The work of induced draft fan of boiler is based on the principle of wings-form. The gas enters the impeller as a power angle,and produces a acting force of equal size and opposite direction,  making the  the gas sprially and axially move forward on the impeller,  so as to be discharged. At the same time, due to the effect of differential pressure at the inlet of fan, the gas is constantly inhaled. As for the fan with adjustable moving blade, the greater the power angle is, the greater the perimeter of the wing back is and the lift force is; the greater the differential pressure of the fan is and the smaller the air volume is. When the power angle of the blade reaches the critical value, the gas will leave the wing back formed line and vortex will happen, at this moment, the pressure of the fan is bigger and the air volume decreases and stall phenomenon occurs. 

The working principle of adjusting mechanism for the moving blade of the fan is  that operated by the servo mechanism, the  hydraulic cylinder control valve and incision channel change, making the oil amount and oil pressure of the both sides of one fixed differential piston change, thus pushing the axial movement of hydraulic cylinder body and driving the regulating elements inside the rotor blade connected witht the hydraulic cylinder body and making the angle of blade change. When the external regulation arm and control valve are in a given position, the hydraulic cylinder moves to the two sides of the differential piston to make the acting forces of hydraulic oil equal, the hydraulic cylinder will automatically be in equilibrium state without swinging . At this moment, the angel of moving blades is no longer to change. 

The adjusting advantages of induced draft fan of boiler are: extensive adjusting range, reachable  maximum volume flow sensitive adjusting device,  reliable in transmission, convenient in operation. It is more than mechanical adjusting device.