WO2011003140A1

WO2011003140A1 - Heat exchanger

Info

Publication number: WO2011003140A1
Application number: PCT/AU2010/000866
Authority: WO
Inventors: Frederick Mark Webb
Original assignee: Frederick Mark Webb
Priority date: 2009-07-06
Filing date: 2010-07-06
Publication date: 2011-01-13

Abstract

A heat exchanger comprises 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. A flow direction control insert is located within each tube and operable to control flow of the first medium to thereby vary the effective path length of the tube and in turn adjust the heat transfer characteristics of the heat exchanger.

Description

HEAT EXCHANGER TECHNICAL FIELD

The present invention relates generally to a heat exchanger and method for forming the same. More specifically, but by no means exclusively, the invention relates to a tubing configuration for improving heat transfer characteristics of a heat exchanger .

BACKGROUND OF THE INVENTION

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 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 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-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 the fins by way of a fan. Clearly, the larger the heat exchanger, the larger the fan required to move the air for 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 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 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 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 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 that other forms of tube or conduit are equally applicable and are not limited to being of cylindrical cross section (e.g. square conduits, hexagonal conduits and the like are envisaged) .

In an embodiment the inventor has developed an alternative configuration of heat exchanger tubing that may exhibit better heat transfer characteristics than conventional exchangers, as previously described. In particular, in one embodiment, the tube configuration provides several

advantages over the art. For example, by providing a flow direction control insert that operates to increase the effective flow path length of the heat exchanger tubes, shorter tube lengths can be employed for achieving the same heat transfer characteristics which may result in reduced manufacturing and running costs. In addition, the amount of circulating fluid may also be reduced, again minimising running costs . For freezer evaporators employing embodiments of the present invention, the fins may be spaced further apart making the evaporator less prone to defrosting issues, as are well known to persons skilled in the art.

Furthermore, it should be appreciated that by implementing the above embodiments the heat exchanger may have a smaller overall dimension than conventional designs for achieving the same heat transfer, making the units more suitable for installation in confined spaces (e.g. roof cavities for air conditioning applications) . In addition, a smaller heat exchanger surface area means that a smaller fan can be used (where required) , which in turn leads to a reduced noise level when compared to larger conventional units . Note that reference has been made to fans given that in preferred embodiments, the fluids are predominantly gases. It should be appreciated that where liquids are used, pumps may be used instead of fans and that similar issues surrounding cost described above applies to pumps as well.

In an embodiment, the flow direction control insert comprises an elongate body having an outer surface which controls the 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 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 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.

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 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 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 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 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 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 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

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 exchanger formed from such a one piece extrusion may

significantly reduce construction time and cost.

In accordance with a second 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 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 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 a third aspect of the present invention 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 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.

In accordance with a fourth 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 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 comprising extruding the 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 transfer characteristic.

In accordance with a fifth 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 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 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 .

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 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 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 .

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 seventh aspect of the present invention there is provided a tube for a heat exchanger, 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 tube,- and a 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 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 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 distance than the tube length. It should be appreciated from the above description that there is described an improved heat exchanger design

including modified tube design, lower mass and overall dimensions, modified methods to assemble the exchanger (or retro-fit an existing heat exchanger) using flow direction control inserts 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 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.

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 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, 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 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 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 present invention;

Figure 8 is a graph showing test results for the Figures 6 and 7 configurations; and Figure 9 is a schematic of a tube carrying a helical flow direction control insert in accordance with a further embodiment of the present invention.

DETAILED DESCRIPTION OF AN EMBODIMENT In the following description, for the purpose of illustration only, the present invention is described in the context of a heat exchanger for a refrigerator, and more particularly to the tube configuration of the refrigerator's condensing unit. It will be appreciated, however, that the present invention 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 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 the referenced figures are not to scale, and only serve to conceptually illustrate the various heat exchanger components and interactions between those

components for varying the effective flow path length for adjusting heat transfer characteristics.

With reference to Figure 1 there is shown a heat exchanger in accordance with an 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.

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) . 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 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 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 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 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 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 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 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 example, different capacity units will require different size 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 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. A method of forming a heat exchanger panel in accordance with an 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 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 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 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 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 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 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 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 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 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 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.

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

incorporating a plurality of tubes including helical flow direction control inserts (hereafter "the helical screw condenser" ) , in accordance with an embodiment of the

invention.

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 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 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 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 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

(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 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 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. It is to be understood that, if any prior art publication is 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 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 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 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.

Claims

CLAIMS ;

1. 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 each tube and operable to control flow of the first medium to thereby vary the effective path length of the tube and in turn adjust the heat transfer characteristics of the heat exchanger.

2. A heat exchanger as claimed in claim 1, whereby the flow direction control insert comprises an elongate body having an outer surface which controls the flow direction.

3. A heat exchanger as claimed in claim 2, wherein the elongate body extends the length of each tube.

4. A heat exchanger as claimed in claim 2 or claim 3 , wherein the elongate body is in the form of a helical screw.

5. A heat exchanger as claimed in claim 4, wherein an outer circumference of the helical screw sealingly contacts an inner surface of the tube to create a helical flow channel.

6. A heat exchanger as claimed in claim 5, wherein the pitch of the helical screw is varied to adjust the effective length of the tube .

7. A heat exchanger as claimed in claim 5 or claim 6, wherein the outer diameter and/or the shank of the helical screw body is varied to adjust the effective length of the tube.

8. A heat exchanger as claimed in any one of the preceding claims, wherein the two mediums are selected from: air, steam, water, oil, beverage, refrigerant and a combination thereof.

9. A heat exchanger as claimed in any one of the preceding claims, wherein the heat exchanger is one of a condenser, evaporator, cooling tower, radiator, Shell and Tube heat exchanger or a Tube in Tube heat exchanger configuration.

10. A heat exchanger as claimed in any one of the above claims wherein a material in the form of copper is extruded to form a thin gauge tube for the heat exchanger tubing.

11. A heat exchanger as claimed in any one of the preceding claims, wherein the insert is formed from a plastic, polymer, elastomer, or rubber material.

12. A heat exchanger as claimed in any one of claims 1 to 10, wherein the insert is formed from a corrosion resilient metal or alloy.

13. A heat exchanger as claimed in any one of the preceding claims, wherein each insert comprises one or more sections.

14. A heat exchanger as claimed in claim 13 , wherein at least one of the one or more sections varies the flow in a different manner to the other sections.

15. A flow direction control insert arranged to be located inside 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, whereby the flow direction control insert is operable to control a flow of the first medium within the tμbe to thereby vary the effective path length of the tube and in turn adjust the heat transfer characteristics of the heat exchanger .

16. A flow direction control insert as claimed in claim 15, further comprising an elongate body having an outer surface which controls the flow direction.

17. A flow direction control insert as claimed in claim 16, wherein the elongate body extends the length of the tube.

18. A flow direction control insert as claimed in claim 16 or claim 17, wherein the elongate member is a helical screw.

19. A flow direction control insert as claimed in claim 18, wherein an outer circumference of the helical screw sealingly contacts an inner surface of the tube to create a helical flow channel .

20. A flow direction control insert as claimed in claim 19, wherein the pitch of the helical screw is varied to adjust the effective length of the tube.

21. A flow direction control insert as claimed in claim 19 or claim 20, wherein the shank and/or outer diameter of the helical screw body is varied to adjust the effective length of the tube.

22. A flow direction control insert as claimed in any one of claims 15 to 21, wherein the two mediums are selected from: air, steam, oil, water, refrigerant, beverage and a combination thereof .

23. A flow direction control insert as claimed in any one of claims 15 to 22, wherein the heat exchanger is one of a

condenser, evaporator, cooling tower, radiator, Shell and Tube heat exchanger and a Tube and Tube heat exchanger .

24. A flow direction control insert as claimed in any one of claims 16 to 23, wherein the body is formed from a plastic, polymer, elastomer or rubber material, or a combination of any of these.

25. A flow direction control insert as claimed in any one of claims 16 to 24, wherein the body is formed from a corrosion resilient metal or alloy or a combination of a ferrous or non- ferrous material .

26. A flow direction control insert as claimed in any one of claims 16 to 25, wherein each insert comprises one or more sections .

27. A flow direction control insert as claimed in claim 26, wherein at least one of the one or more sections varies the flow in a different manner to the other sections to thereby vary the heat transfer characteristics in different parts of the tube.

28. 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 steps of:

locating a flow direction control insert within the tube, the flow 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.

29. 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 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 flow direction of the first medium within the tube to thereby increase the effective path length of the tube and in turn improve the heat transfer characteristic.

30. A method as claimed in claim 28 or claim 29, comprising a flow control insert as claimed in any one of claims 15 to 27.

31. A method for forming a heat exchanger tube, the method comprising extruding the tube with an insert providing a helical internal flow path.

32. A method of forming a heat exchanger comprises 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 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.

33. A method as claimed in claim 32, further comprising extruding the length of heat transmissive material to form 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.

34. A method of forming a heat exchanger comprises extruding a length of heat transmissive material through a die so as to form a tube having an inner surface in which is defined a flow direction control insert, the insert arranged to control the flow of a first heat exchanging medium arranged to be passed through the tube and which medium is arranged to exchange heat with a second heat exchanging medium in thermal contact with an outer surface of the tube .

35. A method as claimed in claim 34, wherein the flow

direction control insert comprises a helical insert as claimed in any one of claims 15 to 27.

36. A method as claimed in any one of claims 32 to 35, wherein the heat transmissive material is aluminium.

37. A tube for a heat exchanger, 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 tube; and a flow direction control device located within the tube and operable to control flow of the first medium.

38. A tube as claimed in claim 37, wherein the flow direction control device is integrally formed with the tube.

39. A tube as claimed in claim 37, wherein the flow direction control device is provided as a separate removably coupled insert.

40. 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 controlling a direction of the flow of the first heat exchanging medium within the tube so that it flows a greater distance than the tube length.

41. A method, exchanger and/or flow direction control insert substantially as hereinbefore described and with reference to the Examples and Figures .