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Fan Basics

Session 9 - Counter Rotating Fan Features

Last time, we learned about axial fans. In this session, we will introduce a unique type of axial fan—Counter Rotating Fans.

Counter Rotating Fan Structure

A Counter Rotating Fan appears to be two axial fans connected in series. Let's call the fan on the intake side the inlet fan and the one on the exhaust side the outlet fan.

Fig.1 Counter Rotating Fan Structure
Fig.1 Counter Rotating Fan Structure

The inlet and outlet fans rotates in opposite directions; as shown in Figure 2, when the inlet fan rotates clockwise, the outlet fan rotates counterclockwise.
Although a Counter Rotating Fan appears to be a combination of two of the same axial fans in series, merely operating two of the same fans in series cannot provide the same performance as a Counter Rotating Fan.

Fig. 2 Visualization of Air Flow
Fig. 2 Visualization of Air Flow

When two axial fans rotating in the same direction are operated in series, the outlet fan further swirls the swirling wind from the inlet fan, generating a spreading flow of air.
However, a Counter Rotating Fan's outlet fan rotating in the opposite direction weakens the swirling components of the wind from the inlet fan, generating a straight air flow.

Counter Rotating Fan Characteristics

Fig. 3 System Impedance and P-Q Performance
Fig. 3 System Impedance and P-Q Performance

Because of the difference in the air flow within the fan, a Counter Rotating Fan has a much higher static pressure than two axial fans rotating in the same direction in series. In the 8th session, we learned that P-Q performance curves of axial fans have a stall region in the middle of the curve where air swirls and becomes unstable. Counter Rotating Fans, however, have a stall region around where the P-Q performance curve is vertical.

Fig. 3 System Impedance and P-Q Performance
Fig. 3 System Impedance and P-Q Performance

Suitable Applications for Counter Rotating Fans

Since Counter Rotating Fans have a stall region more to the left side of the P-Q performance curve than regular axial fans, they have a wider optimum operating range and can blow air strongly even in a device with a high system impedance.


As stated above, Counter Rotating Fans feature a high static pressure and can blow straight air, and therefore are suitable for cooling devices with a high system impedance and for cooling by blowing air directly. They can also be used to blow air to targets at a distance.

Revised: 9/16/2020

< To previous sessionAxial Fan Features

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