GB1347709A - Electric ignition and control systems for fuel burners - Google Patents

Abstract

1347709 Electrical ignition and control systems for burners ROBERTSHAW CONTROLS CO 11 June 1971 [12 June 1970] 27582/71 Headings F4F and F4T A fail-safe ignition control system for a fuel burner 10 employing an electrical igniter 14 with a positive temperature coefficient of resistance as a flame sensor is energized by a transformer 16 when pole 28 of a switch 26 contacts terminal 44 under the action of a thermostatic switch 24 in response to a demand for heat thereby energizing a wire 128 and igniter 14 through a switch 56 and causing wire 128 to expand and pivot a lever 132 anti-clockwise so as to open a switch 90 and close a switch 98 thus completing a circuit through a fuel valve 12 to burner 10 which therefore opens and the burner is ignited. So long as there is a burner flame the igniter 14 presents a high resistance in parallel to a holding coil 168 which is therefore able to retain a lever arm 72, but on flame failure the lever arm is rotated by a bias spring 76 to open switches 56 and 106 and close valve 12 whereupon the system is recycled for re-ignition by the contraction of wire 128 as it cools. Lever 72 may also be released if the current through igniter 14 is too great by the over-expansion of wire 128 and engagement between a leaf-spring 152 and the head of a bolt 80. Use of these two safety mechanisms operate in the event of power failure and faults or short circuits to igniter 14, wire 128, coil 168 and diodes 180 and 182 which are provided to rectify the voltage across coil 168. A resistor 32 is normally connected in series with igniter 14 across a low voltage tap 42 by switch 26 when there is no demand for heat, so that the igniter is always at least partially energized thereby minimizing thermal shocks. In an alternative embodiment a time delay relay coil is provided in parallel with fuel valve 12 to trip a switch a predetermined time interval after ignition has been initiated which, by putting a small resistance in parallel with coil 168, will cut out the fuel supply unless there is a burner flame to activate coil 168. In a further embodiment instead of a time delay relay coil a bi-metal operator is provided wound with a heating coil in series with valve 12 and controlling a double pole double throw switch, and two taps are provided on the secondary of the transformer so that igniter 14 has three different energization levels -for standby, for ignition and for flame proving.