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Application of the fan at Operating Point A would deliver 70 CFM of air if the package resistance imposes a pressure drop of 0.14 inches of water on the air stream.
If the package is modified in some way to reduce its flow resistance so that 70 CFM corresponds to a pressure drop of only 0.04 inches of water (Point B), the fan could be expected to deliver an air flow rate of more than 70 CFM. It also follows that a higher flow rate through the package will increase the pressure drop in the air stream.
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In most cases of forced air cooling, the relationship between the air pressure drop and the air flow rate is a simple parabola:
P = krQ2 (Q Squared), where: P = Pressure Drop Q = Air Flow Rate r = Air Density
The pressure drop/flow rate parabola, often called the system resistance curve, is shown in Figure 1 through Point B. It indicates that a reduction of the package pressure drop from 0.14 to 0.07 inches of water increases this fan's air flow delivery to 92 CFM. At that point, the package requires and the fan provides a pressure of 0.07 inches.
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Figure 2 shows the air performance curve of a dc fan of the same size and configuration, but with lower speed ratings. The same system resistance curve at Point B indicates air flow delivery of 70 CFM, but with substantially lower power requirements and noise.
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Figure 3 summarizes the change from one fan to the other. In some cases, it may be possible to specify a physically smaller fan with lower air flow ratings if package resistance is sufficiently reduced.
Close packaging of electronics to attain minimum package size often results in some obstruction to or deflection of the air stream in a location near the fan. In some designs, the air flow patterns at the entrance and egress are altered enough to change the system resistance and negatively influence performance of the fan.
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Performance curves shown at this web site, and in associated product PDF files, are accurate depictions of fan outputs with unobstructed inlets and outlets. They can be used to choose the most effective starting points for fan performance trials.
It is often impossible, however, to identify from performance curves the exact air flow rate of a fan in a particular package. This can only be done by tests.
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Air Cooling & Air Flow Rates
Sound Generation & Test Standards
