Abstract:
A parallel flow (minichannel or microchannel) evaporator includes a porous member inserted at the entrance of the evaporator channels which provides refrigerant expansion and pressure drop controls resulting in the elimination of refrigerant maldistribution and prevention of potential compressor flooding.

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. 60/649,425, filed Feb. 2, 2005, and entitled PARALLEL FLOW EVAPORATOR INCORPORATING POROUS CHANNEL INSERTS, 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 overcome the problems of the prior art described above. 
         [0011]    The objective of the present invention is to introduce a pressure drop control for the parallel flow (microchannel or minichannel) evaporator that will essentially equalize pressure drop through the heat exchanger circuits 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, which is one of the main causes for refrigerant maldistribution. It has been found that the introduction of a porous media inserted in each parallel flow evaporator channel, or at the entrance of each parallel flow evaporator channel, accomplishes these objectives. For instance, these porous media inserts can be brazed in each channel during furnace brazing of the entire heat exchanger, chemically bonded or mechanically fixed in place. Furthermore, these inserts 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 a thermostatic expansion valve (TXV) or an electronic expansion valve (EXV) is employed as a primary expansion device. 
         [0012]    Any suitable porous insert which accomplishes the above objectives may be used. Suitable and inexpensive porous inserts may be made of sintered metal, compressed metal, such as steel wool, specialty designed porous ceramics, etc. When inexpensive porous media insert is placed in each channel of the parallel flow evaporator, or at the entrance of each parallel flow evaporator channel, it represents a major resistance to the refrigerant flow within the evaporator. In such circumstances, the main pressure drop region will be across these inserts 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 to each channel, a predominantly single-phase liquid refrigerant is flown through the inlet manifold, especially in the case when the porous inserts are utilized as the primary and the only expansion devices. Hence, uniform refrigerant distribution is achieved, evaporator and system performance is enhanced 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. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0013]    For a farther 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: 
           [0014]      FIG. 1  is a schematic illustration of a parallel flow heat exchanger in accordance with the prior art. 
           [0015]      FIG. 2  is a partial side sectional view of one embodiment of the present invention. 
           [0016]      FIG. 3  is an end view of a porous insert positioned at the entrance to a channel of the present invention. 
           [0017]      FIG. 4  is a perspective view of the porous insert illustrated in  FIG. 3 . 
           [0018]      FIG. 5   a  is a side sectional view illustrating a further embodiment of the present invention. 
           [0019]      FIG. 5   b  is a side sectional view illustrating yet a further embodiment of the present invention. 
           [0020]      FIG. 6  is an end view of a plurality of channels in one embodiment of the invention. 
           [0021]      FIG. 7   a  is a perspective view which illustrates a porous cap embodiment of the invention. 
           [0022]      FIG. 7   b  is a perspective view which illustrates a second porous cap embodiment. 
           [0023]      FIG. 7   c  is a perspective view which illustrates a third porous cap embodiment. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0024]    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. 
         [0025]    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. 
         [0026]    According to one embodiment of the present invention, a porous insert  30  is inserted at the entrance of each channel  16 . When the channels  16  have internal structural elements such as support members  16   a  ( FIG. 3 ), usually included for structural rigidity and/or heat transfer enhancement purposes, the porous inserts  30  incorporate slots  32  to accommodate the support members  16   a  when in position at the channel entrance (See  FIG. 4 ). Further, in case a various degree of expansion and/or hydraulic impedance are desired to be provided by the inserts  30  or  32 , for instance, to counter-balance other abovementioned factors effecting refrigerant distribution amongst the channels  16 , characteristics such as porosity values or geometric dimensions (insert depth, insertion depth, etc.) of the inserts can be altered to achieved the desired result for each channel  16 . 
         [0027]      FIG. 5   a  illustrates another embodiment in which all the entrances to the channels  16  are covered by a single porous member  34  positioned within a manifold  40 . Further, a support member  36  may be used to assist in setting up a relative position of the porous member  34  and the channels  16  within the manifold  40 . It should be noted that an assembly of the porous member  34  and support member  36  can be manufactured from and combined in a single member made from porous material. 
         [0028]      FIG. 5   b  is a further embodiment of the structure of  FIG. 5   a  in which the porous member is a composite of two different porous materials  34  and  34   a . Obviously, a number of composite materials within the porous member can be more than two. 
         [0029]      FIG. 6  illustrates a side view of  FIG. 5   a.    
         [0030]      FIG. 7   a  illustrates a unitized elongated porous member  34   b  which seals multiple channels  16  at a predetermined distance from the channel entrance. 
         [0031]      FIG. 7   b  illustrates an elongated porous member  34   c  which caps the ends of multiple channels  16 . 
         [0032]      FIG. 7   c  a modification of the structure of  FIG. 7   b  in which the porous member  34   d  is accurate in shape and caps the ends of the channels  16 . The shape of the porous member  34   d  can be of any suitable configuration, rather than a rectangular in cross-section. Further, the porous member  34   d  is preferably positioned within the manifold  40  in such way that there is a gap between the inner wall of the manifold  40  and the porous member  34   a  allowing for more uniform refrigerant distribution prior to entering the porous member  34   d  and channels  16 . 
         [0033]    It should be understood that any type of porous member and/or material which accomplishes the objectives of the present invention may be used. Similarly, as illustrated by  FIGS. 2-7 , any design or configuration which accomplishes the objectives of the invention may be employed in the use of the present invention. 
         [0034]    Also, it has to be noted that the porous inserts can be used in the condenser and evaporator applications 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 in a two-phase state, and such heat exchanger configurations can similarly benefit from the present invention by incorporating the porous inserts into such intermediate manifolds. Further, the porous inserts can be placed into an inlet manifold of the condenser and an outlet manifold of the evaporator for providing only hydraulic resistance uniformity and pressure drop control and with less effect on overall heat exchanger performance. 
         [0035]    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. 
         [0036]    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.