EP0745208A1 - Improvements relating to fuel-fired burners - Google Patents

Abstract

The flame strip (14) of a fuel fired burner is made up of ceramic plate elements (14A-14Y) stacked together. The elements (14A-14Y) are recessed on the sides so that stacking them together results in the formation of burner ports in the form of deep slots (16A-16F). The plate elements (14A-14Y) are preferably identical and are constant, or dog bone cross section.

Description

IMPROVEMENTS RELATING TO FUEL-FIRED BURNERS

This invention relates to fuel-fired burners, which comprise a flame strip which is a body or plate having burner ports therein. To one side of the flame strip is a plenum chamber to which the mixture of fuel and air is supplied, such mixture passing through the ports and issuing therefrom, and to the other side of the flamestrip in use there will be established a burner flame resulting from the combustion of the mixture issuing from the said ports.

Fuel-fired burners basically take two forms, the first form being termed fully premixed or fully aerated burners in which the quantity of air which is mixed with the fuel is equal to or in excess of the theoretical amount required for complete combustion. In the second type of burner known as partially premixed burners, the air which is mixed with the fuel and is supplied to the plenum chamber is less than the theoretical amount required, and is referred to as the primary air, and the additional air required to complete combustion, which is referred to as the secondary air, is added to the flame by induction from the surrounding atmosphere. The present invention is mainly concerned with fully premixed or fully aerated burners, although it can be applied to the other type of burner as well.

Fully premixed burners are particularly useful in for example appliances such as water heaters and central heating boilers.

Also, the invention has particular application to those burners in which the fuel is a combustible gas, and although the invention can be applied to burners which use ot r fuels, gas is the preferred combustible material, and wl Ϊ reference is made to fuel hereinafter, it is intended maiχ_A_y that such fuel be gas unless the context does not permit of such interpretation.

Fully premixed burners have certain advantages over partially aerated atmospheric burners. Fully premixed burner flames tend to be much more intense, and hence shorter, than those of the partially aerated variety, and this characteristic enables them to be fired sideways or downwards without affecting the combustion quality or causing undue damage to the flamestrip or other burner parts. Also, fully premixed burners can usually be operated with a lower excess air level than is necessary for correct operation of a partially aerated burner at the corresponding full input rate. This tends to result in thermal efficiencies of the appliance during operation being increased. Fully premixed burners are, therefore, ideal for use in boilers and water heaters of the condensing type in which the heat in the products of combustion is removed to such an extent to cause such products to condense. These types of boilers and water heaters are better served by compact flame, downward-firing burners as potentially corrosive moisture produced in the condensing section of the heat exchanger can be easily disposed of, without coming into contact with the burner and its ignition components.

Additionally, fully premixed burners generally produce lower emissions of noxious products, and therefore provide a considerable environmental advantage. Typically, the emission of, for example, nitrogen oxides in fully premixed burners might be in the region of 20 to 40 parts per million (ppm) compared to a figure in excess of 100 ppm for a partially aerated burner at the same fuel input rate.

The performance and operating range of a fully premixed burner is largely determined by its ability to overcome, among others, three potential problems. The first, that of flame instability or flame lift, usually occurs because of high levels of excess air, and the effect is that the flame can become partially or totally detached from the flamestrip, which in turn gives rise to poor combustion, or burner shut down via a safety device, which may be part of the appliance which monitors flame presence.

The second problem occurs at the other end of the performance range and is termed burner overheat. At low gas input rates and/or at low excess air rates, the flame sits very close to the flamestrip surface and can result in overheating of the material of the flamestrip surrounding the burner ports. This can lead to damage to the flamestrip surface, or can cause the flame to propagate back through the ports into the unburnt gas/air mixture in the burner plenum chamber which could, potentially and obviously, have serious consequences.

Thirdly, flames of fully premixed burners are susceptible to the production of resonant combustion noise, which arises, in particular, when the flame end comprises a flat area, which can be affected by flame instabilities, draughts and pressure pulses, to cause the flame to resonate. This occurs especially when the burner is operated within confined combustion chamber/heat exchanger apparatus. The resonant noise can be of an excessive and irritating nature, and, in extreme cases, can lead to damage to burner components.

It has been realised that flamestrip design and in particular the dimensions and shape of the ports of a fully premixed burner flamestrip can have a significant effect on the overall performance of the burner within a particular application. Thur prior patents disclose flamestrips with circular ports of uniform diameter, or groupings of uniform or variable diameter ports. British U.K. Patent 2176588 discloses a flamestrip with deep ports of a rectangular shape.

European patent specification No 0583961 Al discloses a fuel fired burner having a flame strip which is designed to overcome at least some of the problems mentioned above, and in that specification there is disclosed a ceramic flame strip of a burner made up of side and end walls, and between the side walls there are parallel spaced partitions or walls of which opposite ends are secured in recesses in the side walls. The spaces between the partitions form burner slots and they are specially designed to have a tapered followed by a parallel configuration. The construction and design of the ceramic flame strip is therefore rather complicated.

The present invention was developed with a view to providing a simple and effective flamestrip for a fully premixed burner, and at least in its preferred form will obviate or mitigate at least some of the disadvantages which apply to such burners, but in the development of the invention, which has various aspects, it was realised that the flamestrip constructions have wider application and can be applied for example, to partially aerated atmospheric burners. In particular embodiments, the invention addresses the requirement, which is also addressed by the European Patent specification No 0583961, of providing a fully premixed burner which can operate over a sufficently large operating area, with minimal potential for lift, overheat and resonance susceptibility.

In various other prior specifications other burner configurations are disclosed. Thus in European patent application 0375371 there is disclosed a burner arrangement wherein a plurality of bar elements are arranged side by side, so as to define burner slots therebetween which are of a particular convergent/parallel configuration. Th' e bars are held in spacer means and because of the configuration of the bars and slots, difficulties in manufacture and assembly can result.

US patent 228114 discloses a gas burner in which the burner ports are narrow grooves in stacked plates, the objective of the invention in this patent being to provide for a flame distribution in a trough formed in the gas burner. The provision of narrow circular ports formed by the grooves provides too much of a differential between the port area of the flame strip and the non-port area so much so that this burner would suffer from burning inefficiency and would not be suitable for fully premixed operation.

US patent 100995 discloses a gas burner in which specially designed burner bars are stacked face to face, the idea being to create a certain degree of turbulence as the fuel/air mixture passes between the burner bars, but again the fabrication of the burner bar elements represents a difficulty because of their complicated shape.

In US patent 3874599 the burner bar is provided by slicing a burner plate which is provided with circular ports, so that the section of cutting is through the ports. Respective parts of the plate can then be moved relatively in order vary the port dimensions. This form of burner is quite unsuitable for fully premixed operation, because the burner port area of the entire flame strip is too small.

European patent application 0311462 discloses a burner which comprises a block of ceramic material provided with slots therein, but again these slots are of a particular convergent parallel configuration which results in difficui *→y of manufacture of the ceramic block. In any case such a block suffers from thermal stressing which can lead to cracking as explained in the specification.

The present invention is concerned with the provision of a fuel fired burner wherein a flamestrip is provided and it is made up of a number of elements which are of relatively simple construction and are stacked in a particular configuration so that quickly simple and effective flamestrips can be produced and which are design features at least in the preferred forms of the invention giving rise to enhanced operation and overcoming the difficulties referred to herein.

According to one aspect of the present invention, there is provided a fuel fired burner comprising a flame strip body in which burner ports are formed, wherein the body comprises plate elements stacked together face to face, said plate elements having enlarged ends whereby, by bringing of the elements together, there are formed between the elements slots which extend through the body and form burner ports, characterised in that each of said plate elements and therefore each of the burner ports is of constant cross section.

This aspect of the invention provides a particularly simple and suitable construction for the flamestrip body, and it lends itself particularly to fully premixed burners insofar as the ports can be arranged to be elongated passages or "deep" passages, which preferably are in the form of slots of constant cross section. It will also be readily understood that such an arrangement of stacked elements may well be suitable for forming the flamestrip of a partially premixed burner.

It is preferred that the plate elements be formed of heat resistant ceramic material, and it is also preferred that they should be one piece elements. If they are made of identical shape and dimension, then the formation of a flame strip body by stacking the plates in parallel arrangement is particularly simple.

The plate elements may have a cross sectional shape of that of a dog bone and may comprise enlarged ends connected by a slender partition wall. When such plate elements are stacked together to form the flame strip body, elongated and deep burner ports result.

It is preferred that the ratio of the area of the ports to the overall flamestrip area is as high as possible (up to 48%), and a preferred aspect ratio for the burner ports or burner port slots be in the order of 5 to 1 or greater. That is to say the depth of the ports is in the order of five times the width of the port, or greater.

Of each burner element, it is preferred that the partition wall should extend between the enlarged ends in an angled manner, said ends being rectangular, so that the partition wall extends from one side of one face of one of the ends to the other side of the opposite face of the other end so that by placement of two of said elements side by side one way will produce a burner port in the f -πn of a slot with parallel sides and placing the two elements together another way will produce a burner port in the form of a slot having sides which taper towards each other along the length of the slot. A particularly effective configuration therefore results, because the slot configuration can be adjusted by placement of the elements in appropriate dir- osition. In a preferred case, the elements are arranged so that the slots are tapered, but alternate slots are angularly offset by 180°. This arrangement has the advantage that the resulting flame will have an uneven top surface making it less susceptible (more resistant) to the production of resonant noise, as will be explained in more detail hereinafter. It should be mentioned however that it is simplest to produce plate elements of constant cross section which have a central axis of symmetry and although it is not possible to provide tapered cross sectional slots using these elements, they do have the advantage that the stacking of them together is quicker because they do not have to be stacked in any particular angular dispositions.

It is possible to have plate elements of different width or height so that when the elements are stacked, some of the elements stand proud of other elements and if the large and small elements are arranged alternately in the stack, then the individual flames of the burner flame can be contained between pairs of the taller elements in an effective manner.

The invention therefore provides that plate elements are stacked to form the flamestrip and the ports, and in particularly advantageous arrangements, the plates are of the particular configuration above described.

Instead of the plate elements having recesses on both sides there may be a recess on one side only of each element; also each element may be thick enough to accommodate one or more slots which will form one or more of the burner ports in the finished burner.

The concept of providing that the elements are blocks can be extended to the construction that there are no side recesses and in such case each block may have one or more slots, the blocks when assembled being grouped face to face to form the flame strip. Thus also according to the invention there is provided A fuel fired burner comprising an elongated flame strip body in which burner ports are formed, wherein the body comprises block elements stacked together and held in a frame, said block elements having slots therethrough which form burner ports characterised in that the blocks are held so that the slots extend from side to side of the flamestrip body.

Where the flame ports are tapered from one side of the flamestrip to the other end alternate slots are alternately tapered, this produces a non-uniform discharge velocity profile of the gas/air mixture from each slot and along its length, the velocity being greater at wider end compared with that at the narrow end, because the resistance to flow through the port is proportional to port width. Because of this velocity profile, the flame takes up a position with respect to the flamestrip surface which is progressively further away from the flamestrip surface along the length of the port. However, excessive flame lift is prevented by the stabilising influence of the flame at the narrow end of the tapered port, and also by the mutual stabilisation effect on the lifted end of the flame by the more stable flames on the narrow ends of the adjacent ports. The resulting flame profile has advantages in that a well-stabilised fully premixed flame can be produced with a minimum of heat transferred back to the flamestrip, thus minimising the aforementioned problems associated with burner overheat.

Because each flame of each port is of varying height due to varying lift the overall flame surface is irregular which has beneficial effects on the susceptibility of the burner to combustion resonance, in that the pathway for the transmission of pressure waves down through the flame and flamestrip ports and back into the fuel/air supply systems can be eliminated. Thus, forming the flame to have an irregular surface has the effect of breaking of the transmission link between conditions downstream of the flame in the combustion chamber, the flame itself, and the fuel/air supply systems upstream of the flamestrip ports.

The flamestrip body elements can be manufactured by several methods and in any of a number of different materials, notably ceramics. Several flamestrips can be arranged side by side to produce the required heat output from the burner. Thus, the ports are formed from individual pieces of material which have been, for example, cut or, if in ceramic, powder pressed or moulded from a slurry, and assembled to form the port array within a suitable framework or holder, thus offering a degree of versatility in the specification of the size and geometry of the flamestrip. An advantage of producing the flamestrip in elements is that certain physical stresses experienced by the material when produced in an integral block of material would not arise particularly when the burner is subjected to high operating temperatures.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings, wherein;-

Fig. 1 is a perspective view showing a gas burner constructed to embody the present invention;

Fig. 2 is a plan view of a section of the flamestrip of the burner of Fig. 1;

Fig. 3 is an enlarged perspective view showing how the burner elements of the flamestrip of Fig. 2 can be alternately arranged;

Fig. 3A is a view similar to Fig. 3, but on a reduced scale showing a different form of burner element;

Fig. 3B - 3G are plan views showing further forms of burner element;

Fig. 4 is a view somewhat similar to Fig. 1 but showing a burner having two flamestrips arranged side by side;

Fig. 5 is a perspective view of an alternative form of burner construction but embodying a flamestrip similar to that shown in Fig. 2;

Fig. 6 is a sectional view of the burner shown in Fig. 5;

Fig. 7 shows in perspective view an alternative form of flamestrip construction using plate elements;

Fig. 8 is a perspective view of press mould parts for producing the plate elements of Fig. 3A; and

Fig. 9 to 12 are sectional views through the mould parts of Fig. 8 when assembled and show the steps involved in pressing an element.

Referring to the drawings, a gas fired burner 10 is in the form of an individual burner blade which is provided with plenum chamber 12 one side of which is defined by the burner flamestrip 14 which is an elongated body having burner slots or ports 16 which are in the form of slots extending transversely of the length direction of the flamestrip 14.

The flamestrip 14 is held in a casing 18 which defines the plenum chamber, and at the end 20 of the casing 1& conduit 22 provides for the supply of a mixture of air and gas to the interior of the plenum chamber. This mixture will in due course issue from the slots 16, and the issuing gas air mixture is ignited to form the flame on the flamestrip 14 in accordance with known principles of operation. The air and gas or both are supplied under pressure.

In this case the burner 10 is a fully premixed burner and therefore the mixture of air and gas supplied through conduit 22 contains the gas and air in sufficient proportions to support theoretically complete combustion of the gas.

The design of the flamestrip 14 provides for the effective operation of the burner, and if reference is now made to Figures 2 and 3 the flamestrip construction is described in greater detail.

As shown in Figure 2, the flamestrip 14 is made up of flamestrip plate elements 14A to 14G as shown and as many of these elements as are required to form the requisite length of flamestrip will be utilised. The elements 14A to 14G are plate elements and are stacked face to face. The faces of the elements or the sides having regard to the manner of use of the elements, are profiled so that when the elements are brought together as shown, there will be formed therebetween the burner slots 16, indicated in Fig. 2 by references 16A to 16F.

The elements 14A to 14G are identical, and they are preferably formed in ceramic material so as to withstand the heat which they will experience during operation of the burner. The elements may be powder pressed (as hereinafter described) or moulded from a suitable slurry by conventional ceramic article manufacturing techniques.

The elements are constructed so as to be used in a versatile manner, and if reference is now made to Fig. 3, the construction of each element will be readily understood. Fig. 3 illustrates two of elements 14B and 14C from the stack of elements shown in Figure 2. An additional element referred to as 14X is also shown in Fig. 3 for the purpose to be explained hereinafter.

Considering the element 14C shown in Fig. 3 in detail, the element is of constant cross section, and the cross sectional shape is that of a dog bone insofar as the element has enlarged ends 24, 26 of rectangular cross section, and these ends are arranged facing each other with the sides 24A and 26A in a common plane, and sides 24B and 26B in a parallel common plane. The ends 24 and 26 are connected by a slender partition wall 28 which so angled between the opposing faces of the ends 24 and 26 so as to extend from one side of one of said faces 24C to the other side of the other of said faces 26C.

By adopting this configuration, when a pair of elements 14B and 14C are brought together as shown in Fig. 2 and in Fig. 3, the resulting slot 16B is of tapered configuration in a direction across the flamestrip from enlarged end 24 to enlarged end 26.

However, if an element such as element 14X is placed adjacent element 14C in the same configuration as element 14C, the partition wall 28 of element 14X will lie parallel to partition wall 28 of element 14C, and the resulting slot 16X will have its sides parallel.

Reverting to Figure 2, in the arrangement shown, the elements 14A to 14G are arranged so that tapered slots are formed between adjacent elements, but alternate slots taper in opposite directions which as explained herein has the effect of producing a flame across the flamestrip which has an uneven upper surface. This arises in that each flame which stands upon the tapered slot is more lifted at the wider end of the slot than at the narrower end. The advantages previously described herein result from such an arrangement, but it is to be mentioned that it is not necessary to the invention that the slots be configured in this way. The elements could be arranged as shown by 14C and 14X so that all of the slots are parallel sided, or instead of having all the slots either tapered or parallel sided, variations can be achieved by appropriate positioning of the elements either as indicated by elements 14B and 14C or elements 14C and 14X as shown in Fig. 3. By so arranging the elements so the shape of the upper surface of the flame across the strip can be varied.

The element construction illustrated and described provides that the slots are relatively deep and preferably they will be of a depth equal to five times (or greater) the slot width of the maximum slot width in order to achieve good flame characteristics.

As shown in Fig. 3, where the partition walls 28 meet the faces 24C and 26C, the elements are radiused so that the corners of the resulting slots will be rendered curved. This serves to reduce physical stresses which may be experienced by the material as a result of heating. This also minimizes the formation of hot spots in the material surrounding these corners during the operation of the burner.

The slots and elements will be dimensioned to suit the particular application, but for fully premixed burners, it is preferred that the total slot area to the face area of the flamestrip, should be as high as possible (30-40%).

Although not to be considered as specifically limiting the scope of the invention, typical examples of dimensions for the elements, of Fig. 3, are as follows:-

Length of each slot = 30 mm

Width of wide end of tapered slot = 1.5 mm

Width of narrow end of tapered slot = 0.5 mm

Width of constant cross section slot = 1.0 mm

Width of wall 28 = 2.0 mm

Width of ends 24,26 = 3.0 mm

Overall length of each element = 38 mm

It will be appreciated that in any flamestrip construction and burner operation, there has to be adequate provision for satisfactory ignition and cross lighting of the fuel air mixture, together with adequate margins to accommodate the requirements of relevant combustion and safety standards, in particular those relating to flash back or light back of the flame into the fuel air mixture in the plenum chamber. Therefore, close attention must be paid to the choice of the maximum width for the flame port slot, and the width of the partition wall 28 is important in ensuring mutual flame stability between flames from adjacent slots.

Although there are advantages in using the elements configured as shown in Fig. 3, these elements do present a drawback that care has to be exercised in the stacking of the elements together to ensure that the elements form the desired slot pattern. Thus, advantage may be had by using plate elements 14Y shown in Fig. 3A, which are of symmetrical form about their central planes, and the partition 28Y meets the ends 24Y and 26Y centrally, ^' The elements 14Y are also preferably of constant cross section and in all other respects are the same as the elements 14A-14G. The stacking order and disposition of elements 14Y are not critical and stacking will therefore be quicker and easier to automate.

Referring now to Figures 3B to 3G, these figures respectively show different cross sectional shapes for the elements 14 to provide alternative embodiments of the invention. Some of the elements disclosed herein have somewhat deviated from what one might refer to as a plate construction, and they are more similar to locks, but the expression "plates" has been retained for simplicity of description. However, because some of the elements can have a length greater than the width they will be more similar to blocks. The basic principle of the invention however still applies insofar as a plurality of these plates or blocks are used to make up the final flame strip.

In Fig. 3B, an element is shown wherein the sides are recessed similar to the plate elements shown in Fig. 3, except that the element of Fig. 3 is in essence two elements such as elements 14B and 14C formed as a single unit and thereby defining a central slot 100 of tapered configuration.

In the arrangement of Fig. 3C, the element is in fact two elements similar to those shown in Fig. 3A but formed again as a single unit thereby to define a single central slot 100 which also in the finished flamestrip forms a burner port.

In the arrangement of Fig. 3D, a single element has a side recess so that it is of U-configuration as shown. When similar elements are stacked together in the same disposition as the element as shown in Fig. 3D, then a flamestrip having the same characteristic burner slot shape as in Fig. 3A is provided. Fig. 3A shows that a block type element can be provided with a single slot 100 as well as the side recess as in the case of Fig. 3D.

In the arrangement of Fig. 3F, there are no side recesses, but the block 14 has a single central slot 100 which provides a burner port. When similar blocks are stacked side by side, so a row of parallel and similar burner slots is provided. The side walls of the block shown in Fig.3F are half of the required land width between slots.

In the arrangement of Fig. 3G a double block of the form shown in Fig. 3F is provided so that two slots 100 which form burner ports exist in the block, and the central partition is twice the thickness of each of the outer walls.

In the embodiments described herein, the block elements are shown as being of constant cross section, but they could be of varying cross section in a direction through the resulting flame strip.

Although the blocks shown in Figures 3C-3G envisaged the slots 100 in the flamestrip being of equal width, it is not necessary that this should be so nor is it necessary the the slots should be equally spaced throughout the length of the flamestrip.

All of the elements and blocks described herein can be manufactured from ceramic material by a pressing operation described in more detail in relation to one embodiment in Figs. 8 to 12.

Figures 4, 5 and 6 show how the flamestrips described may be embodied in different types of burners. In the arrangement of Fig. 4, the plenum chamber casing 18 is large enough to accommodate and does accommodate two flamestrips 14 in parallel disposition. Each of the flamestrips 14 may be as described hereinbefore. The appropriate mixture of air and gas is supplied to the plenum chamber through end 20 although the supply means is not shown.

The burner disclosed in 5 and 6 comprises a divergent plenum chamber defined by a pair of side plates 30, 32 to the top and wider end which plenum chamber is provided a flamestrip 34 of the construction as hereinbefore described.

The sides 30 and 32 are convergent divergent as shown in Fig. 6 so that the lower ends 34,36 diverge to define a mouth 38 into which gas is injected from a header bar 40. The arrangement is such that the air is drawn in as indicated by arrows 42 and 44 into the aperture throat 38A along with the gas in sufficient quantity to provide for, theoretically, complete combustion of the gas so that the burner is of the fully premixed type.

The side plates 30 and 32 extend between a pair of end plates 46 and 48 which also serve to support the header 40 which has mounting brackets 50 and 52 by which the header is connected to the side plates 46 and 48 by bolts or the like (not shown) .

Instead of using a header 40 with injection nozzles 54 as shown for the gas, a pipe having a plurality of apertures along one side thereof may be used as an alternative with the apertures facing the mouth 38.

This construction of burner, regardless of the flamestrip which is used, constitutes in itself a novel arrangement and the applicant reserves the right to claim protection therefore in the embodiment illustrated, and also in various alternative forms which may be provided.

It will be appreciated that the gas is jetted from the header 40 or the alternative pipe arrangement at sufficient velocity to enable the necessary high levels of primary air to be entrained into the plenum chamber and mixed with the fuel.

Figure 7 shows that it is not necessary that the plate elements be of identical construction, and in some cases as in the case of Figure 7, improvements can be achieved by using differently configured elements.

In the arrangement of Figure 7, the elements are of the same length and have the same cross section, but alternate elements of the series 14A to 14J are taller than the other elements of the series. The advantage of this arrangement is that each pair of adjacent tall elements for example 14A and 14C in having a smaller element 14B stacked therebetween enables the enclosure of the flames from the adjacent slots formed on the one hand between the elements 14A and 14B and on the other hand between elements 14B and 14C in a confined manner. By so confining the flames, in effect in a tunnel,has a beneficial effect on the overall flame stability and has the additional benefit of reducing the susceptibility of the flame to cc ibustion resonance when fired within a compact combustion chamber because the enclosing walls around each section of flame effectively shields the flame from acoustic pressure waves which are generated during a combustion resonance.

It will be understood furthermore that the configuration of the elements can be varied as required, and elements of different length as well as elements of different width may be used. Also the cross sectional shape of the elements may be varied to suit the slot configuration required and indeed the resulting ports may be adapted to be circular or of other form rather than elongated as illustrated and described.

As regards the attachment of the stack of elements to the plenum casing, any suitable arrangement may be adopted. For example the ends of the elements may be provided with shoulders or grooves onto or into which edges of the plenum chamber casing may be bent when as is usual the plenum casing is formed of sheet metal.

A particular advantage of using plate elements stacked together in that burners of different lengths can be created easily by using different numbers of plate elements. Production methods are therefore simplified.

It is of advantage that the plate elements are in one piece, again for simplication of burner construction and assembly, and especially when the elements are solid powder pressings of ceramic material.

Fig. 8 to 12 show a method of producing the elements of the symmetric construction shown in Fig. 3A.

Fig. 8 shows the main parts of a press which comprise a press block 90 of solid steel having the mould cavity 92 therein. The cavity 92 is a slot 92 of constant cross section which is the same as that of the elements 14Y to be produced thereby. The slot 92 extends through the block 92 so as to be able to receive upper 94 and lower 96 die plates which are of the same cross section and which are arranged to be pushed into and pulled out of the slot 92 from above and below as shown by the arrows in Fig. 8. In Fig. 9 which shows the starting position of the pressing cycle, the lower die plate 96 hes penetrated the die slot 92 to lie in a lower initial position. The αpper die plate 94 is retracted. The remainder of the die slot 92 has been filled (by means of a suitable filling/vibrator apparatus - not shown) with the ceramic powder 98 which is to be pressed into a plate element 14Y (Fig. 3A) . In the pressing stage as shown in Fig. 10, the upper die plate 94 is moved down into the die slot 92 as shown and the lower plate 96 is moved up providing an intense compaction of the powder 98 so that it becomes a coherent, if fragile mass in the shape of an element 14Y. This is then ejected by raising the upper plate 94 clear of the mould block 90 and by raising the lower plate 96 until the element 14Y is clear of the die block 90, then it is brushed away, gently, to a collecting chute. The lower die plate 96 returns to the initial position to complete the cycle. The so formed element 14Y is then fired to render it rigid, in known manner. This method of production, with appropriately shaped dies can be used for any of the elements (blocks) described herein.

Claims

1. A fuel fired burner comprising a flame strip body (14) in which burner ports are formed, wherein the body (14) comprises plate elements (14A-14Y) stacked together face to face, said plate elements (14A-14Y) having enlarged ends (24,26) whereby, by bringing of the elements together, there are formed between the elements (14A-14Y) slots (16A-16Y) which extend through the body (14) and form burner ports (16A-16Y), characterised in that each of said plate elements (14A-14Y) and therefore each of the burner ports (16A-16Y) is of constant cross section.

2. A fuel fired burner according to claim 1, characterised in that each plate elements comprises enlarged ends (24, 26) and a partition (28) connecting the ends (24, 26).

3. A fuel fired burner according to claim 2, characterised in that the partition (28) is of constant thickness.

4. A fuel fired burner according to claim 3, characterised in that the thickness of the partition (28) is in the order of 2.0 mm.

5. A fuel fired burner according to claim 2, 3 or 4, characterised in that the partition (28) is angled relative to the central plane of the plate element (14) .

6. A fuel fired burner according to claims 2, 3 or 4 characterised in that the element is symmetrical about the central plane of the partition (28).

7. A fuel fired burner according to claim 1, characterised in that each plate element (14) has one or more slots (100), such forming a burner port.

8. A fuel fired burner comprising an elongated flame strip body (14) in which burner ports are formed, wherein the body (14) comprises block elements (14A-14Y) stacked together and held in a frame, said block elements (14A-14Y) having slots (100) therethrough which form burner ports (16A-16F) characterised in that the blocks (14A-14Y) are held so that the slots (16A-16Y) extend from side to side of the flamestrip body (14) .

9. A fuel fired burner according to claim 8, characterised in that the block elements (14A-14Y) have flat end faces.

10. A fuel fired burner according to any preceding claim, characterised in that the burner ports (16A-16Y) are spaced evenly throughout the flamestrip body and are parallel to each other.

11. A fuel fired burner according to claim 10 characterised in that the burner slots are spaced by the order of 3mm to 3.5mm.

12. A fuel fired burner according to any preceding claim, characterised in that the total burner port area is in the order of 30-40% of the flamestrip body area.

13. A fuel fired burner according to any preceding claim, characterised in that each burner port has a width or average width of the order of 1-1.5mm.

14. A fuel fired burner according to any preceding claim, characterised in that each burner port has a length in the order of 30 mm.

15. A fuel fired burner according to any preceding claim. ceramic material.

16. A fuel fired burner according to any preceding claim, characterised in that each element is of one piece solid member.

17. A fuel fired burner according to any preceding claim, characterised in that all the plate or block elements (14A- 14Y) in the stack perhaps with the exception of the end plates are identical.

18. A fuel fired burner according to any preceding claim, characterised in that alternate plate or block elements (14A- 14Y) in the stack are of a first taller height, and the others are of a second smaller height.