Patent GB-778345-A (GB)
Title: Improvements in tubulous vapour generating and vapour heating units
IPC: F22G5/20
Filed: 1955-08-23
Abstract: 778,345. Water-tube steam boilers; regulating steam temperature. BABCOCK & WILCOX, Ltd. Aug. 23, 1955 [Aug. 25, 1954], No. 24270/55. Classes 123(2) and 123(3) A water-tube steam boiler comprises a slag tap furnace chamber 10 fired by pulverized fuel burners 12, the chamber being lined with steam-generating tubes extending from lower headers to a steam and water drum 44. Some of the tubes 156 lining the firing wall 154 and the tubes 202 lining the opposite wall 200 are bent inwardly and then outwardly to form arches 18, 16 which define the upper limit of the furnace chamber, the arches being separated by a throat 14 through which combustion gases pass into a mixing chamber 20 disposed above the arches. From the mixing chamber the gases flow upwardly through tube screens 192, 206 in two passes separated by a tubulous partition 22, the left-hand pass 26 locating a secondary superheater 56 and an economizer 76, 78 whilst the right-hand pass above a tertiary superheater 68 at the bottom thereof is sub-divided by a vertical partition into a rear pass and a front pass locating respectively low and high temperature reheaters. Above the reheaters is disposed a primary superheater 40. The gas flow in the superheater and reheater passes is controlled by dampers 98, 100 disposed respectively at the outlets of the reheater passes and the outlet of the superheater pass 26. Flue gas discharges through twin ducts 102 and respective air heaters 104 to stack connections 106. Saturated steam from the drum 44 passes successively through the superheater 40, an attemperator 50, the secondary superheater 56, a second attemperator 62 and the superheater 68 to a final outlet header 70. The attemperators 50, 62 are preferably of the spray type, e.g. as described in Specification 656,551. To provide an additional control of steam temperature and to temper the gases from the furnace so that there is no danger of overheating of superheaters 56, 68 or fouling of their tube surfaces by sticky slag particles, cooled flue gas is withdrawn from each of the ducts 102 by respective fans 108 which direct it through twin ducts 114 each controlled by dampers 120 to the ends of chambers 116, 118 whence it passes into the arches 16, 18 and through ports 184<SP>1</SP>, 184 in the arch noses to flow with the fresh combustion gases into the chamber 20 wherein the gases are thoroughly mixed before massing through the screens 192, 206. At full load, sufficient flue gas is recycled to protect the superheater tubes from over-heating &c., and dampers 98 are adjusted automatically to give the desired reheat temperatures, the excessive superheat existing under these conditions being removed by the attemperators 50, 62. As the load is reduced, the 'firing rate, and consequently the gas flow rate through throat 14 is reduced and dampers 120 are automatically adjusted from indications of load or of furnace gas temperature to reduce the recirculated gas flow. Reheat temperatures are then maintained by the automatic opening of dampers 98 and the closing of dampers 100, the superheat temperature being held at the optimum value by the automatic reduction of the water supply to the attemperator sprays. Specification 526,825 also is referred to