METHOD FOR FORMING A GAS BURNER MEMBRANE

Abstract

A method of forming a gas burner membrane. The method comprises forming a plurality of holes in a sheet of material. The holes are formed by laser cutting a required pattern of holes in the sheet of material.

Claims

1-24. (canceled)

25. A method of forming a gas burner membrane, the method comprising forming a plurality of holes in sheet metal, the holes being formed by laser cutting a required pattern of holes in the sheet metal.

26. The method according to claim 25, wherein at least a majority of the holes have a diameter equal to or less than 1.3 times the thickness of the sheet metal.

27. The method according to claim 26, wherein at least a majority of the holes have a diameter equal to or less than the thickness of the sheet metal.

28. The method according to claim 27, wherein at least a majority of the holes have a diameter equal to or less than 0.75 times the thickness of the sheet metal.

29. The method according to claim 26, wherein at least a majority of the holes have a diameter of between 0.1 and 1 mm.

30. The method according to claim 26, wherein at least a majority of the holes have a diameter of between 0.25 and 0.75 mm.

31. The method according to claim 26, wherein the sheet metal has a thickness of between 0.3 and 3 mm.

32. A method according to claim 26, wherein the sheet metal has a thickness of between 0.4 and 1.5 mm.

33. The method according to claim 26, wherein the sheet metal has a thickness of substantially 0.6 mm.

34. The method according to claim 25, wherein the sheet metal is sheet stainless steel.

35. The method according to claim 25, wherein the laser cutting comprises laser drilling.

36. The method according to claim 25, wherein the laser drilling of the required pattern of holes comprises at least one of: single beam laser drilling or multi beam laser drilling.

37. The method according to claim 25, wherein between 500 and 10000 holes are formed in the gas burner membrane.

38. The method according to claim 37, wherein between 1000 and 2000 holes are formed in the gas burner membrane.

39. The method according to claim 25, further comprising: providing a hydrogen gas supply to the gas burner membrane.

40. The method according to claim 39, wherein the hydrogen gas supply is a pure hydrogen gas supply or a hydrogen blend gas supply, wherein the hydrogen blend includes at least 80 vol. % hydrogen gas.

41. A gas burner including: a gas burner membrane comprising a sheet of material with a plurality of holes, with at least a majority of the holes having a diameter equal to or less than 1.3 times the thickness of the sheet of material; and a hydrogen gas supply.

42. The gas burner according to claim 41, wherein the gas burner further comprises a mixing chamber to receive gas from the hydrogen gas supply.

43. A gas burner membrane comprising sheet metal with a plurality of holes, with at least a majority of the holes having a diameter equal to or less than 1.3 times the thickness of the sheet metal.

44. The gas burner membrane according to claim 43, wherein at least a majority of the holes in the gas burner membrane have a diameter of 0.1 mm to 1 mm.

Description

[0048] Embodiments of the present invention will now be described by way of example only, and with reference to the accompanying figures, in which:

[0049] FIG. 1 schematically shows a gas burner according to the invention;

[0050] FIG. 2 schematically shows a further gas burner according to the invention;

[0051] FIG. 3 schematically shows a yet further gas burner according to the invention;

[0052] FIG. 4 is a picture showing an example natural gas burner membrane;

[0053] FIG. 5 is a picture showing a closer view of the holes in the example natural gas burner membrane of FIG. 4;

[0054] FIG. 6 is a picture showing an example gas burner membrane according to the invention;

[0055] FIG. 7 is a picture showing a closer view of the holes in the example gas burner membrane of FIG. 6; and

[0056] FIG. 8 shows an ultraviolet image of a hydrogen flame emitted from the gas burner membrane of FIG. 6, with no flashback, when the burner is used with hydrogen gas.

[0057] FIG. 1 shows a typical gas burner 10 with a mixing chamber 12 with a burner membrane 14 on top. Air and gas is blown into the mixing chamber 12 using a fan 20. The gas is supplied to the fan 20 via a pipe 18 from a supply 16. The burner 10 is controlled by a control unit 22. The fan 20 could for instance operate at 2000 rpm at 5 kW.

[0058] FIG. 2 shows a further typical gas burner 50 with the mixing chamber 12 with the burner membrane 14 on top. The mixing chamber 12 receives gas directly from a supply 16 via a pipe 19, without passing through the fan 20. Air is blown into the mixing chamber 12 using the fan 20, and the burner 10 is controlled by the control unit 22.

[0059] FIG. 3 shows part of a yet further typical gas burner 50 with the mixing chamber 12 with the burner membrane 14 on top. The mixing chamber 12 receives gas from a supply 16 via a pipe 21. A venturi 23 is provided at the mixing chamber 12 entrance, to cause the gas to accelerate prior to entering the mixing chamber 12. The pressure in a narrow passage of the venturi 23 is lower than atmospheric pressure, which causes air surrounding the venturi 23 to be sucked into the venturi 23 via apertures in the venturi 23, and then into the mixing chamber 12.

[0060] The gas burners 10, 50, 60 are intended to be used with gases such as hydrogen or hydrogen blend (greater than 80 vol. %) gas. Hydrogen has a greater flame velocity than natural gas, which increases the risk of flashback (i.e. an uncontrolled upstream propagation of the flame, due to a local imbalance in the flow velocity and the flame speed). If the exit velocity of an air/fuel mixture through the holes in a burner membrane is lower than the flame speed of hydrogen, there is a risk of flashback. Flashback can be explosive and thus dangerous, especially when hydrogen gas is used as a fuel. Increasing the air/hydrogen ratio can reduce the risk of flashback, but at the expense of combustion efficiency and flame stability.

[0061] It has been found that smaller holes in the burner membrane provide for a reduced risk of flashback and efficient burning when used with hydrogen gas or hydrogen blend gas. This is demonstrated by the examples of FIGS. 4 to 8, which show a first example natural gas burner membrane 100 and a second example burner membrane 200.

[0062] The first burner membrane 100 is formed from a 0.6 mm thickness stainless steel sheet 110, and includes holes 120 with a diameter of 0.8 mm. Flashback occurs when this burner membrane 100 is used with hydrogen gas.

[0063] The second burner membrane 200 is also formed from a 0.6 mm thickness stainless steel sheet 210, but includes holes 220 with a diameter of 0.5 mm. In this example the membrane 210 is curved, but in other embodiments the membrane may be flat. The holes 220 are formed by laser drilling, and are evenly spaced in a helical pattern in this example, but could be provided in other patterns in other examples. As demonstrated by the ultraviolet image of FIG. 8, which shows the (normally invisible) hydrogen flame only outside of the membrane, flashback does not occur when hydrogen is used as a fuel for the second burner membrane 200.

[0064] In view of this the burner membrane 14 has been formed with a required pattern of relatively small holes, which could be in accordance with the second burner membrane 200, or such holes could typically have a diameter of 0.1 mm, 0.25 mm, 0.5 mm, 0.75 mm or 1 mm. At least a majority of the holes may have a diameter equal to or less than 1.3 times the membrane thickness. In some examples at least the majority of the holes may have a diameter equal to or less than the membrane thickness, and in some examples at least the majority of the holes may have a diameter equal to or less than 0.75 times the membrane thickness.

[0065] Between 500 and 10000 holes are provided in the burner membrane 14. Preferably between 1000 and 2000 holes are provided.

[0066] The burner membrane 14 of FIGS. 1 to 3 is a single skin membrane and is formed from a sheet of metal, (e.g. ferritic stainless steel), which could for instance be 0.3 mm, 0.5 mm, 0.6 mm, 1.5 mm or 3 mm thick respectively for the hole sizes of 0.1 mm, 0.25 mm, 0.5 mm, 0.75 mm or 1 mm. In some, but not necessarily all examples, the burner membrane 14 of FIGS. 1 to 3 may be the same as the second burner membrane 200.

[0067] Holes of 0.5 mm cannot be formed by traditional sheet metal piercing techniques for a membrane 14 with a thickness of 0.6 mm, as the minimum size of the holes formable by these techniques is around 1.5 times the thickness of the sheet. In some instances using traditional sheet metal piercing techniques, it may be possible to produce holes at a size of 1.3 to 1.5 times the material thickness, but this is considered bad practice. Producing holes at sizes less than 1.3 times the material thickness would result in a high failure rate of the product or tooling. The holes in the first burner membrane 100 of FIGS. 4 & 5 were formed using traditional sheet metal piercing techniques. An option to provide smaller holes could be to reduce the thickness of the burner membrane 14, however this can adversely affect the structural integrity and/or life expectancy of the membrane 14.

[0068] A required pattern of small holes is provided in the sheet of metal of the membrane 14 using laser cutting, wherein one or more lasers are used to cut the sheet of metal. In some examples the laser cutting comprises laser drilling. The laser drilling could include single shot or single pulse laser cutting, where a single laser beam is pulsed onto the material to cause melting thereof, and formation of a required hole. Additionally or alternatively, multi beam laser drilling could be used to form the required holes. This is a simple but fast and efficient technique to provide a required pattern of small holes, within a short space of time. The holes in the second burner membrane 200 of FIGS. 6, 7 and 8 were formed using laser cutting.

[0069] Alternatively a required pattern of holes could be provided in the steel strip using a water jet cutter where a very high pressure jet of water, potentially with an abrasive substance included in the water, is fired in the required pattern at the strip. Using this technique means that no significant heat is produced during the cutting and there are therefore no thermal influences or contamination on the steel strip.

[0070] A further alternative is to provide the required pattern of holes in the strip of material using electron beam drilling as follows. Energy is created and focused precisely on the strip of metal to provide highly localised melting. Here an electrically heated cathode produces electrons that are accelerated by an electrical field applied between a cathode and an anode at a very high voltage. A modulating electrode controls the intensity of the electron beam which is focused on to the strip of material through an electromagnetic lens to power densities of 100 million watts or greater per square centimetre. The electron beam drilling can be used to provide a required pattern accurately and very quickly, and this process can for instance create a hole every single millisecond.

[0071] A yet further alternative is to provide the pattern of holes in in the strip of material using chemical etching as follows. The steel sheet is chemically cleaned and degreased to remove debris, waxes and rolling oils. A photosensitive resist is applied to one side of the sheet. Using laser direct imaging a pattern is formed on the sheet of exposed and unexposed photosensitive material, with the unexposed photosensitive material having the required pattern of holes. The laser direct imaging provides ultraviolet light to expose the photosensitive material other than that which provides the required pattern of holes.

[0072] The unexposed and thus unhardened photosensitive material is removed and etching is carried out as follows. Acid is sprayed on to the sheet, and etching takes place in the parts of the sheet not protected by the exposed photosensitive material. This provides a required pattern of holes, and the exposed photosensitive material is then removed from the strip to provide a burner membrane with a required pattern of holes.

[0073] Obviously the size, shape and pattern of holes can be chosen as required, and in use the burner membrane works well with a hydrogen rich gas to provide ignition immediately above the outer side of the burner membrane.

[0074] This method of forming a burner membrane therefore provides a burner membrane with a pattern of holes to enable use with gases such as hydrogen. This method can also be used to efficiently provide a number of such burner membranes in a bulk manufacturing process. The burner membrane can be used with pre-mix gas burners, post-mix gas burners or naturally aspirated gas burners.

[0075] Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.