Five Airflow Control Strategies for Centrifugal Fans
There are many reasons you would want to control the airflow in your industrial fan application. How you do it is based on your fan curve, whether you need to reduce volume, maintain pressure, change the curve based on frequency in hertz to add efficiency, protect your system against surge, or address multiple issues at once.
There are five primary airflow control strategies for centrifugal fans in industrial settings, including various damper and drive solutions:
- Outlet Damper
- Inlet Box Damper
- Variable Inlet Vane (VIV) Damper
- Variable Frequency Drive (VFD)
- Combination of a VIV and a VFD
The most common airflow control strategy for centrifugal fans is the outlet damper. Sitting right on the outlet of the fan, this is an opposed blade-style outlet damper. We control the flow by incrementally closing the blades to reduce the airflow volume while raising static pressure.
This allows you to get to a more efficient point on the fan curve and cut down on the amp draw from the motor. One note of caution: if you close the damper too much, you can hit the surge danger zone, which you definitely want to avoid to protect your fan.
Inlet Box Damper
Alternatively, the inlet box damper uses a parallel blade design strategy for centrifugal fan airflow control. It goes on the inlet of your fan and starts the prespin of the air to get it moving in the same direction as the blades are turning inside the fan.
In this case, closing the damper incrementally actually moves the curve instead of just moving along the curve. So as you close it, you’ll reduce both volume and pressure. This also can hit the surge danger zone, so you’ll need to be careful not to close it too much.
Variable Inlet Vane (VIV) Damper
Surge is the primary risk with outlet dampers and inlet box dampers. There is an option that eliminates that risk: the variable inlet vane (VIV) damper. That’s why the VIV is the best (by far) for airflow control in centrifugal fans.
The vanes mount with a bullet-nose design, wrapping all the way around the inlet. They shut in the direction of the airflow inside the fan to help with the prespin. The variable inlet vane damper changes the curve, reducing volume while maintaining a similar pressure maximum.
Variable Frequency Drive (VFD)
The fourth strategy isn’t a damper. It’s a variable frequency drive (VFD) on the motor to modulate the speed while the fan is operating. This is by far the most efficient method of controlling flow because you’re not introducing anything into the stream as a pressure source, and you’re gaining energy savings.
How does this constitute an airflow control strategy for centrifugal fans? The VFD rides the system resistance curve, physically changing the fan curve for optimal volume and pressure depending on the frequency of operation in hertz.
Variable Inlet Vane Damper + Variable Frequency Drive
The highest possible control with the least risk of fan surge is to combine the VFD with the VIV. This provides the efficiency of the VFD in riding the system resistance curve while allowing you to go up on speed without increasing flow. By comparison, if you just have a VFD, pressure and volume will go up or down together.
On the curve, the effect is to change the system resistance curve to meet the changed pressure curve for the fullest method of flow control.
Hear it from an Application Engineer
Senior Application Engineer Chet White explains airflow control strategies for centrifugal fans in detail, with specific examples and curve drawings in this 16-minute video.
To determine the right airflow strategy for your centrifugal fan application, reach out and connect with one of our application engineers to discuss the details of your project.
For more information on troubleshooting and application system effects mentioned in this article, check out these additional pages:
- VFD or Damper for Fan Control?
- The Drive to Control Fan Speed
- Overspeeding Industrial Fans
- Do you Need an Inlet Box?