HK1117224B - Parallel flow heat exchangers incorporating porous inserts - Google Patents

Description

Parallel flow heat exchanger with porous insert

CorrelationCross reference to applications

U.S. provisional application No.60/649,425, entitled "PARALLEL FLOW EVAPORATOR with porous channel insert" CHANNEL INSERTS, filed on 2/2005, the entire disclosure of which is incorporated herein by reference, is incorporated herein by reference.

Technical Field

The present invention relates generally to air conditioning, heat pump and refrigeration systems, and more particularly to parallel flow evaporators of such systems.

Background

The definition of so-called parallel flow heat exchangers is widely used in the air conditioning and refrigeration industry and indicates that a heat exchanger has a plurality of parallel channels in which refrigerant is distributed and flows in a direction substantially perpendicular to the flow direction of the refrigerant in an inlet header and an outlet header. This definition is generally applicable in the technical community and will be used throughout this document.

Refrigerant maldistribution in the evaporator of a refrigeration system is a well known phenomenon. Which causes significant degradation of the evaporator and overall system performance over a wide range of operating conditions. Refrigerant maldistribution can occur due to differences in flow resistance inside the evaporator channels, uneven airflow distribution over the external heat transfer surfaces, improper heat exchanger orientation, or poor header and distribution system design. Maldistribution is particularly present in parallel flow evaporators due to their specific design for the refrigerant entering each refrigerant flow path. Attempts to eliminate or reduce the effect of this phenomenon on the performance of parallel flow evaporators have been made with little or no success. The main reasons for these failures are generally related to the complexity and inefficiency of the proposed technique or to the extremely high cost of the solution.

In recent years, parallel flow heat exchangers, particularly furnace-brazed (brazed) aluminum heat exchangers, have received much attention and attention, not only in the automotive field but also in the heating, ventilation, air conditioning and refrigeration (HVAC & R) industry. The main reasons for using the parallel flow technology are related to its superior performance, high degree of compactness and enhanced corrosion resistance. Parallel flow heat exchangers are now used in condensers and evaporators in a variety of products and system designs and configurations. Despite the greater benefits and benefits, the use of evaporators has presented a number of challenges and difficulties. Refrigerant maldistribution is one of the major effects and obstacles to implementing this technology in evaporator applications.

Refrigerant maldistribution occurs in parallel flow heat exchangers, as is known, because of different pressure drops inside the channels and in the inlet and outlet headers and poor header and distribution system design. In the header, the difference in the length of the refrigerant path, phase separation (phase separation), and gravity are major factors causing maldistribution. Inside the heat exchanger channels, variations in heat transfer rate, air flow distribution, manufacturing tolerances and gravity are the main factors. Furthermore, a recent trend towards enhanced heat exchanger performance is to increase the miniaturization of its channels (so-called minichannels and microchannels), which in turn adversely affects the distribution of the refrigerant. Since it is very difficult to control all of these factors, many previous attempts to control refrigerant distribution, particularly in parallel flow evaporators, have failed.

In refrigeration systems employing parallel flow heat exchangers, the inlet and outlet headers or headers (these terms are used interchangeably herein) typically have a generally cylindrical shape. When the two-phase flow enters the header, the vapor phase is generally separated from the liquid phase. Refrigerant maldistribution occurs because the two phases flow independently.

If the two-phase flow enters the inlet header at a relatively high velocity, the momentum of the flow carries the liquid phase away from the header inlet to the remote portion of the header. Thus, the channels closest to the manifold inlet receive predominantly the vapor phase, while the channels further from the manifold inlet receive predominantly the liquid phase. On the other hand, if the velocity of the two-phase flow entering the header is low, there is insufficient 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 enters the furthest channels. Also, the liquid and vapor in the inlet header can be separated by gravity, resulting in similar maldistribution consequences. In both cases, maldistribution phenomena quickly appear in the evaporator and degrade the overall performance of the system.

In addition, maldistribution phenomena can cause a two-phase (zero point superheat) condition at the outlet of some of the channels, creating a potential flooding at the compressor suction that can quickly translate into compressor failure.

Disclosure of Invention

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

It is an object of the present invention to introduce pressure drop control for parallel flow (microchannel or minichannel) evaporators that will substantially balance the pressure drop across the heat exchanger flow paths and thereby eliminate refrigerant maldistribution and the problems associated therewith. Still further, it is an object of the present invention to provide refrigerant expansion at the inlet of each channel to eliminate a predominantly two-phase flow in the inlet header (which is one of the primary causes of refrigerant maldistribution). It has been found that these objects are achieved by inserting a porous medium in each channel of a parallel flow evaporator or introducing a porous medium at the inlet of each channel of a parallel flow evaporator. For example, during furnace brazing of the entire heat exchanger, these porous media inserts may be brazed in each channel, chemically bonded or mechanically fixed in place. Furthermore, for low cost applications, these inserts are used as the primary (and only) expansion device, or, in the case where precise superheat control is required, a thermostatic expansion valve (TXV) or an electronic expansion valve (EXV) is used as the primary expansion device, and these inserts are used as the secondary expansion device.

Any suitable porous insert that achieves the above objectives may be used. Suitable and inexpensive porous inserts may be made of sintered metal, compressed metal, such as steel wool, specially designed porous ceramics, and the like. When an inexpensive porous media insert is placed in or at the entrance to each channel of a parallel flow evaporator, it presents a major resistance to refrigerant flow within the evaporator. In such a case, the primary pressure drop zone will be present on these inserts, while the pressure drop variation in the channels or in the headers of the parallel flow evaporator will play a secondary (insignificant) role. Still further, because refrigerant expansion occurs at the inlet of each channel, a predominantly single-phase liquid refrigerant flows through the inlet header, particularly when the porous insert is used as the primary and sole expansion device. This achieves uniform refrigerant distribution, improving evaporator and system performance, while not losing accurate superheat regulation (whenever needed). Furthermore, the very low cost of the proposed method makes the present invention very attractive.

Drawings

For a further understanding of the objects of the invention, reference is made to the following detailed description of the invention taken in conjunction with the accompanying drawings, in which:

fig. 1 is a schematic diagram of a parallel flow heat exchanger according to the prior art.

FIG. 2 is a partial side sectional view of one embodiment of the present invention.

FIG. 3 is an end view of the porous insert of the present invention at the entrance to the channel.

FIG. 4 is a perspective view of the porous insert shown in FIG. 3.

Fig. 5a is a side cross-sectional view of another embodiment of the present invention.

Fig. 5b is a side cross-sectional view of yet another embodiment of the present invention.

FIG. 6 is an end view of a plurality of channels in one embodiment of the invention.

Fig. 7a is a perspective view showing an embodiment of the porous cap of the present invention.

Fig. 7b is a perspective view showing a second porous cap embodiment.

Fig. 7c is a perspective view showing a third porous cap embodiment.

Detailed Description

Referring to FIG. 1, as an example, a parallel flow (microchannel or minichannel) heat exchanger 10 is illustrated as including an inlet header or manifold 12, an outlet header or manifold 14, and a plurality of parallel arranged channels 16, the channels 16 fluidly connecting the inlet header 12 to the outlet header 14. Typically, the inlet and outlet headers 12, 14 are cylindrical, and the channels 16 are tubes (or extrusions) having a flat or circular cross-section. The channels 16 typically have a plurality of internal and external heat transfer enhancement elements, such as fins. For example, there are typically furnace-brazed (furace-brazed) external fins 18, which external fins 18 are uniformly arranged between the channels and serve to enhance the heat exchange process and structural rigidity. The channels 16 may also have internal heat transfer enhancements and structural elements.

In operation, refrigerant flows into the inlet 20 and into the internal cavity 22 of the inlet header 12. Refrigerant in the form of liquid, vapor or a mixture of liquid and vapor (which is most typically the case where the evaporator has an expansion device located upstream) passes from the internal cavity 22 into the passage openings 24 to pass through the passages 16 into the internal cavity 26 of the outlet header 14. From the internal cavity 26 (which is now typically in vapor form in the case of an evaporator application), the refrigerant exits the outlet 28 and then enters a compressor (not shown). Outside the channels 16, air is caused to flow (preferably uniformly) over the channels and associated fins 18 by air moving means, such as fans (not shown), so that heat transfer interaction occurs between the air flowing outside the channels and the refrigerant in the channels.

According to one embodiment of the invention, a porous insert 30 is inserted at the entrance of each channel 16. When the channel 16 has internal structural elements such as a support member 16a (fig. 3), typically for the purpose of increasing structural rigidity and/or enhancing heat transfer, and when the support member 16a is located at the entrance of the channel, the porous insert 30 has a slot 32 to receive the support member 16 a. Still further, if it is desired to provide varying degrees of expansion and/or hydraulic resistance by the insert 30 or 32, such as to counteract the other factors described above that affect refrigerant distribution among the channels 16, characteristics of the insert, such as porosity values or geometry (insert depth, etc.), may be varied to achieve a desired effect for each channel 16.

Figure 5a shows another embodiment in which all of the inlets of the channels 16 are covered by a single porous block 34 located within a header 40. Still further, the support members 36 may be used to assist in establishing the relative position of the porous member 34 and the channels 16 within the manifold 40. It should be noted that the assembly of the porous member 34 and the support member 36 may be made and combined into a single member made of a porous material.

Fig. 5b is a further embodiment of the structure of fig. 5a, wherein the porous member is a composite of two different porous materials 34 and 34 a. Obviously, the number of composite materials in the porous member may be more than two.

Fig. 6 shows a side view of fig. 5 a.

Fig. 7a shows a single-piece elongate porous member 34b which seals a plurality of channels 16 at a predetermined distance from the channel inlets.

Figure 7b shows an elongate porous member 34c which covers the ends of a plurality of channels 16.

FIG. 7c is a modification of the configuration of FIG. 7b in which the porous member 34d exactly covers the ends of the channels 16 in shape, the porous member 34d may have any suitable configuration, rather than a rectangular cross-section, and further, the porous member 34d is preferably positioned within the header 40 such that there is a gap between the inner wall of the header 40 and the porous member 34a, thereby allowing for a more uniform distribution of refrigerant prior to entering the porous member 34d and channels 16.

It should be understood that any type of porous member and/or material may be used that achieves the objectives of the present invention. Also, as shown in FIGS. 2 through 7, any design or configuration that achieves the objectives of the present invention may be used in the practice of the present invention.

And it is noted that the porous insert may also be used within an intermediate manifold in condenser and evaporator applications. For example, if a heat exchanger has more than one refrigerant pass, an intermediate header (between the inlet header and the outlet header) is added to the design of the heat exchanger. In the intermediate header, the refrigerant generally exists in a two-phase state, and the heat exchanger configuration may likewise benefit from the present invention by incorporating a porous insert into the intermediate header. Furthermore, to provide only fluid resistance uniformity and pressure drop control, porous inserts may be placed into the inlet header of the condenser and the outlet header of the evaporator with less impact on the overall performance of the heat exchanger.

Since the various factors responsible for the maldistribution of refrigerant into the channels 16 are generally known at the design stage for a particular application, the inventors have discovered that these design features can be introduced to counteract these factors to eliminate their adverse effects on the evaporator and overall system performance, and to eliminate compressor flooding and failure that may occur. For example, in many cases, it is generally known whether the refrigerant flows into the inlet header at a high or low rate, and how these velocity values affect the maldistribution phenomenon. Those skilled in the art will recognize how to apply the teachings of the present invention to other system characteristics.

While the invention has been shown and described with reference to the preferred mode as illustrated in the drawing, it will be understood by those skilled in the art that various changes in detail and structure may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.

Claims (40)

1. A parallel flow minichannel or microchannel heat exchanger comprising:

a longitudinally extending inlet header having an inlet for directing fluid flow into said inlet header and a plurality of outlets for directing fluid flow from said inlet header in a transverse direction;

a plurality of channels, each channel having a channel inlet, the plurality of channels being arranged in a substantially parallel relationship and fluidly connected to the plurality of outlets for directing fluid flow from the inlet header; and

an outlet header fluidly connected to the plurality of channels for receiving fluid flow therefrom;

wherein the heat exchanger comprises at least one porous member disposed across the entire flow path of each of the plurality of channels, whereby the heat exchanger exhibits substantially reduced flow maldistribution among the plurality of channels.

2. A parallel flow heat exchanger as set forth in claim 1 wherein said heat exchanger is an evaporator.

3. A parallel flow heat exchanger as set forth in claim 1 wherein said heat exchanger is a condenser.

4. A parallel flow heat exchanger as set forth in claim 1 wherein said porous member is in the form of an insert disposed in at least one channel.

5. A parallel flow heat exchanger as set forth in claim 4 wherein said porous insert is positioned at the channel inlet.

6. A parallel flow heat exchanger as set forth in claim 5 wherein said porous insert is disposed adjacent said channel inlet.

7. A parallel flow heat exchanger as set forth in claim 5 wherein said porous insert is disposed within the interior of said channel.

8. A parallel flow heat exchanger as set forth in claim 1 wherein the porous member is disposed in or in direct fluid communication with the inlet header.

9. A parallel flow heat exchanger as set forth in claim 1 wherein the porous member is disposed in or in direct fluid communication with the outlet header.

10. A parallel flow heat exchanger as set forth in claim 1 wherein the porous member is disposed in or in direct fluid communication with at least one of said headers.

11. A parallel flow heat exchanger as set forth in claim 1 wherein said porous member is made of a material selected from the group consisting of metal and ceramic.

12. A parallel flow heat exchanger as set forth in claim 1 wherein said porous member is made of a material selected from the group consisting of: sintered metal, compressed metal, metal wool or metal wire.

13. A parallel flow heat exchanger as set forth in claim 1 wherein said porous member is disposed longitudinally along the manifold.

14. A parallel flow heat exchanger as set forth in claim 1 wherein there is a gap between said porous member and the header inner wall surface.

15. A parallel flow heat exchanger as set forth in claim 1 wherein said porous member is a composite of at least two different inserts.

16. A parallel flow heat exchanger as set forth in claim 1 wherein said porous member is non-rectangular in cross-section.

17. A parallel flow heat exchanger as set forth in claim 16 wherein said porous member is a portion of a circle in cross-section.

18. A parallel flow heat exchanger as set forth in claim 1 wherein said porous member has a characteristic that is variable between at least two channels.

19. A parallel flow heat exchanger as set forth in claim 1 wherein the performance characteristics of the porous member can be varied by controlling at least one of porosity, depth, insertion depth, and material.

20. A parallel flow minichannel or microchannel heat exchanger comprising:

a longitudinally extending inlet header having an inlet for directing fluid flow into said inlet header and a plurality of outlets for directing fluid flow from said inlet header in a transverse direction;

a plurality of channels, each channel having a channel inlet, the plurality of channels being arranged in a substantially parallel relationship and fluidly connected to the plurality of outlets for directing fluid flow from the inlet header; and

an outlet header fluidly connected to the plurality of channels for receiving fluid flow therefrom;

wherein the heat exchanger comprises at least one porous member disposed across the entire flow path of each of the plurality of channels, wherein the porous member is designed for at least one of expansion control and pressure drop control in the system, and wherein the heat exchanger exhibits a substantially reduced flow maldistribution among the plurality of channels.

21. A parallel flow heat exchanger as set forth in claim 20 wherein said heat exchanger is an evaporator.

22. A parallel flow heat exchanger as set forth in claim 20 wherein said heat exchanger is a condenser.

23. A parallel flow heat exchanger as set forth in claim 20 wherein said porous member serves as the primary expansion device.

24. A parallel flow heat exchanger as set forth in claim 20 wherein said porous member serves as a secondary expansion device.

25. A parallel flow heat exchanger as set forth in claim 20 wherein said porous member is in the form of an insert disposed in at least one channel.

26. A parallel flow heat exchanger as set forth in claim 25 wherein said porous member is disposed at the channel inlet.

27. A parallel flow heat exchanger as set forth in claim 26 wherein said porous member is disposed adjacent said channel inlet.

28. A parallel flow heat exchanger as set forth in claim 26 wherein said porous member is disposed within said channel.

29. A parallel flow heat exchanger as set forth in claim 20 wherein said porous member is disposed in or in direct fluid communication with said inlet header.

30. A parallel flow heat exchanger as set forth in claim 20 wherein the porous member is disposed in or in direct fluid communication with the outlet header.

31. A parallel flow heat exchanger as set forth in claim 20 wherein the porous member is disposed in or in direct fluid communication with an inlet header.

32. A parallel flow heat exchanger as set forth in claim 20 wherein said porous member is made of a material selected from the group consisting of metal and ceramic.

33. A parallel flow heat exchanger as set forth in claim 20 wherein said porous member is made of a material selected from the group consisting of: sintered metal, compressed metal, metal wool or metal wire.

34. A parallel flow heat exchanger as set forth in claim 20 wherein said porous member is disposed longitudinally along the header.

35. A parallel flow heat exchanger as set forth in claim 20 wherein there is a gap between said porous member and a header inner wall surface.

36. A parallel flow heat exchanger as set forth in claim 20 wherein said porous member is a composite of at least two different members.

37. A parallel flow heat exchanger as set forth in claim 20 wherein said porous member is non-rectangular in cross-section.

38. A parallel flow heat exchanger as set forth in claim 37 wherein said porous member is a portion of a circle in cross-section.

39. A parallel flow heat exchanger as set forth in claim 20 wherein said porous member exhibits a performance characteristic that is variable between at least two channels.

40. A parallel flow heat exchanger as set forth in claim 39 wherein said variable performance characteristic is dependent upon at least one of porosity, depth, insertion depth, and material.