Patent Publication Number: US-2010108042-A1

Title: Heat exchanger, method of manufacturing the same, and egr system

Description:
TECHNICAL FIELD 
     The present invention relates to a heat exchanger which has a flow channel through which a corrosive fluid flows, a method of manufacturing a heat exchanger, and an EGR system including the heat exchanger. 
     Priority is claimed on Japanese Patent Application No. 2007-115489, filed Apr. 25, 2007, the content of which is incorporated herein by reference. 
     BACKGROUND ART 
     Due to increased environmental awareness, an EGR (Exhaust Gas Recirculation) system has been proposed in order to suppress generation of NOx in an internal combustion engine (in particular, a diesel engine). With such an EGR system, oxygen concentration in a combustion chamber of the internal combustion engine is decreased by cooling a part of combustion exhaust gas discharged from the internal combustion engine and returning the part to an intake side of the internal combustion engine, thereby trying to reduce NOx. 
     Such an EGR system includes an EGR cooler (heat exchanger) for cooling and returning combustion exhaust gas to an intake side of an internal combustion engine. The EGR cooler cools the combustion exhaust gas by performing heat exchange between the combustion exhaust gas and cooling water supplied from the outside. 
     By the way, in the heat exchanger such as the EGR cooler which has a flow channel through which a corrosive fluid such as combustion exhaust gas flows, the flow channel needs to have corrosion resistance to the corrosive fluid. For this reason, a conventional heat exchanger includes a thick partitioning plate for separating the flow channel, through which the corrosive fluid flows, and a flow channel, through which the cooling water flows, so as to ensure corrosion resistance of the heat exchanger. 
     Such a heat exchanger is manufactured by brazing members (partitioning plates or fins) constituting the flow channel. More specifically, the partitioning plate or fin is formed by pressing a metal (stainless steel) sheet, and after the partitioning plate and the fin are assembled and a brazing material is disposed to a desired portion of the partitioning plate or fin, the brazing material is subjected to a heating process (brazing process) in a vacuum atmosphere, thereby manufacturing the heat exchanger. 
     [Patent Document 1] Japanese Unexamined Patent Application, First Publication No. 2005-49007 
     [Patent Document 2] Japanese Unexamined Patent Application, First Publication No. 2004-317072 
     [Patent Document 3] Japanese Unexamined Patent Application, First Publication No. 2004-335846 
     However, a conventional heat exchanger has the following problems. 
     (1) Since the partitioning plate is formed thick in order to ensure corrosion resistance, it is difficult to reduce the size and enhance the performance of the heat exchanger. 
     (2) Since there is a need for a process for disposing the brazing material on the members constituting the flow channel, such as the partitioning plate, the process of manufacturing the heat exchanger is cumbersome and complicated. Also, the performance of the manufactured heat exchanger is difficult to stabilize. 
     For example, in the case where powder brazing material in powder form is used as the brazing material, the powder brazing material is mixed with a binder and is applied by spraying, and then the brazing material is disposed by volatilizing a part of the binder. However, it is difficult to apply the powder brazing material mixed with the binder in uniform thickness. For this reason, it is difficult to dispose the brazing material uniformly, and more brazing material than is necessary is used in order to reliably perform the brazing. In addition, the binder is attached to peripheral portions after the volatilization of the binder. This requires a vacuum heating process used for volatilizing the binder and a vacuum heating process used for brazing, which makes the time necessary for heating treatment longer, thus increasing manufacturing costs. 
     Alternatively, in the case of utilizing thin sheets of brazing material as the brazing material, it is necessary to cut the large sheet brazing material into shapes corresponding to that of the arrangement portion, and thus the process of manufacturing the heat exchanger is cumbersome and complicated. 
     DISCLOSURE OF INVENTION 
     Therefore, the present invention has been made in view of the above-mentioned problems, and objects of the present invention are as follows: 
     (1) to reduce the size and enhance the performance of a heat exchanger which has a flow channel through which a corrosive fluid flows; and 
     (2) to simplify the manufacturing of a heat exchanger which has a flow channel through which the corrosive fluid flows. 
     A heat exchanger according to the present invention includes a partitioning plate and flow channels of at least two systems which are partitioned by the partitioning plate, in which the partitioning plate is made of a clad sheet having a base material made of stainless steel or a nickel-based alloy, and a clad layer having brazing properties and corrosion resistance to a corrosive fluid, an entire surface of the base material which is exposed to the flow channel of at least one system being coated by the clad layer. 
     In the heat exchanger according to the present invention, the partitioning plate is made of the clad sheet having corrosion resistance and brazing properties. 
     In the heat exchanger according to the present invention, a fin may be installed in the flow channel, and the fin may be made of the clad sheet. 
     In the heat exchanger according to the present invention, the partitioning plate may be formed in the shape of a fin. 
     In the heat exchanger according to the present invention, the corrosive fluid may be combustion exhaust gas from an internal combustion engine, and cooling water may flow through the flow channel other than the flow channel through which the combustion exhaust gas flows. 
     In the heat exchanger according to the present invention, the clad layer may include at least chromium, silicon, phosphorus, and nickel as components. 
     In the heat exchanger according to the present invention, the clad layer may consist of 13 to 18 wt % of chromium, 3 to 4 wt % of silicon, and 4 to 7 wt % of phosphorus, and the remainder being nickel and inevitable impurities. 
     A method of manufacturing a heat exchanger according to the present invention is a method of manufacturing a heat exchanger including a partitioning plate and flow channels of at least two systems which are partitioned by the partitioning plate, the method including: a clad sheet forming step of forming a clad sheet having a base material made of stainless steel or a nickel-based alloy, and a clad layer having brazing properties and corrosion resistance to a corrosive fluid, an entire surface of the base material which is exposed to the flow channel of at least one system being coated by the clad layer; a partitioning plate forming step for forming the partitioning plate by using the clad sheet; and a flow channel forming step for forming the flow channels of at least two systems by using the formed partitioning plate through a brazing process, by which the clad layer of the partitioning plate is molten. 
     In the method of manufacturing a heat exchanger according to the present invention, the partitioning plate is made of the clad sheet having corrosion resistance and brazing properties. 
     In the method of manufacturing a heat exchanger according to the present invention, the method may include a fin forming step for forming a fin by using the clad sheet, and the flow channel forming step may utilize the formed partitioning plate and the formed fin and forms the flow channels of at least two systems through the brazing process by which the clad layers of the partitioning plate and the fin are molten. 
     In the method of manufacturing a heat exchanger according to the present invention, in the partitioning plate forming step, the partitioning plate may be formed in the shape of a fin. 
     In the method of manufacturing a heat exchanger according to the present invention, the clad sheet forming step may include a compression-bonding step for compressively bonding mixture powder, which is obtained by mixing alloy powder including at least any one of chromium, silicon, and phosphorus as its components, and nickel powder, to the base material. 
     In the method of manufacturing a heat exchanger according to the present invention, the mixture powder may include the nickel powder of 10 wt % or more. 
     In the method of manufacturing a heat exchanger according to the present invention, a shape of the nickel powder may have a plurality of protrusions. 
     In the method of manufacturing a heat exchanger according to the present invention, a composition ratio of a sum total of the mixture powder including the plurality of kinds of components may include 13 to 18 wt % of chromium, 3 to 4 wt % of silicon, and 4 to 7 wt % of phosphorus, and the remainder being nickel and inevitable impurities. 
     In the method of manufacturing a heat exchanger according to the present invention, the clad sheet forming step may include a heating step for heating the clad sheet after the compression-bonding step. 
     In the method of manufacturing a heat exchanger according to the present invention, it is preferable that the mixture powder including the plurality of kinds of components may include at least BNi-7 (JIS Standard Z3265; nickel braze material). 
     An EGR (Exhaust Gas Recirculation) system according to the present invention includes an internal combustion engine, an intake flow channel for supplying combustible gas to at least the internal combustion engine, an exhaust flow channel for discharging combustion exhaust gas from the internal combustion engine, and an EGR cooler for cooling a part of the combustion exhaust gas and returning the part to the intake flow channel, in which the heat exchanger according to the present invention is used as the EGR cooler. 
     In the EGR system according to the present invention, the heat exchanger, including the partitioning plate made of at least the clad sheet, of the present invention may be used as the EGR cooler. 
     With the heat exchanger according to the present invention, the partitioning plate is formed of the clad sheet having the clad layer on the entire surface of the flow channel of at least one system, which is exposed in the flow channel, among the fluid channels of at least two systems. For this reason, the surface of the partitioning plate exposed in the flow channel of one system has corrosion resistance and brazing properties. Therefore, it is not necessary to increase the thickness of the partitioning plate in order to ensure corrosion resistance. The heat exchanger, in which the corrosive fluid flows, can be reduced in size, and the performance can be enhanced. Also, it is not necessary to dispose a brazing material on the partitioning plate, and the manufacturing of the heat exchanger can be simplified. 
     With the method of manufacturing a heat exchanger according to the present invention, the partitioning plate is made of the clad sheet. The clad sheet includes the clad layer disposed over the entire surface of the flow channel of at least one system which is exposed in the flow channel, among the fluid channels of at least two systems. For this reason, the surface of the partitioning plate exposed in the flow channel of one system has corrosion resistance and brazing properties. Therefore, it is possible to easily manufacture a heat exchanger with the flow channels having corrosion resistance. In other words, with the method of manufacturing a heat exchanger according to the present invention, it is possible to simplify manufacturing of a heat exchanger in which the corrosive fluid flows. 
     With the EGR system according to the present invention, since the heat exchanger, including the partitioning plate made of at least the clad sheet, of the present invention is used as the EGR cooler, the size of the heat exchanger can be reduced and the performance can be enhanced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view schematically illustrating the configuration of a heat exchanger according to a first embodiment of the present invention. 
         FIG. 2  is a view illustrating a cross section of the heat exchanger according to the first embodiment of the present invention. 
         FIG. 3  is a perspective view of a clad sheet utilized in the heat exchanger according to the first embodiment of the present invention. 
         FIG. 4  is a cross-sectional view of the clad sheet utilized in the heat exchanger according to the first embodiment of the present invention. 
         FIG. 5  is a view schematically illustrating the configuration of an apparatus for manufacturing the clad sheet utilized in the heat exchanger according to the first embodiment of the present invention. 
         FIG. 6  is a view explaining a shape of nickel powder. 
         FIG. 7  is a view explaining a method of manufacturing a heat exchanger according to the first embodiment of the present invention. 
         FIG. 8  is a view explaining the method of manufacturing a heat exchanger according to the first embodiment of the present invention. 
         FIG. 9  is a view explaining the method of manufacturing a heat exchanger according to the first embodiment of the present invention. 
         FIG. 10  is a view schematically illustrating a cross section of a heat exchanger according to a second embodiment of the present invention. 
         FIG. 11  is a view schematically illustrating an EGR cooler according to a third embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE REFERENCE SYMBOLS 
     
         
         
           
               11 : CLAD SHEET 
               2 : METAL PLATE (BASE MATERIAL) 
               3 : CLAD LAYER 
             F: MIXTURE POWDER 
             F 1 : ALLOY POWDER 
             F 2 : NICKEL POWDER 
             F 3 : SILICON POWDER 
             F 4 : BNi-7 POWDER 
             Y: MOLTEN PORTION 
               100 ,  200 : HEAT EXCHANGER 
               110 ,  220 : CORROSIVE GAS FLOW CHANNEL (FLOW CHANNEL) 
               120 ,  230 : COOLING WATER FLOW CHANNEL (FLOW CHANNEL) 
               130 : FIN 
               140 ,  210 : PARTITIONING PLATE 
               300 : EGR SYSTEM 
               310 : INTERNAL COMBUSTION ENGINE 
               320 : INTAKE PIPE 
               330 : EXHAUST PIPE 
               350 : EGR COOLER 
           
         
       
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Hereunder, a heat exchanger, a method of manufacturing the heat exchanger, and an EGR system according to an embodiment of the present invention will be described with reference to accompanying drawings. In the accompanying drawings, the scale of each member is properly altered in order to let each member have a discernable size. 
     First Embodiment 
       FIG. 1  is a perspective view schematically illustrating the configuration of a heat exchanger  100  according to a first embodiment.  FIG. 2  is a view schematically illustrating a cross section of the heat exchanger  100  according to the first embodiment. As shown in the drawings, the heat exchanger  100  according this embodiment includes corrosive gas flow channels  110  through which corrosive gas G such as combustion exhaust gas of an internal combustion engine flows, and cooling water flow channels  120  through which cooling water R flows, the corrosive gas flow channels and the cooling water flow channels being alternatively stacked on each other in plural layers inside of an external frame  150 . 
     The corrosive gas flow channel  110  and the cooling water flow channel  120  are partitioned from each other by a partitioning plate  140  which is provided as a boundary of the respective channels. In other words, the plurality of partitioning plates  140  are disposed in the vertical direction inside the external frame  150 , and it is configured such that spaces between the partitioning plates  140  alternately serve as the corrosive gas flow passages  110  and the coolant water flow passages  120 . As shown in  FIG. 2 , edge portions  141  of the respective partitioning plates  140  are pairwise joined to those of the nearest partitioning plates  140 , the edge portions of the respective pair being bent to the opposite directions, and a closed space formed by the above joint is employed as the corrosive gas flow channel  110 . Meanwhile, an opened space, which is formed by joining the edge portions  141  of each partitioning plate  140 , is formed into a closed space by being closed by the external frame  150 , and the closed space is employed as the cooling water flow channel  120 . 
     The corrosive gas flow channel  110  and the cooling water flow channel  120  are provided therein with fins  130  shaped by pressing a flat plate in a wave shape. The height of the fin  130  is respectively set in compliance with a height of the corrosive gas flow channel  110  or the height of the cooling water flow channel  120 . That is, the fin  131  ( 130 ) installed in the corrosive gas flow channel  110  has a height set in compliance with the height of the corrosive gas flow channel  110 , while the fin  132  ( 130 ) installed in the cooling water flow channel  120  has a height set in compliance with the height of the cooling water flow channel  120 . Each of the fins  130  has a top portion and a bottom portion which are abutted against and joined to the inner wall of each channels  110  and  120 , that is, the partitioning plates  140 . 
     In the heat exchanger  100  according to the present embodiment, the partitioning plate  140  and the fin  130  are made by using a clad sheet  1 .  FIG. 3  is a perspective view of the clad sheet  1  according to the present embodiment.  FIG. 4  is an enlarged sectional-view of the clad sheet  1  according to the present embodiment. As shown in the drawings, the clad sheet  1  is constituted of a sheet-shaped metal plate  2  which serves as a base material, and clad layers  3  compressively bonded onto both entire surfaces of the base material. 
     The metal plate  2  is made of SUS (stainless steel), preferably, SUS304, SUS316, SUS410 or SUS444. The clad layer  3  includes an alloy phase Y 1  (metal phase) containing nickel, chromium, silicon, and phosphorus, a nickel phase Y 2 , and a molten phase Y 3  (alloy phase, metal phase), where BNi-7 (JIS Standard Z3265; nickel brazing material) is molten and then cured. 
     The clad layer  3  consists of nickel, chromium, silicon, phosphorus, and inevitable impurities (not shown). 
     The alloy phase Y 1  is a phase formed by partially melting alloy powder including chromium, silicon, and phosphorus as its components, with the remainder being nickel. 
     Chromium is substance used to improve corrosion resistance of the clad sheet  1 . Silicon and phosphorus are substance to improve brazing properties of the clad sheet  1 . Since there is the alloy phase Y 1  including chromium, silicon and phosphorus as its components in the clad layer  3 , the clad sheet  1  according to the present embodiment has corrosion resistance and brazing properties. 
     The nickel phase Y 2  is a phase formed by partially melting metal powder consisting of nickel which is pure metal of high ductility. Since there is a phase consisting of nickel which is pure metal of high ductility in the clad layer  3 , the clad sheet  1  according to the present embodiment has a high degree of freedom in processing. 
     The molten phase Y 3  is formed by melting BNi-7, as described above, and is filled between the alloy phase Y 1  and the nickel phase Y 2 . The melting point of the molten phase Y 3  is the lowest, as compared with those of the alloy phase Y 1  and the nickel phase Y 2 , and is, preferably, 1150° C. or less. The metal plate  2  used as the base material and the clad layer  3  are fused together by melting the molten phase Y 3 . In other words, the metal plate  2  and the clad layer  3  are bonded to each other, without requiring an adhesive, in the clad sheet  1  according to the present embodiment. 
     As is generally known, BNi-7 is a nickel-based alloy containing approximately 10 wt % of phosphorus. 
     As seen from the above, the clad layer  3  is composed of the alloy phase Y 1  formed by partially melting the alloy powder including chromium, silicon and phosphorus as its component, with the reminder including nickel, and the molten phase Y 3  formed by melting BNi-7 (alloy). In other words, the clad layer  3  includes two alloy phases of different compositions. 
     Also, the clad layer  3  has a composition ratio, which is obtained by taking the mean of the entire clad layer  3 , of 13 wt % of chromium, 4 wt % of silicon, and 6 wt % of phosphorus, and the remainder being nickel and inevitable impurities. 
     In the clad sheet  1 ,  13  wt % of chromium is contained in the clad layer  3 . 
     The clad layer  3  is formed on both entire surfaces of the metal plate  2 . For this reason, the clad layer  3  exercises corrosion resistance to prevent corrosion of the clad sheet  1  itself. 
     In addition, 4 wt % of silicon and 6 wt % of phosphorus are contained in the clad layer  3  of the clad sheet  1 . Therefore, the clad layer  3  has sufficient strength of brazing material, and is able to perform brazing at a low temperature. In other words, the clad layer  3  has superior brazing properties. 
     That is, the clad sheet  1  has corrosion resistance and brazing properties. Accordingly, it is not necessary to dispose an additional brazing sheet or the like, and it is possible to omit treatment involved in processing and disposing the brazing sheet. 
     Furthermore, in the clad sheet  1 , there is the nickel phase Y 2  having high ductility in the clad layer  3 . Therefore, the clad layer  3  is softened, as compared with a clad layer where nickel, chromium, silicon, phosphorus, and inevitable impurities are mixed in the same composition ratio. As a result, the clad sheet  1  according to the present embodiment has a high degree of freedom in processing. 
     Also, in the clad layer  1 , the clad layer  3  is fused with the metal plate  2  used as the base material. In other words, the metal plate  2  and the clad layer  3  are fused together without using an adhesive. 
     For this reason, it is possible to omit a degreasing process in the joining process in which the clad sheet  1  according to the present invention is brazed. 
     In addition, in the clad sheet  1 , the clad layer  3  is formed on both entire surfaces of the metal plate  2 , as described above. Therefore, the whole clad sheet  1  has corrosion resistance. In a conventional manner, since a brazing material is adhered to only a brazed portion of the metal plate or a brazing material sheet is disposed on only the brazed portion, a thickness of the metal plate itself has to be formed thick in order to ensure corrosion resistance for other portions thereof, to which the brazing material is not adhered or on which the brazing material sheet is not disposed. Since the whole of the clad sheet  1  according to the present invention has corrosion resistance, it is possible to suppress the thickness of the metal plate itself. Consequently, for example, apparatuses manufactured by using the clad sheet  1  can be reduced in size or weight. 
     In the heat exchanger  100  according to the present embodiment, the partitioning plate  140  and the fin  130  are made of the above-described clad sheet  1 . In other words, the clad layer  3  having corrosion resistance and brazing properties is formed on the whole of the surface coming in contact with the corrosive gas G passing through the corrosive gas flow channel  110  (including the surface of corrosive gas (corrosive fluid) side of the metal plate  2  (base material) of the clad sheet  1 ). 
     Therefore, the partitioning plate  140  and the fin  130  have corrosion resistance to the corrosive gas G, like the clad sheet  1 . 
     In the heat exchanger  100  with this configuration, the corrosive gas G of high temperature flows through the corrosive gas flow channel  110 , while the cooling water R of low temperature flows through the cooling water flow channel  120 . Indirect heat exchange happens between the corrosive gas G and the cooling water R via the partitioning plate  140  and the fin  130 , so that the corrosive gas G is cooled and discharged. 
     With the heat exchanger  100  according to the present embodiment, the partitioning plate  140  and the fin  130  are made of the clad sheet  1  with the clad layer  3  formed on both surfaces thereof, whereby the clad layer  3  has corrosion resistance and brazing properties. For this reason, it is not necessary to increase the thickness of the partitioning plate in order to ensure corrosion resistance, like a conventional heat exchanger, and it is possible to reduce the thickness of the partitioning plate  140  and the fin  130 , as compared with a conventional heat exchanger. Consequently, the heat exchanger  100  according to the present embodiment has good performance, and is compact. 
     Also, with the heat exchanger  100  according to the present embodiment, both surfaces of the partitioning plate  140  and the fin  130  have brazing properties, and when the partitioning plate  140  and the fin  130  are brazed, it is not necessary to dispose a brazing material on the partitioning plate  140  and the fin  130 , thereby the heat exchanger is easily manufactured. This point will be described in detail when a method of manufacturing the heat exchanger  100  is described hereinafter. 
     The method of manufacturing the heat exchanger  100  according to the present embodiment will now be described. 
     The method of manufacturing the heat exchanger  100  according to the present embodiment includes a clad sheet forming process, a partitioning plate forming process, a fin forming process, and a brazing process. 
     First, the clad sheet forming process will be described. 
       FIG. 5  is a view schematically illustrating the configuration of an apparatus of manufacturing the clad sheet  1 . As shown in the drawing, the manufacturing apparatus includes rolling rollers  10 A and  10 B, belt feeders  20 A and  20 B, and a heating furnace  30 . 
     The rolling rollers  10 A and  10 B are disposed opposite to each other in parallel, with circumferences of the rolling rollers being spaced apart from each other at a desired interval. The metal plate  2  is inserted between the rolling rollers from an upward side to a downward side. 
     The belt feeders  20 A and  20 B are installed over the rolling rollers  10 A and  10 B. The belt feeder  20 A is installed over the rolling roller  10 A so as to supply mixture powder F onto the circumference of the rolling roller  10 A. Also, the belt feeder  20 B is installed over the rolling roller  10 B so as to supply the mixture powder F onto the circumference of the rolling roller  10 B. 
     The heating furnace  30  is installed under the rolling rollers  10 A and  10 B so as to heat the metal plate  2  fed from the rolling rollers  10 A and  10 B at a temperature in the vicinity of the melting point of BNi-7 powder described hereinafter. 
     The mixture powder F supplied from the belt feeders  20 A and  20 B to the rolling rollers  10 A and  10 B is composed of alloy powder F 1 , nickel powder F 2 , silicon powder F 3 , and BNi-7 powder F 4 . 
     The alloy powder F 1  includes 29 wt % of chromium, 4 wt % of silicon, and 6 wt % of phosphorus, as its components, and the remainder being nickel. The alloy powder F 1  is metal powder of low ductility, and is made by, for example, a gas atomizing method. The alloy powder F 1  contains chromium, silicon, and phosphorus as its components. 
     The nickel powder F 2  is metal powder consisting of nickel, which is pure metal, of high ductility, and serves as a binder for compression-bonding the alloy powder F 1  onto the metal plate  2 . Preferably, the nickel powder F 2  is, for example, carbonyl Ni powder with a plurality of protrusions, as shown in  FIG. 6 . By using the nickel powder F 2  having such a shape, adherence properties between the nickel powder F 2  and the allow powder F 1  is improved due to the protrusions, and the alloy powder F 1  can be compressively bonded more firmly onto the metal plate  2 . The nickel powder F 2  of 10 wt % or more is contained in the mixture powder F. 
     The silicon powder F 3  is metal powder consisting of silicon, and is added to adjust a component ratio of the clad layer  3 , that is, to increase silicon contained in the clad layer  3 . 
     The composition ratio of the mixture powder is identical to that of the clad layer  3 , that is, the mixture powder includes 13 wt % of chromium, 4 wt % of silicon, and 6 wt % of phosphorus, and the remainder being nickel and inevitable impurities. 
     With the manufacturing apparatus constituted as described above, the mixture powder F is supplied from the belt feeders  20 A and  20 B to the circumference of each of rolling rollers  10 A and  10 B. The mixture powder F supplied on the circumference of each of rolling rollers  10 A and  10 B is extended by rolling (compression-bonded) onto each surface of the metal plate  2  by further rotation of the rolling rollers  10 A and  10 B. In other words, a compression-bonding process is performed, in which the mixture powder F is compressively bonded onto the metal plate  2  by the rolling rollers  10 A and  10 B. 
     In this embodiment, the nickel powder F 2  is contained in the mixture powder F. For this reason, the nickel powder F 2  is deformed by rolling of the rolling rollers  10 A and  10 B, and serves as a binder for fixing the alloy powder F 1  onto the metal plate  2 . Accordingly, the alloy powder F 1  including chromium, silicon and phosphorus as its components can be compressively bonded onto the metal plate  2 , without using an adhesive containing a resin in a conventional way. 
     Then, the metal plate  2  having both entire surfaces compressively bonded with the mixture powder F is heated in the heating furnace  30  at a temperature in the vicinity of the melting point of the BNi-7 powder F 4 . The BNi-7 powder F 4  having the lowest melting point is molten by the heating. 
     The alloy powder F 1  and the nickel powder F 2  are partially molten by the melting of the BNi-7 powder. Also, the silicon powder F 3  is molten by the melting of the BNi-7 powder. After the respective powder F 1  to F 4  is molten and then cooled, as shown in  FIG. 4 , the clad layer  3  composed of the alloy phase Y 1 , the nickel phase Y 2  and the molten phase Y 3  is formed. 
     Note that the clad sheet is subjected to reheating (a vacuum heating treatment) at brazing. Therefore, the clad layer including the plurality of metal phases comes to have more uniformity. 
     The clad sheet  1  shown in  FIGS. 3 and 4  is formed by the above processes. 
     With the method of forming the clad sheet  1 , the mixture powder F, in which the alloy powder F 1  exercising corrosion resistance and brazing properties, and the nickel powder F 2  exercising adherence properties resulted from the ductility are mixed, is compressively bonded onto the metal plate  2 . Accordingly, when the alloy powder F 1  having corrosion resistance and brazing properties is compressively bonded onto the metal plate  2 , it is possible to firmly fix the allow powder F 1  onto the metal plate  2 . 
     According to the method of forming the clad sheet  1 , the mixture powder F contains the nickel powder F 2  of 10 wt % or more. In such a case, the alloy powder F 1  is properly fixed to the metal plate  2 . For this reason, the method of manufacturing the clad sheet  1  according to the present embodiment can properly fix the alloy powder F 1  to the metal plate  2 . 
     According to the method of manufacturing the clad sheet  1 , since the nickel powder F 2  is, for example, carbonyl Ni powder with a plurality of protrusions, adherence properties between the nickel powder F 2  and the allow powder F 1  are improved due to the protrusions, and the alloy powder F 1  can be more firmly fixed to the metal plate  2 . It is possible to suppress peeling of the mixture powder F from the metal plate  2  by more firmly fixing the alloy powder F 1  to the metal plate  2 . 
     According to the method of forming the clad sheet  1 , the clad layer  3  is formed by melting the BNi-7 powder F 4 . The BNi-7 powder has a low melting point (about 900° C.). Therefore, it is possible to suppress the heating temperature to about 900° C., and to suppress oxidization of chromium or silicon. 
     The partitioning plate forming process is a process for forming the partitioning plate  140  from the clad sheet  1  by pressing the clad sheet  1 , which is manufactured in the clad sheet forming process, so as to bend the edge portion  141 , as shown in  FIG. 7 . 
     The fin forming process is a process for forming the fin  130  by pressing the flat-shaped clad sheet  1 , which is formed in the clad sheet forming process, in a wave shape, as shown in  FIG. 8 . 
     The brazing process is a process for heating the partitioning plates  140  and the fins  130  which are accommodated in the external frame  150 , at a predetermined temperature (vacuum heat treatment) and then cooling the partitioning plates and the fins to braze and firmly fix the partitioning plates  140 , the fins  130  and the external frame  150  to each other. The brazing process utilizes the partitioning plates  140  and the fins  130  each including the clad layer  3  having brazing properties, so that the partitioning plates  140 , the fins  130  and the external frame  150  are brazed and firmly fixed to each other. 
     When the partitioning plate  140  and the fin  130  are heated, a molten solution which is generated by melting a melting portion Y of the clad layer  3  and the alloy powder F 1  is drawn close to the joined portion of the partitioning plates  140  or the jointed portion between the partitioning plate  140  and the fin  130  due to a capillary phenomenon. Therefore, for example, at the joined portion of the partitioning plates  140 , the clad layer  3  becomes thicker at portions where the clad sheets  1  are brazed with each other, as shown in  FIG. 9 . However, it never happens that all the melting solution is attracted to brazing portions, leading to lack of the clad layer at other portions on the clad sheet  1 . Consequently, corrosion resistance can be ensured on all portions in the corrosive gas flow channel  110  and the cooling water flow channel  120 . 
     The heat exchanger  100  shown in  FIGS. 1 and 2  is manufactured by the above process. 
     With the method of manufacturing the heat exchanger  100  according to the present embodiment, the heat exchanger  100  is manufactured by forming the partitioning plate  140  and the fin  130  using the clad sheet  1  including the clad layer  3  having corrosion resistance and brazing properties, and performing a brazing treatment by using the partitioning plate  140  and the fin  130 . Therefore, it is not necessary to perform a process for disposing an additional brazing material on the member (partitioning plate  140  or the fin  130 ) constituting the corrosive gas flow channel  110  or the cooling water flow channel  120 . At the same time, it is not necessary to perform a process for volatizing a binder which is needed when the powder brazing material is used, or a process for cutting the sheet brazing material which is needed when the sheet brazing material is used, as in a conventional method of manufacturing a heat exchanger. According to the method of manufacturing the heat exchanger  100  according to the present embodiment, therefore, the manufacturing process can be simplified. 
     According to the method of manufacturing the heat exchanger  100  according to the present embodiment, since the clad sheet  1  including the clad layer  3  which is formed to have a uniform film thickness due to a rolling treatment is used, it is possible to suppress variation in the thickness of the film which serves as the brazing material, as compared with a conventional method. Accordingly, it is possible to make uniform a channel area of the corrosive gas flow channel  110  and the cooling water flow channel  120 , and to mass-produce heat exchangers which stably exercise desired performance. 
     Also, according to the method of manufacturing the heat exchanger  100  according to the present embodiment, since the clad layer  3  is firmly fixed to the metal plate  2  by the rolling treatment, the metal plate  2  can be subjected to a press process together with the clad layer, that is, the film having brazing properties. 
     In a conventional method, since the brazing material is not fixed to the metal plate, the metal plate with the brazing material can not be subjected to the press process, and thus, the shape of the metal plate subjected to the press process should be simple in view of a process for attaching the brazing material. 
     On the other hand, according to the method of manufacturing the heat exchanger  100  according to the present embodiment, the metal plate with the clad layer can be subjected to the press process, and the clad sheet  1  can be subjected to a fine or complicated processing. Accordingly, it is possible to enhance the degree of freedom in the structure of the heat exchanger. For example, according to the method of manufacturing the heat exchanger  100  according to the present embodiment, it is possible to sufficiently ensure a channel area without the channel being crushed even in the case of manufacturing the heat exchanger with a fin pitch of 1 mm or less. 
     Second Embodiment 
     Next, a second embodiment of the present invention will now be described. In the description of the second embodiment, the same elements as those of the first embodiment will be omitted or will be described in brief. 
       FIG. 10  is a view schematically illustrating a cross section of a heat exchanger  200  according to the second embodiment. As shown in the drawing, the heat exchanger  200  has a configuration in which partitioning plates  210  formed in the shape of a fin are stacked on each other in plural layers inside of the external frame  150 . Top portions and bottom portions of the vertically adjacent partitioning plates  210  come in contact with each other, and are brazed to each other to form a plurality of flow channels. The plurality of flow channels are divided into corrosive gas flow channels  220  and cooling water flow channels  230  in a vertically alternating manner. In order to enhance the visibility thereof, the corrosive gas flow channel  220  is shown by hatching in  FIG. 11 . 
     Contrary to the first embodiment, the fin  130  is not installed in the corrosive gas flow channel  220  and the cooling water flow channel  230 , but the heat exchanger  200  of the present embodiment can exercise a sufficient cooling efficiency, since the partitioning plate  210  is formed in the shape of a fin. 
     With the heat exchanger  200  according to the second embodiment, the partitioning plate  210  is formed from the clad sheet  1 , as is the heat exchanger  100  according to the first embodiment. 
     Accordingly, in the heat exchanger  200  according to the present embodiment, it is not necessary to increase the thickness of the partitioning plate, like a conventional heat exchanger, in order to ensure corrosion resistance, and thus, it is possible to reduce the size of the heat exchanger and improve the performance thereof, as compared with a conventional heat exchanger. 
     Also, in the case of manufacturing the heat exchanger  200  having such a configuration, the clad sheet  1  is formed into the fin-shaped partitioning plate  210 , and the brazing process is performed by using the partitioning plate  210 . When the partitioning plate  210  is brazed, it is not necessary to dispose brazing material on the partitioning plate, thereby easily manufacturing the heat exchanger. 
     Third Embodiment 
     Next, an EGR system according to a third embodiment will now be described, the EGR system being equipped with the heat exchanger  100  according to the first embodiment or the heat exchanger  200  according to the second embodiment. 
       FIG. 11  is a cross-sectional view schematically illustrating the EGR system  300  according to the present embodiment. As shown in the drawing, the EGR system  300  according to the present embodiment includes an internal combustion engine  310 , an intake pipe  320 , an exhaust pipe  330 , a bypass pipe  340 , and an EGR cooler  350 . 
     The internal combustion engine  310  is one for internally combusting air-fuel mixture therein to obtain a power from the combustion energy, and a diesel engine is employed as the internal combustion engine  310  in this embodiment. 
     The intake pipe  320  is a pipe through which combustion air (combustible gas) to be drawn into the internal combustion engine  310  flows, and is connected to the internal combustion engine  310 . 
     The exhaust pipe  330  is a pipe through which combustion exhaust gas to be discharged from the internal combustion engine  310  flows, and is connected to the internal combustion engine  310 . 
     The bypass pipe  340  is a pipe for connecting the intake pipe  320  with the exhaust pipe  330 , and works as a flow channel for returning part of the combustion exhaust gas from the exhaust pipe  330  to the intake pipe  320 . 
     The EGR cooler  350  is installed on an intermediate portion of the bypass pipe  340  so as to cool and discharge the combustible exhaust gas. The heat exchanger  100  according to the first embodiment or the heat exchanger  200  according to the second embodiment is utilized as the EGR cooler  350 . 
     With the EGR system  300 , if the combustion air is drawn into the internal combustion engine  310  through the intake pipe  320 , the combustion air is mixed with fuel sprayed in the internal combustion engine  310  to generate the air-fuel mixture. With the air-fuel mixture being combusted in the internal combustion engine  310 , the combustion energy is taken out as the power, and simultaneously the combustion exhaust gas generated by combustion is discharged through the exhaust pipe  330 . Part of the combustion exhaust gas is returned to the intake pipe  320  through the bypass pipe  340 . However, at that time, the combustion exhaust gas is cooled by the EGR cooler  350  installed on the intermediate portion. After that, the cooled combustion exhaust gas is again drawn into the internal combustion engine  310 . Therefore, oxygen concentration inside of the internal combustion engine  310  can be decreased, and generation of NOx can be suppressed. 
     In the EGR system  300  according to the present embodiment, the heat exchanger  100  according to the first embodiment or the heat exchanger  200  according to the second embodiment is utilized as the EGR cooler  350 . Therefore, the EGR system  300  according to the present embodiment is compact, and has superior cooling performance. 
     While preferred embodiments of the heat exchanger, the manufacturing method of the heat exchanger and the EGR system according to the invention have been described above with reference to the accompanying drawings, obviously these are not to be considered as limitative of the invention. Shapes, combinations and the like of the constituent members illustrated above are merely examples, and various modifications based on design requirements and the like can be made without departing from the spirit or scope of the invention. 
     For example, in the above embodiments, the configuration is described, in which the clad layers  3  are formed on both surfaces of the clad sheet  1 , so that the entire surface of the inner wall of the corrosive gas flow channel  110  and the entire surface of the inner wall of the cooling water flow channel  120  have corrosion resistance. However, the present invention is not limited thereto, and the clad layer  3  may be formed on at least the entire surface of the metal plate  2  of the clad sheet  1  on the side which is exposed to the corrosive gas G so that the clad layer  3  exists on at least the entire surface of the inner wall of the corrosive gas flow channel  110 . 
     In particular, since it is not necessary for the inner wall of the cooling water flow channel  120  to have the high corrosion resistance, only the clad layer  3  for the brazing may be provided. Accordingly, the clad sheet with the clad layer  3  formed on only one surface thereof may be employed in either of two partitioning plates constituting the cooling water flow channel  120 . By employing the configuration, it is possible to reduce the thickness of the partitioning plate and to lower the manufacturing cost, and thus to manufacture an inexpensive, compact and light heat exchanger. 
     In the above embodiments, the heat exchanger is described, in which the corrosive gas flows in only one of two kinds of the flow channels provided in the heat exchanger. However, the present invention is not limited thereto, and, for example, it may be applied to a heat exchanger, in which a corrosive fluid of high temperature flows in one fluid channel, while a corrosive fluid of low temperature flows in another fluid channel. 
     In the above embodiments, the composition in the clad layer  3  of the clad sheet  1  and the mixture powder F is described as 13 wt % of chromium, 4 wt % of silicon, and 6 wt % of phosphorus, and the remainder being nickel and inevitable impurities. However, the present invention is not limited thereto. 
     For example, if the clad layer  3  and the mixture powder F contain 13 to 18 wt % of chromium, the clad layer  3  exercises particularly preferable corrosion resistance. Also, if the clad layer  3  and the mixture powder F contain 3 to 4 wt % of silicon and the clad layer  3  and the mixture powder F contain 4 to 7 wt % of phosphorus, the clad layer  3  exercises particularly preferable brazing properties. For this reason, the composition may be arbitrarily modified so that each component falls in the above range. As one example, the clad layer  3  and the mixture powder F may be composed of 20 wt % of the alloy powder F 1 ,  30  wt % of the nickel powder F 2 , the silicon powder F 3 ,  40  wt % of the BNi-7 powder F 4  (molten portion Y), and 7 wt % of the chromium powder. In this case, the composition of the clad layer  3  and the mixture powder F includes 19 wt % of chromium, 5 wt % of silicon, 5 wt % of phosphorus, and the remainder being nickel. Also, this case can exercise the same effect as that of the clad sheet  1  described in the above embodiments. 
     In the above embodiments, after compression-bonding process is performed, in which the mixture powder F is compressively bonded to the metal plate  2  by the rolling rollers  10 A and  10 B, the BNi-7 powder F 4  is molten by performing the heating process using the heating furnace  30 , so that the alloy powder F 1  is firmly fixed to the metal plate  2 . However, the present invention is not limited thereto, and the heating process may be omitted. In this case, the clad layer  3  is provided with the BNi-7 powder F 4 , instead of the molten portion Y. 
     In the above embodiments, the clad layer  3  is described as being formed on both surfaces of the metal plate  2 . However, the present invention is not limited thereto, and the clad layer  3  may be formed on either one surface of the metal plate  2 . 
     In the above embodiments, the forming apparatus supplies the mixture powder F to the rolling rollers  10 A and  10 B by the use of the belt feeders  20 A and  20 B. However, the present invention is not limited thereto, and anything may be used as long as it can feed the mixture powder F in fixed amounts to the rolling rollers  10 A and  10 B. For example, a screw feeder or roll feeder may be employed, instead of the belt feeders  20 A and  20 B. 
     In the above embodiments, it is described that a ratio of the alloy powder F 1 , the nickel powder F 2  and the silicon powder F 3  contained in the mixture powder F is identical to that of each powder contained in the clad layer  3 , and a ratio of the BNi-7 powder F 4  is identical to that of the molten portion Y included in the clad layer  3 . However, there is a possibility that an amount of the nickel powder F 2  contained in the clad layer  3  may be decreased by reaction of the BNi-7 powder F 4  and the nickel powder F 2  due to the heating. In this case, for example when the mixture powder contains 30 wt % of nickel powder F 2 , there is a possibility that the nickel powder F 2  included in the clad layer  3  is reduced to 20 wt %. 
     INDUSTRIAL APPLICABILITY 
     With the heat exchanger according to the present invention, since it is not necessary to increase the thickness of a partitioning plate so as to ensure corrosion resistance, a heat exchanger having a flow channel through which a corrosive fluid flows can be reduced in size and have high performance. Also, since it is not necessary to dispose a brazing material on the partitioning plate, the heat exchanger can be easily manufactured. 
     By the method of manufacturing a heat exchanger according to the present invention, the process of manufacturing the heat exchanger having a flow channel through which the corrosive fluid flows can be simplified. 
     Also, in the EGR system according to the present invention, since the heat exchanger in the present invention including the partitioning plate made of at least a clad sheet is employed as an EGR cooler, it is possible to reduce the size of the heat exchanger and have high performance.