A gas burner with an improved diffuser

Abstract: A gas burner (5) of the type in which an inflammable mixture is caused to flow out of a diffuser (7,8...20) formed of a plurality of parallel slits (21,22,33,34) separated by grid elements and distributed into a pair of grids separated by an elongate screen (37) extending between the two grids, comprising at least one elongate slit (52,53,61,66) extending perpendicularly to said screen (37) through the breadth of said grid pair, perpendicular to the screen (37), near the grids and adapted to recover the relative dimensional variations of said screen (37) and the grid elements brought about by differential heatings of the screen and the burner (5) walls surrounding the diffuser.

Description: The present invention relates to a gas burner as defined in the preamble of claim 1 and disclosed in document NL-A-8 801 383. Gas burners consist basically of a nozzle through which a fuel gas, such as methane, propane, and the like, under a pressure is injected into a mixing tube, usually a venturi type tube, where a suitable flow of combustion air or primary air is drawn in, due to the suction generated by the flowing fuel gas. The primary air, once admixed to the gas in an appropriate proportion therewith, forms an inflammable mixture which is directed into a mixing and distribution chamber formed of a box-type body of sheet metal provided with openings called diffusers. The mixture flows out of the diffusers, of suitable size, at a suitable velocity which varies with flow rate, and is ignited outside the diffusers either by a pilot flame or suitable igniting devices. The shape and size of the diffusers is particularly critical to an efficient combustion and to the flame stability under different conditions of operation, as well as to the prevention of backfiring. The applicant has developed for this purpose burners with high specific power and low NOx and CO emissions which employ diffusers with pairs of side-by-side grids separated transversely by a screen, whence the inflammable mixture is caused to flow out at a relatively high velocity which is a multiple of the flame rate of propagation at full power, when the air-to-gas ratio of the mixture is approximately stoichiometric or higher (0.8<lambda<1.6). Diffusers of this type, being the subject matter of patent application EP-A-0373157 filed by the applicant, allow laminar flames or particularly stable flame fronts to be generated which extend across divergent surfaces having their origin located close to the central screen and spreading out in a variable manner according to the outflow velocity, inversely therewith. Since the flame develops across two surfaces which are spaced apart from the element or plate whereon the diffusers are formed, the material from which the diffusers are formed is stressed thermally to a negligible extent, is unaffected by ageing due to fatigue and is not subjected to wear or alteration of its structural characteristics. Thus, the useful life of a burner equipped with this type of diffuser (additionally to its efficiency and specific power) is much longer than the life of conventional burners wherein the flame generated by a flow of a mixture of air and fuel at a velocity slightly higher than the flame propagation rate develops in close proximity to the diffuser with local burner overheating. Diffusers of this type can operate at variable flow rates, and therefore, under modulated conditions. However, at reduced flow rates and consequently reduced velocity of the fuel mixture outflow (or when using gas having a higher flame propagation rate than prescribed), the flame tends to approach the diffuser walls causing localized heating of the grid pair and the screen interposed therebetween (similar to that of conventional burners). In these conditions, the differentiated burner heating, with the temperature surge that may attain 300-350°C, causes local internal expansions and stresses which fatigue the material and result in ultimate failure of the same. According to the present invention, this drawback is obviated by providing a gas burner as defined and characterised in general in claim 1. The features and advantages of the invention will be more clearly apparent from the following description of a preferred embodiment of the invention and from the accompanying drawings, in which: Figure 1 is a sectional view of an exemplary structure of a gas burner; Figure 2 is a top view of the burner in Figure 1; Figure 3 is a side view of a further example of a gas burner; Figure 4 is an enlarged view of a diffuser of the burners in Figures 1, 2 and 3; Figure 5 shows a diffuser according to section I-I of Figure 4 and the flame produced thereby at different flow rates; Figure 6 shows schematically according to section I-I of Figure 4 the deformations induced in the diffuser by differential thermal expansions; Figure 7 shows schematically according to section II-II of Figure 4 the deformations induced in the diffuser screen by differential thermal expansions; Figure 8 is a top view of a preferred embodiment of the grid diffuser embodying the present invention; Figure 9 shows two variations of the diffuser in Figure 8; Figure 10 shows a third variation of the diffuser in Figure 8; Figure 11 shows a fourth variation of the diffuser according to the invention. Referring to Figures 1 and 2, a gas burner basically comprises a nozzle 1 which injects a stream of fuel gas (methane, butane, and the like) into a venturi tube formed of a convergent frustum of cone 2, a cylindrical segment 3 of smaller cross-section, and a divergent frustum of cone 4, all enclosed within a box-type burner body 5, for example but not necessarily, of cylindrical shape. The convergent 2 is in communication with the atmosphere and the divergent 4 is open to the box-type body interior through an outlet 6. The box-type body, in turn (Fgure 2), is communicated to the outside through openings or diffusers 7, 8...20. The suction developed by the stream of fuel gas in the throat of the venturi causes combustion air to be drawn in through the convergent. By suitably sizing and positioning the venturi tube relative to the nozzle, an intake of air correctly proportioned to the gas can be developed. The turbulence induced within the divergent 4 and the outlet 6 results in the gas being blended with the air taken in and a combustible mixture being formed which flows out through the diffusers 7...20 at a variable flow velocity with the cumulative input flow rate and in general higher than the flame propagation rate. Quite similar is the operation of the burner of Figure 3, known as a "pipette" or "small ramp" type, wherein the venturi tube, instead of being extended into a mixing distribution chamber of cylindrical shape which surrounds the venturi, is extended into a chamber located on top (or laterally) of the venturi tube and connected thereto by an elbow. On igniting the mixture outside the burner by an appropriate means, each diffuser becomes a source of a flame whose shape is a function of the diffuser shape. Figure 4 is an enlarged view of a preferred embodiment of one of the diffusers 7...20. Each diffuser comprises a plurality of elongate openings or rectilinear slots 21...34 provided in the box-type body and arranged parallel to one another in two rows to form a pair of grids 35, 36 which are separated by an elongate portion 37 of the wall of the box-type body which, on account of the function it serves, can be termed a screen. The slots 21, 34 have a suitable width in the same order of magnitude as the thickness of the wall of the box-type body, for example, of 0.5-0.7 mm, and are separated from each other by a grid element in the same order of magnitude. The length of the slots is of a greater order of magnitude, for example, of 7 mm. The screen which separates the two grids suitably has a width on the order of 1.5-4 times the width of the slots. This type of diffuser is, as shown in Figure 5, unique in that it develops a flame which extends into two divergent fronts arranged like the wings of a butterfly. The combustible mixture flows out through the slotted openings of the diffuser along the direction of arrows 138, 139 in Figure 5. Upon the mixture being ignited, the area 38 overlying the screen 37 becomes saturated with burned gases which are fed continuously by the combustion, while at the diffuser periphery cool air is drawn in which feeds the mixture and makes it leaner, thereby reducing the flame propagation rate. Thus, two opposed flame fronts 39,40 develop which are very stable and whose spatial position depends on the combustible mixture outflow velocity and, hence, on its flow rate. It should be noted that the same type of flame is also generated by burners wherein the two grids are separated by a screen extending in the same direction as the grid elements, as described in the aforementioned patent application. If the flow rate decreases (or if the flame rate of propagation increases), the flame fronts, at maximum flow rate spaced well away from the burner, tend to move down and approach the burner, taking the positions indicated by the references 41, 42, and in the extreme, 43, 44. It can be seen, therefore, that the type of flame developed tends, by thermal radiation of the flame and conduction, to heat the diaphragm 37 more than the walls of the burner which surround the diffuser. These walls are, in fact, swept inside the burner by a stream of low-temperature mixture, represented by arrows 45, 46, which cools them and outside by a stream of cool air represented by arrows 47, 48 induced by the outflow of combustible mixture. On the contrary, within the box-type burner body close to the screen 37, a stagnation zone 49 of the combustible mixture is present which isolates the screen thermally. The same stagnant situation occurs outside the box-type body close to the screen 37. As a result, in operation of the burner, the screen 37 goes to a higher temperature than the burner walls which surround the diffuser, and the grids interposed between the slots are at an intermediate temperature. The difference in temperature is a modest one at large flow rates, but tends to increase up to values on the order of 300°C as the gas flow rate decreases (or as the flame propagation rate increases). The net result is a differential expansion of the screen 37 and the grids, which is greater the smaller the flow rate (or the greater the flame rate of propagation). Figures 6 and 7 illustrate qualitatively the deformations caused by the differential expansions in the grid elements (Figure 6) and the screen 37 (Figure 7). Since the surface of the box-type body is generally convex outwardly and the slots are arranged to lie in the direction of curvature of the body, the elongation of the grid elements relative to the body can be easily accommodated by a greater curvature represented by the dash line 50, at internal elastic stresses of minimal magnitude. However, it should not be overlooked that the grid elements, being attached with one end to the screen, do respond to the screen deformations. Quite different is the situation in which the screen 37, which is subjected to much greater expansion, will find itself. The screen 37, with the ends attached to the walls of the burner body and intermediate points attached to the elements of the grid pairs, undergoes much greater deformation, with considerable internal stresses and flexures and a development, unforeseeable to some extent, of multiple waves as indicated by dash line 51. Due to the flow rate modulation and repeated on/off cycling of the burner, it is fatigue stressed especially at the ends and susceptible of long term failure. The screen deformation also involves the grid elements, one end of which is attached to the screen, with consequent long term failure of these parts as well. This drawback is obviated and the useful life of the burner further extended by providing, as shown in Figure 8, diffusers wherein the grid pair is flanked by an additional pair of slits or slots 52, 53 having, preferably but not necessarily, the same width as the diffuser slits and a length substantially equal to the diffuser width in the perpendicular direction to the screen direction of lay and at the end thereof. In this way, the expansions of the screen 37 in its length direction, indicated by arrows 54, 55, can occur without flexure and corresponding fatigue stressing. By having the screen unconnected to the walls of the diffuser, the added advantage is secured of releasing one end of the grid elements from the diffuser walls imposed with the intermediary of the screen. The resultant structure has side-by-side beams twice as long as the grid elements (with the screen width added). This allows the grid elements to accommodate elongations relative to the diffuser walls with induced bending moments of much reduced magnitude and minimized consequent fatigue effects. While two additional slots 52, 53 are preferable, to also minimize the fatigue stresses of the grid elements, it is apparent that in relation to the length of the screen (and grid elements) a single additional slot located at one side or at the center, if necessary, of the grid pair of the diffuser may suffice. The additional slots contribute toward developing end flame fronts which open fan-like to form, with the flame fronts generated by the other slots, a stable flame which opens wineglass-like and encloses a relatively static burned gas volume to ensure the flame stability. As a further improvement, one or more circular or elongate openings 56, 57, 58, 59 may advantageously be provided opposite the screen 37 with respect to the additional slots 52, 53. These openings function to develop small flames which improve the stability of the flame fronts generated by the diffuser slots. In the instance of the tubular burner body being profiled by a set of rollers, there occur, in the course of the production process, stresses having a transverse component to the direction of slot development which can deform the diffuser elements. According to a further aspect of the present invention, to avoid this problem, the additional slots of increased length are provided with elevations jutting into their interiors which locally reduce the slot width to the minimum required to allow the screen to expand. In fact, it is apparent that with the slots being generally formed by punching, their width cannot be less than a certain limiting value related to the thickness of the material to be punched. As previously mentioned, this width is illustratively of 0.5-0.7 mm, while the relative elongation of the screen at maximum working temperature may be much less. As shown in Figure 9, these elevations are preferably obtained by two different types of processing. The elevation 60 in the slot 61 is obtained, for example, by making in the wall 62 of the burner in close proximity of the slot 61 and at the screen 37 an opening 63 in the form of a slot or a circular opening. The opening 63 is obtained by a punching subsequent to that providing the slot 61, so that the plasticity of the material will develop the bulge or elevation 60 inside the slot 61. Thus, if the screen is stressed, in the course of a subsequent profiling operation, in the direction of arrow 64, it bears on the elevation 60 and further deformations at the limit of plastic yield are prevented. It is apparent that in the instance of plastic yield, the expansion clearance lost at one end of the screen 37 is recovered at the other end. Alternatively, an elevation 65 can be obtained by punching without perforation which squeezes the material plastically driving it into an already formed slot 66. This plastic deformation can be both obtained in the burner wall which surrounds the diffuser and in the screen 37, or both. The foregoing description only covers a preferred embodiment and some variations thereof, but it is apparent that many variations may be introduced. For example, as shown in Figure 10, a burner 67 may be provided, rather than with a plurality of diffusers, with a continuous dual grid diffuser which extends over the whole length of the burner. In this case, the screen intermediate the grids may be divided into a plurality of sections by means of a plurality of elongate slots disposed across the screen a predetermined distance from one another. As a further example, shown in Figure 11, the invention can be applied to diffusers wherein the grid slots are disposed parallel to the direction of screen extension. In this case, the additional slots, preferibly at least two in number, are disposed perpendicularly to the direction of extension of the screen and the grid slots.

Claims: A gas burner (5) of the type in which an inflammable mixture is caused to flow out of at least one diffuser (7, 8, 20) formed of a plurality of parallel slits (21, 22, 33, 34) provided in a sheet metal wall of a burner body (5) and distributed into a pair of grids separated by an elongate wall portion (37) extending between the two grids, wherein said sheet metal wall comprises at least a first elongate slit (52, 61) extending perpendicularly to said elongate wall portion (37) near the grids and being adapted to take up relative dimensional variations of said elongate wall portion (37) brought about by differential heatings of said elongate wall portion (37) and said burner walls (5) surrounding said diffuser (7, 8 ... 20), characterized in that said elongate wall portion (37) has a width on the order of 1.5-4 times the width of said grid slits (21, 22, 33, 34) and said first elongate slit (52, 61) extends only through the breadth of said grid pair perpendicular to said elongate wall portion (37). The burner (5) of Claim 1, wherein said elongate wall portion (37) extends perpendicularly to said grid slits and said first elongate slit (52, 61) extends parallel to said slits at one end of said elongate wall portion (37). The burner of Claim 1, wherein said elongate wall portion (37) extends parallel to said grid slits and said first elongate slit (52, 61) extends perpendicularly to said elongate wall portion (37) and said grid slits. The burner of Claim 1, 2 or 3 comprising a second elongate slit (53, 66) extending perpendicularly to said elongate wall portion (37) at the other end thereof. The burner of Claim 4, comprising at least a third elongate slit parallel to and intermediate said first and second elongate slits, said third slit dividing said elongate wall portion into at least two elongate wall sections. The burner of claim 5, wherein said first (52, 61), second (53, 66) and at least third slits have the same width as said slits and are equispaced therefrom. The burner of any of the preceding claims comprising an elevation (60, 65) jutting into said at least one elongate slit of said diffuser. The burner of Claim 7, comprising an opening (63) formed by punching in the wall around said diffuser near said at least one elongate slit (61), said punching causing said elevation (60) to be formed. The burner of Claim 7, comprising a dimple formed by punching near said at least one elongate slit, said punching resulting in said elevation (65) being formed. The burner of any of the preceding claims comprising at least one opening (56, 57, 58, 63) formed in the wall around said diffuser opposite said elongate wall portion with respect to said first elongate slit to augment, by the outflow of inflammable mixture through seed opening, the flame stability.

Inventors: FOGLIANI GIUSEPPE, SEBASTIANI ENRICO, FOGLIANI, GIUSEPPE, SEBASTIANI, ENRICO
Applicants: WORGAS BRUCIATORI SRL, WORGAS BRUCIATORI S.R.L.
Application year: 1995
Main classification: F23D14
IPC codes: F23D14