Air Duct in a Boiler

Field of the Invention

The present invention concerns a boiler with a combustion chamber for burning fuel, where means for supplying combustion air are provided in the combustion chamber, where the combustion air is preferably supplied via apertures in the bottom of the combustion chamber under the fuel, which is provided on the air supply means, where the combustion chamber further includes at least one air duct for supplying combustion air to the combustion, where the at least one air duct is adapted for supplying combustion air at at least one level above the bottom of the combustion chamber. The, invention furthermore concerns an air duct for such a boiler.

Background of the Invention

It is common knowledge that in a boiler for burning various fuels, including solid fuels as well as liquid or gaseous fuels, a combustion which is as complete as possible is desired under all conceivable conditions. Particularly by using solid fuel where ashes are formed during the combustion, there may arise situations where the supply of air - combustion air - is changed as an ash layer is built up at the bottom of the boiler. In the boilers it is thus common to arrange a grate at the bottom upon which the fuel is provided, and where air is supplied under the grate. However, it has appeared that often a insufficient air supply is effected in this way, and therefore so-called secondary air is often used, typically supplied via an opening in a door of the boiler. Such air supply will partially relieve the problem of insufficient air supply, but the secondary air supplied by this method is not the perfect location in the combustion chamber.

A solution from US 4,343,288 is known, wherein an air duct is led around in the combustion chamber above the location of the fuel. This air duct supplies secondary air to the combustion, however requiring a special aperture for supplying secondary air via this air duct. It is thus a solution which is not immediately implemented on boilers of various known types. The air duct according to US 4,343,288 furthermore has the drawback that due to its size, it is impossible to retrofit on boilers where special fuels advantageously could be supplied secondary air at a position above the fuel. Moreover, it is not possible either to establish an opening through an existing boiler wall for supplying secondary air.

Object of the Invention

It is the object of the invention to indicate a simple solution to supplying secondary air to the combustion chamber, where the supply is effected in a simple way directly at the point where the secondary air is most useful, whereby the combustion is optimised and the CO content of the flue gas is minimised.

Description of the Invention

As mentioned in the introduction, the present invention concerns a boiler with a combustion chamber for burning fuel, where combustion air (primary air) to the combustion chamber is supplied through apertures in the bottom of the combustion chamber under the fuel, where the combustion chamber is further provided with at least one air duct for supplying combustion air (secondary air) to the combustion, where the at least one air duct is adapted for supplying combustion air at at least one level above the bottom of the combustion chamber. By such an air duct it is possible to supply the combustion with an excess of combustion air, whereby a cleaner and better combustion takes place. In an easy and very simple way it thus becomes possible to achieve an optimal combustion in boilers where such a combustion traditionally is not so easy to attain. By ensuring an excess of combustion air at one or more levels above the bottom of the combustion chamber there is achieved the advantage that the air, and thereby the air required for the combustion, is supplied just at location in the combustion chamber where the unburnt flue gases are found. Thus there is provided a better and cleaner combustion, and thereby is attained a higher efficiency in the boiler, all being to the benefit of the environment.

A boiler with a combustion chamber according to the invention is achieved in that the said at least one air duct is with an air intake extending across the at least one air supply aperture at the bottom of the combustion chamber, where the combustion air via this air duct is conducted to the combustion chamber via the apertures at the bottom of the combustion chamber.

The air supply apertures provided in the bottom of the combustion chamber is typically a grate of some kind at the bottom of the combustion chamber, upon which fuel is arranged and burned. By letting the air duct cover one or more of the apertures

in the bottom, there are achieved two advantages, the one being that the covered apertures are not closed by ashes as the boiler is used, and the other that air is conducted via these apertures to and into the air duct for subsequent discharge at a higher level in the combustion chamber.

In one variant of the boiler according to the invention, the at least one air duct extends up along the side of the combustion chamber and is provided with at least one air outlet opening towards the combustion chamber. Alternatively, there may be more air outlet openings, both towards the centre of the combustion chamber and/or towards its top and bottom. Thus is achieved possibility of supplying combustion air right at the point where it is most useful, and there is achieved a cleaner and better combustion of the flue gases formed in the combustion chamber.

In a particularly preferred embodiment of a boiler with a combustion chamber according to the invention, the boiler is a cast iron boiler, where the boiler is built up of cast iron sections, where at least one air duct is mounted at the inner side of the combustion chamber, preferably between two cast iron sections. Such cast iron sections are typically cast with a shape which by joining of two sections appears as an hollow between the sections. The combustion chamber in such a cast iron boiler is thus used as wall in the air duct, as an open profile in the shape of e.g. a U-profile may form the rest of the air duct in that the open profile is mounted with the opening facing the inner wall in the combustion chamber. The mounting may be performed by screws or by another suited method. The end of the open profile may advantageously be closed with a plate element which is adapted to the shape between the two cast iron elements. Another possibility of forming an air duct in a boiler according to the invention is to form it at the casting of the cast iron sections.

A further variant of a boiler with a combustion chamber according to the invention is where the boiler is a plate iron boiler constructed of plate iron, where at least one air duct is mounted at the inner side of the combustion chamber. This air duct can be mounted by screws, welding or by another suitable method.

Irrespective whether the air duct is used in a cast iron boiler or in a plate iron boiler, it can be made of cast iron, sheet iron or other suitable material. However, it is preferred to make air ducts according to the invention of sheet iron and preferably of sectional steel.

An air duct in a boiler with a combustion chamber according to the invention may include a lower plate-shaped part, where this lower part at a distance above the bottom in the combustion chamber extends across at least one air supply aperture in the bottom of the combustion chamber. Such a plate-shaped part ensures inlet of air to the duct part itself. At the same time, it is prevented that ashes cover the air supply apertures provided in the bottom of the combustion chamber, whereby air supply to the duct part is maintained even under extreme conditions by insufficient or lack of cleaning of combustion chamber etc. That the lower projecting part is called plate- shaped is to be construed broadly in this context, and profiled parts are thus also regarded as plate-shaped, such as e.g. U-profϊles in this connection, and therefore regarded as encompassed by the present description.

An embodiment according to the invention concerns an air duct including a duct part with at least one air outlet opening, where this duct part communicates with the lower plate-shaped part, thereby forming a passage for combustion air from apertures in the bottom of the combustion chamber, and which via at least one air outlet opening in the duct part extends to an area of the combustion chamber above the bottom of the chamber, hi one variant, the duct part may be with a plurality of holes for discharge of combustion air. These holes may be provided with threads for mounting screws which thus serve as plugs closing the holes that are unnecessary. Thus is achieved the option of adapting the position of the air outlet openings depending on the needs to be met and the type of fuel applied, which may influence the choice. For example, the air duct may be adapted to the type of fuel which is used in that the number and disposition of the open air supply apertures are regulated according to where the secondary air is desired to be supplied, and how much secondary air needed to be supplied for achieving an optimal and clean combustion. The size of the air supply apertures may advantageously be adapted with regard to the needs present in the individual boiler,

and e.g. there may be holes with a diameter of 8, 10 or 12 mm, but other sizes of openings may also be suitable.

In an embodiment air ducts may be mounted such that there is no projecting part at the bottom, but where the air duct itself is arranged such that it communicates directly with the area under the grate or under apertures in the bottom of the boiler. This type is thus not suited for retrofitting in Standard boilers, but requires adaptation already during the making of the boiler.

A boiler with an air duct according to the invention may be with common natural convection/suction of air without further arrangements, but may also be equipped with a blower or other device that ensures a forced supply of air to the combustion chamber via one or more air ducts in the boiler and possibly via apertures in the bottom of the combustion chamber.

Description of the Drawing

The invention is explained in the following with reference to the drawing, where:

Fig. 1 shows a detail of a boiler with a fitted air duct;

Fig. 2 shows an air duct; and

Fig. 3 shows a cross-section of a boiler where air flows are indicated schematically.

Detailed Description of the Invention hi Fig. 1 appears a detail of a boiler 1 where an air duct 3 is mounted inside the combustion chamber 2. The boiler 1 is a cast iron boiler composed of a series of sections 4, where inside these sections 4 there are not shown channels for water which are heated when firing in the boiler 1. The joined sections 4 appear with apertures 5 in the bottom of the combustion chamber 2 with the nature of a grate upon which the not shown fuel is disposed, and through which primary air is supplied to the combustion chamber 2.

The internal wall 6 in the combustion chamber 2 constitutes a sidewall in the duct in which air is conducted to the combustion. The air duct 3 is here made of a plate material with a projecting part 7 at the lowermost end. This projecting part 7 covers to some degree one or more apertures 5 in the bottom of the combustion chamber, and air

is hereby conducted into the duct formed by the wall 6 in the combustion chamber 2 and by the air duct 3 itself. At the upper end, the air duct 3 is provided with a plate 8 which is adapted to the shape formed by the water-filled cast iron sections 4. In this plate 8 is provided an air outlet opening 9. In the vertical part of the air duct 3, there is shown further six air outlet openings 9, where four of them are blanked off by screws 10. Furthermore, it appears that the air duct 3 is mounted by screws 11 which are fastened in not shown visible threaded holes between respective cast iron sections 4.

In Fig. 2 appears a second air duct 3 which is particularly suited for welding a plate iron boiler. The air duct 3 is made of a U-profile and equipped with air outlet openings 9 in the same way as shown in Fig. 1. The projecting part 7 at the lowermost end is shown here as a closed profile which is therefore insensitive to ashes or anything else in any way at all.

In Fig. 3 is seen a cross-sectional drawing, showing in principle how the air flow is in a boiler 1 according to the invention. The combustion chamber 2 is indicated as an external contour with apertures 5 in the bottom where primary air 12 is supplied. At the sides, air ducts 3 are indicated, here shown with L-shape and with a projecting part 7 at a short distance above the bottom of the combustion chamber. Under the projecting part 7 there are apertures 5 from where the primary air is conducted into the vertical part 13 of the air duct and out through air outlet openings 9 in the air duct 3 as secondary air 14, which is supplied to the combustion as a kind of extra air. The direction of the supplied secondary air is mainly towards the centre of the combustion chamber; however, at the top of the air duct 3, openings 9 are also arranged for supplying secondary air 14.