The present, invention relates to a gas burner apparatus with pre-mixing having the features set out in the preamble of claim 1. Technological background

In the specific technical sector, it is known to construct gas burners of the pre-mixing type, for example, for condensation boilers, whose combustion head is intended to be received in a combustion chamber, to which the air/gas mixture is supplied by means of the delivery of a fan unit. An example of such a burner apparatus is known from Italian Patent Application No. PD2007A000413, in which there is described a burner which is connected to a cover or another similar closure element of the combustion chamber and in which there are coaxially mounted a fan unit and a cap-like combustion head. In such an application, the air/gas mixture which is burnt at the surface of the combustion head of the burner is in the form of a plurality of mixing ducts which are open towards the outer side of the burner in order to draw in air and through which the air, once mixed with the gas, is directed towards the intake section of the fan. Each of those ducts generally functions under reduced pressure, with the fan positioned downstream and is in the form of a Venturi duct, that is to say, which is formed by a convergent portion which is contiguous relative to a divergent portion, between which there is defined a minimum cross-section of the Venturi duct, a reduction in the pressure signal (pressure reduction) being produced in that cross-section. That signal is typically used to draw in from the valve the combustible gas which is then introduced into the mixing duct in order to then become mixed with the air drawn in by the reduced pressure.

The pressure signal which is generated in the minimum cross-section of the

Venturi duct is further a function of various parameters. For example, if the number of revolutions of the fan increases, for a given geometry, a greater flow of air is drawn in with a greater velocity through the minimum cross- section and therefore a greater pressure reduction is consequently generated. Furthermore, if the flow resistances downstream of the fan change, the signal changes as a result. For example, if the flow resistances increase, the signal decreases and, in order to be able to maintain it at the levels before the increase in the flow resistances, it is normally necessary to use another Venturi duct having a smaller minimum cross-section. However, this results in a cost increase because it requires the production and control of additional components in the burner. That problem is particularly apparent in applications in which the range of adjustment of the thermal output means that such components have to be replaced for correct adjustment of the burner. Statement of invention The problem addressed by the present invention is to provide a burner apparatus which is structurally and functionally configured so as to allow the limitations set out with regard to the cited prior art to be overcome. This problem is solved by the invention by means of a burner apparatus constructed in accordance with the appended claims. Brief description of the drawings Further features and advantages of the invention will be better appreciated from the following detailed description of one preferred embodiment thereof which is illustrated by way of non-limiting example with reference to the appended drawings, in which:

Figure 1 is a partially sectioned, perspective view of a burner apparatus constructed in accordance with the present invention,

Figure 2 is a schematic view of a detail of the burner of Figure 1,

Figure 2A is a schematic view of another detail of Figure 1,

Figures 3 and 4 are partially sectioned, perspective views of a detail of the preceding Figures in separate assembly steps, - Figures 5 and 6 are front elevation views of a detail of Figure 4 in two separate construction forms,

Figure 7 is another front elevation view of another construction variant of the detail of Figures 5 and 6. Preferred embodiment of the invention With reference to the Figures set out, a gas burner apparatus with pre- mixing constructed in accordance with the present invention is generally designated 1.

The burner comprises a combustion head 2, preferably having a cap-like construction whose base is advantageously secured to a cover 3 which constitutes a closure element of a combustion chamber (not illustrated) which is associated with the burner.

A fan unit 4, whose impeller 5 of the centrifugal type is coaxial with the combustion head, is further rotatably supported on the cover. The axis of rotation of the impeller, which is actuated by an electric motor 6, is designated X. Behind the combustion head 2 and internally with respect thereto, there is provided a distribution element which is designated 7 and which is also substantially of cap-like form and is provided with through-holes for distributing the air/gas mixture to the combustion head. There remains defined in the body of the burner, between the distribution element 7 and the impeller 5 facing it, an internal space or volume which is generally designated 8 and in which there are conveyed, in a separate and distinct manner, the mixture which is from the delivery cross-section 9 of the fan and which is directed to the distributor 7, and the flow of the air/gas mixture which is directed to the intake section 10 of the fan. Such a burner structure is described in detail in the Italian patent application which is referred to above and filed under number PD2007A000413, to which reference is made here concerning any details which are not expressly set out below. More specifically, there are provided a first plurality of conveying ducts which are designated 11 and which are capable of conveying the combustible air/gas mixture to the combustion head 2 which is supplied by way of the delivery side 9 of the fan and a second plurality of conveying ducts 12 which are suitable for conveying the flow of air and/or gas which is taken from outside the burner body in the direction of the intake section 10 of the fan. That intake section is arranged at the same side of the head 2 and is in communication with a first chamber 13 which is defined in the volume space 8 which remains between the head 2 and the impeller 5. The pluralities of ducts 11 and 12 are further selected in terms of size and structure so that the flow of the air/gas mixture to the delivery side of the fan intersects with the flow of air and/or gas which is directed to the intake section, as schematically illustrated in Figure 2, in which the above- mentioned flows are conveyed so as to be separate and distinct from each other by the ducts 11 and 12, respectively. The ducts 11 of the corresponding plurality extend in the region of the delivery section 9 of the fan, outside the outer diameter of the impeller 5, in order to connect that delivery section to a second chamber 14 for distributing the mixture, which chamber 14 is also defined in the space 8 between the distributor 7 and the impeller and is separated from the chamber 13 by means of a wall 15.

The ducts 11 are distributed circumferentially, downstream of the delivery section 9, with a pre-selected angular interval, in particular with a regular angular interval. Each of the ducts 11 further has, with respect to the direction of flow, an inlet opening 11a for the mixture (provided from the delivery side of the fan), in which the flow has a prevailing radial component, and a discharge opening lib, at the opposite end, in which the flow has a prevailing axial component. Downstream of the discharge opening, the chamber 14 is configured so as to further deviate the flow in order to convey it along the distributor 7 in order to efficiently supply the combustion head 2.

The second plurality of conveying ducts 12 is configured in order to generate pre-mixing of the air, taken from outside, with the gas which is introduced into the duct and thereby to convey a pre-mixed air/gas flow to the intake section 10 of the fan. The ducts 12 are arranged in an alternating position with respect to the ducts 11 in such a manner that, in the region of intersection of the flows, designated M in Figure 2, each duct 12 is interposed between a pair of ducts 11 which are adjacent to each other.

Each duct 12 is open at the outer side of the burner, at the intake section 12a of the duct, by way of which a flow of air is drawn in and extends as far as an opposite discharge section 12b which is placed in communication with the intake section 10 of the fan. Only one of the ducts 12 will be described in detail below, because they are of identical construction. Each duct 12 is provided with mixing means, which comprise a portion of the duct itself that is configured as a Venturi duct, that is to say, comprising a convergent portion 16a which extends into a contiguous, divergent portion 16b, there remaining defined between the portions a restricted section (minimum section) of the Venturi duct, designated 16c. At that minimum section 16c, there is introduced a flow of gas into the Venturi duct, drawn in by the reduced pressure brought about in the minimum section. The duct is designated 17 and continues in the form of a valve 18, which is only schematically illustrated and which is capable of conveying the combustible gas into the corresponding duct 12, at that minimum section 16c. Figure 2A is a purely schematic view of the pre-selected configuration in relation to the Venturi duct.

Each duct 12 is further formed following the engagement of a first region and a second region of the burner, which are separate from each other, along a surface engagement profile, the first region which is generally designated A being defined in the base structure of the cover 3 which extends axially upstream of the impeller, the second region which is generally designated B being arranged downstream of the impeller and in which the wall 15 is arranged in the delivery zone of the burner. There is interposed between the burner regions A and B a fluid-tight sealing element which is designated 20 and which has the main function of preventing gas and/or air from being drawn in between the ducts 11 and 12 in a relative manner.

The sealing element 20 is advantageously of planar annular form with reduced transverse thickness, as clearly shown in Figure 4. According to a main feature of the invention, that sealing element is positioned at the restricted sections 16c of the ducts 12 (minimum section of the Venturi duct). The sealing element 20 is further provided, in the annular wall thereof, with a plurality of through-holes which are all designated 21 and which define the restricted section of the corresponding Venturi duct at least in some of the plurality of ducts 12, once the regions A and B of the burner have been engaged along the engagement profile thereof, with the seal 20 being interposed.

With reference to Figure 5, the through-holes 21 are arranged circumferentially with a regular angular interval along the annular wall of the seal 20. They have, when viewed in cross-section, a substantially drop-like shape. A corresponding through-hole 22 having the shape of the cross- section extending in a prevailing direction, as clearly shown in Figure 5, is provided and is associated in an adjacent position with respect to each hole 21. Those holes 22 define the sections of passage of the gas flow, for each of the ducts 17, at the restricted section of the Venturi duct, where there occurs the introduction of the combustible gas into the air flow which is conveyed by the ducts 12.

In greater detail, each duct 17 is, upstream of the seal 20, in communication with a channel 17a which extends in an annular manner in the burner region A, concentrically relative to the axis X, which serves to distribute and convey the gas from the valve in each of the ducts 17. • The channel 17a is jointly defined by the burner region A and the seal 20 which is contiguous therewith.

There is further defined in the sealing element 20 another plurality of through-holes which are all designated 23 and which delimit the cross- section of passage of the flow of the air/gas mixture which is supplied by the delivery 9 of the fan and which is directed to the distributor 7 of the combustion head. In other words, those holes 23 define the cross-section of each of the ducts .11, in the region of the seal 20, and are positioned along the circumferential profile of the seal, in alignment with the holes 21 with a pre-selected, regular, angular interval. In the preferred embodiment of Figure 5, there is provision for the holes 23 to be provided in an alternating manner with respect to the holes 21, along a common circular profile which is concentric relative to the axis X so that each hole 23 is interposed between a pair of mutually adjacent holes 21, as clearly illustrated in Figure 5.

In accordance with a main feature of the invention, the sealing element 20 is configured so as to be interchangeable with at least a second sealing element which is designated 20' (illustrated in Figures 4 and 6) and which is provided with a plurality of through-holes 21' having the same function as the corresponding holes 21 of the seal 20, the overall size of the cross- sections of those holes 21' being different (greater or smaller than) from the overall size of the cross-sections relating to the holes 21'. In that manner, by the seal 20 being interchanged with the seal 20', it is possible to vary the overall section relating to the flow of air which is drawn into the burner in order to consequently adjust the burner in accordance with the thermal output required and/or the flow resistances which exist in the supply ducts of the air/gas mixture.

It will be appreciated that the invention allows the provision of a plurality of sealing elements with the number and shape of the holes 21 which are advantageously pre-selected in accordance with the requirements for adjustment and the power intervals required for the operation of the burner. Reference will be made below only to a second interchangeable sealing element, it being possible to provide a plurality of mutually interchangeable seals. In a first preferred embodiment, there is provision for there to be the same number of holes 21, 21' and 23, 23' in the seal 20 and 20' and alternating positioning in each of those seals, as described above and illustrated in Figures 5 and 6, respectively. The holes 21 are further equally spaced apart from each other, as are the holes 21', 23 and 23'. The second sealing element 20' differs from the seal 20 in that there are provided holes 21' each having a cross-section of greater size than the size of each corresponding hole 21 of the seal 20.

In a variant of the seal 20', generally designated 20", there can instead be provision for the number of holes 21" to be different from the number of holes 21', but with the holes 21' and 21" having the same cross-section. In Figure 7, which illustrates that variant, it is clearly shown how there are provided only six holes 21", unlike the seal 20' which has nine corresponding holes 21'. As a result, for a given cross-section of the holes 21' and 21", the overall section varies when the seal 20' is interchanged with the seal 20".

Naturally, other construction variants are possible, all directed towards the provision of different configurations of calibrated holes for the passage of intake air, in the restricted section of the Venturi duct, for each of the seals which are provided and which are mutually interchangeable, in such a manner as to generate the desired pressure signal and in order to control the gas valve. Advantageously, it is possible in that manner to ensure wide modulation in accordance with the thermal power required, for each power range, simply by the sealing element being interchanged, without any other additional components and without any modification of the other structural components of the burner both at the side of the cover and at the delivery side. As a result, therefore, control of the burner apparatus is simplified and facilitated, even in the presence of broad modulation ranges that are required for the thermal power of the apparatus. It is further possible, if considered advantageous, to provide for throttling of the passage sections of the gas in the ducts 17, by way of different provision (concerning the number and size of the section) of the holes 22 in each of the sealing elements which may be provided and which can be mutually interchanged. The invention thereby solves the problem set out, resulting in the advantages set out above in relation to known solutions.