Why do parallel fans/blowers show an oscillatory behavior?

[marqram]

I am analyzing an industrial ventilation system that at some point operates two industrial size blowers in parallel. They often show oscillating behaviour: one blower briefly draws more power / generates more flow, and the other performs less. Then the roles reverse. The only practical, known way to reduce the oscillation is to throttle down the vane valve in front of the fan and then open the valve again. The blowers are almost (but not completely) identical.

I seems that both blowers affect each other in some unstable way. Is this a known phenomena? I'm not finding anything when searching for fan/blower/pump oscillation/resonance/hysteresis.

 

[KENAT]

You might want to investigate multi fan 'beat frequencies' - I believe you may be seeing an extreme case of this.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[MikeHalloran]

Get the curves for each fan, and estimate the system curve.

This page explains what to do with that knowledge:

http://www.greenheck.com/library/articles/42

It explains at least one possible mechanism for parallel fans fighting each other, and offers suggestions for mitigation.



Mike Halloran
Pembroke Pines, FL, USA

 

[chicopee]

Have you had any issue with blowing out fuses at their disconnects?

 

[chicopee]

Also, are they similar fans?