Axial and centrifugal fans are two common power devices in industrial ventilation and fluid conveying systems. Careful operators will notice that axial fans require their outlet valves to be fully open when starting, while centrifugal fans require their outlet valves to be closed. This operational difference is not arbitrary but rather determined by the performance characteristics and starting mechanisms of the two fans, embodying a wealth of fluid mechanics and electrical engineering principles.

The performance curve of an axial fan is key to understanding its starting characteristics. The pressure-flow curve of an axial fan exhibits a distinctive hump-shaped curve. In the low-flow region (i.e., when the outlet valve is closed or slightly open), the fan's pressure rises sharply, and power consumption also increases significantly. This is because when the outlet valve is closed, the airflow cannot be discharged smoothly. A large amount of air forms vortices and backflows within the fan, forcing the blades to overcome the strong airflow resistance and work, resulting in a sharp increase in the motor load at startup. Starting the motor at this point can easily overload the motor and even burn out the motor windings. When the outlet valve is fully open, air flows smoothly through the fan, resulting in a high flow rate. At this point, the axial fan's power consumption is low, and the motor's starting load is low, allowing it to start smoothly and gradually reach rated speed. For example, in large tunnel ventilation systems, axial fans must ensure clear air ducts before starting. If the air ducts fail to fully open due to a valve malfunction, this can often cause motor tripping and other startup failures.

Centrifugal fans exhibit different performance characteristics. The pressure-flow curve for a centrifugal fan is relatively flat. As flow increases, air pressure gradually decreases, while power increases slowly. When the outlet valve is closed, the centrifugal fan is in a no-load or light-load state. At this point, the airflow circulating within the fan is minimal, and the resistance the motor must overcome is also low. Starting current is relatively low, effectively avoiding current surges during motor startup. Once the motor is operating normally, the outlet valve is gradually opened, allowing the flow and pressure to slowly increase, and the motor power to steadily increase, ensuring safe and stable operation of the entire system. If a centrifugal fan is started with its outlet valve fully open, the motor's instantaneous power demand will be high due to the significant pipe network resistance it must overcome. The starting current could reach 5-7 times the rated current, potentially damaging the motor itself and causing significant grid voltage fluctuations, impacting the normal operation of other equipment. In central air conditioning water circulation systems, the startup of centrifugal water pumps (which operate on similar principles to centrifugal fans) strictly adheres to the principle of closing the outlet valve first to protect the motor and the entire system.

From a motor starting perspective, the differences in starting methods between axial and centrifugal fans are also determined by their load characteristics. The load torque required to overcome during motor startup is closely related to the fan's power consumption. The high power consumption of axial fans at low flow rates requires reduced load torque during startup, and a fully open outlet valve is an effective means of achieving this goal. The low power consumption of centrifugal fans at low flow rates allows them to be easily started with the outlet valve closed, allowing the load to be gradually increased after the motor stabilizes.

Furthermore, the application scenarios of the two types of fans also impose different requirements on startup operations. Axial flow fans are mostly used in situations that require large flow and low air pressure, such as cooling tower ventilation and mine main ventilation. In these situations, the pipe network resistance is relatively small, and starting with a fully open outlet valve is more in line with the actual operation needs of the system; centrifugal fans are suitable for situations with high air pressure, medium and low flow, such as boiler induced draft and dust removal systems. The pipe network resistance is relatively large, and starting with a closed outlet valve can better adapt to the pressure changes of the system.