WO2009059933A1

WO2009059933A1 - A burner, specifically a premix gas burner

Info

Publication number: WO2009059933A1
Application number: PCT/EP2008/064788
Authority: WO
Inventors: Gianfranco Scribano; Maurizio Beghi; Nicola Lollo; Simone Masiero; Danilo Pulzato; Loris Barduca
Original assignee: Sit La Precisa S.P.A.
Priority date: 2007-11-06
Filing date: 2008-10-31
Publication date: 2009-05-14
Also published as: UA102531C2; US20100316967A1; ITPD20070363A1; CN101849139A; EP2225489A1; CA2704217A1; RU2442078C1

Abstract

What is described is a burner, specifically for gas fuels with premixing, comprising a burner body (2) designed to be housed in a combustion chamber (4) of a heating apparatus, the burner body (2) being provided with a burner head (3) on which an air and gas mixture supplied to the combustion chamber (4) is burnt, in which the burner head (3) has a surface profile (3a) in the form of a dome-shaped spherical segment, the height (H) of the dome being less than half of the diameter (D) of the sphere to which the segment belongs.

Description

A BURNER, SPECIFICALLY A PREMIX GAS BURNER

Technical field

The present invention relates to a burner and specifically to a premix gas burner having the characteristics described in the preamble of Claim 1 which is the principal claim.

Technological background

In this specific technical field, there is a known way of making a premix gas burner whose burner head is intended to be housed in a combustion chamber in a position facing a heat exchanger which, when contacted by the combustion products of the burner, heats the fluid passing through it. Typically, the burner is connected to a shutter or similar element for sealing the combustion chamber. In a first known kind of burner of the aforesaid type, the burner head of the burner is made in a cylindrical shape which can extend into a combustion chamber, which is also of cylindrical shape, the cylinders preferably being coaxial with each other. In this configuration, the cylindrical shell of the burner head faces the coils of the exchanger in the annular space of the chamber which is substantially delimited by the overall dimensions of the cylindrical burner. One of the major problems encountered in the production of burners of the aforesaid type arises from the fact that, during the operation of the heating apparatus, the burner is subjected to a state of stress due to thermal deformation generated by the thermal expansion of the material from which it is made. These states of stress may adversely affect the strength of the material, and in some more severe cases may even cause the fracture of the material. In this context, the burner head area is one of the more critical areas of the burner. The shutter is also typically designed with a diameter greater than that of the combustion chamber. In this case, a further problem is that of reducing the heat losses through the annular space of the shutter which is not occupied by the connection to the burner, this problem generally being tackled by providing this area with a covering of insulating material which reduces the onset of corrosion in the shutter, as well as providing thermal insulation.

In an attempt to make the best use of the available surface in the shutter, burners with flat circular burner heads have also been proposed in the prior art. Although it does not allow the flames to face the coils of the exchanger, this configuration enables the use of insulation on the shutter to be reduced or even eliminated, but it has some limitations. A first limitation is that the burner head does not have a preferred direction of expansion. A second limitation is that the available cross section for the burner head is limited in size by the cross section of the cylindrical combustion chamber. In a further known type of burner, the burner head is hemispherical, and more specifically its diameter is equal to the diameter of the cylindrical combustion chamber. In this configuration, the flames at the base of the hemispherical head are very close to the coils of the exchanger, causing possible damage or premature wear due to the closeness of the flame front, unless the dimensions of the burner are reduced (i.e. by reducing the base diameter of the hemisphere with respect to the diameter of the cylindrical combustion chamber). Furthermore, this shape occupies a considerable volume in the combustion chamber, which is greater than that of an equivalent cylindrical shape. As regards the fixing of the burner, the burner head is typically designed to be fixed by a connecting flange to the structure of the shutter. The system for fastening the burner head to the flange may include, for example, a welded joint. In other known systems, the burner head is fastened to the burner flange by bending over pins or by fixing with screw means. The aforesaid systems have the limitation of opposing the expansion of the burner head as a result of the locking action, thus causing the appearance of states of stress of thermal origin, which may adversely affect the burner head itself. Description of the invention

The fundamental problem tackled by the invention is that of providing a burner, and specifically a premix gas burner, whose structural and functional design is such that the limitations of the aforementioned prior art can be overcome. This problem is resolved by the invention by means of a burner made in accordance with the following claims. Brief description of the drawings

Other features and advantages of the invention will become clear from the following detailed description of some preferred examples of embodiment thereof, illustrated, for the purposes of guidance and in a non-limiting way, with reference to the appended drawings, in which :

Figure 1 is a perspective view of a burner made according to the invention;

Figure 2 is a view in side elevation of the burner of Figure 1, housed in a combustion chamber which is only partially illustrated;

Figure 3 is a partial schematic view of a detail of the burner of the preceding figures;

Figure 4 is a perspective view in partial section of a further detail of the preceding figures; - Figure 5 is a view in partial section of a detail of Figure 4; Figures 6 to 8 are schematic views, in partial section, of different embodiments of the invention. Preferred embodiments of the invention With reference to Figures 1 to 3 initially, the number 1 indicates the whole of a burner, specifically a burner for combustible gas of the premixed type, made according to the present invention.

The burner comprises a burner body 2 with a burner head 3 where the air and gas mixture supplied to the burner is burnt, with the aid of a fan device (not shown) for example. The burner is designed to be housed in a combustion chamber 4, which is shown only schematically, of a heating apparatus (not shown), in which a heat exchanger 5 is also housed, the exchanger being provided with a tube bundle 6 in which a working fluid circulates and is heated by means of the burner. The burner body 2 is fixed to a shutter element 7 which is provided to seal the combustion chamber and which is removably connected to a static structure of the chamber. The number 8 indicates a connecting flange for fixing the burner body to the shutter, as will be described clearly below.

According to a principal feature of the invention, the burner head 3 has a surface profile 3a shaped in the form of a domelike segment of a sphere, as is also shown clearly in Figures 2 and 3. More specifically, given that H indicates the height of the spherical segment, D indicates the diameter of the sphere to which the dome belongs, and Dl indicates the base diameter of the dome, as shown in Figure 3, it is specified that the height H is conveniently chosen to be less than half of the diameter D, and that H is preferably less than half of the base diameter Dl of the dome. It is to be understood that the dome shape can also be produced from similar solid shapes with curved profiles, for example as an ellipsoidal dome instead of a spherical dome. Also identified on the dome are a vertex 9, at the intersection with the principal axis of axial symmetry X, and a circumferential edge 10 at the base of the dome.

It should be noted that, in the positioning of the burner in the combustion chamber, the base diameter Dl of the dome is chosen to have a value suitably smaller than the diameter of the combustion chamber, indicated by D2 in Figure 2.

The spherical dome head 3 can be made from metallic material, from perforated sheet metal for example, or in the form of a metal grid or mesh. In a further alternative, the head 3 can be made from fibre, particularly from metallic fibre. Because of the dome shape of the burner head 3, the flames only face the coils of the tube bundle 6, while a suitable surface 3a of the head is provided and the volume occupied by the chamber 4 is markedly smaller than that which would be occupied by a hemispherical head having a diameter D. A distributor element 11 is also provided under the burner head 3, inside the dome, this element being conveniently made in the form of a platelike element provided with a plurality of through holes, through which the combustible mixture is supplied towards the burner head. The distributor element 11 is preferably made from suitably shaped perforated sheet metal which is located coaxially with the head, with respect to the principal axis X, and which has a convex profile (in the form of a spherical or elliptical dome, for example) whose convexity faces the concave part of the dome, as shown schematically in Figure 5.

The number 12 indicates an additional guide wall positioned below the head 3 and the distributor 11, this wall preferably being coaxial with the head and having a convex profile facing the head. The wall 12, conveniently made in the form of a sheet having a suitable profile (in the shape of a spherical or elliptical dome, for example), has a base edge 12a which is spaced apart radially from the head and from the flange 8, thus delimiting an annular section 12b for the passage of the mixture supplied to the burner. The distributor element 11 is also positioned with respect to the wall 12 so as to form, as shown in the axial sectional view of Figure 6, a kind of guide passage 13 whose cross section decreases towards the axis X. This configuration makes it possible to guide the mixture at substantially constant pressure over the whole surface profile of the burner head 3. In a variant embodiment, shown in Figure 6, it is possible to provide, as an alternative to the distributor element 11, a deflector element 14, positioned at the mouth of the annular section 12b, under the head 3, in the area of the passage 13 having the greatest cross section. More specifically, the deflector 14 can be made from shaped sheet metal which may or may not be perforated and which extends in an annular way, either continuously or discontinuously, from the flange 8, coaxially with the axis X, as shown in Figure 7. Additionally, the flange part 8b can conveniently be made in one piece with the deflector 14. The principal function of the deflector element 14 is that of diverting the flow of combustible mixture supplied by the fan and guiding it into the passage 13 formed between the head 3 and the wall 12. Additionally, by the suitable shaping of the distributor 11 or of the deflector 14, or by a specified positioning of the perforations passing through these components, a low-velocity area 15, which is especially stable, is created on the burner head 3. In Figures 5 and 6, the area 15 is mainly located in the annular space formed by the base portion of the deflector or of the distributor in which there is a substantial change of curvature of the profile of the component concerned.

These specific conditions allow the flame to be ignited and made visible on the burner head. This is because, as a result of the aforesaid properties, relative to the shaping and perforation provided in the distributor or deflector, a pressure drop is created in the area 15 as a result of the pressure losses which are greater than in the rest of the head. The area 15 of the burner, where the velocity is low and the flame stability is greater, can cover a limited portion of the burner head 3, and can, for example, be located in an annular outer ring of the head with a suitable radial width. The configurations of Figures 5 and 6, in which the guide wall 12 is provided, are particularly applicable to cases in which, in order to achieve compactness, it is necessary to reduce the overall space, and in which it is therefore helpful to adopt efficient configurations where the volumes available for the fluid are reduced, in order to achieve a good distribution of the mixture at the burner head. The wall 12 can thus be used in these applications to delimit the guide passage 13.

However, in cases in which it is unnecessary to reduce the volumes available to the fluid, or where large volumes are provided, there is no need for a guide passage, since the volume is sufficient in itself to provide a low-velocity chamber. In this application, therefore, no guide wall is provided, as in the case shown in Figure 7.

Fastening means and counter-means are provided to fix the burner head 3 to the flange 8. These means comprise a pocket-shaped seat 16 in the flange 8 which can be engaged by the edge 10 of the dome that forms the burner head. The pocket-shaped seat 16 is designed to be engaged by the circumferential edge of the dome while allowing limited relative sliding, to enable the head to be locked to the flange, while permitting the free expansion of the head due to thermal deformations developed during operation, thus preventing the creation of states of stress between the head and the flange. More specifically, the pocket-shaped seat 16 extends circumferentially at an axial end of the flange opposite the end at which the flange is fixed to the shutter. The seat extends continuously, without any interruption, in a configuration coaxial with the burner head 3. Said seat is also preferably formed jointly by a pair of corresponding portions 8a and 8b of the flange, which are separate and distinct, and which can be fastened to each other and to the shutter by screw means. As shown in Figure 5, the portion 8b is conveniently made in one piece with the distributor element 11.

The portions 8a and 8b are positioned coaxially with each other, and at a distance from each other, in such a way that corresponding surfaces of said portions which face each other combine to delimit the annular space of the seat 16 which can receive the edge 10 of the dome.

With reference to Figure 5, the (diametric) cross section of the pocket-shaped seat 16 has its main direction of longitudinal extension cutting the principal axis X, in such a way that the edge 10 of the dome can remain engaged by insertion into the pocket. The depth of the pocket 16, in the aforesaid direction of its main longitudinal extension, is chosen so as to allow the edge of the dome engaged in the seat to slide freely relative to the seat, without substantial interface, thus facilitating the free expansion of the head while also maintaining the locking action on the flange. Clearly, alternative shapes of the pocket-shaped seat are possible, provided that it meets the requirement of fixing the head while allowing it to expand freely, at least to a limited extent. Figure 8 shows schematically one such variant embodiment, in which the flange 8 is bent over along one of its axial edges to form the aforesaid pocket shape 16. Thus the invention resolves the initial problem and yields the stated advantages over the known solutions.

A further advantage lies in the overall reduction of the manufacturing costs which can be achieved with the invention, by comparison with the known solutions, together with the greater ease of assembly of the burner, and the greater compactness which can be obtained according to the claimed features. It is also advantageously possible to change an apparatus from a given power rating to a reduced power configuration (even to a ratio of 1 to 4 between the power ratings concerned), simply by replacing the burner head while leaving the rest of the burner structure unaltered.

Claims

1. A burner, specifically for gas fuels with premixing, comprising a burner body (2) designed to be housed in a combustion chamber (4) of a heating apparatus, said burner body (2) being provided with a burner head (3) on which an air and gas mixture supplied to the combustion chamber (4) is burnt, characterized in that said burner head (3) has a surface profile (3a) in the form of a dome-shaped spherical segment, the height (H) of said dome being less than half of the diameter (D) of the sphere to which said segment belongs.

2. A burner according to Claim 1, in which the height (H) of said dome- shaped spherical segment is less than half the base diameter (Dl) of said dome.

3. A burner according to Claim 1 or 2, in which said burner head (3) is made from metallic material.

4. A burner according to Claim 3, in which said burner head (3) is made from perforated sheet metal.

5. A burner according to Claim 1 or 2, in which said burner head (3) is made in the form of a metallic grid or mesh.

6. A burner according to Claim 1 or 2, in which said burner head (3) is made from fibre.

7. A burner according to any one or more of the preceding claims, comprising means and counter-means for fastening the burner head (3) to a support flange (8) of said head, said fastening means and counter- means comprising at least one seat (16) on said flange (8) shaped in the form of a pocket and at least one portion of the perimetric base edge (10) of the dome, said seat (16) being engageable by said portion of dome with the possibility of limited relative sliding, thus locking the head (3) to the flange (8) while allowing the free expansion of the head (3), particularly its thermal expansion, relative to the seat (16).

8. A burner according to Claim 7, in which said pocket-shaped seat (16) extends circumferentially along the flange (8) in a continuous way without any interruption.

9. A burner according to Claim 7 or 8, in which said seat (16) is formed jointly by two corresponding distinct and separate portions (8a, 8b) of said flange (8) which can be fastened together while maintaining a distance between them, in such a way that the pocket of said seat (16) is formed as a result of their locking together.

10. A burner according to any one of Claims 7 to 9, in which said seat (16) has, in a diametric cross section, a predominant longitudinal extension in a direction which cuts the principal axis of the dome.

11. A burner according to Claim 10, in which the depth of said pocket- shaped seat (16) is chosen so as to allow the edge (10) of the head (3) engaged in said seat (16) to slide freely, at least along the said direction of longitudinal extension.

12. A burner according to any one or more of the preceding claims, in which a device is provided to guide the air and gas mixture towards the burner head (3), said device comprising a wall (12) positioned inside the dome, and spaced apart from the inner surface of said dome, the flow of said air and gas mixture being guided between said surface of the head (3) and said wall (12), towards the burner head, said wall being positioned coaxially with the head (3) and having a curved profile with its convexity facing the burner head (3).

13. A burner according to Claim 12, in which said wall (12) is formed by a shaped sheet of metal.

14. A burner according to Claim 12 or 13, in which a deflector element (14) is provided between said wall (12) and the head (3) to divert the flow of the combustible mixture.

15. A burner according to Claim 14, in which said deflector element (14) extends coaxially with the head, (3) and is spaced apart from said wall ( 12) .

16. A burner according to any one of the preceding claims, in which said deflector element (14) extends radially from said flange (8) and runs circumferentially inside the dome of the burner head (3).

17. A burner according to any one of Claims 1 to 13, in which a distributor element (11) is provided between said wall (12) and the head (3), for supplying the flow of the combustible mixture towards the head (3).

18. A burner according to Claim 17, in which said distributor element (11) extends coaxially with the head (3), and is spaced apart from it.

19. A burner according to Claim 17 or 18, in which said distributor element (11) has a plate-like shape with a curved profile, having its convexity facing the burner head (3).

20. A burner according to Claim 19, in which said distributor element (11) comprises a plurality of through holes for distributing the mixture over the surface of the burner head (3).

21. A burner according to Claim 20, in which said distributor element (11) and said guide wall (12) form a guide passage whose annular section, in the direction of the flow, decreases towards the principal axis of the dome.