GB2255169A - Gas boiler with excess air - Google Patents

Abstract

A gas boiler is supplied with at least sufficient air to operate at its maximum output, enough for stoichiometric combustion being delivered to a burner 8 and the excess being discharged from the boiler. The gas fired boiler comprises a casing 5, a burner assembly 8, vortex means 7 for supplying a gas/air mixture for combustion at the burner assembly in approximately stoichiometric proportions, a flue outlet 10, a heat exchanger 9 between the burner assembly 8 and the flue outlet 10 and a fan 6 operable to draw air at 6A into the casing 5 and to discharge combustion products to the flue outlet at 68, air flow passages being provided within the casing which enable air drawn into the casing by the fan 10 to flow both to the fuel supply means 7 and hence to the burner 8 (air A1, A2) or to bypass the burner and heat exchanger and pass directly to the flue outlet (air A3, Fig 1 not shown). The burner 8 comprises a series of water filled tubes 20 interconnected by spaced parallel plates 21 at the surface of which the fuel mixture burns. <IMAGE>

Description

Gas Fired Boilers This invention relates to gas fired boilers.

In boilers fueled by gaseous fuels comprising a mixture of gas and air it is desirable so far as possible that the gas and air should be mixed in the correct stoichiometric ratio. This produces theoretically complete combustion with minimum emission of noxious combustion products such as nitrous oxide. Conventional injectors utilised to mix gas and air in gas boilers do not produce correct stoichiometric conditions. Vortex devices are known which will automatically mix gas and air in the correct proportions for stoichiometric combustion, but such devices present problems in conventional gas boilers where fans are employed to discharge products of combustion through a flue. The fan controls the rate of air flow through the appliance and would thereby prevent a vortex type device from functioning correctly.

Moreover modern gas boilers commonly have a variable rating to enable boiler output to be adjusted to different installation situations, and in order to comply with safety requirements boilers must be capable of operating satisfactorily at up to 15 overload, that is 15 above the rated output of the boiler. These conflicting requirements make it difficult to maintain optimum gas/air ratios during boiler operation with the result that high levels of nitrous oxide can be discharged in some circumstances.

It is an object of the present invention to obviate or mitigate these disadvantages.

According to the invention there is provided a gas fired boiler comprising a casing, a burner assembly, fuel supply means for supplying a gas/air fuel mixture for combustion at said burner assembly in approximately stoichiometric proportions, a flue outlet, a heat exchanger by means of which heat from the combusted fuel is transferred to water to be heated by the boiler, said heat exchanger being disposed between said burner assembly and said flue outlet, fan means operable to draw air into said casing and to discharge combustion products to said flue outlet, and means providing air flow passages within said casing to enable air drawn into the housing by said fan means to flow both to said fuel supply means and hence to said burner and to by-pass said burner and heat exchanger and pass directly to said flue outlet.

Preferably said fuel supply means comprises a vortex type mixing device, for example of the kind shown in U.K.

Patent Specification 1362781 or 1469097. Such devices automatically draw in the necessary volume of air to produce complete combustion of the gas supplied to the device and thereby produce essentially stoichiometric conditions automatically over a range of gas supply volumes and pressures. The device accordingly draws from the total air drawn into the casing the volume required to produce complete combustion of the gaseous fuel, the remainder of the air drawn into the casing by said fan means by-passing said burner and heat exchanger and passing directly to said flue outlet together with the combustion products from the burner.

Preferably also said burner assembly comprises a diffuser comprising an assembly of spaced, finned pipes which form passages for water to be heated, the fins being closely spaced such that the spaces between the fins constitute flame ports of the burner assembly at which the gaseous fuel mixture combusts.

Advantageously said air flow passages include a first passage within which said vortex mixing device is mounted to draw air into said burner assembly, and a second passage incorporating a flue hood disposed above but spaced from said heat exchanger, whereby excess air not required for combustion may pass directly into said flue hood and hence to said flue outlet without passing through said heat exchanger.

The invention also provides a method of operating a gas fired boiler comprising introducing a supply of combustion air at least sufficient to operate the boiler at its maximum rated output, drawing from said supply at any given time sufficient air to produce approximately stoichiometric combustion of a gas supply delivered to the boiler, and discharging the balance of said air supply from the boiler. Preferably the balance of said air supply is discharged with products produced by combustion of said stoichiometric gas/air mixture.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Fig. 1 is a vertical cross-section through one form of boiler according to the invention; Fig. 1A is an enlarged cross-section through a fuel mixing device incorporated in the boiler of Fig. 1; Fig. 2 is a vertical cross-section through an alternative form of boiler; and Fig. 3 is a section on the line III-III in Fig. 2.

Referring to Fig. 1, there is shown a boiler comprising an outer casing 5 surmounted by a fan assembly 6 and incorporating a fuel mixing device 7, burner assembly 8, heat exchanger 9 and flue hood 10.

The fuel mixing device 7 is mounted in the lower region of the casing 5 and comprises a vortex device, for example of the kind described in U.K. Patent Specification 1362781 or 1469097. Gas is supplied to the vortex device through a pipe 12 from a gas supply valve 13. As best seen in Fig. 1A, gas enters an annular chamber 14 in the device 7 from whence it passes through inclined passages 15 to a central passage 16. By virtue of the manner of introduction of the gas through the passages 15, a vortex action is set up within the through passage 16 which is open at both ends. This draws air into the device as indicated by the arrows Al. This air is intimately mixed with the gas due to the vortex action and the resulting gas/air mixture exiting from the device draws in further air as indicated by the arrows A2.The construction of the device is such that as the gas velocity is varied, the gas automatically draws in the correct volume of air required to produce a stoichiometric mixture which is delivered through a chamber 18 to the burner 8.

The burner 8 comprises a series of tubes 20 interconnected by spaced parallel plates 21 defining a series of fins at the surface of which the gaseous fuel mixture is ignited by an ignition device 22. The tubes 20 are connected to headers 23A and 23B, the header 23A being connected to a water supply pipe 24 and the header 23B to a return pipe 25 connected to the heat exchanger 9.

The space 26 above the burner assembly 8 constitutes a combustion chamber above which the heat exchanger 9 is mounted. The heat exchanger comprises a series of tubes 30 of somewhat larger diameter than the tubes 20 and interconnected by a similar series of fins 31 but of larger extent and wider spacing. The tubes 30 are interconnected at their ends by manifold members 32 such that water to be heated passes through the tubes in turn and then leaves the heat exchanger via an outlet pipe 33 from whence it is supplied to a hot water or heating system. Combustion products from the combustion chamber 26 pass over the heat exchanger and thus transfer heat to the water passing through the tubes.

The flue hood 10 is mounted above and spaced from the upper surface of the heat exchanger 9. Combustion products from the heat exchanger travel across this space into the hood 10 under the suction provided by the fan 6 and are then discharged through a flue outlet. By virtue of the spacing of the hood 10 above the heat exchanger 9, excess air drawn into the casing by the fan 6, but not required for combustion purposes, may pass directly into the hood 10 as indicated by the arrows A3 and is then exhausted along with the combustion products. The fan 6 comprises concentric outer and inner pipes 6A and 6B, the pipe 6A being connected to the suction side of the fan and drawing air into the casing and the pipe 6B being connected to the exhaust side of the fan and discharging combustion products and excess air drawn into the flue hood 10.

In operation the fan is set to operate at a speed sufficient to draw into the casing at least sufficient air to enable the boiler to operate at its maximum rating.

Since the vortex mixing device 7 automtically draws in sufficient air to produce complete combustion of the gas supplied to it, under such conditions essentially all of the air drawn into the casing 5 will be used for combustion purposes and little or no air will follow the path A3 directly into the flue 10. However, when the applicance operates at less than its full rating, although the same quantity of air will be drawn into the housing 10 by the fan 6, the mixing device 7 will take only the quantity of air required to combust the gas supplied to it. The excess air will then follow path A3 directly into the flue hood 10 for extraction with the flue gases.

Under all conditions of gas supply, the mixing device 7 will draw in the quantity of air required for complete. combustion and hence essentially stoichiometric combustion is attained under all conditions. The fan 6 is arranged to operate in such a way that its suction effect is essentially confined to the upper part of the casing 5, air for combustion being drawn downwards by the vortex action of the mixing device 7 and the remaining air passing between the heat exchanger 9 and directly into the flue hood 10 under the influence of the exhaust suction of the fan. If the fan were operated in such a way as to draw all the air through the burner assembly and heat exchanger this would interfere with the vortex mixing device 7 and thereby prevent correct operation.

Figs. 2 and 3 of the drawings show a modified boiler in which the same reference numerals have been used to identify parts corresponding to those in Fig. 1. The boiler of Figs. 2 and 3 includes an additional heat exchanger 35 through which products of combustion pass after the heat exchanger 9 and from which the combustion products exhaust to the flue hood 10. Excess air not required for combustion by the mixing device 7 passes through an external passage 36 in the direction of the arrows A3 and is directed over the top of the additional heat exchanger 35 to the flue hood 10. The entrance to the heat exchanger 35 and the passage 36 are interconnected by an opening 37 the purpose of which will be described hereafter.

The boiler of Figs. 2 and 3 is a variable rating boiler which may be adjusted on site to operate at different outputs dependent on site conditions. It also operates as a modulating boiler which self-adjusts its output to meet demand.

When the boiler is operated at its nominal maximum output rating, the mixing device 7 will draw in a substantial proportion of the air drawn into the casing by the fan 6 to produce a stoichiometric combustion mixture.

Combustion products pass through the heat exchanger 9 in which heat is transferred to water to be heated by the boiler. The combustion products are then drawn by the fan 6 through the heat exchanger 35 to extract additional heat. The fan is operated such that the suction effect is just sufficient to ensure that all of the combustion products exiting from the heat exchanger 9 are drawn into the heat exchanger 35 without affecting the supply of combustion air to the vortex device 7. Excess air not drawn in by the vortex device for combustion, passes up the passage 36 and over the top of the heat exchanger 35 to the flue hood 10 from which it is discharged by the fan. Under these conditions very little of the air in the passage 36 is drawn into the heat exchanger 35 through the opening 37.Thus essentially hot combustion products only pass through the heat exchanger and the efficiency of heat exchange is high.

When the boiler is operated under overload conditions, the quantity of air drawn into the casing 5 by the fan 6 remains the same. However as the gas supply to the burner is increased, the vortex mixing device 7 entrains a higher proportion of the air in order to maintain the gas and air mixture in stoichiometric proportions. As a result a larger quantity of combustion products are produced compared with the case where the boiler is operating at its normal rating, and after passing through the heat exchanger 9, excess combustion products are presented at the entrance to the heat exchanger 35 which cannot handle the excess volume as the flow rate through the heat exchanger would exceed that for which the fan has been set.Under these conditions the excess combustion products pass through the opening 37 into the passage 36 and are drawn out with the air flowing through the passage, the volume of which is substantially less than under normal operating conditions since a higher proportion of the incoming air has been used for combustion.

When the boiler is operated at its lowest rated output or modulated setting, the vortex mixing device will again draw in sufficient air to produce a stoichiometric mixture and a larger proportion of the incoming air will pass through the passage 36. Under these conditions the products of combustion all pass through the heat exchangers 9 and 35 in turn and into the flue hood. As there is a lower volume of combustion products the fan 6 will draw a proportion of the air passing through the passage 36 into the heat exchanger 35 through the opening 37. This has the effect of diluting the combustion products passing through the heat exchanger 35 but does not substantially impair efficiency as under these conditions the bulk of the heat is extracted from the combustion products in the heat exchanger 9.

By virtue of the arrangements described, essentially complete combustion of the gaseous fuel mixture is attained over a range of operating conditions since the vortex device automatically takes in the correct proportion of air for complete combustion of the gas supplied to it. By virtue of the provision of alternative air flow paths within the boiler, air in excess of that required for complete combustion by-passes the vortex device and heat exchangers and is drawn out of the casing with the combustion products. Thus, while sufficient air to provide complete combustion under all conditions is drawn into the casing at all times, only the air required for combustion is used for that purpose, thereby maintaining complete combustion under all conditions and reducing the emission of noxious fumes. Tests carried out with boilers constructed according to the invention have produced nitrous oxide emissions as low as 50 ppm.

compared with emissions of 150-200 ppm. in conventional boilers.

A further advantage arising from the arrangements described is that since the excess air which by-passes the heat exchangers is mixed with the combustion products before entering the flue hood 10 and discharge from the boiler, the combustion products are diluted and cooled and it is therefore possible to construct the flue hood and components of the fan from less heat resistant materials.

Such components may, for example, be constructed from suitable plastics materials rather than from metals.

Various modifications may be made without departing from the invention. For example, alternative forms of burner and different types and configuration of heat exchanger may be employed. Alternative forms of mixing device adapted to mix gas and air in correct stoichiometric proportions may be utilised and the type and position of the fan means and the number and arrangement of alternative passages for air flow may be altered. The invention may also be applied to both condensing and non-condensing boilers.

Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.

Claims (17)

Claims:

1. A gas fired boiler comprising a casing, a burner assembly, fuel supply means for supplying a gas/air fuel mixture for combustion at said burner assembly in approximately stoichiometric proportions, a flue outlet, a heat exchanger by means of which heat from the combusted fuel is transferred to water to be heated by the boiler, said heat exchanger being disposed between said burner assembly and said flue outlet, fan means operable to draw air into said casing and to discharge combustion products to said flue outlet, and means providing air flow passages within said casing to enable air drawn into the housing by said fan means to flow both to said fuel supply means and hence to said burner and to by-pass said burner and heat exchanger and pass directly to said flue outlet.

2. A boiler according to claim 1 wherein said fuel supply means comprises a vortex type mixing device.

3. A boiler according to claim 1 or 2 wherein said burner assembly comprises a diffuser comprising an assembly of spaced, finned pipes which form passages for water to be heated, the fins being closely spaced such that the spaces between the fins constitute flame ports of the burner assembly at which the gaseous fuel mixture combusts.

4. A boiler according to any preceding claim wherein said air flow passages include a first passage within which said vortex mixing device is mounted to draw air into said burner assembly, and a second passage incorporating a flue hood disposed above but spaced from said heat exchanger, whereby excess air not required for combustion may pass directly into said flue hood and hence to said flue outlet without passing through said heat exchanger.

5. A boiler according to any preceding claim including a further heat exchanger between said first mentioned heat exchanger and said flue outlet, said passage means enabling air to by-pass both of said heat exchangers.

6. A boiler according to claim 5 including an opening interconnecting said by-pass passage and the zone between said heat exchangers, whereby to enable flow between same in one or other direction under different operating conditions.

7. A gas fired boiler substantially as hereinbefore described with reference to Figs. 1 and 1A of the accompanying drawings.

8. A gas fired boiler substantially as hereinbefore described with reference to Figs. 2 and 3 of the accompanying drawings.

9. A method of operating a gas fired boiler comprising introducing a supply of combustion air at least sufficient to operate the boiler at its maximum rated output, drawing from said supply at any given time sufficient air to produce approximately stoichiometric combustion of a gas supply delivered to the boiler, and discharging the balance of said air supply from the boiler.

10. A method according to claim 9 wherein the balance of said air supply is discharged with products produced by combustion of said stoichiometric gas/air mixture.

11. A method according to claim 9 or 10 wherein said balance of said air supply is introduced into said combustion products after the latter have passed over a heat exchanger to transfer heat to water passing through the heat exchanger.

12. A method according to claim 9 or 10 wherein said balance of said air supply is introduced into said combustion products between heat exchangers over which the combustion products are directed before discharge from the boiler.

13. A method according to claim 12 wherein, under certain conditions of operation, at least a proportion of said combustion products may by-pass one or more of said heat exchangers before discharge from the boiler.

14. A method according to claim 12 or 13 wherein, under certain conditions of operation, at least a proportion of said balance of said air supply, may pass over at least one of said heat exchangers before discharge from the boiler.

15. A method according to any of claims 9 to 14 wherein said supply of combustion air introduced into said boiler is at least sufficient to operate the boiler at 15 over its maximum rated output.

16. A method of operating a gas fired boiler substantially as hereinbefore described.

17. Any novel subject matter or combination including novel subject matter disclosed in the foregoing specification or claims and/or shown in the drawings, whether or not within the scope of or relating to the same invention as any of the preceding claims.