Abstract:
In a parallel flow heat exchanger, which is susceptible to having a non-uniform distribution of a two-phase refrigerant flow to the individual channels, the resultant differences in the refrigerant flow therethrough are compensated and counter-balanced by a corresponding difference in the external heat transfer rate for the respective channels. In one embodiment, these differences are accomplished by variable characteristics of extended heat transfer surface elements such as fin type, fin density, fin geometry and difference in construction materials, and in another embodiment, by varying the airflow distribution over the cross-section area of the heat exchanger.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention relates generally to air conditioning and refrigeration systems and, more particularly, to parallel flow evaporators thereof.  
         [0002]     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.  
         [0003]     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.  
         [0004]     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. 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.  
         [0005]     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 parallel flow evaporators, have failed.  
         [0006]     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.  
         [0007]     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.  
         [0008]     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  
       [0009]     Briefly, in accordance with one aspect of the invention, the uneven distribution of refrigerant to the individual channels from the inlet manifold is overcome and compensated by providing non-uniform external heat transfer characteristics associated with the individual channels, such that the detrimental effects of refrigerant maldistribution are counter-balanced, their effect on the heat exchanger performance is minimized and potential flooding conditions at the evaporator exit are avoided.  
         [0010]     In accordance with another aspect of the invention, the external heat transfer surface parameters such as a number, and/or type and/or size of the fins are varied among the individual channels, which will result in a variable heat transfer rate for the individual channels in such a manner as to counter-balance and compensate the refrigerant maldistribution that would otherwise manifest itself in a variety of applications.  
         [0011]     By yet another aspect of the invention, the airflow rate over the individual channels is selectively made variable such that the variable heat transfer rate is once again obtained to offset the refrigerant maldistribution that would otherwise occur in many applications.  
         [0012]     In the drawings as hereinafter described, preferred and alternate embodiments are depicted; however, various other modifications and alternate constructions can be made thereto without departing from the true spirit and scope of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1  is a schematic illustration of a parallel flow heat exchanger in accordance with the prior art.  
         [0014]      FIGS. 2A and 2B  are illustrations of the design features in accordance with one embodiment of the invention.  
         [0015]      FIGS. 3A and 3B  show the design features in accordance with another embodiment of the present invention.  
         [0016]      FIGS. 4A and 4B  show the design features in accordance with another embodiment of the invention.  
         [0017]      FIG. 5  shows the features in accordance with another embodiment of the invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0018]     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 disposed channels  13  fluidly interconnecting the inlet manifold  11  to the outlet manifold  12 . Generally, the inlet and outlet headers  11  and  12  are cylindrical in shape, and the channels  13  are 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  15 , uniformly 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.  
         [0019]     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 most 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). Externally to the channels  13 , air is circulated uniformly over the channels  13  and associated fins  15  by an air-moving device, such as fan  20 , so that heat transfer interaction occurs between the air flowing outside the channels and refrigerant in the channels.  
         [0020]     Since, for a particular application, 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, for a particular application 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. Although, for illustrative purposes only, the present invention will be described with respect to this particular parameter, a person of ordinarily skill in the art will recognize how to apply the teachings of this invention to other system characteristics.  
         [0021]     In  FIG. 2A , it is seen that the refrigerant flow in the inlet manifold  11  is at a relatively high velocity such that the liquid droplets  21  tend to proceed to the downstream end  22  of the inlet manifold  11 . For that reason, unless the design changes are made, the downstream channels  13  will receive more of the liquid refrigerant and the upstream channels will receive more of the refrigerant vapor to thereby result in an unbalanced and inefficient heat exchanger performance as well as potentially flooding conditions at the evaporator exit, since there may be not enough heat transfer potential to evaporate all the liquid refrigerant in the downstream channels.  
         [0022]     Just oppositely, as shown in  FIG. 2 B , when the refrigerant flow into the in the inlet manifold  11  is at a relatively low velocity, the liquid droplets  21  will tend to remain in the upstream end  23  of the inlet manifold  11  and proceed into the nearby channels, and the downstream channels will tend to receive more vapor. Again, a decreased evaporator performance and flooding will be the likely outcomes.  
         [0023]     In addressing the abovedescribed phenomenon, which exists within the internal confines of the inlet manifold  11  and channels  13 , the inventors have found it feasible to modify the design features of the extended external surfaces of the channels  13  in order to counter-balance the non-uniform conditions within the channels  13 . This can be accomplished in a number of ways, some of which will be described in detail hereinafter.  
         [0024]     Since the pressure drop through all of the parallel paths in the evaporator is substantially equal, the channels flowing predominantly liquid refrigerant (which is at substantially higher density than vapor) receive higher refrigerant flow than the channels flowing vapor refrigerant (assuming equal external heat transfer rate for all the channels) and, as a result of such flow unbalance, performance degradation and possibly flooding conditions occur in the channels, reducing overall system performance and raising reliability concerns for the components such as a compressor.  
         [0025]     One approach to solving the maldistribution problem is that of providing a higher external heat transfer rate (reducing external thermal resistance) by incorporating a higher density of fins, more efficient fin type (e.g. louvered fin) or altering other fin characteristics, such as fin material or height (this will reduce the distance between the channels  13  accordingly) for the channels having the higher refrigerant flow. The precautions have to be made to make sure that airflow over these channels is not appreciably altered, that may diminish the desired effect. That is, in the high velocity refrigerant flow example of  FIG. 2A , for instance the density of the fins  23  associated with the downstream channels is greater than the density of the fins  24  associated with the upstream channels. Although, only two channels are shown, it is understood that there will be a plurality of channels therebetween with different fin densities such that the fin densities increase as the channels proceed toward the downstream end  22 . Furthermore, to reduce manufacturing cost or for the heat exchangers with a sufficient number of channels  13 , the adjacent channels can be combined in sections of an identical fin density, with the fin density increasing from one section to another in the direction of the downstream end  22  of the inlet manifold  11 . In this case, each section is represented by an individual channel  13  in  FIG. 2A .  
         [0026]     Similarly, in the example wherein the refrigerant flow velocity in the inlet manifold  11  is low, as shown in  FIG. 2B , the density of the fins  26  toward the downstream end  22  of the inlet header  11  is less than the density of the fins  27  toward the upstream end of the header  11 .  
         [0027]     In operation, for those channels having a higher density of fins, the heat transfer capability will be enhanced over those having the lower density of fins, such that the refrigerant in those channels evaporates at a higher rate generating more low-density vapor, and the superheat conditions at the channel exit are assured. Consequently, the pressure drop through the channels increases, redirecting the imbalance of the refrigerant flow to the other channels and reducing maldistribution.  
         [0028]     As it was mentioned above, rather than varying the density of the fins across the plurality of channels, the same effect can be achieved by incorporating an enhanced (e.g. louvered) fins, changing the size of the fins, altering fin thickness or providing material differences, in order to selectively vary the heat transfer rate across the channels. Also, internal elements augmenting the heat transfer rate can be applied in a similar manner to achieve similar results. Once again, these design alterations shouldn&#39;t change airflow distribution across the channels, which may diminish the desired outcome.  
         [0029]     Another approach to varying the heat transfer rate across the channels is to vary the flow of air over the respective channels such that those channels having the higher refrigerant flow (i.e. those having more liquid droplets and less vapor) have more air flowing over their outer surfaces than those channels having the lower refrigerant flow (i.e. those having more vapor and less liquid droplets).  
         [0030]     An air-moving device, such as a fan, provides airflow over the external evaporator surfaces to transfer heat from air to refrigerant. Generally, an effort is made to assure that the airflow is uniform over the cross-section area of the heat exchanger. Unfortunately, for some evaporator section constructions, it becomes a difficult task. As a result, different heat transfer rates for different channels result in the same maldistribution phenomenon and flooded conditions as the ones associated with the inlet manifold and discussed above. One embodiment of this invention proposes to utilize a naturally non-uniform airflow or by simple means alter airflow to be non-uniform, in order to counter-balance the maldistribution phenomenon associated with the inlet manifold.  
         [0031]     In  FIG. 3A  a fan within a scroll housing  28  is shown as directing the air, as indicated by the arrows, toward the heat exchanger  41  such that the air flows across the channels  13 . Assuming that the application is for a high velocity refrigerant flow into the inlet manifold, those channels more remote from the inlet  14  will have greater refrigerant flow therethrough. In order to counter-balance this phenomenon, it is therefore desirable to have higher airflow over those channels. This will occur with the arrangement as shown, since, as indicated by the arrows, the airflow associated with the channels adjacent to the downstream end  22  remote from the manifold inlet  14  has, simply saying, lower turning losses than the airflow associated with the channels adjacent to the upstream end near the manifold inlet  14 . Thus, the superior external heat transfer rate will be provided to the downstream channels than to the channels near the opening  14 , as desired. Obviously enough, a sufficient distance is to be provided between the scroll housing  28  and the heat exchanger  41  to obtain the desired results.  
         [0032]     In a similar manner, the  FIG. 3B  illustrates the opposite treatment for an application wherein the refrigerant velocity to the inlet manifold is relatively low. Here, the fan scroll is mounted in an opposite orientation such that the greater heat transfer rate will occur at those channels nearer the opening  14  and lower heat transfer rate will occur at the more remote channels at the downstream end  22  remote from the manifold inlet  14 .  
         [0033]      FIGS. 4A and 4B  embodiments show similar arrangements but include a bank of louvers  29 , which can be selectively positioned in an uniform manner so as to tune to the particular airflow pattern that will bring about the results as desired for a variety of operating conditions. In this case, a conventional fan scroll  28  can be designed and positioned using standard configuration and location, and the airflow distribution over the individual channels is controlled by the louvers  29 .  
         [0034]     In  FIG. 5 , an additional feature is added wherein the bank of louvers  31  are variably angled from one end to the other. Thus, as shown, the louvers nearest the channels associated with the downstream end  22  remote from the manifold inlet  14  provides little or no resistance whereas the louvers adjacent to the channels associated with the upstream end of the opening  14  are turned at a greater angle and therefore act to restrict airflow and reduce the amount of heat transfer that occurs at the channels nearest to the opening  14 .  
         [0035]     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.  
         [0036]     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.