GB842419A - Improvements in tubulous vapour generating and superheating units - Google Patents

Abstract

842,419. Regulating steam temperature in boilers. BABCOCK & WILCOX Ltd. Dec. 13, 1956 [Dec. 16, 1955], No. 38043/56. Class 123(3) In a water-tube steam boiler of the type described in detail in Specification 631,528 consisting of an upright furnace chamber 10 downwardly fired by burners 24 and having a hopper bottom 12 the sloping walls of which, together with the chamber walls, are lined with steam generating tubes, a gas outlet 40 being disposed at an intermediate position relative to the height of the furnace and leading into a pass 42 containing primary and secondary superheaters 44, 46 and an economizer 84, the steam temperature is maintained constant under conditions of decreasing load by recirculating increasing quantities of cooled flue gas to the furnace at a level therein below the level of the outlet 40. The gas is withdrawn from the top of the pass 42 by a fan 90 which delivers it through a duct 96 to a cross-duct located adjacent the hopper throat and having uniformly spaced openings thereinto which distribute the gas evenly throughout the length of the hopper. In a modification, the flue gas is introduced into the furnace through a horizontal row of openings in the front wall thereof, the row of openings being disposed just below the level of the opening 40 in the rear wall. The operation of the gas recirculation system results in the filling of the lower part of the furnace with cool gases thus reducing heat absorption in the furnace and the residence time therein of the fresh combustion gases, thereby increasing the heat content in the gases available for superheating. The rate of gas recirculation is controlled according to variations in steam flow and temperature, for example by a system such as that described in Specification 707,174. Reverse flow into the fan 90 at low loads may be prevented by the use of a control system as described in Specification 803,903.