GB738214A - Improvements relating to vapour generating and superheating units and to the control of the temperature of superheated vapour - Google Patents

Abstract

738,214. Steam superheaters, BABCOCK & WILCOX, Ltd. June 4, 1953 [June 4, 1952], No. 15417/53. Class 123 (3). A steam boiler comprises a furnace chamber 10 divided into two vertical compartments by a mid-partition formed of steam generating tubes, the front and side walls of the chamber being lined with generating tubes whilst the rear or baffle wall 24, which is also formed of generating tubes, serves to separate the furnace from an uppass 32 in which is disposed primary and secondary convection superheaters 90, 96 and an economizer 156. Each furnace compartment is downwardly fired by a group of pulverized fuel burners 100, the burner groups, which are enclosed in windboxes 104, 105, being spaced inwardly from the compartment walls, so that in operation, combustion gases pass down through each compartment without impinging on the walls thereof, the gases then turning laterally for upward flow through pass 32. The steam generating tubes extend between headers such as 20, 38 and a steam and water drum 26 from which steam flows through the primary superheater 90, an interstage attemperator 93 and the secondary superheater 96 to the final outlet header 98. To maintain the temperature of superheat constant over a wide load range, the attemperator 93 is used at high loadings when the superheat temperature tends to rise above the required value, whilst at low loadings when the steam temperature tends to fall, flue gas is recirculated to the furnace to reduce the radiant heat absorption therein and give a corresponding increase in the heat content of the gases entering the superheater pass 32. The flue gas is withdrawn from outlet duct 155 by a fan 150 which discharges it through ducts 140, 142, 144 and damper controlled vertical ducts such as 120, 125 into manifolds 110 ... 113 and 114 ... 117 surrounding respectively the windboxes 104, 105, the gas passing through slots 118 provided along the undersides of the manifolds to form a downwardly moving layer on each wall surface of the furnace compartments thus partly screening the lining tubes thereof from transfer of radiant heat from the combustion products passing from the burners centrally down each compartment. By operation of the dampers in the vertical ducts such as 120, 125, the flue gas layer can be formed on one or more selected surfaces whilst the flow velocity of the layer can be controlled by adjustment of the fan speed. To avoid premature mixing of the flue gas and fresh combustion gas, the flow velocity of the flue gas layer is made approximately the same as or greater than that of the stream of combustion gas. The attemperation and gas recirculation may be automatically controlled by apparatus such as that described in Specification 729,437. Specifications 367,699, 504,114, 516,070, 523,870, 540,119 and 721,155 also are referred to.