IMPROVEMENTS RELATING TO GAS BURNERS

Abstract: A gas burner (1) has a header (3) with flameports (4) from which gas/air mixture (Y) issues as a plurality of jets. A distributor plate (5) is arranged adjacent the header (3) and is provided with apertures (6) interspaced with the said jets. The jets (Y) lie adjacent to the surface of the distributor plate (5) and secondary air is forced through the apertures (6) in jets which are interspaced with and lie transverse to the gas/air mixture jets, mixing with same to increase the aeration of the resulting combustion.

Description: Improvements Relating to Gas Burners This invention relates to gas burners and a means which are designed to operate so as to reduce the levels of toxic emissions which are produced as a result of gas combustion. All forms of gas burner require a certain minimum amount of air to be mixed with the gas so that the latter may be fully combusted. Known forms of gas burner have provision for all or part of this air to be mixed with the gas prior to ignition. Where the gas and all the air required for combustion are mixed in the burner body before exiting the flameports the burners are generally known as premix burners. Flames from such burners are high intensity and low in volume and the mixing of the gas and air generally requires the use of a fan. A consequence of premixing is that highly inflammable gas/air mixtures exist upstream of the flameports and precautions must be taken to ensure that the flames do not light back and burn inside the burner body with resultant destruction of the system. Side reactions in flames can cause formation of undesirable compounds such as oxides of nitrogen known as Nox. The mechanisms of NOX formation are well researched and relate to free radicle concentrations at the flame boundaries and within the flames and to temperatures within the flame envelope. It is known that conventional atmospheric burners used, for example, in cookers, boilers snd central heating systems, generally emit in excess of 100 ppm of nitrogen oxides when functioning under normal operational conditions. Furthermore, it is also known that the reduction in emitted nitrogen oxides is related to the amount of excess air in the gas fuel mixture. Well designed premix burners with intimate mixing of gas with air, in excess of the theoretical requirement needed to effect complete combustion before ignition are capable of producing flames with low Nox concentrations in the combustion products. Conventional aerated burners with usually only 30-70% of the theoretical air requirement in the primary mixture upstream of the flameports with the balance of secondary air requirement diffusing into the flame downstream of the flameports in a largely uncontrolled manner produce considerably higher Nox concentrations in the combustion product. It is possible by the addition of baffles or bluffs around the flame envelope to control secondary air diffusion thus affecting Nox emissions but the resulting flames are usually increased in volume and can produce other combustion problems in the burner application. The present invention concerns a system whereby the gas and a substantial proportion of the air required for combustion are mixed in a controlled manner prior to combustion producing a high intensity flame with substantially reduced Nox concentrations in the combustion product. A method of providing the gas flame in a gas burner comprises creating a forced flow of secondary air in a predetermined pattern by forcing the air through apertures in a distributor plate means, and supplying to the vicinity of the flow of secondary air a supply of combustible gas into which has previously been introduced a quantity of primary air whereby the burner flame which exists in the region of mixing of the primary air/gas mixture and secondary air gives rise to conditions in the flame which suppress side reactions. Preferably, the proportion of primary air which is mixed with the gas is a smaller fraction than the amount of combustion air which is mixed with the primary mixture. Also, according to the invention there is provided apparatus comprising an air distributor plate means having apertures therein, means for supplying air to said distributor member to create a forced flow of secondary air through said apertures, and means for supplying a flow of gas to mix with said forced flow of secondary air to provide a mixture for the creation of the burner flame adjacent the plate means. Also according to the invention there is provided a gas burner comprising a burner header, outlet flame ports in said burner header for the issuance of gas/air mixture jets therefrom, distributor plate means located adjacent said outlet flame ports so that said gas/air mixture jets issuing from the header lie adjacent the plate means, and apertures in said distributor plate means for the passage of secondary air therethrough in a direction transverse to said gas/air mixture jets for the enhancement of the combustion of the gas in said gas/air mixture at said distributor plate means. The distributor plate means may be in the form of a plate which may be flat, 'V shaped, conical, cylindrical or otherwise to suit a particular application. The primary gas/air mixing means may be in the form of a gas injector or injectors feeding the header (or headers) containing a number of primary air openings and also containing a number of said flame ports through which the primary mixture issues to spread uniformly over the downstream surface of the distributor. Air flowing through the distributor plate means mixes intimately with the primary gas/air stream flowing across the surface of the distributor plate means and combusts close to the surface of the distributor plate means each of whose apertures may also be considered to be a flameport. The ratio of air to gas can be changed by varying the airflow through the system and/or the disposition of the distributor orifices and by varying the size and disposition of the primary gas/air openings and/or the gas injector size and/or pressure. Both the heat release and the flame intensity can be varied to suit the particular application. All commonly used fuel gases can be burnt in this manner. Enhanced combustion and therefore substantial reductions in NOX emissions are achieved. The distributor apertures may be in the form of groups of holes or slots and their disposition may be varied according to the required shape of the burner and the distribution of flame or heat release required. In order to obtain optimum mixing of gas and secondary air and resultant attenuation of Nox levels each segment or section of the patterns should have its own individual gas/air primary orifice. Usual application design considerations define total air flows between 100% and 200% of the theoretical requirement to complete combustion for the particular gas used with a preferred range of about 140-190%. With these values in mind and in order to produce the most compact flames it is necessary to ensure the diffusion of the primary gas/air mix into the distributor secondary air columns occurs at the earliest stage. With distributor hole diameters and/or slot widths greater than 1.5mm diffusion is impaired even at low air velocities and the flames can break open with resultant quenching and poor combustion. Uniform spreading of the gas/air mixture over the surface of the distributor plate means is achieved by employing a multiplicity of gas/air orifices arranged to inject the mixture close to the plane of the distributor plate means into low pressure regions between successive flows of the secondary air through the distributor plate transversely and preferably at right angles to the gas/air mixture flow. Said uniform spreading is a function of gas/air stream momentum and the entrainment effect of adjacent distributor plate secondary air flow. The degree of spread obtainable and the consequent maximum spacing between adjacent gas header flameports is governed by the maximum header pressure that can be employed without starvation of the nearest distributor orifices due to excessive gas/air steam velocity and the maximum entrainment effect produced by the distributor secondary air flows at higher velocities. With reference to quiet operation of the burner spreads of about 80mm are possible from a single header. Successive headers may be employed and with contra or opposed flow may be spaced at up to 160mm. For optimum performance the special arrangement of orifices must be such that the primary gas/air stream in the direction of injection follows a low pressure path across the distributor plate without interruption to the furthest orifice. Diffusion of the gas/air mix perpendicular to the flow direction into the distributor air columns becomes ineffective at distances greater than 10mm. At distances less than 2 mm combustion becomes poor due to turbulence and flame quenching. Where long slots are used as distributor orifices air flow characteristics may be modified by obstructions across the airstream on the upstream surface of the distributor to modify the air velocity profile across the slots with resultant increase in flame stability when the burner is being used with low flamespeed cases at higher distributor air velocities. An embodiment of the invention will now be described by way of example only with reference to the accompanying drawings, wherei :- Fig. 1 represents a sectional side elevation view of a gas burner constructed in accordance with the invention; Fig. 2 represents a plan view of the burner illustrated in Fig. 1; Fig. 3 represents a sectional end view of the burner illustrated in Fig. 1; Fig. 4 is an enlarged sectional view of part of the distributor plate; and Figs. 5, 6 and 7 show alternative embodiments. Referring to the drawings, the burner generally comprises a gas injector 1 which opens into a longitudinal header 3 which is provided on opposite sides with a number of gas outlet openings 4 (Fig. 1) evenly spaced along the length thereof. Arranged below the plane containing the gas outlets 4 is an air distributor 5 in the form of a flat plate provided at said opposite sides of the header with a number of elongate slots 6 extending perpendicular to the longitudinal axis of header 3. The distributor plate is flat but this is not essential. It may be 'U' shaped, conical, cylindrical or shaped otherwise to suit a particular application and shape of header. The plate and header may be mounted below a heat exchanger. The distributor can be provided with either slots 6 or holes and preferably the slot width or hole diameter X (Fig. 4) is 1.5mm or less in order to achieve optimum combustion. Openings 4 are positioned such that the gas/air premixture issuing therefrom flows between adjacent slots 6 in the distributor plate 5 as indicated by Y in Figs. 4 and 2. Diffusion of this gas/air premixture the flow of secondary air (5A) becomes ineffective if the gas/air premixture is at a distance more than 10 mm perpendicular to the secondary air flow and so slots 6 are preferably positioned no more than 20mm apart. Moreover, diffusion of the gas/air premixture into the secondary air flow becomes turbulent and flame quenching when said flow is less than 2mm perpendicular to the axis of flow of the gas/air premixture and so slots 6 are preferably positioned no nearer than 4mm apart. The gas/air mixture issuing from header 3 will travel a distance 2 (Fig. 2) over the surface of plate 5 according to the nature of the gas/air stream momentum and the entrainment effects of adjacent distributor plate secondary air flows or columns 5A. Where only a single header is used gas issuing from header 3 will, typically, travel a distance Z of 80mm. However, it will be understood that where a bigger burner is required a number of headers will be employed and in this instance contra or opposed flow with respect to adjacent headers provides for optimum spacing between adjacent headers of 160mm. Primary air openings 9 are provided in the gas injection region of the header 3 and their function will become apparent in the following description. A fan (not shown) is positioned either below or above plate 5 to cause the air flow 5A through the slots 6 by blowing or sucking and in either event air is caused to flow through the distributor plate 5 as indicated by arrows 5A. In operation, gas fuel is injected into header 3 via the gas injector 1. As the gas fuel flows past primary air opening 7 air is entrained or forced into the gas fuel so that a gas/air premixture is produced. This gas/air premixture flows into header 3 and across the distributor plate 5 as it issues from gas fuel outlets 4. As the gas/air premixture flows over the upper surface of distributor plate 5 it is mixed with the secondary air issuing from slots 6. Combination occurs near to the upper surface of the distributor plate and so it will be understood that the distributor slots 6, or orifices, form flame ports. Preferably the gas/air mixture achieved will be between 140- 190% of the theoretical air requirement needed to effect complete combustion. The ratio of air to gas and its amount can be changed by varying: the airflow through the system; the disposition of the distributor orifices; the size and disposition of the primary gas/air openings 4 and/or the injector size and/or pressure. By varying the aforementioned parameters the flame intensity can be varied to suit a particular application. Several alternative embodiments are illustrated diagrammatically in Figs. 5, 6 and 7. In Fig. 5, the header 3 is of the cross sectional shape as shown, and the distributor plate is in two sections 5X and 5Y which form a V on two sides 3A and 3B of the header which are also arranged in the formation. The gas outlet apertures 4 lie adjacent the plate sections 5X so as to issue as jets Y as shown. The secondary air is forced through the plate sections 4X at right angles thereto as will be understood from the drawings. In the arrangement of Fig. 6, a circular header 3 has the outlet apertures 4 arranged on the top surface thereof so that the jets Y issue in the form of a cylindrical array. Surrounding the cylindrical array is a cylindrical distribution plate 5 which has slots or apertures 6 similar to Figs. 1 - 3, and the air is forced horizontally through these apertures and transversely of the gas jets Y so that the flame of the burner will form on the inside of the cylindrical plate 5. In the arrangement of Fig. 7, which is similar to Fig. 6 again, a circular header 3 is provided from which the cylindrical array of jets Y issue. Inside this array is provided a closed cylindrical distribution plate 5, and air to form the secondary air is forced into the inside of the cylinder 5 and issues as curtains or flows at right angles to the jets Y so that the flame exists to the outside of the cylindrical distributor plate, as will be clearly understood from the drawings. The burner units may be constructed as is conventional in metallic materials, and as shown in Fig. 4, the slots 6 are preferably formed by deforming the metal of the plates so as to provide smooth curved intlet sides to provide even air curtains or jets. The burner unit of Figs. 1 - 4 is for use underneath a heat exchanging device, but because the burner unit of the invention is essentially for use in connection with forced air flow through the slots or aperture 6, it can be used above the heat exchanging device in which case the flame will exist to the underside of the plate 5, and the air flow will be forced downwardly through the plate 5. The invention provides in a gas burner that the degree of fuel aeration is augmented. The technique and apparatus described herein is elegant in its approach to the problem of reducing the production of toxic emissions during fuel combustion. The solution, by way of augmenting the gas/fuel air supply, provides for enhanced aeration levels that have a favourable effect on toxic emissions during combustion: incremental increases in the aeration of a gas/fuel mixture has a reciprocal effect on the level of toxic emission during combustion of the fuel. This invention is applicable to all commonly used fuel gases. Thus it can be seen that by following the teaching of the invention a significatnt contribution is made to the art of fuel combustion.

Claims: CLAIMS 1. A gas burner comprising a burner header, outlet flame ports in said burner header for the issuance of gas/air mixture jets therefrom, distributor plate means located adjacent said outlet flame ports so that said gas/air mixture jets issuing from the header lie adjacent the plate means, and apertures in said distributor plate means for the passage of secondary air therethrough in a direction transverse to said gas/air mixture jets for the enhancement of the combustion of the gas in said gas/air mixture at said distributor plate means. 2. A gas burner according to Claim 1, wherein the apertures in said distributor plate means comprise spaced parallel slots, and the outlet flameports in the burner header are similarly spaced. 3. A gas burner according to Claim 2, wherein the said slots are spaced evenly in a range 4 mm to 20 mm. 4. A gas burner according to Claim 1, 2 or 3, wherein said header is a straight manifold and said distributor plate means comprises a flat plate. 5. A gas burner according to Claim 4, wherein the manifold has said outlet flameports in opposite sides thereof and said plate lies to said opposite sides and has said apertures at said opposite sides of the manifold. 6. A gas burner according to Claim 1 or 2, wherein the header is a circular manifold and the distributor plate means is a cylindrical or conical plate. 7. A gas burner according to Claim 1, 2 or 3, wherein the manifold has a pair of adjacent sides which in section define a V shape and the outlet apertures are in each of said sides, and the distributor plate means comprise two plates mounted on said sides respectively and together defining a V-shape in cross section. 8. A gas burner substantially as hereinbefore described with reference to the accompanying drawings. 9. A gas burner according to any preceding claim, in combination with a means for forcing the said secondary air through said apertures. 10. A gas burner assembly according to Claim 9, wherein the said means for forcing comprises a fan which is either arranged to blow or suck the secondary air through said apertures. 11. A method of providing the gas flame in a gas burner comprising creating a forced flow of secondary air in a predetermined pattern by forcing the air through apertures in a distributor plate means, and supplying to the vicinity of the flow of secondary air a supply of combustible gas into which has previously been introduced a quantity of primary air whereby the burner flame which exists in the region of mixing of the primary air/gas mixture and secondary air gives rise to conditions in the flame which suppress side reactions. 12. A method according to Claim 10, wherein the proportion of primary air which is mixed with the gas is a smaller fraction than the amount of secondary air which is mixed with the primary gas/air mixture.

Inventors: CAFFERTY MICHAEL, CAFFERTY, MICHAEL
Applicants: BRAY BURNERS LTD, BRAY BURNERS LIMITED
Application year: 1991
Main classification: F23D14
IPC codes: F23D14