AU2012200524A1

AU2012200524A1 - Heat Exchanger

Info

Publication number: AU2012200524A1
Application number: AU2012200524A
Authority: AU
Inventors: Frederick Mark Webb
Original assignee: Individual
Current assignee: Individual
Priority date: 2009-07-06
Filing date: 2012-01-31
Publication date: 2012-02-23
Anticipated expiration: 2030-07-06
Also published as: EP2549219A1; AU2017206160B2; NZ598010A; AU2012200524B2; AU2017206160A1

Abstract

A heat exchanger comprises a primary flow path arranged to contain a first heat exchanging medium. A secondary flow path 5 arranged to contain a secondary heat exchanging medium, wherein the primary flow path surrounds the secondary flow path for exchanging heat between the two paths. 10 (FIGURE 10)

Description

HEAT EXCHANGER TECHNICAL FIELD The present invention relates generally to heat exchangers methods for forming the same. More specifically, but by no 5 means exclusively, the invention relates to tubing configurations for improving heat transfer characteristics of a heat exchanger. BACKGROUND OF THE INVENTION 10 Heat exchangers can be found in many devices where cooling or heating of fluids, including liquids and gases, is required. The basic principle of any heat exchanger is to provide efficient transfer of heat from one heat exchanging material (e.g. gas, fluid, etc.) to another, without any direct 15 contact between the two. Heat exchangers are commonly found, for example, in refrigeration units, power plants, air conditioning systems, among others. One well-known type of heat exchanger is the Fin and Tube exchanger commonly found, for example, in refrigeration 20 condensers. Fin and Tube exchangers employ a plurality of inter-connected tubes positioned within, and thermally coupled to, a metal structure which is exposed to a flow of air. Often, the metal structure takes the form of a plurality of metal "fins" which run perpendicular to the inter 25 connected tubes and which serve to increase the effective surface area of the heat exchanger. Fluid circulating through the tubes gives off its heat by convection to a flow of air passing through the fins. For certain applications, the flow of air may be forced through 30 the fins by way of a fan. Clearly, the larger the heat exchanger, the larger the fan required to move the air for 1 suitably affecting suitable heat transfer. As may be appreciated by those skilled in the art, despite being well known and used, heat exchangers employing fluid carrying pipes, such as those previously described, have a number of 5 drawbacks. For example, in order to provide sufficient heat transfer for many processes, the interconnected pipes need to be many meters in length leading to the exchangers being relatively large in size when compared to the refrigeration unit (or an equivalent water cooling tower of the same heat 10 load capacity). This in turn not only limits the range of sites that the device can be installed in, but also leads to appreciable manufacturing and operational costs. SUMMARY OF THE INVENTION According to a first aspect of the present invention there is 15 provided a heat exchanger comprising: a primary flow path arranged to contain a first heat exchanging medium; and a secondary flow path arranged to contain a secondary heat exchanging medium, wherein the primary flow path surrounds the secondary flow path for exchanging heat between the two 20 mediums. In an embodiment the primary flow path is helical. In an embodiment the primary flow path is partitioned. In an embodiment the primary flow path is surrounded by one or more heat exchanging fins. 25 In an embodiment the exchanger further comprises a length of tube and wherein the secondary medium is carried within a body of the tube and the first medium is carried within a circumferential outer wall of the body. In an embodiment the tube has a circular cross section. 2 In an embodiment the exchanger further comprises an inlet manifold coupled to a first end of the tube and having a fluid path flow in fluid connection with the primary flow path of the tube for delivery of the first medium. 5 In an embodiment the exchanger further comprises an outlet manifold coupled to a second end of the tube and having a fluid path flow in fluid connection with the primary flow path for expelling the primary medium. In an embodiment the exchanger further comprises a plurality 10 of tubes and wherein the inlet and outlet manifold each comprise a manifold tube having openings defined along their length for receiving corresponding ends of the respective exchanger tubes. In an embodiment an inner surface of the circumferential wall 15 for each tube extends through the manifold tube and meets with a second opening in the manifold tube for receiving/expelling the secondary medium. In accordance with a second aspect there is provided a method of constructing a heat exchanger comprising forming a primary 20 path flow arranged to contain a first heat exchanging medium, so as to surround a secondary path flow arranged to carry a secondary heat exchanging medium. In an embodiment the primary flow path is a helical flow path. 25 In an embodiment the method further comprises forming the helical flow path by winding or extruding a length of a primary tube having a generally elongate cross section such that the length extends along a helical path. In an embodiment the method further comprises 30 winding/extruding the length of tube such that a closed outer 3 circumferential wall is formed so as to define the secondary flow path. In an embodiment the method further comprises locating an inner tube arranged to carry the secondary flow path within 5 the wound length of primary tube. In an embodiment the method further comprises coupling a first end of the primary tube to an opening in an inlet tube arranged to deliver the first medium such that the primary flow path is in fluid communication with the inside of the 10 inlet tube. In an embodiment the method further comprises coupling a second end of the primary tube to an opening in an outlet tube arranged to expel the first medium such that the primary flow path is in fluid communication with the inside of the 15 outlet tube. In an embodiment the method further comprises passing the secondary flow path through a second opening in the respective inlet/outlet tube for delivering/expelling the secondary medium. In accordance with a further aspect there 20 is provided a heat exchanger comprising one or more tubes arranged to carry a flow of a first heat exchanging medium, the first medium arranged to exchange heat with a second heat exchanging medium in thermal contact with the one or more tubes; and a flow direction control insert located within 25 each tube and operable to control flow of the first medium. In an embodiment the flow direction control inserts is operable to vary the effective path length of the conduit. It an embodiment the conduits are in the form of tubes of cylindrical cross section, although it will be understood 30 that other forms of tube or conduit are equally applicable and are not limited to being of cylindrical cross section 4 (e.g. square conduits, hexagonal conduits and the like are envisaged). In an embodiment, the flow direction control insert comprises an elongate body having an outer surface which controls the 5 flow. In an embodiment the outer surface is operable to direct the flow within the tube to increase the effective length of the tube for the purposes of heat exchange. In an embodiment, the elongate body extends the length of each tube. In an embodiment, the elongate body is in the form 10 of a helical screw. The outer circumference of the helical screw may, for example, sealingly contact an inner surface of the tube to create a helical flow channel. In an embodiment, the pitch of the helical screw is varied to adjust the effective length of the tube. Alternatively, the diameter of 15 the tube along with the diameter of the helical screw body may be varied to adjust the effective length. In an embodiment, the two heat exchanging mediums may be selected from air, steam, water, refrigerant, oil, beverage, or any combination thereof. 20 In an embodiment, the heat exchanger is one of a condenser, evaporator, cooling tower, radiator, Shell & Tube and Tube in Tube heat exchanger configuration. In an embodiment, the insert is formed from a plastic, polymer, elastomer, or rubber material. Alternatively, the insert may be formed from 25 a corrosion resilient metal or alloy, or any other suitable material. In an embodiment, each insert comprises one or more sections. The one or more sections may direct the flow in a different manner to other sections. For example, the temperature 30 difference through the first few passes (i.e. tube lengths) may be substantially greater than for the subsequent passes, allowing rapid heat transfer and thus not requiring any form 5 of insert to be implemented (although in an embodiment, an insert may well be provided depending only on the desired implementation). For the remaining passes, a helical insert as previously described may be incorporated within the tubing 5 to account for the loss in heat transfer (i.e. this will effectively reduce the speed of the circulating fluid to allow more time for the circulating fluid to contact the inner wall of the tubing). The flow direction control insert may be implemented at a section of the tubing where the 10 temperature difference is not much different from the second medium, which allows more time for heat transfer. In an embodiment the tube comprises an outer surface having one or more fins located thereon which are in thermal contact with the second heat exchanging medium. In an embodiment the 15 one or more fins are helical outer fins which wrap around the outer surface of the body. In an embodiment a pitch of the helical insert corresponds with a pitch of the helical fins. In an embodiment a plurality of helical fins are located on the outer surface having progressively staged start 20 locations. In an embodiment the tube and at least one of the helical outer fins and helical insert are extruded from a single blank. In an embodiment the tube and helical insert and/or fin are formed from a single aluminium extrusion. A heat 25 exchanger formed from such a one piece extrusion may significantly reduce construction time and cost. In accordance with a further aspect of the present invention there is provided a flow direction control insert arranged to be located inside a heat exchanger comprising a tube arranged 30 to carry a flow of a first heat exchanging medium arranged to exchange heat with a second heat exchanging medium which is in thermal contact with the tubes, whereby the flow direction control insert is operable to control flow of the first 6 medium within the tube to thereby vary the effective path length of the tube and in turn adjust the heat transfer characteristics of the heat exchanger. In accordance with another aspect of the present invention 5 there is provided a method for varying a heat transfer characteristic of a heat exchanger comprising a tube arranged to carry a flow of a first heat exchanging medium arranged to exchange heat with a second heat exchanging medium which is in thermal contact with the tube, the method comprising the 10 steps of: locating a flow direction control insert within the tube, the flow direction control insert having an outer surface which is arranged to control flow of the first medium within the tube to thereby vary the effective path length of the tube and in turn vary the heat transfer characteristic. 15 In accordance with yet another aspect of the present invention there is provided a method of forming a heat exchanger comprising extruding a length of heat transmissive material through a die so as to form a tube having one or more helical fins which extend around an outer surface of the 20 tube, the tube arranged to carry a flow of a first heat exchanging medium, the first medium arranged to exchange heat with a second heat exchanging medium in thermal contact with the one or more helical fins. In an embodiment the method further comprises extruding the 25 length of heat transmissive material to form a flow direction control device within the tube, the flow direction control device having an outer surface which is arranged to control flow of the first medium within the tube to thereby vary the effective path length of the tube and in turn vary the heat 30 transfer characteristic. In accordance with a still further aspect of the present invention there is provided a method of forming a heat exchanger comprising extruding a length of heat transmissive 7 material through a die so as to form a tube having an inner surface in which is defined a flow direction control device, the device arranged to control the flow of a first heat exchanging medium arranged to be passed through the tube and 5 which medium is arranged to exchange heat with a second heat exchanging medium in thermal contact with an outer surface of the tube . In an embodiment the flow direction control device comprises a helical screw as described in accordance with the first aspect. 10 In an embodiment the heat transmissive material is aluminium. In accordance with a sixth aspect of the present invention, there is provided a method of improving a heat transfer characteristic of an existing heat exchanger comprising a tube arranged to carry a flow of a first heat exchanging 15 medium arranged to exchange heat with a second heat exchanging medium which is in thermal contact with the tube, the method comprising the steps of: locating a flow direction control insert within the tube, the flow direction control insert having an outer surface which is arranged to control 20 flow of the first medium within the tube to thereby increase the effective path length of the tube and in turn improve the heat transfer characteristics. In an embodiment the method could be used to adapt existing exchangers. 25 In an embodiment, the flow direction control insert comprises an elongate body and has the characteristics as previously described with reference to a first and/or second aspect. According to a further aspect of the present invention there is provided a tube for a heat exchanger, the tube arranged to 30 carry a flow of a first heat exchanging medium, the first medium arranged to exchange heat with a second heat exchanging medium in thermal contact with the tube, and a 8 flow direction control device located within the tube and operable to control flow of the first medium. In an embodiment the flow direction control device is integrally formed with the tube. In an alternative embodiment 5 the device is provided as a separate removably coupled insert. In accordance with an eighth aspect of the present invention there is provided a method for varying a heat transfer characteristic of a heat exchanger comprising a tube arranged 10 to carry a flow of a first heat exchanging medium arranged to exchange heat with a second heat exchanging medium which is in thermal contact with the tube, the method comprising controlling a direction of the flow of the first heat exchanging medium within the tube so that it flows a greater 15 distance than the tube length. It should be appreciated from the above description that according to at least certain aspects there may be provided an improved heat exchanger design including modified tube design, lower mass and overall dimensions, modified methods 20 to assemble the exchanger (or retro-fit an existing heat exchanger) using flow direction control techniques that are operable to vary the effective length of the heat exchanger tubing. The advantages which should be apparent to those skilled in the art may include an increased heat transfer 25 efficiency, lower manufacturing and running costs through reduced materials, reduced power consumption, simplified installation and the ability to cost effectively retrofit an existing exchanger for improving fluid transfer characteristics. 30 9 BRIEF DESCRIPTION OF DRAWINGS Features and advantages of the present invention will become apparent from the following description of embodiments thereof, by way of example only, with reference to the 5 accompanying drawings, in which: Figure 1 is a schematic of a heat exchanger assembly illustrating installation of a flow direction control insert, in accordance with an embodiment of the present invention; Figures 2a and 2b are sectional top and side elevation views, 10 respectively, of a heat exchanger employing a flow direction control insert, in accordance with an embodiment of the present invention; Figure 3 is a schematic of a helical flow direction control insert, in accordance with an embodiment of the present 15 invention; Figure 4 is a perspective view showing hidden detail of the Figure 1 heat exchanger embodiment; Figure 5 is a process flow diagram showing method steps for varying heat transfer characteristics of a heat exchanger, in 20 accordance with an embodiment of the present invention; Figures 6 and 7 show heat exchanger configurations pre and post insertion of flow direction control insert in a SKOPE 2 door drink merchandising cabinet refrigeration unit model No SK650-C a for test, in accordance with an embodiment of the 25 present invention; Figure 8 is a graph showing test results for the Figures 6 and 7 configurations; Figure 9 is a schematic of a tube carrying a helical flow direction control insert in accordance with a further 30 embodiment of the present invention; 10 Figure 10 is a schematic of an exchanger tube in accordance with yet a further embodiment of the present invention; Figure 11 is a detailed view of section A identified in Figure 10; 5 Figure 12 is an exploded isometric view of a single exchanger tube and manifold, in accordance with an embodiment; Figure 13 is a partially assembled front view of the exchanger tube and manifold of Figure 12; Figure 14 is an assembled isometric view of an exchanger 10 including an inlet and outlet manifold connected to five exchanger tubes, in accordance with an embodiment; Figure 15 is a front view of Figure 14 with a section of the inlet/outlet manifolds shown in hidden detail, illustrating fluid flow directions within the exchanger; 15 Figures 16 and 17 are isometric and end views respectively of an exchanger tube in accordance with another embodiment of the present invention; Figure 18 is a schematic of an exchanger tube in accordance with another embodiment of the present invention; 20 Figure 19 is a detailed view of section A identified in Figure 18; and Figures 20a, 20b and 20c are various schematic views of an extruded length of micro-channel incorporating fins for forming a heat exchanger tube, in accordance with an 25 embodiment. 11 DETAILED DESCRIPTION OF EMBODIMENTS In the following description, for the purpose of illustration only, embodiments of the present invention are described in the context of a heat exchanger for a refrigerator, and more 5 particularly to the tube configuration of the refrigerator's condensing unit. It will be appreciated, however, that embodiments may be implemented for any form of heat exchanger which employs one or more tubes utilised to transfer heat from one medium to another. For example, embodiments could be 10 implemented for small scale applications (such as the refrigeration application described herein) right through to large scale industrial applications including, for example, radiator panels for cooling towers. It should also be appreciated that many of the referenced figures are not to 15 scale, and only serve to conceptually illustrate the various heat exchanger components and interactions between those components for achieving improved heat transfer and condensation draining characteristics when compared to conventional exchanger designs. 20 With reference to Figure 1 there is shown a heat exchanger in accordance with a first embodiment of the present invention. As mentioned above, the heat exchanger is in the form of a fin and tube-type exchanger for a refrigeration condensing unit. 25 According to the first embodiment, the heat exchanger 1 comprises a plurality of tubes 2 which are arranged to carry a flow of a first heat exchanging medium in the form of a refrigerant (e.g. such as R134A-R410, R22, R404A refrigerant that are particularly suited for refrigeration applications). 30 The tubes 2 extend through, and are in thermal contact with, a plurality of stacked fins 3 which are in perpendicular alignment to the tubes 2. As persons skilled in the art will appreciate the configuration of the tubes 2 and fins 3, act 12 to transfer heat from the refrigerant circulating through the pipes to a second medium to thereby cool the refrigerant. In the illustrated embodiment the second medium is air which absorbs the heat from the refrigerant thereby allowing it to 5 cool, condense and turn into a liquid before being recycled to an expansion device and an evaporator unit of the refrigerator. At the bottom left hand section of Figure 1 there is shown a flow direction control insert 4 which is arranged to be 10 located within each tube (as shown in partial hidden detail in the right most tube 2c) and operable to control flow direction of the first medium through the tube to thereby vary the effective path length of the tube. With additional reference to elevation views shown in Figures 2a and 2b and 15 3, the flow direction control inserts are in the form of helical screws 4 that effectively extend the length of each tube (and in turn improve the heat transfer characteristics as will be described in subsequent paragraphs). In the illustrated example, the screws are made of a deformable 20 rubber and are sized such that outer circumference of each helical rib 4a is in direct contact with an inner surface of the tube to thereby form a flowpath (denoted in the drawings as a "gas channel") that serves to increase the effective length of the tube 2. This is best shown in Figure 2a. While 25 in the illustrated example the ribs 4a of the helical screw 4 sealingly engage the tube's inner surface (i.e. an outer edge 5 of each rib 4a is arranged in an interference fit with an inner surface 6 of the tube), in other embodiments the ribs may not extend right the way thereto. According to such an 30 alternative embodiment, the insert 4 may still serve to vary the effective path length, albeit not to the same extent as where they extend right the way. It will be understood that different helical screw configurations and dimensions will have an effect on the extent of the flow path variance. For 35 example, different capacity units will require different size 13 chambers to allow correct flow. Different capacities may be achieved by means of increasing pipe and helical screw diameter and increasing/decreasing the inner diameter (shank) of the helical screw. The helical screw pitch will also 5 adjust the effective length of the flow path; the smaller the pitch of the screw, the longer the effective flow path of the chamber. Furthermore, it will be appreciated that the helical screw may not have a shank but instead be in the form of a spring made from flat rather than a round section. 10 A method of forming a heat exchanger panel in accordance with the first embodiment of the present invention will now be described with additional reference to the flow diagram 500 of Figure 5. With reference to Figure 5 (section A) , a conventional fin 15 and tube heat exchanger is manufactured from a plurality of fins with holes punched evenly, the quantity of which is commensurate with the heat load for the design of the condensing unit. Loose fitting tubes are then inserted through the punched holes and expanded so that the tube is a 20 tight fit in the punched holes (step 502). At step 504, a flow direction control insert in the form of a helical screw is inserted into one or more of the tubes, depending on the heat transfer characteristics required (in the illustrated embodiment it will be noted that all tubes 25 have been used) . Insertion may be achieved by utilising an insert formed of a product that will deform on insertion and reform once in place (e.g. elastomeric type material). An alternative method may be to insert a thin walled metal helical screw with a bore through the centre that will allow 30 a (bullet) to be drawn through the tube expanding the screw to the inner surface of the tube. According to such an embodiment the ends of the tube would need to be sealed prior to soldering the elbows on (described later). To retrofit an 14 existing heat exchanger, the elbows on one end of the heat exchanger would need to be removed, the helical screw inserted and the elbows replaced. At step 506, the ends of the tubes then have elbows soldered 5 to one another to form a continuous serpentine arrangement. This is best illustrated in Figure 6. A fan (not shown) may be added to force air over the fins. EXPERIMENTAL RESULTS A two door drink fridge condensing unit was used for the 10 trial. For expedience, the condenser tubing was split in two sections as can be seen from the Figure 7 schematic. Passes A to I (only some passes are shown in the schematic for illustrative purposes) were modified to accept the helical screw and used as the complete condensing unit, while passes 15 J to U were kept standard (i.e. no flow direction control insert). Due to the halving of the capacity of the condenser, the trial was conducted in a low ambient temperature atmosphere. The results were then compared with the results using the passes J to U again in a low ambient temperature 20 atmosphere. Whilst modifying the left hand part of the condenser some of the passes were damaged and could not be used. Two temperature reading tubes were soldered 50 mm into the gas flow, the end of which was sealed, in the positions marked on tubes A and U of Figures 6 & 7. A temperature probe 25 was then inserted into these tubes for accurate temperature measurements. The test results are shown in Figure 8. It can be observed from the test results that by using a helical screw with fewer passes, a significant positive improvement in relation to efficiency of the heat exchanger is achieved. 30 A further test was carried out in respect of an air conditioning system for a vehicle. A conventional condenser unit from a Holden Astina (hereafter "the Astina condenser") was set up on a test bench alongside a condenser 15 incorporating a plurality of tubes including helical flow direction control inserts (hereafter "the helical screw condenser" ) , in accordance with an embodiment of the invention. 5 The Astina condenser had a block size of 580 mm long x 300 high (i.e. effective fin area) and included a total of 28 tubes having 8 micro-channels defined therein. The micro channels measured 1.7 mm wide x 1.5 mm high. The helical screw condenser on the other hand measured only 490 mm long x 10 310 mm high. 10 tubes formed of 3/4" copper pipe were included in the screw condenser body. Each of the tubes incorporated helical screws of 17.6 mm O/D (outside diameter) 14.9 pitch (i.e. which in this case is the distance in millimeters between the leading edge of each turn of the 15 helical thread), 1 mm blade thickness and centre stem diameter of 2.5 mm. A schematic of the tubing configuration of the helical screw condenser is shown in Figure 9, where the screw body is designated by the reference numeral 10, the thread is designated by the reference numeral 12 and the fins 20 are designated by reference numeral 14. It was demonstrated that the volume of gas through the helical screw condenser body 10 was measured as twice that of the volume through the Astina condenser. From the demonstration it was calculated that a pass of 13.9 mm in the 25 micro channel condenser equated to approximately 57 mm in the new condenser, which increases the effective path length of the helical screw condenser by a factor of 4. Thus, for the same physical size of heat exchanger, the length of the new condenser would be 4 times longer at twice the volume 30 (thereby, by calculation, making the new condenser 8 times bigger in capacity for the same physical size). The above embodiments described the helical insert as being removably coupled to the tubing. However, in an alternative 16 embodiment, the helical insert and outer tubing may be formed as one piece (i.e. integrally formed). For example, the heat exchanger may be formed by extruding a length of heat transmissive material, such as aluminium, through a die so as 5 to form a tube having an inner surface in which the flow direction control insert is formed. Alternatively, or in addition, the outer fin(s) may be extruded with the tubing to minimise construction costs. A second embodiment of an exchanger will now be described 10 with reference to Figures 10 through 17. According to the second embodiment, improved heat transfer characteristics may be achieved without the aid of a spiral insert, as previously described. Instead, and is evident from the figures, the heat exchanger tubes may each comprise a primary flow path 15 15 arranged to carry the flow of the first heat exchanging medium and which surrounds a secondary flow path 16 which carries a second heat exchanging medium. Such a configuration advantageously allows heat from the first heat exchanging medium to not only be exchanged with air (or 20 another suitable medium) passing over the outer wall 18 of the primary flow path 15, but in addition to exchange heat with a medium flowing through the enveloped secondary flow path 16. To further assist with the heat transfer at least one of the primary and secondary flow paths may be helical 25 for increasing their effective path length. Another advantage arising from the aforementioned tubular construction is that condensation is unable to pool on the primary flow path surface. In more detail, and with particular reference to Figures 10 30 and 11, a single heat exchanger tube 17 formed of a suitable heat transmissive material (e.g. copper, aluminium, etc.) comprises an outer circumferential wall 18 which is surrounded by one or more heat exchanging fins 14 in the same manner as previously described with reference to Figure 9. 17 According to the illustrated embodiment, the secondary heat exchanging medium is carried within a separate inner tube 16a located within the tube body, while the first medium is carried within a flow path defined in the outer wall 18. The 5 primary flow path 15 is partitioned by way of internal webs 20 so as to create a plurality of separate helical flow paths which extend along the length of the tube 17. This may serve to increase the heat transfer capabilities, as well as increase the structural strength of the exchanger tube. It 10 will be understood, however, that the primary flow path(s) need not necessarily be helical and could instead, for example, deviate in a serpentine or other suitable non-linear path. Alternatively, the path(s) may be straight and non deviating along the length of the tube as is shown in Figures 15 16 and 17. The exchanger tube 17 (including its partitioned circumferential wall 18) may be formed by an extrusion process (i.e. in a linear fashion). As an alternative technique, and with specific reference to the embodiment 20 shown in Figures 18 and 19, the tube 17 may be formed by coiling/winding a straight length of tubing 19 of generally elongate cross section, such that the length extends along a helical path. Such a technique may advantageously allow manufacturers to utilise readily available straight flow tube 25 lengths which are found in conventional heat exchanger designs (e.g. such as micro-channel tube lengths used in micro-channel heat exchangers) for forming the primary flow paths. Internal webs 20 formed within and extending along the length 19 may advantageously serve to direct the flow in 30 a helical path along the tube (once coiled), for increasing the heat transfer characteristics. As shown in the illustrated embodiment, the tube length 19 is coiled or otherwise formed to create a closed outer circumferential wall which defines a sealed inner flow path for carrying the 35 secondary flow (i.e. such that a separate inner tube is 18 obviated). Figure 20 shows an alternative micro-channel design which could be formed into an exchanger tube as afore described, whereby the fins 14 are integrally extruded with the channel. 5 The exchanger tubes 17 are connected to an inlet and outlet manifold for receiving/expelling the respective heat exchanging mediums. Figure 12 shows an exploded view of the exchanger tube of Figure 11, with an inlet manifold 21a in the form of a copper pipe. As shown, at a first end 19a of 10 the tube 17 the outer wall 18 is paired away, exposing a length of the inner tube 16a which carries the secondary medium. The first end 19a is then inserted into an aperture 22 defined in a wall of the inlet manifold 21 such that the primary flow path is in fluid communication with the 15 inlet manifold for delivering the first heat exchanging medium (in this case refrigerant gas). A portion of the inner tube 16a extends through a slightly smaller opposing aperture 23 in the inlet manifold wall, for receiving the secondary medium (in this case air, which may either be 20 ambient air or alternatively air forced through the secondary flow path using a fan or the like). A second end 19b of the tube 17 is coupled to an outlet manifold 21b (which may, for example, be under vacuum) having the same form as the inlet manifold 21a in an identical manner. An assembled view of a 25 heat exchanger according to an embodiment is shown in Figure 14. Figure 16 shows a sectional view of the manifold coupling through line A-A of Figure 15. It will be understood that in one embodiment the number of flow paths defined in each tube of the exchanger may vary. 30 For example, for an automotive exchanger where the tubes are connected in series, the number of flow paths may reduce for each pass so as to account for changes in the state of the primary heat exchanging medium (e.g. liquid to gas or vice versa). Furthermore, it will be understood that the heat 19 exchanging medium passing through the primary and secondary flow paths may be any suitable medium and should not be seen as being restricted to those described above. For example, rather than the secondary flow path carrying air it could 5 instead carry water such that the primary heat exchanging medium is exchanging heat with two different mediums (i.e. air through the fins and water through the secondary flow path). It is to be understood that, if any prior art publication is 10 referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country. It is acknowledged that the term 'comprise' may, under 15 varying jurisdictions, be attributed with either an exclusive or an inclusive meaning. For the purpose of this specification, and unless otherwise noted, the term 'comprise' shall have an inclusive meaning - i.e. that it will be taken to mean an inclusion of not only the listed 20 components it directly references, but also other non specified components or elements. This rationale will also be used when the term 'comprised' or 'comprising' is used in relation to one or more steps in a method or process. Aspects of the present invention have been described by way 25 of example only and it should be appreciated that modifications and additions may be made thereto without departing from the scope thereof as defined in the appended claims. 20

Claims

1. A heat exchanger comprising: a primary flow path arranged to contain a first heat 5 exchanging medium; a secondary flow path arranged to contain a secondary heat exchanging medium, wherein the primary flow path surrounds the secondary flow path for exchanging heat between the two paths. 10

2. A heat exchanger in accordance with claim 1, wherein the primary flow path is helical.

3. A heat exchanger in accordance with claim 1 or claim 2, wherein the primary flow path is partitioned. 15

4. A heat exchanger in accordance with any one of the preceding claims, wherein the primary flow path is surrounded by one or more heat exchanging fins. 20

5. A heat exchanger in accordance with any one of the preceding claims, further comprising a tube and wherein the secondary medium is carried within a body of the tube and the primary medium is carried within a circumferential outer wall of the body. 25

6. A heat exchanger in accordance with claim 5, wherein the tube has a circular cross section.

7. A heat exchanger in accordance with claim 5 or 6, further 30 comprising an inlet manifold coupled to a first end of the tube and having a fluid path flow in fluid connection with the primary flow path of the tube for delivery of the first medium. 21

8. A heat exchanger in accordance with claim 7, further comprising an outlet manifold coupled to a second end of the tube and having a fluid path flow in fluid connection with the primary flow path for expelling the first medium. 5

9. A heat exchanger in accordance with claim 7 or 8, further comprising a plurality of tubes and wherein the inlet and outlet manifold each comprise a manifold tube having openings defined along their length for receiving corresponding ends of the 10 respective exchanger tubes.

10. A heat exchanger in accordance with claim 9, wherein an inner surface of the circumferential wall for each tube extends through the manifold tube and meets with a second opening in the 15 manifold tube for receiving/expelling the secondary or third medium.

11. A method of constructing a heat exchanger comprising forming a primary path flow arranged to contain a primary heat 20 exchanging medium so as to surround a secondary path flow arranged to carry a secondary heat exchanging medium.

12. A method in accordance with claim 11, wherein the primary flow path is a helical flow path. 25

13. A method in accordance with claim 12, further comprising forming the helical flow path by winding or extruding a length of a primary tube having a generally elongate cross section such that the length extends along a helical path. 30

14. A method in accordance with claim 13, further comprising winding/extruding the length of tube such that a closed outer circumferential wall is formed so as to define the secondary flow path. 22

15. A method in accordance with claim 13, further comprising locating an inner tube arranged to carry the secondary medium within the wound length of primary tube. 5

16. A method in accordance with claim 14 or 15, further comprising coupling a first end of the primary tube to an opening in an inlet tube arranged to deliver the first medium such that the primary flow path is in fluid communication with 10 the inside of the inlet tube.

17. A method in accordance with any one of claims 14 to 16, further comprising coupling a second end of the primary tube to an opening in an outlet tube arranged to expel the first medium 15 such that the primary flow path is in fluid communication with the inside of the outlet tube.

18. A method in accordance with claim 16 or 17, further comprising passing the secondary flow path through a second 20 opening in the respective inlet/outlet tube for delivering/expelling the secondary medium.

19. A heat exchange substantially as hereinbefore described with reference to the accompanying drawings. 25 23