Fin and tube heat exchanger

Abstract

A fin and tube heat exchanger 1 comprises a first fin stack 10 comprising a plurality of first fins 11 stacked in a stacking direction; and a second fin stack 20 comprising a plurality of second fins

Description

The present invention relates to a fin and tube heat exchanger, a heat exchanger unit comprising a fin and tube heat exchanger, a method of manufacturing a fin and tube heat exchanger, an air conditioning unit or heat pump, and a kit for manufacturing a fin and tube heat exchanger. Background Fin and tube heat exchangers are a type of heat exchanger in which a series of tubes are fitted with fins on their outer surface. Fin and tube heat exchangers are typically configured such that a first fluid can pass through the insides of the tubes, whilst a second fluid flows over the outside surface of the tubes and the fins. The fins fitted to the outside surface of the tubes can significantly increase the surface area available for heat transfer between the first and second fluids. Fin and tube heat exchanges are typically used where the second fluid flowing over the outside surface of the tubes and over the fins is a gas, such as air. A common application of fin and tube heat exchangers is in heat pumps and air conditioning units. In some fin and tube heat exchangers, fins are fitted to respective tubes by an interference fit between an aperture through a fin that a respective tube passes through. One way such an interference fit can be provided is by passing a tube having a smaller outer diameter than the diameter of the aperture in the fin through said fin, and subsequently subjecting the tube to an expansion process, whereby a mandrel is forced down the tube to expand its outer diameter to an extent that the tube then forms an interference fit with the aperture in the fin. This expansion process places a large compressive force on the tube along its length. In some fin and tube heat exchangers, multiple stacks of fins with respective arrays of tubes passing therethrough may be present and positioned adjacent each other in the heat exchanger. This can facilitate bending of the tubes of the heat exchanger to give the heat exchanger a certain shape (e.g. an ‘L’ shape), because the fin stacks can move relative to each other during bending. During bending, fins of adjacent stacks may mesh with each other as the fin stacks and tubes deform. The present invention has been devised in light of the above considerations. Summary of the Invention In a first aspect, there is provided a fin and tube heat exchanger, comprising: a first fin stack comprising a plurality of first fins stacked in a stacking direction; a second fin stack comprising a plurality of second fins stacked in the stacking direction; an array of first tubes extending through the first fin stack in the stacking direction, the first tubes spaced apart from each other along a length direction of the first fins; an array of second tubes extending through the second fin stack in the stacking direction, the second tubes spaced apart from each other along a length direction of the second fins; wherein: the first fin stack is positioned adjacent the second fin stack in a width direction of the first fins and / or the second fins. a set of one or more of the second fins are configured such that each second fin of said set crosses over one or more first fins in the stacking direction between outermost first tubes of the array of first tubes and / or between outermost second tubes of the array of second tubes. In this way, when forces and / or moments are applied to the first tubes and / or the second tubes (e.g. a linear compressive force or a bending moment in the width direction of the fins), resistance against the first fin stack meshing with the second fin stack is provided, because the crossing of the first fin with the second fin in the stacking direction between outermost first / second tubes prevents such meshing. This can improve the efficiency of the heat exchanger by providing a lower pressure drop across the fins of the heat exchanger, since the flow path through the first fin stack and second fin stack over the fins and outer surfaces of the tubes has a larger cross-sectional area compared to if fin meshing occurs. Additionally, such resistance against meshing of the fin stacks can facilitate the use of more slender tubes in the heat exchanger, which might otherwise buckle during manufacture of the heat exchanger. By way of example, the tubes of the heat exchanger typically undergo an expansion process to provide an interference fit between the tubes and respective fins. The expansion process may involve mandrels being forced down respective tubes of the heat exchanger to expand the diameter (inner and outer diameter) of the tubes; such an expansion process places a high compressive load on the tubes in the length direction of the tubes and typically results in a high percentage of tubes buckling where there is no arrangement preventing meshing of the fins. This buckling is highly undesirable, as a heat exchanger containing a buckled tube is typically considered scrap (e.g. due to the aforementioned increase in pressure drop due to fin meshing, or because the structural integrity of the tubes is compromised (e.g. a crack through the tube wall may form during buckling)). Accordingly, the present invention can reduce the percentage of heat exchangers with manufacturing defects, and thus reduce manufacturing costs and rejection rate. Furthermore, because the arrangement of the fins provides the heat exchanger with a resistance to buckling, it may be possible to use tubes having a lower moment of inertia (e.g. a smaller outer diameter and / or thinner walls) than would otherwise be used, thereby reducing material usage in the tubes and / or reducing the tube diameter. A reduction in the internal tube diameter reduces the internal volume of the tubes. This can be particularly advantageous where the fluid to flow through the inside of the tubes is hazardous (e.g. toxic, flammable), as it is typically desirable to reduce the volume of that fluid contained within the heat exchanger. A second fin crossing over a first fin in the stacking direction may be understood to mean that at a first position along each second fin of said set, said second fin is positioned on a first side of one or more first fins in the stacking direction and at a second position along said second fin, said second fin is positioned on a second side of said one or more first fins in the stacking direction. Each fin may have a length, width and thickness. The thickness of a fin may be substantially less than the length and width of the fin. The width of the fin may be substantially less than the length of the fin. In a fin stack, the thickness direction of the fin may be substantially aligned with (e.g. parallel to) the stacking direction. A fin may be substantially planar, e.g. the fins in the first fin stack may be substantially planar; that is, their length direction may extend in a plane. The array of first tubes may be spaced apart from each other in the length direction of the first fins. The array of second tubes may be spaced apart from each other in the length direction of the second fins. The outermost first tubes of the array of first tubes may be understood as a pair of first tubes of the array of first tubes that are spaced furthest apart from each other of any pair of first tubes in the first tube array. The outermost second tubes of the array of second tubes may be understood as a pair of second tubes of the array of second tubes that are spaced furthest apart from each other of any pair of second tubes in the second tube array. By the first fin stack being positioned adjacent the second fin stack in the width direction of the first fins and / or second fins, it may be understood that the first fin stack and second fin stack are positioned adjacent each other such that the array of first tubes is substantially parallel to the array of second tubes, but spaced apart in the width direction of the first fins and / or second fins. By way of example, the stacking direction of each fin stack may be substantially parallel and the width direction of the first fins and the second fins may be substantially parallel. The first fin stack and second fin stack may be positioned adjacent each other such that they abut in a plane parallel to the stacking direction and the length direction of the first and / or second fins. The width direction of the first fins and / or the second fins may be substantially perpendicular to the stacking direction and / or the length direction of the first fins and / or second fins. Each second fin in said set of one or more of the second fins may be bent such as to provide the crossing of said second fin over one or more first fins in the stacking direction. That is, each second fin in said set may be non-planar. In some examples, each second fin in said set may be bent into a curved shape (e.g. defining an arc (in a plane through the fin in a plane perpendicular to the width direction), such as a circular arc or an elliptical arc). In other examples, each second fin in said set may be bent into a ‘v’ shape, that is, said fin comprising two planar sections at an angle to each other in a plane through the fin in a plane perpendicular to the width direction. This can provide a convenient manner in which to provide the crossing of the second fins over the first fins in the stacking direction. Said set of one or more of the second fins may be configured such that each second fin of said set crosses back over one or more first fins in the stacking direction between outermost first tubes of the array of first tubes and / or between outermost second tubes of the array of second tubes. In this way, greater resistance to buckling of the tubes and / or meshing of the fins can be provided, because the second fins of the set cross over a given first fin at two positions. A second fin crossing back over a first fin in the stacking direction may be understood to mean that at a third position along each second fin of said set, said second fin is positioned on the first side of said one or more first fins in the stacking direction, the second position being between the first position and third position along the length of said second fin. The first fin stack may comprise a first end spaced apart from a second end in the stacking direction. The second fin stack may comprise a first end spaced apart from a second end in the stacking direction. The first end of the first fin stack may be adjacent the first end of the second fin stack. The second end of the first fin stack may be adjacent the second end of the second fin stack. Where the set of second fins comprises a plurality of second fins, the set of second fins may be configured such that the closer a second fin of said set is to the second end of the second fin stack, the fewer first fins said second fin crosses over in the stacking direction. That is, a second fin of said set proximate the first end of the second fin stack may cross over a greater number of first fins in the stacking direction than a second fin of said set proximate the second end of the second fin stack. In this way, the first fins and second fins may be closer to planar alignment (i.e. closer to lying in the same plane as each other) adjacent the second end of the second fin stack than the first end of the second fin stack, which may reduce the pressure drop across the fin stacks (i.e. the pressure drop for a fluid flowing over the fins and the outside surfaces of the tubes) at the second end of the second fin stack compared to the pressure drop across the fin stacks at the first end of the second fin stack. In some examples, one or more of the second fins may not cross over one or more first fins in the stacking direction; such second fins may be positioned closer to the second end of the second fin stack than the set of one or more second fins crossing over one or more first fins; such second fins may be coplanar with respective first fins. The heat exchanger may further comprise a third fin stack comprising a plurality of third fins stacked in the stacking direction. An array of third tubes may extend through the third fin stack in the stacking direction. The third tubes may be spaced apart from each other along a length direction of the third fins. In some examples, the third fin stack may be positioned adjacent the second fin stack in the width direction of the second fins and / or a width direction of the third fins. The second fin stack may be interposed between the first fin stack and the third fin stack. The third fin stack and third tubes may be configured in the same manner as the first fin stack. That is, the set of one or more second fins may also each cross over one or more third fins in the stacking direction between outermost third tubes of the array of third tubes and / or between outermost second tubes of the array of second tubes. Each second fin in the set of one or more second fins may also cross back over one or more third fins in the stacking direction between outermost third tubes of the array of third tubes and / or between outermost second tubes of the array of second tubes In this way, the meshing of the second fin stack and / or the third fin stack can be prevented, thereby providing greater resistance to buckling of the first tubes, second tubes or third tubes. The third fins may be substantially planar; that is, their length direction may extend in a plane. The first tubes may pass through respective apertures in each of the first fins. The second tubes may pass through respective apertures in each of the second fins. An aperture in the first fin may form an interference fit with a respective first tube. An aperture in the second fin may form an interference fit with a respective second tube. Such an interference fit can improve heat transfer between the tubes and fins of the heat exchanger, thereby improving overall heat transfer efficiency of the heat exchanger. A flange may surround an aperture in a first fin. Said flange may abut an adjacent first fin in the first fin stack in the stacking direction. A flange may surround an aperture in a second fin. Said flange may abut an adjacent second fin in the second fin stack in the stacking direction. Said flange(s) may upstand from a main body of the respective fin in the stacking direction. In this way, the main body of the fins may be separated from each other within the respective fin stack such as to provide a flow path between adjacent fins in the fin stack (e.g. a flow path in the width direction of the fins). This can improve the heat transfer efficiency of the heat exchanger by increasing the surface area of the heat exchanger in contact with a fluid flowing over the fins and the outside surfaces of the tubes. The flanges may have a height of greater than or equal to 1.0 mm and / or less than or equal to 2.0 mm. At the first end of the first fin stack in the stacking direction, adjacent pairs of the first tubes may be connected together to facilitate fluid flow therebetween. At the first end of the second fin stack in the stacking direction, adjacent pairs of the second tubes may be connected together to facilitate fluid flow therebetween. In this way, fluid flowing inside one first / second tube may flow into an adjacent first / second tube. Adjacent pairs of the first tubes and / or second tubes may be fluidly connected together by a respective connector tube. The connector tube may be a hairpin tube, e.g. substantially ‘u’-shaped. In some examples, the first fins and / or the second fins may have a thickness of greater than or equal to 0.05 mm and less than or equal to 0.5 mm, e.g. about 0.1 mm. The thickness of the fins may be the thickness of the main body of the fin (i.e. excluding any flanges). The first fins and / or the second fins may be formed of aluminium, an aluminium alloy, copper, or a copper alloy . In some examples, the first tubes and / or second tubes may have a length less than or equal to about 2.5 meters The first tubes and / or second tubes may have a length greater than or equal to 0.3 meters, e.g. about 1 metre. The first tubes and / or second tubes may have an outer diameter greater than or equal to 4 mm, e.g. about 5 mm, greater than or equal to 7 mm, or greater than or equal to 8 mm. The first tubes and / or the second tubes may have an outer diameter less than or equal to 10 mm, less than or equal to 8 mm, or less than or equal to 6 mm. The first tubes and / or the second tubes may have a wall thickness of less than or equal to 0.3 mm and greater than or equal to 0.15 mm, e.g. less than or equal to 0.25 mm and greater than or equal to 0.18 mm, such as about 0.21 mm. The first tubes and or the second tubes may have a wall thickness of greater than or equal to 0.15 mm. The first tubes and / or the second tubes may have a substantially annular cross section in a plane perpendicular to their length, and / or may be inner-grooved tubes. The first tubes and / or second tubes may be formed of copper or an alloy thereof. The fin and tube heat exchanger may be bent in a direction substantially parallel to the width direction of the first fins and / or the second fins to define a first end of the heat exchanger and a second end of the heat exchanger. The first end of the first fin stack and / or the first end of the second fin stack may be positioned at the first end of the heat exchanger. The second end of the first fin stack and / or the second end of the second fin stack may be positioned at the second end of the heat exchanger. The first tubes and / or the second tubes may have a slenderness ratio greater than or equal to 56, greater than or equal to 115, greater than or equal to 179 greater than or equal to 364, or greater than or equal to 471. The first tubes and / or the second tubes may have a slenderness ratio less than or equal to 959, less than or equal to 471, less than or equal to 364, less than or equal to 179, or less than or equal to 115. The slenderness ratio may be defined as the effective length of the tube divided by the radius of gyration, r, of the cross-section of the tube in a plane perpendicular to its length. The effective length of the tube may be defined as the product of its length, L, and an effective length factor, K, which typically takes a value of K = 0.65. The critical axial stress creat which buckling of the tube is expected to occur may be defined by the equation oc = (KL / r^’ where E is the young’s modulus of the tube material. Such slenderness values may provide a copper tube that cannot withstand an expansion process without further support, because the copper tube has a critical axial stress lower than the axial stress exerted on the tube (absent further support) during expansion. The crossing of the set of one or more second fins with the first fins in the stacking direction can provide the support that such tubes need in order to withstand the expansion process, facilitating the use of such slender tubes in fin and tube heat exchangers. In a second aspect, there is provided a heat exchanger unit comprising a fin and tube heat exchanger according to the first aspect. Any one or more of the optional features of the first aspect are equally applicable to the second aspect, except where such a combination is clearly impermissible or expressly avoided, and are hereby restated in respect of the second aspect. The heat exchanger unit may comprise a tube-side fluid loop, the tube-side fluid loop passing through the first tubes and second tubes of the heat exchanger. The tube-side fluid loop may be charged with a refrigerant. The refrigerant may be a flammable refrigerant, e.g. Propane R-290. The heat exchanger unit may be configured for air to be the fin-side fluid. The heat exchanger may further comprise a fan configured to pass air over the fin and tube heat exchanger. That is, the fan may be configured to pass air over the fins and the outer surface of the tubes. The fan may be configured such that the air (predominantly) passes over the fins and the outer surface of the tubes in the width direction of the heat exchanger (e.g. a flow axis of the fan may be aligned with the width direction of the heat exchanger). The fin and tube heat exchanger may be bent in a direction substantially parallel to the width direction of the first fins and / or the second fins to define a first end of the heat exchanger and a second end of the heat exchanger. This may allow the heat exchanger to fit more efficiently into the space available within the heat exchanger unit (e.g. within a housing of the heat exchanger unit. The bend may be through an angle of about 90 degrees, e.g. an angle of greater than or equal to 80 degrees and less than or equal to 100 degrees. The second end of the heat exchanger may be longer than the first end of the heat exchanger. The fan and heat exchanger may be positioned relative to each other such that a flow axis of the fan is coincident with the second end of the heat exchanger. By way of example, the fan may comprise an impeller configured to rotate about a rotational axis, the rotational axis being parallel to the flow axis and the rotational axis and / or flow axis may intersect the second end of the heat exchanger. The first end of the heat exchanger may extend in a direction substantially parallel to the flow axis of the fan (e.g. the tubes of the heat exchanger may extend in a direction substantially parallel to the flow axis of the fan. By way of example, the fan may comprise a casing at least partially circumscribing the flow axis, and the first end of the heat exchanger may extend along the casing (e.g. along an outside surface of the fan casing) in a direction substantially parallel to the flow axis of the fan. In this way, whilst the space within the heat exchanger unit is being used efficiently by bending the heat exchanger, the flow from the fan is still directed towards the major portion of the heat exchanger. Where the first end of the second fin stack is at the first end of the heat exchanger and the set of second fins is configured such that the closer a second fin of said set is to the second end of the second fin stack, the fewer first fins said second fin crosses over in the stacking direction, then the first end of the heat exchanger being the end extending parallel to the flow axis of the fan may also mean that the flow from the fan predominantly passes through the end of the heat exchanger where the second fins cross over the first fins less, and thus across which the pressure drop it lower. In this way, more efficient heat exchange is facilitated. The heat exchanger unit may further comprise a compressor. The tube-side fluid path may pass through the compressor such that fluid passing therethrough is compressed to increase its pressure. The heat exchanger unit may further comprise an expansion device. The tube-side fluid path may pass through the expansion device such that fluid passing therethrough is expanded to reduce its pressure. In a third aspect there is provided a heat pump, comprising a heat exchanger unit according to the second aspect. Any one or more of the optional features of the first aspect and / or second aspect are equally applicable to the third aspect, except where such a combination is clearly impermissible or expressly avoided, and are hereby restated in respect of the third aspect. The term “heat pump” in this context may be understood to include any system that transfers thermal energy from one location to another. Examples of heat pumps include air-source heat pumps and air conditioning systems, e.g. a domestic air-source heat pump, or a domestic air conditioning system where the heat pump is used to control the temperature of an internal (building interior) space by moving thermal energy to / from an external space (to / from the exterior of the building). In a fourth aspect there is provided a method of manufacturing a fin and tube heat exchanger, the fin and tube heat exchanger comprising: a first fin stack comprising a plurality of first fins stacked in a stacking direction; a second fin stack comprising a plurality of second fins stacked in the stacking direction; an array of first tubes extending through the first fin stack in the stacking direction, the first tubes spaced apart from each other along a length direction of the first fins; an array of second tubes extending through the second fin stack in the stacking direction, the second tubes spaced apart from each other along a length direction of the second fins; wherein the first fin stack is positioned adjacent the second fin stack in a width direction of the first fins and / or the second fins; wherein the method comprises arranging a set of one or more of the second fins such that each second fin of said set crosses over one or more first fins in the stacking direction between outermost tubes of the array of first tubes and / or between outermost second tubes of the array of second tubes. Any one or more of the optional features of the first aspect, second aspect and / or third aspect are equally applicable to the fourth aspect, except where such a combination is clearly impermissible or expressly avoided and are hereby restated in respect of the fourth aspect. The step of arranging said set of one or more of the second fins may be followed by affixing the second fins to the second tubes and / or affixing the first fins to the first tubes. In this way, movement of the first fins relative to the first tubes and / or movement of the second fins relative to the second tubes is then prevented, and the position of the first fins relative to the second fins (i.e. with the set of one or more of the second fins such that each second fin of said set crosses over one or more first fins in the stacking direction) retained. The first tubes may be fixed in position relative to the second tubes at the second end of the heat exchanger by one or more return tubes each connecting a pair of a first tube and a second tube together to facilitate fluid flow therebetween. The return tube may be a hairpin tube, e.g. substantially ‘u’-shaped. The first tubes may be fixed in position relative to the second tubes at the second end of the heat exchanger by a joining plate stacked onto both the first fin stack and second fin stack and through which the first tubes and second tubes both extend, In some examples, affixing the second fins to the second tubes and / or affixing the first fins to the first tubes may be conducted an expansion process whereby the outer diameter of the first tubes and / or second tubes is expanded to contact the respective fins. The expansion process may comprise forcing a mandrel down a respective tube. The mandrel diameter may have a diameter greater than the inner diameter of the respective tube. The expansion process may increase the outer diameter of a tube by about 5%. The expansion process may also result in adjacent fins in the stacking direction being compressed together. The expansion process may form an interference fit between an aperture through a fin and a respective tube that passes therethrough. Where a joining plate is present, the expansion process may affix the first tubes and second tubes to the joining plate by the outer diameter of the first tubes and second tubes being expanded to contact the joining plate. The method may further comprise stacking the plurality of first fins into the first fin stack. The method may further comprise stacking the plurality of second fins into the second fin stack. The method may further comprise passing the plurality of first tubes through the first fin stack. By way of example, each first tube may be passed through respective apertures in the first fins. The method may further comprise passing the plurality of second tubes through the second fin stack. By way of example, each second tube may be passed through respective apertures in the second fins. The method may further comprise positioning the first fin stack adjacent the second fin stack such that the array of the first tubes is substantially parallel to the array of second tubes. The method may comprise bending the set of one or more second fins such that each second fin of said set crosses over one or more first fins in the stacking direction between outermost tubes of the array of first tubes and / or between outermost second tubes of the array of second tubes. The method may comprise bending the heat exchanger. The method may comprise bending the heat exchanger in a direction substantially parallel to the width direction of the first fins and / or second fins In a fifth aspect, there is provided a kit for manufacturing a fin and tube heat exchanger, the kit comprising: a fin stacking tool; and a plurality of first fins and a plurality of second fins; and wherein: the heat exchanger comprises a first fin stack comprising the plurality of first fins stacked in a stacking direction; a second fin stack comprising the plurality of second fins stacked in the stacking direction; the tool comprises a first support surface to support the first fin stack at a first end of the heat exchanger and a second support surface to support the second fin stack at the first end of the heat exchanger; and at least a portion of the second support surface is offset in the stacking direction relative to the first support surface such that, upon stacking the first fins and second fins into the first fin stack and the second fin stack, respectively and supporting said first fin stack and second fin stack on the first support surface and second support surface, respectively, each second fin of a set of one or more of the second fins crosses over one or more first fins in the stacking direction. Stacking the fins using the fin stacking tool positions the first fins relative to the second fins in such a way that the heat exchanger according to the first aspect can be manufactured (e.g. using the method according to the fourth aspect). Any one or more of the optional features of the first aspect, second aspect, third aspect and / or fourth aspect are equally applicable to the fifth aspect, except where such a combination is clearly impermissible or expressly avoided and are hereby restated in respect of the fifth aspect. The kit may further comprise a plurality of first tubes and a plurality of second tubes. The heat exchanger may comprise an array of the first tubes extending through the first fin stack in the stacking direction, the first tubes spaced apart from each other along a length direction of the first fins, and an array of the second tubes extending through the second fin stack in the stacking direction, the second tubes spaced apart from each other along a length direction of the second fins. At least a portion of the second support surface may be offset in the stacking direction from the first support surface such that, upon the tool supporting said first fin stack and second fin stack with the array of first tubes and / or array of second tubes extending therethrough, respectively, each second fin of said set crosses over the one or more first fins in the stacking direction between outermost first tubes of the array of first tubes and / or between outermost second tubes of the array of second tubes. This may provide increased resistance to meshing of the fins and / or buckling of the tubes, since the positions at which the second fins cross the first fins is at a more central position along the length of the fins, and thus closer to the majority of the tubes. The kit may further comprise a plurality of connector tubes configured to fluidly connect adjacent pair of the first tubes and / or the second tubes. The tool may further comprise a plurality of tube support surfaces configured to support the connector tubes at the first end of the heat exchanger when the connector tubes are fluidly connected to adjacent pairs of the first tubes and / or second tubes. In this way, a compressive force can be applied to the tubes (e.g. during an expansion process) from the second end of the heat exchanger and the tube support surfaces can prevent movement of the tubes in their length direction. The tool may comprise one or more first receivers configured to provide first tube support surfaces and the first support surface for the first fins. The tool may comprise one or more second receivers configured to provide second tube support surfaces, and optionally, the second support surface for the second fins. Alternatively, where the second support surface is not provided by the second receivers, the tool may comprise one or more wedge plates configured to support the second support surface (e.g. by extending above the second receivers in the stacking direction). The tool may further comprise a set of expansion mandrels configured to be forced into respective first tubes and / or second tubes from a second end of the heat exchanger. In this way, it can be understood that the tool may be a tube expansion tool. In this way, the tool can be used to expand the outer diameter of the tubes to an extent that each tube then forms an interference fit with the apertures in the fins that it passes through, thereby affixing the fins to the tubes. The tool may further comprise a first sidewall and / or second sidewall configured to abut the first fin stack and / or second fin stack, respectively, along the length direction of the first and / or second fins and the stacking direction of the first and / or second fin stack on opposite sides of the heat exchanger. Such sidewalls may provide support to the fin stacks so that tubes are not able to buckle outwards (e.g. a first tube buckling in a direction away from the second fin stack) when subjected to a compressive force (e.g. during a tube expansion process. The invention includes the combination of the aspects and preferred features described except where such a combination is clearly impermissible or expressly avoided. Summary of the Figures Figure 1A is a schematic of an assembly of fins and tubes for a prior art fin and tube heat exchanger undergoing tube expansion; Figure 1B is a schematic of an assembly of fins and tubes for a prior art fin and tube heat exchanger undergoing tube expansion where a tube has buckled; Figure 2 is a schematic of an assembly of fins and tubes for a fin and tube heat exchanger according to the present disclosure; Figure 3 is a schematic cross section of the assembly of fins and tubes along the section A-A’ in Figure 2; Figure 4 is a schematic of a heat exchanger unit comprising the heat exchanger of Figure 2; and Figures 5A and 5B illustrate a fin stacking tool for use in manufacturing the heat exchanger of Figure 2. Detailed Description of the Invention Aspects and embodiments of the present invention will now be discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art. All documents mentioned in this text are incorporated herein by reference. Figure 1A is a schematic of an assembly of fins 11 and tubes 12 for a prior art fin and tube heat exchanger 1 undergoing tube expansion. The assembly comprises two fin stacks 10 comprising a plurality of fins 11 stacked in the stacking direction S, the fin stacks 10 positioned adjacent to each other in a width direction W of the fins 11 such that the fins 11 in the fin stacks 10 are stacked in the same direction as each other, and the lengths of the fins 11 (extending into the page in Fig. 1A) in the stacks 10 are also parallel to each other. An array of tubes 12 extends through each of the fin stacks 10, the tubes 12 in each array spaced apart from each other along the length direction of the fins 11. In order to fit the fins 11 to the respective tubes by an interference fit between apertures in the fins 11 and the outer surface of the tubes 12, the tubes 12 are initially provided with smaller outer diameters than the diameters of the apertures through the fins 11 that they pass through, and subsequently the tubes 12 undergo an expansion process, whereby a mandrel 50 is forced down each tube 12 to expand its outer diameter to an extent that the tube 12 then forms an interference fit with the aperture in the fin 11. This expansion process places a large compressive force on the tube 12 along its length (in the stacking direction S). Depending on the geometry of the tubes 12, the material they are constructed from, the amount of support the tubes 12 are provided with, and the stress applied by the mandrels 50, the tube 12 may or may not buckle under the compression from the mandrels 50. In Fig. 1 A, the expansion tool comprises sidewalls 51 on the outer sides of the fin stacks 10 that abut the fin stacks 10 and, through the contact between the fins 11 and the tubes 12, provides support for the tubes 12 as they undergo expansion to reduce the likelihood of the tubes 12 buckling outwards in the direction of the adjacent sidewall 51. However, the sidewalls 51 do not provide any support for the tubes 12 in the inward direction to prevent the tubes 12 from buckling towards each other (i.e. away from the adjacent sidewall 51). Because the fins 11 in Fig. 1A are planar, thin, and lie generally parallel to each other, the adjacent fin stacks 10 also provide little support for the tubes 12 in the inward direction during expansion. Fig. 1B provides a schematic of an assembly of fins 11, 11’ and tubes 12, 12’ like that in Figure 1A. However, in the case of Fig. 1B, the right hand tube 12’ has buckled towards the left hand tube 12 due to the compression applied by the mandrel 50 and the lack of support for the right hand tube 12’ in the inward direction towards the adjacent left hand tube 12. As discussed above, the fin stacks 10, 10’ provide little resistance to buckling and Fig. 1B illustrates how the adjacent fin stacks 10, 10’ can simply mesh in the width direction W as the tube 12’ buckles. After a tube 12’ of the heat exchanger 1 has bucked and adjacent fins 11, 11’ have meshed, the heat exchanger 1 is typically considered scrap due to the increase in pressure drop across the external surface of the heat exchanger 1 (due to the fins 11 meshing and disrupting the airflow path across the fins 11) and because the structural integrity of the buckled tube 12’ may be compromised. The likelihood of tubes 12, 12’ buckling during expansion is increased as the tubes 12, 12’ are made smaller in diameter and as the wall thickness of the tubes 12 is reduced, because the radius of gyration of the tubes decreases and thus the slenderness ratio of the tube increases. Figure 2 is a schematic of an assembly of fins 11,21 and tubes 12, 22 for a fin and tube heat exchanger 1 according to the present disclosure, which has been configured to increase the resistance to tube buckling and fin stack meshing. The assembly comprises a first fin stack 10 comprising a plurality of first fins 11 stacked in the stacking direction S, and a second fin stack 20 comprising a plurality of second fins 21 also stacked in the stacking direction S. Similar to the assembly in Fig. 1A and 1B, an array of first tubes 12 extends through the first fin stack 10 in the stacking direction S, the first tubes 12 spaced apart from each other along the length direction L of the first fins 11 and an array of second tubes 22 extends through the second fin stack 20 in the stacking direction S, the second tubes 22 spaced apart from each other along the length direction L of the second fins 21. The first fin stack 10 is positioned adjacent the second fin stack 20 in the width direction W of the first fins 11 and second fins 21, such that the array of first tubes 12 is substantially parallel to the array of second tubes 22 but spaced apart in the width direction W of the first fins 11 and second fins 21. The fins 11,21 have a length, width and thickness, the thickness of a fin 11,21 being substantially less than the length and width of the fin 11,21, and the width being substantially less than the length of the fin 11,21. As illustrated in Figure 2, the first fins 11 are substantially planar, with their length and width extending in a plane containing the length L and width W directions of the heat exchanger 1. In contrast, a set 23 of the second fins 21 are bent (i.e. non-planar) such that each second fin 21 of said set 23 crosses over one or more of the first fins 11 in the stacking direction S between outermost (i.e. in the length direction L) first tubes 12 of the array of first tubes 12 and between outermost (i.e. in the length direction L) second tubes 22 of the array of second tubes 22. Each second fin 21 of said set 23 also crosses back over said one or more of the first fins 11 in the stacking direction S. As illustrated in Figure 2, each second fin 21 is bent into a curved shape and the radius of curvature of the bend in the second fins 21 increases from a first end 30 of the heat exchanger 1 to a second end 40 of the heat exchanger 1 such that the closer a second fin 21 of said set 23 is to the second end 40 of the assembly 1, the fewer first fins 11 said second fin 21 crosses over in the stacking direction S. That is, a second fin 21 of said set 23 proximate the first end 30 of the heat exchanger 1 crosses over a greater number of first fins 11 in the stacking direction S than a second fin 21 of said set 23 proximate the second end 40 of the heat exchanger 1. In this way, the first fins 11 and second fins 21 may be closer to planar alignment (i.e. closer to lying in the same plane as each other) adjacent the second end 40 of the heat exchanger 1 than the first end 30 of the heat exchanger 1. The increase in the radius of curvature of the bend in the second fins 21 towards the second end 40 of the heat exchanger 1 means that the four second fins 21 closest to the second end 40 of the heat exchanger 1 do not cross over any first fins 11 in the stacking direction S. As a result of the set 23 of second fins 21 crossing over first fins 11 in the stacking direction S, resistance against the first fin stack 10 meshing with the second fin stack 20 is provided, because the crossing of the first fins 11 with the second fins 21 in the stacking direction S between outermost first and second tubes 12, 22 prevents such meshing. This in turn provides increased support to the tubes 12, 22 when forces and / or moments are applied to them, such that the tubes 12, 22 are more resistant to buckling. Accordingly tubes 12, 22 having a lower moment of inertia (e.g. a smaller outer diameter and / or thinner walls) can be used, thereby reducing material usage in the tubes and / or reducing the tube 12, 22 diameter. A reduction in the internal tube diameter reduces the internal volume of the tubes 12, 22. This can be particularly advantageous where the fluid flowing through the inside of the tubes 12, 22 is hazardous (e.g. toxic, flammable), as it is typically desirable to reduce the volume of such a fluid contained within the heat exchanger. Finally, as illustrated in Figure 2, adjacent pairs of first tubes 12 are connected together to facilitate fluid flow therebetween by hairpin tubes 14, and similarly adjacent pairs of second tubes 22 are connected together to facilitate fluid flow therebetween by hairpin tubes 24. Figure 3 is a schematic cross section of the assembly 1 of fins 11,21 and tubes 12, 22 along the section A - A’ in Figure 2. Figure 3 further illustrates how the crossing over of the second fins 21 in said set 23 with the first fins 11 prevents the first fin stack 10 meshing with the second fin stack 20 in the width direction when the stacks are pressed together. Figure 3 also further illustrates how the second fins 21 positioned closer to the second end 40 of the assembly 1 have a greater radius of curvature, and therefore either do not cross over a first fin 11 in the stacking direction S, or cross over fewer first fins 11 in the stacking direction S. Figure 4 is a schematic of a heat exchanger unit 100 comprising the heat exchanger 1 of Figure 2. The heat exchanger unit 100 comprises a tube-side fluid loop, the tube side fluid loop passing through the first tubes and second tubes of the heat exchanger 1 discussed with reference to Figures 2 and 3. The tubeside fluid loop is charged with a refrigerant that is used as the working fluid in the heat exchanger unit 100. The heat exchanger unit 100 is configured for air to be the fin-side fluid by comprising a fan 2 configured to pass air over the outer surface of the fin and tube heat exchanger 1. The heat exchanger 1 and fan 2 are assembled together in a heat exchanger unit housing 4. In order to best use the space withing the housing 4, the heat exchanger 1 is bent in a direction substantially parallel to the width direction of the first fins and second fins of the heat exchanger 1 to define a first end 30 of the heat exchanger 1 and a second end 40 of the heat exchanger 1. The bend is through an angle of about 90 degrees, and the position of the bend is such that the first end 30 of the heat exchanger 1 is shorter than the second end 40 of the heat exchanger 1, In this way, the heat exchanger 1 can be positioned within the housing 4 such that the first end 30 of the heat exchanger 1 extends in a direction substantially parallel to the flow axis 3 of the fan 2 and, where the fan 2 comprises a casing at least partially circumscribing the flow axis, the first end 30 of the heat exchanger 1 extends along an outside of the casing. In this way, whilst the space within the heat exchanger unit 100 is being used efficiently by bending the heat exchanger 1, the flow from the fan 2 is still directed towards the major portion of the heat exchanger (i.e. the longer second end 40 of the heat exchanger 1). As illustrated in Fig. 4, the first end 30 of the heat exchanger 1 is the end where the second fins cross over a greater number of first fins in the stacking direction, whilst at the second end 40 of the heat exchanger 1, the second fins cross over fewer first fins in the stacking direction. By configuring the heat exchanger unit 1 in this manner, the airflow from the fan 2 predominantly passes through the second end 40 of the heat exchanger 1 where the pressure drop across the fins is lower, because the first fins and second fins are closer to being in planar alignment. Figures 5A and 5B illustrate a fin stacking tool 200 for use in manufacturing the heat exchanger of Figure 2. The fin stacking tool 200 is configured to support the first fins 11 and second fins 21 during the expansion process such as to provide the crossing over arrangement of the second fins 21 and first fins 11 described with reference to Figures 2 and 3. The tool 200 comprises a first support surface 201 for supporting the first fin stack 10 at the first end 30 of the heat exchanger and a second support surface 202 for supporting the second fin stack 20 at the first end 30 of the heat exchanger. In the case of Fig. 5A, the second support surface 202 is inclined with respect to the first support surface 201 such as to provide the required bending of the second fins 21 such that the set 23 of second fins 21 cross over one or more first fins 11 in the stacking direction S. Examining Fig. 5A, it can be seen that the first support surface 201 is provided by a plurality of first receivers 210 spaced apart from each other in the length direction L, the upper surfaces of which are coplanar such as to provide a planar first support surface 201, upon which a first fin 11 rests. The first receivers 210 also provide first tube support surfaces 203 that are configured to support the connector tubes 14 at the ends of the first tubes 11, as illustrated in Fig. 5A. By preventing movement of the first tubes 12 in the length direction of the first tubes 12, the first tube support surfaces 203 allow a compressive force to be applied to the first tubes 12 during an expansion process from the opposite end of the first tubes 12 to that that the connector tubes 14 are connected to. The tool 200 further comprises a plurality of second receiver 211 spaced apart in the length direction L; the plurality of second receivers 211 are spaced apart from the plurality of first receivers 210 in the width direction W, as with the first fin stack 10 and second fin stack 20. In contrast to the first receivers 210, the second receivers 211 do not provide the second support surface 202 for the second fins 21. Instead, the second receivers 211 just provide a second tube support surface 204 that are configured to support the connector tubes 24 at the ends of the second tubes 21, equivalently to the first tube support surface 203. The second support surface 202 is then provided by two wedge plates 212 positioned on either side of the second receivers 211 in the width direction, as illustrated in Figure 5B. The second support surface 202 is provided by the upper surfaces of the wedge plates 212, which are positioned above the upper surfaces of the second receivers 211 in the stacking direction such that, as illustrated in Fig. 5A, the second fins 21 rest on the support surface 202 of the wedge plates 212, whilst the connector tubes 24 at the end of the second tubes 12 are supported by the second tube support surface 204 of the second receivers 211. The tool 200 in Figures 5A and 5B may be a tube expansion tube; accordingly, whilst not illustrated in Figures 5A and 5B, the tool 200 may further comprise a set of expansion mandrels configured to be forced into respective first tubes 12 and / or second tubes 22 from the second end of the heat exchanger, and may further comprise a first sidewall and a second sidewall configured to abut the first fin stack 10 and second fin stack 20, respectively, along the length direction of the first and second fins 11,21 and along the stacking direction S of the first and second fin stacks 10, 20 on opposite sides of the heat exchanger, in the same manner as the sidewalls illustrated in Figure 1 A. Accordingly, a kit for manufacturing a fin and tube heat exchanger is provided by the tool 200 illustrated in Figures 5A and 5B and a plurality of first fins 11 and second fins 21 for stacking into first fin stacks 10 and second fin stacks 20 on the tool 200. The second support surface 202 is offset in the stacking direction from the first support surface 201 such that, upon stacking the first fins 11 and second fins 21 into the first fin stack 10 and the second fin stack 20, respectively and supporting said first fin stack 10 and second fin stack 20 on the first support surface 201 and second support surface 202, respectively, each second fin 21 of a set of one or more of the second fins 21 crosses over one or more first fins 11 in the stacking direction S. The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof. While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention. For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations. Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described. Throughout this specification, including the claims which follow, unless the context requires otherwise, the word “comprise” and “include”, and variations such as “comprises”, “comprising”, and “including” will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps. It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from “about” one particular value, and / or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and / or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent “about,” it will be understood that the particular value forms another embodiment. The term “about” in relation to a numerical value is optional and means for example + / -10%.

Claims

1. A fin and tube heat exchanger, comprising:a first fin stack comprising a plurality of first fins stacked in a stacking direction;a second fin stack comprising a plurality of second fins stacked in the stacking direction;an array of first tubes extending through the first fin stack in the stacking direction, the first tubes spaced apart from each other along a length direction of the first fins; andan array of second tubes extending through the second fin stack in the stacking direction, the second tubes spaced apart from each other along a length direction of the second fins;wherein:the first fin stack is positioned adjacent the second fin stack in a width direction of the first fins and / or the second fins.a set of one or more of the second fins are configured such that each second fin of said set crosses over one or more first fins in the stacking direction between outermost first tubes of the array of first tubes and / or between outermost second tubes of the array of second tubes.

2. The fin and tube heat exchanger according to claim 1, wherein each second fin in said set is bent such as to provide the crossing of said second fin over one or more first fins in the stacking direction.

3. The fin and tube heat exchanger according to any preceding claim, wherein said set of second fins are configured such that each second fin of said set crosses back over one or more first fins in the stacking direction between outermost first tubes of the array of first tubes and / or between outermost second tubes of the array of second tubes.

4. The fin and tube heat exchanger according to any preceding claim, wherein:the second fin stack comprises a first end spaced apart from a second end in the stacking direction;the set of second fins comprises a plurality of second fins; andthe set of second fins are configured such that the closer a second fin of said set is to the second end of the second fin stack, the fewer first fins said second fin crosses over in the stacking direction.

5. The fin and tube heat exchanger according to any preceding claim, wherein the heat exchanger further comprises:a third fin stack comprising a plurality of third fins stacked in the stacking direction; and an array of third tubes extending through the third fin stack in the stacking direction, the third tubes spaced apart from each other along a length direction of the third fins.

6. The fin and tube heat exchanger according to claim 5, wherein:the third fin stack is positioned adjacent the second fin stack in the width direction of the second fins and / or a width direction of the third fins; andthe second fin stack is interposed between the first fin stack and the third fin stack.

7. The fin and tube heat exchanger according to any preceding claim, wherein:the first tubes pass through respective apertures in each of the first fins; and / orthe second tubes pass through respective apertures in each of the second fins.

8. The fin and tube heat exchanger according to claim 7, wherein:an aperture in a first fin forms an interference fit a respective first tube; and / oran aperture in a second fin forms an interference fit a respective second tube.

9. The fin and tube heat exchanger according to claim 7 or 8, wherein:a flange surrounds an aperture in a first fin, the flange abutting an adjacent first fin in the first fin stack in the stacking direction; and / ora flange surrounds an aperture in a second fin, the flange abutting an adjacent second fin in the second fin stack in the stacking direction.

10. The heat exchanger according to any preceding claim, wherein:at a first end of the first fin stack in the stacking direction, adjacent pairs of the first tubes are connected together to facilitate fluid flow therebetween; and / orat a first end of the second fin stack in the stacking direction, adjacent pairs of the second tubes are fluidly connected together.

11. A heat exchanger unit comprising a fin and tube heat exchanger according to any one of claims 1 to 10.

12. The heat exchanger unit according to claim 11, wherein:the heat exchanger comprising a tube-side fluid loop, the tube-side fluid loop passing through the first tubes and second tubes of the heat exchanger; andthe tube side fluid loop is charged with a refrigerant.

13. The heat exchanger unit according to claim 11 or 12, further comprising a fan configured to pass air over the fin and tube heat exchanger.

14. A heat exchanger unit according to claim 13, wherein:the fin and tube heat exchanger is bent in a direction substantially parallel to the width direction of the first fins and / or the second fins to define a first end of the heat exchanger at an angle to a second end of the heat exchanger.

15. The heat exchanger unit according to claim 14, wherein a flow axis of the fan is coincident with the second end of the heat exchanger.

16. The heat exchanger unit according to claim 15, wherein the first end of the heat exchanger extends in a direction substantially parallel to the flow axis of the fan.

17. A heat pump, comprising a heat exchanger unit according to any one of claims 11 to 16.

18. A method of manufacturing a fin and tube heat exchanger, the fin and tube heat exchanger comprising:a first fin stack comprising a plurality of first fins stacked in a stacking direction;a second fin stack comprising a plurality of second fins stacked in the stacking direction;an array of first tubes extending through the first fin stack in the stacking direction, the first tubes spaced apart from each other along a length direction of the first fins;an array of second tubes extending through the second fin stack in the stacking direction, the second tubes spaced apart from each other along a length direction of the second fins;wherein the first fin stack is positioned adjacent the second fin stack in a width direction of the first fins and / or the second fins;wherein the method comprises arranging a set of one or more of the second fins such that each second fin of said set crosses over one or more first fins in the stacking direction between outermost tubes of the array of first tubes and / or between outermost second tubes of the array of second tubes.

19. The method according to claim 18, wherein the method further comprises, following arranging said set of fins, affixing the second fins to the second tubes and / or affixing the first fins to the first tubes20. A kit for manufacturing a fin and tube heat exchanger, the kit comprising:a fin stacking tool; anda plurality of first fins and a plurality of second fins; and wherein:the heat exchanger comprises a first fin stack comprising the plurality of first fins stacked in a stacking direction; a second fin stack comprising the plurality of second fins stacked in the stacking direction;the tool comprises a first support surface to support the first fin stack at a first end of the heat exchanger and a second support surface to support the second fin stack at the first end of the heat exchanger; andat least a portion of the second support surface is offset in the stacking direction relative to the first support surface such that, upon stacking the first fins and second fins into the first fin stack and the second fin stack, respectively and supporting said first fin stack and second fin stack on the first support surface and second support surface, respectively, each second fin of a set of one or more of the second fins crosses over one or more first fins in the stacking direction.

21. The kit according to claim 20, wherein:the kit further comprises a plurality of first tubes and a plurality of second tubes;the heat exchanger comprises an array of the first tubes extending through the first fin stack in the stacking direction, the first tubes spaced apart from each other along a length direction of the first fins,and an array of the second tubes extending through the second fin stack in the stacking direction, the second tubes spaced apart from each other along a length direction of the second fins; andat least a portion of the second support surface is offset in the stacking direction from the first support surface such that, upon the tool supporting said first fin stack and second fin stack with the array of first tubes and / or array of second tubes extending therethrough, respectively, each second fin of said set crosses over the one or more first fins in the stacking direction between outermost first tubes of the array of first tubes and / or between outermost second tubes of the array of second tubes.

22. The kit according to claim 21, wherein:the kit further comprises a plurality of connector tubes configured to fluidly connect adjacent pair of the first tubes and / or the second tubes; andthe tool further comprises a plurality of tube support surfaces configured to support the connector tubes at the first end of the heat exchanger when the connector tubes are fluidly connected to adjacent pairs of the first tubes and / or second tubes.

23. The kit according to claim 22, wherein the tool further comprises a set of expansion mandrels configured to be forced into respective first tubes and / or second tubes from a second end of the heat exchanger.

24. The kit according to claim 23, wherein the tool further comprises a first sidewall and / or second sidewall configured to abut the first fin stack and / or second fin stack, respectively, along the length direction of the first and / or second fins and the stacking direction first and / or second fin stack on opposite sides of the heat exchanger.

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