Abstract:
A parallel flow heat exchanger includes a plurality of connector tubes which fluidly interconnect the individual flat heat exchange tubes to a refrigerant delivery member such that the refrigerant flows along the lengths of the connector tubes and then flows in a direction orthogonal thereto to enter the flat heat exchange tubes to thereby provide improved refrigerant distribution thereto. The refrigerant distribution member may be an inlet manifold or an entrance port or a refrigerant distributor. The connector tubes may be connected so as to conduct the flow in parallel or in series, and an orifice may be placed at the entrance end thereof to improve refrigerant distribution.

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, sometimes referred to as a flat tube 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 to flow in an orientation generally substantially perpendicular to the refrigerant flow direction in the inlet and outlet manifolds. 
         [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 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. 
         [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, good 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. 
         [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 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. 
         [0006]    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. 
         [0007]    While traditional round tube heat exchangers have a potential to feed each tube or circuit individually, flat tubes have not had such a capability and efforts to improve refrigerant distribution in such heat exchanger have included, for instance, the use of inserts and multiple inlet headers, all of which complicate the design and increase the manufacturing cost. Also, since large diameter headers are replaced with small diameter headers and connectors, operating pressures may be substantially elevated. 
       SUMMARY OF THE INVENTION 
       [0008]    Briefly, in accordance with one aspect of the invention, the individual flat heat exchange tubes of an evaporator are interconnected to a refrigerant delivery member by way of connector tubes such that the two phase refrigerant flows first from the refrigerant delivery member into the connector tubes and then into the individual flat heat exchange tubes to thereby obtain improved distribution of refrigerant flow. 
         [0009]    In accordance with another aspect of the invention the connector tubes are connected to a common inlet manifold and extend generally orthogonally therefrom. 
         [0010]    In accordance with another aspect of the invention, the connector tubes are cylindrical in shape, and the flat heat exchange tubes are inserted into longitudinal slots formed in the connector tubes to form tee joints. 
         [0011]    By yet another aspect of the invention, the connector tubes have orifices at their one end such that the refrigerant entering the connector tube is expanded in the process to thereby improve uniform refrigerant distribution. 
         [0012]    In accordance with another aspect of the invention, each of the connector tubes is fluidly connected directed to a traditional refrigerant distributor by way of an inlet tube. 
         [0013]    In the drawings as hereinafter described, preferred and alternative embodiments are depicted; however, various other modifications and alternate constructions can be made thereto without departing from the spirit and scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a schematic illustration of the present invention as incorporated into a parallel flow evaporator. 
           [0015]      FIG. 2  is a side view thereof. 
           [0016]      FIG. 3  is an end view thereof. 
           [0017]      FIG. 4  is an enlarged view of a portion thereof. 
           [0018]      FIG. 5  is a sectional view as seen along lines  5 - 5  of  FIG. 4 . 
           [0019]      FIGS. 6A and 6B  are respective front and top view of a tee connector. 
           [0020]      FIGS. 7A and 7B  are schematic illustrations of an alterative embodiment thereof. 
           [0021]      FIGS. 8 and 9  are schematic illustrations of another alternative embodiment thereof. 
           [0022]      FIG. 10  is a schematic illustration of another alternative embodiment thereof. 
           [0023]      FIG. 11  is a schematic illustration of another alternative embodiment thereof. 
           [0024]      FIG. 12  is a schematic illustration of another alternative embodiment thereof. 
           [0025]      FIGS. 13A and 13B  are schematic illustrations of another alternative embodiment thereof. 
           [0026]      FIGS. 14 and 15  are schematic illustrations of another alterative embodiment thereof. 
           [0027]      FIG. 16  is a schematic illustration of yet another embodiment thereof. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0028]    Referring to  FIGS. 1-3 , the invention is shown generally at  10  as incorporated into a parallel flow heat exchanger  11  which includes an inlet manifold  12 , a plurality of flat heat exchange tubes  13  and an outlet manifold  14 . 
         [0029]    Each of the flat heat exchange tubes  13  is fluidly connected to a respective connecting tube as shown at  16 ,  17 ,  18  and  19  which are, in turn, fluidly connected to the inlet manifold  12 . 
         [0030]    In operation, two-phase refrigerant flow enters an inlet port  21  of the inlet manifold  12  and flows toward both ends of the inner manifold  12 . It then flows to the individual connector tubes  16 ,  17 ,  18  and  19  and then to the respective flat heat exchange tubes  13 , after which it passes to the outlet manifold  14  and exits from the outlet port  22 . 
         [0031]    Such a design configuration allows for sufficiently small diameters of the inlet manifold  12  and connecting tubes  16 - 19 , which are favorable for refrigerant, distribution among the flat heat exchange tubes  13 . 
         [0032]    As is seen in  FIGS. 4 and 5 , the connector tubes  16 ,  17  and  18  are cylindrical in a cross-section and have linear slots  23 ,  24  and  26 , respectively formed therein for receiving the respective flat heat exchange tubes  13  therein. The degree of the penetration of the flat heat exchange tubes  13  into the respective connector tubes  16 ,  17  and  18  is a matter of a design choice and may be selected to have a significant penetration as shown, or they may have little or no penetration such that the ends of the heat exchange tubes  13  are substantially flush with the inner walls of the connector tubes. Alternatively, the flat heat exchange tubes  13  may have different penetration depths, which may be selected depending on the position of the inlet port  21  to provide substantially equal inlet refrigerant flow impedances among the heat exchange tubes  13 . The flat heat exchange tubes  13  are then fixed in their positions by a process such as welding, furnace brazing or the like. 
         [0033]    As is seen in  FIG. 5 , the flat heat exchange tubes  13  may include a plurality of spaced ports  27  of any suitable cross section and have an overall height of H and an overall width of W. One end  28  of each connector tube, e.g.  17 , is open and connected to the inlet manifold  12  as indicated above. The other end  29  can be sealed as shown in  FIG. 5 , or it may be interconnected to another connector tube as will be described hereinafter. 
         [0034]    As should be understood, the relative sizes of the flat heat exchange tubes  13  and their respective connector tubes  16 - 19  are such that the diameter of the connector tubes is sufficient to allow for the height of the slot  24  to accommodate the height H of the flat heat exchange tube. Similarly, the length of the connector tube, i.e. the distance between the two ends  28  and  29 , should be sufficient to accommodate the width W of the heat exchange tube  13 . 
         [0035]      FIGS. 4 and 5  show connectors  16 ,  17 , and  18  as tubes with a cylindrical cross-section. As should be understood, the connectors may have elliptical, square, rectangular, triangular, or of any other possible shape. Also, the shape of the cross-section and the area may be different along the centerline of the connectors. 
         [0036]      FIGS. 4 and 5  imply one connector per one flat heat exchange tube. As should be understood, a number of adjacent flat heat exchange tubes may be connected to one connector. In this case, multiple slots have to be made in the connectors to accommodate multiple flat heat exchange tubes. 
         [0037]    Further, it may be beneficial to have flat heat exchange tubes of different sizes. For instance, the height or the width of the flat heat exchange tube may be varied. The corresponding slot dimensions of the respective connectors then need to be adjusted accordingly to accept the flat heat exchange tube of different sizes. As one example, the parallel flow heat exchanger may include sections with flat heat exchange tubes of different width to accommodate substantially different airflow amounts passing over these sections. 
         [0038]      FIGS. 4 and 5  show connectors  16 ,  17 , and  18  as straight tubes. Such connectors are called two-end connectors. As should be understood, the connectors may be fabricated as triple-end connectors, particularly as a tee connector shown on  FIGS. 6A and 6B . The tee connector has a first side end  101 , a second side end  102 , and a central end  103 . As should be also understood, each end may have a plurality of ends. Such connectors are called multiple-end connectors. It is obvious that at least one end of the connectors must be active. All remaining ends, if there are any, are inactive and sealed. 
         [0039]      FIGS. 6A and 6B  show the ends  101 ,  102 , and  103  having their centerline in one plane and shaped as the letter T. As should be understood, each end of the two-end, triple-end, and multiple-end connectors may have any possible shape of their centerlines. 
         [0040]    Although the outlet header  14  has been shown as being directly connected to the flat tube channels  13 , it should be understood that connector tubes similar to the connector tubes  16 - 19  may be used to interconnect the flat heat exchange tubes  13  to the outlet manifold  14 . 
         [0041]    The embodiment as described above shows the individual connector tubes  16 - 19  (which are of the two-end connector type) being aligned in parallel arrangement and extending orthogonally from the inlet manifold  12 . It also shows them as being connected such that the flow of refrigerant therein is parallel. It should be understood that, the connector tubes  16 - 19  may be interconnected in serial flow relationship and may be further connected directly to the inlet port, without the need for an inlet manifold  12 . Such an embodiment is shown in  FIGS. 7A and 7B  wherein an elbow  28  interconnects the ends of connector tubes  16  and  17 , an elbow  32  interconnects the ends of connector tubes  17  and  18 , and an elbow  33  interconnects the ends of connector tubes  18  and  19  as shown. 
         [0042]    The refrigerant flow then enters the inlet port  34 , passes through the connector tube  16 , one flat heat exchange tube  13 , the elbow  31 , the connector tube  17 , another flat heat exchange tube  13 , the elbow  32 , the connector tube  18 , the elbow  33  and the connector tube  19 . Eventually, the refrigerant flows out of the outlet port  36 . 
         [0043]      FIGS. 7A and 7B  demonstrate a heat exchanger having tee connectors  16 ,  17 ,  18 , and  19  on one end of the heat transfer tubes  13  and tee-connectors  116 ,  117 ,  118  and  119  on the other end thereof. The connectors each have one active end and two inactive ends. Ultimately, any described connector type is applicable. 
         [0044]      FIGS. 8 and 9  show a heat exchanger having one circuit and four passes. As should be understood, any number of passes per circuit is possible, whatever is appropriate for a particular application. Also, it may be appropriate to have multiple circuits. 
         [0045]      FIG. 10  shows a heat exchanger having three equal parallel circuits. Each circuit has its own inlet port  34   a,    34   b,  and  34   c  and its own outlet port  36   a,    36   b,  and  36   c,  respectively. The refrigerant flow in the  FIG. 10  embodiment is generally downward, as it enters at the top and flows down to the bottom. However, it is possible to have a reversed generally upward (refrigerant enters at the bottom and flows up to the top) or a mixed flow arrangement. The heat exchanger design in  FIG. 10  provides two-end connectors, for the top circuit,  116 ,  16 ,  17 ,  117 ,  118 ,  18 ,  19 , and  119 , and each connector has one active end and one inactive end. 
         [0046]    The heat exchanger design in  FIG. 11  demonstrates a three-circuit, four-pass heat exchanger with tee connectors  116 ,  16 ,  17 ,  117 ,  118 ,  18 ,  19 , and  119 . Each tee connector has one active end and two inactive ends. 
         [0047]      FIGS. 10 and 11  demonstrate the embodiments having the same number of passes in each circuit. As should be understood, the number of passes for each circuit may be different. 
         [0048]    The heat exchangers described above may operate as condensers and evaporators. Usually, condensers have vapor at the inlet and liquid at the outlet. Due to the difference in densities of liquid and vapor phases, the condensers are typically more efficient if they have more inlets and fewer outlets.  FIG. 12  shows a three-circuit heat exchanger having three inlets  34   a,    34   b,  and  34   c;  one outlet  36 ; tee-connectors  116 ,  16 ,  17 ,  117 ,  118 ,  18 ,  119 ; and four-end connector  19  with two sealed side ends.  FIGS. 13A and 13B  show a similar heat exchanger where the four-end connector  19  has one sealed side end. 
         [0049]    The heat exchangers shown on  FIGS. 12 ,  13 A and  13 B may be applied as components of a heat pump system and operate as condensers and evaporators. Evaporators have a two-phase refrigerant at their inlet and typically vapor at the outlet. Due to the differences in densities of liquid and vapor phases, the evaporators may be more efficient if they have fewer inlets and more outlets. Since the operation as a condenser and the operation as an evaporator are reversed, with respect to the refrigerant flow direction, the embodiments in  FIGS. 12 ,  13 A and  13 B should have an appropriate number of inlets and outlets for both operational modes. 
         [0050]    Heat exchangers operating as evaporators should have means for distribution of the two-phase refrigerant. Another embodiment which is applicable for evaporators wherein an inlet manifold is not used is that shown in  FIGS. 14 and 15 , wherein a traditional distributor  40  is fluidly connected to the individual connector tubes  16 - 19  by way of small diameter distributor tubes  38 ,  39 ,  41  and  42  respectively. In this case, an expansion device (not shown) is provided upstream of the distributor  40  such that the two-phase refrigerant flow is passed from the distributor  40  to the individual small diameter distributor tubes  38 ,  39 ,  41  and  42 . The two-phase refrigerant flow then passes to the individual connector tubes  16 - 19  and is further distributed in the manner described hereinabove. 
         [0051]      FIGS. 14 and 15  imply that the number of distributor tubes corresponds to the number of flat heat exchange tubes. It should be understood that, in general, each circuit may have a number of passes with the number of distributor tubes corresponding to the number of circuits. Also, as before with the connector tubes, there is an option to use one distributor for several circuits. 
         [0052]    A variation of the  FIGS. 1-5  embodiment is shown in  FIG. 16  wherein, rather than having an open-end connection between the connector tube  17  and the inlet manifold  12 , as shown in  FIG. 5 , both ends  28  and  29  of the connector tube  19  are closed, and an orifice  42  is provided in the end  28  as shown. Thus, as the refrigerant passes from the inlet manifold  12  through the orifice  42 , expansion occurs so as to provide two-phase lower pressure and temperature refrigerant to the connector tube  19 . The flow of refrigerant from that point is the same as described hereinabove. It should be understood that the orifice  42  may have a plurality of orifices arranged in parallel and/or in series. 
         [0053]      FIG. 16  shows that the number of orifices  42  (or their pluralities) corresponds to the number of flat heat exchange tubes. It should be understood that, in general, each circuit may have several passes with the number of the orifices  42  (or their pluralities) corresponding to the number of circuits. Also, there is an option to use one orifice  42  (or its plurality) for several circuits. 
         [0054]    There are two possible designs. One configuration implies that the manifold  12  operates as a receiver, and the orifices  42  along the manifold  12  operate as expansion devices, providing isenthalpic expansion from a condenser pressure to the evaporator pressure. Another arrangement includes an expansion device attached to the manifold  12 . The expansion device provides isenthalpic expansion from the condenser pressure to a pressure which is higher than the evaporator pressure and lower than the condenser pressure. The orifices  42  function as a refrigerant distributor of the two-phase refrigerant providing single, double, or multiple expansions from the pressure downstream of the expansion device to the evaporator pressure. 
         [0055]    In addition to the advantages discussed hereinabove, the present design features allow for the use of substantially wider heat exchange tubes, reduced fin density and/or increased fin height, without comprising performance characteristics and cost of the heat exchanger. 
         [0056]    It should be understood that the present invention is intended for use with a heat exchanger that can be oriented horizontally, vertically, or inclined. That is, although the flat heat exchange tubes are shown as being horizontally oriented, the present invention would also be useful with vertically oriented and inclined flat heat exchange tubes. 
         [0057]    While certain preferred embodiments of the present invention have been disclosed in detail, it is to be understood that various modifications in its structure may be adopted without departing from the spirit of the invention or the scope of the following claims.