EP0479174B1

EP0479174B1 - Combustion head for gas burners

Info

Publication number: EP0479174B1
Application number: EP91116622A
Authority: EP
Inventors: Flavio Pastorato; Luciano Bianchini
Original assignee: RBL Riello Bruciatori Legnago SpA
Current assignee: RBL Riello Bruciatori Legnago SpA
Priority date: 1990-10-02
Filing date: 1991-09-30
Publication date: 1994-07-06
Anticipated expiration: 2011-09-30
Also published as: EP0479174A2; IT1243147B; EP0479174A3; IT9085622A0; DE69102747D1; IT9085622A1

Description

The invention concerns a combustion head for gas burners, particularly suited for gas combustion.
It is a known fact that inside the burners applied to boilers the combustion of the combustion gas mixed with the combustion air, occurs through a combustion head.
One of the most relevant problems connected with combustion heads from an environmental point of view, is the pollution they cause, because of the dispersion into the atmosphere of the nitric oxides which are produced while the combustion heads are in operation.
Tests have proven that, other conditions remaining the same, the maximum production of nitric oxides occurs when the combustion occurs with an optimum ratio between air and fuel. When, on the other hand, the combustion occurs with an excess of air, or with a defective quantity of the same, the nitric oxides produced are considerably reduced in quantity. By exploiting this concept, one of the most-widely used methods for the reduction of the nitric oxides consists in dividing the flame into two parts, wherein one part of the combustion is done with a defective quantity of air, while the other part is done with an excess of air. Subsequently, the combustion products issuing from the combustion with a defective amount of air are mixed with those issuing from the combustion with an excess of air and the former, thus mixed, are combusted again. This process brings about a final situation of the combustion products which, as a whole, gives the same results, as if the combustion had occurred under optimum conditions. The advantage obtained herewith is that of reducing the percentage of the nitric oxides dispersed into the atmosphere together with other combustion products.
The combustion heads belonging to the known technique, wherein the reduction of the nitric oxides occurs by dividing a flame front, present the inconvenience of a remarkable constructive complexity and, as a consequence, of high costs for the manufacture of the head, due to the presence of a plurality of ducts, suited to achieve the described division of the flame. The presence of these duct systems within the combustion head entails also the inconvenience of creating within the combustion head itself high load losses on the gas side, which cause the reduction of the combustion head power, when the pressure in the pipeline is low.
In some cases it is not possible to recycle the burnt gases within the combustion head and it becomes, therefore, necessary to achieve said recycling outside the head. This occurs by recycling a certain percentage of the burnt gases (approx. 20 to 30%) by drawing them from the stack and sending them back into the combustion head through a fan. This system, too, presents a remarkable construction complexity and, as a consequence, rather high manufacturing costs.
Another inconvenience of the gas combustion heads of the known types consists in the value of modulation ratio which can be obtained, or better said, of the ratio between the minimum and the maximum power which the head can reach, while maintaining unaltered the optimum combustion parameters, both from the point of view of the fume outlet and from the combustion stability.
It is a known fact that in the gas combustion heads available on the market, the modulation ratio can reach a maximum value of 1:10 and this value is only reached in particularly accurate models.
Another inconvenience of the gas combustion heads of the known types is the length of the flame, which can reach a remarkable development in the axial direction. Particularly when the goal is to reach high combustion powers, it is necessary to obtain within the head such pressures as to create a particularly long flame, which can cause damages to the boiler in those cases, when the latter presents a not-too-long combustion chamber, and this because the flame touches the bottom of the combustion chamber itself.
Not the least inconvenience consists in the fact that in the gas combustion heads of the known types the flame stabilization is obtained either through mechanical stabilizing devices or through privileged ducts, which mechanically force the motion field of the flame. It is obvious that this entails a further increase of the load losses, as well as an increase of the manufacturing costs and an increase of the difficulties in the construction of the combustion head.
It is the purpose of the present invention to overcome the described inconveniences by disclosing a gas combustion head which, the performance level remaining the same in relation to known combustion heads, reaches the following purposes:

a lower dispersion of nitric oxides and of carbon dioxide into the atmosphere;
lower load losses on the gas side;
higher modulation ratios;
a reduction of the axial length of the flame.

The just described purposes and others, which will be better illustrated hereinafter, are reached by a combustion head of a gas burner which, in accordance with the main patent claim is fed by a blower unit connected with an external tube-shaped body through a manifold flange conveying the combustion air and comprising:

an inner tubular duct arranged co-axially within the external tube-shaped body and solidly attached to the manifold flange, into which a mixture of air and gas, or just gas is conveyed;
distributors for the outlet of the air-gas mixture or of the gas, arranged in the front part of the inner tubular duct;
an intermediate tubular body, co-axial both with the outer tube-shaped body and with the inner tubular duct and comprised within their air space, attached through mechanical means which allow the adjustment of its position in the axial direction;
at least one element for the ignition of the flame, and is characterized in that the outlet distributors of the inner tubular duct for the air-gas mixture or for the gas, are annular air spaces which are perpendicular to the axis of the inner tubular body and are formed by rings and/or discs, co-axial with each other and connected with each other through spacers and/or through at least one stub pipe.

According to a preferred embodiment the distributors protrude axially in relation to the front end of the outer tube-shaped body and are divided into two ranges, one range being intermediate. It is in this intermediate range that the combustion with an excess of air occurs and it is formed by a pair of distributors obtained in the junction area between the front end of the inner tubular duct and the rear end of the stub pipe. The other range is at the terminal end and it is in this range that the combustion with a defective amount of air occurs. It consists of a single distributor positioned near the front end of the stub pipe.
The main advantage obtained with the combustion head according to the invention is a decrease of pollution caused by nitric oxides and, because of the low load losses, it can also be used in those cases, when the gas pressures in the pipelines are low.
Another advantage consists in the possibility of producing burners with a higher application flexibility, thanks to the higher modulation ratio which it is possible to obtain with the combustion head of the invention.
Moreover, the absence of mechanical means within the head, which in other models are necessary to obtain the division of the flame front, makes it possible to produce a combustion head at lower manufacturing costs.
Another advantage consists in that the particular radial conformation of the distributors permits to obtain a flame developing in the radial direction, so that this head can be used with the purpose of obtaining high thermal powers even in boilers presenting combustion chambers with limited lengths.
The head manufactured according to the dictates of the invention yields also the advantage that it can conveniently be used also in room heaters, thanks to the essentially radial arrangement of the flame.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter,

Fig. 1 represents the combustion head according to the invention applied to a burner unit;
Fig. 2 represents an axionometric magnified view of the combustion head of Fig. 1;
Fig. 3 shows the combustion head according to the invention in its longitudinal cross-section;
Fig. 4 shows the magnified longitudinal cross-section of the detail concerning the radial distributors of the combustion head.

As can be observed in Fig. 1, the gas combustion head according to the invention is indicated as a whole with 1, and it consists of an external tube-shaped body 2, which is connected with the blower unit 3 of a burner, which is indicated as a whole with 4, through a manifold flange 5. By observing Fig. 2 and Fig. 3 it will be seen that within the outer tube-shaped body 2 and co-axial with it there is an inner tubular duct 6, which is attached to the manifold flange 5 by means of an elbow sleeve 7 applied at the rear end 16. Finally, the intermediate tubular body 8 is comprised between the outer tube-shaped body 2 and the inner tubular duct 6 and it is co-axial with them. Said intermediate tubular body 8 is attached to the manifold flange 5 by mechanical fastening means, such as, for instance, the screws 9, which make it possible to adjust its position along the direction of the longitudinal axis 10 of the combustion head 1.
Combustion air 12 is let through the rear opening 11 of the manifold flange 5 into the intermediate tubular body 8. Moreover, a gas/air mixture 14 or gas alone is let into the inner tubular duct 6 through the opening 13, at the bottom of the manifold flange 5, which communicates with the interior of the elbow sleeve 7.
As can be observed in Fig. 2, Fig. 3 and also in the magnified detail of Fig. 4, a stub pipe 40 is present in the front part of the inner tubular duct 6. This stub pipe 40 presents a rear end 41 with an annular disc 36 attached to it and a front end 42 with a circular disc 23 attached to it by means of the spacer 24. The annular disc 36 of the stub pipe 40, in turn, is attached to another annular disc 35, which, in turn, is connected with the front end 17 of the inner tubular duct 6, through the spacers 53 and the fastening screws 33, while between said annular discs 35 and 36 there is an intermediate annular disc 32.
Thus two annular air spaces are obtained, which form the distributors, through which the air-gas mixture or the gas comes out of the inner tubular duct 6 and, after mixing itself with more combustion air coming from the intermediate tubular body 8 it produces the flame.
More specifically, the distributor 31 is formed between the intermediate annular disc 32 and the annular disc 35, while the distributor 51 is formed between the intermediate annular disc 32 and the annular disc 36, belonging to the stub pipe 40.
On the other hand, the distributor 21, constituting the range of end-distributors 21 is formed between the front end 42 of the stub pipe 40 and the circular disc 23.
Such air-gas mixture , or the gas, 14, which streams into the inner tubular duct 6, streams out through the distributors 31, 51 and 21 and mixes itself with the combustion air 12, which streams through the intermediate tubular body 8, thus producing the combustion which is ignited by the ignition element 15.
Thanks to the special configuration described, wherein the distributors 31, 51 and 21 of the combustion head are arranged in two ranges 30 and 20 respectively, which are axially spaced from each other by the stub pipe 40, the flame front splits up and generates two combustion areas differing from each other, wherein the combustion of one area occurs with an excess of air, while the combustion of the other area occurs with a defective amount of air. More specifically, the combustion corresponding to the range 30 of the distributors occurs with an excess of air, while the combustion corresponding to the distributor range 20 occurs with a defective amount of air. The products resulting from the combustion with an excess of air - the direction of their flow being indicated by arrow 44 - are re-cycled following the direction of arrow 44. Thereafter, they are mixed with the products of the combustion occurring with a defective amount of air, which flow in the direction indicated with arrow 25. The products of the combustion which has occurred with a defective amount of air are then burnt again, so that the combustion process, once the combustion has been completed, produces the same results as if it had occurred under conditions in a stoichiometric ratio. However, thanks to the division of the flame front, the amount of nitric oxides is considerably decreased.
The air flow 45 streaming out of the intermediate tubular body 8 within the outer tube-shaped body 2, can be adjusted by suitably changing the axial position of the intermediate tubular body 8 along axis 10, so as to change its distance 43 from the outlet end of the outer tube-shaped body 2.
On the basis of what has been described, it can be concluded that the combustion head according to the invention achieves the division of the flame front and the subsequent re-cycling of the combustion products with an ensuing completion of the combustion process, thereby achieving a considerable reduction of the nitric oxide component in the exhaust fumes.
It has been described how the distributors 31, 51 and 21 are formed by placing face to face and co-axial with each other the discs 35 and 36 and an intermediate disc 32 comprised between the first two and by placing a disc 23 in front of and co-axial with the front end 42 of the stub pipe 40. It is understandable that these distributors generate low load losses of the flow of the air-gas mixture or of the gas, which flows into them and which comes from the inner tubular duct 6, so that the gas combustion head according to the invention also fulfills the purpose of achieving in the gas area load losses which are lower than those occurring in combustion heads presenting the same characteristics and wherein the combustion occurs with a division of the flame front.
Experimental tests have also shown that in the combustion head according to the invention modulation ratios can be reached which are much higher than the modulation ratios obtainable with equivalent combustion heads available on the market. It has specifically been said in the introduction that the modulation ratios obtainable with the available combustion heads reach a maximum value of 1:10. On the other hand, with the combustion head according to the invention the modulation ratios obtainable can reach a maximum value of 1:20, which goes to the advantage of a higher flexibility of application of the combustion head according to the invention.
In the description it has been remarked that the flows 34 and 25 streaming out of the distributors 31, 51 and 21 generate a flame presenting an essentially radial development located in the area comprised between the two ranges 20 and 30 of distributors. Whenever, because of the need of the user, it becomes necessary to increase the combustion pressures, in order to increase the thermal power of the boiler, there is an increase in the radial dimension of the flame and not, as is the case in the combustion heads of the known types, an increase of its axial length. This yields the advantage that it is possible to use the combustion head of the invention, the power remaining the same, in boilers having combustion chambers with a low development in their axial direction, without causing any damage to the combustion chambers.
Therefore, as has already been said, the combustion chamber according to the invention presents the following advantages: a decreased pollution factor, the possibility of being used even with low gas pressures in the pipeline, a higher flexibility of application and also a simplified manufacturing process with the resulting advantage of decreased manufacturing costs.
Moreover, because the combustion head according to the invention presents a flame with an essentially radial development, it can be used also in room heaters for the heating of storage rooms, drying rooms and similar.
During the manufacturing process, the combustion head according to the invention, may undergo changes and modifications concerning some of its specific constructive components. For instance, the distributors present in each range can be one or more according to the constructive needs or to the type of performance required from the head.

Claims

A combustion head for a gas burner fed by a blower unit (3) connected with an external tube-shaped body (2) through a manifold flange (5) conveying the combustion air (12) and comprising;
- an inner tubular duct (6), arranged co-axially within the external tube-shaped body (2) and attached to the manifold flange (5) and into which a combustion mixture consisting of air and gas or gas alone (14) is conveyed;

- distributors (51, 31, 21) letting out the air-gas combustion mixture or the gas alone (14) arranged in the front part of the inner tubular duct (6);

- an intermediate tubular body (8), co-axial both with the external tube-shaped body (2) and with the inner tubular duct (6) and comprised within their air space, attached through mechanical means (9), which allow the adjustment of its position in the axial direction (10);

- at least one flame igniting element (15),
characterized in that the distributors (51, 31, 21) letting out of the inner tubular duct (6) the air-gas combustion mixture or the gas alone are annular air spaces perpendicular to the axis of the inner tubular body (6), formed by rings (35, 36, 32) and/or discs (23) which are co-axial with each other and connected with each other by means of spacers (33, 24) and/or through at least one stub pipe (40).
A combustion head according to claim 1, characterized in that the distributors (51, 31, 21) protrude axially in relation to the front end of the external tube-shaped body (2).
A combustion head according to claim 1 or 2, characterized in that the distributors (51, 31, 21) are divided into two ranges, one of which is an intermediate range (30) wherein a combustion with an excess of air occurs and it consists of at least one distributor (51, 31) obtained near the junction area between the front end (17) of the inner tubular duct (6) and the rear end (41) of the stub pipe (40), and the other is an end range (20) wherein a combustion with a defective amount of air occurs and it consists of another distributor (21) obtained near the front end (42) of the stub pipe (40).
A combustion head according to claim 3, characterized in that the distributors (51, 31) belonging to the intermediate range (30) are a pair of distributors and they are formed by an intermediate annular disc (32) comprised between the other two annular discs (35, 36) with which it is connected through spacers (53)
A combustion head according to claim 3, characterized in that the distributor (21) belonging to the end range (20) is a single distributor formed by a circular disc (23) applied co-axially through spacers (24) to the front end (42) of the stub pipe (40).