GB998317A - Steam generating unit - Google Patents

Abstract

998,317. Controlling steam temperature in boilers. RILEY STOKER CORPORATION. Jan. 14, 1963 [Jan. 23, 1962], No. 1555/63. Heading F4A. In a steam generator having an upwardlyfired, vertically elongated furnace discharging gases into a convection section, superheat and reheat temperatures are controlled by combining the effects of flue gas recirculation and flame placement in the furnace, flue gas being gathered from two spaced locations so that its temperature may be regulated and introduced into the furnace at upper and lower locations thereof whilst the burners are provided with means whereby the main stream of combustion gas may be positioned relative to the front and rear walls of the furnace. In the form shown the generator furnace 11 is lined with water tubes which extend between lower headers and a drum 31 and is fired by burners 26, 27 located on the lower surfaces of opposed abutments 19, 23, the burners being provided with vanes whereby the flames therefrom can be directed downwardly or upwardly into a high temperature cell 28, an upward and downward inclination respectively of the vanes of the burners 26, 27 causing the main stream of combustion gases to flow upwardly against the rear wall 15 of the upper part 29 of the furnace and vice versa. Above a nose 41 is a lateral offtake 42 leading to parallel downpasses 37, 38, the gas flow through which is controlled by dampers 51, 44. Gases leave the boiler through an air heater 56 and stack 57. Steam from the drum 31 flows successively through a primary convection superheater 33 located in the downpass 38, a desuperheater 116, a radiant superheater 30 located on the front wall 14 of the furnace and a pendent final superheater 34 to a turbine through an outlet main 59 from which signals representative of temperature and flow are transmitted from respective pickoffs 76, 128 to a main controller 49. A convection reheater is located in the pass 37 and is connected to a pendent section 43 whence steam returns to the turbine through a main 58 having a temperature pick-off 78 transmitting reheat temperature signals to the controller. Recirculated gas is withdrawn from the downpass 35 at 106 above an economizer 96 from a point below the dampers 51 through ducts 105, 68 by a fan 67 and delivered through a duct 97 to the interior of the nose 41 whence it enters the furnace 29 through openings 104 and also through a duct 65 to manifolds 61, 62 whence it flows into the cell 28 through openings disposed between the burners 26, 27, these openings being provided with gas directing vanes. At low load, when the superheat temperature tends to be below the desired value, the burner vanes are set by the controller 49 to cause the hot gases to rise adjacent the front wall of the furnace in contact with the superheater 30, their residence time in the furnace being shortened so that more heat is available for convection superheating. Simultaneously, the damper 107 is opened and the damper 95 closed and the fan 67 draws flue gas at the highest temperature through the duct 105, the dampers 70, 98 being opened so that most of the flue gas enters the furnace through the nose openings 104, the effect being further to raise the superheat temperature. At high loads when the superheat temperature tends to be too high the flame is directed against the rear furnace wall 15, the amount of recirculated gas is low and it is all withdrawn through the lower offtake so that its temperature is the minimum. Furthermore, most of the gas is introduced at burner level. At loads exceeding 100% the desuperheater is used to reduce the steam temperature and at all loads the controller 49 adjusts the dampers 44, 51 to maintain the desired relationship between superheat and reheat temperatures. Specifications 765,298 and 998,316 are referred to.