Abstract:
A method is provided for mitigating two-phase refrigerant maldistribution in heat exchange channels of a parallel flow evaporator. An inlet manifold of the evaporator includes a first stage, at least one intermediate stage, and a final stage. Each stage includes an expansion chamber, a contraction chamber, and at least one heat exchange channel interconnecting the stage to an outlet manifold of the evaporator. The method includes throttling the two-phase refrigerant in the expansion chamber of each stage, flowing a portion of the throttled refrigerant through the at least one heat exchange channel to the outlet manifold, mixing and jetting the two-phase refrigerant in each contraction chamber to increase the velocity of the refrigerant, and passing the refrigerant out an exit of the outlet manifold.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuing application of U.S. patent application Ser. No. 10/987,961, filed Nov. 12, 2004, entitled “Parallel Flow Evaporator with Shaped Manifolds,” which application is incorporated herein in its entirety by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    This invention generally relates to air conditioning and refrigeration systems and, more particularly, to parallel flow evaporators thereof. 
         [0003]    A definition of a so-called parallel flow heat exchanger is widely used in the air conditioning and refrigeration industry now 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. 
         [0004]    Refrigerant maldistribution in refrigerant system evaporators is a well-known phenomenon. 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 evaporator 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. 
         [0005]    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, structural rigidity and enhanced resistance to corrosion. Parallel flow heat exchangers are now utilized in both condenser and evaporator applications for multiple products and system designs and configurations. The evaporator applications, although promising greater benefits and rewards, are more challenging and problematic. Refrigerant maldistribution is one of the primary concerns and obstacles for the implementation of this technology in the evaporator applications. 
         [0006]    As known, 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, gravity and turbulence 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 the parallel flow evaporators, have failed. 
         [0007]    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. 
         [0008]    If the two-phase flow enters the inlet manifold at a relatively high velocity, the liquid phase (i.e. droplets of liquid) is carried by the momentum of the flow further away from the manifold entrance to the remote portion of the header. Hence, the channels closest to the manifold entrance receive predominantly the vapor phase and the channels remote from the manifold entrance receive mostly the liquid phase. If, on the other hand, the velocity of the two-phase flow entering the manifold is low, there is not enough momentum to carry the liquid phase along the header. As a result, the liquid phase enters the channels closest to the inlet and the vapor phase proceeds to the most remote ones. Also, the liquid and vapor phases in the inlet manifold can be separated by the gravity forces, causing similar maldistribution consequences. In either case, maldistribution phenomenon quickly surfaces and manifests itself in evaporator and overall system performance degradation. 
       SUMMARY OF THE INVENTION 
       [0009]    Briefly, in accordance with one aspect of the invention, a method is provided for mitigating two-phase refrigerant maldistribution in heat exchange channels of a parallel flow evaporator. An inlet manifold of the evaporator includes a first stage, at least one intermediate stage, and a final stage. Each stage includes an expansion chamber, a contraction chamber, and at least one heat exchange channel interconnecting the stage to an outlet manifold of the evaporator. The method includes throttling the two-phase refrigerant in the expansion chamber of each stage, flowing a portion of the throttled refrigerant through the at least one heat exchange channel to the outlet manifold, mixing and jetting the two-phase refrigerant in each contraction chamber to increase the velocity of the refrigerant, and passing the refrigerant out an exit of the outlet manifold. 
         [0010]    By another aspect of the invention, the outlet manifold of the evaporator includes a first stage, at least one intermediate stage, and a final stage. Each stage includes an expansion chamber and a contraction chamber, and the heat exchange channels interconnect the stages in the inlet manifold to the stages in the outlet manifold. 
         [0011]    By yet another aspect of the invention, in a parallel flow evaporator comprising an inlet manifold, an outlet manifold, and a plurality of channels fluidly connecting the inlet manifold to the outlet manifold, a method is provided for mitigating two-phase refrigerant maldistribution in the channels. The method includes the steps of partially evaporating the two-phase refrigerant through a repetitive series of stages in the inlet manifold. Each stage comprises an expansion chamber, a contraction chamber, and at least one of the channels. The method further includes the step of balancing hydraulic resistances between each stage and the associated channel. 
         [0012]    By still another aspect of the invention, balancing resistances includes configuring the hydraulic resistance of the contraction chamber to be at least one and a half times lower than the hydraulic resistance of the associated channel. 
         [0013]    By still another aspect of the invention, the expansion chambers of the repetitive series of stages in the inlet manifold include progressively smaller cross-sectional areas defining a cross-sectional area reduction ratio. 
         [0014]    By still another aspect of the invention, the method for mitigating two-phase refrigerant maldistribution in heat exchange channels of a parallel flow evaporator further includes the step of partially evaporating the refrigerant through a repetitive series of stages in the outlet manifold. The stages comprise an expansion chamber, a contraction chamber, and at least one of the channels. 
         [0015]    By still another aspect of the invention, the expansion chambers of the repetitive series of stages in the outlet manifold comprise progressively larger cross-sectional areas. 
         [0016]    By still another aspect of the invention, the progressively larger cross-sectional areas of the expansion chambers in the outlet manifold are proportional to the progressively smaller cross-sectional areas of the expansion chambers in the inlet manifold. 
         [0017]    In the drawings as hereinafter described, preferred and alternate embodiments are depicted; however, various other modifications and alternate designs and constructions can be made thereto without departing from the true spirit and scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  is a schematic illustration of a parallel flow heat exchanger in accordance with the prior art. 
           [0019]      FIG. 2  is a schematic illustration of one embodiment of the present invention. 
           [0020]      FIG. 3  is a schematic illustration of an alternative embodiment of the present invention. 
           [0021]      FIG. 4  is a schematic illustration of yet another embodiment of the present invention. 
           [0022]      FIG. 5  is a schematic illustration of still another embodiment of the present invention. 
           [0023]      FIG. 6  is a schematic illustration of still another embodiment of the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0024]    Referring now to  FIG. 1 , a parallel flow heat exchanger is shown to include an inlet header or manifold  11 , an outlet header or manifold  12  and a plurality of parallel channels  13  fluidly connecting the inlet manifold  11  to the outlet manifold  12 . Generally, the inlet and outlet manifolds  11  and  12  are cylindrical in shape, and the channels  13  are usually tubes (or extrusions) of flattened or round shape. Channels  13  normally have a plurality of internal and external heat transfer enhancement elements, such as fins. For instance, external fins, disposed therebetween for the enhancement of the heat exchange process and structural rigidity are typically furnace-brazed. Channels  13  may have internal heat transfer enhancements and structural elements as well. 
         [0025]    In operation, two-phase refrigerant flows into the inlet opening  14  and into the internal cavity  16  of the inlet header  11 . From the internal cavity  16 , the refrigerant, in the form of a liquid, a vapor or a mixture of liquid and vapor (the latter is a typical scenario) enters the tube openings  17  to pass through the channels  13  to the internal cavity  18  of the outlet header  12 . From there, the refrigerant, which is now usually in the form of a vapor, passes out the outlet opening  19  and then to the compressor (not shown). 
         [0026]    As discussed hereinabove, it is desirable that the two-phase refrigerant passing from the inlet header  11  to the individual channels  13  do so in a uniform manner (or in other words, with equal vapor quality) such that the full heat exchange benefit of the individual channels can be obtained and flooding conditions are not created and observed at the compressor suction (this may damage the compressor). However, because of various phenomena as discussed hereinabove, a non-uniform flow of refrigerant to the individual channels  13  (so-called maldistribution) occurs. In order to address this problem, the applicants have introduced design features that will create a mixing and jetting effects in the two-phase refrigerant flow in the inlet manifold  11  to thereby bring about a more uniform homogeneous flow into to the channels  13 . 
         [0027]    Referring to  FIG. 2 , the heat exchanger is formed with a conventional outlet manifold  12  and channels  13 , but with a differently shaped inlet manifold  21 , as shown. Rather than being cylindrical in the usual manner, the inlet manifold  21  is hour-glass shaped (i.e. with a plurality of alternating expansion and contraction chambers). For simplicity, the inlet manifold  21  is shown to include three expansion chambers  22 ,  23  and  24  with interconnecting contraction chambers  26  and  27 . Each of the expansion chambers  22 ,  23  and  24  is preferably interconnected to an associated channel  13 , as shown. In actual practice, a larger number of expansion chambers and associated channels  13  would be provided. Further, each of the expansion chambers may be connected to more than one channel  13 . It is preferred, however, that none of the channels  13  would be connected directly to a contraction chamber. 
         [0028]    In operation, the two-phase refrigerant enters the inlet opening  28  and enters the first expansion chamber  22  where it is partially expanded with a portion thereof entering the associated channel(s). The remaining two-phase refrigerant is then forced through the contraction chamber  26  such that when it enters the expansion chamber  23  in more homogeneous manner due to increased velocity, partial evaporation (or throttling) occurs, thereby presenting a homogeneous condition for the refrigerant mixture flowing to the associated channel(s) and to the downstream channels. The remaining refrigerant then passes through the contraction chamber  27 , where more mixing and jetting of the two (liquid and vapor) refrigerant phases occurs and into the expansion chamber  24 , wherein, once again, a partial evaporation process is taking place, thereby presenting a homogenous mixture to the associated channel(s). In this way, the partial evaporation process is incrementally (i.e. progressively) maintained through the length of the inlet manifold  21 , so as to result in a more uniform distribution of refrigerant among the channels. 
         [0029]    Although the refrigerant flow in the inlet manifold  21  is progressively diminishing, it is essential not to introduce excessive flow impedance in the inlet manifold  21  relative to other flow resistances in the heat exchanger. Thus, the cross-section areas of the contraction chambers  26  and  27  must be properly sized for a particular application and for a particular configuration of the heat exchanger to maintain the balance between the desired partial evaporation process and undesired additional hydraulic resistance for the refrigerant flowing to the downstream channels. Generally, flow impedance of the contraction chamber should be at least one and a half times lower than the hydraulic resistance of the associated channels  13 . It is also desirable to balance the impedance of the contraction chambers in the inlet manifold  21  with corresponding pressure drops in the outlet manifold as will be further described hereinafter. 
         [0030]    An alternative embodiment of the present invention is shown in  FIG. 3 , wherein the inlet manifold  31  is again hour-glass shaped but with expansions chambers  32 ,  33  and  34  being progressively smaller in a cross section to accommodate the reduced refrigerant flow, as it moves from the inlet  38  toward the downstream end thereof. Further, the contraction chambers  36  and  37  are also preferably formed of a progressive smaller size for the same reasons. Generally, the cross-section area reduction ratio of the expansion chambers is proportional to the refrigerant flow rate ratio reduction entering and leaving the chamber. If this ratio is not uniform, the average value should be used instead for the estimates. The contraction chambers can be sized by employing an identical procedure and values. 
         [0031]    In  FIG. 4 , a further embodiment is shown wherein the inlet manifold  21  is identical to that as described in respect to  FIG. 2  or that shown in  FIG. 3 , but the outlet manifold  41  also being hour-glass shaped with expansion chambers  42 ,  43  and  44  and contraction chambers  46  and  47  alternately disposed as shown. These expansion and contraction chambers are not necessarily and most likely will not be of the same sizes as those of the inlet manifold  21 , since the refrigerant flowing within the outlet manifold  41  in a completely different thermodynamic state. Although if the chamber of the inlet manifold  21  are progressively smaller in size toward the downstream end thereof, the chambers of the outlet manifold  41  should preferably be progressively larger toward the downstream end thereof, as shown in  FIG. 5 , and identical aspect ratio can be utilized in sizing the outlet manifold chambers. In this way, the impedances that are presented in the inlet manifold  21  are matched by those in the outlet manifold  41  such that, the most favorable conditions for the uniform refrigerant flow distribution among the parallel channels  13  are created throughout the heat exchanger, enhancing the system performance and improving compressor reliability, by preventing flooded conditions at the compressor suction. 
         [0032]    An alternative embodiment of the present invention is shown in  FIG. 6 , wherein the inlet manifold  51  is again hour-glass shaped with the expansions chambers  52 ,  53  and  54  and the contraction chambers  55  and  56  disposed in between the expansion chambers. Additionally, refrigerant-mixing inserts  57  are placed within the contraction chambers  55  and  56  to promote mixing and even more homogeneous conditions at the entrance of the adjacent downstream expansion chambers. Although inserts  57  can be spiral in shape or have internal fins or indentations, any other configurations promoting mixing are also acceptable. In all other aspects this embodiment is similar to the embodiments discussed above. 
         [0033]    In has to be understood that the expansion and contraction chambers may be of any shape, cross-section area and configuration as long as a repetitive process of partial evaporation is created and a proper balance of hydraulic resistances is maintained. 
         [0034]    Furthermore, it should be noted that both vertical and horizontal channel orientations will benefit from the teaching of the present invention, although higher benefits will be obtained for the latter configuration. Also, although the teachings of this invention are particularly advantageous for the evaporator applications, refrigerant system condensers may benefit from them as well. 
         [0035]    While the present invention has been particularly shown and described with reference to preferred and alternate embodiments as illustrated in the drawings, it will be understood by one skilled in the art that various changes in detail may be effected therein without departing from the true spirit and scope of the invention as defined by the claims.