Sizing Exhaust to Minimize Overspray

You want a fan large enough to move lots of air, but without creating turbulence. Here are some useful formulas. September 26, 2006

I have a small custom picture framing shop and I spray water base lacquer in a small room, 13x11x9 feet, so I use a 12 inch fan that delivers 1.650 CFM. It seems that this may not be powerful enough to capture all the over spray. Any suggestions?

Forum Responses
(Finishing Forum)
From contributor J:
The amount of air that is required to exhaust is: (height of room) X (width of room) X 100. This is assuming you use the whole room to spray.

From contributor D:
If the overspray is landing on your surfaces and causing a rough finish, then you need a strategy to deal with the overspray. But if this is not happening, then I think that you might open a can of worms, and for what?

If you increase the airflow to suck away your overspray, you could create other finish problems that many booth owners do not anticipate until long after their finishers complain that their booth design sucks eggs, so to speak. I am talking about ridiculous drafts and vortices of air which affect layout, flowout and flash off.

Also, the air that is being sucked out has to be replaced and that introduces the prospect of getting dust, trash, debris and even lint in your flashing off finishes. As long as your code allows you, as long as your overspray is not ruining your already finished surfaces, leave well enough alone.

From contributor C:

Your fan seems about right per the formula provided by contributor J. Too much velocity or suction is a bigger problem bringing in dust and turbulence. I like a slow moving air mass to get even drying. You should always place the parts to be sprayed carefully and always spray with your spray heading downwind toward the fan and try not to go (or spray) behind any previously sprayed surfaces. This is hard with picture frames, but a turntable could help. Maybe a lazy susan with a cross of some sort to lay frames on and spray through.

From contributor G:
Your post made me think of my first spray room. With filters in the door, I would go in there (with a good respirator, usually) and spray and soon it would fog up, for the too small 24" fan in the ceiling couldn't move enough air and push it outside. What was I thinking? Oh, I remember now - large spray booth fans are expensive! I'm not sure if you're actually in a room like I was, and certainly not sure where your fan is pulling air from - maybe filters in a wall or door like I did - but if I were going to calculate the fan necessary for a spray booth that was 13 feet wide and 9 feet tall, for example, it would go - 13x9=117x125=14,625 cfm necessary, which pretty much calls for a 36" fan. Turn it around 1000rpm - so a 1750 motor, 2.5-3 horse.

Build a small booth (with filter bank and buffer area behind with your 24" fan) inside this room, spray into it. Use the formula above to derive its possible height/width sizes, knowing your fan does about 7000cfm. So maybe a 7' wide by 8' tall (7' deep, 3' buffer space, plus filter bank wall and 4 feet of spray depth = 7000cfm requirement. Make everything metal.

From contributor A:
The general rule for exhausting overspray from an area is that you will need to move 1000 cfm for every square foot of exhaust filter you have and have the air change over twice to four times a minute. Our waterborne booth moves about 6,000 cfm in our new shop, which is a fully enclosed pressurized system with four stage filtration including active carbon bio filters, heated air makeup, and AC.

You'll want to change the air over twice per minute to have effective overspray removal for spraying smaller things like frames and cabinet doors. Our booth is 12x20x9 = 2160 cubic feet. With 6000 cfm we effectively change the air in the booth more than twice each minute.

The questioner's booth is 1287 cubic feet and will need to move a minimum of 2600 cfm to effectively exhaust the overspray. I would recommend a 3500-4000 cfm fan for this application. Depending on where the exhaust is going, you might want to filter it well if it comes back in the shop. Also, any air intake areas to the spray room should have filters (automotive booth sticky filters), or you will notice a lot of dust coming in with the 4000 cfm of air flowing through.

You might also consider making sure your gun is set up right. We use high quality guns with good transfer efficiency and they produce only minimal overspray even when spraying 4' x 8' sheets of plywood.