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What's New in HVAC Fan Applications - Part 2

Posted June 20, 2012 12:00 PM by larhere

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 -12345678
6-18.25" DDP Fans:Octave Bands -12345678
4-22.25" DDP Fans:Octave Bands -12345678
2-27.0" DDP Fans:Octave Bands -12345678
1-40.25" DDP Fan:Octave Bands -12345678
1-33.0" DWDI Fan:Octave Bands -12345678
1-40.25" BDP Fan:Octave Bands -12345678


  1. These sound power levels reflect the number of fans in each array.
  2. 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- DDP6-DDP4-DDP2-DDP1-DDP1-DWDI1-BDP
Section Length: 58"66"74"87"114"92"83"


  1. The housed DWDI fan has the highest sound power levels in the two lowest octave bands.
  2. 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.
  3. 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:


  1. 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.
  2. 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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