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(11) | EP 0 892 213 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | A filtering-bed burner and a gas combustion method carried out by it |
| (57) A filtering-bed burner is described which comprises two concentric holding nets (9,
10) between which a space filled with granular elements of irregular shape (8) made
of crystallized silicon dioxide is defined, said granular elements having a particle
size included between 2.5 and 7 mm and defining a filtering bed (3) of a thickness
included between 10 and 20 mm. An inlet header (4) and a closing element (5) are associated
with the opposite ends of the filtering bed (3) and fastened to the corresponding
edges of the holding nets (9, 10). The closing element (5) has a connecting portion
(6) of the same form and size as the inlet header and a closing wall (7) axially delimiting
a pre-mixing chamber (2) defined within the filtering bed. A hyperstoichiometric air-gas
mixture is conveyed to the pre-mixing chamber (2) at a flow rate adapted to determine
a specific flame power that can be modulated between about 25 W/cm2 and 350 W/cm2. |
[0001] The present invention relates to a filtering-bed burner of the type comprising a pre-mixing chamber to be associated with feeding means to supply an air-combustible gas mixture, and a filtering bed formed of granular elements disposed to define a delimitation wall for the pre-mixing chamber, said filtering bed being arranged so as to be passed through by the air-gas mixture that is fired close to an outer surface provided on the filtering bed itself on the opposite side relative to the pre-mixing chamber.
[0002] The invention also relates to a method of combustion of a gas carried out by said burner, said method comprising the steps of: pre-mixing the gas with a predetermined amount of air to form an air-gas mixture; feeding said air-gas mixture through a filtering bed defined by granular elements to cause an intimate mixing between the air and the gas forming said mixture, firing the air-gas mixture close to an outlet surface of said filtering bed.
[0003] It is known that burners currently used in boilers and water heaters or similar apparatuses for household and/or industrial use, are essentially comprised of a mixing chamber in which a combustive air-combustible gas mixture is formed. This mixture is then ejected from the burner through a plurality of holes or shaped openings conveniently distributed on a burner wall, intended to delimit the mixing chamber.
[0004] At the burner outlet, the air-gas mixture is fired, thereby forming a plurality of small flames correspondingly distributed at the location of said through holes or openings provided in the burner wall.
[0005] The above described perforated-wall burners have the special feature of being of relatively cheap construction, so that use of same is widespread in different application fields.
[0006] However these burners have some drawbacks of technical and operational character that are essentially due to the fact that they do not enable an intimate mixing between the air and gas before ejection and firing of the mixture formed by them. This fact can give rise to a defective burning of the mixture which results in production of polluting discharges, and CO and NOx emissions.
[0007] This situation has a tendency to become worse when the air-gas mixture flow rate is wished to be modulated in order to adjust the power of the flame produced by the burner within a desired value range. This modulation, on the other hand, can take place only within a relatively limited value range. Actually, when the mixture flow rate is increased beyond certain values, depending on the burner construction features, undesired phenomena may arise that involve detachment or separation of the flame from the perforated burner wall. On the contrary, when the mixture flow rate tends to be reduced beyond given values, the flame can move too close to the perforated burner wall thereby causing overheating of said mixture. Due to thermal expansions, a deformation of the through openings or holes may occur that will lead to a consequent loss of control in the outgoing mixture flow, as well as to the risk of dangerous backfires.
[0008] Presently, one of the possible alternatives to burners provided with a perforated wall of the type described above involves employment of the so-called "filtering-bed" burners. In this type of burners, the air-gas mixture conveyed to the inside of the burner is forced to pass through a filtering bed formed of granular elements disposed in several superposed layers, to define a labyrinth formed with hollow spaces promoting an intimate mixing between the air and gas forming the mixture, said mixture being fired close to an outlet surface of the filtering bed.
[0009] For example, US Patent 3,322,179 describes a burner having a filtering bed formed of a plurality of spherules or granular elements of different shape made of glass, quartz, silicon carbide or ceramic material, mutually joined at the points of mutual tangency by a sintering process and distributed to form two different superposed layers.
[0010] A first layer, turned towards the pre-mixing chamber, is formed of spherules or granular elements of different shape having a particle size included between about 0.15 and 0.3 mm. A second layer, defining the outlet surface of the filtering bed, is formed of spherules or granular elements of a particle size included between 0.5 and 1 mm.
[0011] Another filtering-bed burner is described in US Patent 3,947,233. In the burner disclosed in this document the filtering bed is essentially defined by a plate-like element of sintered metal particles, defined by spherules or irregular bodies. In this case too, particles are distributed to form two distinct layers. A first layer, of a thickness in the order of 3 mm, is formed of spherules the size of which is such selected that the interspaces defined between them have a size in the order of 0.1 mm. The spherules forming the second layer, of a thickness in the order of 1 mm and defining the outlet surface of the filtering bed, have such sizes that the interspaces defined between them have a diameter in the order of 0.02 mm.
[0012] US Patent 5,591,095, in the name of the same Applicant, discloses a burner the filtering element of which is formed of spherules of ceramic material of predetermined sizes, included by way of example between 1 and 10 mm, optionally welded together by a sintering process. The mutual divergence of the spherule surfaces at the burner outlet causes the outgoing flow speed to slow down immediately downstream of the interspaces defined between the spherules themselves, even at high flow rates of the air-gas mixture conveyed through the filtering bed, thereby generating a continuous flame front confined between the spherule surfaces.
[0013] In other words, the flame stays always adherent to the spherule surfaces, so that a localized heating produced on said surfaces may cause incandescence thereof and consequent heat emission by radiation.
[0014] In accordance with the present invention it has been found that by making the filtering bed of granular elements of irregular shape having a particle size included between 2.5 and 7 mm, preferably between 3 and 5 mm and randomly laid down upon each other so as to form an overall thickness included between 10 and 20 mm, the possibility of making the burner operate in an optimal manner, also modulating the flow rate value of the air-gas mixture within exceptionally wide value ranges, is advantageously achieved.
[0015] Such a burner can be in addition produced at very reduced costs, comparable to those of conventional burners provided with a perforated plate, even in the case in which the burner is of cylindrical form.
[0016] In particular, the invention relates to a filtering-bed burner characterized in that said filtering bed of a thickness included between 10 mm and 20 mm, is made up of granular elements of irregular shape, having a particle size included between 2.5 and 7 mm, randomly disposed upon each other.
[0017] Still in accordance with the present invention, a preferential solution of a burner of cylindrical form is obtained according to a process which is characterized in that it comprises the following steps: forming an inlet header having a base wall in the form of an annulus, as well as an outer perimetric wall and an inner perimetric wall substantially extending at right angles from respectively opposite perimetric edges of the base wall; associating first and second concentrically-disposed holding nets with said outer perimetric wall and said inner perimetric wall respectively; filling a space defined between said holding nets at least partly with granular elements disposed randomly against each other so as to define a filtering bed; forming a closing element having a connecting portion of shape and sizes substantially identical with those of the inlet header, and a closing wall; causing engagement of an inner perimetric wall and an outer perimetric wall of said connecting portion respectively with the first and second holding nets on an opposite side relative to the inlet header.
[0018] Further features and advantages will become more apparent from the detailed description of a preferred but non-exclusive embodiment of a filtering-bed burner and a process carried out by it in accordance with the present invention. This description will be taken hereinafter with reference to the accompanying drawings, given by way of non-limiting example, in which:
- Fig. 1 is a diametrical sectional view of a filtering-bed burner in accordance with a first embodiment of the invention;
- Fig. 2 shows a different embodiment of the burner in question.
[0019] With reference to the drawings, a filtering-bed burner in accordance with the present invention has been generally identified by reference numeral 1.
[0020] Burner 1 comprises a pre-mixing chamber 2 to be operatively associated with feeding means for supply of an air-gas mixture, said means being only diagrammatically shown in that it can be made in a manner known per se and conventional.
[0021] Burner 1 further comprises at least one filtering bed 3 arranged to define, by its inlet surface 3a, at least one wall delimiting the pre-mixing chamber 2. In more detail, in the embodiment shown the pre-mixing chamber 2 is directly defined within the filtering bed 3 and circumscribed by the inlet wall 3a of the filtering bed, the latter being made of a cylindrical tubular form, preferably of circular section.
[0022] In a manner known per se, the filtering bed 3 is adapted to be passed through by the air-gas mixture introduced into the pre-mixing chamber 2 which is fired at the outside of the burner, close to an outlet surface 3b provided by the filtering bed on the opposite side relative to the pre-mixing chamber 2.
[0023] The pre-mixing chamber 2 is connected to the feeding means 1a by an inlet header 4 associated with one end 3c of the filtering bed 3. The inlet header 4 is preferably made of a die-cut and drawn metal sheet and has a base portion 4a in the form of an annulus which is such disposed that the first end 3c of the filtering bed 3 is delimited thereby. Header 4 further has an outer perimetric portion 4b of cylindrical conformation extending in a substantially perpendicular direction from an outer perimetric edge of the base portion 4a. Furthermore, extending from an inner perimetric edge of the base portion 4a, preferably in the opposite direction relative to the outer perimetric portion 4b, there is an inner perimetric portion 4c which is also extended in a direction substantially perpendicular to the base portion 4a and has a substantially cylindrical conformation.
[0024] Burner 1 further has a closing element 5 associated with a second end 3d of the filtering bed 3, on the opposite side relative to the inlet header 4. Advantageously, this closing element 5 has a connecting portion 6 the shape and sizes of which are substantially identical with those of the inlet header 4, and a closing wall 7 arranged to close the end of the pre-mixing chamber 2. In more detail, the connecting portion 6 of the closing element 5 has a respective base wall 6a in the form of an annulus, so as to delimit the second end 3d of the filtering bed 3, as well as an outer perimetric wall 6b and an inner perimetric wall 6c extending at right angles and in opposite directions from an outer perimetric edge and an inner perimetric edge respectively of the base wall 6a. The closing wall 7 extends in a diametrical plane from an end edge of the inner perimetric wall 6c.
[0025] It is to note that advantageously the closing element 5, of a conformation and structure substantially identical with those of the inlet header 4, is adapted to be made with the aid of the same machines and equipment as employed for making the header itself. In particular, since both elements are obtained by carrying out die-cutting and drawing of a metal sheet, it will be possible to produce these two components with one and the same mould by arranging a movable punch in said mould, which is adapted to selectively cause either forming of the closing wall 7 so as to obtain the closing element 6, or removal of the metal sheet portion corresponding to such a wall so as to form the inlet header 4.
[0026] As can be easily recognized by comparing the accompanying drawings, header 4 can have the same orientation as the connecting portion 6 of the closing element 5, as shown in Fig. 1, or it may be disposed in mirror image relationship relative to this connecting portion 6, as shown in Fig. 2.
[0027] The filtering bed 3 is substantially formed of a homogeneous mixture of granular elements 8 of irregular shape, preferably of crystallized silicon dioxide, having differentiated particle sizes of a value included between 2.5 and 7 mm, and preferably included between 3 and 5 mm.
[0028] The embodiment involving granular elements of different size homogeneously mixed with each other is preferred in that it has been found that it enables a more efficient mixing between the air and gas forming the mixture admitted to the pre-mixing chamber 2. However, employment of granular elements of same size, the particle size of which is included within the above specified value ranges, may be also taken into account.
[0029] The granular elements 8 are freely and randomly disposed against each other to fill a space defined between a first holding net 9 extending at the inlet surface 3a of the filtering bed 3 and a second holding net 10 extending at the outlet surface 3b. In the embodiment shown the first and second nets 9 and 10 have a cylindrical configuration and are concentrically disposed with respect to each other.
[0030] Each of the holding nets 9, 10 can be made either of a metal wire for example, or of a die-cut and optionally stretched metal sheet so as to form the net meshes by spreading apart cuts produced by the die-cutting operation.
[0031] Preferably the holding nets have the same mesh width as the particle size of the smallest granular elements or a slightly smaller width.
[0032] In this manner, the holding nets are adapted to carry out an efficient holding action of the granular elements 8 without putting up too much resistance to the air-gas mixture passage.
[0033] It is to note that, since the granular elements 8 are of random and irregular shape, the interspaces defined between them and passed through by the air-gas mixture have lower sizes than the particle size values of the granular elements themselves.
[0034] In order to eliminate any risk of backfire, without hindering the air-gas mixture passage through the filtering bed 3 too much, thickness of said bed is provided to be included between 10 and 20 mm.
[0035] In the embodiment shown in Fig. 1, the first holding net 9 is fastened by its opposite end edges 9a, 9b to the inner perimetric walls 4c, 6c provided by the inlet header 4 and the connecting portion 6 of the closing element 5, respectively. The outer holding net 10, in turn, has its opposite end edges 10a, 10b fastened to the outer perimetric walls 4b, 6b belonging to the inlet header 4 and the closing element 5, respectively.
[0036] In more detail, in the embodiment shown in Fig. 1 the first holding net 9 is internally in engagement both with the inner perimetric wall 6c of the closing element 5 and the inner perimetric wall 4c of header 4. The second holding net 10, in turn, is engaged externally of the outer perimetric wall 6b of the closing element 5 and externally of the outer perimetric wall 4b of header 4.
[0037] In the embodiment shown in Fig. 2, on the contrary, both holding nets 9, 10 are engaged internally of the corresponding perimetric walls 4b, 4c, 6b, 6c belonging to the header 4 and closing element 5.
[0038] Connection between nets 9, 10 and header 4, as well as the closing element 5, can be carried out by welding, for example.
[0039] At least one insulating element 11, preferably made of an elastically-yielding refractory material, a ceramic fibre for example, may be interposed between at least one of the filtering bed 3 ends and the corresponding base wall 6a, 4a of the closing element 5 and/or header 4.
[0040] In accordance with a further preferential feature of the present invention, burner 1 further comprises at least one shielding element 12 extending from the closing element 5 close to the inlet surface 3a of the filtering bed 3, to the inside of the pre-mixing chamber 2. In more detail, this shielding element 12 extends axially over at least 1/10 of the axial length of the filtering bed past the corresponding end 3d of the filtering bed 3 itself. Owing to the presence of the shielding element 12, distribution of the air-gas mixture through the filtering bed 3 is made still more homogeneous, thereby eliminating the undesired effect of overpressures that are likely to be created close to the corresponding end 3d of the filtering bed, due to the presence of the closing wall 7 that suddenly stops mixture advancing along the pre-mixing chamber 2.
[0041] A second shielding element 13 may be also provided and it extends from the inlet header 4 close to the inlet surface 3a of the filtering bed 3 to the inside of the pre-mixing chamber 2. This second shielding element 13 can be either defined by the inner perimetric wall 4c of header 4, as shown in Fig. 1, or made as an extension of said wall. Alternatively, as provided in Fig. 2, the second shielding element 13 can be coaxially inserted in the inner perimetric wall of header 4.
[0042] The burner in accordance with the invention can be advantageously obtained by a process first providing fastening, by welding for example, of the first and second holding nets 9, 10 to the inlet header 4. Subsequently the interspace defined between nets 9, 10 is filled with the granular elements 8 forming the filtering bed 3. When filling has been completed, the closing element 5 is fitted to the holding nets 9, 10 and fastened to the first holding net 9 by welding, for example. Concurrently with, or before engagement of the closing element 5, insertion of the optional shielding element or elements 12 is carried out.
[0043] Obviously, assembling can be also executed with a reverse sequence with reference to the above described one, that is the holding nets 9, 10 are first engaged in the closing element 5, whereas mounting of header 4 is executed once the filtering bed 3 has been formed by introduction of the granular elements 8.
[0044] If arrangement of the insulating elements 11 is provided, important tolerances in the axial size of the filtering bed 3 are enabled. Actually, the elastically-yielding insulating element or elements 11 act on the filtering bed ends causing compacting of said bed on mounting of the closing element 5 and/or header 4. In addition, the insulating elements 11 prevent the flame produced by combustion from being propagated internally of the outer perimetric wall 6b, 6c of the closing element 5 and/or header 4, thereby giving rise to an undesired overheating of said element 5 and header 4.
[0045] In addition, the insulating elements enable compensation for thermal expansions undergone by the material during operation of the burner.
[0046] In accordance with a further feature of the present invention, the feeding means 1a is advantageously arranged to feed the air-gas mixture at a specific flame power included between a minimum value of about 25 W/cm2 and a maximum value of about 350 W/cm2.
[0047] To the ends of the present description, by specific flame power it is intended a power delivered by the flame for each cm2 of the outlet surface 3b of the filtering bed 3.
[0048] Advantageously, the feeding means 1a may be of a type capable of modulating the air-gas mixture flow rate so as to adjust the specific power flame at each moment depending on requirements within a value range included between said minimum and maximum values.
[0049] It is also preferably provided for the feeding means to be of the type capable of supplying a greatly hyperstoichiometric air-gas mixture, with an oxygen excess in the combustion fumes included between 3.5% and 9%.
[0050] Use of plenty of air in excess enables the flame temperature to he lowered to advantage of the combustion quality and also the flow rate and consequently the flow speed of the mixture passing through the filtering bed 3 to be increased.
[0051] It is to note that feeding of excess air would tend to make achievement of a perfectly homogeneous mixing of gas in air quite difficult. However, due to the typology, size features and arrangement of the granular elements 8 forming the filtering bed 3 in accordance with the present invention, homogeneity of gas-mixing in air is in any case excellent, at any flow rate value and also when the excess air is brought to the maximum values. Therefore an excellent quality of combustion is ensured, with a very low production of CO and NOx, under any operating conditions. The optimal mixing quality also eliminates any risks of flame detachment even under the highest running conditions, as well as any risks of backfire under the lowest running conditions. During operation at low running conditions, that is at 25 W/cm2, the outgoing flow rate can be in any case maintained, by increasing the air percentage in the air-gas mixture for example, to such a level that the flame stays slightly detached from the outlet surface of the filtering bed, so that undesired overheating of the second holding net 10 and the granular elements 8 located close to the outlet surface 3b is not caused.
[0052] By using granular elements 8 of ceramic material as well as known materials adapted to the purpose in the manufacture of the second holding net 10, it is in any case possible to reduce the air excess and/or the mixture flow rate so that the flame stays in contact with the outlet surface of the filtering bed to bring the latter to an incandescence condition and dissipate heat by radiation.
[0053] Heat dissipation by radiation can be also achieved by arranging an auxiliary radiating net-like element 14 (shown in chain line in Fig. 1) to a position conveniently spaced apart from the outlet surface of the filtering bed 3.
[0055] The burner in reference enables operation to be carried out in a greatly adjustable manner and ensures an excellent combustion at any load level, while at the same time having production costs comparable with those of standard perforated-plate burners.
[0056] The characteristic parameters of an embodiment of a burner made in accordance with the present invention are set forth herebelow:
- diameter of the outlet surface 3b of the filtering bed 3:60 mm;
- material forming the granular elements 8: crystallized silicon dioxide;
- particle size of the granular elements 8: 3 mm to 5 mm;
- thickness of the filtering bed: 14 mm;
- mesh width of the holding nets 9 and 10: 3 mm;
- possibility of modulation in operation: 30 W/cm2 to 300 W/cm2;
- feeding: air-methane gas mixture with a ratio λ equal to 1.56 corresponding to an oxygen excess of 7.5%.
1. A filtering-bed burner, comprising:
- a pre-mixing chamber (2) to be associated with feeding means (1a) for supplying an air-combustible gas mixture;
- a filtering bed (3) formed of granular elements (8) such disposed as to define a
delimitation wall of the pre-mixing chamber (2), said filtering bed (3) being arranged
to be passed through by an air-gas mixture that is fired close to an outer surface
(3b) provided by the filtering bed on the opposite side relative to the pre-mixing
chamber (2);
characterized in that
said filtering bed (3), of a thickness included between 10 mm and 20 mm, is formed
of granular elements (8) of irregular form, of a particle size included between 2.5
and 7 mm, randomly disposed against each other.
2. A burner as claimed in claim 1, wherein the filtering bed (3) is formed of a homogeneous
mixture of granular elements (8) of differentiated particle size.
3. A burner as claimed in claim 1, wherein said granular elements (8) are made of crystallized
silicon dioxide.
4. A burner as claimed in claim 1, wherein said granular elements (8) are randomly disposed
in respect of each other so as to fill a space defined between a first holding net
(9) and a second holding net (10) extending from the filtering bed (3) at an inlet
surface (3b) and an outlet surface (3a) of the air-gas mixture, respectively.
5. A burner as claimed in claim 4, wherein at least one of said holding nets (9, 10)
is made of a metal wire.
6. A burner as claimed in claim 4, wherein at least one of said holding nets (9, 10)
is made of a die-cut and stretched metal sheet.
7. A burner as claimed in claim 4, wherein the mesh width of each of said holding nets
(9, 10) is the same as or smaller than the particle size of the granular elements
(8) of the smallest size.
8. A burner as claimed in claim 1, wherein said feeding means (1a) is arranged to send
an air-gas mixture at a specific flame power included between a minimum value substantially
equal to 25 W/cm2 and a maximum value substantially equal to 350 W/cm2.
9. A burner as claimed in claim 8, wherein the air-gas mixture flow rate can be modulated
within a value range included between said minimum value and said maximum value.
10. A burner as claimed in claim 1, wherein said air-gas mixture is a hyperstoichiometric
mixture having an oxygen excess in the combustion fumes included between 3.5% and
9%.
11. A burner as claimed in claim 1, wherein said filtering bed (3) has a cylindrical tubular
conformation.
12. A burner as claimed in claim 11 further comprising an inlet header (4) associated
with a first end (3c) of said filtering bed (3) and a closing element (5) associated
with a second end (3d) of the filtering bed (3).
13. A burner as claimed in claim 12, wherein the inlet header (4) is formed of a die-cut
and drawn metal sheet and has a base wall (4a) in the form of an annulus, as well
as an outer perimetric wall (4b) and an inner perimetric wall (4c) substantially extending
at right angles from respectively opposite perimetric edges of the base wall.
14. A burner as claimed in claim 12, wherein said closing element (5) has a connecting
portion (6) of shape and sizes that are substantially identical with those of the
inlet header (4), and a closing wall (7) intended for closing the pre-mixing chamber
(2).
15. A burner as claimed in claim 12, further comprising at least one insulating element
(11) interposed between the filtering bed (3) and a base wall (6a) of said closing
element (5).
16. A burner according to claim 12, further comprising at least one insulating element
(12) interposed between the filtering bed (3) and a base wall (4a) of said inlet header
(4).
17. A burner as claimed in claims 4 and 11, wherein said first holding net (9) is fastened
by its opposite end edges, to inner perimetric walls (6b, 4b) belonging to the closing
element (5) and the inlet header (4), respectively.
18. A burner as claimed in claims 4 and 11, wherein said second holding net (10) is fastened,
by its opposite end edges, to outer perimetric walls (6c, 4c) belonging to the closing
element (5) and the inlet header (4), respectively.
19. A burner as claimed in claim 12, further comprising at least one first diffuser element
(12) extending from the closing element (5) close to the inlet surface (3a) of the
filtering bed (3) towards the pre-mixing chamber (2).
20. A burner as claimed in claim 12, further comprising at least one second diffuser element
(13) extending from the inlet header (4) close to the inlet surface (3a) of the filtering
bed (3) towards the pre-mixing chamber (2).
21. A method of burning combustible gas comprising the steps of:
- pre-mixing the gas with a predetermined amount of air to form an air-gas mixture;
- feeding said air-gas mixture through a filtering bed (3) defined by granular elements (8) to cause an intimate mixing between the air and gas forming said mixture;
- firing the air-gas mixture close to an outlet surface (3a) of said filtering bed
(3),
characterized in that
said filtering bed (3) has a thickness included between 10 mm and 20 mm, said granular
elements (8) being of irregular shape and having a particle size included between
2.5 and 7 mm.
22. A method as claimed in claim 21, wherein mixture feeding through the filtering bed
is carried out at a flow rate corresponding to a specific flame power substantially
included between 25 W/cm2 and 350 W/cm2.
23. A method as claimed in claim 21, wherein in the pre-mixing step excess air is fed
so that with said gas it forms a hyperstoichiometric mixture with an oxygen excess
in the combustion fumes included between 3.5% and 9%.
24. A process for making a filtering-bed burner characterized in that it comprises the
following steps:
- forming an inlet header (4) having a base wall (4a) in the form of an annulus, as well as an outer perimetric wall (4b) and an inner perimetric wall (4c) substantially extending at right angles from respectively opposite perimetric edges of the base wall;
- associating first and second concentrically-disposed holding nets (9, 10) with said outer perimetric wall (4b) and said inner perimetric wall (4c), respectively;
- filling a space defined between said holding nets (9, 10) at least partly with granular elements (8) disposed randomly against each other so as to define a filtering bed (3);
- forming a closing element (5) having a connecting portion (6) of shape and sizes substantially identical with those of the inlet header (4), and a closing wall (7);
- engaging an inner perimetric wall (6b) and an outer perimetric wall (6c) of said connecting portion (6) with the first and second holding nets (9, 10) respectively, on an opposite side relative to the inlet header (4).