Can centrifugal blowers experience surge?

Surge, as the name implies, is similar to asthma in humans; it involves periodic flow of air and reverse flow in the fan. Axial flow fans are more prone to surge. Severe surge can cause fatigue and damage to the fan blades. The typical symptoms of a fan experiencing surge are as follows: 1. The current decreases and fluctuates frequently, and the outlet air pressure also fluctuates. 2 The fan produces abnormal noise at high levels, experiences significant vibration, its casing temperature rises, and the surging vibration of the supply and exhaust fans causes fluctuations in the negative pressure inside the furnace, leading to unstable combustion. Common causes: 1. Excessive dust accumulation in the smoke ducts, blockages in these ducts, or insufficient opening of the dampers in the smoke ducts, all of which lead to high system resistance. (We have encountered this occasionally, but not frequently.) When two fans operate in parallel, excessive differences in the opening degrees of their guide vanes can cause the fan with a smaller opening degree to operate in the surge zone. (A common scenario we face is where the linkage of the fan’s guide vane actuator comes loose under varying loads, resulting in asynchronous adjustment of the guide vanes of the two fans and thus large discrepancies.) 4. The fans operate at low capacity for extended periods of time. The general principle for handling this situation is to adjust the load and reduce the blade opening of the fans with higher output so that their outputs are similar; thereafter, investigate based on the possible causes mentioned above and take appropriate action. So-called surge occurs when a fan with a Q-H performance curve in the shape of a hump operates to the left of the critical point on that curve, that is, in the unstable region; in such cases, the fan’s flow rate and head experience unstable, periodic fluctuations in an instant. The maximum head generated by the fan will be less than the losses in the pipeline, causing the fluid to flow in the opposite direction, back from the pipeline into the fan (resulting in negative flow). Since the fan continues to operate, it starts to deliver flow again when the pressure in the pipeline decreases. As long as the flow rate required by the outside system remains below the critical value, this process repeats itself, resulting in surge. The left half of the performance curve of an axial flow fan features a saddle-shaped region. Operating within this region can sometimes lead to significant fluctuations in the fan’s flow rate, pressure head, and power; intense vibrations occur in the fan and ducts, and noise levels rise considerably. Such abnormal operating conditions are generally referred to as \"surge,\" and this unstable region is called the surge zone. In fact, surge is merely a phenomenon that can occur within the region of unstable operating conditions, whereas what inevitably occurs in that region is rotating flow separation, also known as rotating stall. These two operating conditions are different, but they are also related to some extent. As shown in Figure 17 below: it is the Q-H performance curve of an axial flow fan. If the fan’s flow rate is reduced using throttling, then as long as the fan’s operating point is to the right of point K, the fan will operate stably. When the fan’s flow rate Q is less than QK, the maximum pressure head generated by the fan decreases and becomes less than the pressure in the duct system. Since the capacity of the duct system is large, the pressure in the ducts remains at HK at that moment; therefore, this pressure is greater than the pressure head generated by the fan, causing the airflow to flow in the opposite direction back into the fan. As a result, the operating point shifts rapidly from point K to point C. However, the reverse airflow reduces the air volume in the duct system, causing the pressure within the ducts to drop rapidly. The operating point moves swiftly along the CD line to point D, where the flow rate is Q=0; at this point, the fan supplies no air at all. Since the fan continues to operate, when the pressure in the air duct drops to point D, the fan starts to deliver flow again. In order to balance with the pressure in the air duct, the operating point shifts from D to the corresponding point F. As long as the flow rate required by the outside world remains less than QK, the above process repeats itself. If the operating condition of the fan repeats in a cycle of F-K-C-D-F, and the frequency of this cycle coincides with the oscillation frequency of the fan system, resonance will occur, causing the fan to surge. When a fan operates in the surge zone, the flow rate fluctuates sharply, resulting in air flow impacts that cause intense vibration in the fan, increased noise, and fluctuations in air pressure. The greater the capacity and head of the fan, the more severe the effects of surge become. Therefore, for a fan to experience surge, the following conditions must be met: a) The operating point of the fan lies within the unstable region of a Q-H performance curve that has a hump shape ； b) The air duct system has a sufficiently large volume, and together with the fan it forms an elastic aerodynamic system ； c) Resonance occurs when the frequency of the entire cycle matches the airflow oscillation frequency of the system. Both rotating stall and surge occur in the unstable region to the left of the Q-H performance curve; therefore, they are closely related to the Q-H performance curve of axial flow fans. However, there is a fundamental difference between rotating stall and surge. Rotational backflow occurs in the entire unstable region to the left of the peak of the fan’s Q-H performance curve shown in Figure 5-18 ； And surge occurs only in the part of the Q-H performance curve that slopes upward to the right. The occurrence of rotational deflection is determined solely by factors such as the structural properties of the impeller blades and the airflow conditions, and is unrelated to the capacity and shape of the duct system. Rotation has a less severe impact on the proper operation of the fan compared to surge. 　The situation is different when the fan experiences surge during operation. During surge, the flow rate, total pressure, and power of the fan experience fluctuations or significant variations, accompanied by noticeable noise, sometimes even high-decibel noise. Vibrations during surge can sometimes be very severe, damaging the fan and piping system. Therefore, when surge occurs, the fan cannot operate. An oscillation warning device is installed at the inlet of the impeller in axial flow fans; this device consists of a pitot tube placed in front of the impeller, with its opening facing the direction of rotation of the impeller, as shown in Figure 5-19. A pitot tube consists of a straight tube whose end is bent at 90° (with the opening of the pitot tube facing the direction of the airflow); it is connected to a U-tube, and the reading on this U-tube (pressure gauge) should represent the sum of the kinetic energy (dynamic pressure) and static pressure of the airflow (total pressure). Under normal conditions, the airflow pressure measured by the pitot tube is negative, as it measures the pressure before the impeller. However, when the fan operates in the surge region, due to the significant fluctuations in airflow pressure, the pressure measured by the pitot tube is also a fluctuating value. In order to enable the pulse pressure sent by the pitot tube to trigger an alarm signal through the pressure switch, the alarm value for the pitot tube is set as follows: when the moving blade is at its minimum angle position (–30°), the pressure in front of the fan impeller, measured using a U-tube, plus an additional 2000 Pa, serves as the setting value for the surge alarm device. When the operating conditions exceed the surge limit, a sound and light alarm is sent to the control panel via a pitot tube and a differential pressure switch, prompting operators to take action promptly to restore the fan to normal operating conditions. 　To prevent the axial flow fan from operating within the regions of rotational flow separation and surge, it is necessary to carefully verify when selecting such a fan that its normal operating point falls within the stable region. When choosing a method of regulation, attention must be paid to any changes in the operating point. Axial flow fans with adjustable vanes use changes in the installation angle of these vanes for regulation; therefore, when the fan’s flow rate decreases, the reduction in axial velocity leads to a change in the angle of attack of the airflow. This change is compensated for by adjusting the vanes’ installation angle, thereby preventing an increase in the angle of attack and avoiding rotational flow separation or surge. As the blade installation angle decreases, the unstable region of the fan becomes smaller, which is highly beneficial for the stable operation of the fan. Specific measures to prevent surge: 1) Keep the flow rate of the pump or fan consistently greater than QK. If the flow rate required by the system is less than QK, a recirculation pipe or an automatic discharge valve can be installed to ensure that the exhaust flow rate of the fan remains greater than QK. Surge alarm device: 2) If the performance curve of the pipeline does not pass through the origin, changing the speed of the fan may also result in stable operating conditions. By using the parabola of the highest pressure point in the performance curve of the fan at various rotation speeds, the fan’s performance curve is divided into two parts: the right side represents the stable operating area, while the left side represents the unstable operating area. When the pipeline’s performance curve passes through the origin, changing the rotation speed has no effect, as the operating points at each rotation speed are under similar conditions. 3) Adjustable blades are used for axial flow fans. When the flow rate required by the system decreases, its installation angle is reduced, the performance curve shifts downward, the critical point moves to the lower left, and the output flow rate also decreases accordingly. 4) The most fundamental measure is to avoid using fans with a hump-shaped performance curve, and instead opt for fans whose performance curve slopes downward in a straight line. 　Stall and surge are two distinct concepts. Stall is a hydrodynamic phenomenon caused by the structural characteristics of the blades; its basic characteristics, such as the rotational speed in the stall region, the starting point of flow separation, and the point where it disappears, follow their own rules and are not influenced by the volume or shape of the fan system. 　Surge is a manifestation of the oscillatory behavior that arises from the interaction between the performance of a fan and the piping system. Its basic characteristics, such as amplitude and frequency, are determined by the volume of the fan-piping system. Fluctuations in flow rate, pressure, and power occur due to unstable operating conditions. However, experimental studies show that surge always occurs in close association with flow separation within the blade channels, and an increase in the attack angle is also related to a decrease in flow rate. Therefore, rotational flow separation must occur within the unstable operating region where surge takes place. This post was last edited by kingberg on 2009-3-22 15:17.]

For ordinary centrifugal fans, backflow measures are sufficient to prevent surging. It involves running a pipe from the outlet of the centrifugal fan to its inlet; by opening this pipe when surge occurs in the fan, surge can be prevented. Leaving the pipeline empty can also prevent surging.

As mentioned on the second floor, any fluid can experience surge, and gas is a type of fluid

I’m learning about it – before I only knew the phenomena but never studied the reasons, so I’ve come to make up for that