CN1293745A - Flue and hot water heater - Google Patents

Abstract

A flue is adapted to pass exhaust gas from one end of a heater to the other. The flue includes a central core and an elongated, generally cylindrical outer tube surrounding the central core. The cross-section of the outer tube is corrugated to form a plurality of corrugations. The inner surface of the outer tube is in contact with the inner core so that the corrugations form a series of air passages around the inner core. In use exhaust gas flows through these channels. A series of different corrugated structures are given. The invention extends to a water heater provided with a flue, and to a water heater comprising a central flue leading into an annular space surrounding the outside of the water container through which exhaust gas flows upwards to into the annulus and out of the water heater.

Description

Flue and Hot Water Heaters

field of invention

The present invention relates to flues for appliances such as gas and oil fired space, water heaters or other appliances utilizing exhaust gas to provide heating, and to water heaters.

Background of the invention

In the case of water heaters, the flues are usually metal cylinders which may be glazed and which may contain heat exchanging elements such as in US Patent 2,950,740 published and issued August 30, 1960 or October 31, 1967 U.S. Patent 3,349,754 issued and published on .

Fluid heaters, such as water heaters, have a flue disposed within a container of fluid to be heated and extending from the lower end of the container to the upper end. The flue has a passage for heated exhaust gases to move from the combustion chamber below the vessel to the exhaust above the vessel.

It is important for the flue to provide a large heat transfer surface relative to the cross-sectional area of the flue passage. This will ensure an increased conduction surface area enabling more heat to be transferred to the fluid to be heated.

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

The creation of hot spots can crack or chip the enamel on the fluid side of the flue, corroding the flue, and reducing the life of the heater.

Heaters of the above-mentioned type are well known and widely used for heating water in both industrial and domestic applications. For reasons of efficiency, it is important that a high proportion of the calorific value of the heat released by the combustion be transferred to the water to be heated. For this reason, different types of flue designs and water container designs have been used in the past, some of which are relatively efficient.

One arrangement which has proven to be more successful in the past has been to use an internal flue, ie upwardly through the center of the water container. However, it has been found that with this type of design, the gases exiting the top of the flue are still relatively hot. When such a gas is vented into the atmosphere, one can imagine the amount of heat lost.

Summary of the invention

According to one aspect of the present invention, there is provided a flue suitable for passing exhaust gas from one end of a heater to the other, the flue comprising:

a central core;

a substantially cylindrical elongated outer tube of thermally conductive material surrounding the central core, the outer tube is corrugated in cross-section to form a plurality of corrugations, the outer tube has an inner surface and a The outer surface;

the inner surface is in contact with the inner core over at least part of the length of the outer tube, the outer surface is in contact with the fluid to be heated by the heater; and

One or more air passages are formed between the corrugations and the inner core, through which exhaust gas flows in use.

Preferably, the inner core is a solid or closed tubular structure so that in use exhaust gas is constrained upwardly through the air passage.

Preferably, said channels formed by said corrugations provide a greater ratio of heat transfer surface area to flue passage volume than a finned flue.

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

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

The inner core may be a tubular structure having an internal passage blocked by one or more baffles therein. The inner core is deformable corresponding to the corrugations on the outer tube, the corrugations nesting into the deformation of the inner core.

Alternatively, the corrugations are formed in the outer tube by a cold working process with the inner core in place within the outer tube, the radially inner portions of the corrugations being made in contact with the outer surface of the inner core . The inner core itself can be deformed during said process to receive said radially inner part of the corrugations.

Preferably, said corrugations have a contact surface with said inner core between adjacent corrugations, said contact surface being provided in one or more of the following forms: a concave surface, a convex surface, a welded surface, a radius and A surface that matches the outer surface of the inner core, a surface that is substantially concentric with the outer surface of the inner core.

Preferably, said flue has an upright cylindrical top and bottom.

Preferably, the flue is suitable for installation in a water heater. The invention also extends to a water heater incorporating a corrugated flue as defined herein.

Preferably, said corrugations are substantially evenly distributed around the circumference of the core.

Preferably, the corrugations have an amplitude greater than the wall thickness of the outer tube.

The corrugation may have a sinusoidal cross-section.

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

Preferably, the radially inner portion of the corrugation is connected to the core along a length of the core corresponding to the length at which the core is normally at a scaling temperature in use by means of a connection which prevents the corrugation from coming out of contact with the core. Inner core.

Preferably, the connecting device is one or more of the following: embossing, welding, spot welding, vacuum welding.

According to a second aspect of the present invention there is provided a water heater comprising:

a water container having an internal flue extending substantially vertically through the container from the bottom of the container to a flue outlet at the container portion;

Heating means for applying heat adjacent to the bottom of the vessel, the heating means having a burner and a fan directing exhaust gases from the burner through the flue;

an outer shell surrounding and spaced from at least a portion of the vessel to define a substantially annular space around the vessel, said annular space being in fluid communication with said flue outlet; and

an exhaust means for exhausting air from said annular space.

It is constructed so that exhaust gases pass through the inner flue and through the annular space before exiting the heater through the exhaust.

Preferably, at least part of the surface of the housing facing the annular space has flow guiding structures. These flow directing structures may be in the form of a series of reliefs or bumps on the surface. Alternatively, the flow directing structure may be in the form of vanes, fins, corrugations or other fluid flow directing shapes. Another option for the flow directing structure is to surface treat only the inner surface of the housing. Alternatively, the outer surface of the container may also have vanes, fins or other structures.

At least part of the housing may be insulated, and the housing may be insulated at least over that portion forming said annular space.

Preferably both the container and the housing are substantially right cylindrical and the annular space preferably has a substantially constant width over at least part of its length and around the entire circumference of the container.

The above and other features of the present invention will become more apparent from the following description of an embodiment presented by way of example. In the description reference is made to the drawings, but specific features shown in the drawings should not be considered as limiting the scope of the invention.

Brief description of the drawings

Fig. 1 is the front view of the flue of an embodiment of the present invention;

Fig. 2 is a top view of the flue shown in Fig. 1;

Fig. 3 shows that the flue of Fig. 1 and 2 is arranged on a forced draft water heater;

Fig. 4 is the top view of the flue of the second embodiment;

Fig. 5 is the top view of the flue of the third embodiment;

Fig. 6 is the plan view of the flue of the fourth embodiment;

Fig. 7 is the front view of the flue of the sixth embodiment;

Figure 7A is a top view of the flue shown in Figure 7;

Fig. 8 is the front view of the flue of the sixth embodiment;

Figure 8A is a top view of the flue shown in Figure 8;

Fig. 9 is the front view of the flue of the seventh embodiment;

Fig. 10 is the front view of the flue of the eighth embodiment;

Fig. 11 shows a cross-section of the flue of the ninth embodiment;

Figure 12 represents a cross-section of the tenth embodiment; and

Figure 13 is a side sectional view of a water heater of the present invention.

Detailed description of the embodiment

In Figures 1 and 2 there is shown a flue 2 having a right cylindrical top and bottom, indicated by reference numerals 4 and 6 respectively. Extending between the ends 4 and 6 is a central region 7 which is contoured or shaped to form a plurality of corrugations 8 which are generally sinusoidal about an inner core 10 as shown in the top view of FIG. 2 . The troughs or radially inner portions 12 of the corrugations 8 are in contact with the inner core 10 along at least part of the length of the flue.

The corrugations 8 together with the inner core form a series of separate channels 9 along the flue. The channels 9 shown in Figures 1 and 2 are straight and aligned with the longitudinal axis of the flue.

The contact of the core 10 with the corrugations 8 ensures that the heat absorbed by the core 10 is transferred to the material of the corrugations 8 and thus to the fluid surrounding the corrugations 8 in a heater.

The inner core 10 comprises a closed tube 11 . One end of the closed tube 11 is closed by a disc 14 welded to the end of the closed tube 11 . The lower end 16 is open so that exhaust gases can move into the inner volume of the inner tube 10 . Once the exhaust gas encounters the disc 14 any other gas attempting to enter will instead enter the channel 9 formed by the corrugations 8 along the route indicated by the line 18 . In this way, no communicable passages are provided through the inner core 10 from the lower cylindrical end 6 of the flue to its upper cylindrical end 4, thereby ensuring upward flow of exhaust gases through the passages 9. As mentioned above, the inner core 10 effectively forms a baffle, thereby directing exhaust gases along a particular path.

As shown in Figure 3, the flue 2 is installed in a water container 20 by welding the cylindrical ends 4 and 6 to the ends 24 and 26 of the container respectively. Container ends 24 and 26 are often referred to as positive ends because they provide additional volume to container 20 . In the vessel 20 shown in FIG. 3, the vessel end 26 has a combustion chamber 28 therein. The combustion chamber 28 is surrounded by water which is filled into the volume 30 of the container 20 .

In the embodiment shown in Figure 3, the exhaust gas exits the flue 2 through the cylindrical end 4 and enters an outer annular exhaust passage 32 which directs the exhaust gas along a route 34 out of the flue 2 and then in the container 20 flows downwards and exits the exhaust 36.

Shown in FIG. 4 is a top view of a fluted flue 2A which is similar to that of FIG. 2 except that the corrugations 8A have a bottom 12A concentric with the outer surface or circumference of the inner core 10A. Surface 12A may provide surface contact with the inner core 10A along the length of corrugations 8A.

Shown in FIG. 5 is a top view of flue 2B with grooves. In this embodiment, the fluted flue 2B is such that the bottom 12B of the corrugations 8B protrude into the inner volume of the inner core 10B. This will provide an arcuate contact surface area which is greater than that of the flue 2A of FIG. 4 .

Shown in Figure 6 is a fluted flue 2C in which the bottom 12C is welded to the inner core 10C along the entire length of the corrugation.

Shown in FIG. 7 is a fluted flue 2D similar to that shown in FIG. The corrugations 8D are welded by their bases 12D to the inner core 10D (preferably a tubular structure made of mild or carbon steel). The weld may comprise a series of spot welds 50 spaced along a longitudinal line along the length of the base 12D. While such spot welds 50 may extend entirely along the length of the flue 2D, they are more important in the upstream region of the flue 2D. If there were no spot welds 50, this area of the flue 2 would normally be subjected to scaling temperatures.

In approximately the lower or upstream 10% to 20% of the tubular core 10D, or the first 100 mm to 200 mm of the core 10D, scaling may occur if the bottom 12D separates from the inner tube 10D. If the bottoms 12D and 10D were separated, the inner core 10D would not be able to dissipate or transfer the heat it absorbed to the water or other medium outside the outer tube 11A. If this separation occurs, the temperature of the core in this upstream region will rise above 500°C and the mild or carbon steel will start to gas scale. Scaling reduces the heat transfer capacity of the flue 2D by causing the material to expand. This expansion may result in plugging or partial plugging of one or more channels formed by the corrugations. Another problem with descaling is the danger of metal scale or particles, which may fall onto the burner and block its orifice causing damage, or cause some other problem.

Spot welds 50 may be replaced by continuous welds in the same area, if desired. Alternatively, the connection between the inner core 10D and the outer tube 11D at the bottom 12D may be accomplished by any one or combination of: vacuum welding, embossing, intermittent welding, spot welding or continuous welding.

Another solution to the scaling problem is to form a fluted flue 2E as shown in FIG. 9 . This flue 2E can eliminate the possibility of scaling by sealing the upstream end of the tubular core 10E with a ceramic plug 13E which occupies the length of the tubular core 10E where scaling is likely. The ceramic plug 13E eliminates the need for a downstream end disc 14 as shown in FIG. 1, because the inner tube 10E is sealed at its lower or upstream end by the ceramic plug 13E.

Ceramic plug 13E may have a tapered end 15E that is conical, conical, hemispherical, or otherwise conical in shape to provide less resistance to the path of exhaust gas flow through end 15E.

Another solution to prevent scaling is to use a length of stainless steel tube to manufacture the tubular inner core 10 to form the part of the inner core 10 that may be scaled, and the rest of the inner core 10 is made of a soft tube with the same inner and outer diameters. Steel tube composition. Mild steel and stainless steel tubes may be welded together to form a single continuous core closed at any point along its length.

The stainless steel selected for this occasion should be temperature-resistant, such as AISI310 or AISI321 or AISI430 or AISI316. The inner core 10 manufactured in this way can be sealed with a mild steel disc 14 at the mild steel end, or with a similar disc made of stainless steel at the stainless steel end of the inner tube. If desired, the stainless steel end may have a tapered configuration similar in shape to that of the ceramic plug 13E of FIG. 9 .

Shown in FIG. 8 is a fluted flue 2F having corrugations 8F starting at the downstream end of the flue 2F and parallel to the longitudinal axis of the inner core 10 . At some distance along the length of the flue 2F, the corrugations 8F become helical, as shown at 17F. The purpose of this is to allow the exhaust gases to have a longer route to leave the flue 2F, so as the heat is transferred from the exhaust gases and their temperature decreases, the rest of the heat can be removed from it by a longer route, allowing the heat transfer process to be efficient longer time. The helical corrugations may be parallel to each other at a fixed angle relative to the core axis, as shown in FIG. 8 .

Alternatively, corrugations 17F may start with a small helix angle that gradually increases as corrugations 17F progress upstream. The rate of change or increase in the helix angle can be chosen to complement the rate at which heat is removed from the exhaust so that the exhaust leaving flue 2F will have a predetermined temperature and the efficiency of the backflow free flue can be maintained.

As shown in Figure 10, there are only two helical corrugations 8J on the flue 2J. These corrugations will have a relatively large cross-sectional area. Since there are only two corrugations 2J, each corrugation is made two full revolutions around the inner core along a helical course. If there are more corrugations, the number of turns or helix angle can be reduced.

To ensure maximum heat transfer from any of the corrugations described above, it is desirable to construct the corrugations such that the cross-sectional area is sized such that the maximum distance separating the sides of the corrugations or the corrugations from the core is no greater than about 12 mm. It has been found that if these dimensions are greater than 12 mm, the boundary layer effects produced by exhaust gas passing through corrugations of this size reduce the effectiveness of the assembled flue. The optimum maximum distance between the surfaces of the corrugations or between a surface of the corrugations and the inner core is 8 mm.

Preferably, there are 2 to 8 corrugations on a flue that replaces a prior art flue, ie a four inch or 100 mm flue. However, we believe that the most efficient structure is one with about six corrugations, as shown in Figures 1 to 9.

An advantage of these embodiments is that, for an external dimension of about 100 mm at the cylindrical ends 4 and 6, the flue 2 can be used with a burner producing about 100 MJ of energy. Operating the fluted flues 2, 2A, 2B and 2C at about 80% efficiency means that about 80 MJ of heat will pass through the flues to heat the water adjacent to them. Whereas for a conventional flue such as the finned flue shown in U.S. Patent 2,950,740 published and published on August 40, 1960 or in U.S. Patent 3,349,754 issued and published on October 31, 1967, A typical 4 inch diameter flue of this configuration can pass only about 45 megajoules of maximum energy.

If more heat is required to pass through mild steel enamelled flues such as those disclosed in the aforementioned US patents, this added energy will often cause the enamel on these flues to crack due to hot spots created at the fin attachment points. It is conceivable that the corrugated flue described herein suffers from this problem to a lesser extent.

Another advantage of the present invention is that only the outer surfaces of the fluted flues 2 and 2A to 2J need to be glazed. This is because, by making the flue work at about 80% efficiency, there is no danger of condensation occurring inside the flue, which would otherwise corrode the device, or possibly produce water droplets, which fall on hot surfaces and cause Steam, the presence of which in turn clogs the flue.

Another advantage of the flues 2 and 2A to 2J is that substantially no hot spots are formed on the flues due to the increased surface area of the corrugations. This is an advantage because hot spots are often the cause of enamel cracking, which quickly produces corrosion and other deterioration on the water side of the flue.

Another advantage of the flues 2 and 2A to 2J is that, due to their shape and configuration, such flues can be used at a much higher Work at heat transfer rates. This means that the overall height of a water heater embodying the invention having the above-mentioned flue is reduced and thus less material is required for its manufacture.

Another advantage is that the water heater can be provided with a smaller storage volume, therefore the water heater will have a quicker recovery time to bring the water back up to the desired water temperature.

The water heater shown in Figure 3 is of the all-condensing type. However, the above described flue embodying the invention could be used in a non-condensing fan or forced draft water heater and would provide the advantage (provided the corrugations are sufficiently far apart from each other) by the flue that the water heater would not have standby loss. This is because natural ventilation or convection does not pass through the corrugations due to the resistance exerted by the corrugations. This effect is more pronounced when the corrugations at least partially have a helical course around the inner tube.

Shown in FIGS. 11 and 12 are cross-sections of the other two flues 2K and 2L. The flues 2K and 2L are made from a first plate 70 which extends into and out of the sheet of drawing to provide a length of flue. The ends may have a transition piece to provide a cylindrical end for connecting the flues 2K and 2L to the end of a water container.

The flue 2K has a second plate 72 which is corrugated and overlies the plate 70 so as to provide three parallel channels 71 , 73 and 75 . The flue 2L is similarly constructed, but it has a third plate 74 which also overlies the plate 70 but on the opposite side on which the plate 72 is placed. The third plate 74 is corrugated and forms three further parallel longitudinally extending channels 77 , 79 and 81 between itself and the first plate 70 . Channels 71 and 77, like channels 73 and 77 and channels 75 and 81, are also in back-to-back positions.

Compared to finned flues, these embodiments of the present invention produce flues with a high ratio of heat transfer surface to flue passage volume and thus higher heat transfer capabilities than finned flues. This is not because the heat transfer surface area is increased, but because a smaller channel is provided (compared to the heat transfer surface area it provides), resulting in an increased convective velocity of the exhaust gas passing through the channel. The increase in convective velocity is produced by the induced or forced draft provided by the fan or blower, by the increase in the velocity of the exhaust air through the channels. Directing the exhaust air only through the channels created by the corrugations provides a certain amount of heat transfer surface area to maximize the use and extraction of as much heat as possible from the fast-flowing exhaust air.

Turning next to FIG. 13 , the thermal heater 100 includes a vessel 112 having a generally cylindrical sidewall 114 , a bottom 116 and a top 118 . Container 112 is preferably made of mild steel and is glazed on the inside and outside. The enamel located on the outside can be replaced by an epoxy coating if desired.

An internal flue 120 extends from the bottom 116 to the top 118 and terminates in a flue outlet 122 at the top of the vessel. The flue 120 in a preferred embodiment is made of corrosion-resistant material, and may have a corrugated structure as described above. The flue 120 is formed by a pair of tubular members coaxially nested within each other, the outer tubular member being corrugated, forming the corrugations indicated at 124 . As already described above, such a corrugated structure may provide excellent heat exchange properties to the inner heating surface of the container. The vessel has a water inlet 126 for introducing cold water into the vessel and a water outlet 128 disposed near the top of the vessel for hot water to exit the vessel.

The container 112 is surrounded by an outer casing 130, which is constructed of a thermally insulating material. The housing 130 is spaced from the top 118 and side walls 114 of the container such that a space 132 is formed between the inner surface of the housing 130 and the outer surface of the container 112 . The space 132 has a substantially annular structure, which forms a flow channel for the exhaust gas to flow out of the flue outlet 122 .

The water heater has a burner 134 which is supplied with gas via a gas supply device 136 and a gas pipeline 38 . The fan 140 forces the combusted gases up through the flue 120 towards the flue outlet 122 so that the flow of gas through the flue is somewhat forced.

The gas flowing out from the flue outlet 122 flows in the direction of the arrow 42 toward an exhaust outlet 144 disposed at the lowest point of the annular space 132 . In this way, the heated gas contacts the vessel 112 during its upward passage through the flue 120 and contacts the vessel again as the heated gas passes downwardly through the annular space 132 towards the exhaust outlet 144 . This ensures that a large proportion of the available heat in the gas has been transferred to the vessel 112 before the gas exits the exhaust outlet 144, thereby ensuring relatively efficient heating of the water contained within the vessel.

We have found that the performance of the water heater can be improved by forming flow directing structures on the inner surface of the housing, ie, the surface facing the annular space 132 . In particular, the flow guiding structure has the effect of disrupting the boundary layer of the passing exhaust gas. In this example, these flow directing structures take the form of ridges or bumps. The effect of these ridges or bumps is that the laminar flow of gas down through the annulus is somewhat disturbed by the bumps, thus ensuring that the heated gas remains in contact with the outer surface of the vessel, allowing heat to be transferred from the gas to the vessel. As previously mentioned, it would also be advantageous to provide some form of flow directing structure on the outer surface of the vessel to assist in the transfer of heat from the gas to the vessel.

The location of the exhaust outlet in the enclosure 130, i.e. the distance below the top of the enclosure 130, depends on the input of the burner to the heater measured in MJ/hr and the surface area required for the auxiliary heat exchanger to condense at the dew point .

The gas exits the main flue (central flue tube) above the dew point and then condenses on the outer surface of the vessel 112 shell. To achieve sufficient condensation, the temperature must drop below the dew point to allow the latent heat of condensation to escape from the flue gas. In this case, the dew point is typically around 65°C and requires cooling to only a fraction of a degree above the water temperature. Preferably, condensation is confined to the outside of vessel 112, as this makes it easier to control condensate and prevent corrosion.

Therefore, if the heat input is increased, either a larger heat exchanger area (ie, vessel surface area bounding the annular space 132) is required, or a smaller cross-sectional area of the annular space 132 is required. A smaller cross-sectional area increases the exhaust gas velocity, but requires a larger fan as a prerequisite.

Many variations may be made to the above-described embodiments without departing from the scope of the invention. For example, the container may be formed slightly differently from that described above, and the housing may likewise be constructed differently than shown herein. However, the applicant believes that the structure shown in the drawings is relatively cheap to manufacture and provides a water heater with 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 described or shown in the description or drawings. All of these different combinations constitute various alternative aspects of the invention.

The embodiments of the present invention have been described above, and it is obvious to those skilled in the art that various modifications can be made without departing from the scope of the present invention.

Claims (32)

1. A flue adapted to pass exhaust gas from one end of a heater to the other, the flue comprising: a central core; a substantially cylindrical elongated outer tube of thermally conductive material surrounding the central core, the outer tube is corrugated in cross-section to form a plurality of corrugations, the outer tube has an inner surface and a The outer surface; the inner surface is in contact with the inner core over at least part of the length of the outer tube, the outer surface is in contact with the fluid to be heated by the heater; and One or more air passages are formed between the corrugations and the inner core, through which exhaust gas flows in use. 2. 3. A flue as claimed in claim 1 wherein the inner core is a solid or closed tubular structure such that in use exhaust gas is constrained upwardly through the air passage. 3. A flue as claimed in any preceding claim wherein said channels formed by said corrugations provide a greater ratio of heat transfer surface area to flue passage volume than a finned flue. 4. A flue as claimed in any preceding claim, wherein the corrugations are axially aligned with the flue axis over at least part of the length of the flue. 5. A flue as claimed in any preceding claim wherein the corrugations extend along at least part of the length of the flue in a helical course around the outside of the inner core. 6. A flue as claimed in any preceding claim, wherein the inner surface of the outer tube is joined to the inner core at the point of contact between the outer tube and the inner core. 7. 3. A flue as claimed in any preceding claim wherein the inner core is a tubular structure having an internal passage blocked by one or more baffles therein. 8. 7. A flue as claimed in claim 7, wherein the internal passage is closed by a plate disposed at or near the upper end of the flue in operation. 9. A flue as claimed in any preceding claim, wherein the inner core is deformed corresponding to the corrugations on the outer tube, the corrugations nesting into the deformations of the inner core. 10. A flue as claimed in any preceding claim, wherein the corrugations are formed in the outer tube by a cold working process with the inner core in place within the outer tube, the corrugations being The radially inner portion is made in contact with the outer surface of the inner core. 11. 10. The flue of claim 10 wherein the inner core itself is deformed during said cold working process to receive said radially inner portions of the corrugations. 12. A flue according to any one of the preceding claims, wherein the corrugations have a contact surface with the inner core between adjacent corrugations, the contact surface being configured as one or more of the following form: A concave surface, a convex surface, a welded surface, a surface having a radius matching the outer surface of the inner core, a surface substantially concentric with the outer surface of the inner core. 13. A flue as claimed in any preceding claim, wherein the flue has a top and a bottom end in the shape of a right cylinder. 14. 3. A flue as claimed in any one of the preceding claims, which is particularly suitable for installation in a water heater. 15. A flue as claimed in any preceding claim wherein the corrugations are evenly distributed around the circumference of the inner core. 16. A flue as claimed in any preceding claim, wherein the corrugations have an amplitude greater than the wall thickness of the outer tube. 17. A flue as claimed in any one of the preceding claims, characterized in that the cross-section of the corrugations is sinusoidal. 18. A flue as claimed in any preceding claim, wherein the corrugations have a first portion parallel to the axis of the flue and a second portion of helical configuration, the second portion being downstream of the first portion. 19. A flue as claimed in any one of the preceding claims, wherein the radially inner portion of the corrugations is, along one of the cores normally in use, connected by means of a connection which prevents the corrugations from coming out of contact with the core. The length corresponding to the length at the scale temperature is connected to the inner core. 20. The flue according to claim 19, wherein the connecting device is one or more of the following: Embossing, welding, spot welding, vacuum welding. twenty one. A water heater equipped with a corrugated flue according to any one of the preceding claims. twenty two. A water heater comprising: a water container having an internal flue extending substantially vertically through the container from the bottom of the container to a flue outlet at the container portion; Heating means for applying heat adjacent to the bottom of the vessel, the heating means having a burner and a fan directing exhaust gases from the burner through the flue; an outer shell surrounding and spaced from at least a portion of the vessel to define a substantially annular space around the vessel, said annular space being in fluid communication with said flue outlet; and an exhaust means for exhausting air from said annular space. It is constructed so that exhaust gases pass through the inner flue and through the annular space before exiting the heater through the exhaust. twenty three. 22. The water heater of claim 22, wherein at least part of the surface of the housing facing the annular space has flow directing structure. twenty four. 23. A water heater as claimed in claim 23 wherein the flow directing structures are in the form of a series of reliefs or bumps on said surface. 25． 23. The water heater of claim 23, wherein said flow directing structure is any one of vanes, fins, corrugations or other fluid directing shapes. 26． 23. The water heater of claim 23, wherein the flow directing structure includes a surface treatment of the inner surface of the housing. 27． 26. A water heater as claimed in any one of claims 22 to 26 wherein the vessel has flow directing structures on an outer surface. 28． 27. A water heater as claimed in any one of claims 22 to 27 wherein at least part of the housing is insulated. 29． A water heater as claimed in any one of claims 22 to 28 wherein both the vessel and the housing are substantially in the shape of a right cylinder and the annular space has a substantially constant width over at least part of its length and around the entire circumference of the vessel . 30． A water heater as claimed in any one of claims 22 to 29 wherein said internal flue is of the type defined in any one of claims 1 to 21. 31． A flue substantially as hereinbefore described with reference to any of the embodiments shown in the accompanying drawings. 32． A water heater substantially as hereinbefore described with reference to any of the embodiments shown in the accompanying drawings.

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