IT202000002209A1 - PREMIX BURNER - Google Patents

Description

DESCRIPTION

Attached to a patent application for INDUSTRIAL INVENTION having the title

PREMIX BURNER

The present invention relates to a premix burner for the combustion of a mixture of a fuel and an comburent. Burners of this type are frequently used in domestic heating boilers and have a typically circular section body, on the surface of which flames are generated by combustion of a mixture generally obtained by mixing air and gas in a predetermined quantitative ratio.

The value of this ratio for which the mass of fuel present in the mixture reacts completely with the mass of comburent? defined stoichiometric; values of the ratio higher than the unit ?, that is? values for which the quantity? d? air? in excess of the stoichiometric ratio, they are defined as hyperstichiometric. The optimal value of the ratio, usually used in domestic boilers,? equal to about 1.3: this value? therefore hyperstichiometric and allows to contain the level of noxious emissions, compared to the level obtained in correspondence with the stoichiometric ratio, and at the same time to obtain a good yield.

Particularly in the case of domestic applications? Is it necessary to modulate the heat output required from the burner according to the needs? of users: consequently, unless measures are taken to dynamically adjust the relationship between the quantities? of air and gas, during the operation of a premix burner the ratio differs from the optimal value according to the variations in the thermal load of the burner.

In order to obviate this drawback, which results in an increase in the emissions of polluting or noxious unburnt residues, in the known art the regulation of the quantity is used. of air to be mixed with the gas by means of suitable pneumatic regulation systems.

A further development of these regulation systems? represented by the system described in the European patent EP 1 036 984 B1, in which the air-gas ratio (generally indicated with the Greek letter?)? electronically regulated by means of a control circuit that receives a signal, called ionization, from an electrode located in the vicinity? of the external surface of the burner on which the flames develop during the combustion of the mixture. The signal detected by the electrode? proportional to the intensity? of the flames and allows to estimate the value of the relationship?; the value cos? estimated pu? be used to dynamically adjust the quantity? of gas and maintain the value of? equal to the optimal nominal value. In order to improve the sensitivity? of the detection of the ionization signal, the patent EP 1 036 984 B1 proposes to increase the intensity? of the flames in the burner area in front of which? placed the ionization electrode. A solution similar to that illustrated in EP 1 036 984 B1? proposed in patent application EP 0339 499 A2 (see in particular figure 3 of the latter document).

The increase in intensity? of the flames produces a temperature gradient on the surface of the prior art burners; such a gradient? likely to cause mechanical breakage and to reduce in general the reliability? of the burner itself. Thermal gradients are accentuated when a burner? used in modulation mode, that is? when the required heat output is dynamically varied.

To reduce the thermal stresses produced by these gradients,? I notice the use of fiber or metal mesh coverings placed on the external surface of the burner. This solution, in addition to entailing high costs due to the fact that the coating material, having to withstand high temperatures,? made of expensive alloys (for example based on iron and chromium or silicon carbide), it also involves the formation of gas pockets between the coating and the support of the coating itself; finally, due to the relative elasticity? of the coating, the behavior of the burner? subject to change and not? therefore stable.

Purpose of the present invention? making available a premix burner which overcomes the drawbacks of the prior art mentioned above.

In particular, ? purpose of the present invention to provide a premix burner capable of allowing the detection of an ionization signal during the operation of the burner in a reliable, stable and economical way, in particular when the thermal power is dynamically modulated, avoiding at the same time the formation of gas pockets.

Said purposes are fully achieved by the burner according to claim 1.

In particular, the burner according to this claim comprises an external body on whose surface, called combustion, the flames develop during the combustion of the mixture; the external body? equipped with openings, called flame, arranged in an area of this surface called the flame area. The flame zone therefore indicates, in the present invention, the portion of the combustion surface in which flame openings are present and in which, consequently, flames can develop during burner operation: the parts of the combustion surface without openings therefore do not belong to the flame zone. The flame zone can? be constituted by a single region or, alternatively, by a multiplicity? of regions separated by portions without flame openings.

The burner also comprises a distributor located inside the external body and also equipped with openings called passageways, arranged on a surface opposite the external body, said distribution surface, in an area also called distribution, so to allow the passage of the mixture of fuel and comburent from inside the distributor towards the flame openings. Similarly to what already? indicated for the flame zone, the distribution zone indicates, in the present invention, the portion of the distribution surface in which passage openings are present; the parts of the distribution surface without passage openings do not belong to the distribution area. The distribution area can? be constituted by a single region or, alternatively, by a multiplicity? of regions separated by portions without passage opening.

The distribution zone and the flame zone are arranged opposite each other: in other words, the distribution zone and the flame zone face each other.

The external body and the distributor extend at least in a first and second direction orthogonal to each other. By way of example, it is mentioned that the external body and the distributor can have the shape of parallelepipeds or cylinders.

The passage openings on the distribution surface of the distributor are geometrically configured in a portion of the distribution area in such a way that the ratio between the specific flow rate of the mixture in this portion, called detection, is greater than the specific flow rate in the remaining part of the area distribution.

In the context of the present invention, the expression specific flow rate (indicated below with the symbol q) designates the volumetric flow of mixture through the unit? of surface (that is through an area of area equal to one square meter) in the unit? of time (ie in a second) and is therefore measured in m <3> / (m <2>? s): the specific flow rate has therefore the dimensions of a velocity? (m / s).

The burner is characterized by the fact that the distributor? placed inside the external body and? separated from the external body by a cavity of non-zero thickness, through which the mixture flows from the passage openings towards the flame openings. The gap extends in a third direction orthogonal to the first and second directions.

The burner according to the present invention differs from the burners described in the aforementioned documents EP 1036 984 B1 and EP 0339 499, in which a flat perforated burner (Brennerplatte, in the original German terminology of the two documents)? arranged on the top? of a duct for the mixture. The mixture is not distributed by means of a distributor placed inside the burner but flows directly from the duct through the holes of the flat burner, on whose external surface the flames are formed during the combustion of the mixture itself.

In the present invention, a distributor? placed inside the burner body and? separated from the latter by an interspace: as a consequence of this arrangement, the mixture leaving the distributor reaches the flame openings only after passing through the internal distributor and the interspace itself.

The use of the internal distributor in the present invention allows the mixture to be distributed more easily. homogeneous on the surface of the burner and significantly reduces the onset of thermal gradients, which on the contrary are very pronounced on the combustion surface of the burners of the prior art, in which they often cause breakages and, more? in general, they cause a reduction in the operating life.

Furthermore, the use in the present invention of an interspace between the distributor and the external body of the burner, on the surface of which the flames are formed by combustion of the mixture which is transited through the aforementioned cavity, it allows to reduce the breakage phenomena due to thermal stress even with respect to prior art devices equipped with an internal distributor: while in fact in such devices the distributor is known? typically in contact with the aforementioned metal mesh on which combustion takes place, with the result that a strong temperature gradient is created between the distributor and the mesh itself, in the burner of the present invention the distributor does not? never in contact with the surface on which the combustion takes place, thanks to the presence of the cavity. There? it allows to avoid the formation of sudden thermal gradients between the distributor and the surface on which the high temperature flames are formed during combustion, with a consequent significant reduction in the phenomena of breakage and degradation of the burner.

A further advantage of the technical solution adopted in the present invention? consisting of the fact that the gi? mentioned and expensive metal mesh nets capable of withstanding high temperatures. The absence of such networks, in addition to allowing a significant reduction in the production costs of the burner, advantageously allows to avoid the formation of gas pockets, typically present between the mesh network and the body of known burners, and also improves stability. ? of operation of the device, whose behavior is not? pi? subject to variations due to flexibility? of the knitted nets used in the prior art.

A significant benefit of the technical solution adopted in the present invention? represented by the substantial repeatability? of the ionization curve obtainable by varying the heat output of the burner and for a predetermined value of the air-gas ratio? regardless of the type of gas used in the mixture.

Figures 7A and 7B illustrate the trend of the ionization curves measured respectively for a burner according to the invention and for a conventional burner as the heat input varies, for a mixture of air and gas of the G20 family (methane; curve with diamonds) and a mixture of air and liquid propane gas (LPG; curve with squares). In the case of Figure 7A, relating to a burner according to the present invention, the trend of the ionization current as the heat input Q? qualitatively identical for the two blends. On the contrary, in the case of a conventional burner, the ionization curve relating to the mixture of air and gas of the G20 family has a very different behavior from that relating to the mixture of air and LPG: while the first curve? substantially flat, the second has a peak in the low flow region, around 3.5-4 kW, and then drops to significantly higher values? low compared to the first curve on the rest of the thermal flow range.

The substantial similarity of the ionization curves for mixtures of air with gases of different families allows the operation of a boiler equipped with a burner according to the present invention to be controlled in a simple way, even in the case, frequent in practice, of variations in the composition of the available gases. in the gas distribution network.

The burner according to the present invention? also characterized by a high slope and by the monotony of the sensitivity curve, that is? of the function that describes the trend of the ionization current as the air-gas ratio varies (?), for a predetermined value of the number of revolutions per minute of the fan that supplies the air for the mixture (i.e. for a flow rate value fixed thermal). The high slope of the curve and its substantially monotonous descending character over the entire operating range guarantee a unique relationship between the ionization current and the air-gas ratio and allow the value of this ratio to be adjusted precisely; on the contrary, in conventional burners the sensitivity? can present the so-called? inversion ?, that is? a region in which the curve has an inflection point and in which two values of the ratio correspond to an ionization current value, with a consequent difficulty? of the electronic control system of the ratio value to correctly regulate the operation of the boiler.

In the present invention, the dimensions of the sensing portion in the first and second directions are selected in such a way as to be smaller than the corresponding dimensions of the dispenser in the same directions: the sensing portion therefore occupies a region of the distribution zone on the surface of the dispenser. area lower than that of the area itself and limited in two perpendicular directions, respectively coinciding with the first and second directions.

The detection portion allows to locally increase the flow of mixture which, through the surface of the distributor, reaches the external body. In correspondence of the circumscribed region in which the flow is cos? ? amplified? the flames develop with greater intensity? compared to the rest of the device and generate, in any operating mode of the burner, an intensity ionization signal? such as to be easily detectable, for example by means of an ionization electrode known per se, so? allowing a reliable regulation of the air-gas ratio.

The limitation of the spatial extension of the sensing portion to a region of limited dimensions in the first and second direction of development of the distributor and of the external body allows to concentrate the amplified flow of mixture in a restricted region, avoiding thermal stress on the rest of the distributor . The extension limitation also allows to adapt the shape of the detection portion to the geometry of the ionization electrode, typically of elongated shape, so as to maximize the detected signal. The present finding is not? however limited to the use of elongated electrodes, nor does it necessarily require that the sensing portion has an elongated shape: on the contrary, this portion can? take on a square, rhomboid, oval or circular shape. These forms are purely illustrative: the detection portion can? assume further geometric shapes, provided that the shape and size of the portion allow the ionization electrode, facing this portion, to detect the ionization signal with maximum sensitivity. possible.

The location of the locally increased flow detection portion on the distributor surface inside the burner body, instead of on the surface of this body, as taught in the prior art, and the presence of an interspace separating the distributor from the external body therefore allow reduce the thermal stresses normally produced by a local increase in intensity? of the flame caused by an increase in the flow rate of the mixture.

The local increase of the specific range in the sensing portion can? advantageously be achieved by adjusting the ratio between the sum of the areas of the passage openings formed on the surface of the distributor and the total area of this portion, so that it is greater than the ratio between the sum of the areas of the openings in the rest of the distribution area and the area of that remainder. In other words, the local increase in the specific flow rate can? be advantageously controlled by regulating the porosity? of the sensing portion with respect to the porosity? of the rest of the distributor surface.

Since? the area of the passage openings? controllable in a precise and repeatable manner, given that these openings are made on the surface of the distributor by means of high-precision mechanical machining, with the present invention? It is possible to guarantee a reliable and stable regulation of the ionization signal and, consequently, a stable behavior of the burner itself.

Preferably the area of each of the passage openings present in the sensing portion on the surface of the distributor? greater than the area of each of the flame openings on the combustion surface of the outer body. Since? the area in which the flame openings are arranged, that is? the so-called flame zone,? arranged in front of the detection portion in which the passage openings are arranged, the increased area of the latter with respect to the area of each flame opening makes s? that the flow out of each passage opening is divided between pi? flame openings, instead of reaching only one flame opening. So ? possible to create a? bed? of sealing flames on the surface of the burner and the phenomenon of flame detachment is advantageously avoided.

In particular, in the case in which the passage openings and the flame openings are chosen to have a circular shape, it becomes particularly simple to adjust the relative ratio between the area of each passage opening and the area of each flame opening. in that case? it is sufficient to adjust the diameters of the respective openings when manufacturing the distributor and the external body. The use of circular openings therefore allows to control the operation of the burner in a particularly reliable, simple and economical way. The diameter of the flame openings?, In the case of circular openings, preferably less than 1.5 mm; this value advantageously reduces the backfire phenomenon.

The passage openings and the flame openings can be advantageously arranged in a regular manner on the surface of the distributor and on that of the external body of the burner, for example by arranging the passage openings according to a periodic pattern having a first pitch P1 and the flame openings according to a periodic pattern having a second step P2, different from the first step and preferably smaller. The arrangement of the openings according to a periodic reason? particularly advantageous in terms of manufacturing, given that these openings are made by repeatedly perforating a metal strip, usually made of steel, from which two perforated portions intended to form the distributor and the external body are then cut, by means of programmable mechanical machines that perform a ribbon pitch: making regular motifs? in fact, it can be obtained simply by setting the advancement step of the machine equal to the value of the first or second step of the periodic pattern.

It is understood that the purposes of the present invention can also be achieved by means of a non-regular and non-periodic distribution of the passage and / or flame openings, as long as the distributor and the external body are kept spaced apart by means of a gap. a detection portion with increased specific flow rate is provided on the surface of the distributor, as explained above.

The thickness of the cavity must be different from zero; preferably, the thickness of the cavity? less than 4 mm and, even more? preferably, this thickness? chosen equal to 0.6 mm. The presence of a cavity of non-zero thickness improves the distribution effect of the mixture exiting each opening of passage on pi? openings of flame and therefore reduces the flame detachment phenomenon; the value of 0.6 mm yes? experimentally proven to be optimal in reducing this phenomenon. The burner object of the present invention is advantageously used in boilers equipped with a combustion chamber, preferably in combination with a detection electrode to detect the ionization signal generated by the flames that develop on the burner surface, when the mixture is burned.

Object of the present invention? also a method of regulating the flow rate of a mixture of comburent and fuel in a premix burner as described above. The method includes the following steps:

1) introduction of the mixture inside the distributor through the burner head;

2) diffusion of the mixture from the distribution area located on the distribution surface of the distributor to the flame area located on the combustion surface of the external body through the cavity of not zero thickness;

3) combustion of the mixture in the flame zone;

4) detection of an ionization signal, produced by the combustion of the mixture, by means of a detection electrode disposed outside the flame zone of the external body in correspondence with the detection portion of the distributor;

5) sending the ionization signal to a control device for adjusting the ratio between the quantity? of comburent and the quantity? fuel mixture;

6) adjustment of said ratio by means of the control device so that the ratio is equal to a predetermined value.

The aforementioned characteristics will be better understood from the following description of a preferred embodiment, illustrated purely by way of non-limiting example in the accompanying drawings, in which: - Figure 1 illustrates the parts of a premix burner according to the present invention;

Figure 2 illustrates a side view of the burner of Figure 1 in the state in which the parts are assembled together;

figure 3 illustrates the burner of figure 2 with the external body partially raised with respect to the internal distributor;

figure 4 illustrates the internal distributor of the burner of figure 2;

Figure 5 illustrates another side view of the burner of Figure 2 with the external body partially raised with respect to the internal distributor, together with an enlargement showing the gap between the external body and the internal distributor;

Figure 6 illustrates a perforated flat belt usable for manufacturing the external body of a burner according to the invention, with an alternative distribution of the flame openings with respect to that illustrated in Figures 2;

- Figures 7A and 7B illustrate the trend of the ionization current as the heat input varies for a predetermined value of the air-gas ratio, respectively in a burner according to the patent and in a conventional burner, for two different mixtures of air with gas of different families (methane and LPG).

Figure 1 illustrates, purely by way of example, an exploded view of a premix burner (100) according to a preferred embodiment of the present invention. The burner comprises an external body (3) provided on its surface (31), called combustion, with openings (33) called flame; the combustion surface (31) represents the surface on which, during the operation of the burner (100), the flames develop, in particular in correspondence with the flame openings (33). The region of the combustion surface (31) on which the flame openings (33) are present substantially constitutes the so-called flame zone.

In the example, the external body (3) has a cylindrical shape and? equipped with openings (33) of circular shape; it is understood that the purposes of the present invention can also be achieved by means of external bodies having a different shape, for example in the shape of a parallelepiped, and by using flame openings (33) of a different shape from the circular one, for example elongated slits; these slits can be combined with circular openings and can be distributed on the combustion surface according to periodic or irregular geometric patterns, depending on the desired flame distribution.

Purely by way of example, it should be remembered that the flame openings (33) can be distributed on the combustion surface (31) along the axial direction of the cylindrical body (3) in areas that have different geometric patterns: for example, the diameter of the openings (33) and / or the distance between them inside each zone pu? be different from the diameter and distance in each? or even just in some - of the other areas. The criteria for selecting the dimensions (diameter of the openings and / or distance between them in each zone, width of each zone in the axial direction, number of zones) are known to the person skilled in the art and will not be repeated here. In the example of the embodiment illustrated in the figures, the flame openings (33) are circular holes with a diameter of 0.6 mm, repeated periodically both in the tangential direction and in the axial direction of the cylindrical body (3). The holes (33) are periodically arranged in sequence along the tangential direction at a mutual distance (pitch) equal to 1.4 mm; sequences of holes (33) adjacent in the axial direction are offset along the tangential direction; the periodic distance in the axial direction between the sequences of adjacent holes (33)? equal to 1.2 mm. In general, ? it is preferable to keep the diameter of the holes below 1.5 mm to avoid flame detachment phenomena.

The outer body (3) of the burner (100)? arranged around a distributor (2), placed inside the external body (3) itself; in the example of figure 1 the distributor (2) (which will also be referred to with the expression internal distributor) also has a cylindrical shape and? arranged coaxially with the external body (3), how can you? appreciate from figure 3, in which the external body (3)? partially raised with respect to the distributor (2) and allows a glimpse of the lower part of the latter. How already? indicated for the external body (3), also the distributor (2) can? have other geometric shapes: for example, in the aforementioned case of a parallelepiped-shaped external body (plane burner), the internal distributor (2) also preferably has the shape of a parallelepiped, contained inside the parallelepiped pi? large that forms the external body.

The external body (3)? manufactured starting from a flat strip which is perforated by means of a punching machine, so as to achieve the desired spatial distribution of flame openings (33); in the case of an external cylindrical body, the flat belt, once pierced, is folded back on s? itself to form the cylindrical body (3). Although the outer body (3) illustrated in Figure 1 has a homogeneous spatial distribution of flame openings (33),? Other distributions can be applied. By way of example, figure 6 shows a flat ribbon, usable to manufacture a cylindrical outer body, characterized by a distribution of flame openings (33) gradually less dense in the axial direction proceeding from the base (34) of the body (3) towards the summit? (32) of the body itself (3). How can you? see from figures 1 and 2, the base (34) of the body (3)? next, in the mounted burner, to a flange (12), while the top? (32)? adjacent to a plug (4) which closes the cylindrical body (3). Thanks to the gradual decrease in density? of openings (33) along the surface (31) of the outer body (3),? possible to reduce the deformations of the external body (3) due to the abrupt passage from a perforated region, that is? the flame zone equipped with flame openings (33), to a region without holes. An abrupt transition between the flame zone and the zone without holes in fact creates an abrupt thermal gradient, due to the fact that the flames essentially develop only in the flame zone, and can? thus cause deformation. In a further variant, not illustrated, the gradual decrease of the densities? of openings (33) along the surface (31) of the external body (3) can be made in the opposite direction to that illustrated in figure 6, that is? proceeding from the top? (32) towards the base (34).

How can you? better appreciated from figure 4, in which the internal distributor (2)? shown without the external body (3) surrounding it, the distributor (2) has on its surface (21) (called distribution) a plurality of of openings (23, 26), called passage; in the example of Figure 4 these openings have a circular shape. The region of the distribution surface (21) on which the passage openings (23, 26) are present substantially constitutes the so-called distribution zone.

The main function of the distributor (2)? to allow the passage and diffusion of a fluid mixture introduced into the burner (100) towards the combustion surface of the burner, coinciding with the combustion surface (31) of the aforementioned external body (3). The introduction of the mixture, typically consisting of air and gas, occurs through one or more? openings (11) arranged on the surface of a head (1) placed at the base of the burner (100), how can you? see from the exploded view of Figure 1, and fixed to the latter by means of a flange (12), shown in perspective view in Figure 1 and from the side in Figure 4. not explicitly illustrated in the figures, both the internal distributor (2) and the external body (3) of the burner (100) are fixed to the head (1). The number, shape and size of the inlet openings (11) present on the head (1) and their spatial distribution can be determined by the copper technician on the basis of commonly known design principles. The passage of the mixture, introduced through the aforementioned head (1), from the inside of the distributor (2) to the combustion surface (31) of the external body (3)? ensured by the aforementioned passage openings (23), whose shape, size and spatial distribution? except for the openings (26) in the region of said sensing portion (200), described below with reference to Figure 4 - can? generally be determined by the person skilled in the art on the basis of known principles, so as to ensure a predetermined fluid-dynamic distribution of the mixture inside the burner. As an example, the distribution of the openings (23) on the surface of the distributor (2) can? be determined? with the exception of the openings (26) in the sensing portion (200)? according to the teachings of the European patent EP 1 914 476 B1 of which? owner the applicant of this application. In the example of the embodiment illustrated in Figure 4, the passage openings (23) have a circular shape and, with the exception of the aforementioned sensing portion (200), partially hidden in the figure by an electrode (5) described below and used for detecting an ionization signal, these openings are regularly distributed on the distribution surface (21) with constant pitch and diameter.

In the latter, as can be clearly seen from Figure 4, the passage openings (26) are geometrically configured in such a way that the specific flow rate of mixture flowing through the sensing portion (200) is greater than in the remaining parts (201 , 202) of the distribution area, that is? in the remaining part of the distribution surface (21) in which there are passage openings (23), formed in the example shown in the figure by two regions designated by numbers 201 and 202. As already? indicated, is meant by specific flow rate q the volumetric flow of mixture that crosses the unit? of surface (that is an area of area equal to one square meter) in the unit? of time (ie in a second).

In the example of Figure 4, the sensing portion (200)? a region of narrow and elongated shape along the axial direction of the distributor (2) and inside which the passage openings (26), of circular shape in the example, have a larger diameter than the passage openings (23) in the remaining part (201, 202) of the distribution area. The increased size of the passage openings (26) in the sensing portion (200) facilitates the passage of the mixture in this portion and therefore increases the specific volume of mixture it reaches in the unit. of time the combustion surface (31) of the external body (3) (not visible in figure 4), where it is burned on the flame openings (33) during the combustion process. The increased size of the passage openings (26) in the sensing portion (200) thus produces locally larger flames. intense on the part of the combustion surface (31) which overhangs the underlying sensing portion (200).

In general, the local increase of the mixture flow in the sensing portion (200)? reached if the ratio between the sum of the areas of the passage openings (26) present in the aforementioned detection portion (200) and the total area of this portion (200)? greater than the ratio between the sum of the areas of the openings (23) in the remainder (201, 202) of the distribution area and the total area of that remaining area. In other words, the local increase in the flow of the mixture? guaranteed if the porosity? local? R in the detection portion (200)? greater than on the remainder (201, 202) of the distribution surface (21). The term porosity? ? used in the current meaning in the technique of burners and generally indicates, with reference to a surface that has openings or "voids", the ratio between the sum of the empty areas and the total area of the surface.

The porosity? can be increased in different ways by acting on the geometric configuration of the passage openings:? It is possible, in the case of circular openings arranged in a regular manner, to increase the diameter of each passage opening (26) in the sensing portion (200) with respect to the diameter of the openings (23) in the rest (201, 202) of the distribution, as shown in figure 4; always in the case of circular openings arranged in a regular manner,? alternatively, it is possible to keep the diameter of the passage openings (26) unchanged, increasing instead their density? (i.e. by reducing the pitch) in the sensing portion (200) with respect to the rest of the distribution zone. ? in fact, it is easy to show how, in the case of passage openings regularly arranged with a pitch pR in the detection portion (200) and with a pitch pD in the rest (201, 202) of the distribution area, the respective diameters of the openings are called d1 and d2 (26) and (23), the relationship between the porosity? ? R of the detection portion (200) and that? D of the rest (201, 202) of the distribution zone? equal to (d1 / d2)? (pD / pR) and pu? be made greater than the unit, so as to guarantee the condition of increased flow required by the present invention, by acting on the diameters and / or on the pitches.

The objects of the present invention can also be achieved by arranging the passage openings in the sensing portion (200) and / or on the rest (201, 202) of the distribution surface (21) in an irregular manner, provided that the condition that the specific flow rate of mixture in the detection portion (200) is greater than on the rest (201, 202) of the distribution zone.

Thanks to the increased local flow of mixture at the detection portion (200), the intensity? of the flames that develop on the part of the flame surface (31) of the external body (3) which covers the aforementioned portion (200)? always greater than on the rest of the flame surface (31); consequently, the intensity? of the ionization signal detectable in the vicinity? of this part of the flame surface (31) by means of the detection electrode (5)? greater than in correspondence with the rest of the flame surface, given that the intensity? of the ionization signal? directly proportional to the intensity? of the flames. By suitably selecting the geometric parameters of the passage openings (26) in the portion (200) according to the principles illustrated above,? therefore it is possible to guarantee an intensity ionization signal over the entire operating range of the burner? sufficient to dynamically adjust, by means of a feedback circuit, the ratio between the quantity? of air and the quantity? of gas of the mixture introduced into the burner. The present invention therefore allows the value of the so-called ratio? when the heat output required by the burner varies, that is? depending on the modulation regime. Are the control loops known per se? and will not be described; an example of such a circuit? illustrated in the aforementioned European patent EP 1036 984 B1.

By using a conventional regulation circuit in combination with a premix burner according to the present invention and with a detection electrode, arranged outside the flame zone of the external body at the detection portion,? It is possible to regulate the flow of the mixture of comburent (for example air) and fuel (for example gas) in an optimal way during the operation of the burner, even if the heat output required by the burner is modulated.

The method for regulating the flow of mixture according to the present invention comprises the following steps:

1) introduction of the mixture inside the distributor (2) through the head (1) of the burner (100);

2) diffusion of the mixture from the distribution area (200, 201, 202) placed on the distribution surface (21) of the distributor (2) to the flame area located on the combustion surface (31) of the external body (3) through the? interspace (300) of non-zero thickness G;

3) combustion of the mixture in the flame zone on the combustion surface (31);

4) detection of the ionization signal produced by the combustion of the mixture by means of the detection electrode (5), arranged outside the flame zone located in correspondence with the detection portion (200) of the distributor (2);

5) sending of the ionization signal to a control device, placed in the regulation circuit, for the regulation of the ratio (?) Between the quantity? of comburent and the quantity? fuel mixture;

6) adjustment of said ratio by means of the control device so that the ratio (?) Is equal to a predetermined value.

The control device can? be, for example, a valve that regulates the inflow of gas entering the chamber in which the air is premixed with the gas, before being introduced into the burner (100) through the head (1). The predetermined value of the relationship? depends on the type of comburent and fuel used: in general terms, this value? chosen equal to the so-called stoichiometric value, that is? the value for which the combustion reaction of the fuel? it completes and does not produce residues, such as carbon monoxide, in the case of a fossil gas type fuel.

Preferably the dimensions of the sensing portion (200) are chosen according to the dimensions of the sensing electrode (5), so as to concentrate the increased flow of mixture in a region placed in front of the electrode (5) and so on. increase the sensitivity? the measurement of the ionization signal detected by the electrode itself. Since? the ionization electrode (5)? usually elongated, how can you? see from Figures 1, 4 and 5, the sensing portion (200) typically also has an elongated shape in a first direction, essentially parallel to the projection of the electrode (5) on the external surface (31) of the burner. How already? explained above, the shape of the sensing portion (200) is not? limited to an elongated shape and can, by way of example, be square. In the example shown in the figure, the size of the sensing portion (200) in the first direction? smaller than the size of the distributor (2) in the same direction; also the size of the sensing portion (200) in a second direction perpendicular to the first dimension? limited and? smaller than the corresponding distributor size in the same direction. In the case of a cylindrical ionization electrode (5) of length L and diameter D arranged parallel to the axis of the burner, as in the example of figure 4, the sensing portion (200) can? for example extend in a direction parallel to the length of the electrode (5)? therefore parallel to the axis of the burner (100) - on a section of height L of the distribution surface (21), while the size of the portion in the direction perpendicular to the electrode, that is? the width W of the distribution portion (200), can? be equal to 3D. In any case, the length and width of the distribution portion (200) are smaller than the corresponding dimensions of the distributor (2).

The sensing portion (200)? limited to a region of dimensions smaller than the corresponding dimensions of the distributor (2) in the two aforementioned directions. In the case illustrated in Figure 4, the detection portion (200) extends in height (i.e. in the direction of the burner axis along which the mixture spreads) along a longer section. short of the total height of the distributor (2): how can you? see from the figure, the internal distributor (2) also extends into a lower area located below the regions 200, 201 and 202 and does not, in the example, have passage openings (23) and (26). Figure 4 further shows that the sensing portion 200? also limited in width, that is? in the circumferential direction of the cylinder (2). It is understood that the purposes of the present invention can also be achieved by using a detection portion whose height? different from that of the remaining parts (201, 202) of the distribution area. As indicated above, the distribution area? defined as the part of the distribution surface (21) equipped with passage openings (23, 26): therefore, in the example of figure 4, the distribution zone substantially corresponds to the regions 200, 201, 202 arranged above the part lower surface (21) without openings (the so-called blind zone).

How ? possible to appreciate from figure 5, the internal distributor (2)? separated from the external body (3) of the burner (100) by means of an interspace (300) with a thickness G that is not zero. The thickness G? preferably less than 4 mm: in the example shown in the figure, G? equal of 0,6 mm. The interspace (300)? empty and contains air.

The experiments carried out have indicated that the presence of a gap with a thickness G that is not zero and preferably not exceeding 4 mm improves the distribution effect of the mixture exiting each passage opening (23, 26) towards the flame openings ( 31): the mixture leaving each passage opening (23, 26) in a direction substantially perpendicular to the area of the opening, that is? in the radial direction, in the case of the cylindrical burner illustrated in the figures, by crossing the cavity of non-zero thickness it is also distributed in directions other than the aforementioned direction perpendicular to the area of each opening (23, 26). In particular, the crossing of the cavity allows the mixture coming out of each single passage opening to spread also in the axial direction and to reach, consequently, more? of a flame opening (33). Thanks to the distribution of the mixture coming out of each passage opening (23, 26) on pi? flame openings (33), a? bed? of flames of non-zero thickness, which contributes to the reduction of the flame detachment phenomenon. The value of the thickness G equal to 0.6 mm yes? experimentally proven to be optimal in reducing this phenomenon.

The flame failure phenomenon, the effects of which are generally negative on the operation of the burner, has particularly deleterious consequences if it occurs on the region of the combustion surface (31) which the detection electrode (5) faces, given that in this case can? the ionization signal necessary to optimally adjust the air-gas ratio is lost. In order to avoid flame failure in the aforementioned region of the combustion surface (31),? it is preferable to choose the area of each of the passage openings (26) present in the detection portion (200) so that it is greater than the area of each of the flame openings (31) present on the combustion surface (31) of the external body (3). In this way the partitioning effect of the mixture leaving each passage opening (26) on pi? flame openings (33)? further strengthened.

Claims (25)

CLAIMS 1. Premix burner (100) comprising: an external body (3) equipped with a combustion surface (31) equipped with flame openings (33) arranged in a first zone called flame, a distributor (2) equipped with a distribution surface (21) equipped with passage (23, 26) arranged in a second zone called distribution and a head (1) connected to the external body and to the distributor and provided with at least one opening (11) to introduce a mixture of comburent and fuel inside the distributor (2), in which burner (100) the external body (3) and the distributor (2) extend at least in a first and a second direction and the passage openings (26), in a portion (200) of the distribution zone called the detection portion, are geometrically configured in such a way that the specific flow rate (q) of the mixture, defined as the flow of mixture through a unit surface in the unit? of time, both greater in the detection portion (200) than in the remaining part (201, 202) of the distribution area, the area of the detection portion (200) being smaller than the area of the distribution area, characterized by the fact that the distributor (2)? placed inside the external body (3) and the external body (3) and the distributor (2) are separated along a third direction, orthogonal to the first and second, by means of an interspace (300) of non-zero thickness (G) through which the mixture exiting the openings passage (23, 26) flows towards the flame openings (33). 2. Burner according to claim 1, wherein the distribution zone equipped with the passage openings (23, 26) and the flame zone equipped with the flame openings (33) are arranged opposite each other. 3. Burner according to claim 1 or claim 2, wherein the non-zero thickness of the cavity? less than 4 mm and preferably equal to 0.6 mm. 4. Burner according to claim 1 or claim 2, wherein the non-zero thickness of the cavity? less than 0.5 mm and preferably equal to 0.3 mm. 5. Burner according to any one of the preceding claims, comprising spacer elements arranged in the interspace, preferably integral with the distributor. 6. Burner according to any one of the preceding claims, wherein the sensing portion has an elongated shape, the dimension of said portion in one of the two aforesaid directions being significantly greater than the dimension in the other direction and preferably equal to at least three times the dimension minor. 7. Burner according to any one of the preceding claims, in which the passage openings are geometrically configured in the detection portion so that the ratio between the sum of the areas of the passage openings and the total area of said portion is greater than the ratio between the sum of the areas of the openings in the remainder of the distribution zone and the area of said remainder of the distribution zone. 8. Burner according to any one of the preceding claims, wherein the area of each of the passage openings in the sensing portion? greater than the area of each of the flame openings. 9. Burner according to any one of the preceding claims, in which the passage openings have a circular shape and have a first diameter (D1) and the flame openings have a circular shape and have a second diameter (D2) smaller than the first (D1). 10. Burner according to claim 9, wherein the second diameter? equal to or less than 1.5 mm. 11. Burner according to any one of the preceding claims, in which the flame openings are distributed in a regular manner on the combustion surface with a first periodic step (P1), the passage openings are distributed in a regular manner on the distribution surface at least in the sensing portion with a second periodic step (P2), and the first step (P1)? greater than the second step (P2). 12. Burner according to any one of the preceding claims, in which the external body and the distributor are shaped as coaxial cylinders closed at one end, arranged in the opposite position to the burner head, by means of a plug. 13. Burner according to claim 12, wherein the flame zone? constituted by a plurality? of longitudinal sectors that extend in a direction parallel to the axis of the coaxial cylinders and mutually separated in the circumferential direction by non-perforated sectors. 14. Burner according to claim 13, wherein the number (N) of non-perforated sectors? equal to, or greater than, 18 and the width (x) of each of these sectors in the circumferential direction? equal to or less than 1.8 mm. 15. Burner according to claim 13, wherein the number (N) of non-perforated sectors? less than 18 and equal to, or greater than 8, and the width (x) in millimeters of each of those sectors in the circumferential direction? equal to, or less than, 1.8? 18 / N, where N? the number of non-perforated sectors. 16. Burner according to claim 13, wherein the number (N) of non-perforated sectors? less than 8 and the width (x) of each of these sectors in the circumferential direction? less than 10 mm. 17. Burner according to claim 12, wherein the flame zone? constituted by a plurality? of circumferential sectors that extend parallel to the direction of the circumference of the coaxial cylinders and are contiguous in the axial direction. 18. Burner according to claim 17, wherein: if the number (M) of circumferential sectors? equal to, or greater than, 3, the width (y) of each of these sectors in the axial direction? less than 5 mm; if the number (M) of circumferential sectors? less than 3, the width (y) of each of these sectors in the axial direction? less than 30 mm. 19. Burner according to any one of the preceding claims, not including metal mesh nets. 20. Burner according to any one of the preceding claims, wherein the material of which? constituted the distributor (2)? rigid and has a coefficient of thermal expansion equal to or greater than 11.5x10-6 /? C at least in the temperature range between 400? C and 900? C. 21. Burner according to any one of the preceding claims, comprising a detection electrode disposed outside the flame zone of the external body in correspondence with the detection portion of the distributor. 22. Burner according to claim 21, comprising a control device for adjusting the ratio between the quantity? of comburent and the quantity? of fuel of the mixture, the control device being configured in such a way as to receive from the sensing electrode an ionization signal, indicative of this ratio. 23. Boiler comprising a combustion chamber and a burner according to any one of the preceding claims. 24. Process for regulating the flow rate of a mixture of comburent and fuel in a premix burner according to any one of claims 1 to 20, comprising the following steps: 1) introduction of the mixture inside the distributor through the burner head; 2) diffusion of the mixture from the distribution area located on the distribution surface of the distributor to the flame area located on the combustion surface of the external body through the cavity of not zero thickness; 3) combustion of the mixture in the flame zone; 4) detection of an ionization signal, produced by the combustion of the mixture, by means of a detection electrode disposed outside the flame zone of the external body in correspondence with the detection portion of the distributor; 5) sending the ionization signal to a control device for adjusting the ratio between the quantity? of comburent and the quantity? fuel mixture; 6) adjustment of said ratio by means of the control device so that the ratio is equal to a predetermined value. 25. Process for regulating the flow rate of a mixture of comburent and fuel according to claim 24, in which diffusion and combustion of the mixture take place without the use of metal mesh.