The present invention relates to a premix gas burner having the characteristics set out in the preamble to main Claim 1. Background art

The invention falls in particular within the specific technical field of modulating gas burners in which the burner power can be modulated within a predetermined modulation range in dependence on specific operating requirements. Conventional burners can be classified on the basis of combustion type, that is, they may have a diffuse flame or a premixed flame. "Diffuse flame" means a combustion process in which the reagents are mixed by a fluid- dynamic effect downstream of the burner head. "Premixed flame" means a process in which the mixture reaches the burner head already ready for combustion.

With diffuse-flame combustion, there are two principal phenomena, a mixing phenomenon (characterized by a mixing time) and a combustion phenomenon (characterized by a combustion time). In premix combustion, the mixing time is zero and only the combustion time remains so that flames of this type are generally faster. Moreover, for a given burnt power, a premix flame occupies less space than a diffuse flame because the reaction is quicker since it is not dependent on the mixing time. As far as the combustion products are concerned, nitrogen oxide emissions are connected with various aspects, for example, with the mixing of the reagents, with the time spent by the combustion products in "hot spots", with the general temperature within the flame, and with its uniformity. The premixing of the reagents upstream of the burner head leads to the cancelling-out of "hot spots" in the combustion zone and hence to a more uniform temperature. This is not possible in diffuse flames in which the thermal field is not uniform.

Moreover, in the diffuse-flame field, combustion conditions have been developed which lead to a reduction in fluid-dynamically acting nitrogen oxide emissions, that is, which succeed in rendering the thermal field more uniform and reducing the average temperature in the combustion chamber. The burners that have been developed, in the diffuse combustion field, in order to reduce the nitrogen oxides acting on the fluid dynamics of the combustion are known as "swirl" or recirculation burners. This is because they cause the combustion products to recirculate in the combustion zone; this recirculation tends to reduce the average temperature of the combustion zone, reducing the nitrogen oxides of thermal origin. These burners are usually operated within limited modulation ratios, for example, with a ratio of 1 to 2, because the recirculation phenomenon is a fluid- dynamic phenomenon which is present solely in certain reagent-velocity conditions. Burners of this type are not generally used in the domestic field because of the limited modulation range and because of the space occupied by the flame. In the domestic field, the use of premix burners is preferred in condensation boilers. These burners have a valve and a mixing system upstream of the burner. With conventional premix burners, it is possible to reach modulation ratios of 1 to 5 and also to limit the nitrogen oxides because the thermal field in the flame is rendered uniform and because the time spent in the combustion zone is short.

The facility to have wider modulation ranges, that is, to reduce the power in the burner head, is connected with the presence of a burner head having apertures that are designed to stabilize the flame above them and to cancel out the flashback or flame detachment effect. With currently-known condensation-boiler technology, one of the possible ways of reducing nitrogen oxides is to increase the amount of air whilst keeping the amount of fuel constant. The maximum temperature that can be reached within the flame is thus reduced and the nitrogen oxides of thermal original are therefore reduced. However, in this way, two problems are encountered; the first lies in the fact that the heat exchanger associated with the burner works optimally only for particular air-fuel ratios and, if the ratio is changed, the exchanger no longer works with maximum efficiency; the second is connected with the fact that flames which are too weak, that is with a large excess of air, are unstable.

To solve the above-mentioned problems, the Applicant has proposed burners constructed in accordance with the subjects of Italian patents Nos. PD2008A000021 and PD2008A000112. These are directed towards burners comprising a device for guiding the flow of the mixture, which device can impart rotational components to the velocity of the flow in the burner head, and a flashback prevention device positioned in the burner head. In the solutions proposed, the structure of the burner comprises substantially four basic, separate portions. There is a first element which is shaped with a flange for connection to the shutter, a second, cylindrical element carrying the finned deflector zone, a third, conical element adjacent the cylindrical portion, and a fourth element provided with through apertures and constituting the flashback prevention device.

Description of the invention

The main objective of the invention is to provide a burner which ensures the above-mentioned functions in order to stabilize the flame and to cancel out the flashback or flame detachment effect, and which at the same time has a simpler structure with a reduction in the number of components to be assembled and consequently easier and quicker assembly of the burner.

This object is achieved by the invention by means of a burner formed in accordance with the appended claims.

Brief description of the drawings

Further characteristics and advantages of the invention will become clearer from the following detailed description of a preferred embodiment thereof which is described by way of non-limiting example with reference to the appended drawings, in which:

Figure 1 is a schematic, partially-sectioned, side elevational view of a burner formed in accordance with the invention,

Figure 2 is a partially-sectioned perspective view of the burner according to the invention, Figure 3 is an exploded perspective view of the burner of the preceding drawings, and

Figure 4 is a partially-sectioned perspective view of a detail of the burner of the preceding drawings.

Preferred embodiment of the invention With reference initially to Figure 1, a burner, in particular a premix, modulating combustible-gas burner formed in accordance with the present invention, is generally indicated 1.

The burner 1 comprises a burner body 2 with a burner head 3, downstream of which the combustion of the air-gas mixture takes place; the air-gas mixture is supplied to the burner, for example, by means of a fan, not shown.

The burner is arranged to be housed in a combustion chamber Ia, shown only schematically, of a heating apparatus, which also houses a heat exchanger device Ib inside which an operative fluid circulates and is heated by means of the burner.

The combustion chamber Ia is delimited and sealed in an openable manner by a shutter element, indicated 5, having the configuration of a plate-like, disk-shaped base. The shutter element 5 has a central through aperture 6 in the region of which the burner body 2 is mounted. The combustible air- gas mixture directed towards the burner head 3 is supplied through the aperture, the configuration of which will be described in greater detail below.

The burner body 2 also has an axially symmetrical cylindrical portion 7 which has a principal axis X and at one end of which an annular flange 8 is provided; the portion 7 is extended, at the opposite axial end, by a portion 9 with a conical wall defining the end portion of the burner head 3. The burner body is fixed to the shutter element 5, coaxially with the aperture 6, by means of the flange 8.

A device, generally indicated 10, for guiding the flow of the mixture, which device can impart rotational components to the velocity of the flow supplied through the burner head, and a flashback prevention device, indicated 11, are also provided in the burner head 3.

The above-mentioned devices are constructed structurally independently of one another and the flashback prevention device is advantageously arranged downstream of the flow guide device.

According to a principal characteristic of the invention, the flow guide device 10 is produced integrally with the shutter element 5, in the region of the aperture 6. More particularly, the flow guide device comprises a plurality of radial deflector fins 12 which are integral with the shutter and spaced apart angularly at regular intervals, with a spoke-like arrangement extending from a central support zone 13 which forms part of the shutter element and extends in the region of the axis X.

The fins 12 are preferably inclined to the axial direction of the flow through the head so as to impart a rotational component to the flow that is conveyed through respective passageway sections 14 which are defined between each pair of adjacent fins 12. The sections 14 are defined within the through aperture 6 formed in the shutter element. In greater detail, each fin 12 comprises a first surface portion 12a which extends radially between the support 13 and a circumferential edge of the aperture 6, and a second surface portion 12b which forms an extension of the first portion and is inclined to the axis X, as shown in the drawings. The surface portion 12b of each fin also has a substantially flat surface which is inclined at 45° to a plane perpendicular to the axis X. The flash-back prevention device comprises a plate-like element 15 provided with a plurality of through apertures 16 which extend through its thickness and have a slot-like and/or circular shape in accordance with a preselected configuration, and through which the air-gas mixture intended for combustion on the surface of the head is supplied. The configuration of the apertures 16 is also selected so as to prevent flash-back when the burner is operating at low power and thus to ensure stable attachment of the flame to the burner.

Preferably, the plate-like element 15 has a flat, disk-like configuration and is formed so as to be structurally independent of the burner body and is capable of being held thereon by being interposed between the shutter element and the flange of the burner body, the fixing of the flange to the shutter holding the flash-back prevention device in position. Moreover, the element 15 may be made of blanked sheet metal with apertures 16 formed by drilling or punching. In operation, the air-gas reagent mixture, premixed upstream of the burner head 3, is supplied through the sections 14 of the aperture 6 and is deflected beforehand by the inclined profiles of the deflector fins 12. In the centre of the burner, in the zone downstream of the head, a reduced pressure is created which, in predetermined conditions, can draw the flow back into the burner, thus forming a toroidal recirculation bubble which serves to stabilize the flame and to reduce the average temperature of the flame front, thus reducing nitrogen oxide emissions (NOx). In this phase, the operation of the burner is the typical operation of a recirculation burner with a predetermined number of "swirls" and this operation is typical, within the modulation range, as the power is gradually increased, up to the maximum power permitted for the burner. Conversely, as the power, and hence the flow-rate of the reagents, is reduced, the reduced pressure in the centre of the burner can no longer draw the flow back towards it and the burner tends to operate as a conventional premix burner without recirculation. Moreover, in this phase of operation, the burner is not subject to problems due to flash-back, by virtue of the operation of the flash-back prevention device provided in the burner, or to problems of instability owing to flame detachment, by virtue of the stabilizing function of the same device. As a result, the burner can adapt to the combustion regime, within the modulation range in which it is used, as the burnt power varies. During the transition from burner with recirculation to conventional premixed burner, there may also be a fluid-dynamically unstable transition zone with fluctuations of the reagent flow which may lead to pulsations or extinction of the flame in this phase. To ensure the presence of a stabilized flame even in this transition zone, a taper is provided in the side wall of the burner body (the conical portion 9) as described above. The presence of this conical wall creates a flame anchorage zone and thus renders the transition between the two combustion regimes stable. This technical solution affords many advantages over previously known solutions in the technological aspects of the construction and of the assembly of the burner.

By virtue of the fact that the flow guide device is formed integrally with the shutter element, the number of components to be produced and assembled is advantageously reduced with consequent easier and quicker assembly of the burner. In fact it is necessary merely to fix of the burner head to the shutter element with the preliminary interposition of the flash-back prevention device to complete the assembly of the burner. The invention thus achieves the objectives proposed, affording the advantages described over known solutions.