729,437. Fluid-pressure setvomotor-control systems. BABCOCK & WILCOX, Ltd. Dec. 6, 1951 [Dec. 6, 1950], No. 28636/51. Class 135. [Also in Groups XII and XIII] In a steam generating and superheating unit, the final temperature of the superheated steam is maintained substantially constant over a wide load range, say 100 to 50 per. cent, by attemperating the superheated steam in an upper load range adjacent the normal load in dependence on variations in the load and the final superheat temperature and by the recirculation of furnace gases to the steam generating section of the unit over a lower load range mainly or entirely below the load range in which attemperation takes place, regulation of the recirculated gases also being dependent on the variations in load and in the final superheat temperature. Fig. 8 shows an automatic fluid pressure control system for effecting the above method of regulation. Pulverized fuel burners 24 supplied with secondary air through a duct 28 feed a combustion chamber 10 lined with water tubes connected to a steam and water drum 20 and a water drum 22. The gases pass through an outlet 34 into a tube lined gas pass 32 whence they pass upwardly again through a gas pass 42 to an air heater 30. The pass 42 contains banks of tubes 62, 64, 66, 68, 70, 72, 74, 76, 78, 80, 82, banks 66, 74, 76, 78 comprising the primary superheater, banks 62, 64, the secondary superheater, banks 68, 70, 72, the reheater and banks 80, 82 the economizer. Steam from the drum 20 passes through the primary superheater and out through a header 114 to the attemporator 115, the supply of water to which is regulated by a valve 338. Superheated steam from the attemporator 115 enters the secondary superheater by a header 116 and leaves for use in a turbine or other by a header 120. Some of the gases from the top of the pass 42 are recirculated through a flue 350 and a duct 160 by a fan 150, these gases being passed into the secondary air or into the chamber 10 at a point 134. The gases are regulated by a damper 352 operated by a servo-mechanism 378. The valve 338 is controlled by the load on the plantas represented by the flow of secondary air in the duct 28, by the superheat outlet temperature in the header 120, and by the temperature at the outlet of the attemperator. The damper 352 on the other hand is controlled by the two first-mentioned factors and also by the flow of the recirculated gases in the flue 350. Thus the pressure difference across the orifice 314 in the duct 28 operates a pilot valve 340 controlling an averaging relay 318 which is also controlled by a further pilot valve forming part of an attemporator outlet temperature controller 316. The relay 318 controls a standardizing relay 320 which is also connected through lines 326, 324 to a pilot valve 344 operated by a controller 322 responsive to the final superheat temperature in the header 120. A selector valve 332 is interposed between.the relay 320 and the valve actuator 336. The secondary air flow also influences through the meter 310 a pilot valve 354 controlling an averaging relay 358 which is also controlled by a further pilot valve 366 operated by a meter 360 responsive to the flow of recirculated gas in the flue 350. The combined effect of the flow of secondary air and of recirculated gas is transmitted as a fluid pressure from the relay 358 to a further relay 370 where it is combined with a response from final steam temperature controller 322. A selector valve 374 is interposed between the relay 370 and the servo-mechanism 378. The starting and stopping of the fan 150 is controlled by a ratio controller 400. A control circuit 412 for the fan is made by the engagement of a movable contact 414 with one of the contacts 416, 418. The contact 414 is carried by a spring-loaded lever 415 pivoted at 417 and subject to the opposing action of two diaphragms or bellows 402, 406 subject to the control pressures from the relays 320 and 370 respectively. The fan may be arranged to start at a temperature different from that at which it stops by suitable adjustment of the contacts 416, 418. Thus the ratio controller and its associated control elements may be set so that as the load increases the fan is stopped either before or after the control point is reached. Similarly, water may be supplied to the attemperator as the load decreases down to a point slightly below the control point, and reversely the water supply may be restarted before the load reaches the control point. The pilot valves 340, 354, 366 and 344 are preferably as described in Specification 425,507, [Group XII]. The standardizing relays are as described with reference to Fig. 4 of Specification 461,389, the averaging relays being similar but slightly modified. The selector valves 332, 374 are preferably as described in U.S.A. Specification 2,202,485.