Makeup Air Volume for a Spray Booth

Quick tips on sizing the inflow air fan capacity to match a spray booth exhaust fan. November 8, 2007

We recently added on to our finishing building. We always have just had an 18" exhaust fan in the wall. The new building has a spray room that is 12w x 10l x 12h with closing doors. Next to that room, we built a flash off room that measures 12w x 25l x 12h with its own closing doors. We are wondering how many cfm fan and makeup air we need, and/or how many times the cubic feet of air need to be turned over per minute to meet guidelines. In our area, there are no inspectors, other than electrical. We plan on putting a couple of filtered openings between the spray room and flash off room.

Forum Responses
(Dust Collection Forum)
From contributor L:
12w x 10L x 12h, I assume you have the air coming the 10' direction, which means your air is flowing across the 12 width. With this configuration, you need a fan that can pull at least 14,400 CFM to get the required 100 fpm. If the cross-sectional flow is 10 x 12, then you need a 12,000 cfm fan. I don't think your 18" fan is cutting it. An 18" tube axial fan with a 1 1/2HP motor will pull about 5300 cfm.

Your flash off area is about 3600 cu ft. I would go at minimum a change over rate of 10 times an hour, once every 6 minutes. Which would require a 600 cfm fan. If it were me I would go with a 3600 cfm fan to evacuate the flash once every minute. Remember to use explosion proof fans, or bare minimum a hazardous location fan.

From Curt Corum, forum technical advisor:
Typically, a make up air unit should provide 10% more than the air exhausted. If you are exhausting 10,000 cfm, make up air unit should provide 11,000 cfm
(slightly over pressurize).