Adjustment and flow rate of centrifugal blowers

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For large centrifugal blowers, the main option is Siemens’ HV-turbo single-stage high-speed centrifugal blowers. In recent years, the use of air-suspended centrifugal blowers has been increasing gradually. 1. HV-turbo centrifugal blowers can be adjusted in three main ways: through inlet vanes, through outlet vanes, or by using both inlet and outlet vanes simultaneously. Typically, centrifugal blowers rely on pressure regulation via the air ducts; by comparing the set pressure with the actual pressure, a PID control system built into the blower’s LCP control cabinet adjusts the opening degree of the vanes. This is the most common approach. But is it possible to adjust the blower’s vane opening directly based on the set and actual air flow rates? Recently, I noticed that it seems the blower is equipped with an Aba flow meter. In the LCP control cabinet, in addition to automatic pressure regulation, there is also automatic regulation based on air flow rate, but I didn’t take a close look ； 2. Regarding the issue of flow meters for centrifugal fans, why are some Aba flow meters installed at the fan inlet? Aren’t they usually installed on the outlet pipeline of the fan? Is this because the associated centrifugal fan uses inlet vanes for flow regulation – that is, when the inlet vanes are used to control the airflow, the flow meter is installed at the inlet; while when outlet vanes are used for flow regulation, the flow meter is installed at the outlet? 3. Air-suspended centrifugal blowers do not regulate the air volume by adjusting the opening of the guide vanes; instead, they use variable frequency to control the motor speed. Then why is a flow meter installed at the inlet?