 2A ). As an example, in parallel-flow heat exchangers, the two mediums may enter the heat exchanger  100  generally at the same end ( 118 ,  126 ) and flow in the same general direction, relatively parallel to one another (arrows of  FIG. 1 ), to the other end ( 120 ,  132 ) of the heat exchanger  100 . An advantage of a parallel-flow configuration is that the hottest point of the hot medium is adjacent to the coldest point of the cold medium. Accordingly, the two mediums start at the highest temperature difference and approach the same temperature when they exit the heat exchanger. Advantageously, in the case of aircraft fuel, a parallel-flow configuration can prevent icing at the point that the fuel is at it coldest by locating the hottest temperature oil in proximity to the coldest fuel. 
         [0023]    In an alternative configuration, one of the mediums flows from right to left in  FIG. 1 , i.e., the fluids flow opposite to each other. This is an example of a counter-flow, or contraflow, configuration (see  FIG. 3A ). In counter-flow heat exchangers the mediums enter the heat exchanger from opposite ends, for example, and flow in opposite directions. This results in the temperature at the outlet/exit of each medium approaching the temperature at the inlet/entry of the other medium. An advantage of counter-flow heat exchangers is that they can optimize the thermal energy transfer efficiency between the mediums for given heat exchanger sizes. Thus, a counter-flow configuration is preferred when size is a constraint or factor. 
         [0024]      FIGS. 2A, 2B, 3A, and 3B  illustrate the differences between parallel-flow and counter-flow configurations. 
         [0025]    Turning to  FIG. 2A , a parallel-flow heat exchanger  200  is shown. Although schematically shown, elements of heat exchanger  200  are substantially similar to heat exchanger  100  of  FIG. 1 ; thus like features are preceded with a “2” rather than a “1.” In the parallel-flow heat exchanger  200 , a first medium  201  is a relatively hot fluid that enters on the left side of  FIG. 2A  at point  203 , cools off as it transfers thermal energy to the second medium  207  while passing through the shell space  212 , and exits the heat exchanger  200  on the right side at point  205 . The medium fluid  207  is a relatively cold fluid that enters on the left side of  FIG. 2A  at point  209 , warms up as thermal energy is transferred to it from the relatively hot first medium  201  while passing through tubes  204 , and exits the heat exchanger  200  on the right side at point  211 . This configuration enables the hottest point of the hot fluid to be in thermal contact with the coldest point of the cold fluid. As the mediums  201 ,  207  pass through the heat exchanger  200 , they will approach the same temperature, as shown in  FIG. 2B . 
         [0026]    A relative temperature gradient representative of the first and second mediums  201 ,  207  passing through the parallel-flow heat exchanger  200  is shown in  FIG. 2B . The solid line represents a relative temperature of the first medium  201  as it passes through the heat exchanger  200 , from point  203  (inlet/entry) to point  205  (outlet/exit). The dashed line represents the temperature of the second medium  207  as it passes from point  209  (inlet/entry) to point  211  (outlet/exit). The arrows indicate relative direction of flow of the two mediums  201 ,  207  through heat exchanger  200 . As shown, the first medium  201  starts at a relatively high temperature at point  203  and then decreases in temperature to point  205  as thermal energy is transferred away from the first medium  201 . In contrast, as thermal energy is transferred to the second medium  207 , the temperature of the second medium  207  increases from point  209  to point  211 . The parallel fluid flow enables a high transfer rate of energy from the hot medium to the cold medium quickly, and thus prevents icing, e.g., the hot medium is provided at the coldest location in the heat exchanger to prevent icing in the cold medium. Specifically, when both mediums enter the heat exchanger, the hottest temperature of the first medium  201  at point  203  is adjacent to the coldest temperature of the second medium  207  at point  209 . This presents the highest temperature gradient between the two mediums, and thus the best solution to counter icing. 
         [0027]    Turning now to  FIG. 3A , a counter-flow heat exchanger  300  is shown. Although schematically shown, elements of heat exchanger  300  are substantially similar to heat exchanger  100  of  FIG. 1 ; thus like features are preceded with a “3” rather than a “1.” In the counter-flow heat exchanger  300 , a first medium  301  is a relatively hot fluid that enters on the left side of  FIG. 3A  at point  303 , cools off as it transfers thermal energy to the second medium  307  while passing through the shell space  312 , and exits the heat exchanger  300  on the right side at point  305 . The second medium  307  is a relatively cold fluid that enters on the right side of  FIG. 3A  at point  309 , warms up as thermal energy is transferred to it from the relatively hot first medium  301  while passing through tubes  304 , and exits the heat exchanger  300  on the left side at point  311 . This configuration enables the mediums to maintain a relatively constant temperature gradient as they pass through the heat exchanger  300 , as shown in  FIG. 3B . 
         [0028]    A relative temperature gradient representative of the first and second mediums passing through the counter-flow heat exchanger  300  is shown in  FIG. 3B . The solid line represents a relative temperature of the first medium  301  as it passes through the heat exchanger  300 , from point  303  (inlet/entry) to point  305  (outlet/exit). The dashed line represents the temperature of the second medium  307  as it passes from point  309  (inlet/entry) to point  311  (outlet/exit). The arrows indicate relative direction of flow of the two mediums  301 ,  307  through heat exchanger  300 . As shown, the first medium  301  starts at a relatively high temperature at point  303  and then decreases in temperature to point  305  as thermal energy is transferred away from the first medium  301 . In contrast, the second medium  307  flows in the opposite direction, as indicated by the arrows, and is at the coldest temperature at point  309  and the warmest temperature at point  311 . The counter fluid flow enables a consistent thermal energy transfer that is efficient and enables the heat exchanger  300  to be optimized for sizing. 
         [0029]    Regardless of the type of heat exchanger, the principle of operation is to have two mediums of different temperatures brought into close contact but prevent the mediums from mixing. This allows for cold mediums to be warmed and warm mediums to be cooled without energy being added or removed from the system; it is merely an exchange of thermal energy between the mediums. Further, there is also a change in pressure in the mediums, as the temperature changes, which transfers energy, e.g., a pressure drop occurs as each fluid moves from the entrance of the heat exchanger to the exit of the heat exchanger, transferring energy. In the example of heat exchangers employed in aircraft, size and weight constraints apply, in additional to the requirement of providing a vessel for high pressure mediums. Due to the size and weight constraints, a counter-flow shell and tube heat exchanger provides the best advantage, but due to icing problems during flight, parallel flow may be preferred. 
         [0030]    Turning now to  FIG. 4 , a heat exchanger  400  in accordance with an exemplary embodiment of the invention is shown. Heat exchanger  400  includes similar features as heat exchanger  100  of  FIG. 1 ; thus like features are preceded with a “4” rather than a “1.” Similar to heat exchanger  100  of  FIG. 1 , heat exchanger  400  includes a shell and tube assembly, with similar components as described above and is arranged as a parallel-flow configuration. The primary difference between heat exchanger  400  and the embodiments described above is the inclusion of heat pipes  450 ,  452 , which may be dimpled heat pipes. Heat pipes as used herein refer to thermal-transfer devices that combine the principles of both thermal conductivity and phase transition to efficiently manage the transfer of thermal energy between two solid interfaces. At a hot interface of a heat pipe a liquid in contact with a thermally conductive solid surface turns into a vapor by absorbing heat from that surface. The vapor then travels along the heat pipe to the cold interface and condenses back into a liquid—releasing the latent thermal energy. The liquid then returns to the hot interface through capillary action, centrifugal force, gravity, or other process, and the cycle repeats. 
         [0031]    The addition of heat pipes  450 ,  452  allows for a parallel-flow heat exchanger to include the benefits of a counter-flow heat exchanger, i.e., optimization of thermal energy transfer efficiency, and thus the size of the heat exchanger can be optimized with the benefits/advantages of both parallel-flow and counter-flow heat exchanger configurations. The materials and mediums of the heat pipes are configured such that the mediums of the heat exchanger will cause a phase transition of the heat pipe medium, thus enabling efficient intra-medium thermal transfer. 
         [0032]    As shown in  FIG. 4 , heat pipes  450  are included within the tubes  404  of the heat exchanger  400 . The heat pipes  450  allow for thermal energy transfer within the fluid that passes through the tubes  404 . Similarly, heat pipes  452  are included within the shell space  412  and allow for thermal energy transfer within the fluid that passes through the shell space  412 . Accordingly, in heat exchanger  400 , there are two types of thermal energy transfer. First, thermal energy transfer occurs between the first and second mediums through the tubes  404  without mixing of the first and second mediums, similar to that described above (inter-medium thermal transfer). Second, thermal energy transfer occurs within the first medium and within the second medium because of the heat pipes  450 ,  452  (intra-medium thermal transfer). 
         [0033]    In operation, in the parallel-flow heat exchanger  400  of  FIG. 4 , the temperature extremes of the two mediums occur at the entry point to the heat exchanger  400 , which are adjacent. The first medium enters at the first port  418  at a high temperature (hot fluid), and the second medium enters at the third port  426  at a low temperature (cold fluid). Thus, the hottest temperature of the first medium is adjacent to the coldest temperature of the second medium, which prevents icing, as discussed above with respect to a parallel-flow configuration. With the addition of the heat pipes  452 , located in shell space  412 , the high temperature of the first medium within the shell space  412  is transferred toward the portions of the shell space  412  where the first medium is cooler. Similarly, in the tubes  404 , the heat pipes  450  allow for the warm thermal conditions of the second medium located toward the second cavity  430  to be carried back toward the first cavity  428 , thus providing additional heat to the cold second medium. 
         [0034]    As shown in  FIG. 5 , a relative temperature plot representative of the temperatures of the first and second mediums  401 ,  407  as they flow through heat exchanger  400  is shown. The entry points of first port  418  and third port  426  are shown on the left side of the plot and indicate the largest temperature difference between the two mediums. However, because the heat pipes  450  and  452  are included, the temperature difference between the first medium  401  and the second medium  407  equalizes very quickly, and provides a relatively constant temperature gradient between the first and second mediums  401 ,  407  throughout heat exchanger  400 . This enables an optimized thermal energy transfer similar to a counter-flow configuration, but also includes the inlet temperature advantages of a parallel-flow configuration. 
         [0035]    Advantageously, embodiments of the invention provide maximum thermal energy transfer and maximum absolute pressure capability for a given volume. Furthermore, advantageously, icing within a fuel line, such as on an aircraft, can be efficiently prevented. Moreover, heat pipes added to a shell and tube heat exchanger provide a uniform temperature gradient and thermal energy transfer throughout the heat exchanger while maintaining the benefit of icing prevention and optimizing the heat exchanger size. 
         [0036]    While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions, combination, sub-combination, or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. 
         [0037]    For example, although described herein as a particular shell and tube heat exchanger in each of the embodiments, other types of shell and tube heat exchangers may employ heat pipes without departing from the scope of the invention. One such alternative configuration is a U-shaped shell and tube heat exchanger, with heat pipes located within the U-shaped tubes and within the shell space of the heat exchanger. Furthermore, variations of shell and tube heat exchangers may include any number of tubes, shapes, sizes, and/or configurations without departing from the scope of the invention. Moreover, although described above in  FIG. 4  with heat pipes located within both the tube space and the shell space, alternative embodiments may include heat pipes in only one of the tube space and the shell space. Further, although shown as having a heat pipe in each tube, this is merely an example, and any number of heat pipes may be used in each of the tube space and the shell space of the heat exchanger. The mediums discussed above are also not limiting, and other mediums beside fuels and oils may be employed, either as the hot medium or as the cold medium, and the type or composition of the medium is not intended to be limiting. Moreover, different types of heat exchangers that are not tube and shell may employ similar heat pipes or heat transfer devices without departing from the scope of the invention. 
         [0038]    Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.