Patent GB-806917-A (GB)
Title: Improvements in heat utilising apparatus
IPC: F22G5/04
Filed: 1955-08-04
Abstract: 806,917. Recirculating combustion gases in furnaces. BABCOCK & WILCOX Ltd. Aug. 4, 1955 [Aug. 5, 1954], No. 22424/55. Class 51(1). [Also in Groups XIII and XXIX] In a heat-utilizing apparatus, such as a steam generator, having a gas supply system including a fan which delivers relatively cool gas from a region of low pressure to a high-pressure region containing high-pressure gas, automatically controlled means are provided for preventing reverse flow through the fan of relatively high temperature gas from the region of high pressure, such means being rendered effective on the occurrence of an abnormal condition in the gas supply system indicative of reverse flow or impending reverse flow. These means may be in the form of a damper arranged in a duct between the fan and the high-pressure region. The damper is arranged to close when the pressure drop across it falls to a predetermined value, say 1 inch of water. Means are also provided for rendering the automatically controlled means ineffective if the load on the fan exceeds a predetermined fraction, say a half, of the full load. The automatically controlled means is also responsive to other variable factors such as the flow and final temperature of the superheated steam. Fig. 1 shows a steam generator having a furnace chamber 10, a secondary superheater 34, a primary superheater 45, 47, 48, 49, and an economizer 60. The furnace gases ascending in the chamber 10 pass into a gas pass 44 and descend over the primary superheater into a duct 62 connected by way of an air heater 78 to a stack. A portion of the gases are bypassed through ducts 64, 66 for re-circulation by a fan 68 the outlet duct 70 of which opens at the throat 72 of the furnace chamber. A forced-draught fan 74 supplies secondary air to a wind-box 84 adjacent the burners 42 via ducts 76, 81, 82 and the air heater 78. A by-pass duct 86 controlled by a damper 88 supplies air to the duct 70. The cooling air damper 88 is operatively connected with a shut-off damper 120, Fig. 4, in the duct 70 through a linkage 176 so that when one is open the other is closed and vice versa. A control damper 112 in the duct 66 is operated by a fluid-pressure actuator 170 as described in U.S.A. Specification 2,536,184. A similar actuator 174 operates the dampers 88, 120. The actuators 170, 174 are controlled by solenoid-operated valves 222, 220 respectively. The solenoids 165, 166, Fig. 3, of the valves 220, 222 are energized under the control of a device 214 operated by a diaphragm 114 responsive to the pressure drop across the control damper 112. The device 214 has associated therewith a pushbutton switch 500 for starting and stopping the motor of the fan 68. The fan motor is in circuit with lines 153, 154, Fig. 3, connected by fuses 151<SP>1</SP>, 1521 to bus-bars 151, 152 and when the fan motor circuit breaker is closed auxiliary contacts 158 close and complete a circuit through the coil 159 of a time delay relay resulting after a delay in the closure of contacts 160. The solenoids 165, 166 are therefore energized through the normally closed contacts 164 so that the damper 88 partly closes and the dampers 112, 120 open. If the pressure drop across the damper 112 is less than 1 inch of water the contacts 168 close also and provided the load on the fan motor is less than say 50 per cent the contacts 167 are closed and circuit is completed through the coil 169 of a trip relay. The latter relay causes the contacts 164 to open and the solenoids 165, 166 to be de-energized so that the damper 88 opens and the dampers 112, 120 close. This, however, will not take place if the load on the fan motor is above 50 per cent since in that case the contacts 167 will be open. Provided that the pressure drop across the damper 112 is above the minimum value and hence the solenoid valve 222 is open, this damper is subject to further controls through a pneumatic line 210. Thus recorder-controllers 180, 192, 198 respectively responsive to the final steam temperature, the steam outflow, and the product of the mass flow of furnace gases and the gas temperature at the furnace outlet operate on pilot valves 182, 194, 200 as described in Specification 425,507. The control pressures governed by the three pilot valves are applied to a totalizing relay 190 similar to that described in Specification 461,389, [Group XXIX], the pressure from the pilot valve 182 being modified by a standardizing relay 186 also described in Specification 461,389. The resultant control pressure is applied to a relay 204 the output of which is applied to a manualautomatic selector valve 208 as described in U.S.A. Specification 2,202,485 the valve 208 being connected directly to the line 210. The state of the damper 112 is indicated by lights 234, 236 controlled by pressure-operated switches 230, 232. When the temperature in the duct 70 exceeds the normal full load value by say 50‹ F., a temperature-responsive element 240 transmits a signal through a line 242 to an interlock which is effective to de-energize the solenoid 165 so that the damper 88 opens and the damper 120 closes. Fig. 5 shows a further arrangement having shut-off dampers 346 operated automatically by an actuator 344 under the control of a solenoid-operated three-way valve 350 and control dampers 310 operated by actuators 320, 334 conjointly controlled by a solenoid-operated three-way valve 340. The cool air supply damper 372 in the duct 370 is independently operated by an actuator 376 under the control of three-way solenoid valves 382, 384. Hand-operated dampers 300, 302, 306 are also provided. In normal operation, the solenoid valve 340 connects the lines 322, 372 and the control dampers 310 are subject to the influence of recorder-controllers 400, 402, 404, 406 and a differential pressure responsive controller 360 acting through pilot valves 416, 430, 442, 448, 464, relays 420, 434, 436, 460, 456, and a manual-automatic selector valve 470. The recorder-controllers 400, 402, 404, 406 are respectively responsive to final superheat temperature, primary superheat temperature, steam flow, and the product of the outlet temperature and mass flow of the furnace gases. The responses of the controllers 400, 402 are co-ordinated by a selector relay 420 the output of which is applied to a standardizing relay 434. The output of the latter together with the responses of the controllers 404, 406 is applied to a ratio relay 436 as described in Specification 744,566, [Group XXIX]. The output of the latter is applied to a difference relay 456 which also receives the output of a standardizing relay 460 controlled by the pilot valve 464. The controller 360 is responsive to the pressure drop across the dampers 310 and as well as operating the pilot valve 464 controls a circuit similar to that in Fig. 3 whereby the solenoids of the valves 340, 350, 382 and 384 are energized and de-energized, the arrangement being such that the damper 372 is closed when the dampers 310, 346 are open and vice versa. Specification 758,230, [Group XXIX], also is referred to.