The present invention relates to gaseous fuel combustion systems with either natural or forced draft, in particular to a method and apparatuses in accomplishment of same, for controlling the outflow of the air-gas mixture through the flame openings of burners, totally or partially premixed, in order to obtain: a stable flame, detached from the flame openings area, low emissions, in the most wide field of burner capacity modulation required in practice, even in feeding conditions with test gases foreseen from Standards for extreme conditions of use, in a so simple and practical way to be used also for apparatus with capacity of only few kW.
The gas combustion system is the assembly of the burner, with the combustion chamber, the heat exchanger, the means for the circulation of air and exhausts, if existing, as well as the control apparatus with its sensors; more elements of the assembly can form a sole body therefore a distinction is only possible considering the function of each element.
The gas combustion systems are the main functional assembly of domestic and industrial appliances as central heating boilers, water heaters, of two main types: instantaneous and storage water heater, room heater and furnaces, gas cookers etc.
For burner it is understood the functional assembly of parts which create the mixture of air (primary air) and fuel-gas (hereafter simply said mixture), making possible the flowing out of them in the combustion chamber through the flame openings.
The invention applies in particular to fuel-gas combustion systems, where:
Slots obtained in sheet metal of thin thickness, so close to create an almost homogeneous sole jet of mixture, are considered as a single flame opening.
The front of flame is recognisable because it emits in the visible, even if the specific maximum emission, due to OH and CH ions, is respectively in the wavelength between 305 and 320 mm and around 431.5 and 438 mm.
The apparatuses of the kind disclosed in the preamble of the claims arise when the burner capacity is modulating down therefore the mixture exit velocity reduce or when instead of the standard fuel-gas for which the apparatus is set, a fuel-gas from the same family as said standard fuel-gas but prone on flame blow-off or prone to flash back are fed. Specifically, the burner flame openings surface may attain critical temperature value, and in some other occasion, the flame may become unstable, resulting in poor combustion of fuel-gas. Generally said, the flame openings surface, which reach temperature up to 500°C, must be of costly materials and moreover the thermal stress reduces the possibility of flame openings design.
In 1979 Kawamura T. carried on experimental studies on a lamellar flame substantially detached from the area occupied by the flame openings using a burner designed for laboratory purpose, operating in free air, the outflow was forced through two parallel long slots punched in sheet metal.
The WO-A-8 901 116, of the same inventor, claims the following method: in an atmospheric burner, A) inside the burner it is sucked a quantity of primary air which is at least 80% of the air stoichiometrically necessary to the combustion; B) the secondary air, from the very first combustion steps, laps all sides of every single bladed small flame exiting from each group of slots foreseen on the burner diffuser, in such a way that the total air involved exceeds the stoichiometrical air value; C) the combustion starts and is completed within the bladed thickness of the flame,
The burners considered are only atmospheric, the only foreseen burner diffuser is in sheet metal with thickness of around 0.5 mm., the group of slots is defined "each group of slots being appropriately subdivided longitudinally into two subgroups of slots lying not so close together" to anchor the flame on the unpunched surface between subgroups of slots.
The PCT/IT 91/00056, perfecting the previous one, considers also the forced burners and the fully premixed burners with no need of secondary air, but it expects the sole use of sheet metal diffuser and of the sole anchorage of the flame on the flame openings surface.
The anchorage zone reaches high temperatures particularly in limit conditions, moreover the thermal stress, caused from temperature differences of 200-300°C in few millimetres, forces the use of costly materials and constrains special shape and arrangement of the slots which limits the specific capacity of said burner. Furthermore because the exit velocity is limited by the inefficacy of the flame anchorage, the specific capacity of the burner itself is again limited. The difficulty of the anchorage of this type of flame imposes also the introduction of more or less expensive regulation systems as per EP0606527A1 deposited on 16 August 1993. In other hyperstoichiometric burners the very low exit velocity of the mixture generates the flame in the proximity of the flame openings surface therefore such surface is water cooled to maintain temperature compatible with not too expensive materials, but the water cooling system doubles the cost of the burner and the absence of flame anchorage leaves the possibility of unstable conditions.
The PCT/GB 90/01485 Cafferty filed by BRAY BURNERS LIMITED considers a burner with a maximum primary aeration λ=0.7, "...enabling the issuance from the body of a curtain of said air-gas mixture, wall means adjacent the aperture means arranged so as to lie substantially parallel thereto and adjacent one side therefore...» the invention doesn't consider the hyperstoichiometric mixture with consequent flame of large surface and minimum thickness, doesn't consider the fluids dynamic obstacle immersed on the mixture flow and completely detached and down stream from the flame opening/s. Said invention doesn't solve the problems described previously particularly the difficulty to maintain the flame stable and the level (50 ppm) of emissions reached, originated by the inadequate premixture, which is not acceptable.
WO-A-9736135 (prior art under Article 54(3) EPC) describes and claims a method and apparatuses for the fulfilment of the same to maintain the flame, around an optimum position, regulating at least one of the three variable quantities of the mixture exiting the flame port/s: the value of the premixture rate λ, the outflow velocity, the temperature upstream the flame front. To obtain such results WO-A-9736135 uses modification of the fan speed, of the flame openings cross section and heat elements. Fluids dynamic obstacles are foreseen to create downstream a stagnation zone, anchorage of the flame, but not formed and positioned in a way to create downstream divergent fluids threads. The dilution of the mixture downstream the flame openings to locally reduce the flame speed is not foreseen
The aim of this invention is to provide a method and apparatuses in fulfilment of same, to eliminate the aforesaid difficulties, to make possible the proper combustion in very compact combustion systems, even formed by a sole body, to reduce the emission to a very minimum, to maintain the temperature of the flame opening/s surface low to allow the use of steel, aluminium, and other inexpensive materials, all previously described goals must be achieved in all operating conditions caused by the capacity modulation or/and the use of limit gases. The feasibility must be so simple, to be used also for combustion systems of capacity of only few hundred watt.
This aim is reached by applying the method so that, in a gaseous fuel combustion system with either natural or forced draft, a premixed, atmospheric or forced burner form the fuel-gas air mixture, from almost stoichiometric to strongly hyperstoichiometric, which flows in combustion chamber from at least one flame opening with velocity, distribution such as to obtain a substantially laminar flow and velocity over 0.4 m/sec. An almost homogeneous single jet of mixture exits from each flame opening, whereby, immediately down-stream of said opening, the gaseous content of the volume surrounding the jet is sucked into said jet and mixed therewith. The jet velocity is, in all operating conditions, higher than the combustion velocity of the mixture in said condition, at least for a distance equal to that of at least one fluids dynamic obstacle which is immersed in said jet, down stream the flame opening/s. Said obstacle is always completely detached from said flame opening/s, having such shape and dimensions to create down stream a stagnation zone, anchorage of the flame in all operating conditions, the distance of the down-stream side being at least big as the width of the at least one flame opening. The flow field, down stream and on the side of the obstacle, is almost laminar with progressively divergent fluids threads, to obtain a flame at the beginning ignited from a specific device, from laminar to wrinkled, characterised by a large surface and very low thickness anchored to the obstacle at least in some points by vortexes related to the stagnation zone, said flame doesn't touch at all the flame openings surface.
If the flame openings on the burner head are distributed close each other and disposed on a surface in an homogeneous way, the jets flowing out from different openings, join together after few to some ten mm. remaining the jet velocity over the combustion velocity at the given conditions of mixture and temperature; in this case mainly recycled mixture, is sucked into said jet/s just down stream the flame openings and the fluids dynamic obstacles, no longer related with the shape and arrangement of flame openings, may form a mesh net or, for instance, an assembly of parallel rods having centreline up to perpendicular to the axes of elongated flame openings.
If the flame openings are homogeneously distributed over a surface but spaced each other, the jets flowing out from different openings don't join together before the jet velocity becomes equal to the combustion velocity at the given conditions of mixture and temperature; in this case and in case of single mixture jet expressly supplied secondary air, pure or mixed with recycled mixture, is sucked into said jet/s just down stream the flame opening increasing locally the λ of the mixture up to 30% and the fluids dynamic obstacle must closely follow the shape of the related opening.
In case of spaced flame openings, following the previously described criteria the body/s forming the obstacle need to have the centreline of the assembly or of the body in the centre, or slightly out of the centre (for instance when near the side walls of the combustion chamber) of the mixture jet in the observed point. The distance obstacle opening and its shape may vary along the opening length to follow flow-rate changes along said opening or to obtain particular flame shapes to comply with the heat exchanger. The position of said obstacles follow the flame openings surface.
It is possible to define the position of the flame as the distance between the barycentre of the flame front and the surface of at least one flame opening which generates this front, hereafter said quantity will be called flame distance.
If the temperature, the aeration ratio λ, of the mixture remain stable but flow-rate decreases (the exit velocity decreases), the flame distance decreases and vice-versa if the flow rate increases.
The flame distance optimum value can generally be predetermined arbitrary constant, but can have different values according to the fuel-gas flow rate; in any case, during the on periods on the combustion system, the instantaneous ratio, which is the detected flame distance/optimum flame distance, have the value -1- for the reached conditions considered as optimum, values over 1 show the tendency to the flame blow-off increasing as the ratio increases, values under 1 show the tendency to overheat the burner head (means the flame openings zone) increasing as the ratio decreases.
The instantaneous ratio: detected flame distance/optimum flame distance, will be hereafter called flame ratio.
To maintain the flame ratio near the optimum in all the operating conditions, the modification on the flame distance will cause a change of the cross section of the obstacle changing the flow conditions on the side of said obstacle and therefore shape of the flame, for example: bimetal or sealed bulbs filled with expansible fluid can be used.
Following the teaching of the Patent demand MI/96/A00058 of 23-03-96 of the same inventor, the outflow cross-section of the flame opening/s is varied according to the flame ratio, between a minimum and a maximum cross-section, causing a flame ratio > 1 an outflow section increase and vice versa, so as to maintain the flame ratio around a pre-fixed value, except for a different regulation during the transient periods, for example of starting, when needed.
By adopting the flame ratio, or any variable quantity which follows the same variation law, as control-parameter for the variation of the outflow cross-section of the flame opening/s, it is possible to reduce at the minimum the variation of the outflow velocity at the change of the burner capacity by maintaining the flame stable and low emissions.
For instance, the distance of a V shaped obstacle can be varied in such a way that the vertex of the V would enter the flame opening reducing the open surface.
By using any variant of the described method, the temperature of the mixture outflow zone of the burner head remains within acceptable limits (even below 200 °C), at any capacity condition of the burner, type of feeding gas, temperature of the inlet air; the flame remain stable; the harmful emissions are reduced to minima values.
In order to better understand the invention, its characteristic and the advantages it provides, few embodiments thereof are described hereinafter, by way of non limitative example and with the assistance of the appended drawings, in which
FIG. 1 shows, in vertical cross section, a combustion system 1 with a heat exchanger 2, a combustion chamber 3, a fan 4 for the air gas and exhausts circulation, put upstream the combustion chamber for which this is in over pressure compared to the outside of shell 5 representing the body of the combustion system, which, in this case, is a water cooled aluminium casting, whose inferior part forms the burner head 6 and the burner body 8, inside which a distributor 8C is installed; flame openings 7B obtained from the aluminium casting have lips, are lengthened, perpendicularly to the drawing surface. On the centre axe of the jet out flowing from said openings 7B, inside the combustion chamber 3, at a distance, from the flame openings surface, which can reach ten times the width of said openings, according to the exit velocity, coplanar with the jet flowing out, are disposed fluids dynamic obstacles 12A, made on small ceramic bars, with isosceles triangle cross section, having sharpest edge upstream and a side perpendicular to the jet axe downstream; the width of the obstacles cross section is of the same magnitude of the width of the openings, in this case between 2 and 4 mm., the height of the lips of the openings can vary between 7 and 20 mm., in the length the openings are interrupted every 25-50 mm.
The lamellar flame 19, ignited by a device not in the figure, generates and remains firmly anchored to triangle section obstacles 12A, downstream of the flame openings 7B, becoming alike a flying carpet, wave shaped because the openings are each other spaced so to create separate jets flowing out joining together only after the combustion for the expansion due to the reached temperature; a certain amount of exhaust gases is recycled from the doubled vortex formed between two flame openings and sucked inside the mixture jet just down stream the openings edges.
The process controller 15 according to the signals transmitted by the ionisation current sensor 14A operates the fuel gas valve 11 and the fan 4 speed.
FIG. 2 shows, in vertical cross section a combustion system operating in forced draft with the fan 4 mounted downstream the heat exchanger 2 so the inside of the shell 5 is in depression compared to the outside. The burner, the body 8 of which is the bottom part of the shell 5, is a fully premixed atmospheric one, the air-fuel gas mixture is obtained in a Venturi type tube 10 from the fuel gas exiting the injector 23 and the air from outside the shell 5 entering the mouth 9. Under the vacuum created by the fan in the combustion chamber 3 with respect to the region outside the shell 5 the mixture is drawn through the Venturi 10 and the mixing chamber to the flame openings 7A, better described in fig. 15, 16, obtained on the sheet metal, for example, of 0.4-0.6 mm thickness, of the burner head 6.
The flame openings 7A, each made of a row of slots, where the slots are 0.5 to 0.75 mm. width and 5 to 15 mm. long, parallel to each other on the long side with centre distance between 0.9 and 1.5 mm., rows are spaced centre to centre from 15 to 60 mm to obtain a flying carpet type lamellar flame 19 anchored to external obstacles 12B, made on stainless steel sheet metal, visible in V shaped cross section with upstream vertex and centreline of the V, perpendicular to the surface and in centre of the flame openings, parallel to the rows and distant to the slot surface from few to some ten mm according to the exit velocity of the mixture, V shaped obstacles of this type are better described in fig 7, 8, 9. The distance among flame openings and shape and position of the obstacles don't permit any recycling of exhaust gases, therefore the jets down stream the burner head surface suck only recycled air gas mixture.
The lamellar flame covers the plan of the combustion chamber 3, lying at level of the optical sensor 14B. The process controller, not shown, varies the gas flow through the valve 11, according to the heat request.
The optical device 14B, based on photo sensor/s, transmits a signal corresponding to the detected position of the flame to the process controller, not shown, which compares said position to a pre-fixed one (flame ratio), in order to change the fan speed.
FIG. 3 shows a natural draft combustion system 1, which employs a partially premixed atmospheric burner with body 8A of the extractable type; the secondary air is introduced from the inlet openings 18 on the shell 5. Flame openings 7A are formed of rows (perpendicularly lengthened to the drawing) of slots as in fig.2, 15, 16, punched on burner head 6; The external fluids dynamic obstacles have a V shaped cross section, as in fig.2, but made from bimetallic sheets cut and bent as described in fig.15, 16, 17. Being the centre distance among exits 7A big, the flame, ignited by a device not seen, divides itself in long separate V shaped lamellar flames 19A (perpendicularly lengthened to the drawing), at the same time the secondary air, introduced through the apertures 18, is sucked from the mixture jets just down stream the flame opening increasing the λ of the mixture up to 30%.
At the same time, the external bimetallic sheet V shaped obstacle, according to the temperature reached changes their cross section, therefore changing the outflow velocity field so to favour the stability. For higher temperatures (continuous line) smaller cross section, the contrary (dashed line) for lower temperatures.
The ignition is obtained from the heat element 14 and the flame presence is controlled by a thermocouple not shown.
FIG.4 show a natural draft combustion system with partially premixed atmospheric burner, without any specific process control. The burner head 6 is of perforated sheet metal having the circular holes diameter from 2 to 5 mm. and lips from 1.5 to 7 mm. high.as better described in fig. 10, 11, uniformly positioned on the burner head as in fig. 11, so spaced each other to obtain a ratio open/total surface between 0.2 and 0.5 as to form a single substantially homogeneous mixture jet and to build a carpet shaped lamellar flame 19 anchored to a mesh net fluids dynamic obstacle 13 B, been the net made of wires of high nickel steel, regularly distributed to cover more/over the complete flame openings surface. Through calibrated passages 18 the outside and the inside of the shell 5 are in pressure balance, secondary air inlets only at the maximum capacity, at the ignition some air goes out. The presence of flame is detected from a combined device 14 A, which is an ionisation current sensor also capable to cause the spark ignition.
FIG. 5 shows an atmospheric burner with tubular body 8A with a sole flame opening 7B between two lips, with an external fluids dynamics obstacle having a triangle cross section positioned with the central line on centreplane of the burner at a distance from the flame opening edges up to 10 times the width of said flame opening with a cross dimension of the same magnitude of said width, anchoring the large V shaped flame. The fluid dynamics obstacle, of triangle section ceramic rod, is supported (the support is not shown) on the extremities leaving the possibility of expansion.
FIG. 6 shows the atmospheric burner of fig. 5 in vertical cross section to better understand the reciprocal dimension and position of the flame opening in this particular case.
FIG. 7 shows a tubular atmospheric burner with the body 8A forming a sole flame opening 7B with two mobile lips 7B2, which define the outflow cross section of it, moved by the deformation (temperature function of the flame opening edges) of two bimetallic sensors-actuators 17. The lips 7B2-a position full line drawn and 7B2-b the dashed drawn one correspond to two different conditions of the flame opening temperature obviously higher the one corresponding to the dashed line, the same for the position 17-a and 17-b of the bimetallic arches; the cross section of the burner body 8A is similar to that of fig.5 but the external obstacle have a V shaped cross section 12B.
FIG. 8 and 9, show the vertical cross sections of the tubular burner of fig. 7 in the two different operating conditions, shown with dashed and continuos lines for the same conditions represented there in the same way.
FIG. 10 shows, on plan view from the top, round holes having an external lip around, punched on sheet metal, arranged in groups of three holes in line, the groups parallel and so near to each other, forming a row which is one flame opening 7C, in fact the mixture exiting from all the slots of a row form a sole jet, the three rows shown have centre distance as in fig. 3 to obtain separate V shaped flames; dimensions of the holes are as defined in fig. 4. The projection of external trilobated elongated obstacles 12D are shown with dashed lines.
FIG. 11 shows, on plan view from the top, round holes having an external lip around, punched on sheet metal as those shown in fig.10 disposed regularly and close together, with a ratio open /total surface around 30%, jets flowing out from the flame openings are separate and join together around the level of the trilobated obstacles 12D, but because in this case the centre distance of said external fluids dynamic obstacles (projection is shown in dashed line), is big enough, a flame shaped alike a wave flying carpet is obtained, similar to that of fig. 1 obtained with a different lay out of flame openings and obstacles.
This fact demonstrate that when jets of air-gas mixture exiting from different flame openings join together before the external obstacle and maintain, there, a velocity higher than the combustion velocity for that mixture in that condition, the shape and position can be independent from that of the flame openings; the obstacles my assume various forms, parallel elongated rods, trilobe shaped 12D as here, V shaped 12B, triangular 12A, also round, nets of different construction and design as the mesh net 13B of fig. 4 and the laid on small rods 13C of fig. 14.
FIG.12 shows the vertical section of two aligned elongated openings so close together that a sole obstacle (12C), in this case a bimetallic V shaped one, the vertex is near the flame openings surface. Any variation of the form and or position of the flame cause a change of the cross section obstacle such to tend to re-establish the previous conditions, particularly at any increase of the flame distance causes an increase of the transversal dimension of the obstacle (12C) cross section, the opposite for a reduction. Also the flame and the wall design is visible.
FIG. 13 shows an enlarged plan view of the burner head in a first level where are visible slots parallel each other combined in groups of three and four, these said groups (the flame openings) been distributed in a check pattern to obtain a flying carpet shape lamellar flame 19 in the second level the external obstacle is visible formed of a net 13C made of parallel ceramic rods in one direction and wires perpendicularly, regularly covering the combustion chamber plan.
FIG. 14 shows the vertical section corresponding with the application of fig. 13 where the reciprocal position of openings, obstacle and flame are visible.
FIG. 15 shows the vertical section C-C of fig. 16 where the reciprocal position of openings, obstacle and flame are visible. In this case the composition of the gaseous content from the volume surrounding the jet sucked, immediately down stream the flame opening, inside said jet, is a mixture of secondary air and recycled exhaust gases in different ratio following different operating conditions.
FIG. 16 is a top view in two levels, of a part of the burner's head 6 of fig.3, two flame openings are represented, made of a row each of parallel slots having width from 0.5 to 0.75 and length from 5 to 15 mm. adjacent on the long side, spaced centre to centre from 0.9 to 1.5 mm., the external bimetallic obstacle 12C V shaped is visible on the left side on the second level.
FIG. 17 show the plan view of part of a sheet metal rectangle with alternate parallel cuts which bent would form the V shaped obstacle of fig.3, 12, 15, 16, the cuts are foreseen to prevent arc formed bending of the obstacle caused by the temperature distribution during the heating periods of the combustion system, if the sides of the V are longer than some mm. and the total length of the obstacle more than about 100 mm.
FIG.18 shows a forced draft combustion system, the burner, the body 8A of which is bottom part of the shell 5, the air-fuel gas mixture is obtained in a Venturi type tube 10 from the fuel gas exiting the injector 23 and the air from outside the shell 5 entering the mouth 9.
An optical device 14B is used to detect the flame ratio as to permit to the controller 15 the variation of the outflow velocity of the mixture according to said flame ratio using a step by step motor 25 moving up and down the obstacles 13A through an eccentric 28 as better described in fig 19 and 20. The flame openings 7B, lips formed, are lengthened, perpendicularly to the drawing surface; on the centre axe of the jet out flowing from said openings 7B, inside the combustion chamber 3, at a variable distance, from the flame openings surface, parallel to the openings, are disposed (displaced) fluids dynamic obstacles, V shaped made of stainless steel sheet metal 12Aa, supported each by a ceramic rod 12Ab,; the openings and the obstacles have the same length. The lamellar flame 19, ignited by a device not in the figure, generates and remains firmly anchored to obstacles 12B, downstream of the flame openings 7B becoming alike a wave shaped flying carpet because the openings are each other spaced about five times the openings width. No recirculation of exhaust gases is foreseen because the openings are too close to each other.
FIG. 19 shows the vertical section A-A of fig. 18, looks clear how the eccentric 28 moved by the motor 25 can push down the obstacles 12A against the springs 30 actuating the desired changes of the cross section of the flame openings 7B visible in fig. 18 and 20.
FIG. 20 shows a vertical section of lips formed flame opening 7B with movable obstacle 12B which modify the outflow cross-section of the flame opening and therefore the outflow velocity, according to the flame ratio, between a minimum and a maximum cross-section, causing a flame ratio > 1 an outflow section increase and vice versa, so as to maintain the flame 19A distance around a pre-fixed value, except for a different regulation during the transient periods, for example of starting, when needed. Two position of the obstacle and that correlate of the flame, are shown, with dashed line 12B-a the one for smaller outflow cross-section continuos line 12B-b those for higher.