Patent Publication Number: US-10767605-B2

Title: Heat exchanger

Description:
TECHNICAL FIELD 
     The present disclosure relates to a heat exchanger that exchanges heat between gas and a cooling medium. 
     BACKGROUND ART 
     Patent Literatures 1 and 2 disclose heat exchangers. Hereinafter, the heat exchangers described in Patent Literatures 1 and 2 will be briefly explained with the reference signs used in Patent Literatures 1 and 2 being given in parentheses. 
     In the heat exchanger described in Patent Literature 1, flat rectangular-tubular tubes ( 110 ) are stacked, and gas passes through inside the tubes ( 110 ). Protruding portions ( 112 ) are formed at outer edges of a bonding surface of the tube ( 110 ), and the protruding portions ( 112 ) of the tubes ( 110 ) adjacent to each other are joined together such that a flow path ( 115 ) surrounded with the protruding portions ( 112 ) is formed between the adjacent tubes ( 110 ). The protruding portions ( 112 ) are not formed in four portions ( 113   a ,  113   b ) in the outer edges of the bonding surface of the tube ( 110 ), and these portions ( 113   a ,  113   b ) form opening portions in which two opening portions ( 113   a ) serve as entrances to the flow path ( 115 ) and the other two opening portions ( 113   b ) serve as exits from the flow path ( 115 ). The stacked body of the tubes ( 110 ) is housed in a tubular water tank ( 130 ), and the tubular water tank ( 130 ) bulges out around the opening portions ( 113   a ) serving as the entrances. A pipe hole ( 132   d ) is formed in a part facing the opening portions ( 113   a ) of a bulging portion ( 132   b ), and cooling water is introduced into the bulging portion ( 132   b ) through the pipe hole ( 132   b ). Accordingly, the cooling water flows from the bulging portion ( 132   b ) to the flow paths ( 115 ) through the opening portions ( 113   a ). 
     In the heat exchanger described in Patent Literature 2, flat rectangular-tubular tubes ( 110 ) are stacked, and gas passes through inside the tubes ( 110 ). Protruding portions ( 112 ) are formed at outer edges of a bonding surface of the tube ( 110 ), and the protruding portions ( 112 ) of tubes ( 110 ) adjacent to each other are joined together such that a flow path ( 113 ) surrounded by the protruding portions ( 112 ) is formed between the adjacent tubes ( 110 ). The protruding portions ( 112 ) are not formed in two portions ( 113   a ,  113   b ) in the outer edges of the bonding surface of the tube ( 110 ), and these portions ( 113   a ,  113   b ) form opening portions in which one opening portion ( 113   a ) serves as an entrance to the flow path ( 113 ) and the other opening portion ( 113   b ) serves as an exit from the flow path ( 113 ). The stacked body of the tubes ( 110 ) is housed in a tubular water tank ( 130 ). An end portion of the stacked body of the tubes ( 110 ) is fitted in an opening portion ( 146 ) of an inner gas tank ( 140 B), and an outer peripheral surface of the end portion is joined to an inner peripheral surface of the opening portion ( 146 ) of the inner gas tank ( 140 B). This allows gas introduced into the inner gas tank ( 140 B) to flow into the tubes ( 110 ). The inner gas tank ( 140 B) is housed in an outer tank ( 140 A), and cooling water is introduced into the outer tank ( 140 A). A joint part at which the stacked body of the tubes ( 110 ) and the inner gas tank ( 140 B) are joined together is arranged in an opening of the outer tank ( 140 A). The opening of the outer tank ( 140 A) is connected with an opening of the tubular water tank ( 130 ). For the cooling water introduced into the outer tank ( 140 A), a flow path ( 150 ) is formed between the outer surfaces of the inner gas tank ( 140 B) and the stacked body of the tubes ( 110 ) and the inner surfaces of the outer tank ( 140 ) and the tubular water tank ( 130 ), and the cooling water introduced into the outer tank ( 140 A) flows into the above-described opening portions ( 113   a ) through the flow path ( 150 ). Accordingly, the cooling water flows into the flow paths ( 113 ) each between the tubes ( 110 ) adjacent to each other. 
     CITATION LIST 
     Patent Literature 
     [PTL 1] Japanese Patent No. 5500399 
     [PTL 2] Japanese Patent Application Publication No. 2014-169857 
     SUMMARY 
     Technical Problem 
     However, in the heat exchanger described in Patent Literature 1, the cooling water having passed through the opening portions ( 113   a ) close to the pipe hole ( 132   d ) is likely to stagnate around the opening portions ( 113   a ) on the opposite side. Also in the heat exchanger described in Patent Literature 2, the cooling water flowing into the flow paths ( 113 ) from the opening portions ( 113   a ) is likely to stagnate in a part distant from the openings ( 113   a ). 
     Accordingly, both of the heat exchangers of Patent Literatures 1 and 2 have a risk that the stagnated cooling water is heated and boiled by the heat of the gas to damage the heat exchanger due to boiling. 
     The present disclosure has been achieved in view of the above-described circumstances. An issue to be solved by the present disclosure is to prevent stagnation of a cooling medium such as cooling water. 
     Solution to Problem 
     A main aspect of the present disclosure for achieving an object described above is a heat exchanger, comprising: a stack formed by stacking a plurality of tubes through which gas flows; a tubular inner tank in which the stack is housed; and a tubular outer tank that is mounted on the outside of the inner tank so as to define an inner space between the outer tank and an outer peripheral surface of the inner tank, wherein each of both end portions of the tubes has a thickness greater than each of middle portions of the tubes, the both end portions of the tubes adjacent to each other in the stack are joined together so as to form a clearance between the middle portions of the adjacent tubes in the stack, outer peripheries of both end portions of the stack are joined to an inner peripheral surface of the inner tank, an introduction hole for introducing a cooling medium is formed in the outer tank, a discharge hole for discharging the cooling medium is formed at a location between the both end portions of the tubes in the inner tank, and a communication hole allowing the clearance and the inner space to communicate with each other is formed in each of both side surfaces of the inner tank positioned inside the outer tank. 
     According to the above, since a tubular outer tank defines an inner space between an outer peripheral surface of an inner tank and an inner peripheral surface of the outer tank, a cooling medium flowing into the inner space through an introduction hole easily reach the whole inner space. In addition, since the cooling medium having flown into the inner space flows into a clearance between the middle portions of tubes adjacent to each other from communication holes formed in both sides of the clearance. Accordingly, the cooling medium is not stagnated in the clearance between the middle portions of the tubes. 
     Advantageous Effects 
     According to the present disclosure, it is possible to inhibit stagnation of a cooling medium. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a plan view illustrating a heat exchanger. 
         FIG. 2  is a right side view illustrating a heat exchanger. 
         FIG. 3  is a cross-sectional view taken along line III-III of  FIG. 1 . 
         FIG. 4  is a cross-sectional view taken along line IV-IV of  FIG. 2 . 
         FIG. 5  is a cross-sectional view taken along line V-V of  FIG. 2 . 
         FIG. 6  is an exploded perspective view illustrating a heat exchanger. 
         FIG. 7  is an exploded perspective view illustrating a heat exchanger. 
         FIG. 8  is an exploded perspective view illustrating a tube and an inner fin. 
         FIG. 9  is an enlarged view illustrating a IX region of  FIG. 3 . 
         FIG. 10  is an exploded perspective view illustrating a heat exchanger of a comparative example. 
         FIG. 11  is a cross sectional view of a heat exchanger of a comparative example. 
         FIG. 12  is a cross sectional view illustrating a heat exchanger of a comparative example. 
         FIG. 13  is a graph for comparing an analysis result of an embodiment with an analysis result of a comparative example. 
         FIG. 14  is a graph for comparing an analysis result of an embodiment with an analysis result of a comparative example. 
         FIG. 15  is a side view illustrating an inner tank of a heat exchanger in a first modification. 
         FIG. 16  is a side view illustrating an inner tank of a heat exchanger in a second modification. 
         FIG. 17  is a side view illustrating an inner tank of a heat exchanger in a third modification. 
         FIG. 18  is a side view illustrating an inner tank of a heat exchanger in a fourth modification. 
         FIG. 19  is a side view illustrating an inner tank of a heat exchanger in a fifth modification. 
         FIG. 20  is a side view illustrating an inner tank of a heat exchanger in a sixth modification. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     An embodiment of the present disclosure will be described below with reference to the drawings. Various limitations that are technically preferable to implement the present disclosure are made in the embodiment which will be described later, however, they are not intended to limit the scope of the present disclosure to the following embodiment and the illustrated examples. 
     1. Configuration of Heat Exchanger 
       FIG. 1  is a plan view illustrating a heat exchanger  1 , and  FIG. 2  is a side view illustrating the heat exchanger  1 .  FIGS. 3, 4, and 5  are a cross-sectional view taken along line III-III, a cross-sectional view taken along line IV-IV, and a cross-sectional view taken along line V-V, respectively.  FIGS. 6 and 7  are exploded perspective views illustrating the heat exchanger  1 . 
     The heat exchanger  1  is provided in an exhaust gas recirculation system, for example, and used as a gas cooler. Specifically, exhaust gas from an internal combustion engine such as a diesel engine and a gasoline engine is cooled by the heat exchanger  1  and then supplied again to the inlet side of the internal combustion engine. 
     As illustrated in  FIGS. 1 to 7 , the heat exchanger  1  includes plural tubes  10 , plural inner fins  18 , an inner tank  20 , an entrance tank  30 , an exit tank  40 , an outer tank  50 , an inlet pipe  60 , and an outlet pipe  70 . A material of these members  10 ,  18 ,  20 ,  30 ,  40 ,  50 ,  60 , and  70  is a SUS material and the like, for example, and these members  10 ,  18 ,  20 ,  30 ,  40 ,  50 ,  60 , and  70  have high heat conductivity. Joint parts which will be described later are joined by welding or brazing, for example. 
     In the following descriptions, the side of the entrance tank  30  refers to the “front side”, the side of the exit tank  40  refers to the “rear side”, the side to which the inlet pipe  60  and the outlet pipe  70  protrude refers to the “upper side”, the side opposite thereto refers to the “lower side”, and the right side and the left side when viewing from the front side to the rear side refer to the “right side” and the “left side”, respectively. Note that, the direction from the upper side to the lower side is not necessarily the direction of gravity. 
     1-1. Tube and Inner Fin 
       FIG. 8  is an exploded perspective view illustrating the tube  10  and the inner fin  18 . As illustrated in  FIGS. 4 and 8 , the tube  10  is formed in a tubular shape that has a flat rectangular-shaped cross-section orthogonal to the longitudinal direction (front-rear direction) of the tube  10 , and the width (right-left length) of the tube  10  is greater than the thickness (top-bottom length) of the tube  10 . Specifically, the tube  10  is configured such that two tube plates  10 A,  10 B each having a square U-shaped (U-shaped, groove-shaped) cross-section formed by presswork, rolling processing, and/or the like are joined together with their openings facing each other. The inner space of the tube  10  forms a flow path through which the gas flows. 
     A wavy inner fin  18  is disposed inside the tube  10 , and the inner fin  18  and the inner surfaces of the tube  10  are joined together. In this embodiment, the inner fin  18  is an offset fin; however, the inner fin  18  may be a corrugated fin, a wavy fin, or a louver fin. 
     As illustrated in  FIGS. 6 and 7 , a front end portion  11  and a rear end portion  12  of the tube  10  has a thickness (top-bottom direction) greater than a middle portion  13  located therebetween. Thus, upper surfaces and lower surfaces of the both end portions  11  and  12  of the tube  10  bulge out more than the upper surface and the lower surface of the middle portion  13 , and the upper surface and the lower surface of the middle portion  13  are recessed. Plural protruding portions  14  are formed on the upper surface and the lower surface of the middle portion  13  of the tube  10 , and the back sides of the protruding portions  14  are formed such that corresponding parts on the inner surface of the tube  10  are recessed. 
     As illustrated in  FIGS. 4 to 7 , these tubes  10  are stacked in the thickness direction (top-bottom direction). In the tubes  10  adjacent to each other, the lower surface of the upper tube  10  and the upper surface of the lower tube  10  face each other. The end portions  11  of the adjacent tubes  10  are joined together and the end portions  12  of the adjacent tubes  10  are joined together, while the middle portions  13  of the adjacent tubes  10  (in the parts thereof except the protruding portions  14 ) are apart from each other in the top-bottom direction. Thus, a clearance  91  is formed between the middle portions  13  of the adjacent tubes  10 , and the clearance  91  forms a flow path that allows a coolant (cooling liquid) to flow therethrough. 
     Hereinafter, the stack body of the tubes  10  is referred to as a tube stack  19 . 
     1-2. Inner Tank 
     As illustrated in  FIGS. 4 to 7 , the inner tank  20  is formed in a rectangular-tubular shape. The inner tank  20  is a joined body including two half bodies  20 A and  20 B. Specifically, the half bodies  20 A and  20 B are each formed to have a square-U shaped (U shaped, groove shaped) cross-section by presswork, rolling processing, and/or the like, and the half bodies  20 A and  20 B are joined together in a state where the openings of the half bodies  20 A and  20 B face each other and the lower end portion of the upper half body  20 A nests in the upper end portion of the lower half body  20 B. 
     The inner tank  20  houses a tube stack  19 . A front end portion  21  and a rear end portion  22  of the inner tank  20  are open, the inner peripheral surface of the front end portion  21  is joined to the entire periphery of the outer peripheral surface of the front end portion in the tube stack  19 , and the inner peripheral surface of the rear end portion  22  is joined to the entire periphery of the outer peripheral surface of the rear end portion in the tube stack  19 . The upper surface of the middle portion  13  of the uppermost tube  10  is partially apart from the inner surface of the inner tank  20   s  as to form a clearance  92  therebetween. This clearance  92  forms a flow path that allows the coolant to flow therethrough. Likewise, the lower surface of the middle portion  13  of the lowermost tube  10  is partially apart from the inner surface of the inner tank  20  so as to form a clearance  93  therebetween. This clearance  93  forms a flow path through which the coolant flows. 
     Plural communication holes  24  are formed in the front part of the upper surface of the inner tank  20 , and plural communication holes  25  are formed in the front part of the lower surface of the inner tank  20 . Plural communication holes  26  are formed in the front part of the left side surface of the inner tank  20 , and plural communication holes  27  are formed in the front part of the right side surface of the inner tank  20 . 
     These communication holes  24  to  27  are arranged in a peripheral direction at slightly rear of the joint part of the front end portion of the tube stack  19  and the front end portion  21  of the inner tank  20 . 
     As illustrated in  FIGS. 1 to 3 and 5 , a bulging portion  23  bulging outward is formed on rear parts of the upper surface, left side surface, and lower surface of the inner tank  20 . The bulging portion  23  is arranged on the front side relative to the joint part of the rear end portions  12  of the tubes  10  and the rear end portion  22  of the inner tank  20 . A distance between the inner surface of the bulging portion  23  and the outer surface of the tube stack  19  is greater than a distance between the inner surface of the inner tank  20  other than the bulging portion  23  and the outer surface of the tube stack  19 . 
     A discharge hole  29  is formed in the upper surface of the bulging portion  23 . The discharge hole  29  is arranged close to the left edge of the upper surface of the bulging portion  23 . Thus, as illustrated in  FIGS. 1 and 5 , the discharge hole  29  partially protrudes to the left from the left side-surface of the tube stack  19 , and the left side-surface of the middle portion  13  of the tube  10  extends in the front-rear direction across the discharge hole  29  when viewed from above. 
     1-3. Outlet Pipe 
     As illustrated in  FIGS. 1, 5 , and the like, the outlet pipe  70  is coupled to the discharge hole  29  of the inner tank  20 . The outlet pipe  70  protrudes upward from the upper surface of the inner tank  20 . 
     1-4. Entrance Tank 
     As illustrated in  FIGS. 1 to 3, 6, and 7 , the entrance tank  30  is formed in a hollow pyramid shape. The front-side top portion of the entrance tank  30  is open, and a rear-side bottom portion of the entrance tank  30  is open as well. The exhaust gas from the internal combustion engine is introduced into the entrance tank  30  through a front-side opening  31  of the entrance tank  30 . 
       FIG. 9  is an enlarged view illustrating the IX region of  FIG. 3 . As illustrated in  FIGS. 3 and 9 , the inner peripheral surface of a rear end portion  32  of the entrance tank  30  is joined to the outer peripheral surface of the front end portion  21  of the inner tank  20 , in a state where the front end portion  21  of the inner tank  20  nests in the rear end opening of the entrance tank  30 . 
     Note that a flange (not shown) is mounted to the outer peripheral portion of the front end portion of the entrance tank  30 . 
     1-5. Outer Tank 
     As illustrated in  FIGS. 1 to 4, 6, and 7 , the outer tank  50  is formed in a rectangular-tubular shape. The outer tank  50  is a joined body including two half bodies  50 A and  50 B. Specifically, the half bodies  50 A and  50 B are each formed to have a square U-shaped (U-shaped, groove-shaped) cross-section by presswork, rolling processing, and/or the like, and the half bodies  50 A and  50 B are joined together in a state where the openings of the half bodies  50 A and  50 B face each other and the lower end portion of the upper half body  50 A nests in the upper end portion of the lower half body  50 B. 
     As illustrated in  FIGS. 1 to 3 , the inner tank  20  is inserted into the outer tank  50 , and the inner peripheral surface of the rear end portion of the outer tank  50  is joined to the outer peripheral surface of the inner tank  20 . Since the total length of the outer tank  50  is shorter than that of the inner tank  20 , a rear portion of the inner tank  20  protrudes and is exposed from the rear end portion of the outer tank  50 . 
     As illustrated in  FIGS. 3 and 9 , the outer peripheral surface of the rear end portion  32  of the entrance tank  30  is joined to the inner peripheral surface of the front end portion of the outer tank  50  in a state where the rear end portion  32  of the entrance tank  30  nests in the opening of the front end portion of the outer tank  50 . As illustrated in  FIGS. 3 and 4 , the middle portion of the outer tank  50  bulges outward more than the front end portion and the rear end portion thereof, and an inner space  55  is formed between the middle portion of the outer tank  50  and the inner tank  20 . Thus, as illustrated in  FIGS. 3 and 9 , the rear end portion  32  of the entrance tank  30  is exposed to the inner space  55 , and the front portion of the inner tank  20  is exposed to the inner space  55  as well. 
     The communication holes  24  to  27  allow the inner space  55  of the outer tank  50  and the interior of the inner tank  20  to communicate with each other. Specifically, the communication holes  24  allow the inner space  55  and the clearance  92  between the uppermost tube  10  and an inner surface of the outer tank  50  to communicate with each other. The communication holes  25  allow the inner space  55  and the clearance  93  between the lowermost tube  10  and the inner surface of the outer tank  50  to communicate with each other. The communication holes  26  and  27  are arranged at positions corresponding to the clearances  91  between the tubes  10  adjacent to each other, while the communication holes  26  are arranged on the left of the clearances  91  and the communication holes  27  are arranged on the right of the clearances  91  so that the communication holes  26  and the communication holes  27  face each other with the clearances  91  arranged therebetween (see  FIG. 4 ). 
     An introduction hole  51  is formed in the upper surface of the outer tank  50 . The introduction hole  51  is arranged close to the left edge of the upper surface of the outer tank  50 . Thus, as illustrated in  FIGS. 1 and 4 , the introduction hole  51  partially protrudes to the left from the left side surface of the inner tank  20 , and the left side surface of the inner tank  20  extends in the front-rear direction across the introduction hole  51  when viewed from above. 
     Any of the communication holes  24  to  27  formed in the inner tank  20  is also offset from a position at which the communication hole faces the introduction hole  51 . 
     1-6. Inlet Pipe 
     As illustrated in  FIGS. 1, 4 , and the like, the inlet pipe  60  is coupled to the introduction hole  51  of the outer tank  50 . The inlet pipe  60  protrudes upward from the upper surface of the outer tank  50 . The coolant is introduced into the outer tank  50  through the inlet pipe  60 . 
     1-7. Exit Tank 
     As illustrated in  FIGS. 1 to 3, 6, and 7 , the exit tank  40  is formed in a hollow pyramid shape. The front-side bottom portion of the exit tank  40  is open, and the rear-side top portion of the exit tank  40  is open as well. 
     The inner peripheral surface of the front end portion of the exit tank  40  is joined to the outer peripheral surface of the rear end portion  22  of the inner tank  20 , in a state where the rear end portion  22  of the inner tank  20  nests in the front-side opening of the exit tank  40 . 
     Note that a flange (not shown) is mounted to the outer peripheral portion of the rear end portion of the exit tank  40 . 
     2. Gas Flow 
     The exhaust gas from the internal combustion engine is introduced into the entrance tank  30  through the front-side opening  31  of the entrance tank  30  (see the arrow A shown in  FIG. 3 ). The exhaust gas is distributed to the inside of each tube  10 . In the tube  10 , the exhaust gas flows from the front end portion  11  to the rear end portion  12  of the tube  10  while the exhaust gas is in contact with the inner fin  18 . The exhaust gas is then discharged from the exit tank  40  through the rear-side opening  41  (see the arrow B shown in  FIG. 3 ) and is supplied again to the inlet side of the internal combustion engine. 
     3. Coolant Flow 
     The coolant is introduced into the outer tank  50  through the inlet pipe  60  and the introduction hole  51 . Since the inlet pipe  60  and the introduction hole  51  partially protrudes to the left from the left side-surface of the inner tank  20 , the coolant introduced to the outer tank  50  flows downward along the side of the left side-surface of the inner tank  20  (see the arrow C shown in  FIG. 4 ) and flows to the right after hitting the upper surface of the inner tank  20  (see the arrow D shown in  FIG. 4 ). Accordingly, the coolant reaches the whole inner space  55  of the outer tank  50 . 
     As illustrated in  FIGS. 3 and 9 , since the rear end portion  32  of the entrance tank  30  is in contact with the coolant in the inner space  55 , heat is exchanged between the gas in the entrance tank  30  and the coolant in the inner space  55 , thereby cooling the gas before flowing into the tubes  10 . 
     Since the outer tank  50  surrounds the front portions of the inner tank  20  and the tube stack  19 , and the coolant reaches the whole inner space  55  of the outer tank  50 , heat is exchanged between the gas inside the front portions of the tubes  10  and the coolant in the inner space  55 . 
     Incidentally, since the coolant introduced into the heat exchanger  1  has the lowest temperature in the inner space  55 , the rear end portion  32  of the entrance tank  30  in contact with the coolant in the inner space  55  is likely to be cooled. On the other hand, since the gas is introduced into the entrance tank  30 , the temperature of the front portion of the entrance tank  30  is high. Accordingly, the entrance tank  30  has a temperature gradient in which the temperature thereof decreases from the front side thereof to the rear side thereof. In addition, as illustrated in  FIG. 9 , the rear end portion  32  of the entrance tank  30  that is likely to be cooled by the coolant is in contact with not only the coolant but also the outer tank  50  and the inner tank  20 , and thus the temperature gradient in the entrance tank  30  is gentle. This can prevent damage to the entrance tank  30  due to the temperature gradient. 
     The coolant introduced into the outer tank  50  flows into the inner tank  20  through the communication holes  24  to  27 . Specifically, the coolant flows into the clearance  92  between the uppermost tube  10  and the inner surface of the outer tank  50  through the communication holes  24 . The coolant flows into the clearance  93  between the lowermost tube  10  and the inner surface of the outer tank  50  through the communication holes  25 . The coolant flows into the clearances  91  each between the tubes  10  adjacent to each other through the communication holes  26  and  27 . 
     Here, the inner space  55  of the outer tank  50  is formed along the entire periphery of the inner tank  20 , and the communication holes  24  to  27  are arranged in the peripheral direction as described above, and thus the coolant passes through any of the communication holes  24  to  27  at a uniform flow rate. Since neither of the communication holes  26  on the left nor the communication holes  27  on the right face the introduction hole  51 , the flow rate of the coolant passing through the communication holes  26  and the flow rate of the coolant passing through the communication holes  27  are equal to each other. 
     The coolant having flown in the clearances  91 ,  92 , and  93  flows toward the rear side. Heat is exchanged between the coolant in the clearances  91 ,  92 , and  93  and the gas in the tubes  10 , thereby cooling the gas in the tubes  10 . 
     Since the flow path of the coolant is narrowed by the communication holes  24  to  27 , a flow speed of the coolant in the clearances  91 ,  92 , and  93  is higher. This makes it possible to inhibit the coolant from being stagnated in the clearances  91 ,  92 , and  93 . Particularly, since the coolant flows into the clearances  91  from the communication holes  26  and  27  on both sides, the coolant is hardly stagnated in the clearances  91 . In addition, since the flow rates of the coolant in the communication holes  26  and  27  are equal to each other, it is possible to further inhibit occurrence of such stagnation. 
     Accordingly, the coolant in the clearances  91 ,  92 , and  93  is not excessively heated, thereby being able to inhibit boiling of the coolant. Further, the temperature distribution in the tubes  10  becomes uniform, thereby being able to prevent damage to the tubes  10  due to non-uniformity of the temperature distribution can be prevented. 
     4. Verification 
     By comparing the heat exchanger  1  of the above-described embodiment with a heat exchanger  101  of a comparative example illustrated in  FIGS. 10 to 12 , it is verified that the heat exchanger  1  has higher cooling efficiency than the heat exchanger  101 . 
     Differences between the heat exchanger  1  of the above-described embodiment and the heat exchanger  101  of the comparative example will be described in the following. Except for the differences described below, the heat exchanger  1  of the embodiment and the heat exchanger  101  of the comparative example are similarly configured. Note that the portions in the heat exchanger  101  of the comparative example that correspond to those in the heat exchanger  1  of the embodiment are given the reference numbers that have common numbers in the last two digits. 
     Although the heat exchanger  1  of the embodiment includes the outer tank  50 , the heat exchanger  101  of the comparative example includes no such a component as to be equivalent to the outer tank  50 . That is, as illustrated in  FIGS. 10 to 12 , in the heat exchanger  101  of the comparative example, a bulging portion  180  bulging outward is formed on the front parts of the upper surface, left side surface, and lower surface of an inner tank  120 , and a pipe hole  129  is formed in the upper surface of the bulging portion  180 , and an inlet pipe  160  is coupled to the pipe hole  129 . The pipe hole  129  is arranged close to the left edge of the upper surface of the bulging portion  180 . 
     In the heat exchanger  1  of the embodiment, the communication holes  24  to  27  are formed in the outer tank  50 , whereas, in the heat exchanger  101  of the comparative example, those corresponding to the communication holes  24  to  27  are not formed in the outer tank  150 . 
     Fluid analysis/heat exchange analysis of the heat exchangers  1 ,  101  described above have been conducted. Conditions of the analyses are as follows: the temperature of the gas introduced into openings  31 ,  131  of entrance tanks  30 ,  130  is set at 780° C.; a mass flow rate of the gas is set at 10 g/s; the temperature of the coolant (cooling water) introduced into inlet pipes  60 ,  160  is set at 90° C.; and a volume flow rate of the coolant is set at 8 L/min. 
     The maximum temperatures in temperature distributions in a to g parts (front ends of tubes  10 ,  110 ) illustrated in  FIGS. 3 and 11  are calculated by the fluid analysis/heat exchange analysis. The calculated results are shown in  FIG. 13 . As apparent from  FIG. 13 , it can be seen that the temperatures in the a to g parts are lower in the heat exchanger  1  of the embodiment than the heat exchanger  101  of the comparative example. Thus, the heat exchanger  1  of the embodiment is superior in cooling of the gas. 
     In addition, differences between the maximum temperatures and the minimum temperatures in the temperature distributions in the a to g parts are calculated by the fluid analysis/heat exchange analysis. The calculated results are shown in  FIG. 14 . As apparent from in  FIG. 14 , it can be seen that the temperature differences in the c to g parts are smaller in the heat exchanger  1  of the embodiment than in the heat exchanger  101  of the comparative example. Thus, the heat exchanger  1  of the embodiment has more uniform temperature distributions in the tubes  10  and higher effects of preventing damage to the tubes  10  than the heat exchanger  101  of the comparative example. 
     5. Modifications 
     Although an embodiment of the present disclosure is described above, an embodiment described above is simply to facilitate understanding of the present disclosure and are not in any way to be construed as limiting the present disclosure. An embodiment of the present disclosure may variously be changed or altered without departing from its gist and encompass equivalents thereof. Modifications made from an embodiment described above will be explained as follows. 
     (1)  FIGS. 15 to 20  are right side views illustrating the inner tank  20  inside the outer tank  50 . 
     As illustrated in  FIG. 15 , any of the communication holes  27  may have the same area (front-rear length and top-bottom length). The same applies to the communication holes  26  on the opposite side. 
     As illustrated in  FIG. 16 , the areas of the communication holes  27  decrease in the order from bottom to top. The same applies to the communication holes  26  on the opposite side. Note that all the communication holes  26  and  27  corresponding to each other have the same front-rear length, respectively. 
     As illustrated in  FIG. 17 , one of the communication holes  27  arranged in the center has the greatest area, the areas of the communication holes  27  above the center communication hole  27  increase in the order from top to bottom, and the areas of the communication holes  27  below the center communication hole  27  increase in the order from bottom to top. The same applies to the communication holes  26  on the opposite side. Note that all the communication holes  26  and  27  corresponding to each other have the same front-rear length, respectively. 
     As illustrated in  FIGS. 18 to 20 , a single communication hole  27  may be formed to be elongated in the top-bottom direction, and the communication hole  27  may communicate with plural clearances  91 . The same applies to the communication hole  26  on the opposite side. In this case, the front-rear lengths of the communication hole  27  illustrated in  FIG. 18  and the communication hole  26  on the opposite side are uniform. The front-rear lengths of the communication hole  27  and the opposite communication hole  26  illustrated in  FIG. 19  gradually decrease from bottom to top. The front-rear lengths of the communication hole  27  illustrated in  FIG. 20  and the communication hole  26  on the opposite side gradually increase from top to the center and gradually decrease from the center to bottom. 
     (2) In an embodiment described above, the heat exchanger  1  is used as a gas cooler in an exhaust gas recirculation system, however, the heat exchanger  1  may be provided in a system other than the exhaust gas recirculation system as long as the heat exchanger  1  is used as a gas cooler for cooling gas using a cooling medium that is cooler than the gas. 
     REFERENCE SIGNS LIST 
     
         
           1  heat exchanger 
           10  tube 
           11  front end portion of tube 
           12  rear end portion of tube 
           13  middle portion of tube 
           19  tube stack 
           20  inner tank 
           21  front end portion of inner tank 
           22  rear end portion of inner tank 
           26 ,  27  communication hole 
           29  discharge hole 
           30  entrance tank 
           50  outer tank 
           51  introduction hole 
           55  inner space 
           91  clearance