Patent Publication Number: US-2010116226-A1

Title: Boiler with a combustion head cooling system

Description:
PRIORITY CLAIM 
     The present application is a United States national phase application filed pursuant to 35 USC §371 of International Patent Application Serial No.: PCT/IB2007/003398, filed Nov. 7, 2007; which application claims priority to Italian Patent Application Serial No.: B02006A000758, filed Nov. 8, 2006; which foregoing applications are incorporated herein by reference in their entireties. 
    
    
     TECHNICAL FIELD 
     An embodiment of the present invention relates to a boiler with cooling system of the combustion head of the burner. Specifically, the present invention relates to a heater with air-gas premixing burner, to which explicit reference will be made in the following description without loosing in generality. 
     BACKGROUND 
     It is known that boilers with total air-gas premixing burner are designed to work with burners using only combustion air mixed with combustible gas upstream of the combustion head of the burner and, thus, without any contribution of secondary air downstream of the combustion head of the burner. 
     The known boilers with air-gas premixing burners comprise a mixer in which the total air-gas premixing is performed, a fan, a burner, a combustion chamber downstream of the burner, and a heat exchanger which comprises, in turn, a plurality of tubes defining the path of the water in a heating system. 
     The mixer may be located upstream or downstream of the fan according to the choice made by the manufacturer. 
     The boilers of the above-described type may present the severe problem related to the excessive heating of the combustion head of the burner, and specifically the overheating of the diffuser element which represents the part of the combustion head in contact with the flame of the burner. 
     It is indeed known that when total air-gas premixing burners run at the minimum of the heat supply thereof, the flame front progressively approaches the surface of the diffuser heating it to very high temperatures such as to require solutions adapted to withstand such high temperatures. 
     In order to overcome such a problem of the diffuser overheating there have been used diffusers formed by materials presenting a high resistance to high temperatures, which however imply other disadvantages. Specifically, the diffusers formed by high alloy steel or high-temperature-resistant metallic mesh present the disadvantage of being too expensive, while the diffusers formed by ceramic material, such as the diffusers of porous matrix or ceramic matrix, in addition to being costly, present the disadvantage of being particularly fragile. 
     SUMMARY 
     An embodiment of the present invention is a cooling system for the combustion head of a total air-gas premixing burner, the technical features of which are such to overcome the drawbacks of the known art. 
     Furthermore, according to an embodiment of the present invention, the external element of the combustion head of the burner is a diffuser. 
     According to a further embodiment, a plurality of seats adapted to accommodate part of the tubes are obtained in the diffuser element. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following examples are provided by way of non-limiting illustration for a better understanding of one or more embodiments of the invention with the aid of the figures in the accompanying drawing, in which: 
         FIG. 1  is a section of a condensing boiler with total air-gas premixing burner according to an embodiment of the present invention with parts removed for the sake of simplicity; 
         FIG. 2  is an enlarged detail of  FIG. 1 ; 
         FIG. 3  is an exploded view of part of  FIG. 1 ; and 
         FIG. 4  is a section of a non-condensing boiler with total air-gas premixing burner according to an embodiment of the present invention with parts removed for the sake of simplicity. 
     
    
    
     DETAILED DESCRIPTION 
     In  FIG. 1 , numeral  1  indicates an embodiment of a condensing boiler as a whole. 
     The boiler  1  comprises a fan  2  adapted to blow the air-gas mixtures into a burner  3  arranged underneath the fan  2  it is connected to, a combustion chamber  4  faced by the burner  3 , and a heat exchanger  5  comprising a plurality of tubes  6  in which the water of a heating system (not shown) runs. Specifically, the burner  3 , the combustion chamber  4  and the heat exchanger  5  are accommodated inside a containing structure  7 , the side walls of which at the combustion chamber  4  are protected by ceramic fiber panels  12 . 
     As shown in  FIGS. 2 and 3 , the burner  3  comprises a covering plate  8  adapted to close the containing structure  7  and presenting a hole  9  for the input of the air-gas mixture from the fan  2 , a baffle  10  arranged at the hole  9  and fixed to the covering plate  8 , and a combustion head  11 , which faces the combustion chamber  4  and comprises a distributor element  14  and a diffuser element  15 . 
     Specifically, the covering plate  8  is fixed to the structure  7  in reversible manner to open and close the structure  7  itself, while the combustion head  11  is fixed to the covering plate  8  by means of fastening devices  24  and rests on a part of the tubes  6  as described below. In this manner, after having lifted the burner  3 , the combustion chamber  4  may be easily inspected. 
     The distributor element  14  and the diffuser element  15  are constituted by two corresponding appropriately perforated plates, in which the sum of the holes obtained in the distributor element  14  has a total passage surface of generally smaller size than that of the holes obtained in the diffuser element  13 . 
     In the distributor  14  and in the diffuser  15  there are obtained a plurality of longitudinal grooves  16 , having an essentially semi-circular profile and defining corresponding semicircular profile protrusions  16   a  on the opposite surface. 
     Specifically, as shown in  FIG. 2 , in an embodiment, the distributor  14  and the diffuser  15  are reciprocally coupled with the protrusions  16   a  of the diffuser  15  which engage the grooves  16  of the distributor  14 . 
     The heat exchanger  5 , at least in the specific embodiment shown in  FIG. 1 , comprises a first plurality of circular section tubes  6   a  arranged in the upper part of the combustion chamber  4 , and accommodated inside the longitudinal grooves  16  obtained in the diffuser  15 . Specifically, the boiler  1  comprises a plurality of interposition elements  17  formed by heat-conducting material, such as for example metallic meshes, each of which is positioned between each tube  6   a  and the corresponding groove  16  in which it is accommodated. 
     The heat exchanger  5  further comprises a second plurality of elliptical section tubes  6   b  arranged in two reciprocally offset rows, and a third plurality of circular section tubes  6   c  arranged underneath the tubes  6   b  and also arranged on two reciprocally offset rows. Specifically, the second plurality of tubes  6   b  is designed to exploit the heat coming directly from the flame and from the fumes and the latent condensation heat of the combustion fumes, and the third plurality of tubes  6   c  is designed to further exploit the latent condensation heat of the combustion fumes. 
     As known, a plurality of fins  18 , only one of which is shown in  FIGS. 1 and 2 , is coupled to the tubes  6   b  to assist the heat exchange between the fumes and the water. Furthermore, the boiler  1  comprises ignition means of the flame  19  (known and not described in detail). 
     Finally, in a lower part of the containing structure  7  there are obtained (known and not shown for the sake of simplicity) a drain tap, through which the fume condensate is evacuated, and an inlet mouth of an evacuation stack of the fumes themselves. 
     In  FIG. 4 , numeral  20  shows a non-condensing boiler as a whole, with total premixing according to an embodiment of the present invention. 
     The parts of the boiler  20  equal to those of the boiler  1  will be indicated with the same numerals and will not be described again. 
     The boiler  20  differs from the boiler  1  both for the total exchanging surface of the exchanger which, having a smaller number of tubes and a smaller exchange fin surface, does not allow the fumes to cool down to the extent of condensing the vapor contained therein, both for the arrangement of the burner and of the heat exchanger, which, the condensate production ceasing, may possibly be arranged with the heat exchanger located over the burner. 
     Given the different spatial arrangement with respect to the boiler  1 , in the boiler  20  the combustion chamber  4  may be easily inspectable after extracting the burner  3  downwards, since the burner is also in this case easily removable from the containing structure  7 . 
     Unlike the boiler  1 , the boiler  20  comprises a heat exchanger  21  comprising, in turn, a plurality of tubes  22  in which the water of a heating system (not shown) runs. The plurality of tubes  22  comprise a first type of circular section tubes  22   a  accommodated inside the longitudinal grooves  16  obtained in the diffuser  15  with the interposition of interposition elements  17  formed by heat conducting material, and a second type of elliptical section tubes  22   b  designed to exploit the heat coming directly from the flame and from the fumes. As known, a plurality of fins  23 , only one of which is shown in  FIG. 4 , is coupled to the tubes  22   b  to assist the heat exchange between the fumes and the water. Finally, the boiler  20  comprises a pipe  24  for discharging the fumes which in this case will not undergo a condensation process. 
     It is apparent to a person skilled in the art that one or more embodiments of the present invention, ensuring a constant cooling of the diffuser, offer the advantage of using diffusers formed by low-cost steel, without, because of this, increasing the complexity of the burner components. Indeed, the hydraulic circuit responsible for the cooling of the diffuser belongs to the heat exchanger as a whole without, therefore, adding a new hydraulic circuit and the corresponding components. 
     The boiler of an embodiment of the present invention, comprising part of the tubes of the heat exchanger in contact with the combustion head of the burner, contemplates a heat exchange directly from the burner estimatable in a fraction ranging from 5% to 15% according to the heat supply developed by the burner itself, with a consequent increase of the total exchange capacity of the exchanger-burner system. 
     Other advantages of one or more embodiments of the present invention relate to the reduction of polluting emissions, specifically NO x , due to the presence in the burner of only rather cold surfaces, and the possibility by the operators to inspect the combustion chamber and the heat exchanger without need to disconnect any part of the hydraulic circuit. Indeed, it is sufficient to lift the covering plate  8  and extract from the containing structure  7  the combustion head  11  of the burner  3 , generally integral with the containing structure  7 , to gain direct access to the combustion chamber  4  and to the heat exchanger  5 . 
     The tubes  6   a  and  22   a  responsible for the cooling of the diffuser  15  may have a different shape from that described above and, above all, may be in direct contact with the diffuser  15 , which may either present different seats from those shown in the description or be entirely free thereof. 
     The tubes  6   a  and  22   a  responsible for the cooling of the diffuser  15  may be in different number from those described above and at a reciprocally different distance, providing that the number and distance are such to sufficiently cool the diffuser  15 . 
     The reciprocal distance between distributor  14  and diffuser  15  may be different from that shown in the figures, for example, according to the known art. 
     The dimensions of the chamber formed by the covering plate  8  and the distributor  14  may be different from that shown in the figures, for example, according to the known art. 
     The surfaces of the diffuser  15  and of the distributor  14  may be, taken individually, either flat or curved with curving radius facing either inwards or outwards with respect to the burner chamber, in order to allow the displacements deriving from the thermal expansions in a known and predetermined direction. 
     The distributor  14  may be made in one piece or in several pieces coupled to the diffuser  15  so as to prevent the escape of air-gas mixture directly from the chamber inside the burner  3  to the diffuser. 
     The number of holes or slots and the shape of the holes and the slots of the distributor  14  and the diffuser  15  may be variable at will, for example, according to the known art. 
     The distributor  14  and the diffuser  15  may be either rigidly fixed to the covering plate  8 , e.g., by means of the fastening devices  24 , or be coupled to the covering plate  8  by means of the same fastening devices which are used to fasten the covering plate  8  onto the structure  7 , for example, according to the known art. 
     Finally, the circulation of water in the tubes  6   a  and  22   a  may be in series and/or in parallel according to the specific needs of the water circuit of the heating system. 
     Naturally, in order to satisfy local and specific requirements, a person skilled in the art may apply to the embodiments described above many modifications and alterations. Particularly, although one or more embodiments have been described with a certain degree of particularity, it should be understood that various omissions, substitutions and changes in the form and details as well as other embodiments are possible. Moreover, it is expressly intended that specific elements and/or method steps described in connection with any disclosed embodiment may be incorporated in any other embodiment as a general matter of design choice.