WO2002103262A1

WO2002103262A1 - Brazed heat transfer element

Info

Publication number: WO2002103262A1
Application number: PCT/GB2002/002503
Authority: WO
Inventors: Andrew John Rothwell; Bengt Viklund; Göte BERGGREN; Remo Prato
Original assignee: Ti Group Automotive Systems Limited
Priority date: 2001-06-15
Filing date: 2002-06-14
Publication date: 2002-12-27
Also published as: GB0114579D0; EP1399699A1

Abstract

A heat transfer element assembly (501) for use within a refrigeration system comprising tubing (502) brazed to aluminium sheet (503). In a preferred embodiment the tubing (502) comprises an oblong cross-section and further comprises a serpentine shape. In alternative embodiments the tubing (502) and/or the aluminium sheet (503) are fabricated from an aluminium alloy. A method of manufacturing a heat transfer element assembly (501) is also disclosed.

Description

Brazed Heat Transfer Element

Field of the Invention The present invention relates to a brazed heat transfer element for use within a refrigeration system, a refrigeration unit comprising an evaporator and a method of manufacturing a refrigeration unit comprising an evaporator.

Introduction

Refrigeration systems operate to maintain a temperature below the temperature of the surroundings within a refrigeration cavity in which, for example, food may be preserved. Such a refrigeration system may be found in a domestic refrigeration unit, for example, a domestic refrigerator or a domestic freezer. In such a refrigeration unit, heat is absorbed from the refrigeration cavity by means of an evaporator and is rejected to the surrounding environment by means of a condenser.

Refrigeration systems comprising one or more heat transfer elements are known, for example within a vapour-compression refrigeration system. According to performance, efficiency and design specifications, heat transfer elements may be mounted within the refrigeration cavity or within the walls defining the refrigeration cavity within a refrigeration unit. For example, a refrigeration unit may comprise a visible evaporator contained within the refrigeration cavity to maximise performance and efficiency, or alternatively for aesthetic reasons, a refrigeration unit may comprise a hidden evaporator contained within the refrigeration cavity walling. Assembly of a hidden refrigeration evaporator within a refrigeration unit typically comprises the steps of mounting the hidden evaporator within the refrigeration cavity walling before injecting foam into the refrigeration cavity walling to seal in the hidden evaporator. For maximised performance, the hidden evaporator is position immediately behind the inner refrigeration cavity walling at the rear of the refrigeration cavity.

A disadvantage with foaming around a refrigeration evaporator is that any contours or indentations associated with the refrigeration evaporator may become visible within the inner refrigeration cavity walling. This effect is clearly undesirable for commercial reasons.

To achieve the desired aesthetic quality of an assembled hidden refrigeration system, hidden evaporators are required to have one side that will remain flat and rigid during installation. A refrigeration evaporator that can function either as a hidden or a visible evaporator is required to have one side that will remain flat and rigid during construction and operation of the refrigeration unit.

Brief Summary of the Invention

According to a first aspect of the present invention, there is provided an evaporator for a refrigeration system comprising: tubing having an oblong cross-section and formed into a shape having at least one substantially planar face; and an aluminium sheet, wherein said substantially planar face of said tubing is brazed onto a side of said aluminium sheet. According to a second aspect of the present invention, there is provided a heat transfer element comprising tubing brazed to aluminium sheet wherein said tubing comprises an oblong cross-section. Oblong is herein defined as a geometrical shape being longer than broad. The preferred oblong cross-section is a geometrical shape being longer than broad, having substantially parallel long plane sides and rounded ends. The present invention provides a heat transfer element that is suitable for use as a visible or as a hidden heat transfer element.

In a preferred embodiment, the tubing is fabricated from an aluminium alloy. In a second preferred embodiment, the aluminium sheet is fabricated from an aluminium alloy. According to the preferred embodiment, the tubing is fabricated from a first aluminium alloy series and the aluminium sheet is fabricated from a second aluminium alloy series such that the second aluminium alloy series is sacrificial to the first aluminium alloy series.

In the preferred embodiment, the tubing comprises an oblong cross- section providing an increased contact area between the tubing and the aluminium sheet.

According to the preferred method of manufacture, the tubing is furnace brazed to the aluminium sheet. The aluminium sheet is preferably received clad with solder, more preferably clad with solder and flux. The invention will now be described by way of example only, with reference to the accompanying drawings in which;

Figure 1 illustrates a refrigeration unit incorporating a visible evaporator;

Figure 2 illustrates a section of a refrigeration evaporator manufactured by a prior art roll bond process; Figure 3 illustrates a section of an evaporator manufactured by a prior art one side flat roll bond process;

Figure 4 shows a plan view of a section of a prior art tube on plate evaporator manufactured by a clinching technique, partially surrounded by foam;

Figure 5 shows a heat transfer element assembly according to the present invention;

Figure 6 shows a simplified cross-section of a refrigerator incorporating on the rear wall of the refrigeration cavity, a heat transfer element assembly according to the present invention mounted as a visible evaporator;

Figure 7 shows a simplified cross-section of a refrigerator incorporating within the rear wall of the refrigeration cavity a heat transfer element assembly according to the present invention mounted as a hidden evaporator;

Figure 8 shows tubing which can be used in the manufacture of a heat transfer element assembly according to the present invention formed into a desired shape;

Figure 9 shows tubing which can be used in the manufacture of a heat transfer element assembly according to the present invention, comprising the preferred oblong cross-section and formed into a desired shape;

Figure 10 shows a coil of aluminium sheet which may be used in the manufacture of a heat transfer element assembly according to the present invention; Figure 11 details a first method of manufacturing a heat transfer element assembly according to the present invention; Figure 12 details a second method of manufacturing a heat transfer element assembly according to the present invention;

Figure 13 shows a pre-brazed assembly of a heat transfer element assembly according to the present invention; Figure 14 illustrates a section of tubing comprising the preferred oblong cross-section brazed to aluminium sheet;

Figure 15 illustrates a section of tubing comprising a circular cross- section brazed to aluminium alloy sheet;

Figure 16 shows a refrigeration unit in the process of defrosting; Figure 17 illustrates second generation ice build up around a section of an evaporator manufactured by a prior art clinching technique, comprising tubing comprising a circular cross-section;

Figure 18 illustrates second generation ice build up around a section of a heat transfer element assembly according to the present invention mounted as a refrigeration evaporator, manufactured with tubing comprising the preferred oblong cross-section;

Figure 19 illustrates galvanic corrosion of a heat transfer element assembly according to the present invention comprising tubing having a circular cross-section; Figure 20 illustrates galvanic corrosion in the aluminium sheet of a heat transfer element assembly according to the preferred embodiment of the present invention.

A preferred embodiment and associated methods of manufacture according to the present invention will now be described by way of example only with reference to the accompanying drawings identified above. Figure 1

Figure 1 illustrates a refrigeration unit 101 incorporating a refrigeration evaporator 102. Refrigeration evaporator 102 is mounted to the rear wall of inner refrigeration cavity walling 103 within the refrigeration cavity

104, so that one side of the refrigeration evaporator 102 is visible within refrigeration cavity 104, when refrigeration unit door 105 is open. Refrigeration cavity 104 is typically used to temporarily store and preserve perishable items, for example, consumable food goods. Refrigeration unit 101 is fitted with removable shelves 106 and 107 within the refrigeration cavity 104 to maximise the available storage space. According to the design of the refrigeration unit 101, removable shelves 106 and 107 may be able to contact the visible face of the refrigeration evaporator 102, although contact is undesirable for hygiene reasons.

Figure 2

A known method of manufacturing a refrigeration evaporator is by a roll bond process. A section of a refrigeration evaporator 201 manufactured by a standard roll bond process is shown in Figure 2. A first aluminium alloy sheet 202 has on one side a pattern, typically comprising graphite and applied by a silk screen process. A second aluminium alloy sheet 203, having a similar composition to the first aluminium alloy sheet 202 is then layered on to the one side of the first aluminium alloy sheet 202 comprising the aforementioned pattern. This pattern prevents the mechanical bonding of the first aluminium alloy sheet 202 and the second aluminium alloy sheet 203, in the areas it covers, during processing. The layered panel is hot rolled and cold rolled, resulting in areas 204, 205, 206, and 207 where the first aluminium alloy sheet 202 and the second aluminium alloy sheet 203 are mechanically bonded.

The bonded sheet panel is then inflated under high pressure, forcing the areas of the first aluminium alloy sheet 203 and the second aluminium alloy sheet 203 that are not bonded to contour outwards, resulting in the formation of hollow channels 208, 209 and 210. The resulting refrigeration evaporator is suitable for use as a visible refrigeration evaporator. Figure 3 A refrigeration evaporator suitable for use as a hidden refrigeration evaporator can be manufactured by a roll bond process. Figure 3 illustrates a section of a refrigeration evaporator 301 manufactured by a one side flat roll bond process. This process is similar to the previously described standard roll bond process, but differs in that during inflation of the bonded sheet panel, one side is retained flat. In order to achieve this, a first sheet 302, which is required to remain flat during inflation of the bonded sheet panel, is manufactured from an alloy having greater rigidity characteristics than the alloy from which a second sheet 303 is manufactured.

In addition, the described standard one side flat roll bond process requires specific inflation press equipment. Due to these differences, the described standard one side flat roll bond process is significantly more expensive than the previously described standard roll bond process.

Figure 4 Figure 4 illustrates an alternative method, known as tube on plate, of manufacturing a refrigeration evaporator having one flat side. A section of a typical tube on plate refrigeration evaporator 401 is shown in plan view in Figure 4. Aluminium sheet 402 comprises pre-positioned protrusions 403,404,405 and 406. These protrusions function to mechanically secure tubing 407 to aluminium sheet 402. Protrusions 403 and 404 are shown in the initial undeformed state. Protrusions 405 and 406 are shown in the final position around tubing 407, following mechanical formation around tubing 407. This tube on plate technique, known as clinching, is a less expensive manufacturing process than the previously described standard one side flat roll bond process. One problem associated with the described prior art clinching technique is that metal to metal contact between tubing 407 and aluminium sheet 402 occurs only in the regions protrusions are clinched. Metal to metal contact between the components of a heat transfer element assembly is desired to maximise performance efficiency. A further disadvantage associated with this technique is that handling of the refrigeration evaporator 401 prior to installation is that clinched protrusions 405 and 406 may loosen. This is a particular problem if refrigeration evaporator 401 is to mounted as a hidden refrigeration evaporator. During installation, foam 408 is injected, typically at high pressure, into the area surrounding refrigeration evaporator 401. Foam 408 can penetrate region 409 between aluminium sheet 402 and tubing 407 within the mechanical fixing provided by clinched protrusion 405. Clearly this problem is amplified if the mechanical fixing has been loosened by handling, resulting in an enlarged region 409. As foam 408 expands on cooling within region 409, the tubing 407 will become insulated. This effect deteriorates the performance of refrigeration evaporator 401 by restricting heat transfer from aluminium sheet 402 to tubing 407.

Similarly, if refrigeration evaporator 401 is mounted as a visible evaporator, region 409 is exposed to penetrating water, which on freezing will insulate tubing 405. This effect deteriorates the performance of refrigeration evaporator 401 by restricting heat transfer from aluminium sheet 402 to tubing 407. Furthermore, penetration of any substance in region 409 can further loosen the mechanical fixing provided by protrusion 405, which can result in the loss of contact between tubing 407 and aluminium sheet 402.

Alternatively, tubing may be secured to aluminium sheet by use of an adhesive. However, adhesives are generally insulating and may deteriorate the performance of the refrigeration evaporator as described previously. Non- insulating adhesive, for example a silver-based adhesive, is available but this type of adhesive is expensive and to minimise waste on application of the adhesive, it is advantageous to use a machine application, which further increases the cost of manufacture of the heat transfer element.

A second alternative method of securing tubing to aluminium sheet is spot welding. However, this requires monitoring of the spacing and quality of the spot welds, thus increasing the expense of the process. Furthermore, as indicated with reference to the described clinching technique, spot welding provides intermittent metal to metal contact between the tubing and the aluminium sheet. A substantially continuous contact is desired for efficient heat transfer.

The present invention provides a heat transfer element that may be manufactured for use as a condenser or an evaporator. Figure 5

Figure 5 shows a heat transfer element assembly 501 according to the preferred embodiment of the present invention. Heat transfer element assembly 501 comprises tubing 502 fabricated aluminium alloy, preferably AA1070 series alloy, comprising an oblong cross-section, and further comprises aluminium sheet 503, fabricated from aluminium alloy, preferably AA3003 series alloy. Tubing 502 is fixedly secured to aluminium sheet 503 by means of a furnace brazing process, more preferably a controlled atmosphere furnace brazing process. A furnace brazing process advantageously provides a substantially continuous metal to metal contact between tubing 502 and aluminium sheet 503.

In an alternative embodiment, tubing 502 is fabricated from aluminium. According to an alternative embodiment of the present invention, tubing 502 is a 1000 series aluminium alloy, preferably a 1070 series aluminium alloy. In a further alternative embodiment aluminium sheet 503 is fabricated from aluminium. In a further alternative embodiment, aluminium sheet 503 is fabricated from a 3000 series alloy, preferably a 3003 series alloy. In a further alternative embodiment, aluminium sheet 503 is fabricated from a 7000 series alloy. According to a further alternative embodiment tubing 502 is fabricated from any material, comprising any cross-section and is brazed to aluminium sheet 503.

Referring to Figures 2 and 3, showing heat transfer elements manufactured by a described prior art standard roll bond process, differences in the residual strengths of batches of materials, and in the pressure of inflation of bonded sheet panels, can result in an internal volumetric difference of approximately +/- 10% between individual heat transfer elements.

It is therefore necessary to determine and introduce an amount of refrigerant, known as an overcharge, into each heat transfer element, by means of an accumulator effect, in order to ensure that a minimum, more preferably an optimum specification for operation of individual heat transfer element is achieved.

The internal volume of heat transfer element assembly 501 is, in effect, the internal volume of tubing 502, and can advantageously be more accurately determined than the internal volume of heat transfer elements manufactured by a described prior art standard roll bond process. This feature of the present invention provides a financial advantage through the reduction in refrigerant overcharge introduced into heat transfer element 501.

In addition, this feature provides for consistency of operation between individual heat transfer elements. A substantially continuous direct contact between tubing 502 and aluminium sheet 503 provides for an increase in the performance efficiency associated with heat transfer element assembly 501.

Furthermore, this direct contact advantageously eliminates the possibility of region 409 wherein a substance, more specifically foam 408 or water, can penetrate between tubing 502 and aluminium sheet 503. Thus, the present invention addresses the problem of undesired insulation of tubing

502.

In addition, a direct contact between tubing 502 and aluminium sheet

503 formed during brazing increases the rigidity of heat transfer element assembly 501 This is advantageous in reducing the risk of damage to heat transfer element assembly 501 during transportation, installation or operation. For example, if heat transfer element assembly 501 is installed as a visible evaporator, there is a possibility that aluminium sheet 503 may become damaged during cleaning, or for example undesired contact with removable shelves 106, 107. According to the preferred embodiment, tubing 502 and aluminium sheet 503 are fabricated from aluminium alloy. Post-brazed pure aluminium is known to be soft and wavy in form, and this is expensive to correct. Aluminium alloy can advantageously be selected to provide improved post- brazed rigidity characteristics in comparison with pure aluminium. According to the preferred embodiment, tubing 502 comprises a serpentine shape 504, but may alternatively comprise any desired shape.

Figure 6

Figure 6 shows a simplified cross-section of heat transfer element assembly 501 mounted as a visible evaporator within refrigeration unit 101.

Heat transfer element assembly 501 is mounted to the rear wall of inner refrigeration cavity walling 103 within refrigeration cavity 104 such that the plane side of heat transfer element assembly 501 is visible within refrigeration cavity 104 when the refrigeration unit door 105 is open. Alternatively heat transfer element assembly 501 may be mounted as a hidden evaporator within refrigeration unit 101 shown in simplified cross- section in Figure 7. Heat transfer element assembly 501 is concealed with the rear wall of inner refrigeration cavity walling 103 of refrigeration cavity 104 such that it is not visible when the refrigeration unit door 105 is open. Heat transfer element assembly 501 absorbs heat from within refrigeration cavity

104 the rear wall of inner refrigeration cavity walling 103. Through the flow of refrigerant 701, heat is transferred away from refrigeration cavity 104. Aluminium sheet 503 is positioned immediately behind the internal surface of rear wall 103 for maximised performance efficiency.

Energy efficiency of many utilities is important both commercially and legally. Regulations and guidelines regarding energy efficiency are increasingly introduced and enforced with the objective of providing consumers with increasingly energy efficient goods.

Hidden heat transfer elements absorb heat through inner refrigeration cavity walling 103 from one surface only and typically have lower performance efficiency than visible heat transfer elements that have an increased, exposed surface area through which heat can be directly absorbed.

Revised energy efficiency regulations are imminent, and may be sufficient to restrict the current use of hidden heat transfer elements. The present invention anticipates this possibility and provides a heat transfer element assembly 501 that can function as a hidden heat transfer element or a visible heat transfer element.

Due to the nature of the heat transfer described above, the performance efficiency of a heat transfer element assembly 501 installed as a hidden evaporator is dependant on material and physical variables. For maximised performance against cost, it is desirable to decrease the material thickness of tubing 502 and aluminium plate 503.

Decreasing the material thickness of aluminium plate 503 decreases the distance through which heat is to be transferred to tubing 502. Decreasing the material thickness of tubing 502 advantageously decreases the distance heat is to be transferred to refrigerant 701, which advantageously flows in closer proximity to refrigeration cavity walling 103 due the decreased material thicknesses.

Thus, this feature provides for an increase in performance efficiency of heat transfer element assembly 501 and for the refrigeration system comprising heat transfer element 501. This feature further provides for a reduction in the operation costs of the refrigeration unit 101 comprising the aforementioned refrigeration system.

As stated previously, aluminium alloy can be selected to provide improved post-brazed rigidity characteristics in comparison with pure aluminium. Due to the improvement in post-brazed rigidity, the thickness of tubing 502 and aluminium sheet 503 may be further reduced, resulting in performance and financial advantages.

Figure 8 Figure 8 shows tubing 502 comprising a circular cross-section, which can be used in the manufacture of heat transfer element assembly 501. A circular cross-section is herein defined as a cross-section comprising a substantially circular shape such that when the tubing 502 is assembled onto the aluminium sheet 503, there is an associated one dimensional area of contact.

Tubing 502 fabricated from aluminium, or aluminium alloy, can advantageously be easily formed into a desired shape 801 as shown in Figure 8. According to the preferred embodiment of the present invention, the desired shape 801 is a serpentine shape 504 comprising at least one substantially planar face and a serpentine shape 504 in two dimensions. Figure 9

According to the preferred embodiment of the present invention, tubing 502 comprises the preferred oblong cross-section 901 as illustrated in Figure 9. Thus, when tubing 502 is assembled onto aluminium sheet 503, there is an associated two dimensional area of contact.

An advantage of tubing 502 comprising an oblong cross-section is an increased surface area in contact with aluminium sheet 503 during the brazing process. This results in an improved fixing between tubing 502 and aluminium sheet 503. Furthermore, an increase in contact area provides for an increase in heat transfer between aluminium sheet 503 to tubing 502. In addition, an increase in contact area provides for an increase in the post- brazed rigidity of heat transfer element assembly 501. According to alternative embodiments, tubing 502 may have any shape cross-section, for example a square or a triangular cross-section.

Figure 10

Aluminium sheet 503 that can be used in the manufacture of heat transfer element assembly 501 is shown in Figure 10. According to the preferred method of manufacture, aluminium sheet 503 is supplied clad with aluminium solder 1001 , for example 4045 solder, and is further clad with flux. According to alternative methods of manufacture, aluminium sheet 503 is clad with solder, or is received without any cladding. According to a preferred method of manufacture, aluminium sheet 503 is clad with a single layer comprising aluminium solder 1001 and flux 1002. Traditionally a computer numerically coded, CNC, application is used to apply aluminium solder 1001 along the length of tubing 502 contacting aluminium sheet 503. CNC applications are pre-programmed to follow a predetermined path, in this case corresponding to the shape of tubing 502. This operation may result in the uneven or inconsistent application of aluminium solder 1001. Alternatively, a flux rod may be formed into the same shape as tubing 502 and secured into position. In these processes, tubing 502 must be accurately aligned on aluminium sheet 503. This is traditionally done using jigging. When furnace brazing, jigging will absorb heat from within the furnace and create a heat sink. It is therefore desirable to reduce the amount of jigging used to temporarily secure the assembly and consequently reduce the financial expense of, in effect, heating the jigging.

Manufacturing heat transfer element assembly 501 using aluminium sheet 503 clad with aluminium solder 1001 provides for a less time- consuming and less expensive method of aligning tubing 502 and applying aluminium solder 1001. An advantage of aluminium sheet 503 clad with aluminium solder 1001, is that the aluminium solder 1001 is automatically available to form a brazed joint around tubing 502. Therefore, the consistency and quality of a brazed joint formed from aluminium sheet 503 that is clad with aluminium solder 1001 is more reliable than a brazed joint produced from aluminium solder 1001 applied to aluminium sheet 503 using a CNC application. Furthermore, as it is not necessary to accommodate other machinery in this process the present invention provides for a less complex and less expensive jigging requirement In addition, as the position and quantity of the cladding comprising aluminium solder 1001 is predetermined, no further consideration of this element is necessary during the manufacturing process.

Figure 11

Figure 11 details a first process for manufacturing heat transfer element assembly 501 according to the preferred embodiment of the present invention. At step 1101 , tubing 502 is received, preferably in the form of a coil, is straightened and then cut, preferably with a guillotine, to the required length. Alternatively, tubing 502 may be received in the form of individual lengths. Tubing 502 is formed into a serpentine shape 504, at step 1102. At step 1103, tubing 502 is processed by a hydraulic press such that following processing, tubing 502 comprises the preferred oblong cross-section 901. At step 1104, tubing 502 is cleaned and any grease is removed. Tubing 502 is dried at step 1105 prior to assembly with aluminium sheet 503, clad with aluminium solder 1001' which at step 1106 is received in the form of a coil, cut to the desired length, preferably with a guillotine, and then levelled.

Alternatively, aluminium sheet 503 may be received in the form of individual lengths. Aluminium sheet 503 is preferably cleaned, as described above, prior to the assembly of tubing 502 with aluminium sheet 503.

At step 1107, tubing 502 is temporarily secured on aluminium sheet 503 by use of brazing jigs. This stage produces a pre-brazed assembly corresponding to step 1108. Flux is then applied to the pre-brazed assembly at step 1109. Flux is applied to the side of aluminium sheet 503 in contact with tubing 502. Flux may be applied by an electrostatic or flux solution spray application. After completion of the flux application, the pre-brazed assembly enters the brazing furnace at step 1110. The brazing furnace is preferably a continuous controlled atmosphere (nitrogen) tunnel furnace. Such a continuous controlled atmosphere tunnel furnace comprises a preheat/drying/thermal degreasing zone, a plurality of brazing zones, the number of which is dependant upon product output and product mass, and a cooling zone. Alternatively the brazing furnace may be a batch furnace. The pre-brazed assembly is passed through a brazing furnace on a production line, for example on a conveyor belt. During step 1110, tubing 502 is brazed onto aluminium sheet 503. On leaving the brazing furnace, heat transfer element assembly 501 is cooled and at step 1111 the brazing jigs are removed. If heat transfer element assembly 501 is to be mounted as a visible evaporator, the surface of aluminium sheet 503 which will be visible following installation, is coated at step 1112, to improve the aesthetic quality of heat transfer element assembly 501. At step 1113, heat transfer element assembly 501 is inspected for any defects and/or safety tested.

Figure 12

Figure 12 details a second process for manufacturing heat transfer element assembly 501. At step 1101 , tubing 502 is received in the form of a coil, straightened and cut, preferably with a guillotine, to the required length. Alternatively, tubing 502 may be supplied in the form of individual lengths. Tubing 502 is formed into a serpentine shape 504 at step 1102. Tubing 502 is processed at step 1103 such that following processing, tubing 502 comprises the preferred oblong cross-section 901. At step 1106 aluminium sheet 503, received clad with solder, is cut, preferably with a guillotine, to the desired length and levelled. Alternatively, aluminium sheet 503 may be supplied in the form of individual lengths. Tubing 502 and aluminium sheet

503 may be cleaned, as described previously, prior to assembly. Aluminium sheet 503 is loaded into an electrostatic flux application booth at step 1201 and flux is applied, following which aluminium sheet 503 is dried at step 1202. At step 1107, shaped tubing 502 is temporarily secured by use of brazing jigs to aluminium sheet 503 to form a pre-brazed assembly corresponding to step 1108. The pre-brazed assembly 1203 is then passed through a brazing furnace at step 1110. During step 1110, tubing 502 is brazed onto aluminium sheet 503. On leaving the brazing furnace, heat transfer element assembly 501 is cooled and the brazing jigs are removed at step 1111. If heat transfer element assembly 501 is to be mounted as a visible refrigeration evaporator, the outer surface of aluminium sheet 503, which will be visible following installation, is coated at step 1112. Heat transfer element assembly 501 is then inspected at step 1113 for any defects and/or safety tested.

According to alternative methods of manufacture, tubing 502 is formed into any desired shape 801.

According to a second method of manufacture, the application of flux is omitted. According to alternative methods of manufacture, aluminium sheet 503 is received clad with flux, or flux is not necessary.

According to a third alternative method of manufacture, step 1112 is omitted.

According to further alternative methods of manufacture, tubing 502 comprises any shape cross-section and is not processed.

Figure 13 A pre-brazed assembly 1301 corresponding to step 1203 is shown in

Figure 13. Tubing 502 is temporarily secured by jigging 1302 to aluminium sheet 503 which is clad with aluminium solder 1001 and flux 1002. Jigging 1302 is shown to provide a clamping effect but may be of any suitable construction, for example comprising one or more weights. Depending upon the construction of the brazing furnace used in the manufacture of heat transfer element assembly 501 , a plurality of jigs or fixtures of any type may be utilised, to increase the quality or the production rate of heat transfer element assemblies according to the present invention.

Figure 14 A section 1401 of pre-brazed assembly 1301 after brazing according to step 1110 is shown in Figure 14. Brazed joint 1402 secures tubing 502 to aluminium sheet 503, and comprises aluminium solder 1001. During brazing, aluminium solder 1001 flows into the areas surrounding the points of contact between tubing 502 and aluminium sheet 503. The flow of aluminium solder 1001 during brazing typically occurs by capillary action. The curvature of the outer surface of brazed joint 1402 between tubing 502 and aluminium sheet 503 can be seen in area 1403.

Figure 15 Figure 15 shows a section 1501 of tubing 502 comprising a circular cross-section, brazed to aluminium sheet 503. Brazed joint 1502 secures tubing 502 to aluminium sheet 503 and comprises aluminium solder 1001. During brazing, aluminium solder 1001 flows into the area of contact between tubing 502 and aluminium sheet 503. The curvature of the outer surface of brazed joint 1502 between tubing 502 and aluminium sheet 503 can be seen in area 1503. From comparison of Figure 14 and Figure 15, it can be seen that tubing 502 comprising the preferred oblong cross-section 901 provides an increased area of contact between tubing 502 and aluminium sheet 503. This increase in contact area provides for an increase in heat transfer between tubing 502 and aluminium sheet 503. Thus, this feature provides for an increase in the performance efficiency of heat transfer element assembly 501. Furthermore, an increase in contact area between tubing 502 and aluminium sheet 503 provides for an increase in the post-brazed rigidity of heat transfer element assembly 501. From comparison of Figure 14 and Figure 15, it can be observed that brazed joint 1402 differs to brazed joint 1502 in that the outer contour formed by tubing 502 comprising a circular cross-section and brazed joint 1602, shown in area 1503, is more recessed than the outer contour formed by tubing 502 comprising an oblong cross-section and brazed joint 1402, shown in area 1403. Furthermore, the diameter of tubing 502 comprising a circular cross-section extends over brazed joint 1502 further than the diameter of tubing 502 comprising an oblong cross-section extends over brazed joint 1402.

A problem with mounting a heat transfer element assembly 501 as a visible evaporator within a refrigeration unit 101 occurs when the refrigeration unit 101 is defrosted.

Figure 16

Figure 16 shows existing ice 1601 within refrigeration cavity 104. The presence of ice 1601 can deteriorate the performance of heat transfer element assembly 501 by insulating tubing 502 and aluminium sheet 503. As shown in Figure 16, existing ice 1601 inside refrigeration cavity

104, for example around heat transfer element assembly 501, melts and water 1602 can drain around the exposed surfaces of the heat transfer element assembly 501. Hence, there is a potential for water 1602 to become trapped in regions between tubing 502 and aluminium sheet 503.

Water 1602 that becomes trapped while refrigeration unit 101 is defrosted, will consequently freeze when refrigeration unit 101 returns to normal operation. Figure 17 A particular problem occurs when water 1602 becomes trapped adjacent existing ice 1601. When refrigeration unit 101 is returned to normal operation, water 1602 that is trapped adjacent existing ice 1601 freezes to form a second layer of ice 1701 as shown in Figure 17. This is known as a second generation ice build up. Figure 17 shows second generation ice build up around a section of a tube on plate refrigeration evaporator 401 manufactured by the described prior art clinching technique. Heat transfer elements manufactured by this prior art technique are particularly sensitive to ice formation, in particular, as shown, in the regions between protrusion 405 and tubing 407, which are exposed to the penetration of water 1602. Penetration of water 1602 in these regions creates a damaging effect on the mechanical fixing of tubing 407 to aluminium sheet 402 provided by protrusion 405, which as a consequence of the expansion of water 1602 during freezing, is forced away from aluminium sheet 503. Furthermore, the presence of a second layer of ice 1701 increases the detrimental insulating effect provided by existing ice 1601. The present invention addresses these problems and the further problem of the escalation of second generation ice build up. This can occur when refrigeration unit 101 is defrosted a subsequent time and any water 1601 trapped adjacent the second layer of ice 1701 and/or existing ice 1601 which has not defrosted, consequently freezes, as previously described on the return of refrigeration unit 101 to normal operation. Thus, the severity of the aforementioned problems, and the severity of second generation ice build up can escalate rapidly, further decreasing the performance efficiency of the affected heat transfer element.

Figure 18

Figure 18 shows a section of heat transfer element assembly 501 according to the preferred embodiment of the present invention installed as a visible evaporator. As indicated previously, an advantage of tubing 502 comprising an oblong cross-section is that brazed joint 1402 in section 1401 is less deeply recessed in comparison with brazed joint 1502 formed in section 1501, wherein tubing 502 comprises a circular cross-section. This feature provides for a less severe formation of existing ice 1601 , which further provides for a decrease in the detrimental insulating effect provided by existing ice 1601. Furthermore, a decrease in the formation of existing ice

1601 provides for a decrease in the time taken for existing ice 1601 to defrost. This advantageously decreases the potential for a second layer of ice 1701 to form adjacent existing ice 1601 , and further provides for a decrease in the potential of the escalation of a second generation ice build up. A second advantage provided by tubing 502 comprising the shown preferred oblong cross-section 901, shown in the preferred orientation, is an increase in the area of contact with aluminium sheet 503. Thus, the preferred embodiment of the present invention is less sensitive to the detrimental insulating effects caused by the formation of ice such as existing ice 1601 and/or a second layer of ice 1701 , due an increase in the area through which heat can be directly transferred.

A further advantage provided by the present invention is an increase in the area of direct contact between tubing 502 and aluminium sheet 503 provided by brazed joint 1502. This feature advantageously prevents the penetration of water 1602 between tubing 502 and aluminium sheet 503, and thus prevents any consequent formation of ice such as existing ice 1601 and/or a second layer of ice 1701. Thus, the present invention addresses the problem of the degradation of fixings within heat transfer element assembly 501.

Figure 19

The present invention addresses a further problem created by the formation of ice such as existing ice 1601 which can create an electrolytic cell between, for example, tubing 502 and aluminium sheet 503. As shown in

Figure 19, this can consequently initiate galvanic corrosion 1901 , shown in tubing 502. Clearly, this is an undesirable effect that can result, over time, in degradation of tubing 502. In the event of complete degradation of tubing 502, heat transfer element assembly 501 will cease to operate. Complete degradation is herein defined as degradation resulting in the loss of at least one area of a component. This may be evident, for example by leakage of refrigerant 701.

Figure 20

According to the preferred embodiment of the present invention, aluminium sheet 503 is fabricated from an aluminium alloy series which is sacrificial to the aluminium alloy series from which tubing 502 is fabricated. Thus, any galvanic corrosion 1901 resulting from the presence of ice 1601 between tubing 502 and aluminium sheet 503 will occur only in aluminium sheet 503, as illustrated in Figure 20. This is advantageous in that degradation of aluminium sheet 503 caused by galvanic corrosion 1901 will not damage the internal operation of heat transfer element assembly 501.

Claims

1. An evaporator for a refrigeration system comprising: tubing having an oblong cross-section and formed into a shape having at least one substantially planar face; and an aluminium sheet, wherein said substantially planar face of said tubing is brazed onto a side of said aluminium sheet.

2. An evaporator for a refrigeration system according to claim 1 , wherein said shape is substantially two dimensional.

3. An evaporator for a refrigeration system according to claim 1 or claim 2, wherein said shape is a serpentine shape.

4. An evaporator for a refrigeration system according to any of claims 1 to 3, wherein said aluminium sheet comprises aluminium alloy.

5. An evaporator for a refrigeration system according to any of claims 1 to 4, wherein said tubing is manufactured from a first aluminium alloy series and said aluminium sheet is manufactured from a second aluminium alloy series.

6. An evaporator for a refrigeration system according to any of claims 1 to 5, wherein said second aluminium alloy series is sacrificial to said first aluminium alloy series.

7. A refrigeration unit comprising an evaporator according to any of claims 1 to 6.

8. A refrigeration unit according to claim 7, wherein said refrigeration unit comprises an inner refrigeration cavity walling defining a refrigeration cavity, and said evaporator is located within the refrigerator cavity.

9. A refrigeration unit according to claim 7, wherein said refrigeration unit comprises an inner refrigeration cavity wall defining a refrigeration cavity, and said evaporator is located immediately behind the inner refrigeration cavity wall.

10. A method of manufacturing a refrigeration unit comprising an evaporator, said method comprising the steps of: forming tubing into a desired shape such that said tubing has an oblong cross-section and a shape having at least one substantially planar face; assembling said tubing onto aluminium sheet such that said substantially planar face of said tubing is against a side of said aluminium sheet; and brazing said assembly to form the evaporator.

11. A heat transfer element assembly comprising tubing brazed to aluminium sheet wherein said tubing comprises an oblong cross-section.

12. A heat transfer element assembly comprising aluminium tubing brazed to aluminium sheet wherein said aluminium tubing is an aluminium alloy.

13. A heat transfer element assembly comprising aluminium tubing brazed to aluminium sheet wherein said aluminium sheet is an aluminium alloy.

14. A heat transfer element assembly comprising tubing brazed to aluminium sheet wherein said aluminium sheet is clad with solder or solder and flux.

15. A method of manufacturing a heat transfer element comprising the steps of forming tubing into a desired shape, assembling said tubing onto aluminium sheet and brazing said assembly.

16. A heat transfer element assembly according to any of claims

11 , 14 or 15 wherein said tubing is aluminium.

17. A heat transfer element assembly according to any of claims 11 , 13, 14 or 15 wherein said tubing is an aluminium alloy.

18. A heat transfer element assembly according to any of claims 11, 12, 14, 15 or claims 16 or 17 dependent on claim 15 wherein said aluminium sheet is an aluminium alloy.

19. A heat transfer element assembly according to claim 18 and claim 17 dependent on any of claims 11 , 14 or 15, or claim 18 dependent on claim 12, or claim 17 dependent on claim 13, wherein said tubing is manufactured from a first aluminium alloy series and said aluminium sheet is manufactured from a second aluminium alloy series.

20. A heat transfer element assembly according to claim 19, wherein said second aluminium alloy series is sacrificial to said first aluminium alloy series.

21. A method of manufacturing a heat transfer element assembly according to claims 16 to 20 dependent on claim 15 wherein said tubing has an oblong cross-section.

22. A method of manufacturing a heat transfer element assembly according to claim 11 wherein said tubing initially has a circular cross- section and is processed to form tubing comprising an oblong cross- section.

23. A method of manufacturing a heat transfer element assembly according to claim 22 wherein said tubing is processed prior to assembly onto said aluminium sheet

24. A method of manufacturing a heat transfer element assembly according to any of claims 16 to 23 dependent on claim 15 wherein said aluminium sheet is clad with solder.

25. A method of manufacturing a heat transfer element assembly according to claim 24 wherein flux is applied to said aluminium sheet prior to the assembly of said tubing onto said aluminium sheet.

26. A method of manufacturing a heat transfer element assembly according to claim 24 wherein flux is applied after the assembly of said tubing onto said aluminium sheet.

27. A method of manufacturing a heat transfer element assembly according to claims 24 wherein said aluminium sheet is additionally clad with flux.

28. A method of manufacturing a heat transfer element assembly according to any of claims 16 to 27 dependent on claim 15 further comprising the step of applying a post-brazed coating to said heat transfer element.

29. A heat transfer element assembly according to any of claims 11 to 28 wherein said heat transfer element is an evaporator.

30. A heat transfer element assembly according to any of claims 11 to 28 wherein said heat transfer element is a condenser.

31. A heat transfer element assembly according to any of claims 11 to 30 wherein said tubing comprises a serpentine shape.

32. A heat transfer element assembly according to any of claims 11 to 31 wherein said tubing is brazed to said aluminium sheet by a furnace brazing process.

33. A heat transfer assembly according to claim 32 wherein said furnace brazing process is a controlled atmosphere furnace brazing process.

34. A refrigeration system comprising a heat transfer element according to any of claims 11, 12, 13, 14 or any preceding claim dependent on any of claims 11 , 12, 13 or 14.

35. A refrigeration system comprising a heat transfer element manufactured according to claim 15 or any preceding claim dependent on claim 15.

36. A heat transfer element assembly substantially as herein described with reference to and as shown in Figure 5.

37. A method of manufacturing a heat transfer element assembly substantially as herein described with reference to and as shown in Figure 12 or Figure 13.