Ask the Expert Question-and-Answer Archive

by Ron Joseph

August, 2008

CFM Issues

Q: I'm the safety supervisor at OXBO Intl. We currently are having issues with our air flow. We are currently at exhaust is 37600 CFM and out Air make up is 22000CFM. That is a difference of 15600 CFM which isn't good for our painters. I need to know what we can do and if there are any OSHA Regs on our air flow restricitions?

A: If the air flows rates are as different as you state, your spray booth must be operating under extreme negative pressure. In themselves the numbers don't tell me if you are within OSHAs recommended range of air velocity. For that you need to know the cross-sectional area of the booth.

In any case, at the very least, you should balance the two air flow rates and this can be done by increasing the RPM of the air supply fan, or decreasing the RPM of the exhaust fan. Can one assume that the two fans were balanced when the booth was originally installed? If so, something has gone out of whack and you might want to check if there are any new air restrictions on the air supply side, or if someone has changed something on the exhaust side.

To prevent this inequality from occurring again, you might want to consider installing a VFD (variable frequency drive) on the air supply fan. Ideally, the pressure difference between the inside of the booth and the outside should be +/- 0.003 - 0.007 inches WC.

I hope this helps.

Regards,

Ron Joseph