Parallel Flow Heat Exchanger With Crimped Channel Entrance

Abstract

A parallel flow (minichannel or microchannel) evaporator includes channels which are crimped at or adjacent to their entrance location which provides for a refrigerant expansion and pressure drop control resulting in the elimination of refrigerant maldistribution in the evaporator and prevention of potential compressor flooding. Progressive crimping to counter-balance factors effecting refrigerant distribution is also disclosed.

Description

CROSS-REFERENCE TO RELATED APPLICATION

Reference is made to and this application claims priority from and the benefit of U.S. Provisional Application Ser. No. 60/649,383, filed Feb. 2, 2005, and entitled PARALLEL FLOW EVAPORATOR WITH CRIMPED CHANNEL ENTRANCE, which application is incorporated herein in its entirety by reference.

BACKGROUND OF THE INVENTION

This invention relates generally to air conditioning, heat pump and refrigeration systems and, more particularly, to parallel flow evaporators thereof.

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.

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.

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&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.

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.

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.

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.

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

It is therefore an object of the present invention to provide for a system and method which overcomes the problems of the prior art described above.

The objective of the invention is to introduce a pressure drop control for the parallel flow evaporator that will essentially equalize pressure drop through the heat exchanger channels 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 and preventing phase separation, which is one of the main causes for refrigerant maldistribution.

In accordance with the present invention, each of the channels is crimped at or adjacent to their entrance location such that a desired restriction for each of the channels is provided. The restriction size may be varied from channel to channel, if desired, in order to accommodate other non-uniform factors (such as different heat transfer rates) affecting the maldistribution phenomenon. The channels may be crimped at the very end/entrance or some distance away from the entrance in order not to interfere with the brazing joint to the inlet manifold. Additionally, internal rigidity (and/or heat transfer enhancement) fins can be simply compressed during crimping process or machined down prior to crimping. Furthermore, these restrictions 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 another fixed area restriction device (such as a capillary tube or an orifice) or a thermostatic expansion valve (TXV) or an electronic expansion valve (EXV) is employed as a primary expansion device. Also, the precision of crimping doesn't have to be of extremely high tolerance in a latter case.

In both cases outlined above, but especially if the crimping restrictions are provided as primary expansion devices at the entrance of each channel of the parallel flow evaporator, they represent a major resistance to the refrigerant flow within the evaporator. In such circumstances, the main pressure drop region will be across these restrictions 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 of each channel, a predominantly single-phase liquid refrigerant is flown through the inlet manifold and no phase separation occurs prior to entering individual evaporator channels. Hence, uniform refrigerant distribution is achieved, evaporator and system performance is enhanced, flooding conditions at the compressor suction are avoided 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.

Any suitable means of crimping may be employed such as a crimping tool in the form of pliers having the desired crimping face geometry or the use of stamping die having the desired geometry.

BRIEF DESCRIPTION OF THE DRAWINGS

For a further 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:

FIG. 1 is a schematic illustration of a parallel flow heat exchanger in accordance with the prior art.

FIG. 2 is an enlarged partial side sectional view of a parallel flow heat exchanger illustrating one embodiment of the present invention.

FIG. 3a is a view of FIG. 2 illustrating a second embodiment of the present invention.

FIG. 3b is a view of FIG. 2 illustrating a third embodiment of the present invention.

FIG. 3c is a view of FIG. 2 illustrating a fourth embodiment of the present invention.

FIG. 3d is a view of FIG. 2 illustrating a fifth embodiment of the present invention.

FIG. 4 is an end view of an uncrimped channel.

FIG. 5 is a view of FIG. 4 after crimping to a predetermined configuration.

FIG. 6 is a view of FIG. 4 after crimping to a second configuration.

FIG. 7 is an end view of a second uncrimped channel.

FIG. 8 is a view of FIG. 7 after crimping to a predetermined configuration.

DESCRIPTION OF THE PREFERRED EMBODIMENT

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 (See FIGS. 4-6).

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.

According to one embodiment of the invention, as illustrated in FIG. 2, the channels 16 have been crimped at least at the entrance end 30 to provide for a restriction in each channel and to assure refrigerant expansion directly at each channel entrance which results in a pressure drop across the restriction and reduction and/or elimination of phase separation and refrigerant maldistribution in the system.

In a second embodiment of the invention, as illustrated in FIG. 3a, the channels are crimped at the very end 32 and at a point 34, some distance away from the end and the attachment point to the manifold 12.

In a third embodiment, as illustrated in FIG. 3b, the channels are crimped at a single location 36, a predetermined distance from the channel end and, once again, away form the attachment point to the manifold 12, in order not to interfere with the attachment process.

In a fourth embodiment, as illustrated in FIG. 3c, the channels are crimped for a predetermined length or distance “L” near the channel ends but with less cross-section area alteration/reduction than in FIGS. 2, 3a and 3b.

In a fifth embodiment of the invention, as illustrated in FIG. 3d, the channels are crimped at multiple locations 38, 40 and 42 near the channel ends, forming a passage of alternating contractions and expansions, but, once again, with less cross-section area alteration/reduction than in FIGS. 2, 3a and 3b.

FIG. 4 illustrates a cross section of an uncrimped channel 50 having flattened shape and integral vertical support members 52.

FIG. 5 illustrates channel 50 crimped to a predetermined configuration 60 which would be suitable for use in the present invention. In this case, crimping occurs around support members 52 and leaves them unaltered.

FIG. 6 illustrates channel 50 crimped to a more flattened configuration 70 which would also be suitable for use in the present invention. In this case, crimping occurs uniformly and alters support members 52 to a different shape and cross-section 72. Obviously, different support members can be utilized within the scope of the present invention to divide channels 16 internally into multiple refrigerant passes of triangular, trapezoidal, circular or any other suitable cross-section. In all these cases, support members can be altered during the crimping process or left unchanged.

FIG. 7 illustrates a cross section of an uncrimped channel 80 of a flattened shape (no internal support members are present in this design configuration).

FIG. 8 illustrates channel 80 crimped to a more flattened configuration 90 suitable for use in the present invention.

Also, it has to be noted that crimping doesn't have to be uniform throughout all the channels but instead can progressively change from one channel to another or from one channel section to another, for instance, to counter-balance other factors effecting refrigerant maldistribution.

Further, it has to be noted that the crimping can be used in the condenser and evaporator applications at the channel entrance 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 flown in a two-phase state, and such heat exchanger configurations can similarly benefit from the present invention by incorporating channel crimping at the entrance ends directly communicating with intermediate manifolds. Further, the crimping can be done at the exit end of the channels 16 or at some intermediate location along the channel length providing only hydraulic resistance uniformity and pressure drop control and with less effect on overall heat exchanger performance.

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.

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.

Claims

1. A parallel flow (minichannel or microchannel) heat exchanger 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 at least one of said channels is crimped to change the cross-section of said channel.

2. A parallel flow heat exchanger as set forth in claim 1 wherein said channels are crimped at their respective ends.

3. The heat exchanger of claim 2 wherein the crimped end is an entrance end.

4. The heat exchanger of claim 2 wherein the crimped end is an exit end.

5. The heat exchanger of claim 2 wherein the crimped end is in direct fluid communication with at least one of inlet, outlet or intermediate manifold.

6. A parallel flow heat exchanger as set forth in claim 2 wherein said channels are crimped at a predetermined distance from at least one channel end.

7. The heat exchanger of claim 1 wherein the channels are crimped in at least one intermediate position along channel length.

8. A parallel flow heat exchanger as set forth in claim 1 wherein at least one of said channels is crimped at two separate locations along its length.

9. A parallel flow heat exchanger as set froth in claim 1 wherein all of said channels are crimped at least at one location at their respective ends.

10. A parallel flow heat exchanger as set forth in claim 8 wherein all of said channels are crimped at two predetermined locations between their respective ends.

11. The structure of claim 1 wherein said heat exchanger is an evaporator.

12. The structure of claim 1 wherein said heat exchanger is a condenser.

13. A parallel flow (minichannel or microchannel) heat exchanger 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 each of said channels is crimped to change the cross-section of said channel.

14. A parallel flow heat exchanger as set forth in claim 13 wherein each of said channels is crimped at their respective ends.

15. The heat exchanger of claim 13 wherein the crimped end is an entrance end.

16. The heat exchanger of claim 13 wherein the crimped end is an exit end.

17. The heat exchanger of claim 13 wherein the crimped end is in direct fluid communication with at least one of inlet, outlet or intermediate manifold.

18. The heat exchanger of claim 13 wherein the channels are crimped in at least one intermediate position along channel length.

19. A parallel flow heat exchanger as set forth in claim 14 wherein said channels are crimped at a predetermined distance from at least one channel end.

20. A parallel flow heat exchanger as set forth in claim 13 wherein at least one of said channels is crimped at two separate locations along its length.

21. A parallel flow heat exchanger as set froth in claim 13 wherein a plurality of said channels are crimped at least at one location at their respective ends.

22. A parallel flow heat exchanger as set forth in claim 13 wherein all of said channels are crimped at multiple predetermined locations between their respective ends.

23. The heat exchanger of claim 1 wherein the crimping is progressive along the channel length.

24. The heat exchange of claim 13 wherein the crimping is progressive along the channel length.

25. The heat exchanger of claim 1 wherein the crimping is progressive amongst the channels.

26. The heat exchange of claim 13 wherein the crimping is progressive amongst the channels.

27. The heat exchanger of claim 1 wherein the crimping is limited to an external channel wall.

28. The heat exchanger of claim 13 wherein the crimping is limited to an external channel wall.

29. The heat exchanger of claim 1 wherein the crimping modifies an external channel wall and an internal support member.

30. The heat exchanger of claim 13 wherein the crimping modifies an external channel wall and an internal support member.

31. The heat exchanger of claim 1 wherein the crimping changes the channel cross-section uniformly.

32. The heat exchanger of claim 13 wherein the crimping changes the channel cross-section uniformly.

33. The heat exchanger of claim 1 wherein crimping changes the channel cross-section non-uniformly.

34. The heat exchanger of claim 13 wherein crimping changes the channel cross-section non-uniformly.

35. The heat exchanger of claim 1 wherein the crimping results in at least one of a pressure drop control and an expansion control.

36. The heat exchanger of claim 13 in which the crimping results in at least one of a pressure drop control and an expansion control.