EP0291372A1 - Gas burner of the cold nozzle type - Google Patents

Abstract

The burner according to the invention comprises an injection device capable of injecting a flow of combustible gas mixture into a burner head comprising successively: a pressure recovery chamber (10) having a divergent profile, an ignition chamber (11 ) having two parallel walls (18, 19) extended by two deflectors (30, 31) whose front edges form between them a passage of reduced width, and a diffusion grid forming between the aforesaid two chambers a curved partition. The front edges of the deflectors (30, 31) and the aforesaid diffusion grid have circular shapes that are substantially coaxial. The invention makes it possible to avoid discontinuities in the shape of the front face of the flame and the harmful effects of defective orientation of the flame. burner.

Description

The present invention relates to a gas burner, of the cold nozzle type, capable of carrying out external combustion of a gas mixture at high speed.

It applies in particular but not exclusively to the heating at low temperature of plastic films, for example polyethylene films with a view to their shrinking, by means of a gaseous flow originating from the combustion of a combustible gas such as propane and air.

To satisfy this type of application, the burner must therefore be designed so as to produce a gas flow having a temperature of the order of 120 ° to 540 ° C at a predetermined distance from the burner (distance at which the plastic film treat must be arranged).

At this distance, the temperature must be relatively homogeneous and the gas flow free of burning material, if one wishes to avoid any possibility of burning, scorching and blistering of the film.

With a view to achieving this result, a burner has already been proposed comprising an injection device capable of producing a flow of combustible gas mixture at high speed and of injecting this flow into a burner head of tubular shape comprising successively:
a pressure recovery chamber having, in the plane of symmetry of the head, a divergent shape and inside which the gas mixture coming from the injection device develops in a fan configuration,
- an ignition chamber of substantially constant and rectangular section,
two deflectors which respectively extend the two long sides of the ignition chamber, and which converge from one to the other, these two deflectors having two rectilinear front edges forming between them a passage of reduced width, and
- A diffusion grid, of substantially semi-cylindrical shape fixed inside the head at the junction of the two chambers, this grid forming a curved partition, oriented parallel to said edges, in said plane of symmetry, and whose concavity is oriented towards the inside of the pressure recovery chamber.

It turns out that, thanks to the structure described above, the burner head is not licked by the flame and therefore does not undergo significant heating.

However, contrary to what was foreseeable, the flame produced does not have a rigorously rectilinear and homogeneous front front.

In fact, it is observed in use that the front front of this flame has in its central region a substantially straight front bordered, laterally by two respective projections in the shape of a horn.

It is clear that the presence of these two lateral horns prevents taking advantage of the advantages which this type of burner otherwise presents.

A first object of the invention is therefore to eliminate this drawback.

Furthermore, it turns out that burners of this kind are most commonly used as "hand burners" and consequently include a handle which the operator holds in his hand.

During his work, he must sweep the film to be treated, keeping the burner oriented so as to keep the straight front of the flame parallel to the zone of the film in treatment, in order to obtain heating. homogeneous of this area.

However, this parallelism is difficult to acquire and then to maintain.

Consequently, it is difficult to avoid untimely orientations which induce temperature heterogeneities which risk damaging the film.

The invention therefore also aims to eliminate this drawback.

To achieve these results, the invention provides a burner of the aforementioned type, more particularly characterized in that the front edges of the two long sides of the ignition chamber, on which the deflectors are connected, the front edges of the deflectors and the grate. diffusion have substantially coaxial circular shapes.

The tests carried out on this burner show that a flame is obtained, the front face of which has a circular shape free of horns.

In addition, by choosing an appropriate radius of curvature of the abovementioned edges and of the grid, it becomes possible to cause a variation in orientation of the burner, does not modify the heating effected on the area of the film being treated.

• La figure 1 est une coupe axiale schématique illus­trant la structure d'un brûleur selon l'invention ;
• La figure 2 est une vue schématique, en perspective d'une tête de brûleur selon l'invention ;
• La figure 3 est une vue en perspective schématique de la grille utilisée dans le brûleur de la figure 1 ;
• La figure 4 est une vue de côté d'un brûleur à main de type pistolet muni d'une tête de brûleur du type de celle représentée figure 2 ;
• La figure 5 est une représentation schématique montrant l'action d'un brûleur à grille rectiligne et dont les bords antérieurs de la chambre d'ignition et des déflecteurs sont droits ;
• La figure 6 est une représentation schématique montrant l'action d'un brûleur selon l'invention ; et
• Les figures 7 à 9 sont des coupes schématiques de grilles munies chacune d'un obstacle servant à engen­drer des turbulences en vue de transmettre le front de la flamme sur toute l'étendue de la section de sortie du brûleur.

An embodiment of the invention will be described below, by way of nonlimiting example, with reference to the appended drawings in which:

• Figure 1 is a schematic axial section illustrating the structure of a burner according to the invention;
• Figure 2 is a schematic perspective view of a burner head according to the invention;
• Figure 3 is a schematic perspective view of the grate used in the burner of Figure 1;
• Figure 4 is a side view of a pistol type hand burner with a burner head of the type shown in Figure 2;
• FIG. 5 is a schematic representation showing the action of a burner with a rectilinear grate and the front edges of the ignition chamber and deflectors are straight;
• Figure 6 is a schematic representation showing the action of a burner according to the invention; and
• Figures 7 to 9 are schematic sections of grids each provided with an obstacle used to generate turbulence in order to transmit the flame front over the entire extent of the burner outlet section.

As previously mentioned, the burner according to the invention comprises an injection device 1 capable of transmitting to the head of the burner 2, a flow of combustible gaseous mixture.

Thus, in the example shown in FIG. 1, the injection device 1 more particularly involves:
a tube 3 formed in two sections, namely a convergent rear section 4 and a substantially divergent front section 5,
an injection nozzle 6 mounted in the converging section 4, this nozzle 6 being connected to a source of flammable gas having a pressure of the order of 3 to 4 bars, and
- at least one air passage opening 7 located in the annular zone between said nozzle 6 and said section 4.

This device therefore forms a jet pump which drives the air coming from the opening 7 and generates in the converging portion 4 of the pipe 3 (point I) a flow of gaseous mixture at high speed, of the order of 12660 meters. /minute.

Inside the divergent portion 5, this speed gradually decreases (point J₁ at 6000 meters / min, point J₂ at 5000 meters / min, point K at 3500 meters / min) to reach the inlet of the burner head 2 at a speed which however remains relatively high (point L of the order of 1000 meters / min).

The burner head 2, of tubular shape, is made up of two parts which successively delimit a pressure recovery chamber 10 which is connected to the pipe and an ignition chamber 11 which opens into the open air.

The pressure recovery chamber 10 has, beyond its zone of connection to the pipe 3, a flared shape delimited by two converging walls 12, 13, of increasing width and two diverging side walls 14, 15 of decreasing width. As can be seen in Figures 2 and 3, the front edges 16, 17 of the two walls converging 12, 13 have coaxial circular shapes.

The ignition chamber 11 also has a flared shape. However, in this example, it is delimited by two parallel walls 18, 19 which extend the two converging walls 12, 13, and two divergent side walls 20, 21 which extend the side walls 14, 15, respectively along the same orientations. The front edges 22, 23 of the two walls 18, 19 are circular and extend coaxially to the edges 16, 17.

These two chambers 10, 11 are separated from each other by a diffusion grid 24 which consists of a piece of perforated sheet metal having the shape of a toric sector of substantially semi-cylindrical section, and whose large radius of curvature corresponds substantially to that of the front edges 16, 17 of the converging walls 12, 13. The lateral edges 24 ′, 24˝ of this grid form, with the two parallel walls 18, 19, gas flow flow corridors in which, as will be explained later, combustion cannot be established.

The fixing of this diffusion grid 24 inside the burner head 2 is carried out in the junction zone of the chambers 10 and 11, the concavity of this grid being oriented towards the pressure recovery chamber 10.

To this end, the grid 24 comprises two flanges 25, 26 giving it a shape substantially in Ω (omega), that is to say of which the section has a curved shape bordered by two lateral wings.

The walls 18 and 19 of the ignition chamber 11 are extended by two deflectors 30, 31 of circular shape which substantially converge towards each other and have two respective edges 32, 33 coaxial forming between them a passage space of width less than the width of the lateral sides 20, 21 of the ignition chamber 11.

The operating principle of this burner head is then as follows:

Inside the pressure recovery chamber 10, the speed of the gas mixture undergoes a slight decrease as it develops in a fan configuration. At the level of the diffusion grid 24, a relatively homogeneous pressure zone is then formed, due to a conversion of the kinetic energy of the gas flows.

Through the perforations of the grid 24, the combustible mixture forms a succession of again accelerated jets (point N at 2400 m / min).

In the middle part of the grid 24, these jets are oriented axially while in the lateral parts, they are substantially radial and abut on the walls 18, 19 and on the deflectors 30, 31.

It can be seen that in the middle part of the grid 24, the gas flow formed by the jets undergoes a deceleration ΔV₁ (point O at 1600 m / min). This is due to the fact that at the exit from the perforations in the middle zone of the grid 24, an expansion of the fuel mixture occurs, this expansion being favored by the distribution of the gas flow as jets. This reduction in the flow speed allows the combustion to be established at a slight distance from the grate 24.

Beyond point O, the combustion of the gas mixture is established and there is a slight acceleration of the gases (expansion due to combustion). However, the speed at point OO remains lower than that measured at point N.

Furthermore, the gas flow coming from the perforations located in the lateral parts of the perforated grid 24 undergoes a deceleration ΔV₂ much greater than the deceleration ΔV₁. Indeed, the gas flow strikes the walls 18, 19 of the ignition chamber 11 and is braked. This gas flow also creates, along the walls 18, 19 and on the deflectors 30, 31, two flow zones at higher pressure with low acceleration (points F, G, H respectively at 600, 650, 800 m / min) in which the flame does not propagate (dead space). The walls 18, 19 of the ignition chamber 11 and the deflectors 30, 31 which are not licked by the flame and are ventilated by the flow of combustible mixture therefore do not heat up and are, on the contrary, constantly cooled.

This explains the reason why the burner does not undergo any significant rise in temperature (burner with cold nozzle).

As previously mentioned, when leaving the diffusion grid 24, the speed of the gas flow is not homogeneous, which should, in principle, lead to heterogeneity of the flame.

This drawback is eliminated by the use of deflectors 30, 31 which serve to deflect the flow zones of the fuel mixture coming from the lateral parts of the grid 24 and to make them slightly converge towards the central region of the gas flow.

At the same time, the speed of this gas flow is reduced, by knock-on effect, substantially to that of the central part of the flow. The combustion of the gas mixture then forms, beyond the front edges of the deflectors 30, 31, a fan-shaped flame.

Thanks to the shape of the burner head 2 and in particular the circular shape of the front edges 32, 33 of the deflectors 30, 31 and of the diffusion grid 24, the front face 40 of the flame 41 also has a coaxial circular shape (FIG. 6).

At this front front 40, the temperature of the gas flow is substantially homogeneous and isothermal. Beyond this front edge 40, the temperature gradually decreases, with circular isotherms 42, 43, 44 of the type of those which have been shown in FIG. 6.

In this FIG. 6, it can be seen that the isotherm 44 is tangent to the surface of the film to be treated 45 and that this feature is maintained even if the head of the burner 2 undergoes a variation in inclination such as that which is indicated by dashed lines.

It is therefore clear that this variation in inclination does not incur the risk of having, on a determined area of the film 45, a rise in temperature which can lead to burning, russeting or even blistering.

This would not be the case if a burner of a type similar to that described above is used, but in which the front edges of the ignition chamber and deflectors as well as the diffusion grid are rectilinear.

Indeed, as this appears in FIG. 5, the front front 46 of the flame 47 produced by this burner 2 ′ has a straight central part bordered by two lateral horns 48, 49 projecting.

It is clear that the isotherms of the gas flow generated by the flame 47 have a similar appearance and that there is therefore a risk of burning the film 45 at the lateral regions of the flow.

In addition, this risk of burns is considerably aggravated during a variation in orientation of the burner 2 ′ inducing a lack of parallelism between the central part of the front front of the flame 47 and the area of the film being treated (illustrated in broken lines in FIG. 5).

In this case, the film will undergo overheating in the lateral region 50 of the gas flow.

The invention therefore makes it possible to eliminate these two disadvantages thanks to the fact that it makes it possible to obtain a flame front free of horns and that the circular shape of this flame front eliminates the harmful effects of a variation in orientation of the burner head.

It should be noted that the burner according to the invention can be associated with a body 53 in the form of a gun comprising a trigger 54 which controls both the admission of the combustible gas and the ignition circuit of the burner.

Advantageously, this ignition circuit may comprise a piezoelectric generator actuated by the trigger 54 and a spark plug 55 opening into the ignition chamber 11.

It turns out, however, that due to the circular shape of the burner head 2 and the high exit speed of the gas flow, the transmission of the flame over the entire extent of the exit section of the burner does not take place. does not always perform correctly at the time of ignition, especially when the length of this outlet section is important.

The invention therefore proposes to solve this problem by placing, in the central area of the ignition chamber 11, a low-rise obstacle pressed tightly against the grid 24, so as to create a local disturbance of the gas streams itself generating areas at low speed where combustion can be established and propagated without generating a combustion zone under the obstacle.

This obstacle can extend over all or part of the central area of the diffusion grid 24 or, possibly, be arranged obliquely with respect thereto.

Figures 7, 8 and 9 show various possibilities for achieving this obstacle.

Thus, FIGS. 7 and 8 show in cross section obstacles formed by a round wire 60 (FIG. 7) and by a wire of square section 61 (FIG. 8) fixed on the diffusion grid 24.

In the example shown in FIG. 9, the obstacle is simply obtained by a fold 62 formed on the diffusion grid 24.

Of course, the invention is not limited to the embodiments described above.

Thus, for example, the diffusion grid 24 could comprise at least over part of its surface a double wall.

Likewise, this grid 24 could be doubled by a finer grid, extending in the pressure distribution chamber or even in the ignition chamber, to avoid possible flashbacks.

Claims (9)

1. Gas burner, of the cold nozzle type, comprising an injection device (1) capable of producing a flow of combustible gas mixture, at high speed, and of injecting this flow into a burner head (2) tubular comprising successively:
- a pressure recovery chamber (10) having, in a plane of symmetry of the head (2), a divergent profile, and inside which the gas mixture coming from the injection device develops in a configuration in fan,
- an ignition chamber (11) having two walls (18, 19) substantially parallel to said plane of symmetry,
- two deflectors (30, 31) which respectively extend the two said walls (18, 19) and which converge towards one another, these two deflectors (30, 31) having two parallel front edges forming between them a passage of width reduced, and
- a diffusion grid (24) of concave shape forming inside the burner head (1) a curved partition which separates the pressure recovery chamber (10) and the ignition chamber (11), the concavity of this partition being oriented towards the interior of the pressure recovery chamber (10), so that the lateral edges (24 ′, 24˝) of this grid form, with the two parallel walls (18, 19) , gas flow corridors in which combustion cannot be established,
characterized in that the front edges (23, 22) of the above walls (18, 19) to which the deflectors (30, 31) are connected, the front edges (32, 33) of these deflectors (30, 31), and the diffusion grid (24) have circular shapes. 2. Burner according to claim 1,
characterized in that the above circular shapes are coaxial. 3. Burner according to claim 1,
characterized in that the above diffusion grid (24) has the shape of a toric sector of substantially semi-cylindrical section, the large radius of curvature of which corresponds substantially to that of the above circular shapes. 4. Burner according to one of the preceding claims,
characterized in that the said diffusion grid (24) has a section comprising a curved part bordered by two lateral wings. 5. Burner according to one of the preceding claims,
characterized in that the above diffusion grid (24) comprises on at least part of its surface a double wall. 6. Burner according to one of the preceding claims,
characterized in that it comprises, in the median zone of the ignition chamber (11), a low height obstacle pressed tightly against the diffusion grid (24), so as to create a local disturbance of the gas streams injected by the grid (24). 7. Burner according to claim 6,
characterized in that the above obstacle consists of a wire (60, 61) fixed on the diffusion grid (24). 8. Burner according to claim 6,
characterized in that the above obstacle is obtained by a fold (62) formed on the diffusion grid (24). 9. Burner according to one of the preceding claims,
characterized in that it further comprises a fine grid extending against the diffusion grid (24).

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