Reasons for stalling axial fans

Xiao Bian continues to popularize the end of the article for everyone. The axial fan stall is followed by the first two articles, and it is also the end of the knowledge of Xiaobian popular axial flow machine. I hope that through the three articles shared by Xiaobian, everyone can Have a clear understanding of the three major problems that may occur with the axial flow machine, then look down.

The axial fan blades are usually streamlined. When operating under design conditions, the airflow angle (ie, the difference between the direction of the inlet airflow relative to the speed w and the blade mounting angle) is about zero, the airflow resistance is small, and the fan efficiency is high. When the fan flow decreases, the direction angle of w changes and the airflow angle increases. When the angle of attack increases to a certain critical value, the vortex zone is generated at the end of the blade back, the so-called de-flow condition (stall), the resistance increases sharply, and the lift (pressure) decreases rapidly; the angle of attack increases again, The flow phenomenon is more serious, and even some leaf channel obstruction may occur.

The stall characteristic of the axial fan is determined by the characteristics of the blade type of the fan, and is also affected by the system characteristics such as the air duct resistance. The characteristic curve of the axial flow type fan is adjusted by the blade, wherein the saddle curve M is different for the fan. The stall point of the angle is connected, and the operating point falls on the upper left of the saddle curve. Both are unstable working conditions. This line is also called the stall line.

From the figure we can easily see:

1 Under the same blade angle, the greater the pipeline resistance, the higher the wind pressure at the fan outlet, and the closer the fan operates to the unstable working condition zone;

2 Under the condition that the resistance characteristics of the pipeline are constant, the opening degree of the fan blade is larger, and the operating point of the fan is closer to the unstable working condition zone.

When the following phenomenon occurs in the axial flow fan operating in parallel, it indicates that the fan has stalled:

1 The head, flow and current of the stall fan are greatly reduced;

2 The noise of the stall fan is obviously increased, and the vibration occurs in the casing, air duct and flue when it is serious;

3 In the case of “automatic” input, the current and volume ratio of the other fan operating in parallel with the stall fan can be greatly increased;

4 Unlike the fan “surge”, after the fan stalls, the wind pressure and flow rate will not pulsate.

The stall phenomenon of the fan is an unstable operating condition of the fan, which is very harmful to the operation safety of the fan:

1When the fan stalls, the air volume and wind pressure are greatly reduced, causing the furnace to burn with drastic changes, which is prone to fire accidents;

2 When another fan in parallel operation is put into "automatic", the output will increase, which will easily cause the motor to be overloaded;

3 The vibration of the stall fan is obviously increased, and the vibration of the fan equipment and the air duct may be greatly damaged;

4 When the process is not correct, it is easy to cause the fan to “surge” and damage the equipment.

The resistance of the outlet channel of the blower is too large, and the opening degree of the blade is large. Falling into the unstable working condition zone of the fan is the real cause of the stall of the B blower. The ash accumulation of the air preheater should be removed, and the wind resistance of the air preheater should be reduced. It is the fundamental measure to solve the stall of the blower. When one blower stalls, the blower blade should be quickly shut down, so that the blower can return to the stable condition as soon as possible. Zone operation.

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