However, to enlarge the fan blade angle opening should not be arbitrary. Because when the opening angle is too large, the air reflected off the fan blade will no longer be uniform. At that time the reflected air and pressure will stop rising and normally will fall. This is what is called a Stall Point.
Big fans in the industrial world do have a large blade pitch which is adjusted according to the required air flow . And generally also a stall will occur at 95% blade pitch opening or greater.
If you can imagine, the fan stalls as if the fan blade is just "banging" the air without creating the expected airflow.
There is 1 type of fan which turns out to be designed to work in a stall condition. Namely the Centrifugal Fan. This fan is considered to always operate in a stall condition because of the difference in the direction of the fan blade speed and the direction of the intake air.
A fan that is in a stall condition usually makes a lot of noise. In fact, it usually sounds like the impeller of a fan is hitting a solid object (hammering).
What really needs to be paid attention to in the fan stall phenomenon is the potential for material damage that is very likely to occur. Axial fans that often stall will usually experience fatigue (fatigue). However, it is different from a centrifugal fan, which is designed to operate in a stall, so the damage that may occur is minimal.
Apart from being able to detect a stall by looking at parameters such as vibration, noise, and a decreased motor ampere value, there is an instrument called the Petermann Probe which functions to detect stall with more precision. In fact, we can even find out an indication of a stall or commonly known as a surge condition (for a moment where a stall will occur ).
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| Turbulent flow areas on the axial fan when a stall is about to occur |
Variations in air pressure that appear in the fan blade area at the time of stall , indicate that the air flow caused by the fan is blocked ( blockage ) or even flows in the opposite direction.
This phenomenon is used by one tool to identify a stall on an axial fan , namely the Petermann Probe . Petermann Probe works by comparing the amount of air pressure between the total air pressure in the opposite direction to the direction of rotation of the impeller, with the static air pressure ( static pressure ) at the same point. For more details, it can be seen in the image below.
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| The concept of using the Petermann Probe |
The concept of the Petermann Probe is that the magnitude of the measured pressure difference will be close to zero when the fan is under normal operating conditions. However, at stall conditions the value of this pressure difference will be even greater and more positive. This is in accordance with the chart below.
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| Graph of the pressure difference from the Petermann Probe with air flow |