GB835232A - Improvements in tubulous vapour generating and vapour heating units and a method of operating such units - Google Patents

Abstract

835,232. Water-tube steam boilers. BABCOCK & WILCOX Ltd., Sept. 4, 1956 [Sept. 7, 1955], No. 25605/55. Classes 123(2) and 123(3) A high-capacity water-tube boiler which may be accommodated in a space affording relatively little headroom for example, a tunnel and which may be brought rapidly from idling to full load without endangering the unit or its associated turbine includes a furnace chamber 1 lined with steam generating tubes extending from lower headers such as 5 to a steam and water drum 9, combustion gases leaving the furnace through an outlet 17 and passing downwardly in an open pass 19 which they leave through an inclined tube screen 25. The gases then enter a horizontally extending gas passage 27 formed by a lining of refractory material within a steel casing and spanned by a secondary superheater 57, the passage thereafter being divided by a wall 37 into parallel passes 39, 41 respectively containing a reheater 43 and a primary superheater 49, economiser sections 59 being disposed downstream in each pass. Gas flow in the passes is controlled by outlet dampers 61 and, beyond the dampers, outlets 63 are provided through which cooled flue gas is withdrawn by fans 67, 69 and supplied through damper-controlled ducts to the bottom of the furnace and to the top and bottom of the open pass pass 19. For improving the operation of the unit when it is fired to give an output of 5 to 6% of its maximum when the turbine is idling, auxiliary gas recirculating means are provided comprising a fan 101 which withdraws gases from the passes 39, 41 downstream of the steam heaters 43, 49 and supplies them to the bottom of the furnace through a damper-controlled duct 109. When operating at full load gases are recirculated to the bottom of the pass 19 to mix with and temper the fresh furnace gas in the zone upstream of the superheater 57. As the load decreases, the recycled gas flow to the bottom of the pass 19 is decreased and that to the top of the pass increased to reduce radiation in the pass and maintain the superheater temperature, the reheat temperature being controlled by adjustment of the dampers 61. In the lower range of loads, gases are recycled to the furnace chamber. At turbine idling load the fan 101 recycles sufficient gas to the furnace to ensure that condensation in the steam heaters does not occur and that the temperatures of the heating surfaces of the unit and the turbine are such that the plant may be rapidly placed under load.