EP2257755A1

EP2257755A1 - Heat exchanger tube configuration for improved flow distribution

Info

Publication number: EP2257755A1
Application number: EP09716438A
Authority: EP
Inventors: Alexander Chen; Jules R. Munoz; Young K. Park; Parmesh Verma; Silivia Miglioli; Yirong Jiang
Original assignee: Carrier Corp
Current assignee: Carrier Corp
Priority date: 2008-03-07
Filing date: 2009-02-05
Publication date: 2010-12-08
Also published as: US20110132585A1; WO2009111129A1; EP2257755A4; CN101965496A

Abstract

A microchannel heat exchanger includes for each channel, a serpentine shaped tube for providing a plurality of parallel flow passes for successively conducting fluid flow therethrough, and being fluidly interconnected between an inlet and an outlet manifold. Multiple circuits are obtained by the individual serpentine shaped tubes. Various methods are provided for forming the serpentine shaped tubes.

Description

Heat Exchanger Tube Configuration for Improved Flow Distribution

Technical Field

[0001] This invention relates generally to air conditioning systems and, more particularly, to parallel flow heat exchangers.

Background of the Invention

[0002] 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 is particularly pronounced in parallel flow evaporators due to their specific design with respect to refrigerant routing. Attempts to eliminate/reduce the effects of this phenomenon on the performance of brazed aluminum heat exchangers have been made with little or no success. The primary reasons for such failures have generally been complexity/inefficiency or prohibitively high cost of the solution. [0003] In recent years, parallel flow heat exchangers have received much attention and interest, not just in the automotive industry but also in the heating, ventilation, air conditioning and refrigeration (HVAC&R) industry. The primary reasons for the employment of the parallel flow technology deals with 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/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 evaporator applications.

[0004] As known, refrigerant maldistribution in parallel flow heat exchangers occurs because of unequal pressure drops inside the mini-channels or microchannels as well as in the inlet and outlet manifolds. In the manifolds or headers, the difference in length of refrigerant paths, phase separation, gravity and turbulence are the primary factors responsible for maldistribution. Inside the heat exchanger mini-channels, variation in the heat transfer rate, airflow rate and gravity are the dominant factors. Because 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.

[0005] In the refrigerant systems utilizing parallel flow heat exchangers, the inlet and outlet headers 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 move independently, refrigerant maldistribution tends to occur.

[0006] The problems of unequal flow distribution are particularly evident in multi-pass mini-channel heat exchangers wherein the inlet and outlet headers are commonly divided into longitudinally spaced sections which are interconnected by straight tubes. One approach to solving these problems is shown and described in U.S. Patent No.: 7,143,605, wherein an inlet manifold includes an internally disposed distribution tube with a plurality of orifices formed therein. [0007] Serpentine, multiple pass heat exchangers are known in the art as shown by U.S. Patents 7,069,980; 4,962,811; 5,036,909; 6,705,386 and U.S. 2005/0217834 Al. Generally, they do not incorporate the feature of multiple circuits. U.S. Patent 5,036,909 does include multiple circuits but they are constructed to be in a nested, one inside the other, relationship. Such a design presents problems of inflexibility in design, manufacture and use. The present invention overcomes these problems.

Disclosure of the Invention

[0008] Briefly, in accordance with one aspect of the invention, the plurality of parallel mini-channels are serpentine in shape so as to thereby provide a plurality of parallel flow passes but which are connected to the inlet and outlet manifolds only at the respective inlet and outlet ends. In this way, the inlet manifold can be relatively short and be directly connected to fewer inlet ends of the microchannels for uniform flow distribution. Further, each circuit has all of its flow passes laterally spaced from all of the flow passes of the adjacent circuits. [0009] In accordance with another aspect of the invention, a method of promoting uniform refrigerant flow from an inlet manifold to a plurality of parallel mini-channels, including the steps of providing a flat tube shaped in a serpentine manner to form a plurality of flow passes for successively conducting fluid flow therethrough and fluidly connecting an end thereof to an inlet manifold and the other end thereof to an outlet manifold, with each circuit having all of its flow passes spaced laterally from all of the flow passes of the adjacent circuits. [0010] In the drawings as hereinafter described, preferred and modified 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

[0011] FIG. 1 is a schematic illustration of a multi-pass microchannel heat exchanger in accordance with the prior art.

[0012] FIG. 2 is a perspective view of single three pass parallel mini-channel member in accordance with the present invention.

[0013] FIG. 2A is a perspective view of a single four-pass parallel mini- channel member in accordance with the present invention. [0014] FIG. 3 is a perspective view of a single component thereof.

[0015] FIG. 3A is an alternative embodiment thereof.

[0016] FIG. 4 is an exploded view of components of another embodiment thereof.

[0017] FIG. 4A is an alterative embodiment thereof.

[0018] FIG. 5A is a schematic illustration of a heat exchanger in accordance with the prior art.

[0019] FIG. 5B is a schematic illustration of a heat exchanger in accordance with the present invention.

[0020] FIG. 6 is an alternative embodiment thereof.

[0021] FIG. 7 is yet another alternative embodiment thereof.

[0022] FIG. 8A, 8B and & 8C are schematic illustrations of various possible embodiments of the inlet manifold. Detailed Description of the Invention

[0023] A multi-pass mini-channel heat exchanger in accordance with the prior art is shown in Fig. 1 and includes a primary manifold 11 , a secondary manifold 12 and a plurality of mini-channel tubes 13 fluidly interconnected therebetween. The primary manifold 11 has dividers 14 and 16 provided therein to thereby form independent sections 17, 18 and 19 that are fluidly isolated from each other. The section 17 functions as an inlet manifold and the section 19 functions as an outlet manifold. Similarly, the secondary manifold 12 has a divider 21 which forms the sections 22 and 23 which are so mutually isolated. [0024] The heat exchanger as shown comprises a four pass, seven circuit configuration. That is, there are seven tubes in each of the four pass groupings 24, 26, 27 and 28. The tubes in the pass grouping 24 thus fluidly interconnects the section 17 of the primary manifold 11 to the section 22 of the secondary manifold 12, with the pass grouping 26 then fluidly interconnecting the section 22 to the section 18 of the primary manifold. Similarly, the pass grouping 27 fluidly interconnects the section 18 in the primary header 11 to the section 23 of the secondary manifold 12, and the pass grouping 28 fluidly interconnects the section 23 of the secondary manifold 12, to section 19 of the primary manifold 11. The refrigerant then flows through the assembly as indicated by the arrows. [0025] It should be understood that, with such a configuration, uniform distribution of refrigerant flow to the individual channels is very difficult to obtain. The primary reason is that the distribution to the seven tubes has to be made at the entrance of each of the pass groupings 24, 26, 27 and 28. During each pass transition, such as in section 22, two-phase mixture exiting pass grouping 24 will be allowed to mix, and will have the tendency to phase separate, leading to maldistribution to pass grouping 26. It should be pointed out that, as in the conventional configuration, the mini-channel tubes are spaced with fins in between. [0026] In Fig. 2 there is an illustration of a single parallel mini-channel tube that is applied to obtain a three pass heat exchanger. It comprises three planar portions 29, 31 and 32 and the two arcuate portions 33 and 34. The planar portions 29, 31 and 32 are arranged in parallel relationship, with the planar portions 29 and 31 being fluidly interconnected by the arcuate portion 33, and with the respective ends of the planar portions 31 and 32 being fluidly interconnected by the arcuate portion 34. An inlet end 36 is fluidly connected to an inlet manifold, and the outlet end 37 is fluidly connected to an outlet manifold. Thus, the refrigerant passes from the inlet manifold and through the entire three passes to the outlet manifold without requiring any redistribution of the refrigerant when entering the next pass. [0027] It should be understood that the flat tube structure as shown represents a single circuit in a three pass configuration, and a multi-circuit heat exchanger can be obtained by simply juxtaposing other identically shaped tubes in parallel relationship with the tube as shown. These features will be more fully described hereinafter.

[0028] It should be recognized that although the tube is shown as being flat in its configuration, it may be formed in other shapes such as round, oval, or racetrack shaped in cross-section, for example. An advantage to the flat shape as shown is that this is conventional geometry for microchannel or mini-channel heat exchangers. Further, the flat tubes enable the design of a small inactive heat exchanger area at the top and bottom due to their flat profile. [0029] The tube as shown in Fig. 2 represents a finished three-pass tube which may be fabricated by any of various manufacturing processes. One method that can be applied is to simply form the three pass tube from a single unitary member which is bent around to form the 180° turns at the arcuate portions 33 and 34. With such an approach, care must be taken not to crimp the tube so as to restrict the flow of refrigerant through the arcuate portions 33 or 34. The distance between the planar portions 29, 31 and 32 can be selected to fit the design of the overall heat exchanger.

[0030] Fig. 2A shows another tube which is formed in a four-pass configuration with combination of two long bends and one short bend. Here, it will be seen that the bends are substantially 90° bends rather than curvilinear bends as shown in Fig. 2. Accordingly, the considerations for preventing crimping are different and probably more critical than with the arcuate sections of the Fig. 2 embodiment. Critical in this regard is the type of material that is used (e.g. preferably a more ductile material), the bend radius, the wall thickness, and the internal parallel arrangements inside the tubes, which are all factors that can influence the bend shape and form.

[0031] Another approach to fabrication is that shown in Fig. 3 wherein a shorter section of tube is bent around a 180° turn near its one end to form a J-shaped member 38 comprising a planar element 39 and an arcuate element 41. This provides only a single pass from the inlet manifold 42 but can easily be combined with other similar J-shaped members to obtain a multi-pass arrangement. That is, to add a second pass to that shown, one can easily connect an end of the planar element 39 of a second J-shaped member to the end of the arcuate element 41 of the member as shown to obtain a second pass. A third pass then can be obtained by connecting a planar element to connect its one end to the end of the arcuate element 41 of the second J-shaped member, with the other end thereof being fluidly connected to the outlet manifold. Connections between individual members can be made by brazing or the like.

[0032] Another possible fabricating process that may be used that shown in

Fig. 4 wherein the arcuate sections 45 and 43 may be formed from shorter portions of a tube and then connected to the planar elements to obtain a three pass tube. That is, arcuate section 45 fluidly interconnects the ends of planar elements 44 and 46, and arcuate section 43 fluidly interconnects the ends of planar elements 46 and 47. [0033] The applicants have recognized that, as the refrigerant is expanded as it successively flows through the various passes, it is desirable to progressively increase the cross sectional areas of the tubes in the downstream direction. Ideally, this would be accomplished on a continuous basis but, as a practical matter, such a design would be difficult to implement. Accordingly, this may also be accomplished in a step wise manner. Such a step wise approach can easily be implemented in the methods of fabrication as shown in Figs. 3 and 4 by gradually increasing the cross sectional area of the successive planar elements within any particular circuit as shown in Figs. 3A and 4A.

[0034] Considering now the manner in which the tubes may be combined to form a multiple circuit heat exchanger, a prior art, nested, approach is shown in Fig. 5A wherein circuits 48 and 49 are fluidly interconnected between inlet header 51 and 52. Each of the circuits 48 and 49 is formed in a serpentine shape so as to provide five passes between the inlet header 51 and the outlet header 52. This arrangement allows the headers 51 and 52 to be relatively small with the inlet header 51 providing for a single distribution between the two circuits, and with the distribution in each circuit remaining throughout the flow of refrigerant through the heat exchanger. However, in order for the tubes of the circuit 49 to be nested within the tubes of the circuit 48 as shown, their size/shape needs to be selected accordingly. Further, if one wants to add a third circuit, it would be necessary to provide a third differently shaped tube that could be nested outside of the circuit 48 or inside the circuit 49. Such a change, in turn, may require the redesigning of the entire heat exchanger when considering the features of the fin density, fin height, tube details, etc.

[0035] Referring to Fig. 5B, the heat exchanger of the present invention is shown to include circuits 53 and 54, with each having five passes between the inlet header 56 and outlet header 57. However, rather than having the tubes of the circuit 54 nested within the tubes of the circuit 53 as in the prior art, the entire five passes of the circuit 54 are grouped together with the group being laterally spaced from the entire group of five passes of the circuit 53. This arrangement allows the tubes of the circuit 54 to be substantially identical to the tubes of the circuit 53, with only the lengths of the inlet lines 58 and 59 and the lengths of outlet lines 61 and 62 being different. That is, the five passes of the circuit 53 are substantially identical to the five passes of the circuit 54. This allows them to be mass produced to reduce cost. It also allows them to be stacked vertically, horizontally or in the airflow directions for optimal performance. Further, additional circuits can be easily added by simply placing one or more circuits in spaced relationship to the circuit 54. [0036] In Fig. 6 there is shown an alternative embodiment of a heat exchanger having a five pass, four circuit arrangement to again obtain a total of twenty tubes. Here, the four circuits 63, 64, 66 and 67 are fluidly connected between an inlet header 68 and an outlet header 69, with each of the circuits containing five groups of passes between its inlet and outlet ends. [0037] Referring now to Fig. 7, there are shown two heat exchanger units 70 and 71 in spaced relationship with respect to the direction of airflow therethrough. Unit 70 has circuits 72 and 73 fluidly interposed between inlet header 74 and outlet header 76. Unit 71 has circuits 77 and 78 fluidly connected between inlet header 79 and outlet header 81. As will be seen, the inlet and outlet headers of the respective units 70 and 71 are substantially reversed. The purpose is to obtain better efficiency when considering the operation of the two units in combination. That is, in the heat exchanger unit 70, the refrigerant entering from the left side of each of the circuits 72 and 73 will tend to be cooler than the refrigerant near to the downstream ends of those circuits (i.e. toward the right side). Similarly, with the inlet header 79 on the right side of the unit 71, the refrigerant flowing in the passes nearer to the right side of circuits 77 and 78 will be cooler than the refrigerant in those passes on the left side of those circuits. Because of this counterflow relationship between the flow in the units 70 and 71, a more balanced heat transfer and better efficiency will result. The arrangement of circuits as set forth in the present invention facilities such a design.

[0038] The applicants have recognized that if a heat exchanger is arranged in such a manner that the tubes emanating therefrom are in a parallel horizontal arrangement, but with the tubes being vertically spaced, then gravity will tend to cause more of the heavier liquid refrigerant to flow to the lower tubes and more of the lighter vapor to the upper tubes, thereby causing maldistribution. Accordingly, one of the arrangements of 8A, 8B or 8C is preferable, wherein the inlet manifold is shown at 82 and the mini-channels are shown at 83. As will be seen in Figs. 8A and 8B, the incoming fluid is flowing upwardly or downwardly, respectively, and therefore each of the tubes is effected the same as the other tubes with respect to the force of gravity. Thus, even distribution is more likely to occur. [0039] In Fig. 8C, in order to further enhance the uniform distribution of refrigerant to the tubes 83, a distributor 84 is installed within the inlet header 82 as shown.

[0040] While the present invention has been particularly shown and described with reference to the preferred mode 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.

Claims

We Claim:

1. A heat exchanger of the type having at least one unit having inlet and outlet manifolds fluidly interconnected by a plurality of circuits with each circuit having separate parallel mini-channels for conducting the flow of refrigerant therebetween; wherein said parallel mini-channels are each formed in a serpentine shape so as to provide a plurality of parallel flow passes for successively conducting fluid flow therethrough and with each circuit having an inlet end fluidly connected to the inlet manifold and an outlet end fluidly connected to the outlet manifold and with each circuit having all of its parallel flow passes grouped together, and with each group being laterally spaced from all of the groups of the adjacent circuits.

2. A heat exchanger as set forth in claim 1 wherein said parallel mini- channels are each formed of a unitary member which is bent into the desired serpentine shape.

3. A heat exchanger as set forth in claim 1 wherein said parallel mini- channels are formed from a plurality of planar tubes with U-shaped members interconnected at the ends of adjacent planar tubes to provide the serpentine shape.

4. A heat exchanger as set forth in claim 1 wherein said parallel mini- channels are formed, in part, by fluidly interconnected J-shaped members.

5. A heat exchanger as set forth in claim 1 wherein said plurality of parallel flow passes have cross sectional areas which increase or decrease toward the downstream passes.

6. A heat exchanger as set forth in claim 5 wherein said increases or decreases are in a step wise fashion.

7. A heat exchanger as set forth in claim 1 wherein said inlet manifold includes a distributor disposed therein to facilitate the uniform distribution of refrigerant to the individual mini-channels.

8. A heat exchanger as set forth in claim 1 wherein said parallel mini- channels have their respective inlet ends oriented vertically.

9. A heat exchanger as set forth in claim 1 including a pair of units arranged in spaced relationship in the direction of airflow therethrough and with the respective directions of refrigerant flow being in counterflow relationship.

10. A method of promoting uniform refrigerant flow from an inlet manifold of a heat exchanger to a plurality of parallel multi-channel, mini-channels fluidly connected thereto, comprising the steps of: providing a plurality of tubes shaped in a serpentine manner and arranged to form a plurality of circuits with each circuit having a plurality of parallel flow passes for successively conducting fluid flow therethrough and with each circuit having all of its parallel flow passes grouped together, and with each group being laterally spaced from all of the groups of the adjacent circuits; and fluidly connected each circuit at one end thereof to an inlet manifold and at the other end thereof to an outlet manifold.

11. A method as set forth in claim 10 wherein said at least one flat tube is formed of a unitary member which is bent into the desired serpentine shape.

12. A method as set forth in claim 10 wherein said at least one flat tube is formed from a plurality of planar tubes with U-shaped members being interconnected at the ends of adjacent planar tubes to provide the serpentine shape.

13. A method as set forth in claim 10 wherein said parallel mini-channels are formed, in part, by fluidly interconnecting J-shaped members.

14. A method as set forth in claim 10 wherein said plurality of parallel flow passes have cross sectional areas which increase or decrease toward the downstream passes.

15. A method as set forth in claim 14 wherein said increases or decreases are in a step wise fashion.

16. A method as set forth in claim 10 wherein said inlet manifold includes a distributor disposed therein to facilitate the uniform distribution of refrigerant to the individual mini-channels.

17. A method as set forth in claim 10 and including the step of orienting the inlet ends of said plurality of flat tubes vertically with respect to one another.

18. A method as set forth in claim 10 and including the steps of providing another such heat exchanger in spaced relationship in the direction of air flow to said one heat exchanger and causing the respective directions of refrigerant flow through the heat exchangers to be in counterflow relationship.