Patent Publication Number: US-11022340-B2

Title: Enhanced heat transfer surfaces for heat exchangers

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority from and the benefit of U.S. Provisional Application Ser. No. 62/369,553, entitled “ENHANCED INTERNAL/EXTERNAL HEAT TRANSFER SURFACES FOR TUBULAR HEAT EXCHANGERS,” filed Aug. 1, 2016, which is hereby incorporated by reference. 
    
    
     BACKGROUND 
     The present disclosure relates generally to heating, ventilating, and air conditioning systems. A wide range of applications exist for heating, ventilating, and air conditioning (HVAC) systems. For example, residential, light commercial, commercial, and industrial systems are used to control temperatures and air quality in residences and buildings. Such systems often are dedicated to either heating or cooling, although systems are common that perform both of these functions. Generally, these systems operate by implementing a thermal cycle in which fluids are heated and cooled to provide the desired temperature in a controlled space, typically the inside of a residence or building. Similar systems are used for vehicle heating and cooling, and as well as for general refrigeration. 
     Many HVAC systems include furnace systems. For instance, an HVAC system may include a furnace system with a burner assembly and a heat exchanger to produce hot air to heat an enclosed space, such as a room in a residential, commercial, or industrial building. Generally, furnace systems operate by burning or combusting a mixture of air and fuel in the burner assembly to produce combustion products. The combustion products may pass through tubes or piping in the heat exchanger, where air passing over the tubes or pipes extracts heat from the combustion products. The heated air may be exported from the furnace system for heating a load (e.g., a room). The heat exchanger, which in some cases may be a multi-pass heat exchanger (e.g., a two-pass or four-pass heat exchanger), may include surface features on the second pass (as well as the third and fourth passes in a four-pass heat exchanger) to enhance heat transfer. 
     SUMMARY 
     The present disclosure relates to a furnace system that includes a burner assembly that includes a burner configured to produce a flame and a heat exchanger that includes a plurality of tube passes. The plurality of tube passes cooperatively forms a conduit for flowing combustion products generated by the burner assembly. Each tube pass of the plurality of tube passes overlaps with other tube passes of the plurality of tube passes. A first tube pass of the plurality of tube passes is configured to receive the flame, and the first tube pass includes a first plurality of surface enhancements extending radially outward from an outer surface of the first tube pass relative to a central axis of the first tube pass. The furnace system also includes a baffle that is coupled to the burner assembly, extends toward the first tube pass, and is configured to contact the flame and the first tube pass. 
     The present disclosure also relates to a furnace heat exchanger that includes a first tube pass. The first tube pass includes an outer surface. The furnace heat exchanger also includes a second tube pass. The first tube pass and the second tube pass are fluidly coupled to one another in a U-shaped configuration. The first tube pass is configured to receive a flame and combustion products from a furnace system. Also, the first tube pass includes a surface enhancement extending radially outward from the outer surface of the first tube pass. 
     The present disclosure further relates to a heating, ventilating, and air conditioning (HVAC) unit that includes a furnace system and a burner assembly of the furnace system. The burner assembly includes a plurality of burners, and each burner of the plurality of burners is configured to produce combustion products and a flame. The HVAC unit also includes a heat exchanger of the furnace system. The heat exchanger includes a plurality of first tube passes. Each first tube pass of the plurality of first tube passes is configured to receive the combustion products and the flame from one burner of the plurality of burners, and each first tube pass of the plurality of first tube passes includes a surface enhancement. Additionally, the HVAC unit includes a plate of the furnace system. The plate is disposed between the plurality of burners and the heat exchanger, and the plate includes a plurality of openings. Each opening of the plurality of openings is aligned with a respective burner of the plurality of burners and a respective first tube pass of the plurality of first tube passes. Moreover, the HVAC unit includes a plurality of baffles of the furnace system. Each baffle of the plurality of baffles is disposed in a respective opening of the plurality of openings. Also, each baffle of the plurality of baffles is configured to contact the flame of a respective burner of the plurality of burners and contact a respective first tube pass of the plurality of first tube passes. 
    
    
     
       DRAWINGS 
         FIG. 1  is a perspective view a heating, ventilating, and air conditioning (HVAC) system for building environmental management, in accordance with embodiments described herein; 
         FIG. 2  is a perspective view of the HVAC unit of  FIG. 1 , in accordance with embodiments described herein; 
         FIG. 3  is a perspective view of a residential heating and cooling system, in accordance with embodiments described herein; 
         FIG. 4  is a schematic diagram of a vapor compression system that may be used in the HVAC system of  FIG. 1  and the residential heating and cooling system  FIG. 3 , in accordance with embodiments described herein; 
         FIG. 5  is a schematic diagram of a furnace system, in accordance with embodiments described herein; 
         FIG. 6  is a perspective view of a furnace system, in accordance with embodiments described herein; 
         FIG. 7  is a perspective view of a baffle that may be included in the furnace system of  FIG. 6 , in accordance with embodiments described herein; 
         FIG. 8  is a perspective view of a portion of a heat exchanger, in accordance with embodiments described herein; 
         FIG. 9  is an axial view of the portion of the heat exchanger of  FIG. 8 , in accordance with embodiments described herein; 
         FIG. 10  is a perspective view of a portion of a heat exchanger, in accordance with embodiments described herein; 
         FIG. 11  is an axial view of the portion of the heat exchanger of  FIG. 10 , in accordance with embodiments described herein; 
         FIG. 12  is a perspective view of a portion of a heat exchanger, in accordance with embodiments described herein; 
         FIG. 13  is an axial view of the portion of the heat exchanger of  FIG. 12 , in accordance with embodiments described herein; 
         FIG. 14  is a perspective view of a portion of a heat exchanger, in accordance with embodiments described herein; 
         FIG. 15  is an axial view of the portion of the heat exchanger of  FIG. 14 , in accordance with embodiments described herein; 
         FIG. 16  is a perspective view of a portion of a heat exchanger, in accordance with embodiments described herein; and 
         FIG. 17  is an axial view of the portion of the heat exchanger of  FIG. 16 , in accordance with embodiments described herein. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure is directed to heating, ventilating, and air conditioning (HVAC) systems and components thereof. More specifically, the present disclosure relates to HVAC units with a furnace system having a multi-pass heat exchanger (e.g., 2-pass or 4-pass heat exchangers) that receives combustion products from the furnace system. In accordance with present embodiments, the first pass of the heat exchanger may include enhanced surface features (e.g., dimples, fins, protrusions) that increase the transfer of heat to air in the HVAC unit used to heat a space (e.g., a room) without impinging on the flame of the furnace system. Additionally, the furnace system may include a baffle that reduces the production of certain gases and increases heat transfer without impinging on the flame of the furnace system. 
     Turning now to the drawings,  FIG. 1  illustrates a heating, ventilating, and air conditioning (HVAC) system for building environmental management that may employ one or more HVAC units. In the illustrated embodiment, a building  10  is air conditioned by a system that includes an HVAC unit  12 . The building  10  may be a commercial structure or a residential structure. As shown, the HVAC unit  12  is disposed on the roof of the building  10 ; however, the HVAC unit  12  may be located in other equipment rooms or areas adjacent the building  10 . The HVAC unit  12  may be a single package unit containing other equipment, such as a blower, integrated air handler, and/or auxiliary heating unit. In other embodiments, the HVAC unit  12  may be part of a split HVAC system, such as the system shown in  FIG. 3 , which includes an outdoor HVAC unit  58  and an indoor HVAC unit  56 . 
     The HVAC unit  12  is an air cooled device that implements a refrigeration cycle to provide conditioned air to the building  10 . Specifically, the HVAC unit  12  may include one or more heat exchangers across which an air flow is passed to condition the air flow before the air flow is supplied to the building. In the illustrated embodiment, the HVAC unit  12  is a rooftop unit (RTU) that conditions a supply air stream, such as environmental air and/or a return air flow from the building  10 . After the HVAC unit  12  conditions the air, the air is supplied to the building  10  via ductwork  14  extending throughout the building  10  from the HVAC unit  12 . For example, the ductwork  14  may extend to various individual floors or other sections of the building  10 . In certain embodiments, the HVAC unit  12  may be a heat pump that provides both heating and cooling to the building with one refrigeration circuit configured to operate in different modes. In other embodiments, the HVAC unit  12  may include one or more refrigeration circuits for cooling an air stream and a furnace for heating the air stream. 
     A control device  16 , one type of which may be a thermostat, may be used to designate the temperature of the conditioned air. The control device  16  also may be used to control the flow of air through the ductwork  14 . For example, the control device  16  may be used to regulate operation of one or more components of the HVAC unit  12  or other components, such as dampers and fans, within the building  10  that may control flow of air through and/or from the ductwork  14 . In some embodiments, other devices may be included in the system, such as pressure and/or temperature transducers or switches that sense the temperatures and pressures of the supply air, return air, and so forth. Moreover, the control device  16  may include computer systems that are integrated with or separate from other building control or monitoring systems, and even systems that are remote from the building  10 . 
       FIG. 2  is a perspective view of an embodiment of the HVAC unit  12 . In the illustrated embodiment, the HVAC unit  12  is a single package unit that may include one or more independent refrigeration circuits and components that are tested, charged, wired, piped, and ready for installation. The HVAC unit  12  may provide a variety of heating and/or cooling functions, such as cooling only, heating only, cooling with electric heat, cooling with dehumidification, cooling with gas heat, or cooling with a heat pump. As described above, the HVAC unit  12  may directly cool and/or heat an air stream provided to the building  10  to condition a space in the building  10 . 
     As shown in the illustrated embodiment of  FIG. 2 , a cabinet  24  encloses the HVAC unit  12  and provides structural support and protection to the internal components from environmental and other contaminants. In some embodiments, the cabinet  24  may be constructed of galvanized steel and insulated with aluminum foil faced insulation. Rails  26  may be joined to the bottom perimeter of the cabinet  24  and provide a foundation for the HVAC unit  12 . In certain embodiments, the rails  26  may provide access for a forklift and/or overhead rigging to facilitate installation and/or removal of the HVAC unit  12 . In some embodiments, the rails  26  may fit into “curbs” on the roof to enable the HVAC unit  12  to provide air to the ductwork  14  from the bottom of the HVAC unit  12  while blocking elements such as rain from leaking into the building  10 . 
     The HVAC unit  12  includes heat exchangers  28  and  30  in fluid communication with one or more refrigeration circuits. Tubes within the heat exchangers  28  and  30  may circulate refrigerant (for example, R-410A, steam, or water) through the heat exchangers  28  and  30 . The tubes may be of various types, such as multichannel tubes, conventional copper or aluminum tubing, and so forth. Together, the heat exchangers  28  and  30  may implement a thermal cycle in which the refrigerant undergoes phase changes and/or temperature changes as it flows through the heat exchangers  28  and  30  to produce heated and/or cooled air. For example, the heat exchanger  28  may function as a condenser where heat is released from the refrigerant to ambient air, and the heat exchanger  30  may function as an evaporator where the refrigerant absorbs heat to cool an air stream. In other embodiments, the HVAC unit  12  may operate in a heat pump mode where the roles of the heat exchangers  28  and  30  may be reversed. That is, the heat exchanger  28  may function as an evaporator and the heat exchanger  30  may function as a condenser. In further embodiments, the HVAC unit  12  may include a furnace for heating the air stream that is supplied to the building  10 . While the illustrated embodiment of  FIG. 2  shows the HVAC unit  12  having two of the heat exchangers  28  and  30 , in other embodiments, the HVAC unit  12  may include one heat exchanger or more than two heat exchangers. 
     The heat exchanger  30  is located within a compartment  31  that separates the heat exchanger  30  from the heat exchanger  28 . Fans  32  draw air from the environment through the heat exchanger  28 . Air may be heated and/or cooled as the air flows through the heat exchanger  28  before being released back to the environment surrounding the rooftop unit  12 . A blower assembly  34 , powered by a motor  36 , draws air through the heat exchanger  30  to heat or cool the air. The heated or cooled air may be directed to the building  10  by the ductwork  14 , which may be connected to the HVAC unit  12 . Before flowing through the heat exchanger  30 , the conditioned air flows through one or more filters  38  that may remove particulates and contaminants from the air. In certain embodiments, the filters  38  may be disposed on the air intake side of the heat exchanger  30  to prevent contaminants from contacting the heat exchanger  30 . 
     The HVAC unit  12  also may include other equipment for implementing the thermal cycle. Compressors  42  increase the pressure and temperature of the refrigerant before the refrigerant enters the heat exchanger  28 . The compressors  42  may be any suitable type of compressors, such as scroll compressors, rotary compressors, screw compressors, or reciprocating compressors. In some embodiments, the compressors  42  may include a pair of hermetic direct drive compressors arranged in a dual stage configuration  44 . However, in other embodiments, any number of the compressors  42  may be provided to achieve various stages of heating and/or cooling. As may be appreciated, additional equipment and devices may be included in the HVAC unit  12 , such as a solid-core filter drier, a drain pan, a disconnect switch, an economizer, pressure switches, phase monitors, and humidity sensors, among other things. 
     The HVAC unit  12  may receive power through a terminal block  46 . For example, a high voltage power source may be connected to the terminal block  46  to power the equipment. The operation of the HVAC unit  12  may be governed or regulated by a control board  48 . The control board  48  may include control circuitry connected to a thermostat, sensors, and alarms (one or more being referred to herein separately or collectively as the control device  16 ). The control circuitry may be configured to control operation of the equipment, provide alarms, and monitor safety switches. Wiring  49  may connect the control board  48  and the terminal block  46  to the equipment of the HVAC unit  12 . 
       FIG. 3  illustrates a residential heating and cooling system  50 , also in accordance with present techniques. The residential heating and cooling system  50  may provide heated and cooled air to a residential structure, as well as provide outside air for ventilation and provide improved indoor air quality (IAQ) through devices such as ultraviolet lights and air filters. In the illustrated embodiment, the residential heating and cooling system  50  is a split HVAC system. In general, a residence  52  conditioned by a split HVAC system may include refrigerant conduits  54  that operatively couple the indoor unit  56  to the outdoor unit  58 . The indoor unit  56  may be positioned in a utility room, an attic, a basement, and so forth. The outdoor unit  58  is typically situated adjacent to a side of residence  52  and is covered by a shroud to protect the system components and to prevent leaves and other debris or contaminants from entering the unit. The refrigerant conduits  54  transfer refrigerant between the indoor unit  56  and the outdoor unit  58 , typically transferring primarily liquid refrigerant in one direction and primarily vaporized refrigerant in an opposite direction. 
     When the system shown in  FIG. 3  is operating as an air conditioner, a heat exchanger  60  in the outdoor unit  58  serves as a condenser for re-condensing vaporized refrigerant flowing from the indoor unit  56  to the outdoor unit  58  via one of the refrigerant conduits  54 . In these applications, a heat exchanger  62  of the indoor unit functions as an evaporator. Specifically, the heat exchanger  62  receives liquid refrigerant (which may be expanded by an expansion device, not shown) and evaporates the refrigerant before returning it to the outdoor unit  58 . 
     The outdoor unit  58  draws environmental air through the heat exchanger  60  using a fan  64  and expels the air above the outdoor unit  58 . When operating as an air conditioner, the air is heated by the heat exchanger  60  within the outdoor unit  58  and exits the unit at a temperature higher than it entered. The indoor unit  56  includes a blower or fan  66  that directs air through or across the indoor heat exchanger  62 , where the air is cooled when the system is operating in air conditioning mode. Thereafter, the air is passed through ductwork  68  that directs the air to the residence  52 . The overall system operates to maintain a desired temperature as set by a system controller. When the temperature sensed inside the residence  52  is higher than the set point on the thermostat (plus a small amount), the residential heating and cooling system  50  may become operative to refrigerate additional air for circulation through the residence  52 . When the temperature reaches the set point (minus a small amount), the residential heating and cooling system  50  may stop the refrigeration cycle temporarily. 
     The residential heating and cooling system  50  may also operate as a heat pump. When operating as a heat pump, the roles of heat exchangers  60  and  62  are reversed. That is, the heat exchanger  60  of the outdoor unit  58  will serve as an evaporator to evaporate refrigerant and thereby cool air entering the outdoor unit  58  as the air passes over outdoor the heat exchanger  60 . The indoor heat exchanger  62  will receive a stream of air blown over it and will heat the air by condensing the refrigerant. 
     In some embodiments, the indoor unit  56  may include a furnace system  70 . For example, the indoor unit  56  may include the furnace system  70  when the residential heating and cooling system  50  is not configured to operate as a heat pump. The furnace system  70  may include a burner assembly and heat exchanger, among other components, inside the indoor unit  56 . Fuel is provided to the burner assembly of the furnace  70  where it is mixed with air and combusted to form combustion products. The combustion products may pass through tubes or piping in a heat exchanger (that is, separate from heat exchanger  62 ), such that air directed by the blower  66  passes over the tubes or pipes and extracts heat from the combustion products. The heated air may then be routed from the furnace system  70  to the ductwork  68  for heating the residence  52 . 
       FIG. 4  is an embodiment of a vapor compression system  72  that can be used in any of the systems described above. The vapor compression system  72  may circulate a refrigerant through a circuit starting with a compressor  74 . The circuit may also include a condenser  76 , an expansion valve(s) or device(s)  78 , and an evaporator  80 . The vapor compression system  72  may further include a control panel  82  that has an analog to digital (A/D) converter  84 , a microprocessor  86 , a non-volatile memory  88 , and/or an interface board  90 . The control panel  82  and its components may function to regulate operation of the vapor compression system  72  based on feedback from an operator, from sensors of the vapor compression system  72  that detect operating conditions, and so forth. 
     In some embodiments, the vapor compression system  72  may use one or more of a variable speed drive (VSDs)  92 , a motor  94 , the compressor  74 , the condenser  76 , the expansion valve or device  78 , and/or the evaporator  80 . The motor  94  may drive the compressor  74  and may be powered by the variable speed drive (VSD)  92 . The VSD  92  receives alternating current (AC) power having a particular fixed line voltage and fixed line frequency from an AC power source, and provides power having a variable voltage and frequency to the motor  94 . In other embodiments, the motor  94  may be powered directly from an AC or direct current (DC) power source. The motor  94  may include any type of electric motor that can be powered by a VSD or directly from an AC or DC power source, such as a switched reluctance motor, an induction motor, an electronically commutated permanent magnet motor, or another suitable motor. 
     The compressor  74  compresses a refrigerant vapor and delivers the vapor to the condenser  76  through a discharge passage. In some embodiments, the compressor  74  may be a centrifugal compressor. The refrigerant vapor delivered by the compressor  74  to the condenser  76  may transfer heat to a fluid passing across the condenser  76 , such as ambient or environmental air  96 . The refrigerant vapor may condense to a refrigerant liquid in the condenser  76  as a result of thermal heat transfer with the environmental air  96 . The liquid refrigerant from the condenser  76  may flow through the expansion device  78  to the evaporator  80 . 
     The liquid refrigerant delivered to the evaporator  80  may absorb heat from another air stream, such as a supply air stream  98  provided to the building  10  or the residence  52 . For example, the supply air stream  98  may include ambient or environmental air, return air from a building, or a combination of the two. The liquid refrigerant in the evaporator  80  may undergo a phase change from the liquid refrigerant to a refrigerant vapor. In this manner, the evaporator  80  may reduce the temperature of the supply air stream  98  via thermal heat transfer with the refrigerant. Thereafter, the vapor refrigerant exits the evaporator  80  and returns to the compressor  74  by a suction line to complete the cycle. 
     In some embodiments, the vapor compression system  72  may further include a reheat coil in addition to the evaporator  80 . For example, the reheat coil may be positioned downstream of the evaporator relative to the supply air stream  98  and may reheat the supply air stream  98  when the supply air stream  98  is overcooled to remove humidity from the supply air stream  98  before the supply air stream  98  is directed to the building  10  or the residence  52 . 
     It should be appreciated that any of the features described herein may be incorporated with the HVAC unit  12 , the residential heating and cooling system  50 , or other HVAC systems. Additionally, while the features disclosed herein are described in the context of embodiments that directly heat and cool a supply air stream provided to a building or other load, embodiments of the present disclosure may be applicable to other HVAC systems as well. For example, the features described herein may be applied to mechanical cooling systems, free cooling systems, chiller systems, or other heat pump or refrigeration applications. 
     As discussed below, the HVAC unit  12  may include a furnace system that includes heat exchangers with enhanced surfaces that enable greater heat transfer to air that is heated by the HVAC unit  12 . Additionally, the heat exchangers discussed below may also be included in the furnace system  70  of the residential heating and cooling system  50 . For instance, the heat exchangers  60  and  62  may include the features discussed below. Furthermore, the furnace or furnace system  70  may include one or more baffles that reduce the production of certain gases and increase heat transfer without impinging on flames produced by the furnace and/or furnace system  70 . 
     Keeping the discussion of HVAC unit  12  in mind,  FIG. 5  illustrates a schematic block diagram of a furnace system  124  that may be included in the HVAC unit  12 . However, it should be noted that the furnace system  124  may also be included in other HVAC systems and unit, such as those used in residential settings. The furnace system  124  includes a housing  126  having a burner assembly  128  and a heat exchanger  130 , among other components, inside the housing  126 . Depending on the embodiment, the burner assembly  128 , the heat exchanger  130 , and other components of the housing  126  may be housed in separate housings, separate portions of the housing  126 , or in a single portion of the housing  126 . Additionally, the various components of the furnace system  124  may be coupled to a surface of the housing  126 , whether external or internal to the housing  126 . 
     In the present embodiment, a fuel source  132  provides fuel to individual burners within the burner assembly  128 . The fuel may include natural gas, liquefied petroleum gas, fuel oil, coal, wood, or the like. Air, or some other oxidant, is also provided to the burners in the burner assembly  128  from an oxidant or combustion air source  134 . For example, combustion air from the combustion air source  134  may be drawn into each individual burner of the burner assembly  128  to mix with the fuel drawn into each individual burner of the burner assembly  128 , as set forth above. The combustion air source  134  may be a container with compressed oxidant (e.g., compressed air), or the combustion air source  134  may be an atmosphere within or surrounding the HVAC unit  12 . For example, the combustion air source  134  may be an area within the burner assembly  128  external to the individual burners of the burner assembly  128 . In certain embodiments, the air may be sucked from atmosphere or some area proximate the burners into the burners of the burner assembly  128  via a pressure difference generated by a combustion air blower  136 , which may also be responsible for drawing combustion products through the heat exchanger  130 . In other words, a flow path exits between the burners of the burner assembly  128  and the combustion air blower  136 , such that the combustion air blower  136  assists in both drawing oxidant (e.g., air) into the burners of the burner assembly  128  and drawing combustion products through the flow path between the combustion air blower  136  and the burner assembly  128 . The oxidant, as previously described, mixes with the fuel in the burners to form a combustible mixture, which may be referred to herein as “the mixture.” The mixture may be ignited in a primary combustion zone  138  of the burner assembly  128  via an igniter  140 , where the primary combustion zone  138  refers to all the zones in each of the burners together. For example, an embodiment including four burners may include four total zones, i.e., one zone within each burner, where all four zones together are cumulatively referred to as the primary combustion zone  138 . 
     An electrical pulse (e.g., a signal or electricity) may be sent through the igniter  140  to instruct the igniter  140  to produce a spark adjacent to or within the burners of the burner assembly  128 . In some embodiments, a spark may be provided to the primary combustion zone  138  of each burner of the burner assembly  128 , such that the mixture within each burner is ignited. In other embodiments, the mixture may be ignited by other means, such as a hot surface igniter or a pilot light flame. 
     In the illustrated embodiment, once ignited, the mixture in the primary combustion zone  138  burns and forms combustion products. The combustion products, along with a flame, exits the burners of the burner assembly  128  and passes through openings in a venturi plate  142  (e.g., shoot-through plate) of the burner assembly  128  (e.g., downstream of the burners within the burner assembly  128 ). Additional combustion air is provided to the flame for enhanced combustion downstream of the venturi plate  142  via a secondary combustion air gap  144 . 
     The secondary combustion air may be pulled into the path of the flame from the secondary combustion air gap  144  via a pressure difference generated by the combustion air blower  136 . Upon combustion, combustion products and/or a corresponding flame may pass through openings in the venturi plate  142 . Secondary combustion air may then be provided from the secondary combustion air gap  144  (e.g., via the combustion air blower  136 ) for additional combustion downstream of the venturi plate  142  (e.g., secondary combustion in a secondary combustion zone downstream of the venturi plate  142 ). Combustion air provided from the secondary combustion air gap  144  may enhance combustion of the mixture in the burner assembly  128 , outside of the burner assembly  128 , or a combination thereof, and may reduce the overall noise of the combustion process. It should be noted that a space may exist between the outlets of the individual burners of the burner assembly  128  and the openings in the venturi plate  142  of the burner assembly  128 , and that secondary combustion may take place within this space even before the flame and/or combustion products pass through the venturi plate  142 . In other words, secondary combustion may take place upstream of the venturi plate  142  (e.g., between the venturi plate  142  and the outlets of the burners of the burner assembly  128 ), downstream the venturi plate  142  (e.g., after receiving additional secondary combustion air from the secondary combustion air gap  144 ), or a combination thereof. The inclusion of the secondary combustion air gap  144  enables secondary combustion to occur at some point downstream of the venturi plate  142 , such that combustion is enhanced and such that velocity of the flow through the venturi plate  142  is reduced, as set forth above, which reduces noise. 
     The openings of the venturi plate  142  are generally aligned with openings of tubes of the heat exchanger  130 . In some embodiments, the openings in the venturi plate  142  are also aligned with openings in a panel  146  (e.g., vestibule panel) coupled to the tubes of the heat exchanger  130 , where the panel  146  is positioned between the venturi plate  142  and the tubes. Although the boundaries along the openings in the venturi plate  142  may not be directly coupled with or otherwise engaging the tubes, the openings may be generally aligned to facilitate flow of combustion products therethrough. In some embodiments, the secondary combustion air gap  144  may partially separate the venturi plate  142  from the tubes or from a component that includes the tubes (e.g., the panel  146 ), as will be discussed in detail below. However, during operation, the combustion products still generally pass through the openings in the venturi plate  142  and extend into and through the tubes of the heat exchanger  130  via entry into the openings of the panel  146 . In some embodiments, secondary combustion may occur in the area between the venturi plate  142  and the panel  146  and may be enhanced via added combustion air from the secondary combustion air gap  144 . However, in other embodiments, secondary combustion may not occur in this area, and this area may only be included to draw secondary combustion air into the path of the combustion products exiting the venturi plate  142 , such that secondary combustion may occur just inside the tubes of the heat exchanger  130  (e.g., after passing through the openings in the panel  146 ). 
     The furnace system  124  may also include one or more baffles  148 . More specifically, the baffles  148  may be colocated with the flames produced by burners of the burner assembly  128 , extend through the secondary combustion air gap  144 , and contact the heat exchanger  130 . In some embodiments, the baffles  148  may extend into the heat exchanger  130 . The baffles  148  may quench the flame and reduce levels of nitrous oxide produced from combusting the mixture. However, it should be noted that the flames produced by the burners may travel along and/or through the baffles  148  and enter the heat exchanger  130 . Moreover, as a result of being placed in the flames, the baffles  148  may generate infrared and/or radiant heat, which may be transferred to the heat exchanger  130 . Additionally, the baffles  148  may be made from iron-chromium-aluminum alloys, nickel-chromium alloys, iron-chromium-cobalt-nickel alloys, nickel-copper alloys, nickel-cobalt alloys and other alloys configured to withstand high temperatures (e.g., temperatures greater than 1,000° C.) and/or promote heat transfer. 
     A fan  150 , such as an air blower or some other flow-motivating device, forces a medium (e.g., air) over the tubes in the heat exchanger  130  to generate a heated medium by transferring heat from the combustion products to the medium. In some embodiments, the fan  150  may be the same as the fan  32  of  FIG. 2 . The fan  150  operates to blow air over the tubes to generate hot air, and the hot air may be exported to a load  152  (e.g., a room) for heating the load  152 . It should be noted that the fan  150 , in some embodiments, may be a separate component from the heat exchanger  130  and may blow air across the heat exchanger  130  to generate the hot air. In another embodiment, the fan  150  may be located inside the heat exchanger  130  (e.g., as a combined component) and may operate to blow the air directly over the tubes of the heat exchanger  130 , as previously described. Further, it should be noted that the fan  150  may reside in any appropriate portion of the heat exchanger  130 . For example, the fan  150  may be at a bottom of the heat exchanger  130  and blow air upwards over the tubes, the fan  150  may be at the left or right of the heat exchanger  130  and blow air cross-wise over the tubes, or the fan  150  may be at the top of the heat exchanger  130  and blow air downwards over the tubes. Further still, the fan  150  may be a mechanical fan, a centrifugal fan, or some other type of fan. 
     Heat may be transferred more efficiently to the medium (e.g., air) that passes over the tubes of the heat exchanger  130  when the heat exchanger includes surface enhanced surfaces. For example, and as discussed below in greater detail, the tubes of the heat exchanger  130  may include various surface enhancements, such as protrusions that may extend outwards from or into the heat exchanger  130 . It is to be appreciated that, in presently disclosed embodiments, the first pass of a multi-pass heat exchanger may include such surface enhancements and not impinge on any flames produced by the burner assembly  128 . Moreover, the first pass of a multi-pass heat exchanger may also contact and/or include a portion of the baffle  148 . 
     Combustion products passing through the tubes of the heat exchanger  130  may be motivated through the tubes via the combustion air blower  136 . Indeed, the combustion air blower  136  may generate a pressure difference between an area surrounding the burner assembly  128  and a flow path from the burner assembly  128  to an external environment  154 . In other words, the combustion air blower  136  may draw air into the burners of the burner assembly  128 , draw the combustion products from the burners of the burner assembly  128  into the tubes of the heat exchanger  130 , and draw the combustion products through the tubes of the heat exchanger  130 . Additionally, the combustion air blower  136  may be configured to pull the combustion products from the heat exchanger  130  and blow the combustion products into an exhaust stack  156  of the furnace system  124 , which may be configured to export the combustion products from the furnace system  124  into the environment  154  or some other area external to the furnace system  124 . Further still, the combustion air blower  136  may be responsible for drawing secondary combustion air from the secondary combustion air gap  144  into the path of the flame and combustion products as they travel through the venturi plate  142  and through the panel  146  into the heat exchanger  130 . 
     With the discussion of  FIG. 5  in mind,  FIG. 6  is perspective view of an embodiment of the furnace system  124 . In the illustrated embodiment, the burner assembly  128  is located near a bottom surface  38  of the furnace system  124 . Four burners  158  are located within the burner assembly  128 . However, in other embodiments the furnace system  124  may include more or less than four burners  158  (e.g., one, two, three, five, six, or more burners  158 ). As previously described, each burner  158  is configured to combust a mixture of air and fuel. Additionally, in the illustrated embodiment, fuel is routed from a fuel source through a gas inlet  160  of a control valve  162 . The control valve  162  is coupled to a manifold  164 , which distributes the fuel to each burner  158 . In some embodiments, the fuel may be distributed via the manifold  164  to each burner  158  evenly. The control valve  162  may control a flow of fuel to the burners  158 , such that the control valve  162  controls a quantity (e.g., volume) of fuel in the mixture of each burner  158 . 
     The igniter  140  provides a spark to the burners  158  for igniting the mixture in each burner  158 . The combustion/burning occurring within each burner  158  may be considered to be occurring in the primary combustion zone  138 . As previously described, the mixture includes air drawn into an interior of each burner  158  and fuel provided to each burner  158  via the manifold  164 . However, additional oxidant (e.g., air) may be introduced via the secondary combustion air gap  144  for enhancing combustion/burning. The secondary combustion air gap  144  is located downstream of the burners  158 . In the illustrated embodiment, the secondary combustion air gap  144  is located between the burner assembly  128  and the heat exchanger  130 . Specifically, the secondary combustion air gap  144  is located downstream of the venturi plate  142  of the burner assembly  128  and upstream of the vestibule panel  146  of the heat exchanger  130 , which may serve as an entire front panel of the furnace system  124 . 
     In the illustrated embodiment, combustion products, including the flames of the burners  158 , may pass through tubes  166  of the heat exchanger  130 . More specifically, the combustion products and/or the flame are routed through the openings in the venturi plate  142  of the burner assembly  128 , through the vestibule panel  146 , and into tubes  166  of the heat exchanger  130 , where the secondary combustion air gap  144  provides additional secondary combustion air to the flame and/or combustion products downstream of the venturi plate  142 . The fan  150  in the illustrated embodiment is located near the bottom surface of the housing  126  of the furnace system  124 . The fan  150  is configured to blow air over and/or across the tubes  166  of the heat exchanger  130 , such that the air extracts heat from the combustion products routed through the heat exchanger  130 . The hot air is may be routed through a duct that delivers the hot air to a load (e.g., the load  152 ), such as a room of a building. The combustion products may be pulled through, and blown from, the tubes  166  of the heat exchanger  130  into an exhaust stack  156  (e.g., a chimney), where the combustion products may be exported from the furnace system  124  to the environment  154 . 
     The heat exchanger  130  may be a multi-pass heat exchanger. For instance, as illustrated, the heat-exchanger is a four-pass heat exchanger. In other words, the tubes  166  of the heat exchanger  130  have a first tube pass  168 , a second tube pass  170 , a third tube pass  172 , and a fourth tube pass  174  that overlap with at least one of the other tube passes  168 ,  170 ,  172 ,  174  and cooperatively form a conduit. For instance, the tube passes  168 ,  170 ,  172 ,  174  may be fluidly coupled to at least one other of the tube passes  168 ,  170 ,  172 ,  174  in a U-shaped configuration (e.g., a U-shaped bend). Combustion products, including flames produced by the burners  158 , may enter the heat exchanger  130  via openings  175  in the first tube pass  168  of the tubes  166 , and the combustion products may continue to travel through the second tube pass  170 , third tube pass  172 , and fourth pass  174  of the heat exchanger  130 . More specifically, the combustion products, including the flame, may travel through the venturi plate  142  and the vestibule panel  146  along and/or through a baffle (e.g., baffle  148 ) before entering the first tube pass  168  of the heat exchanger  130 . Additionally, a combustion air blower  136  may be coupled to the fourth tube pass  174  of the heat exchanger to draw air and the combustion products through the heat exchanger  130 . The contents of the heat exchanger  130  may exit the heat exchanger  130  and the furnace system  124  via an exhaust stack (e.g., exhaust stack  156 ). 
     While the illustrated embodiment of the heat exchanger  130  is a four-pass heat exchanger, it should be noted that, in other embodiments, different heat exchangers may be used. For example, a two-pass heat exchanger, which may include a first pass and a second pass, may be used instead of a four-pass heat exchanger. For instance, a two-pass heat exchanger may generally have the shape of a “U,” with the first pass receiving the combustion products, including the flame(s), from the burner assembly  128 . Moreover, the heat exchanger  130  may be made from various materials. For example, the heat exchanger  130  may be made from aluminized steel, such as steel that has an aluminum coating or an aluminum-silicon alloy coating. Additionally, in some embodiments, the heat exchanger  130  may be made from aluminum or copper. 
     In any case, the first tube pass  168 , as well as the other passes (i.e., the second tube pass  170 , third pass  172 , and fourth tube pass  174 ) may include surface enhancements. The surface enhancements may improve the transfer of heat from the heat exchanger  130  to the air surrounding the heat exchanger  130  that is to be delivered to a load (e.g., a room to be heated). As discussed below with regard to  FIGS. 8-17 , the surface enhancements may include features that increase the surface area of the tubes  166  of the heat exchanger  130 . For example, the surface enhancements may include features that extend outwards from and/or into the tubes  166  of the heat exchanger. In any case, the surface enhancements of the first tube pass  168  are configured such that the heat exchanger  130  will not impinge on the flame(s) produced by the burner assembly  128 . 
     Continuing with the drawings,  FIG. 7  is a perspective view of one embodiment of the baffle  148 . As described above, the baffle  148  may be placed in a common location where a flame is produced by the burner  158 . More specifically, a front portion  176  of the baffle  148  may be placed in a flame produced by one of the burners  158 . The baffle  148  includes a mesh structure to enable the flame (and other combustion products) to travel through faces  178  of the baffle  148  and enter the heat exchanger  130 . The heat exchanger  130  may contact and/or be coupled to the baffle  148  via an end portion  180  of the baffle  148 . More specifically, the end portion  180  of the baffle  148  may contact and/or be coupled to the opening  175  of the heat exchanger  130  through which the heat exchanger  130  receives the combustion products, including the flame(s). In some embodiments, some or all of the end portion  180  may be disposed within the heat exchanger  130  (e.g., within one of the tubes  166  of the heat exchanger  130 ). For example, in the embodiment illustrated in  FIG. 6 , the end portion  180  of the baffle  148  may contact and/or be partially disposed within the first tube pass  168  of the heat exchanger  130 . 
     Inclusion of the baffle  148  in the furnace system  124  may increase the efficiency of the furnace system  124 . For instance, as discussed above, the baffle  148  may allow for increased heat transfer to the heat exchanger  130 , which may allow for air that is to be sent to a load (e.g., a room supplied with air by the furnace system  124 ) to be more efficiently heated. Heat may be transferred even more efficiently in embodiments where the furnace system  124  includes the baffle  148  as well as a multi-pass heat exchanger (e.g., heat exchanger  130 ) that includes surface enhancements on the first tube pass  168 . Indeed, in such embodiments the heat exchanger  130  may be a more compact size yet still enable efficiencies observed in furnace systems that do not include both the baffle  148  as well as a multi-pass heat exchanger with surface enhancements on the first tube pass  168 . Additionally, while the illustrated embodiments of the baffle  148  has a repeating “U” shape, in other embodiments, the baffle  148  may be a different shape. For example, in another embodiment, the baffle  148  may have a repeating “V” shape. 
       FIGS. 8-17  show various views of different embodiments of portions of the heat exchanger  130 . More specifically,  FIGS. 8-17  show different surface enhancements that may be present on any of the passes of the heat exchanger  130 , including the first tube pass  168 . Moreover, each of the embodiments associated with  FIGS. 8-17  may be used on the first tube pass  168  of the heat exchanger  130  without causing impingement of the flame(s) produced by the burner(s)  158 . In each of the illustrated embodiments, some or all of the surface enhancements extend radially outward from an exterior surface of the heat exchanger  130 , which enables the flame(s) from the burner assembly  128  to enter the first tube pass  168  of the heat exchanger  130  without being impinged. For example, in some embodiments, the surface enhancements increase an interior volume of the first tube pass  168 , which may reduce and/or eliminate a likelihood of flame impingement occurring. Furthermore, when combined in a furnace system (e.g., furnace system  124 ) that includes one or more baffles  148 , the flame(s) will not be impinged by a heat exchanger  130  that includes the surface enhancements illustrated in  FIGS. 8-17 . That is, even though the baffle(s)  148  may quench the flame(s) produced by the burner assembly  128 , the flame(s) can enter the heat exchanger  130  without being impinged. Generally speaking, and as discussed below, the surface enhancements of the heat exchanger  130  may include various features that may extend away from and/or into the heat exchanger  130 . Moreover, the surface enhancements may increase the transfer of heat from the heat exchanger  130  to the medium (e.g., air) passing over the heat exchanger  130 . Increased heat transfer may increase the efficiency of the furnace system  124  and/or an HVAC unit (e.g., HVAC unit  12 ) in which the furnace system  124  may be disposed. 
       FIG. 8  is a perspective view of the first tube pass  168  of the tube  166  of the heat exchanger  130 , illustrating surface enhancements in the form of dimples  182  formed in the first tube pass  168 . Specifically, the dimples  182  extend radially outwards, relative to a central axis  183  (i.e., longitudinal axis) of the tube  166 , from an outer surface  185  of the tube  166 .  FIG. 9  is an axial view of the first tube pass  168  of the tube  166  shown in  FIG. 8 . As shown, the dimples  182  extend a distance  187  from the outer surface  185  of the tube  166 . In certain embodiments, the distance  187  may measure one inch or less, though in other embodiments, the dimples  182  may be larger (i.e., extend a greater distance, such as three or four inches). 
     Additionally, as shown in  FIG. 10 , the first tube pass  168  of the tube  166  of the heat exchanger  130  may include some dimples  182  that extend radially outwards from an outer surface  185  of the first tube pass  168  of the heat exchanger  130  relative to the central axis  183  and some dimples  182  that extend radially inward relative to the central axis  183 . Additionally, it should be noted that, in some embodiments, the dimples  182  may only extend radially into the heat exchanger  130 .  FIG. 11  is an axial view of the first tube pass  168  of the tube  166  shown in  FIG. 10 . As illustrated, the dimples  182  extend outward a distance  189  or extend inward a distance  190  from the outer surface  185  of the tube  166 . The distances  189 ,  190  may be similar or the same as the distance  187 . However, it should be noted that in some embodiments, the distance  190  may be less than the distance  190  to reduce the likelihood of impingement of the flame(s). 
     Regarding the embodiments illustrated in  FIGS. 8-11 , the dimples  182  may cover various amounts of the outer surface  185  of the tube  166 . For example, the dimples  182  may cover five to seventy-five percent of the outer surface  185  of the tube in various embodiments. 
     Continuing with the illustrated embodiments of the heat exchanger  130 ,  FIG. 12  is perspective view of an embodiment of a portion of the first tube pass  168  of the tube  166  of the heat exchanger  130  that includes another type of surface enhancement, i.e., protrusions  184  (e.g., longitudinal fins). The protrusions  184  extend longitudinally along the heat exchanger  130  relative to the central axis  183 . However, in other embodiments, the protrusions  184  may extend circumferentially around the heat exchanger  130 . As illustrated, the protrusions may be formed on the heat exchanger  66  in such a manner than increases the volume within the heat exchanger  130 . In other words, a space  186  may be formed by the protrusions  184  on an interior surface  192  of the heat exchanger  130 . Additionally, while the present embodiment includes four protrusions  184 , it should be noted that other embodiments may include a different number of protrusions  184  (e.g., one, two, three, five, six, seven or more protrusions  184 ).  FIG. 13  is an axial view of the embodiment of the first tube pass  168  of the tube  166  shown in  FIG. 12 . As illustrated, the protrusions  184  may extend a distance  194  from the outer surface  185  of the tube  166 . The distance  194  may be greater than the distances  187 ,  189 ,  190  of the dimples  182  discussed above. For example, the distance  194  may be greater than four inches. However, in other embodiments, the distance  194  may be similar to any of the distances  187 ,  189 ,  190  of the dimples  182 . 
       FIG. 14  is a perspective view of a portion of another embodiment of the first tube pass  168  of the tube  166  of the heat exchanger  130  that includes fins  188  that extend circumferentially around the heat exchanger  130  relative to the central axis  183  (i.e., longitudinal axis). More specifically, the fins  188  are segmented partial fins. In other words, the fins  188  extend only partially about the circumference of the first tube pass  168 . However, in other embodiments, the fins  188  may be fully or partially circular. In any case, the fins  188  may be formed as part of the heat exchanger  130 . For instance, the fins  188  may be formed by making protrusions  196  (e.g., via a mandrel) on the interior surface  192  of the tube  166 .  FIG. 15  is an axial view of the embodiment of the first tube pass  168  of the tube  166  shown in  FIG. 14 . The fins  188  may extend a distance  198  radially outward from the exterior surface  185  of the tube  166 . The distance  198  may vary at various portions of the tube  166 . Moreover, the distance  198  may be similar to the distances  187 ,  189 ,  190 ,  194  described above. It should be noted that the distance  198  may also vary based on a thickness  199  of the tube  166 . For instance, for a greater thickness  199 , a greater distance  198  may be possible. 
     However, in other embodiments, the fins  188  may be attached to the outer surface  185  of the first tube pass  168  of the tube  166  of the heat exchanger  130 . For example, as illustrated in  FIG. 16 , the fins  188  are coupled to the heat exchanger  130  via a mechanical securement. The mechanical securement may include a mechanical connection (e.g., screws, rivets, or clamps such as toggle-locking clamps) or expanding the tubing  66  of the heat exchanger  130  in such a manner as to promote heat transfer. Additionally, the fins  188  may be made from materials such as aluminum, copper, and steel. The fins  188  are arranged circumferentially around the outer surface  185  of the tube  166  relative to a central axis  183 . The fins  188  extend radially outward from the first tube pass  168  of the tube  166  relative to the central axis  183  (i.e., longitudinal axis).  FIG. 17  is an axial view of the embodiment of the first tube pass  168  of the tube  166  shown in  FIG. 16 . As illustrated, the fins  188  may extend radially a distance  200  from the outer surface  185  of the tube  166 . The distance  200  may be similar to the distances  187 ,  189 ,  190 ,  194 ,  198  described above. It should also be noted that the distance  200  may differ between two different fins  188 . For example, one fin  188  may extend a distance  200  that is different from another fin  188  that is positioned on first tube pass  168  of the tube  166 . 
     In any case, it should be noted that the surface enhancements of the heat exchanger  130  illustrated in the embodiments of  FIGS. 8-17  may be distributed evenly (e.g., in a staggered arrangement), unevenly, in a pattern, without a pattern, circumferentially, longitudinally, or any combination thereof. Additionally, the heat exchanger  130  may include a higher or lower density of the surface enhancements in other embodiments. That is, in other embodiments, the heat exchanger  130  may include a number of surface enhancements that is greater than or less than the amount of surface enhancements shown in  FIGS. 8-17 . Moreover, the heat exchanger  130  may include more than one of the illustrated types of surface enhancements. For instance, any of the tube passes of the heat exchanger  130 , including the first tube pass  168 , may include dimples  182 , protrusions  184 , fins  188 , or any combination thereof. Moreover, the various passes of a multi-stage heat exchanger may include different surface enhancements. For example, in a two-pass heat exchanger, a first pass may include features that extend outwards from the first pass (e.g., dimples, protrusions, and/or fins), while a second pass may include features that extend into the second pass (e.g., dimples). As another example, in a four-pass heat exchanger, the first and second passes may be the same as the first and second passes of the two-pass heat exchanger described in the last example, and the third and fourth passes may also include features that extend into the third pass (e.g., dimples). However, in any multi-stage heat exchanger, any of the passes other than first pass may not include surface enhancements. For example, in a four-pass heat exchanger, it may be the case that only the first and second passes include surface enhancements. 
     While only certain features and embodiments of the present disclosure have been illustrated and described, many modifications and changes may occur to those skilled in the art (e.g., variations in sizes, dimensions, structures, shapes and proportions of the various elements, values of parameters (e.g., temperatures, pressures, etc.), mounting arrangements, use of materials, colors, orientations, etc.) without materially departing from the novel teachings and advantages of the subject matter recited in the claims. The order or sequence of any process or method steps may be varied or re-sequenced according to alternative embodiments. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the present disclosure. Furthermore, in an effort to provide a concise description of the exemplary embodiments, all features of an actual implementation may not have been described (i.e., those unrelated to the presently contemplated best mode of carrying out the present disclosure, or those unrelated to enabling the claimed embodiments). It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation specific decisions may be made. Such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure, without undue experimentation.