GB631894A - Improvements in or relating to protection apparatus for controlling the supply of fuel to furnaces fired by means of liquid or gaseous fuels - Google Patents

Abstract

631,894. Supplying fuel to liquid and gaseous fuel burners. CRAWFORD, A. E., and JOYNER, H. D. May 15, 1947, No. 13117. [Class 75(i)] Protection apparatus for controlling the supply of liquid or gaseous fuels to a furnace in the event of the flame being extinguished comprises a circuit containing two triodes, the grid of the first being connected to a point at earth potential by way of a gap lying in the path of the flame of a furnace, a relay device controlling the fuel valve being connected in the anode circuit of the second triode, the anode current of which is dependant on the gridcathode potential of the first triode, the said potential depending on the value of the current flowing across the gap. With the mains 15 switched on and the burner 12 not ignited, the path between the electrode 10 and the burner 12 is non-conducting. The grid and cathode of V1 being connected by the resistor R1 are at the same potential and permit anode current to flow during the positive half cycle of the anode voltage of V1. This current passes through resistor R3 and charges condenser C2 to hold the grid of V2 at a potential sufficiently negative to restrict the anode current of V2 and prevent the operation of relay K. The anode current of V2 flows via the bye-pass condenser C3 and the resistor R2 creating a potential across R2 and hence between the electrode 10 and burner 12 by way of R1. With relay K de-energized its contacts 19 and 20 connect a warning horn 21 across the mains 15 and disconnect an indicator lamp 22. When the fuel valve 13 is opened manually and the flame ignited, ionisation takes place to form a conductor path in parallel with R1 and R2 causing a current to flow through R1 and charge condenser C1 to apply a negative bias to V1 and reduce its anode current; this reduces the negative bias applied to V2 and increases its anode current thus increasing the potential difference across the burner electrodes and further increasing the negative bias on V1. This process continues until sufficient current is flowing in the anode of V2 to operate relay K to change over contacts 19 and 20 so disconnecting the warning horn 21, connecting indicator lamp 22, and the solenoid 14 to maintain fuel valve 13 open. Should the flame be extinguished the circuit reverts to the first described condition and the fuel valve 13 is closed. If a short or low resistance circuit develops between the burner 12 and electrode 10 the negative bias en V1 is reduced and so closes fuel valve 13. A spring loaded push button switch may be provided to supply current to the solenoid 14 for initial starting.