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In Part 1 we briefly looked at the efficiency and redundancy of six
different options for a fan array, but we did not look at the sound and unit
length advantages of a fan array.
 Fan Sound Power Levels
One of the real advantages of a fan array with direct driven plenum
fans is the sound power reduction in the lower octave bands. In the following table we will look at the
six options in Part 1 plus a single belt driven plenum fan.
The table below shows the outlet sound power levels, in the eight
octave bands, for all seven options at 30,000 CFM @ 6.0 " TSP. Outlet sound levels are normally more
critical that inlet sound levels.
| 12-15.0"
DDP Fans: | Octave Bands - | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |
| Outlet | 95 | 94 | 98 | 104 | 97 | 94 | 94 | 89 |
| 6-18.25" DDP Fans: | Octave Bands - | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |
| Outlet | 93 | 93 | 103 | 103 | 96 | 93 | 89 | 83 |
| 4-22.25" DDP Fans: | Octave Bands - | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |
| Outlet | 93 | 94 | 101 | 98 | 92 | 87 | 85 | 83 |
| 2-27.0" DDP Fans: | Octave Bands - | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |
| Outlet | 94 | 97 | 103 | 100 | 94 | 88 | 84 | 79 |
| 1-40.25" DDP Fan: | Octave Bands - | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |
| Outlet | 94 | 99 | 96 | 91 | 88 | 84 | 79 | 74 |
| 1-33.0" DWDI Fan: | Octave Bands - | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |
| Outlet | 103 | 102 | 102 | 93 | 89 | 86 | 82 | 76 |
| 1-40.25" BDP Fan: | Octave Bands - | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |
| Outlet | 97 | 102 | 101 | 96 | 92 | 87 | 81 | 75 |
Notes:
- These
sound power levels reflect the number of fans in each array.
- The
above plenum fans are 9 bladed. If
desired you could select 12 bladed fans to improve the blade pass tone into one
higher octave band.
The above sound levels show the sound power of each fan option, but
does not take into account the attenuation due to the perforated panels or the
attenuation by the air handling unit.
Bare fan sound power levels are useful in picking the quietest fans, but
if sound is truly critical then the only way to get accurate unit sound data is
to have the unit tested in an AMCA 300 reverberant room.
In general the most critical octave bands are the lowest. Acoustical reduction inside the air handling
unit, in the duct systems and sound attenuators have a much harder time
attenuating the lower octave bands.
| No. of Fans: | 12- DDP | 6-DDP | 4-DDP | 2-DDP | 1-DDP | 1-DWDI | 1-BDP |
| Section Length: | 58" | 66" | 74" | 87" | 114" | 92" | 83" |
Observations:
- The
housed DWDI fan has the highest sound power levels in the two lowest octave
bands.
- The
direct driven plenum fan has lower sound power levels that the belt driven
plenum fan due to the open inlet of the direct driven fan.
- The 4
fan array is quietest with the 6 fan array a close second. The 4 fan array has 96% redundancy and the 6
fan array has 100% redundancy if one fan fails.
General
statement - A fan array with direct driven plenum fans is the best solution
when sound in an important criteria, but the quantity of fans will vary from
application to application. More is not
always better.
Unit
Length
On some applications the unit length of the air handling unit may be a
problem and going to a fan array may be the solution. When you go from a single fan to a fan array
the length of the fan section can be shorten.
Using the 30,000 CFM @ 6" TSP as an example we offer the following table
of fan section unit length:
Observations:
- If you
want to use a direct driven plenum fan for acoustical reasons and you have air
handling unit length issue then going from a single DDP fan to twelve DDP fans
could save up to 56" in unit length.
- Going
from a single DDP fan to a dual DDP fans would save 27", but going from six DDP
fans to twelve DDP fans would save only
8".
General
Statement - A fan array can save unit length, but if it's the only reason for
selecting a fan array then it would be much more efficient and cost effective
to find other parts of the air handling
unit to shorten or to have the space allotted expanded.
We will address the issues of fan isolation and variable frequency
drive redundancy in a future blog.
Editor's Note: CR4 would like to thank Holcombe Kelley, of Air Handling Solutions LLC, for contributing this blog entry.
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