Patent Publication Number: US-11639793-B2

Title: Gas furnace

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority from Korean Patent Application No. 10-2019-0141388, filed on Nov. 7, 2019, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present disclosure relates to a gas furnace, and more particularly to a gas furnace in which nitrogen oxide (NOx) emissions may be greatly reduced by premixing air and a fuel gas before combustion and by controlling an air ratio while increasing a mixing rate of the air and the fuel gas. 
     2. Description of the Related Art 
     Generally, a gas furnace is a heating device which heats the indoor air by supplying air heat-exchanged with a flame and high-temperature combustion gas which are produced during combustion of a fuel gas.  FIG.  1    illustrates a general gas furnace. 
     Referring to  FIG.  1   , a flame and high-temperature combustion gas may be produced in the burner assembly  4  during combustion of a fuel gas and air. Here, the fuel gas is fed from a gas valve (not shown) into the burner assembly  4  through a manifold  3 . The high-temperature combustion gas may pass through a heat exchanger  5  to be discharged to the outside through an exhaust pipe  8 . In this case, the indoor air, introduced by a blower  6  through an internal air duct D 1 , may be heated while passing through the heat exchanger  5 , and then may be guided to an indoor space through the supply air duct D 2 , thereby heating the indoor space. 
     An inducer  7  induces the flow of the combustion gas passing through the heat exchanger  5  and the exhaust pipe  8 , and condensate, which is generated as the combustion gas is condensed while passing through the heat exchanger  5  and/or the exhaust pipe  8 , may be discharged to the outside through a condensate trap  9 . 
     Thermal NOx (hereinafter briefly referred to as NOx) is formed as a result of chemical reaction between atmospheric nitrogen and oxygen at high temperature (more specifically, at a flame temperature of about 1800 K or higher) during combustion of the fuel gas in the gas furnace. As a typical air pollutant, the NOx emissions are regulated by the air quality management office. 
     For example, in the United States, NOx emissions are regulated by the South Coast Air Quality Management District (South Coast AQMD), which has recently tightened the regulations on a NOx emission limit from 40 ng/J (nano-grams per Joule) to less than 14 ng/J. 
     Accordingly, there has been active research on techniques for reducing NOx emissions in the gas furnace. U.S. 20120247444A1 discloses a premix gas furnace for premixing air and a fuel gas before combustion, in which flame temperature may be reduced by increasing the air ratio, thereby reducing NOx emissions. 
     However, the premix gas furnace has problems in that by only controlling the air ratio, there is a limitation in reducing the flame temperature, and an excessive increase in the flame temperature may cause flame instability. 
     Further, the premix gas furnace fails to disclose a structure for preventing an increase in local flame temperature by increasing a mixing rate of the air and fuel gas. 
     SUMMARY OF THE INVENTION 
     It is a first object of the present disclosure to provide a gas furnace capable of reducing nitrogen oxide emissions by providing a full premixing mechanism. 
     It is a second object of the present disclosure to provide a gas furnace, in which by increasing a mixing rate of the fuel gas and air, the increase in local flame temperature may be prevented, thereby reducing the nitrogen oxide (NOx) emissions. 
     The objects of the present disclosure are not limited to the aforementioned objects and other objects not described herein will be clearly understood by those skilled in the art from the following description. 
     In accordance with an aspect of the present disclosure, the above and other objects can be accomplished by providing a gas furnace, including: a mixer for mixing air and a fuel gas, which are introduced through an intake pipe and a manifold, respectively, to form a mixture; a mixing pipe through which the mixture, having passed through the mixer, flows; a burner assembly for producing a combustion gas by burning the mixture having passed through the mixing pipe; a heat exchanger through which the combustion gas flows; an exhaust pipe through which an exhaust gas, as the combustion gas having passed through the heat exchanger, is discharged outside of the gas furnace; and an inducer for inducing a flow of a fluid through the intake pipe, the mixer, the mixing pipe, the burner assembly, the heat exchanger, and the exhaust pipe. In this case, the mixer may have a front end connected to the intake pipe, a rear end connected to the mixing pipe, and a side surface connected to the manifold. 
     The mixer may include: a mixer housing forming an exterior of the mixer; and a Venturi tube disposed inside the mixer housing. 
     The Venturi tube may include: a converging section having on one end an air inlet, through which the air, having passed through the intake pipe, is introduced; a throat connected to the converging section and having a fuel inlet hole, through which the fuel gas having passed through the manifold is introduced, and which is formed on at least a portion of a side surface of the throat; and a diverging section which is connected to the throat, and in which the air and the fuel gas, having passed through the converging section and the fuel inlet hole, respectively, are mixed and flow as a mixture, and which has on one end a discharge port through which the mixture is discharged to the mixing pipe. 
     The converging section may have a diameter which decreases toward a downstream side; and the diverging section may have a diameter which increases toward a downstream side. 
     The fuel inlet hole may include a plurality of fuel inlet holes which are spaced apart from each other at predetermined intervals in a circumferential direction of the throat. 
     The plurality of fuel inlet holes may include first fuel inlet holes which pass through the side surface of the throat in a radially inward direction of the throat. The plurality of fuel inlet holes may include second fuel inlet holes which obliquely pass through the side surface of the throat in the circumferential direction of the throat relative to the radially inward direction of the throat. 
     Other unmentioned technical solutions can be clearly understood from the following description by those having ordinary skill in the technical field to which the present disclosure pertains. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a perspective view of a general gas furnace. 
         FIG.  2    is a perspective view of a gas furnace according to an embodiment of the present disclosure. 
         FIG.  3    is a partial perspective view of a gas furnace according to an embodiment of the present disclosure. 
         FIG.  4    is a partial sectional view of a gas furnace according to an embodiment of the present disclosure. 
         FIG.  5    is a perspective view of a mixer according to an embodiment of the present disclosure. 
         FIG.  6    is a perspective view of a mixer according to another embodiment of the present disclosure. 
         FIGS.  7 A and  7 B  are diagrams explaining a discharge direction of a fuel gas through fuel inlet holes according to an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Advantages and features of the present disclosure and methods for accomplishing the same will be more clearly understood from exemplary embodiments described below with reference to the accompanying drawings. However, the present disclosure is not limited to the following embodiments but may be implemented in various different forms. The embodiments are provided only to complete disclosure of the present disclosure and to fully provide a person having ordinary skill in the art to which the present disclosure pertains with the category of the present disclosure, and the present disclosure will be defined by the scope of the appended claims. Wherever possible, like reference numerals generally denote like elements through the specification. 
     The present disclosure may also be described based on a spatial orthogonal coordinate system with X, Y and Z axes mutually crossing at right angles, as illustrated in  FIG.  2    and the like. In the present disclosure, the X, Y and Z axes are defined based on a Z axis direction being defined as an up-down direction and an X axis direction being defined as a front-rear direction. Each axis direction (X-, Y-, and Z-axis directions) may indicate both directions in which each of the axes extends; and +X-, +Y-, and +Z-axis directions having a plus sign (“+”) may indicate a positive direction, which is either one of both directions in which each of the axes extends. Further, −X-, −Y-, and −Z-axis directions having a minus sign (“−”) may indicate a negative direction, which is the other one of both directions in which each of the axes extends. 
     Hereinafter, a gas furnace according to embodiments of the present disclosure will be described in further detail with reference to  FIGS.  2  to  7   . 
       FIG.  2    is a perspective view of a gas furnace according to an embodiment of the present disclosure. 
     The gas furnace  10  according to an embodiment is a device for heating an indoor space by heat exchanging air with flames and a high-temperature combustion gas C, which are produced during combustion of a fuel gas F, and supplying the heat-exchanged air to the indoor space. 
     Referring to  FIG.  2   , the gas furnace  10  includes: a mixer  32  in which air A and the fuel gas F and/or an exhaust gas E are mixed; a mixing pipe  33  through which the mixture, having passed through the mixer  32 , flows; a burner assembly  40  for producing the combustion gas C by burning the mixture having passed through the mixing pipe  33 ; and a heat exchanger  50  through which the combustion gas C flows. 
     In addition, the gas furnace  10  includes: an inducer  70  for inducing the flow of the combustion gas C to pass through the heat exchanger  50  to be discharged through an exhaust pipe  80 ; a blower  60  for blowing the air, to be supplied to the indoor space, around the heat exchanger  50 ; and a condensate trap  90  for collecting a condensate, generated in the heat exchanger  50  and/or the exhaust pipe  80 , and discharging the collected condensate to the outside. 
     The air A may be introduced into the mixer  32  through an intake pipe  31 , and the fuel gas F may flow from a gas valve  20  and a nozzle  21   a  into the mixer  32  through a manifold  21 . Here, as the fuel gas F, Liquefied Natural Gas (LNG) may be used, which is natural gas that has been cooled down to liquid form, or Liquefied Petroleum Gas (LPG) may be used, which is a by-product of crude oil refining and is pressurized into liquid form. 
     By the opening and closing of the gas valve  20 , the fuel gas F may be supplied to the manifold  21  or may be blocked from flowing to the manifold  21 . By controlling a degree of opening of the gas valve  20 , it is possible to control an amount of the fuel gas F supplied to the manifold  21 . Accordingly, the gas valve  20  may control the heating power of the gas furnace  10 . 
     The mixture of the air and the fuel gas F may flow through the mixing pipe  33 , as will be described later. The mixing pipe  33  may guide the mixture to the burner assembly  40  which will be described later, and the mixture of the air and the fuel gas F may be continuously mixed even while being guided to the burner assembly  40  by the mixing pipe  33 . 
     The mixture, fed into the burner assembly  40 , may be burned by ignition provided by an igniter. In this case, the flames and high-temperature combustion gas C may be produced during combustion of the mixture. 
     The heat exchanger  50  may include a flow passage through which the combustion gas C may flow. The following description is given of an example in which the gas furnace  10  includes the heat exchanger  40  including a primary heat exchanger  51  and a secondary heat exchanger  52 , but the heat exchanger  50  may include only the primary heat exchanger  51  depending on embodiments. 
     One end of the primary heat exchanger  51  may be disposed adjacent to the burner assembly  40 . The other end thereof, which is opposite the one end of the primary heat exchanger  51 , may be connected to a Hot Collect Box (HCB)  14 . The combustion gas C, flowing from the one end to the other end of the primary heat exchanger  51 , may be transferred to the secondary heat exchanger  52  through the HCB  14 . 
     One end of the secondary heat exchanger  52  may be connected to the HCB  14 . The combustion gas C, having passed through the primary heat exchanger  51 , may flow through the one end of the secondary heat exchanger  52  to pass therethrough. The secondary heat exchanger  52  may heat exchange, once again, the combustion gas C, having passed through the primary heat exchanger  51 , with air passing around the secondary heat exchanger  52 . That is, as heat energy from the combustion gas, having passed through the primary heat exchanger  51 , may be used again by the secondary heat exchanger  52 , the efficiency of the gas furnace  10  may be improved. 
     Condensate is generated as the combustion gas C, passing through the secondary heat exchanger  52 , is condensed by heat transfer with air passing around the secondary heat exchanger  52 . In other words, water vapor contained in the combustion gas C may be condensed and transformed into condensate. For this reason, the gas furnace  10 , including the primary heat exchanger  51  and the secondary heat exchanger  52 , is called a condensing gas furnace. In this case, the generated condensate may be collected in a Cold Collect Box (CCB)  16 . To this end, the one end of the secondary heat exchanger  52  and the other end thereof, which is opposite the one end, may be connected to a one side surface of the CCB  16 . 
     The condensate generated by the secondary heat exchanger  52  may flow out of the condensate trap through the CCB  16  and then may be discharged outside of the gas furnace  10  through a discharge port. In this case, the condensate trap  90  may be connected to the other side surface of the CCB  16 . Further, the condensate trap  90  may collect and discharge not only the condensate, generated in the secondary heat exchanger  52 , but also condensate generated in the exhaust pipe  80  connected to the inducer  70 . That is, condensate, which is generated when the combustion gas C, which has not yet been condensed at the other end of the secondary heat exchanger  52 , is condensed by passing through the exhaust pipe  80 , may also be collected in the condensate trap  90 , and may be discharged outside of the gas furnace  10  through the discharge port. 
     The inducer  70 , which will be described later, may be connected to the other side surface of the CCB  16 . For convenience explanation, the following description is given of an example in which the inducer  70  is connected to the CCB  16 , but the inducer  70  may also be connected to a mounting plate  12  having the CCB  16  connected thereto. 
     The CCB  16  may have an opening. The other end of the secondary heat exchanger  52  and the inducer  70  may communicate with each other through the opening formed at the CCB  16 . That is, the combustion gas C, having passed through the other end of the secondary heat exchanger  52 , may flow out of the inducer  70  through the opening formed at the CCB  16 , and may be discharged outside of the gas furnace  10  through the exhaust pipe  80 . 
     The inducer  70  may communicate with the other end of the secondary heat exchanger  52  through the opening formed at the CCB  16 . One end of the inducer  70  is connected to the other side surface of the CCB  16 , and the other end of the inducer  70  may be connected to the exhaust pipe  80 . The inducer  70  may induce the flow of the combustion gas C to pass through the primary heat exchanger  51 , the HCB  14 , and the secondary heat exchanger  52 , to be discharged through the exhaust pipe  80 . In this regard, the inducer  70  may be called an Induced Draft Motor (IDM). 
     The blower  60  may be disposed at a lower part of the gas furnace  10 . The blower  60  may cause the air, to be supplied to the indoor space, to move upward from the lower part of the gas furnace  10 . In this regard, the blower  60  may be called an Indoor Blower Motor (IBM). 
     The blower  60  may cause the air to pass around the heat exchanger  50 . Temperature of the air, passing around the heat exchanger  50  by the blower  60 , may be increased as the air receives heat energy transferred from the combustion gas C via the heat exchanger  50 . As the air with increased temperature is supplied to the indoor space, the indoor space may be heated. 
     As in a general gas furnace  1  illustrated in  FIG.  1   , the gas furnace  10  may include a case (not numbered). The above components of the gas furnace  10  may be accommodated in the case. 
     The case may have a lower opening (not numbered), which is formed at a lower side of the case on a side surface adjacent to the blower  60 . An internal air duct D 1 , through which the air introduced from the indoor space (hereinafter referred to as indoor air RA) passes, may be installed at the lower opening. A supply air duct D 2 , through which the air to be supplied to the indoor space (hereinafter referred to as supply air SA) passes, may be installed at an upper opening (not numbered) which is formed at an upper side of the case. 
     That is, once the blower  60  operates, temperature of the air increases while the air, introduced from the indoor space as the internal air RA through the internal air duct D 1 , passes through the heat exchanger  50 , such that the air with increased temperature may be supplied to the indoor space as the supply air SA through the supply air duct D 2 , thereby heating the indoor space. 
     When compared to the gas furnace  10  according to the above embodiment and the following embodiments of the present disclosure which will be described later, the general gas furnace  1  illustrated in  FIG.  1    is different from the gas furnace  10  of the present disclosure, and the differences are as follows. 
     That is, in the general gas furnace  1 , a fuel gas, having passed through a manifold  3 , is injected into a burner assembly  4  through a nozzle installed at the manifold  3 , and the fuel gas may pass through a Venturi tube (not numbered) of the burner assembly  4  to be mixed with air naturally aspirated into the burner assembly  4 , to form a mixture. However, the general gas furnace  1  may have difficulty in reducing nitrogen oxide (NOx) emissions for the following reasons. 
     First, it can be understood that the general gas furnace  1  provides a partial pre-mixing mechanism having diffusion combustion characteristics, in which the fuel gas, injected through the nozzle, passes through the Venturi tube along with primary air, introduced through a space between a lower side of the burner assembly  4  and the nozzle, to be mixed as a mixture, and then the mixture is burned with secondary air introduced through a space between an upper side of the burner assembly  4  and the heat exchanger  5 . 
     However, due to the diffusion combustion characteristics that a flame spread speed is considerably slower than the speed of combustion chemical reaction, the gas furnace having such partial pre-mixing mechanism may have difficulty in reducing the flame temperature even by oversupplying the secondary air. Furthermore, the gas furnace may also have difficulty in controlling an air ratio (i.e., ratio of actual air supply to the theoretical amount of air), such that there is a limitation in reducing nitrogen oxide (NOx) emissions. 
     In order to solve the above problems, the present disclosure has been devised to provide the gas furnace  10  including a full premixing mechanism, in which by increasing a mixing rate of the fuel gas and air, the increase in local flame temperature may be prevented, thereby reducing the nitrogen oxide (NOx) emissions. The gas furnace  10  will be described in further detail below. 
       FIG.  3    is a partial perspective view of a gas furnace according to an embodiment of the present disclosure. 
     Referring to  FIGS.  2  and  3   , the gas furnace  10  includes a mixer  32 , a mixing pipe  33 , a burner assembly  40 , a heat exchanger  50 , an exhaust pipe  80 , an inducer  70 , and a blower  60 . 
     The inducer  70  induces the air A to flow into the mixer  32  through the intake pipe  31 , induces a mixture, which will be described below, to flow from the mixing pipe  33  to the burner assembly  40 , and induces a combustion gas C, which will be described below, to flow from the burner assembly  40  to the heat exchanger  50  and the exhaust pipe  80 . Further, the blower  60  may cause the flow of air passing around the heat exchanger  50 . 
     The mixer  32  forms a mixture by mixing the air A, introduced from each of the intake pipe  31  and the manifold  21 , and a fuel gas F. Here, the intake pipe  31  is a pipe having one side exposed to the outside, such that air participating in combustion may be drawn through the intake pipe  31 ; and the manifold  21  is a pipe having one side connected to the gas valve  20 , such that the fuel gas F participating in combustion may flow through the manifold  21 . The amount of the fuel gas F, flowing through the manifold  21 , may be controlled by opening and closing the gas valve  20  or by controlling the degree of opening of the gas valve  20 , as described above. In addition, the gas furnace  10  may further include a controller for controlling the opening and closing of the gas valve  20  or the degree of opening of the gas valve  20 . 
     The mixture, formed by the mixer  32 , may pass through the mixing pipe  33  to be fed into the burner assembly  40 , and the air A participating in combustion may be fully pre-mixed with the fuel gas F to be fed into the burner assembly  40 , such that the flame temperature may be reduced easily by controlling the air ratio (i.e., controlling the amount of drawn air so that air may be oversupplied for combustion). Further, the intake pipe  31 , the mixer  32 , the mixing pipe  33 , the burner assembly  40 , and the heat exchanger  50  communicate with each other, such that the flame temperature may be reduced easily by controlling the air ratio by the operation of the inducer  70 , thereby greatly reducing nitrogen oxide (NOx) emissions. In other words, conditions of combustion in a lean area for reducing the NOx emissions may be readily achieved. 
     The present disclosure may provide the Venturi effect to increase a mixing rate of the air A and the fuel gas F in the mixer  32 , which will be described in further detail below. 
       FIG.  4    is a partial sectional view of a gas furnace according to an embodiment of the present disclosure. 
     Referring to  FIG.  4   , the mixture, having passed through the mixer  32 , may flow through the mixing pipe  33 . The mixing pipe  33  may guide the mixture to the burner assembly  40 . The burner assembly  40  may produce a flame and high-temperature combustion gas C by burning the mixture having passed through the mixing pipe  33 . 
     The burner assembly  40  may include a mixing chamber  41 , a burner  42 , a burner plate  43 , combustion chambers  44 :  441 ,  442 ,  443 , and  444 , and a burner box  45 . The gas furnace  10  may include a plurality of primary heat exchangers  51 . In this case, the gas furnace  10  may include a plurality of burners  42  and a plurality of combustion chambers  44 , the number of the burners  42  and the number of the combustion chambers  44  being equal to the number of the primary heat exchangers  51 . For example, the gas furnace  10  may include four primary heat exchangers  51  disposed parallel to each other, and four burners  42  and four combustion chambers  44  corresponding to the four primary heat exchangers  51 . 
     The mixing chamber  41  may serve as a medium for delivering the mixture from the mixing pipe  33  to the burner  42 . That is, the mixing pipe  33  may be connected to a connector provided at one side of the mixing chamber  41 , such that the mixture, having passed through the mixing pipe  33 , may flow into the mixing chamber  41  through the connector  411  and then may be fed into the burner  42 . The mixture of the air and the fuel gas may be continuously mixed even while being guided to the burner  42  through the mixing chamber  41 . 
     The flame, produced by the combustion of the mixture, may be anchored to the burner  42 . For example, the burner  42  may include a perforated burner plate  42   a  and a burner mat  42   b.    
     The perforated burner plate  42   a  may have a plurality of ports, through which the mixture is ejected. For example, the perforated burner plate  42   a  may be made of a stainless material. The perforated burner plate  42   a  may serve to uniformly distribute the mixture to the burner mat  42   b  which will be described below. In this case, the flow of the mixture is redistributed between the perforated burner plate  42   a  and the burner mat  42   b , thereby effectively providing a uniform flow of the mixture. In addition, compared to a case where the burner  42  includes only the burner mat  42   b , the burner  42  includes the perforated burner plate  42   a  as well as the burner mat  42   b  as described above, such that flame stability may be improved. Further, the perforated burner plate  42   a  may also serve to support the burner mat  42   b.    
     The burner mat  42   b  may be connected to an upper part of the perforated burner plate  42   a , and may distribute the mixture, ejected through the ports of the perforated burner plate  42   a , more uniformly, thereby allowing the flame to be stably anchored to the burner mat  42   b . For example, the burner mat  42   b  may be made of a metal fiber with gaps smaller than the diameter of the ports. The burner mat  42   b  may be understood as a circular array of cylinders in which the ejection speed of the mixture is close to zero, such that the flames may be stably anchored to the surface of the burner mat  42   b . As a result, flame stability may be greatly improved, and the heating power of the gas furnace may be efficiently adjusted in a wide range. That is, the burner mat  42   b  may be effective in preventing flame flash-back in the case where the heating power of the gas furnace is considerably reduced, and in preventing flame blow-out in the case where the heating power of the gas furnace is considerably increased. 
     A plurality of burners  42  may be connected to one side of the burner plate  43 . A plurality of burner holes, communicating with the plurality of combustion chambers  44 , may be formed on a body of the burner plate  43 . 
     One end of the combustion chamber  44  may be connected to the other side of the burner plate  43 , and the other end of the combustion chamber  44  may be disposed adjacent to the primary heat exchanger  51 . The mixing chamber  41  may be connected to one end of the burner box  45 , and one side of the mounting plate  12  may be connected to the other end of the burner box  45 . Further, the burner  42 , the burner plate  43 , and the combustion chamber  44  may be provided inside the burner box  45 . 
     In addition, the gas furnace  10  may further include an igniter  451  disposed inside the combustion chamber  44 . For example, the igniter  451  may be installed on an inner surface of the burner box  45 , to be inserted into a hole of the combustion chamber  44 . Once the mixture, fed into the burner  42  through the connector  411 , is burned by the ignition provided by the igniter  451 , flames and high-temperature combustion gas C may be produced, and the produced flames may be anchored to the burner  42 . 
     Even when the igniter  451  is disposed in only any one of the plurality of combustion chambers  44  (i.e., first combustion chamber  441 ), the flames may be spread to adjacent burners through flame spread holes  435 :  435   a ,  435   b , and  435   c  formed in the burner plate  43 . In this case, the burner assembly  40  may include flame spread tunnels  445 :  445   a ,  445   b , and  445   c  which are formed between adjacent combustion chambers  44  at positions corresponding to the flame spread holes  435 , thereby forming a flame spread path with the flame spread holes  435 . 
     The flame spread tunnels  445  may prevent the mixture, ejected from the flame spread holes  435 , from leaking to the outside, thereby allowing the flame spread holes  435  to serve a function in spreading the flames between the adjacent individual burners. 
     The mixture, having passed through the mixing pipe  33 , may pass through the mixing chamber  41  to be distributed not only to the plurality of burners  42  but also to the flame spread holes  435 , and the flames may be distributed between the adjacent burners  42  along the flame spread path formed between the flame spread holes  435  and the flame spread tunnels  445 . 
     That is, flames may be spread between individual burners via the flame spread holes  453  according to the mechanism in which the flame, anchored to any one of the burners  42  adjacent to the flame spread holes  435 , burns the mixture ejected from the flame spread holes  435  to produce a flame, and the produced flame burns the mixture ejected from another one of the burners  42  adjacent to the flame spread holes  435  to produce a flame. 
     The high-temperature combustion gas C, having passed through the combustion chambers  44 , may be fed into the heat exchanger  51 . That is, the high-temperature combustion gas C, produced by each of the burners  42 , may pass through each of the plurality of combustion chambers  44  to be guided to each of the plurality of heat exchangers  51 , such that heat loss may be reduced compared to the case of integrated burners facing a plurality of heat exchangers (i.e., a case where some of the flames and high-temperature combustion gases C, produced by the integrated burners, pass out of the plurality of heat exchangers, thereby causing heat loss). 
     In addition, the gas furnace  10  may further include a flame detector  452  disposed inside the combustion chamber  44 . For example, the flame detector  452  may be installed on an inner surface of the burner box  45 , to be inserted into a hole formed at the combustion chamber  44 . Based on the characteristics that flames may be spread sequentially through the flame spread holes of the present disclosure, it is possible to detect whether flames are formed in response to operation of the gas furnace even when the flame detector  452  is disposed in only any one of the plurality of combustion chambers  44 . When the flame detector  452  detects that flames are not formed in response to operation of the gas furnace, which may cause a safety risk, such that it is required to block the supply of fuel gas F to the manifold  21  by closing the gas valve  20 . 
     The heat exchanger  50  may include a gas passage, through which the high-temperature combustion gas C flows which is produced by the combustion. The combustion gas (hereinafter referred to as an exhaust gas E), having passed through the heat exchanger  50 , may pass through the inducer  70  to be discharged to the outside through the exhaust pipe  80 . In this case, the condensate generated in the heat exchanger  50 , particularly the secondary heat exchanger  52  and the exhaust pipe  80 , may be collected in the condensate trap  90  to be discharged to the outside. 
       FIG.  5    is a perspective view of a mixer according to an embodiment of the present disclosure.  FIG.  6    is a perspective view of a mixer according to another embodiment of the present disclosure.  FIG.  7    is a diagram explaining a discharge direction of a fuel gas through fuel inlet holes according to an embodiment of the present disclosure. 
     Referring to  FIGS.  4  and  5   , the mixer  32  includes a mixer housing  32   a  and a Venturi tube  32   b . An intake pipe  31  is connected to a front end of the mixer housing  32   a , a mixing pipe  33  may be connected to a rear end of the mixer housing  32   a , and a manifold  21  may be connected to a side surface of the mixer housing  32   a . Here, the intake pipe  31  may be connected to the mixer housing  32   a  via an intake pipe connector  31   a , and the mixing pipe  33  may be integrally connected to the rear end of the mixer housing  32   a , but the arrangement is not limited thereto. 
     That is, the air A and the fuel gas F may flow into the mixer  32  through the intake pipe  31  and the manifold  21 , respectively, to be mixed and then fed into the mixing pipe  33 . 
     The Venturi tube  32   b  may be provided inside the mixer housing  32   a . An outer circumferential surface of each of a converging section  321 , a throat  322 , and a diverging section  323  of the Venturi tube  32   b , which will be described below, may be disposed on an inner circumferential surface of the mixer housing  32   a  at positions spaced apart from each other at predetermined intervals. 
     In addition, the Venturi tube  32   b  may include a flange  326  which extends outwards from the outer circumferential surface of the Venturi tube  32   b , to be pressed against the inner circumferential surface of the mixer housing  32   a , such that the Venturi tube  32   b  may be fixed to the inside of the mixer housing  32   a.    
     The Venturi tube  32   b  may include the converging section  321 , the throat  322 , and the diverging section  323 . 
     The converging section  321  has on one end an air inlet, through which the air A, having passed through the intake pipe  31 , is introduced; and a flange  328  may be formed on an outer circumferential surface of the one end. A pressure sensor may be installed at the flange  328 , to sense the pressure of air flowing through the Venturi tube  32   b.    
     The converging section  321  may have a diameter which decreases toward a downstream side. Accordingly, the pressure of air passing through the converging section  321  may drop (further, flow rate increases) and a negative pressure may be formed, which is known as the Venturi effect. In this case, the air pressure drop may allow the fuel gas F to easily flow through a fuel inlet hole  322   a  of the throat  322 . Further, as the flow rate of the air increases, a turbulence intensity in the air flow increases, thereby increasing a mixing rate of the air A and the fuel gas F, which will be described below. 
     The throat  322  may be connected to the converging section  32 , and may have the fuel inlet hole  322   a , through which the fuel gas F having passed through the manifold  21  is introduced, and which is formed on at least a portion of the side surface of the throat  322 . 
     In the gas furnace  10  illustrated in  FIG.  5    according to an embodiment of the present disclosure, the throat  322  may have a diameter which is maintained constant. In the gas furnace illustrated in  FIG.  6    according to another embodiment of the present disclosure, the diameter of a throat  322 ′ may decrease toward a downstream side until a predetermined point is reached, and the diameter of the throat  322 ′ may increase from the point toward the downstream side. Alternatively, depending on embodiments, the diameter of the throat  322 ′ may be maintained at a predetermined value until a predetermined point of the downstream side is reached, and the diameter may increase from the predetermined point toward the downstream side. Further, depending on embodiments, the diameter of the throat  322 ′ may decrease toward a downstream side until a predetermined point is reached, and the diameter of the throat  322 ′ may be maintained at a predetermined value from the predetermined point. 
     The fuel inlet hole  322   a  may include a plurality of fuel inlet holes  322   a  which are spaced apart from each other at predetermined intervals in a circumferential direction of the throat  322 , thereby allowing the fuel gas F to be easily flow through the Venturi tube  32   b . The plurality of fuel inlet holes  322   a  may be symmetrical to each other with respect to a longitudinal direction of the Venturi tube  32   b . For example, the diameter of the plurality of fuel inlet holes  322   a  may be in a range of 1/10 to 1/25 of the diameter of the throat  322 . 
     The fuel inlet holes  322   a  may be formed at positions corresponding to a space between the flange  326  as the side surface of the throat  322  and a connection portion of the manifold  21  in the mixer housing  32   a . In this manner, compared to the case where the fuel inlet holes  322   a  are formed at a connection portion of the manifold  21  in the mixer housing  32 , it is possible to prevent the fuel gas F from being supplied intensively through some of the fuel inlet holes  322   a , thereby allowing the fuel gas F to be supplied uniformly through all the fuel inlet holes  321   a.    
     The diverging section  323  may be connected to the throat  322 , and the air A and the fuel F, having passed through the converging section  321  and the fuel inlet holes  322   a , respectively, may be mixed and may flow through the diverging section  323  as a mixture. 
     The diverging section  323  may have a diameter which increases toward the downstream side. Accordingly, the pressure, which drops while passing through the converging section  321 , may be recovered to a predetermined value while passing through the diverging section  323 , and the air A and the fuel gas F may be mixed more easily. Further, the diverging section  323  may have at one end a discharge port, through which the mixture is discharged to the mixing pipe  33 . 
     In addition, the Venturi tube  32   b  may include the flange  326  which extends outwards from an outer circumferential surface of a portion connected to the throat  322  in the converging section  321 , to be pressed against the inner circumferential surface of the mixer housing  32   a . The flange  326  not only fixes the Venturi tube  32   b  to the inside of the mixer housing  32   a  but also prevents the fuel gas F, having passed through the manifold  21 , from flowing outside of the converging section  321 . 
     Referring to  FIG.  7   , the plurality of fuel inlet holes may include first fuel inlet holes  322   a , which pass through the side surface of the throat  322  in a radially inward direction of the throat  322  (see (a) of  FIG.  7   ). As a result, the fuel gas F may flow through the first fuel inlet holes  322   a  in a vertical direction or a radially inward direction relative to the flow of fluids (i.e., air and/or mixture) passing through the Venturi tube  32   b , such that the fuel gas F may penetrate deep into the flow of the fluids, thereby increasing a mixing rate of the air and the fuel gas F. 
     Depending on embodiments, the plurality of fuel inlet holes may include second fuel inlet holes  322   a ′ which obliquely pass through the side surface of the throat  322  in a circumferential direction of the throat  322  relative to the radially inward direction of the throat  322  (see (b) of  FIG.  7   ). For example, the direction in which the second fuel inlet holes  322   a ′ are formed may be a direction inclined at an angle of 35 degrees to 85 degrees in the circumferential direction of the throat  322  relative to the radially inward direction of the throat  322 . As a result, the flow of the fuel gas F through the second fuel inlet holes  322   a ′ may impart a swirl effect to the flow of the fluids (i.e., air and/or mixture) passing through the Venturi tube  32   b , thereby increasing a mixing rate of the air and the fuel gas F. 
     Further, depending on embodiments, the plurality of fuel inlet holes may include both the first fuel inlet holes  322   a  and the second fuel inlet holes  322   a ′, in which case the first and second fuel inlet holes  322   a  and  322   a ′ may be disposed alternately on the side surface of the throat  322 , and may be provided in even numbers such as 2, 4, 6, 8, 10, and the like. 
     In this case, the fuel gas F may be discharged through the first fuel inlet holes  322   a  in a manner that produces a momentum effect in a radial direction to the flow of the fluids passing through the Venturi tube  32   b , and the fuel gas F may be discharged through the second fuel inlet holes  322   a ′ in a manner that produces the swirl effect to the flow of the fluids passing through the Venturi tube  32   b , thereby greatly increasing a mixing rate of the air and the fuel gas F. Accordingly, it is possible to prevent an increase in local flame temperature, and the nitrogen oxide (NOx) emissions may be greatly reduced. 
     The gas furnace according to the present disclosure has one or more of the following effects. 
     First, by fully premixing the air and fuel before combustion in the burner assembly, it is possible to easily control an amount of air intake for operation in the lean region, such that the nitrogen oxide emissions may be easily reduced. 
     Secondly, the air and fuel gas are mixed in the mixer while passing through the Venturi tube, such that a mixing rate thereof may be increased and nitrogen oxide emissions may be greatly reduced, compared to the case where flame temperature is locally increased due to a relatively low mixing rate. 
     Thirdly, the fuel inlet holes, formed at the Venturi tube, are formed in a radially inward direction of the throat or are formed obliquely in a circumferential direction of the throat relative to the radially inward direction of the throat, such that the mixing ratio of the air and fuel gas in the Venturi tube is increased, thereby preventing the creation of nitrogen oxide caused by the locally increased flame temperature. 
     While the gas furnace according to the embodiments of the present disclosure has been described above with reference to the accompanying drawings, it should be understood that the present disclosure is not limited to the aforementioned embodiments, and various modifications and equivalent embodiments may be possible without departing from the scope and spirit of the disclosure as defined by the appended claims. Therefore, the scope of the present disclosure should be limited only by the accompanying claims and equivalents thereof.