WO2002037047A1

WO2002037047A1 - Heat exchanger and/or fluid mixing means

Info

Publication number: WO2002037047A1
Application number: PCT/GB2001/004858
Authority: WO
Inventors: Keith Thomas Symonds
Original assignee: Chart Heat Exchangers
Priority date: 2000-10-31
Filing date: 2001-10-31
Publication date: 2002-05-10
Also published as: AU2002210758A1; GB2372948A; GB0026552D0

Abstract

A heat exchanger or fluid mixing means comprises a bonded stack of plates (Figure 3), the stack comprising at least one group of main perforated plates (10A, 10B, 10C), wherein at least two adjacent plates of the group of main perforated plates (10A, 10B, 10C) have their perforations (12A, 12B, 12C) aligned in rows with continuous ribs (16A, 16B, 16C) between adjacent rows and the adjacent plates (10A, 10B, 10C) are aligned whereby the rows of perforations in one plate overlap in the direction of the rows with the rows of perforations of an adjacent plate and the ribs (16A, 16B, 16C) of adjacent plates lie in correspondence with each other to provide discrete fluid channels extending across the plates (10A, 10B, 10C) and being characterised in that at least some of the continuous ribs (16A, 16B, 16C) are shaped to have turbulator means (18A, 18B, 18C) in the form of projections into the flow channels, whereby turbulence is induced in a fluid flowing along those flow channels.

Description

HEAT EXCHANGER AND/OR FLUID MIXING MEANS

This invention relates to a compact heat exchanger and/or fluid mixing means which incorporates a series of plates having apertures which define a plurality of passages through which fluid may flow.

Compact heat exchangers are characterised by their high "area density" which means that they have a high ratio of heat transfer surface to heat exchanger volume. Area density is typically greater than 300m²/m³ and may be more than 700 m²/m³. Such heat exchangers are typically used to cool (or heat) process fluids.

One well known, but expensive to manufacture type of heat exchanger, is the so-called tube and shell heat exchanger. Essentially such heat exchangers consist of an exterior tubular shell through which run a number of longitudinally-extending smaller diameter tubes carrying one or more fluids. Other fluids, with which heat is to be exchanged, typically pass transversely across the heat exchanger such that heat is exchanged through the tube walls. A large number of tubes may be needed and they each have to be individually and accurately fixed/secured onto a header plate at each end of the shell. In each case, holes need to be drilled in the header plates very accurately to locate the tubes. High quality tested tubing then needs to be assembled into the plates and brazed or welded or mechanically-expanded into position. As the tubes are reduced in diameter to increase surfaces available for heat transfer and hence performance/compactness, the more difficult and expensive such configurations become to manufacture. A second known type of heat exchanger is the so-called primary plate/secondary plate type exchanger in which a stack of plates is assembled, the stack having primary plates which directly separate two different fluid streams and secondary plates between adjacent primary plates. The secondary plates act as fins which add to the strength of structure and may be provided with perforations to provide additional flow paths for the fluids. The plates are usually bonded together by brazing but this may have the disadvantage of affecting the physical properties of the plates in the brazed regions or may introduce into the system, by means of the braze material, a potentially less satisfactory structure in terms of strength and corrosion resistance. It has been proposed to bond the plates together by diffusion bonding but a satisfactory construction that can withstand the high pressures involved has not been achieved and the fins may buckle during the bonding process.

In our international patent application no. PCT/GB98/01565 (publication no. WO 98/55812) we have described and claimed an improved construction of this second type of heat exchanger, which can be satisfactorily made by, for example, diffusion bonding or by brazing. It also aims to provide a heat exchanger construction which can also be readily adapted for use as a fluid mixing means, e.g. it can be used as a chemical reactor in which fluids which are to react together are mixed. Thus, where a reaction is exothermic, the invention may provide a means whereby the exothermic heat of reaction may be removed efficiently or, alternatively, it may be used to supply heat to an endothermic reaction. The products of the invention are also useful in a variety of applications, for example as fuel-cell reformers and gas clean-up units associated with fuel cell technology. Accordingly, in WO 98/55812 is described a heat exchanger or fluid mixing means comprising a bonded stack of plates, the stack comprising at least one group of main perforated plates, wherein at least two adjacent plates of the group of main perforated plates have their perforations aligned in rows with continuous ribs between adjacent rows and the adjacent plates are aligned whereby the rows of perforations in one plate overlap in the direction of the rows with the rows of perforations of an adjacent plate and the ribs of adjacent plates lie in correspondence with each other to provide discrete fluid channels extending across the plates, a channel corresponding to each row of perforations, the channels together forming one or more fluid passageways across the plates and the passageway (s) in the group of main perforated plates being separated from passageway(s) in any adjacent group of perforated plates by an intervening plate.

In the heat exchangers according to WO 98/55812, the perforations of a plate are preferably slots and adjacent slots in a row of slots are separated by a transverse bar which act as flow interrupters as fluid flows along each discrete fluid channel extending across the plates. Thus, the transverse bars aid heat transfer and fluid mixing as they cause the fluid(s) in their travel across the plates to flow under and over the bars. The direction of fluid flow, therefore, which is essentially parallel to the plates is provided with turbulent motion out of the plane of the plates.

It is an object of the present invention to provide further improvement to the heat transfer and fluid mixing abilities of these structures by increasing the turbulence caused to the fluid flow paths across the plates. Accordingly.a first aspect of the present invention provides a heat exchanger or fluid mixing means comprising a bonded stack of plates, the stack comprising at least one group of main perforated plates, wherein at least two adjacent plates of the group of main perforated plates have their perforations aligned in rows with continuous ribs between adjacent rows and the adjacent plates are aligned whereby the rows of perforations in one plate overlap in the direction of the rows with the rows of perforations of an adjacent plate and the ribs of adjacent plates lie in correspondence with each other to provide discrete fluid channels extending across the plates, a channel corresponding to each row of perforations, the channels together forming one or more fluid passageways across the plates and the passageway( s) in the group of main perforated plates being separated from passageway(s) in any adjacent group of perforated plates by an intervening plate and being characterised in that at least some of the continuous ribs are shaped to have turbulator means in the form of projections into the flow channels, whereby turbulence is induced into the flow of a fluid along the flow channels.

The projections may be positioned along the ribs such that they form vertical columns in the stack of plates or they may be arranged to be non-coincident between adjacent plates in the stack.

The projections provide the means to give turbulence in directions parallel to the planes of the plates in addition to the turbulence out of those planes caused by the transverse bars and hence can result in greatly improved heat exchange and fluid mixing capabilities. In one preferred embodiment the projections extend in the plane of their respective plates and are angled to the direction of their respective flow channels, i.e. ignoring any out of planar flow caused by the transverse bars, whereby they provide an increasing obstruction to flow in the downstream direction.

If desired, the projections may additionally protrude from the plane of their respective plate.

The angle at which the projection extends from its rib may be constant or may vary. The projection may, therefore, be linear, curved or of irregular outline.

^• Preferably, at their widest extent the projections extend to obstruct no more than half of the width of their respective flow channel.

The rib portion between each successive pair of transverse bars may contain one or more turbulator means or, depending on the degree of turbulence desired, there may be turbulator means between some pairs of transverse bars and not between others.

In another preferred embodiment, the turbulator means are offset from those of adjacent plates. It will be appreciated that in order for the rows of slots of adjacent plates to overlap in the direction of the rows, the transverse bars of adjacent plates are offset from each other, thereby providing the flow channels over and under the transverse bars. In this preferred embodiment, the turbulator means are additionally offset from one plate to the next so that, when the plates are stacked together, the turbulator means effectively can form a series of steps. These can be shaped to produce a vortex shedding effect as will be well understood by the skilled man of the art. For example, each group of main perforated plates in the stack may comprise three plates, each plate having its turbulator means offset in relation to those of the other two plates, thereby producing the desired step effect. Each group of main perforated plates may contain more or less plates, as required.

The intervening plate separating the fluid passageways in one group of main perforated plates from passageways in an adjacent group of plates may be unperforated to provide complete separation of the passageways of the respective groups of plates. Such an intervening plate will be referred to below as a "separator plate". Alternatively, as is described in more detail below, the intervening plate may contain holes positioned and sized to provide controlled mixing of the fluids in those passageways. Such an intervening plate will be referred to below as a mixing plate.

The passageways formed by the rows of discrete channels across the plates may simply traverse across the plates once from one side to the other. However, in a first specific embodiment, the perforations at one or both ends of each row are shaped to turn their respective channels through an angle whereby the passageway defined by the channels continues in a different direction through the stack.

In a second specific embodiment, two or more separate passageways are provided across a group of plates whereby streams of different fluids may flow parallel to each other in the same layer provided by said group of plates. This embodiment can provide improved temperature profiles across the plates and reduced thermal stress. Because the plates are stacked with the main perforated plates of each group aligned with their perforations in parallel rows, it will be appreciated that the solid regions (i.e. ribs) of those plates between the rows of perforations are also aligned in parallel rows. As the perforated plates, therefore, are stacked one above each other, the parallel ribs are aligned through the stack and hence this not only provides the discrete channels referred to above, it provides strength through the assembled stack whereby the pressures generated in the bonding process can be withstood. The invention, therefore, provides a stack structure that can be bonded without the risk of the fins of the secondary plates collapsing under the pressures generated. The fins also provide the means of withstanding internal pressures in the operating streams.

The perforations may be of any desired shape but are preferably elongated slots. In the aforementioned first embodiment the slots at the end of a row are preferably "L " or "V" shaped with the angle of the "V" being determined by the desired change of direction of the passageway.

The plates may be rectangular, for example square, or circular for example or of any other shape.

Where the plates are square or rectangular, each row of slots may extend from a first edge of the plate parallel to a second edge of the plate and for substantially the whole length of that second edge. It will be appreciated that a substantially unperforated edge or border will normally be required around the perimeter of the major faces of the plate to enable the plates of the stack to be bonded together and to provide pressure containment for the stream or streams. However, a completely unperforated border is not essential and slots in the border may be required for inlet and outlet means, for example. A plurality of rows of slots may, therefore, extend across the plate from the first edge towards the opposite, third, edge. In respect of the first embodiment described above, adjacent that opposite third edge the slots at the end of the row may be "L" shaped whereby each row then extends at right angles to its original direction, i.e. extends parallel to the third edge. A second right angle turn may then be arranged whereby the rows of slots then extend back across the plate parallel to the first plurality of rows and so on.

Depending on the number and width of the rows in each plurality of rows and on the width of the plate, this change of direction can be repeated several times across the plate. Thus a passageway defined by at least a pair of perforated plates may extend backwards and forwards across the plates, i.e. a multi-pass arrangement.

Where the plates are circular the rows and passageways may extend from the outer perimeter as a segment of the circle towards the centre and then turn through an angle " α " to extend back towards the perimeter and so on. The rows and passageways (and hence the slots) can narrow as they get closer to the centre and the number of segments and hence turns will, of course, be determined by α°, e.g. where α° = 45°, there will be eight segments.

The perforations or slots together with the desired turbulator means are preferably photochemically etched through the plates by known methods, although spark erosion, punching or any other suitable means may be used, if desired. Each of the plurality of fluid channels forming an individual passageway may pass through the stack without any communication with another channel of the passageway. No mixing of fluid in those channels can, therefore, take place and the stack may function purely as a heat exchanger with fluids at different temperatures passing through different groups of perforated plates or passing through different passageways in the same group of perforated plates.

In another embodiment there is provided intercommunication at selected positions between the channels of a passageway. Thus cross-channels or vents may be etched or otherwise formed in the plates to provide access between adjacent channels. The vents may be formed at any desired position along the flow channels. Thus, fluids flowing through separate channels may be mixed at pre-arranged positions on their journey through the passageways through the stack and this mixing may be employed to improve heat exchange capability.

Alternatively or additionally, inlets for a further fluid may be provided through the peripheral borders of the plates. Thus, reactant may be introduced and mixed via the peripheral border inlets whereby the stack may be employed as a chemical reactor.

In another embodiment the invention provides a stack in which a fluid stream from one group of main perforated plates may be injected into a fluid stream in an adjacent group of main plates. Injection holes for this purpose are provided in an intervening mixing plate which separates the two groups of main perforated plates. So-called "process intensification" can be achieved by this means, and any reaction caused by the injection of a first fluid into a second fluid can be controlled by the pressure differential between the two streams, the size, numbers and spacing of the injection holes and by sandwiching the second stream between the first stream and a coolant or heating stream, as appropriate.

The density of the slots, and hence of the ribs or fins between each row of slots, may be varied, as required. Thus the number of slots per unit width or per unit length of a plate may be arranged to suit any particular flow/pressure drop/distribution change requirements.

The rows of slots may extend linearly across the plate but this is not essential and they may be arranged in other desired patterns, e.g. herringbone or chevron.

The plates of a stack are preferably of the same material and are preferably thin sheets of metal, e.g. of 0.5 mm thickness or less. The material is preferably stainless steel but other metals, e.g. aluminium, copper or titanium or alloys thereof, may be used.

Inlet and outlet headers or manifolds for the different fluids may be secured to the stack after bonding together of the stack plates or, alternatively, may be formed from integral features on the plates.

As indicated above, the components of a stack may be bonded together by diffusion bonding or by brazing. Diffusion bonding, where possible, may be preferred but, in the case of aluminium, which is difficult to diffusion bond, brazing may be necessary. It is then preferable to clad the aluminium surfaces, e.g. by hot-roll pressure bonding, with a suitable brazing alloy, in order to achieve satisfactory bonding by the brazing technique, although other means to provide the braze medium may be used, e.g. foil or vapour deposition.

Further aspects of the invention provide methods of exchanging heat between fluids or mixing fluids, comprising forcing a first fluid through a stack of bonded plates, the stack comprising at least one group of main perforated plates, wherein at least two adjacent plates of the group of main perforated plates have their perforations aligned in rows with continuous ribs between adjacent rows and the adjacent plates are aligned whereby the rows of perforations in one plate overlap in the direction of the rows with the rows of perforations of an adjacent plate and the ribs of adjacent plates lie in correspondence with each other to provide discrete fluid channels extending across the plates, a channel corresponding to each row of perforations, the channels together forming one or more fluid passageways across the plates through which the first fluid flows and the^' passageway(s) in the group of main perforated plates being separated from passageway(s) in any adjacent group of perforated plates through which a second fluid flows by an intervening plate, being either unperforated or perforated respectively for exchanging heat or mixing fluid, characterised by inducing turbulence in at least one of the fluids by forcing that fluid passed turbulator means present in the form of projections into the flow channels on at least some of the continuous ribs.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying, drawings in which:

Figure 1 is a plan view of a portion of each of three main perforated plates for use in the invention; Figure 2 is an elevation of a portion of a stack comprising the three plates of Figure 1 ;

Figure 3 is a diagrammatic illustration of a stack of plates according to the invention for use as a chemical reactor; and

Figures 4 to 8 are diagrammatic plan views of the individual types of plates making up the stack of Figure 3.

In Figure 1 are shown portions of three main perforated plates 10A, 10B, and 10C. The plates are rectangular, thin plates of metal each having a series of perforations in the form of elongated slots 12A, 12B, 12C through their thickness. The slots lie in parallel rows extending across the plates.

Transverse bars or flow interceptors 14A, 14B, 14C separate each slot from adjacent slots in the same row. Ribs 16A, 16B, 16C extending in the direction of the slots separate each slot in a row from a slot in an adjacent row.

As can be seen, transverse bars 14A of plate 10A are positioned to be offset from transverse bars 14B of plate 10B, which are in turn offset from transverse bars 14C of plate 10C. Thus when plates 10A, 10B, 10C are stacked together, as is shown in Figure 2, the transverse bars of the three plates do not coincide but are spaced from each other. Each rib is shaped to have a triangular shaped turbulator 18A, 188, 18C positioned between adjacent pairs of transverse bars. The turbulators, therefore, each provide an angled projection into their respective slots. It will be noted that the turbulators between one pair of transverse bars extend from the opposite side of their ribs to those between the next adjacent pair of turbulators. In other words, the direction of projection of the turbulators alternates across the plates. (It will be appreciated that this alternating arrangement may be modified, as desired. For example, a turbulator might project from each side of each or of some of the ribs.)

As can be seen in Figure 2, when the plates are stacked together the turbulators form a stepped obstruction to fluid flow. One fluid flow channel across the stack is indicated between pairs of parallel ribs 16A, 16B 16C by single headed arrows. The flow is essentially parallel to the plane of the plates but rises and falls out of the plane of the plates to pass under and over the transverse bars 14A, 14B, 14C.

The turbulators 18A, 18B, 18C are positioned to give a gradually increasing obstruction to the flow along the channel to provide turbulence as indicated by the double-headed arrows. This turbulence is stepped in effect and is generally in the plane of the plates in contrast to the turbulence caused by the transverse bars.

It will be appreciated that the turbulators need not overlap with those of adjacent plates, i.e. they may be spaced from each other in the direction of flow along the channel. In Figure 3 is shown a stack of plates of the invention particularly suitable for use as a chemical reactor. The plates used to make the stack are shown diagrammatically in Figures 4 to 8.

In Figure 4 plate P represents a group of at least two perforated plates having the rows of slots required to provide the fluid channels. As shown, the group has a passageway 61 defining a four-pass arrangement between inlet 62 and outlet 63. In other words, the passageway defined by the channels passes from one edge of the plate to a second opposite edge is turned to return to the first edge, turned again to return to the second edge and, finally turned again to return to the first edge. Process fluid will pass through this group of plates in the stack of Figure 3. It will be appreciated that each plate in this group will have rows of slots with transverse bars, ribs and turbulators as described above.

In Figure 5 is shown a group of plates C having a passageway 64 defining a four-pass arrangement between inlet 65 and outlet 66. Coolant will pass through this group of plates in the stack of Figure 3 and plates C will be, as shown, rotated clockwise through 90° relative to plates P. The plates of this group need not be provided with turbulators.

In Figure 6 is a group of plates R similar to group P, except that turbulators are not necessary in these plates. Again, this group has a passageway 67 defining a four-pass arrangement between an inlet 68 and an outlet 69. Reactant will pass through this group of plates in the stack of Figure 3. As shown, plates R are rotated clockwise through 90° relative to plates C and 180° relative to plates P. Figure 7 shows a single, unperforated intervening separator plate TS.

Figure 8 shows a single intervening mixing plate TP. Plate TP has groups of circular holes TPP through its thickness, although holes of other shapes maybe used, if desired. As indicated above, the holes are of size, number and position to control mixing of fluids passing through passageways separate by mixing plate TP.

The plates or groups of plates are assembled and bonded into a stack in the order shown in Figure 3. It will be noted that the stack repeats in the sequence TS-P- TP-R- TS-C- TS-P and so on. Thus the reactant and process layers are separated by the mixing plate TP whereby the reactant can be mixed into the process layer at the predetermined positions governed by fluid pressure and the size and number of the holes. The turbulators in plates P then ensure good mixing between the process fluid and the reactant. Coolant fluid passes between adjacent pairs of unperforated plates TS.

The invention provides a heat exchanger construction that can provide a chemical reactor of considerably increased efficiency. The exact profiles sizes and positions of the turbulators can be varied widely but may be readily determined for any particular circumstances by the skilled man of the art. Whilst the turbulators are shown as projections of a regular triangular form this need not be the case.

The heat exchanger/fluid mixing means may find particular utility in the exchanging of heat or fluid mixing wherein at least one of the fluids is viscous, the added turbulence caused by the turbulator means in the flow thereof will aid the process of fluid mixing and/or of heat exchange.

Claims

1. A heat exchanger or fluid mixing means comprising a bonded stack of plates (Figure 3), the stack comprising at least one group of main perforated plates

(10A, 10B, 10C), wherein at least two adjacent plates of the group of main perforated plates (10A, 10B, 10C) have their perforations (12A, 12B, 12C) aligned in rows with continuous ribs (16A, 16B, 16C) between adjacent rows and the adjacent plates (10A, 10B, 10C) are aligned whereby the rows of perforations in one plate overlap in the direction of the rows with the rows of perforations of an adjacent plate and the ribs (16A, 16B, 16C) of adjacent plates lie in correspondence with each other to provide discrete fluid channels extending across the plates (10A, 10B, 10C), a channel corresponding to each row of perforations, the channels together forming one or more fluid passageways across the plates (10A, 10B, 10C) and the passageway(s) in the group of main perforated plates (10A, 10B, 10C) being separated from passageway(s) in any adjacent group of perforated plates (10A, 10B, 10C) by an intervening plate (TP; TS), characterised in that at least some of the continuous ribs (16A, 16B, 16C) are shaped to have turbulator means (18A, 18B, 18C) in the form of projections into the flow channels, whereby turbulence is induced in a fluid flowing along those flow channels.

2. A heat exchanger or fluid mixing means according to Claim 1 , wherein the projections (18A, 18B, 18C) extend in the plane of their respective plates and are angled to the direction of their respective flow channels, whereby they provide an increasing obstruction to flow in the downstream direction.

3. A heat exchanger or fluid mixing means according to Claim 1 or 2, wherein the projections (18A, 18B, 18C) additionally protrude from the plane of their respective plate.

4. A heat exchanger or fluid mixing means according to any of Claims 1 , 2 or 3, wherein the angle at which each projection extends from its rib is constant.

5. A heat exchanger or fluid mixing means according to any of Claims 1 to 3, wherein the angle at which each projection extends from its rib varies, providing each projection with a curved or irregular outline.

6. A heat exchanger or fluid mixing means according to any preceding Claim, wherein, at their widest extent the projections (18A, 18B, 18C) extend to obstruct no more than half of the width of their respective flow channel.

7. A heat exchanger or fluid mixing means according to any preceding Claim, wherein the rib portion (16A, 16B, 16C) between a successive pair of transverse bars (14A, 14B, 14C) contains one or more turbulator means (18A, 18B, 18C).

8. A heat exchanger or fluid mixing means according to any of Claims 1 to 6, wherein turbulator means (18A, 18B, 18C) are absent between some pairs of transverse bars (14A, 14B, 14C) of a plate.

9. A heat exchanger or fluid mixing means according to any preceding Claim, wherein the projections (18A, 18B, 18C) are positioned along the ribs (16A, 16B, 16C) such that they form vertical columns in the stack of plates.

10. A heat exchanger or fluid mixing means according to any of Claims 1 to 8, wherein the projections (18A, 18B, 18C) are arranged to be non-coincident between adjacent plates in the stack, such that the turbulator means (18A, 18B, 18C) form a series of steps.

11. A heat exchanger or fluid mixing means according to Claim 10, wherein the series of steps of turbulator means (18A, 18B, 18C) produce, during use, a vortex shedding effect.

12. A heat exchanger or fluid mixing means according to any preceding Claim, wherein the intervening plate (TS) separating the fluid passageways in one group of main perforated plates (10A, 10B, 10C) from passageways in an adjacent group of plates is unperforated to provide complete separation of the passageways of the respective groups of plates.

13. A heat exchanger or fluid mixing means according to any of Claims 1 to 11, wherein the intervening plate (TP), separating the fluid passageways in one group of main perforated plates (10A, 10B, 10C) from passageways in an adjacent group of plates, is perforated, the perforations being positioned and sized to provide controlled mixing of the fluids between those passageways.

14. A heat exchanger or fluid mixing means according to any preceding Claim, wherein the plates are either rectangular or circular.

15. A heat exchanger or fluid mixing means according to Claim 14, wherein the plates are rectangular and a row of perforations (12A, 12B, 12C) extends from a first edge of the plate parallel to a second edge of the plate and for substantially the whole length of that second edge towards a third edge.

16. A heat exchanger or fluid mixing means according to any preceding Claim, wherein the perforations (12A, 12B, 12C) at one or both ends of each row of two or more adjacent plates are shaped to turn their respective channels through an angle whereby the passageway defined by the channels continues in a different direction through the stack.

17. A heat exchanger or fluid mixing means according to Claim 14 or 15, or Claim 16 when dependent thereon, wherein the perforations (12A, 12B, 12C) at the end of a row are "L " or "V" shaped with the angle of the "V" being determined by the desired change of direction of the passageway.

18. A heat exchanger or fluid mixing means according to Claim 17, wherein adjacent the third edge the perforations (12A, 12B, 12C) at the end of the row are "L" shaped whereby each row then extends at right angles to its original direction, i.e. extends parallel to the third edge.

19. A heat exchanger or fluid mixing means according to Claim 18, wherein the rows of perforations (12A, 12B, 12C) are arranged to provide a multi-pass flow arrangement.

20. A heat exchanger or fluid mixing means according to any preceding Claim, wherein two or more separate passageways are provided across a group of plates whereby streams of different fluids may flow parallel to each other in the same layer provided by said group of plates.

21. A heat exchanger or fluid mixing means according to Claim 14, wherein the plates are circular and the rows and passageways extend from the outer perimeter as a segment of the circle towards the centre and then turn through an angle " α " to extend back towards the perimeter.

22. A heat exchanger or fluid mixing means according to Claim 21 , wherein the rows and passageways, and hence the slots, narrow as they get closer to the centre of the plate.

23. A heat exchanger or fluid mixing means according to Claim 21 or 22, wherein α° = 45°.

24. A heat exchanger or fluid mixing means according to any preceding Claim, wherein there is provided, at selected positions, intercommunication between the channels of a passageway.

25. A heat exchanger or fluid mixing means according to any preceding Claim, wherein the plates (10A, 10B, 10C) of a stack are thin sheets of metal.

26. A heat exchanger or fluid mixing means according to Claim 25, wherein the metal is stainless steel, aluminium, copper or titanium or alloys thereof.

27. A heat exchanger or fluid mixing means according to any preceding Claim, wherein inlet and outlet headers or manifolds, for the different fluids, which communicate with the passageways, are secured to the stack after bonding together of the stack plates.

28. A heat exchanger or fluid mixing means according to any of Claims 1 to 26, wherein inlet and outlet headers or manifolds, for the different fluids, which communicate with the passageways are formed from extension portions of the plates.

29. A heat exchanger or fluid mixing means according to any preceding Claim, wherein cross-channels or vents are provided in at least some ribs (16A, 16B, 16C) of one or more plates (10A, 10B, 10C) to provide fluid communication between the channels of adjacent passageways.

30. A method of forming a heat exchanger or fluid mixing means according to any preceding Claim, comprising bonding together the stack of plates by diffusion bonding or by brazing.

31. A method of forming a plate for a heat exchanger or fluid mixing means according to any preceding Claim, comprising forming perforations or slots together with the desired tubulator means by photochemically etching, spark eroding or punching through a sheet material.

32. A method of exchanging heat between fluids, comprising forcing a first fluid through a stack of bonded plates, the stack comprising at least one group of main perforated plates, wherein at least two adjacent plates of the group of main perforated plates have their perforations aligned in rows with continuous ribs between adjacent rows and the adjacent plates are aligned whereby the rows of perforations in one plate overlap in the direction of the rows with the rows of perforations of an adjacent plate and the ribs of adjacent plates lie in correspondence with each other to provide discrete fluid channels extending across the plates, a channel corresponding to each row of perforations, the channels together forming one or more fluid passageways across the plates through which the first fluid flows and the passageway(s) in the group of main perforated plates being separated from passageway(s) in any adjacent group of perforated plates through which a second fluid flows by an intervening unperforated plate, wherein heat is exchanged between the first and second fluids across the intervening plate and characterised by inducing turbulence in at least one of the fluids by forcing that fluid passed turbulator means present in the form of projections into the flow channels on at least some of the continuous ribs.

33. A method of mixing fluids, comprising forcing a first fluid through a stack of bonded plates, the stack comprising at least one group of main perforated plates, wherein at least two adjacent plates of the group of main perforated plates have their perforations aligned in rows with continuous ribs between adjacent rows and the adjacent plates are aligned whereby the rows of perforations in one plate overlap in the direction of the rows with the rows of perforations of an adjacent plate and the ribs of adjacent plates lie in correspondence with each other to provide discrete fluid channels extending across the plates, a channel corresponding to each row of perforations, the channels together forming one or more fluid passageways across the plates through which the first fluid flows and the passageway(s) in the group of main perforated plates being separated from passageway(s) in any adjacent group of perforated -plates through which a second fluid flows by an intervening perforated plate, whereby one of the first or second fluids will pass into the flow channel of the other through the perforations in the intervening plate and characterised by inducing turbulence in at least one of the fluids by forcing that fluid passed turbulator means present in the form of projections into the flow channels on at least some of the continuous ribs.