Why are the control devices for intake and exhaust fans mostly installed at the inlet rather than at the outlet?

During operation, the flow rate of the fan always needs to be adjusted in accordance with changes in the boiler load. There are generally two methods of adjustment: one is to use adjustable baffles installed at the fan outlet, and the other is to use guides or simple flow directors installed at the inlet. However, the economic efficiency of adjustment at the outlet differs from that at the inlet. When using outlet regulation to reduce flow rate, it is necessary to close the regulating damper, thereby increasing the resistance in the air duct. This causes the pipeline characteristic curve Q—ΔP to shift from OA to OB; the operating point of the fan moves from A to B, the fan’s flow rate decreases from QA to QB, and the fan’s pressure head increases by ΔP. However, this increased pressure head serves to compensate for the throttling losses that arise as a result of the reduced flow rate. When inlet regulation is used, the opening degree of the guide vane is changed to alter the preliminary swirl at the fan inlet, thereby modifying the P–Q curve of the fan itself, while the characteristics of the outlet duct remain unchanged. When the guide vane is fully open, the fan’s characteristic curve is BA with the operating point at A; when the guide vane is reduced, the swirl at the fan inlet increases first, and the outlet pressure drops accordingly. The fan’s characteristic curve then changes to BA1, with the operating point shifting to A1. At this point, both the flow rate and head of the fan decrease, and the power consumption of the fan also drops accordingly; therefore, using inlet regulation is more economical than outlet regulation. The control device for the intake and exhaust fans is installed at the inlet rather than at the outlet; the damper at the outlet can only be in an fully open or fully closed position, serving to connect or disconnect the fans from the flue gas system

I agree with the view from the second floor