Patent Publication Number: US-2011056668-A1

Title: Modular heat exchanger

Description:
FIELD OF THE INVENTION 
     This invention relates generally to heat exchangers having a plurality of heat transfer tubes extending between a first header and a second header, also sometimes referred to as manifolds, and, more particularly, to modular multi-channel tube heat exchangers. 
     Refrigerant vapor compression systems are well known in the art. Air conditioners and heat pumps employing refrigerant vapor compression cycles are commonly used for cooling or cooling/heating air supplied to a climate-controlled comfort zone within a residence, office building, hospital, school, restaurant or other facility. Refrigerant vapor compression systems are also commonly used for cooling air or other secondary fluid to provide a refrigerated environment for food items and beverage products within, for instance, display cases in supermarkets, convenience stores, groceries, cafeterias, restaurants and other food service establishments. 
     Commonly, these refrigerant vapor compression systems include a compression device, a refrigerant heat rejection heat exchanger, an expansion device and a refrigerant heat absorption heat exchanger connected in serial refrigerant flow communication in a refrigerant vapor compression cycle. In a subcritical refrigerant vapor compression cycle, the refrigerant heat rejection heat exchanger functions as a condenser. In a transcritical refrigerant vapor compression cycle, however, the refrigerant heat rejection heat exchanger functions as a gas cooler. In either a subcritical or transcritical refrigerant vapor compression cycle, the refrigerant heat absorption heat exchanger functions as an evaporator. Additionally, conventional refrigerant vapor compression systems sometimes include one or more refrigerant-to-refrigerant heat exchangers, for example, an economizer heat exchanger or a suction line-to-liquid line heat exchanger, or air-to-refrigerant heat exchanger, such as a reheat heat exchanger or an intercooler. 
     Historically, the refrigerant heat rejection heat exchanger and the refrigerant heat absorption heat exchanger used in such refrigerant vapor compression systems have been round tube and plate fin heat exchangers constituting a plurality of round tubes, typically having a diameter of ½ inch, ⅜ inch or 7 millimeters, disposed in a desired circuiting arrangement, with each circuit defining a refrigerant flow path extending between a pair of headers or manifolds. Thus, a round tube and plate fin heat exchanger with conventional round tubes will have a relatively small number of large flow area refrigerant flow paths extending between the headers. Generally, both the tubes and headers of round tube heat exchangers are made of copper, which facilitates assembly of these heat exchangers, and also simplifies connection to the copper refrigerant lines of the refrigerant vapor compression system, by simple brazing or soldering. Additionally, leaks in copper tubes or their connections may be easily repaired both in the factory and in the field by either brazing or soldering or potentially removing and replacing the leaking tube or the leaking section of a tube. The round tubes of the round tube and plate fin heat exchangers are typically expanded to make a good mechanical and thermal contact with the plate fins. The plate fins are typically made from aluminum or copper and represent a secondary extended heat transfer surface. 
     More recently, flat, rectangular or oval shape, multi-channel tubes are being used in heat exchangers for refrigerant vapor compression systems. Sometimes, such multi-channel heat exchanger constructions are referred to as microchannel or minichannel heat exchangers as well. Each multi-channel tube has a plurality of flow channels extending longitudinally in parallel relationship the length of the tube, each channel defining a small cross-sectional flow area refrigerant path. Thus, a heat exchanger with multi-channel tubes extending in parallel relationship between a pair of headers or manifolds of the heat exchanger will define a relatively large number of small cross-sectional flow area refrigerant paths extending between the two headers. To provide a multi-pass flow arrangement within a multi-channel heat exchanger core, the headers, which in some embodiments may be intermediate manifolds, may be divided into a number of chambers, which depends on a desired number of refrigerant passes. 
     Great Britain Patent No. 938,888 discloses a heat exchanger plate made up of a plurality of elongated hollow box-section sub-units secured together in side-by-side contact by welding, epoxy resin adhesive or clamping to provide a complete plate. Header means at the ends of the sub-units connect the sub-units in series, parallel, or a series-parallel combination with respect to the flow of coolant or refrigerant through the sub-units. 
     U.S. Pat. No. Re. 35,502 discloses an evaporator for a refrigeration system which is a heat exchanger having a plurality of hydraulically parallel flow paths defined by heat exchange tubes, an inlet header, an outlet header and a pair of intermediate headers. A first row of tubes extends between the inlet header and a first intermediate header and a second row of tubes extends between a second intermediate header and the outlet header, the first and second intermediate headers are disposed in side-by-side relationship and interconnected in flow communication at the respective ends by U-shaped tubes. 
     To reduce the cost of the multi-channel heat exchanger, it is known to assemble the heat exchanger from extruded or welded aluminum tubes and aluminum headers/manifolds, and, if desired, aluminum fins disposed between adjacent tube pairs. Once the multi-channel, flat tube heat exchanger has been assembled, the entire assembled heat exchanger must be placed in a brazing furnace to bond the aluminum components together. As a consequence, the overall size of the heat exchanger is limited by the size of the available brazing furnaces. 
     SUMMARY OF THE INVENTION 
     A modular multi-channel tube heat exchanger comprises a plurality of aluminum heat exchanger modules selectively connected in fluid communication by tubing, such as copper or aluminum tubing, in a parallel flow configuration, a series flow configuration or a combined parallel/series flow configuration. Each heat exchanger module includes at least a first aluminum header, at least a second aluminum header and at least one aluminum heat exchange tube extending longitudinally therebetween. Each header of the heat exchanger module may function as an inlet header or an outlet header or an intermediate header, depending on the refrigerant flow path configuration within that heat exchange module. The at least one heat exchange tube may comprise a plurality of heat exchange tubes. Each tube may have a plurality of flow paths extending longitudinally in parallel relationship from an inlet end thereof in fluid communication with the first header to an outlet end thereof in fluid communication with the second header. A plurality of heat transfer tubes within each heat exchanger module may have straight or serpentine configuration, multiple or single flow channels, and round or flattened cross-section. The first aluminum header and the second aluminum header of each one of the plurality of heat exchanger modules is connected by a copper or aluminum tube in fluid flow communication to at least one of the first aluminum header and the second aluminum header of another one of the plurality of heat exchanger modules. 
     In an embodiment, each of the first and second aluminum headers of the at least one of the plurality of heat exchanger modules includes an aluminum or brass nipple that is attached to the connecting tube by a mechanical connection, such as a threaded connection, a compression connection or an adhesive bonding connection. In an embodiment, each of the first and second aluminum headers of the at least one of the plurality of heat exchanger modules includes an aluminum or brass nipple that is attached to the connecting tube by a thermal bonding connection, such as a solder connection, a brazed connection or a metal-to-metal thermal diffusion connection. 
     In an embodiment, each of the first and second aluminum headers of the at least one of the plurality of heat exchanger modules includes a copper nipple that is connected to a cooper connecting tube by a thermal bonding connection, such as a solder connection, a brazing connection, or a metal-to-metal thermal diffusion connection. In an embodiment, each of the first and second aluminum headers of the at least one of the plurality of heat exchanger modules includes a copper nipple that is connected to a cooper connecting tube by a mechanical connection, such as a threaded connection, a compression connection or an adhesive bonding connection. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a further understanding of the invention, reference will be made to the following detailed description of the invention which is to be read in connection with the accompanying drawing, wherein: 
         FIG. 1  is a schematic illustration of a first exemplary embodiment of a modular, multi-channel heat exchanger in accord with the present invention; 
         FIG. 2  is a schematic illustration of a second exemplary embodiment of a modular, multi-channel heat exchanger in accord with the present invention; 
         FIG. 3  is a schematic illustration of a third exemplary embodiment of a modular, multi-channel heat exchanger in accord with the present invention; 
         FIG. 4  is a side elevation view of a fourth exemplary embodiment of a modular, multi-channel heat exchanger in accord with the present invention; 
         FIG. 5  is a schematic illustration of a fifth exemplary embodiment of a modular, multi-channel heat exchanger in accord with the present invention; 
         FIG. 6  is a schematic illustration of a sixth exemplary embodiment of a modular, multi-channel heat exchanger in accord with the present invention; 
         FIG. 7  is a schematic illustration of a seventh exemplary embodiment of a modular, multi-channel heat exchanger in accord with the present invention; 
         FIG. 8  is a schematic illustration of a eighth exemplary embodiment of a modular, multi-channel heat exchanger in accord with the present invention; 
         FIG. 9  is a perspective view of a section of a heat exchanger module of a modular, multi-channel heat exchanger in accord with the present invention; and 
         FIG. 10  is a schematic illustration of an exemplary variable arrangement of heat exchanger modules dependant on a mode of operation for a refrigerant system in accord with the present invention 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The modular, multi-channel heat exchanger  10  of the invention will be described in general herein with reference to the various exemplary embodiments depicted in  FIGS. 1-8 . However, the depicted embodiments are illustrative and are not intended to limit the invention. It is to be understood that the modular, multi-channel tube heat exchanger  10  of the invention may be comprised of any number of a plurality of heat exchanger modules arranged in various configurations. 
     In each of the illustrated exemplary embodiments shown in  FIGS. 1-4 , the heat exchanger  10 , in its simplest configuration, includes a plurality, that is two or more, single pass, parallel-tube, multi-channel tube heat exchanger modules  15 . In the exemplary embodiment depicted in  FIG. 1 , the heat exchanger  10  comprises three heat exchanger modules  15  arranged in a parallel configuration, with respect to refrigerant flow. In the exemplary embodiment depicted in  FIG. 2 , the heat exchanger  10  comprises a pair of heat exchanger modules  15  arranged in a series flow configuration, with respect to refrigerant flow. In the exemplary embodiment depicted in  FIG. 3 , the heat exchanger  10  comprises a pair of heat exchanger modules  15  arranged in a parallel configuration, with respect to refrigerant flow, with respect to each other, and arranged in a series flow configuration, with respect to refrigerant flow, with an additional heat exchanger module  15  disposed downstream, with respect to refrigerant flow, thereof. In the exemplary embodiment depicted in  FIG. 4 , the heat exchanger  10  comprises three heat exchanger modules  15 A arranged in a parallel configuration, with respect to refrigerant flow, with respect to each other and arranged in a series flow configuration, with respect to refrigerant flow, with an additional heat exchanger module  15 B disposed downstream, with respect to refrigerant flow, thereof. 
     Referring now to  FIGS. 1-4  in particular, each single-pass heat exchanger module  15  includes a plurality of longitudinally extending multi-channel heat exchange tubes  40  thereby providing a plurality of fluid flow paths between an inlet header  20  and an outlet header  30 . The inlet header  20  defines a fluid chamber for collecting refrigerant, or other primary heat exchange fluid, and distributing the collected refrigerant, or other primary heat exchange fluid, amongst the plurality of heat exchange tubes  40 . The outlet header  30  defines a fluid chamber for collecting refrigerant or other primary heat exchange fluid having traversed the heat exchange tubes  40  and passing the collected refrigerant, or other primary heat exchange fluid, from the heat exchanger module  15  downstream. Each heat exchange tube  40  has an inlet at one end in fluid flow communication to the inlet header  20  and an outlet at its other end in fluid flow communication to the outlet header  30 . In the exemplary embodiments of the heat exchanger  10  depicted in  FIGS. 1-3 , each of the heat exchange modules  15  is depicted with their respective heat exchange tubes  40  arranged in parallel relationship extending generally horizontally between a generally vertically extending inlet header  20  and a generally vertically extending outlet header  30 . In the exemplary embodiment of the modular heat exchanger  10  depicted in  FIG. 4 , each of the heat exchange modules  15  is depicted with their respective heat exchange tubes  40  arranged in parallel relationship extending generally vertically between a generally horizontally extending inlet header  20  and a generally horizontally extending outlet header  30 . 
     Referring now to  FIGS. 5 and 6 , each heat exchanger  10  includes a plurality of multi-pass, parallel-tube, multi-channel tube heat exchanger modules  15 ′. In the exemplary embodiment depicted in  FIG. 5 , the heat exchanger  10  includes two heat exchanger modules  15 ′ disposed in a parallel flow configuration, with respect to fluid flow through the heat exchange tubes  40  thereof, with the heat exchange tubes  40  of each of the heat exchanger modules  15 ′ being arranged in a two-pass configuration. In this embodiment, each module  15 ′ has a first portion of the heat exchange tubes  40  that extend between an inlet header  20  and an intermediate header  80  and a second portion of the heat exchange tubes  40  that extend between the intermediate header  80  and an outlet header  30 . The inlet header  20  and the outlet header  30  comprise separate sections of a common manifold structure. 
     In the exemplary embodiment depicted in  FIG. 6 , the heat exchanger  10  includes three heat exchanger modules  15 ′ disposed in a parallel flow configuration, with respect to fluid flow through the heat exchange tubes  40  thereof, with the heat exchange tubes  40  of each of the heat exchanger modules  15 ′ being arranged in a four-pass configuration. In this embodiment, each module  15 ′ has a first portion of the heat exchange tubes  40  that extend between an inlet header  20  and a first section of a first intermediate header  80 , a second portion of the heat exchange tubes  40  that extend between the first section of the first intermediate header  80  and a second intermediate header  90 , a third portion of the heat exchange tubes  40  that extend between the second intermediate header  90  and a second section of the first intermediate header  80 , and a fourth portion of the heat exchange tubes  40  that extend between the second section of the first intermediate header  80  and an outlet header  30 . The inlet header  20 , the second intermediate header  90  and the outlet header  30  may comprise separate sections of a common manifold structure, as depicted in  FIG. 6 . 
     Referring now to  FIGS. 7 and 8 , each heat exchanger  10  depicted therein includes a plurality of multi-pass, serpentine, multi-channel tube heat exchanger modules  15 ″. In the exemplary embodiment depicted in  FIG. 7 , the heat exchanger  10  includes two heat exchanger modules  15 ″disposed in a parallel flow configuration with respect to fluid flow through the serpentine heat exchange tubes  40 ′ thereof, with the two serpentine heat exchange tubes  40 ′ of each of the heat exchanger modules  15 ″ extending between an inlet header  20  and an outlet header  30  in a four-pass arrangement. In the exemplary embodiment depicted in  FIG. 8 , the heat exchanger  10  includes three heat exchanger modules  15 ″disposed in a parallel flow configuration, with respect to fluid flow through the serpentine heat exchange tubes  40 ′ thereof, with the three serpentine heat exchange tubes  40 ′ of each of the heat exchanger modules  15 ″ extending between an inlet header  20  and an outlet header  30  in a three-pass arrangement. 
     Referring now to  FIG. 9 , each heat exchange tube  40  has a plurality of parallel flow channels  42  extending longitudinally, i.e. parallel to the axis of the tube, along the length of the tube thereby providing multiple, independent, parallel flow paths between the inlet of the tube and the outlet of the tube. Each multi-channel heat exchange tube  40  is a “flat” tube of, for instance, rectangular, racetrack or oval cross-section, defining an interior which is subdivided to form a side-by-side array of independent flow channels  42 . The flat, multi-channel tubes  40  may, for example, have a width of fifty millimeters or less, typically twelve to twenty-five millimeters, and a height of about two millimeters or less, as compared to conventional prior art round tubes having a diameter of ½ inch, ⅜ inch or 7 mm. The tubes  40  are shown in the drawing hereof, for ease and clarity of illustration, as having ten channels  42  defining flow paths having a rectangular cross-section. However, it is to be understood that in practical applications, such as, for example, refrigerant vapor compression systems, each multi-channel tube  40  will typically have about ten to twenty flow channels  42 , but may have a greater or a lesser multiplicity of channels, as desired. Generally, each flow channel  42  will have a hydraulic diameter, defined as four times the flow area divided by the “wetted” perimeter, in the range from about 200 microns to about 3 millimeters. Although depicted as having a rectangular cross-section in the drawings, the channels  42  may have a circular, triangular, trapezoidal cross-section or any other desired cross-section. Although multi-channel flattened aluminum heat exchange tubes such as the tubes depicted in  FIG. 9  are discussed in connection with each of the exemplary embodiments of the invention illustrated in  FIGS. 1-8 , other tube configurations may be employed. For example, the heat exchange tubes  40  comprising single channel, flattened aluminum heat exchange tubes or multi-channel, round aluminum heat exchange tubes or single channel, round aluminum heat exchange tubes, may also be employed in the heat exchanger modules of the modular heat exchanger  10  of the invention. 
     Depending on the particular application in which the modular heat exchanger  10  is to be used, the heat exchanger modules  15  may include heat transfer fins  50  positioned between adjacent heat transfer tubes  40  for heat transfer enhancement. The presence of fins  50  also enhances structural rigidity and heat exchanger design compactness. The fins  50  may be flat, as depicted in  FIG. 9 , or may have a wavy, corrugated or louvered design and typically form triangular, rectangular, offset or trapezoidal airflow passages. In the configuration of the heat exchanger module  15  depicted in  FIG. 9 , the heat exchange tubes  40  extend longitudinally in a generally horizontal direction and the fins  50  extend longitudinally in a generally vertical direction. However, in another configuration, the heat exchange tubes  40  may extend longitudinally in a generally vertical direction, and the fins  50  may extend longitudinally in a generally horizontal direction. In other embodiments, the heat exchange tubes  40  may extend longitudinally at an angle to the horizontal/vertical direction between a pair of headers that also extend at an angle to the horizontal/vertical direction. 
     In applications, whether the heat exchange tubes  40  are orientated horizontally or vertically or otherwise, a secondary fluid, such as air, flows through the heat exchanger module  15  and over the external surfaces of the heat exchange tubes  40  and the associated fins  50 . The heat exchange tubes  40  extend transversely across the flow path of the secondary fluid with the leading edge  41  of each heat exchange tube  40  facing upstream into the incoming flow of secondary fluid. As the secondary fluid passes over the external surfaces of the heat exchange tubes  40  and the associated fins  50 , heat exchange takes place between the secondary fluid and a primary fluid, such as refrigerant, water or glycol solution, flowing through the channels  42  of the multi-channel heat exchange tubes  40 . When the heat exchanger  10  is used in a refrigerant vapor compression system, such as in refrigeration and air conditioning applications, the primary fluid is a refrigerant and the secondary fluid is generally air to be cooled if the heat exchanger  10  is employed as an evaporator, or refrigerant heat absorption heat exchanger, or air to be heated if the heat exchanger  10  is employed as a condenser or a gas cooler and functions as a refrigerant heat rejection heat exchanger. 
     To reduce cost and simplify assembly, each multi-channel tube heat exchanger module  15  may be made of aluminum, as opposed to copper. The heat exchange tubes  40  are typically extruded or welded aluminum tubes. The headers/manifolds  20  and  30  are formed of aluminum, and, the fins  50 , if provided, are made from an aluminum sheet as well. After the heat exchanger module  15  has been assembled with brazing compound applied as in conventional practice at contacting surfaces between the heat exchange tube  40  and associated fins  50  and between the ends of the heat exchange tubes  40  and the respective headers  20  and  30 , the entire assembled heat exchanger module  15  is placed in a brazing furnace to permanently bond the aluminum components together. Some other components, such as manifold caps, connecting tube stubs and brackets, can be also permanently attached during the furnace brazing of the heat exchange module  15 . 
     As noted previously, the modular, multi-channel tube heat exchanger  10  is constructed from a plurality of heat exchanger modules  15  connected in refrigerant flow communication with each other. For example, two or more heat exchanger modules  15  may be connected together by means of copper or aluminum refrigerant lines  60 . In an embodiment, the aluminum inlet header  20  is provided with a copper or brass inlet nipple  25  attached permanently to the aluminum header, during the brazing operation conducted at the manufacturing plant after the furnace brazing operation, during manufacturing of the heat exchanger module  15 . Similarly, in this embodiment, the aluminum outlet header  30  is provided with a copper or brass outlet nipple  35  attached permanently to the aluminum header, during the brazing operation conducted at the manufacturing plant after the furnace brazing operation, during manufacturing of the heat exchanger module  15 . With this construction, the heat exchanger  10  may be readily assembled in the field or at the manufacturing plant by soldering copper refrigerant lines  60  to the appropriate copper inlet and outlet nipples of the respective heat exchanger modules  15 . In this manner, the heat exchanger  10  may be readily assembled in any desired size and configuration simply by linking the appropriate number of aluminum heat exchanger modules  15  in the desired refrigerant circuit flow arrangement. Further, the inlet nipple  25  and the outlet nipple  35  may be made from aluminum and attached respectively to the headers  20  and  30  during the same furnace brazing process. In this case, connecting refrigerant lines  60  made from copper or aluminum may be attached to the nipples  25  and  35  by the brazing process after the furnace brazing operation. Other thermal bonding connections, such as a metal-to-metal thermal diffusion connection, may also be used to join nipples and connecting tubes made of similar metals, such as a copper nipple to a copper tube or an aluminum nipple to an aluminum tube. 
     In an embodiment, the aluminum inlet header  20  is provided with a threaded inlet nipple  25  attached permanently to the aluminum header, during the brazing operation conducted at the manufacturing plant, generally after the furnace brazing operation, during manufacturing of the heat exchanger module  15 . Similarly, in this embodiment, the aluminum outlet header  30  is provided with a threaded outlet nipple  35  attached permanently to the aluminum header, during the brazing operation conducted at the manufacturing plant after the furnace brazing operation, during manufacturing of the heat exchanger module  15 . With this construction, the heat exchanger  10  may be readily assembled in the field or at the factory by mechanically connecting the refrigerant lines  60 , generally made out of copper, aluminum or stainless steel, to the appropriate threaded inlet or outlet nipples of the respective heat exchanger modules. In this manner, the heat exchanger  10  may again be readily field or factory assembled in any desired size and configuration simply by linking the appropriate number of aluminum heat exchanger modules  15  connected by refrigerant lines in the desired refrigerant circuit flow arrangement using threaded mechanical connections. Other mechanical connections, such as a compression connection or a chemical bonding connection, such as for example glue or other adhesive, may be used to join nipples and connecting tubes made of dissimilar metals, such a copper nipple to an aluminum tube or an aluminum nipple to a copper tube. 
     Referring now to  FIG. 1  in particular, the heat exchanger  10  depicted therein comprises three heat exchanger modules  15  arranged in a parallel configuration, with respect to refrigerant flow. The aluminum inlet headers  20  of the three heat exchanger modules  15  are connected in refrigerant flow communication by refrigerant lines  60  connected to the respective inlet nipples  25  of the aluminum inlet headers  20  at connection points  70  to receive refrigerant from the refrigerant circuit via a refrigerant line  65  that is in flow communication with each of the refrigerant lines  60 . The aluminum outlet headers  30  of the three heat exchanger modules  15  are connected in refrigerant flow communication by refrigerant lines  62  connected to the respective outlet nipples  35  of the aluminum outlet headers  30  at connection points  70  to return refrigerant to the refrigerant circuit via a refrigerant line  75  that is in flow communication with each of the refrigerant lines  62 . In this manner, the flow of refrigerant, or other primary fluid, flows in parallel through the three heat exchanger modules  15 . The heat exchanger modules  15  are generally disposed in a planar arrangement so that the air, or other secondary fluid secondary fluid, flowing therethrough has the same, or about the same, temperature passing through the respective heat exchanger modules. 
     Referring now to  FIG. 2  in particular, the heat exchanger  10  depicted therein comprises a pair of heat exchanger modules  15  arranged in a series flow configuration, with respect to refrigerant flow. The two aluminum heat exchanger modules  15  are connected in refrigerant flow communication by a refrigerant line  64  extending between the outlet nipple  35  of the aluminum outlet header  30  of the upstream heat exchanger module  15  and the inlet nipple  25  of the aluminum inlet header  20  of the downstream heat exchanger module  15  at connection points  70  so that the flow of refrigerant, or other primary fluid, is arranged in series through the two heat exchanger modules  15 . The inlet nipple  25  of the inlet header  20  of the upstream heat exchanger module  15  is connected to the refrigerant line  65  of the refrigerant circuit to receive refrigerant from the refrigerant circuit. The outlet nipple  35  of the outlet header  30  of the downstream heat exchanger module  15  is connected to the refrigerant line  75  of the refrigerant circuit to return refrigerant to the refrigerant circuit. The heat exchanger modules  15  are disposed in a planar arrangement so that the air, or other secondary fluid, flowing therethrough preferably has the same, or about the same, temperature passing through the respective heat exchanger modules. 
     Referring now to  FIG. 3  in particular, the heat exchanger  10  comprises a pair of upstream heat exchanger modules  15  arranged in a parallel configuration, with respect to refrigerant flow, with respect to each other and arranged in a series configuration, with respect to refrigerant flow, with an additional heat exchanger module  15  disposed downstream, with respect to refrigerant flow thereof. The aluminum inlet headers  20  of the first two upstream heat exchanger modules  15  are connected in refrigerant flow communication by refrigerant lines  60  connected to the respective inlet nipples  25  of the aluminum inlet headers  20  at the connection points  70  to receive refrigerant from the refrigerant circuit via a refrigerant line  65  in flow communication with each of the refrigerant lines  60 . The aluminum outlet headers  30  of the two heat exchanger modules  15  are connected in refrigerant flow communication by the refrigerant lines  62  connected to the respective outlet nipples  35  of the aluminum outlet headers  30  at the connection points  70 . The refrigerant lines  62  are in flow communication with a refrigerant line  64  which is connected to the inlet nipple  25  of the inlet header  20  of the third downstream heat exchanger module  15 . The outlet nipple  35  of the outlet header  30  is connected to a refrigerant line  75  to return refrigerant to the refrigerant circuit via the refrigerant line  75 . In this manner, the flow of refrigerant, or other primary fluid, passes in parallel arrangement through the first and second upstream heat exchanger modules and thence in series through the third downstream heat exchanger module. The three heat exchanger modules  15  may be disposed in a planar arrangement so that the air, or other secondary fluid, flowing therethrough has the same, or about the same, temperature passing through the respective heat exchanger modules. 
     Referring now to  FIG. 4  in particular, the heat exchanger  10  comprises three upstream heat exchanger modules  15 A arranged in a parallel configuration, with respect to refrigerant flow, with respect to each other and collectively arranged in a series flow configuration, with respect to refrigerant flow, with a fourth heat exchanger module  15 B disposed downstream, with respect to refrigerant flow thereof. In the depicted embodiment, the three upstream heat exchanger modules  15 A are disposed in a stack arrangement, rather than a planar arrangement, whereby air, or other secondary fluid, passes in series through the stacked heat exchanger modules as indicated in  FIG. 4 . Refrigerant, or other primary fluid, flows through the downstream-most (with respect to air flow) three heat exchanger modules  15 A in parallel and thence proceeds through the upstream-most (with respect to air flow) heat exchanger module  15 B which is disposed in series refrigerant flow relationship with the other three heat exchanger modules  15 A. This arrangement of the heat exchanger modules  15 A and  15 B is exemplary, and in many cases, it would be desirable to have a combination of planar and serial configurations, with respect to the airflow, of the heat exchanger modules  15 A and  15 B. For instance, for the condenser or gas cooler applications, the highest efficiency may be achieved if the heat exchanger modules  15 A are arranged in a planar configuration with each other, with respect to the airflow, and positioned downstream, with respect to the airflow, of the heat exchanger module  15 B. Many other configurations accommodating various applications would be also feasible and within the scope of the invention. 
     The aluminum inlet headers  20  of the three heat exchanger modules  15 A are connected in refrigerant flow communication by refrigerant lines  60  connected to the respective inlet nipples  25  of the aluminum inlet headers  20  at connection points  70  to receive refrigerant from the refrigerant circuit via the refrigerant line  65  in flow communication with each of the refrigerant lines  60 . The aluminum outlet headers  30  of the three heat exchanger modules  15 A are connected in refrigerant flow communication by the refrigerant lines  62  connected to the respective outlet nipples  35  of the aluminum outlet headers  30  at connection points  70 . The refrigerant lines  62  are in flow communication with the refrigerant line  64  which is connected to the inlet nipple  25  of the inlet header  20  of the fourth heat exchanger module  15 B. The outlet nipple  35  of the outlet header  30  of the fourth heat exchanger module  15 B is connected to the refrigerant line  75  to return refrigerant to the refrigerant circuit. 
     Referring now to  FIG. 5 , as noted before, the heat exchanger  10  includes two heat exchanger modules  15 ′ disposed in a parallel flow configuration, with respect to fluid flow through the heat exchange tubes  40  thereof, with the heat exchange tubes  40  of each of the heat exchanger modules  15 ′ being configured in a two-pass arrangement. The aluminum inlet headers  20  of the two heat exchanger modules  15 ′ are connected in refrigerant flow communication by refrigerant lines  60  connected to the respective inlet nipples  25  of the aluminum inlet headers  20  at connection points  70  to receive refrigerant from the refrigerant circuit via a refrigerant line  65  that is in flow communication with each of the refrigerant lines  60 . The aluminum outlet headers  30  of the two heat exchanger modules  15 ′ are connected in refrigerant flow communication by refrigerant lines  62  connected to the respective outlet nipples  35  of the aluminum outlet headers  30  at connection points  70  to return refrigerant to refrigerant line  75  of the refrigerant circuit that is in flow communication with each of the refrigerant lines  62 . Additionally, the two intermediate headers  80  are interconnected in refrigerant flow communication by refrigerant equalization lines  66  connected at one end to the nipple  85  of the first of the intermediate headers  80  and at its other end to the nipple  85  of the second of the intermediate headers  80 . 
     The refrigerant equalization lines  66  provide several important functions improving performance of the modular heat exchanger  10 . The most significant two functions are the equalization of the refrigerant pressure in the intermediate headers  80  of different heat exchanger modules  15 ′, and proper redistribution of the liquid refrigerant phase of the two-phase refrigerant mixture accumulated in the intermediate headers  80 . The former function allows for the uniform operation of the heat exchange tubes  40  within the first and second passes positioned within different heat exchanger modules  15 ′. The later function allows for the reduction or elimination of refrigerant maldistribution associated with the second refrigerant pass. Refrigerant maldistribution between the heat exchange tubes  40  significantly reduces performance of the heat exchangers, and may be particularly pronounced for microchannel or minichannel heat exchangers. Therefore, dividing the refrigerant flow between the heat exchanger modules  15 ′ and providing refrigerant flow communication means, such as the refrigerant equalization lines  66 , between the intermediate headers  80  of different heat exchanger modules  15 ′, equalizes the liquid refrigerant phase content within the intermediate headers  80 , improves refrigerant distribution amongst the heat exchange tubes  40  within the downstream pass and enhances overall performance of the modular heat exchanger  10 . 
     Referring now to  FIG. 6 , as noted before, the heat exchanger  10  depicted therein comprises three heat exchanger modules  15 ′ arranged in a parallel configuration, with respect to refrigerant flow. Each of the heat exchanger modules  15 ′ comprises a four-pass heat exchanger module having a first manifold sectioned into an inlet header  20 , an outlet header  30  and an intermediate header  90  disposed therebetween and a second manifold sectioned into a pair of intermediate headers  80 A and  80 B. The aluminum inlet headers  20  of the three heat exchanger modules  15 ′ are connected in refrigerant flow communication by refrigerant lines  60  connected to the respective inlet nipples  25  of the aluminum inlet headers  20  at connection points  70  to receive refrigerant from the refrigerant circuit via a refrigerant line  65  that is in flow communication with each of the refrigerant lines  60 . The aluminum outlet headers  30  of the three heat exchanger modules  15 ′ are connected in refrigerant flow communication by refrigerant lines  62  connected to the respective outlet nipples  35  of the aluminum outlet headers  30  at connection points  70  to return refrigerant to refrigerant line  75  of the refrigerant circuit that is in flow communication with each of the refrigerant lines  62 . Additionally, the two intermediate headers  80 A are interconnected in refrigerant flow communication by a refrigerant equalization line  64  connected to the respective nipples  85  of the two intermediate headers  80 A and the three intermediate headers  80 B are interconnected in refrigerant flow communication by refrigerant equalization lines  68  connected to the respective nipples  85  of each of the intermediate headers  80 B. All benefits provided by the refrigerant equalization line  64  of the  FIG. 5  embodiment are applicable to the refrigerant equalization lines  64  and  68  of the  FIG. 6  embodiment as well. 
     Referring now to  FIG. 7 , as noted before, the heat exchanger  10  includes two heat exchanger modules  15 ″ disposed in a parallel flow configuration, with respect to fluid flow through the serpentine heat exchange tubes  40  thereof, with the serpentine heat exchange tubes  40  of each of the heat exchanger modules  15 ″ being configured in a four-pass arrangement. The aluminum inlet headers  20  of the two heat exchanger modules  15 ″ are connected in refrigerant flow communication by refrigerant lines  60  connected to the respective inlet nipples  25  of the aluminum inlet headers  20  at connection points  70  to receive refrigerant from the refrigerant circuit via a refrigerant line  65  that is in flow communication with each of the refrigerant lines  60 . The aluminum outlet headers  30  of the two heat exchanger modules  15 ″ are connected in refrigerant flow communication by refrigerant lines  62  connected to the respective outlet nipples  35  of the aluminum outlet headers  30  at connection points  70  to return refrigerant to a refrigerant line  75  of the refrigerant circuit that is in flow communication with each of the refrigerant lines  62 . 
     Referring now to  FIG. 8 , as noted before, the heat exchanger  10  includes three heat exchanger modules  15 ″ disposed in a parallel flow configuration, with respect to fluid flow through the serpentine heat exchange tubes  40  thereof, with the serpentine heat exchange tubes  40  of each of the heat exchanger modules  15 ″ being configured in a three-pass arrangement. The aluminum inlet headers  20  of the three heat exchanger modules  15 ″ are connected in refrigerant flow communication by refrigerant lines  60  connected to the respective inlet nipples  25  of the aluminum inlet headers  20  at connection points  70  to receive refrigerant from the refrigerant circuit via a refrigerant line  65  that is in flow communication with each of the refrigerant lines  60 . The aluminum outlet headers  30  of the three heat exchanger modules  15 ″ are connected in refrigerant flow communication by refrigerant lines  62  connected to the respective outlet nipples  35  of the aluminum outlet headers  30  at connection points  70  to return refrigerant to a refrigerant line  75  of the refrigerant circuit that is in flow communication with each of the refrigerant lines  62 . 
     The heat exchanger configurations discussed hereinbefore are exemplary. Because the heat exchanger  10  of the invention is of a modular in design, that is, composed of two or more heat exchanger modules  15 ,  15 ′ or  15 ″ the heat exchanger  10  can be selectively sized by connecting any desired number of heat exchanger modules  15 ,  15 ′ or  15 ″ together in either parallel flow or series flow or mixed parallel and series flow arrangements, with respect to refrigerant flow, and also, independently of the refrigerant flow configuration, connecting the heat exchanger modules  15 ,  15 ′ or  15 ″ together in either parallel flow configuration or series flow configuration or mixed parallel and series flow configuration, with respect to air flow. The heat exchanger modules  15 ,  15 ′ or  15 ″ may also be configured in parallel flow, counter-flow or mixed parallel/counter-flow arrangements, with respect to heat exchange between the refrigerant flow and the air flow. The modular nature of the heat exchanger  10  also facilitates the design of a heat exchanger  10  of selectively variable configuration having a first configuration for operation in a first mode and a second configuration for operation in a second mode, such as, for example, for in heat pump applications wherein the heat exchanger functions as a condenser (or a gas cooler) in one of the cooling or heating modes of operation and as an evaporator in the other of the cooling or heating modes of operation. 
     An exemplary variable configuration of the modular heat exchanger  10  including two heat exchanger modules  15  is shown in  FIG. 10 , where, for instance, the modular heat exchanger  10  either functions as a condenser (the refrigerant flow is shown by solid arrows) in one of the cooling or heating modes of operation or functions as an evaporator (the refrigerant flow is shown by dashed arrows) in the other cooling or heating modes of operation. When the modular heat exchanger  10  operates as a condenser, a check valve  102  allows the refrigerant flow through the heat exchanger modules  15  in sequence, while check valves  104  and  106  are closed, by pressure differentials, preventing refrigerant flow through refrigerant lines  108  and  110 . When the modular heat exchanger  10  operates as an evaporator, the check valve  102  is closed, by the pressure differential, and the check valves  104  and  106  are open, due to pressure differentials, allowing refrigerant flow through the heat exchanger modules  15 , connected by the refrigerant lines  110  and  108  at the inlet and outlet respectively, in parallel. The check valves  102 ,  104  and  106  can be replaced by solenoid valves, if desired. Other refrigerant flow control devices as well as their locations and arrangements are also feasible. 
     Other examples when variable heat exchanger configurations are highly desirable and conveniently provided by the modular heat exchanger  10  include activation and deactivation of the heat exchanger modules  15 ,  15 ′ or  15 ″ depending on the environmental conditions and the type of cooling mode of operation. The former applications comprise, but are not limited to, reduction of a number of parallel heat exchanger modules  15 ,  15 ′ or  15 ″ to prevent oil retention within the heat exchanger with reduced refrigerant flow rate, for instance, at lower temperatures, or bypassing some of the sequential heat exchanger modules  15 ,  15 ′ or  15 ″ when the pressure drop for the heat exchanger  10  becomes excessive or when different sensible and latent loads are required for the evaporator. The latter applications include, for instance, sharing the heat exchanger modules  15 ,  15 ′ or  15 ″ between the evaporator and reheat heat exchanger while switching between cooling and reheat modes of operation, or between the condenser (or gas cooler) and intercooler while operating refrigerant systems with multiple compression stages. As in the embodiment depicted in  FIG. 10 , all these various arrangements and configuration can be provided, for instance, by appropriate refrigerant flow control devices such as valves located in the refrigerant conduits interconnecting the heat exchanger modules  15 ,  15 ′ or  15 ″ so as to permit individual modules to be selectively activated and deactivated for various functions depending upon the configuration desired for a particular operating mode and environmental conditions. Valves may also be installed as appropriate in the conduits connecting the heat exchanger modules  15 ,  15 ′ or  15 ″ to permit the refrigerant flow configuration between the heat exchanger modules  15 ,  15 ′ or  15 ″ to be selectively reconfigured and to permit the heat exchange relationship between refrigerant flow and air flow to be selectively reconfigured from parallel flow to counter-flow through any particular module. 
     Additionally, the heat exchanger modules  15 ,  15 ′ and  15 ″ may be standardized, allowing optimization of the manufacturing process, increase of production volumes and subsequent price reduction. The modular design of the heat exchanger  10  of the invention also facilitates field repairs in that if a leak develops in a heat exchange tube  40  or  40 ′ or a header of an individual heat exchanger module  15 ,  15 ′ or  15 ″, that heat exchanger module may be removed by un-soldering/un-brazing or un-threading at the connections  70 . 
     In each of the exemplary embodiments of the heat exchanger  10  depicted in  FIGS. 1-8 , each header is depicted as having one flow connection nipple. However, if desired, multiple flow connection nipples may be provided on any header, whether inlet, outlet or intermediate, for better refrigerant distribution or other purpose. Further, in heat pump applications, the modular heat exchanger  10  has to accommodate flow reversals throughout the refrigerant system while operational modes are switched between cooling and heating. Under such circumstances, the heat exchanger performing the heat rejection function in the cooling mode of operation has to perform the heat absorption function in the heating mode of operation, and vice versa. Thus, the arrows in the Figures are pointing in one way only depicting the direction of flow of refrigerant, or other primary heat exchange fluid, in a single mode of operation for illustration purposes only. 
     The modular heat exchanger  10  could be easily configured to accommodate straight-through refrigerant pass configuration of the parallel heat exchanger modules  15 , e.g. of  FIG. 1 , as well as converging/diverging refrigerant pass arrangement of the sequential heat exchanger modules  15 , e.g. of  FIG. 2 , and parallel-series configuration of the heat exchanger modules  15 , e.g. of the  FIG. 3 . As known, in many cases, for optimal performance, it is desired to have converging refrigerant pass arrangement for the condenser applications and diverging refrigerant pass configuration for the evaporator applications. 
     As noted previously, the connections  70  may be made by thermal bonding, for example soldering or brazing or metal-to-metal diffusion, if the inlet and outlet nipples  25  and  35  are made out of compatible metals, such as copper or aluminum, or by mechanical or chemical connection, for example a threaded connection, a compression connection or a chemical bonding connection, if the inlet and outlet nipples  25  and  35  are made, for instance, out of aluminum, copper, brass or stainless steel, whether similar or dissimilar metals are involved. 
     While the present invention has been particularly shown and described with reference to the exemplary embodiments as illustrated in the drawing, it will be understood by one skilled in the art that various changes in detail may be effected therein without departing from the spirit and scope of the invention as defined by the claims.