Abstract:
A plate fin heat transfer device utilizes titanium plate members and aluminum dividers. The solid bar may be titanium, aluminum or an alloy of either. The titanium plate members may have a thermal conductivity of approximately 50 or 100 BTU/Hr/ft/F/in and dramatically reduce matrix conduction of heat within the plate members. The plate members may be as thin as approximately 0.002 inches while providing the necessary strength to avoid leakage during or after the manufacturing process. The advantageous thinness satisfies weight and volume parameters critical to an aircraft.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention generally relates to apparatus and methods for improving the effectiveness of plate heat exchangers and, more particularly, to apparatus and methods of reducing matrix conduction effects in plate heat exchangers. 
         [0002]    In the aerospace industry, an aircraft typically has an environmental control system that may include various heat transfer devices. These heat transfer devices may include plate fin heat exchangers. The desire to utilize high thermal conductivity materials for the heat exchanger and minimize the weight of the aircraft has lead to the use of aluminum plate members in the plate fin heat exchangers. Light weight high effectiveness heat exchangers are often made entirely of aluminum for its high thermal conductivity. In this regard the term “aluminum” means commercially pure aluminum or an alloy where aluminum is the largest constituent. 
         [0003]    There is a greater driving force for heat transfer when the temperature difference of two mating fluids is greater. As a result of the high thermal conductivity of aluminum, as seen from  FIG. 1  wherein arrows represent heat flux and wherein the wavy lines represent matrix conduction of heat within the plate members, total heat transfer across the plate members  12  from fluid  11  to fluid  13  is reduced and the functioning of the heat exchanger is reduced when the matrix conduction is high because the sum of the average local temperature difference between fluid  11  and fluid  13  over the entire heat exchanger is reduced by the matrix conduction. Overall effectiveness of the heat exchanger is reduced when heat runs along the plate member  12  away from the local area toward another area in the heat exchanger matrix such as the solid bars resulting in a less advantageous path from fluid  11  to fluid  13 . This can occur when heat runs along plate member  12  from a hot end of plate member  12  to a cold end of plate member  12  instead of crossing plate member  12 . Another parameter that must be satisfied in constructing heat transfer systems in aircraft is minimizing volume since space is limited. 
         [0004]    As can be seen, there is a need to have a heat transfer device in an aircraft that minimizes volume while still minimizing weight and that avoids the deleterious effects of matrix conduction. 
       SUMMARY OF THE INVENTION 
       [0005]    In one aspect of the present invention, there is presented a plate fin heat transfer device, comprising a first titanium plate member and a second titanium plate member, the first and second titanium plate members each having a thickness of between approximately 0.002 inches and approximately 0.125 inches and having a top edge; a solid bar joining the top edges of the first and second titanium plate members; a set of aluminum dividers joined between the first and second titanium plate members, the dividers defining a series of passageways; and a series of alternate passageways alongside an outside surface of the first titanium plate member and alongside an outside surface of the second titanium plate member. 
         [0006]    In a further aspect of the invention, there is presented a method of exchanging heat using a plate fin heat transfer device, comprising directing a heating fluid alongside a surface of a first titanium plate member so as to cause convection of heat energy from the heating fluid to the first titanium plate member, the first titanium plate member joined to a second titanium plate member by aluminum dividers that define a series of passageways between the first and second titanium plate members, a solid bar of aluminum joined to a top edge of the first and second titanium plate members; directing a heating fluid alongside a surface of the second titanium plate member so as to cause convection of heat energy from the heating fluid that is alongside the surface of the second titanium plate member to the second titanium plate member; and directing a cooling fluid through the series of passageways to cause a heat flux from the first and second titanium plate members to the cooling fluid with matrix conduction that is reduced compared to a plate member made of aluminum such that overall heat transfer conductance of the heat transfer device is improved by approximately 5% to 50% compared to a heat transfer device with plate members made of aluminum. 
         [0007]    In another aspect of the invention, there is presented a plate fin heat transfer device, comprising a plurality of titanium plate members having a thermal conductivity of no more than approximately 120 BTU/Hr/ft/F/in, each of the plate members also being thinner than 0.012 inches, the plate members having a top edge; a set of aluminum fins brazed between two of the plurality of titanium plate members, the fins defining a series of passageways for a first fluid to pass through; solid bars joining top edges of any two of the plate members, the solid bars made of aluminum, titanium, an alloy of aluminum or an alloy of titanium; and a second fluid having a different temperature than the first fluid, the second fluid directed alongside an outside surface of a first titanium plate member and directed alongside an outside surface of a second titanium plate member. 
         [0008]    These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, descriptions and claims 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a sectional view of a prior art heat transfer device showing matrix conduction through plate members; 
           [0010]      FIG. 2  is a partially cut perspective view of a plate fin heat transfer device core with its housing partially cut away, in accordance with the present invention; 
           [0011]      FIG. 3  is a sectional view of the heat transfer device of  FIG. 2 ; 
           [0012]      FIG. 4  is a sectional view of plate fin heat transfer device of the present invention showing reduced matrix conduction; and 
           [0013]      FIG. 5  is a flow chart showing a method of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]    The following detailed description is of the best currently contemplated modes of carrying out the invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention, since the scope of the invention is best defined by the appended claims. 
         [0015]    The present invention generally provides a plate fin heat exchanger for use where matrix conduction is a problem because the heat exchanger is required to have very high effectiveness usually in systems where there is low flow velocity. In the plate fin heat exchanger of the present invention, the plate members may be entirely titanium. The remainder of the heat exchanger may be aluminum. 
         [0016]    In contrast to the prior art, which does not use titanium plate members in an otherwise aluminum heat exchanger, the heat exchanger of the present invention may utilize plate members that are titanium. In further contrast to the prior art, in which matrix conduction within the aluminum plate members negatively affects the performance of the heat transfer device, the plate fin heat exchanger of the present invention may minimize matrix conduction. In further contrast to the prior art, which utilizes plate members that maximize thermal conductivity, which is intuitive for heat transfer devices, the plate fin heat exchanger of the present invention may utilize plate members made of a lower thermal conductivity material than aluminum. In further contrast to the prior art, in which the entire heat exchanger may be aluminum, the present invention may utilize plate members of titanium such that lower matrix conduction results in greater overall heat transfer efficiency so that volume may be reduced an important consideration for an aircraft. Furthermore, in contrast to the prior art plate fin heat exchangers, for example those made of all-aluminum, in which the weight cannot be fully minimized because, despite aluminum being lightweight, aluminum plate members need to be thicker to have the necessary load capacity, the plate fin heat exchanger of the present invention saves weight, since, although its titanium plate members are denser, they can be thinner than conventional aluminum plate members due to their superior strength. Moreover, the overall size of the plate fin heat exchanger of the present invention may be reduced by an amount in the order of approximately 5% to approximately 20% by reducing the size of any of its three dimensions (i.e. stack height, etc.) and this may save a significant amount of weight. Specifically, in further contrast to the prior art, in which leakage and holes would likely arise during brazing of plate members if such plate members were manufactured having a thickness of only 0.01 inches, in the plate members of the heat exchanger of the present invention, the plate members are durable enough that leakage may not occur (or may more easily meet the acceptance limit of leakage for the device) even though the plate members may be as thin or thinner than 0.01 inches. 
         [0017]      FIG. 2  shows a partially cut perspective view of a plate fin heat transfer device  10  whose housing has been partially cut away. Device  10  may comprise a series of plate members  20 , as well as dividers  30  and solid bars  40 . Dividers  30  may be called fins and are made in a myriad of configurations known to those skilled in the art. Dividers may be made typically from formed sheet metal but may also be fabricated from other structures. Plate members  20  may be made entirely of titanium. In this regard, the term “titanium” means commercially pure titanium or an alloy of titanium where titanium is the largest constituent. It is understood that plate members  20  are sometimes also referred to in the industry as “tubesheets”, as “tube plates”, as “parting sheets” or as “separator plates”. 
         [0018]    As seen from  FIG. 2 , each of titanium plate members  20  may have a top edge  29  to which a solid bar  40  may attach. Thus, solid bar  40  may join top edges  29   a,    29   b  of the two titanium plate members  22 ,  24  and may join top edges of any two other titanium plate members  20 . Solid bar  40  may also be made of aluminum, titanium or an alloy of aluminum or titanium. Of course, plate fin heat transfer device  10  may contain much more than two plate members. Accordingly, when plate members  20  are referred to as including first and second titanium plate members, these may be arbitrarily chosen to represent any two titanium plate members that have fins adjoining them. 
         [0019]    As seen from  FIG. 2 , plate fin heat transfer device  10  may also have a set of aluminum fins  30  brazed between the first and second titanium plate members  22 ,  24 . Fins  30 , for example vertical fins  30   a,  may define a series of passageways  31  for a first fluid  51 , such as a cooling fluid  60  to pass through. As seen from  FIG. 2 , alternate sets of fins, for example horizontal fins  30   b  on the other side of first plate member  22  or second plate member  24 , may define a series of alternate passageways  33  and may have a second fluid  52  such as a heating fluid  50  passing through these fins  30   b.  It should be appreciated that while the first fluid may be a heating fluid and the second fluid may be a cooling fluid it may also be true that the first fluid may be a cooling fluid and the second fluid may be a heating fluid. 
         [0020]    As can be seen from  FIG. 3 , first plate member  22  and second plate member  24  may each contain a plate member layer  26 , which may be made entirely of titanium, and may each also contain braze alloy layers  28  which may be made of an aluminum braze alloy. First and second titanium plate members  22 ,  24  each may have a thickness from about 0.002 inches to about 0.125 inches and typically approximately 0.006 inches or even less. Titanium plate members  20 , even at a thickness of 0.002 inches, may have the strength needed to withstand breakage and leakage during brazing and afterwards. 
         [0021]    As seen further from  FIG. 3 , heating fluid  50  may be directed alongside an outside surface  21   a  of the first titanium plate member  22  and invention also envisions a method  100  of exchanging heat using a plate fin heat exchanger. One step  110  of method  100  comprises directing a first fluid, such as a heating fluid, alongside a surface of a first titanium plate member so as to cause convection of heat energy from the heating fluid to the first titanium plate member. In step  110  the first titanium plate member may be joined to a second titanium plate member by aluminum dividers that define a series of passageways between the plate members. Furthermore, a sold bar, such as made of aluminum, may be joined to a top edge of the first and second titanium plate members. An additional step  120  of method  100  involves directing a heating fluid, which may or may not be the same heating fluid, alongside a surface of a second titanium plate member so as to cause convection of heat energy from that heating fluid to the second titanium plate member. A further step  130  of method  100  involves directing a second fluid, which may be a cooling fluid, through the series of passageways to cause a heat flux from the first and second titanium plate members to the cooling fluid with matrix conduction that may be less than if the plate member were aluminum such that heat transfer conductance of the heat exchanger may be improved by 5% or more. For example, heat transfer conductance may be improved in certain cases by approximately 10%, 20%, 30%, 40% or 50%. 
         [0022]    It should be appreciated that the fluid running through fins  20  may also be a heating fluid and the cooling fluid may be the fluid that is directed alongside the outer surface of the titanium plate members. In that case, the cooling fluid alongside one titanium plate member may or may not be the same cooling fluid that is directed alongside an outer surface of the second titanium plate member. 
         [0023]    It should be understood, of course, that the foregoing relates to exemplary embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention as set forth in the following claims. alongside an outside surface  21   b  of the second titanium plate member  24 . As seen from  FIG. 4  wherein arrows represent heat flow by convection, heat from heating fluid  50  may thereby conduct into first and second titanium plate members  22 ,  24  and may then conduct from plate members  20  into cooling fluid  60 . 
         [0024]      FIG. 4  shows that with the titanium plate members  20  there may be less matrix conduction of heat within the first and second titanium plate members  20  from the hot end of plate member  20  to the cold end of plate member  20 . The amount of matrix conduction within titanium plate members  20  may be significantly less than would be the case were the plate members made of aluminum, as in the prior art heat transfer device of  FIG. 1 . 
         [0025]    Titanium plate members  20  may be made of various types of titanium such as titanium-CP-70 having a thermal conductivity of approximately 118 BTU/Hr/ft/F/in, titanium-6-4 having a thermal conductivity of approximately 50 BTU/Hr/ft/F/in or titanium-21S, which has a thermal conductivity of approximately 53 BTU/Hr/ft/F/in. Accordingly, the thermal conductivity of the plate members  20  may be significantly lower than for aluminum plate members, which may have a thermal conductivity of over 1000, for example approximately 1070 BTU/Hr/ft/F/in for 6061 aluminum and approximately 1370 BTU/Hr/ft/F/in for 6951 aluminum. Notwithstanding that, each of the plate members  20  may be significantly stronger than aluminum plate members of equal size. Size here refers to the length and width of the plate member (i.e. the dimensions other than the thickness of the plate member). Each of the plate members  20  may also be thinner than 0.012 inches for example, as thin as from about 0.002 inches. 
         [0026]    As is known in the industry, the heating and cooling fluids passages in the plate fin heat transfer device may be in various configurations, including a counterflow configuration, a crossflow configuration, a multi-pass crossflow configuration, or any other well known configuration. 
         [0027]    As can be seen from  FIG. 5 , which is a flow chart, the present