What causes surge in dust extraction fans?

What causes surge in dust extraction fans?

Causes of surge in dust extraction fans. Due to varying technical performance, frequent adjustments to performance parameters are required. If the flow rate of the dust extraction fan continuously decreases to an extremely low value, the airflow will violently form vortices within the duct. This significantly degrades flow conditions. At this point, the continuous airflow deviates from the blade passages, forming discontinuous airflow clusters at the impeller outlet. These discontinuous airflow clusters are expelled from the fan outlet. Dust extraction fans always operate within ductwork systems. After one airflow cluster exits the fan outlet, the next cluster creates negative pressure. However, the pressure within the ductwork does not drop immediately. It is well known that dust extraction fans always flow from high-pressure zones to low-pressure zones. Therefore, as the airflow moves forward, a back pressure zone forms behind it. The dust extraction fan will flow until it reaches a point where the pressure equals positive pressure, at which point it ceases to flow. The instantaneous pressure at the impeller outlet becomes higher than the negative pressure zone behind it, causing backflow. This backflow quickly encounters the second airflow group, initiating another cycle of backflow. This periodic airflow oscillation repeatedly occurs throughout the system. In fluid dynamics, this is termed separation, and in turbines, it is also known as "surge." This phenomenon, characterized by periodic vibrations and rhythmic pulsations, is often referred to as "squeaking." The whine is closely related to the ductwork system. The larger the duct capacity, the greater the surge amplitude and the lower the frequency. Conversely, smaller duct capacity results in smaller surge amplitude and higher frequency. Thus, two prerequisites exist for dust extraction fan surging: First, when fan flow is extremely low, significant deviation occurs between the air inlet angle and the blade installation angle. This substantially increases the impact angle value, boosting efficiency. Even if no deviation is detected, efficiency will rapidly decline. Second, the influence of the ductwork. When the duct network's resistance coefficient is high, its performance curve may intersect with the lower-left portion of the fan's performance curve. Surging occurs in this "surging zone." If the duct network has low resistance or short ducts, surging is unlikely to occur, but the risk of fan vibration becomes severe. Following a surge in dust extraction fans, noise intensifies due to strong airflow pulsations and periodic vibrations. Blade stresses not only surge dramatically but also exert immense pressure on propeller welds, riveted joints, the main shaft, bearings, and bearing housings. Axial forces in the bearing housings directly damage the propeller, main shaft, bearings, bearing housings, and foundation. If the dust extraction fan is not immediately stopped, the entire machine may become unusable. Alongside foundation damage, employee personal safety is also threatened, necessitating an immediate shutdown. Dust extraction fan noise is assessed based on airflow noise at the fan outlet pipe. Under normal operating conditions, fan noise remains relatively low during stable, continuous operation. The airflow generated by the entire system and the resulting noise exhibit peaks and troughs, fluctuating periodically. Upon entering reverse state, noise levels immediately increase, and the dust extraction fan emits a banging sound. Verify the pressure and flow rate at the respirator outlet. When the respirator operates stably, changes in outlet pressure and inlet flow are small and regular, with measured data fluctuating around an average value within a narrow range. Approaching reverse state causes significant fluctuations in both parameters.