CH653115A5 - GAS BURNER. - Google Patents

Abstract

1. A gas burner comprising a premixer space (25) having inlets for air and gas connected respectively to a fan (17) and to a pressurized gaseous fuel source, and an outlet opening into a distributor space (27) one wall of which is formed by a distribution grid (28) for communication with a combustion zone, characterized in that said spaces are contained in a housing (18) having a symmetry axis and divided in a plane substantially parallel to said grid by a partition (26) delimiting said spaces (25, 27) and forming at least one communication passage therebetween extending at its periphery, said gas inlet (20) comprising nozzles (21) distributed about the symmetry axis of said housing (18) for injecting the gas in a general direction perpendicular to said axis, said nozzles (21) opening into an air inlet chamber (24) arranged within said premixer space (25) at a centered position with respect to said axis, at least one opening extending through the portion of the wall of said housing adjacent said chamber (24) for communication thereof with the atmosphere, at least one partition (23) coaxial with said symmetry axis and substantially parallel to the side wall of said housing (18) separating said chamber (24) from said premixer space (25) and being formed with distribution openings (22) extending therethrough at centered positions with the axes of respective ones of said nozzles (21).

Description

The present invention relates to a gas burner comprising a premix enclosure having air and gas intakes intended to be connected respectively to a fan and to a source of pressurized gaseous fuel.

Such burners have already been proposed to replace the burners in which the combustion air is entrained by a venturi effect created by the pressurized gas projected through a distribution nozzle. In such burners, only part of the air necessary for combustion is thus mixed with the gaseous fuel, so that the gas must be distributed in spaced out ramps to allow the passage of secondary air. This leads to relatively large burners, noisy and operating with a large excess of air.

The burners proposed to replace these burners commonly used in water heaters in particular, include a premix enclosure associated with forced air circulation, as illustrated for example in patent application FR No. 2481415. Despite the improvement brought by such a solution, the volume occupied by such a burner is still relatively large and the quality of the air-gas mixture leaves something to be desired, so that the quantity of excess air required remains relatively high. However, it is known that the higher the excess air, the more the yield decreases in proportion to the mass flow rate of still fairly hot gases discharged through the chimney.

There are currently compact high-efficiency boilers in which the combustion gases are cooled below the condensation temperature of the water vapor contained in the flue gases, which makes it possible to discharge gases to the atmosphere at low temperatures. . However, we know that the condensation temperature of the vapors called dew point is inversely proportional to the rate of excess air, so that if we want to be able to reach such a temperature in a domestic heating boiler for example, it is necessary the excess air rate is sufficiently low. Such a requirement also poses a problem of temperature resistance of the materials. We know, in fact, that the combustion temperature of a given fuel is maximum at stoichiometric conditions.

It has certainly already been proposed radiant burners such as that described in patent CH No. 567690. However, such burners require the use of ceramics, expensive and fragile materials, suitable for use in industrial installations in particular, but not for domestic heaters due to their price and fragility and the fact that in boilers, water is mainly heated by convection of combustion gases.

The object of the present invention is to provide a compact burner, made of non-refractory material, capable of operating at a low rate of excess air.

To this end, the subject of the present invention is a gas burner comprising a premix enclosure having air and gas inlets intended to be connected respectively to a fan and to a source of pressurized gaseous fuel and an outlet opening into a distribution enclosure, one wall of which is formed by a distribution grid intended to communicate with a combustion zone. This burner is characterized in that these enclosures are contained in a housing with an axis of symmetry, divided along a plane substantially parallel to said grid by a partition delimiting these enclosures and providing at least one communication passage between them extending at its periphery, said gas inlet comprising nozzles distributed around the axis of symmetry of the housing for injecting the gas into the premixing enclosure, in a general direction perpendicular to this axis, these nozzles opening into an inlet chamber air formed before the premixing enclosure and centered with respect to said axis, at least one opening passing through the wall portion of the housing adjacent to this chamber to put it in communication with the atmosphere, and a partition coaxial with this axis of symmetry and substantially parallel to the side wall of the housing separating this chamber from the premix enclosure and being traversed by distribution openings centered on the axes respective nozzles.

The ratio between the total section of the perforated wall and the inlet of the distribution enclosure ensures a flow speed of the gas and air mixture which makes it possible to lower the temperature of the perforated wall to a value compatible with that which a chromium-rich steel can withstand for a relatively low rate of excess air, that is to say for a fairly low mass flow of gas and a fairly high combustion temperature. As will also be seen later, the dimensions of the burner become very small compared to its power. Combustion at a low rate of excess air also leads to a very reduced flame height

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so that in the case of a boiler or a water heater, it is possible to make a very compact wall appliance.

The appended drawing illustrates, schematically and by way of example, various embodiments of the gas burner which is the subject of the present invention.

Fig. 1 is a perspective view of the first embodiment.

Fig. 2 is a sectional view along II-II of the fìg. 1 of the burner associated with a combustion chamber of a boiler.

Fig. 3 is a diametrical sectional view of a second embodiment.

Fig. 4 is a top view of FIG. 3.

Fig. 5 is a perspective view of a variant of FIG. 1.

Figs. 6 and 7 show two explanatory diagrams.

The burner illustrated in fig. 1 and 2 is in the general form of a relatively thin parallelepipedic casing 1, the interior of which is divided into two premixing chambers 2 respectively of distribution 3 by a partition 4 extending substantially at mid-height of the casing 1. The face of the housing 1 located opposite the partition 4, delimiting with it the distribution enclosure 3 is essentially constituted by a perforated grid 5 for making the distribution enclosure 3 communicate with a combustion zone adjacent to the face external of this perforated grid 5.

The other wall 6 of the housing 1 parallel to the perforated grid 5 forms a channel 7 extending longitudinally to the axis of the housing 1. The bottom of this channel is adjacent to a portion of the partition 4 and its side walls 8 and 9 by portions folded at 90 ° from the wall 6 and joining the partition 4. The side 10 intended to be connected to a source of pressurized gaseous fuel extends longitudinally in the channel 7. This conduit 10 is crossed by orifices distribution 11 constituting gas injection nozzles, distributed longitudinally along this conduit 10. The injection axes of these orifices are contained in a diametral plane parallel to the large faces of the housing 1. The walls 8 and 9 of the channel 7 serve as a partition between the premix enclosure 2 and this channel 7. These walls 8 and 9 are pierced with openings 12, each coaxial with the injection axis of one of the distribution orifices 11. The diameter of these openings 12 is chosen so that the injection cone of s distribution orifices 11 pass through them, providing an annular section between the cone and the corresponding opening 12 for the passage of air.

The burner according to the invention is a burner with forced air circulation. In the example of figs. 1 and 2, this forced circulation is carried out by vacuum thanks to a fan 13 located at the inlet of an evacuation duct 14 for combustion gases from a combustion chamber 15 with which the burner is associated, the intake of this fan being turned towards this combustion chamber. A heat exchanger 16 with the combustion gases produced in this combustion chamber 15 extends through this chamber between the burner 1 and the fan 13. Of course, in the case where the burner is subjected to the vacuum of the fan 13, the combustion chamber 15 is hermetically closed so that only the air intake chamber, formed here by the channel 7, is connected,

on the one hand, with the atmosphere, by the opening of this channel 7 and,

on the other hand, with the admission of the fan through the openings 12.

The partition 4 progressively departs from the grid 5 from its portion forming the bottom of the channel 7 and this symmetrically on either side of this channel which is coaxial with the axis of symmetry of the partition 4. At its two lateral ends, this partition 4 has a rim 4a directed towards the grid 5 and the edge of which delimits the entry section in the distribution enclosure 3.

Before examining in detail the choice of certain dimensions chosen for the burner which has just been described, we want to analyze here some choices which have been made in the design thereof.

The diagram in fig. 6 shows the evolution curves of the combustion gases in C02 as a function of the percentage of oxygen remaining in these combustion gases, this percentage being closely linked to the rate of excess air. The higher the CO 2 level, the higher the yield. However, we note that this rate of C02 increases in inverse function to the rate of 02-

In burners in which only a portion of the combustion air is premixed with combustible gas, the rest of the combustion air being taken up by the flame in the atmosphere which surrounds it, the rate of excess air is relatively high, because the air-gas mixture takes place partly at the very place of combustion and is therefore quite bad. When all the air necessary for combustion is mixed beforehand with the gas, the rate of excess air can decrease in a proportion which is a function of the homogeneity of the mixture obtained, thereby increasing the yield.

It is also known that the flame temperature with respect to the proportion of oxygen is in the form of a curve which culminates in a stoichiometric ratio between the combustible gas and the air. Therefore, by reducing the excess air rate to increase the efficiency of the burner, the flame temperature reaches a value at which it is no longer possible to use steel. Ceramics have certainly already been used for radiation burners. However, it is a type of burner more specifically used in the industry. In a boiler or a water heater for example, radiant heating is of no interest. The use of ceramics of a high price and which moreover are fragile is not justified for domestic appliances and for a mode of heating essentially by convection. The use of Kantal type alloy resistant to higher or lower temperatures of the order of 1000 ° -1200 ° C is also a relatively expensive solution, all the more expensive as the limit temperature will be higher and can pose corrosion problems with combustion gases at these temperatures. In addition, this alloy has poorer mechanical properties than steel.

This is the reason why the burner which is the subject of the invention and in particular its distribution enclosure 3 have been studied with a view to preferably ensuring a certain cooling of the grate 5 to which the flames are attached and this taking account of '' an excess air rate X <1.5. This cooling is obtained by ensuring a speed of flow of the gases in the distribution enclosure 3 which makes it possible to ensure a certain cooling of the grid 5. This speed is given by the speed of flow of the gases through the grid 5 and by the ratio between the passage section of this grid 5 and the entry section in the distribution enclosure 3. In addition, the section of the distribution enclosure decreases from the entry towards the central axis of the burner to take account of the gas flow through the grid 5 and ensure a substantially constant flow speed through the entire distribution enclosure 3. This cooling of the grid 5 allows, if it is sufficient, to use a steel with 17% chromium for example. The fact that this grid 5 is maintained at a temperature substantially lower than that of the flame has a second advantage, that of preventing combustion from going upstream of the grid 5.

With an excess air rate X of between 1.2 and 1.3 (fig. 7), the gas flow speed through the grid 5 is between 0.5 and 5 m / s depending on the power. To obtain appreciable cooling of the grid 5, the flow speed of the gas mixture near this grid 5 must be at least of the order of 2 m / s which, for the lower speed of the range speed through grid 5 which is 0.5 m / s, would give a speed ratio of Mt, so that the inlet section of the distribution enclosure must be four times smaller than that of the grid 5. To take into account the fact that the power range can, in certain cases, lead to a minimum flow speed through the grid of the order of 2 m / s instead of 0.5, we can admit of in general that this ratio of the inlet and outlet sections of the distribution enclosure 3 should not be greater than 1.

The grid 5 is made of perforated sheet steel whose perforations have, in this example, 0.8 mm in diameter and are regularly distributed, their surface representing 15% of the total surface of this grid.

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The shape of the housing 1 containing the enclosures 2 and 3 being generally parallelepiped, and the partition 4 separating these enclosures deviating from the grid 5 from the center towards the edges of the enclosure 1 by forming a distribution enclosure 3 with section progressively increasing in direction from the edges of this box, it follows that the corresponding section of the premix enclosure 2 decreases in the same proportion. This reduction in section of the premix enclosure 2 promotes the homogenization of the mixture which can be further improved by placing a grid 16 at its outlet so as to split the flow. It should be noted that this mixture is already favored by the walls 8 and 9 and the openings 12 through which the gas is injected so that vortices form downstream of these walls between the gas and the air, promoting their mixing.

It can be seen that if the burner described requires the use of a fan to supply it with the necessary proportion of air, unlike conventional burner burners in which a proportion of this air is entrained by the gas through an injector. tor, the rest of this air being supplied by the atmosphere surrounding the flames, in return, this burner works with an excess of air less than 1.5 and preferably between 1.2 and 1.3, therefore with higher yield. If we look at the diagram in fig. 7, it is noted that, at this low rate of excess air, the condensation temperature of the water vapors in the combustion gases is of the order of 54 ° C. for natural gas. In a central heating circuit in which the water returns to the boiler at around 45 ° C, there is therefore a difference of the order of 10 ° C between the condensation temperature and that of the water in the heating circuit . This is a second advantage resulting from the presence of the fan since,

without it, it is not possible to lower the rate of excess air below 1.5, which, according to the diagram in fig. 7, lowers the temperature of the dew point below 50 ° C so that the difference between the return water temperature and that of the dew point becomes too small.

In addition to these performance advantages, we also note the extreme compactness of the burner compared to conventional ramp burners. This compactness comes in part from the symmetry that allows all of the elements to be inscribed in a very flat parallelepiped housing. Finally, given the small excess of air, the length of the flames is very reduced, which makes it possible to have a heat exchanger near the burner and to produce a device of small volume compared to its power. This is how it is possible to produce very compact wall-mounted boilers with a power of the order of 20 kW.

Of course, the fan could very well be upstream with its outlet directed towards the burner, like the fan 17

illustrated in fig. 3. In this variant, the housing 18 of the burner is cylindrical, the axis of symmetry being constituted by any one of its diameters. Aside from this difference in shape, the general design of this burner is identical to that of the burner described previously. A gas supply pipe 19 is mounted coaxially with the fan distribution pipe 17a and passes through its wall so as to be able to connect it to a source of pressurized gas. This supply conduit 19 ends in a cylindrical housing 20 whose side wall is traversed by nozzles 21 whose injection axes io are concentric with respective openings 22 passing through a circular partition 23 concentric with the cylindrical housing 20 and which separates a air supply chamber 24, to which leads the distribution duct 17a of the fan 17, of the premix enclosure 25. As in the embodiment of FIGS. 1 and 2, a partition 26 separates this premix enclosure 25 from the distribution enclosure 27. In this variant, a slightly convex distribution grid 28 is attached to the housing, against the edge of which it is clamped by a ring 29 and clamping screws 30. Between the distribution chambers 27 and the premix 25, two perforated sheets 20 and 31 serve to homogenize the gas-air mixture.

In this variant, as in the previous embodiment, various modifications can still be envisaged. Thus, the center of the partition 4 or 26 separating the bottom of the supply chamber 7 or 24 can be crossed by perforations 25 intended to cool the grid 5 or 28.

Fig. 5 illustrates yet other variants, one of which is applicable to one or the other of the preceding embodiments. It consists of a flap 34 for adjusting the air intake in the channel 7, this flap being slidably mounted by means of elongated openings 35 associated with fixing screws 36, making it possible to advance or retract the flap 34 above the channel 7. Another variant is illustrated by this fig. 5. It consists in forming in the center of the distribution grid 5 a rib 5a fixed to the center of the partition 4 and axially dividing the distribution compartment 3 into two equal parts. Furthermore, a slide 37 is mounted adjacent to one of the walls 8 or 9 of the channel 7. This slide has openings 38 of the same diameter and the same spacing as the openings 12 passing through these walls 8 and 9. As a result , by simply moving this slide 37, it is possible to bring the openings 38 in front of the openings 12 40 of the wall 8 or 9 against which the slide 37 is mounted or to plug the openings 12 by masking the openings 12 by the solid parts of the slide 37. As a result, it is possible to work only with half of the grid 5 and to increase the power range covered by the same burner. Furthermore, the rib 5a 45 constitutes a cooling fin for the grid 5.

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Claims (8)

653,115 1. Gas burner comprising a premix enclosure (2; 25) having air and gas inlets intended to be connected respectively to a fan and to a source of pressurized gaseous fuel and an outlet opening into a distribution enclosure (3; 27), one wall of which is formed by a distribution grid (5; 28) intended to communicate with a combustion zone, characterized in that these enclosures are contained in a housing (1; 18) with axis of symmetry, divided along a plane substantially parallel to said grid by a partition (4; 26) delimiting these enclosures and providing at least one communication passage between them extending at its periphery, said gas inlet comprising nozzles distributed around the axis of symmetry of the housing for injecting the gas into the premixing chamber in a general direction perpendicular to this axis, these nozzles opening into an air intake chamber (7; 24) formed before the premixing chamber and centered with respect to said axis, at least one opening passing through the wall portion of the housing adjacent to this chamber to put it in communication with the atmosphere, and a partition (8, 9; 23) coaxial with this axis of symmetry and substantially parallel to the side wall of the housing separating this chamber from the premix enclosure and being traversed by distribution openings centered on the respective axes of the nozzles. 2. Gas burner according to claim 1, characterized in that the partition (4; 26) delimiting these enclosures has an axis of symmetry substantially coinciding with that of the housing and gradually moves away from said grid, from said axis towards the periphery to provide a distribution enclosure whose cross section decreases from the periphery towards the center. 2 3. Gas burner according to claim 1, characterized in that the partition (4; 26) delimiting these enclosures has nozzles (33) for communicating said chamber with the central part of said distribution enclosure, these nozzles being arranged to direct an air jet against said perforated wall. 4. Gas burner according to claim 1, characterized in that said communication passage between the speakers comprises a grid. 5. Burner according to claim 1, characterized in that the ratio between the total section of the openings of the distribution grid and the section of said communication passage between the speakers is greater than 1. 6. Burner according to claim 5, characterized in that the said ratio is greater than 2. 7. Burner according to claim 1, characterized in that an adjustable flap (34) is associated with said opening passing through the portion of the wall of the housing adjacent to said intake chamber. 8. Burner according to claim 1, characterized in that said housing is of generally parallelepipedal shape with a distribution enclosure divided axially into two parts and comprises means for closing the distribution openings passing through a part of the partition separating the the premix enclosure.