| Application Considerations
FigureA-14-Comparision of Discharge Sound Power by Fan Type
FigureA-15-Comparision of Inlet+Casing Sound Power by
fan type
Optimizing an Arrangement for Acoustical Performance
Air handlers designed for IAQ-related cleaning usually
employ hard metal surfaces that do little to attenuate
the primary source of noise: the fan. Designers are challenged to select and arrange the air handler components so that the
inlet discharge and casing- radiated sound power levels
are less than the target value. To accomplish this, the designer must choose the right fan and determine whether additional unit
attenuation is necessary.
Choose the Right Fan
Obviously, the quieter the noise source (in this case, the fan), the less attenuation
is needed along the sound transmission paths. Although
selecting a quieter fan often increases the initial
cost of the air handler, it can be a cost- effective system solution because it:
- Reduces the need for path attenuation
(e.g., silencers) by diminishing the sound level along all transmission paths
- Reduces energy consumption (i.e., a
fan is normally quietest when running at the most efficient point on its operating curve), providing operating cost
savings to offset the initial cost.
Acoustical tests indicate that a
fan’s sound power (Lw) level depends on three factors: its type/design, its operating point on the fan curve, and its
application requirements (e.g., critical octave band
frequency). Figure A-14 and Figure A-15 compare the
inlet and outlet sound power levels of four different fans. Together, these charts demonstrate that fan inlet sound is not equivalent to fan outlet sound. More importantly, they underscore the need to obtain accurate sound data from the manufacturer
so that appropriate source and/or path attenuation
methods can be applied.
The Modular Climate Changer® gives designers a choice of five
different fan types. "AHU Components: Fan Module" later in this section summarizes each fan’s application considerations
and acoustical characteristics.
If Needed, Provide Additional Unit
Attenuation
The Modular Climate Changer selection program
accurately predicts the sound power level at the
boundary of the air handler based on its fan(s) and arrangement. Use this software tool to:
- Compare various casing options (e.g.,
solid versus perforated double-wall liners).
- Determine the attenuating effect of
adding a discharge plenum.
- Evaluate the effectiveness of
deadening discharge sound with lined turning modules rather than more expensive rectangular duct silencers.
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