Patent GB-631660-A (GB)
Abstract: 631,660. Automatic pressure control systems. BURNS, F. D. March 15, 1946, No. 8156. [Class 38 (iv)] [Also in Group XXXVIII] In a stabilizer for a process having control elements at primary and secondary sequential control stages, and respectively actuated in response to metering units responsive to process conditions at the primary and secondary stages, the operation of a primary power member in response to its associated control device actuates simultaneously an impulse generator connected to the secondary metering unit and, before any change occurs in the process conditions at the secondary stage, applies temporarily to the metering unit thereof a preacting impulse proportional in amount and direction to the changes at the primary stage. The example comprises a stabilizer system for use with a boiler or furnace of the type employing a turbine-driven forced-draught fan at the air intake of the combustion chamber, controlled to respond to variations in the steam pressure, and a turbine-driven induced-draught fan at the outlet or stack of the combustion system. The primary metering unit 10 comprises a chamber 14 sub-divided by a transverse metal diaphragm 13, with pipe connections 28, 29 on opposite sides leading to the inlet and outlet respectively of a pre-heater, and a lever 18 connected to the centre of the diaphragm and extending into a housing 17 where it is connected to a link 22 which extends in an air-tight manner through a rubber diaphragm and is connected through a spring 27 to a steam pressure gauge or other means responsive to the process under control. The lever 22 also extends through a pivoted yoke from which depends a weigh-beam 31 with a magnet 32 at its lower end, the arrangement being such that, under normal conditions with the system balanced, the magnet 32 is intermediate a pair of pivoted mercury switches 33, 34 connected to terminals on a reversible motor 36. Movement of lever 22 produced by variation in the tension of spring 27 due to a pressure change in the steam gauge, thus operates to turn the motor 36 in one direction or the other. The motor shaft is connected to radial arms 38, 39, the latter having an extension 40 connected by a link 41 to means for controlling the forced draught fan, while arms 38, 39 operate the connecting-rods 42, 46 of pistons 43, 47 to move the latter in opposite directions within open-ended cylinders 44, 48. Cylinder 44 is connected by a feed-back pressure (or vacuum) pipe 45 to the pipe 29 leading to one side of diaphragm 13, so that operation of motor 36 to actuate link 41 also produces a change in the pressure in the part 15 of metering unit 10 and effects a follow-up movement on weigh-beam 31, tending to return it to the neutral position. An adjustable bleeder valve 291 may be provided in the conduit 29 to regulate the time required to dissipate pressure received from cylinder 44. Either dead-beat regulation or a hunting regulation with greater speed of response may be obtained according to the rate at which diaphragm 13 is rebalanced. Counter-clockwise movement of motor 36 operates link 41 to increase the air flow to the combustion chamber, and simultaneously creates a sub-atmospheric pressure in cylinder 48 to produce a secondary control on the furnace draught by way of a conduit 49 associated with a metering unit 50 similar to unit 10. The pressure reduction upsets the balance of unit 50, and a weigh-beam 63 is moved to close a -motor switch 65, thereby operating motor 68 and link 72 to regulate the secondary control device which produces an increase in the speed of the induced-draught fan. At the same time a piston 74 is moved to create a positive pressure which is applied to the chamber 53 of unit 50 to rebalance the diaphragm 54 and stop the motor 68. The conduit connection between cylinder 75 and unit 50 includes a lag chamber 82 with a bleedvalve 84 opening to the atmosphere, and thus dissipates the pressure produced in cylinders 48 and 75 in a time governed by the setting of the bleed-valve and proportional to the time required for the induced draught fan to respond to the actuation of link 72. In a modification, Fig. 2 (not shown), the lag chamber is connected to a conduit joining the primary metering unit and the control cylinder 44.