Abstract:
A parallel flow (minichannel or microchannel) evaporator includes channels which are crimped at or adjacent to their entrance location which provides for a refrigerant expansion and pressure drop control resulting in the elimination of refrigerant maldistribution in the evaporator and prevention of potential compressor flooding. Progressive crimping to counter-balance factors effecting refrigerant distribution is also disclosed.

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. 60/649,383, filed Feb. 2, 2005, and entitled PARALLEL FLOW EVAPORATOR WITH CRIMPED CHANNEL ENTRANCE, which application is incorporated herein in its entirety by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    This invention relates generally to air conditioning, heat pump 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 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 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. 
         [0005]    In recent years, parallel flow heat exchangers, and furnace-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. 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 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. 
         [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 (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. 
         [0009]    Moreover, maldistribution phenomenon may cause the two-phase (zero superheat) conditions at the exit of some channels, promoting potential flooding at the compressor suction that may quickly translate into the compressor damage. 
       SUMMARY OF THE INVENTION 
       [0010]    It is therefore an object of the present invention to provide for a system and method which overcomes the problems of the prior art described above. 
         [0011]    The objective of the invention is to introduce a pressure drop control for the parallel flow evaporator that will essentially equalize pressure drop through the heat exchanger channels and therefore eliminate refrigerant maldistribution and the problems associated with it. Further, it is the objective of the present invention to provide refrigerant expansion at the entrance of each channel, thus eliminating a predominantly two-phase flow in the inlet manifold and preventing phase separation, which is one of the main causes for refrigerant maldistribution. 
         [0012]    In accordance with the present invention, each of the channels is crimped at or adjacent to their entrance location such that a desired restriction for each of the channels is provided. The restriction size may be varied from channel to channel, if desired, in order to accommodate other non-uniform factors (such as different heat transfer rates) affecting the maldistribution phenomenon. The channels may be crimped at the very end/entrance or some distance away from the entrance in order not to interfere with the brazing joint to the inlet manifold. Additionally, internal rigidity (and/or heat transfer enhancement) fins can be simply compressed during crimping process or machined down prior to crimping. Furthermore, these restrictions can be used as primary (and the only) expansion devices for low-cost applications or as secondary expansion devices, in case precise superheat control is required, and another fixed area restriction device (such as a capillary tube or an orifice) or a thermostatic expansion valve (TXV) or an electronic expansion valve (EXV) is employed as a primary expansion device. Also, the precision of crimping doesn&#39;t have to be of extremely high tolerance in a latter case. 
         [0013]    In both cases outlined above, but especially if the crimping restrictions are provided as primary expansion devices at the entrance of each channel of the parallel flow evaporator, they represent a major resistance to the refrigerant flow within the evaporator. In such circumstances, the main pressure drop region will be across these restrictions and the variations in the pressure drop in the channels or in the manifolds of the parallel flow evaporators will play a minor (insignificant) role. Further, since refrigerant expansion is taking place at the entrance of each channel, a predominantly single-phase liquid refrigerant is flown through the inlet manifold and no phase separation occurs prior to entering individual evaporator channels. Hence, uniform refrigerant distribution is achieved, evaporator and system performance is enhanced, flooding conditions at the compressor suction are avoided and, at the same time, precise superheat control is not lost (whenever required). Furthermore, low extra cost for the proposed method makes this invention very attractive. 
         [0014]    Any suitable means of crimping may be employed such as a crimping tool in the form of pliers having the desired crimping face geometry or the use of stamping die having the desired geometry. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0015]    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: 
           [0016]      FIG. 1  is a schematic illustration of a parallel flow heat exchanger in accordance with the prior art. 
           [0017]      FIG. 2  is an enlarged partial side sectional view of a parallel flow heat exchanger illustrating one embodiment of the present invention. 
           [0018]      FIG. 3   a  is a view of  FIG. 2  illustrating a second embodiment of the present invention. 
           [0019]      FIG. 3   b  is a view of  FIG. 2  illustrating a third embodiment of the present invention. 
           [0020]      FIG. 3   c  is a view of  FIG. 2  illustrating a fourth embodiment of the present invention. 
           [0021]      FIG. 3   d  is a view of  FIG. 2  illustrating a fifth embodiment of the present invention. 
           [0022]      FIG. 4  is an end view of an uncrimped channel. 
           [0023]      FIG. 5  is a view of  FIG. 4  after crimping to a predetermined configuration. 
           [0024]      FIG. 6  is a view of  FIG. 4  after crimping to a second configuration. 
           [0025]      FIG. 7  is an end view of a second uncrimped channel. 
           [0026]      FIG. 8  is a view of  FIG. 7  after crimping to a predetermined configuration. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0027]    Referring now to  FIG. 1 , a parallel flow (minichannel or microchannel) heat exchanger  10  is shown which includes an inlet header or manifold  12 , an outlet header or manifold  14  and a plurality of parallel disposed channels  16  fluidly interconnecting the inlet manifold  12  to the outlet manifold  14 . Typically, the inlet and outlet headers  12  and  14  are cylindrical in shape, and the channels  16  are tubes (or extrusions) of flattened or round cross-section. Channels  16  normally have a plurality of internal and external heat transfer enhancement elements, such as fins. For instance, external fins  18 , uniformly disposed therebetween for the enhancement of the heat exchange process and structural rigidity are typically furnace-brazed. Channels  16  may have internal heat transfer enhancements and structural elements as well (See  FIGS. 4-6 ). 
         [0028]    In operation, refrigerant flows into the inlet opening  20  and into the internal cavity  22  of the inlet header  12 . From the internal cavity  22 , the refrigerant, in the form of a liquid, a vapor or a mixture of liquid and vapor (the most typical scenario in the case of an evaporator with an expansion device located upstream) enters the channel openings  24  to pass through the channels  16  to the internal cavity  26  of the outlet header  14 . From there, the refrigerant, which is now usually in the form of a vapor, in the case of evaporator applications, flows out of the outlet opening  28  and then to the compressor (not shown). Externally to the channels  16 , air is circulated preferably uniformly over the channels  16  and associated fins  18  by an air-moving device, such as fan (not shown), so that heat transfer interaction occurs between the air flowing outside the channels and refrigerant within the channels. 
         [0029]    According to one embodiment of the invention, as illustrated in  FIG. 2 , the channels  16  have been crimped at least at the entrance end  30  to provide for a restriction in each channel and to assure refrigerant expansion directly at each channel entrance which results in a pressure drop across the restriction and reduction and/or elimination of phase separation and refrigerant maldistribution in the system. 
         [0030]    In a second embodiment of the invention, as illustrated in  FIG. 3   a , the channels are crimped at the very end  32  and at a point  34 , some distance away from the end and the attachment point to the manifold  12 . 
         [0031]    In a third embodiment, as illustrated in  FIG. 3   b , the channels are crimped at a single location  36 , a predetermined distance from the channel end and, once again, away form the attachment point to the manifold  12 , in order not to interfere with the attachment process. 
         [0032]    In a fourth embodiment, as illustrated in  FIG. 3   c , the channels are crimped for a predetermined length or distance “L” near the channel ends but with less cross-section area alteration/reduction than in  FIGS. 2 ,  3   a  and  3   b.    
         [0033]    In a fifth embodiment of the invention, as illustrated in  FIG. 3   d , the channels are crimped at multiple locations  38 ,  40  and  42  near the channel ends, forming a passage of alternating contractions and expansions, but, once again, with less cross-section area alteration/reduction than in  FIGS. 2 ,  3   a  and  3   b.    
         [0034]      FIG. 4  illustrates a cross section of an uncrimped channel  50  having flattened shape and integral vertical support members  52 . 
         [0035]      FIG. 5  illustrates channel  50  crimped to a predetermined configuration  60  which would be suitable for use in the present invention. In this case, crimping occurs around support members  52  and leaves them unaltered. 
         [0036]      FIG. 6  illustrates channel  50  crimped to a more flattened configuration  70  which would also be suitable for use in the present invention. In this case, crimping occurs uniformly and alters support members  52  to a different shape and cross-section  72 . Obviously, different support members can be utilized within the scope of the present invention to divide channels  16  internally into multiple refrigerant passes of triangular, trapezoidal, circular or any other suitable cross-section. In all these cases, support members can be altered during the crimping process or left unchanged. 
         [0037]      FIG. 7  illustrates a cross section of an uncrimped channel  80  of a flattened shape (no internal support members are present in this design configuration). 
         [0038]      FIG. 8  illustrates channel  80  crimped to a more flattened configuration  90  suitable for use in the present invention. 
         [0039]    Also, it has to be noted that crimping doesn&#39;t have to be uniform throughout all the channels but instead can progressively change from one channel to another or from one channel section to another, for instance, to counter-balance other factors effecting refrigerant maldistribution. 
         [0040]    Further, it has to be noted that the crimping can be used in the condenser and evaporator applications at the channel entrance within intermediate manifolds as well. For instance, if a heat exchanger has more than one refrigerant pass, an intermediate manifold (between inlet and outlet manifolds) is incorporated in the heat exchanger design. In the intermediate manifold, refrigerant is typically flown in a two-phase state, and such heat exchanger configurations can similarly benefit from the present invention by incorporating channel crimping at the entrance ends directly communicating with intermediate manifolds. Further, the crimping can be done at the exit end of the channels  16  or at some intermediate location along the channel length providing only hydraulic resistance uniformity and pressure drop control and with less effect on overall heat exchanger performance. 
         [0041]    Since, for particular applications, the various factors that cause the maldistribution of refrigerant to the channels are generally known at the design stage, the inventors have found it feasible to introduce the design features that will counter-balance them in order to eliminate the detrimental effects on the evaporator and overall system performance as well as potential compressor flooding and damage. For instance, in many cases it is generally known whether the refrigerant flows into the inlet manifold at a high or low velocity and how the maldistribution phenomenon is affected by the velocity values. A person of ordinarily skill in the art will recognize how to apply the teachings of this invention to other system characteristics. 
         [0042]    While the present invention has been particularly shown and described with reference to the preferred 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.