Patent GB-709419-A (GB)
Abstract: 709,419. Automatic boiler control systems. SMOOT, C. H. Oct. 2, 1950, No. 24021/50. Class 38(4) A number of related variables such as fuel and air supply to a boiler are controlled by automatic systems, and the datum setting of the systems is varied in accordance with e.g. steam pressure which is thereby maintained constant. General. As shown, Fig. 1, a furnace. 11 heating a boiler 10 is supplied with primary fuel (coal) at 12, secondary fuel (gas) at 13, and primary and secondary air at 16, 18, from forced draught fans 21, 22. Each of the above supplies, i.e. forced draught pressure, secondary fuel, primary and secondary air flow, and induced draught are controlled automatically by detectors 51, 62, 73, 29 and actuators 51, 55. 58, 77, 78, 26 respectively to maintain them at present level as determined by the loading on the detector unit. The induced draught is controlled manually from panels 32, 34 independent of the other variables. The loading on the other detectors is varied automatically to maintain the steam pressure in a header 37 constant. The steam pressure signal from detector 38 controls the primary fuel actuator 57 and varies the loading on the secondary-fuel detector 59 and forced-draught-pressuredetector 52. The sum of the primary and secondary fuel signals varies the loading on the forced-draught-air-flow-detector 73. The proportion of the control signal passed on to each detector is manually adjustable by potentiometers on control panels such as 46 ... 49. Metering regulator (or detector unit)', Fig. 5. Comprises a pressure-responsive bellows 157 acting on a spring-loaded, pivoted beam 156 to move a plunger 158 and vary the exhaust outlet through pipe 159 from a differential fluid-pressure servomotor 163 which varies the tapping on a resistor 168 to provide an output voltage varying with pressure. The tension in the loading spring 155 may be adjusted by a motor driving a cam 151 bearing on a bell crank 152. The motor is also coupled to an anti-hunting velocity feedback generator and a displacement feedback potentiometer, Fig. 6 (not shown). Position regulator (or actuator), Fig. 7. Comprises a motor and feedback generator 116 and potentiometer 121 in a unit similar to the detector adjustment unit. The motor drives a link 172 to actuate a valve 174, a fluid-pressure, power-follow-up motor 182. Movement of the piston 181 (operating e.g. the louvres) rocks a pivoted cam 178 connected to a bell-crank 175 pivoted at 176, to reset the valve 174 when the follow-up position is reached. Control panel, Fig. 2 (not shown). Each panel comprises a manual/automaticcontrol, change-over switch, manual control switch, a proportioning potentiometer by which the proportion of the automatic control signal sent on may be adjusted and two voltmeters showing the automatic control signal and the actual condition signal. (These are normally the same under autocontrol.) Control system, Fig. 3. Each control system comprises a fairly conventional follow-up between the movement of the main control potentiometer arm and the actuator or loading adjustment. Steam pressure detector 38 varies a potentiometer 41 supplying proportioning potentiometers 101, 101<SP>1</SP> on the control panels. The signal from the wiper arm 102 is applied to a servo amplifier together with displacement and velocity feedback signals from potentiometer 121 and generator 125 driven by the motor 116. The sum signal for the air-flow control is derived from a transformer having its primary in parallel and secondary in series with the displacement potentiometers 121 for primary and secondary fuel. The amplifier incorporates a rectifier limiter, Fig. 2 (not shown).