WO1997046831A1

WO1997046831A1 - Improvements in or relating to a burner head

Info

Publication number: WO1997046831A1
Application number: PCT/GB1997/001499
Authority: WO
Inventors: Brendan Kemp
Original assignee: Autoflame Engineering Ltd.
Priority date: 1996-06-07
Filing date: 1997-06-03
Publication date: 1997-12-11
Also published as: GB9611965D0; GB2313906A

Abstract

A burner head including a blast tube (3) and a device for controlling the air flow through the blast tube (3), the flow controlling device comprising a flow constrictor (11) which is alterable between a first configuration and a second configuration in which the flow constrictor (11) extends radially across an annular portion of the blast tube (3) to reduce the cross-sectional area in the blast tube (3) available for unobstructed air flow.

Description

IMPROVEMENTS IN OR RELATING TO A BURNER HEAD

The invention relates to a burner head which includes a flow controlling device for controlling the air flow through the blast tube, to the flow controlling device and to a method of operating a burner head.

In a burner head, fuel, typically, oil or gas, is mixed with air and ignited to produce a flame. It is important that the burner operates with combustion that is efficient and that the production of unwanted waste products is kept to a low level.

Pπor art burner heads have been designed with those objectives in mind. Factors that affect the efficiency of the combustion and the production of pollutants include the ratio of the fuel to air, the quality of the mixing of the fuel and air, the rate of supply of fuel and air, the nature of the air flow, and the shape of the burner head in the region where mixing of air and fuel and ignition takes place. Burner heads can be operated over a range of settings so the characteristics of those factors affecting the combustion may vary with the setting on the burner. A problem with conventional prior art burner heads is that they were designed for good operation only at the top of the operating range. When the burner is turned down, the conditions for combustion deteriorate and the operation of the burner becomes unsatisfactory.

One of the factors affecting the efficiency of combustion and production of pollutants is the air flow in the region where mixing of fuel and air takes place. Combustion is improved if the mixing of the tuel and air is good. In the conventional burner head, as the burner is turned down, there is a decrease in the pressure drop across the head, the velocity of the air is reduced, because the rate of supply of air is reduced, and the kinetic energy of the air in the mixing region is reduced; hence the quality of the mixing of fuel and air is also reduced.

There have been some proposals intended to overcome the above-mentioned problem. For example, the provision of a wedge around the inner surface of the blast tube of a burner head, which has a diffuser centrally disposed in the blast tube, has been proposed. During operation at one setting of the burner, the wedge is positioned in a - position well upstream from the diffuser. When the burner is to be used at a lower setting, the wedge is moved axially along the tube to the region where the diffuser is located, so that the wedge extends into the blast tube and reduces the cross-sectional area of the passageway around the diffuser at that point, thereby increasing the velocity of the air at that point. It has also been proposed to provide a sleeve that is spaced inwardly from the blast tube concentrically therewith and is axially movable along the tube; air flows both inside and outside the sleeve but airflow outside the sleeve can be restricted when the burner is to be used at a lower setting by moving the sleeve axially forwards.

One disadvantage of those proposals is that, in each case, the wedge or sleeve interferes with the air flow both in its rest position and in the operating position. A further disadvantage is that the shape of the sleeve and the wedge are fixed. In each case, it is not possible to vary the extent to which the device projects into the blast tube.

As an alternative, a device resembling the shutter in a camera, known as an 'iris damper', has been proposed for changing the geometry of the blast tube. Again, one disadvantage of such a device is that it introduces an abrupt obstacle to the air flow, disturbing the flow and affecting combustion. It is also of rather complex construction. An object of the present invention is to mitigate the disadvantages of the prior art and provide a simple and effective way of promoting efficient combustion throughout the range of settings for a burner.

Accordingly, the invention provides a burner head including a blast tube and a device for controlling the air flow through the blast tube, the flow controlling device comprising a flow constrictor which is alterable between a first configuration and a second configuration in which the flow constrictor extends radially across an annular portion of the blast tube to reduce the cross- sectional area in the blast tube available for unobstructed air flow.

As a result of the invention, depending on the setting of the burner, the configuration of the flow constrictor can be chosen to result in a suitable air flow to achieve the desired mixing conditions.

Preferably, the flow constrictor is such that the extent to which the flow constrictor extends radially across an annular portion of the blast tube is continuously variable. In that way, the flow constrictor -can be altered, continuously, to many different configurations, so that the desired air flow conditions in the mixing region can be achieved for each setting of the burner .

Preferably, the shape of the flow constrictor is alterable by deformation thereof. A defor able flow constrictor provides a simple and effective way of attaining the objects of the invention.

The configuration of the flow constrictor may be alterable between a first configuration in which the flow constrictor is disposed substantially around the inner wall of the blast tube and a second configuration in which the flow constrictor projects into the blast tube to increase the velocity of air flow through the tube. In the first configuration, the flow constrictor does not interfere with the air flow.

Preferably, the burner head includes an actuator which moves a part of the flow constrictor axially along the blast tube to change the configuration of the constrictor.

Advantageously, in at least one configuration the surface of the flow constrictor is curved. A curved surface interferes less with the air flow and results in a smoother air flow, requiring less fan power. The flow constrictor may comprise a plurality of elongate elements. Preferably, at least some of the elements are integral portions of the same member and are joined together at at least one end. The circumferential spacing of the elongate members in one configuration is constant along their length. Alternatively, the circumferential spacing of the elongate members in one configuration may be variable along their length. The elongate elements are preferably laminar.

The burner head will usually comprise a diffuser for promoting the mixing of fuel and air, the diffuser being disposed in the blast tube and the flow constrictor being disposed radially outwardly of the diffuser at approximately the same axial position as the diffuser.

The flow constrictor may include a plurality of elongate elements that in one configuration are inwardly bowed so as to project into the flow path in the blast tube.

The invention also provides a method of operating a burner head including a device for controlling the air flow through the blast tube of the burner head comprising adjusting the con iguration of a flow constrictor device to alter the cross-sectional area available for air flow. Preferably, the configuration of the flow constrictor is varied to substantially maintain or increase the velocity of the air flow in the region of the flow constricting device as the rate of supply of volume of air is varied.

The invention further provides a device for controlling the air flow through the blast tube of a burner head comprising a flow restrictor which is alterable between a first configuration and a second configuration in which, in use, the flow constrictor extends radially into the blast tube to obstruct the air flow through the blast tube, to alter the cross-sectional area of the passage for air flow.

Preferably, the shape of the flow constrictor is deformable.

The flow constrictor may comprise a plurality of elongate elements. Preferably, at least some of the elements are integral portions of the same member and are joined together at at least one end. The circumferential spacing of the elongate members in one configuration may be constant along their length. Alternatively, the circumferential spacing of the elongate members in one configuration may be variable along their length. Preferably, the elongate elements are laminar. By way of example, an embodiment of the invention will now be described, with reference to the accompanying drawings, of which:

Fig. 1 is a front view of a burner head, with a flow constrictor in a first configuration;

Fig. 2 is a side view of the burner head corresponding to Fig. 1;

Fig. 3 is a front view of the flow constrictor as shown in Fig. 1; Fig. 4 is a side view of the flow constrictor corresponding to Fig. 3;

Fig. 5 is a front view of the burner head, with the flow constrictor in a second configuration;

Fig. 6 is a side view of the burner head corresponding to Fig. 5;

Fig. 7 is a front view of the flow constrictor as shown in Fig. 5;

Fig. 8 is a side view of the flow constrictor corresponding to Fig. 7.

Figs. 1 and 2 show a burner head, indicated generally by the reference numeral 1, for burning fuel, which may be either liquid (for example, oil) or gas, in air.

The burner head 1 comprises a main body 2 and a blast tube 3, where mixing of the fuel and air, ignition and combustion take place.

A duct 4 for the air supply is connected to the main body 2. The burner head 1 also comprises pipes 5 for an oil supply and a duct 6 for a gas supply. The pipes 5 are connected to a central tubular member 7 extending axially along part of the blast tube 3 and of the main body 2 of the burner head 1. The duct 6 is connected to a duct 8 of annular cross-section extending co-axially with the tubular member 7 along most of the length of the tubular member.

The blast tube 3 has a circular cross-section and a tapered outlet 9. A diffuser 10 in the form of an apertured annular plate with various shapes formed in its forwardly directed face is provided in the centre of the blast tube. The diffuser 10 is situated close to the outlet ends of the tubular member 7. The annular gas duct 8 terminates at its outlet end in a plurality of separate outlet pipes 8A which are connected to the diffuser 10 and define a ring of gas outlets on the forward face of the diffuser 10.

The parts of the burner head described above are -conventional, but the blast tube also comprises a flow controlling device including a flow constrictor 11 which, in the configuration shown in Figs. 1 and 2, is in the form of a sheet disposed around the inner surface of the blast tube in the vicinity of and at the same axial position as the diffuser 10. The flow controlling device includes actuators 12 connected at one end to the rear edge of the sheet and situated around the inner surface of the blast tube. The actuators 12 are connected to actuating means (not shown) at the other end. The flow constrictor 11 is described in more detail below with reference to Figs. 3 and 4. The actuating means comprise a mechanical transducer which receives an electrical input signal from a control system for controlling the flow of fuel and air to the burner head and produces a mechanical output in the form of a controlled displacement of the ends of the actuators 12. The control system may be for example the Micro

Modulation system sold by Autoflame Engineering Ltd and described in GB 2 138 610 and GB 2 169 726. The mechanical transducer may include linear or rotary prime movers. In a particular example of the invention the mechanical transducer comprises a stepper motor drivingly connected to rotate a disc about an axis perpendicular to the longitudinal axis of the blast tube and a drive member rotatably mounted on the disc at a position displaced from the axis of rotation of the disc. The drive- member is mechanically coupled to the actuators 12 and thus a controlled longitudinal displacement of the actuators 12 is obtained from a controlled angle of rotation of the stepper motor.

With reference to Figs. 3 and 4, the flow constrictor 11 is made from a thin, rectangular sheet 13 of stainless steel. A plurality of parallel slots 14 have been cut into the sheet using laser-cutting to form a plurality of rectangular, laminar, spaced apart elements 15. The slots 14 are approximately 1mm wide and 4mm apart. The sheet 13 is bent and secured at the ends to form a ring, with the slots 14 extending across the width of the ring from one edge to the other. Actuators 12 are attached to the rear edge of the sheet 13 and the forward edge of the sheet, referring back to Fig. 2, is attached to the inner surface of the blast tube 3.

In operation, fuel in the form of liquid or gas is supplied to the outlet of the duct 6 or the tubular member 7 in the vicinity of the diffuser 10. Air is supplied through the duct 4 and flows through the main body 2 of the burner head 1 and into the blast tube 3. Most of the air passes through an annular passageway 16 formed between the inner surface of the blast tube 3 and the perimeter of the diffuser 10 but some air also passes through the central opening and the apertures in the -diffuser. The fuel and the air mix in a low-pressure region formed immediately downstream of the diffuser 10, where combustion takes place. At the top of the heating range for the burner head 1, the flow constrictor 11 is in the first configuration shown in Figs. 1 to 4. In that configuration, the sheet of stainless steel 11 is lying substantially flat against the inner surface of the blast tube 3 (the sheet is in fact slightly bent so that it is lifted away slightly from the inner surface of the blast tube to enable it to be more easily deformed) so that the cross-sectional area of the passageway 15 through which air is flowing between the diffuser 10 and the blast tube 3 is substantially the same as if the flow constrictor was not present, or, in other words, the effective internal diameter of the blast tube 3 is unchanged. The design of the blast tube 3, and the conditions within the burner head are such that the conditions for combustion are very good.

When the burner is turned down by a control system, the actuating means are operated by the control system so that the actuators 12 are displaced axially in the direction of the outlet 9 of the blast tube 3, in turn displacing the rear edge of the stainless steel sheet 13 forming part of the flow constrictor 11. The rear edge of the sheet 13 slides along the inner surface of the blast tube 3 and the sheet 13 deforms, rather like a "magic lantern", into a second configuration where each of the elements 15 are inwardly bowed and the edges remain on the inner surface of the blast tube 3, as shown in Figs. 5 to 8 , so that the flow constrictor 11 projects into the air flow.

As shown in Figs. 5 to 8, in the second configuration of the flow constrictor 11, the flow constrictor produces a smooth curved inner surface in the blast tube 3, with the peak of the curve substantially at the same axial position as the diffuser 10 (see Fig. 6) . Consequently, in the second configuration of the flow constrictor 11, the effective internal diameter of the blast tube is reduced along the length of the flow constrictor and in particular at the diffuser 10. Thus, the cross-sectional area of the passage along which the air is flowing in the region of the diffuser 10, and before entering the mixing region, is reduced. That means that, although the burner has been turned down, so that the rate of supply of fuel and air has been reduced, in the region of the flow constrictor 11 the velocity of the air flow has not been correspondingly reduced as it would have been had the effective internal diameter of the blast tube remained the same. The quality of the mixing of the fuel and air, and hence the nature of the combustion, depends on the velocity of the air at the ^"diffuser; thus, the use of the flow constrictor 11 enables combustion conditions at the bottom of the heating range to be very good. To obtain efficient combustion and to avoid or diminish the production of pollutants, harmful emissions and to reduce waste deposits such as carbon on the burner head, especially on the diffuser, it is important to control the nature of the air flow. Because the internal surface of the flow constrictor is smoothly curved, the air flow in the blast tube is not significantly disturbed by the flow constrictor 11 with the result that better mixing of fuel and air takes place, with reduced emissions of waste products.

In the second configuration of the flow constrictor 11, there are gaps between adjacent elements 15 of the constrictor, through which small amounts of air can pass. It may be advantageous to permit such air flow to promote transfer of heat away from the flow constrictor.

The amount of the displacement of the actuators 12 determines the extent to which the flow constrictor 11 is deformed, and hence the cross-sectional area of the passageway 16 through which air flows. The magnitude of the displacement can be varied continuously between the two extreme positions referred to above and the construction of the flow constrictor is such that the - cross-sectional area of the passageway 15 is then also varied continuously. The flow constrictor 11 can be used to maintain a substantially constant air velocity as the burner is turned up or down through its range. The actuating means and the burner control may be connected together in such a way that alteration of the burner setting automatically results in deformation of the flow constrictor to the correct configuration for that setting to maintain a desired air velocity for efficient mixing.

As a variation to the flow constrictor described above, the slots may be narrower at each end than they are at the middle, so that in deformed configurations there are no gaps between adjacent elements, or the gaps are smaller than in the embodiment described above.

Alternatively, the flow constrictor may comprise two overlaid sheets of stainless steel, each formed with a plurality of straight slots as described above, so that in a deformed configuration, there is no direct passage for the air from one surface of the flow constrictor to the other.

The flow constrictor may be manufactured by methods other than laser-cutting, for example by using a stamping tool.

Flow constrictors which assume shapes other than a smoothly curved surface may also be used depending on the desired air flow characteristics.

Claims

Cla ims

1. A burner head including a blast tube and a device for controlling the air flow through the blast tube, the flow controlling device comprising a flow constrictor which is alterable between a first configuration and a second configuration in which the flow constrictor extends radially across an annular portion of the blast tube to reduce the cross-sectional area in the blast tube available for unobstructed air flow.

2. A burner head as claimed in claim 1, the flow constrictor being such that the extent to which the flow constrictor extends radially across an annular portion of the blast tube is continuously variable.

3. A burner head as claimed in claim 1 or claim 2 in which the shape of the flow constrictor is alterable by deformation thereof.

4. A burner head as claimed in any one of claims 1 to 3 in which the configuration of the flow constrictor is alterable between a first configuration in which the flow constrictor is disposed substantially around the inner -wall of the blast tube and a second configuration in which the flow constrictor projects into the blast tube to increase the velocity of air flow through the tube.

5. A burner head as claimed in any one of claims 1 to 4 in which the burner head further includes an actuator which moves a part of the flow constrictor axially along the blast tube to change the configuration of the constrictor.

6. A burner head as claimed in any one of claims 1 to 5 in which in at least one configuration the surface of the flow constrictor is curved.

7. A burner head as claimed in any one of claims 1 to 6 in which the flow constrictor comprises a plurality of elongate elements.

8. A burner head as claimed in claim 8 in which at least some of the elements are integral portions of the same member and are joined together at at least one end.

9. A burner head as claimed in claim 8 in which the circumferential spacing of the elongate members in one configuration is constant along their length.

10. A burner head as claimed in claim 8 in which the - circumferential spacing of the elongate members in one configuration is variable along their length.

11. A burner head as claimed in any one of claims 7 to

10 in which the elongate elements are laminar.

12. A burner head as claimed in any one of claims 1 to

11 further including a diffuser for promoting the mixing of fuel and air, the diffuser being disposed in the blast tube and the flow constrictor being disposed radially outwardly of the diffuser at approximately the same axial position as the diffuser.

13. A burner head as claimed in any one of claims 1 to 12 in which the flow constrictor includes a plurality of elongate elements that in one configuration are inwardly bowed so as to project into the flow path in the blast tube.

14. A method of operating a burner head including a device for controlling the air flow through the blast tube of the burner head comprising adjusting the configuration of a flow constrictor to alter the cross- sectional area available for air flow.

15. A method of operating a burner head according to claim 14 in which the configuration of the flow

-constrictor is varied to substantially maintain or increase the velocity of the air flow in the region of the flow constricting device as the rate of supply of volume of air is varied.

16. A method of operating a burner according to claim 14 or claim 15 using a burner head according to any one of claims 2 to 13.

17. A device for regulating the air flow through the blast tube of a burner head comprising a flow restrictor which is alterable between a first con iguration and a second configuration in which, in use, the flow constrictor extends radially across an annular portion of the blast tube to reduce the cross-sectional area in the blast tube available for unobstructed air flow.

18. A device as claimed in claim 17 in which the flow constrictor comprises deformable material.

19. A device as claimed in claim 17 or claim 18 in which the flow constrictor comprises a plurality of elongate elements.

20. A device as claimed in claim 19 in which at least some of the elements are integral portions of the same member and are joined together at at least one end.

21. A device as claimed in claim 20 in which the circumferential spacing of the elongate members in one con iguration is constant along their length.

22. A device as claimed in claim 20 in which the circumferential spacing of the elongate members in one configuration is variable along their length.

23. A device as claimed in any one of claims 19 to 22 in which the elongate elements are laminar.