GB772451A - Improvements relating to tubulous vapour generating and vapour heating units and methods of operation thereof - Google Patents

Abstract

772,451. Controlling steam temperature in water-tube boilers. BABCOCK & WILCOX, Ltd. May 24, 1955 [May 24, 1954], No. 15303/54. Classes 123(2) and 123(3) A water-tube boiler comprises twin vertical furnace chambers 1, 1x lined with steam generating tubes and separated by a tubulous partition 2, the lining tubes and those of the partition extending between lower headers such as 12, 24 and' a steam and water drum 50 common to both halves of the unit. The furnaces 1, 1x are fired by respective pulverized fuel burner groups 14, 14x and have hoppershaped bottom portions. An arch 21, 21x projects from the common rear wall 20 of the furnaces to define, with the front wall 10, an outlet throat for combustion gases. In the description below, since the two halves of the unit are identical only that associated with furnace 1 is referred to, reference numerals for corresponding parts in the other half being suffixed "x" in the Figures. From a level just below the gas outlet throat to the roof 27 of the unit, the side and front walls 32, 10 are :lined, with radiant superheater tubes 44, 45. Above the arch 21 a lateral gas pass 28, locating a secondary superheater 53 comprising pendent tube platens widely spaced across the pass, conducts combustion gases to a turning space 70 whence they flow downwardly in parallel passes 71, 72 lined with superheating tubes, and respectively housing a primary superheater 110 and a reheater 120. The gases leave passes 71, 72 through respective openings controlled by dampers 140, 150 and unite for downward flow in a pass 141 containing an economiser 142. From the bottom 162 of pass 141 the gases flow to the stack through a duct 163. Each half of the unit is provided with a flue gas recirculation system for controlling reheat and superheat temperatures. The gas is withdrawn from the bottom of pass 141 through a duct 167 and delivered by a fan 168 to a manifold 171 whence damper-controlled ducts lead to openings on each side of the hopper bottom of the furnace chamber and a damper-controlled extension leads to a branch duct 177 disposed on the furnace front wall at the level of arch 21 and provided with openings 42 into the furnace. The boiler is designed and the dampers 140, 150 set so that at or near full load the reheat temperature is at the desired value whilst the superheat temperature is too high; under these load conditions, the superheat temperature is reduced as necessary by a spray attemperator 49. With decreasing load, when the temperatures of reheat and superheat tend to fall, the reheat temperature is maintained' by recirculating flue gas to ports 42 to increase the mass flow rates through passage 28 and hence the heat available for reheating. Both the decreasing load and the gas recirculation to ports 42 tend to cause a reduction in superheat temperature which is compensated firstly by reducing the water supply to the attemperator and, at loads below attemperation range, by recirculating flue gas to the furnace bottom portions to reduce the amount of heat absorbed by the furnace lining tubes and make more available for superheating.