Question re: Fan Surge

[ChasBean1]

Greetings all, can anyone briefly (or I guess even lengthily) explain fan surge? My knowledge about this is general, such that a designer shouldn't select a fan in the upper left region of the fan curve chart. I've heard explanations that since the fan curve levels off in this area, there may be two or more air flows that match the given static pressure and the fan may go wildly back and forth between these flows. I've also found a general definition indicating cyclical backflow causing large intermittent forces on the fan blades.

Does anyone know more about fan surge or has anyone experienced this? If this is a real problem, wouldn't closing volume inlet vanes on a VAV fan put the fan into surge region by raising the system curve? If anyone has further insight into this it would be appreciated.

Tx, Chas

 

[LeaderP]

Chas:

You've basically hit the nail on the head. With inlet vanes, you're essentially creating a series of fan curves parallel to the wide open inlet vane curve and it results in moving the surge point to the left as you throttle the inlet vane.

I've heard a large fan surging as well as a centrifugal air compressor(acts much like a fan)and you feel like you don't want to be in the same room.

For further reading, I'd suggest the FAN ENGINEERING handbook published by Buffalo Forge Co. (See chapt. 14)

 

Blast furnace blowers can surge. I've not experienced it, but the operators say they can rip them selves right out of the foundation, very destructive to the fan and associated equipment. Lots of air, 100,000 scfm.