US3177937A - Spirally-finned heat exchanger - Google Patents

Description

April 13, 1965 J. M. SLOUGH SPIRALLY-FINNED HEAT EXCHANGER Filed Dec.

INVENTOR. JACK M. SLOUGH ATTO RN EY 3,177,937 SlllRALLY-FMNED HEAT EXCHANGER Jack M. Slough, West Covina, Califi, assignor of fifty percent to James G. De Flon, Whittier, Calif. Filed Dec. 10, 1962, Ser. No. 245,348 s Claims. (Cl. 165-184) This invention relates to tubular heat exchangers and more particularly to an improved structure of this type featuring improved means for dissipating heat from the interior of the tube to a surrounding medium and wherein the heat transfer efficiency is maintained at a high level over a wide range of operating temperature conditions.

Tubular heat exchangers have many desirable characteristics. One of the characteristic problems which has received the attention of designers over a long period of years has been that of providing high eificiency means for transferring heat between the exterior of the tube and a surrounding medium. A commonly employed expedient involves the use of thin radial fins in intimate contact with the exterior of the tube. However, the provision of a high efficiency heat transfer path between the tube and these fins has presented many difficulties. Previously the fins were provided with central openings dimensioned to have a snug fit with the exterior of the tube and these were then assembled endwise over the tube from one end. The movement of the fins into assembled position unavoidably upsets the material around the rims of the openings so enlarging these openings that a very poor heat conducting path results and necessitating dipping the assembly in a tinning or brazing bath. This technique is costly and frequently does not provide a satisfactory bond and heat transfer path between the tube and the fin.

Other proposals involve slotting the fin from one edge and pressing them into position laterally from one side of the tube. However, this necessitates cutting away fin material leaving one edge of the tube without fin material thereby losing the advantage of heat dissipating finning along one entire side of the tube. Another proposal to avoid this undesirable condition involves installing alternate fins from opposite sides of the tubing. Still another proposal to improve on this latter expedient requires slitting the fins and bend a tab outwardly of sufficient width to permit assembly of the fins from one side of the tubing. Thereafter the tab is restored to its initial position to fill the assembly gap. However, this expedient also requires dipping the finished structure in a tinning bath in order to provide a heat conducting path between the inner edge of the tab and the juxtaposed surface of the tubing.

Other proposals have been to edge-wind straight stripping in a spiral path into a spiral groove formed in the exterior of the tubing following which portions of the tube are crushed against the edges of the fin. These proposals have presented new problems owing to the cost of preparing the grooved tubing, the dificulty of edge-winding soft ductile stripping onto the tube and simultaneously crushing the groove walls against the fin to provide a good heat transfer path between the tube and the fin. Also the spiral grooving greatly weakens the tube or requires the use of tubes with excessively thick walls.

The foregoing and other serious shortcomings and disadvantages of prior heat exchange structures of the type referred to are overcome to a highly satisfactory degree by the expedients employed in practicing the present invention, the novel features of which include both the method of fabricating the exchanger and the resulting product. By the invention a spirally finned tube is produced at high speed in completely finished condition withfitates atet out need for dipping or brazing the assembled components and featuring the simultaneous application of the spiral finning along with a thermal lock wrapping found to provide an excellent heat transfer between the tubing and the spiral finning. Both the radial fin and the thermal lock strip are applied to the tube in close proximity to one another and under high tension with certain areas and edges of each component in high pressure intimate contact with one another and with surfaces of the tubing. More specifically, the present invention contemplates a heat exchanger of high efficiency, particularly at higher operating temperatures, and utilizing tubing of ferrous material surrounded with a spiral wrapping of radially disposed strip of good heat conducting material and an intervening inwardly crowned locking strip. The latter strip preferably has substantially the same coefficient of expansion as the tube and is wrapped about the tube under tension with its lateral edges press-wedged against the side walls of the edge-wound spiral fin. Because of the closely related coeflicients of expansion of the tube and of the crowned locking strip, and the temperature gradient across the wall of the tubing, it is found that under high temperature operating conditions, the crown portion of the strip is actually forced into firmer higher pressure contact with the tube thereby enhancing the efliciency of heat transfer. Additionally, the greater width of the crowned strip than the underlying portion of the tube, taken together with the higher coeflicient of the radial spiral finning, results in higher pressure contact between the lateral edges of the crowned strip and the finning with resultant better heat transfer.

Accordingly, it is a primary object of the invention to provide an improved helically finned tubular heat exchanger wherein the exposed surface of'the tube is covered with a transversely and inwardly crowned strip of heat conductive material having a transverse width greater than the axial spacing between convolutions of radial finning and wedged between these convolutions with the 1 crowned portion thereof in intimate heat transfer relation with the tube surface.

Another object of the invention is the provision of an improved heat exchange structure produced by wrapping tubing between convolutions of radially disposed spiral finning with a crowned strip of heat conductive material having substantially the same coefficient of expansion as the tubing and with its lateral edges in pressure contact with the side walls of said spiral firming.

Another object of the invention is the provision of a new technique for construction of a superior heat exchanger featuring the simultaneous formation of a transversely crowned heat transfer strip and the wrapping of said strip about the exterior of a spirally finned heat exchange tube in a manner to wedge the strip between the spiral finning with the intervening inwardly crowned portion of the strip held in hoop tension contact with the exterior surface of the tube.

'Another object of the invention is the provision of a technique for processing a crowned thermal locking strip from wire stock and simultaneously wrapping the finished product under tension about tubing with the lateral edges thereof wedged between the side walls of adjacent convolutions of spiral finning as the latter is wrapped about the tubing.

These and other more specific objects will appear upon reading the following specification and claims and upon considering in connection therewith the attached drawing to which they relate.

Referring now to the drawing in which a preferred embodiment of the invention is illustrated.

FIGURE 1 is a general schematic view in perspective showing essential steps employed in fabricating a tubular or We heat exchanger in accordance with the principles of this invention;

FIGURE 2 is a fragmentary view on an enlarged scale taken along line 22 on FIGURE 1 showing the manner in which radial finning applicator rolls are utilized to provide lateral support to these fins during application of the thermal lock strip;

FIGURE 3 is a fragmentary transverse View taken along line 33 on FIGURE 1 showing the heat conducs tive ribbon being transversely crownedimrnediately prior to being wrapped about the tubing; and 7 FIGURE 4 is an enlarged transverse sectional view through the tubing and finning after the crowned ribbon has been wrapped therebetween and filled with a heat conductive filler.

Referring more particularly to FIGURE 1, there is shown a short section of a heat exchanger designated generally 19 and made in accordance with the principles of the present invention.

This heat exchanger will be understood as comprising an elongated tube 11 of any desired length and formed of any suitable material and preferably one having good heat conducting properties such as copper, brass, aluminum and steel. Although a complete heat exchange structure is not illustrated, it will be understood that tubes finned in the manner to be described below may be assembled in any of a great variety of ways well known to those skilled in this art to provide a heat exchanger structure suited for a particular application. In some cases, straight sections of the finned tube comprise the entire heat exchanger while in other cases the tubing will be sinuous and provided with return bend portions, or sealed in multiple between connecting headers, or assembled in numerous other modes well known to the prior art.

Owing to novel technique and mode of assembly of the finning and the fin-locking structure to the tubing, it is pointed out and emphasized that the tubing need not have a smooth exterior surface, nor need this surface be finished to a high degree of accuracy and roundness as has been a primary requisite in making most prior finned tubular heat exchangers. This is because the radial fin and the locking strip for holding these fins in place are simultaneously wrapped about the tube under tension thereby assuring that the surfaces of each are pressed into high pressure contact with the tube and with one another without need for any substantial relative movement between the contacting surfaces.

A preferred mode of assembling heat exchanger 10 is illustrated in FIGURE 1. Tube 11 is supported rotatably between three identical rollers 12 carried by independent shafts 13 arranged in known manner for adjustment toward and away from tube 11. Of importance is the factthat the parallel axes of shafts 13 are inclined to the axis of tube 11 with the axis of each shaft 13 lying normal to a plane representing the angle of pitch of the spiral finning 15 to be wrapped about the tube. The means provided for adjusting shafts 13 toward and away from tube 11 preferably include compression springs capable of holding the rims of rollers 12 in resilient pressure contact with tube 11.

At least one of the shafts 13 is power driven through a suitable motor. Because of the pressure contact between the serrated rims of the roller and the tube, power rotation of one of the rollers is effective to rotate the tube in the direction indicated by the arrows in FIGURE 1. By virtue of the inclination of the planes of rollers 12 to the axis of tube 11 it will be apparent that the tube is progressively advanced axially past the rollers in the direction indicated by arrow 16.

Rollers 12 are best shown in FIGURE 2 as having a deep narrow groove 18 proportioned to receive therebetween strip 15 to be wound helically about tube 11. The inner portion of groove 18 should be sufficiently wide to receive the thickness of strip 15 with a close fit and the outer open end of the groove may be slightly wider to facilitate feeding of the strip into the. groove while at the same time lending support to its opposite lateral sides in areas adjacent tube 11. The side walls of groove 18 will therefore be understood as providing lateral support for.

axial width corresponding substantially to the spacing desired between-adjacent convolutions of fin 15. The

rear face 20 of roller 12 will be understood as providing a firm and powerful backup for the adjacent radial face of the immediately adjacent convolution of finning 15 thereby positively safeguarding against axial displacement of this convolution under pressure exerted by the assembly thereagainst of the thermal lock strip as will be explained in detail presently.

Pinning strip 15 is supplied from a supply roll 22 suitably supported on a spindle, not shown, positioned laterally of tube 11 and provided with a suitable slip clutch device effective to maintain a desired high tension in strip 15 as it is dispensed edgewise into groove 18 of rollers 12 and wound about the tube as it is rotated.

An important feature of the invention involves the provision of a thermal lock strip 25 formed of metallic material such as copper, aluminum, stainless steel, or other material having good heat conducting properties,

highstrength, and immunity to corrosion and attack by moisture and other substances in the operating environment in which the heat exchanger is to be employed. Desirably both spiral finning 15 and thermal lock strip 25 are formed of a material having the same or substantially the same coefficient of expansion as tubing 11. This assures that the components of the heat exchanger will expand similarly in response to similar temperature rises. It'has been found practical and desirable to form looking strip 25 from continuous wire stock, the Wire being progressively flattened into ribbon of the desired thickness and width simultaneously with the application of the resulting product between the convolutions of spiral finning 15.

To this end there is shown in FIGURE l a coil 26 of the selected wire material supported on a supply mandrel or other suitable support. This wire passes through pairs of ribbon forming pressure rolls 27, 28 in accordance with known technique effective in progressively shaping this wire into thin ribbon 30 of the requisite width for assembly between convolutions of finning 15. In this connection it is pointed out that simple adjustment of the pressure acting on the forming rolls acts to vary the ribbon width to the precise value'required to provide high-pressure contact with the surfaces of finning 15.

The resulting ribbon is then allowed to form a long take-up loop 31 before passing through tensioning rolls 32 suitably spring-biased toward one another and adjustable until the ribbon is placed under a desired tension as it is wrapped about tube 11. Ribbon 30 will also be understood as passing through a pair of crowning rolls 34, 35 best shown in FIGURE 3. One roll 34 has a crowned rim whereas the other is formed with a complementally shaped concave rim cooperable therewith to crown ribbon 30 transversely thereof as it is fed between these rollers. The width of the crowned ribbon is preferably slightly greater than the distance between the facing surfaces of adjacent convolutions of radial finning 15. In consequence, the pressure application of ribbon 39 between adjacent convolutions of fins 15 results in further arcuate deformation of this ribbon as its rim edges are forced into high pressure contact with the fin surfaces. This pressure contact is prevented from spreading the fins apart owing in part :to the supporting action of the previous convolution of strip 30 on finished portions of the heat exchanger assembly and in part, to the back-up action of radial faces 20 of rollers 12. The tension applied to thermal lock strip 30 will also be understood as adequate to force the inwardly facing crowned portion 3% of strip 349 into pressure contact with the outer surface of tube 11.

Desirably, but not necessarily, the voids between the interior surface of lock strip 39 and the juxtaposed surface of tube 11 is fi led with material 46 having good heat conducting properties and etiective in facilitating the transfer of heat between tube 11, strip 39 and finning 15. Desirably filling 4% comprises a metal alloy having a relatively low melting temperature and preferably not higher than 700 degrees F. Various alloys meeting this requirement are available commercially.

Filling the voids with material 40 is carried out simultaneously with the application of strip 31? to the assembly. For this purpose alloy 46 is heated to a temperature appreciably above its melting temperature and supplied in any suitable manner, not shown, into the voids in such manner as to exclude air and to maintain the voids fully described is continued progressively and automatically,

until a length of tubing of desired length has been completed. It is, of course, understood that the opposite ends of the spiral fin 15 as well as of thermal lock strip 30 are rigidly and fixedly secured to tubing 11 While being held under tension and in pressure contact with tube 11. Such securement may be accomplished by welding, brazing, or the use of mechanical fasteners forming no part of the present invention.

The manner in which the components of the completed assembly cooperate to transfer heat between a medium confined to the tubing and another in contact with the exterior surfaces will be quite apparent from the foregoing. Assuming, for example, that steam at some temperature such as 700 degrees F. is passing through tube 11, it will be recognized that the hot tube will expand as will fin 15 and thermal lock strip 25. However, since each of these components is preferably of the same material or of materials having substantiallythe same coeflicients of expansion, there is no tendency for the two thin strips to expand away from the tube; on the contrary, the portions of both fin 15 and strip 25 in contact with the exterior of tube 11 are of the same diameter illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details of construction or design herein shown other than as defined in the appended claims. r

I claim:

1. In a tubular heat exchanger of the type comprising an elongated tube having a wide strip of heat conducting material spirally edge-wrapped to the exterior thereof under tension and projecting radially therefrom, that improvement which comprises a heat'conductive surface covering for the exposed exterior portion of said tube, said covering comprising a continuousthin metal ribbon crowned transversely thereof and Wrapped with the crown thereof pressed against the tube surface and with the opposite lateral edges of said ribbon in pressure contact with the juxtaposed surfaces of said wide strip of radial finning. a

2. A tubular heat exchanger of the type defined in claim 1 characterized in that said continuous ribbon is suficiently wider than the axial distance between adjacent convolutions of said spirally wrapped 'finning that the lateral edges of said ribbon are deflected outwardly into pressure contact with said finning along continuous spiral zones of contact spaced radially from the exterior surface of said tube.

3. A tubular heat exchanger of the type defined in claim 2 characterized in that the void space between the inner lateral edge portions of said ribbon and the underlying portions of said tube are filled with heat conductive metal having a low melting temperature.

4. A finned tubular heat exchanger comprising an elongated tubular body, heat dissipating fin means encircling said tube helically and comprising a continuous narrow ribbon of good heat conducting material wound on edge ribbon and the underlying portion of said tube between as this surface'and each will expand similarly to the tube;

In consequence, both will remain in high pressure contact with tube 11.

As respects the crowned portion of the thermal lock: ing strip, however, it will be appreciated that the portion in direct pressure contact with tube 11 will have a higher temperature than its opposite lateral edges. As a result, the locking strip tends to flatten causing its lateral edges to bite more forcibly into fins 15 and-increasing the heat transfer efiiciency between these components. The heat conducting material 40 filling the space between tube 11 and strip 25 also becomes molten at higher temperatures and is then especially effective in transferring heat to both strip 25 and firming 15.

It is also pointed out and emphasized that the thermal lock strip, taken with fin 15, entirely cover and conceal the exterior surface of the tubing protecting the latter from corrosion and substituting the smooth surfaces .of the convolutions of said fin means is charged with heat conducting material. I I

6. A heat exchanger as defined in claim 5 characterized in that said charge of heat conducting material conceals and is nonfluid at room temperaturebut melts and becomes fluid at a predetermined higher temperature at which said heat'exchanger is intended to operate.

References Cited by the Examiner UNITED STATES PATENTS CHARLES SUKALO, Primary Examiner.

FREDERICK L. MATTESON, JR., JAMES W. WEST- HAVER, Examiners.

Claims (1)

1. IN A TUBULAR HEAT EXCHANGER OF THE TYPE COMPRISING AN ELONGATED TUBE HAVING A WIDE STRIP OF HEAT CONDUCTING MATERIAL SPIRALLY EDGE-WRAPPED TO THE EXTERIOR THEREOF UNDER TENSION AND PROJECTING RADIALLY THEREFROM THAT IMPROVEMENT WHICH COMPRISES A HEAT CONDUCTIVE SURFACE COVERING FOR THE EXPOSED EXTERIOR PORTION OF SAID TUBE, SAID COVERING COMPRISING A CONTINUOUS THIN METAL RIBBON CROWNED TRANSVERSELY THEREOF AND WRAPPED WITH THE CROWN THEREOF PRESSED AGINST THE TUBE SURFACE AND WITH THE OPPOSITE LATERAL EDGES OF SAID RIBBON IN PRESSURE CONTACT WITH THE JUXTAPOSED SURFACES OF SAID WIDE STRIP OF RADIAL FINNING.

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