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(11) | EP 0 204 505 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Gas burner element |
| (57) A surface-combustion radiant element, in which series of aligned slots extending
in the element body are provided to pass gas/air mixture, the said slots comprising
a first series of slots, opening at the combustion face to secure combustion without
striking back of flame into the body, themselves fed with the gas/air mixture by an
intermediate series of slots, aligned at an angle to the first series and extending
from a distribution face of the body towards the combustion face so that the slots
of the two series intersect within the body and form gas passages at the positions
of intersection. |
[0002] For certain applications cylindrical elements for surface combustion of a gas/air mixture are required. We have had difficulty in making them, at economic cost, with the necessary provision for passage of the gas mixture. We have for example not succeeded with self porous foam ceramic, made by impregnation of an open cell polymer foam with ceramic slip and subsequent firing, because stresses in the resulting elements result in their early breakage in service. Casting from bonded ceramic fibres also proved difficult if pins were used as conventional for the necessary passages for the gas mixture. Such pins, as used for example in the manufacture of flat elements in our U.K. Patent Specification No. 1 436 842, are complicated and in practice unworkable for a cylindrical element, in the customary conditions of vacuum forming from a slurry of fibre and bonding clay.
[0003] We have looked at the problem afresh in the light of the essentials of a surface combustion element, which are provision for passage of the gas mixture, mechanical strength for service, and prevention of striking back of the flame through the element. We have realised that certain slotted structures can both meet these requirements and allow for ready manufacture, not only of the cylindrical elements giving the initial problem, but of elements generally, all capable of operating in self aerating burners with gas at ordinary mains pressure.
[0004] The invention accordingly provides a surface-combustion radiant element, particularly for self aerating burners, in which series of aligned slots extending in the element body are provided to pass gas/air mixture, the said slots comprising a first series of slots, opening at the combustion face, themselves fed with the gas/air mixture by an intermediate series of slots, aligned at an angle to the first series and extending from a distribution face of the body towards the combustion face so that the slots of the two series intersect within the body and form gas passages at the positions of intersection. The slots are dimensioned to secure combustion without striking back of flame into the body, preferably 0.2 to 0.4mm to 1.2 or 1.3mm wide, allowing for different gases and throughputs, without the flame front propagating back in the gas mixture or the heat front creeping back in the ceramic itself.
[0005] The manufacturing requirements are conveniently met by a cast or moulded construction with the slots made in situ or cut afterwards, according to the element shape. A preferred angle of the two series to each other is 90° though other angles can be used. Large elements can be made without the cost of custom made vacuum forming tools for each design.
[0006] Conveniently in a cylindrical element the slots of one series are generally axially disposed of the element and those of the other series generally circumferentially disposed. Preferably, to minimise hoop stress, the slots at the combustion face are aligned substantially axially of the element. Such cylindrical burners can be used with the gas/air mixture passing to burn at the outer face or, for example for an annealing furnace for wire or tube, with the mixture passing inwards to burn at the inner face. A plenum chamber surrounding the burner is then required, instead of gas/air mixture passing outwards, but no other change.
[0007] Cylindrical burners can also be made in a modified form wherein the two series of slots are aligned parallel to each other whereby they coincide and the gas passages run the length of the slots. A form of such burners most effectively combining mechanical strength and resistance to thermal cycling is one with slots axially of the element and interrupted in their length and disposed with the interruptions staggered from one slot to the next so that there is no continuous band of material round the cylinder at any one axial position.
[0008] Desirably in all forms of the element the edges of the slots at the combustion face are chamfered to bring the flame front back into the slot and improve visible production of radiant energy.
[0009] Suitable materials include bonded refractory ceramic fibre which is well known in the art and both as such and in relation to the microstructure formed on casting and firing is described in detail in our U.K. Patent Specification No. 1 436 842 the disclosure of which is incorporated herein by reference. The elements retain the high thermal shock resistance, low thermal conductivity and low thermal capacity referred to in that specification.
[0010] The elements may be mounted in metal or ceramic holders fed with gas and air by a jet and venturi in the conventional way for a self aerating burner. When cylindrical, they may be of any generally cylindrical shape, round or not, the term being indicative of radiation of heat over substantially 360° or at least a large arc.
[0011] Gas burner elements according to the invention are illustrated by way of example in the accompanying drawings in which:-
Fig. 1 is a schematic part sectional view of an element mounted for feeding from a gas jet through a venturi;
Figs. 2 and 3 are detail sectional views of the body of the burner element, Fig. 2 being in a plane through the longitudinal axis of the element and
Fig. 3 in a plane transverse of the axis;
Fig. 4 is a partial view of a tool for vacuum casting of the element;
Fig. 5 is a view of the end cap of such a tool;
Fig. 6 is a part view of an alternative burner element;
Fig. 7 is a fragmentary view of part of the element of Fig. 6 from the interior; and
Fig. 8 is a view of a burner of the modified form referred to above.
[0012] In Fig. 1 there is shown a burner body 1 of vacuum cast bonded refractory ceramic fibre such as is referred to above. This particular example is 10.2cm outside diameter and 7.8cm internal diameter. The body has a distribution bore 2, and is mounted on a schematically indicated mounting plate of steel 3 carrying a venturi tube 4 which is 26cm long and 5cm diameter and is also made of steel. The venturi tube is fed with natural gas (methane) at 8" (ca. 20cm) water gauge from a conventional jet 5 of which the manufacturers reference is "Amal 470" (1.9mm diameter). Externally of the body schematically indicated slots 6 are provided in the form of 59 turns of a continuous spiral groove machined by a high speed slitting wheel traversed past the body 1 while the body is rotated. A closed cap 7 of the same material as the body closes the open end of the body.
[0013] Details of the slots are seen in Figs. 2 and 3 where the width is seen as 0.9mm in the axial direction of the body, the radial depth as 6.5mm, the axial spacing 4mm. The wall thickness of the body is 1.2cm. Internally the body has formed in it 36 equidistantly spaced vertical slots 0.9mm wide in the circumferential direction and 7mm deep in the radial direction. These slots are referenced 8. The sum of the depthsof the slots is 1.35cm so that they overlap or intersect within the body to the extent of 0.15mm and form in effect a rectangular array of small gas passages at positions indicated at 9 in Figs. 2 and 3.
[0014] The element of Fig. 1 is manufactured on a tool which is shown in Fig. 4. A per se conventional filter casting mesh 10 in cylindrical form is surrounded by a schematically indicated array of metal strips 11 to form the slots 8. The mesh and strips are fitted into end caps 12 one of which is shown in Fig. 5. Filter casting proceeds in the ordinary way and it is found surprisingly that on removal of one of the end caps the tool can be removed without undue friction or damage from the green body formed, which is fired in the ordinary way.
[0015] In Fig. 6 an alternative element is shown in which the slots 8 are replaced by much wider slots 13 which in this particular example are machined into the filter cast body rather than cast in situ, though they can also be cast. This construction gives a gas passage in the form of a slot fully open to the exterior as shown at 14 in Fig. 7 rather than the small gas passages 9 referred to in relation to Fig. 1. It is found that both constructions give an effective control of the passage of the gas/air mixture entering the distribution bore.
[0016] What is in effect a further development of the above is seen in Fig. 8, where simple axially directed slots 15 are formed, discontinuous so that the integrity of the body is maintained. The body consists of a slotted central portion and unslotted portions present at each end, and the pattern of slots is arranged so that there is no continuous band of material around the cylinder circumference at any one point. Such a design is mechanically flexible and withstands thermal cycling well.
1. A surface-combustion radiant element, in which series of aligned slots extending
in the element body are provided to pass gas/air mixture, the said slots comprising
a first series of slots, opening at the combustion face to secure combustion without
striking back of flame into the body, themselves fed with the gas/air mixture by an
intermediate series of slots, aligned at an angle to the first series and extending
from a distribution face of the body towards the combustion face so that the slots
of the two series intersect within the body and form gas passages at the positions
of intersection.
2. A radiant element formed as a cylinder, whereof the outer face or the inner face
is the combustion face, said element being as set out in claim 1 or in a modified
form wherein the two series of slots are aligned parallel to each other whereby they
coincide and the gas passages run the length of the slots.
3. A radiant element according to claim 2 wherein, to minimise hoop stress, the slots
at the combustion face are aligned substantially axially of the element.
4. A radiant element in the modified form, according to claim 3, wherein the slots
are interrupted in their length and disposed with the interruptions staggered from
one slot to the next so that there is no continuous band of material round the cylinder
at any one axial position.
5. A radiant element according to any preceding claim, wherein the slots at the combustion
face are 0.2 to 1.3mm wide.
6. A radiant element according to any preceding claim, wherein the edges of the slots
at the combustion face are chamfered to bring the flame front back into the slot and
improve visible production of radiant energy.
7. A self aerating burner comprising an element according to any of claims 1 to 4,
fed with gas/air mixture by means of a gas jet directed into a venturi, the mixture
entering a distribution chamber for passage through the element.