Which damper you should use for proper fan’s flow rate control?

The most common mistake when you want to control the flow rate of a fan is to install a simple butterfly damper on the inlet.
It is true that an effective reduction in the amount of air delivered by the fan is achieved. But the risk of fan breakage that this method entails must be evaluated. As long as the damper is fully open, there is no appreciable distortion to the flow entering the impeller.

Diego Perfettibile Co Owner of PBN the industrial fans built to last

As the damper is progressively closed, however, the velocity profile at the fan inlet will be increasingly uneven and characterized by vortices and dead zones . (see
Figure 1 and Figure 2)
Using dampers other than control dampers (IGV or VIV) to modulate the fan flow rate is therefore detrimental in two different respects.
First, the more you close the damper, the more you actually drive the fan to work closer and closer to the surge zone, that is, the part of the curve to the left of the pressure maximum.
As if that were not enough, progressing to ever greater closures also introduces vortices and dead zones in the inlet zone in the impeller, thus generating further instability and pulsations in the flow rate delivered by the fan.

-Figure 1 Streamlines with butterly damper open at 50%                                                                                  -Figure 2 Streamlines with butterly damper open at 20%

Just to give you an idea of the effects of this type of regulation, prolonged operation of a fan with a 20% open butterfly damper was the cause of very frequent bearing failures at one of our customers . When we went to the field to reset the fan and verify what had happened, we found vibration values going up by an order of magnitude when the damper was placed at a 20 percent opening compared to the 100 percent open condition.
For these reasons, I never cease to reiterate that a butterfly or guillotine damper used to regulate, especially if placed at the intake, is one of the most serious mistakes that can be made in fan installation.

At PBN, we always recommend- and install on our fans-dampers that are specifically designed to allow flow rate control while avoiding the problems discussed above.
To request a PBN technician’s advice on flow rate regulation in your systems, fill out the contact form found on the web page