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158 Appendix B: Technical information
If the enclosure has smaller openings than the fan area, you must add this resistance
to the enclosure pressure. If you have a 80 mm x 80 mm fan and only the
equivalent of a 60 mm x 60 mm enclosure opening, you have an area ratio of
(60 x 60)/(80 x 80) =3600/6400 = 0.5625. So your fan will already be at 56% of
its maximum supported pressure. You must add this value to the turbulent
contribution of the enclosure.
With this information, you can now select a fan. If you decide that you need more
than one fan, putting them in parallel will double the flow. Putting them in series
(one intake and one exhaust or two stacked intakes/exhausts) will double the
pressure.
Fan example
The following provides an example of how to calculate the amount of air flow
required. It uses an enclosure in which you are dissipating 300 W. In addition, it
uses a typical 120 mm x 120 mm x 25 mm 12 V fan that is operating at high
speed. Note that very high speeds are not recommended because they significantly
reduce the life of the fan.
0.48
12.0
0.40
10.0
0.32
8.0
0.24
6.0
0.16
4.0
0.08
2.0
0
0
0.8
1.6
0
32.0
0
64.0
Air Flow
Fan settings
Super high
Very high
Higher than high
High
Medium
Low
3.2
4.0 (M /MIN)
2.4
96.0
128.0
(CFM)
3
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