Roger Dickinson, Torrington Research Company, Torrington,
There are three basics to creating low-noise axial fan applications:
- airflow path design
- initial fan design
- analysis and correction of specific acoustical responses in
the physical model
Each is important and contributes to the overall sound levels of
Figure 1. Representative flow-versus-pressure
characteristic of system air path.
A flow-versus-pressure characteristic of the system air path is
determined first (Figure 1). It is common to find areas in
the system that, due to air direction changes or reduced cross sections,
increase airflow resistance. Minimizing losses not only will
ultimately reduce noise but also reduce power. Another issue
is the optimization of the amount of airflow needed. The evaluation
should include any fan-to-system interference problems, such as
a wall directly upstream or downstream of the fan. During
this process it is important to conceptualize the number and size
of fans needed. These choices help initiate the next step.
Figure 2. Representative data
used to determine optimal fan performance.
With an optimized system design the flow-versus-pressure point
(operating point) is determined and used to design a fan (Figure
2). While it is possible to select a fan from a product catalog,
the best approach is to design a new one that will operate at or
near its peak efficiency point. The design process allows
the choice of a fan operating speed that can be reduced for any
given diameter by increasing characteristics, like blade pitch and
blade geometry. Reduced operating speed correlates to low
noise. Once a design is completed, a model that can achieve
the desired operating point can be built.
The final step is confirming that the proper flow is achieved
and then analyzing the fan's noise signature on a decibel-versus-frequency
graph (Figure 3). This review allows the designer to find
and correct noise "spikes", such as blade pass, imbalance
and natural frequencies. While this last step is often overlooked,
it can help greatly, especially if modification can be made to the
design that was created in step two above.
Figure 3. Representative axial
fan sound signature.
These three basic steps are essential to making major reductions
in noise levels.