GB1582831A

GB1582831A - Provision of extended external surfaces on tubes

Info

Publication number: GB1582831A
Application number: GB10619/76A
Authority: GB
Original assignee: GKN Birwelco Ltd
Current assignee: GKN Birwelco Ltd
Priority date: 1977-06-15
Filing date: 1977-06-15
Publication date: 1981-01-14

Description

(54) IMPROVEMENTS RELATING TO THE PROVISION OF EXTENDED EXTERNAL SURFACES ON TUBES (71) We, GKN BIRWELCO LIMITED, a British Company of Mucklow House, Mucklow Hill, Halesowen, West Midlands B62 8DQ, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to the provision of extended external surfaces on tubes to produce finned tube assemblies for use in heat transfer equipment.

The provision of extended surfaces on long tubes (i.e. over lOft long) is conventionally achieved with a static machine which forms a continuous fin around the rotating tube whilst the tube is fed end-wise through the machine. Such a process enables a small compact machine to be used which, with suitable feeder and receiving racks, can deal with tubes of 100 ft or more in length. Such a machine may form the extended surface by extruding metal radially outwardly from the thick base tube or, alternatively, by wrapping a deformed strip of ductile metal edgewise around the tube in a continuous helix.

The processes and machines are suitable for a soft material such as aluminium which can be obtained in a quality that can be easily worked and which meets the other requirements of the extended surface. However, such materials in many applications are either expensive or unavailable due to a variety of reasons.

If it is required to provide extended surfaces in less ductile and tougher materials, or if the shape of surface desired requires excessive deformation of the material, the above mentioned processes become difficult and expensive. This either complicates the machine due to the excessive forces which have to be exerted during the process of fin formation or alternatively makes the material expensive due to the raw material specification being upgraded to ease the finforming process whilst still retaining other desired properties.

When the problem has been met in the past it has been conventional to deal with less ductile and tougher materials, or with difficult shapes of extended surfaces, by pressing, forming and/or cutting etc., onepiece fins from a sheet of material in such a way that a hole is formed in each fin which is then threaded over the tube. The use of such a method with short tubes (say below lOft) is relatively simple but, due to the fact that the one-piece fin has to be slid along the full length of the tube, the machine to execute this process becomes progressively more complicated when it is required to deal with long tubes. This is especially true if damage or deformation of the relatively lightly constructed one-piece fins is to be avoided and if a tight fit between the tube and the fins is to be achieved.

An object of one aspect of the invention is to provide a finned tube assembly which may include fins of said less ductile and tougher materials, e.g. steel, and which may be more easily manufactured than heretofore.

According to this aspect of the invention we provide a finned tube assembly comprising a tube and a number of series of external fins on the tube, the fins being mutually parallel and generally normal to the longitudinal axis of the tube, each fin embracing the tube over part of the periphery thereof and each fin of each series being joined to a fin of another series, the joints between adjacent edges of fins of two series extending outwardly from said tube and being formed by interlocking hook formations along said edges.

Joints between the fins cause some obstruction of the free space between the fins. This need not be a serious problem if the joints are arranged with consideration to the intended direction of fluid flow over the tube assembly. Thus, for example, the joints could lie parallel to said intended direction or could be arranged to assist in directing the fluid flow over the tube assembly. By suitably arranging the joints, a finned tube assembly embodying the invention in a heat exchanger would give similar heat-transfer and pressure-drop characteristics to a similar finned tube assembly having a continuous fin or a plurality of onepiece fins.

In a preferred construction there are two oppositely disposed series of fins. Each fin may embrace the tube over substantially half the periphery thereof and, as described above, the joints between the fins from the two series may be mutually parallel.

The fins of each series may be individual, or they may be formed in pairs, or they may be formed as one of a plurality of fins.

Such a pair or plurality will be formed from a single piece of metal, e.g. steel, by cutting and folding.

An object of another aspect of the invention is to provide a method of making a finned tube assembly which is particularly applicable to the manufacture of long tube assemblies and is simpler than methods at present used.

According to this aspect of the invention we provide a method of making a finned tube assembly comprising applying to the external surface of a tube a number of series of parallel fins each of which is generally normal to the longitudinal axis of the tube and embraces the tube over part of the periphery thereof and joining each fin of one series to a fin of another series by means of interlocking hook formations provided on adjacent edges of such fins which extend outwardly from said tube.

This method is simpler and more convenient than conventional methods using one-piece fins since it is not necessary to thread a closely fitting fin down a long length of tube.

The invention will now be described in detail by way of example with reference to the accompanying drawings in which: Figure 1 is a view of a blank used to make a single fin for the invention; Figure 2 is a perspective view of an individual fin made from the blank of Figure 1; Figure 3 is a view of part of a finned tube assembly employing the fins of Figure 2; Figure 4 is a detail view showing the interconnection of the fins of the two series in the assembly of Figure 3.

Figure 5 is a view of a blank used for making a pair of interconnected fins; Figure 6 is a perspective view of a pair of fins made from the blank of Figure 5; Figure 7 is a view of part of a finned tube assembly made using the fin pairs of Figure 6; Figure 8 is a detail view showing the interconnection between fins of the two series in the assembly of Figure 7; Figure 9 is a view similar to Figure 5 but of a slightly different blank for forming interconnected pairs of fins; Figure 10 is a view of part of a blank for making a plurality of interconnected fins; Figure 11 is a perspective view of part of a series of interconnected fins made from the blank of Figure 10; Figure 12 is a detail view showing the interconneotion between two pluralities of fins such as shown in Figure 10; Figures 13 to 16 show different shapes of fin which are possible; and Figure 17 is a view of a heat exchanger assembly including a plurality of finned tube assemblies as described.

Referring to Figures 1 to 4, these show an embodiment in which each fin is an individual fin rather than being one of a fin pair. Figure 1 shows a blank 32 from which a fin is made. The blank is generally rectangular and has a semi-circular cut-out 33 and two U-shaped cuts 34 adjacent two corners thereof. There are six radial cuts 35 extending inwardly from the periphery of the semi-circular cut-out 33 and two notches 36 at the ends of the circular cut-out. The blank is bent to form the fin shown in Figure 2. Thus the material within the Ushaped cuts 34 is bent to provide spacing projections 37 which project from one surface of the fin 38. The material between adjacent cuts 35 is bent in opposite directions to form flanges 39 and 40 and the edge portions 41 of the blank are curled to form hooks 42.

As shown in Figure 3, the individual fins 38 are applied to the external surface of a tube 43 and the hooks 42 of oppositely directed fins interengage as shown in detail in Figure 4. The flanges 39 and 40 are resilient so that a pair of fins 38 can be applied to the external surface of the tube 43 and pushed together thus resiliently bending the flanges 39 and 40 until the hooks 42 can engage as shown in Figure 4. The fins may then be released and the natural resilience of the flanges 39 and 40 will cause the hooks to remain in engagement. The fins are retained parallel to one another by the spacing projections 37 as is clear from Figure 3. For further security if desired, the hooked connection can be soldered as indicated at 44. The solder, being in compression, will ensure that the hooks 42 do not become disengaged.

Referring now to Figures 5 to 8, these show a construction similar to that of Figures 1 to 4 except that the fins are arranged in pairs. Each pair of fins is made from a blank 45 shown in Figure 5. The blank is generally rectangular and has, at its ends, semi-circular cut-outs 46. Radiating from each cut-out are a number of radial cuts 47. The centre of the blank is slotted at 48, the slot having widened ends 49. Inter connecting straps 50 are provided between the parts of the blank which form the fins and the blank is slit at 51 and triangular areas of material 52 are removed. The blank is then bent into shape about parallel lines 54 thus forming two fins 54 and 55 shown in Figure 6, the fins being interconnected by the straps 50 which, it will be noted, are inset from the edges 56 of the fins.This inset is due to the removal of the triangular areas of material 49 and 52. The edge portion of each semi-circular cut-out 46 is bent inwardly to form a discontinuous flange 57 and the edge portions of the blank on either side of each semi-circular cut out are rolled to form hooks 58. At one end of the fin pair the material is bent out to form a spacing projection 59 extending from the fin 54 while at the other end, similarly, material is bent out to form a spacing projection 60 projecting from the fin 55. The projections 59 and 60 extend in opposite directions.

Figure 7 shows four fin pairs applied to a tube 61. The fins of the pairs are designated 54a, 55a; 54b, 55b; 54c, 55c and 54d, 55d. The hooks 58 on the fin pairs are engaged as described in relation to Figures 1 to 4. That is to say that the fin pairs are pushed towards one another on the tube so as elastically to deflect the flanges 57 to enable the hooks 58 on opposite fin pairs to be engaged whereupon the fin pairs are released and the natural resilience of the flanges retains the hooks engaged. It will be noted, however, from Figure 7 that the fin pairs are staggered relative to one another on opposite sides of the tube.Thus the fin 55a of a pair on one side is aligned with the fin 55b of the pair on the other side whereas the fin 54b on one side is aligned with the fin 54c on the other side and the fin 55c on one side is aligned with the fin 55d on the other side. This is clearly shown in Figure 8 which is an enlarged view of the way the hooks 58 are interengaged, the hooks on the fins 54a and 55a being designated at 58a, the hooks on the fins 54b and 55b being designated at 58b and the hooks on the fins 54c and 55c being designated at 58c.

As before, the hooks may be additionally held in engagement by solder such as that indicated at 62.

Figure 9 shows a slightly different form of blank to that shown in Figure 5 but intended to produce a similar fin pair. In Figure 9 less material has been removed from the blank thus keeping waste of material to a minimum. There is thus, in the blank 63 of Figure 9, a central slot 64 having cross slots 65 at its ends. Extending inwardly from the opposite edges of the fins are Tshaped slots 66. Between the cross members of the slots 66 and the slots 65 are left interconnecting straps 67 similar to the straps 50 of Figure 6. The fin pair is made by bending the blank into U-shape about the lines 68 and it will be seen that the roots, i.e. the parts of the straps 67 where they join the fins, will be inset from the edges of the fins remote from the tube formed by the edges of the slot 64.This will be similar to the arrangement of Figure 6 where the roots 50a of the straps 50 are inset from the edges 56 of the fins which, in the finished assembly are remote from the tube.

Instead of the fins being arranged in pairs and each series consisting of a number of pairs of fins, each series of fins may comprise a plurality of interconnected fins.

Figure 10 shows part of a blank for making such a plurality. The blank 69 shown in Figure 10 has a repeating pattern of cut-out and slots. Thus there is a plurality of slots 70 having cross slots 71 at the ends thereof and opposite the cross slots 71 T-shaped slots 72 extending inwardly from the edge of the blank. Between the slots 71 and the cross members of the slots 72 is left material for interconnecting straps 73. Between each adjacent pair of slots 70 is a generally circular cut-out 74 having oppositely directed notches 74a which lead into T-shaped slots having intersecting limbs 75 and 76. Between each limb 76 and an edge 77 of the blank is left material for an interconnecting strap 78.

The blank is bent in concertina fashion about lines 79 passing through the ends of the slots 71, and lines 80 passing through the centre of the notches 74.

Figure 11 shows the resulting plurality of fins arranged in concertina shape and showing at the edges of the fins remote from the tube the interconnecting straps 73 whose roots are inset from the edges 77 of the fins. This also shows that the interconnecting straps 78 at the edges of the fins adjacent to the tube are bowed. The edges of the slots 75 are rolled to form hooks 81 and the peripheries of the circular cut-outs 73 are bent to form flanges 82 to engage the tube 90 in a manner similar to that described in relation to Figures 6 to 8.

As before, the fins are applied to the tube by being moved towards one another with a tube between them, the flanges 82 are deflected to enable the hooks 81 to engage with one another and thus hold the series on the tube. As shown in Figure 12, the interconnecting straps 78 of the two series are mutually intercalated along the length of the tube. In Figure 12, the straps 78 of the upper series of fins in that figure are indicated at 78a while the straps of the lower series in that figure are indicated at 78b.

In the embodiments of Figures 1 to 12 the joints between the fins of the two series formed by the interengaging hooks such as 41, 58 and 81 will be arranged to provide a minimum of obstruction to fluid flow over the tube assembly which will, in Figures 3 and 7, be perpendicular to the plane of the paper and, in Figure 11, in the direction of the arrow A in that figure. As shown the joints are mutually parallel and also extend radially of the longitudinal axis of the tube.

It is to be noted also that the interconnecting straps such as 50, 67, and 78 are parallel to the joints and therefore provide no substantial interruption to fluid flow across the assembly and the spacing projections 37 in the embodiment of Figures 1 to 4 are also parallel to the joints. The interconnecting straps 78 of the embodiment of Figures 10 to 12 are also substantially radial and do not constitute any substantial interruption to the flow.

The invention has been described in relation to a finned tube assembly which, when viewed parallel to the axis of the tube, has a generally square shape. Figures 13 to 16 show some modifications. Figure 13 shows a round tube surrounded by semicircular fins so that the assembly is circular in cross section. Figure 14 illustrates the fact that one may have fins arranged to embrace part of the periphery of each of a number of tubes. In Figure 14 there are three tubes 83 with elongated fins 84 each having three cut-outs to receive the tubes 83. Figure 15 shows rectangular fins and circular tube and Figure 16 shows rectangular fins with an elliptical tube. In each of the embodiments of Figures 13 to 16 it will be seen that the joints between the fins indicated by the lines 85 to 88 respectively are generally radial of the tube axis.However, as shown by the chain lines 85a and 85b in Figure 13 the joints could be parallel but not radial. Also the joints could be as shown at 85a and 85c in Figure 13 in which case they would help to divert fluid flow over the assembly in the required directions.

Figure 17 shows a heat exchanger comprising a block 89 of finned tubes embodying the invention arranged over a fan 90 in a tubular portion 91 which diverges into a frusto-conical duct. Some of the joints between the fins of the two series on each tube are indicated at 92 and it will be seen that they are parallel to the direction of air flow over the tube assemblies. The tube assemblies in each row may be aligned as shown or may be staggered. The joints may be arranged to assist in directing the air flow over the tubes and need not all be parallel.

WHAT WE CLAIM IS:- 1. A finned tube assembly comprising a tube and a number of series of external fins on the tube, the fins being mutually parallel and generally normal to the longitudinal axis of the tube, each fin embracing the tube over part of the periphery thereof and each fin of each series being joined to a fin of another series, the joints between adjacent edges of fins of two series extending outwardly from said tube and being formed by interlocking hook formations along said edges.

2. An assembly according to Claim 1 wherein there are two oppositely disposed series of fins.

3. An assembly according to Claim 2 wherein each fin embraces the tube over substantially half the periphery thereof.

4. An assembly according to Claim 2 or Claim 3 wherein the joints between the fins from the two series are mutually parallel.

5. An assembly according to any one of claims 1 to 4 wherein parts of the fins which engage the tube are resilient and urge said hook formations into engagement.

6. An assembly according to Claim 5 wherein said tube-engaging parts are discos: tinuous flanges.

7. An assembly according to any one of the preceding claims wherein each fin of each is connected to at least one other fin of the same series.

8. An assembly according to Claim 7 wherein the fins of each series are interconnected in pairs and the fins of each pair are interconnected at their edges remote from the tube by integral strap portions.

9. An assembly according to Claim 8 wherein the roots of said strap portions are inset from said remote edges.

10. An assembly according to any one of the preceding claims wherein at, or adjacent to, the edges of the fins remote from the tube are provided integral spacing projections.

11. An assembly according to Claim 7 wherein each series consists of at least one plurality of fins interconnected both at their edges remote from the tube and at their edges adjacent to the tube, the interconnection at the edges remote from the tube being by integral strap portions whose roots are inset from said remote edges and said edges adjacent to the tube having said hook formations which interlock with complementary hook formations on the fins of another series and, outside said hook portions, integral, bowed interconnecting straps.

12. An assembly according to Claim 11 wherein said bowed interconnecting straps of the two series are mutually intercalated.

13. A method of making a finned tube assembly comprising applying to the external surface of a tube a number of series of parallel fins each of which is generally normal to the longitudinal axis of the tube and embraces the tube over part of the peri piety thereof and joining each fin of one series to a fin of another series by means of

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. formed by the interengaging hooks such as 41, 58 and 81 will be arranged to provide a minimum of obstruction to fluid flow over the tube assembly which will, in Figures 3 and 7, be perpendicular to the plane of the paper and, in Figure 11, in the direction of the arrow A in that figure. As shown the joints are mutually parallel and also extend radially of the longitudinal axis of the tube. It is to be noted also that the interconnecting straps such as 50, 67, and 78 are parallel to the joints and therefore provide no substantial interruption to fluid flow across the assembly and the spacing projections 37 in the embodiment of Figures 1 to 4 are also parallel to the joints. The interconnecting straps 78 of the embodiment of Figures 10 to 12 are also substantially radial and do not constitute any substantial interruption to the flow. The invention has been described in relation to a finned tube assembly which, when viewed parallel to the axis of the tube, has a generally square shape. Figures 13 to 16 show some modifications. Figure 13 shows a round tube surrounded by semicircular fins so that the assembly is circular in cross section. Figure 14 illustrates the fact that one may have fins arranged to embrace part of the periphery of each of a number of tubes. In Figure 14 there are three tubes 83 with elongated fins 84 each having three cut-outs to receive the tubes 83. Figure 15 shows rectangular fins and circular tube and Figure 16 shows rectangular fins with an elliptical tube. In each of the embodiments of Figures 13 to 16 it will be seen that the joints between the fins indicated by the lines 85 to 88 respectively are generally radial of the tube axis.However, as shown by the chain lines 85a and 85b in Figure 13 the joints could be parallel but not radial. Also the joints could be as shown at 85a and 85c in Figure 13 in which case they would help to divert fluid flow over the assembly in the required directions. Figure 17 shows a heat exchanger comprising a block 89 of finned tubes embodying the invention arranged over a fan 90 in a tubular portion 91 which diverges into a frusto-conical duct. Some of the joints between the fins of the two series on each tube are indicated at 92 and it will be seen that they are parallel to the direction of air flow over the tube assemblies. The tube assemblies in each row may be aligned as shown or may be staggered. The joints may be arranged to assist in directing the air flow over the tubes and need not all be parallel. WHAT WE CLAIM IS:-

1. A finned tube assembly comprising a tube and a number of series of external fins on the tube, the fins being mutually parallel and generally normal to the longitudinal axis of the tube, each fin embracing the tube over part of the periphery thereof and each fin of each series being joined to a fin of another series, the joints between adjacent edges of fins of two series extending outwardly from said tube and being formed by interlocking hook formations along said edges.

interlocking hook formations provided on adjacent edges of such fins which extend outwardly from said tube.

14. A method according to Claim 13 wherein there are two series of fins which are applied to the tube from opposite directions.

15. A method according to Claim 14 wherein the joining is effected so that the joints between the fins from the two series are mutually parallel.

16. A method according to any one of Claims 13 to 15 wherein the fins of each series are applied as individual fins.

17. A method according to any one of Claims 13 to 15 wherein the fins of each series are applied as two or more interconnected fins made by cutting and folding a single sheet of metal.

18. A method according to any one of Claims 13 to 17 including forming said fins resilient parts to engage the tube, compressing said resilient parts to permit interengagement of said hook forma tions on fins of one series with hook formations on the fins of another series, interengaging said hook formations and then allowing the resilient parts to hold the hook formations in engagement.

19. A heat exchanger comprising a plurality of finned tube assemblies as claimed in any one of Claims 1 to 12.

20. A finned tube assembly substantially as hereinbefore described with reference to and as shown in Figures 5 to 8 of the accompanying drawings.

21. A finned tube assembly substantially as hereinbefore described with reference to, and as shown in Figures 9 to 12 of the accompanying drawings.

22. A finned tube assembly according to Claim 21 but modified substantially as hereinbefore described with reference to and shown in Figure 13 of the accompanying drawings.

23. A finned tube assembly substantially as hereinbefore described with reference to and as shown in Figures 14 to 16 of the accompanying drawings.

24. A heat exchanger comprising a plurality of finned tube assemblies according to any one of Claims 20 to 23.

25. A heat exchanger according to Claim 24 and substantially as hereinbefore described with reference to and as shown in Figure 21 of the accompanying drawings.