Understanding Centrifugal Fans

Every fan has a stable and unstable operating range. Effects of an unstable operating range vary greatly, depending on wheel design. For simplicity, stable flow is defined as the condition under which enough air flows through the fan wheel to fill the spaces between the blades. Also, let’s say there is only one static pressure output for a given flow and that it does not vary.

As pressure is increased and flow reduced, the fan rides the curve to the left of the original operating point. At some point, airflow is reduced so that there is not enough air to fill the blade passages. On a centrifugal blower fan, the entire wheel rarely stalls at the same moment. Because of slight manufacturing variations, one blade or section of blades does not produce the same pressure as the others and stalls.

Under certain conditions, a cascade effect occurs. The original blade stall disrupts flow into the blade next to it, and the stall travels from blade to blade. Occasionally, the blade that originally stalled recovers and the stall zone travels around the wheel. This condition is called a rotating stall. The stall zone typically rotates opposite the fan rotation at a third of the fan speed.

As the flow continues to decrease, the entire wheel goes into stall. The effects, most noticeable on FC and BC types, vary greatly with system design, amount of resistance, and wheel type. Signs of stall include vibrating ductwork and fans, and failed bearings.

Vibration may increase only slightly while additional rumbling is audible in the ductwork. In some cases, the system itself exhibits an airflow- induced resonance that coincides with the stall frequency in the fan wheel. As a result, whole sections of ductwork appear to be “breathing.” causing substantial damage. If any fan or system exhibits any of these symptoms and the operating point has changed, there is a good chance the fan is running at an unstable operating point.

On the other hand, the radial bladed fan is the most rugged construction type. Its performance curve usually rises continuously and may have a small flat area near the top (see Fig. 2b). On some designs, flow selections can be made all the way to zero. Because of the small number of blades, each creates its own stall one with little effect on adjacent blades. 

Even with each blade in stall, overall fan output remains stable. BC fans rely on a different mechanism to provide lift. Air moves over the blade in a motion similar to airplane wings. When a blade stalls, it has a pronounced affect on adjacent blades.

The unstable operating region on the fan curve extends farther out on the curve (see Fig. 2c) than it does on a radial blade design. Selections should be approximately 30% to the right of the peak on the curve.
The stable operating range depends both on the fan wheel design and the precision with which the unit was manufactured. The more consistent the blade construction, the wider the stable operating range.