US 3288209 A
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Description (OCR text may contain errors)
Nov. 29, 1966 A. J. WALL ETAL 3,288,209
HEAT TRANSMITTING TUBES HAVING HELICAL FIN MEANS Filed April 15, 1964 2 Sheets-Sheet 1 FIG. 7.
IMVEA/TORS ALAN James WALL: ALAN PALMER Nov. 29, 1966 A. J. WALL ETAL 3,288,209
HEAT TRANSMITTING TUBES HAVING HELICAL FIN MEANS Filed April 13, 1964 2 Sheets-Sheet 2 wTcRS ALAN JAMES WALL AlJA/V PAL/HER B a Mg 1M1 't QAZ'MMW United States Patent 3,288,209 HEAT TRANSMITTING TUBES HAVING HELICAL FIN MEANS Alan J. Wall, Lincoln, and Alan Palmer, London, England,
assignors to Clayton Dewandre Company Limited, Lincoln, England Filed Apr. 13, 1964, Ser. No. 359,099 4 Claims. (Cl. 165-184) This invention relates to heat transmitting tubes and particularly to tubes provided with extended surface for heat transfer and consisting of metal strip or ribbon wound helically onto the tube.
It is common practice in the manufacture of such tubes to wind 21 copper, aluminium or other metal ribbon edgewise on to the tube, the ribbon being crimped radially towards its inner edge or stretched towards the outer edge to enable it to be wrapped around the tube, and the wound-on ribbon being secured to the tube by a soldering, welding, or equivalent operation, or by mechanical means.
A plain ribbon stretched towards its outer edge has the disadvantage that a static or boundary layer of the fluid (usually a gas) flowing at right angles to the tube can form along the unbroken surface of the fin formed by the ribbon, whose surface is virtually parallel to the fluid flow. Such a boundary layer offers substantial resistance to the transfer of heat between the fin and the fluid, this resistance increasing as the fin spacing decreases due to a tendency towards the formation of thicker boundary layers. Radial crimping of the fin tends to reduce or disrupt this boundary layer, but has the disadvantage that the depth of the crimp limits the number of turns that may be applied to a given length of tube, by placing a minimum limit on the fin pitch. Further, the depth of the crimp increases progressively from the outer edge of the fiin towards the tube wall, thus offering increased resistance to fluid flow at the root of the fin. This results in a reduction in the so-called fin efficiency as most of the effective heat transfer takes place where the fluid velocity is highest, namely towards the outer edge of the fin. Moreover, the total resistance to fluid flow is greater with crimped fins than with plain fins similarly pitched.
The main object of the present invention is to disrupt the above-mentioned boundary layer to improve heat transfer, without offering significant extra resistance to fluid flow over the tube.
According to the invention a heat transmitting tube has an extended surface in the form of a metallic strip or ribbon folded longitudinally to substantially U, L or similar cross section and wound upon the tube to form an upstanding fin or fins, the strip or ribbon being provided with a series of slots extending partly across its width whereby the completed fin consists of spaced fingers interconnected by a continuous outer edge portion.
Reference will now be made to the accompanying drawings which illustrate examples of heat transmitting tubes constructed according to the invention and in which:
FIG. 1 is a view showing the initial form of the metallic strip to form the fins,
FIG. 2 is a perspective view showing another stage in the formation of the fins,
FIG. 3 is a perspective view showing part of a completed tube,
FIGS. 4 and 5 are fragmentary sectional views showing two methods of arranging fins on the surfaces of tubes, and
FIG. 6 is a perspective view of a modified construction.
Referring first to FIG. 1, the fin is formed from a flat metallic ribbon 0 whose width is slightly greater than twice the desired height of the final fin as applied to the tube. The flat ribbon a is pierced with an equispaced series of identical substantially rectangular slots b, extending across the width of the ribbon, but terminating approximately .050" from each edge of the ribbon. The slots may be at right angles to the ribbon axis (as shown in FIG. 1), or the angle may depart from a right angle by any convenient angle. The dimension of each slot b parallel to the ribbon axis is substantially equal to the distance between the edges of adjacent slots, and will generally be of the order of 1/16"1/20".
To form fins on the exterior of a tube, the perforated ribbons a, b, is folded about its longitudinal axis into a U-shape, as shown in FIG. 2, the limbs of the U being preferably of equal length. The base 0 of the U is substantially flat, and is applied to the tube wall virtually parallel to the tube axis. The folded ribbon thus formed is applied as a helical winding to the surface of a tube d as shown in FIG. 3 to constitute a number of radial spaced fingers e interconnected by a continuous outer helical edge portion 1. The mounting of the folded ribbon as a helical winding brings the components of the base 0 which lie between the slots 12 at the base of the U-fold into close edge contact forming a substantially continuous base for attachment to the surface of the tube and as shown in FIG. 4. The helical winding may be attached to the tube by various known means, for example, by a stout tension wire passing along within the base 0 of the U-folded strip, or by welding, brazing, or soldering, with or without the tension wire.
To enable a fin as above described to be applied to a tube whose diameter is equal to or less than the final fin height, the base portions 0 of the fingers of the perforated ribbon may be twisted a few degrees, all in the same direction, about an axis at right angles to the edges of the ribbon. This arrangement will cause the bases 0 and the portions of the fingers e adjacent to the tube wall to overlap at the tube periphery as shown in FIG. 5, enabling the fin in its folded form to be applied successfully to the smaller diameter tubes.
Alternatively, the fingers Q can be made narrower at the centre of the strip, so that they can be more closely packed to enable small diameter tubes to be used.
The slots b prevent formation of any appreciable thickness of boundary layer by continually disrupting the fluid flow as it passes across the fins. Moreover, as the fin is not crimped, it offers very little more resistance to fluid flow than a plain uncrimped fin, whilst the slight extra turbulence induced as the flow of fluid passes from a slot to a spoke or finger e of the fin will aid heat transmission between metal and fluid.
The unbroken connection 1 at the outer extremity of the fin gives mechanical support to the individual radial fingers e of the fin, and reduces the number of sharp edges liable to cause accumulation of dust or lint.
In the alternative construction shown in FIG. 6, a single fin, L-shaped in cross-section may be formed from a fiat ribbon whose width is slightly greater than the desired height of the final fin as applied to a tube. This fin is provided with slots b as in the previous construction and these slots extend through the base portions 0 of fingers e between the slots, the outer ends of the fingers being connected by the edge portion 7. The base portions 0 are attached to the surface of a tube by any known means and may also be peened into a helical groove in the tube surface.
1. In assembled combination, a heat transfer tube and means providing an extended surface heat transfer fin structure on said tube comprising a preformed longitudinally folded metallic strip wound substantially helically edgewise upon and along said tube, said strip having a spaced series of continuous slots extending partly across its width to form a number of spaced fiat fingers that in the folded fin structure have free inner ends and are integrally interconnected at their outer ends by a longitudinally continuous portion, said strip being so folded as to provide at the inner end of each finger an angularly extending relatively fiat base portion that is attached to the periphery of said tube, and means securing said free inner ends of said fingers along the periphery of said tube whereby the fin structure comprises radially extending circumferentially spaced flat fingers interconnected by a longitudinally continuous outer edge portion with said slots being continuously open substantially between said tube and said outer edge portion.
2. In the assembly defined in claim 1, said strip being folded to substantially U-shaped cross section with said finger base portions forming the bridge of the U-shape.
3. In the assembly defined in claim 1, said free ends of the fingers being substantially L-shaped in cross section.
4. In the assembly defined in claim 1, all of said base portions being twisted a few degrees in the same direc- 4 tion about an axis at right angles to the preformed strip for allowing said base portions to overlap on the tube periphery when the preformed strip is wound upon a tube whose diameter is equal to or less than the radial distance from the tube periphery to the outer periphery of said fin structure.
References Cited by the Examiner UNITED STATES PATENTS 2,680,009 6/1954 Nekut 16547 FOREIGN PATENTS 1,298,197 5/1962 France.
839,508 5/1952 Germany. 799,391 8/1958 Great Britain.
ROBERT A. OLEARY, Primary Examiner.
N. R. WILSON, Assistant Examiner.