GB2071839A

GB2071839A - Burner with fan-assisted premixing of air and gas

Info

Publication number: GB2071839A
Application number: GB8108303A
Authority: GB
Original assignee: Saunier Duval Eau Chaude Chauffage SDECC SA
Current assignee: Saunier Duval Eau Chaude Chauffage SDECC SA
Priority date: 1980-03-17
Filing date: 1981-03-17
Publication date: 1981-09-23
Also published as: DE3110301C2; FR2478265A1; IT8120373A0; DE3110301A1; ES269625U; ES269625Y; FR2478265B1; IT1136937B; GB2071839B

Abstract

A cylindrical burner with fan- assisted premixing of air and of fuel-gas is in the form of a tubular barrel (1) having flame outlet openings (3) obtained by punching of the metal and folding back of the edges of the opening to form radially outwardly extending projections or teeth (4) to form punchings spread along the whole periphery of the tubular barrel (1). A baffle (20) is arranged within the tubular barrel (1) to transform the iongitudinal motion of the gaseous mixture into a uniform radial motion. <IMAGE>

Description

SPECIFICATION Burner with fan-assisted premixing of air and gas The present invention relates to a burner with fan-assisted air/fuel-gas premixing for domestic hot water central heating boilers of the "room sealed" type.

This type of burner frequently consists of a perforated cylindrical element, closed at one end and having the other open for the introduction of a fuel-gas and air mixture under pressure imposed by an upstream fan. In these known burners, the mixing rate in the element is not constant, and the static pressure outside the element, while the air/fuelgas mixture is being burned, is not uniform.

Consequently, the flow of the mixture through the perforations, as well as the height of the flames clinging to the periphery of the cylinder, are not uniform.

Moreover, these burners always function with a certain excess of air over a stoichiometric ratio which diminishes the rate of combustion and increases the possibilities of the flame blowing out, in particular in the case of fuel gases whose inherent combustion rate is very low.

It will thus be seen that for this type of burner, it is necessary to make provision, on the one hand, for a device which will make it possible to maintain a constant rate of flow of the fuel-gas/air mixture within the burner in order to establish a uniform static pressure outside the wall where the gas/air mixture is being burned and, on the other hand, for a flame attraction device.

For this purpose, it is known to provide a burner with a premixing stage which consists of a tubular barrel, pierced by smooth holes evenly distributed over the whole periphery of this barrel and within which barrel there is placed a fine mesh grating for attracting the flames. Moreover a cylindrical casing, pierced by holes distributed all over its length, is also positioned within the tubular barrel for a regular distribution of the fuel-air/gas mixture.

This burner is thus extremely complicated and expensive, for its requires the use of three concentric elements. Moreover, it is not possible to obtain the same horizontal velocities for the fuel-gas/air mixture at the top, and bottom parts of the burner using a cylindrical casing whose holes have the same diameter at the top and at the bottom.

There is also known a burner formed by a perforated metal sheet, arranged in the form of a cylinder closed at one end and fed with the fuel.gas/air mixture via its other open end. A conical box, whose tip is orientated towards the mixture intake at the base of the burner, is placed in the cylinder to maintain a constant ratio for the mixture. For this conical box to be effective, it is necessary that the flow of the combustible mixture between the cylinder and the cone should be uniform. It is therefore necessary to create a considerable drop in pressure which can only be achieved by an additional filter arranged at the cylinder intake.

Moreover, the perforations in the cylinder are as small as possible in order to avoid a blow-back of the flames; but these perforations must nevertheless be sufficiently great to avoid the flame blowing out, particularly in the case where fuel-gases are used which have a very low rate of flame propagation.

The object of the invention is to provide a cylindrical burner with fan-assisted air and fuel-gas premixing, which avoids the abovementioned drawbacks in that it facilitates an excellent attraction of the flames and an even distribution of the gaseous mixture in the burner whilst diminishing the number of components constituting this burner.

According to the present invention, we provide a cylindrical burner with fan-assisted premixing of air and fuel-gas comprising a tubular barrel provided with evenly distributed flame outlet openings, having their edges extending radially outwardly to form with the said openings punchings; and a perforated frusto-conical baffle arranged within the tubular barrel, wherein the said edges of each opening of the flame outlets form irregular projections which are variable in height and which are orientated differently from one row to another to facilitate the clinging of the flames thereon, and the frusto-conical baffle is perforated along all its periphery and its length to transform from its base to its top the longitudinal motion of the gaseous mixture into uniform radial motion.

The frusto-conical permeable baffle within the tubular barrel may, for example, be formed of metal foil, and may have its smallest diameter nearer the closed end of the burner barrel and its largest diameter open towards the mixture intake to transform the longitudinal motion of the gaseous mixture into uniform radial motion.

In order that the present invention may more readily be understood the following description is given, merely by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic perspective view of a burner in accordance with the invention; Figure 2 is a perspective view on an enlarged scale of the flame outlets of the burner of Fig. 1; Figure 3 is a schematic cross-sectional view of the outlet openings for the flames in the burner of Fig. 1; Figure 4 is a longitudinal cross-sectional view of the burner of Fig. 1; Figure 5 is a schematic view showing the orientation and permutation of the opening outlets for the flames; Figure 6 is a view in perspective of the flame outlets on a variant of the burner according to the present invention.

In the embodiment represented in Figs. 1 to 4, the burner is in the form of a tubular barrel 1 closed at its top end by a disc 2 and fed with a fuel-gas/air mixture via the lower open end.

Evenly distributed along its whole periphery, the tubular barrel 1 comprises flame outlet openings 3 which have the shape of substantially equilateral triangles. These openings 3 are obtained by means of a press whose die bed is larger than the punch so that the metal tears without breaking off and forms, at each opening 3, three projections or teeth 4 (Fig. 2) whose tips are orientated towards the outside of the tubular barrel and whereto the flames cling.

There is thus provided, around each opening 3, a pyramid-shaped punching whose deoth is equal to < 3/4 times the side of the equilateral triangle defining the said opening; this depth represents the average flame-arresting distance in order to avoid flame blowback.

Projections 4 have irregularities, and the height of the punchings 5 is variable so as to have a point where the rate of the gas flow is equal to the fundamental combustion speed.

Moreover a controlled irregularity in the dimensions of the openings 3 is permitted, so as to obtain strictiy the total required surface but with openings having tolerances in their dimensions so that the outlet velocities of the gaseous mixture differ per orifice. The distance "b" (Fig. 3) between the base and the tip of each projection 4 of the same opening 3 (measured radially of the barrel 1) is between 0.3 and 1.3 mm and the height of the projections is between 1.5 mm and 2 mm.

Referring to Fig. 5, it will be seen that the punchings 5 form sloping (i.e. helical) symmetrical longitudinal rows. The tips of the flame outlet opening 3 are subjected to a circular permutation in such a way that the intervals "a" between each adjacent flame outlet should be essentially irregular.

Since the flames 7 (Fig. 3) are close to each other and are under pressure, a low pressure is created between each flame outlet 3 which is accentuated by the circular permutations of the openings to produce turbulences by divergence of the flow at the flame base and hence eddies 8 which bring about a process of recirculation and reintroduction of the gas and of the hot combustion products coming from the part of the flame situated further downstream as indicated by reference 9 in Fig. 3.

This has the effect of increasing the fundamental combustion speed at this point and to ensure a large area of flame stability.

Within the tubular barrel 1, is a device 20 (Fig. 4) transforming the longitudinal motion of the gaseous mixture into a uniform radial motion. This device 20 is a baffle which has a surface of revolution whose generatrix may be expressed by an equation of the form: S S = Ax + K where A, K and n are respectively two constants and an exponent taking into account the physical characteristics of the fluids and the pressure losses, S is the radius of the internal surface and x is the height of the cone frustum measured axially of the burner.

Mathematical and physical considerations related to the desire for a greater ease of manufacture make it possible to truncate the very flat summit of this generatrix curve. The remaining sides, which are symmetrical in relation to an axis, may hence be assimilated to two straight lines and the surface of revolu- tion hence takes a frusto-conicai shape whose smallest diameter d, and largest diameter d2 are linked by the relation: d2 - d, = = A, 2x where A, is a new constant deriving from the transformation set out above and which takes into account the physical characteristics of the fluids and the pressure losses.

This simplification thus leads to a much greater ease of production without thereby disturbing the transformation of the longitudi nal velocity into a radial velocity of the gaseous mixture and the uniformity of the pressure.

This truncated cone, made of a fine metal foil, has its smaller base orientated towards the top part of the tubular barrel 1 and its larger base substantially of the same diameter as the said barrel so that the whole of the air and gas mixture must pass through the holes of the metal foil to reach the tubular barrel 1.

It does allow, in the case of a turbuient rough motion of the gaseous mixture, to have the vertical velocity of this mixture transformed into a uniform horizontal velocity over the whole length of the tubular barrel. Moreover, it serves as the ultimate barrier against flame blow-back.

In a variant of the invention represented in Fig. 6 the flame outlet openings 10 are circular, and each is surrounded by two projections or teeth 11 so as to constitute, with the said openings, cylindrical punchings 12.

The cylindrical burner with fan-assisted air and fuel-gas premixing in accordance with the present invention has many advantages.

First of all, it makes it possible to avoid blowing out and flame blow-back, on the one hand, by means of irregular projections or teeth at the level of each opening which increase the flame-arresting effect and, on the other hand, by the shape and variable height of the punchings which increase the wall effect against flame blow-back. Finally, it makes it possible to have uniformity of velocity field in the flow of the gaseous mixture.

All these advantages can be obtained simply by using a tubular burner barrel and a perforated frusto-conical baffle 20.

Claims

1. A cylindrical burner with fan-assisted premixing of air and fuel-gas comprising a tubular barrel provided with evenly distributed flame outlet openings, having their edges extending radially outwardly to form with the said openings punchings; and a perforated frusto-conical baffle arranged within the tubular barrel, wherein the said edges of each opening of the flame outlets form irregular projections which are variable in height and which are orientated differently from one row to another to facilitate the clinging of the flames thereon, and the frusto-conical baffle is perforated along all its periphery and its length to transform from its base to its top the longitudinal motion of the gaseous mixture into uniform radial motion.

2. A burner according to claim 1, wherein the edges of said flame outlet openings are triangular and the said edges of the flame outlet openings include three said projections constituting, with the openings, pyramidal punchings.

3. A burner according to claim 1, wherein the said flame outlet openings are circular and the said edges of the flame outlet openings include two said projections constituting, with the openings, cylindrical punchings.

4. A burner according to claim 2 or 3, wherein the respective distances separating the tip of each said flame outlet opening from the nearest tip of an adjacent flame outlet opening are non-uniform.

5. A burner according to claim 2 or 3, wherein the distance, measured radially of the burner barrel, between the base and the top of each said projection of the same flame outlet opening is between 0.3 and 1.3 mm.

6. A burner according to any one of the preceding claims, wherein the height of the said projections measured radially of said tubular burner barrel is between 1.5 mm and 2 mm.

7. A burner according to any one of the preceding claims, wherein the ratio of the smallest diameter d, in relation to the largest diameter d2 of said frusto-conical baffle substantially corresponds to the expression: d2 - d ------= A1 2x where A, is a constant taking account of the physical characteristics of the fluids and of the pressure losses, and x is the height of the cone frustum, measured axially of said tubular burner barrel.

8. A burner according to any one of the preceding claims, wherein the frusto-conical baffle is made of a fine metal foil.

9. A burner with fan-assisted premixing of air and fuel-gas, such burner being substantially as hereinbefore described with reference to, and as illustrated in, Figs. 1 to 4 of the accompanying drawings.

1 0. A burner according to claim 9, when modified as described with reference to Fig. 6 of the accompanying drawings.

11. A gas-fired room-sealed water heater including an inlet fan and a burner according to any one of the preceding claims.