PILOT BURNER

Abstract: A pilot burner comprises a burner body (7) including a mixing chamber, the mixing chamber having a gas supply inlet (5) and a primary air inlet (8) arranged to permit, in use, introduction of gas and air respectively into the mixing chamber. The chamber is provided with an outlet having an outlet port (9) in the form of a slot with an enlarged portion (10) disposed substantially centrally thereof. The pilot burner can be used with a thermocouple (3) and spark electrode (4) in a pilot burner and atmospheric sensing assembly for a gas-fired appliance.

Description: Title - Pilot Burner This invention relates to a pilot burner, in particular to a pilot burner suitable for use in gas-fired appliances. Pilot burners are used in gas-fired appliances, eg gas fires, boilers and water heaters, to ignite the main burner flame. The pilot flame needs to be highly stable. A problem which may occur with gas appliances is that, for example in a poorly ventilated environment, they may result in an increase in the atmospheric concentration of noxious or toxic gases such as carbon dioxide and carbon monoxide (or a corresponding decrease in oxygen concentration). This may be dangerous for persons in the vicinity of the appliance. Pilot burner assemblies therefore commonly produce a secondary flame which is arranged to heat a thermoelectric device such as a thermocouple. So long as the secondary flame acts on the thermocouple, an e.m.f. is generated and this maintains a safety valve, eg a solenoid valve, in an open condition, thereby maintaining the gas supply to the pilot and the main burner. If for any reason the secondary flame is extinguished, for example as a result of a substantial fall in the gas supply pressure or as a result of a rise in ambient carbon dioxide concentration, the thermocouple cools and the solenoid valve closes, cutting off the gas supply. The secondary flame thus acts as a safety device. A disadvantage of known forms of such pilot burners is that the response to depletion of ambient oxygen (so-called "vitiation") may be inadequate, leading to shut-off of the gas supply only after the composition of gases in the room has reached a dangerous level. Such pilot burners may also perform inconsistently, ie the cut-off characteristics of different burners of the same nominal design may vary substantially. There has now been devised an improved form of pilot burner, which overcomes or substantially mitigates the disadvantages associated with known pilot burners. According to the invention, a pilot burner comprises a burner body including a mixing chamber, the mixing chamber having a gas supply inlet and a primary air inlet arranged to permit, in use, introduction of gas and air respectively into the mixing chamber, the chamber being provided with an outlet having an outlet port, the port comprising a slot with an enlarged portion disposed substantially centrally thereof. The pilot burner according to the invention is advantageous primarily in that the shape of the outlet port is such that the flame created at the burner has three distinct regions - a central region which is the largest and serves as the main pilot flame, and lateral regions which are smaller. One of the lateral flame regions may serve to act upon a thermoelectric device for the maintenance of the gas supply, while the other may be directed towards, for instance, a spark electrode for ignition of the flame. It is found that the lateral regions of the flame are particularly sensitive to vitiation or reduction in gas supply pressure. Either deviation from normal operating conditions results in a marked change in the flame pattern and this results in a movement of the lateral flame region away from the thermoelectric device. Consequential cooling of the thermoelectric device results in shutting off of the gas supply. This occurs before there is any significant diminution of the main, central, flame region. The pilot burner of the present invention may be more sensitive to increasing levels of, for example, carbon dioxide, than is the case for other known pilot burners. This may lead to shut off of the gas supply at lower atmospheric carbon dioxide levels, resulting in greater safety. Nonetheless the robust and vigorous central flame region stabilizes the flame and persists until shut-off occurs. The pilot burner according to the invention may also be manufactured very reproducibly so that the performance of the burner is extremely consistent. The outlet port is preferably part-circular, eg a semicircular slot and is preferably formed in an outlet head of annular cross-section, eg a generally cylindrical pilot burner head. In other arrangements, the slot can be disposed at any appropriate angle from horizontal to vertical, with the external shape of the outlet head modified to accommodate this. The slot is preferably a deep slot, i.e. its depth is preferably greater than its width. This may be achieved by forming the slot in a thick-walled body. Such an arrangement facilitates induction of secondary combustion air at the outlet port and provides a suitable gas exit velocity profile. It is particularly preferred that the slot be sharp-edged, ie that the walls of the slot be disposed substantially orthogonally to the surface of the body in which the slot is formed. In particular, it is found that in a pilot burner suitable for use, for example, in a domestic gas-fired appliance such as a gas fire, boiler or water heater, the slot is preferably semicircular and is formed in a cylindrical body. The cylindrical body will generally have an outside diameter in the range 5mm to 15mm, and more preferably 7mm to 13mm, and an internal bore of 3mm to 6mm. more preferably 3.5mm to 5.5mm. The depth of the slot in such a case will be the thickness of the hollow cylindrical body, ie it will be typically 1.5mm to 6.5mm. The width of the slot is preferably 0.5mm to 1.3mm. The ratio of the slot width to its depth is typically in the range 1 :2 to 1 :4, more preferably 1 :2.5 to 1 :3. The enlarged portion of the slot is most preferably circular in cross-section, though it may have any other suitable shape, and most preferably extends radially within the burner body. In one particular arrangement the outlet port can be disposed at right angles to the longitudinal axis of the mixing chamber. Other configurations with the plane of the outlet slot disposed at any angle between 90° and 0° to the axis of the mixing chamber may also be effective without diminution of performance. This enables a burner mounted on a fixed plate or bracket, together with a thermocouple and spark electrode to have an outlet slot configuration which optimises the projection of the pilot flame to reliably ignite the main burner(s) of a wide range of appliances. Thus, according to another aspect of the invention, there is provided a pilot burner and atmospheric sensing assembly for use in a gas-fired appliance, the assembly comprising a pilot burner with a burner body including a mixing chamber, the mixing chamber having a gas supply inlet and a primary air inlet arranged to permit, in use, introduction of gas and air respectively into the mixing chamber, the chamber being provided with an outlet having an outlet port comprising a slot with an enlarged portion disposed substantially centrally thereof, a thermoelectric device fixedly mounted adjacent a first end of said slot and a spark electrode fixedly mounted adjacent the other end of said slot. The thermoelectric device, most commonly a thermocouple, is positioned such that it is acted upon by a lateral region of the flame formed at the slot. Most preferably, the thermocouple is mounted a short distance forward of the centre of the burner body. The spark electrode is positioned with its tip an appropriate distance from the burner body to suit the voltage source, most commonly a piezoelectric device, to which it is connected. The electrode most preferably comprises a rod of metal, commonly steel, which can be physically deformed to adjust the spark gap between the tip of the electrode and the burner body. Embodiments of the pilot burner according to the invention will now be described, by way of illustration only, with reference to the accompanying drawings, in which Figure 1 is a front elevational view of a first embodiment of a pilot burner assembly incorporating a burner according to the invention; Figure 2 is a rear view of the pilot burner assembly of Figure 1 ; Figure 3 is a side view of the assembly of Figure 2; Figure 4 is a plan view of the assembly of Figure 2; Figure 5 is a sectional view along the line A-A in Figure 1 ; Figure 6 is a plan view of the burner in operation, showing the normal flame condition; Figure 7 is a view similar to Figure 3, but showing the flame in a vitiated condition; Figure .8 is a view similar to Figure 1 of a second embodiment of a pilot burner assembly; Figure 9 is a plan view of the assembly of Figure 8; Figure 10 is a view similar to Figure 1 of a third embodiment of a pilot burner assembly; and Figure 1 1 is a plan view of the assembly of Figure 10. Referring first to Figures 1 and 5, a pilot burner comprises an injector tube (1) with an internal convergent cylindrical bore of suitable dimensions which is provided with a gas inlet connection (5), a jet nozzle (6) with a diameter in the range 0.2 to 0.5mm, a fixed primary air inlet of one or more holes (8) and a cylindrical burner head (7) with a discharge port configured as a semicircular slot (9) with a central circular enlargement (10). The slot (9) has a depth d (corresponding to the wall thickness of the burner head (7) - see Figure 5) which is several times greater than its width w. This burner is mounted on a bracket (2), together with a thermocouple (3) and a spark electrode (4) positioned for optimum performance and fixed to prevent displacement. The electrode (4) is mounted within an insulating sleeve (41) and is provided at its lower end with a tab (42) by which it can be electrically connected to a voltage source such as a conventional piezoelectric ignition device. The thermocouple (3) is connected, in use, by a conductor (31 ) to a solenoid shut-off valve (not shown) in conventional fashion. In use, the gas supply is turned on, eg by manual rotation of a control knob on the appliance, against the action of a spring. Rotation of the control knob also activates the piezoelectric ignition device, causing a spark to be generated in the gap between the electrode (4) and the burner head (7). This ignites a flame at the slot (9) which in turn heats the thermocouple (3). Once the thermocouple (3) has reached its operating temperature, the control knob may be released without shutting off the gas supply. If the knob is released before the operating temperature is reached then the gas supply is shut off by the solenoid valve. With an injected primary air premix in the range 35 to 60% the fan shaped flame produced by the specially profiled burner port (9,10) is stable and vigorous with a spread of approximately 180 degrees but featuring three predominant jets of flame, one at the centre and one each at the extreme edges of the fan. This is shown in Figure 6. The generation of the predominant central jet of the flame stabilises and controls the overall shape of the flame. In a vitiating atmosphere and rising CO 2 concentration the fan shaped flame starts to narrow in spread causing that portion of the flame normally impinging on the thermocouple (3) to shift away and thus cause the thermocouple (3) to cool and the solenoid shut-off valve to close before a dangerous CO or CO 2 level is reached. This is shown in Figure 7. Referring now to Figures 8 to 11, other embodiments of the pilot burner assembly according to the invention are broadly similar to that described above. However, these embodiments differ from the previously-described embodiment and from each other in the orientation of the slot. In the embodiment of Figures 8 and 9, the slot (110) is oriented vertically, ie co-axially with the bore of the burner head (107). In the third embodiment, shown in Figures 10 and 1 1, the slot (210) is inclined at approximately 45° to the vertical. In both cases, other components of the assembly are as described above.

Claims: Claims 1. A pilot burner comprising a burner body including a mixing chamber, the mixing chamber having a gas supply inlet and a primary air inlet arranged to permit, in use, introduction of gas and air respectively into the mixing chamber, the chamber being provided with an outlet having an outlet port, the port comprising a slot with an enlarged portion disposed substantially centrally thereof. 2. A pilot burner as claimed in Claim 1 , wherein the outlet port is part-circular. 3. A pilot burner as claimed in Claim 2, wherein the outlet port is semi-circular. 4. A pilot burner as claimed in any preceding claim, wherein the outlet port is of annular cross-section and is formed in a generally cylindrical burner head. 5. A pilot burner as claimed in any preceding claim, wherein the slot is a deep slot with a depth greater than its width. 6. A pilot burner as claimed in any preceding claim, wherein the slot is sharp-edged, the walls of the slot being disposed substantially orthogonally to the surface of the body in which the slot is formed. 7. A pilot burner as claimed in any preceding claim, wherein the slot is semicircular and is formed in a cylindrical body. 8. A pilot burner as claimed in any preceding claim, wherein the burner body has an outside diameter in the range 5mm to 15mm. 9. A pilot burner as claimed in any preceding claim, wherein the burner body has an internal bore of 3mm to 6mm. 10. A pilot burner as claimed in any preceding claim, wherein the slot has a depth of 1 ,5mm o 6.5mm. 11. A pilot burner as claimed in any preceding claim, wherein the slot has a width of 0.5mm to 1.3mm. 12. A pilot burner as claimed in any preceding claim, wherein the ratio of the slot width to its depth is in the range 1 :2 to 1 :4. 13. A pilot burner as claimed in any preceding claim, wherein the enlarged portion of the slot is circular in cross-section. 14. A pilot burner as claimed in any preceding claim, wherein the outlet port is disposed at right angles to the longitudinal axis of the mixing chamber. 15. A pilot burner and atmospheric sensing assembly for use in a gas-fired appliance, the assembly comprising a pilot burner as claimed in any preceding claim, a thermoelectric device fixedly mounted adjacent a first end of the slot and a spark electrode fixedly mounted adjacent the other end of the slot. 16. An assembly as claimed in Claim 15, wherein the thermocouple is mounted forward of the centre of the burner body.

Inventors: HEFFERNAN WILLIAM JOSEPH, RING ALLAN ARTHUR, HEFFERNAN, WILLIAM, JOSEPH, RING, ALLAN, ARTHUR
Applicants: HEFFERNAN WILLIAM JOSEPH, RING ALLAN ARTHUR, SEAGAS COPILOT LIMITED, HEFFERNAN, WILLIAM, JOSEPH, RING, ALLAN, ARTHUR
Application year: 1997
Main classification: F23Q9
IPC codes: F23Q9