WO1997025577A1

WO1997025577A1 - Composite metal fin and method for producing the same

Info

Publication number: WO1997025577A1
Application number: PCT/US1996/020750
Authority: WO
Inventors: Jerry E. Ryan
Original assignee: Fintube Limited Partnership
Priority date: 1996-01-11
Filing date: 1996-12-13
Publication date: 1997-07-17

Abstract

The invention is a bi-metal or multi-metal fin (10) and method for making the same for use on a finned heat exchange tube (20). The fin (10) is created by completely encasing one or more internal metals (38) within an external metal (20) so once the fin (10) is welded to the heat exchanger tube (16), only the external metal (20) is exposed externally on the fin (10). The external metal (20) is selected for its weldability to the tube material (16) and for its corrosion resistance while the internal metal or metals (38) are selected for thermal conductivity and other desirable characteristics, depending on the application where the finned heat exchange tube (20) is to be employed.

Description

COMPOSITE METAL FIN AND METHOD FOR PRODUCING THE SAME

Background Of The Invention

1. Field of the Invention

The present invention relates to a fin for attachment to a finned heat exchanger tube. More specifically, the fin and the method for producing the fin involve use of two types of metals, a more heat resistant weldable metal externally on the fin and a more heat conductive metal internally on the fin.

2. Description of the Related Art

Different metals, such as for example, stainless steel, carbon steel, aluminum and copper, differ in their thermal conductivity and in their corrosion resistance. The thermal conductivity and corrosion resistance of metals are two critical criteria in deciding what type of metal to employ in producing a fin for use on a finned tube for service in a given heat exchange application. Other criteria which may also affect the choice of metal to be employed include whether the metal is magnetic or non-magnetic, the density or weight of the metal, the structural strength of the metal at elevated temperatures, the price of the metal, and welding characteristics.

In order to optimize the thermal efficiency and performance of the fin, it would be desirable to create a fin comprised of two or more metals, with an external metal completely encasing the other internal metal or metals. Preferably, the external metal would be structurally strong at the elevated temperatures to which it would be constantly exposed in a heat exchange environment, often in the range of 600° F - 800" F, and would also be highly resistant to corrosion so that it would be able to withstand the corrosive gases or liquids often encountered in certain heat exchange environments. Metals such as stainless steel or carbon steel are each good candidates for use as an external metal for such a bi-metal fin.

However, neither of these metals are as thermally conductive as other softer metals. Thus, it is desirable to use more thermally conductive metals for the internal metal or metals, such as for example, aluminum, copper or even carbon steel when stainless steel is used as the external metal. By employing an internal metal of higher thermal conductivity with a more corrosion resistant, weldable, external metal, the resulting fin is able to perform at a higher efficiency than a fin comprised of the external metal alone yet is still able to remain structurally strong and resist corrosion better than a fin comprised solely of the internal metal.

The desirability of employing combinations of different metals in heat exchange applications is widely known. However, until now there has not been an economical method for producing a bi-metal fin with an external metal which completely protects the inner metal from exposure to corrosion and heat.

It is an object of the present invention to provide a bi-metal fin composed of one or more metals sandwiched between and fully protected externally by another external metal.

It is a further object of the invention to provide a bi-metal fin which can achieve better corrosion resistance and better thermal conductivity than possible with a fin composed of a single metal.

A still further object of the invention is to provide a bi-metal fin whose welding properties, magnetic properties, weight and cost can be controlled by proper selection of an internal metal or metals. For heat absorbing tubes, another object of the invention is to keep the tips of the fin cooler. By keeping the tips cooler, the external metal of the fin can be selected from metals which are less expensive than the alloys which are capable of withstanding higher temperatures without oxidizing.

SUMMARY OF THE INVENTION

The present invention is a bi-metal or multi-metal fin and method for making the same. The fin is provided with a base side which can be welded by means of high frequency current, to the external surface of a heat transfer tube. In order to create the fin, first a strip of external metal is passed longitudinally between several pairs of rollers in order to form the external metal into a cross-sectionally U-shaped configuration, having a void formed between two upwardly extending side walls. A shorter strip of internal metal is then inserted longitudinally within the void. Then the external and internal metals are passed between another set of rollers in order to physically bond the metals together to form the fin. The uppermost surface of the internal metal is somewhat recessed from the edges of the external metal side walls so that the internal metal will not be involved when the base side of the fin is subsequently welded to the tube. This permits steel tubes welded with steel base sided fins. Any one of several methods may be used to manufacture the multi-metal fin. A first alternate fin is created by welding a plug onto the base end of the fin before welding the fin to the tube.

A second alternate fin is created by substituting powder internal metal for the shorter strip and thereafter heating the internal and external metals so as to melt the internal metal within the void.

A third alternate fin is created by first flame spraying an internal metal onto one side of the external metal strip, preferably leaving the edges of the strip free of sprayed internal metal, then folding the strip longitudinally via rollers to form the third alternate fin. A fourth alternate fin is created by a method similar to the method employed to create the third alternate fin, only instead of flame spraying, employing diffusion cladding to clad one side of the external metal strip with clad internal metal, preferably leaving the edges of the strip free of clad internal metal. The strip is then folded longitudinally via rollers to form the fourth alternate fin. A fifth alternate fin is created by inserting a rod of internal metal within a void provided within hollow tubing comprised of external metal. The tubing encased rod is then passed through a series of rollers to flatten and physically bond the external and rod internal metals together. Either side of the resulting fifth alternate fin may be designated as the base side to be welded to the heat transfer tube.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a perspective view illustrating the steps of a preferred method for making a bi-metal fin in accordance with the present invention.

Figure 2 is a cross-sectional view of the finned tube of Figure 1 taken along line 2-2.

Figure 3 is a cross-sectional view taken along line 3-3 of Figure 1.

Figure 4 is a cross-sectional view taken along line 4-4 of Figure 1.

Figure 4A is a cross-sectional view taken along line 4A-4A of Figure 1.

Figure 5 is a cross-sectional view taken along line 5-5 of Figure 1.

Figure 6 is a cross-sectional view taken along line 6-6 of Figure 1.

Figure 7 is a cross-sectional view taken along line 7-7 of Figure 1.

Figure 8 is a cross-sectional view of a fin similar to the one illustrated in Figure 7, except with the open side of the fin welded closed.

Figure 9 is a cross-sectional view of a fin similar to the one illustrated in Figure 7, except employing a granular or powdered internal metal. Figure 10 is a cross-sectional view of a fin similar to the one illustrated in Figure 7, except that the internal metal was flame sprayed onto one side of the external metal prior to the steps illustrated in Figures 3 and 4.

Figure 11 is a cross-sectional view of a fin similar to the one illustrated in Figure 10, except that the internal metal was clad onto one side of the external metal prior to the steps illustrated in Figures 3 and 4.

Figure 12 is a cross-sectional view of a rod inserted within hollow tubing to illustrate the initial steps of an alternate method for creating a bi-metal fin.

Figure 13 is a cross-sectional view of a bi-metal fin resulting when the tubing and encased rod of Figure 12 are roll formed in order to flatten them.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings and initially to Figure 1 , there is illustrated a process for making a bi-metal fin 10 according to a preferred embodiment of the present invention. As illustrated, the bi-metal fin 10 has a base side 12 which may be welded helically by means of high frequency current 13 to an external surface 14 of a heat exchange tube 16, as is commonly done to increase heat transfer efficiency between the external surface 14, and an opposite internal surface 18 of the tube 16. A portion of the resulting finned tube 20 is shown in cross section in Figure 2. Referring now to Figures 1 , and 3 through 7, the bi-metal fin 10 is produced by first creating a void 21 in an external metal 20. One method of creating the void 21 is to begin with a flat strip 22 of external metal 20 of a width "W". The strip 22 of external metal 20 is passed between first upper and lower rollers 24 and 26 in order to form from the external metal 20 a longitudinal trough with two upwardly extending side walls 27A and 27B, one on either side of the trough, as shown in cross section in Figure 3. The trough and the side walls 27A and 27B are parallel with a longitudinal axis 23 of the strip 22, and the trough will eventually be deepened until the side walls 27A and 27B are parallel with one another, as illustrated in Figure 4A and as will be described hereafter. Next, the external metal 20 is passed between second upper and lower rollers 28 and 30 in order to further deepen the trough formed in the external metal 20, as shown in cross-section in Figure 4.

The external metal 20 is then passed between third upper and lower rollers 32 and 34 in order to form the external metal 20 into a U-shaped configuration with the void 21 formed between the opposite and now parallel side walls 27A and 27B of the U-shaped external metal 20, shown in cross section in Figure 5.

Once the void 21 is formed in the external metal 20, a shorter strip 36 of internal metal 38 is inserted into the void 21 between the side walls 27A and 27B, as shown by the arrow in the cross sectional view appearing in Figure 5.

The shorter strip 36 has a height H which is slightly less than one-half of the width W of the original strip 22 of the external metal 20 so that when the shorter strip 36 is fully inserted within the void 21 , an uppermost surface 39 of the shorter strip 36 is recessed within side walls 27A and 27B at the base side 12, as best seen in Figure 7. This recessing of the internal metal 38 allows the external metal 20 of the side walls 27A and 27B to be welded to the heat transfer tube 16 without significantly involving the internal metal 38, thus resulting in a stronger weld.

Once the shorter strip 36 of internal metal 38 begins to enter the void 21 , the external and internal metals 20 and 38 are passed between fourth upper and lower rollers 40 and 42 in order to physically bind together the external and internal metals 20 and 38 in tight mechanical contact, as shown in cross section in Figure 6. The resulting bi-metal fin 10, which is shown in cross section in Figure 7, is then ready to be welded by its base side 12 to the external surface 14 of the heat transfer tube 16, as previously described and as illustrated in

Figure 1.

A leading end 41 A and a trailing end 41 B of the fin 10 are welded closed either before or after the fin 10 is welded to the tube 16 so that, if welded together prior to the fin 10 being welded to the tube 16, the internal metal 38 is thus encased within the external metal 20 on five sides of the internal metal 38, the uppermost surface 39 remaining the only exposed side of the internal metal 38. Once the fin 10 is welded to the tube 16, the uppermost surface 39 is sealed against the external surface 14 of the tube 16, thus totally sealing the internal metal 38 within the external metal 20.

Alternately, if the fin 10 is welded to the tube 16 prior to welding closed the leading and trailing ends 41 A and 41 B, the internal metal 38 is encased on three sides in the fin 10, encased on four sides when the fin 10 is welded to the tube 16, and totally encased on all six sides when the leading and trailing ends

41 A and 41 B are thereafter welded closed.

Although the term "bi-metal" is used throughout this document, the invention is not limited to use of only one external metal 20 and one internal metal 38. It is contemplated that one or more different metals may be used simultaneously as internal metals 38 in order to achieve the desired thermal efficiency and other desired physical properties in the bi-metal fin 10.

A first alternate embodiment of the bi-metal fin 10A, which is illustrated in Figure 8, is created by a method essentially identical to the one previously described for the bi-metal fin 10, but additionally welds a plug 44 onto the base side 12 in order to seal the internal metal 38 completely within the external metal 20 immediately after exiting the fourth rollers 40 and 42 and just prior to being welded to the heat transfer tube 16.

A second alternate embodiment of the bi-metal fin 10B, which is illustrated in Figure 9, is created essentially identically to the previously described method for creating the bi-metal fin 10 except a granular internal metal 38B is substituted for the shorter strip 36 of internal metal 38. After the granular internal metal 38B is placed into the void 21 and passed between the fourth rollers 40 and 42, but before the second alternate bi-metal fin 10B is welded to the heat transfer tube 16, the external metal 20 and the granular internal metal 38B are heated to a temperature exceeding the melting point of the granular internal metal 38B, but not up to the melting point of the external metal 20, which is a higher melting point than the melting point of the granular internal metal 38B. Prior to heating, the external and the granular internal metals 20 and 38B are maintained in an upright position, i.e., with the base side 12 upward, so that the granular internal metal 38B remains within the void 21 and does not spill out under the influence of gravity. After the heating is completed, the now melted granular internal and unmelted external metals 38B and 20 are maintained in an upright position until the granular internal metal 38B cools below its melting point and anneals within the void 21. In order to facilitate rapid cooling, the second alternate bi-metal fin 10B can be subject to any of a variety of commonly available cooling processes. After sufficient cooled, the second alternate bi-metal fin 10B is then ready to be welded to the heat transfer tube 16, as previously described.

Figures 10 and 11 illustrate third and fourth alternate embodiments of the bi-metal fin, 10C and 10D respectively. The third alternate bi-metal fin 10C is created by first flame spraying a sprayed internal metal 38C onto one side of the flat strip 22 of external metal 20 prior to passing the strip 22 through consecutively, the first, second, third and fourth rollers; 24 and 26; 28 and 30; 32 and 34; and 40 and 42, to form the third alternate fin 10C.

The fourth alternate fin 10D is created identically to the third alternate fin 10C except that initially a clad internal metal 38D is diffusion clad, i.e., bonded, onto one side of the flat strip 22 of external metal 20, instead of flame spraying, prior to passing the strip 22 through consecutively the first, second, third and fourth rollers; 24 and 26; 28 and 30; 32 and 34; and 40 and 42, to form the fourth alternate fin 10D. As shown in Figures 10 and 11 respectively, it is preferable that the sprayed internal metal 38C and the clad internal metal 38D provided on the third and fourth alternate bi-metal fins 10C and 10D, respectively, be omitted on both edges 45A and 45B of the flat strip 22 in order that the prayed internal metal 38C and the clad internal metal 38D are recessed within the external metal 20 on the base side 12 of the completed third and fourth alternate bi-metal fins 10C and 10D. Still a fifth alternate fin 10E is shown in Figure 13. As illustrated in Figure

12, the fifth alternate fin 10E is created by first inserting a rod 46 of rod internal metal 38E within a circular void 21E formed in hollow tubing 48, as illustrated in cross section in Figure 12. Then the tubing 48 and the enclosed rod internal metal 38E are passed between a series of rollers (not illustrated) in order to gradually flatten them into the fifth alternate fin 10E, as shown in cross section in Figure 13. Either side 50 or 52 of the fifth alternate fin 10E may be designated as the base side 12 since both sides 50 and 52 of the fifth alternate fin 10E are identical. Once flattened, as shown in Figure 13, the fifth alternate fin 10E is then ready to be welded onto the heat transfer tube 16. Thus, the bi-metal fin 10, the first alternate fin 10A, and the second alternate fin 10B all are created by the steps of forming the void 21 in the external metal 20 between side walls 27A and 27B, inserting internal metal 38 (or, in the case of the second alternate fin 10B, granular internal metal 38B is used instead of internal metal 38) into the void 21 and then processing the external and internal metals 20 and 38 (or in the case of the second alternate fin 10B, 20 and 38B) so that metals 20 and 38 (or alternately 20 and 38B) are physically bound together. This physical binding together is accomplished in fins 10 and 10A by passing the external and internal metals 20 and 38 between fourth upper and lower rollers 40 and 42; physically binding together is accomplished in fin 10B by heating the external and granular internal metals 20 and 38B to melt the granular internal metal 38B and then cooling the metals 20 and 38B so the granular internal metal 38B anneals. On the other hand, the third and fourth alternate fins 10C and 10D physically bind, respectively, the sprayed internal metal 38C and the clad internal metal 38D to one side of the external metal 20 before the external metal

20 is folded lengthwise so the external metal 20 encases, respectively, the sprayed internal metals 38C and the clad internal metal 38D.

The fifth alternate fin 10E is created by forming the external metal 20 into hollow tubing 48, inserting the rod 46 of rod internal metal 38E into a circular void 21 E within the hollow tubing 48 and then passing the external and rod internal metals 20 and 38E through consecutive sets of rollers (not illustrated) in order to flatten the metals 20 and 38E into, respectively, the side walls 27A and 27B and the flattened rod internal metal 38E which is located between the side walls 27A and 27B, thus physically binding them together to create the fifth alternate fin 10E.

Each of the alternate fins 10A, 10B, 10C, 10D and 10E is provided with a leading end 41 A and an opposite traveling end 41 B which are welded closed either immediately prior to or immediately after welding the fin 10A, 10B, 10C, 10D or 10E to the heat transfer tube 16.

While the invention has been described with a certain degree of particularity, it is manifest that many changes may be made in the details of construction and the arrangement of components without departing from the spirit and scope of this disclosure. It is understood that the invention is not limited to the embodiments set forth herein for the purposes of exemplification, but is to be limited only by the scope of the attached claim or claims, including the full range of equivalency to which each element thereof is entitled.

Claims

What is claimed is:

1. A composite metal fin for use on a finned heat exchange tube comprising:

at least one internal metal encased within at least one external metal to form a fin so that only external metal is exposed externally on said fin when said fin is attached externally to a finned heat exchange tube.

2. A composite metal fin according to Claim 1 wherein said fin is attached to said tube by means of a base end of said fin.

3. A composite metal fin according to Claim 2 wherein said internal metal is recessed within said external metal at said base end.

4. A composite metal fin according to Claim 3 further comprising a plug welded on said base end to seal said internal metal within said external metal prior to welding said fin to said tube.

5. A composite metal fin according to Claim 1 wherein said internal metal is flame sprayed onto said external metal.

6. A composite metal fin according to Claim 1 wherein said internal metal is diffusion clad onto said external metal. 7. A composite metal fin according to Claim 1 wherein said internal metal is powdered, then melted and cooled to annealing within said external metal which is not melted.

8. A composite metal fin according to Claim 1 wherein said internal metal is placed inside tubing of external metal and then rolled to flatten.

9. A method for creating a composite metal fin comprising the following steps:

a. Folding a strip of external metal lengthwise to form a pair of external side walls therefrom;

b. inserting at least one internal metal between said side walls;

c. Physically bonding said internal metal to said external metal;

d. Welding closed a beginning end and a trailing end of said fin;

e. Welding a base side of said fin externally to a heat exchange tube.

0. A method according to Claim 9 wherein:

Step d is performed after step e. 11. A method according to Claim 9 further comprising the following step which is performed before step e.

f. Welding a plug onto a base side of said fin.

12. A method according to Claim 11 wherein:

Step d is performed after step e.

13. A method according to Claim 9 wherein:

Step c is performed before step a, and step b is omitted.

14. A method according to Claim 13 wherein step c is performed by flame spraying said internal metal to one side of said external metal.

15. A method according to Claim 14 wherein:

Step d is performed after step e.

16. A method according to Claim 13 wherein step c is performed by diffusion cladding said internal metal to one side of said external metal.

17. A method according to Claim 16 wherein:

Step d is performed after step e. 18. A method according to Claim 9 wherein in step c the internal and external metals are physically bonded together by passing said metals between at least one pair of rollers.

19. A method according to Claim 18 wherein:

Step d is performed after step e.

20. A method according to Claim 9 wherein step c further comprises the following steps:

a. Heating said internal metal and said external metal to a temperature which exceeds a melting point of said internal metal but does not reach a melting point of said external metal; and

b. cooling said internal and external metals to bond said internal metal to said external metal.

21. A method according to Claim 20 wherein:

Step d is performed after step e.

22. A method according to Claim 9 further comprising the following step which is substituted for steps a and b:

a. inserting a rod comprised of internal metal within hollow tubing comprising of external metal. 23. A method according to Claim 22 wherein:

Step d is performed after step e.

24. A method according to Claim 22 wherein in step c the internal metal and external metal are physically bonded together by passing said metals between pairs of rollers in order to flatten said metals.