Q. We are designing a lab exhaust fan system with multiple fans on a plenum. We are using Strobic Air Fans. We are using VFDs on the project. Do we need bypass dampers?
A. We typically advise that bypass dampers only be used when necessary. In a lot of applications where VFDs are used, bypass dampers are not necessary. If the fans can turn down to the minimum design point and still maintain 3,000 FPM outlet velocity then we don’t need the dampers. This will vary depending on the fan and number of fans selected. Best just to look at the fan curves.
We build the control sequence to activate the bypass only when the VFDs have reached their minimum turn-down point. That point can be roughly calculated in design and then adjusted during the balance of the system.
In some cases we will provide the bypass damper as a backup in case a VFD will fail. In this case we will still only provide one damper.
Note that when using VFD’s on a multiple fan system that, for best performance, the fans need to be ramped up and down together.