GB2126331A - Burner and method of burning a fuel - Google Patents

Abstract

In a recuperative burner, the wall through which heat is conducted between hot products of combustion and combustion air is provided near to the combustion chamber with fins (39,40) disposed in the air passage. Further from the combustion chamber, the wall has no fins. <IMAGE>

Description

SPECIFICATION Burner and method of burning a fuel From one aspect, the present invention relates to a burner of the kind, hereinafter called the kind specified, defining a fuel passage along which, in use, fuel flows towards a combustion chamber, an air passage along which, in use, air flows towards the combustion chamber, an exhaust passage along which, in use, hot products of combustion flow from the combustion chamber and defining an exhaust inlet to the exhaust passage from the combustion chamber and which burner comprises a wall structure separating the air passage from the exhaust passage and through which wall structure, in use, heat flows from the hot products of combustion to the air.

The wall structure is required to transmit heat and is usually formed entirely of metal. However, attempts to use the fuel more efficiently result in a higher temperature of the products of combustion which pass through the exhaust inlet into the exhaust passage. Metal wall structures subjected to higher temperatures in this way tend to fail and it has therefore been proposed that a part of the wall structure nearest to the exhaust inlet should be formed of a ceramic material better adapted to withstand high temperatures than is the metal which has been used heretofore. However, ceramics are generally less satisfactory conductors of heat than are metals and this proposed modification of burners of the kind specified will impair the transfer of heat from the hot products of combustion to the air in the air passage.Ceramics of sufficiently thin section to give satisfactory rates of heat conduction are difficult and costly to fabricate and are fragile once fabricated.

According to a first aspect of the invention, a first portion of said wall structure of a burner of the kind specified, which portion is adjacent to the exhaust inlet, is better adapted to transmit heat to gases in the air passage than is a further portion of the wall structure, which further portion is spaced further along the exhaust passage from the exhaust inlet than is the first portion.

With this arrangement, the cooling effect of the air on the first portion of the wall structure can be greater than the cooling effect of the air on the further portion of the wall structure, so that the temperature achieved by the first portion is lower than would be the case if ail parts of the wall structure were similarly adapted to transmit heat.

Alternatively expressed, the invention provides a burner of the kind specified wherein a first portion of said wall structure adjacent to the exhaust intet is so formed or constructed that at its relatively colder side facing the air passage, it presents a greater surface area per unit length of the air or exhaust passage than it presents at its relatively hotter side facing the exhaust passage.

The ratio of surface area per unit length presented by said wall structure, at its relatively colder side to that presented at its relatively hotter side, may be greater in respect of said first portion of the wall structure than in respect to the succeeding further portion situated downstream of the exhaust passage.

The first portion of the wall structure is preferably adapted to transmit heat by the provision on the first portion of a plurality of fins which extend into the air passage.

According to a further aspect of the invention, there is provided a method of burning a fuel wherein air is fed along an air passage to a combustion chamber, fuei is fed along a fuel passage to the combustion chamber, the fuel and air are burned in the combustion chamber, products of the combustion are led from the combustion chamber along an exhaust passage in counter-current flow with respect to the flow of air in the air passage, heat is transferred from the products of combustion in the exhaust passage to air in the air passage via heat transfer means, and the ratio of the rate of heat transfer to air in the air passage from the relatively colder side of a first part of the heat transfer means near to the downstream end of the air passage to the rate of heat absorption from the exhaust gases at the relatively hotter side of the heat transfer means is higher than the corresponding ratio subsisting in respect of a further part of the heat exchange means near the upstream end thereof, so that the temperature of the first part of the heat exchange means is kept lower than would be the case were the two ratios to be equal.

An example of a burner embodying the first aspect of the invention and which can be used in a method according to the second aspect of the invention, will now be described, with reference to the accompanying drawings wherein Figure 1 shows diagrammatically a cross-section of the burner in a plane containing an axis of the burner; and Figure2 is a graph in which the temperature of the heat exchange means separating the exhaust gas and combustion air passageways is plotted against distance from the exhaust inlet.

The burner comprises a structure 10 formed mainly of metal and defining a number of co-axial passages, the axis of which is indicated at 11.

Adjacent to one end, the structure 10 includes a flange 12 by means of which the burner can be mounted on the wall of a combustion chamber (not shown) in the usual way. The burner further includes an annular structure 13 of ceramic materials defining a central space 14through which the axis 11 extends and an annular passage 15 which is separated from the space 14 by a ceramic wall 16 and has open ends facing along the axis 11. The radially outer boundary of the passage 15 is defined by a further wall 17 of ceramic material. Blocks 18 of ceramic material positioned in the annular passage 15 and spaced from each other circumferentially of the axis 11 maintain the required positional relation of the walls 16 and 17.When the burner is mounted on the wall of a combustion chamber, the space 14 forms a part of the combustion chamber and the annular passge 15 communicates with the combustion chamber.

The structure 10 of the burner defines a central passage 19 through which the combustion chamber can be viewed during operation of the burner.

Around the passage 19, there is an annular fuel passage 20 with an outlet opening 21 leading radially outwardly from the fuel passage into the space 14. A fuel inlet 22 communicates with an end portion of the fuel passage 20 remote from the space 14.

Between the fuel passage 20 and the annular passage 15, there are defined a number of air outlets 23 from an air passage 24 through which air flows, as shown by the arrows, from an air inlet 25. An exhaust passage 26 leads from the annular passage 15, an end of which remote from the combustion chamber constitutes an exhaust inlet to the exhaust passage 26, to an exhaust outlet 27. A first part 28 of the exhaust passage 26 is annular and co-axial with the fuel passage 20. A second part 29 of the exhaust passage 26 is annular, perpendicular to the axis 11 and leads in a direction away from the axis 11 from one side of the first part 28. The second part 29 of the exhaust passage is surrounded by an annular first part 30 of the air passage 24.A second part 31 of the air passage is annular, is co-axial with the fuel passage 20 and leads from the first part 30 ofthe air passage in a direction towards the combustion chamber to openings 32 into a third part 33 of the air passage which is also annular and lies between the second part 31 of the air passage and the first part 28 of the exhaust passage. At a position remote from the combustion chamber, the third part 33 of the air passage communicates with a fourth annular part 34 of the air passage, this fourth part lying adjacent to the radially inner boundary of the first part 28 of the exhaust passage and communicating with the air outlets 23.

Between the fourth part 34 of the air passage and the fuel passage 20, there is an annular space 35 through which there extend to positions adjacent to the air outlets 23 devices 36 used in operation and control of the burner, for example a pilot burner, radiation-sensing devices and one or more further observation ports.

The wall structure which separates the exhaust passage 26 from the third and fourth parts 33 and 34 of the air passage is formed of metal, for example stainless steel, and includes a radially inner wall 37 and a radially outer wall 38. Respective first portions 37a, 38a of these walls which lie near to the annular passage 15 through which exhaust gases enter, are adapted by the provision of fins 39, 40 from their relatively colder faces to transmit heat to air in the air passage 24 more effectively than portions 37b, 38b of the walls 37 and 38 spaced further from the annular passage 15 absorb heat at their relatively hotter faces from the exhaust gases.The inner wall 37 has a number of fins 39 which extend from the wall into the fourth part 34 of the air passage and which extend from the end of the air passage adjacent to the air outlets 23 in a direction along the axis 11 for approximately one third of the length of the fourth part of the air passage. Fins 40 of substantially the same length of the outer wall 38 project into the third part 33 of the air passage and extend therealong from an end thereof adjacent to the annular passage 15.

When the burner is in use, fuel and air emerge from the outlets 21 and 23 respectively into the space 14, where they mix and burn, the combustion continuing into a part of the combustion chamber adjacent to the space 14. Hot exhaust gases flow from the combustion chamber through the annular passage 15 into the first part 28 of the exhaust passage. The temperature of the exhaust gases, as they enterthe exhaust passage, is high. Heat is absorbed from exhaust gases at the faces of the portions 37a, 38a of the walls 37 and 38 and the exhaust gases flow onwards to the second part 29 of the exhaust passage, where further heat absorption takes place transferred to faces of the walls 37b and 38b.

Since the exhaust gases are in counter-current flow with respect to the air, air which approaches the air outlets 23 is hot, and the more effectively the air is heated, the smaller the temperature difference between the wall 37 and air adjacent thereto.

Nevertheless, a high rate of heat transfer from the wall portions 37a, 38a which bears the fins 39,40 respectively to the air is maintained by these fins, so that the temperature of the wall portion 37a is kept lower. The ratio of the surface area taking into account the fins 39 at the relatively colder side of the first portion 37a facing passage 34, to the ratio of surface area at the relatively hotter side facing passage 38, is higher than the corresponding ratio at the upstream portion of the passage 38 bounded by wall portions 37b, 38b. The ratios of the corresponding rates of heat transfer at the surfaces are similarly, although not necessarily, proportionally related.The improved ratio of heat transfer provided by the fins 30 and 40 does tend to increase the temperature difference between incoming exhaust gases and the wall portions 37a, 38a, but the cooling effect of the fins more than compensates for this increased temperature difference and does not prejudice the attainment of reduced temperature over the wall portions 37a, 38a. Thus, the walls 37 and 38 can be formed of metal.

It will be noted that the exhaust passage is of convergent form from the inlet and has its greater cross-sectional area in the vicinity of the inlet. Thus, exhaust gas velocity is lowest in this region and this contributes to relatively thick thermal boundary layers of exhaust gas at the relaively hot side of the portions 37a and 38a of the walls 37, 38. This assists in establishing the reduction of temperature described in respect of the wall portions 37a, 38a.

Figure 2 shows graphically in curve Athe operating temperature profile which would exist in the wall 37 in the absence of the fins 39 and 40 and in curve B the operating temperature when the fins are provided, both being plotted against distance upstream from the exhaust inlet.

Claims (9)

1. A burner of the kind specified wherein a first portion of said wall structure adjacent to the exhaust inlet is better adapted to transmit heat to gases in the air passage than is a further portion of the wall structure, which further portion is spaced further along the exhaust passage from the exhaust inlet than is the first portion.

2. A burner of the kind specified wherein a first portion of said wall structure adjacent to the exhaust inlet is so formed or constructed that at its relatively colder side facing the air passage, it presents a greater surface area per unit length of the air or exhaust passage than it presents at its relatively hotter side facing the exhaust passage.

3. A burner according to Claim 2 wherein the ratio of surface area per unit length presented by said wall structure at its relatively colder side to that presented at its relatively hotter side is greater in respect of said first portion of the wall structure than in respect of the succeeding further portion situated downstream of the exhaust passage.

4. A burner according to any one of Claims 1 to 3 wherein, at its side facing the air passage, the first portion of the wall structure has a plurality of fins which extend into the air passage.

5. A burner according to any one of Claims 1 to 4 wherein the exhaust passage, at least adjacent to the exhaust inlet, said wall structure comprises inner and outer walls defining respectively radially inner and radially outer boundaries between the exhaust passage adjacent to the exhaust inlet, a plurality of fins project radially inwardly from the inner wall and further fins project radially outwardly from the outer wall into respective portions of the air passage situated respectively inwardly and outwardly of the exhaust passage.

6. A burner according to Claim 5 wherein the exhaust gas inlet is spaced radially from both the inner and outer walls of the exhaust passage.

7. A method of burning fuel wherein air is fed along an air passage to a combustion chamber, fuel is fed along a fuel passage to the combustion chamber, the fuel and air are burned in the combustion chamber, products of the combustion are led from the combustion chamber along an exhaust passage in counter-current flow with respect to the flow of air in the air passage, heat is transferred from the products of combustion in the exhaust passage to air in the air passage via heat transfer means and wherein the ratio of the rate of heat transfer to air in the air passage from the relatively colder side of a first part of the heat transfer means near to the downstream end of the air passage to the rate of heat absorption from the exhaust gases at the relatively hotter side of the heat transfer is higher than the corresponding ratio subsisting in respect of a further part of the heat exchange means near the upstream end thereof, so that the temperature of the first part of the heat exchange means is kept lower than would be the case were the two ratios to be equal.

8. A burner substantially as herein described with reference to and as illustrated in the accompanying drawing.

9. Any novel feature or novel combination of features disclosed herein and/or in the accompanying drawing.