US2261137A

US2261137A - Heat exchange conductor

Info

Publication number: US2261137A
Application number: US380690A
Authority: US
Inventors: Jr John W Brown
Original assignee: Brown Fintube Co
Current assignee: Brown Fintube Co
Priority date: 1941-02-26
Filing date: 1941-02-26
Publication date: 1941-11-04
Anticipated expiration: 1958-11-04

Description

No, 4, 194k 3, W, ERQWNy JR 2,261,137

HEAT EXCHANGE CONDUCTOR Filed Feb. 26, 1941.1

V INVENTOR. w `TDH/v wow/v; y@

l A NORA/YS APatented Nov. 4, 1941 HEAT EXCHANGE CONDUCTOR John W. Brown. Jr., Lakewood, Ohio. aolignor to Brown Fintube Company, Elyria,

poration o! Ohio Ohio, a cor- Application February 28, 1941, No. 380,690

Claims. (Cl. 257-262) This invention relates to finned tubes adapted particularly for use as heat exchange conduits, although the tubes are also useful as structural members. This application constitutes,vinsofar as common subject matter is concerned, a continuation in part of my co-pending application Serial No. 323,593 filed March 12, 1940.

In the manufacture of heat exchangers it is desirable to augment the area of the fluid conducting tubes employed by securing fins of various types to the exterior of the tubes. Fins extending longitudinally of the tubes are particularly advantageous in some types of installations because of the desirable flow characteristics which can be obtained in the fluid surrounding the tube. For other types of services, helical or circumferential fins may be preferable. Various schemes have been proposed for securing such fins to the exterior of tubes but so far as I am aware, all prior constructions have been open to serious objections in that they were expensive and did not connect the iins to the tubes in such manner that full advantage could be taken of the heat transferring capacity of the fins; that is, in prior types of finned tubes the transfer of heat from the tube to the fins has not been as enlcient as desired.

It Is accordingly among the objects of my invention to provide a finned tube wherein an adequate path is provided for the flow of heat from the tube to the fins. Another object is to provide such a tube which can be rapidly and efliciently manufactured. Further and more specific objects of my invention will become apparent from the following description of preferred forms thereof, reference being made to the accompanying drawing. The essential characteristics are summarized in the claims.

Briefly, my invention, in its preferred form, is directed to finned tubes in which each iin member comprises a fin portion and a base portion welded to the tube; for example, each fin member may be of sheet metal preformed to either a U-shaped or an L-shaped cross-section with the base of the U or L, as the case may be, welded to the exterior of the tube. Preferably a plurality of longitudinally extending n members are secured to the exterior of the tube, the

bases of two oppositely disposed fin members being welded simultaneously to the tube by electric resistance welding to produce substantially identical rows of welded areas, the welding current being interrupted to produce a series of spot welds disposed in such a manner that the welded area between the bases of the fln members and the tube is such that the path for the transfer of heat from the tube through the welded areas and into the base of the fin member will be at least substantially as great as the path for the transfer of heat from the base of the fin member to the fins themselves.

Preferably the welding operation is carried out with opposite pairs of welds made by the use of two. electrodes pressed against the bases of two oppositely disposed fin members, with the current flowing through the tube from the weld in one fin base to that in the other fin base, a preferred method and apparatus being described in detail in my said application Serial No. 325,593.

Referring now to the drawing, Fig. 1 is a perspective view illustrating a section of finned tube made according to one preferred form of my invention; Fig. 2 is a fragmentary plan view on an enlarged scale, illustrating one of the finned members secured to the tube; Figs. 3and 4 are fragmentary sections through the tube as indicated by the lines 3-3 and 4--4 respectively on Fig. 2; and Figs. 5 and 6 are views similar to Figs. 2 and 4 but showing a slightly modified form of tube. v

As shown in Fig. 1 of the drawing, a finned tube made according to a preferred form of my invention may comprise a tubular member T to which a number of fin members II are secured. Each fin member is preferably ofchannel section as shown in Fig. 3 and may comprise a base portion I2 and fins I4 formed integrally therewith, the base and fins being of substantially the same thickness. The base portions may be curved slightly, as shown, to conform substantially to the outer surface of the tube.

It will be evident to those skilled in the art that the transfer of heat from the tube T to the fins I4 must take place through the base portion I2. Further, the limiting factor in the flow of heat from the base portion I2 to the ns I4 is found in the longitudinal sectional area of the fins adjacent the base; i. e., the regions indicated at a-a in Fig. 3. Per unit of length, this area is of course proportional to the thickness of these sections.

It follows, then, that if full advantage is to be taken of the heat transferring capacity of the fins Il, the path or paths for the flow of heat from the'tube T to the base I2 must be at least equal in area to the combined areas of the ns I4 in the regions a-a. To accomplish this result I secure the channel members I I to the tube by a series of resistance welds indicated diagrammatically at I6 and having a combined area per unit of length at least substantially equal to the longitudinal sectional area of the ns I4 at the junctures between the fins and the base portion I2.

The desired area can be readily secured by a series of resistance welds spaced as shown in the drawing and any increase in area is of no material advantage so far as heat transfer efficiency is concerned, for, in any event, the transfer of heat to the fins I4 will be limited to the conductive capacity of the portions a-a of the fins. An important advantage in securing the ns to the tube by intermittent welds is found in the fact that if the tube is subjected to a sudden heat shock in service the fins can buckle or stretch in the regions between welds without subjecting the tube itself to unduly large forces, whereas if the ns were welded continuously to the tube such an action would not be possible and a heat shock might have the effect of damaging the tube.

A further and important advantage of utilizing intermittent welds will be apparent from a consideration of Figs, 2, 3 and 4. As shown in a somewhat exaggerated fashion in Figs. 3 and 4 the production of the weld between the base portion I2 and the tube T results in a thinning of the base portion in the region of the weld because of the electrode pressure required to produce a welded connection, the amount which the metal has been thinned being indicated at I'I. The area available for transfer of heat from a Welded area to the base is determined by the product of the thickness of the welded area at its juncture with the remainder of the base and the perimetric length of the welded area.

While the thickness of the base is reduced in the zone I8 to a thickness less than the thickness of the flanges in the regions a-a, adequate paths for the flow of heat into the basethat is, paths with areas at least substantially as great as the paths forflow of heat from the base into the fin portions-are provided by my arrangement wherein the fins are secured to the tube by a series of equally spaced resistance welds. As shown in Fig. 2, by utilizing a series of resistance welds, the limiting cross-sectional area becomes the peripheral area of the minute cylinders which constitute projections of the VWelded areas through the base. The axial length of eachof the cylinders is the distance indicated at I8 While the periphery of each is substantially the same as the periphery of the adjacent Welded area I 6. The welding operation is carried out so that the combined perimetric lengths of the welded areas and, correspondingly, of the cylinders in the arrangement shown in Fig. 2 exceeds the total length of the flanges on section of tubing illustrated, the total Aperimetric length of the Welded areas having at least substantially the same proportional relationship to the total length of the flanges as the' original thickness of the base bears to the thickness of the welded areas. Thus the increase in perimetric length as compared to the length of the flanges, at least offsets the decrease in section due to the Welding operation and there is no loss in efliciency of heat transfer at this point. For example, in a one inch length of tube with a channel section n member having a thickness of 0.031 inch, the cross-sectional area at the base of the fins will be 0.062 square inch. If the welding operation is carried out in such a manner as to produce about three uniformly spaced welds to the inch with each welded area being of slightly oval form, a little more than 1A inch long and %4 inch wide, the periphery of each spot will be about 3/4". With three such spots to the inch. and the thickness of the metal in the region of the welds being reduced to 0.028 inch, the limiting area for the transfer of heat from the welded areas to the remainder of the base of the tube will be 3 x :V4 x 0.028=0.063 square inch.

As an example of a procedure which I have found to be satisfactory in welding low carbon steel channel section fins of a thickness of about .031", to a low carbon steel twelve-gauge tube having an outside diameter of 2", with the welded area substantially as described above, I engage the bases of the channel members with electrode rolls having a width of about %4 inch with an electrode pressure of 900 to 1,000 pounds. The current employed is preferably about 12,000 amps. at 6-8 volts. Each weld is made with a duration of current of 3 cycles of 60-cycle, single phase, alternating current, the current being shut oi for a period of 3 cycles between welds, the ns being welded to the tube at a speed of 18 to 20 feet per minute, this speed being amply suillcient to prevent the tube from being deformed by the pressure of the electrodes.

As described above, with this procedure the total welded area between the bases of the tins and the tubel is suflicient to give an ample path for conduction of heat to the bases and the periphery of the welded areas in the bases is such that there is an ample path for the flow of heat from the region of the welds into the adjacent areas of the bases.

As another example of a procedure which gives satisfactory results, fins composed of stainless steel containing 4% to 6% of chromium and having a thickness of .031" may be welded to a twelve-gage tube composed of the same material with a current of about 9600 amperes, the control being arranged to have the current flow to produce the welds for a period of 2 cycles, the current being shut off for a like period between welds. An electrode pressure of 900 to 1,000 pounds is employed with electrodes 964" Wide, the operation being carried on at a speed of from 12 4feet to 14 feet per minute. 'I'his procedure gives about seven welds to the inch, the welds overlapping slightly.

A section of finned tubing made according to the above outlined method is illustrated in Figs. 5 and 6 of the drawing wherein the channel member 2| is secured to the tube T by a series of welded areas 26 joining the base 22 of the channel to the tube. It will be noted that these welded areas are more closely spaced than those illustrated in Figs. 2 and 4, with the result that the welds overlap, producing a scalloped weld extending longitudinally of the fin. The area of these welds exceeds the longitudinal cross-sectional area of the fins 24 in the region adjacent the juncture of fln and base and by reason of the scalloped edges the perimetric length of the welded areas exceeds the total length of the flanges 24, so that in spite of the fact that the thickness of the fin base is reduced by the welding operation, an adequate path for the flow of heat from the welded areas to the adjacent portions of the base is provided by reason of the increased length of the weld giving an adequate cross-sectional area for the flow of heat. This type of weld also produces a tube that is resistant to heat shock, for the scalloped welds provide zones where the welded connections are quite narrow, thus permitting some buckling of the fins to take place and preventing damage to the tube.

It will be evident that the greatest increase in length can be obtained when the welded areas are just tangent to each other. Welds of this type are particularly advantageous in conjunction with the welding of fins of thin section where the thickness of the nn base is reduced by a considerable percentage by the welding operation so that a material extension of the perimetric length of the welded areas is required in order to insure an adequate path for the flow of heat. It is to be understood that welds of this type are included within the terms spaced or intermittent as the terms are used herein, for while the welded areas overlap, their centers are spaced and they are produced by intermittent currents and the intermittent character of the welds can readily be determined by an inspection of a welded sample.

As those skilled in the art will appreciate, the speed, pressure, current and timing may be varied to give the desired results with different materials, sizes and conditions of operation. The procedures outlined are given merely by way of example.

From the foregoing description of preferred forms of my invention it will be evident that I have provided an efficient and economical finned tube member which is particularly useful in heat exchangers and in which full advantage is taken of the heat transfer capacity of the fins. Tubes embodying my invention not only are eflicient from'the standpoint of heat transfer but also have great structural strength and are not liable to failure because o heat shocks. Further, my finned tubes can be produced economically and rapidly, and can be made of materials and in sizes and types to meet a wide variety of uses.

Various changes and modifications in my invention will be apparent to those skilled in the art. It is therefore to be understood that my patent is not limited to the preferred form of my invention described herein and that it is contemplated that changes and modifications Within the scope and spirit of my invention may be made by those skilled in the art. Therefore it is to be understood that the description herein is given by way of example and not by way of limitation and that my patent is not limited to the forms described herein or in any manner other than by the appended claims when given the range of equivalents to which my patent may be entitled.

Reference is hereby made to my copending application Serial No. 240,328 filed November 14, 1938, which contains related subject matter, and to my copending application Serial No. 388,835 filed April 16, 1941, which relates to methods of making linned tube heat exchange conductors.

I claim:

1. A heat exchange conductor comprising a pipe, a plurality of channel shaped iin members extending longitudinally of the pipe with their base or web portions engaging the outer surface oi.' the pipe, each of said channel members being united to the pipe by a series of substantially evenly spaced spot welds, said spot welds overlapping to produce a welded area having scalloped edges, the combined area of said welds being at least substantially equal to the combined longitudinal cross sectional area of the two ilanges of the channel, and the total perimetric length of said welds exceeding twice the length of the channel member.

2. A heat exchange conductor comprising a pipe, a iin member secured to the outer surface of the pipe, said iin member comprising a base portion and at least one flange portion formed integrally therewith and being united to the pipe by a series of substantially evenly spaced spot welds, said spot welds overlapping to produce a welded area having scalloped edges, the oombined area of said welds being at least substantially equal to the longitudinal cross sectional area of the flange or flanges, and the total perimetric length of said welds exceeding the total length of said flange or flanges.

3. A heat exchange conductor comprising a pipe, a n member secured to the outer surface of the pipe, said 1in member comprising a base and at least one flange portion formed integrally therewith, the base being secured t0 the tube by a resistance welded area extending therealong, the base being reduced in thickness in said welded area, said welded area having scalloped edges, the perimetric length thereof being increased as compared to its lineal length by a percentage at least substantially as great as the percentage of reduction of thickness of said area.

4. A heat exchange conductor comprising a pipe, a fin member secured to the outer surface of the pipe, said fin member comprising a base and at least one flange portion formed integrally therewith, the base being secured to the tube by a series oi interfused tube and base portions,

ythe total area of said interfused portions being at least substantially equal to the longitudinal cross sectional area of the juncture of the base with the flange or flanges, the thickness of 4the interfused portions being less than the thickness of the base at the juncture of the base and said flange or ilanges, and the total perimetric length of said interfused portions being suiliciently greater than the total length of said ilange or flanges that the product of lthe thickness 01' said interfused portions and the total perimetric length thereof is at least substantially as great as the said longitudinal cross sectional area of the juncture of the base with said flange or flanges.

5. A heat exchange conductor comprising aV pipe, a iin member secured to the outer surface of the pipe, said iin member comprising a base and two ilanges formed integrally therewith and extending therefrom, the base being secured to the tube by a series of interfused tube and base portions, said portions overlapping to produce a longitudinally extending welded area having scalloped edges and at least substantially equal in area to the combined longitudinal cross sectional area of the flanges at the juncture of the base therewith, the thickness of the base within said welded area being less than the thickness of the base at the juncture of the base and said flanges, and the total perlmetric length of said welded area being suillciently greater than the total length of said flanges that the product of the thickness of said base within said welded area and the total perimetrlc length of said welded area is at least substantially as great as the said combined longitudinal crossrsectlonal area of the flanges.

JOHN W. BROWN, Jn.