WO2007082901A1

WO2007082901A1 - Finned heat exchanger

Info

Publication number: WO2007082901A1
Application number: PCT/EP2007/050466
Authority: WO
Inventors: Johannes Antonius Maria Reinders
Original assignee: Oxycom Beheer B.V.
Priority date: 2006-01-17
Filing date: 2007-01-17
Publication date: 2007-07-26
Also published as: GB0600819D0; EP1977183A1

Abstract

A heat exchanger, compring a primary fluid flow path (19); a secondary fluid flow path (20) adjacent the primary path (19) and isolated therefrom for the purposes of fluid exchange; a first plurality of spaced elongate thermally conductive fins (6) having subsantially U-shaped cross-section, projecting into the primary path (19); and a second plurality of spaced elongate thermally conductive fins (6) having substantially U-shaped cross-section, projecting into the secondary path (20); wherein the first and second plurality of thermally conductive fins (6) are partially nested togehter so that portions of sidewalls of the first plurality of fins (6) and portions of sidewalls of the second plurality of fins (S) are overlapped and held in thermally conductive relationship.

Description

FINNED HEAT EXCHANGER

BACKGROUND OF THE INVENTION

1. Field of the Invention

[0001] The present invention relates to heat exchange devices and more particularly to evaporative cooling devices of the type that can cool a primary or product air stream by evaporation of a fluid into a secondary or working air stream. Such devices can also operate to provide heat recovery in combination with ventilation.

2. Description of the Related Art

[0002] An evaporative cooler is a device that uses the latent heat of evaporation of a liquid to provide cooling. The principle of evaporative cooling has been known for many centuries. For example, a damp cloth placed over an object will keep the object cool by evaporation of liquid from the cloth. By continuously adding liquid to the cloth, the cooling effect may be maintained indefinitely without input of electrical energy. The lowest temperature that can be reached by evaporation of moisture in this way into an air stream defines the wet-bulb temperature for that air. An indirect evaporative cooler makes use of this principle. A product air stream passing over a primary surface of a heat exchange element may be cooled by a working air stream passing over and absorbing moisture from a secondary wetted surface of the heat exchanger.

[0003] According to theory, if a quantity of air is cooled by direct evaporation its absolute humidity increases due to the uptake of moisture. Its relative humidity also increases due to its lowered temperature until at the wet bulb temperature it is fully saturated with water vapour. If the air is cooled without direct evaporation however, its absolute humidity remains the same. As its temperature decreases only the relative humidity increases until full saturation of the air is reached at the so-called dew point. The dew point is thus lower than the wet bulb temperature and is in fact defined as the temperature to which a body of air must be cooled to reach saturation or 100% relative humidity. At this point, water vapour in the air condenses.

[0004] Attempts have been made to improve on the principle of indirect evaporative cooling by cooling or drying the working air stream prior to evaporation taking place. A O5589.0026.O0PC00 -2-

particularly convenient way of cooling the working air stream is to feedback a portion of the cooled product air. Such devices are often referred to as dew point coolers as they may lower the temperature of the product air to below its wet bulb temperature and close to the dew point. By optimising the surfaces with which the air streams exchange heat, highly effective heat transfer can be achieved. This has been found especially significant in the case of the heat transfer from the wetted secondary surface. In order to provide moisture to the working air stream, the wetted secondary surface may be provided with some form of liquid supply e.g. in the form of a hydrophilic layer. The presence of such a layer can however result in increased thermal isolation of the secondary surface from the working air stream, thus reducing heat transfer.

[0005] A particularly efficient form of dew point cooler is known from PCT publication WO03/091633, the contents of which are hereby incorporated by reference in their entirety. The device uses a membrane having heat transfer elements on its primary and secondary surfaces. These heat transfer elements are in the form of fins and are believed to improve transmission of heat from the primary surface to the secondary surface. The fins act both to directly conduct heat to the membrane and also to break up the various boundary layers that develop in the flow. They also serve to increase the total area available for heat exchange on the relevant surfaces. Further important features of the wetted second surface are known from that document and also from PCT publication WO05/019739, the contents of which are also incorporated by reference in their entirety. Accordingly, by careful choice of the material used as a water retaining layer, optimal evaporation may be achieved without thermal isolation of the secondary surface from the working air stream.

[0006] The driving temperature differential between the primary and secondary flows of an evaporative cooler of this type must be very low in order to achieve cooling down to the dew point. As a consequence, in order for good heat transfer to occur, the heat conduction coefficient across the heat exchanger must be high. In the case of WO03/091633, the point of attachment of the fins to the membrane is believed to be an area of poor heat transmission. In particular, the adhesives and coatings provided between the layers substantially increases the thermal resistance. Furthermore, despite attempts to maximise the contact area between fin and membrane, manufacturing techniques have achieved actual contact over a relatively O5589.OO26.00PCO0 -3-

limited area. According to PCT publication WO 03/091648 A, attempts have been made to improve heat transmission by connecting the fins on opposing sides of a membrane directly through the membrane. According to PCT publication WO 01/57461, the fins are formed as convolutions in the membrane itself.

[0007] Metals are generally good conductors of heat and a device described in PCT publication WO04/040219, the contents of which are hereby incorporated by reference in their entirety, uses a heat sealable metal laminate for forming both the fins and the membrane. These are then heat sealed together.

[0008] Many other configurations have also been suggested for evaporative cooling devices. In such configurations a heat transferring membrane divides the wet region, where liquid is provided for evaporation, from the dry region. A number of constructions by Maisotsenko et al are shown in US6581402, in which primary and working streams across a plate are separated by channel guides. The secondary stream is diverted to the opposite side of the plate and receives heat by evaporation and by heat transfer from the plate.

BRIEF SUMMARY OF THE INVENTION

[0009] In order to improve heat transmission between a primary and secondary flow, there is provided according to the invention a heat exchanger comprising a primary fluid flow path; a secondary fluid flow path adjacent the primary fluid flow path and isolated therefrom for the purposes of fluid exchange; a first plurality of spaced elongate thermally conductive fins having substantially U-shaped cross-section, projecting into the primary fluid flow path; and a second plurality of spaced elongate thermally conductive fins having substantially U-shaped cross-section, projecting into the secondary fluid flow path; wherein the first plurality of thermally conductive fins are partially nested or overlapping with the second plurality of thermally conductive fins so that portions of the sidewalls of the first plurality of fins and portions of the sidewalls of the second plurality of fins are overlapped and held in thermally conductive relationship.

[0010] This is in contrast to conventional arrangements where strips of fins having a generally wave-like cross-section are provided stacked one on top of the other, the wavelike cross-section of the stacked strips being out of phase so that the troughs of one strip oppose 05589.0026.00PCOO -A-

the peaks of the other and no nesting takes place. Such an arrangement is illustrated in WO03/091633, Heat transfer between such conventional fins takes place between the opposed peaks and troughs of the stacked fins.

[0011] In order to direct the primary and secondary fluid flows, there may be provided a primary inlet duct forming an inlet fluid connection to supply air to a set of primary flow paths and a secondary inlet duct forming an inlet fluid connection to supply air to a set of secondary paths. The inlet ducts may be formed integrally with the fins or from additional elements.

[0012] There may furthermore be provided a water distribution system to provide water to the secondary paths in order to wet the walls thereof. In this manner a primary air flow through the primary paths may be cooled by heat conduction from the fins in the primary paths to the fins in the secondary paths to cause evaporation of the water into a secondary air flow through the secondary paths. In the present context, reference to primary and secondary paths is, unless otherwise specified, intended to cover both the paths in their entirety and also individual path segments within the device.

[0013] According to a further embodiment of the invention, the overlapping regions of the first and second plurality of thermally conductive fins are joined to form a gastight seal. In this form, the bases of the U-shaped fins effectively form a continuous membrane dividing the primary and secondary flow paths.

[0014] According to an alternative further embodiment of the invention, a membrane is provided between the first and second plurality of thermally conducting fins to form a gastight barrier between the primary and secondary fluid flow paths.

[0015] According to a further embodiment of the invention, the thermally conducting fins may comprise boundary layer disrupting formations. Such formations or elements are important in preventing the build up of laminar flow along the paths, in particular in the secondary paths. Laminar flow is generally undesirable for good heat transfer from the surface of the fins. By disrupting the boundary layers, local turbulent flow and better mixing of the saturated air may be encouraged, leading to a higher heat transfer coefficient. It is noted that turbulent flow throughout the heat exchanger is usually undesirable, as the increase in pressure drop along the paths would outweigh the benefits due to increased heat transfer. The 05589.0026.00PCOO -5-

formations may be provided on the surfaces of the fins or may be formed by local distortions or contours of the fins themselves.

[0016] Preferably the device comprises a plurality of pairs of partially nested heat conducting fins stacked together in further partially nested relationship or in non-nested relationship. In this manner a large number of flow paths can be built up in a simple manner.

[0017] Preferably openings may be provided in the fins, particularly the sidewalls thereof, for directing flow from one surface of the fins to the other. The openings may have a number of important functions. Firstly, they can act to disrupt boundary layers and break up local laminar flow, thus increasing the heat transfer coefficient. Secondly, by directing the secondary flow over both surfaces of the fins, if water or a water retaining layer is provided on one of the surfaces of the fins, the secondary flow can be alternately exposed to thermal heat transfer and latent heat. The openings are preferably in the form of louvres or similar flow directing vents. Louvres have been found to be most effective in directing saturated air away from the boundary layer while minimising pressure drop due to excess turbulence.

[0018] According to one embodiment of the invention the flow paths are all generally aligned with the elongation of the fins and the direction of flow in the primary paths is counter to the flow in the secondary paths. Counter flow configuration has been recognised as the most optimum for efficient dew point cooling.

[0019] According to another embodiment of the invention the direction of flow in the primary paths is counter to the flow in the secondary paths and generally perpendicular to the elongation of the fins. Such a configuration can be achieved if the louvres or openings through the sidewalls of the fins are sufficiently large to allow flow to take place through the fins. A significant advantage of such a configuration is that the effective heat conducting length, per unit length of a corrugated strip of fins, is longer in this direction than in the direction of elongation of the fins and hence heat conduction in the direction of the primary flow is reduced.

[0020] In an alternative embodiment, the direction of flow in the primary paths may be generally perpendicular to the flow in the secondary paths. The device will then operate in cross flow. One of the flows may be parallel to the elongation of the fins and the other flow may take advantage of openings or louvres to pass through the fins. Alternatively, both flows O5589.O026.O0PC00 -6-

may be partially through and partially parallel to the fins. It is noted that a considerable advantage of the present invention is the versatility that it provides in allowing different flow configurations.

[0021] According to a preferred embodiment of the invention the above described heat exchanger is an evaporative cooling device.

[0022] According to a preferred embodiment of the invention the invention, the device further comprises a hydrophilic layer or outer surface at least partially covering the fins in the secondary flow paths. The hydrophilic layer acts as a water retaining and releasing layer. In this context, reference to water is understood to cover any other evaporative fluid that may be used in the operation of the device as an evaporative cooler. Most preferably, the hydrophilic layer is provided on one surface of the fins only. The hydrophilic layer need not be a separate layer but may also be formed as a surface treatment of the plate to improve its hydrophilicity. Cementitious materials such as Portland cement have in the past been found highly desirable. Alternatively, fibre materials may be used. It has been found to be of great importance that the water retaining layer should not obstruct heat transfer from the plate by insulating it from the secondary flow.

[0023] According to an important aspect of the invention, the fins should be good thermal conductors. Preferably the fins comprise aluminium, which is also light and easy to fabricate. The fins may also comprise other metals, in particular as alloys. The fins may if necessary be provided with protective layers e.g. to prevent corrosion or fouling. Nevertheless, such layers should not unduly inhibit heat transfer to the fins.

[0024] According to a preferred embodiment of the invention, outlets from the primary paths are in fluid connection with inlets to the secondary paths. In this manner part of the flow through the primary paths may be subsequently directed through the secondary paths. Operation in this manner as a dew point cooler is believed to be beneficial in achieving the highest efficiency of operation and the lowest outlet temperature from the primary paths. The fluid connection between primary outlet and secondary inlet may be on a one to one basis with one primary path providing inlet flow to one secondary path. Alternatively, the combined primary flow may be split and a part thereof returned and distributed to the secondary paths. In a further alternative, certain primary paths may be directed exclusively to 05589.OO26.00PC00 -7-

providing secondary air to all of the secondary paths. In this context, reference to outlets from the primary paths is intended to include any suitable connection, whether internal or external that can deliver part of the primary flow to supply flow through the secondary paths.

[0025] According to a yet further aspect of the invention, there is provided an evaporative cooler comprising an evaporative cooling device as described above having a housing for receiving the evaporative, inlet ducts connecting to the primary channels, outlet ducts connecting from the primary and secondary paths, an air circulation device for causing circulation of air through the primary and secondary paths, a water supply providing water to the water distribution system and a controller for controlling operation of the cooler. Such a cooler may then operate as a stand alone device or may be integrated into a larger heating and ventilation system. Additionally, temperature, pressure, humidity and other such sensors may be provided within the housing for monitoring operation and where necessary providing feedback to the controller.

[0026] According to yet a further aspect of the current invention there is provided a method of making a heat exchanger; comprising the steps of, providing a first thermally conducting body in the form of a corrugated strip to act as a primary fluid flow path; providing a second thermally conducting body in the form of a corrugated strip to act as a secondary fluid flow path; at least partially nesting the first corrugated strip into the second corrugated strip so that sidewalls of folds in the corrugated strips at least partially overlap; and fixing the corrugated strips together so that the overlapping portions of the sidewalls are in thermally conductive relationship, and the primary and secondary fluid flow paths are isolated for the purposes of fluid exchange. The corrugated strips may comprise fins as described above, wherein preferably the second thermally conducting body has a water retaining surface.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The features and advantages of the invention will be appreciated upon reference to the following drawings of a number of exemplary embodiments, in which:

[0028] Figure 1 shows a top perspective of an example of a series of thermally conductive fins provided in a corrugated strip; 05589.OO26.O0PC00 -8-

[0029] Figure 2 shows an underneath perspective of an example of a series of thermally conductive fins provided in a corrugated strip;

[0030] Figure 3 shows a partial schematic cross-section through a heat exchanger; [0031 ] Figure 4 shows an enlarged partial view of the heat exchanger of figure 3 ;

[0032] Figure 5 shows a partial schematic cross-section through a plurality of pairs of stacked fins in a first configuration;

[0033] Figure 6 shows a perspective view of the stacked fins of figure 5;

[0034] Figure 7 shows a partial schematic cross-section through a plurality of pairs of stacked fins in a second configuration;

[0035] Figure 8 shows a perspective view of the stacked fins of figure 7;

[0036] Figure 9 shows a schematic perspective view of part of an evaporative cooling device;

[0037] Figure 10 shows a schematic cross-section through a strip of fins.

[0038] Figure 11 shows a schematic perspective view of an evaporative cooling device; and

[0039] Figure 12 shows a schematic perspective view of a plurality of nested fins.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0040] A corrugated strip 6 comprising a series of elongate thermally conductive fins 7 is illustrated in figures 1 and 2. Each of the fins 7 is provided with sidewalls 8, an end wall 9 and has a generally U-shaped cross-section. Adjacent fins 7 are spaced and joined together at their open ends by connector walls 10 to make up the corrugated strip 6. Inter-fin channels 16 for fluid flow are formed between the spaced fins 7 and intra-fin channels 17 for gas flow are formed within the fins 7.

[0041] Figures 3 and 4 illustrate a pair of corrugated sheets 6 for use in a heat exchanger. As illustrated the corrugated sheets 6 are partially nested with one another. The fins 7 of one corrugated strip 6 partially protrude into the intra-fin channels 17 of the other corrugated strip 6. In this manner, portions of the sidewalls 8 of the corrugated strips 6 overlap with one another. The external and internal dimensions of the fins 7 are such that the overlapping O5589.0O26.O0PC00 -9-

portions of the sidewalls 8 fit snugly together to form a good thermally conductive connection therebetween.

[0042] The generally wave-like form of the cross- sections of the paired corrugated strips 6 are arranged in-phase with one another. This allows the above-described nesting of the strips 6.

[0043] A gas-tight seal separates the nested corrugated strips 6 into a primary fluid flow path 19 and a secondary fluid flow path 20. The primary 19 and secondary 20 paths are isolated from one another for the purposes of fluid exchange by the gas-tight seal. The primary path 19 is comprised of the inter-fin 16 and intra-fm 17 channels of one of the corrugated strips 6 and the second path is comprised of the inter-fin 16 and intra-fm 17 channels of the other corrugated strip 6.

[0044] In the illustrated embodiment a membrane 18 is provided to form the gas-tight seal between the two corrugated strips 6. In an alternative preferred embodiment the gas-tight seal is achieved by joining the overlapping sidewall 8 portions together, for example by welding, gluing or other methods.

[0045] Direct metal to metal joining of the sidewall portions to the membrane and/or to each other is particularly advantageous because no adhesive layer is required. This increases the thermal contact between the sidewalis because the thermally insulating adhesive layer is no longer present. The fins 7 in one example are directly metal to metal joined to the membrane or to one another by a formfitting connection.

[0046] In the illustrated embodiment the direction of fluid flow along the fluid flow paths 19, 20 is parallel to the elongation of the fins 7.

[0047] In use, a first medium, such as a gas or liquid, having a particular temperature, flows along the primary path 19. A second medium having a different temperature to the first medium flows along the secondary fluid flow path 20. The thermally conductive fins 7 act as thermal conduits between the two media so as to bring them toward thermal equilibrium. The overlapping portions of the sidewalls 8 ensure a good thermal connection between the fins 7 in the primary path and the fins 7 in the secondary path increasing the speed of thermal 05589.0026.00PCOO -10-

transference between the two media and as a result the efficiency of the heat exchanger in which they are located.

[0048] As illustrated in figures 5 and 6 a plurality of stacked pairs of nested corrugated strips 6 are provided. Each pair is separated from the adjacent pair by a substantially planar spacer membrane 26 extending therebetween. The spacer membrane need not be gas-tight if adjacent channels carry similar flows.

[0049] In an alternative preferred embodiment, as illustrated in øgures 7 and 8, a plurality of corrugated strips 6 are partially nested together in order to form a series of partially nested strips 6.

[0050] The extent of overlap of the sidewalls 8 of the nested strips 6 is adjusted according to the particular conditions of intended use. Equally the ratio of the width of the end walls 9 to the height of the sidewalls 8 of the fins may be adjusted according to the requirements of particular uses. Factors which may be important in determining the particular dimensions of the fins 7 and strips 6 include the nature of the media passing along the flow paths; the thermal gradient between the different media; and the material composition of the fins and membrane.

[0051 ] The fins of the primary and secondary paths may be identical in form or may take different forms so long as the required nesting is still achieved. For example, in a particular embodiment (not shown) the fins of the primary and secondary paths may have sidewalls 8 of different heights. This can advantageously produce primary and secondary channels of different cross-sectional areas.

[0052] Preferably and as illustrated in figures 1 and 2 the sidewalls 8 of the fins 7 are provided with louvres 27 in the form of elongate slots penetrating through the sidewalls 8. The louvres 27 are arranged in groups. A first group 28 serves to direct fluid flow from the inter-fin channels 16 into the intra-fin channels 17 and a second group 29 serves to direct the flow from the intra-fin channels 17 into the inter-fin channels 16. By directing the flow between both surfaces of the fins 7 in this manner, louvres 27 serve to increase the heat transfer coefficient by breaking up the boundary layers that develop in the fluid as it flows. It will be clear that other break up elements may be provided in addition to or instead of the 05589.0026.00PCOO -1 1-

louvres 27. For example a surface profile on the fins 7, such as dimples or corrugation, may be provided to encourage flow disruption

[0053] In addition to this function, on the second flow path secondary air can be caused to alternately flow over first an outer surface of the fins 7, where it can receive moisture by evaporation from a liquid retaining layer (to be described in more detail later), followed by the inner surface of the fin 7 where it can receive direct thermal energy to raise its temperature.

[0054] Fins 7 are also provided with conduction bridges 30. These bridges 30 are in the form of cuts through the fins 7 over substantially their whole height. They reduce unwanted transport of heat along the fins 7 in the direction of the air flow which could otherwise reduce the temperature difference between inlet and outlet. The louvres also act to reduce heat transfer along the length of the fins 7.

[0055] While the fins 7 of figures 1 and 2 have sidewalls 8 which are straight in the elongate direction, curvilinear or zig-zig fins may also be provided. It is believed that such fin shapes are advantageous in breaking up the boundary layers that develop in flow along the fins 7, since with each change in direction of the sidewalls 8, turbulent flow is established.

[0056] Various cross-sectional shapes are possible for the fins as long as there is sufficient overlap of the sidewalls for the purposes of thermal transference.

[0057] The fins 7, are preferably formed from metal such as soft annealed aluminium. Although metal has been found preferable for manufacture of the membrane 18 it is noted that other materials including plastics materials may be used as described in prior applications WO 03/091648 A and WO 01/57461 A, the contents of which are hereby incorporated by reference in their entirety.

[005S] The above-described heat exchanger is preferably used as an evaporative cooling device. For such use it is preferable that the strips 6 in the secondary path are provided with a liquid retaining layer in the form of an outer surface or covering of a hydrophilic material. An important factor for the efficient operation of an evaporative cooler is the nature of the liquid retaining layer. Although reference is made to a liquid retaining layer, it is clearly understood that the layer is in fact a liquid retaining and releasing layer. A requirement of such a layer is 05589.0026.O0PC00 -12-

that it easily gives up its water such that no resistance to evaporation is encountered. It is also important that it should distribute water quickly and effectively to all relevant surfaces. It should thus be hydrophilic without being hygroscopic, preferably retaining water primarily by surface tension effects.

[0059] In a preferred embodiment, the liquid retaining layer is formed from a fibrous material. The layer preferably has a very open structure such that the metal of the fins 7 can be clearly seen through the spaces between the fibres of the layer. This is believed to encourage direct heat transfer from the fins 7 without smothering them. Prior art devices using thick wicking layers have effectively insulated the heat transmitting layer preventing transfer of thermal heat. An exemplary material for forming the water retaining layer is a 20g/m² polyester/viscose 50/50 blend, available from Lantor B.V. in The Netherlands. Another exemplary material is a 30g/m² polyamide coated polyester fibre available under the name Colback™ from Colbond N. V. in The Netherlands. Other materials having similar properties including synthetic and natural fibres such as wool may also be used. Where necessary, the liquid retaining layer may be coated or otherwise treated to provide anti bacterial or other anti fouling properties.

[0060] The liquid retaining layer may be adhesively attached to the fins 7. For use with aluminium and Lantor fibres, a 2 micron layer of two-component polyurethane adhesive has been found to provide excellent results. When present as such a thin layer, its effect on heat transfer is negligible. It should furthermore be noted that the presence of the liquid retaining layer only influences heat transfer from fins 7 into the media flowing along the secondary path and does not have any significant influence on heat conduction between nested fins 7 and hence between the primary 19 and secondary 20 paths. The above-described fibrous layers have been found ideal for the purposes of manufacturing since they can be provided as a laminate that can be formed into louvres and other shapes in a continuous process. Other liquid retaining layers such as Portland cement may also be used and have in fact been found to provide superior properties although as yet, their production is more complex since there is a tendency to crack or flake if applied prior to forming of the heat exchange element It is nevertheless believed that other surface finishes such as aluminium oxide may themselves be adequate for providing the water retention and wicking required. O5589.O026.00PC00 -13-

[0061] Preferably the covering of hydrophilic material is present on the surface of the fins facing the inter-fin channels 16 but not on the surface of the fins facing the intra-fm channels

17.

[0062] Further preferably the hydrophilic material is provided in intermittent strips such that air flow passes alternately over the metallic surface of the fins 7 and the wetted surface of the water retaining layer.

[0063] Figure 9 shows a section of an embodiment of an evaporative cooling device comprising the above described nested fms 7 having a square "U" configuration. Arrows A and B give an indication of the direction of air flows for use as a dew point cooler. Arrow A represents the flow of primary air over the first path 19. Arrow B represents the flow of secondary air over the second path 20.

[0064] The heat exchange element preferably comprises a membrane forming a gastight seal between the primary and secondary flow paths 19, 20. The membrane is formed from a thin gauge aluminium sheet. In this case the fins 7 sandwich the membrane as they partially nest together. Preferably the fms are affixed to the membrane by heat seal adhesive. To this end, the fms are formed from aluminium laminated with a heat seal adhesive.

[0065] hi evaporative cooling devices of this type, heat exchange takes place primarily on the surfaces of the fins 7 in the second path 20 rather than at the membrane itself.

[0066] A water distribution system 116, as shown in figure 9, is preferably provided. The water distribution system illustrated is in the form of a series of conduits 118 leading from the water supply 119 to outlets 122 for ejecting droplets 124 of water into the secondary paths. The louvres 27 allow the droplets 124 to pass through the sidewalls of the fins 7 to the further inter and intra-fm channels below. Alternative water distribution systems may also be used. A preferred arrangement is the system presently used in the Oxycell Rooftop 400 evaporative cooler substantially as described in International Patent Publication No. WO04/076931, the content of which is hereby incorporated by reference in its entirety. Both the water supply 1 19 and the circulation device 115 are controlled by a controller 130. The device may be enclosed in an appropriate housing (not shown). O5589.0026.OOPC00 -14-

[0067] Figure 10 shows the different layers forming the construction of the fins 7. They comprise a layer of soft annealed aluminium 48 provided with layers of primer 50 for adhesion to a membrane having an anti-corrosive adhesive for activation by heat and pressure for coupling. The fins 7 are also provided with a liquid retaining layer 37 on their outer surface, which serves to retain and subsequently release the water for evaporation.

[0068] Operation of the device according to Figure 9 will now be explained in further detail. Water or another evaporable liquid is supplied to the water retaining layer 37 by the water distribution system 1 16. A flow of secondary air B is caused to flow through secondary path 20. As the secondary air passes over the water retaining layer 37, it takes up water by evaporation. The louvres 27 direct the air through the sidewalls 8 of the fins 7 where it is then warmed by direct heat transfer from the surface of the fins 7. A flow of primary air A is caused to flow through the primary path 19 in counter flow to the secondary flow B. The primary flow A is cooled by direct heat transfer to the fins 7. The heat transferred to the fins 7 in the region of the primary channel 19 is conducted within the fin strip 7 to the fin strip 6 with which it is nested and then to the region of the secondary path 20.

[0069] Operation of the device in a dew point cooler configuration will now be described based on the principle described. A primary air flow A enters an inlet to the primary paths 19 at a temperature Tl and flows through primary paths 19. The flow A is driven by a circulation device 115. The flow A is cooled by heat transfer to the fins 7 to a temperature T2 close to its dew point. On exit from the primary channel 19 the cooled primary flow A is split to form a cooled product flow C and secondary flow B. The product flow C is delivered by appropriate ducts to wherever the cooled air is required. The secondary flow B is returned through the secondary paths 20. As the secondary flow returns, it is heated by heat transfer from the fins 7 and takes up moisture by evaporation from the water retaining layer 37. On exit from the secondary path 20, the flow B will have relumed to close to its original temperature Tl but will be almost 100% saturated. The difference in enthalpy between the flows A and B represents the amount of cooling available for the product flow C.

[0070] In order to function effectively as a dew point cooler, heat transmission between the fins 16 on the first surface 12 and the second surface 14 must be maximized by ensuring O5589.0026.00PCO0 -15-

appropriate joining techniques. In order to also maximise the area of heat transfer the fins 7 are partially nested so that their sidewalls are held in thermally conductive relationship.

[0071] In alternative preferred embodiments the direction of flow in the primary paths is counter to the flow in the secondary paths and generally perpendicular to the elongation of the fins_* Such a configuration can be achieved if the louvres or openings through the sidewalls of the fins are sufficiently large to allow flow to take place through the fins. A significant advantage of such a configuration is that the effective heat conducting length, per unit length of a corrugated strip of fins, is longer in this direction than in the direction of elongation of the fins and hence heat conduction in the direction of the primary flow is reduced.

[0072] In an alternative embodiment, the direction of flow in the primary paths may be generally perpendicular to the flow in the secondary paths. The device will then operate in cross flow. One of the flows may be parallel to the elongation of the fins and the other flow may take advantage of openings or louvres to pass through the fins. Alternatively, both flows may be partially through and partially parallel to the fins. It is noted that a considerable advantage of the present invention is the versatility that it provides in allowing different flow configurations.

[0073] A further alternative arrangement of the fins according to the principles of the present invention is depicted in Figure 11. According to Figure 11, a heat exchanger is shown having the same format as a heat exchanger known from WO03/091633, referenced above. The heat exchanger comprises a plurality of strips 6 of thermally conductive fins 7 as shown in Figure 1. The strips 6 are applied to both sides of a membrane 18 using heat and pressure sufficient to cause the membrane 18 to assume partially the shape of the fin 7. Unlike the prior art arrangement of WO03/091633, the fins 7 in Figure 11 are at least partially nested together in the manner described in relation to Figures 3 and 4. It has been found that the resulting heat exchanger has a significantly higher heat transfer coefficient when operated as an evaporative cooler.

[0074] A further alternative arrangement of fins is depicted in Figure 12. According to Figure 12, a similar arrangement to that of Figures 3 and 4 is shown in which individual fins 7 are joined on either side of a membrane 18, defining a primary fluid flow path 19 and a secondary- fluid flow path 20. Each of the fins 7 is provided with sidewalls 8 and an end wall 9 forming O5589.OO26.O0PC00 -16-

a generally U-shaped cross-section. Unlike the embodiment of Figures 3 and 4, fins 7 are not joined together at their open ends by connector walls. The spacing of adjacent fins 7 on one side of the membrane is selected to allow them to nest with a fin on the opposite side of the membrane. In this manner, an overlapping region can be formed in which a good heat conducting connection can be formed between the fins 7 on either side of the membrane.

[0075] In a particularly preferred embodiment of the invention when used as an evaporative cooler such as a dewpoint cooler, the sidewalls 8 of the fins in the primary path 19 are taller than the sidewalls 8 of the fins in the secondary path 20. In this manner the cross-sectional area of the primary path is greater than that of the secondary path allowing a greater volume of gas to flow along it. This is advantageous since a smaller volume of air evaporating water (the air in the secondary path) can be used to cool a larger volume of air (the air in the primary path) because the latent heat of evaporation of water into air is greater than the specific heat capacity of air.

[0076] Thus, the invention has been described by reference to certain embodiments discussed above. It will be recognized that these embodiments are susceptible to various modifications and alternative forms well known to those of skill in the art. Many modifications in addition to those described above may be made to the structures and techniques described herein without departing from the spirit and scope of the invention. Accordingly, although specific embodiments have been described, these are examples only and are not limiting upon the scope of the invention.

[0077] It should be noted that the term "comprising" as used in the claims or description of this application does not exclude other elements or steps; and the terms "a" and "an" do not exclude a plurality. Any reference signs in the claims shall not be construed as limiting the scope of the claims.

[0078] It is to be understood that any feature described in relation to one embodiment may also be used in other of the embodiments. Furthermore, equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.

Claims

O5589.0026.00GB00 -17-What is claimed is:

1. A heat exchanger, comprising a primary fluid flow path; a secondary fluid flow path adjacent the primary path and isolated therefrom for the purposes of fluid exchange; a first plurality of spaced, elongate, thermally conductive fins having substantially U- shaped cross-section, projecting into the primary path; and a second plurality of spaced elongate thermally conductive fins having substantially U-shaped cross-section, projecting into the secondary path; wherein the first and second plurality of thermally conductive fins are partially nested together or overlapping so that portions of sidewalls of the first plurality of fins and portions of sidewalls of the second plurality of fins are overlapped and held in thermally conductive relationship.

2. A heat exchanger according to claim 1 wherein the overlapping regions of the first and second plurality of thermally conductive fins are in direct contact and form a gastight seal.

3. A heat exchanger according to claim 1 wherein a membrane is provided between the first and second plurality of thermally conducting fins to form a gastight barrier between the primary and secondary fluid flow paths.

4. A heat exchanger according to any one of the preceding claims wherein the sidewalls of at least one of the first or second plurality of thermally conducting fins is provided with disruptors to disrupt laminar fluid flow.

5. A heat exchanger according to any one of the preceding claims, comprising openings in the sidewalls of the fins for directing fluid flow therethrough.

6. A heat exchanger according to claim 5 wherein the openings are formed as louvres

05589.OO26.O0PC00 -18-

7. A heat exchanger according to any one of the preceding claims wherein one of the first or second plurality of thermally conductive fins is provided with a hydrophilic layer covering at least a part of its surface.

8. A heat exchanger according to any one of the preceding claims, wherein an outlet from the primary path is in fluid connection with an inlet to the secondary path whereby at least part of the flow through the primary path may be subsequently directed through the secondary path.

9. A heat exchanger according to any one of the preceding claims, wherein a plurality of pairs of partially nested primary and secondary fluid flow paths are provided adjacent to one another.

10. A heat exchanger according to any one of the preceding claims, wherein the first or second plurality of fins are joined together to form a substantially continuous corrugated strip.

11. An evaporative cooler comprising a heat exchanger according to any one of claims 1 to 10, further comprising; a housing for receiving the heat exchanger; an inlet duct connecting to the primary path; outlet ducts connecting from primary and secondary paths; an air circulation device for causing circulation of air along the primary and secondary paths; a water supply providing water to the secondary path; and a controller for controlling operation of the cooler,

12. A method of making a heat exchanger; comprising: providing a first thermally conducting body in the form of a corrugated strip to act as a primary fluid flow path; providing a second thermally conducting body in the form of a corrugated strip to act as a secondary fluid flow path; O5589.O026.00PC00 -19-

at least partially nesting the first corrugated strip into the second corrugated strip so that sidewalls of folds in the corrugated strips at least partially overlap; and fixing the corrugated strips together so that the overlapping portions of the sidewalls are in thermally conductive relationship, and the primary and secondary fluid flow paths are isolated for the purposes of fluid exchange.

13. The method of claim 12, further comprising providing a water retaining outer surface over at least a part of the second thermally conducting body.

14. The method of making a heat exchanger according to claim 12 or 13, further comprising providing a membrane between the first and second thermally conducting bodies and fixing the corrugated strips together by connecting them to the membrane.

15. The method of making a heat exchanger according to claim 14, wherein nesting of the first corrugated strip into the second corrugated strip takes place by substantially deforming the membrane to conform to the first and second corrugated strips.

16. An evaporative cooling device substantially as described hereinbefore, with reference to the description and figures.