GB791452A - Improvements relating to tubulous vapour generating and vapour heating units - Google Patents

Abstract

791,452. Regulating steam temperature in water-tube boilers. BABCOCK & WILCOX, Ltd. Dec. 2, 1955 [Dec. 2 1954], No. 34948/54. Classes 123(2) and 123(3) A water-tube boiler has a vertical furnace 1 lined with steam generating tubes and fired through the front wall 3 by pulverized fuel burners 2, the hot gases flowing through a lateral pass 11 locating a secondary superheater 22 and then turning in a chamber 25 for downward flow through parallel passes 26, 27 locating respectively a reheater 35 and a primary superheater 40. Below the reheater and the superheater 40 the front and rear walls 28, 29 of the passes converge at 28a, 29a to the entrance of a common downpass 47 containing an economizer 48. The converging walls 28a, 29a are formed with respective flue gas recirculation outlets 49, 59 from which ducts 50, 60 lead to the inlet of a fan 52 which delivers the flue gas either through a duct 63 controlled by a damper 65 into the ash collection chamber 17 disposed below the furnace 1 whence it rises into the furnace, or through a duct 66 controlled by a damper 68 to ports 67 located in the front wall 3 of the furnace at the level of an arch 10 projecting from the rear wall 7 thereof. The effective size of the openings 49, 59 can be regulated by respective independently operable dampers 53, 61 which can each be swung from a position wherein it completely closes the gas recirculation opening, leaving the flow path from the respective downpass 26, 27 fully open, to one in which it closes off the downpass and fully opens the gas recirculation opening. When operating at a predetermined design load and without gas recirculation, the dampers 53, 61 being fully open, i.e. in the position shown for the damper 61, both the superheated and reheated steam have the desired temperature. At loads above the design load, the reheat temperature, which would otherwise be too high is held at the correct value by reducing gas flow in the pass 26 by adjusting the dampers 53, 61, excessive superheat being corrected by an interstage attemperator 46 and overheating of the superheater 22 being avoided by recirculating gas to the ports 67. At loads below the design load, gas is recirculated to the bottom of the furnace to increase the heat available for superheating and reheating, the dampers 53, 61 being adjusted as necessary to maintain the required relationship between the two temperatures. With this arrangement the gases to be recirculated are withdrawn from the parallel passes at a location on the higher pressure side of the proportioning control dampers 53, 61, the lower pressure drop across the fan 52 correspondingly reducing the power required to drive the fan. When the fan 52 is of the overhung type with radial blades developing a pressure head which rises with an increase in mass flow, the pressure-head at zero-mass flow may be insufficient to overcome the pressure difference between the furnace and the passes 26, 27 so that recirculation cannot be initiated. This is overcome by closing damper 61 for a short period until the fan 52 establishes a gas now against the reduced pressure difference, when the damper is reset.