Patent Publication Number: US-11022382-B2

Title: System and method for heat exchanger of an HVAC and R system

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a U.S. Non-Provisional application claiming priority from and the benefit of U.S. Provisional Application Ser. No. 62/640,469, entitled “SYSTEM AND METHOD FOR HEAT EXCHANGER OF AN HVAC&amp;R SYSTEM,” filed Mar. 8, 2018, which is hereby incorporated by reference in its entirety for all purposes. 
    
    
     BACKGROUND 
     This disclosure relates generally to heating, ventilating, and air conditioning (HVAC) systems. Specifically, the present disclosure relates to heat exchangers for HVAC units. 
     This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present techniques, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as an admission of any kind. 
     A heating, ventilating, and air conditioning (HVAC) system may be used to thermally regulate an environment, such as a building, home, or other structure. The HVAC system may include a vapor compression system, which includes heat exchangers such as a condenser and an evaporator, which transfer thermal energy between the HVAC system and the environment. A refrigerant may be used as a heat transfer fluid that is directed through the heat exchangers of the vapor compression system. In some cases, the HVAC system may cool a flow of fluid by directing the fluid across a heat exchange area of an evaporator. For example, the refrigerant flowing through the evaporator may absorb thermal energy from the flow of fluid to be cooled, and thus decrease the thermal energy of the flow of fluid to be cooled. In many cases, the thermal energy absorbed by the refrigerant may heat the refrigerant to a hot, gaseous phase. The gaseous refrigerant may be directed through a condenser, which may remove the absorbed thermal energy the refrigerant and transfer the thermal energy to a cooling fluid. 
     Due to spatial constraints, typical condensers are unable to remove a sufficient amount of thermal energy from the refrigerant that enables the refrigerant to completely change phase within the condenser. In many cases, typical condensers may thus exhaust a two-phase mixture of refrigerant that is insufficiently cooled, which is subsequently recirculated through the HVAC system. The two-phase refrigerant may be unable to effectively absorb heat from the fluid to be cooled. Unfortunately, this may decrease the ability of the HVAC system to transfer thermal energy between the fluid to be cooled and the refrigerant, which decreases the efficiency of the HVAC system. 
     SUMMARY 
     The present disclosure relates to a heat exchanger for a heating, ventilating, and air conditioning (HVAC) system that includes a first slab having a first plurality of tubes extending between a first manifold and a second manifold and a second slab having a second plurality of tubes and a third plurality of tubes. The second plurality of tubes extends between a third manifold and a fourth manifold and the third plurality of tubes extends between the fourth manifold and a fifth manifold, such that the heat exchanger defines a refrigerant path sequentially through the first plurality of tubes, the second plurality of tubes, and the third plurality of tubes. 
     The present disclosure also relates to a heating, ventilating, and air conditioning (HVAC) heat exchanger including a first slab extending along a length of the HVAC heat exchanger having a first manifold and a second manifold and a first plurality of tubes extending between the first manifold and the second manifold to define a first pass of the HVAC heat exchanger. The HVAC heat exchanger also includes a second slab extending along the length of the HVAC heat exchanger having a third manifold and a fourth manifold. The third manifold is divided into an upper chamber and a lower chamber, such that a second plurality of tubes extends between the upper chamber and the fourth manifold to define a second pass of the HVAC heat exchanger and a third plurality of tubes extend between the lower chamber and the fourth manifold to define a third pass of the HVAC heat exchanger. 
     The present disclosure also relates to a method for operating a heat exchanger, including directing a refrigerant through a first plurality of tubes in a first direction, in which the first plurality of tubes is disposed within a first slab of the heat exchanger. The method also includes directing the refrigerant through a second plurality of tubes in a second direction, in which the second plurality of tubes is disposed within a second slab of the heat exchanger and the second direction is opposite of the first direction. The method further includes directing the refrigerant through a third plurality of tubes in the first direction, in which the third plurality of tubes is disposed within the second slab. 
     The present disclosure also relates to a heat exchanger including a first network of heat exchanger tubes having a first inlet manifold and a first outlet manifold, in which the first network of heat exchanger tubes includes a first length, a first height, and a first width. The heat exchanger also includes a second network of heat exchanger tubes having a second inlet manifold and a second outlet manifold, in which the second network of heat exchanger tubes includes second length, a second height, and a second width. The heat exchanger further includes a third network of heat exchanger tubes having a third inlet manifold and a third outlet manifold, in which the third network of heat exchanger tubes includes a third length, a third height, and a third width and the second network of heat exchanger tubes and the third network of heat exchanger tubes are stacked along their respective height dimensions. The first width of first network of heat exchanger tubes is adjacent the second width of the second network of heat exchanger tubes and the third width of the third network of heat exchanger tubes. The first outlet manifold of the first network of heat exchanger tubes is coupled to the second inlet manifold of the second network of heat exchanger tubes and the second outlet manifold of the second network of heat exchanger tubes is coupled to the third inlet manifold of the third network of heat exchanger tubes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various aspects of this disclosure may be better understood upon reading the following detailed description and upon reference to the drawings in which: 
         FIG. 1  is a perspective view of an embodiment of a building that may utilize a heating, ventilating, and air conditioning (HVAC) system in a commercial setting, in accordance with an aspect of the present disclosure; 
         FIG. 2  is a perspective view of an embodiment of a vapor compression system, in accordance with an aspect of the present disclosure; 
         FIG. 3  is a schematic of an embodiment of the vapor compression system of  FIG. 2 , in accordance with an aspect of the present disclosure; 
         FIG. 4  is a schematic of an embodiment of the vapor compression system of  FIG. 2 , in accordance with an aspect of the present disclosure; 
         FIG. 5  is a perspective view of an embodiment of a heat exchanger that may be used in the vapor compression system of  FIGS. 2 and 3 , in accordance with an aspect of the present disclosure; 
         FIG. 6  is a perspective view of an embodiment of a first slab of the heat exchanger of  FIG. 5 , in accordance with an aspect of the present disclosure; 
         FIG. 7  is a perspective view of an embodiment of a second slab of the heat exchanger of  FIG. 5 , in accordance with an aspect of the present disclosure; 
         FIG. 8  is a front view of an embodiment of a heat exchanger system including the heat exchanger of  FIG. 5 , in accordance with an aspect of the present disclosure; 
         FIG. 9  is a rear view of an embodiment of the heat exchanger system of  FIG. 8 , in accordance with an aspect of the present disclosure; 
         FIG. 10  is a perspective view of an embodiment of a heat exchanger unit that may be used with the vapor compression system of  FIG. 2 , in accordance with an aspect of the present disclosure; and 
         FIG. 11  is an embodiment of a method that may be used to operate the heat exchanger of  FIG. 5 , in accordance with an embodiment in the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     One or more specific embodiments of the present disclosure will be described below. These described embodiments are only examples of the presently disclosed techniques. Additionally, in an effort to provide a concise description of these embodiments, all features of an actual implementation may not be described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that 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. 
     A vapor compression system includes heat exchangers, such as a condenser and an evaporator, that transfer thermal energy between a heat transfer fluid, such as a refrigerant and a fluid to be conditioned, such as air. A compressor is used to circulate the refrigerant through conduits of the vapor compression system, which fluidly couple the condenser, the evaporator, and the compressor. In some cases, the vapor compression system may be configured to cool a flow of air by directing the flow of air across the evaporator of the vapor compression system. A refrigerant flowing through the evaporator may absorb heat from the flow of air, and thus change phase within the evaporator. The refrigerant may exit the evaporator in a hot, gaseous state. In many cases, the condenser is used to remove the absorbed thermal energy from the refrigerant, such that the refrigerant may change phase before being recirculated through the conduits of the vapor compression system. Typical condensers may be unable to sufficiently condense the refrigerant, such that a two-phase mixture of liquid and gaseous refrigerant exits the condenser and is recirculated in the vapor compression system. Unfortunately, this may decrease the efficiently of the vapor compression system. 
     Embodiments of the present disclosure are directed to a heat exchanger, such as a condenser, that may increase the efficiency of thermal energy transfer between the refrigerant and a flow of air by enabling the refrigerant to complete multiple passes through the condenser. For example, the heat exchanger may include a plurality of tubes, such as micro-channel tubes, that enable the refrigerant to complete a predetermined amount of passes through the heat exchanger. In some embodiments, the heat exchanger may include a first slab and a second slab disposed adjacent to one another, which each include a plurality of micro-channel tubes. The refrigerant may complete a first pass through a first plurality of tubes disposed within the first slab. The refrigerant may complete a second and third pass through a second plurality of tubes and a third plurality of tubes, respectively, which are disposed within the second slab. In some cases, gaseous refrigerant from the vapor compression system may flow into the first slab of the heat exchanger and condense, or partially condense, within the first plurality of tubes. The refrigerant may enter the second slab and fully condense while completing the second pass through the second plurality of tubes. Finally, the refrigerant may be sub-cooled while completing the third pass through the third plurality of tubes. Accordingly, embodiments of the heat exchanger disclosed herein may efficiently remove thermal energy from the refrigerant, and thus improve an efficiency of the HVAC 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 through the heat exchangers  28  and  30 . For example, the refrigerant may be R- 410 A. 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 of these components may be 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, 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, 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, or the set point 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, or 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 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 above, embodiments of the present disclosure are directed to a heat exchanger, such as a micro-channel heat exchanger that includes multiple slabs disposed adjacent to one another. Each slab may include a plurality of tubes or micro-channel tubes that extend along a length of the slab. The heat exchanger may be configured to enable the refrigerant to complete a first pass through the first slab and a second and third pass through the second slab. A heat exchange fluid, such as cooling air, may be directed across cooling fins of the first and second slabs of the heat exchanger. As such, the heat exchange fluid may remove thermal energy from the refrigerant during each pass. 
     With the foregoing in mind,  FIG. 5  illustrates a perspective view of an embodiment of a multi-pass heat exchanger  100  that may be used in the embodiments of the HVAC unit  12  shown in  FIG. 1 , the residential heating and cooling system  50  shown in  FIG. 3 , or any suitable HVAC system. To facilitate discussion, the multi-pass heat exchanger  100  and its components may be described with reference to a longitudinal axis or direction  102 , a vertical axis or direction  104 , and a lateral axis or direction  106 . The multi-pass heat exchanger  100  includes a first slab  108  or first heat exchanger and a second slab  110  or second heat exchanger that are disposed adjacent and parallel to one another along the longitudinal direction  102 . For example, a width of the first slab  108  may be disposed adjacent and parallel to a width of the second slab  110 . The first slab  108  and the second slab  110  may be coupled together via fasteners, such as bolts or clamps, adhesives, such as bonding glue, welding, or any suitable method known in the art. While the first and second slab  108  and  110  may be integrally formed or joined with one another, one of ordinary skill in the art would appreciate that such a configuration would include two slabs. 
     The first slab  108  and the second slab  110  may each have a length  112  and a height  114  that extends along the longitudinal direction  102  and the vertical direction  104 , respectively. A heat exchange fluid  116 , such as air, may flow transversely along the lateral direction  106  across the first and second slabs  108 ,  110 . As described in greater detail herein, the heat exchange fluid  116  may be used to transfer thermal energy between the refrigerant flowing through the multi-pass heat exchanger  100  and an ambient environment. 
     The multi-pass heat exchanger  100  may be fluidly coupled to the conduits of the vapor compression system  72  at a main inlet  118  and a main outlet  120 . The refrigerant from the vapor compression system  72  may flow through the main inlet  118  and enter a distribution manifold  120  of the first slab  108 . The distribution manifold  120  may distribute the refrigerant to a first plurality of tubes  122  or a first network of heat exchanger tubes, such as micro-channel tubes, that extend along the length  112  of the first slab  108 . The distribution manifold  120  may extend across the full height  114  of the first slab  108 , such that the refrigerant is directed to each tube  123  of the first plurality of tubes  122 . The distribution manifold  120  also extends along a width that is generally parallel to the lateral direction  106 . In certain embodiments, the width of the distribution manifold  120  is indicative of the width of the first plurality of tubes  122 . The refrigerant may flow through the first plurality of tubes  122  from a first end portion  124  to a second end portion  126  of the multi-pass heat exchanger  100 , and thus complete a first pass through the multi-pass heat exchanger  100 . The refrigerant is collected in a collection manifold  128  of the first slab  108  before being directed into the second slab  110 . 
     In some embodiments, the second slab  110  may include a split manifold  130  that extends along the height  114  of the second slab  110 . The split manifold  130  may be divided into an upper distribution manifold  132 , or an upper chamber, and a lower collection manifold  134 , or a lower chamber, via a cap plate  135 . The cap plate  135  may be coupled to an interior region of the split manifold  130  via an adhesive, welding, or other manner, and thus divide the split manifold  130  into the upper distribution manifold  132  and the lower collection manifold  134 . In some embodiments, the upper distribution manifold  132  and the lower collection manifold  134  may be separate manifolds that each extend along the vertical direction  104 , such that the upper distribution manifold  132  and the lower collection manifold  134  may be axially coupled to one another with respect to the vertical direction  104  via the adhesive and/or fasteners. The upper distribution manifold  132  may extend along a first length  136  or a first portion of the height  114 , and the lower collection manifold  134  may extend along a second length  138  or a second portion of the height  114 . The upper distribution manifold  132  and the lower collection manifold  134  each also extend along a respective width that is generally parallel to the lateral direction  106 . As described in greater detail herein, the split manifold  130  enables the refrigerant to complete two passes through the second slab  110  of the multi-pass heat exchanger  100 . A manifold tube  140  may fluidly couple an outlet  142  of the collection manifold  128  to an inlet  144  of the upper distribution manifold  132 . The manifold tube  140  may be coupled to the outlet  142  and the inlet  144  via brazing, welding, or any other suitable method. 
     The upper distribution manifold  132  may be in fluid communication with a second plurality of tubes, as shown in  FIG. 7 , that extend from the second end portion  126  to the first end portion  124  of the multi-pass heat exchanger  100 . The refrigerant may flow through the second plurality of tubes from the upper distribution manifold  132  toward a collection manifold  146  of the second slab  110 . The collection manifold  146  may extend across the full height  114  of the second slab  110 , and direct the refrigerant into a third plurality of tubes, as shown in  FIG. 7 , that are in fluid communication with the lower collection manifold  134 . The refrigerant may flow from the collection manifold  146  near the first end portion  124  of the multi-pass heat exchanger  100  to the lower collection manifold  134  near the second end portion  126  of the multi-pass heat exchanger  100 , and thus complete a third pass. The refrigerant may exit the multi-pass heat exchanger  100  and return to the vapor compression system  72  via the main outlet  120 . 
       FIG. 6  is a perspective view of the first slab  108  of the multi-pass heat exchanger  100 . As discussed above, the refrigerant may be distributed across the full height  114  of the first slab  108  via the distribution manifold  120 . In some embodiments, the refrigerant flowing into the distribution manifold  120  from the main inlet  118  may be in the gaseous phase. The gaseous refrigerant may be directed through the first plurality of tubes  122  along the longitudinal direction  102  to complete the first pass. As such, the gaseous refrigerant may transfer thermal energy to the first plurality of tubes  122  and cooling fins  150  disposed between each tube  123  of the first plurality of tubes  122 . The heat exchange fluid  116 , such as cooling air, may flow transversely along the lateral direction  106  across the first slab  108  and between the cooling fins  150 . The cooling fins  150  increase a heat transfer surface area of the first plurality of tubes  122 , which may enable the gaseous refrigerant within the first plurality of tubes  122  to exchange thermal energy with the heat exchange fluid  116  more effectively. 
     In some embodiments, the gaseous refrigerant may change phase while flowing through the first pass of the multi-pass heat exchanger  100 . For example, a portion of the gaseous refrigerant may condense such that a mixture of gaseous refrigerant and liquid refrigerant may exit the first plurality of tubes  122 . In other embodiments, substantially all of the gaseous refrigerant may condense, such that the refrigerant may exit the first plurality of tubes  122  in a substantially liquid phase. The collection manifold  128  may collect the refrigerant exiting the first plurality of tubes  122 , indicated by arrows  152 , and direct the liquid refrigerant towards the outlet  142  of the collection manifold  142 . The refrigerant may subsequently flow into the second slab  110  through the manifold tube  140 . 
       FIG. 7  is a perspective view of the second slab  110  of the multi-pass heat exchanger  100 . As discussed above, refrigerant may enter the upper distribution manifold  132  of the second slab  110  via the inlet  144 . The upper distribution manifold  132  may distribute the refrigerant to a second plurality of tubes  154 , such as a second network of heat exchanger tubes, which is in fluid communication with the upper distribution manifold  132 . Accordingly, a height of the second plurality of tubes  154  is indicative of the first length  136  or a height of the upper distribution manifold  132 . As discussed above, the upper distribution manifold  132  also includes a width extending along the lateral direction  106 , such that the width of the upper distribution manifold  132  may be indicative of a width of the second plurality of tubes  154 . The refrigerant may complete the second pass by flowing through the second plurality of tubes  154  by from the second end portion  126  of the multi-pass heat exchanger  100  to the first end portion  124  of the multi-pass heat exchanger  100 . While completing the second pass, the refrigerant may exchange thermal energy with the heat exchange fluid  116  flowing across the fins  150  of the second plurality of tubes, before flowing into the collection manifold  146  of the second slab  110 , as indicated by arrows  156 . As such, the refrigerant within the collection manifold  146  may be of a lower thermal energy than the refrigerant within the upper distribution manifold  132 . For example, the refrigerant may enter the first plurality of tubes  154  as a two-phase mixture and condense while flowing through the second pass, such that the refrigerant may exit the second plurality of tubes  154  in a substantially liquid phase. In some embodiments, the refrigerant may already enter the first plurality of tubes  154  in the liquid phase, such that the second pass may sub-cool the liquid refrigerant. 
     The collection manifold  146  may be in fluid communication with a third plurality of tubes  158 , or a third network of heat exchanger tubes, which extend between the collection manifold  146  and the lower collection manifold  134 . Accordingly, a height of the third plurality of tubes  158  is indicative of the third length  138  or a height of the lower collection manifold  134 . In certain embodiments, the width of the lower collection manifold  134  is indicative of a width of the third plurality of tubes  158 . The collection manifold  146  may distribute the refrigerant exiting the second plurality of tubes  154  to the third plurality of tubes  158 , as indicated by arrows  160 . The refrigerant may thus flow through the third plurality of tubes  158  from the first end portion  124  of the multi-pass heat exchanger  100  to the second end portion  126  of the multi-pass heat exchanger  100  to complete the third pass. The refrigerant may transfer thermal energy to the heat exchange fluid  116  via the fins  150  when completing the third pass. As such, the third plurality of tubes  158  may sub-cool the refrigerant. The refrigerant may exit the lower collection manifold  134  through the main outlet  120 , and be directed through the vapor compression system  72 . It should be noted that in certain embodiments, the collection manifold  146  may include a pair of separate manifolds that are associated with the second plurality of tubes  154  and the third plurality of tubes  158 , respectively. For example, a first manifold of the pair of manifolds may couple to the second plurality of tubes  154 , while a second manifold of the pair of manifolds may couple to the third plurality of tubes  158 . In such embodiments, the first and second manifolds are placed in fluid communication with one another, such that refrigerant may flow from the second plurality of tubes  154  to the third plurality of tubes  158  by flowing through the first manifold and the second manifold. 
     The first length  136  of the upper distribution manifold  132  and the second length  138  of the lower collection manifold  134  adjusts a proportion of tubes  123  within the second pass and the third pass of the multi-pass heat exchanger  100 , respectively. For example, increasing the first length  136  and decreasing the second length  138  while the height  114  remains substantially constant may increase a quantity of tubes  123  in the second pass and decrease a quantity of tubes  123  in the third. A ratio between the quantity of tubes  123  in the second pass and the quantity of tubes  123  in the third pass may be optimized to increase the efficiency of the multi-pass heat exchanger. 
     For example, experimental tests may be used to determine which ratio of tubes between the first pass and the second pass results in the largest temperature drop or the most efficient rate of heat transfer between refrigerant entering the second slab  110  through the inlet  144  and refrigerant exiting the second slab  110  through the main outlet  120 . The experimental test may include the collection of empirical data, such as temperature measurements of the refrigerant taken near the inlet  144  and the main outlet  120 , to determine the optimal ratio of tubes  123  between the second pass and the third pass. As a non-limiting example, it may be determined that an optimal heat transfer efficiency of the second slab  110  is achieved when the second plurality of tubes  154  includes seventy percent of the tubes  123  within the second slab  110  and the third plurality of tubes  158  includes the remaining thirty percent of the tubes  123  within the second slab  110 . In other embodiments, the second plurality of tubes  154  may include more than fifty percent of the tubes  123  within the second slab  110 , more than sixty percent of the tubes  123  within the second slab  110 , or any other suitable percentage of the tubes  123  within the second slab  110 , while the third plurality of tubes  158  includes the respective remaining portion of the tubes  123 . 
     In some embodiments, a radial dimension of the first plurality of tubes  122 , the second plurality of tubes  154 , and/or the third plurality of tubes  158  may each be the same or different. For example, each tube  123  of the first plurality of tubes  122  may have a radial dimension of twenty five millimeters, while each tube  123  of the second plurality of tubes  154  and the third plurality of tubes  158  may have a radial dimension of eighteen millimeters. In some embodiments, all tubes of the first plurality of tubes  122 , the second plurality of tubes  154 , and the third plurality of tubes  158  may have radial dimension that is substantially similar. For example, in one embodiment, the first plurality of tubes  122 , the second plurality of tubes  154 , and the third plurality of tubes  158  may each have an inside diametral that is less than one millimeter (mm). In some embodiments, the radial dimensions of the tubes  123  may be used to optimize the heat transfer efficiency of the multi-pass heat exchanger  100 , using experimental trials similar to those described above. For example, it may be determined that gaseous refrigerant flowing through the first plurality of tubes  122  flows more effectively in a larger diameter tube  123 , while liquid refrigerant flowing through the second plurality of tubes  154  and/or the third plurality of tubes  156  flows more effectively in a smaller diameter tube  123 . It should be noted that the tubes  123  within the first plurality of tubes  122 , the second plurality of tubes  154 , the third plurality of tubes  158  are not limited to an oval or a circular cross section, but can be square, triangular, or any other suitable cross-sectional shape. 
       FIG. 8  illustrates a front view an embodiment of a heat exchanger system  168 . The heat exchanger system  168  may be used to couple two multi-pass heat exchangers  100  together in a parallel flow path. For example, a frame  170  may be used to support a first multi-pass heat exchanger  172  and a second multi-pass heat exchanger  174 . The first and second multi-pass heat exchangers  172 ,  174  may be positioned at an angle  176  relative to one another. In some embodiments, the angle  176  may be between zero and ninety degrees, such that the first and second multi-pass heat exchangers  172 ,  174  are positioned in a “V-shape” configuration. A mounting bracket  178  may be used to couple a lower portion the first and second multi-pass heat exchangers  172 ,  174  to a cross-member  180  of the frame  170 . An upper portion of the first and second multi-pass heat exchangers  172 ,  174  may couple to a shroud  182  of the frame  170 . As discussed in greater detail herein, the shroud  182  may include a fan  186 , such as the fan  32 , which is configured to direct a cooling fluid across the first and second slabs  108 ,  110  of each multi-pass heat exchanger  100 . 
     An inlet manifold  190  may receive a flow of refrigerant from the vapor compression system  72  and direct the refrigerant toward the multi-pass heat exchangers  100 . The inlet manifold  190  may split the flow of refrigerant into two separate flows, such that a first flow of refrigerant may enter the main inlet  118  of the first multi-pass heat exchanger  172  and a second flow of refrigerant may enter the main inlet  118  of the second multi-pass heat exchanger  174 . The first and second flows of refrigerant may each complete a first pass through the first plurality of tubes  122  within first slab  108  of the first multi-pass heat exchanger  174  or the second multi-pass heat exchanger  176 , respectively. 
     With the foregoing in mind,  FIG. 9  illustrates a rear view of an embodiment of the heat exchanger system  168 . When each of the first and second flows of refrigerant complete the first pass through the respective first slabs  108 , the first and second flows of refrigerant are directed to respective second slabs  110  via the manifold tubes  140 , and complete respective second and the third passes through the multi-pass heat exchangers  100 . The first and second flow of refrigerant may exit the main outlet  120  of the first multi-pass heat exchanger  172  and the second multi-pass heat exchanger  174 , respectively, and combine into a single refrigerant flow via a return manifold  188 . In some embodiments, the refrigerant may be redirected back toward the vapor compression system  72 . 
     As discussed above, the fan  186  may direct cooling fluid across the first and second slabs  108 ,  110  of each multi-pass heat exchanger  100 . The heat exchanger system  168  may include forward and rear shrouds, as shown in  FIG. 10 , which may block heat exchange fluid  116  from bypassing the multi-pass heat exchangers  100  and entering the fan  186  directly. As such, a pressure drop between the ambient environment and an interior region  191  between the first multi-pass heat exchanger  172  and the second multi-pass heat exchanger  174  may be generated. The heat exchange fluid  116  may thus be directed through the multi-pass heat exchangers  100  and across the cooling fins  150 , such that the heat exchange fluid may absorb thermal energy from the second slab  110 , and subsequently absorb thermal energy from the first slab  108 . After flowing through the multi-pass heat exchangers  100 , the heat exchange fluid  116  may be exhausted as heated waste fluid  192  near an upper and portion  194  of the frame  170 . 
     In some embodiments, the efficiency of each multi-pass heat exchanger  100  may be optimized by directing the heat exchange fluid  116  through the second slab  110  and before directing the heat exchange fluid  116  through the first slab  108 . For example, refrigerant into the first slab  108  from the vapor compression system  72  may in a hot, gaseous state, which is of high thermal energy. As discussed above, thermal energy may be extracted from the refrigerant during the first pass through the first slab  108 , such that the refrigerant exits the first slab  108  in a two-phase mixture or a completely liquid phase. The cooled, two-phase or liquid refrigerant subsequently enters the second and third passes within the second slab  110 , which enables the multi-pass heat exchanger  100  to extract additional thermal energy from the refrigerant. 
     Because the refrigerant within the second slab  110  is of lower thermal energy than the refrigerant within the first slab  108 , it is desirable to direct the heat exchange fluid  116  across the second slab  110  before directing the heat exchange fluid  116  across the first slab  108 . For example, the heat exchange fluid  116  may increase in temperature due to thermal energy absorbed from the refrigerant after flowing through the second slab  110  and the first slab  108 . Therefore, directing the refrigerant through the second slab  110  before directing the refrigerant through the first slab  108  may enable the second slab  110  to contact fresh, unheated heat exchange fluid  116  flowing directly from the ambient environment. The heat exchange fluid  116  may absorb thermal energy from the pre-cooled refrigerant within the second slab  110  that has already been cooled while completing the first pass within the first slab  108 . The heat exchange fluid  116  may thus increase in temperature when absorbing thermal energy from the refrigerant within the second slab  110 , however, the thermal exchange fluid  116  may still be cooler than the refrigerant within the first slab  108 . The warmed heat exchange fluid  116  exiting the second slab  110  may thus absorb additional thermal energy from the refrigerant within the first slab  108 . The heat exchange fluid  116  may exit the first slab  108  as the heated waste fluid  190  that is directed to the ambient environment via the fan  186 . 
       FIG. 10  illustrates an embodiment of a heat exchanger unit  200  that includes multiple exchanger systems  168 . While two heat exchanger systems  168  are shown in the illustrated embodiment of the heat exchanger unit  200 , the heat exchanger unit may include 1, 3, 4, 5, 6, 7, 8 or more heat exchanger system  168 . As discuses above, a forward shroud  202  and a rear shroud  204  may be used to enclose an opening near the first end portion  124  and the second end portion  126 , respectively, of each of the multi-pass heat exchangers  100 . In some embodiments, hot, gaseous refrigerant may be directed from the vapor compression system  72  toward the heat exchanger unit  200 , as indicated by arrow  206 , via an inlet conduit  208  that may couple to the inlet manifold  190  of each heat exchanger system  168 . The gaseous refrigerant may be cooled and condensed by flowing through a respective multi-pass heat exchanger  100  of the heat exchanger unit  200  and return the vapor compression system  72 , as indicated by arrow  210 , via an outlet conduit  212  that may couple to the return manifolds  188  of each heat exchanger system  168 . 
       FIG. 11  is an embodiment of a method  220  that may be used to operate the multi-pass heat exchanger  100 . The heat exchange fluid  116  may be directed, as indicated by process block  222 , across the first slab  108  and the second slab  110  of the multi-pass heat exchanger  100  using a fan  186 . For example, the heat exchange fluid  116  may be configured to flow across the cooling fins  150  of the first slab  108  and the cooling fins  150  of the second slab  110 . In some embodiments, the heat exchange fluid  116  may be configured to flow across the cooling fins  150  of the second slab  110  prior to flowing across the cooling fins  150  of the first slab  108 . As discussed above, directing the cooling fluid  116  across the second slab  110  prior to the first slab  108  may enable the cooling fluid to absorb thermal energy from the substantially cool refrigerant within the second slab  110  before absorbing thermal energy from the substantially hot refrigerant within the first slab  108 . 
     In some embodiments, gaseous refrigerant from the vapor compression system  72  may be directed, as indicated by process block  224 , through the first plurality of tubes  122  of the first slab  108  and condense into a two-phase mixture of liquid refrigerant and gaseous refrigerant. For example, the cooling fluid  116  flowing across the first slab  108  may absorb thermal energy from the gaseous refrigerant, such that the gaseous refrigerant may condense into the two-phase state. In some embodiments, the gaseous refrigerant may condense into a substantially liquid state after completing the first pass. The two-phase or liquid refrigerant may be directed, as indicated by process block  226 , through the second plurality of tubes  154  of the second slab  110 , such that the cooling fluid  116  may absorb additional thermal energy from the two-phase and/or liquid refrigerant. If the refrigerant enters the second slab  110  in the substantially liquid state, the refrigerant may be sub-cooled while completing the second pass. The liquid refrigerant may be directed, as indicated by process block  228 , through the third plurality of tubes  158 , such that the liquid refrigerant may be sub-cooled while additional thermal energy is removed from the refrigerant. The sub-cooled refrigerant may be directed, as indicated by process block  230 , toward the vapor compression system  72  for reuse in the vapor compression system  72 . 
     The aforementioned embodiments of the multi-pass heat exchanger  100  may be used on the HVAC unit  12 , the residential heating and cooling system  50 , or in any suitable vapor compression system. Additionally, the specific embodiments described above have been shown by way of example, and it should be understood that these embodiments may be susceptible to various modifications and alternative forms. It should be further understood that the claims are not intended to be limited to the particular forms disclosed, but rather to cover all modifications, equivalents, and alternatives falling within the spirit and scope of this disclosure.