Abstract:
A parallel flow heat exchanger system ( 10, 50, 100, 200 ) for heat pump applications in which single and multiple paths of variable length are established via flow control systems which also allow for refrigerant flow reversal within the parallel flow heat exchanger system ( 10, 50, 100, 200 ), while switching between cooling and heating modes of operation. Examples of flow control devices are an expansion device ( 80 ) and various check valves ( 70, 72, 74, 76 ). The parallel flow heat exchanger system may have converging or diverging flow circuits and may constitute a single-pass or a multi-pass evaporator together with and a multi-pass condenser.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    Reference is made to and this application claims priority from and the benefit of U.S. Provisional Application Ser. No. 60/649,382, filed Feb. 2, 2005, and entitled PARALLEL FLOW HEAT EXCHANGERS FOR HEAT PUMP APPLICATIONS, which application is incorporated herein in its entirety by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    This invention relates generally to refrigerant heat pump systems and, more particularly, to parallel flow heat exchangers thereof. 
         [0003]    A definition of a so-called parallel flow heat exchanger is widely used in the air conditioning and refrigeration industry and designates a heat exchanger with a plurality of parallel passages, among which refrigerant is distributed and flown in the orientation generally substantially perpendicular to the refrigerant flow direction in the inlet and outlet manifolds. This definition is well adapted within the technical community and will be used throughout the text. Parallel flow heat exchangers started to gain popularity in the air conditioning installations but their application in the heat pump field is extremely limited for the reasons outlined below. 
         [0004]    Refrigerant heat pump systems typically operate in either cooling or heating mode, depending on thermal load demands and environmental conditions. A conventional heat pump system includes a compressor, a flow control device such as a four-way reversing valve, an outdoor heat exchanger, an expansion device, and an indoor heat exchanger. The four-way reversing valve directs refrigerant flown out of a compressor discharge port to either outdoor or indoor heat exchanger as well as routes it back to a compressor suction port from another of these heat exchangers, while the heat pump system is operating in the cooling or heating mode respectively. In the cooling mode of operation, the refrigerant is compressed in the compressor, delivered downstream to a four-way reversing valve and then routed to the outdoor heat exchanger (a condenser in this case). In the condenser, heat is removed from the refrigerant during heat transfer interaction with a secondary fluid such as air, blown over the condenser external surfaces by an air-moving device such as fan. As a result, the refrigerant is desuperheated, condensed and typically subcooled. From the outdoor heat exchanger, the refrigerant flows through the expansion device, where it is expanded to a lower pressure and temperature, and then to an indoor heat exchanger (an evaporator in this case). In the evaporator, refrigerant, during heat transfer interaction, cools air (or other secondary fluid) delivered to a conditioned space by an air-moving device such as fan. While the refrigerant, that is evaporated and superheated, cools the air flowing over the indoor heat exchanger, typically, moisture is also taken out of the air stream, thus the air is dehumidified as well. From the indoor heat exchanger, the refrigerant, once again, passes through the four-way reversing valve and is returned to the compressor. 
         [0005]    In the heating mode of operation, the refrigerant flow through the heat pump system is essentially reversed. The refrigerant flows from the compressor to the four-way reversing valve and is routed to the indoor heat exchanger. In the indoor heat exchanger, which now serves as a condenser, the heat is released to the air to be delivered to the indoor environment by the fan to heat the indoor environment. The desuperheated, condensed and typically subcooled refrigerant then flows through the expansion device and to the downstream outdoor heat exchanger, where heat is transferred from a relatively cold ambient environment to the refrigerant, which is evaporated and generally superheated. The refrigerant is then directed to the four-way reversing valve and is returned to the compressor. 
         [0006]    As known to a person skilled in the art, a simplified operation of the basic heat pump system has been described above, and many variations and optional features can be incorporated into the heat pump schematics. For instance, separate expansion devices can be employed for the heating and cooling modes of operation or an economizer or reheat cycle can be integrated into a heat pump design. Further, with the introduction of natural refrigerants such as R744, the high pressure side heat exchanger can potentially operate in the supercritical region (above the critical point), and a single-phase refrigerant will be flowing through its heat exchange tube instead of predominantly two-phase fluid such as at subcritical conditions. In this case, the condenser becomes a single-phase cooler type heat exchanger. 
         [0007]    As can be seen from a simplified description of the heat pump operation, both heat exchangers typically serve a double duty as a condenser and as an evaporator, depending on the mode of operation. Further, a refrigerant flow through the heat pump heat exchangers is typically reversed (unless specific piping arrangements are made) during aforementioned modes of operation. Consequently, heat exchanger and heat pump system designers face a challenge to optimize the heat exchanger circuiting configuration for performance in both cooling and heating modes of operation. This becomes a particularly difficult task, since an adequate balance between refrigerant heat transfer and pressure drop characteristics is to be maintained throughout the heat exchanger. Therefore, many heat pump heat exchanges are designed with an equal, although not optimal, number of straight-through circuits for both cooling and heating modes of operation. 
         [0008]    In general, the more vapor is contained in the two-phase refrigerant mixture flowing through the heat exchanger and the higher refrigerant flow rate the larger number of parallel circuits is required for efficient heat exchanger operation. Thus, the efficient condensers typically incorporate converging circuits and efficient evaporators employ either straight-through or diverging circuits. In other words, the heat exchanger circuits are either combined or split at some intermediate locations along the refrigerant paths to accommodate the changes in the refrigerant density and improve characteristics of condensing or evaporating refrigerant flows respectively. In conventional plate-and-fin heat exchangers, such circuit alterations, along with the refrigerant flow direction reversal, can be accomplished by utilizing the tripods and intermediate manifolds, as known in the industry. In the parallel flow heat exchangers, due to the design particulars as well as manifold design and refrigerant distribution specifics, the number of parallel circuits can be altered only at the manifold locations, restricting heat exchanger design flexibility, especially in the heat pump applications. Consequently, implementation of a variable number of parallel circuits along the heat exchanger length as well as variable length circuits for cooling and heating modes of operation represent a significant obstacle for heat exchanger and heat pump system designers and is not known in the art of parallel flow heat exchangers. 
         [0009]    Another challenge a heat exchanger designer faces is refrigerant maldistribution, especially pronounced in the refrigerant system evaporators. It causes significant evaporator and overall system performance degradation over a wide range of operating conditions. Maldistribution of refrigerant may occur due to differences in flow impedances within evaporator channels, non-uniform airflow distribution over external heat transfer surfaces, improper heat exchanger orientation or poor manifold and distribution system design. Maldistribution is particularly pronounced in parallel flow evaporators due to their specific design with respect to refrigerant routing to each refrigerant circuit. Attempts to eliminate or reduce the effects of this phenomenon on the performance of parallel flow evaporators have been made with little or no success. The primary reasons for such failures have generally been related to complexity and inefficiency of the proposed technique or prohibitively high cost of the solution. 
         [0010]    In recent years, parallel flow heat exchangers, and brazed aluminum heat exchangers in particular, have received much attention and interest, not just in the automotive field but also in the heating, ventilation, air conditioning and refrigeration (HVAC&amp;R) industry. The primary reasons for the employment of the parallel flow technology are related to its superior performance, high degree of compactness and enhanced resistance to corrosion. As mentioned above, in the heat pump systems, each parallel flow heat exchanger is utilized as both a condensers and an evaporator, depending on the mode of operation, and refrigerant maldistribution is one of the primary concerns and obstacles for the implementation of this technology in the evaporators of the heat pump systems. 
         [0011]    Refrigerant maldistribution in parallel flow heat exchangers occurs because of unequal pressure drop inside the channels and in the inlet and outlet manifolds, as well as poor manifold and distribution system design. In the manifolds, the difference in length of refrigerant paths, phase separation and gravity are the primary factors responsible for maldistribution. Inside the heat exchanger channels, variations in the heat transfer rate, airflow distribution, manufacturing tolerances, and gravity are the dominant factors. Furthermore, the recent trend of the heat exchanger performance enhancement promoted miniaturization of its channels (so-called minichannels and microchannels), which in turn negatively impacted refrigerant distribution. Since it is extremely difficult to control all these factors, many of the previous attempts to manage refrigerant distribution, especially in parallel flow evaporators, have failed. 
         [0012]    In the refrigerant systems utilizing parallel flow heat exchangers, the inlet and outlet manifolds or headers (these terms will be used interchangeably throughout the text) usually have a conventional cylindrical shape. When the two-phase flow enters the header, the vapor phase is usually separated from the liquid phase. Since both phases flow independently, refrigerant maldistribution tends to occur, potentially causing the two-phase (zero superheat) conditions at the exit of some heat transfer tubes and promoting flooding at the compressor suction that may quickly translate into the compressor damage. 
         [0013]    Thus, a designer of parallel flow heat exchangers for the heat pump applications faces the following challenges: implementation of the variable length diverging and conversing circuits for improving performance characteristics in the heating and cooling modes of operation, handling the reversed flow and avoiding maldistribution (as well as and other reliability issues such as oil holdup). Therefore, there is a need for improved parallel flow heat exchanger hardware and heat pump system designs which address and overcome the challenges described above. 
       SUMMARY OF THE INVENTION 
       [0014]    It is the object of the present invention to provide for a parallel flow heat exchanger construction which exhibits performance advantages, particularly in the heat pump installations, by employing converging and/or diverging circuits and consequently providing adequate balancing of refrigerant heat transfer and pressure drop characteristics. It is another object of the present invention to provide for a parallel flow heat exchanger system design incorporating variable length circuits, including the capability for a refrigerant flow reversal, to enhance heat pump system performance while switching between and operating in both cooling and heating modes. 
         [0015]    In one embodiment, a heat exchanger system design includes a parallel flow heat exchanger having two refrigerant passes while operating as a condenser and a single refrigerant pass while operating as an evaporator. In the condenser operation, the refrigerant is delivered to an inlet manifold and distributed to a larger number of parallel heat exchange tubes in the first path, collected in the intermediate manifold and then delivered to the outlet manifold through a smaller remaining number of parallel heat exchange tubes as will be described in greater detail hereinafter. In the evaporator operation, by utilizing a check valve system and routing piping, the refrigerant flow through the parallel flow heat exchanger is reversed and arranged in a single-pass configuration, while a single expansion device is provided to expand refrigerant to a lower pressure and temperature upstream of the evaporator. Therefore, the aforementioned benefits of enhanced performance and improved reliability are achieved in both cooling and heating modes of operation due to an optimal balance between refrigerant heat transfer and pressure drop characteristics inside the heat exchange tubes. 
         [0016]    In another embodiment, a heat exchanger system includes a separate intermediate manifold and a parallel flow heat exchanger operating as a three-pass condenser and a single-pass evaporator. Operation and obtained advantages of this system are analogous to the previous embodiment. Furthermore, multiple expansion devices are provided to avoid or diminish effects of refrigerant maldistribution. 
         [0017]    In still another embodiment, a heat exchanger system incorporates a parallel flow heat exchanger having three passes in the condenser operation while having only a single pass in the evaporator duty. This embodiment includes a single expansion device and a distributor system that can improve refrigerant distribution as well. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    For a further understanding of the objects 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, where: 
           [0019]      FIG. 1A  is a schematic illustration of a parallel flow heat exchanger adapted for two-pass condenser applications. 
           [0020]      FIG. 1B  is a view of  FIG. 1A  adapted for two-pass evaporator applications. 
           [0021]      FIG. 2A  is a schematic illustration of a second embodiment of a parallel flow heat exchanger system adapted for two-pass condenser applications. 
           [0022]      FIG. 2B  is a view of  FIG. 2A  adapted for single-pass evaporator applications. 
           [0023]      FIG. 3A  is a schematic illustration of a third embodiment of a parallel flow heat exchanger system adapted for three-pass condenser applications. 
           [0024]      FIG. 3B  is a view of FIG.  3 Aa adapted for single-pass evaporator applications. 
           [0025]      FIG. 4A  is a schematic illustration of a fourth embodiment of a parallel flow heat exchanger system of the present invention adapted for three-pass condenser applications. 
           [0026]      FIG. 4B  is a view of  FIG. 4A  adapted for single-pass evaporator applications. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0027]    In the operation of a conventional parallel flow heat exchanger, refrigerant flows through the inlet opening and into the internal cavity of an inlet manifold. From the inlet manifold, the refrigerant, in a single-pass configuration, enters and passes through a series of parallel heat transfer tubes to the internal cavity of an outlet manifold. Externally to the tubes, air is circulated over the heat exchange tubes and associated airside fins by an air-moving device such as fan, so that heat transfer interaction occurs between the air flowing outside the heat transfer tubes and refrigerant inside the tubes. The heat exchange tubes can be hollow or have internal enhancements such as ribs for structural rigidity and heat transfer augmentation. These internal enhancements divide each heat exchange tube into multiple channels along which the refrigerant is flown in a parallel manner. The channels typically have circular, rectangular, triangular, trapezoidal or any other feasible cross-section. Furthermore, the heat transfer tubes can be of any cross-section, but preferably are either predominantly rectangular or oval. The heat exchanger elements are usually made from aluminum and attached to each other during furnace brazing operations. 
         [0028]    In a multi-pass arrangement, the heat transfer tubes are divided into tube banks and the refrigerant is flown from one tube bank to another in a parallel manner through a number of intermediate manifolds or manifold chambers associated with inlet and outlet manifolds. A number of heat transfer tubes in each tube bank can be varied based on performance and reliability requirements. 
         [0029]    As mentioned above, in general, the more vapor is contained in the two-phase refrigerant mixture flowing through the heat exchanger and the higher refrigerant flow rate the larger number of parallel circuits is required for efficient heat exchanger operation. Thus, the condensers typically incorporate converging circuits and evaporators employ either straight-through or diverging circuits. In other words, a number of parallel heat exchanger circuits is altered at the intermediate manifold locations to accommodate the changes in refrigerant density and improve characteristics (balance the heat transfer and pressure drop) of condensing or evaporating refrigerant flows. 
         [0030]    As also explained above, in the heat pump operation, each heat exchanger typically serves a double duty as a condenser and as an evaporator, depending on the mode of operation (cooling or heating). Further, the refrigerant flow through the heat pump heat exchangers is typically reversed during aforementioned modes of operation. Consequently, heat exchanger and heat pump system designers face a challenge to optimize heat exchanger circuiting configuration for performance and reliability in both cooling and heating modes of operation. It becomes a particularly difficult task, since an adequate balance between refrigerant heat transfer and pressure drop characteristics is to be maintained throughout the heat exchanger at a variety of operating conditions. Therefore, many heat pump heat exchanges are designed with an equal, although not optimal, number of straight-through circuits for both cooling and heating modes of operation. 
         [0031]    Referring now to  FIGS. 1A and 1B , in one embodiment of the invention, a parallel flow heat exchanger  10  is shown to include an inlet header or manifold  12 , and adjoining outlet header or manifold  14 , and a plurality of parallel disposed heat exchange tubes  22  fluidly interconnecting the inlet manifold and the outlet manifold with an intermediate manifold  20  disposed on an opposite side of the heat exchanger  10 . Typically, the inlet and outlet manifolds  12  and  14  are circular or rectangular in cross-section, and the heat exchange tubes  22  are tubes (or extrusions) of flattened or round shape. As mentioned above, the heat exchange tubes  22  normally have a plurality of internal and external heat transfer enhancement elements, such as fins. For instance, external fins  24 , uniformly disposed therebetween for the enhancement of the heat exchange process and structural rigidity, are typically furnace-brazed. The heat transfer tubes  22  may also have internal heat transfer enhancements and structural elements dividing each tube into multiple channels among which the refrigerant is flown is a parallel manner. As known, these channels may be of a rectangular, circular, triangular, trapezoidal or any other feasible cross-section. 
         [0032]    In the condenser operation, as shown in  FIG. 1A , the refrigerant is delivered to the manifold  12  through a refrigerant line  16  positioned downstream of a four-way reversing valve (not shown) and distributed to a relatively large number of parallel heat exchange tubes in the first path or tube bank  22 A (approximately ⅔ of the total number of tubes), collected in the intermediate manifold  20  and then delivered to the manifold  14  through a relatively small remaining number of parallel heat exchange tubes in the second path or tube bank  22 B (approximately ⅓ of the total number of tubes). From the manifold  14  refrigerant flows out to a refrigerant line  18  communicating with a downstream expansion device of the heat pump system (not shown). During heat transfer interaction with the air blown over external heat transfer surfaces of the heat exchanger  10  by an air-moving device such as fan, the refrigerant is desuperheated and partially condensed in the first tube bank  22 A and completely condensed and then subcooled in the second tube bank  22 B. A smaller number of heat transfer tubes in the second bank reflects higher density refrigerant flowing through the bank and is needed to maintain an appropriate balance between refrigerant heat transfer and pressure drop characteristics. In this embodiment, manifolds  12  and  14  are adjacent, share the same general construction member  26  and are separated by a rigid partition  28 . 
         [0033]    In the evaporator operation, the refrigerant flow through the heat exchange tubes  22  is reversed (see  FIG. 1B ). In  FIG. 1B , the parallel flow heat exchanger  10  has identical manifold construction to the  FIG. 1A  embodiment but a number of the parallel heat exchange tubes in the first pass or tube bank  32 A is smaller now (approximately ⅓ of the total number of tubes) than a number of the parallel heat exchange tubes in the second pass or tube bank  32 B (approximately ⅔ of the total number of tubes). In the evaporator operation, refrigerant is partially evaporated in the first pass  32 A and completely evaporated and then superheated in the second pass  32 B, once again, due to heat transfer interaction with the air blown over the heat exchanger external surfaces. Now, a larger of number of heat exchange tubes in the second bank (than in the first bank) reflects higher density refrigerant flowing through the bank and is desired to maintain an appropriate balance between refrigerant heat transfer and pressure drop characteristics. 
         [0034]    Therefore, an appropriate split in a number of heat exchange tubes  22  into the first and second passes can be designed for optimal enhanced performance of the parallel flow heat exchanger  10  in both cooling and heating modes of operation of the heat pump system. It has to be noted, that although the orientation of the parallel flow heat exchanger  10  is shown horizontally, other orientations such as vertical or at an angle are also within the scope of the invention. Further, parallel flow heat exchanger  10  can be straight, as shown in  FIGS. 1A and 1B  or can be bent or otherwise formed into any desired shape. 
         [0035]    In the embodiments shown in  FIGS. 2A and 2B , the heat exchanger system  50  includes a parallel flow heat exchanger  90  and an associated refrigerant flow control system. In the condenser operation depicted in  FIG. 2A , the refrigerant enters the parallel flow heat exchanger  90  through a refrigerant line  58  and flows through a check valve  70 , located on a refrigerant line  82 , into a manifold  54 , while a check valve  72  prevents refrigerant from immediately entering an intermediate manifold  60  through a refrigerant line  66 . Thereafter, the refrigerant flows through a first pass or tube bank  52 A containing a relatively large number of heat exchange tubes (approximately ⅔ of the total number of tubes), enters intermediate manifold  60  and is directed to a second pass or tube bank  52 B containing a relatively small number of heat exchange tubes (approximately ⅓ of the total number of tubes). A higher pressure acting on an apposite side of the check valve  72  prevents the refrigerant flowing out of the intermediate manifold  60  from entering into the refrigerant line  66 . In case there are any concerns regarding operation of the check valve  72 , it can always be replaced with a solenoid valve. After leaving the second tube bank  52 B, refrigerant is entering manifold  52 , that shares the same general construction  84  with the manifold  54 , and is leaving the manifold  52  through a refrigerant line  62  and a check valve  74  to be delivered to an expansion device through a refrigerant line  56 . A check valve  76  positioned on a refrigerant line  64  prevents refrigerant flowing through an expansion device  80 , in case separate expansion devices are utilized for cooling and heating modes of operation. 
         [0036]    During heat transfer interaction with the air blown over external heat transfer surfaces of the heat exchanger  90  by an air-moving device, the refrigerant is desuperheated and partially condensed in the first tube bank  52 A and completely condensed and then subcooled in the second tube bank  52 B. Once again, a smaller number of heat transfer tubes in the second bank reflects higher density refrigerant flowing through the bank and is needed to maintain an appropriate balance between refrigerant heat transfer and pressure drop characteristics. In this embodiment, manifolds  52  and  54  are also adjacent, share the same general construction member  84  and are separated by a check valve  78 . Once again, higher pressure acting on an opposite side of the check valve  78  prevents refrigerant from entering the manifold  54  from the manifold  52 . The advantages similar to the benefits of the  FIG. 1A  embodiment are obtained here as well. 
         [0037]    In the evaporator operation depicted in  FIG. 2B , the refrigerant flows from the refrigerant line  56  into the refrigerant line  64  through the check valve  76  and expansion device  80 , while the check valve  74  prevents the refrigerant to enter the refrigerant line  62  and to bypass the expansion device  80 . In the expansion device  80 , that can be of a fixed orifice type (e.g. a capillary tube, an accurator or an orifice) or a valve type (e.g. thermostatic expansion valve or electronic expansion valve), the refrigerant is expanded to a lower pressure and temperature and enters the manifolds  52  and  54  in a parallel manner, since the check valve  78  doesn&#39;t prevent refrigerant from entering the manifold  54  now. Form the manifolds  52  and  54 , the refrigerant simultaneously flows through all heat exchange tubes  22  in a single-pass arrangement, enters manifold  60  and leaves the parallel flow evaporator  90  through the check valve  72  and refrigerant lines  66  and  58  to be delivered to the four-way reversing valve and returned to the compressor. The check valve  70 , installed in the refrigerant line  82 , prevents the refrigerant from immediately leaving the manifold  54  and parallel flow heat exchanger  90  without passing through the heat exchange tubes  22 . As in the  FIG. 1B  embodiment, in the evaporator operation, refrigerant is evaporated and then superheated, although in a single pass, due to heat transfer interaction with the air blown over the heat exchanger external surfaces. Since in many cases, a higher number of refrigerant circuits is beneficial for the evaporator operation, a performance augmentation is achieved in the  FIG. 2B  embodiment. Therefore, variable length refrigerant circuits provided for the parallel flow heat exchanger system  50  assure optimal enhanced performance in both cooling and heating modes of operation of the heat pump system. Also, it has to be noted that if the expansion device  80  is of an electronic type, then the check valve  76  is not required. 
         [0038]    In the embodiments shown in  FIGS. 3A and 3B , the heat exchanger system  100  includes a parallel flow heat exchanger  110  and an associated refrigerant flow control system. In the condenser operation depicted in  FIG. 3A , the refrigerant enters the parallel flow heat exchanger  110  through a refrigerant line  112  and flows into a manifold  114 , while a check valve  118  prevents refrigerant from immediately entering an intermediate manifold  116 . Thereafter, the refrigerant flows through a first pass or tube bank  152 A containing a relatively large number of heat exchange tubes, enters intermediate manifold  120  and is directed to a second pass or tube bank  152 B containing a smaller number of heat exchange tubes. A higher pressure acting on an apposite side of the check valve  118  prevents the refrigerant flowing out of the intermediate manifold  116  from re-entering the manifold  114 . After leaving the second tube bank  152 B, refrigerant enters a third pass or tube bank  152 C containing even smaller number of heat exchange tubes and is directed through a refrigerant line  128  and a check valve  130  to be delivered to an expansion device through a refrigerant line  136 . A check valve  134  positioned on a refrigerant line  132  prevents refrigerant from flowing through expansion devices  124 , in case there is a concern that the expansion devices  124  themselves will not create high enough hydraulic resistance to refrigerant flow. Thus, in some situations, the check valve  134  may not be required. Analogously, the high hydraulic resistance created by the expansion devices  124  predominantly prevents refrigerant flow communication between manifolds  120  and  126 . 
         [0039]    As before, during heat transfer interaction with the air blown over external heat transfer surfaces of the heat exchanger  110  by an air-moving device, the refrigerant is desuperheated and partially condensed in the first tube bank  152 A, completely (or almost completely) condensed in the second tube bank  152 B and then subcooled in the third tube bank  152 C. Once again, a progressively smaller number of heat exchange tubes in the second and third tube banks reflects higher density refrigerant flowing through the bank and is needed to maintain an appropriate balance between refrigerant heat transfer and pressure drop characteristics. Similarly, a higher number of refrigerant passes in the condenser operation can be implemented if desired. 
         [0040]    In the evaporator operation depicted in  FIG. 3B , the refrigerant flows from the refrigerant line  136  into the refrigerant line  132  through the check valve  134  and into the manifold  126  to be distributed among the expansion devices  124  positioned on connecting lines  122 , while the check valve  130  prevents the refrigerant from entering the refrigerant line  128  and to bypass the expansion devices  124 . In the expansion devices  124 , that are typically of a fixed orifice type (e.g. a capillary tube, an accurator or an orifice), the refrigerant is expanded to a lower pressure and temperature and enters the manifold  120  and all the heat exchange tubes  22  in a parallel manner, since the check valve  118  doesn&#39;t prevent direct refrigerant flow communication between the manifolds  114  and  116 . The refrigerant simultaneously flows through all heat exchange tubes  22  in a single-pass arrangement, enters manifold  114  and  116  and leaves the parallel flow evaporator  110  through the refrigerant line  112 . As in the  FIG. 2B  embodiment, in the evaporator operation, refrigerant is evaporated and then superheated in a single pass, due to heat transfer interaction with the air blown over the heat exchanger external surfaces. Once again, in many cases, a higher number of refrigerant circuits is beneficial for the evaporator operation, and a performance augmentation is achieved in the  FIG. 3B  embodiment. Therefore, variable length refrigerant circuits provided for the parallel flow heat exchanger system  100  assure optimal enhanced performance in both cooling and heating modes of operation of the heat pump system. 
         [0041]    Additionally, the connecting lines  122  may be installed to penetrate inside the intermediate manifold  120  to face the opposite ends of the heat exchange tubes  22  defining relatively narrow gaps between the heat exchange tubes  22  and connecting lines  122 . These narrow gaps improve refrigerant distribution in the evaporator operation and may be uniform for all the heat exchange tubes  22  or alternatively may change from one heat exchange tube to another or from one heat exchange tube section to another, depending on the heat exchanger design and application constraints. 
         [0042]    In the embodiments shown in  FIGS. 4A and 4B , the heat exchanger system  200  includes a parallel flow heat exchanger  210  and an associated refrigerant flow control system. In the condenser operation depicted in  FIG. 4A , the refrigerant enters the parallel flow heat exchanger  210  through a refrigerant line  212  and flows into a manifold  214 . A check valve  218  prevents refrigerant from immediately entering an intermediate manifold  216 . Thereafter, the refrigerant flows through a first pass or tube bank  252 A containing a relatively large number of heat exchange tubes, enters an intermediate manifold  220  and is directed to a second pass or tube bank  252 B containing a smaller number of heat exchange tubes. A higher pressure acting on an opposite side of the check valve  218  prevents the refrigerant from re-entering the manifold  214  from the manifold  216 . After leaving the second tube bank  252 B and the manifold  216 , refrigerant enters a third pass or tube bank  252 C containing an even smaller number of tubes and then passes through a refrigerant line  228  and a check valve  230  to be delivered to a refrigerant line  236  and a downstream expansion device (in case separate expansion devices are utilized for heating and cooling operations). At the same time, a check valve  234  prevents refrigerant from flowing through a distribution device (or so-called distributor)  240 , distributor tubes  222 , refrigerant line  232  and an expansion device  224 . As before, if the expansion device  224  is of electronic type, then the check valve  234  may not be required. 
         [0043]    As before, during heat transfer interaction with the air blown over external heat transfer surfaces of the heat exchanger  210  by an air-moving device, the refrigerant is desuperheated and partially condensed in the first tube bank  252 A, completely (or almost completely) condensed in the second tube bank  252 B and then subcooled in the third tube bank  252 C. Once again, a progressively smaller number of heat exchange tubes in the second and third tube banks reflects higher density refrigerant flowing through the bank and is needed to maintain an appropriate balance between refrigerant heat transfer and pressure drop characteristics. As noted above, a higher number of refrigerant passes in the condenser operation can be implemented if desired. 
         [0044]    In the evaporator operation depicted in  FIG. 4B , the refrigerant flows from the refrigerant line  236  through the check valve  234  and the expansion device  224 , through the refrigerant line  232  and to the distributor  240 . From the distributor  240  the refrigerant is simultaneously distributed between the distributor tubes  222  to be delivered to the manifold  220  and through all the heat exchange tubes  22  in a single-pass arrangement. Thereafter, the refrigerant simultaneously enters the manifolds  214  and  216  directly fluidly connected to each other (since the refrigerant flows through the check valve  218  in an opposite direction now) and leaves the parallel flow evaporator  210  through the refrigerant line  212 . As in the  FIG. 3B  embodiment, in the evaporator operation, refrigerant is evaporated and then superheated in a single pass, due to heat transfer interaction with the air blown over the heat exchanger external surfaces. As was noted before, in many cases, a higher number of refrigerant circuits is beneficial for the evaporator operation, a performance augmentation is achieved in the  FIG. 4B  embodiment. Therefore, variable length refrigerant circuits provided for the parallel flow heat exchanger system  200  assure optimal enhanced performance in both cooling and heating modes of operation of the heat pump system. 
         [0045]    Additionally, the distributor tubes  222  are preferably installed to penetrate inside the intermediate manifold  220  to face the opposite ends of the heat exchange tubes  22  forming relatively narrow gaps between the heat exchange tubes  22  and distributor tubes  222 . These narrow gaps improve refrigerant distribution in the evaporator operation and may be uniform for all the heat exchange tubes  22  or alternatively may change from one heat exchange tube to another or from one heat exchange tube section to another, depending on the heat exchanger design and application constraints. In case refrigerant maldistribution is not a concern, the entire distribution system  240 - 222  can be eliminated, with the refrigerant line  232  extending directly to the manifold  220 . 
         [0046]    It has to be understood that that the presented schematics are exemplary and many arrangements and configurations are possible to achieve variable length circuits in cooling and heating modes of operation for the heat pump system with the parallel flow heat exchangers. Further, various multi-pass arrangements are feasible for the condenser and evaporator applications with the manifolds or manifold chambers positioned on the same or opposite sides of the parallel flow heat exchanger. 
         [0047]    While the present invention has been particularly shown and described with reference to the preferred mode 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.