Serving western Canada's sheet metal industry

By: Norm Grusnick

Almost all of the information on SYSTEM EFFECT places major emphasis on pressure losses and higher energy consumption that results from aerodynamically inadequate connections between a fan and the duct work system. The pressure losses capture the greatest attention in a system. There is, however, another penalty—the “acoustical penalty.” That is, the sound output of the fan is affected by aerodynamically inadequate fan-to-system connections.

There are several examples that illustrate how the acoustical SYSTEM EFFECT happens. First let us shift the performance point. The aerodynamic efficiency of a fan is directly related to its acoustical efficiency; as this efficiency rises and falls so does the fan’s acoustical efficiency. A fan is quietest at its peak aerodynamic efficiency—this may be taken as fan mechanical efficiency. What this means to the client is that when the fan is most efficient it is also the most quiet. It is in the designer’s best interest to ensure that the duct system the fan is attached to will enable the fan to operate at its peak efficiency. A change in the operating point cause changes in sound output. This must be taken into consideration with fans controlled with frequency drives.

For centrifugal fans, a “rumble” is usually associated with low airflow and turbulence. This rumble is associated with low acoustical efficiency in the lower octave bands, and results in higher sound levels in those octave bands. Higher octave bands produce less sound. The overall impression is that the fan sound level is louder because you hear the low frequency rumble. A fan is less efficient at low airflow thus it will have low acoustical efficiency. As airflow increases from shut-off to about the middle of the capacity range, a fan becomes more efficient, both aerodynamically and acoustically. Fan sound levels are usually at a minimum at fan peak efficiency. As fan airflow increases towards wide open, less low frequency sound is produced. This decrease is offset now by upper frequency sound; the overall impression is a much louder fan.

While the example referenced is a centrifugal fan, the same can is generally true for axial fans. One example of axial fans – the vaneaxial fan – deserves a bit more attention. Many vaneaxial fans stall at somewhere between 30-35 per cent of wide-open air flow capacity. The acoustic SYSTEM EFFECT in this case may be both severe and pronounced. The severity depends on the the fan-to-system connections. These inadequate connections would be something like a hard elbow right at the inlet of the fan. A bad design can create pressure changes within the impeller blades. Stall, for a vaneaxial fan impeller, is the situation where the blades are unable to generate enough lift to incoming air to overcome the downstream pressure acting against the impeller. While in stall, the fan sound levels over the entire spectrum are dramatically increased: stall is usually heard as an unmistakable howl. If a vaneaxial fan operates in stall for too long a time, the results will not be good. It is possible for SYSTEM EFFECT to cause catastrophic failure of the impeller through excessive vibration.

Problems such as the ones described above are most easily avoided through careful system design at the beginning of a project. Retrofit systems, of course, require greater care in design as much of the duct system already exists. NG