Noisy Fan

[Truckdriver2]

It was required to replace a previously centrifugal engineroom supply fan with an axial flow fan. The new fan may be oversized to some extent. The headroom over the fan is poor. The fire damper therefore strikes an angle of about 45 degrees to the fan intake. Also the inlet vent is angled at 45 degrees upwards. The air entering the fan is therefore, I believe, quite turbulent.

The fan is producing an unacceptable level of noise. A VSD was fitted to try out different RPM. At lower RPM the fan noise is considerably less but the fan develops a considerable singing noise.

It is not immediately feasible to reposition the fan in order to develop better flow characteristics. The way I see it the noise has to be accepted but silenced.

The inlet duct is large, say 2.5 meters by 2.5 meters but there is little thickness to fit a silencer.

Can anyone suggest a company, product or technique that may be fitted to silence this noise or any other method to reduce the noise?

 

[zekeman]

How is it mounted? Can you tweak the speed/
1)Perhaps softening the mounting for openers.
2)Change the speed slightly up or down from that point.
3)Coating the ductwork with insulation material.

 

[STYMIEDPIPER]

Oversized may pose a problem
Fire damper hitting fan may pose a problem
Fan inlet of 45 deg is probably not a problem

The basis of your problem is probably due to fan selection. Most, if not all, fan catalogs list the sones rating of the fan. It appears that your fan has a high sone rating that is not acceptable for your application.

I am confident that someone will respond to your problem that has a plausible solution.

It may be more cost effective (with respect to time and manpower with trial and error approach)to bite the bullet and purchase new fan that is right for the application

 

[walkes]

There is a product from Mascoat that helps in reducing breakout noise from the duct work. A local contractor here used it successfully to reduce noise in an exposed duct where lining the duct was not feasible.

http://www.mascoat.com/dB.htm

However, you will still have to deal with air noise traveling within the duct.

 

[MintJulep]

Sounds possibly like "beating". Essentially setting up a standing wave between each fan blade as it passes the inlet vanes or fire damper blades.

The singing could then be that you have found the natural frequency of the system.

A higher or lower speed might help.

More distance between the impeller and the inlet vanes might help.

Non-uniform blade spacing might help.

Active noise redecution would probably work.