Patent Publication Number: US-2023152007-A1

Title: Heat exchanger unit and condensing boiler using the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 16/973,025 filed on Dec. 7, 2020, and entitled “HEAT EXCHANGER UNIT AND CONDENSING BOILER USING THE SAME,” which is the U.S. National Phase entry of International Patent Application No. PCT/KR2019/006543 filed May 30, 2019, which claims priority to Korean Patent Application Nos. 10-2018-0064669, filed on Jun. 5, 2018; 10-2018-0064666 filed Jun. 5, 2018; 10-2018-0064668 filed Jun. 5, 2018; and 10-2018-0156356 filed Dec. 6, 2018, all of the above listed applications are herein incorporated by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to a condensing boiler and a heat exchanger unit used therein. 
     BACKGROUND ART 
     A boiler is an apparatus for heating a desired area by heating fluid in a container. Accordingly, to heat up heating-water of the boiler, the boiler generally has a heat source, a burner including the heat source, and a heat exchanger unit for heating the heating-water using combustion gas. In a condensing boiler that comprehensively uses heat of combustion gas, sensible heat generated from a burner is supplied to heating-water, and the sensible heat of the combustion gas generated from the burner and latent heat caused by a phase change of the combustion gas are supplied to the heating-water. Accordingly, the heating-water is heated. 
     To supply the sensible heat and the latent heat to the heating-water, a method of locating a container for storing the heating-water in a position close to an area in which the combustion gas flows and a heat source for supplying sensible heat is mainly used. Heat is indirectly transferred to the heating-water through the container to raise the temperature of the heating-water to a temperature appropriate for heating, and thereafter the heating-water is supplied to an area that has to be heated. 
     A plate type heat exchanger unit having a plurality of stacked plates is mainly used for the heat transfer. However, the plate type heat exchanger unit has problems of a difficulty in a manufacturing process and high cost despite excellent thermal efficiency. 
     DISCLOSURE 
     Technical Problem 
     The present disclosure has been made to solve the above-mentioned problems occurring in the prior art. An aspect of the present disclosure provides condensing boiler and a heat exchanger unit used therein that has excellent thermal efficiency while using a fin-tube type heat exchange device. 
     Technical Solution 
     A heat exchanger unit according to an embodiment of the present disclosure includes a sensible heat exchanging part including a sensible heat exchange pipe that is disposed in a sensible heat exchange area for receiving sensible heat generated by a combustion reaction and heating heating-water and that receives the heating-water and allows the heating-water to flow therethrough and a sensible heat fin disposed in the sensible heat exchange area and formed in a plate shape across the sensible heat exchange pipe such that the sensible heat exchange pipe passes through the sensible heat fin, and a latent heat exchanging part including a latent heat exchange pipe that is disposed in a latent heat exchange area for receiving latent heat generated during a phase change of combustion gas and heating the heating-water and that receives the heating-water and allows the heating-water to flow therethrough and a latent heat fin disposed in the latent heat exchange area and formed in a plate shape across the latent heat exchange pipe such that the latent heat exchange pipe passes through the latent heat fin, the latent heat exchange area being located downstream of the sensible heat exchange area based on a reference direction that is a flow direction of the combustion gas generated during the combustion reaction. 
     A condensing boiler according to an embodiment of the present disclosure includes a burner assembly causes a combustion reaction, a combustion chamber located downstream of the burner assembly on the basis of a flow direction of a combustion gas generated by the combustion reaction, and in which flame generated by the combustion reaction is located and a heat exchanger unit configured to receive sensible heat and the combustion gas generated by the combustion reaction to heat heating-water. The heat exchanger unit includes a sensible heat exchanging part including a sensible heat exchange pipe disposed in a sensible heat exchange area and configured to receive the heating-water and allow the heating-water to flow therethrough and a sensible heat fin disposed in the sensible heat exchange area and formed in a plate shape across the sensible heat exchange pipe such that the sensible heat exchange pipe passes through the sensible heat fin, the sensible heat exchange area being configured to receive sensible heat generated by a combustion reaction and to heat the heating-water, the heat exchanger unit includes a latent heat exchanging part including a latent heat exchange pipe disposed in a latent heat exchange area and configured to receive the heating-water and allow the heating-water to flow therethrough and a latent heat fin disposed in the latent heat exchange area and formed in a plate shape across the latent heat exchange pipe such that the latent heat exchange pipe passes through the latent heat fin, the latent heat exchange area being located downstream of the sensible heat exchange area based on a reference direction that is a flow direction of combustion gas generated during the combustion reaction and configured to receive latent heat generated during a phase change of the combustion gas and to heat the heating-water. 
     Advantageous Effects 
     Accordingly, heat transfer efficiency may not be deteriorated despite the use of an inexpensive and easy-to-manufacture fin-tube type heat exchanger unit. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG.  1    is a vertical sectional view of part of an exemplary heat exchanger unit. 
         FIG.  2    is a vertical sectional view of a heat exchanger unit and a condensing boiler using the same according to a first embodiment of the present disclosure. 
         FIG.  3    is a side view of the heat exchanger unit and the condensing boiler using the same according to the first embodiment of the present disclosure. 
         FIG.  4    is a plan view of a combustion chamber of the heat exchanger unit according to the first embodiment of the present disclosure. 
         FIG.  5    is a plan view of a sensible heat exchanger of the heat exchanger unit according to the first embodiment of the present disclosure. 
         FIG.  6    is a view illustrating an area where a sensible heat exchange pipe and a sensible heat fin are disposed in the vertical sectional view of the heat exchanger unit according to the first embodiment of the present disclosure. 
         FIG.  7    is a view illustrating an area where a sensible heat exchange pipe and a sensible heat fin are disposed in a vertical sectional view of a heat exchanger unit according to one modified example of the first embodiment of the present disclosure. 
         FIG.  8    is a view illustrating a second general sensible heat side plate of the heat exchanger unit according to the first embodiment of the present disclosure and flow passage caps included in a second flow passage cap plate when viewed from the outside along a predetermined direction. 
         FIG.  9    is a view illustrating a first general sensible heat side plate of the heat exchanger unit according to the first embodiment of the present disclosure and flow passage caps included in a first flow passage cap plate when viewed from the inside along the predetermined direction. 
         FIG.  10    is a view illustrating a heat exchanger unit according to another modified example of the first embodiment of the present disclosure when viewed from outside a second connection flow passage cap plate. 
         FIG.  11    is a view illustrating a first connection flow passage cap plate of the heat exchanger unit according to the other modified example of the first embodiment of the present disclosure. 
         FIG.  12    is a view illustrating a partial area of a second main general side plate of the heat exchanger unit according to the other modified example of the first embodiment of the present disclosure together with flow passage caps included in the second connection flow passage cap plate when viewed from the outside along a predetermined direction. 
         FIG.  13    is a view illustrating a first main general side plate of the heat exchanger unit according to the other modified example of the first embodiment of the present disclosure together with flow passage caps included in the first connection flow passage cap plate when viewed from the inside along a predetermined direction. 
         FIG.  14    is a perspective view illustrating a sensible heat flow passage and a latent heat flow passage of the heat exchanger unit according to the other modified example of the first embodiment of the present disclosure. 
         FIG.  15    is a vertical sectional view of a heat exchanger unit according to a second embodiment of the present disclosure. 
         FIG.  16    is a front view illustrating a flow passage cap plate of a heat exchanger unit according to a modified example of the second embodiment of the present disclosure together with pipes. 
         FIG.  17    is a vertical sectional view of a heat exchanger unit and a condensing boiler using the same according to a third embodiment of the present disclosure. 
         FIG.  18    is a side view of the heat exchanger unit and the condensing boiler using the same according to the third embodiment of the present disclosure. 
         FIG.  19    is a plan view of the heat exchanger unit according to the third embodiment of the present disclosure. 
         FIG.  20    is a vertical sectional view of the heat exchanger unit according to the third embodiment of the present disclosure. 
         FIG.  21    is a perspective view illustrating a plurality of downstream fins according to the third embodiment of the present disclosure and condensate located therebetween. 
         FIG.  22    is a vertical sectional view of a heat exchanger unit according to a first modified example of the third embodiment of the present disclosure. 
         FIG.  23    is a vertical sectional view of a heat exchanger unit according to a second modified example of the third embodiment of the present disclosure. 
         FIG.  24    is a vertical sectional view of a heat exchanger unit according to a third modified example of the third embodiment of the present disclosure. 
         FIG.  25    is a vertical sectional view of a heat exchanger unit according to a fourth modified example of the third embodiment of the present disclosure. 
         FIG.  26    is a view illustrating a second general side plate of the heat exchanger unit according to the third embodiment of the present disclosure together with flow passage caps included in the second flow passage cap plate. 
         FIG.  27    is a view illustrating a first general side plate of the heat exchanger unit according to the third embodiment of the present disclosure together with flow passage caps included in the first flow passage cap plate. 
         FIG.  28    is a perspective view illustrating all flow passages included in the heat exchanger unit according to the third embodiment of the present disclosure. 
         FIG.  29    is a perspective view illustrating a situation in which the connection flow passage cap plates are separated from each other in the heat exchanger unit according to the other modified example of the first embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Hereinafter, some embodiments of the present disclosure will be described in detail with reference to the exemplary drawings. In adding the reference numerals to the components of each drawing, it should be noted that the identical or equivalent component is designated by the identical numeral even when they are displayed on other drawings. Further, in describing the embodiment of the present disclosure, a detailed description of well-known features or functions will be ruled out in order not to unnecessarily obscure the gist of the present disclosure. 
     In describing the components of the embodiment according to the present disclosure, terms such as first, second, “A”, “B”, (a), (b), and the like may be used. These terms are merely intended to distinguish one component from another component, and the terms do not limit the nature, sequence or order of the components. When a component is described as “connected”, “coupled”, or “linked” to another component, this may mean the components are not only directly “connected”, “coupled”, or “linked” but also are indirectly “connected”, “coupled”, or “linked” via a third component. 
     As a method of arranging a burner, heat exchangers, and a combustion chamber constituting a condensing boiler, a method of configuring the condensing boiler by locating the burner at the lowermost position and sequentially arranging the combustion chamber surrounded by insulation of a dry type, a sensible heat exchanger of a fin-tube type, and a latent heat exchanger of a plate type in an upward direction may be considered. This type of condensing boiler is referred to as a bottom-up boiler. In the case of the bottom-up boiler, condensate generated by condensation of combustion gas in the latent heat exchanger may fall onto the sensible heat exchanger and the combustion chamber. Therefore, the sensible heat exchanger and the insulation of the dry type that surrounds the combustion chamber may be easily corroded by the condensate with high acidity. Furthermore, as the different types of heat exchangers are connected with each other, manufacturing costs may be increased due to additional connecting parts. 
     To solve the problems caused by the condensate, a method of configuring a condensing boiler by locating a burner at the uppermost position and sequentially arranging a combustion chamber thermally insulated by being surrounded by a heat insulation pipe, a sensible heat exchanger of a fin-tube type, and a latent heat exchanger of a plate type in a downward direction may be considered. This type of condensing boiler is referred to as a top-down boiler. In this case, as the latent heat exchanger is located at the lowermost position, condensate is immediately discharged through a condensate receiver and does not reach the sensible heat exchanger or the combustion chamber, and thus a problem of corrosion may be solved. However, many parts including the heat insulation pipe used to cool the combustion chamber are used, and due to this, the number of assembly steps is increased, which leads to an increase in manufacturing costs. Furthermore, as the different types of heat exchangers are connected with each other, manufacturing costs may be increased due to additional connecting parts. 
       FIG.  1    is a vertical sectional view of part of an exemplary heat exchanger unit. As illustrated in  FIG.  1   , a top-down boiler may be used, and a method of performing thermal insulation in a dry type by surrounding a combustion chamber  102  and a sensible heat exchanger  103  with insulation  101  may be considered. That is, a case where the insulation of a dry type, which is used for the combustion chamber  102 , is disposed to insulate heat radiated from the area of the sensible heat exchanger  103  may be considered. However, in this case, due to the sensible heat exchanger  103 , flame generated through a combustion reaction, and excessive heat generated from combustion gas, the insulation  101  may be damaged, and the durability may be decreased. Furthermore, condensate is more likely to be generated in a position adjacent to the sensible heat exchanger  103  than in the combustion chamber  102 , and therefore when the insulation  101 , as in the drawing, further extends downward beyond the position that the combustion chamber  102  extends downward and reaches, the condensate may make contact with the insulation  101  of a dry type so that the insulation  101  may be damaged. 
     First Embodiment 
       FIG.  2    is a vertical sectional view of a heat exchanger unit and a condensing boiler  1  using the same according to the first embodiment of the present disclosure.  FIG.  3    is a side view of the heat exchanger unit and the condensing boiler  1  using the same according to the first embodiment of the present disclosure. 
     Referring to the drawings, the heat exchanger unit according to the first embodiment of the present disclosure includes a sensible heat exchanger  30 , a latent heat exchanger  40 , and sensible heat insulation pipes  34 . The components constituting the heat exchanger unit may be fixed in positions as illustrated. 
     Furthermore, the condensing boiler  1  including the heat exchanger unit according to the first embodiment of the present disclosure includes a combustion chamber  20  and a burner assembly  10  including a burner  11 . The burner assembly  10  and the heat exchanger unit are disposed in sequence along a flow direction D 1  of combustion gas, and components are arranged in the heat exchanger unit along the same direction in the order of the combustion chamber  20 , the sensible heat exchanger  30 , the latent heat exchanger  40 , and the sensible heat insulation pipes  34  disposed together with the sensible heat exchanger  30 . Accordingly, the components of the condensing boiler  1  will be described below in the above-described order of arrangement. 
     The heat exchanger unit and the condensing boiler  1  using the same according to the first embodiment of the present disclosure will be described below based on a top-down condensing boiler  1  in which combustion gas flows vertically downward. Accordingly, the flow direction D 1  of the combustion gas that is represented by an arrow may be the same as the vertical downward direction at the position where the condensing boiler  1  is installed. As the top-down condensing boiler  1  is selected, condensate produced by condensation of the combustion gas may be generated only at the lowermost side of the condensing boiler  1  and may be immediately discharged to the outside through a lower end of the condensing boiler  1 . Accordingly, components constituting the condensing boiler  1  may be prevented from being corroded. However, the configuration of the present disclosure may be used for a bottom-up condensing boiler capable of naturally forming a path of heating-water in a downward direction by using a property by which heated combustion gas is moved upward by convection. 
     The condensing boiler  1  according to the first embodiment of the present disclosure may include a condensate receiver  55  located at the most downstream side along the flow direction D 1  of the combustion gas. When condensate generated from the latent heat exchanger  40  drops in the vertically downward direction by the weight of the condensate, the condensate receiver  55  may collect the condensate. To allow the collected condensate to be discharged through a condensate outlet  53  extending in the vertically downward direction, the condensate receiver  55  may have an inner surface inclined toward the condensate outlet  53 . 
     Furthermore, to allow residual combustion gas to be discharged at the same time that the condensate is discharged, an exhaust duct  52  may be formed so as to be in communication with the condensate receiver  55 . The exhaust duct  52  extends in the vertically upward direction and discharges the residual combustion gas to the outside. 
     Burner Assembly  10   
     The burner assembly  10  is a component that includes the burner  11  radiating heat and that causes a combustion reaction of injected fuel and air to generate combustion gas. 
     A premix burner may be used as the burner assembly  10  used in the condensing boiler  1  according to the first embodiment of the present disclosure. The premix type burner is a device that mixes injected air and fuel at a predetermined ratio and burns the mixed air and fuel using radiating heat to generate combustion gas. For this operation, the burner assembly  10  according to the first embodiment of the present disclosure may include a mix chamber  12  that is a space in which mixed fuel for a combustion reaction is prepared by mixing injected fuel and air at a predetermined ratio, and the burner  11  that applies heat to the mixed fuel mixed by the mix chamber  12 . The burner assembly  10  having the above-described structure is provided to obtain optimal fuel efficiency and thermal efficiency by causing a combustion reaction by heating air and fuel mixed at a ratio appropriate for the combustion reaction. 
     To supply air into the mix chamber  12  and blow combustion gas generated in the burner assembly  10  in the vertically downward direction, the condensing boiler  1  of the present disclosure may further include a blower  54 . The blower  54  may include a pump that is connected with the mix chamber  12  and that forcibly delivers air toward the burner assembly  10  that is connected to the mix chamber  12  in the vertically downward direction. 
     Combustion Chamber  20   
       FIG.  4    is a plan view of the combustion chamber  20  according to the first embodiment of the present disclosure. 
     The combustion chamber  20  will be described below with reference to  FIG.  4    together with  FIGS.  2  and  3   . The combustion chamber  20  is a component that includes an interior space  22  provided such that flame that a combustion reaction by the burner assembly  10  generates is located. Accordingly, the combustion chamber  20  is formed by surrounding the interior space  22  with sidewalls. The burner assembly  10  and the combustion chamber  20  are coupled such that the burner  11  of the burner assembly  10  is located upstream of the interior space  22  based on the flow direction D 1  of the combustion gas. 
     The burner assembly  10  applies heat to air and fuel to cause a combustion reaction. Flame and combustion gas accompanied by thermal energy are generated as products of the combustion reaction. The flame is located in the interior space  22  of the combustion chamber  20 , but extends from the burner assembly  10  along the flow direction D of the combustion gas. The combustion gas flows through the interior space  22 . The interior space  22  of the combustion chamber  20  may be connected in a direction parallel to the flow direction D 1  of the combustion gas. In the first embodiment of the present disclosure, the flow direction D 1  of the combustion gas is the vertically downward direction, and therefore the interior space  22  of the combustion chamber  20  is formed to be connected in the vertical direction. 
     A combustion chamber heat insulation part  24  may be formed on at least a partial area of an inner surface of a combustion chamber sidewall  21  constituting the combustion chamber  20 . The combustion chamber sidewall  21  may be constituted by two general side plates  211  parallel to each other and two heat insulation side plates  212  perpendicular to the general side plates  211  and parallel to each other and may be formed in a rectangular parallelepiped shape. The combustion chamber heat insulation part  24  may be disposed on the insides of the heat insulation side plates  212 . The combustion chamber heat insulation part  24  may be formed of insulation blocking a heat flow and may reduce the amount by which heat generated by the combustion reaction is transferred outside the combustion chamber  20  through inner surfaces of the combustion chamber  20 . The amount of heat transferred from the interior space  22  of the combustion chamber  20  to the outside of the combustion chamber  20  may be reduced by the combustion chamber heat insulation part  24 . A porous polystyrene panel decreasing a heat flow or a needle mat made of silica, which is an inorganic material, may be used as an example of the insulation. However, the type of the insulation is not limited thereto. 
     The combustion chamber heat insulation part  24  may be disposed on the general side plates  211  of the combustion chamber  20  as well, and thus an additional thermal insulation effect may be obtained by surrounding the entire interior space  22  of the combustion chamber  20  with the insulation. 
     A heat insulation pipe through which fluid flows may be disposed around the combustion chamber  20  for thermal insulation. However, in a case where a large number of heat insulation pipes are used, a lot of cost is consumed in production. However, because the heat exchanger unit of the present disclosure is implemented in the top-down type, condensation of condensate does not occur in the combustion chamber  20 , and there is no risk of corrosion. Accordingly, the combustion chamber  20  of a dry type that uses insulation cheaper than a heat insulation pipe as a material of which the combustion chamber heat insulation part  24  is made may be configured. 
     The length of the combustion chamber heat insulation part  24  may be determined such that the combustion chamber heat insulation part  24  surrounds only the interior space  22  of the combustion chamber  20  without surrounding the sensible heat exchanger  30 , which will be described below, based on the flow direction D 1  of the combustion gas. That is, the combustion chamber heat insulation part  24  may be provided so as not to be located inside a sensible heat exchanger case  31  that will be described below. Accordingly, in the case where the insulation  101  is disposed as illustrated in  FIG.  1   , the insulation  101  may be damaged by excessive heat and condensate, whereas in the first embodiment of the present disclosure, the combustion chamber heat insulation part  24  is disposed as illustrated in  FIG.  2   , and thus excessive heat generated from the sensible heat exchanger  30  may not be transferred to the combustion chamber heat insulation part  24 . 
     Sensible Heat Exchanger  30   
       FIG.  5    is a plan view of the sensible heat exchanger  30  of the heat exchanger unit according to the first embodiment of the present disclosure.  FIG.  6    is a view illustrating an area where a sensible heat exchange pipe  32  and a sensible heat fin  33  are disposed in the vertical sectional view of the heat exchanger unit according to the first embodiment of the present disclosure. 
     A basic configuration of the sensible heat exchanger  30  will be described below with reference to  FIGS.  2 ,  3 ,  5 , and  6   . 
     The sensible heat exchanger  30  is disposed downstream of the combustion chamber  20  based on the flow direction D 1  of the combustion gas. The sensible heat exchanger  30  is a component that receives, by radiant heat and convection of the combustion gas, sensible heat generated by a combustion reaction triggered by the burner assembly  10  located over the sensible heat exchanger  30  and that heats heating-water flowing in the sensible heat exchanger  30 . 
     Specifically, the sensible heat exchanger  30  includes the sensible heat exchange pipe  32  through which the heating-water flows and around which the combustion gas flows, and the sensible heat exchanger case  31  into which opposite ends of the sensible heat exchange pipe  32  are inserted. The sensible heat exchange pipe  32  is located in the sensible heat exchanger case  31 , and the combustion gas flows around the sensible heat exchange pipe  32  to indirectly exchange heat with the heating-water. 
     The sensible heat exchange pipe  32  extends along a predetermined direction D 2  in a space formed in the sensible heat exchanger case  31 . The predetermined direction D 2  may preferably be a direction perpendicular to the flow direction D 1  of the combustion gas. The sensible heat exchange pipe  32  may include a plurality of straight portions  321 ,  322 ,  323 , and  324  arranged to be spaced apart from each other along an orthogonal direction perpendicular to the one direction and the flow direction D 1  of the combustion gas. 
     The plurality of straight portions  321 ,  322 ,  323 , and  324  are arranged, and flow passage cap plates  361  and  362  that will be described below exist to connect end portions of the straight portions  321 ,  322 ,  323 , and  324  inserted into insertion holes formed in general sensible heat side plates  311  of the sensible heat exchanger case  31  that will be described below. The set of the straight portions  321 ,  322 ,  323 , and  324  forms the one sensible heat exchange pipe  32 . Accordingly, a continuous winding flow passage of the heating-water may be formed by the arrangement of the sensible heat exchange pipe  32 . 
     For example, in a case where the straight portions  321 ,  322 ,  323 , and  324  of  FIG.  5    are connected in series, the heating-water may be heated by receiving sensible heat of the combustion gas and the burner assembly  10  while passing through the sensible heat exchange pipe  32  in such a manner that the heating-water introduced in the direction of the arrow illustrated in  FIG.  5    is discharged by flowing to the right in the drawing along the first outer straight portion  321  included in the sensible heat exchange pipe  32 , flowing to the left in the drawing along the intermediate straight portion  323  located under the first outer straight portion  321  in the drawing, flowing, in a discharge step, to the right in the drawing along the intermediate straight portion  324  located over the second outer straight portion  322  in the drawing, and moving to the left in the drawing along the second outer straight portion  322 . 
     A turbulator (not illustrated) having a shape that hampers a flow of the heating-water to make the flow of the heating-water turbulent may be disposed in the sensible heat exchange pipe  32 . 
     The sensible heat exchanger case  31  may be constituted by two general side plate portions spaced apart from each other in the predetermined direction D 2  and parallel to each other and two heat insulation side plate portions spaced apart from each other along an orthogonal direction perpendicular to the predetermined direction D 2  and parallel to each other and may be formed in the form of a rectangular parallelepiped. The general side plate portions and the heat insulation side plate portions may be general side plates and heat insulation side plates that are separate from each other and may be partial areas of a side plate of an integrated heat exchanger case. In this disclosure, it will be exemplified that the general side plate portions and the heat insulation side plate portions are formed of general side plates and heat insulation side plates that are separate from each other. 
     The general sensible heat side plates  311  and sensible heat insulation side plates  312  form the interior space of the sensible heat exchanger case  31 . Here, the sensible heat insulation side plates  312  are used with the meaning of side plates to which the sensible heat insulation pipes  34  are disposed to be adjacent, rather than the meaning of side plates that reduce the amount of heat transferred to the outside, thereby achieving thermal insulation. 
     The general sensible heat side plates  311  may include a first general sensible heat side plate  3111  and a second general sensible heat side plate  3112  spaced apart from each other along the predetermined direction D 2 . The opposite ends of the straight portions  321 ,  322 ,  323 , and  324  constituting the sensible heat exchange pipe  32  may be inserted into the first general sensible heat side plate  3111  and the second general sensible heat side plate  3112 , and thus the straight portions  321 ,  322 ,  323 , and  324  may be accommodated in the sensible heat exchanger case  31 . The combustion gas flows in the space formed in the sensible heat exchanger case  31  and moves from the combustion chamber  20  to a latent heat exchanger case  41  that will be described below. 
     The sensible heat insulation pipes  34  may be disposed adjacent to the sensible heat exchanger  30 . The sensible heat insulation pipes  34  are pipe type components that are disposed to thermally insulate the sensible heat exchanger  30  by allowing the heating-water to flow through the components. Here, the thermal insulation includes both confining heat in any position to prevent heat transfer and absorbing heat released from any position to the outside so as to decrease the amount of heat finally released to the outside. The meaning of the thermal insulation may be identically applied to other embodiments of the present disclosure and modified examples thereof. 
     Specifically, the sensible heat insulation pipes  34  may be disposed adjacent to the outsides of the sensible heat insulation side plates  312 . The sensible heat insulation pipes  34  may be disposed adjacent to the two sensible heat insulation side plates  312 , respectively. The sensible heat insulation pipes  34  may be disposed to make contact with the outsides of the sensible heat insulation side plates  312 , or the sensible heat insulation pipes  34  may be disposed in positions spaced apart from the outsides of the sensible heat insulation side plates  312 . 
     Referring to the drawings, in the heat exchanger unit according to the first embodiment of the present disclosure, a first sensible heat insulation pipe  341  and a second sensible heat insulation pipe  342  are spaced apart from each other and are disposed along the outsides of the sensible heat insulation side plates  312 . In  FIG.  5   , the sensible heat insulation pipes  34  are illustrated as being located inward of the sensible heat insulation side plates  312 . However, the sensible heat insulation side plates  312  cover the sensible heat insulation pipes  34  at the same time as being located inward of the sensible heat insulation pipes  34  inside the sensible heat exchanger  30 , and the positions of the sensible heat insulation pipes  34  are illustrated for convenience of description. Accordingly, the sensible heat insulation pipes  34  covered by the sensible heat insulation side plates  312  are actually located in the area where the sensible heat insulation pipes  34  are illustrated in  FIG.  5   , and in the plan view, the sensible heat insulation pipes  34  do not appear. 
     Accordingly, the sensible heat insulation pipes  34  are located outside the sensible heat exchanger case  31  through which the combustion gas passes, and therefore the sensible heat insulation pipes  34  may not cross or meet the combustion gas. The sensible heat insulation pipes  34  may not be used for heat exchange between the combustion gas and the heating-water, but may perform only a thermal insulation function of blocking release of heat from the sensible heat exchanger  30  to the outside by using the heating-water. 
     The sensible heat insulation pipes  34  may be disposed to be spaced apart from the combustion chamber  20  along the flow direction D 1  of the combustion gas without making contact with the combustion chamber  20 . Accordingly, the sensible heat insulation pipes  34  may not be used for thermal insulation of the combustion chamber  20 , but may be used only for thermal insulation of the sensible heat exchanger  30 . 
     The sensible heat insulation pipes  34 , together with the sensible heat exchange pipe  32 , form a sensible heat flow passage through which the heating-water flows. 
     The shape of interior spaces of the sensible heat insulation pipes  34 , as illustrated in  FIGS.  2  and  6   , may be formed in an oval shape in a cross-section obtained by cutting the sensible heat insulation pipes  34  with a plane perpendicular to the direction in which the sensible heat insulation pipes  34  extend. Specifically, the interior spaces of the sensible heat insulation pipes  34  may be formed in an oval shape having a long axis parallel to the flow direction D 1  of the combustion gas. 
     The sensible heat insulation pipes  34  may be located adjacent to the sensible heat insulation side plates  312  of the sensible heat exchanger  30  and may be disposed at an upstream side based on the flow direction D 1  of the combustion gas. That is, the sensible heat insulation pipes  34  may be disposed closer to the combustion chamber  20  than the latent heat exchanger  40  that will be described below. Flame generated by the burner assembly  20  in the combustion chamber  20  may reach downstream of the combustion chamber  20  based on the flow direction D 1  of the combustion gas, and therefore the upstream side of the sensible heat exchanger  30  may have a highest temperature while making contact with the combustion chamber  20 . Accordingly, the sensible heat insulation pipes  34  may be disposed adjacent to the upstream side of the sensible heat exchanger  30  and may thermally insulate the upstream side of the sensible heat exchanger  30  from which a large amount of heat is released due to the largest temperature difference between the interior space of the sensible heat exchanger  30  and the outside. However, the sensible heat insulation pipes  34  may be located in the center based on the flow direction D 1  of the combustion gas. 
     The sensible heat exchanger  30  may further include the sensible heat fin  33  capable of raising the thermal conductivity of the sensible heat exchange pipe  32 , and thus the sensible heat exchanger  30  of a fin-tube type may be formed. The sensible heat fin  33  is formed in a plate shape that is perpendicular to the direction in which the sensible heat exchange pipe  32  extends, and the sensible heat exchange pipe  32  passes through the sensible heat fin  33 . A plurality of sensible heat fins  33  may be disposed to be spaced apart from each other at predetermined intervals along the predetermined direction D 2  in which the sensible heat exchange pipe  32  extends. The sensible heat exchange pipe  32  and the sensible heat fin  33  may be formed of a metallic material with high thermal conductivity to increase the surface area of the sensible heat exchange pipe  32  from which the sensible heat fin  33  receives sensible heat, thereby transferring a larger amount of sensible heat to the heating-water. 
     In a cross-section obtained by cutting the sensible heat exchange pipe  32  with a plane perpendicular to the predetermined direction D 2  in which the sensible heat exchange pipe  32  extends, the interior space of the sensible heat exchange pipe  32  may be formed in the shape of a long hole that extends along the flow direction D 1  of the combustion gas. As can be seen in  FIG.  6   , the sensible heat exchange pipe  32  according to the first embodiment of the present disclosure may have a flat long hole shape formed such that a value obtained by dividing the length of the interior space of the sensible heat exchange pipe  32  in the cross-section based on the flow direction D 1  of the combustion gas by the width according to the direction perpendicular to the flow direction D 1  of the combustion gas equals 2 or more. 
     When the flat type pipe having the above-described shape is employed for the sensible heat exchange pipe  32 , due to a wider heat exchange area in the relationship with combustion gas, the heating-water may receive a larger amount of heat and may be sufficiently heated even though flowing along the sensible heat exchange pipe  32  having the same length, as compared with when a pipe having a different shape, such as a circular shape or an oval shape, is employed for the sensible heat exchange pipe  32 . 
     A through-hole through which the sensible heat exchange pipe  32  passes may be formed in the sensible heat fin  33 . The area of the through-hole may be equal to or smaller than the area of the sensible heat exchange pipe  32 , and the sensible heat exchange pipe  32  may be firmly inserted into the through-hole. Furthermore, the sensible heat fin  33  may be integrally coupled with the sensible heat exchange pipe  32  through brazing welding. 
     However, the sensible heat insulation pipes  34  are not coupled with the sensible heat fin  33 . The sensible heat insulation pipes  34  are not fastened with the sensible heat fin  33 , and the sensible heat insulation pipes  34  and the sensible heat fin  33  may be disposed on opposite sides with the sensible heat insulation side plates  312  therebetween. The sensible heat fin  33  and the sensible heat insulation pipes  34  may make contact with the sensible heat insulation side plates  312 , but the sensible heat fin  33  and the sensible heat insulation pipes  34  do not make direct contact with each other. Because the sensible heat insulation pipes  34 , as described above, are disposed for thermal insulation of the sensible heat exchanger  30  rather than for heat exchange between the combustion gas and the heating-water, the sensible heat fin  33  and the sensible heat insulation pipes  34  are not directly connected with each other. Accordingly, the sensible heat fin  33  and the sensible heat insulation pipes  34  are disposed so as not to cross each other. 
     A common louver hole  331  may be additionally formed in the sensible heat fin  33  along the predetermined direction D 2  in which the sensible heat exchange pipe  32  extends. The louver hole  331  may be formed by punching. The louver hole  331  includes a burr raised along the periphery thereof. When the combustion gas flows, the burr blocks the combustion gas to cause the combustion gas to flow around the sensible heat exchange pipe  32 , thereby facilitating heat exchange between the combustion gas and the heating-water. 
     A plurality of louver holes  331  may be formed. The louver holes  331 , as illustrated in  FIG.  6   , may include a plurality of first louver holes  3311  that extend in an oblique direction with respect to the flow direction D 1  of the combustion gas and that are formed in the outermost portions of the sensible heat fin  33 , and a plurality of second louver holes  3312  that are formed between the sensible heat exchange pipes  32  adjacent to each other and that extend in the direction perpendicular to the flow direction D 1  of the combustion gas. The louver holes  331  may be disposed to be spaced apart from each other at predetermined intervals along the flow direction D 1  of the combustion gas. 
     The sensible heat fin  33  may further include valleys  334  and protruding portions  333 . The sensible heat fin  33  may be basically formed to surround the sensible heat exchange pipe  32 . The sensible heat fin  33  may surround areas corresponding to a predetermined width from the peripheries of upstream-side end portions of the sensible heat exchange pipe  32  based on the flow direction D 1  of the combustion gas such that the areas are distinguished from the remaining areas of the sensible heat exchange pipe  32 . Accordingly, between the adjacent upstream-side end portions of the sensible heat exchange pipe  32 , the valleys  334  may be concavely formed in the sensible heat fin  33  along the flow direction D 1  of the combustion gas. The areas of the sensible heat fin  33  that are adjacent to the upstream-side end portions of the sensible heat exchange pipe  32  relatively protrude to form the protruding portions  33 . Unnecessary areas are open by forming the valleys  334 , and thus the combustion gas may more freely flow between the sensible heat fin  33  and the sensible heat exchange pipe  32 . 
     The sensible heat fin  33  may further include concave portions  332 . The concave portions  332  are concavely formed from a downstream-side edge of the sensible heat fin  33  toward downstream-side end portions of the sensible heat exchange pipe  32  based on the flow direction D 1  of the combustion gas. The purpose of forming the concave portions  332  is similar to the purpose of forming the valleys  334 . 
     According to one modified example of the first embodiment, the shapes of a sensible heat exchange pipe  62 , sensible heat insulation pipes  64 , and a sensible heat fin  63  may be deformed.  FIG.  7    is a view illustrating an area where the sensible heat exchange pipe  62  and the sensible heat fin  63  are disposed in a vertical sectional view of a heat exchanger unit according to the one modified example of the first embodiment of the present disclosure. 
     According to the one modified example of the first embodiment, the sensible heat insulation pipes  64  may be disposed adjacent to an upstream side of a sensible heat exchanger  60  based on the flow direction of the combustion gas, which is one of directions in which the cross-section of the illustrated sensible heat exchange pipe  62  extends, and a cross-section obtained by cutting the sensible heat insulation pipes  64  with a plane perpendicular to a predetermined direction in which the sensible heat insulation pipes  64  extend may have a circular shape. Furthermore, unlike in  FIG.  6   , the sensible heat insulation pipes  64  may be disposed adjacent to an inner surface of a heat insulation side plate  65 . Unlike in the first embodiment of  FIG.  6   , six sensible heat exchange pipes  62  may be provided in the one modified example of the first embodiment of  FIG.  7   . However, the number of sensible heat exchange pipes  62  is not limited thereto. 
     According to the one modified example of the first embodiment, likewise to second louver holes  6312 , first louver holes  6311  of the sensible heat fin  63  may extend in a direction perpendicular to the flow direction of the combustion gas. Various modifications can be made to the shape of louver holes  631 . 
     The flow passage cap plates  361  and  362  of the sensible heat exchanger  30  according to the first embodiment will be described below with reference to  FIGS.  2 ,  3 ,  5 ,  6 ,  8 , and  9   .  FIG.  8    is a view illustrating the second general sensible heat side plate  3112  according to the first embodiment of the present disclosure and flow passage caps included in the second flow passage cap plate  362  when viewed from the outside along the predetermined direction D 2 .  FIG.  9    is a view illustrating the first general sensible heat side plate  3111  of the heat exchanger unit according to the first embodiment of the present disclosure and flow passage caps included in the first flow passage cap plate  361  when viewed from the inside along the predetermined direction D 2 . 
     Referring to  FIG.  29    for explaining another modified example of the first embodiment of the present disclosure,  FIG.  8    is view in which flow passage caps  3621 ,  3622 , and  3623  of the second flow passage cap plate  362  are illustrated by dotted lines on a view of the second general sensible heat side plate  3112 , the straight portions  321 ,  322 ,  323 , and  324  of the sensible heat exchange pipe  32 , and the sensible heat insulation pipes  341  and  342  of the first embodiment of the present disclosure that corresponds to a view of a second main general side plate  5112  and pipes  32 ,  42 ,  341 , and  342  coupled thereto when viewed from a second connection flow passage cap plate  72  of  FIG.  29    along line H-H′.  FIG.  9    is a view in which flow passage caps  3611  and  3612  of the first flow passage cap plate  361  are illustrated by dotted lines on a view of the first general sensible heat side plate  3111 , the straight portions  321 ,  322 ,  323 , and  324  of the sensible heat exchange pipe  32 , and the sensible heat insulation pipes  341  and  342  of the first embodiment of the present disclosure that corresponds to a view of a first main general side plate  5111 , into which a first connection flow passage cap plate  71  is inserted, when viewed along line G-G′ of  FIG.  29    for explaining the other modified example of the first embodiment of the present disclosure. 
     The heat exchanger unit may include the plurality of flow passage cap plates  361  and  362  including a plurality of flow passage caps that connect the sensible heat insulation pipes  34  and end portions of the sensible heat exchange pipe  32  adjacent to the sensible heat insulation pipes  34  or connect the straight portions  321 ,  322 ,  323 , and  324  adjacent to each other among the plurality of straight portions  321 ,  322 ,  323 , and  324 . The flow passage cap plates  361  and  362  may include the flow passage caps and may connect the straight portions  321 ,  322 ,  323 , and  324  spaced apart from each other, thereby forming a flow passage through which the heating-water flows in the sensible heat exchanger  30 . 
     Specifically, opposite ends of the straight portions  321 ,  322 ,  323 , and  324  included in the sensible heat exchange pipe  32  and opposite ends of the sensible heat insulation pipes  34  are inserted into the general sensible heat side plates  311  of the sensible heat exchanger case  31 , but are open without being closed. The straight portions  321 ,  322 ,  323 , and  324  included in the sensible heat exchange pipe  32  and the sensible heat insulation pipes  34  extend from one of the general sensible heat side plates  311  to the other, and the opposite ends thereof are exposed outside the general sensible heat side plates  311 . The flow passage cap plates  361  and  362  are coupled to the general sensible heat side plates  311  while covering the general sensible heat side plates  311  from the outside. Accordingly, the flow passage caps of the flow passage cap plates  361  and  362 , together with the general sensible heat side plates  311 , form a connection space that surrounds the ends of the straight portions  321 ,  322 ,  323 , and  324  and the ends of the sensible heat insulation pipes  34 . 
     The flow passage caps included in the flow passage cap plates  361  and  362  form an empty connection space, in which fluid is able to flow, between the general sensible heat side plates  311  and the inner surfaces of the flow passage caps. The flow passage caps having the connection space therein may connect two straight portions adjacent to each other among the plurality of straight portions  321 ,  322 ,  323 , and  324  inserted into the general sensible heat side plates  311 , or may connect the sensible heat insulation pipes  34  and straight portions adjacent to the sensible heat insulation pipes  34 . The flow passage cap plates  361  and  362  may be coupled to the general sensible heat side plates  311  through brazing welding, or may be fit into the general sensible heat side plates  311 . However, the coupling method is not limited thereto. 
     The number of straight portions  321 ,  322 ,  323 , and  324  or the sensible heat insulation pipes  34  that the flow passage caps simultaneously connect is not limited to the content illustrated in the drawing. Accordingly, the number of flow passage caps included in one flow passage cap plate  361  or  362  is also not limited to the content illustrated, and a modification can be made to the number of flow passage caps. 
     A flow passage cap may form a series flow passage in which an inlet of one pipe and an outlet of another pipe are connected, or may form a parallel flow passage in which inlets and outlets of connected pipes are common. Here, an inlet refers to an opening at one end of a pipe through which the heating-water is introduced into the pipe, and an outlet refers to an opening at an opposite end of the pipe through which the heating-water is released from the pipe. The pipes include the straight portions  321 ,  322 ,  323 , and  324  and the first and second sensible heat insulation pipes  341  and  342 . In a case of forming a series flow passage using the pipes, acoustic boiling noise generated by the heating-water that flows slow and is overheated may be reduced, and the heating-water may be allowed to flow fast. In a case where a parallel flow passage is at least partially included in the series flow passage, the load of a pump forcibly delivering the heating-water may be decreased. 
     A straight portion in which one end of the sensible heat exchange pipe  32  is located and that is located at the outermost position based on the orthogonal direction is referred to as the first outer straight portion  321 . A sensible heat insulation pipe adjacent to the first outer straight portion  321  is referred to as the first sensible heat insulation pipe  341 . 
     Furthermore, a sensible heat insulation pipe located on the opposite side to the first sensible heat insulation pipe  341  in the orthogonal direction is referred to as the second sensible heat insulation pipe  342 , a straight portion adjacent to the second sensible heat insulation pipe  342  is referred to as the second outer straight portion  322 , and straight portions located between the first outer straight portion  321  and the second outer straight portion  322  are referred to as the intermediate straight portions  323  and  324 . 
     The first sensible heat insulation pipe  341 , the first outer straight portion  321 , the intermediate straight portions  323  and  324 , the second outer straight portion  322 , and the second sensible heat insulation pipe  342  may be sequentially connected to form one series sensible heat flow passage, or may form a parallel flow passage in which inlets and outlets of at least some thereof are common. One intermediate straight portion  323  and the other intermediate straight portion  324  may also be connected in series. 
     The pipes may be connected only in series to form a sensible heat flow passage. For example, inlets and outlets of pipes adjacent to each other among the pipes may be connected in series to form a sensible heat flow passage through which the heating-water is delivered from the first sensible heat insulation pipe  341  to the first outer straight portion  321 , the intermediate straight portion  323  adjacent to the first outer straight portion  321 , the intermediate straight portion  324  adjacent to the second outer straight portion  322 , the second outer straight portion  322 , and the second sensible heat insulation pipe  342  in sequence. A sensible heat flow passage configured only in series will be described below in detail in the description of a sensible heat flow passage included in a heat exchanger unit according to another modified example of the first embodiment of the present disclosure that will be described with reference to  FIGS.  10  to  14   . 
     A sensible heat flow passage may partly include a parallel flow passage, and therefore a case where some of the straight portions  321 ,  322 ,  323 , and  324  are connected in parallel will be described in the description of a sensible heat flow passage according to an embodiment of the present disclosure that is described with reference to  FIGS.  8  and  9   . 
     For example, a parallel flow passage may be configured as follows. The first sensible heat insulation pipe  341  and the first outer straight portion  321  may form a parallel flow passage, the second sensible heat insulation pipe  342  and the second outer straight portion  322  may form a parallel flow passage, the intermediate straight portions  323  and  324  may form a parallel flow passage, the first outer straight portion  321  and the intermediate straight portion  323  may form a parallel flow passage, and the second outer straight portion  322  and the intermediate straight portion  324  may form a parallel flow passage. 
     Furthermore, an entire sensible heat flow passage may be configured by combining a plurality of parallel flow passages among the parallel flow passages with a series flow passage. For example, when the first sensible heat insulation pipe  341  and the first outer straight portion  321  form a parallel flow passage, the parallel flow passage, the intermediate straight portions  323  and  324 , the second outer straight portion  322 , and the second sensible heat insulation pipe  342  may be sequentially connected to form one sensible heat flow passage. In contrast, when the second sensible heat insulation pipe  342  and the second outer straight portion  322  form a parallel flow passage, the first sensible heat insulation pipe  341 , the first outer straight portion  321 , the intermediate straight portions  323  and  324 , and the parallel flow passage may be sequentially connected to form one sensible heat flow passage. Furthermore, in a case where parallel flow passages are formed in the above-described two portions, the parallel flow passages may be connected with the intermediate straight portions  323  and  324  located therebetween to form one sensible heat flow passage. 
     A case where a parallel flow passage receives the heating-water first when the heating-water is introduced into the sensible heat exchanger  30  will be described in the first embodiment of the present disclosure. The first outer straight portion  321  and the first sensible heat insulation pipe  341  may be connected in parallel and may receive and discharge the heating-water together. A heating-water supply hole  371  may be formed in the inlet flow passage cap  3621  among the flow caps included in the second flow passage cap plate  362  that covers the second general sensible heat side plate  3112 . The heating-water supply hole  371  may be an opening that receives the heating-water from a heating-water pipe and delivers the heating-water to the inlet flow passage cap  3621 . The heating-water supply hole  371  may connect a sensible heat flow passage and a latent heat flow passage by receiving the heating-water discharged from the latent heat exchanger  40 . 
     The inlet flow passage cap  3621  connects one end of the first outer straight portion  321  and one end of the first sensible heat insulation pipe  341  that is adjacent to the one end of the first outer straight portion  321 . While the heating-water is supplied to the inlet flow passage cap  3621  through the heating-water supply hole  371 , the heating-water is introduced into the one end of the first outer straight portion  321  and the one end of the first sensible heat insulation pipe  341  that are connected to the inlet flow passage cap  3621 . 
     The heating-water passes through the first outer straight portion  321  and the first sensible heat insulation pipe  341  and reaches the first flow passage cap  3611  of the first flow passage cap plate  361  located on the opposite side to the second flow passage cap plate  362  based on the sensible heat exchange pipe  32 . The first flow passage cap  3611  connects an opposite end of the first sensible heat insulation pipe  341 , an opposite end of the first outer straight portion  321 , and the intermediate straight portion  323  adjacent to the first outer straight portion  321 . Accordingly, the first outer straight portion  321  and the first sensible heat insulation pipe  341  is connected with the adjacent intermediate straight portion  323  in series in the first flow passage cap  3611  and receives the heating-water passing through the first outer straight portion  321  and the first sensible heat insulation pipe  341 . 
     The intermediate straight portion  323  adjacent to the first outer straight portion  321  and the intermediate straight portion  324  adjacent to the second outer straight portion  322  that will be described below may be connected in the intermediate flow passage cap  3623  located on the second flow passage cap plate  362  and may deliver the heating-water from one intermediate straight portion  323  to the other intermediate straight portion  324 . The two intermediate straight portions  323  and  324  form part of a heating-water flow passage in series in the intermediate flow passage cap  3623 . 
     A case where the heating-water is discharged through a parallel flow passage from the sensible heat exchanger  30  will be described. A straight portion disposed adjacent to the second sensible heat insulation pipe  342 , which is the sensible heat insulation pipe  34  through which the heating-water is discharged, is the second outer straight portion  322 . 
     The second outer straight portion  322  and the second sensible heat insulation pipe  342  may be connected in parallel and may receive and discharge the heating-water together. One end of the second outer straight portion  322  and one end of the second sensible heat insulation pipe  342  that is adjacent to the one end of the second outer straight portion  322  are connected with the straight portion  324  adjacent to the second outer straight portion  322  in series in the second flow passage cap  3621  among the flow passage caps included in the first flow passage cap plate  361  that covers the first general sensible heat side plate  3111 . Accordingly, the heating-water delivered to the second flow passage cap  3612  through the adjacent straight portion  324  is introduced into the one end of the second outer straight portion  322  and the one end of the second sensible heat insulation pipe  342 . 
     The heating-water passes through the second outer straight portion  322  and the second sensible heat insulation pipe  342  and is discharged to an opposite end of the second outer straight portion  322  and an opposite end of the second sensible heat insulation pipe  342 . The opposite end of the second outer straight portion  322  and the opposite end of the second sensible heat insulation pipe  342  are connected to the outlet flow passage cap  3622 , which is one of the flow passage caps formed on the second flow passage cap plate  362 , and therefore the heating-water is located in the outlet flow passage cap  3622 . The outlet flow passage cap  3622  includes a heating-water outlet  372 , and the heating-water released to the outlet flow passage cap  3622  is discharged through the heating-water outlet  372 . The heating-water pipe may receive the heated heating-water through the heating-water outlet  372  and may deliver the heating-water to a main flow passage. 
     The description of the configuration of the sensible heat flow passage of the first embodiment may be applied to other embodiments of the present disclosure and modified examples thereof. 
     Latent Heat Exchanger  40   
     The latent heat exchanger  40  will be described below with reference to  FIGS.  2  and  3   . The latent heat exchanger  40  may be disposed downstream of the sensible heat exchanger  30  based on the flow direction D 1  of the combustion gas. The latent heat exchanger  40  receives latent heat generated during a phase change of the combustion gas and heats the heating-water using the latent heat. Accordingly, the combustion gas passing through the sensible heat exchanger  30  is delivered to the latent heat exchanger  40 , the heating-water flows in the latent heat exchanger  40 , and the heating-water and the combustion gas indirectly exchange heat with each other. 
     Similarly to the sensible heat exchanger  30 , the latent heat exchanger  40  may include a latent heat exchange pipe  42  through which the heating-water flows and around which the combustion gas flows. The latent heat exchange pipe  42  may deliver latent heat by a phase change of the combustion gas to the heating-water. The latent heat exchanger  40  may include the latent heat exchanger case  41  into which opposite ends of the latent heat exchange pipe  42  are inserted. The latent heat exchange pipe  42  may be formed to be similar to the sensible heat exchange pipe  32 , and the latent heat exchanger case may also be formed to be similar to the sensible heat exchanger case  31 . Therefore, exceptional characteristics will be described below, but the overall description is replaced with the description of the sensible heat exchanger  30 . However, a phenomenon may arise in which condensate is generated by a phase change of the combustion gas around the latent heat exchange pipe  42  and falls into the condensate receiver  55  by the gravity. 
     Likewise to the sensible heat exchanger  30 , the latent heat exchanger  40  may be of a fin-tube type. Accordingly, a latent heat fin  43  is formed in a plate shape perpendicular to the predetermined direction D 2  in which the latent heat exchange pipe  42  extends, and the latent heat exchange pipe  42  passes through the latent heat fin  43 . The latent heat fin  43  may transfer a larger amount of latent heat to the heating-water by increasing the surface area of the latent heat exchange pipe  42  capable of receiving latent heat. 
     A plurality of latent heat fins  43  may be disposed to be spaced apart from each other at predetermined intervals along the predetermined direction D 2  in which the latent heat exchange pipe  42  extends. The intervals at which the latent heat fins  43  are spaced apart from each other may be intervals by which condensate formed between the adjacent latent heat fins  43  is easily discharged. The intervals by which the condensate is easily discharged refer to intervals between the latent heat fins  43  in a state in which the weight of the condensate formed between the latent heat fins  43  is greater than the vertical resultant force of tensions acting between the latent heat fins  43  and the condensate. The height of the condensate formed between the latent heat fins  43  is inversely proportional to the minimum interval between the latent heat fins  43  by which the condensate is easily discharged. Therefore, the intervals by which the condensate is easily discharged may be determined by selecting an appropriate height of the condensate desired to be discharged from the latent heat exchanger  40 . 
     The number of latent heat fins  43  may be smaller than the number of sensible heat fins  33 . Accordingly, the intervals at which the adjacent latent heat fins  43  are spaced apart from each other may be greater than or equal to the intervals at which the adjacent sensible heat fins  33  are spaced apart from each other. Specific descriptions of the numbers and intervals of the sensible heat fins  33  and the latent heat fins  43  are replaced with contents that will be described below in a third embodiment. The cross-sectional area of the interior space of the latent heat exchange pipe  42  obtained by cutting the latent heat exchange pipe  42  with a plane perpendicular to the direction in which the latent heat exchange pipe  42  extends may be smaller than the cross-sectional area of the interior space of the sensible heat exchange pipe  32  obtained by cutting the sensible heat exchange pipe  32  with a plane perpendicular to the direction in which the sensible heat exchange pipe  32  extends. The direction in which the latent heat exchange pipe  42  extends may also be the predetermined direction D 2 . As in the description of the latent heat fin  43  described above, by making the size of the latent heat exchange pipe  42  smaller than the size of the sensible heat exchange pipe  32 , the latent heat exchange pipe  42  may have a larger surface area in the same volume than the sensible heat exchange pipe  32 . As the surface area of the latent heat exchange pipe  42  is increased, a larger amount of heat may be exchanged between the heating-water flowing along the latent heat exchange pipe  42  and the condensate. 
     The cross-section of the latent heat exchange pipe  42  obtained by cutting the latent heat exchange pipe  42  with a plane perpendicular to the predetermined direction D  2  may have a long hole shape similarly to the cross-section of the sensible heat exchange pipe  32 . 
     In the first embodiment of the present disclosure, the latent heat exchanger  40  is illustrated as having no means for thermal insulation. However, in various modified examples, the latent heat exchanger  40  may also have latent heat insulation pipes (not illustrated) that are disposed in the same manner as the sensible heat insulation pipes  34 . The latent heat insulation pipes may be disposed adjacent to the latent heat exchanger case, and the heating-water may flow through the latent heat insulation pipes to thermally insulate the latent heat exchanger  40 . 
     Although the sensible heat exchanger case  31  and the latent heat exchanger case  41  have been described as separate from each other, the sensible heat exchanger case  31  and the latent heat exchanger case  41  may be integrally formed with each other as illustrated in the drawing. In this case, an integrated main case  51  including both the sensible heat exchanger case  31  and the latent heat exchanger case  41  may be considered. Accordingly, the sensible heat insulation side plates  312  of the sensible heat exchanger  30  and latent heat insulation side plates  412  of the latent heat exchanger  40  may integrally form main heat insulation side plates  512 , and the general sensible heat side plates  311  of the sensible heat exchanger  30  and general latent heat side plates  411  of the latent heat exchanger  40  may integrally form main general latent heat side plates  511 . Likewise, a first main general side plate  5111  included in the main general side plates  511  may include the first sensible heat insulation side plate  3111  and a first latent heat insulation side plate  4111  located at the same side along the predetermined direction D 2 , and a second main general side plate  5112  included in the main general side plates  511  may include the second sensible heat insulation side plate  3112  and a second latent heat insulation side plate  4112  located on the opposite side along the predetermined direction D 2 . 
     Hereinafter, a situation in which heat exchangers  30  and  40  of a heat exchanger unit according to another modified example of the first embodiment of the present disclosure are connected by connection flow passage cap plates  71  and  72  to form a sensible heat flow passage and a latent heat flow passage connected together will be described below with reference to  FIGS.  10  to  14  and  29   . 
       FIG.  10    is a view illustrating the heat exchanger unit according to the other modified example of the first embodiment of the present disclosure when viewed from outside the second connection flow passage cap plate  72 .  FIG.  11    is a view illustrating the first connection flow passage cap plate  71  of the heat exchanger unit according to the other modified example of the first embodiment of the present disclosure.  FIG.  12    is a view illustrating a partial area of a second main general side plate  5112  of the heat exchanger unit according to the other modified example of the first embodiment of the present disclosure together with flow passage caps included in the second connection flow passage cap plate  72  when viewed from the outside along a predetermined direction.  FIG.  13    is a view illustrating a first main general side plate  5111  of the heat exchanger unit according to the other modified example of the first embodiment of the present disclosure together with flow passage caps included in the first connection flow passage cap plate  71  when viewed from the inside along a predetermined direction.  FIG.  14    is a perspective view illustrating the sensible heat flow passage and the latent heat flow passage of the heat exchanger unit according to the other modified example of the first embodiment of the present disclosure.  FIG.  29    is a perspective view illustrating a situation in which the connection flow passage cap plates are separated from each other in the heat exchanger unit according to the other modified example of the first embodiment of the present disclosure. 
       FIG.  12    is view in which flow passage caps  722 ,  723 ,  724 , and  725  of the second connection flow passage cap plate  72  are illustrated by dotted lines on a view of the second main general side plate  5112 , straight portions  321 ,  322 ,  323 , and  324  of a sensible heat exchange pipe  32 , and sensible heat insulation pipes  341  and  342  according to the other modified example of the first embodiment of the present disclosure when viewed from the second connection flow passage cap plate  72  of  FIG.  29    along line H-H′  FIG.  13    is view in which flow passage caps  712 ,  713 , and  714  of the first connection flow passage cap plate  71  are illustrated by dotted lines on a view of the first main general side plate  5111 , the straight portions  321 ,  322 ,  323 , and  324  of the sensible heat exchange pipe  32 , and the sensible heat insulation pipes  341  and  342  according to the other modified example of the first embodiment of the present disclosure when viewed along line G-G′ of  FIG.  29   . 
     In the other modified example of the first embodiment of the present disclosure, the latent heat flow passage that is connected to the sensible heat flow passage and through which heating-water flows is formed by a latent heat exchange pipe  42 , and the sensible heat flow passage through which the heating-water flows is formed by the sensible heat exchange pipe  32  and sensible heat insulation pipes  34 . In  FIG.  14   , the latent heat flow passage is represented in the form of an arrow passing through the latent heat exchange pipe  42 , and the sensible heat flow passage is represented in the form of an arrow passing through the sensible heat exchange pipe  32  and the sensible heat insulation pipes  341  and  342 . For a better understanding of areas through which the flow passages pass, the flow passage caps of the connection flow passage cap plates  71  and  72  are not illustrated in  FIG.  14    in a state in which general side plates, heat insulation side plates, and fins of the heat exchanger unit are removed. The sensible heat flow passage and the latent heat flow passage are connected to form an integrated heating-water flow passage. The sensible heat flow passage may include a series flow passage in at least a partial section, and the latent heat flow passage may include a parallel flow passage in at least a partial section. In the other modified example of the first embodiment of the present disclosure illustrated in  FIGS.  10  to  14  and  29   , the sensible heat flow passage is configured to include only a series flow passage, and the latent heat flow passage is configured to include a parallel flow passage. 
     To form the heating-water flow passage without connection by a separate tube body, the connection flow passage cap plates  71  and  72  connecting the sensible heat exchanger  30  and the latent heat exchanger  30  may be disposed in the other modified example of the first embodiment of the present disclosure. 
     To connect openings of the latent heat exchange pipe  42 , the sensible heat exchange pipe  32 , and the sensible heat insulation pipes  34  exposed outside the two main general side plates  5111  and  5112  of the main case ( 51  of  FIG.  2   ), the connection flow passage cap plates  71  and  72 , a kind of a flow passage cap plate, include flow passage caps having, between the main general side plate  511  and the flow passage caps, a connection space surrounding the openings. 
     To connect an outlet of the latent heat flow passage that is exposed outside a reference side plate, which is one of the two main general side plates  5111  and  5112 , and is formed by the latent heat exchange pipe  42  and an inlet of the sensible heat flow passage that is exposed outside the reference side plate and that introduces the heating-water into the sensible heat insulation pipes  34 , one of the connection flow passage cap plates  71  and  72  that is located on one side along the predetermined direction D 1  includes a connection flow passage cap having, between the reference side plate and the connection flow passage cap, a connection space surrounding the outlet of the latent heat flow passage and the inlet of the sensible heat flow passage. 
     In the other modified example of the first embodiment of the present disclosure, the reference side plate is the second main general side plate  5112 , and the one connection flow passage cap plate is the second connection flow passage cap plate  72  including the connection flow passage cap  722 . However, the position where the reference side plate is disposed is not limited thereto. 
     The connection flow passage cap  722  extends along the flow direction D 1  of the combustion gas to connect the sensible heat exchanger  30  and the latent heat exchanger  40  stacked on each other. Furthermore, as the connection flow passage cap  722  connects a plurality of straight portions included in the latent heat exchange pipe  42  and the sensible heat insulation pipes  34 , the connection flow passage cap  722  may extend into the latent heat exchanger  40  while extending along the flow direction D 1  of the combustion gas. Accordingly, the connection flow passage cap  722  that is not completely parallel to the flow direction D 1  of the combustion gas and that has a portion in an inclined form may be formed. 
     An inlet flow passage cap  721  having a heating-water supply hole  7211  formed therein and the outlet flow passage cap  725  having a heating-water discharge hole  7251  that is the outlet of the sensible heat flow passage are formed in the second connection flow passage cap plate  72 . The outlet of the sensible heat flow passage is implemented by the outlet of the second sensible heat insulation pipe  342 . In the other modified example of the first embodiment of the present disclosure, it is assumed that the heating-water is introduced into the latent heat exchanger  40  through the heating-water supply hole  7211 , the heating-water flows to the sensible heat exchanger  30  through the connection flow passage cap  722 , and the heating-water is heated and discharged through the heating-water discharge hole  7251  from the sensible heat exchanger  30 . However, the inlet flow passage cap  721  and the heating-water supply hole  7211  may be disposed to be connected with the sensible heat exchanger  30 , the outlet flow passage cap  725  and the heating-water discharge hole  7251  may be disposed to be connected with the latent heat exchanger  40 , and the heating-water flow passage may be formed in an opposite direction such that the heating-water passing through the sensible heat exchanger  30  faces toward the latent heat exchanger  40 . 
     The plurality of straight portions included in the latent heat exchange pipe  42  may be connected to the inlet flow passage cap  721  in parallel, and the heating-water introduced through the heating-water supply hole  7211  may move along the parallel flow passage. The outlet of the second sensible heat insulation pipe  342  may be connected to the outlet flow passage cap  725 , and the outlet flow passage cap  725  may receive, from the second sensible heat insulation pipe  342 , the heating-water heated via the sensible heat flow passage and may discharge the heating-water. 
     When it is assumed that a virtual rectangular parallelepiped accommodates both the latent heat exchanger  40  and the sensible heat exchanger  30 , the heating-water supply hole  7211  that is the inlet of the latent heat flow passage and the heating-water discharge hole  7251  that is the outlet of the sensible heat flow passage may be provided together on a reference surface that is one of the six surfaces of the rectangular parallelepiped. In other words, both the heating-water supply hole  7211  and the heating-water discharge hole  7251  may be provided in a flow passage cap plate that covers one of side plates constituting the main case ( 51  of  FIG.  1   ). In the other modified example of the first embodiment of the present disclosure, the one side plate may be the second main general side plate  5111  that forms a connection space together with the flow passage caps of the second connection flow passage cap plate  72 , and the flow passage cap plate covering the one side plate is the second connection flow passage cap plate  72 . Accordingly, the heating-water is introduced into and discharged from the heat exchanger unit through a side surface on which the second connection flow passage cap plate  72  is disposed among side surfaces of the heat exchanger unit. However, the reference surface may be differently disposed without being limited thereto. 
     As the heating-water supply hole  7211  and the heating-water discharge hole  7251  are disposed in the same side surface of the heat exchanger unit, the direction in which the heating-water is introduced through the heating-water supply hole  7211  and the direction which the heating-water is discharged through the heating-water discharge hole  7251  may be opposite to each other. As the heating-water is introduced and discharged through the same side surface, a space required for arranging a heating-water pipe connected to the heating-water supply hole  7211  and the heating-water discharge hole  7251  may be saved. However, the heating-water supply hole  7211  and the heating-water discharge hole  7251  may be disposed in opposite side surfaces. 
     To locate the heating-water supply hole  7211  and the heating-water discharge hole  7251  in the same side surface, the heating-water flow passage may include an even number of sections in which the heating-water faces from one side to an opposite side of the predetermined direction D 2  or from the opposite side to the one side. That is, the number of times that the heating-water faces from one side surface to another side surface of the heat exchanger unit based on the predetermined direction D 2  may be an even number in the entire heating-water flow passage. In other words, when only a change in a progress direction from one side to an opposite side of the predetermined direction D 2  or from the opposite side to the one side is counted, the heating-water flow passage may change the direction an odd number of times. In the other modified example of the first embodiment of the present disclosure, the entire heating-water flow passage changes the direction a total of seven times, but the number of times is not limited thereto. In other words, in the latent heat flow passage and the sensible heat flow passage, the number of sections connecting the reference surface and the surface opposite to the reference surface along the predetermined direction D 2  may be an even number such that the heating-water flowing from the reference surface to the surface located on the opposite side to the reference surface returns to the reference surface again. 
     The description of the positions of the heating-water supply hole  7211  and the heating-water discharge hole  7251  of the first embodiment may be applied to other embodiments of the present disclosure and modified examples thereof. 
     The second connection flow passage cap plate  72  includes the second sensible heat flow passage cap  723  and the fourth sensible heat flow passage cap  724  that connect the straight portions  321 ,  322 ,  323 , and  324  included in the sensible heat exchange pipe  32 . The second sensible heat flow passage cap  723  may connect the first outer straight portion  321  and the intermediate straight portion  323  in series, and the third sensible heat flow passage cap  724  may connect the second outer straight portion  322  and the intermediate straight portion  324  in series. 
     The first connection flow passage cap plate  71  is coupled to the first main general side plate  5111  on the opposite side to the second connection flow passage cap plate  72  based on the sensible heat exchanger  30  and the latent heat exchanger  40 . Accordingly, the connection flow passage cap  722  is not formed, and the first connection flow passage cap plate  71  includes the latent heat flow passage cap  722  connecting the straight portions included in the latent heat exchange pipe  42 , the first sensible heat flow passage cap  712  connecting the straight portions included in the sensible heat exchange pipe  32 , the third sensible heat flow passage cap  713 , and the fifth sensible heat flow passage cap  714 . In  FIG.  11   , one latent heat flow passage cap  711  is formed. However, the number of latent heat flow passage caps  711  is not limited thereto, and a plurality of latent heat flow passage caps  711  may be formed. 
     The latent heat flow passage cap  711  may be connected with ends of the plurality of straight portions included in the latent heat exchange pipe  42 . Accordingly, the plurality of straight portions included in the latent heat exchange pipe  42  may form a parallel flow passage. The first sensible heat flow passage cap  712  may connect the first sensible heat insulation pipe  341  and the first outer straight portion  321 , the third sensible heat flow passage cap  713  may connect the intermediate straight portions  323  and  324 , and the fifth sensible heat flow passage cap  714  may connect the second outer straight portion  322  and the second sensible heat insulation pipe  342 . 
     The description of the configuration of the latent heat flow passage including the parallel flow passage of the first embodiment may be applied to other embodiments of the present disclosure and modified examples thereof. 
     The heating-water flow passage formed by the first connection flow passage cap plate  71  and the second connection flow passage cap plate  72  according to the other modified example of the first embodiment of the present disclosure will be described along a flow of heating-water. The heating-water is introduced into the latent heat exchanger  40  through the heating-water supply hole  7211  formed in the inlet flow passage cap  721  of the second connection flow passage cap plate  72 . As the inlet flow passage cap  721  connects the plurality of straight portions of the latent heat exchange pipe  42  in parallel, the heating-water is delivered to the latent heat flow passage cap  711 , which is formed on the second connection flow passage cap plate  72 , along the parallel flow passage through the plurality of latent heat exchange pipes  42  connected to the inlet flow passage cap  721 . 
     As the latent heat flow passage cap  722  connects the latent heat exchange pipes  42  in parallel, the heating-water is delivered to the connection flow passage cap  713  through the plurality of latent heat exchange pipes  42  not connected with the inlet flow passage cap  712  and connected with the connection flow passage cap  713  in parallel. That is, the heating-water flows in parallel in the area of the heating-water flow passage that corresponds to the latent heat exchanger  40 . 
     The connection flow passage cap  722  is connected with the first sensible heat insulation pipe  341 . The heating-water flows through the first sensible heat insulation pipe  341  and is delivered to the first sensible heat flow passage cap  712  of the first connection flow passage cap plate  71 , and heat loss of the sensible heat exchanger  30  is interrupted. 
     The heating-water is delivered to the first outer straight portion  321  connected to the first sensible heat flow passage cap  712 . The heating-water is delivered to the second sensible heat flow passage cap  723 . As the intermediate straight portion  323  is connected to the second sensible heat flow passage cap  723 , the heating-water flows along the intermediate straight portion  323  and is delivered to the third sensible heat flow passage cap  713 . As the intermediate straight portion  324  is connected to the third sensible heat flow passage cap  713 , the heating-water flows along the intermediate straight portion  324  and is delivered to the fourth sensible heat flow passage cap  724 . As the second outer straight portion  322  is connected to the fourth sensible heat flow passage cap  724 , the heating-water flows along the second outer straight portion  322  and is delivered to the fifth sensible heat flow passage cap  714 . As the second sensible heat insulation pipe  342  is connected to the fifth sensible heat flow passage cap  714 , the heating-water flows along the second sensible heat insulation pipe  342  and is delivered to the outlet flow passage cap  725 . 
     That is, the heating-water flows along the sensible heat flow passage in series. The heating-water is heated by sensible heat while reciprocating between the first connection flow passage cap plate  71  and the second connection flow passage cap plate  72  and is delivered to the second sensible heat insulation pipe  342 . 
     The second sensible heat insulation pipe  342  interrupts heat loss of the sensible heat exchanger  30  while delivering the heating-water to the outlet flow passage cap  725 , and the heating-water is discharged through the heating-water discharge hole  7251  and is used for heating. 
     Main Flow Passage 
     The condensing boiler  1  including the heat exchanger according to the first embodiment of the present disclosure includes the main flow passage. The main flow passage is a pipe that is directly or indirectly connected to a heating flow passage for providing heating and that supplies the heating-water to the heating flow passage. The main flow passage is directly or indirectly connected to the sensible heat exchanger  30  or the latent heat exchanger  40  and supplies the heating-water to the heat exchanger to heat the heating-water or supplies the heated heating-water from the heat exchanger to the heating flow passage. Accordingly, the heating-water pipe connected with the sensible heat exchanger  30  and the latent heat exchanger  40  to supply or receive the heating-water may be included in the main flow passage. 
     Second Embodiment 
       FIG.  15    is a vertical sectional view of a heat exchanger unit according to the second embodiment of the present disclosure. 
     Referring to  FIG.  15   , the heat exchanger unit according to the second embodiment of the present disclosure may have a sensible heat exchanger  81  and a latent heat exchanger  82  having two rows of latent heat exchangers. A first latent heat exchanger  821  located at an upstream side based on a flow direction of combustion gas may have a greater width in an orthogonal direction than a second latent heat exchanger  822 . 
     Furthermore, the heat exchanger unit according to the second embodiment of the present disclosure may have a larger number of straight portions  8211  included in a latent heat exchange pipe and a larger number of straight portions  811  included in a sensible heat exchange pipe than the heat exchanger units according to the first embodiment of the present disclosure and the one modified example of the first embodiment of the present disclosure. The number of straight portions that the first latent heat exchanger  821  has may be larger than the number of straight portions that the second latent heat exchanger  821  has. 
       FIG.  16    is a front view illustrating a flow passage cap plate  90  of a heat exchanger unit according to a modified example of the second embodiment of the present disclosure together with pipes. The pipes are illustrated by dotted lines. 
     Referring to  FIG.  16   , the flow passage cap plate  90  of the heat exchanger unit according to the modified example of the second embodiment of the present disclosure includes a heating-water discharge hole  91  not formed through a flow passage cap and directly formed through the flow passage cap plate  90 . The heating-water discharge hole  91  may not be located downstream of a sensible heat exchange pipe  95  along a flow direction D 1  of combustion gas and may be disposed on the same line along an orthogonal direction so as to be adjacent to the sensible heat exchange pipe  95 . 
     The flow passage cap plate  90  according to the modified example of the second embodiment of the present disclosure may include a modified connection flow passage cap  92 . When the modified connection flow passage cap  92  is compared with the connection flow passage cap ( 722  of  FIG.  10   ) according to the other modified example of the first embodiment of the present disclosure, the length of an inclined portion  922  formed to be inclined with respect to the orthogonal direction and the flow direction D 1  of the combustion gas is smaller than the length of a portion  923  extending along the flow direction D 1  of the combustion gas and the length of a portion  921  extending along the orthogonal direction. Furthermore, when the modified connection flow passage cap  92  is compared with the connection flow passage cap ( 722  of  FIG.  10   ) according to the other modified example of the first embodiment of the present disclosure, the width of the inclined portion  922  is smaller than the width of the portion  921  extending along the orthogonal direction. 
     Due to the position of the heating-water discharge hole  912  and the shape of the connection flow passage cap  92 , the flow passage cap plate  90  may have an asymmetrical structure that does not have line symmetry with respect to a line parallel to the flow direction D 1  of the combustion gas. The flow passage cap plate  90  may have a tapered shape having a decreasing width along the flow direction D 1  of the combustion gas. In  FIG.  16   , a left inclined portion  93  and a right inclined portion  94  may be configured to have tapered outer surfaces from different locations to other different locations based on the flow direction D 1  of the combustion gas. This is to reduce a waste of material by cutting a portion corresponding to an unnecessary area. 
     The shapes of the sensible heat exchanger  81  and the latent heat exchanger  82  according to the second embodiment or the shape of the flow passage cap plate  90  according to the modified example of the second embodiment may be applied to other embodiments of the present disclosure and modified examples thereof. 
     Third Embodiment 
       FIG.  17    is a vertical sectional view of a heat exchanger unit and a condensing boiler  2  using the same according to the third embodiment of the present disclosure.  FIG.  18    is a side view of the heat exchanger unit and the condensing boiler  2  using the same according to the third embodiment of the present disclosure. 
     Referring to the drawings, the condensing boiler  2  according to the third embodiment of the present disclosure includes a combustion chamber  20  and the heat exchanger unit. 
     Furthermore, the condensing boiler  2  including the heat exchanger unit according to the third embodiment of the present disclosure includes a burner assembly  10  including a burner  11 . The burner assembly  10  and the heat exchanger unit are disposed in sequence along a reference direction D 1 , which is a flow direction of combustion gas, and components are arranged in the heat exchanger unit along the same direction in the order of the combustion chamber  20  and the heat exchanger unit. Accordingly, the components of the condensing boiler  2  will be described below in the above-described order of arrangement. 
     The heat exchanger unit according to the third embodiment of the present disclosure, and the burner assembly  10 , the combustion chamber  20 , a condensate receiver  55 , a condensate outlet  53 , and an exhaust duct  52  included in the condensing boiler  2  using the heat exchanger unit are identical or very similar to the corresponding components of the first embodiment. Therefore, descriptions thereof are replaced with the above-described contents of the first embodiment. 
     Heat Exchanger Unit 
       FIG.  19    is a plan view of the heat exchanger unit according to the third embodiment of the present disclosure.  FIG.  20    is a vertical sectional view of the heat exchanger unit according to the third embodiment of the present disclosure. 
     Referring to the drawings, the heat exchanger unit according to the third embodiment of the present disclosure includes a sensible heat exchanging part  300  and a latent heat exchanging part  400 . Furthermore, the heat exchanger unit of the present disclosure may include a housing  510  that defines heat exchange areas inside by surrounding a sensible heat exchange area and a latent heat exchange area in which the heat exchanging parts  300  and  400  are disposed. 
     The sensible heat exchanging part  300  and the latent heat exchanging part  400  may be disposed in the sensible heat exchange area and the latent heat exchange area, respectively. The sensible heat exchange area and the latent heat exchange area may be connected, and the combustion gas delivered from the combustion chamber  20  may flow in the sensible heat exchange area and the latent heat exchange area along the reference direction D 1  that is the flow direction. 
     Heat Exchanger Unit—Sensible Heat Exchanging Part  300   
     The sensible heat exchange area is an area that is located downstream of the combustion chamber  20  based on the reference direction D 1  and that receives sensible heat generated at the upstream side and heats heating-water. The size of the sensible heat exchange area is determined by a space from the most upstream side to the most downstream side of the sensible heat exchanging part  300  along the reference direction D 1  in the space surrounded by the housing  510 . Accordingly, the sensible heat exchange area may be connected with an interior space  22  of the combustion chamber  20  and may receive radiant heat from the burner  11 , and the combustion gas may flow in the sensible heat exchange area. Furthermore, because the sensible heat exchange area has to be able to transfer sensible heat to the heating-water, the sensible heat exchanging part  300  including a sensible heat exchange pipe  320  and a sensible heat fin  330  is disposed in the sensible heat exchange area. 
     The sensible heat exchange pipe  320  is a pipe type component through which the heating-water flows and around which the combustion gas flows. The sensible heat exchange pipe  320  extends along a predetermined direction D 2  in the sensible heat exchange area  32 . The predetermined direction D 2  may preferably be a direction perpendicular to the reference direction D 1 . The sensible heat exchange pipe  320  may extend along the predetermined direction D 2  and may be coupled to the housing  510 . 
     The sensible heat exchange pipe  320  may include a plurality of sensible heat straight portions. The sensible heat straight portions may be arranged to be spaced apart from each other along an orthogonal direction perpendicular to the predetermined direction D 2 . The plurality of sensible heat straight portions of the sensible heat exchange pipe  320  may be coupled to flow passage cap plates  363  and  364  of the housing  510 , which will be described below, to form one sensible heat flow passage through which the heating-water flows. 
     The sensible heat fin  330  is formed in a plate shape across the direction in which the sensible heat exchange pipe  320  extends, and the sensible heat exchange pipe  320  passes through the sensible heat fin  330 . As the sensible heat fin  330  has a shape through which the sensible heat exchange pipe  320  passes, the sensible heat exchanging part  300  may configure a heat exchanger of a fin-tube type. 
     As the sensible heat exchanging part  300  includes the sensible heat fin  330 , the thermal conductivity of the sensible heat exchange pipe  320  may be raised. A plurality of sensible heat fins  330  may be disposed to be spaced apart from each other at predetermined intervals along the predetermined direction D 2  in which the sensible heat exchange pipe  320  extends. The sensible heat fin  330  may transfer a larger amount of sensible heat to the heating-water by increasing the surface area of the sensible heat exchange pipe  320  capable of receiving sensible heat. Accordingly, for effective heat transfer, the sensible heat exchange pipe  320  and the sensible heat fin  330  may be formed of metal having a high thermal conductivity. 
     A cross-section obtained by cutting the sensible heat exchange pipe  320  with a plane perpendicular to the predetermined direction D 2  in which the sensible heat exchange pipe  320  extends may be formed in the shape of a long hole that extends along the reference direction D 1 . As can be seen in the drawings, the sensible heat exchange pipe  320  according to the third embodiment of the present disclosure may have a flat shape formed such that a value obtained by dividing the length of the interior space in the cross-section based on the reference direction D 1  by the width according to the direction perpendicular to the reference direction D 1  equals 2 or more. When the flat type pipe having the above-described shape is employed for the sensible heat exchange pipe  320 , due to a wider heat exchange area in the relationship with the combustion gas, the heating-water may receive a larger amount of heat and may be sufficiently heated even though flowing along the sensible heat exchange pipe  320  having the same length, as compared with when a pipe having a different shape, such as a circular shape or an oval shape, is employed for the sensible heat exchange pipe  320 . 
     A through-hole through which the sensible heat exchange pipe  320  passes may be formed in the sensible heat fin  330 . The area of the through-hole may be equal to or smaller than the area of the sensible heat exchange pipe  320 , and the sensible heat exchange pipe  320  may be firmly inserted into the through-hole. Furthermore, the sensible heat fin  330  may be integrally coupled with the sensible heat exchange pipe  320  through brazing welding. A method of coupling the sensible heat fin  330  and the sensible heat exchange pipe  320  through brazing welding will be described in detail in the descriptions of  FIGS.  15  and  16   . 
     Common louver holes  3303  and  3304  may be additionally formed in the sensible heat fin  330  along the direction in which the sensible heat exchange pipe  320  extends. The louver holes  3303  and  3304  may be formed by punching. The louver holes  3303  and  3304  include a burr raised along the periphery thereof. When the combustion gas flows, the burr blocks the combustion gas to cause the combustion gas to flow around the sensible heat exchange pipe  320 , thereby facilitating heat exchange between the combustion gas and the heating-water. A plurality of louver holes  3303  and  3304  may be formed. The louver holes  3303  and  3304 , as illustrated in the drawings, may include the first louver holes  3303  that extend in an oblique direction with respect to the reference direction D 1  and the second louver holes  3304  that are formed between the adjacent sensible heat straight portions of the sensible heat exchange pipe  320  and that extend in the orthogonal direction perpendicular to the reference direction D 1 . The louver holes  3303  and  3304  may be disposed to be spaced apart from each other at predetermined intervals along the reference direction D 1 . 
     The sensible heat fin  330  may further include valleys  3302  and protruding portions  3301 . The sensible heat fin  330  may be basically formed to surround the sensible heat exchange pipe  320 . The sensible heat fin  33  may surround areas corresponding to a predetermined width from the peripheries of upstream-side end portions of the sensible heat exchange pipe  320  based on the reference direction D 1  such that the areas are distinguished from the remaining areas of the sensible heat exchange pipe  320 . Accordingly, between the adjacent upstream-side end portions of the sensible heat exchange pipe  320 , the valleys  3302  may be concavely formed in the sensible heat fin  330  along the reference direction D 1 . The areas of the sensible heat fin  330  that are adjacent to the upstream-side end portions of the sensible heat exchange pipe  320  relatively protrude to form the protruding portions  3301 . Unnecessary areas are open by forming the valleys  3302 , and thus the combustion gas may more freely flow between the sensible heat fin  330  and the sensible heat exchange pipe  320 . 
     Heat Exchanger Unit—Latent Heat Exchanging Part  400   
     The latent heat exchange area is an area that is located downstream of the sensible heat exchange area based on the reference direction D 1  and that receives latent heat generated during a phase change of the combustion gas and heats the heating-water. The size of the latent heat exchange area is determined by a space from the most upstream side to the most downstream side of the latent heat exchanging part  400  along the reference direction D 1  in the space surrounded by the housing  510 . The latent heat exchanging part  400  that includes a latent heat exchange pipe  420  through which the heating-water flows and around which the combustion gas flows and a latent heat fin  430  that is formed in a plate shape across the predetermined direction D 2 , in which the latent heat exchange pipe  420  extends, and through which the latent heat exchange pipe  420  passes is disposed in the latent heat exchange area. 
     The configurations of the latent heat exchange pipe  420  and the latent heat fin  430  are similar to the configurations of the sensible heat exchange pipe  320  and the sensible heat fin  330 . Therefore, descriptions of basic structures of the latent heat exchange pipe  420  and the latent heat fin  430  are replaced with the above descriptions of the structures of the sensible heat exchange pipe  320  and the sensible heat fin  330 . Accordingly, the latent heat exchanging part  400  may also be configured in a fin-tube type. 
     The latent heat exchange pipe  420  may include a plurality of upstream straight portions  421  and a plurality of downstream straight portions  422  located downstream of the upstream straight portions  421  based on the reference direction D 1 . One of the plurality of downstream straight portions  422  may be connected with one upstream straight portion  421  among the plurality of upstream straight portions  421 . That is, the latent heat exchange pipe  420  may be disposed in two rows. The latent heat exchange pipe  420  may be disposed to have more than two rows. As the latent heat exchange pipe  420  has the plurality of rows of straight portions, the latent heat exchange pipe  420  may improve thermal efficiency that is likely to be degraded when a fin-tube type is used. 
     In  FIG.  20   , four upstream straight portions  421  and three downstream straight portions  422  are disposed. This is because the reference cross-sectional area of the latent heat exchange area is decreased along the reference direction D 1  as will be described below. However, the number of latent heat straight portions  421  and  422  constituting the latent heat exchange pipe  420  and extending in the predetermined direction D 2  is not limited thereto. 
     As the latent heat exchange pipe  420  is disposed in two rows, the latent heat fin  430  may also be disposed to be separated depending on the latent heat exchange pipe  420 . That is, an upstream fin  431  included in the latent heat fin  430  may be coupled to the upstream straight portions  421 , and a downstream fin  432  included in the latent heat fin  430  may be coupled to the downstream straight portions  422 . 
     As the latent heat exchange pipe  420  is disposed in two rows, a situation in which the combustion gas fails to sufficiently transfer heat to the heating-water due to a deficiency of a heat transfer area in the latent heat exchange area may be prevented, and as sufficient heat exchange occurs over a wide area for the entire combustion gas, a fraction at which the combustion gas is discharged without experiencing a phase change may be reduced. 
     The cross-sectional area of the interior spaces of the latent heat straight portions  421  and  422  of the latent heat exchange pipe  420  may be smaller than the cross-sectional area of the interior spaces of the sensible heat straight portions of the sensible heat exchange pipe  320 . Instead of making the cross-sectional area of the latent heat straight portions  421  and  422  smaller than the cross-sectional area of the interior spaces of the sensible heat straight portions, the total number of sensible heat straight portions may be made smaller than the total number of latent heat straight portions  421  and  422  such that the product of the cross-sectional area of the interior spaces of the sensible heat straight portions and the total length of the sensible heat exchange pipe  320  remains at a numerical value corresponding to the product of the cross-sectional area of the interior spaces of the latent heat straight portions  421  and  422  and the total length of the latent heat exchange pipe  420 . 
     In other words, the latent heat exchange pipe  420  may be formed such that in a cross-section obtained by cutting the sensible heat exchange pipe  320  with a plane perpendicular to the direction in which the sensible heat exchange pipe  320  extends, the number of closed curves formed by the peripheries of the sensible heat straight portions is smaller than the number of closed curves formed by the peripheries of the latent heat straight portions  421  and  422 . In a case where the same number of or more pipes having a larger cross-sectional area than the latent heat straight portions  421  and  422  are disposed in the sensible heat exchanging part  300 , when the heating-water moves to the adjacent sensible heat exchange pipe  320  via the flow passage cap plates  363  and  364 , the heating-water may not be efficiently circulated due to a rapid pressure drop in the heating-water that occurs in a section where a flow passage is sharply bent. Accordingly, the cross-sectional areas and the total numbers of the sensible heat exchange pipes  320  and the latent heat exchange pipes  420  are adjusted as described above. The contents regarding the cross-sectional areas and the total numbers of the heat exchange pipes may be applied to other embodiments and modified examples thereof. Likewise to the sensible heat fin  330 , a plurality of latent heat fins  430  are disposed to be spaced apart from each other at predetermined intervals along the direction in which the latent heat exchange pipe  420  extends. 
     One or more layers in which the latent heat fins  430  located in the same position based on the reference direction D 1  are disposed may be formed. The total number of latent heat fins  430  disposed in the layer most adjacent to the sensible heat fin  330  among the layers may be smaller than the total number of sensible heat fins  330 . 
     Referring to the drawings, a total of two layers including one layer formed by the upstream fin  431  and one layer formed by the downstream fin  432  may be disposed. The upstream fin  431  is disposed in the layer most adjacent to the sensible heat fin  330  among the layers. The total number of upstream fins  431  may be smaller than the total number of sensible heat fins  330 . 
     The distance by which the adjacent two latent heat fins  430  are spaced apart from each other may be longer than the distance by which the adjacent two sensible heat fins  330  are spaced apart from each other. To prevent condensate from being easily formed between the latent heat fins  430  and hampering a movement of the combustion gas, the interval between the latent heat fins  430  is greater than the interval between the sensible heat fins  330 . In the latent heat fin  430 , the distance by which the adjacent two downstream fins  432  are spaced apart from each other may be longer than the distance by which the adjacent two upstream fins  431  are spaced apart from each other. 
     A predetermined interval at which the adjacent latent heat fins  430  are spaced apart from each other along the predetermined direction D 2  may be a distance by which condensate formed by condensation of the combustion gas between the adjacent latent heat fins  430  does not connect the adjacent latent heat fins  430 . That is, the distance between the adjacent latent heat fins  430  may be an interval by which condensate is easily discharged. 
       FIG.  21    is a perspective view illustrating a plurality of downstream fins  432  according to the third embodiment of the present disclosure and condensate W located therebetween. The distance between the adjacent latent heat fins  430  will be described with reference to  FIG.  21   , with the downstream fins  432  among the latent heat fins  430  as an example. 
     Drops of condensate may be formed and attached to surfaces of the downstream fins  432 . The drops of condensate formed on the surfaces of the adjacent downstream fins  432  may be combined to form a large drop of condensate W that blocks the space between the latent heat fins  430 , and the combustion gas may not smoothly move along the reference direction D 1  due to the large drop of condensate W. Accordingly, the downstream fins  432  are disposed to be spaced apart from each other at a predetermined interval or more such that the drops of condensate are not combined with each other and the combustion gas flows between the adjacent downstream fins  432 . 
     Specifically, the interval by which the condensate W is easily discharged refers to the interval between the adjacent downstream fins  432  in a state in which the weight of the condensate W formed between the downstream fins  432  is greater than the vertical resultant force of tensions T acting between the downstream fins  432  and the condensate W. 
     Referring to the drawing, the condensate W exists between the downstream fins  432  that are spaced apart from each other by a distance of d and adjacent to each other and that have a width of b in the predetermined direction D 2 . At this time, the weight (or, the body force) of the condensate W formed to a height of h is represented by the product of the volume of the condensate W (the distance d×the width b×the height h) and the specific gravity Y of the condensate W. The weight acts on the condensate in the vertically downward direction. 
     Meanwhile, the force acting on the condensate W in the vertically upward direction is formed by the resultant force of surface tensions. The distance d satisfying Equation 1 below is the interval by which the condensate W is easily discharged, where θ is the angle that a line extending from the surface of the condensate W forms with each of the downstream fins  432  and T is surface tension by which the condensate W is pulled by the downstream fin  432 . 
       γ× b×d×h×g≥ 2( T×b ×cos θ)  [Equation 1]
 
     Here, g is the acceleration of gravity. When Equation 1 above is balanced under the assumption that other conditions are equal, the height h of the condensate W and the interval d between the downstream fins  432  by which the condensate W is easily discharged have an inverse relationship. Accordingly, the interval by which the condensate is easily discharged may be determined by selecting an appropriate height of the condensate desired to be discharged from the latent heat exchanger  40 . 
     Tension T measured in one situation is 0.073 N/m. At room temperature, the specific gravity of the condensate is 1000 kg/m 3 , θ may be approximated to 0 degrees, and g may be approximated to 9.8 m/s 2 . As the height h of the condensate mainly ranges from 5 mm to 8 mm, the predetermined interval d of 1.9 mm to 3 mm may be obtained in the one situation by substituting the values into Equation 1. The descriptions of the number of fins and the interval may be applied to other embodiments of the present disclosure and modified examples thereof 
     Heat Exchanger Unit—Housing  510  and Flow Passage Cap Plates  363  and  364   
     The housing  510  will be described below with reference to  FIGS.  17  to  20   . The housing  510  is a component that surrounds and defines the sensible heat exchange area and the latent heat exchange area and may include a heat insulation side plate  5120  and a general side plate  5110 . The general side plate  5110  may include a first general side plate  5113  and a second general side plate  5114  spaced apart from each other along the predetermined direction D 2  and covered by the flow passage cap plates  363  and  364 . The heat insulation side plate  5120  is a plate-shaped component extending along the reference direction D 1  and the predetermined direction D 2 . Two heat insulation side plates  5120  may be disposed to be spaced apart from each other in the orthogonal direction. Accordingly, the heat insulation side plates  5120  form two side surfaces of the heat exchanger unit. The lateral shapes of the sensible heat exchange area and the latent heat exchange area are determined depending on the shape of inner surfaces of the heat insulation side plates  5120 . 
     Here, the heat insulation side plates  5120  are used with the meaning of side plates to which sensible heat insulation pipes  340  are disposed to be adjacent, rather than the meaning of side plates that reduce the amount of heat transferred to the outside, thereby achieving thermal insulation. The sensible heat insulation pipes  340  may be additionally disposed adjacent to the heat insulation side plates  5120 . The sensible heat insulation pipes  34  are pipe type components that are disposed adjacent to the housing  510  surrounding the sensible heat exchange area and that allow the heating-water to flow therethrough to reduce the amount by which heat of the sensible heat exchange area is released outside the housing  510 . As illustrated, two sensible heat insulation pipes  340  may extend in the predetermined direction D 2  that is the same as the direction in which the sensible heat exchange pipe  320  extends. 
     As illustrated in the drawings, the sensible heat insulation pipes  340  may be formed in an oval shape in a cross-section obtained by cutting the sensible heat insulation pipes  340  with a plane perpendicular to the direction in which sensible heat insulation pipes  340  extend. Specifically, the sensible heat insulation pipes  340  may be formed in an oval shape having a long axis parallel to the reference direction D 1 . The description of the sensible heat insulation pipes ( 34  of  FIG.  2   ) of the first embodiment may be identically applied to the sensible heat insulation pipes  340  of the third embodiment. 
     The general side plates  51110  and the flow passage cap plates  363  and  364  are plate-shaped components extending along the reference direction D 1  and the orthogonal direction. The two general side plates  5110  may be disposed to be spaced apart from each other in the predetermined direction D 2  in which the sensible heat exchange pipe  320  or the latent heat exchange pipe  420  extends. The two general side plates  5110 , when being disposed, may be disposed at opposite ends of the sensible heat straight portions and the latent heat straight portions  421  and  422 . The opposite ends of the sensible heat straight portions and the latent heat straight portions  421  and  422  may be coupled through the two general side plates  5113  and  5114 . Likewise, the two flow passage cap plates  363  and  364  may be coupled while covering the general side plates  5110  from the outside. Accordingly, the general side plates  5110  and the flow passage cap plates  363  and  364  may form the remaining two side surfaces of the heat exchanger unit that the heat insulation side plates  512  do not cover. Other lateral shapes of the sensible heat exchange area and the latent heat exchange area are determined depending on the shape of inner surfaces of the general side plates  5110 . 
     The flow passage cap plates  312  and  313  may include the second flow passage cap plate  364  and the first flow passage cap plate  363  on which a plurality of flow passage caps are formed. The second flow passage cap plate  364  and the first flow passage cap plate  363  may cover the second general side plate  5114  and the first general side plate  5113  and may be disposed adjacent to the opposite ends of the sensible heat straight portions or the latent heat straight portions  421  and  422 . A heating-water supply hole  3710  and a heating-water discharge hole  3720  may be disposed in the second flow passage cap plate  364 . The heating-water supply hole  3710  may be an opening through which the heating-water is supplied from the outside to one end of an integrated latent heat flow passage formed by the latent heat exchange pipe  420  and may be an inlet of the latent heat flow passage, and the heating-water discharge hole  3720  may be an opening through which the heating-water is discharged to the outside from one end of an integrated sensible heat flow passage formed by the sensible heat exchange pipe  320  and may be an outlet of the sensible heat flow passage. 
     The heating-water may be introduced from the outside through the heating-water supply hole  3710  located at a relatively downstream side based on the reference direction D 1  and may be delivered to the latent heat exchange pipe  420 . The heating-water heated in the sensible heat exchange pipe  320  may be discharged to the outside through the heating-water discharge hole  3720  located at a relatively upstream side based on the reference direction D 1 . However, the positions of the heating-water supply hole  3710  and the heating-water discharge hole  3720  are not limited thereto. 
     To connect the outlet of the latent heat flow passage exposed outside one of the side plates constituting the housing  510  and the inlet of the sensible heat flow passage exposed outside the one side plate, one of the flow passage cap plates  363  and  364  may include, between the one side plate and the one flow passage cap plate, a flow passage cap having a connection space surrounding the outlet of the latent heat flow passage and the inlet of the sensible heat flow passage. In the third embodiment of the present disclosure, the flow passage cap may be a second flow passage cap  3642  provided on the second flow passage cap plate  364 . Accordingly, one of the side plates is the second general side plate  5112  that forms the connection space together with the second flow passage cap plate  364 . However, the side plate and the flow passage cap plate that connect the inlet of the sensible heat flow passage and the outlet of the latent heat flow passage are not limited thereto. 
     The descriptions of the heating-water pipe and the main flow passage of the first embodiment may be applied to a heating-water pipe and a main flow passage that are connected to the heating-water supply hole  3710  and the heating-water discharge hole  3720  of the third embodiment of the present disclosure. 
     Shape of Heat Exchange Area Formed by Housing  510   
     The cross-sectional area of each heat exchange area defined on a plane perpendicular to the reference direction D 1  is referred to as a reference cross-sectional area. The housing  510  may be provided such that the reference cross-sectional area at the most downstream side is smaller than the reference cross-sectional area at the most upstream side based on the reference direction D 1 . The housing  510  may be provided such that at least one section in which the reference cross-sectional area of the heat exchange area gradually decreases along the reference direction D 1  is formed to allow the combustion gas to flow at higher speed in the latent heat exchange area than in the sensible heat exchange area. 
     The housing  510  may be formed to include at least one section in which the reference cross-sectional area gradually decreases along the reference direction D 1 . Accordingly, the heat exchange area may have a tapered shape along the reference direction D 1  as a whole. As the housing  510  is formed such that the reference cross-sectional area of the heat exchange area decreases, the occurrence of a dead zone where heat transfer efficiency is deteriorated due to a sharp reduction in flow speed at a specific position when the combustion gas flows in the latent heat exchange area may be prevented by Bernoulli&#39;s principle. In particular, in a case where the latent heat exchange pipe  420  is formed of two or more layers as in the third embodiment of the present disclosure, the condensate may block the space between the latent heat fins  430 , or the length of the latent heat exchange area along the reference direction D 1  may become longer, and thus thermal efficiency may be deteriorated. However, the problem may be overcome because the heat exchange area has a tapered shape due to the housing. Specifically, the housing  510  may be formed to include at least one section in which the width of the heat exchange area in the orthogonal direction gradually decreases along the reference direction D 1 , and the width of the heat exchange area in the predetermined direction D 2  may be formed to remain constant along the reference direction D 1 . That is, the reference cross-sectional area is decreased by reducing only the width in the orthogonal direction in a state in which the width in the predetermined direction D 2  is maintained along the reference direction D 1 . To form the above-described shape, the general side plates  5110  may be formed in a general plate shape, and the heat insulation side plates  5120  may be formed to be bent as illustrated. 
     Specifically, referring to  FIG.  20   , a section corresponding to the latent heat exchange area is a section from a second point A 2  at which the inlet end of the upstream fin  431  is located to a point at which the outlet end of the downstream fin  432  is located. A section in which the reference cross-sectional area decreases along the reference direction D 1  in the latent heat exchange area is formed between the second point A 2  and a third point A 3  and between a fourth point A 4  and a sixth point A 6 . A section in which the reference cross-sectional area is maintained is formed between the third point A 3  and the fourth point A 4  and between the sixth point A 6  and the outlet end of the downstream fin  432 . Furthermore, a section between a first point A 1  and the second point A 2  that does not correspond to the latent heat exchange area, but is part of the heat exchange area is also a section in which the reference cross-sectional area decreases along the reference direction D 1 . 
     In  FIG.  20   , it can be seen that the heat exchange area is formed to include at least one section in which the width in the orthogonal direction decreases along the reference direction D 1  and at least one section in which the width in the orthogonal direction remains constant. 
     Specifically, it can be seen that the width of the latent heat exchange area in the orthogonal direction decreases along the reference direction D 1  in the section from the second point A 2  to the third point A 3  and in the section from the fourth point A 4  to the sixth point A 6 . In contrast, it can be seen that the width in the orthogonal direction remains constant in the section from the third point A 3  to the fourth point A 4  and in the section from the sixth point A 6  to the most downstream side of the housing  510 . 
     It can be seen that in the section in which the straight portions  421  and  422  of the latent heat exchange pipe  420  are located, the width in the orthogonal direction approximately remains constant and heat exchange is sufficiently performed, and in the section located between the straight portions, the reference cross-sectional area decreases along the reference direction D 1  to increase flow speed. 
     The shape of the heat exchange area may be described by defining the most upstream side of each fin  330 ,  432 , or  432  based on the reference direction D 1  as an inlet end and defining the most downstream side as an outlet end. The housing  510  may be provided such that the reference cross-sectional area gradually decreases from the outlet end of the sensible heat fin  330  to the inlet end of the latent heat fin  430  along the reference direction D 1 . That is, in  FIG.  20   , the housing  510  may be formed such that the reference cross-sectional area gradually decreases along the reference direction D 1  in the section from the first point A 1  at which the outlet end of the sensible heat fin  330  is located to the second point A 2  at which the inlet end of the latent heat fin  430  is located. 
     The housing  510  may be provided such that the reference cross-sectional area of the inlet end of the downstream fin  432  is smaller than the reference cross-sectional area of the inlet end of the upstream fin  431 . That is, the section between the second point A 2  and a fifth point A 5  includes at least one section in which the reference cross-sectional area decreases along the reference direction D 1 , such that the reference cross-sectional area at the fifth point A 5  at which the inlet end of the downstream fin  432  is located is smaller than the reference cross-sectional area at the second point A 2  at which the inlet end of the upstream fin  431  is located. 
     Referring to  FIG.  20   , the housing  510  may be formed such that the reference cross-sectional area of a partial section of the sensible heat exchange area also decreases along the reference direction D 1 . 
     As the width of the heat exchange area is changed as described above, each fin may have a section in which the width in the orthogonal direction decreases along the reference direction D 1 . 
     The area of the sensible heat fin  330  or the latent heat fin  430  in the heat exchange area that makes contact with the inner surface of the housing  510  may be provided such that the width gradually decreases along the reference direction D 1  to correspond to a gradual reduction in the reference cross-sectional area based on the width of a fin defined in a direction perpendicular to the reference direction D 1 . Referring to  FIG.  20   , it can be seen that the area adjacent to the outlet end of the sensible heat fin  330  and the width of the upstream fin  431  located in the section from the fourth point A 4  to the outlet end of the upstream fin  431  decrease along the reference direction D 1  depending on the shape of the inner surface of the housing  510 . 
       FIG.  22    is a vertical sectional view of a heat exchanger unit according to a first modified example of the third embodiment of the present disclosure. 
     In  FIG.  22   , the form of the heat exchanger unit according to the first modified example of the third embodiment that has one row of sensible heat exchange pipes  320  and two rows of latent heat exchange pipes  420  as in the third embodiment of the present disclosure may be identified. 
     A housing  510   b  according to the first modified example of the third embodiment may also be provided such that the reference cross-sectional area at the most downstream side is smaller than the reference cross-sectional area at the most upstream side based on a reference direction D 1 . 
     The housing  510   b  may be provided such that at least one section in which the reference cross-sectional area gradually decreases along the reference direction D 1  is formed to allow combustion gas to flow at higher speed in a latent heat exchange area than in a sensible heat exchange area. Descriptions of effects obtained by the heat exchanger unit as the section in which the reference cross-sectional area decreases is disposed are replaced with the contents described above with reference to  FIG.  20   . 
     The housing  510   b  may be provided such that the reference cross-sectional area gradually decreases from the outlet end of a sensible heat fin  330   b  to the inlet end of a latent heat fin  430   b . The housing  510   b  may be formed such that the reference cross-sectional area gradually decreases along the reference direction D 1  in the section from a first point B 1  at which the outlet end of the sensible heat fin  330   b  is located to a second point B 2  at which the inlet end of the latent heat fin  430   b  is located. 
     In the section from the second point B 2  at which the inlet end of the latent heat fin  430   b  is located to the outlet end of a downstream fin  432   b , a heat exchange area may have only a section in which the reference cross-sectional area gradually decreases along the reference direction D 1  and a section in which the reference cross-sectional area is maintained. Accordingly, the reference cross-sectional area at the outlet end of the downstream fin  432   b  may be smaller than the reference cross-sectional area at the second point B 2 . 
     The housing  510   b  may be provided such that the reference cross-sectional area gradually decreases from the inlet end of the latent heat fin  430   b  to the outlet end of the latent heat fin  430   b . The housing  510   b  may be formed such that the reference cross-sectional area of the section from the second point B 2  at which the inlet end of an upstream fin  431   b , which is a kind of the latent heat fin  430   b , is located to a third point b 3  at which the outlet end of the upstream fin  431   b  is located gradually decreases along the reference direction D 1 . 
     The housing  510   b  may be provided such that the reference cross-sectional area at the inlet end of the downstream fin  432   b  is smaller than the reference cross-sectional area at the inlet end of the upstream fin  431   b . The housing  510   b  may be provided such that the reference cross-sectional area gradually decreases along the reference direction D 1  in the section from the second point B 2  at which the inlet end of the upstream fin  431   b  is located to a fourth point B 4  at which the inlet end of the downstream fin  432   b  is located, and the condition may be satisfied. 
     Referring to the drawing, the latent heat exchange area may have a section from the second point B 2  to a fifth point B 5  in which the reference cross-sectional area gradually decreases along the reference direction D 1  and a section from the fifth point B 5  to the outlet end of the downstream fin  432   b  in which the reference cross-sectional area remains constant. 
     The area of the latent heat fin  430   b  that makes contact with the inner surface of the housing  510   b  may be provided such that the width gradually decreases along the reference direction D 1  to correspond to a gradual reduction in the reference cross-sectional area based on the width of the fin defined in an orthogonal direction. Referring to the drawing, the housing  510   b  is provided such that the reference cross-sectional area of the section from the second point B 2  at which the inlet end of the upstream fin  431   b  is located to the third point B 3  at which the outlet end is located gradually decreases along the reference direction D 1 . Accordingly, a tapered shape may be formed such that the width of the upstream fin  431   b  defined in the orthogonal direction gradually decreases along the reference direction D 1 . 
       FIG.  23    is a vertical sectional view of a heat exchanger unit according to a second modified example of the third embodiment of the present disclosure. 
     In  FIG.  23   , the form of the heat exchanger unit according to the second modified example of the third embodiment that has one row of sensible heat exchange pipes  320   c  and two rows of latent heat exchange pipes  420   c  as in the third embodiment of the present disclosure may be identified. The heat exchange pipes of the second modified example differ from the heat exchange pipes of the third embodiment in that in this modified example, the sensible heat exchange pipes  320   c  include a total of five straight portions and a total of six upstream straight portions  421   c  are disposed. 
     A housing  510   c  according to the second modified example of the third embodiment may also be provided such that the reference cross-sectional area at the most downstream side is smaller than the reference cross-sectional area at the most upstream side based on a reference direction D 1 . 
     The housing  510   c  may be provided such that at least one section in which the reference cross-sectional area gradually decreases along the reference direction D 1  is formed to allow combustion gas to flow at higher speed in a latent heat exchange area than in a sensible heat exchange area. Descriptions of effects obtained by the heat exchanger unit as the section in which the reference cross-sectional area decreases is disposed are replaced with the contents described above with reference to  FIG.  20   . 
     The housing  510   c  may be provided such that the reference cross-sectional area gradually decreases from the outlet end of a sensible heat fin  330   c  to the inlet end of a latent heat fin  430   c . The housing  510   c  may be formed such that the reference cross-sectional area of the section from a first point C 1  at which the outlet end of the sensible heat fin  330   c  is located to a second point C 2  at which the inlet end of the latent heat fin  430   c  is located gradually decreases along the reference direction D 1 . 
     The housing  510   c  may be provided such that in the section from the second point C 2  at which the inlet end of the latent heat fin  430   c  is located to the outlet end of a downstream fin  432   c , a heat exchange area has only a section in which the reference cross-sectional area gradually decreases along the reference direction D 1  and a section in which the reference cross-sectional area is maintained. Accordingly, the reference cross-sectional area at the outlet end of the downstream fin  432   c  may be smaller than the reference cross-sectional area at the second point C 2 . 
     By more specifically limiting the shape of the latent heat exchange area, the housing  510   c  may be provided such that the reference cross-sectional area at the inlet end of the downstream fin  432   c  is smaller than the reference cross-sectional area at the inlet end of an upstream fin  431   c . The housing  510   c  may be provided such that the reference cross-sectional area gradually decreases along the reference direction D 1  in the section from the second point C 2  at which the inlet end of the upstream fin  431   c  is located to a fifth point C 5  at which the inlet end of the downstream fin  432   c  is located, and the condition may be satisfied. 
     Referring to the drawing, the latent heat exchange area may have a section from the second point C 2  to a third point C 3  and a section from the fifth point C 5  to a sixth point C 6 , which are sections in which the reference cross-sectional area gradually decreases along the reference direction D 1 , and a section from the third point C 3  to a fourth point C 4  and a section from the sixth point C 6  to the outlet end of the downstream fin  432   c , which are sections in which the reference cross-sectional area remains constant. 
     According to the second modified example of the third embodiment of the present disclosure, the inlet end of one of the latent heat fins  430   c  may be formed to be flat without having a plurality of valleys and protruding portions, like another fin. 
       FIG.  24    is a vertical sectional view of a heat exchanger unit according to a third modified example of the third embodiment of the present disclosure. 
     Referring to  FIG.  24   , the heat exchanger unit according to the third modified example of the third embodiment of the present disclosure includes one row of sensible heat exchange pipes  320   e  and one row of latent heat exchange pipes  420   e . The sensible heat exchange pipes  320   e  include four straight portions, and the latent heat exchange pipes  420   e  include six straight portions. However, the numbers of straight portions are not limited thereto. 
     A housing  510   e  according to the third modified example of the third embodiment may also be provided such that the reference cross-sectional area at the most downstream side is smaller than the reference cross-sectional area at the most upstream side based on a reference direction D 1 . 
     The housing  510   e  may be provided such that at least one section in which the reference cross-sectional area gradually decreases along the reference direction D 1  is formed to allow combustion gas to flow at higher speed in a latent heat exchange area than in a sensible heat exchange area. Descriptions of effects obtained by the heat exchanger unit as the section in which the reference cross-sectional area decreases is disposed are replaced with the contents described above with reference to  FIG.  20   . 
     The housing  510   e  may be provided such that the reference cross-sectional area gradually decreases from the outlet end of a sensible heat fin  330   e  to the inlet end of a latent heat fin  430   e . The housing  510   e  may be formed such that the reference cross-sectional area is maintained in the section from a first point E 1  at which the outlet end of the sensible heat fin  330   e  is located to a second point E 2  located downstream of the first point E 1  and the reference cross-sectional area of the section from the second point E 2  to a third point E 3  at which the inlet end of the latent heat fin  430   e  is located gradually decreases along the reference direction D 1 . Accordingly, the reference cross-sectional area does not increase from the outlet end of the sensible heat fin  330   e  to the inlet end of the latent heat fin  430   e.    
     The housing  510   e  may be provided such that in the section from the third point E 3  at which the inlet end of the latent heat fin  430   e  is located to a fifth point E 5  at which the outlet end of the latent heat fin  430   e  is located, a heat exchange area has only a section in which the reference cross-sectional area gradually decreases along the reference direction D 1  and a section in which the reference cross-sectional area is maintained. In the section from the third point E 3  at which the inlet end of the latent heat fin  430   e  is located to a fourth point E 4  located downstream of the third point E 3 , the reference cross-sectional area may gradually decrease along the reference direction D 1 , and in the section from the fourth point E 4  to the fifth point E 5 , the reference cross-sectional area remains constant. Accordingly, the reference cross-sectional area at the fifth point E 5  at which the outlet end of the latent heat fin  430   e  is located may be smaller than the reference cross-sectional area at the third point E 3  at which the inlet end of the latent heat fin  430   e  is located. 
     The housing  510   e  may be provided such that a first section in which the reference cross-sectional area gradually decreases from the outlet end of the sensible heat fin  330   e  to the inlet end of the latent heat fin  430   e  and a second section in which the reference cross-sectional area is maintained between the most upstream side and the outlet end of the latent heat fin  430   e  based on the reference direction D 1  in the area where the latent heat fin  430   e  makes contact with the housing  510   e  are formed. The first section is a section from the second point E 2  adjacent to the outlet end of the sensible heat fin  330   e  to the third point E 3  at which the inlet end of the latent heat fin  430   e  is located, and the second section is a section from the fourth point E 4  to the fifth point E 5 . The area of the latent heat fin  430   e  that makes contact with the inner surface of the housing  510   e  may be provided such that the width of a portion corresponding to the second section remains constant based on the width of a fin defined in an orthogonal direction perpendicular to the reference direction D 1 . 
     Referring to the drawing, the latent heat exchange area may have a section from the second point E 2  to the fourth point E 4 , which is a section in which the reference cross-sectional area gradually decreases along the reference direction D 1 , and a section from the first point E 1  to the second point E 2  and a section from the fourth point E 4  to the fifth point E 5 , which are sections in which the reference cross-sectional area remains constant. 
       FIG.  25    is a vertical sectional view of a heat exchanger unit according to a fourth modified example of the third embodiment of the present disclosure. 
     Referring to  FIG.  25   , the heat exchanger unit according to the fourth modified example of the third embodiment of the present disclosure includes one row of sensible heat exchange pipes  320   f  and one row of latent heat exchange pipes  420   f . The sensible heat exchange pipes  320   f  include six straight portions, and the latent heat exchange pipes  420   f  include six straight portions. However, the numbers of straight portions are not limited thereto. 
     A housing  510   f  according to the fourth modified example of the third embodiment may also be provided such that the reference cross-sectional area at the most downstream side is smaller than the reference cross-sectional area at the most upstream side based on a reference direction D 1 . 
     The housing  510   f  may be provided such that at least one section in which the reference cross-sectional area gradually decreases along the reference direction D 1  is formed to allow combustion gas to flow at higher speed in a latent heat exchange area than in a sensible heat exchange area. Descriptions of effects obtained by the heat exchanger unit as the section in which the reference cross-sectional area decreases is disposed are replaced with the contents described above with reference to  FIG.  20   . 
     The housing  510   f  may be provided such that the reference cross-sectional area gradually decreases from the outlet end of a sensible heat fin  330   f  to the inlet end of a latent heat fin  430   f . The housing  510   f  may be formed such that the reference cross-sectional area of the section from a first point F 1  at which the outlet end of the sensible heat fin  330   f  is located to a second point F 2  at which the inlet end of the latent heat fin  430   f  is located gradually decreases along the reference direction D 1 . Accordingly, the reference cross-sectional area does not increase from the outlet end of the sensible heat fin  330   f  to the inlet end of the latent heat fin  430   f.    
     The housing  510   f  may be provided such that in the section from the second point F 2  at which the inlet end of the latent heat fin  430   f  is located to the outlet end of the latent heat fin  430   f , a heat exchange area has only a section in which the reference cross-sectional area gradually decreases along the reference direction D 1  and a section in which the reference cross-sectional area is maintained. In the section from the second point F 2  at which the inlet end of the latent heat fin  430   f  is located to a third point F 3  located downstream of the second point F 2 , the reference cross-sectional area may gradually decrease along the reference direction D 1 , and in the section from the third point F 3  to the outlet end of the latent heat fin  430   f , the reference cross-sectional area may remain constant. Accordingly, the reference cross-sectional area at the outlet end of the latent heat fin  430   f  may be smaller than the reference cross-sectional area at the second point F 2  at which the inlet end of the latent heat fin  430   f  is located. 
     Referring to the drawing, the latent heat fin  430   f  according to the fourth modified example of the third embodiment of the present disclosure may include a pointed portion  4210   f  at the most downstream-side end thereof. The pointed portion  4210   f  is a portion in which the width in an orthogonal direction perpendicular to the reference direction D 1  decreases along the reference direction D 1 , and a plurality of pointed portions  4210   f  may be provided along the orthogonal direction. The pointed portion may have the above-described shape such that condensate formed on the latent heat fin  430   f  by a phase change of the combustion gas is collected. 
     The descriptions of the configurations of the housings according to the modified examples of the third embodiment may be applied to other embodiments of the present disclosure and modified examples thereof. 
       FIG.  26    is a view illustrating the second general side plate  5114  of the heat exchanger unit according to the third embodiment of the present disclosure together with flow passage caps included in the second flow passage cap plate  364 .  FIG.  27    is a view illustrating the first general side plate  5113  of the heat exchanger unit according to the third embodiment of the present disclosure together with flow passage caps included in the first flow passage cap plate  363 .  FIG.  28    is a perspective view illustrating all the flow passages included in the heat exchanger unit according to the third embodiment of the present disclosure. 
     Flow passages formed by the sensible heat exchange pipe  320 , the latent heat exchange pipe  420 , and the flow passage cap plates  363  and  364  of the heat exchanger unit according to the third embodiment of the present disclosure will be described below with reference to  FIGS.  26  to  28   . For a better understanding of areas through which the flow passages pass, the flow passage caps of the flow passage cap plates  363  and  364  are not illustrated in  FIG.  28    in a state in which the general side plates  5110 , the heat insulation side plates  5120 , and the fins of the heat exchanger unit are removed. 
     Referring to the other modified example of the first embodiment of  FIG.  29   ,  FIG.  26    is view in which flow passage caps  3641 ,  3642 ,  3643 ,  3644 , and  3645  of the second flow passage cap plate  364  are illustrated by dotted lines on a view of the second general side plate  5114 , the sensible heat exchange pipe  320 , the latent heat exchange pipe  420 , and sensible heat insulation pipes  3410  and  3420  of the third embodiment of the present disclosure that corresponds to a view of the heat exchanger unit when viewed from the second connection flow passage cap plate  72  of  FIG.  29    along line H-H′.  FIG.  27    is a view in which flow passage caps  3631 ,  3632 ,  3633 , and  3634  of the first flow passage cap plate  363  are illustrated by dotted lines on a view of the first general sensible heat side plate  5111 , the sensible heat exchange pipe  320 , the latent heat exchange pipe  420 , and the sensible heat insulation pipes  3410  and  3420  of the third embodiment of the present disclosure that corresponds to a view of the first main general side plate  5111 , into which the first connection flow passage cap plate  71  is inserted, when viewed along line G-G′ of  FIG.  29   . 
     The sensible heat straight portions may form a sensible flow passage through which the heating-water flows, and the latent heat straight portions  421  and  422  may form a latent heat flow passage through which the heating-water flows and that is connected to the sensible heat flow passage. The sensible heat flow passage may include a series flow passage in at least a partial section, and the latent heat flow passage may include a parallel flow passage in at least a partial section. 
     As described above, the flow passage cap plates  363  and  364  may include the first flow passage cap plate  363  and the second flow passage cap plate  364 . The second flow passage cap plate  364  may have the first flow passage cap  3641 , the second flow passage cap  3642 , the third flow passage cap  3643 , the fourth flow passage cap  3644 , and the fifth flow passage cap  3645  formed thereon, and the first flow passage cap plate  363  may have the sixth flow passage cap  3631 , the seventh flow passage cap  3632 , the eighth flow passage cap  3633 , and the ninth flow passage cap  3634  formed thereon. The flow passage caps formed on the flow passage cap plates  363  and  364  are formed in a convex shape toward the outside of the heat exchanger unit and are connected with the ends of the straight portions included in the sensible heat exchange pipe  320  or the ends of the straight portions  421  and  422  included in the latent heat exchange pipe  420  to allow the heating-water to flow inside. When the flow passage caps of the flow passage cap plates  363  and  364  cover the general side plates ( 5110  of  FIG.  17   ), the heating-water flows in the space formed by the general side plates and the flow passage caps. 
     The heating-water supply hole  3710  is formed in the first flow passage cap  3641  located at the most downstream side of the second flow passage cap plate  364  based on the reference direction D 1 . The heating-water is introduced into the heat exchanger unit through the heating-water supply hole  3710 . The introduced heating-water flows through the downstream straight portions  422 , each having one end connected to the first flow passage cap  3641 . Accordingly, the downstream straight portions  422  may form a parallel flow passage. 
     The heating-water reaches the sixth flow passage cap  3631 , to which an opposite end of each downstream straight portion  422  is connected, through the downstream straight portion  422 . The opposite end of the downstream straight portion  422  and one end of each upstream straight portion  421  are connected to the sixth flow passage cap  3631 . Accordingly, the heating-water is introduced into the upstream straight portions  421  from the sixth flow passage cap  3631  and flows along the upstream straight portions  421 . Accordingly, the upstream straight portions  421  may form a parallel flow passage. 
     An opposite end of each upstream straight portion  421  is connected to the second flow passage cap  3642 , and the heating-water flowing along the upstream straight portion  421  is delivered to the second flow passage cap  3642 . The second flow passage cap  3642  is connected with the first sensible heat insulation pipe  3410  and delivers the heating-water to the first sensible heat insulation pipe  3410 . 
     The heating-water moving along the first sensible heat insulation pipe  3410  reaches the seventh flow passage cap  3632  to which the first sensible heat insulation pipe  3410  is connected. A sensible heat flow passage in a zigzag form is formed along the sensible heat straight portions arranged in sequence from the seventh flow passage cap  3632  and connected in series, and the heating-water flows along the sensible heat flow passage from the seventh flow passage cap  3632  to the third flow passage cap  3643 , from the third flow passage cap  3643  to the eighth flow passage cap  3633 , from the eighth flow passage cap  3633  to the fourth flow passage cap  3644 , and from the fourth flow passage cap  3644  to the ninth flow passage cap  3634 . In a case where the sensible heat insulation pipes  3410  and  3420  are arranged as in the third embodiment of the present disclosure, the sensible heat flow passage may be implemented by connection of the straight portions included in the sensible heat insulation pipes  3410  and  3420  and the sensible heat exchange pipe  32 . 
     The ninth flow passage cap  3634  is also connected with the second sensible heat insulation pipe  3420 , and the heating-water flows along the second sensible heat insulation pipe  3420  and reaches the fifth flow passage cap  3645 . The fifth flow passage cap  3645  is connected with the heating-water discharge hole  3720 , and the heating-water delivered through the second sensible heat insulation pipe  3420  is discharged in a heated state through the heating-water discharge hole  3720 . The entire flow passage in which the heating-water is delivered between the downstream straight portions  422  and the upstream straight portions  421  connected with each other and the heating-water is delivered between the upstream straight portions  421  and the latent heat flow passage connected with each other is illustrated by arrows in  FIG.  28   . The heating-water is heated and discharged while flowing along the entire flow passage. 
     Hereinabove, even though all of the components are coupled into one body or operate in a combined state in the description of the above-mentioned embodiments of the present disclosure, the present disclosure is not limited to these embodiments. That is, all of the components may operate in one or more selective combination within the range of the purpose of the present disclosure. It should be also understood that the terms of “include”, “comprise” or “have” in the specification are “open type” expressions just to say that the corresponding components exist and, unless specifically described to the contrary, do not exclude but may include additional components. Unless otherwise defined, all terms used herein, including technical and scientific terms, have the same meaning as those generally understood by those skilled in the art to which the present disclosure pertains. Such terms as those defined in a generally used dictionary are to be interpreted as having meanings equal to the contextual meanings in the relevant field of art, and are not to be interpreted as having ideal or excessively formal meanings unless clearly defined as having such in the present application. 
     Hereinabove, although the present disclosure has been described with reference to exemplary embodiments and the accompanying drawings, the present disclosure is not limited thereto, but may be variously modified and altered by those skilled in the art to which the present disclosure pertains without departing from the spirit and scope of the present disclosure claimed in the following claims. Therefore, the exemplary embodiments of the present disclosure are provided to explain the spirit and scope of the present disclosure, but not to limit them, so that the spirit and scope of the present disclosure is not limited by the embodiments. The scope of the present disclosure should be construed on the basis of the accompanying claims, and all the technical ideas within the scope equivalent to the claims should be included in the scope of the present disclosure.