AU760683B2 - A flue and hot water heater - Google Patents

Description

WO 99/49268 PCT/AU99/00200 A Flue and Hot Water Heater Field of the invention The present invention relates to flues for such apparatus as gas and oil fired heaters for space, heating water or other appliances utilising exhaust gases to provide heating and to water heaters.

Background of the invention In the case of water heaters, flues are generally cylinders of metal which may be enamelled and which may have heat exchange elements located therein such as those described in US patent 2950740 which was published and issued on 30 August 1960 or US patent 3349754 which was issued and published on 31 October 1967.

Fluid heaters, such as water heaters, have flues which are generally located within a tank of fluid to be heated and extend from a lower end to the upper end of the tank. The flue provides a passage for hot exhaust gases to move from a combustion chamber located below the tank, to an exhaust located above the tank.

It is important for the flue is to provide a large heat transfer surface relative to the cross sectional area of the flue way. This will ensure that the flue will have an increased conduction surface area and thus is able to transfer more heat to fluid to be heated.

A problem with prior art flues is that hot spots tend to occur at different points in the length of the flue. This can be caused by the temperature in fins and such like being subjected to too much heat compared to the ability of the fins to transfer the heat to the flue wall and thus to the fluid to be heated.

The creation of hot spots have the effect of breaking down or fracturing of enamel on the fluid side of the flue, and this in turn leads to corrosion of the flue, which in turn reduces the serviceable life of the heater.

Heaters of the aforementioned type are well-known and are used extensively to heat water for both industrial and domestic use. It is important for reasons of efficiency that a high percentage of the calorific value of the heat released as a consequence of combustion is transferred to the water to be heated. For this reason different types of flue designs and water vessel designs have 2 been employed in the past some of which are more efficient than others.

One type of arrangement which has proved to be reasonably successful in the past employs an internal flue, that is, a flue which passes up the centre of the water vessel.

It is found that with that type of design, however, gases exhausting from the top of the flue are still relatively hot.

When those gases are exhausted to atmosphere that heat is, of course, lost.

SUMMARY OF THE INVENTION According to a first aspect of the invention there is provided a flue adapted to communicate exhaust gases from one end of a heater to the other, the flue comprising.

a central internal core; an elongate outer tube surrounding the central core formed of a thermally conductive material, the outer tube having a corrugated configuration in cross section thereby defining a plurality of corrugations, the outer tube having an internal and an external surface; the internal surface being in contact with the core over at least part of the length of the outer tube, and the external surface being in 15 contact with fluid to be heated by the heater; and the corrugations and the core defining between them one or more air passages through which exhaust gases flow in •use.

Preferably the internal core is closed such that exhaust gases are constrained to pass •0•o up the air passages in use.

Preferably said corrugations are axially aligned with the flue axis over at least part of the length of the flue. Optionally the corrugations extend along at least a part of the length of said flue in a helical path around the outside of the core.

The inner surface of the outer tube may be joined to the core at points of contact between the outer tube and the core.

The core may be of tubular construction having an internal passage blocked of by one or more barriers therein. The core may be deformed in registry with the corrugations on the external tube, the corrugations nesting into the deformations in the core.

Optionally the outer tube has said corrugations formed therein by means of a cold working process performed with the core in place within the outer tube, the radially inner sections of the corrugations so formed being in contact with the outer surface of the core. The core may itself be deformed during said process to thereby receive said radially inner sections of the corrugations.

Preferably said corrugations include between adjacent ones of said corrugations a contact surface to contact said core, said contact surface being provided as one or more of the following: a concave surface, a convex surface, a welded surface, a surface having a radius which matches the surface of said core outer surface; a surface 10 which is substantially concentric with the outer surface of said core.

Preferably said flue and or said outer tube have or has a cylindrical top and bottom end.

The flue is Preferably adapted to be installed in a water heater. The invention extends to a water heater incorporating a corrugated flue as defined herein.

15 Preferably said corrugations are substantially uniformly distributed about the circumference of the core.

Preferably the corrugations are of an amplitude which is greater than the wall thickness of the outer tube.

S The corrugations may have a sinusoidal profile in a cross section.

The corrugations may have a first section which is parallel to the flue axis and a second section of helical configuration, the second section being located downstream of the first section.

Preferably the radially inner sections of the corrugations are joined to the core by a joining means which prevents the corrugations from breaking contact with the core along a length of the core which corresponds to the length of the core which would be in use typically subject to scaling temperatures.

Preferably said Joining means is one or more of the following: coining, welding, spot S welding, vacuum brazing.

According to a second aspect of the invention there is provided a water heater comprising a vessel for water having an internal flue extending generally vertically through the vessel from the base thereof to a flue outlet at the top thereof; heating means for applying heat adjacent to the base of the vessel, the heating means including a burner and a fan for directing exhaust gases from the burner through the flue; a surrounding at least part of the vessel and spaced therefrom to define a generally annular space around the vessel, said annular space being in flow communication with said flue outlet; and an exhaust from said annular space, the arrangement being such that exhaust gases pass through the internal flue and through 10 the annular space prior to exiting the heater through the exhaust, wherein at least part of the surface of the jacket which faces towards the annular space has flow directing *formations thereon.

The flow directing formations may be in the form of a series of convex dimples or buttons on said surface. Alternatively, said flow directing formations maybe in the form of vanes, fins, corrugations, or other fluid flow directing shapes. A further alternative is for the flow directing formations to simply comprise a surface treatment of the inner surface of the jacket. Optionally, the outer surface of the vessel may, S"likewise, have vanes, fins or other formations thereon.

At least part of the jacket may be insulated and the jacket may be insulated over at 20 least that part there of which forms said annular space.

Both the vessel and the jacket are preferably of substantially right circular cylindrical shape, and the annular space is preferably of substantially constant width over at least part of the length thereof, and around the entire circumference of the vessel.

These and further features of the invention will be made more apparent from the description of an embodiment thereof given below by way of example. In the description reference is made to the accompanying drawing but the specific features shown in the drawing should not be construed as limiting the ambit of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 illustrates a front elevation of a flue of an embodiment of the present psT1 invention; WO 99/49268 PCT/AU99/00200 Figure 2 illustrates a plan view of the flue of Figure 1; Figure 3 illustrates the flue of Figures 1 and 2 located in a force draft water heater; Figure 4 illustrates a plan view of a flue of a second embodiment; Figure 5 illustrates a plan view of a flue of a third embodiment; Figure 6 illustrates a plan view of a flue of a fourth embodiment; Figure 7 illustrates a front elevation of a flue of a sixth embodiment; Figure 7A illustrates a plan view of the flue of Figure 7; Figure 8 illustrates a front elevation of a flue of a sixth embodiment; Figure 8A illustrates a plan view of the flue of Figure 8; Figure 9 illustrates a front elevation of a flue of an seventh embodiment; Figure 10 illustrates a front elevation of a flue of a eighth embodiment; Figure 10A illustrates a plan view of the flue of Figure Figure 11 illustrates a cross section through a flue of a ninth embodiment; Figure 12 illustrates a cross section through a flue of a tenth embodiment; and Figure 13 illustrates a cross sectional side view of a water heater according to the invention.

Detailed description of the embodiments Illustrated in Figure 1 and 2 is a flue 2 which has a top and bottom right circular cylindrical ends 4 and 6 respectively. Extending between each of the ends 4 and 6 is a central region 7 which is contoured or shaped to define a plurality of corrugations 8 which are generally, as depicted in the plan view of Figure 2, in the form of sinusoidal path around an inner core 10. The troughs or radially inner sections 12 of the corrugations 8 are, at least along a portion of the length of the flue, in contact with the inner core The corrugations 8 together with the inner core form a series of discrete passages 9 along the flue. The passages 9 illustrated in Figures 1 and 2 are straight and aligned with the longitudinal axis of the flue.

The inner core 10, being in contact with the corrugation 8, ensures that heat absorbed by the WO 99/49268 PCT/AU99/00200 6 inner core 10 can pass out into the material of the corrugations 8 and thus into fluid which would surround the corrugations 8 in a heater.

The inner core 10 comprises a blanked off tube 11. The blanking off is achieved at one end of tube 11 by a disc 14 which is welded to the terminus of the tube 11. The lower end 16 is open so that exhaust gases can move into the internal volume of the inner tube 10. Once the exhaust gases engage the disc 14 any further gases attempting to enter will instead be forced along paths indicated by the lines 18 into the passages 9 formed by the corrugations 8. Thus by providing no communicable passage through the core 10 from the lower cylindrical end 6 of the flue to the upper cylindrical end 4 thereof ensures that the exhaust gases flow up through passages 9. The core 10 effectively forms a baffle to direct exhaust gases along a particular path, as is described above.

As illustrated in Figure 3, the flue 2 may be mounted inside a water tank 20 by welding of the cylindrical ends 4 and 6 to respectively tank ends 24 and 26. The tank ends 24 and 26 are generally known as plus ends due to the fact that they provide additional volume to the tank In the tank 20 illustrated in Figure 3, the tank end 26 includes a combustion chamber 28 within the tank end 26 The combustion chamber 28 is surrounded by water which fills the volume of the tank In the embodiment illustrated in Figure 3, the exhaust gases exit the flue 2 via cylindrical end 4 and into an outer annular exhaust passage 32 which directs the exhaust gases along the path 34 to pass outwardly of the flue 2 and then downwardly around the outside of the tank 20 and out through the exhaust 36.

Illustrated in Figure 4 is a plan view of a fluted flue 2A which is similar to that of Figure 2 except that the corrugations 8A have a base portion 12A which is concentric with the outside surface or circumference of the core 10A. The surfaces 12A will provide surface contact along the longitudinal length of the corrugations 8A with the core Illustrated in Figure 5 is a plan view of a fluted flue 2B. In this embodiment the fluted flue 2B is such that the base portion 12B of the corrugations 8B project into the internal volume of the core 10B. This will provide a curved surface area of contact which is greater surface area of contact compared to that of the flue 2A of Figure 4.

WO 99/49268 PCT/AU99/00200 7 Illustrated in Figure 6 is a fluted flue 2C, wherein along the full length of the corrugations, the base portions 12C are welded to the core Illustrated in Figure 7 is a fluted flue 2D similar to that of Figure 1, except that at the end of the fluted flue 2D, namely that end at the end thereof opposite to the disc 14D, the corrugations 8D are welded via their base portions 12D, to the core O10D (which is preferably of tubular construction manufactured from mild steel or carbon steel). The welding may comprise a series of spaced apart spot welds 50 in a longitudinal line along the length of the base portions 12D.

While such spot welds 50 could be extended the entire way along the length of the flue 2D, they are more important in the upstream region of the flue 2D. This region of the flue 2 which would typically be subject to scaling temperatures, if the spot welds 50 were not present.

Scaling may occur in approximately the lower or upstream 10% to 20% of the tubular core or the first 100mm to 200mm, of the core 10D if the base portions 12D separate away from the inner tube 10D. If separation of the base portions 12D and 10D occurs, the core 10D will not be able to dissipate or transmit the heat it absorbs, to water or other medium on the external side of the outer tube 11A. If this separation occurs the temperature of the inner core in this upstream region will rise above 500 whereupon mild or carbon steels will begin scaling. Such scaling may reduce the heat transfer capabilities of the flue 2D by producing expansion of material undergoing scaling. This expansion may result in a blockage or partial blockage of one or more of the passages formed by the corrugations. Another difficulty with scaling is the risk of scales or particles of metal, which may fall onto the burner and thereby damage it by bocking ports, or cause some other problem.

If desired, the spot welds 50 can be replaced by continuous welds in the same region.

Alternatively, the join between the inner core O10D and outer tube 11D at the base portion 12D can be made by any one or combination of one or more of the following: vacuum brazing, coining, intermittent welding, spot welding or continuous welding.

An alternative solution to the scaling difficulties is to produce a fluted flue 2E as illustrated in Figure 9. Such a flue 2E counteracts the potential for scaling by sealing the upstream end of the tubular core 10E with a ceramic plug 13E which occupies that length of the tubular core which may be subject to scaling. The ceramic plug 13E removes the need to have a downstream end disc 14 as in Figure 1, as the inner tube IOE is sealed by the plug 13E at its lower or WO 99/49268 PCT/AU99/00200 8 upstream end.

The ceramic plug 13E can include a tapered end 15E, which is conical, pyramidal, hemispherical or other tapered formation so as to provide less resistance to the flow path of exhaust gases passing over the end Another solution (not illustrated) for the prevention of scaling is to manufacture the tubular core from a length of stainless steel tubing to form that portion of the core 10 that may be subject to scaling, with the rest of the core 10 being constructed from a mild steel tube of the same inside and outside diameter. The mild steel and stainless steel tubes can be welded together to form an a single continuous core which is blocked off at any appropriate position along its length.

A stainless steel selected for this application should be of a type which is temperature resistant, such as AISI 310 or AISI 321 or AISI 430 or AISI 316. A core 10 manufactured in this way can be sealed at the mild steel end by a mild steel disc 14, or a similar disc made of stainless steel on the stainless steel end of the inner tube. If desired the stainless steel end can have a tapered formation of a similar shape to that of the end of plug 13E of Figure 9.

Illustrated in Figure 8 is a fluted flue 2F, which has corrugations 8F which begin at the downstream end of the flue 2F and are parallel to the longitudinal axis of the core 10. At some distance along the length of the flue 2F, the corrugations 8F become helical, as indicated at numeral 17F. The purpose for this is to allow the exhaust gases a longer path to exit the flue 2F, so that as heat is extracted from the exhaust gases and their temperature decreases, the remaining heat can be withdrawn therefrom by the longer path to allow greater time for the heat transfer process. The helical corrugations can be parallel to each other at a fixed angle to the core axis as illustrated in Figure 8.

Alternatively the corrugations 17F could start out with a small helix angle which progressively increases as the corrugations 17F progress upstream. The rate of change or increase of the helix angle can be chosen so as to complement the rate at which heat is extracted from the gases, so that the exhaust gases leaving the flue 2F will do so with a predetermined temperature, and the efficiency of flowthrough flue is maintained.

As illustrated in Figure 10, the flue 2J has only 2 two helical corrugations 8J thereon. These corrugations will have a relatively large cross sectional area. As only two corrugations 2J are WO 99/49268 PCT/AU99/00200 9 present, each corrugation is made to revolve around the core in a helical path for two full revolutions. If more corrugations were present, then the number of revolutions or the helix angle can be reduced.

To ensure maximum heat transfer from any of the corrugations mentioned above, the corrugations will need to be constructed so that the dimensions of the cross sectional area are such that the maximum distances separating the sides of the corrugations or the corrugations and the inner core, are no greater than some 12 mm. It has been found that if these dimensions are greater than 12 mm, the boundary layer effects produced by the exhaust gases passing through corrugations of this size reduce the effectiveness of the assembled flue. The most preferred maximum distance between the surfaces of the corrugations or a surface of the corrugation and the inner core is 8mm.

Preferably, on a flue to be used in replacement of a prior art, four inch or 100mm flue, there are between 2 and 8 corrugation. However, the most effective configurations are believed to be those with some six corrugations as illustrated in Figures 1 to 9.

One of the advantages of the embodiments is that for an outside dimension of approximately 100mm at the cylindrical ends 4 and 6 the flue 2 is able to be used with a burner which can generate approximately 100 megajoules of energy. By operating the fluted flues 2, 2A, 2B and 2C at appropriately 80% efficiency means that approximately 80 megajoules of heat will pass through the flue to heat the water adjacent thereto. Whereas for a regular flue such as a finned flue of the type illustrated in US patent 2950740 which was published and issued on 30 August 1960 or US patent 3349754 which was issued and published on 31 October 1967, a typical 4 inch diameter flue of this construction allows a maximum energy passing through that flue of approximately 45 megajoules.

If it was desired to pass greater heat through mild steel enamelled flues, such as those disclosed in the US patents mentioned above, such additional energy would normally result in the break down of the enamel on those flues, due to the hot spots created at the points of attachment of the fins. It is envisaged that the corrugated flues described herein will suffer to a lesser extent from this problem.

Another advantage of the present invention is that only the external surface of the fluted flues 2 and 2A to 2J, need be enamelled. This is because by operating the flue at some 80% efficiency WO 99/49268 PCT/AU99/00200 there is no risk of condensation occurring on the inside of the flue which would otherwise corrode the unit or could produce water droplets which will hit hot surfaces creating steam which in turn will block the flue with the presence of the steam.

Another advantage of the flues 2, and 2A to 2J is that because of the increased surface area of the corrugations substantially no hot spots form on the flue. This is an advantage because hot spots are normally the cause of enamel breakdown and once enamel is broken down, corrosion and other deterioration will rapidly occur on the water side of the flue.

A further advantage of the flues 2 and 2A to 2J is that a flue such as these, can operate at a much higher speed of heat transfer because of its shape and construction by comparison to round flues, even those with finned internal surfaces. This means that water heaters having flues as described above, which embody the present invention, can be of reduced overall height, which will require less materials to manufacture.

Another advantage is that a water heater will result which has a smaller storage volume and because of this the water heater will have quicker recovery time, to get the water back to a desired water temperature.

The water heater illustrated in Figure 3 is of a fully condensing variety. However, the flues described above which embody the present invention can be used in non-condensing fan or forced draft water heaters and will provide the advantage (providing that the corrugations are sufficiently far apart from each other that there will be no stand-by losses from the water heater, via the flue. This is because natural draft or convection currents will not pass through the corrugations due to the resistance provided thereby. This effect is even more noticeable when the corrugations have, at least in part, a helical path around the inner tube.

Illustrated in figures 11 and 12 are the cross sections of two further flues 2K and 2L. The flues 2K and 2L are produced from a first sheet 70 which extends into and out of the page of the drawing to provide a length of flue. The ends can have a transition piece to provide a cylindrical end for connecting the flues 2K and 2L to the ends of a water tank.

The flue 2K has a second sheet 72 which is corrugated and which overlays the sheet 70, so as to provide three parallel passages 71, 73 and 75. The flue 2L is similarly constructed but includes a third sheet 74 which also overlays the sheet 70, but on the other side thereof to the side on which sheet 72 is located. The third sheet 74 is corrugated and forms between itself and the first WO 99/49268 PCT/AU99/00200 11 sheet 70, three more parallel longitudinally extending passages 77, 79 and 81. The passage 71 and 77 are in back to back locations as are the passages 73 and 77, and passages 75 and 81.

The embodiments of the present invention produce a flue with a higher heat transfer surface area to flue way volume ratio, when compared to a finned flue, and thus a higher heat transfer capability by comparison to finned flues. This happens, not because of an increase in the surface areas of heat conduction, but rather by the provision of relatively small passages, (by comparison to the heat transfer surface area provided thereby) which results in an increase in the rates of convection of the exhaust through the passages. The increased rates of convection result from an increase in the speed of the exhaust gases passing through the passages, due to the induced or forced draft provided by a fan or blower. The directing of the exhaust gases to pass only through the passages formed by the corrugations provides an amount of heat transfer surface area to take best advantage of and withdraw as much heat as possible out of, the fast flowing exhaust gases.

Turning now to Figure 13 of the drawings, a water heater 110 comprises a vessel 112 having a generally cylindrical side wall 114, a base 116 and top 118. The vessel 112 is preferably fabricated from mild steel and enamelled both internally and externally. If desired the externally located enamel could be substituted by an epoxy coating.

An internal flue 120 extends from the base 116 to the top 118 and terminates in a flue outlet 122 at the top of the vessel. The flue 120 in the preferred embodiment is of a corrosion resistant material and may be of corrugated construction in the manner described above. The flue 120 is defined by a pair of tubular members co-axially nested one-within the other, the outer tubular member being of corrugated configuration forming corrugations as indicated at numeral 124.

This corrugated arrangement provides a superior heat exchange characteristics for the interior heating surface of the vessel as has been described above. The vessel has a water inlet 126 to which cold water is introduced into the vessel, and a water outlet 128 located near the top of the vessel through which hot water is extracted from the vessel.

A jacket 130 surrounds the vessel 112, the jacket 130 being comprised of a thermally insulating material. The jacket 130 is spaced away from the top 118 and side wall 114 of the vessel so that a space 132 is formed between the interior surface of the jacket 130 and outer surface of the vessel 112. That space 132 is of generally annular configuration and forms a flow passage for WO 99/49268 PCT/AU99/00200 12 exhaust gases which pass out of the flue outlet 122.

The water heater is provided with a burner 134 which is supplied with gas through a gas supply arrangement 136 and gas conduit 38. A fan 140 urges the combusted gases up the flue 120 towards the flue outlet 122 so that flow of gas through the flue is, to some extent, a forced flow.

Gas from the flue outlet 122 passes in the direction of arrows 42 towards an exhaust outlet 144 located at the lower most point of the annular space 132. Thus, heated gas is in contact with the vessel 112 during its passage up the internal flue 120 and again as that heated gas passes down the annular space 132 towards the exhaust 144. This ensures that by the time the gas exits through the exhaust 144 a substantial percentage of the useable heat in the gas has been conducted to the vessel 112 thereby ensuring that water contained within the vessel is heated relatively efficiently.

It is found that the performance of the water heater is enhanced by forming flow directing formations 146 on the inner surface of the jacket, that is, the surface which faces towards the annular space 132. Flow directing formations, inter alia, have the effect of breaking up the boundary layers of the exhaust gases passing through. These flow directing formations, in this instance, take the form of the convex dimples or buttons. The effect of these dimples or buttons is that laminar flow of gas down the annular space is, to some extent, interrupted by the buttons which thereby ensures that heated gas is kept in contact the outer surface of the vessel so that heat is transferred from the gas to the vessel. As mentioned previously, it may be advantageous to also have flow directing formations of some form or another on the outer surface of the vessel to assist in heat transfer from the gas to the vessel.

The location of the exhaust outlet in the jacket 130, being that distance down from the top of the jacket 130, is dependent upon the input to the heater by the burner measured in Mj/hr and the surface area required by the secondary heat exchanger to condense below the dew point.

The gases exit from the primary flue (central flue pipe), above the dew point, and then condense on the external surface of the vessel 112 shell. To achieve full condensation the temperature must fall below the dew point, so that the latent heat of condensation is extracted from the flue gases. Typically the dew point in these circumstances would be about 65 deg C, with a desire to cool to just a few degrees above the water temperature. It is preferable to restrict the condensation to the outside of the vessel 112 as it is easier to control the condensate and WO 99/49268 PCT/AU99/00200 13 provide precautions against corrosion.

Thus if heat input is increased either a larger secondary heat exchange area (being the vessel surface area which bounds the annular space 132), or a smaller cross sectional area of the annular space 132 is required. The smaller cross sectional area increases the exhaust gas velocity but with the penalty of a larger fan being required.

There may be many variations to the above described embodiment without departing from the scope of the invention. For example, the tank may be somewhat differently formed to that described herein and the arrangement of the jacket may, likewise be different to that depicted herein. However, applicant considers that the arrangement as depicted in the drawing will be relatively inexpensive to manufacture and will provide a water heater of relatively high efficiency characteristics.

It will be understood that the invention disclosed and defined herein extends to all alternative combinations of two or more of the individual features mentioned or evident from the text or drawings. All of these different combinations constitute various alternative aspects of the invention.

The foregoing describes embodiments of the present invention and modifications, obvious to those skilled in the art can be made thereto, without departing from the scope of the present invention.

Claims (29)

1. A flue adapted to communicate exhaust gases from one end of a heater to the other, the flue comprising: a central internal core; an elongate outer tube surrounding the central core formed of a thermally conductive material, the outer tube having a corrugated configuration in cross section thereby defining a plurality of corrugations, the outer tube having an internal and an external surface; the internal surface being in contact with the core over at least part of the length of the outer tube, and the external surface being in contact with fluid to be heated by the heater; and the corrugations and the core defining between them one or more air passages through which exhaust gases flow in use.

2. A flue as claimed in claiml wherein the internal core is closed such that exhaust gases are constrained to pass up said air passages in use.

3. A flue as claimed in any one of the preceding claims, wherein the said corrugations are axially aligned with the flue axis for at least part of the length of the flue.

4. A flue as claimed in any one of the preceding claims, wherein the corrugations extend at least along apart of the flue in a helical path around the outside of the core. 20

5. A flue as claimed in any one of the preceding claims, wherein the outer tube is ooooo joined to the core at points of contact between the outer tube and the core.

6. A flue as claimed in any one of the preceding claims, wherein the core is of tubular construction having an internal passage blocked off by one or more barriers therein.

7. A flue according to claim 6, wherein the internal passage is blocked off by a plate located at or near the operatively upper end of the flue. o.

8. A flue as claimed in any one of the preceding claims, wherein the core is deformed in registry with the corrugations on the external tube, the So corrugations nesting into the deformations in the core.

9. A flue as claimed in any one of the preceding claims, wherein the outer tube has said corrugations formed therein by means of a cold working process performed with the core in place within the outer tube, whereby the radially inner sections of the corrugations so formed are in contact with the outer surface of the core.

A flue according to claim 9, wherein the core is deformed during said cold working process to thereby receive said radially inner sections of the corrugations.

11. A flue as claimed in any one of the preceding claims wherein said corrugations include between adjacent ones of said corrugations, a contact surface to contact said core, said contact surface being provided at any one or more of the following: a concave surface, a convex surface, a welded surface, a surface having a radius which matches the surface of said core surface, a surface which is substantially concentric with outer surface of said core.

12. A flue as claimed in any one of the preceding claims, wherein said flue and or said outer tube have or has a cylindrical top and bottom end.

13. A flue as claimed in any one of the preceding claims, which is specifically adapted to be installed in a water heater.

14. A flue as claimed in any one of the preceding claims, wherein said 20 corrugations are uniformly arranged around the circumference of the core.

A flue as claimed in any one of the preceding claims, wherein the corrugations are of an amplitude which is greater than the wall thickness of the outer tube.

16. A flue as claimed in any one of the preceding claims, wherein the corrugations are of a sinusoidal profile in cross section.

17. A flue as claimed in any one of the preceding claims, wherein the corrugations have a first section which is parallel to the flue axis and a second section of helical configuration, the second section being located downstream of the first S""section.

18. A flue as claimed in any one of the preceding claims, wherein the radially inner sections of the corrugations are joined to the core by joining means which prevent the corrugations breaking contact with the core along a length of the core which corresponds to the length of the core which would, in use, typically be subject to scaling temperatures.

19. A flue according to claim 18 wherein said joining means is one or more of the following processes: coining, welding, spot welding, vacuum brazing.

A water heater incorporating a corrugated flue according to any preceding claim.

21. A water heater comprising: a vessel for water having an internal flue extending generally vertically through the vessel from the base thereof to a flue outlet at the top thereof; heating means for applying heat adjacent to the base of the vessel, the heating means including a burner and a fan for directing exhaust gases from the burner through the flue; a jacket surrounding at least part of the vessel and spaced therefrom to define a generally annular space around the vessel, said annular space being in flow communication with said flue outlet; and an exhaust from said annular space, the arrangement being such that exhaust gases pass through the internal flue and through the annular space prior to exiting the heater through the exhaust, and wherein at least part of the surface of the jacket which faces towards said annular space has flow directing formations thereon.

22. A water heater according to claim 21 wherein the flow directing formations are S •in the form on a series of convex dimples or buttons on said surface.

23. A water heater according to claim 21 wherein said flow directing formations are in the form of any one of vanes, fins, corrugations, or other fluid directing shapes.

24. A water heater according to claim 21 wherein said flow directing formations comprise a surface treatment of the inner surface of the jacket.

25. A water heater according to any one of claims 21 to 24 wherein the outer ",rT R surface of the vessel has flow directing formations thereon.

26. A water heater according to any one of claims 21 to 25 wherein at least part of the jacket is insulated.

27. A water heater according to any one of claims 21 to 26 wherein both the vessel and the jacket are of substantially right circular cylindrical shape, and the annular space is of substantially constant width over at least part of the length thereof, and around the entire circumference of the vessel.

28. A water heater according to any one of claims 21 to 27 wherein said internal flue is of a type defined by any one of claims 1 to

29. A flue substantially as hereinbefore described with reference to any one of the embodiments shown in the accompanying drawings. A water heater substantially as hereinbefore described with reference to any one of the embodiments shown in the accompanying drawings. DATED THIS 17 th DAY OF FEBRUARY 2003 RHEEM AUSTRALIA PTY LIMITED BY ITS PATENT ATTORNEYS HALFORD CO o SS S S. S S.