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Fan Basics

Session 7 - System Impedance of Devices

In previous sessions we looked at fan characteristics. In this seventh session we will examine system impedance in devices where fans are mounted.
The electrical resistance (Ω) of a device is the criterion for determining its power capacity (A). Based on the same principle, the system impedance of a device is the criterion used for selecting a fan.

System Impedance

System impedance is the resistance to the flow of air within a device. Even if the fan that is mounted is changed, the system impedance will barely change as long as the position and quantity of the components within the device stays the same.
In general, the system impedance can be approximated with a quadratic curve proportional to the square of the airflow. The quadratic curve tends to slope toward the X-axis when system impedance is low (air flows easily), and slope toward the Y-axis when system impedance is high (air flows with difficulty).

Figure 1: Relationship between system impedance and mounting density inside devices

By knowing the point where the "system impedance" of a device and the "P-Q performance" of a fan intersect, you can determine whether the fan is suitable for the device. This intersection is called the "operating point," and the airflow at the operating point is called a fan's "operating airflow."

System Impedance and P-Q Performance

Measuring system impedance

Although system impedance can be calculated from a formula, when doing so it will be necessary to measure the operating airflow, so a measuring device will still be required. Usually, multi-nozzle double chamber measuring devices are used to evaluate the characteristics of fans with high accuracy.
This measurement method converts the pressure differential generated in chambers A and B of the figure into the air velocity of the nozzles cross-section, and multiplies this air velocity by the cross-sectional area of the nozzle to obtain the airflow. By selecting nozzles with different openings according to the airflow range of the object being measured, accurate measurement is possible for a wide range of airflow.

Fig. 3: Multi-nozzle double chamber measuring device

In general, since double chamber measuring devices are large and stationary, it is necessary to bring the device to be measured to the place where the double chamber measuring device is installed.

However, there is also a portable measuring device. That is the SANYO DENKI "Airflow Tester." Since it uses the same multi-nozzle double chamber measurement method, it can easily measure system impedance and operating airflow with high precision. By measuring system impedance with this Airflow Tester, one can efficiently design housings that take measures against heat into account.

Double chamber measuring device Airflow Tester
Mass Approx. 600 kg Approx. 6 kg
Size [mm] 1000×1000×6000 250×250×600
Airflow(m3/min) 0.05 to 20 0.2 to 8.0
Static pressure [Pa] 0 to 2000 0 to 1000
Measurement method Double chamber
Measurable criteria Operating airflow / system impedance / P-Q performance
Measurement accuracy ±2% ±7%

Date of publication: June 28, 2018

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