Patent Publication Number: US-2017356700-A1

Title: Frost tolerant microchannel heat exchanger

Description:
BACKGROUND 
     This invention relates generally to heat pump and refrigeration applications and, more particularly, to a microchannel heat exchanger configured for use in a heat pump or refrigeration system. 
     Heating, ventilation, air conditioning and refrigeration (HVAC&amp;R) systems include heat exchangers to reject or accept heat between the refrigerant circulating within the system and surroundings. One type of heat exchanger that has become increasingly popular due to its compactness, structural rigidity, and superior performance, is a microchannel or minichannel heat exchanger. A microchannel heat exchanger includes two or more containment forms, such as tubes, through which a cooling or heating fluid (i.e. refrigerant or a glycol solution) is circulated. The tubes typically have a flattened cross-section and multiple parallel flow channels. Fins are typically arranged to extend between the tubes to assist in the transfer of thermal energy between the heating/cooling fluid and the surrounding environment. The fins have a corrugated pattern, incorporate louvers to boost heat transfer, and are typically secured to the tubes via brazing. 
     Conventional microchannel heat exchangers commonly have substantially identical fins throughout the heat exchanger core. In the heat pump and refrigeration applications, when the microchannel heat exchanger is utilized as an evaporator, moisture present in the airflow provided to the heat exchanger for cooling may condense and then freeze on the external heat exchanger surfaces. The ice or frost formed may block the flow of air through the heat exchanger, thereby reducing the efficiency and functionality of the heat exchanger and HVAC&amp;R system. Microchannel heat exchangers tend to freeze faster than the round tube and plate fin heat exchangers and therefore require more frequent defrosts, reducing useful heat exchanger utilization time and overall performance. Consequently, it is desirable to construct the microchannel heat exchanger with improved frost tolerance and enhanced performance. 
     SUMMARY OF THE INVENTION 
     A heat exchanger is provided including a first manifold, a second manifold, and a plurality of heat exchange tube segments fluidly coupling the first and second manifold. The heat exchange tube segments include a bend defining a first slab and a second arranged at an angle to one another. Each of the heat exchange tube segments includes at least a first heat exchange tube and a second heat exchange tube at least partially connected by a web extending there between. The first heat exchange tube and the second heat exchange tube are asymmetrical such that a cross-sectional flow area of the first heat exchange tube is different than that of the second heat exchange tube. A fluid flows sequentially through the first heat exchange tubes of the first slab and the second slab, and then through the second heat exchange tubes of the second slab and first slab. 
     In addition to one or more of the features described above, or as an alternative, in further embodiments an airflow across the heat exchanger moves from the first slab toward the second slab. 
     In addition to one or more of the features described above, or as an alternative, in further embodiments an airflow across the heat exchanger moves from the second slab toward the first slab. 
     In addition to one or more of the features described above, or as an alternative, in further embodiments the cross-sectional flow area of the first heat exchange tubes is smaller than the cross-sectional area of the second heat exchange tubes. 
     In addition to one or more of the features described above, or as an alternative, in further embodiments the fluid within the first heat exchange tubes includes a liquid or liquid-vapor mixture including less than 20% vapor by mass. 
     In addition to one or more of the features described above, or as an alternative, in further embodiments the fluid within the second heat exchange tubes includes a vapor or liquid-vapor mixture including at least 50% vapor by mass. 
     In addition to one or more of the features described above, or as an alternative, in further embodiments the cross-sectional flow area of the first heat exchange tubes is larger than the cross-sectional area of the second heat exchange tubes. 
     In addition to one or more of the features described above, or as an alternative, in further embodiments the fluid within the second heat exchange tubes includes a liquid or liquid-vapor mixture including less than 20% vapor by mass. 
     In addition to one or more of the features described above, or as an alternative, in further embodiments the fluid within the first heat exchange tubes includes a vapor or liquid-vapor mixture including at least 50% vapor by mass. 
     According to yet another embodiment of the invention, a heat exchanger is provided including a first manifold, a second manifold, and a plurality of heat exchange tube segments fluidly coupling the first and second manifold. The heat exchange tube segments include a bend defining a first slab and a second arranged at an angle to one another. Each of the heat exchange tube segments includes at least a first heat exchange tube and a second heat exchange tube at least partially connected by a web extending there between. A fluid flow sequentially through the first heat exchange tubes and the second heat exchange tubes of the heat exchanger such that the fluid within the first heat exchange tubes is a liquid and the fluid within the second heat exchange tubes is a vapor. 
     In addition to one or more of the features described above, or as an alternative, in further embodiments the first heat exchange tube and the second heat exchange tube are asymmetrical such that a cross-sectional flow area of the first heat exchange tube is different than a cross-sectional flow area of the second heat exchange tube. 
     In addition to one or more of the features described above, or as an alternative, in further embodiments the cross-sectional flow area of the first heat exchange tubes is smaller than the cross-sectional area of the second heat exchange tubes. 
     In addition to one or more of the features described above, or as an alternative, in further embodiments an airflow across the heat exchanger moves from the first slab toward the second slab. 
     In addition to one or more of the features described above, or as an alternative, in further embodiments wherein an airflow across the heat exchanger moves from the second slab toward the first slab. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a schematic diagram of an example of a vapor refrigeration cycle of a refrigeration system; 
         FIG. 2  is a side view of a microchannel heat exchanger according to an embodiment of the invention prior to a bending operation; 
         FIG. 3  is a cross-sectional view of a tube segment of a microchannel heat exchanger according to an embodiment of the invention; 
         FIG. 4  is a cross-sectional view of a tube segment of a microchannel heat exchanger according to an embodiment of the invention; 
         FIG. 5  is a perspective view of a microchannel heat exchanger according to an embodiment of the invention; 
         FIG. 6  is a cross-sectional view of a microchannel heat exchanger according to another embodiment of the invention; 
         FIG. 7  is a cross-sectional view of a microchannel heat exchanger according to yet an embodiment of the invention; and 
         FIG. 8  is a cross-sectional view of a microchannel heat exchanger according to yet an embodiment of the invention. 
     
    
    
     The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawings. 
     DETAILED DESCRIPTION 
     Referring now to  FIG. 1 , a vapor compression refrigerant cycle  20  of an air conditioning or refrigeration system is schematically illustrated. Exemplary air conditioning or refrigeration systems include, but are not limited to, split, packaged, chiller, rooftop, supermarket, and transport refrigeration systems for example. A refrigerant R is configured to circulate through the vapor compression cycle  20  such that the refrigerant R absorbs heat when evaporated at a low temperature and pressure and releases heat when condensed at a higher temperature and pressure. Within this cycle  20 , the refrigerant R flows in a counterclockwise direction as indicated by the arrow. The compressor  22  receives refrigerant vapor from the evaporator  24  and compresses it to a higher temperature and pressure, with the relatively hot vapor then passing to the condenser  26  where it is cooled and condensed to a liquid state by a heat exchange relationship with a cooling medium (not shown) such as air. The liquid refrigerant R then passes from the condenser  26  to an expansion device  28 , wherein the refrigerant R is expanded to a low temperature two-phase liquid/vapor state as it passes to the evaporator  24 . The low pressure vapor then returns to the compressor  22  where the cycle is repeated. It has to be understood that the refrigeration cycle  20  depicted in  FIG. 1  is a simplistic representation of an HVAC&amp;R system, and many enhancements and features known in the art may be included in the schematic. In particular, the heat pump refrigerant cycle includes a four-way valve (not shown) disposed downstream of the compressor with respect to the refrigerant flow that allows reversing the refrigerant flow direction throughout the refrigerant cycle to switch between the cooling and heating mode of operation for the environment to be conditioned. 
     Referring now to  FIG. 2 , an example of a heat exchanger  30  configured for use in the vapor compression system  20  is illustrated in more detail. The heat exchanger  30  may be used as either a condenser  24  or an evaporator  28  in the vapor compression system  20 . The heat exchanger  30  includes at least a first manifold or header  32 , a second manifold or header  34  spaced apart from the first manifold  32 , and a plurality of tube segments  36  extending in a spaced, parallel relationship between and connecting the first manifold  32  and the second manifold  34 . In the illustrated, non-limiting embodiments, the first header  32  and the second header  34  are oriented generally horizontally and the heat exchange tube segments  36  extend generally vertically between the two headers  32 ,  34 . However, other configurations, such as where the first and second headers  32 ,  34  are arranged substantially vertically are also within the scope of the invention. 
     As illustrated in the cross-sections of  FIGS. 3 and 4 , each of the plurality of tube segments  36  extending between the first manifold  32  and the second manifold  34  is a multiport extruded (MPE) tube segment  36  and includes at least a first heat exchange tube  38  and a second heat exchange tube  40  connected by a web  42  extending at least partially there between. In one embodiment, the web  42  arranged at the outermost tube segments  36  includes a plurality of openings. 
     An interior flow passage of each heat exchange tube  38 , 40  may be divided by interior walls into a plurality of discrete flow channels  44   a ,  44   b  that extend over the length of the tube segments  36  and establish fluid communication between the respective first and second manifolds  32 ,  34 . The interior flow passages of the first heat exchange tubes  38  may be divided into a different number of discrete flow channels  44  than the interior flow passages of the second heat exchange tubes  40 . The flow channels  44   a ,  44   b  may have any shape cross-section, such as a circular cross-section, a rectangular cross-section, a trapezoidal cross-section, a triangular cross-section, or another non-circular cross-section for example. The plurality of heat exchange tube segments  36  including the discrete flow channels  44   a ,  44   b  may be formed using known techniques, such as extrusion for example. 
     Each first heat exchange tube  38  and second heat exchange tube  40  has a respective leading edge  46   a ,  46   b , a trailing edge  48   a ,  48   b , a first surface  50   a ,  50   b , and a second surface  52   a ,  52   b  ( FIG. 3 ). The leading edge  46   a ,  46   b  of each heat exchange tube  38 ,  40  is upstream of its respective trailing edge  48   a ,  48   b  with respect to an airflow A through the heat exchanger  30 . 
     The first heat exchange tubes  38  and the second heat exchanger tubes  40  are substantially different or asymmetric. In the illustrated, non-limiting embodiment, the second heat exchange tubes  40  are wider and have a greater number of discrete flow channels  44  than the first heat exchange tube  38 , resulting in a larger cross-sectional flow area. Although the second heat exchange tube  40 , as illustrated in  FIG. 3 , is wider than the first heat exchange tube  38 , other configurations, such as where the plurality of first heat exchange tubes  38  have a greater cross-sectional flow area than the plurality of second heat exchange tubes  40  for example, are within the scope of the invention. The ratio of asymmetry between the first heat exchange tubes  38  and the second heat exchanger tubes  40  may depend on any of a variety of parameters of the heat exchanger, such as capacity, 
     Referring now to  FIG. 5 , each tube segment  36  of the heat exchanger  30  includes at least one bend  60 , such that the heat exchanger  30  has a multi-pass configuration relative to the airflow A. The bend  60  is generally formed about an axis extending substantially perpendicular to the longitudinal axis or the discrete flow channels  44   a ,  44   b  of the tube segments  36 . In the illustrated embodiment, the bend  60  is a ribbon fold; however other types of bends are within the scope of the invention. In the illustrated, non-limiting embodiment, the bend  60  is formed at an approximate midpoint of the tube segments  36  between the opposing first and second manifolds  32 ,  34 . 
     The bend  60  at least partially defines a first section or slab  62  and a second section or slab  64  of the plurality of tube segments  36 . As shown in the FIG., the bend  60  can be formed such that the first slab is positioned at an obtuse angle with respect to the second slab  64 . Alternatively, or in addition, the bend  60  can also be formed such that the first slab  62  is arranged at either an acute angle or substantially parallel to the second slab  64 . The bend  60  allows for the formation of a heat exchanger  30  having a conventional A-coil or V-coil shape. In embodiments where the first slab  62  and the second slab  64  are arranged substantially parallel, the lengths of the first slab  62  and the second slab  64  may vary to offset the position of the first manifold  32  relative to the second manifold  34 . Alternatively, the free ends of the first slab  62  and the second slab  64  may angle or flare away from one another to accommodate the manifolds  32 ,  34 . 
     As previously stated, the heat exchanger  30  includes a multi-pass configuration as a result of the bend  60  formed therein. In one embodiment, illustrated in  FIG. 6 , the heat exchanger  30  is configured such that both the first heat exchanger tube  38  and the second heat exchanger tube  40  of a tube segment  36  within the first slab  62  define a first pass relative to an airflow A. Similarly, both the first heat exchanger tube  38  and the second heat exchanger tube  40  within the second slab  64  of the same tube segment  36  define a subsequent pass relative to the airflow. Although in the illustrated FIG., the fluid or refrigerant has a counter flow orientation relative to the direction of the airflow, other embodiments where the refrigerant has a parallel flow orientation are also within the scope of the invention. 
     In another embodiment, as illustrated in  FIGS. 7 and 8 , the first heat exchanger tube  38  and the second heat exchanger tube  40  within the same first slab  62  or second slab  64  are configured as different passes within the refrigerant flow path of the heat exchanger  30 . For example, as shown in  FIG. 7 , the heat exchanger  30  may be configured such that refrigerant flows sequentially through the first heat exchanger tube  38  of both the first slab  62  and the second slab  64  prior to flowing through the second heat exchanger tube  40  of the second slab  64  and the first slab  62 . However, other flow configurations such as where the refrigerant flows through the second heat exchanger tubes  40  before flowing the first heat exchanger tubes  38 , as shown in  FIG. 8 , is within the scope of the invention. In addition, the refrigerant may enter the heat exchanger  30  at the same slab as the airflow, as shown in the embodiments of  FIGS. 7 and 8 , or alternatively, may enter the heat exchanger at a different slab as the airflow. 
     Depending on the direction of the airflow A relative to the heat exchanger  30  and which slab the refrigerant is configured as an inlet to the heat exchanger  30 , the flow through the first heat exchanger tube  38  has a first configuration and the flow through the second heat exchanger tube  40  has a second configuration, different from the first configuration. As shown in the illustrated, non-limiting embodiment of  FIG. 7 , with the airflow A flowing from the first slab  62  toward the second slab  64 , the flow within the first heat exchanger tube  38  is parallel to the direction of the airflow A, and the flow within the second heat exchanger tube  40  is counter to the airflow A. In embodiments where the refrigerant is first provided to the second heat exchanger tubes  40 , as shown in  FIG. 8 , the flow within the second heat exchanger tubes  40  is parallel to the direction of the airflow A, and the flow within the first heat exchanger tubes  38  is counter to the airflow A. 
     To minimize the formation of frost on the heat exchanger  30 , the flow path of the refrigerant through the heat exchanger  30  may be configured such that the liquid or two phase portion of the refrigerant flows through the heat exchanger tube having a smaller cross-sectional flow area and the vapor portion of the refrigerant flows through the heat exchanger tube having a larger cross-sectional flow area. For example, in the embodiment illustrated in  FIG. 8 , the second heat exchanger tube  40  has a smaller cross-sectional flow area than the first heat exchanger tube  38 . The airflow is configured to flow from the first slab  62  to the second slab  64 , and the liquid or two-phase refrigerant is input to the second heat exchanger tubes  40  of the first slab  62 . By the time the refrigerant reaches first heat exchange tubes  38  of the first slab  62 , the refrigerant is a superheated vapor which is at a higher temperature than the saturation temperature. As a result, the amount of heat transfer that occurs between the airflow A and first heat exchange tubes  38  of the first slab  62  is limited. In such embodiments, the liquid or liquid vapor mixture within the second heat exchange tubes  40  is less than 20% vapor by mass and the vapor or liquid-vapor mixture within the first heat exchanger tubes  38  is at least 50% vapor by mass. 
     In other embodiments, refrigerant may be provided to the first heat exchange tubes  38  then the second heat exchange tubes  40 , as shown in  FIG. 7 . In such embodiments, first heat exchanger tubes  38  may have a cross-sectional flow area smaller than that of the second heat exchanger tubes  40  such that the liquid or liquid vapor mixture within the first heat exchange tubes  38  is less than 20% vapor by mass and the vapor or liquid-vapor mixture within the second heat exchanger tubes  40  is at least 50% vapor by mass. 
     Presence of superheated vapor and reducing the amount of heat transfer between an airflow A and a fluid R in the pass of the refrigerant where the airflow initially contacts the heat exchanger leads to reduced rate of frost accumulation and improved frost tolerance. As a result, the formation of frost, and therefore a number of defrost cycles required to maintain the operational efficiency of the heat exchanger  30  are reduced. Because the operational efficiency of the heat exchanger  30  is improved (due to a lower number of defrost cycles and increased heat transfer in the second slab), the size of the heat exchanger  30  required for a desired application may also be reduced. Alternatively, size of other components, such as a compressor may be reduced, which in turn would cause even higher evaporation temperature and further reduction of defrost cycles as well as the system performance boost. 
     While the present invention has been particularly shown and described with reference to the exemplary embodiments as illustrated in the drawing, it will be recognized by those skilled in the art that various modifications may be made without departing from the spirit and scope of the invention. Therefore, it is intended that the present disclosure not be limited to the particular embodiment(s) disclosed as, but that the disclosure will include all embodiments falling within the scope of the appended claims. In particular, similar principals and ratios may be extended to the rooftops applications and vertical package units.