| Air Performance and Package Resistance |  |
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| The air performance curve of a typical cooling fan is shown in green at right. It is customary in the trade to plot the air flow rate as an abscissa, with static pressure as the primary ordinate and speed, power and electrical current flow plotted on a secondary axis. Application of the fan at Operating Point A delivers 70 CFM of air as 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. <more> |
| Air Performance and Package Resistance |  |
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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:
The pressure drop/flow rate parabola, often called the system resistance curve, is drawn through Point B. It indicates that a reduction of the package pressure drop from 0.14 to 0.04 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. <more> |
| Air Performance and Package Resistance |  |
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The graph at right shows the air performance curve of a fan of the same size and configuration, but with lower speed ratings. The same system resistance curve at Point B now indicates air delivery of 70 CFM at Point B, but that air flow rate is attained with substantially lower power requirements and less noise. |
| Air Performance and Package Resistance |  |
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At right is a summary of 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. <top> |