EP1029215A1

EP1029215A1 - Heat exchanger

Info

Publication number: EP1029215A1
Application number: EP98952902A
Authority: EP
Inventors: Martin Booth; Barry Borgi; Edward Rodney Beldon
Original assignee: Marconi Communications Inc
Current assignee: Marconi Communications Inc
Priority date: 1997-11-12
Filing date: 1998-11-12
Publication date: 2000-08-23
Also published as: CA2309862A1; WO1999024772A1; AU1044499A; KR20010015811A; GB9723812D0; BR9814170A

Abstract

A heat exchanger comprising a plurality of plates (10A) is described. The plurality of stackable plates (10A), when stacked one upon the other, from fluid channels between adjacent plates (10A) allowing heat exchange between fluid flowing on one side of a plate (10A) and fluid flowing on the other side of the same plate (10A), each plate (10A) comprising interengaging means (3A, 3B) on at least one pair of opposing edge regions (32, 34, 36, 38) for engaging with interengaging means (3A, 3B) on a corresponding pair of edge regions of a neighbouring plate (10A) to form seals between these edges (32, 34, 36, 38) when the plates (10A) are stacked together.

Description

HEAT EXCHANGER

Field of the Invention

This invention relates to a plate-type heat-exchanger and a plurality of plates for a heat exchanger. It relates particularly, but not exclusively, to a heat exchanger for supplying cool fluid such as air to an electrical equipment cabinet .

Background to the Invention

Plates heat exchangers generally comprise a stack of thin plates which are typically four sided. The plates are stacked to form a three dimensional structure in which cool external air and warm extracted air traverse opposite sides of each plate to allow exchange of heat. Typically the flow of cool air on one side of a plate is roughly at right angles to the flow of warm air across the other side of the plate. Travelling down one side of the stack the edges of the plates are usually sealed together in pairs. On a neighbouring side, the edges of the plates are also sealed together in pairs but in an inverted arrangement so that where one plate is sealed to its upper neighbour along one edge on one side of the stack, it is sealed to its lower neighbour along another edge on an adjacent side of the stack. This allows cross-flow of warm air and cool air on opposite sides of each plate, the two flows typically remaining unmixed.

The heat exchanger plates are typically formed from metal such as aluminum or copper which are good heat conductors. Such metal plates are, however, expensive to manufacture and difficult to assemble into stacks for heat exchangers particularly because of the necessity of forming seals between the edges of alternating pairs of plate edges. This results in a lengthy and awkward assembly process. Moreover, such plates are also vulnerable to corrosion and can be heavy.

Plastic plates have been developed to alleviate the problem of corrosion. Such plastic plates may also be difficult and time-consuming to assemble. The plates, whether plastic or metal, are typically welded or glued to seal the edges together in the required manner, for example, to allow cross-flow. Sometimes separate clamps and even sealing means, such as sealing gaskets, are used. Nevertheless, such arrangements are unsatisfactory, particularly because of the time it takes to assemble the plates in the stack with the necessary seals in place.

It is therefore an object of the present invention to alleviate the problems mentioned above.

Summary of the Invention

According to a first aspect of the present invention, there is provided a plurality of stackable plates for a heat exchanger, which, when stacked one upon the other, form fluid channels between adjacent plates allowing heat exchange between fluid flowing on one side of a plate and fluid flowing on the other side of the same plate, each plate comprising interengaging means on at least one pair of opposing edge regions for engaging with interengaging means on a corresponding pair of edge regions of a neighbouring plate to form seals between these edges when the plates are stacked together.

Preferably, one or both fluids are gaseous such as air.

Preferably, the interengaging members are arranged so that when the plates are stacked together with directions of the fluid flow on opposite sides of any one plate cross .

Preferably, the interengaging means comprises an elongate member extending along an edge region.

Preferably, at least one pair of opposing edge regions are offset at a higher level with respect to a central region of at least one plate.

Preferably, at least one pair of opposing edge regions is offset at a lower level with respect to a central region of at least one plate.

Preferably, every other plate has edge regions which are at substantially the same level as a central region of the plate.

Preferably, each plate is identical . In another embodiment, every other plate may be identical, there being two kinds of plates .

Preferably, the interengaging means comprises a ridge along the edge regions projecting from one surface so as to form a recess on the other surface for accommodating a corresponding ridge on an adjacent plate so that a seal is formed between the plates along that edge region when the plates are stacked together.

Preferably, each edge region of each plate comprises at least one such ridge along it .

Preferably, one or more edge regions comprise two or more of the ridges spaced apart from one another.

Preferably, the ridges form a substantially continuous ridge structure about the periphery of the plate. Preferably, each ridge is of the same cross section. Preferably, the cross section is constant along the ridge. Preferably, the cross section of the ridges is generally rectangular or generally square. Preferably, the width of the uppermost surface of the ridge is greater than that of the base of the ridge. For example, the sides of the ridge may be angled with respect to the plate in opposite directions so that the uppermost surface of the ridge is wider than the base of the ridge. These sides may be straight. In one embodiment, there may be a lip formed at the junction between the uppermost surface and the sides of the ridge, the lip extending lateral from the sides of the ridge. Preferably, the resilience of the ridges of neighbouring plates enables one ridge to be snap-fitted into the ridge of the neighbouring plate.

Preferably, at least one pair of opposing edge regions of each plate comprises a projection from one surface of the edge region for preventing the interengaging means on that surface of the plate from engaging with corresponding interengaging means on a neighbouring plate adjacent that surface of the edge region.

Preferably, interengaging means on one or more edge regions in the form of two ridges are provided and in which the projections are positioned in between the ridges .

Preferably, one or more corners of each plate has an interengaging corner structure for interengaging with a corresponding corner structure on a neighbouring plate. Preferably, a corner structure comprises a projecting member on one surface of the plate and a recess on the other surface of the plate. Typically, the corner structures provide a means of locking plates together, via internal or external components such as clamps and so on.

Preferably, a central region of the plate contains a plurality of upstands. The upstands preferably protrude from one side of the plate, but upstands protruding from the other side of the plate may also be provided. Preferably, at least some of the upstands are lower than the separation of the central regions of the plates in the stack when the plates are stacked together. Preferably, one or more of the upstands comprise elongate recesses across an uppermost surfaces arranged to enable air to flow through the recess. Preferably, the elongate recesses are as deep as the height of some of the upstands. The upstands may form part of the plate, for example, they may be formed by pressing the plate. Or, the upstands may be attached to the plate by gluing, welding or the like.

Preferably, the opposing edge regions are substantially parallel to one another.

In a further aspect there is provided a heat exchanger comprising a plurality of plates according to the invention. Preferably, the heat exchanger comprises a plurality of identical plates.

According to a second aspect of the invention there is provided a plurality of stackable plates for a heat exchanger, which when stacked one upon the other, form fluid channels between adjacent plates allowing heat exchange between fluid flowing on one side of a plate and fluid flowing on the other side of the same plate, each plate being welded on at least one pair of opposing edge regions to a corresponding pair of edge regions of a neighbouring plate to form seals between those edges. Preferably the weld is formed by sonic welding. Preferably, the plates are made from a polymeric material .

The heat-exchanger plates may comprise plastics or engineering polymers. Such plates have the advantage of being so inexpensive that they may simply be thrown away and replaced with new plates at regular intervals. The plates may be an alloy of polymers and metal, whether ferrous or non-ferrous, or alloys of plastics, or metal, or a combination of both.

Brief Description of the Drawings

The present invention will now be described by way of example with reference to the accompanying drawings.

Figure 1 is a schematic plan view of a basic version of a heat exchanger plate in accordance with the invention.

Figure 2 is a schematic exploded cross-sectional view from the side of a stack formed from a number of plates like that seen in figure 1.

Figure 3 is a close-up schematic cross-sectional view mid-way along an edge of a stack of plates similar to the plate in figure 1.

Figure 4 is a close up schematic cross-sectional view mid-way along an edge of a stack of first and second improved plates in accordance with a preferred embodiment of the invention. The dotted line shows an alternative improved version of one of the plates. Figures 5A and 5C are schematic cross-sectional exploded views through improved ridges in the edge region of neighbouring plates according to the invention, the ridges being adapted to snap-fit into one another shown in figures 5B and 5D .

Figure 6 is a schematic perspective view from above of a first improved heat exchanger plate. The inserts show neighbouring edges of this first plate in cross-section.

Figure 7 is a schematic perspective view from above of a second heat exchanger plate.

Figure 8 is a side view of the corner region of plate 10A of figure 6 on top of plate 10B of figure 7, seen from the direction of arrow 110.

Figure 9 is a side view of the corner of plate 10A of figure 6 on top of plate 10B of figure 7 seen from the direction of arrow 120.

Figure 10 is a close up perspective view of upstands 45 and 50 showing air flow channel 60.

Detailed Description of the Drawings

Figure 1 shows a basic heat exchanger plate in accordance with the principles of the invention. A central region 2 is bounded by, in this embodiment, a continuous ridge of generally square cross-section 3. Plate 1 is typically formed from injection moulding and ridge 3 is formed when a recess is pressed into the rear of plate 1. Two opposite sides 3A of ridge 3 are at an approximately constant height along their length.

The remaining two sides 3B have pillars or projections 4 which are also pressed out during injection moulding. Pillars 4 are spaced at intervals part way along sides 3B. Pillars 4 which are taller than the ridge on which they are formed serve to space that ridge of one plate from a corresponding ridge of a neighbouring plate when the plates are stacked together to form a heat exchanger.

This can be seen more clearly in figure 2 in which a series of identical plates, 201 to 206, are seen in an exploded cross-sectional view, every other plate having been turned 90° with respect to its neighbours. As can be seen from figure 2, the lower most plate 201 has upstanding pillars 4 on its outermost edges upstanding pillars are higher than ridge 3B so preventing the underside of ridge 3A of the corresponding edge of plate

202 just above it from sitting on ridge 3B.

Midway between the outer edges of the lower plate 201 is ridge 3A which does not have any pillars 4. Therefore, there is nothing to prevent the recess at the rear of ridge 3B of plate 202 just above it from slotting over ridge 3A to form a seal at the side of the plate when the plates are stocked together.

Similarly for the next pair of plates 202, 203 at the outer edges, ridge 3A slots into the recess behind ridge 3B on the plate just above it when the plates are stacked. However, two pillars 4 midway across ridge 3B at the rear edge of plate 202 prevent ridge 3A of plate

203 from slotting over ridge 3B of plate 202. In this way, each edge of each plate forms a seal with a corresponding edge region of one but not the other of its neighbouring plates in the stack.

This can be seen more clearly in figure 3, in which a cross-sectional side view through a mid portion of stack of four plates 101, 102, 103, 104 is seen. The outermost edge of plate 101 forms a ridge 3A. Ridge 3A slots into the recess formed to the rear of ridge 3B of plate 102. However, ridge 3B also includes spaced upstanding projections 4 which prevent the recess to the rear of ridge 3A of plate 103 from slotting onto ridge 3B of plate 102. Therefore, fluid such as air can flow between plates 102 and 103 from left to right, or vice versa, as seen in the drawing. However, no fluids such as air can flow between plates 101 and 102 from left to right or vice versa because of the effective seal formed by the circuitous path provided between plates 101 and 102 where ridge 3A fits in the recess behind ridge 3B.

However, fluid such as air can flow between plates 101 and 102 in a transverse direction, ie into or out of the paper because of upstanding pillars 4 on the transverse edges of plate 101 (not shown) and upstands 60 which are periodically placed in the central region 2 of plate 101.

It will be apparent that whilst the following description refers to air other fluids, even liquids, may be used.

As will be appreciated by those skilled in the art, whereas projections four are positioned so as to extend from ridges 3B on one surface of the heat exchanger plate, an alternative, or even an additional arrangement, is such that the projections are placed on the rear surface of the plate, say below ridges 3A. Where both from and rear projections are provided, these would be on neighbouring edges of the same plate, though depending from opposite surfaces of that plate.

The arrangement in which identical plates are used is not ideal since there is some stain induced in plate 102 at bend 6 where it bends to pass over upstand 60 of plate 101. Therefore, whilst this embodiment provides many of the advantages of the invention, and also reduces production costs because it uses identical plates throughout the stack, it has some disadvantages.

The presence of bend 6 in plate 102, particularly if plate 102 is formed from a resilient material such as plastic, will tend to cause ridge 3B of plate 102 to rise off and become separated from ridge 3A of plate 101. This is clearly undesirable since air would then be able to flow in both transverse directions between plates 101 and 102. One solution is to provide a snap-fit mechanism between ridges 3A and 3B to prevent their separation. Indeed, such a snap-fit arrangement can be used with any of the embodiments of the invention and will be described later.

However, a further option is to provide two kinds of plates in the stack. For example, one or both kinds can be provided with an additional bend just inside the outer ridges 3A and 3B, formed in opposite directions, to enable the plates to sit more naturally upon each other. This can be seen in figure 4 in which plate 10B has a central section 26 which is lower than an edge region containing ridge 3A. The edge region containing ridge 3A and central region 26 are separated by an intermediate region 27B. Similarly, plate 10A has a central region 26 which is generally higher than the edge region containing ridge 3B and upstands 4. Ridge 3B and central region 26 of plate 10A are separated by a region 27A, which is preferably, of the same dimensions as region 27B. If regions 27A and 27B were not present in plates 10A and 10B, the circuitous path forming the seal between ridges 3A and 3B and preventing air flow between plates 10B and 10A would be less undulating and hence less effective. This would be particularly marked on the inner edge of ridges 3A and 3B. Because of the presence of folds 28A and 28B in plates 10A and 10B, central region 26 of plates 10A and 10B are of a generally constant separation over the central region 26. Moreover, ridges 3A and 3B are not subject to a resilient force causing them to them to move apart opening an air flow between them.

An alternative arrangement is one in which plate 10B is generally level over both edge and central regions as indicated by the dotted line in figure 4. In this case, intermediate region 27B is at the same level as the remainder of plate 10B. Other alternatives can be envisaged, for example, the inner edge of ridge 3A of plate 10B could extend below the level of the edge region. This would have the advantage of lengthening the undulating route between plates 10B and 10A, improving the degree of sealing. Additional ridges and recesses or other shapes can also be provided to lengthen and improve the route, the only requirement being that whatever shape is selected the structure is able to fit to the structure of a neighbouring plate to provide a seal . The advantage of a structure chosen to sit within or on a neighbouring structure is one of simplicity. In addition, ridges and recesses are especially simple to manufacture by injection moulding.

Further improvements seen in figure 4 include the introduction of two different kinds of upstands or protrusions 60. Plates 10B have a taller upstand 45 sized and shaped to sit within a recess formed to the rear of a lower upstand 50. The interaction of upstands 45 and 50 space and support central regions 26 of plates 10A and 10B. The upstands are typically recessed on their reverse side, both the upstands and recesses disturbing the air flow across the plate. However, the upstands could simply be glued or welded to one or both surfaces of a plate. Typically, the upstands cover around 3% (usually 3.11%) of the surface area of one side of the plate to provide the right degree of turbulence without excessive restriction of the flow. The percentage area coverage of upstands can be varied between about 1 or 2% and 12%. For example, where the upstands are recessed on the other side of the plate, the effective area of the upstands doubles to around 6% of the area of the plate surface.

Referring now to figures 5A and 5B, one possible snap-fit mechanism is shown. Here, a ridge of initially square cross-section is pressed out by injection moulding. On cooling, the ridge deforms ever so slightly under its own weight, for example, or by an additional deformation force to produce a slight enlargement at the rear of recess 6. When two ridges 3 with recesses 6 are pressed together the swelling on the enlarged outer surface of one ridge 3 presses and snap- fits into the inner portion of recess 6 of a neighbouring ridge 3. It will be understood that press or zip lock structures such as those used to seal plastic folders are particularly suitable .

This method of forming ridges 3 in which natural deformation on cooling produces a snap- fit mechanism is particularly advantageous since no additional steps are required in producing the plates. Indeed, one major advantage of a preferred embodiment of the invention is that all the features necessary to provide the required airflow through the stack are formed in the plate on injection moulding without the need for further processing steps. Of course, other processing steps may be introduced if desired, for example, if alternative snap-fit mechanisms are required. Such alternative snap- fit mechanisms include press studs spaced about the outer and inner surfaces of neighbouring ridges or edge regions of the plates. Another advantage is that on stacking the plates together, sealing of the plate edge in the required locations happens automatically as ridges slot into recesses up and down the stack. This dramatically reduces the construction time for a plate-type heat exchanger.

A further type of ridge incorporating a snap- fit mechanism is shown in figures 5C and 5D. Here, the sides and uppermost surface of the ridge are flat, the sides being turned outwards from one another with respect to the main plate surface. In the embodiment shown the sides are at an angle of approximately 80° to the plate and the uppermost surface of the ridge.

Up to press embodiments of the invention have only been shown using one ridge, either 3A or 3B, around the outer edges of the heat exchanger plate. A more undulating route and therefore a better seal is provided if two or more ridges are used. One example of a plate 10A including two substantially parallel ridges 3A, 3B in the edge regions 32, 34, 36 and 38 of plate 10A is seen in figure 6. Here, ridges 30 are not continuous around the plate but are bonded by corner recesses 20 and corresponding projections 22. Corner recesses 20 and projections 22 in conjunction with similar recesses on plate 10B, as will be described later, are used to locate plates 10A on 10B and 10B on 10A.

In figure 6, first heat exchanger plate 10A includes a recess 20 at each of its corners. Whilst a square plate is shown, and is preferred, a rectangular plate can be used. The walls of each corner recess 20 form projection 22, which depends from the underside of the plate 10A. Between each pair of corner recesses 20 are, in this embodiment, two substantially parallel ridges indicated at 3A and 3B. The parallel ridges 3A, 3B are formed along respectively edges 32, 34 and 36, 38 of the plate.

Upstands 40 are provided on opposite edges 36, 38 for separating one plate from its neighbour along those edges. Upstands 40 are spaced apart evenly along edges 36, 38 and are positioned in the gap between the substantially parallel ridges 30.

Two sets of centrally located upstands 45, 50 are also formed in the central region 26 of plate 10A. Upstands 45, which project further than upstands 50, are provided for structural strength and plate separation. They also promote turbulence as do lower upstands 50. Both upstands 45 and 50 have semi-cylindrical channels or ducts 60 formed across their upper surface for reducing form drag or stagnation ie the accumulation of material such as dust, particles, insects etc in the still air which forms behind any body when air or fluid flows around it. The semi-cylindrical shape of channels 60 is seen in figure 10. In upstands 50, channels 60 are of almost the same depth as the height of the upstands. Nevertheless, the channels 60 can be varied in shape, size and orientation. Typically, the channels or recesses 60 are elongate though this is not necessary.

Upstands 45 and 50 are formed in alternate rows. In plate 10A, the shorter upstands 50 form the outermost row on all four edges. The rows are aligned in this embodiment so that each upstand 45 is located substantially half way between two of the taller upstands 50.

The cross-sections AA' and BB ' shown in the inserts in figure 6 are through, respectively, edge region 32 and parallel ridges 30A and edge section 38 and parallel ridges 3B. As can be seen from the insert, the two ridges are, in this particular embodiment, of approximately the same height and width as one another. The main section of plate 26 is slightly lower than the level of edge section 32. Region 27B just inward of parallel ridges 3A, 3B is the same level as the region between ridges 3A, 3B and the outer portion of edge section 32. This is also true for edge section 34. However, edge sections 36 and 38 are slightly lower than central region 26. Indeed, section 27A which is just inward of ridges 3A 3B on edge sections 36 and 38 and edge sections 36 and 38 are positioned just below the level of the central section 26. For both kinds of edge section the height of ridges 30 is approximately a distance d above the base level of section 27A and 27B as appropriate. This means that relative to the central section 26 of plate 10A opposite edges 36, 38 are lower whereas opposite edges 32, 34 are higher. When a place 10A is positioned overlaying a plate 10B, lower edge sections 36, 38 engage plate 10B. When a plate 10A is positioned beneath a plate 10B, higher edge sections 32, 34 engage plate 10B.

Figure 7 shows a second heat exchanger plate 10B according to the invention. Plate 10B includes a recess 24 at each of its corners. Each recess 24 forms a projection 26, which depends from the underside of plate 10B. At one side of each recess 24 is a raised rectangular wall 28. Each recess 24 is spaced from its neighbouring corner recesses 24 by two substantially parallel ridges 3 (shown schematically only in figure 7) . Ridges 3 are formed on edge regions 40, 42, 44 and 46 and span the length between the raised walls 28, where present or corner recesses 24.

Upstands 4 are provided on edge regions 40, 42 of the plate 10B for plate separation. Upstands 4 are spaced apart evenly along the ends 40, 42 and are positioned in the gap between ridges 3.

A plurality of centrally located upstands 45, 50 are also formed on the central region 26 of plate 10B. As with plate 10A, upstands 45 and 50 are provided for structural strength, plate separation and for turbulence promotion. Both upstands 45 and 50 include channels 60 for reducing drag as shown in figure 10. The upstands typically cover around 3% of one side of each plate in this case (or 3% of each side where the upstands are recessed) . The upstands promote variation in fluid velocity, induce differential pressures across the plates so providing for turbulence in the flow.

Upstands 45 and 50 are also formed in alternating rows on plate 60, upstands 45 forming the outermost rows. Each projection 50 is positioned at a point substantially parallel to the midpoint between two projections 45. In other words the pattern of upstands on plate 10B is the reverse of that on plate 10A.

In the preferred heat-exchanger according to the present invention, both kinds of plates are used and stocked in alternating fashion on top of each other. Each pair of plates comprises a plate 10A and a plate 10B.

Figures 8 and 9 show how the plates 10A and 10B are aligned in each pair, in this case with 10A uppermost. The projections 22 of the plate 10A are located in the corner recesses 24 of the plate 10B. The ridge 3A of edge region 46 of the plate 10 corner slots, and preferably, snap-fits into the recess behind ridge 3B of edge region 38 of plate 10A.

The alignment of plates 10A and 10B in this manner creates a sandwich in which the edges of pairs of plates are sealed tightly together along opposite edges allowing airflow between intervening edges. Thus, stacking the plates in this manner allows air to flow between a first and second plate in one direction and the second and a third plate in another, preferably transverse, direction. Of course, the plates could be arranged and/or shaped so that air flow between a first and second plate is at an acute angle or even in a parallel direction with respect to air flow between the second and third plates, as would be understood by someone skilled in the art.

The heat-exchanger plates according to the present invention are preferably made from plastic material such as engineering polymers. Engineering polymers yield plates which are durable yet cheap to produce. The low cost of the plates means they can simply be thrown away and replaced with new ones when they become old or the heat transfer characteristics deteriorate due to particle build up. This is far simpler than the cleaning methods mandated by conventional arrangements. Advantageously, the plates may be recycled to form new plates.

Thus, with a very simple design, the present invention provides a heat-exchanger which is quick and easy to assemble. A tight seal is provided between heat exchanger plates as the plates slot into one another at selected edges when stacked so that there is no need for sealing gaskets, gluing or welding. The plates are arranged to provide a cross-flow or counter flow heat- exchanger, with unmixed flow.

The structural simplicity reduces manufacturing and production costs and also improves productivity. This contrasts with conventional heat exchangers which are difficult and time-consuming to assemble and did not provide such a tight seal between plates.

It will be appreciated that the interengaging members whilst preferably formed from interlocking structures such as a ridge/recess structure, they could be formed from a series of discrete neighbouring structures, though this is less preferred. These structures could be a series of shorter ridges interspersed with snap- fit mechanisms, or a series of discrete neighbouring snap-fit mechanisms such as press studs. Indeed, the interengaging members could simply be provided by substantially planar but offset edges of all or every other plate as these will provide a degree of sealing at the edges, for example, in figure 4 where regions 27A and 27B meet.

Alternatively or in addition to the provision of interengaging means as in the first aspect of the invention, opposing edges of neighbouring pairs of plates may be welded together by sonic welding using sonitrodes. Sonitrodes are typically made from steel having slightly roughened ends which grip a material to be welded. The sonitrodes send a high frequency vibration into the material say at 95-150dB which agitates the material sufficiently to form a weld. The frequency of the vibration is adjusted to the material to be welded as well as the length shape and strength of weld to be formed .

In either aspect of the invention, a minimum gap of 3mm between neighbouring plates is preferred, though not essential. In practice preferred embodiments of the plates have been found to withstand the pressure of 10 psi (pounds per square inch) before the seals between edges are compromised.

Claims

1. A plurality of stackable plates for a heat exchanger, which, when stacked one upon the other, form fluid channels between adjacent plates allowing heat exchange between fluid flowing on one side of a plate and fluid flowing on the other side of the same plate, each plate comprising interengaging means on at least one pair of opposing edge regions for engaging with interengaging means on a corresponding pair of edge regions of a neighbouring plate to form seals between these edges when the plates are stacked together.

2. A plurality of plates according to claim 1, in which the interengaging members are arranged so that when the plates are stacked together the directions of the fluid flow on opposite sides of any one plate cross .

3. A plurality of stackable plates according to claim 1 or 2 , in which the interengaging means comprises an elongate member extending along an edge region.

4. A plurality of plates according to any preceding claim, in which at least one pair of opposing edge regions are offset at a higher level with respect of a central region of at least one plate.

5. A plurality of plates according to any preceding claim, in which at least one pair of opposing edge regions is offset at a lower level with respect to a central region of at least one plate.

6. A plurality of plates according to any preceding claim in which every other plate has edge regions which are at substantially the same level as a central region of the plate.

7. A plurality of plates according to any preceding claim in which each plate is identical.

8. A plurality of stackable plates according to any preceding claim, in which the interengaging means comprises a ridge along the edge regions projecting from one surface so as to form a recess on the other surface for accommodating a corresponding ridge on an adjacent plate so that a seal is formed between the plates along that edge region when the plates are stacked together.

9. A plurality of plates according to claim 8, in which each edge region of each plate comprises at least one such ridge along it .

10. A plurality of plates according to claim 9, in which one or more edge regions comprise two or more of the ridges spaced apart from one another.

11. A plurality of plates according to claim 9 or 10, in which the ridges form a substantially continuous ridge structure about the periphery of the plate.

12. A plurality of plates according to any of claims 8 to 11, in which each ridge is of the same cross section.

13. A plurality of plates according to any of claims 8 to 12, in which the cross section of the ridges is generally rectangular or generally square.

14. A plurality of plates according to any preceding claim in which at least one pair of opposing edge regions of each plate comprises a projection from one surface of the edge region for preventing the interengaging means on that surface of the plate from engaging with corresponding interengaging means on a neighbouring place adjacent that surface of the edge region.

15. A plurality of plates according to claim 14, in which interengaging means on one or more edge regions in the form of two ridges are provided and in which the projections are positioned in between the ridges .

16. A plurality of plates according to any preceding claim in which one or more corners of each plate has an interengaging corner structure for interengaging with a corresponding corner structure on a neighbouring plate.

17. A plurality of plates according to claim 16, in which a corner structure comprises a projecting member on one surface of the plate and a recess on the other surface of the plate.

18. A plurality of plates according to any preceding claim in which a central region of the plate contains a plurality of upstands.

19. A plurality of plates according to claim 18, in which at least some of the upstands are lower than

> the separation of the central regions of the plates in the stack when the plates are stacked together.

20. A plurality of plates according to claim 19, in which the upstands are substantially cylindrical.

21. A plurality of plates according to any claim 18, 19, or 20 in which one or more of the upstands comprise an recess across an uppermost surfaces arranged to enable fluid to flow through the recess.

22. A plurality of plates according to claim 21, in which the recess is elongate.

23. A plurality of plates according to claim 21 or 22, in which the recess is substantially semi- cylindrical in cross-section.

24. A plurality of plates according to claim 21, 22 or 23, in which the elongate recesses are as deep as the height of some or all of the upstands.

25. A plurality of stackable plates for a heat exchanger or a plurality of stackable plates according to any preceding claim which when stacked one upon the other, form fluid channels between adjacent plates allowing heat exchange between fluid flowing on one side of a plate and fluid flowing on the other side of the same plate, each plate being welded on at least one pair of opposing edge regions to a corresponding pair of edge regions of a neighbouring plate to form seals between those edges.

26. A plurality of plates according to claim 25 in which the weld is formed by sonic welding.

27. A heat exchanger comprising a plurality of plates according to any preceding claim.

28. A heat exchanger substantially as described with reference to the accompanying figures.

29 A plurality of plates for a heat exchanger substantially as described herein with reference to the accompanying figures.