Abstract:
A heat exchanger includes a heat exchanger wall that bounds a passage. A coating lines the heat exchanger wall. The coating has a thickness that varies according to location on the heat exchanger wall.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The present disclosure claims priority to U.S. Provisional Application 61/733,804 filed Dec. 5, 2012. 
     
    
     BACKGROUND 
       [0002]    This application relates to corrosion protection of heat exchangers. 
         [0003]    Heat exchangers are known and used in various types of thermal management systems. For example, thermal management systems in aircraft utilize air/liquid coolant heat exchangers to control the temperature of power electronics. Generally, such heat exchangers are subject to corrosive conditions because of exposure to air. For instance, corrosion can cause leaks in the heat exchanger and reduction in thermal management control. The heat exchanger therefore includes a corrosion-resistant coating. 
       SUMMARY 
       [0004]    Disclosed is a heat exchanger that includes a heat exchanger wall that bounds a passage. A coating lines the heat exchanger wall. The coating has a thickness that varies according to location on the heat exchanger wall. 
         [0005]    In another aspect, a heat exchanger includes an inlet face arranged to receive ram air, an exit face opposed from the inlet face and a plurality of heat exchanger walls extending between the inlet face and the exit face. The heat exchanger walls define a plurality of passages that open at respective leading edges at the inlet face and at respective trailing edges at the exit face. A coating lines the plurality of heat exchanger walls. The coating has a thickness that varies according to location on the plurality of heat exchanger walls. 
         [0006]    Also disclosed is a method of protecting a heat exchanger from corrosion. The method includes providing a heat exchanger wall that bounds a passage and identifying at least one location on the heat exchanger wall that is more susceptible to corrosion than at least one other location on the heat exchanger wall. A coating is provided on the heat exchanger wall in a thickness that varies according to the identified locations such that the coating has a first thickness T 1  at the location on the heat exchanger wall that is more susceptible to corrosion and a second, different thickness T 2  at the least one other location on the heat exchanger wall, where T 1  is greater than T 2 . 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIGS. 1 and 2  illustrate different perspective views of an example heat exchanger. 
           [0008]      FIG. 3  shows a cross-section through a portion of the heat exchanger of  FIGS. 1 and 2  at a ram face. 
           [0009]      FIG. 4  shows a cross-section through a passage of the heat exchanger of  FIGS. 1 and 2 . 
           [0010]      FIG. 5  shows a sectioned view of the heat exchanger  20  with plates and fins. 
           [0011]      FIGS. 6 ,  7  and  8  show corrosion testing results. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]      FIGS. 1 and 2  show perspective views from different angles of an example heat exchanger  20 . In this example, the heat exchanger  20  is an air/liquid heat exchanger that includes an inlet  22  and an outlet  24  for conveying liquid coolant through the heat exchanger  20 . Air  26 , such as ram air in an aircraft end-use, can be conveyed through the heat exchanger  20  to remove heat from the liquid coolant. 
         [0013]    In this example, the heat exchanger  20  is a plate and fin arrangement that includes a plurality of plates  28  that define internal channels (not shown) for the conveyance of the liquid coolant. The plates  28  are separated from one another by a plurality of fins  30 , which facilitate thermal transfer between the air  26  and the liquid coolant through the walls of the plates  28 . The plates  28  and fins  30  are formed of a metallic material, as described in further detail below. It is to be understood that the examples herein are not limited to plate and fin arrangements. 
         [0014]    The air  26  conveyed through the heat exchanger  20  can carry moisture or other substances that can contribute to corroding the metallic material of the heat exchanger  20 . In this regard, as the air  26  encounters the heat exchanger  20 , the moisture or other substances can deposit on the heat exchanger  20 , causing potentially corrosive conditions at that location. The moisture or other substances tend to deposit or accumulate in the locations at which the air  26  first encounters the heat exchanger  20 . 
         [0015]    For example, referring to  FIG. 2 , the air  26  enters the heat exchanger  20  from the right-hand side and travels through the heat exchanger  20  exiting at the left-hand side. Thus, the heat exchanger  20  includes a ram face  32  where the air  26  enters the heat exchanger  20  and an exit face  34  ( FIG. 4 ) where the air  26  exits the heat exchanger  20 . 
         [0016]      FIG. 3  shows a portion of the ram face  32  and two of the fins  30  at the ram face  32 . The heat exchanger  20  includes passages  36  between adjacent ones of the fins  30  through which the air  26  travels between the ram face  32  and the exit face  34  ( FIG. 4 ). The passages  36  are bounded by the plates  28  and the fins  30 . Thus, the plates  28  and the fins  30  are walls that define and bound the passages  36 . 
         [0017]    The passages  36  include an initial or inlet section  38  at the ram face  32  that first encounters the incoming air  26 . It is at this location that the heat exchanger  20  can be most susceptible to the deposition and accumulation of moisture or other substances than can contribute to corrosion. In this regard, the heat exchanger  20  includes a corrosion-resistant coating  40  that lines, by fully or substantially covering, the fins  30 . The coating  40  can also cover the plates  28  and thus the examples herein are also applicable to the plates  28 . 
         [0018]    As shown, the coating  40  has a variable thickness with a first thickness T 1  and a second, different thickness T 2 . In this example, T 1  is greater than T 2 . As also shown, the location of T 1  is at the leading edge of the fins  30  in the inlet section  38  of the ram face  32  and the location of T 2  is at the interior of the passage  36 , spaced inwards from the leading edge. As an example, T 2  can be in the middle third of the passage  36 . The thickness of the coating  40  can gradually change between the locations at T 1  and T 2 . In this example, the inlet section  38  is encapsulated in a relatively thicker part of the coating  40  and the thickness then gradually decreases into the passage  36 . 
         [0019]    At the ram face  32  where the moisture or other substances can primarily deposit or accumulate, the coating  40  is thus thicker to provide a greater degree of corrosion protection. At the location of T 2  inside of the passage  36 , the coating  40  is thinner because less moisture or other substances deposit and less corrosion protection is therefore needed. Such locations where relatively greater and lesser corrosion protection is needed can be identified experimentally via observations from corrosion testing of heat exchangers, corrosion testing of test pieces and/or testing simulations, for example. 
         [0020]    In a further example, the thickness of the coating  40  can be represented by a maximum thickness and a minimum thickness. In one example, the maximum thickness is T 1  and the minimum (non-zero) thickness is T 2 . For example, a ratio of T 1 /T 2  is equal to or greater than 2. In a further example, the ratio of T 1 /T 2  is 3-7. In a further example, the ratio of T 1 /T 2  is 10 or greater. In a further example, the thickness T 1  is not less than 25.4 micrometers (i.e., the thickness T 1  is greater than or equal to 25.4 micrometers). In a further example, the thickness T 2  is no less than 2.54 micrometers (i.e., the thickness T 1  is greater than or equal to 2.54 micrometers). 
         [0021]    While  FIG. 3  only shows the leading edge of the fins  30  in the inlet section  38  at the ram face  32 ,  FIG. 4  also shows the exit face  34 . In this example, the fins  30  include a trailing edge at an outlet portion  42 , at which the coating  40  has a thickness T 3 . For example, T 3  is equal to or approximately equal to T 1  and is thicker than T 2  by any of the above-described ratios. As can be appreciated however, in other examples, the thickness T 3  may be equal to or approximately equal to T 2  such that the coating  40  is nominally thicker only at the location of T 1  at the inlet section  38 . 
         [0022]    The coating  40  can be an organic coating and the plates  28  and fins  30  of the heat exchanger  20  can be a metallic material. In one example, the coating  40  is an epoxy-based organic coating and can be deposited onto the heat exchanger  20  using an electrodeposition technique. In such a technique, the voltage and time used to deposit the coating  40  can be controlled to accentuate or change the variable thickness of the coating  40 . The metallic material of the fins  30  can be aluminum or aluminum alloy. Likewise, the plates  28  can also be aluminum or aluminum alloy and may be brazed or otherwise bonded to the fins  30  in a known manner. 
         [0023]    By providing the coating  40  with a thicker portion at the location of thickness T 1  and a thinner portion at the location of thickness T 2 , good corrosion protection is locally provided while reducing weight of the heat exchanger  20 . For example, a heat exchanger that has a relatively uniform thickness coating approximately equivalent to the thickness T 1  has a greater weight than the heat exchanger  20  that uses locally thick portions of the coating  40  only where needed. 
         [0024]      FIG. 5  shows a sectioned view of the heat exchanger  20  with the fins  30 .  FIGS. 6 ,  7 , and  8  show corrosion testing conducted under ASTM B117 salt spray for 2,016 hours. In  FIG. 6  a portion of one of the fins  30  tested is shown. The fin  30  shows no corrosion with the coating  40  at the thinner thickness T 2 .  FIG. 7  shows a comparative fin  30  with the coating  40  at the thicker thickness of T 1  and also shows no corrosion.  FIG. 8  shows a brazed intersection of the fins  30  and plates  28  with no corrosion. 
         [0025]    Aircraft air management systems are required to operate in corrosive environments. The corrosive environments can cause corrosion damage to heat exchangers and other components within the aircraft air management system. The corrosion damage can lead to leaks within the air management system. In a liquid cooled air management system, leakage of the liquid coolant can cause a malfunction of the liquid cooling system. Malfunction of the liquid cooling system can cause aircraft downtime due to unscheduled maintenance activity. An electrodeposited organic coating has been demonstrated to provide superior corrosion protection. However, existing coating processes can result in a weight increase of approximately 2 to 3 pounds. 
         [0026]    Corrosion damage can occur as a result of the corrosive environment present in aircraft air management systems. Liquid coolant leakage can result from corrosion damage to liquid to air heat exchangers. In order to address the corrosion issue, an electrodeposited organic coating has been applied to the heat exchanger. This electrodeposited organic coating has been demonstrated to provide a level of corrosion protection superior to the corrosion protection offered by the previously used silicone aluminum coating. However, the process used to apply the electrodeposited organic coating results in a very thorough coating resulting in a weight increase of approximately 2 to 3 pounds. In the examples herein, reduced thickness of the coating  40  results in a heat exchanger with variable coating thickness. The reduced coating thickness results in an estimated weight reduction of 1.5 pounds per heat exchanger compared with a uniform coating. Coating thickness is reduced at the core of the heat exchanger, but thickness is greater at the heat exchanger core face where corrosion is likely to occur. A section of heat exchanger coated with the variable thickness coating has been demonstrated to exhibit no corrosion when subjected to salt spray testing for 2000 hours. Therefore, the variable thickness coating will provide excellent corrosion resistance while reducing the weight of aircraft heat exchangers. Furthermore, the process associated with the thinner coating results in reduced processing time and reduced coating material consumption. 
         [0027]    There are a number of benefits associated with the examples disclosed herein. By coating the heat exchanger  20  with a variable thickness coating, the maximum corrosion resistance is provided at the heat exchanger core face where corrosion is likely to occur. The coating thickness is reduced in the interior of the heat exchanger core where corrosion is less likely to occur. The reduced thickness in the interior of the heat exchanger core results in an estimated weight reduction of 1.5 pounds per heat exchanger compared with the current process. The thinner coating in the heat exchanger core is expected to result in improved heat transfer and reduced pressure drop. The process associated with the thinner coating results in reduced processing time in a coating bath. The thinner coating also results in reduced coating material consumption. Reduced processing time and reduced coating material consumption are expected to result in lower costs. 
         [0028]    An electrodeposition coating process can be used to coat heat exchangers with a greater thickness at the inlet and exit and lesser thickness thinner on the inside. Thicker coating can be beneficial at the inlet, since corrosion typically occurs at the ram face. Weight reduction can be achieved. Testing conducted on the alloys actually used to construct a heat exchanger has shown that this variable thickness coating approach provides excellent corrosion protection. 
         [0029]    Although a combination of features is shown in the illustrated examples, not all of them need to be combined to realize the benefits of various embodiments of this disclosure. In other words, a system designed according to an embodiment of this disclosure will not necessarily include all of the features shown in any one of the Figures or all of the portions schematically shown in the Figures. Moreover, selected features of one example embodiment may be combined with selected features of other example embodiments. 
         [0030]    The preceding description is exemplary rather than limiting in nature. Variations and modifications to the disclosed examples may become apparent to those skilled in the art that do not necessarily depart from the essence of this disclosure. The scope of legal protection given to this disclosure can only be determined by studying the following claims.