GB862147A - Improvements in and relating to apparatus for smoke detection - Google Patents

Abstract

862,147. Electrostatic precipitators. ASSOCIATED ELECTRICAL INDUSTRIES Ltd. May 20, 1957 [May 23, 1956], No. 15962/56. Class 39(1). Apparatus for detecting smoke particles carried in a main flow of gas 1, comprises a duct 2 for branching-off from the main flow a test flow, the duct containing a charging electrode 5 connected to a high voltage, and, positioned downstream thereof, a collecting electrode 10 connected to a circuit 21, 22 for detecting a charge on the collecting electrode. The circuit 21, 22 comprises a voltmeter V for ascertaining the voltage drop across a load resistance 21 connected between the collector electrode 10 and earth to provide a continuous indication of the rate of charge transfer to the collecting electrode. The charging electrode 5 is in the form of a wire which is suspended from a trap electrode 8 with the intermediary of an insulator 9, the electrode 8 being connected to the wall of the tube 4 which is earthed. The trap electrode 8 diverts gas ions of the test flow from the collecting electrode 10. The collector electrode 10 is in the form of a tube detachably con- -nected with the tube 4 together with an insulating gasket 13. The tube 10 contains a central electrode 14 supplied with a high-ve potential which is less-ve than the potential on the electrode 5. An elbow tube 15, connected by flanges 16, 17 and an insulating gasket 18 to the tube 10, leads to a fan for extracting the test flow. In the construction shown in Fig. 2 in which the test flow is located inside the main flow of gas a portion of the gas flow enters the test duct 2a through an opening 24 to flow towards a high-ve voltage charging electrode 26 of needle shape mounted on a pear-shaped ceramic body 28 mounted within a bulb-shaped tube 31 of glass. A concave coarse wire mesh 27 earthed through the metal section 25 determines the shape of the field provided by the electrode 26. A second electrode 32 has applied to it a voltage, less-ve than the voltage applied to the electrode 26, by a conductor ring 33 and conductor 34. The collector electrode is formed by a cylindrical section 35 detachably connected to tube 31 and connected by a conductor 36 to a measuring circuit as in Fig. 1 which may also include an amplifier. An earthed outer casing 37 is provided. The electrode 32 is mounted on a ceramic insulator 38 resting on a spider 39. An air ejector 42 may be provided to facilitate the test flow. Dampers or valves 43, 44 can be provided at the ends of the ejector and shutters at the entrance and exit ends of the test duct 2a to prevent passage of gases and deposition of dust when smoke tests are not being carried out. The air ejector 42 may be detachable from the pipe leading to the damper 44 and the opening in the flue closed by the plate P may be large enough to allow removal of the test duct 2a through the opening. The duct 2a can be supported, through the intermediary of a bracket, by plate P to be portable with the plate, or without plate P if the bracket 23 includes a clamping seating for the duct 2a as shown in Fig. 2. A rapping device comprising a rod 45 can be arranged to hit the collector electrode 35, for example automatically when the shutters, previously referred to, are opened. In a modification the second high voltage electrode is omitted and porous or multi-channel material, such as steel or metal wool, arranged to act as a collector electrode, the smoke particles discharging on to the material and thence through a surrounding wall of conducting material, insulated from the remainder of the test duct, to the measuring circuit.