GB2547455A

GB2547455A - A Boiler

Info

Publication number: GB2547455A
Application number: GB1602885.4A
Authority: GB
Inventors: Butler Paul
Original assignee: Chilltecnologies Ltd
Current assignee: Chilltecnologies Ltd
Priority date: 2016-02-18
Filing date: 2016-02-18
Publication date: 2017-08-23
Anticipated expiration: 2036-02-18
Also published as: GB2547455B; GB201602885D0

Abstract

A boiler 12 suitable for an absorption chiller comprises a vessel 36 suitable for storing a working fluid having a longitudinal axis. The vessel comprises first 38 and second 40 ends, a side wall 42 and a plurality of heat receivers (heat exchangers) 46 located in first 52 and second 54 regions. The heat receivers are formed from first and second materials having a first and second thermal conductivities. The first region may be below the second region and the first conductivity is less than the second conductivity. Heat receivers in each region may be of different length to the other region(s) (fig 5), or each heat receiver can be rotationally offset from a subjacent heat receiver and comprises an elongate member in the form of a loop defined by a centre point rotationally offset from the centre point of the subjacent loop. The side wall can be formed from a plurality of stacked lamina or further comprise a plurality of recessed sections. The heat receivers can mounted by brazing and the first and second materials can be steel and copper respectively.

Description

A Boiler

FIELD OF THE INVENTION

The present invention relates to a boiler for an absorption chiller.

BACKGROUND OF THE INVENTION

Absorption chillers are commonly used cooling systems which use thermal energy sources to produce chilled water. Typically, the main components of an absorption chiller are a boiler, a condenser, an evaporator and an absorber. The refrigerant (working fluid) is first heated in the boiler to evaporate it and is subsequently condensed and cooled as it flows around the absorption chiller prior to being absorbed by a water rich solution of the working fluid before flowing back to the boiler. The working fluid is reheated when it is in the boiler, so as to begin the cycle again.

The boiler typically comprises a vessel so as to store the working fluid, and heat from a heat source flows over the surface of the vessel so as to heat the working fluid contained therein. The transfer of heat from the heat source to the working fluid stored in the vessel can be poor resulting in reduced efficiency of the system. Additionally, the distribution of heat transfer along the surface of the vessel is often poor resulting in damage occurring to parts of the vessel faster than others, thus reducing the lifespan of the absorption chiller.

The invention seeks to overcome or at least mitigate the problems associated with the prior art.

SUMMARY OF THE INVENTION A first aspect of the invention provides a boiler for an absorption chiller, the boiler comprising: a vessel for storing a working fluid, the vessel having a longitudinal axis and comprising a first end and a second end, and a side wall extending therebetween the vessel further comprising a plurality of heat receivers extending from the side wall, wherein the vessel defines a first region and a second region along the longitudinal axis and wherein the heat receivers are formed from a first material having a first thermal conductivity in the first region and are formed from a second material having a second thermal conductivity in the second region.

Advantageously, this arrangement enables the tailoring of the amount heat transfer that occurs at different locations along the side wall within the boiler, and may work to improve the overall lifespan of the absorption chiller.

The first region may be provided below the second region, and the first conductivity may be less than the second thermal conductivity.

Advantageously, this enables a more even distribution of heat transfer throughout the boiler. This prevents the majority of the heat from being absorbed by the heat receivers at the bottom of the boiler and improves the durability of the boiler.

The first region may extend approximately half of the longitudinal length of the side wall, and the second region may extend approximately half of the longitudinal length of the side wall.

The first region may extend approximately a quarter of the longitudinal length of the side wall, and the second region may extend approximately three quarters of the longitudinal length of the side wall.

Each heat receiver may be rotationally offset from the heat receiver subjacent thereto.

Advantageously, the offset heat receivers produce more turbulence in the air flowing through the boiler and this results in more efficient heat transfer from the air flowing through the boiler to the heat receivers.

Each heat receiver may include an elongate member configured to form at least one loop therein.

Advantageously, the loop in the heat receiver produces more turbulence in the air flowing through the boiler and this results in more efficient heat transfer from the air flowing through the boiler to the heat receivers when compared to a standard fin heat receiver.

The diameter of the loop may be in the range of 10mm-50mm.

Advantageously, this range of diameters enables the extension of the heat exchanger away from the side wall to be varied. The amount of extension can be varied at different locations within the boiler or can be selected to suit the application.

The diameter of the elongate member may be in the range of 0.5mm to 5mm.

Advantageously, this range of diameters of the member enables the amount of thermal transfer to be varied. The amount of transfer is dependent on the thickness of the elongate member and can be varied at different locations within the boiler or can be selected to suit the application.

The spacing between adjacent loops may vary at different locations along the longitudinal axis of the vessel.

Advantageously, varying the amount of the loops at different locations along the longitudinal axis of the vessel enables the adjustment of thermal transfer a different locations on the side wall.

Each loop defines a centre point and the centre point of each loop may be rotationally offset from the loop subjacent thereto.

Each loop defines a diameter and the offset distance may be smaller than the diameter of the loop.

The side wall may be formed from a plurality of stacked lamina and each lamina may include at least one of the heat receivers extending therefrom.

Advantageously, this provides an easy method of varying the heat receiver material within the boiler, i.e. the heat receiver of each lamina could have different thermal properties to the one subjacent thereto.

The heat receiver of each lamina may have a higher thermal conductivity than the heat receiver on the lamina subjacent thereto.

Advantageously, this produces a gradual change in the thermal conductivity of the heat receivers along the longitudinal axis of the vessel, more evenly distributing the heat transfer.

The vessel may define a first section and a second section along the longitudinal axis and the heat exchangers may extend from the side wall by a first distance in a first section and by a second distance in a second section.

Advantageously, this allows further control of the distribution of heat transfer as the heat exchangers that extend further from the side wall will interact with the flowing through the boiler over a larger surface area, thus increasing the heat transfer.

The first section may be below the second section, and the first distance may be less than the second distance.

The extension of the heat exchangers may be in the range of 10mm and 50mm.

The heat receivers may be mounted to the side wall, e.g. by brazing.

The side wall surface may be provided with a plurality of recessed sections so as to mount the loops of the heat exchanger thereto.

Advantageously, this provides an easy location on which to mount the heat exchangers.

The surface of the recessed regions may be configured to conform to the outer surface of the loop of the heat exchanger.

Advantageously, this arrangement provides a larger contact surface and hence a mounting surface between the side wall of the vessel and the outer surface of the heat exchanger, resulting in a stronger bond between the two.

The heat receivers may be formed integrally with the side wall.

The first material may be steel and the second material may be copper.

The boiler may also be provided with a heat source to supply heat to the boiler. The heat source may be a gas burner. A second aspect of the invention provides a boiler for an absorption chiller, the boiler comprising: a vessel for storing a working fluid having a longitudinal axis; the vessel comprising a top and a bottom and a side wall extending therebetween; wherein the side wall comprises a heat receiver extending from the vessel, wherein the heat receiver comprises an elongate member configured to form at least one loop therein.

Advantageously, the loop in the heat receiver produces more turbulence in the air flowing through the boiler and this results in more efficient heat transfer from the air flowing through the boiler to the heat receivers when compared to a standard fin heat receiver. Also, providing a heat receiver in the form a loop of material enables a single length of material to have multiple loops, and hence multiple heat receivers from a single piece of material and hence is more reliable.

Each loop may be rotationally offset from the heat receiver subjacent thereto.

Advantageously, this increases the turbulence that is induced in the flow of heat produced by the heat source.

The at least one side wall may define a cylindrical surface and the heat receiver may substantially spiral around said cylindrical surface.

All of the heat exchangers may be formed from a single piece of material., e.g. from a single elongate member.

The diameter of the loop may be in the range of 10mm-50mm.

The diameter of the elongate member may be in the range of 0.5mm to 5mm.

The spacing between adjacent loops may vary along the longitudinal axis of the vessel.

Advantageously, varying the number density of the loops enables the adjustment of thermal transfer a different locations on the side wall.

The side wall may be formed from a plurality of stacked lamina and each lamina may include at least one heat receiver extending therefrom.

The vessel may define a first section and a second section along the longitudinal axis, and wherein the heat exchangers may extend from the side wall by a first distance in a first section and by a second distance in a second section.

The first region may be below the second region and the first distance may be less than the second distance.

The heat receivers may be mounted to the side wall.

The side wall surface may have a plurality of recessed sections so as to mount the loops of the heat exchanger thereto.

The boiler may include a heat source to supply heat to the boiler. The heat source may include a gas burner.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference to the accompanying drawings, in which:

Figure 1 is a front view of an absorption chiller comprising a boiler according to an embodiment of the present invention;

Figure 2 is a schematic flow path of the absorption chiller of Figure 1;

Figure 3 is a cross sectional view of a boiler according to an embodiment of the present invention;

Figure 4 is a cross sectional view of a boiler according to an embodiment of the present invention;

Figure 5 is a cross sectional view of a boiler according to an embodiment of the present invention;

Figure 6 is a front view of a boiler according to an embodiment of the present invention;

Figure 7 is an end view of a section of the boiler of Figure 6; and

Figure 8 is a plan view of a section of the boiler according to an embodiment of the present invention. DETAIFED DESCRIPTION OF EMBODIMENT(S)

Referring firstly to Figure 1, an absorption chiller is illustrated generally at 10. The absorption chiller 10 comprises a boiler 12 which includes a vessel (not shown) for storing a working fluid therein. The boiler 12 also includes a heat source 13, for heating the working fluid stored in the vessel of the boiler 12. Additionally or alternatively, the boiler 12 may be provided with a secondary heat source in the form of waste heat gases which enter the boiler 12 via a waste heat inlet 115 (Figure 2). The working fluid is transported around a high pressure flow path of the absorption chiller 10 by the pressure that is generated in the boiler 12. The working fluid is then transported around a low pressure flow path of the absorption chiller 10 by means of a working fluid pump, e.g. in the form of a diaphragm solution pump 34. In this embodiment, the working fluid is a water-ammonia solution but any suitable working fluid, such as a lithium bromide-water solution or a lithium chloride-water solution may be used. Following evaporation of the working fluid in the boiler 12, the working fluid (now gaseous) flows into a levelling chamber 14 to stabilise the pressure of the working fluid vapour. The working fluid then passes through a first condenser 16 to remove water from the working fluid and then a second condenser 18. In the condenser 18, air is drawn through the condenser coils resulting in a working fluid that has been cooled and returned to a liquid state. A heat exchanger 20 is located downstream of the second condenser 18 and the working fluid flows in thermal communication with the heat exchanger 20, so as to further cool the working fluid. The working fluid then passes through a restrictor 22, which causes an expansion of the working fluid, resulting a drop in pressure and temperature of the working fluid. A further heat exchanger 24 is provided downstream of the restrictor 22. The heat exchanger 24 is maintained at a constant temperature by a chilled water circuit 26, whereby the heat exchanger 24 is configured to cause heating of the working fluid. Due to the low pressure of the working fluid, the raise in temperature results in evaporation of the working fluid.

The absorption chiller 10 further includes two absorbers 28, 30 which are located downstream of a third heat exchanger 23. In this embodiment, absorber 28 is in the form of a standard plate heat exchanger. However, any suitable heat exchanger may be used. Internal to absorber 28, the working fluid vapour is mixed with a water rich ammonia solution that works to maximise absorption of the working fluid in the absorbers 28 and 30. Following exiting the absorber 30, the working fluid then flows through a working fluid store 32 constantly feeding the diaphragm solution pump.

The working fluid is transported around the low pressure working fluid flow path of the absorption chiller 10, by means of the diaphragm solution pump 34, through which the working fluid passes before re-entering the boiler 10 to begin the cycle again.

Referring to Figure 3, a boiler 12 according an embodiment of the present invention is indicated generally at 12. The boiler 12 comprises a primary heat source 13, in the form of a gas burner or alternatively an oil burner, and a vessel 36 configured to store a working fluid therein. The vessel 36 is located proximate to the outer wall of the boiler 12 and is substantially ring shaped in plan view, to define a flue extending vertically through the vessel. The vessel 36 includes a top wall 38, a bottom wall 40, and two side walls in the form of an inner wall 42 and an outer wall 44. In the illustrated embodiment, the outer wall 44 of the vessel 36 forms part of the wall of the boiler 12. In an alternative embodiment, the outer wall 44 of the vessel 36 is spaced apart from the wall of the boiler 12. The inner side wall 42 includes a plurality of heat exchangers 46 mounted thereto. The heat exchangers 46 are in a form which substantially extends away from the surface of the side wall 42 of the vessel 36 in to the flue. The heat exchangers 46 are configured for heating the working fluid contained within the vessel 36 by conducting heat generated by the primary heat source 13 to heat the inner wall 42 of the vessel 36.

In the illustrated embodiment, the heat exchangers 46 are all identical. However, in such a configuration, the majority of the heat from the heat source 13 will be received by the heat exchangers 46 that are closer to the bottom 40 of the vessel 36. This produces poor distribution of heat transfer to the working fluid. Additionally, due to disparity of heat transfer between the heat receivers 46 near the bottom 40 and the top 38 of the vessel 36, the heat receives 46 near bottom 40 will degrade quicker, shortening the lifespan of the boiler 12.

In an alternative embodiment, the heat exchangers may be divided into groups, as indicated by 52, 54, 56 in Figure3. The heat receivers 46 contained within each group 52, 54, 56 have the same thermal conductivity properties, but the thermal conductivity of each group may be different from the others. This enables thermal properties of the heat exchangers to be tailored at different locations in the boiler. In one embodiment, the thermal conductivity of the heat exchangers 46 in a first group 52, located immediately below the second group 54 proximate the bottom 40 of the vessel 36, is lower than that of the second group 54. Similarly, the heat exchangers 46 of the second group 54, located below a third group 56, is lower than that of the third group 56. This arrangement advantageously improves the distribution of thermal transfer to the vessel 36. The heat exchangers of the different groups 52, 54, 56 may be formed from different materials, e.g. steel, copper. It will also be appreciated that the relative longitudinal lengths of the groups 52, 54, 56 may be varied to suit the application, and the heat exchanger may be provided in two, four or any suitable number of different groups where each group may be formed from different materials.

Referring to Figure 4, a boiler according to an embodiment of the invention is indicated generally at 112. Like features with respect to the boiler 12 of Figure 3 are labelled with the prefix "1" and only differences are discussed.

In this embodiment, the outer wall 144 of the vessel 136 is spaced apart from the outer wall of the boiler 112. The vessel 136 is ring shaped in plan view and defines a flue extending vertically through the vessel 136. However, the boiler 112 is configured so that flue gases also flow between the wall of the boiler 112 and the outer wall 144 of the vessel 136. In addition to the heat exchangers 146, the boiler 112 is also provided with heat exchangers 148, which extend substantially away from the side wall 144 of the vessel 136 towards the outer wall of the boiler 112. In alternative embodiments, the boiler 112 may only include the heat exchangers 148 and not the heat exchangers 146. The heat exchangers 146, 148 may also be grouped into groups 152, 154, 156 having differing thermal conductivities, e.g. as has been described with reference to Figure 3.

Referring to Figure 5, an alternative boiler according to an embodiment of the invention is indicated generally at 212. Some of the features with respect to figure 3 are referenced with the prefix 2 and only differences are discussed.

In the illustrated embodiment, the boiler 212 includes a variety of heat exchangers 246, which extend substantially away from the side wall 242 of the vessel 236. The heat exchangers 246 are divided into 3 groups; a first group 258 located proximate to the bottom 240 of the vessel 236; a second group 260 located immediately above the first group of heat exchangers 258 proximate to the middle of the vessel 236; and a third group 262 located immediately above the second group of heat exchangers 260 and proximate to the top 238 of the vessel 236. Each of the heat exchangers 246 extends substantially away from the side wall 242 of the vessel 236, but the amount of extension away from said side wall differs between the groups 258, 260, 262. The first group of heat exchangers 258 extend into the flow of the boiler 212 by a distance which is greater than that of the heat exchangers of the second group 260. The extension of the heat exchangers of the second group 260 is in turn greater than the distance of extension of the third group of heat exchangers 262. This arrangement of the heat exchangers 246 enables an improved distribution of thermal transfer from the heat flowing past the heat exchangers 246 from the heat source 213 to be transferred to the vessel 236 so as to heat the working fluid. The amount of extension of the heat exchangers 236 may vary between 10mm and 50mm, and more specifically may vary between 20mm and 40mm. Extensions of between 10mm and 50mm would typically be used for a flue having a diameter of between 90mm and 270mm, but it will be appreciated that the amount of extension of the heat exchangers may be varied to suit the application. It will be further appreciated that the extension of the heat exchangers away from the side wall may be varied to suit the fuel that is used in the boiler, the flow rate of the working fluid through the boiler and/or the temperature generated by the boiler. The external heat exchangers may also have the same features as the internal heat exchangers of Figure 5.

Referring to Figures 6 to 8, a boiler according to an embodiment of the invention is indicated generally at 312. Similar Features with respect to Figures 3 to 5 are labelled with the prefix “3” and only differences are discussed. In the illustrated embodiment, an alternative configuration of the heat exchangers is provided in comparison to those described in reference to Figures 3 to 5.

The heat exchangers 346 are provided in the form of an elongate member which is configured to form a loop therein. In this configuration, each loop essentially forms a heat exchanger 346, but one piece of material, extending up to around 1000m, may be produced to include several of the loops. Advantageously, the loops provide a larger contact surface area for the heat produced by the heat source to flow over, thus increasing the heat transfer from the heat source to the working fluid stored within the vessel. Additionally, the loops induce a great amount of turbulence in the heat flow produced by the heat source 313. The elongate member may be formed from steel, copper or any other material having a suitable thermal properties.

The elongate member is positioned so as to spiral around the outer wall of the vessel 336 and this allows all of the heat exchangers 346, in the form of loops, to be formed from a single piece of material. Typically, conventional boilers comprising heat exchangers are composed a series of stacked lamina with each lamina having at least one fin. However, the configuration as illustrated in Figure 6 to 8, enables the boiler to be produced separately to the heat exchangers, where the heat exchangers can then be mounted to the boiler at a later stage. This enables the boiler to be formed from a single sheet of material, which reduces the cost and time required to manufacture a boiler and results in no failure points between layers that are present between the stacked lamina.

Furthermore, the loops may be rotationally offset from the loops immediately above and below, to prevent there from being a direct flow path from the bottom to the top of the boiler. Each loop may be configured to overlap the loop that is subjacent thereto. The distance that a subjacent loop is offset with respect to a first loop will be less than the diameter of the loop; more specifically the offset distance will be a half, a quarter or a tenth of the diameter of the loop. This arrangement increases the turbulence induced in the heat flow, thus increasing the heat transfer of heat from the heat source 313 to the working fluid stored in the vessel 336.

The diameter of the loops 346 produced in the elongate member may be varied between 10mm and 50mm, and more specifically between 20mm and 40mm. However, it will be appreciated that the diameter of the loops may be varied to suit the dimensions of the boiler that is being used. The diameter of the loops 346 determine the amount of extension of the heat exchanger, i.e. loop, away from the side wall of the vessel 336.

It will be appreciated that the separation between adjacent loops along the elongate member may be varied at different positions within the boiler. For example, the spacing between adjacent loops may be at a first spacing in a region close to the bottom 340 of the vessel 336 and may be at a second spacing in a region close to the top 338 of the vessel 336, where the first spacing in shorter than the second spacing.

This arrangement results in a variation in the amount of loops attached the vessel along the length of the wall 342, 344 of the vessel 336 and results in fewer heat exchangers closer to the bottom 340 of the vessel 336 than closer to the top 338 of the vessel 336. This results in a smaller surface area of the heat exchangers for heat transfer to occur over closer to the bottom than closer to the top. This works to improve the distribution of heat transfer along the length of the vessel 336 of the boiler 312.

In this embodiment, the thickness of the elongate member may be varied to suit the application as required, with a thicker diameter of wire providing an improved thermal transfer. The elongate member is typically manufactured from steel with a diameter of between 0.5mm to 5mm. However, alternative thicknesses may also be suitable depending on the type of material that is used.

When the loops are mounted to an external surface of the vessel, the contact area between the loop and the vessel (where the loop is mounted to the vessel) is small. Due to this, the bonding strength may be limited. In order to increase this bonding strength, the surface of the vessel may be provided with recesses so as to accommodate each of the loops.

The recessed regions may be configured to conform to the outer surface of the loop. That is, each recessed region defines a curved surface has substantially the same radius of curvature as the loop that is to be attached thereto. This maximises the contact area between the vessel and the loop, thus maximises the bonding strength. In the illustrated embodiment, the loops are welded to the vessel walls but any alternative bonding method such as braising etc. may be used.

The loops as illustrated in Figures 6 to 8 may also be incorporated into the boilers as illustrated in Figures 3 to 5. That is, the thermal conductivity of the heat exchanger and thus the loops may be varied at different locations within the boiler to further improve the distribution of thermal transfer from the heat source to the working fluid.

Although the invention has been described above with reference to one or more preferred embodiments, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims.

Claims

1. A boiler for an absorption chiller, the boiler comprising: a vessel for storing a working fluid, the vessel having a longitudinal axis and comprising a first end and a second end, and a side wall extending therebetween; the vessel further comprising a plurality of heat receivers extending from the side wall, wherein the vessel defines a first region and a second region along the longitudinal axis and wherein the heat receivers are formed from a first material having a first thermal conductivity in the first region and are formed from a second material having a second thermal conductivity in the second region.

2. The boiler according to claim 1, wherein the first region is below the second region, and wherein the first conductivity is less than the second thermal conductivity.

3. The boiler according to claim 2, wherein the first region extends approximately half of the longitudinal length of the side wall, and wherein the second region extends approximately half of the longitudinal length of the side wall.

4. The boiler according to claim 2, wherein the first region extends approximately a quarter of the longitudinal length of the side wall, and wherein the second region extends approximately three quarters of the longitudinal length of the side wall.

5. The boiler according to any preceding claim, wherein each heat receiver is rotationally offset from the heat receiver subjacent thereto.

6. The boiler according to any preceding claim, wherein each heat receiver comprises an elongate member configured to form at least one loop therein.

7. The boiler according to claim 6, wherein the diameter of the loop is in the range of 10mm-50mm.

8. The boiler according to claim 6, wherein the diameter of the elongate member is in the range of 0.5mm to 5mm.

9. The boiler according to any one of claims 6 to 8 when dependent upon claim 5, wherein the spacing between adjacent loops varies at different locations along the longitudinal axis of the vessel.

10. The boiler according to any one of claims 6 to 9 when dependent upon claim 5, wherein each loop defines a centre point and the centre point of each loop is rotationally offset from the loop subjacent thereto.

11. The boiler according to claim 6 when dependent upon claim 5, wherein each loop defines a diameter and the offset distance is smaller than the diameter of the loop.

12. The boiler according to any preceding claim, wherein the side wall is formed from a plurality of stacked lamina and each lamina comprises at least one of the heat receivers extending therefrom.

13. The boiler according to claim 12, wherein the heat receiver of each lamina has a higher thermal conductivity than the heat receiver on the lamina subjacent thereto.

14. The boiler according to any preceding claim, wherein the vessel defines a first section and a second section along the longitudinal axis and wherein the heat exchangers extend from the side wall by a first distance in a first section and by a second distance in a second section.

15. The boiler according to claim 14, wherein the first section is below the second section, and wherein the first distance is less than the second distance.

16. The boiler according to claim 14 or claim 15, wherein the extension of the heat exchangers is in the range of 10mm and 50mm.

17. The boiler according to any preceding claim, wherein the heat receivers are mounted to the side wall, e.g. by brazing.

18. The boiler according to any one of claims 6 to 17, wherein the side wall surface further comprises a plurality of recessed sections so as to mount the loops of the heat exchanger thereto.

19. The boiler according to claim 18, wherein the surface of the recessed regions is configured to conform to the outer surface of the loop of the heat exchanger.

20. The boiler according to any one of claims 1 to 16, wherein the heat receivers are formed integrally with the side wall.

21. The boiler according to any preceding claim, wherein the first material is steel and the second material is copper.

22. The boiler according to any preceding claim, further comprising a heat source to supply heat to the boiler.

23. The boiler according to claim 22, wherein the heat source comprises a gas burner.

24. A boiler for an absorption chiller, the boiler comprising: a vessel for storing a working fluid having a longitudinal axis; the vessel comprising a top and a bottom and a side wall extending therebetween; wherein the side wall comprises a heat receiver extending from the vessel, wherein the heat receiver comprises an elongate member configured to form at least one loop therein.

25. The boiler according to claim 24, wherein each loop is rotationally offset from the heat receiver subjacent thereto.

26. The boiler according to claim 24 or claim 25, wherein each loop defines a centre point and the centre point of each loop is rotationally offset from the loop subjacent thereto.

27. The boiler according to claim 25 or claim 26, wherein each loop defines a diameter and the offset distance is smaller than the diameter of the loop.

28. The boiler according to any one of claims 24 to 27, wherein the at least one side wall defines a cylindrical surface and the heat receiver substantially spirals around said cylindrical surface.

29. The boiler according to claim 28, wherein all of the plurality of heat exchangers are formed from a single piece of material.

30. The boiler according to any one of claims 24 to 29, wherein the diameter of the loop is in the range of 10mm-50mm.

31. The boiler according to any one of claims 24 to 30, wherein the diameter of the elongate member is in the range of 0.5mm to 5mm.

32. The boiler according to any one of claims 24 to 31, wherein the spacing between adjacent loops varies along the longitudinal axis of the vessel.

33. The boiler according to any one of claims 24 to 32, wherein the side wall is formed from a plurality of stacked lamina and wherein each lamina comprises at least one heat receiver extending therefrom.

34. The boiler according to any one of claims 24 to 33, wherein the vessel defines a first section and a second section along the longitudinal axis, and wherein the heat exchangers extend from the side wall by a first distance in a first section and by a second distance in a second section.

35. The boiler according to claim 34, wherein the first region is below the second region and the first distance is less than the second distance.

36. The boiler according to any one of claims 24 to 35, wherein the heat receivers are mounted to the side wall.

37. The boiler according to claim 36, wherein the side wall surface further comprises a plurality of recessed sections so as to mount the loops of the heat exchanger thereto.

38. The boiler according to claim 37, wherein the surface of the recessed regions is configured to conform to the outer surface of the loop of the heat exchanger.

39. The boiler according to any one of claims 24 to 38, further comprising a heat source to supply heat to the boiler.

40. The boiler according to claim 39, wherein the heat source comprises a gas burner.

41. A boiler substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.