GB791239A - Improvements in or relating to steam generators - Google Patents

Abstract

791,239. Combustion apparatus. COMBUSTION ENGINEERING, Inc. March 31, 1955, No. 9409/55. Class 51 (1). [Also in Group XIII] A water-tube steam boiler, wherein the steam temperature at loads belowthe maximum is held at the desired value by the recirculation of flue gas comprises a furnace 10 lined with steam generating tubes 14 and fired by burners 12, the combustion gases leaving the furnace at 28 and flowing through a passageway 29 locating successively a superheater 32, an economizer 34, a main air heater 36, the lower compartment 38 of a dust collector 40 and an induced draught fan 42 from which the gases are conveyed to a stack 30. Connected in parallel with the economizer the air heater, the compartment 38 and the fan 42 is a duct 48 having dampers 50, 52 at its inlet and outlet respectively. In the duct 48 is positioned successively an auxiliary air preheater 54, the upper compartment 56 of the dust collector 40 and a fan 58. At the respective outlets from the air heaters 36, 54, the passageway 29 and the duct 48 are connected by a duct 60 controlled by a valve 62. Branching from the outlet of the fan 58 is a duct 76 through which cooled combustion gases may be recirculated to the furnace, the flow in the duct being controlled by a damper 78. The two fans 42, 58 are driven by a single motor 75. Combustion air is supplied to the furnace by a fan 64 which delivers the air through ducts 66, 68, the air heater 36 and a duct 70, the auxiliary air heater 54 being connected in by-pass relation with the duct 66. Flow through the duct 66 and the air heater 54 is respectively controlled by dampers 72. 74. At or near maximum load the boiler is designed to give the proper steam temperature without gas recirculation. Under these conditions the valves 50, 74, 78 are closed and the valves 52, 62, 72 open, whereby both of the fans 42, 58 act as parallel-connected induced draught fans. As the load decreases the valves 50, 74, 78 are moved towards the open position and the valves 52, 62, 72 correspondingly closed, the adjustment being such that the outlet pressure of the fan 58 is increased to above that in the furnace and gas is recirculated to the latter through the duct 76 the flow being controlled by the automatic adjustment of the valves 52, 78' in response to variations in the temperature of steam in the superheater outlet header 44. With this arrangement, a completely separate recirculating fan is not required and the motor 75 need only have sufficient power to handle the volume of combustion gas generated at full load.