High pressure header and heat exchanger and method of making the same

Abstract

A method of providing volume production of highly pressure resistant headers (10), (12) is provided and allows the headers (10), (12) to be formed of a header structure (10), (12) with a relatively thin wall portion (32) and a relatively thick wall portion (30). A strip (40) is utilized to provide the desired thickness at the thin wall portion (32) while allowing both the thin wall portion (32) and the strip (40) to have tube slots (34), (42) formed therein by a one step punching operation.

Description

FIELD OF THE INVENTION

This invention relates to headers for heat exchangers, and more particularly, to headers and heat exchangers incorporating such headers which are designed for extremely high pressure applications.

BACKGROUND OF THE INVENTION

Concern for global warming and the deterioration of the ozone layer as a result of the escape of fluorine containing refrigerants from refrigeration systems, including air conditioning systems, has prompted a new look at refrigeration systems utilizing more environmentally friendly refrigerants. One such system under study is a carbon dioxide (CO₂) based system wherein CO₂ is employed as the refrigerant. CO₂ systems operate at significantly higher internal pressure than do conventional systems employing fluorine based refrigerants and as a consequence, there is a need to improve the pressure resistance of heat exchangers used in such applications as, for example, the gas cooler and the evaporator of such systems.

At the same time, these systems have the potential for extensive use in vehicular air conditioning systems where weight, because of its impact on fuel economy, is of considerable concern. This consideration makes it impossible to achieve the desired pressure resistance simply by expanding wall thickness of conventional heat exchangers used in such systems without other major changes because of the added weight of thicker walled elements. Furthermore, this solution is not an economically viable one because taking existing components without changing their size other than to increase wall thickness to achieve pressure resistance means more material will have to go into the heat exchanger, most notably in the headers, thereby increasing the cost of the resulting heat exchanger.

Various solutions to this problem have been proposed. For example, many of the heat exchangers employ tubular headers which are generally cylindrical in shape. Conventional flattened tubes have their ends fitted in tube slots in the headers, which tube slots are transverse to the direction of elongation of the header. It has been proposed to reduce the diameter of the header and reorient the tube slots so that they are elongated in the direction of elongation of the header. The tubes are then provided with a twist near where their ends enter the header so as to present a desired orientation of the tubes for air flow between the tubes through the heat exchanger.

One primary difficulty in this approach is that with smaller diameter headers, the process of forming the tube slots in the headers has become increasingly difficult. In order to have a desired wall thickness in the smaller diameter headers, it has been necessary to form the tube slots by machining procedures as, for example, by milling. Unfortunately, these machining operations are time consuming and expensive and are particularly more costly than the various punching techniques that have been used to form transverse tube slots in cylindrical headers in conventional heat exchangers utilizing conventional refrigerants.

Thus, there is a real need for a less costly header for use in high pressure heat exchangers, such as those used as condensers, gas coolers and/or evaporators in high pressure refrigeration systems. The present invention is directed to meeting that need.

SUMMARY OF THE INVENTION

It is the principal objects of the invention to provide a) a new and improved method for making a header for a high pressure heat exchanger, b) a new and improved header with high pressure resistance for use in high pressure heat exchangers, and c) a new and improved heat exchanger having improved pressure resistance enabling it to function in a high pressure system as, for example, a high pressure refrigeration system such as a CO₂ refrigeration system.

According to one facet of the invention, there is provided a method of making a high pressure resistant header for a heat exchanger which includes the steps of a) providing an elongated header structure including a pair of side-by-side longitudinally extending passages, the passages surrounded by a wall of sufficient thickness to resist deformation when a fluid is placed within the passages at an operating pressure at which deformation is to be resisted, b) thinning the wall along its length by providing a first mating exterior surface on a part thereof so that the wall, at the first mating surface is sufficiently thin that tube slots may be formed therein by punching as opposed to more expensive machining procedures, c) punching tube slots at predetermined spaced intervals of the wall at the first mating surface, d) providing an elongated strip having a second mating surface complimentary to the first mating surface and of a thickness such that the combined thickness of the strip and the wall at its first mating surface is about equal to or greater than the desired thickness of the wall, e) punching tube slots in the strip at the predetermined spaced intervals, f) abutting the second mating surface of the strip to the first mating surface of the header structure with the tube slots in each being aligned with one another and g) thereafter bonding the strip to the header structure along their respective lengths to provide a unitary header with tube slots therein.

In a preferred embodiment, both of the mating surfaces are flat surfaces.

A preferred embodiment also contemplates that steps a) and b) are performed simultaneously by extrusion of the header structure.

In one embodiment, step b) is performed by providing a strip receiving groove in that part of the header structure exterior surface and the groove has a flat bottom surface defining the first mating surface.

According to another aspect of the invention, a header for a high pressure heat exchanger is provided. The header includes an elongated tubular like element having a pair of side-by-side longitudinally extending passages, and a tube receiving side. The element is a unitary structure and has a relatively thick wall partially surrounding the passage and a relatively thin wall at the tube receiving side. A first exterior mating surface defined by a relief is located at the relatively thin wall of the element and a plurality of punched first tube receiving slots are located at the first mating surface and are in fluid communication with the passage and are located at predetermined spaced intervals. An elongated strip having a second mating surface complementary to and abutted against the first mating surface is provided such that the thickness of the strip and the thin wall is substantially equal to or greater than the thickness of the thick wall. A second plurality of tube receiving slots are located in the strip and are punched therein and located at the same predetermined intervals as the tube slots in the first mating surface. They are aligned with the first tube receiving slots. A joint is provided that bonds the element and the strip together.

Preferably, the joint is a brazed joint.

According to still another facet of the invention, a high pressure heat exchanger is provided and includes a header as described previously. A plurality of tubes, each of flattened cross section, are provided and have their ends disposed within corresponding ones of the tube slots.

In one form, the ends of the tubes are twisted about 90° to the remainder of the corresponding tube and fins extend between and are bonded to adjacent ones of the remainders of the tubes.

Preferably, the fins are serpentine fins.

Other objects and advantages will become apparent from the following specification taken in connection with the accompanying drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevational view of a heat exchanger made according to the invention;

FIG. 2 is a cross-section of one embodiment of a header made according to the invention;

FIG. 3 is a plan view of a header made according to the invention;

FIG. 4 is a plan view of a strip that is applied to the header element shown in FIG. 3 to form a header made according to the invention;

FIG. 5 is a cross-section of a modified embodiment of the header;

FIG. 6 is a cross-section of still another modified embodiment;

FIG. 7 is a cross-section of another embodiment of a header made according to the invention;

FIG. 8 is a cross-section of still another embodiment of a header made according to the invention; and

FIG. 9 is a view similar to FIG. 7, but showing an alternate tube construction for use with the header;

FIG. 10 is a cross-section of still another embodiment of the invention in a condition just prior to final assembly prior to brazing; and

FIG. 11 is a cross-section of the embodiment of FIG. 10 at a subsequent step in its assembly and prior to brazing.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A heat exchanger made in accordance with the invention is illustrated in FIG. 1 and will be described in the context of a refrigeration system. However, the invention, in each of its facets, is applicable to high pressure heat exchangers, generally; and no limitation to refrigeration systems is intended except as set forth in the claims. The heat exchanger is seen to include opposed, spaced headers 10 and 12. The headers 10 and 12 are tubular as will be seen hereinafter and receive the ends 14 of straight flattened tubes 16. The ends 14 are in fluid communication with the interior of the headers 10, 12 and spaced from one another. Alternatively, the headers 10, 12 could be closely adjacent to one another with U-shaped flattened tubes (not shown) placed in fluid communication with the interiors of the headers 10, 12.

Fins 18, preferably serpentine fins, extend between and are bonded to adjacent ones of the tubes 16 intermediate between the ends 14.

The tubes 16 are flattened tubes and between the ends 14, have their major dimension running from front to back of the heat exchanger. That is to say, the fins 18 are bonded to the sides of the tubes 16 along their major dimensions. The minor dimension faces forward to minimize the obstruction to air flow imposed by the tubes 16 themselves.

Adjacent to the ends 14, the tubes 16 include a twist 20 allowing the ends 14 to be inserted into tube slots (not shown in FIG. 1) that are elongated and extend in the direction of elongation of the headers 10, 12. In the usual case, the twist 20 will be 90°, although other angles could be employed if desired.

One of the headers 10 may be provided with an inlet schematically indicated by an arrow 22 while the opposite header is provided with an outlet, schematically illustrated by an arrow 24. Of course, in some instances, the heat exchanger may be a so-called multi-pass heat exchanger, in which case baffles to direct the flow back and forth between the headers 10, 12 at least once may be provided. When the number of passes is an even number, both the inlet 22 and the outlet 24 will be in the same header 10, 12 whereas, for a single pass heat exchanger, or a multiple pass heat exchanger having an odd number of passes, the inlet 22 and outlet 24 will be in different ones of the headers 10, 12. Moreover, if desired, a multiple row heat exchanger could be made using a plurality of the structures shown in FIG. 1 in stacked relation with the headers 10 and/or 12 connected by manifolds which in turn can be baffled as well to provide any desired flow circuit.

Turning now to FIGS. 2-4, the headers 10, 12 will be described. As both are identical to each other, only the header 10 will be described in detail, it being understood that the same description applies to the header 12.

As seen in FIG. 2, the header 10 is a cylindrical tube 26 having a central, cylindrical bore 28 which serves as a passageway for one of the heat exchange fluids used with the heat exchanger. The header 10 has a relatively thick walled portion 30 and a relatively thin walled portion 32. The thick walled portion 30 is provided with a thickness sufficient to withstand, without deformation, the typical operating pressures encountered within the passage 28 during operation of the heat exchanger within a refrigeration system, plus an appropriate safety factor. The thin walled portion 32, at its thinnest point, has a thickness about half of that of the thick walled portion 30; and this thickness is such that a series of elongated tube slots 34 may be provided in the thin walled portion by a simple punching operation. In the embodiment illustrated in FIG. 2, the thin walled portion 32 is defined by the flat bottom 36 of a relief in the form of a groove 38 formed along the length of the header 10. The bottom 36 serves as a first mating surface and typically will be flat but may take on other configurations if desired.

According to the invention, an elongated strip 40 is bonded in the groove 38 as by brazing or soldering. To this end, the strip 40 typically will be braze clad. Such bonds are generically referred to herein as metallurgical bonds. The strip 40 has a plurality of elongated tube receiving slots 42 which are preferably of the same size and shape as the slots 34 in the header 10. They are also located at the same predetermined intervals as the slots 34. Thus, the elongated strip 40 may be inserted within the groove 38 and the tube slots 34 and 42 aligned with one another preliminary to forming the aforementioned metallurgical bond. While it is preferred that the slots 42 be of the same size and shape as the slots 34 in the header 10, in some applications it may be desirable for one set of the slots 34 or 42 to be of a size and shape suitable for receiving and forming a suitable bond joint with the ends 14 of the tubes and the other of the set of slots 34 or 42 being of a different size and/or shape that will not necessarily be suitable for forming a bond joint with the ends 14 of the tube 16.

The strip 40 has a flat surface 44 which is a second mating surface to mate with the bottom 36 of the groove 38. When other than flat surfaces are used as the bottom of the groove 36, the surface 44 will be configured to be complementary to the shape of the bottom 36 of the groove 38.

The strip 40 has a thickness approximately equal to or greater than half the thickness of the thick walled portion 30 of the header 10, or vice versa, so that the tube slots 42 may be formed therein by a simple punching operation. When assembled as illustrated in FIG. 2, the minimum total thickness of the header 10 at its thin walled portion 32 and the strip 40 will be equal to or exceed the thickness of the thick walled portion 30 of the remainder of the header.

In the usual case, aluminum will be utilized as the material for forming both the header 10 and the strip 40 because of its light weight so as to minimize the mass of the heat exchanger in which the header is used. However, other materials could be utilized if desired.

Significantly, the thickness of the thin walled portion 32 and the strip 40 are both chosen so that the tube slots 34, 42 may be punched in the respective elements 10, 40, rather than requiring forming by machining operations such as milling. As a result, the resulting tube slots, which are a combination of the slots 34 and 42, may be inexpensively formed thereby reducing the cost of the resulting header.

In a preferred embodiment, the headers 10 are formed by extrusion although it is possible to form them by other means as, for example, roll forming out of a strip of suitable material.

Typically, the headers 10 will have their thin walled portion 32 on the exteriors thereof for ease of application and alignment of the strip 40 thereto. However, it is possible to provide the relatively thin area 32 on the interior of the header, that is, as part of the inner wall defining the passage 28.

FIG. 5 shows an alternative embodiment where the header 10 is formed with a cross-section of a semi oval having sides terminating at points 50. In the embodiment of FIG. 5, the thin walled portion is located between the sides 50 and again is in the form of a relief provided by a groove 38 for receipt of a strip 40. The relative dimensions are the same as mentioned previously and therefore allow punching of the tube slots 34, 42 in both the header 10 and the strip 40. The header 10 and the strip 40 are, of course, metallurgically bonded to one another as mentioned previously.

FIG. 6 illustrates still another embodiment of the header 10 and again one wherein its cross-section is that of a semi oval. In this case, the groove 38 is not formed in favor of a simple, planar surface 52 serving as a relief extending between the ends 50 of the semi oval. In this case, a somewhat wider strip 40 may be employed to extend from one side 50 to the other of the header semi oval shape.

Again, the arrangement is such that a thin walled portion 32 is provided to be covered by the strip 40.

In general, the embodiments shown in FIGS. 2 and 5 are preferred in that the groove 38 provides for ease of positioning of the strip 40. And of those two, the embodiment illustrated in FIG. 2 is preferred because, as a comparison of the cross-sections of each of the embodiments illustrated in FIGS. 2, 5 and 6 will show, less material is required to form the embodiment illustrated in FIG. 2 than either of the others, thereby assuring a minimum of cost.

It is also preferred that the tube slots 34 and 42 be elongated, thereby accommodating the use of flattened tubes such as the tube 16. It is also preferred that the direction of elongation of the tube slots 34 and 42 be in the direction of elongation of the headers 10 and 12 as this allows a reduction in the diameter of both the passage 28 and the header 10,12. This reduction in diameter in turn allows the use of a thinner walled header 10, even at its relatively thick portion 30 while still meeting pressure resistance requirements for a system. It also minimizes the amount of material employed, all the while allowing the use of a fairly generous major dimension in the tubes 16.

By way of example, the embodiment illustrated in FIG. 2 may be made of a header having an outer diameter approximately 0.500 inches and with a diameter of the passage 28 of 0.25 inches. This provides a wall thickness of 0.125 inches for the relatively thick portion 30. The groove 38 may have a depth of about 0.062 inches while the strip 40 may have a thickness of 0.063 inches. The width of the strip 40 may also be approximately 0.250 inches.

In general, it is desirable that the thin walled portion 32, at its thinnest, be equal to approximately one-half the wall thickness of the header 10 and that the strip 40 have the same approximate thickness. When this is done, the ability to satisfactorily punch the tube slots 34 and 42 is maximized because both the thin walled portion 32 and the strip 40 will be at minimum thickness to facilitate punching.

FIGS. 7 and 8 show still a further modification of the invention. In the interest of brevity, the components common to the embodiment of FIG. 2 are given like reference numerals and will not be redescribed but it should be understood that previously described features of these components apply equally to the embodiments of FIGS. 7 and 8. In these embodiments, a pair of side-by-side bores or passages 54 are provided rather then a single cylindrical bore 28 as in FIG. 2. FIG. 7 illustrates the bores 54 as being cylindrical, whereas FIG. 8 shows the bores 54 as being non-cylindrical. The headers 10 of FIGS. 7 and 8 are desirable for use in multiple row heat exchanger having two rows of the tubes 16 (as shown in FIG. 7), rather than a single row of the tubes 16 as it has been previously described.

Each of the bores 54 has a set of the tube slots 34 and a set of tube slots 42 associated with the bore 54. It follows that the strip 40 has two sets of the slots 42, one set aligned with one of the bores 54 and the other set aligned with the other bore 54. For purposes of illustration, each of the tubes 16 is shown with its major dimension extending transverse to the longitudinal axis of the associated bore 54. However, it should be understood that in some applications it may be desirable for the major dimensions to extend parallel to the longitudinal axis of the associated bore 54.

A pair of the headers 10 of either FIGS. 7 and 8 can be provided in spaced, parallel relation, as shown for the headers in FIG. 1, with two rows of the tubes 16 extending between them, or each of the tubes 16 can be provided with a bend portion 55 remote from a single one of the headers 10 of FIG. 7 or 8 such that the two rows are formed from parallel legs 56 of the same tube with the ends of the tube 16 being received in a single one of the headers 10, as shown in FIG. 9. As yet another alternative, a single row of flattened tubes 16 having relatively large major dimensions can be used with either of the headers 10 of FIGS. 7 and 8, with a notch 57 provided in the end of the tube and portions 58 and 59 located on opposite sides of the notch 57 being received in respective ones of the tube slots 34,42 in fluid communication with respective ones of the bores 54.

FIGS. 10 and 11 show still a further modification of the invention. It is illustrated in the context of the embodiment illustrated in FIG. 2, but it will be readily appreciated that it is applicable to the embodiment of FIG. 5 as well. In the interest of brevity, the components common to the embodiment of FIG. 2 are given like reference numerals and will not be redescribed. In this embodiment, the groove 38 is flanked by tabs 60 along its length. As seen in FIG. 10, the ends 62 of the tabs extend beyond the radially outer side 64 of the strip 40 such that strip 40 nests within the groove 38 inwardly of the end 62 of the tabs 60. As seen in FIG. 11, the ends 62 of the tabs 60 may be crimped or otherwise deformed over the ends of the radially outer surface 64. This crimping may be along the entire length of the tabs 60 or may occur intermittently at desired locations along their length. In any event, the tabs 60, and specifically their ends 62 provide self-fixturing of the header tube assembly during brazing.

It is to be particularly noted that while the foregoing description is made with reference to the embodiment illustrated in FIG. 2, it is equally applicable to the embodiment illustrated in FIG. 5 and could actually even be employed with the embodiment of FIGS. 6, 7 and 8 if the width of the strip 40 were slightly reduced in the tabs placed on opposed sides of the surface 52. This structure assures that the strip 40 is firmly held within the notch 36 during brazing to assure a leak-free interface between the tube 10 and the strip 40.

From the foregoing, it will be appreciated that the invention can provide a low cost, low mass header for volume production in systems such as CO₂ refrigeration systems having minimum burst pressures of about 6,500 psi or more. The invention allows the use of a one step punching operation for each of the headers and the strips and thus eliminates the currently required milling process for forming tube slots and headers of the thicknesses of concern.

Claims

1. A method of making a high pressure resistant header for a heat exchanger, comprising the steps of:

(a) providing an elongated header structure including a pair of side-by-side longitudinally extending passages, the passages surrounded by a wall of sufficient thickness to resist deformation when a fluid is placed within said passages at an operating pressure at which deformation is to be resisted;

(b) thinning the wall along its length by providing a first mating exterior surface on a part thereof so that the wall, at said first mating exterior surface, is sufficiently thin that tube slots may be punched in said wall at said first mating surface extending through said wall into said passages;

(c) punching tube slots at predetermined spaced intervals in said wall at said first mating surface, with some of the slots extending into one of the passages and other of the slots extending into the other of the passages;

(d) providing an elongated strip having a second-mating surface complementary to said first mating surface and of a thickness such that the combined thickness of the strip and said wall at its first mating surface is about equal to or greater than said sufficient thickness of said wall;

(e) punching tube slots in said strip at said predetermined spaced intervals;

(f) abutting the second mating surface of said strip to the first mating surface of said header structure with the tube slots in each being aligned with one another; and

(g) bonding the strip to the header structure along their respective lengths to provide a unitary header with tube slots therein.

2. The method of claim 1 wherein both said mating surfaces are flat surfaces.

3. The method of claim 2 wherein steps (a) and (b) are performed simultaneously by extrusion of said header structure.

4. The method of claim 1 wherein steps (a) and (b) are performed simultaneously by extrusion of said header structure.

5. The method of claim 1 wherein step (b) is performed by providing a strip receiving groove in said part of said header structure exterior surface.

6. The method of claim 5 wherein said groove has a flat bottom surface defining said first mating surface and said second mating surface is flat.

7. The method of claim 1 wherein step (e) is performed by punching the tube slots in said strips to be of substantially the same size and shape as the tube slots in the first mating surface.

8. The method of claim 1 wherein step (b) is performed by forming a strip receiving groove in said wall.

9. The method of claim 8 wherein step (f) is preceded by the step of providing tabs on opposite sides of the strip receiving groove and succeeded by the step of deforming the tabs over the strip.

10. The method of claim 1 wherein step (f) is preceded by the steps of providing tabs on opposite sides of said first mating exterior surface and step (f) succeeded by and step (g) preceded by the step of deforming the tabs over opposite edges of the strip.

11. The method of claim 1 wherein said passages of step (a) are provided with a cylindrical shape.

12. The method of claim 1 wherein said passages of step (a) are provided with a non-cylindrical shape.

13. A header for a high pressure heat exchanger, comprising:

an elongated tubular like element having a pair of side-by-side longitudinally extending passages and a tube receiving side, said element being a unitary structure having a relatively thick wall partially surrounding said passages and a relatively thin wall at said tube receiving side;

a first mating exterior surface defined by a relief at said relatively thin wall of said element;

a plurality of punched first tube receiving slots at said first mating surface in fluid communication with said passages and located at predetermined spaced intervals;

an elongated strip having a second mating surface complementary to and abutted against said first mating surface such that the thickness of said strip and said thin wall is substantially equal to or greater than the thickness of said thick wall;

a plurality of second, punched tube receiving slots in said strip and located therein at said predetermined intervals, said second tube receiving slots being aligned with said first tube receiving slots; and

a joint bonding said element and said strip together.

14. The header of claim 13 wherein both said first and second mating surfaces are flat.

15. The header of claim 13 wherein said joint is a brazed joint.

16. The header of claim 13 wherein both said first and second mating surfaces are flat, and said joint is a brazed joint.

17. The header of claim 16 wherein said first mating surface is defined by the bottom of a groove formed in said exterior and said strip is located in said groove.

18. The header of claim 13 wherein said passages are cylindrical passages.

19. The header of claim 13 wherein said passages are non-cylindrical passages.

20. The header of claim 13 wherein said tube slots are elongated in the direction of elongation of said element.

21. The header of claim 13 including tabs on opposite sides of said relief and deformed over opposite sides of said elongated strip.

22. A high pressure heat exchanger comprising:

at least one header defined by an elongated tubular like element having a pair of side-by-side longitudinally extending passages and a tube receiving side, said element being a unitary structure having a relatively thick wall partially surrounding said passages and a relatively thin wall at said tube receiving side;

a first mating exterior surface defined by a relief at said relatively thin wall of said element;

a plurality of punched first tube receiving slots at said first mating surface in fluid communication with said passages and located at predetermined spaced intervals;

an elongated strip having a second mating surface complementary to and abutted against said first mating surface such that the thickness of said strip and said thin wall is substantially equal to or greater than the thickness of said thick wall;

a plurality of second, punched tube receiving slots in said strip and located therein at said predetermined intervals, said second tube receiving slots being aligned with said first tube receiving slots;

a joint bonding said element and said strip together;

a plurality of tubes, each of flattened cross section, having their ends disposed within said tube slots; and

fins extending between and bonded to adjacent ones of said tubes.

23. The heat exchanger of claim 22 wherein said fins are serpentine fins.

24. The heat exchanger of claim 22 wherein both said first and second mating surfaces are flat.

25. The heat exchanger of claim 22 wherein said first mating surface is on the exterior of said element.

26. The heat exchanger of claim 22 wherein said joint is a brazed joint.

27. The heat exchanger of claim 22 wherein both said first and second mating surfaces are flat, said joint is a brazed joint.

28. The heat exchanger of claim 27 wherein said first mating surface is defined by the bottom of a groove formed in said exterior and said strip is located in said groove.

29. The heat exchanger of claim 22 wherein said passages are cylindrical passages.

30. The heat exchanger of claim 22 wherein said passages are non-cylindrical passages.

31. The heat exchanger of claim 22 including tabs on opposite sides of said relief and deformed over opposite sides of said elongated strip.

32. The heat exchanger of claim 22 wherein said plurality of the tubes comprises a first group of tubes having their ends disposed within said tube slots in fluid communication with one of said passages, and a second group of tubes having their ends disposed within said tube slots in fluid communication with the other of said passages.

33. The heat exchanger of claim 22 wherein each of said ends of each of said plurality of tubes is notched with a first part on one side of said notch disposed within said tube slots in fluid communication with one of said passages, and a second part on the other side of said notch disposed within said tube slots in fluid communication with the other of said passages.

34. The heat exchanger of claim 22 wherein each of said tubes of said plurality of tubes include a pair of parallel legs and a bend joining said legs at a location remote from said at least one header, one of said legs having an end disposed within said tube slots in fluid communication with one of said passages, and the other of said legs having another end disposed within said tube slots in fluid communication with the other of said passages.