How to Adjust Air Performance in the Field without Sheave Changes or VFDs on Adjustable Blade Fans
Air performance adjustments in the field are sometimes necessary for a variety of reasons. For example, the system’s design looks perfect on paper, but the realities of installation can require airflow changes. Other factors include unaccounted or overestimated pressures, or building renovations. It is possible to make performance changes for inline axial fans (Figure 1) using one of three methods.
 |
| Figure 1 - Axial fan with adjustable blades |
The use of variable frequency drives (VFDs) or sheave changes on belt drive fans will adjust air performance. However, adjusting the pitch of the impeller may be the best option for inline axial fans having the adjustment capability.
All three methods increase or slow airspeed but changing the blade pitch offers advantages over the other two methods. Changing the blade pitch requires no additional components. So long as the electrical amp draw of the new performance point is within the nameplate of the supplied motor, there is no need for sheaves or VFDs.
Before You Begin
After determining the new required airflow performance point for the fan, contact your manufacturer’s local representative or use a computer selection program to determine the new pitch required. Note: if using the selection program, confirm the model (size, hub size, blade quantity) performance is identical to the unit in need of adjustment.
- Before adjusting the pitch, confirm that the new performance electrical amp draw does not exceed that on the motor nameplate
- Turn off all power before working on equipment
- To avoid flexing of blades during operation, keep the blades as close to a uniform pitch as possible
- Tighten each of the blades to the manufacturers recommended torque setting
The video below provides more detail to the steps required for adjusting blade pitch.
from