GB786962A - Improvements relating to tubulous vapour generating and vapour heating units - Google Patents

Abstract

786,962. Regulating steam temperature in boilers. BABCOCK & WILCOX, Ltd. Nov. 30, 1955 [Dec. 2, 1954], No. 34947/54. Classes 123(2) and 123(3) A water-tube boiler comprises a furnace 1 lined with steam generating tubes extending between lower headers 9, 11, 13 and an upper steam and water drum 17 and fired through the front wall 3 by pulverized fuel burners 19, the furnace gases flowing from the top of the furnace through a lateral pass 33 containing a secondary superheater 55 into a turning space 61 whence they flow downwardly in parallel passes 63, 65 locating respectively a primary superheater 115 and a reheater 101. The respective bottom outlets 143, 159 from the passes 63, 65 are controlled by dampers 145, 161, the gas streams from the outlets uniting for downward flow in a pass 147 containing economiser sections 149, a lateral duct 167 conveying the gases to an air heater, an induced draught fan and chimney (!not shown). Saturated steam from the drum 17 passes downwardly in a cage of superheater tubes forming the basis of the downpasses 63, 65 and the dividing wall 67 therebetween, the steam then flowing upwardly in the primary superheater 115, through a spray attemperator 129 and the secondary superheater 55 to the final outlet header 59. To control the temperatures of reheat and superheat, cool flue gas is withdrawn from the bottom of downpass 147 by a fan 177 which delivers it into a manifold 183 from which damper-controlled ducts 187, 189 convey the gas to ports disposed in the front wall 3 of the furnace at the level of the furnace nose 23 and to ports at the upper end of the furnace ash collecting chamber 49 respectively. An additional branch duct 193 extends upwards from the manifold 183 and has a lateral extension 194 to two ducts 195, 197, Fig. 2, having respective control dampers 211, 213 and ducts 199, 203 which convey the gases into the upper ends of downpassses 65, 63 through ports 201, 205. In operation at high loads, the temperature of reheat, which would otherwise be too high, is limited by recycling gas to ports 201, the resulting increased flow of hot gas in the pass 63 causing increased superheat which is then offset by attemperation. As the load falls flue gases are recirculated to the bottom of the furnace to increase the heat available for steam heating and, since the reheat temperature will then be too low, flue gas is introduced into pass 63 to increase the proportion of hot gas flowing in the reheater pass 65. At or near full load, flue gases are also delivered to the top of the furnace to limit the temperature of the gases entering the pass 33 to avoid deposition of sticky slag on the superheater 55. The proportioning of the furnace gases between passes 63, 65 is thus achieved by gas recirculation but minor adjustments of the dampers 145, 161 may be made to vary the relative temperatures of the reheat and superheat. If it is desired to start recirculation to the furnace when the pressure drop between the latter and downpass 147 is greater than the pressure developed by the fan 177, the dampers in ducts 187, 189 are closed and dampers 211, 213 opened, gas being recycled to the downpasses 63, 65 until the fan pressure is sufficient to overcome the said pressure drop, when dampers in the ducts 187, and 189 if desired, are opened and the dampers 211, 213 closed.