EP1809968A2

EP1809968A2 - Parallel flow evaporator with shaped manifolds

Info

Publication number: EP1809968A2
Application number: EP05821090A
Authority: EP
Inventors: Allen C. Kirkwood; Michael F. Taras; Robert A. Chopko
Original assignee: Carrier Corp
Current assignee: Carrier Corp
Priority date: 2004-11-12
Filing date: 2005-11-14
Publication date: 2007-07-25
Also published as: EP1809968A4; US20060101850A1; WO2006053311A2; WO2006053311A3; US20100071392A1

Abstract

In a parallel flow evaporator, the inlet manifold construction consists of alternating expansion and contraction chambers to promote homogeneous conditions of the refrigerant, as it flows longitudinally through the inlet manifold, as a result of partial evaporation (throttling) and mixing and jetting effects (due to velocity augmentation). In a preferred embodiment, the parallel channels are fluidly connected to the expansion chambers so as to receive a homogeneous refrigerant mixture therefrom. In one embodiment, the expansion and contraction chambers are progressively smaller in size toward a downstream end, so as to accommodate the diminishing refrigerant flow as it progresses longitudinally along the inlet manifold. In another embodiment, the outlet manifold also consists of a repetitive pattern of alternating expansion and contraction chambers, so as to balance the impedances of the inlet manifold. In still another embodiment, these chambers are progressively larger in size toward a downstream end of the outlet manifold. In yet another embodiment, the flow-mixing inserts are introduced into the contraction chambers to further promote homogeneous conditions within the manifold. As a result, maldistribution in the heat exchanger is avoided, resulting in system performance augmentation and compressor reliability enhancement.

Description

Parallel Flow Evaporator with Shaped Manifolds

Background of the Invention

[0001] This invention generally relates 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 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&R) industry. The primary reasons for the employment of the parallel flow technology are related to its superior performance, high degree of compactness, 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, 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 rum 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 the 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 (i.e. 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.

Summary of the Invention

[0008] Briefly, in accordance with one aspect of the invention, rather than being cylindrical in form, the inlet manifold is hour-glass shaped along its longitudinal axis such that alternate expansion and contraction chambers are provided, resulting in a mixing effect of the two phases of the refrigerant flowing therethrough and thereby providing a homogenous mixture of refrigerant entering the individual channels of the heat exchanger.

[0009] By another aspect of the invention, the individual channels are connected to the inlet manifold at its expansion chambers, and the contraction chamber portions are disposed between adjacent channels. [0010] By yet another aspect of the invention, the expansion chambers are progressively smaller toward the downstream end of the inlet manifold to accommodate the progressively diminishing refrigerant flow in the inlet header. [0011] By still another aspect of the invention, the contraction chambers are equipped with the flow mixing devices, promoting homogeneous conditions at the entrance of the adjacent downstream expansion chambers. [0012] In the drawings as hereinafter described, preferred and alternate embodiments are depicted; however, various other modifications and alternate designs and 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] FIG. 2 is a schematic illustration of one embodiment of the present invention.

[0015] FIG. 3 is a schematic illustration of an alternative embodiment of the present invention. [0016] FIG. 4 is a schematic illustration of yet another embodiment of the present invention.

[0017] FIG. 5 is a schematic illustration of still another embodiment of the present invention.

[0018] FIG. 6 is a schematic illustration of still another embodiment of the present invention.

Description of the Preferred Embodiment

[0019] 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 channels 13 fluidly connecting the inlet manifold 11 to the outlet manifold 12. Generally, the inlet and outlet manifolds 11 and 12 are cylindrical in shape, and the channels 13 are usually 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, 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.

[0020] 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 latter is a 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).

[0021] As discussed hereinabove, it is desirable that the two-phase refrigerant passing from the inlet header 11 to the individual channels 13 do so in a uniform manner (or in other words, with equal vapor quality) such that the full heat exchange benefit of the individual channels can be obtained and flooding conditions are not created and observed at the compressor suction (this may damage the compressor). However, because of various phenomena as discussed hereinabove, a non-uniform flow of refrigerant to the individual channels 13 (so-called maldistribution) occurs. In order to address this problem, the applicants have introduced design features that will create a mixing and jetting effects in the two- phase refrigerant flow in the inlet manifold 11 to thereby bring about a more uniform homogeneous flow into to the channels 13.

[0022] Referring to Fig. 2, the heat exchanger is formed with a conventional outlet manifold 12 and channels 13, but with a differently shaped inlet manifold 21, as shown. Rather than being cylindrical in the usual manner, the inlet manifold 21 is hour-glass shaped (i.e. with a plurality of alternating expansion and contraction chambers). For simplicity, the inlet manifold 21 is shown to include three expansion chambers 22, 23 and 24 with interconnecting contraction chambers 26 and 27. Each of the expansion chambers 22, 23 and 24 is preferably interconnected to an associated channel 13, as shown. In actual practice, a larger number of expansion chambers and associated channels 13 would be provided. Further, each of the expansion chambers may be connected to more than one channel 13. It is preferred, however, that none of the channels 13 would be connected directly to a contraction chamber.

[0023] In operation, the two-phase refrigerant enters the inlet opening 28 and enters the first expansion chamber 22 where it is partially expanded with a portion thereof entering the associated channel(s). The remaining two-phase refrigerant is then forced through the contraction chamber 26 such that when it enters the expansion chamber 23 in more homogeneous manner due to increased velocity, partial evaporation (or tlirottling) occurs, thereby presenting a homogeneous condition for the refrigerant mixture flowing to the associated channel(s) and to the downstream channels. The remaining refrigerant then passes through the contraction chamber 27, where more mixing and jetting of the two (liquid and vapor) refrigerant phases occurs and into the expansion chamber 24, wherein, once again, a partial evaporation process is taking place, thereby presenting a homogenous mixture to the associated channel(s). In this way, the partial evaporation process is incrementally (i.e. progressively) maintained through the length of the inlet manifold 21, so as to result in a more uniform distribution of refrigerant among the channels. [0024] Although the refrigerant flow in the inlet manifold 21 is progressively diminishing, it is essential not to introduce excessive flow impedance in the inlet manifold 21 relative to other flow resistances in the heat exchanger. Thus, the cross- section areas of the contraction chambers 26 and 27 must be properly sized for a particular application and for a particular configuration of the heat exchanger to maintain the balance between the desired partial evaporation process and undesired additional hydraulic resistance for the refrigerant flowing to the downstream channels. Generally, flow impedance of the contraction chamber should be at least one and a half times lower than the hydraulic resistance of the associated channels 13. It is also desirable to balance the impedance of the contraction chambers in the inlet manifold 21 with corresponding pressure drops in the outlet manifold as will be further described hereinafter.

[0025] An alternative embodiment of the present invention is shown in Fig.

3, wherein the inlet manifold 31 is again hour-glass shaped but with expansions chambers 32, 33 and 34 being progressively smaller in a cross section to accommodate the reduced refrigerant flow, as it moves from the inlet 38 toward the downstream end thereof. Further, the contraction chambers 36 and 37 are also preferably formed of a progressive smaller size for the same reasons. Generally, the cross-section area reduction ratio of the expansion chambers is proportional to the refrigerant flow rate ratio reduction entering and leaving the chamber. If this ratio is not uniform, the average value should be used instead for the estimates. The contraction chambers can be sized by employing an identical procedure and values. [0026] In Fig. 4, a further embodiment is shown wherein the inlet manifold

21 is identical to that as described in respect to Fig. 2 or that shown in Fig. 3, but the outlet manifold 41 also being hour-glass shaped with expansion chambers 42, 43 and 44 and contraction chambers 46 and 47 alternately disposed as shown. These expansion and contraction chambers are not necessarily and most likely will not be of the same sizes as those of the inlet manifold 21, since the refrigerant flowing within the outlet manifold 41 in a completely different thermodynamic state. Although if the chamber of the inlet manifold 21 are progressively smaller in size toward the downstream end thereof, the chambers of the outlet manifold 41 should preferably be progressively larger toward the downstream end thereof, as shown in Fig. 5, and identical aspect ratio can be utilized in sizing the outlet manifold chambers. In this way, the impedances that are presented in the inlet manifold 21 are matched by those in the outlet manifold 41 such that, the most favorable conditions for the uniform refrigerant flow distribution among the parallel channels 13 are created throughout the heat exchanger, enhancing the system performance and improving compressor reliability, by preventing flooded conditions at the compressor suction.

[0027] An alternative embodiment of the present invention is shown in Fig.

6, wherein the inlet manifold 51 is again hour-glass shaped with the expansions chambers 52, 53 and 54 and the contraction chambers 55 and 56 disposed in between the expansion chambers. Additionally, refrigerant-mixing inserts 57 are placed within the contraction chambers 55 and 56 to promote mixing and even more homogeneous conditions at the entrance of the adjacent downstream expansion chambers. Although inserts 57 can be spiral in shape or have internal fins or indentations, any other configurations promoting mixing are also acceptable. In all other aspects this embodiment is similar to the embodiments discussed above. [0028] In has to be understood that the expansion and contraction chambers may be of any shape, cross-section area and configuration as long as a repetitive process of partial evaporation is created and a proper balance of hydraulic resistances is maintained.

[0029] 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. [0030] 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.

Claims

We Claim:

1. A parallel flow evaporator comprising: an inlet manifold extending longitudinally and having an inlet opening for conducting the flow of a fluid into said inlet manifold and a plurality of outlet openings for conducting the flow of fluid transversely from said inlet manifold; a plurality of channels aligned in substantially parallel relationship and fluidly connected to said plurality of outlet openings for conducting the flow of fluid from said inlet manifold; and an outlet manifold fluidly connected to said plurality of said channels for receiving the flow of fluid therefrom; wherein said inlet manifold consists of a longitudinally extending repetitive pattern of expansion and contraction chambers.

2. A parallel flow evaporator as set forth in claim 1 wherein said expansion and contraction chambers are collectively hour-glass shaped.

3. A parallel flow evaporator as set forth in claim 1 wherein said outlet openings are formed in said expansion chambers.

4. A parallel flow evaporator as set forth in claim 1 wherein said expansion chambers are progressively smaller toward a downstream end of said inlet manifold.

5. A parallel flow evaporator as set forth in claim 1 wherein contraction chambers are progressively smaller toward a downstream end of said inlet manifold.

6. A parallel flow evaporator as set forth in claim 1 wherein said outlet manifold consists of a longitudinally extending repetitive pattern of expansion and contraction chambers.

7. A parallel flow evaporator as set forth in claim 6 wherein said outlet manifold expansion chambers are progressively larger toward a downstream end of said outlet manifold.

8. A parallel flow evaporator as set forth in claim 6 wherein said outlet manifold contraction chambers are progressively larger toward a downstream end of said outlet manifold.

9. A parallel flow evaporator as set forth in claim 1 wherein said contraction chambers of said inlet manifold are equipped with flow-mixing devices.

10. A parallel flow heat exchanger of the type having an inlet manifold extending longitudinally and fluidly interconnected to an outlet manifold by a plurality of parallel channels for conducting the flow of the first fluid therethrough and adapted for having a second fluid circulated thereover for purposes of exchange of heat between the two fluids; wherein said inlet manifold is formed of a plurality of alternately disposed expansion and contraction chambers.

11. A parallel flow heat exchanger as set forth in claim 10 wherein said inlet manifold is hour-glass shaped in longitudinal cross-section.

12. A parallel flow heat exchanger as set forth in claim 10 wherein said plurality of parallel channels are fluidly connected to said expansion chambers.

13. A parallel flow heat exchanger as set forth in claim 10 wherein said expansion chambers are progressively smaller toward a downstream end of said inlet manifold.

14. A parallel flow heat exchanger as set forth in claim 10 wherein said contraction chambers are progressively smaller toward a downstream end of said inlet manifold.

15. A parallel flow heat exchanger as set forth in claim 10 wherein said outlet manifold is formed of alternating expansion chambers and contraction chambers.

16. A parallel flow heat exchanger as set forth in claim 15 wherein said outlet manifold is hour-glass shaped.

17. A parallel flow heat exchanger as set forth in claim 15 wherein said expansion chambers are fluidly connected to said channels.

18. A parallel flow heat exchanger as set forth in claim 15 wherein said expansion chambers are progressively larger toward a downstream end of said outlet manifold.

19. A parallel flow heat exchanger as set forth in claim 15 wherein said contraction chamber are progressively larger toward a downstream end of said outlet manifold.

20. A parallel flow heat exchanger as set forth in claim 10 wherein said contraction chambers of said inlet manifold are equipped with flow-mixing devices.