EP0005959B1

EP0005959B1 - Heat exchanger fins and apparatus for making same

Info

Publication number: EP0005959B1
Application number: EP19790300923
Authority: EP
Inventors: Timothy Anton Turton Cowell
Original assignee: Armstrong Engineering Ltd
Current assignee: Armstrong Engineering Ltd
Priority date: 1978-05-31
Filing date: 1979-05-24
Publication date: 1982-03-17
Also published as: EP0005959A1; DE2962272D1; DE7928310U1; EP0005959B2

Description

This invention relates to heat exchangers and in particular to secondary surface elements for heat exchangers.
Louvred sheet material is already known for use in heat exchangers and the material may take several forms, such as in a fin and tube heat exchanger in which the tubes pass through apertures in a plurality of secondary surface fins. Corrugated sheet material is also used in which such material is located between tubes and louvres are formed in the flanks of the corrugations. In such arrangements the louvres are inclined or twisted relative to the plane of the associated sheet and gaps are formed in the material through which heat exchange medium can pass.
In our French patent No. 2380828 there is proposed corrugated secondary surface material for heat exchangers in which there is formed a plurality of side by side strips in which the corrugations are displaced from one another in their longitudinal direction. In this arrangement the flanks of the corrugations lie perpendicular to said longitudinal direction and parallel to the flow of heat exchange medium passing over the material.
French patent specification No. 2270544 discloses a heat exchange material formed with a series of projections of which the flanks lie obliquely to a direction at right angles to the rows of projections. In this case the material is intended to be employed in an oil cooler to provide an annular passage in which a high degree of turbulence is created in the fluid and to achieve this the fluid is passed around the annulus in the direction of the rows of projections. The material is located around a cylindrical passage.
The various known forms of secondary surface elements for heat exchangers suffer from disadvantages either in relation to their heat exchange properties or their ease of manufacture or both. Moreover prior elements have failed to combine the attributes of high heat transfer characteristics with low energy loss in the fluid passing therethrough. An object of the invention is to overcome these disadvantages and provide a heat exchanger incorporating a secondary surface element which is easily made and which has good heat exchange properties and low energy loss.
According to one aspect of the invention a heat exchanger comprises at least one secondary surface element formed of a plurality of corrugated parallel strips each strip being similarly shaped and corresponding portions of the flanks of the corrugations in adjacent strips being displaced from one another in the longitudinal direction of the strips whereby the leading edge of each flank is displaced in one direction from the trailing edge of the corresponding flank of an immediately adjacent strip, the strips being joined to one another to form a unitary structure, and the flanks of the corrugations each being inclined in the opposite direction to said displacement at an acute angle to the longitudinal direction of the strips, characterised in that the heat exchanger further comprises tubes for a heat exchange medium, the lengthwise direction of the tubes lying parallel to the longitudinal direction of the strips in said unitary structure, the tubes being in contact with the crests of the corrugated strips, the arrangement of the tubes and secondary surface element being such as to provide for the flow of fluid over the element in a direction transverse to the longitudinal direction of the strips.
Such an arrangement provides large gaps of substantially constant cross section between the strips for the fluid to passs through the element. At the same time a good heat transfer rate is achieved by good contact of the fluid with the element and the provision of a large number of leading edges contacted by the fluid in passing over the element. Accordingly fluid passing through the tubes can give up heat to the fluid passing over the element at a high rate without undue energy loss in either fluid.
Preferably the direction of displacement of the edges of the strips is reversed across the element to provide at least two groups of strips, the displacement of the edges of each group being in opposite directions to one another and the direction of inclination of the flanks of each group being opposite to the direction of displacement of the strips of that group, and the angle of the flanks of the corrugations to the longitudinal direction of the strips is in the range 60°-80°. In one arrangement the displacement between the leading edges and the trailing edges of adjacent strips is such that the corrugations are in alignment in a direction at a right angle to the longitudinal direction of the strips. Alternatively the displacement between the leading edges and the trailing edges of adjacent strips is of the order of one third the pitch of the corrugations.
According to a further aspect of the invention apparatus for forming a secondary surface element for the heat exchanger of the invention comprises a pair of rolls and drive means for driving at least one of the rolls so that the rolls rotate in opposite directions, each roll being formed of a set of laminae arranged side by side along the axis of the associated roll and each lamina in a set having a corrugated profile with circumferentially spaced teeth, corresponding edges of the teeth of adjacent laminae being angularly displaced from one another about the axis of the associated roll, and the teeth of one set of laminae being arranged in mesh with the teeth of the other set of laminae, characterised by the teeth of the laminae lying on helical paths extending around the associated roll, and the element being formed by feeding sheet material between the rolls.
By this arrangement the teeth form the flanks of the corrugations at an acute angle to the longitudinal direction of the strips and the elements are formed simply and effectively.
Further features of the invention will appear from the following description of an embodiment of the invention given by way of example only and with reference to the drawings in which:

Fig. 1 is a side elevation of part of one form of secondary surface element,
Fig. 1 A is an enlarged view of part of the element of Fig. 1,
Fig. 2_'is a cross-section on the line 2-2 in Fig. 1,
Fig. 2A is a cross-section on the line 2A-2A in Fig. 1 A,
Fig. 3 is a cross-section corresponding to that of Fig. 2 of part of a complete secondary surface element,
Fig. 4 is a perspective view of the element of Fig. 3 in combination with heat exchanger tubes,
Fig. 5 is a side elevation of apparatus for making the secondary surface elements of Figs. 1 to 4, and
Fig. 6 is an enlarged view of part of the apparatus of Fig. 5.

Referring to the drawings and firstly to Figs. 1, 1A, 2 and 2A, a secondary surface element is shown for use sandwiched between heat exchanger tubes in the manner shown in Fig. 4. The element of Figs. 1, 1A, 2 and 2A is of corrugated form and is made from metal sheet or foil, the corrugations in this case being of truncated triangular form each with flat crests 17. It will, however, be appreciated that corrugations of other shapes can be formed, for example of generally sinusoidal shape or with relatively wider or narrower flat crests, provided that these are such that the material retains a unitary structure.
In Figs. 1, 1 A, 2 and 2A only three strips 11, 12 and 13 are shown as making up the element but it will be appreciated that a unitary sheet will normally have a greater number of strips.
Each of the strips 11, 12 and 13, is displaced from its adjacent strips by an amount d and the extent of the displacement d may be selected according to pitch P of the corrugations. The height of the secondary surface is given as h.
The flanks 15 of the corrugations are each inclined at an acute angle a to the longitudinal direction of the strips. The angle α is preferably in the range 60^0-80^0. The direction of displacement of the strips is opposite to the direction of inclination of the flanks 15. It will be appreciated that each strip 11, 12 and 13 has a leading and trailing edges, such as at 11a, 12a and 11 b, 12b, having regard to a direction of flow A of fluid over the material which direction A is at a right angle to the longitudinal direction of the strips. Adjacent strips are interconnected at the crests 17 to 17a to form a unitary structure.
The displacement of adjacent strips from one another can be of any desired amount, ranging from a small displacement d by which the corrugations are aligned with one another in rows lying perpendicular to the longitudinal direction of the strips, to a larger displacement d of the order of a third the pitch P of the corrugations.
In use of the material in heat exchangers it is generally advantageous to maintain a large displacement or gap between the leading eges 11 a a of the flanks 15 of the strips and the corresponding trailing edges 116 of the flanks of the preceding strips. At the same time the widths W of the strips should be kept narrow, but the achievement of these characteristics may have to be a compromise with the requirements of the manufacturing process.
Referring now to Fig. 3 a complete secondary surface element is shown in which the direction of displacement d of the adjacent strips is reversed centrally of the element. At the same point in the strips as the reversal of displacement takes place the direction of inclination of the flanks of the corrugations is also reversed. The central strip 3) may be parallel to the general direction of flow A, as shown, or it may be of V-shaped profile, limbs of the V lying parallel to the sets of strips to each side of the strip 30. As seen in Fig. 3, the element is of symmetrical configuration about the strip 30 and this, together with the inclination of the flanks, enables the material to be readily formed, as will be described.
It will be appreciated that, with the material having corrugations of the kind described and shown, the free edges 11 a, 11 b of the strips can extend the full height h of the material. This compares with louvred material in which the louvres do not extend the full height but are formed along only a portion of the flanks of the corrugations.
Referring now particularly to Fig. 4, the material of Fig. 3, is assembled with tubes 31 so that an element is bounded on each side by a row of tubes 31. The lengthwise direction of the tubes is parallel to the plane of symmetry of the strips. The tubes 31 are in contact with the crests of the corrugations. A heat exchange fluid is passed through the tubes 31 and a flow of further fluid, usually air, passes over the element in the general direction of arrows A, transverse to the longitudinal direction of the strips, to set up a heat exchange relationship between the two fluids.
Apparatus for forming the corrugated material of Figs 1-4 is shown in Figs. 5 and 6 of the drawings and it comprises a pair of rolls 20 and 21 mounted on parallel shafts 22 and 23 for rotation in opposite directions, each roll being made up of laminae 24 arranged side by side in contact with one another and each lamina having an outer profile formed with circumferentially-spaced teeth 25. Adjacent laminae in each roll are angularly displaced from one another about the axis of the associated roll by a predetermined amount, as will be explained, and the teeth mesh together on rotation with a suitable space between them to receive the metal sheet or foil to be corrugated. The teeth 25 are helically arranged, that is, each tooth on each lamina lies along a helical path extending around the associated roll. This gives rise to the flanks of the teeth extending at an acute angle to lines parallel to the axis of the associated roll, which angle is the same for all the teeth so that the teeth may mesh with one another. Moreover, the helical paths on which the teeth of each lamina lie are equally spaced from one another circumferentially of the lamina.
One way of imagining the constructions of the rolls is to consider a pair of matching helical gear wheels. Divide each gear wheel into a plurality of laminae along lines at right angles to the axes of the wheels and then displace each lamina a small amount angularly in relation to its next adjacent lamina in the opposite direction to the inclination of the flanks of the teeth and in a manner such that the gear wheels can mesh together. This results in a pair of rolls in accordance with the present invention and indeed the apparatus may be constructed generally as described. Alternatively helical teeth may be machined in an assembly of generally disc like laminae which are then angularly displaced from one another.
Upon feeding a sheet of heat-conducting material, for example metal sheet or foil, between these rolls, corrugated sheet material, such as shown in Figs. 1-4 of the drawings, is formed. In this case the teeth 25 of the rolls are shaped to produce the truncated triangular corrugations shown.

Claims

1. A heat exchanger comprising at least one secondary surface element formed of a plurality of corrugated parallel strips (11, 12, 13) each strip being similarly shaped and corresponding portions of the flanks (15) of the corrugations in adjacent strips being displaced from one another in the longitudinal direction of the strips whereby the leading edge (12a) of each flank (15) is displaced (d) in one direction from the trailing edge ( 1 1 b) of the corresponding flank of an immediately adjacent strip, the strips (11, 12, 13) being joined to one another to form a unitary structure, and the flanks (15) of the corrugations each being inclined in the opposite direction to said displacement at an acute angle (α) to the longitudinal direction of the strips, characterised in that the heat exchanger further comprises tubes (31) for a heat exchange medium, the lengthwise direction of the tubes lying parallel to the longitudinal direction of the strips in said unitary structure, the tubes being in contact with the crests (17) of the corrugated strips (11, 12, 13), the arrangement of the tubes (31) and secondary surface element being such as to provide for the flow of fluid (A) over the element in a direction transverse to the longitudinal direction of the strips.

2. A heat exchanger according to claim 1 characterised in that the direction of displacement of the edges of the strips (11, 12, 13) is reversed across the element to provide at least two groups of strips, the displacement (d) of the edges of each group being in opposite directions to one another and the direction of inclination of the flanks (15) of each group being opposite to the direction of displacement of the strips of that group.

3. A heat exchanger according to claim 2 characterised in that the element is arranged symmetrically about a central longitudinal strip (30).

4. A heat exchanger according to any one of claims 1 to 3 characterised in that the angle (α) of the flanks (15) of the corrugations to the longitudinal direction of the strips is in the range 60°-80°.

5. A heat exchanger according to any one of claims 1 to 4 characterised in that the displacement (d) between the leading edges and the trailing edges of adjacent strips is such that the corrugations are in alignment in a direction at a right angle to the longitudinal direction of the strips.

6. A heat exchanger according to any one of claims 1 to 4 characterised in that the displacement (d) between the leading edges (12a) and the trailing edges (12b) of adjacent strips is of the order of one third the pitch (P) of the corrugations.

7. Apparatus for forming a secondary surface element of the heat exchanger according to any one of claims 1 to 6 comprising a pair of rolls (20, 21) and drive means for driving at least one of the rolls so that the rolls rotate in opposite directions, each roll being formed of a set of laminae arranged side by side along the axis (22; 23) of the associated roll and each . lamina in a set having a corrugated profile with circumferentially spaced teeth (25), corresponding edges of the teeth of adjacent laminae being angularly displaced from one another about the axis of the associated roll, and the teeth of one set of laminae being arranged in mesh with the teeth of the other set of laminae, characterised by the teeth (25) of the laminae (24) lying on helical paths extending around the associated roll (20; 21), and the element being formed by feeding sheet material between the rolls.