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Power Plant Engineering 3.19 Steam Power Plants, ° A trimming signal from fuel and _ airflow a Tow Arr ta, sensor maintains the proper air-fuel ratio. sensor sensor, ¢ A steam flow sensor is substituted for the ] 1, , fuel-flow sensor, when it is difficult to obtain, accurate fuel flows. Steam r, ¢ Usually about a 5 second delay is allowed, ae - san, when changing flows of coal and combustion l, air to ensure the prevention of a momentary Pulverized coal Forced draft, rich mixture and thereby obtaining smoke- power drive) fan drive), free combustion. t 1, , Fig. 3.11: Steam pressure control, , , , , , , , 3.19 AUTOMATIC COMBUSTION CONTROL, , , , By means of automatic combustion control, it becomes easy to maintain a constant steam pressure and, uniform furnace draught and supply of air or fuel can be regulated to meet the changes in steam, demand. The boiler operation becomes more flexible and better efficiency of combustion is achieved., This saves manual labour also., , Hagan system of automatic combustion control is shown in Fig. 3.12., , , , Hand €, regulator /, , , , Steam pressure, gauge, , , , Hand, , regulator, \ ~% Rie, , ! Master, Boiler | Air supply I relay, , I, , \, , 1, , 1, , ', , 1, , 1, 1, , , , , , , , , , , , Draft, , Seconda, connections ry, , air fan, Combustion |=." ~, chamber, , , , Hand regulator, , , , = Air supply, , 2] Air supply, , Relay, , , , , , Ged, , , , , , , , , , , , , , , , , , , , Fig. 3.12: Hagan system of automatic combustion control, Master relay R; is sensitive to small variations in steam pressure and it is connected to steam pressure, gauge., Decrease in pressure of steam operates the master relay Ri, which in turn, operates the servo-motor, coupled to the vanes of induced draught (I.D.) fan to open them slightly and simultaneously., The secondary air fan damper gets opened proportionately., By this readjustment of induced draught, stabilised conditions in the combustion chamber get changed., These changes operate relay R2, which in turn, operates the servo-motor coupled to the vanes of forced, draught (F.D.) fan to adjust them, so that, stable conditions in combustion chamber are maintained.