Patent Publication Number: US-6907734-B2

Title: Heat exchanger

Description:
FIELD OF THE INVENTION 
   The present invention relates to a heat exchanger for recovering, with a heat medium flowing through a heat medium passage, the thermal energy of a high temperature fluid flowing through the interior of a fluid passage extending from a heat source. 
   BACKGROUND ART 
   An evaporator that carries out heat exchange between exhaust gas of an internal combustion engine and water so as to heat the water by the heat of the exhaust gas and generate high temperature, high pressure steam is known from Japanese Patent Application Laid-open Nos. 2001-207910 and 2001-207839. 
   Japanese Patent Application Laid-open No. 2001-207910 discloses an arrangement in which an evaporator is disposed in each of a plurality of exhaust ports of a multicylinder internal combustion engine, thus giving a high efficiency of heat exchange with a high temperature exhaust gas while avoiding the occurrence of exhaust interference and thereby ensuring the output of the internal combustion engine, and a single evaporator is disposed in a section where a plurality of exhaust passages are combined, thereby improving the efficiency of heat exchange by using exhaust gas that has decreased pulsations after being merged and thus has a uniform temperature. Japanese Patent Application Laid-open No. 2001-207839 discloses an arrangement in which a plurality of heat exchangers are disposed in a layered state in an exhaust passage of an internal combustion engine, thus lowering the heat transfer density of a heat exchanger on the upstream side where the flow rate of exhaust gas is high and increasing the heat transfer density of a heat exchanger on the downstream side where the flow rate of exhaust gas is low, and thereby ensuring a uniform heat transfer performance across all of the heat exchangers. 
   However, since the above-mentioned conventional heat exchanger has a structure in which heat exchange is carried out by contacting the exhaust gas with the external surface of a spiral- or zigzag-shaped pipe member within which water flows, the heat transfer area is limited to the surface area of the pipe member, and there is a limit to the improvement of the heat exchange efficiency. 
   DISCLOSURE OF INVENTION 
   The present invention has been achieved under the above-mentioned circumstances, and it is an object thereof to improve the heat exchange efficiency by maximizing the heat transfer area of a heat exchanger. 
   In order to attain this object, in accordance with a first aspect of the present invention, there is proposed a heat exchanger for recovering, with a heat medium flowing through a heat medium passage, the thermal energy of a high temperature fluid flowing through the interior of a fluid passage extending from a heat source, characterized in that the fluid passage is formed by arranging a large number of heat transfer plates at intervals from each other and partitioning the gaps between adjacent heat transfer plates using a partition wall that is formed integrally with the heat transfer plates, and the heat medium passage is formed from a large number of pipe members running through the heat transfer plates and being connected in a zigzag shape. 
   In accordance with this arrangement, since the fluid passage is formed by partitioning, using the partition wall, the gaps between the large number of heat transfer plates arranged at intervals from each other, and the heat medium passage is formed by the large number of pipe members running through the heat transfer plates and being connected in a zigzag shape, it is possible to carry out heat exchange between the exhaust gas and the heat medium via the large surfaces of the large number of heat transfer plates and the large number of pipe members, thereby greatly improving the heat exchange efficiency. Moreover, since the partition wall partitioning the gaps between adjacent heat transfer plates is formed integrally with the heat transfer plates, a fluid passage having any shape can be constructed with a simple structure while suppressing any increase in the number of components. 
   Furthermore, in accordance with a second aspect of the present invention, in addition to the first aspect, there is proposed a heat exchanger wherein the packing density of the pipe members disposed on the upstream side of the fluid passage is sparse and the packing density on the downstream side of the fluid passage is dense. 
   In accordance with this arrangement, since the packing density of the pipe members disposed on the upstream side of the fluid passage is sparse and the packing density of the pipe members disposed on the downstream side of the fluid passage is dense, it is possible to reduce the pressure loss caused by the pipe members on the upstream side of the fluid passage, where the flow rate is high because the high temperature fluid has high temperature and a large volume, and it is possible to ensure the efficiency of heat exchange between the heat medium and the high temperature fluid on the downstream side of the fluid passage, where the flow rate is low because the high temperature fluid has low temperature and a small volume. 
   Moreover, in accordance with a third aspect of the present invention, in addition to the first or second aspect, there is proposed a heat exchanger wherein the heat source is a combustion chamber of an internal combustion engine, the high temperature fluid is exhaust gas discharged from the combustion chamber, and the heat transfer plates support an exhaust gas purification catalyst. 
   In accordance with this arrangement, since the exhaust gas purification catalyst is supported on the heat transfer plates, when the exhaust gas discharged from the combustion chamber of the internal combustion engine carries out heat exchange with the heat medium via the heat transfer plates, the exhaust gas can be cleaned up by the exhaust gas purification catalyst. Moreover, since the heat transfer plates supporting the exhaust gas purification catalyst have a large surface area, the exhaust gas purification efficiency can be increased. 
   A combustion chamber  24  of an embodiment corresponds to the heat source of the present invention, first to third exhaust gas passages  87 ,  88 , and  89  of the embodiment correspond to the fluid passages of the present invention, and a water passage W 3  of the embodiment corresponds to the heat medium passage of the present invention. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  to  FIG. 23  show one embodiment of the present invention; 
       FIG. 1  is a diagram showing the overall arrangement of a Rankine cycle system; 
       FIG. 2  is a vertical sectional view of the surroundings of a cylinder head of an internal combustion engine; 
       FIG. 3  is an enlarged view of a part  3  in  FIG. 2 ; 
       FIG. 4  is a view from arrowed line  4 — 4  in  FIG. 2 ; 
       FIG. 5  is a sectional view along line  5 — 5  in  FIG. 4 ; 
       FIG. 6  is a sectional view along line  6 — 6  in  FIG. 4 ; 
       FIG. 7  is a partially cutaway perspective view of an independent exhaust port; 
       FIG. 8  is a view from arrow  8  in  FIG. 7 ; 
       FIG. 9  is a view from arrow  9  in  FIG. 8 ; 
       FIG. 10  is a view from arrow  10  in  FIG. 8 ; 
       FIG. 11A  and  FIG. 11B  are schematic views showing the flow of water in a grouped exhaust port; 
       FIG. 12  is an enlarged sectional view of an essential part in  FIG. 2 ; 
       FIG. 13  is a view from arrowed line  13 — 13  in  FIG. 12 ; 
       FIG. 14  is a view from arrow  14  in  FIG. 12 ; 
       FIG. 15  is a sectional view along line  15 — 15  in  FIG. 12 ; 
       FIG. 16  is an enlarged view of a part  16  in  FIG. 15 ; 
       FIG. 17  is a sectional view along line  17 — 17  in  FIG. 14 ; 
       FIG. 18  is a sectional view along line  18 — 18  in  FIG. 14 ; 
       FIG. 19  is a sectional view along line  19 — 19  in  FIG. 14 ; 
       FIG. 20  is a sectional view along line  20 — 20  in  FIG. 12 ; 
       FIG. 21  is a sectional view along line  21 — 21  in  FIG. 12 ; 
       FIG. 22  is a diagram showing the flow of water in a main evaporator; and 
       FIG. 23  is a diagram showing the flow of exhaust gas in the main evaporator. 
   

   BEST MODE FOR CARRYING OUT THE INVENTION 
   An embodiment of the present invention is explained below with reference to  FIG. 1  to  FIG. 23 . 
     FIG. 1  shows the overall arrangement of a Rankine cycle system to which the present invention is applied. 
   The Rankine cycle system, which recovers the thermal energy of an exhaust gas of an internal combustion engine E and converts it into mechanical energy, includes a main evaporator  11  that heats water with exhaust gas discharged from the internal combustion engine E so as to generate high temperature, high pressure steam, an expander  12  that is operated by the high temperature, high pressure steam generated by the main evaporator  11  so as to generate mechanical energy, a condenser  13  that cools decreased temperature, decreased pressure steam that has completed work in the expander  12  so as to turn it back into water, a reservoir tank  14  for collecting water discharged from the condenser  13 , and a supply pump  15  for pressurizing the water collected in the reservoir tank  14 . A portion of the water discharged from the supply pump  15  is supplied to the main evaporator  11 , which is provided downstream of an exhaust port  16  of the internal combustion engine E, turns into high temperature, high pressure steam in the main evaporator  11 , and is supplied to the expander  12 , and the rest of water discharged from the supply pump  15  is heated while passing through an auxiliary evaporator  17  provided on the outer periphery of the exhaust port  16 , and then merges into the main evaporator  11  at a predetermined position. 
   The main evaporator  11  carries out heat exchange mainly with the exhaust gas discharged from the exhaust port  16  and generates steam, but the auxiliary evaporator  17  carries out heat exchange not only with the exhaust gas flowing through the exhaust port  16  but also with the exhaust port  16  itself, which is in contact with a high temperature exhaust gas, thus generating steam and simultaneously cooling the exhaust port  16 . 
   As shown in  FIG. 2 , a cylinder head  20  and a head cover  21  are joined to a cylinder block  19  of the in-line four-cylinder internal combustion engine E, and four combustion chambers  24  are formed between the lower face of the cylinder head  20  and the upper face of each of four pistons  23  slidably fitted in four cylinder sleeves  22  housed in the cylinder block  19 . Formed in the cylinder head  20  are intake ports  26  and exhaust ports  16 , which communicate with the corresponding combustion chambers  24 . An intake valve seat  27  at the downstream end of the intake port  26  is opened and closed by a head  28   a  of an intake valve  28 , and an exhaust valve seat  29  at the upstream end of the exhaust port  16  is opened and closed by a head  30   a  of an exhaust valve  30 . Whereas the intake port  26  is formed directly in the cylinder head  20 , the exhaust port  16  is formed from four independent exhaust ports  16 A and one grouped exhaust port  16 B, each thereof being made of a member that is separate from the cylinder head  20  and fitted in the cylinder head  20 . 
   Supported on the cylinder head  20  are a single camshaft  31 , a single intake rocker arm shaft  32 , and a single exhaust rocker arm shaft  33 . One end of an intake rocker arm  34  rockably supported by the intake rocker arm shaft  32  abuts against an intake cam  35  provided on the camshaft  31 , and the other end thereof abuts against a stem  28   b  of the intake valve  28 , which is slidably supported by an intake valve guide  36  provided in the cylinder head  20  and is urged upward by a valve spring  37 . Furthermore, one end of an exhaust rocker arm  38  rockably supported by the exhaust rocker arm shaft  33  abuts against an exhaust cam  39  provided on the camshaft  31 , and the other end thereof abuts against the upper end of a stem  30   b  of the exhaust valve  30 , which is slidably supported by an exhaust valve guide  40  provided in the cylinder head  20  and is urged upward by a valve spring  41 . 
   The exhaust port  16  is formed from the four independent exhaust ports  16 A, which are positioned on the upstream side of the flow of exhaust gas, and the single grouped exhaust port  16 B, which communicates with the downstream side of the independent exhaust ports  16 A, and an end portion on the upstream side of the main evaporator  11  is fitted into the inside of the grouped exhaust port  16 B. The auxiliary evaporator  17  is provided so as to straddle the independent exhaust ports  16 A and the grouped exhaust port  16 B communicating with the downstream side thereof. 
   The structure of the independent exhaust ports  16 A is first explained in detail with reference to  FIG. 3  and  FIG. 7 . 
   The independent exhaust port  16 A is formed from a first port member  51 , a first cover member  52 , a second port member  53 , and a second cover member  54 . The first port member  51  and the first cover member  52  form an upstream portion  55  of the independent exhaust port  16 A that communicates with the combustion chamber  24 , and have a structure in which the first port member  51 , which is on the inside, is covered by the first cover member  52 , which is on the outside, and a labyrinth-shaped water passage W 2  is formed between the inner face of the first cover member  52  and a channel formed on the outer face of the first port member  51 . The lower faces of the first port member  51  and the first cover member  52  abut against the upper face of the exhaust valve seat  29 , which is formed in the cylinder head  20 , via a seal  56 . Moreover, an opening  51   a  through which the stem  30   b  of the exhaust valve  30  runs is formed in an upper wall of the first port member  51 , and the lower end of the exhaust valve guide  40  is fitted via a seal  57  in an opening  52   a  formed on an upper wall of the first cover member  52 . 
   The second port member  53  and the second cover member  54  form a downstream portion  58  of the independent exhaust port  16 A, which communicates with the grouped exhaust port  16 B, and have a structure in which the second port member  53 , which is on the inside, is covered by the second cover member  54 , which is on the outside, and the labyrinth-shaped water passage W 2  is formed between the inner face of the second cover member  54  and a channel formed on the outer face of the second port member  53 . An end portion of the second cover member  54  is fitted in an opening  52   b  formed in a side face of the first cover member  52 , thereby joining the first port member  51  and the second port member  53  smoothly so as to define a curved passage for the exhaust gas. The water passage W 2  defined by the second port member  53  and the second cover member  54  includes a water inlet  59  on the lower side thereof and a water outlet  60  on the upper side thereof. 
   The shape of the water passage W 2  of the independent exhaust port  16 A is now explained with reference to  FIG. 8  to  FIG. 10 . 
   The water passage W 2  is formed with lateral symmetry relative to a plane of symmetry P 1  of the independent exhaust port  16 A; immediately after the water inlet  59  the water passage W 2  branches into two lines so as to sandwich the plane of symmetry P 1  and the two lines merge again immediately before the water outlet  60 . To explain in more detail, the water passage W 2  extends linearly from the water inlet  59  along a lower face of the downstream portion  58  (part a), moves therefrom to the upstream portion  55 , extends in a semicircular shape around the head  30   a  of the exhaust valve  30  (part b), extends therefrom linearly upward along the stem  30   b  of the exhaust valve  30  up to the vicinity of the lower end of the exhaust valve guide  40  (part c), extends therefrom toward the head  30   a  of the exhaust valve  30  while bent in a zigzag shape (part d), returns therefrom back to the downstream portion  58 , and extends toward the water outlet  60  while bent in a zigzag shape (part e). 
   The structure of the grouped exhaust port  16 B is now explained in detail with reference to  FIG. 2  to  FIG. 6 . 
   The grouped exhaust port  16 B includes a rectangular frame-shaped flange  61 , and by tightening a plurality of bolts  62  running through a flange  11   a  of the main evaporator  11  to the cylinder head  20  the main evaporator  11  and the grouped exhaust port  16 B are together secured to the cylinder head  20  (see  FIG. 2 ). The downstream end of a pressed sheet material third port member  63  is welded to the flange  61  of the grouped exhaust port  16 B, and four openings  63   a  formed in the upstream end of the third port member  63  communicate with exits of the four independent exhaust ports  16 A. The downstream end of a pressed sheet material fourth port member  64  is welded to an inner face of the third port member  63 , and the upstream end of the fourth port member  64  is superimposed on the four openings  63   a  of the third port member  63  and welded. The exhaust gases discharged from the four independent exhaust ports  16 A are therefore merged in the grouped exhaust port  16 B, and guided evenly to the main evaporator  11 . 
   Water passages W 1 , which are formed from a pipe material, are disposed in a space surrounded by the third port member  63  and the fourth port member  64  of the grouped exhaust port  16 B. Since the water passages W 1  have a symmetrical structure relative to a plane of symmetry P 2 ,  FIG. 4  to  FIG. 6 ,  FIG. 11A , and  FIG. 11B  show the water passage W 1  on one side of the plane of symmetry P 2 . The water passage W 1  has a first line passing through the independent exhaust port  16 A( 1 ) on the side close to the plane of symmetry P 2  and a second line passing through the independent exhaust port  16 A( 2 ) on the side far from the plane of symmetry P 2 . 
   That is, the water passage W 1  starting at a water inlet  65  provided on an end portion of the flange  61  extends linearly along an inner face of the fourth port member  64  (part f), and extends linearly therefrom along an inner face of the third port member  63  (part g). A coupling  66  is provided in the part Q, and the water inlet  59  of the independent exhaust port  16 A( 1 ) is connected to this coupling  66 . The water passage W 1  extending from a coupling  67  to which the water outlet  60  of the independent exhaust port  16 A( 1 ) is connected extends linearly along the inner face of the third port member  63  (part h), extends therefrom along the inner face of the third port member  63  in a zigzag shape (part i), extends linearly therefrom along the inner face of the third port member  63  (part j), turns downward through 90°, and communicates with the water outlet  68 . The water outlet  68  communicates with an intermediate portion of the main evaporator  11  via a connecting pipe  106 , which will be described later. 
   The water passage W 1  extending through the coupling  66  further extends along the inner face of the third port member  63  in a zigzag shape (part k), extends linearly along the inner face of the fourth port member  64  (part m), turns through 90°, extends linearly (part n), further turns through 90°, extends linearly along the inner face of the third port member  63  (part o), and is connected to the water inlet  59  of the independent exhaust port  16 A( 2 ) via a coupling  69  provided therein. A coupling  70  to which the water outlet  60  of the independent exhaust port  16 A( 2 ) is connected merges with the part j of the water passage W 1 . 
   The structure of the main evaporator  11  is now explained in detail with reference to  FIG. 12  to  FIG. 21 . 
   The main evaporator  11 , which communicates with the downstream side of the auxiliary evaporator  17 , has a casing  81  fixed to its flange  11   a , the cross section of the casing  81  being substantially rectangular, and an exhaust exit  11   b  communicating with an exhaust pipe  82  (see  FIG. 13 ) is formed on a lower face of the casing  81 . A large number of thin metal heat transfer plates  83  are disposed parallel to each other at a predetermined pitch within the casing  81 . An exhaust gas purification catalyst for cleaning up the exhaust gas is supported on the surface of all of the heat transfer plates  83 . 
   As is clear from  FIG. 16 , the heat transfer plate  83  is formed from a first heat transfer plate  83 ( 1 ) and a second heat transfer plate  83 ( 2 ) having plane-symmetric concavoconvex portions, and they are alternately superimposed. The first heat transfer plate  83 ( 1 ) and the second heat transfer plate  83 ( 2 ) thus make contact and are brazed to each other at abutment sections  84  and  85 , and a partition wall  86  for blocking the circulation of exhaust gas is formed in this section. 
   The partition wall  86  is arranged in the shape shown in  FIG. 12 , and forms a bent exhaust gas passage between adjacent heat transfer plates  83 . The exhaust gas passage is formed from a first exhaust gas passage  87 , a second exhaust gas passage  88 , and a third exhaust gas passage  89 , the first exhaust gas passage  87  communicating with the downstream end of the auxiliary evaporator  17  and extending linearly in a direction away from the cylinder head  20 , the second exhaust gas passage  88  bending through 180° at the downstream end of the first exhaust gas passage  87  and extending linearly toward the cylinder head  20 , and the third exhaust gas passage  89  bending through 180° at the downstream end of the second exhaust gas passage  88 , extending in a direction away from the cylinder head  20 , further bending through 90°, and extending downward so as to form an overall L-shape. An exhaust gas-combining section  81   a  formed within the casing  81 , which the downstream end of the third exhaust gas passage  89  faces, is connected to the exhaust pipe  82  via the exhaust exit  11   b . Furthermore, a gap  86   a  is provided by cutting away a portion of the partition wall  86  of the heat transfer plate  83  on the high temperature side of the first, second, and third exhaust gas passages  87 ,  88 , and  89 , thereby blocking heat transfer from a high temperature portion to a low temperature portion of the heat transfer plates  83  and enabling the high temperature portion and the low temperature portion to be maintained at desired temperatures. 
   A large number of pipe members  90 , through which water circulates, run through all the heat transfer plates  83  and are joined integrally thereto by brazing so that heat transfer is possible therebetween. 
   As is clear by referring additionally to  FIG. 12 ,  FIG. 15 , and  FIG. 19 , an oxygen concentration sensor  91  is mounted on the middle of the lower face of the main evaporator  11 , and a detection portion  91   a  at the extremity thereof faces the first exhaust gas passage  87 . Provided on the lower face of the main evaporator  11  on which the oxygen concentration sensor  91  is mounted is an oxygen concentration sensor cooling portion  92 , which is partitioned off beneath the first exhaust gas passage  87  via the partition wall  86 . A flat upper face of the oxygen concentration sensor cooling portion  92  faces the first exhaust gas passage  87  via the partition wall  86 , and a lower face thereof faces the atmosphere via the casing  81 . The oxygen concentration sensor cooling portion  92  includes a plurality of pipe members  93 , which run through the heat transfer plates  83  and are joined thereto by brazing. 
   As is most clearly shown in  FIG. 15 , left and right headers  96 L and  96 R are provided at longitudinally opposite ends of the casing  81  of the main evaporator  11 , the left and right headers  96 L and  96 R being formed by integrally connecting inner plates  94  and outer plates  95  with a predetermined gap therebetween. Each of the headers  96 L,  96 R has the inner plate  94  thereof superimposed on the heat transfer plate  83  that is layered on the outermost side. A water inlet pipe  97  communicating with the downstream side of the supply pump  15  runs through a rear face of the casing  81  of the main evaporator  11 , reaches the outer face of the outer plate  95  of the left-hand (when facing the cylinder head  20 ) header  96 L, and is connected via a bifurcated coupling  98  to two of the pipe members  90  positioned at the downstream end of the third exhaust gas passage  89 . 
   These two pipe members  90  form the beginning of two lines of water passages W 3 , and adjacent pipe members  90  of each line are sequentially connected via U-shaped couplings  99  in the left and right headers  96 L and  96 R, thus forming the water passages W 3  in a zigzag shape. As is clear from  FIG. 22 , the direction of flow of water in the water passages W 3  is opposite to the direction of flow of exhaust gas, which is in the direction first exhaust gas passage  87 →second exhaust gas passage  88 →third exhaust gas passage  89 , that is, the flow of water is from the third exhaust gas passage  89  to the first exhaust gas passage  87  via the second exhaust gas passage  88 . That is, the exhaust gas and the water have a countercurrent arrangement. 
   As is clear from  FIG. 12 , the density of the pipe members  90  is the most sparse in the first exhaust gas passage  87 , which is on the upstream side of the flow of exhaust gas, moderate in the second exhaust gas passage  88 , which is in the middle, and the most dense in the third exhaust gas passage  89 , which is on the downstream side. 
   As is clear from  FIG. 15 , a water inlet pipe  100  communicating with the downstream side of the supply pump  15  runs through a rear face of the casing  81  of the main evaporator  11  and reaches an outer face of the outer plate  95  of the left-hand (when facing the cylinder head  20 ) header  96 L, and is connected to two of the pipe members  93  via a bifurcated coupling  101 . These two pipe members  93  form the beginning of two lines of water passages W 4 , and adjacent pipe members  93  of each line are connected via U-shaped couplings  102  in the left and right headers  96 L and  96 R and via five couplings  103  in a space surrounding the oxygen concentration sensor  91 , thus forming the water passages W 4  in a zigzag shape. The downstream ends of the two line water passages W 4  communicate via couplings  104  and connecting pipes  105  with the water inlets  65  (see  FIG. 5 ) of the auxiliary evaporator  17  formed within the flanges  11   a  and  61 . 
   As shown in  FIG. 17 ,  FIG. 18 , and  FIG. 21 , the two connecting pipes  106  communicating with the water outlets of the water passages W 2  of the auxiliary evaporator  17  extend to the exterior of the casing  81  through the outside of the headers  96 L and  96 R, bend through 180°, re-enter the interior of the casing  81 , and are connected to the pipe members  90  of the cooling water passages W 3  via bifurcated couplings  107  provided in the headers  96 L and  96 R. The position of the pipe members  90  connected to the connecting pipes  106  is in the vicinity of the upstream end of the second exhaust gas passage  88  as shown by the reference numerals  90 ( 1 ) and  90 ( 2 ) in  FIG. 22 . 
   As shown in  FIG. 21 , in the right-hand header  96 R, the two pipe members  90  (shown by the reference numerals  90 ( 3 ) and  90 ( 4 ) in  FIG. 22 ) positioned at the downstream end of the water passages W 3  are connected via a bifurcated coupling  108  to a water outlet pipe  109  that communicates with the expander  12 . 
   The operation of the embodiment of the present invention having the above-mentioned arrangement is now explained. 
   In  FIG. 1 , a portion of the water discharged from the supply pump  15  of the Rankine cycle system is supplied to the main evaporator  11 , which is provided downstream of the exhaust port  16  of the internal combustion engine E, and the rest of the water discharged from the supply pump  15  passes through the auxiliary evaporator  17  provided on the outer periphery of the exhaust port  16  and merges into the main evaporator  11  at a predetermined position. 
   The operation in the main evaporator  11  is first explained. A portion of the low temperature water discharged from the supply pump  15  flows to the left header  96 L of the casing  81  of the main evaporator  11  via the water inlet pipe  97  (see  FIG. 15 ), and the flow is divided into the two lines of water passages W 3  via the coupling  98 . Each of the water passages W 3  is formed from the large number of pipe members  90  connected in a zigzag shape, and carries out heat exchange with the exhaust gas passing through the gaps between the large number of heat transfer plates  83 , through which the pipe members  90  run, thereby depriving the exhaust gas of thermal energy and increasing in temperature. The two pipe members  90  at the downstream end of the two lines of water passages W 3  are merged with the water outlet pipe  109  (see  FIG. 21 ) via the coupling  108 . The water is heated and turns into high temperature, high pressure steam while flowing through the water passages W 3 , and is supplied to the expander  12 . 
   Since the heat of the exhaust gas is transferred from the large number of heat transfer plates  83 , which have a large surface area and are arranged at a small pitch, to the water flowing through the large number of pipe members  90 , it is possible to ensure that there is a sufficient area of heat exchange between the exhaust gas and the water. Accordingly, even when the flow rate of the exhaust gas is reduced, that is, when the cross-sectional area of the exhaust gas flow path in the main evaporator  11  is increased, sufficient heat exchange efficiency can be obtained, and suppressing an increase in the back pressure of the exhaust passage can prevent any decrease in the output of the internal combustion engine E. Furthermore, when pressing the heat transfer plates  83 , the partition wall  86  can be provided in any shape by forming only the abutment sections  84  and  85 , and the first to third exhaust gas passages  87 ,  88 , and  89 , which are bent, can be formed without employing any special component for providing the partition wall  86 . 
   Moreover, as is clear from  FIG. 22  and  FIG. 23 , whereas the exhaust gas flows from the first exhaust gas passage  87  to the third exhaust gas passage  89  via the second exhaust gas passage  88 , water within the water passages W 3  flows from the third exhaust gas passage  89  to the first exhaust gas passage  87  via the second exhaust gas passage  88  so as to oppose the direction of flow of the exhaust gas, thereby ensuring that there is sufficient temperature difference between the water and the exhaust gas along the whole length of the water passages W 3  and improving the heat exchange efficiency of the main evaporator  11 . 
   Furthermore, since the density of the pipe members  90  is low in the first exhaust gas passage  87 , which is on the upstream side of the flow of the exhaust gas, and the density of the pipe members  90  gradually increases therefrom toward the third exhaust gas passage  89 , which is on the downstream side, by reducing the density of the pipe members  90  in the upstream section where the exhaust gas has a high temperature and a large volume and the flow rate is high it is possible to minimize the pressure loss due to impingement of the exhaust gas on the pipe members  90 , and by increasing the density of the pipe members  90  in the downstream section where the exhaust gas has a low temperature and a small volume and the flow rate is low it is possible to ensure that there is sufficient contact between the exhaust gas and the pipe members  90  and improve the heat exchange efficiency. 
   Moreover, since the exhaust gas purification catalyst is supported on the heat transfer plates  83 , which have a large surface area, it is possible to ensure that the exhaust gas makes sufficient contact with the exhaust gas purification catalyst, thereby cleaning the exhaust gas effectively. 
   The rest of the low temperature water discharged from the supply pump  15  enters the interior of the left-hand header  96 L of the casing  81  of the main evaporator  11  via the water inlet pipe  100  (see  FIG. 15 ), and the flow thereof is divided into the two lines of water passages W 4  via the coupling  101 . Water that has flowed in a zigzag shape through the interior of the pipe members  93  forming each of the water passages W 4  is first merged in the H-shaped coupling  103  in the vicinity of the oxygen concentration sensor  91 , is then divided again, further flows in a zigzag shape through the interior of the pipe members  93 , then flows from the left and right headers  96 L and  96 R through the couplings  104 , the connecting pipes  105 , and the water inlets  65 , and is then supplied to the auxiliary evaporator  17 . 
   In this way, since the surroundings of the oxygen concentration sensor  91 , which passes through the oxygen concentration sensor cooling portion  92 , are cooled by the low temperature water flowing through the water passages W 4 , the heat of the high temperature exhaust gas flowing through the first exhaust gas passage  87 , which the detection portion  91   a  of the oxygen concentration sensor  91  faces, can be prevented from escaping to the outside of the main evaporator  11  via the oxygen concentration sensor  91 , thereby improving the efficiency of recovery of waste heat of the internal combustion engine E. 
   Furthermore, since the first exhaust gas passage  87  and the second exhaust gas passage  88 , which are positioned on the upstream side of the flow of exhaust gas and through which the high temperature exhaust gas flows, are disposed in a radially inner portion of the main evaporator  11 , the third exhaust gas passage  89 , which is positioned on the downstream side of the flow of exhaust gas and to which water having the lowest temperature is supplied, is disposed in a radially outer portion of the main evaporator  11 , and the oxygen concentration sensor cooling portion  92 , to which water having the lowest temperature is supplied, is disposed in a radially outer portion of the main evaporator  11 , that is, since the outsides of the first exhaust gas passage  87  and the second exhaust gas passage  88 , which reach a high temperature due to the passage of high temperature exhaust gas, are surrounded by the third exhaust gas passage  89  and the oxygen concentration sensor cooling portion  92 , which reach a low temperature due to the passage of low temperature water, it is possible to minimize the dissipation of thermal energy to the outside of the main evaporator  11 , thereby improving the waste heat recovery efficiency. 
   A gap that maintains an air layer is formed between the inner periphery of the casing  81  and the outer periphery of the heat transfer plates  83 , and the heat insulating effect of this air layer can further reduce the dissipation of thermal energy to the outside of the main evaporator  11 . 
   The operation in the auxiliary evaporator  17  is now explained. In  FIG. 11A  and  FIG. 11B , water discharged from the oxygen concentration sensor cooling portion  92  flows into the water passage W 1  from the water inlet  65  of the grouped exhaust port  16 B and the flow is divided into the first line and the second line. The first line shown in  FIG. 11A  has a route that reaches the water outlet  68  via the part f and the part g of the water passage W 1 , the coupling  66 , the water passage W 2  of the independent exhaust port  16 A( 1 ), the coupling  67 , and the part h, the part i, and the part j of the water passage W 1 . On the other hand, the second line shown in  FIG. 11B  has a route that reaches the water outlet  68  via the part f, the part g, the part k, the part m, the part n, and the part o of the water passage W 1 , the coupling  69 , the water passage W 2  of the independent exhaust port  16 A( 2 ), and the part j of the water passage W 1 . Since in the first line the first half of the water passage W 1  is short and the second half thereof is long, and in the second line the first half of the water passage W 1  is long and the second half thereof is short, the overall length of the water passage W 1  in the two lines is equalized, thus making the amount supplied substantially equal, preventing an imbalance in the waste heat recovery, and improving the heat exchange efficiency. 
   As shown in  FIG. 7  to  FIG. 10 , the structures of the water passages W 2  provided in the two independent exhaust ports  16 A( 1 ) and  16 A( 2 ) are identical, and water supplied from the water inlet  59  branches so as to sandwich the plane of symmetry P 1 , passes through the part a, the part b, the part c, the part d, and the part e, is merged, and is then discharged via the water outlet  60 . 
   In this way, since the auxiliary evaporator  17  is arranged so that the surroundings of the exhaust port  16 , which reach a high temperature due to the passage of exhaust gas, are surrounded by the water passages W 1  and W 2 , the exhaust gas heat dissipated from the exhaust port  16  via the cylinder head  20  can be recovered effectively as high temperature, high pressure steam. In particular, since the water supplied to the water passages W 1  and W 2  is comparatively low temperature water that has only passed through the oxygen concentration sensor cooling portion  92  after being discharged from the supply pump  15 , the surroundings of the exhaust port  16  can be cooled effectively, and high temperature, high pressure steam can be generated, thus enhancing the waste heat recovery effect of the internal combustion engine E. Furthermore, although the heat of exhaust gas easily escapes to the outside via the exhaust valve  30 , intensive cooling, with low temperature water, of the section that requires cooling of the internal combustion engine E, that is, the exhaust valve seat  29 , with which the head  30   a  of the exhaust valve  30  makes contact, and the vicinity of the exhaust valve guide  40 , with which the stem  30   b  of the exhaust valve  30  makes contact, enables the escape of heat via the exhaust valve  30  to be suppressed, thus further enhancing the waste heat recovery effect, and enables thermal expansion of the exhaust valve  30 , the exhaust valve seat  29 , and the exhaust valve guide  40 , etc. to be suppressed, thus maintaining dimensional and positional precision and thereby maintaining desired functions thereof. 
   Water that has passed through the auxiliary evaporator  17  passes from the connecting pipes  106  (see  FIG. 17  and  FIG. 18 ) through the couplings  107  (see  FIG. 20 ) provided in the left and right headers  96 L and  96 R, and is merged in the pipe members  90  of the second exhaust gas passage  88  of the main evaporator  11 . In this arrangement, by making, in the merging section, the temperature of water passing through the water passages W 3  on the main evaporator  11  side substantially equal to the temperature of water supplied from the auxiliary evaporator  17 , the waste heat recovery effect can be further improved. This control of water temperature can be carried out by regulating the flow rate ratio when splitting the flow of water discharged from the supply pump  15  into the main evaporator  11  side and the auxiliary evaporator  17  side. 
   Although an embodiment of the present invention is explained in detail above, the present invention can be modified in a variety of ways without departing from the spirit and scope of the present invention. 
   For example, in the embodiment the heat exchanger is exemplified by the main evaporator  11 , but the heat exchanger of the present invention is not limited to an evaporator. 
   Furthermore, in the embodiment water is illustrated as the heat medium, but the heat medium of the present invention is not limited to water. 
   INDUSTRIAL APPLICABILITY 
   Although the present invention can be suitably applied to an evaporator for a Rankine cycle system and, in particular, to an evaporator for a Rankine cycle system that recovers the thermal energy of the exhaust gas of an internal combustion engine of an automobile and converts it into mechanical energy, the present invention can also be applied to a heat exchanger for any purpose.