What Size Exhaust Fan Under Bench?

[Dennis Towns]

My mentor has a vent fan with louvers mounted under bench in his outside wall. It does a really good job removing handle dust. Too nasty for me to climb under there to try to read. Anybody have an idea how many CFM and box size a decent fan needs to be? I believe his looks to be about 20"x20" I have a couple of new in box Fantech 12" that says moves 800 CFM , they have sealed motors. Just don't want to do the work and not work. Thanks for any input.

 

[Stacy E. Apelt - Bladesmith]

Your 12" 800CFM would be fine. A 12" round duct handles a little over 700CFM, 12X12" square will handle 1000CFM.

Most folks get a used AC blower fan of whatever size is available. A louvered exhaust is a very good idea for a shop. It can be used for light dust extraction but is also good for airflow. One important thing to make sure of is that there is a FRESH AIR IN port somewhere that is equivalent in size to the exhaust. This can be as simple as an open door or window, vented eves, a louver in the roof peak, or an incoming fan.

I use a 6" round duct and 150CFM blower for AIR IN in the grinding room. That is sufficient for a 10'X8' room. There is an independent dust extraction system with its own exhaust vent, and a hanging air filter.

 

[Dennis Towns]

Thank you, had not thought about the IN AIR part. Good information for sure.