Patent GB-654729-A (GB)
Title: Improvements in or relating to electric safety control systems for fuel burners
IPC: F23N5/20
Filed: 1948-08-26
Abstract: 654,729. Controlling burners. ROBERTSHAW-FULTON CONTROLS CO. Aug. 26, 1948, No. 22547. Addition to 610,145. [Classes 75(i) and 75(iii)] An improvement in or modification of any embodiment of the safety control apparatus for fuel burners claimed in the parent Specification which comprises an operating coil of a control relay the armature contacts of which control an electrically operable fuel control device, e.g. an electromagnetic valve, to which is electrically connected a thermal resistor having a preselected temperature coefficient of resistance located to be heated by a flame at the burner, and adapted when unheated by a flame in cooperation with a resistance means to cause energy of insufficient value to be supplied to the fuel control so that fuel supply is prevented, a switch for delaying co-operation for a predetermined period during which said thermal resistor has time to become heated, and a timing device controlled by the said armature contacts and operating said switch to vary the total circuit resistance so as to maintain the energy supplied to the fuel control device at a value sufficient to permit the supply of fuel during said predetermined period, consists in that said armature contacts.control the operation of the timing device only, and that, in addition to said switch, a second switch is provided which is also operated by said timing device and which controls the movement of said fuel control device. The electrical bridge circuit comprises arms CD, DA containing the thermal resistor 52 and the control resistor 54, respectively, an arm CB containing the operating coil 44 of a relay 40, and arm BA containing a thermally actuated switch 58 having contacts 62, 64, a resistance igniter 24 and a series resistance 26, and a detector arm containing a cut-out 78, the bridge being connected at the points AC across the secondary winding of a transformer 30. The switch 58 is actuated by a bi-metallic element 66 heated by a coil 68 connected in series with the relay switch 40 and across the primary winding of the transformer 30, and the operating coil of the electromagnetic valve 18 controlling the supply of fuel to the main burner 10 is connected across said primary winding in series with contacts 76, 74 adapted to be closed by the bi-metallic element 66. Thermostatic or manual opening of the main fuel valve 14 closes a main switch to energize the transformer 30, and allows fuel as far as the closed valve 18 and to the pilot burner 20 where it is ignited by the igniter 24. The bridge circuit is initially balanced and operates as a parallel mesh carrying sufficient current to energize the relay 40 to close the contacts 48, 50 and complete the circuit of the heater coil 68, so that after a predetermined period the switch 58 opens to de-energize the igniter 24 and convert the bridge circuit into a circuit including the resistor 54 in series with a parallel mesh comprising the resistor 52 in parallel with the relay coil 44 and the cut-out 78. Further warping of the bi-metallic element 66 causes contacts 74, 76 to close to energize the operating coil of the valve 18 to permit fuel to pass to the main burner 10. In the event of failure of the pilot burner flame the resistor 52 will cool and the current through the relay coil 44 will become insufficient to maintain the contacts 48, 50 closed so that the coil 68 is de-energized. The arm 66 then moves to disconnect the contacts 76, 74 and to close the switch 58, such movement occupying a period of time to effect a delay between the closing of the valve 18 and the re-energization of the igniter 24. Specification 589,873, is referred to.