GB755457A - Improvements in and relating to safety control devices for burners when firing furnaces - Google Patents

Abstract

755,457 Automatic circuit-breakers, thermal switches. RHEOSTATIC CO., Ltd., FLIGHT, T. N., and SAWYER, W. A. April 27, 1954, No. 12149/54. Class 38(5) [Also in Group XI] An automatic circuit-breaker in an electrical safety control system for a furnace burner (see Group XI), and serving to shut down the burner if a flame is not established in the furnace, comprises a bimetallic strip 7, Fig. 1, fixed to a metal block 8 on which a heating resistance 9 is wound, and the lower end 5 of the strip serves as a catch for engaging the end of an insulating bar 3 carrying a contact 4 and a resilient blade 2 pressing a contact 1 into engagement with the contact 4. The strip 3 is pivoted at its other end which is carried by an ambient temperature compensating strip 10. In the event of failure to establish the flame the heating resistance 9 will cause the bimetallic strip 7 to disengage the catch 5 from the arm 3, so that the latter will be drawn upwardly by a spring 14. The blade 2 is held, however, by the end of a push rod 15, the upward movement of which is restricted, so that the contacts 1, 4 become separated. The contact 1 is then held in engagement with a contact 16 through which an alarm circuit may be energized, and the heat stored in the metal block maintains the deflection of the bimetallic strip 7 so that the catch cannot be re-engaged until a safety or purge period has expired. When such period has expired the rod 15 may be pushed manually downwardly to re-engage the arm 3 with the catch 5. The control system also includes a bimetallic element 20, Fig. 2a, heated during the operation of the burner to hold open switches in the burner starting circuit for a predetermined time after each stepping of the burner. The switches comprise three contacts 24, carried by the bimetallic element 20, associated with three contacts 28, carried by an ambient temperature compensating bimetallic element 21. When the heating winding 22 is de-energized the contacts are held closed by a permanent magnet 27 carried by the strip 20, which attracts twin armatures 31, 32 carried by the strip 21, the relative inclination of the strips 20, 21 being such that the greater attraction is exerted on the armature 31. When the heating winding 22 is energized the warping of the strip 20 causes the three pairs of contacts to open with snap action, and on de-energization of the winding 22 the magnet 27 effects snap closure of the contacts. The temperature of the strip 20 at this instant is lower than that at which the contacts are opened due to the differential action of the magnetic attraction exerted on the two armatures 31, 32. The delay in closing the switches is variable by altering this differential action by adjustment of the relative inclination of the strips 20, 21 by a screw 35. Two of the contacts 24 are resiliently mounted and are so positioned that they engage the contacts 28 immediately before the engagement of the other contacts 24, 28.