EP0055711B1

EP0055711B1 - Low profile heat exchanger and method of making the same

Info

Publication number: EP0055711B1
Application number: EP81900865A
Authority: EP
Inventors: Alexander Goloff
Original assignee: Caterpillar Tractor Co
Current assignee: Caterpillar Inc
Priority date: 1980-07-07
Filing date: 1980-07-07
Publication date: 1985-10-09
Also published as: JPS57500945A; DE3071178D1; WO1982000194A1; EP0055711A1; EP0055711A4; CA1140531A

Abstract

A low profile heat exchanger module (46) and method for forming the same wherein the heat exchanger module (46) is formed from one or more compact, single sheet primary surface heat exchanger core units (38a and 38b). Each single sheet primary surface core unit (38a and 38b) is made from a rectangular sheet of a suitable heat exchange material (10) which has been serrated along the longitudinal edges (18) to provide entrance ramps (20) and to minimize flow blockage. Fluid flow is controlled by closures (42, 44) in the ends of alternate flow passages (58, 64) which serve to isolate the fluid in one passage from the fluid in an adjacent passage.

Description

Technical field

This invention relates to a low profile heat exchanger module for use in heat furnaces, steel melting furnaces, gas turbines which use recuperators and the like, and a method for forming the same.

Background art

United States Patent 3,759,323 issued to Harry J. Dawson et al describes an important prior art primary surface heat exchanger for use as a recuperator core of a gas turbine. Dawson et al discloses a heat exchanger core made from a multiplicity of thin metal sheets which have been individually corrugated, or folded in a wavy pattern according to the method described in United States Patent 3,892,119 issued to Miller et al. A large number of these metal plates are stacked on top of each other and the edges of the sheets are crushed to form the flat sections necessary to encase the assembled and to allow the attachment of suitable manifolding for conveying hot and cold fluids. The necessity for crushing the edges results in blockage, and hence the restriction of fluid flow. In addition, the depth of the corrugations in the unit described by Dawson et al is limited by the need to crush the edges. Deep corrugations or pleats cannot be crushed in an organized or predictable manner without causing severe blockage.
The pattern of the corrugations and the relatively thick crushed edges of the individual sheets described by Dawson et al result in rigidity in all directions in the heat exchanger unit. This may lead to the development of high thermal strains, especially when transient loads are characterised by steep gradients.
Another example of a primary surface heat exchager is illustrated in FR-A-2315674 in which a number of heat exchange cores are provided, each core being formed from a number of stacked sheets with upturned edges to maintain the sheets spaced apart. This requires a large number of welds to connect the sheets together which is undesirable.
A major drawback of the prior art heat exchanger construction has been the presence of high stress concentration factors which have resulted from the need to crush the edges. In some applications the effect has been that of producing a multiplicity of cracks. Another problem has been the failure of the weld to penetrate at certain junctions, which results in a preformed crack. While such stress concentration factors may not be significant when the assembly is preloaded in compression, as intended, and when the transients are not steep, high stresses which lead to premature failures may appear under severe operating conditions and after prolonged periods of operation during which the preload is likely to be relaxed.
DE-A-2523151 discloses a heat exchange core formed from a plurality of walls connected together in series to define fluid flow passages between adjacent pairs of walls, the ends of alternate passages being closed. In this construction the relatively thin walls are only supported at each end and this can lead to problems during use when the walls may flex and inhibit fluid flow along the passages.
In accordance with one aspect of the present invention, a heat exchange core comprises a unitary strip providing a plurality of walls in series to define fluid flow passages between adjacent pairs of walls, a portion of each end of every alternate passage, forming a first group of passages, being closed, the closed portions on the end of the core being in alignment and a portion of each end of the remaining passages, forming a second group of passages, being closed, the closed portions of the second group on each end of the core being in alignment with each other whereby the closed end portions of each group of passages are adjacent open end portions and offset from closed end portions of the other group of passages, the walls being undulated between the open ends of the fluid passages, and is characterised in that a plurality of rows of spaced apart bosses are provided between the open ends of the fluid passages, the bosses separating adjacent walls and guiding fluid flow along the passages in use.
In accordance with a second aspect of the present invention, a method of forming a heat exhange core comprises pleating a unitary, elongate strip of heat conducting material to form a plurality of walls defining fluid passages therebetween, whereby the open side of each fluid passage opens in a direction opposite to that of the next adjacent fluid passages on either side thereof; closing a portion of each open end of every alternate fluid passage to form a first group of fluid passages having adjacent aligned closed portions and aligned open portions at either end thereof; closing a portion of each open end of the remaining fluid passages to form a second group of fluid passages having adjacent aligned closed portions and aligned open portions at either end thereof whereby the closed end portions of each group of fluid passages are adjacent the open end portions and offset from the closed end portions of the other group of fluid passages; and forming undulations in the unitary strip prior to pleating the strip, the undulations extending between the side edges thereof, characterised in that a plurality of rows of spaced apart bosses are provided between the open ends of the fluid passages, the bosses seprating adjacent walls and guiding fluid flow along the passages in use.
With this invention, rows of spaced apart bosses are formed in the walls of the heat exchange core and have the dual function of maintaining the walls of the passages spaced apart while at the same time assisting in guiding fluid flow along the passages. A further advantage is that the spaces between the bosses induce fluid turbulence in the flow path and thus enhance still further the . heat transfer characteristics in the core. In this way, maximum heat exchange efficiency is achieved.
Conveniently, a low profile heat exchanger may be constructed from one or more heat exchange cores in accordance with the invention. Each core is formed from a single sheet of thin material which has been pleated to any depth desired. Before pleating, the longitudinal edges of thin sheet may be serrated to provide fluid entrance ramps that minimise blockage, and the surface of the sheet may be embossed to define flow channels and to provide a means for directing flow and controlling turbulence.
With reference to British Patent No. 2,079,437, the applicants have voluntarily limited the scope of the present application and submitted separate claims for Great Britain.
Thus, in accordance with this aspect of the present invention, a heat exchange core comprises a unitary strip providing a plurality of walls in series to define fluid flow passages between adjacent pairs of walls, a portion of each end of every alternate passage, forming a first group of passages, being closed, the closed portions on each end of the core being in alignment and a portion of each end of the remaining passages, forming a second group of passages, being closed, the closed portions of the second group on each end of the core being in alignment with each other whereby the closed end portions of each group of passages are adjacent open end portions and offset from closed end portions of the other group of passages, the walls being undulated between the open ends of the fluid passages, wherein a plurality of rows of spaced apart bosses are provided between the open ends of the fluid passages, the bosses separating adjacent walls and guiding fluid flow along the passages in use; and is characterised in that a strip is attached to the walls and extends across at least one end of the heat exchange core between the closed and open end portions of the fluid passages.
In accordance with a further aspect of the present invention, a method of forming a heat exchange core comprises pleating a unitary, elongate strip of heat conducting material to form a plurality of walls defining fluid passages therebetween, whereby the open side of each fluid passage opens in a direction opposite to that of the next adjacent fluid passages on either side thereof; closing a portion of each open end of every alternate fluid passage to form a first group of fluid passages having adjacent, aligned closed portions and aligned open portions at either end thereof; closing a portion of each open end of the remaining fluid passages to form a second group of fluid passages having adjacent closed portions and aligned open portions at either end thereof whereby the closed end portions of each group of fluid passages are adjacent the open end portions and offset from the closed end portions of the other group of fluid passages, and forming undulations in the unitary strip prior to pleating the strip, the undulations extending between the side edges thereof and comprising a plurality of rows of spaced apart bosses between the open ends of the fluid passages, the bosses separating adjacent walls and guiding fluid flow along the passages in use and is characterised by securing an elongate strip across at least one end of the unitary strip after the pleating step.
The provision of one or more strips, which are preferably corrugated, assists in holding each core in its pleated form.
Some examples of heat exchange cores and methods for forming such cores in accordance with the present invention will now be described with reference to the accompanying drawings, in which:-

Figure 1 is a plan view of a portion of a single sheet of heat exchange material employed to form a heat exchange core;
Figure 2 is a plan view of a portion of the single sheet of Figure 1 after embossing to show a "U" flow concept.
Figure 2' is a plan view of a portion of the single sheet of Figure 1 after embossing to show a "Z" flow concept.
Figure 3 is a view in side elevation of the single' sheet of Figure 2 during the pleating thereof;
Figure 4 is a diagrammatic illustration of a side view of a pleated assembly;
Figure 5 is a diagrammatic partial perspective view of a pleated assembly;
Figure 6 is a cross sectional view of a heat exchanger;
Figure 7 is a diagrammatic illustration of an end view of the heat exchanger;
Figure 8 is a plan view of a portion of a single sheet of heat exchange material of a second embodiment of the present invention;
Figure 9 is a view in side elevation of the single sheet of the embodiment of Figure 8;
Figure 10 is a view in side elevation of the single sheet of the embodiment of Figure 8 during pleating thereof;
Figure 11 is a diagrammatic illustration of a third embodiment of the present invention; and
Figure 12 is a diagrammatic illustration of the method of producing the embodiment of Figure 11.

Figure 1 illustrates a single sheet used to form one of the pleated assemblies of a heat exchange core. The single sheet, indicated generally at 10, is a long rectangular strip of heat exchange material, for example a suitable thin metal, such as heat resistant steel. The width of the sheet 10 indicated at 12 determines the length of the resulting pleated assembly, and may be varied by the designer to fit the desired heat exchanger application. The longitudinal edges of the sheet are serrated or cut in a sawtooth pattern so that the distance 14 between notches 16 is about equal to the desired height of the pleated assembly. The apexes or points of the serrated edge are shown at 18, and the distance of an edge 20 extending between notch 16 and point 18 should preferably be equal to the height of the pleated assembly as indicated at 14 to eliminate fluid flow blockage. For example, the assembly may be about 1-2 inches (2.3-5 cm) in height at 14 and the width 12 may be about 6―7 inches (15―17 cm).
Obviously, the configuration of the serrated edges of the sheet 10 can be altered to meet varying structural requirements. For example, the apexes 18 may be rounded instead of pointed, and the edge 20 can be curved rather than straight.
The pleated assembly is formed in a manner depicted in Figures 2 and 3 or 2' and 3. The serrated sheet of heat exchange material 10 is divided into sections or walls 22 and 24 which are embossed by means of conventional dies, shaped rollers or any other embossing techniques. The embossing serves to separate the subsequently formed pleats of the heat exchanger accurately and to guide the fluid flow through the completed heat exchanger. The embossing may space the pleates for a distance of about .030 inches (0.076 cm).
As illustrated in Figures 2 and 2', each section 22 and 24 may be embossed in a U-shaped or Z-shaped configuration respectively as shown by the rows of bosses 26 on section 22 and the rows of bosses 28 on section 24. While only four rows of bosses 26 and four rows of bosses 28 are shown, the number of rows could be much higher and is limited only by the size of the sections 22 and 24. Fluid flow channels 27 are defined on the face of each section 22 and 24 between the rows of bosses 26 and 28. The interruptions between bosses in a row induce fluid turbulence in the flow path defined thereby and thus enhance the heat transfer characteristics of the pleated assembly formed from the sheet 10.
Since the sheet 10 will be pleated into a pleated assembly which forms part of the core of the heat exchanger module depicted in Figure 6, it is necessary to embosss the sheet so that the pleats are spaced far enough apart to allow fluid flow between them. This is accomplished by embossing sections 22 and 24 in opposite directions. Thus in Figure 2, the bosses 26 on section 22 project upwardly, while the bosses 28 on section 24 project downwardly. This alternate. arrangement is maintained throughout the length of the sheet 10.
Once the sheet 10 is embossed, the sheet is pleated by folding it along lines or crest portions 34 between the sections or walls 22 and 24. Pleating may be accomplished mechanically in any conventional manner, such as on machines utilizing dull-edged knife blades like those used for pleating filter paper in the manufacture of air cleaners and oil filters, but which have been modified to pleat thin metal or heat exchange material rather than paper. Figure 3 shows the embossed sheet 10 being pleated and compressed at the lower end 36 to form the pleated assembly shown in Figure 4. The sections 22 and 24 of the sheet are compressed together until the raised bosses 26 and 28 contact the next adjacent section of the sheet. Thus the height of such bosses accurately controls the spacing between adjacent sections when the sheet 10 is pleated and compressed. To enable the bosses to perform this spacing function, it is imperative that the bosses on adjacent sections 22 and 24 be precluded from nesting when the sections are compressed together. This may be accomplished, as illustrated in Figure 2, by placing the bosses on section 24 so that they fall in the spaces between the bosses on section 22 when the two sections are pleated. In addition, the bosses may be of different depths, and it is possible to have both deep and shallow bosses on the same sheet.
When sheet 10 is pleated and compressed as at 36 in Figure 3, a side view of the pleated assembly would appear as illustrated in Figure 4, but without items 40 in which the pleated assembly is indicated generally as 38. The embossing is omitted for clarity, but in actuality the fluid flow paths are defined by the bosses previously described. Inclined fluid entrance ramps are formed by edges 20, between notches 16 and points, 18 and the pleated assembly is in part held together by means of corrugated strips 40 which are welded to points 18 across each end of the assembly. This is shown in Figure 5, which depicts an end view of the pleated assembly, not showing the corrugated strip 40. The pleated assembly is also held together by welds 42 and 44 which plug one side of the open ends of the fluid passages between sections 22 and 24, leaving the remaining sides 43 and 45 open. The passages are plugged at both ends, although only one end is shown in Figure 5. Additional external clamping may be provided to preload the heat exchanger pleats.
The junction of points 18 and welds 42 and 44 creates a relatively inelastic, rigid assembly. However, if such an inelastic area is attached to a relatively elastic area, high thermal stresses cannot be generated. Since the corrugated strip permits limited flexibility along the overall width of the edge which is distant from the weld zone and provides flexibility, and therefore high thermal stresses are substantially eliminated.
It will be noted with reference to Figure 5, that the welds 42 above the corrugated strip 40 are staggered with relation to the welds 44 below the strip. Thus fluid entry to every other fluid passage occurs above the strip while fluid entry to the intermediate fluid passages occurs below the strip. However, since the edges 20 borderning the entry to a fluid passage form an inclined ramp having a length equal to the total height of the pleated assembly, unrestricted fluid flow into each passage is assured.
The heat exchanger module 46 may be formed by stacking at least two pleated assemblies 38 within a housing 48 as shown in Figure 6. The upper pleated assembly 38a is placed over the lower pleated assembly 38b with a spacer 50 between them. The spacer 50 is essentially either a solid sheet or a mesh or perforated strip and is shown extending along the entire length of line 34 between sections of the pleated assemblies from point 52 to point 54. However, the spacer 50 may be placed so that it stops short of points 52 and 54. It is possible, by varying the thickness and the length of the spacer 50, to reduce fluid flow blockage beyond the reduction achieved by means of the fluid entry ramps defined by edges 20. As previously mentioned in discussing Figure 1, the length of edges 20 should be equal to pleat depth 14 to minimize fluid flow blockage. Although the ramps are shown to be straight, longer ramps may be achieved within the same dimensions by curving edges 20, thus lengthening the ramps while maintaining the compactness of the unit.
Hot and cold fluid manifolds are attached to the ends of the two stacked pleated assemblies as shown in Figure 6, and result in a low profile heat exchanger. Inlet manifolds 56 provide hot fluid, for example hot exhaust gas from a gas turbine, through fluid passages 58. This hot fluid follows the path shown by the white arrows 58 to outlet manifolds 60 which collect the previously hot fluid after heat has been transferred therefrom in the heat exchanger core. Since the heat exchanger of the present invention is a counterflow type heat exchanger, cool fluid, as, for example air from the compressor of a gas turbine engine, is supplied to the path shown by arrows 64 by a cool fluid inlet manifold 62. This cool fluid flows through the pleated assemblies 38a and 38b along the paths indicated by the dark arrows, is heated, and then is collected by an outlet manifold 66.
The corrugated strips 40 are preferably welded to the housing 48 to form the manifolding. The width of corrugated strip 40 introduces a desired flexibility into the heat exchanger unit, as it allows the reduction of stresses in the presence of thermal gradients. It is also possible, but less desirable, to weld rigid manifolding directly to the pleated assemblies, which is not shown in Figure 6. However, this means of attaching the manifolding to the pleated assembly does not provide the flexibility and consequent dissipation of thermal stresses possible with the arrangement shown in Figure 6.
Figure 7 illustrates an end view of the heat exchanger module 46 used as a recuperator for receiving hot exhaust uunand compressed air from a gas turbine engine. Hot gas input manifolds 56 are at the top and bottom and the hot combustion air output manifold 66 is in the center. Hot gas flows in alternate passages 58 while air to be heated flows in the opposite direction through the intermediate passages 64. The housing 48 closes the open ends of the passages 58.
A second embodiment of the present invention is diagrammatically shown in Figures 8, 9, and 10. Heat exchanger sheet material 72, which has been serrated along the longitudinal edges in the same manner as sheet 10 in Figure 1 and subsequently embossed, is bent between points 68 and 70 by passing it over shaped rollers, which are not shown in Figure 8, prior to pleating to form a sheet which is rippled in cross section (Figure 9). This structure provides flexibility to the pleats in the direction 68-70. Figure 10, which corresponds to Figure 3, shows the rippled sheet 72 being pleated and compressed into a bent pleated assembly 74. The method depicted in Figures 8, 9, and 10 results in increased flexibility, not only in the pleats themselves, but also in the welded plugs 42 and 44 used to seal alternate flow passages as shown in Figures 5 and 7.
Additional flexibility in the direction of sheet width 12 shown in Figure 1 can be introduced in still another embodiment of the present invention. The compressed pleated assembly shown at 36 in Figure 3 can be made to assume an arcuate shape or even an "S" shape, (not shown) when viewed in the direction of the crest portions or edges 34, as depicted in Figure 11.
Finally, the sheet 10 can be treated in an alternate method after pleating to enhance fluid flow and flexibility. This is accomplished as shown in Figure 12. A pleated and compressed sheet, indicated generally at 76, is passed over a roller 78, which causes the pleats to separate as at 80 while remaining compressed at 82, thus allowing cams, rollers, pawls, or other suitable mechanisms to be introduced into the wide gaps at 80 to spread the pleats at edge 34 to any desired distance. After passing over roller 78, the pleated and compressed sheet passes over roller 84 where the wide gaps 80 become compressed at 86 and the pleats which were compressed at 82 become separated at 88, thus permitting the same type of cams, rollers or pawls to bow the pleats as at 34, resulting in a compact and bowed, pleated assembly after the core passes over the roller 84 as shown in Figure 11. This introduces flexibility in the direction of the width of the strip thus minimizing thermal stresses.
The heat exchanger module 46 may be effectively employed as a recuperator for a gas turbine engine or for other heat exhange applications, as, for example, in steel heat treating or melting furnaces. The inlet manifold 62 is connected to a source of cool fluid to be heated while the inlet manifolds 56 are connected to a source of heated fluid. In a gas turbine engine, the inlet manifolds 56 would be connected to receive hot exhaust gases from the engine while the inlet manifold 62 would be connected to receive compressor discharge air from the engine. As the cooler discharge air passes through the pleated assemblies 38a and 38b with the counter flowing hot exhaust gas, the air is heated by the heat transfer provided by heat exchange sections 22 and 24. The exhaust gas then passes out of the outlet manifolds 60 and is normally vented to the atmosphere while the heated air passes to the outlet manifold 66. This outlet manifold is connected to the combustor of the gas turbine engine and proceeds on through the engine in the conventional manner.

Claims

1. A heat exchange core (38) comprising a unitary strip (10) providing a plurality of walls (22, 24) in series to define fluid flow passages (58, 64) between adjacent pairs of walls, a portion (44) of each end of every alternate passage, forming a first group of passages (58), being closed, the closed portions on each end of the core being in alignment and a portion (42) of each end of the remaining passages, forming a second group of passages (64), being closed, the closed portions of the second group on each end of the core being in alignment with each other whereby the closed end portions of each group of passages are adjacent open end portions (45, 43) and offset from closed end portions of the other group of passages, the walls being undulated between the open ends of the fluid passages, wherein a plurality of rows of spaced apart bosses (26, 28) are provided between the open ends of the fluid passages, the bosses separating adjacent walls and guiding fluid flow along the passages in use; characterised in that a strip (40) is attached to the walls (22, 24) and extends across at least one end of the heat exchange core (38) between the closed and open end portions (42, 43, 44, 45) of the fluid passages (58, 64).

2. A core (38) according to claim 1, wherein the bosses (26, 28) are formed on only one side of each wall (22, 24), the bosses on adjacent walls (22, 24) extending from opposite sides thereof.

3. A core (38) according to claim 2, wherein the bosses (26, 28) on adjacent walls (22, 24) are misaligned to prevent nesting of the walls.

4. A core (38) according to any of claims 1 to 3, wherein the bosses (26, 28) are elongate.

5. A core (38) according to any of the preceding claims, wherein the strip (40) is corrugated to provide flexibility along its length.

6. A primary surface heat exchanger (46) comprising at least one heat exchange core (38), the or each core comprising a unitary strip (10) providing a plurality of walls (22, 24) in series to define fluid flow passages (58, 64) between adjacent pairs of walls, a portion (44) of each end of every alternate passage, forming a first group of passages (58), being closed, the closed portions on each end of the core being in alignment and a portion (42) of each end of the remaining passages, forming a second group of passages (64), being closed, the closed portions of the second group on each end of the core being in alignment with each other whereby the closed end portions of each group of passages are adjacent open end portions (45, 43) and offset from closed end portions of the other group of passages, the walls being undulated between the open ends of the fluid passages, wherein a plurality of rows of spaced apart bosses (26, 28) are provided between the open ends of the fluid passages, the bosses separating adjacent walls and guiding fluid flow along the passages in use; first fluid inlet means (56) extending across the open portions (43) of the first group of fluid passages (58) and the closed portions (42) of the second group of fluid passages (64) at one end of the heat exchange core (38); first fluid outlet means (60) positioned at the other end of the heat exchanger core (38) and extending across the open portions (43) of the first group of fluid passages (58) and the closed portions (42) of the second group of fluid passages (64) at that end, the first fluid inlet and outlet means (56, 60) communicating with the first group of fluid passages (58); second fluid inlet means (62) extending across the open portions (45) of the second group of fluid passages (64) and the closed portions (44) of the first group of fluid passages (58) at one end of the heat exchange core (38); and second fluid outlet means (66) positioned at the end of the heat exchange core (38) opposite to the second fluid inlet means (62), extending across the open portions (45) of the second group of fluid passages (64) and across the closed portions (44) of the first group of fluid passages (58), the second inlet and outlet means (62, 66) communicating with the second group of fluid passages (64), characterised in that for the or each heat exchange core a strip (40) is attached to the walls (22, 24) and extends aross at least one end of the heat exchange core (38) between the closed and open end portions (42, 43, 44, 45) of the fluid passages (58, 64).

7. A primary surface heat exchanger (46) according to claim 6, wherein the first and second fluid inlet means (56, 68) are at opposite ends of the core (38).

8. A primary surface heat exchanger (46) according to claim 6 or claim 7, comprising two heat exchange cores (38a, 38b) according to any of claims 1 to 5, the heat exchange cores (38a, 38b) being relatively positioned so that the portions (45) of each core at which the respective second groups of fluid passages (64) are open are adjacent and in alignment; wherein the first and second fluid inlet means (56, 62) are in communication with the first and second groups of fluid passages (58, 64) respectively of each core, and the first and second fluid outlet means (60, 66) are in fluid communication with the first and second groups of fluid passages (58, 64) respectively of each core.

9. A primary surface heat exchanger (46) according to any of claims 6 to 8, further including a housing (48) partially defining the first and second fluid inlet and outlet means, spaced from and enclosing opposite ends of the or each heat exchange core (38), wherein a strip (40) is attached to each end of the or each core (38) and extends outwardly from the respective core end to the housing (48) to divide the interior of the housing into the first and second fluid inlet and outlet means.

10. A method of forming a heat exchange core (38), the method comprising pleating a unitary, elongate strip (10) of heat conducting material to form a plurality of walls defining fluid passages (58, 64) therebetween, whereby the open side of each fluid passage opens in a direction opposite to that of the next adjacent fluid passages on either side thereof; closing a portion (44) of each open end of every alternate fluid passage (58) to form a first group of fluid passages (58) having adjacent, aligned closed portions (44) and aligned open portions (43) at either end thereof; closing a portion (42) of each open end of the remaining fluid passages (64) to form a second group offluid passages (64) having adjacent aligned closed portions (42) and aligned open portions (45) at either end thereof whereby the closed end portions (42, 44) of each group of fluid passages are adjacent the open end portions (43, 45) and offset from the closed end portions of the other group of fluid passages, and forming undulations in the unitary strip (10) prior to pleating the strip (10), the undulations extending between the side edges thereof and comprising a plurality of rows of spaced apart bosses (26, 28) between the open ends of the fluid passages, the bosses separating adjacent walls and guiding fluid flow along the passages in use; characterised by securing an elongate strip (40) across at least one end of the unitary strip (10) after the pleating step.

11. A method according to claim 10, further including cutting the unitary strip of heat conducting material (10) along each side thereof to a sawtooth configuration prior to the pleating step to provide an apex (18) at the end of each wall (22, 24) the end edges (20) being inclined away from either side of the apex (18) to points (16) at the dividing lines (34) between the respective walls and its adjacent walls.

12. A method according to claim 10 or claim 11, further including closing the ends of each fluid passage in the first group of fluid passages (58) along the inclined edges (20) extending in a first direction away from each apex (18), and closing the ends of each fluid passage in the second group of fluid passages (64) along the inclined edges (20) extending in a second, opposite direction away from the apex (18).

13. A method according to any of claims 10 to 12 further including partially separating the walls (22, 24) of the pleated strip (10), and spreading the separated walls by a preselected distance.