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Ask the Expert Question-and-Answer Archive

by Ron Joseph

August, 2005

Air Velocity - Air Flow Rate in Paint Spray Booth

Q. I have a Binks cross draft booth over here, and the industrial hygenist told me that it is pulling 80 CFM right now. And in order to Paint polyurethane paint, I need to pull 100 CFM. The booth is 25'x14'and has a 5 horsepower exhaust motor 1735 rpm. The filters on the doors look like filters from an a/c unit at your house and the exhaust filters are paper/cardboard with holes . What would I have to do to raise the CFM to 100. Would I have to replace the exaust motor, maybe get a 7.5 or 10 hp motor? I know the motor is a 60 hz motor and here they have 220/50 hz power supply so it might be slowing down the motor by running off of 50 hz. Any advice or help you have would be awesome. The paint we use now turns into chalk in the sun and it would be great to shoot the poly !!

A. First, I need to know whether your industrial hygienist (IH) measured the air velocity at the face of the filters, or in various locations inside the paint spray booth. The difference is enormous, and if he/she measured the velocities at the face of the filters, I can assure you that the velocity inside the spray booth where the painters stand, is well below the 80 ft/min reported.

Second, I assume that your IH is referring to the linear air velocity (ft/min) and not the volumetric air flow rate (cubic ft/min)?

There are several reasons why you are not pulling 100 ft/min. However, based on what you've told me, you are using very inefficient particulate filters to remove the overspray. My guess is that a huge amount of overspray has deposited on the sheet metal walls and ducts of the exhaust system. In fact, it is possible that overspray has also deposited on the fan blades thus slowing them down.

My fist recommendation is that you ask your IH to tell you whether the 80 ft/min was measured at the face of the filters or inside the booth. Depending on the answer you might need to increase the air flow rate considerably or only marginally. Then, I suggest that you look inside the exhaust plenum and especially on the fan blades to see if they are being hindered by overspray. If so, clean out the booth, but TAKE ALL SAFETY PRECAUTIONS to prevent sparks and don't undertake this until you have evacuated all solvent vapors from the booth.

If the air velocity after cleaning out the exhaust plenum is still inadequate, you might need to increase the fan speed and/or increase the horsepower to the motor. Your fan supplier can calculate the energy required to increase the RPM using your existing motor, but changing out the pulley to a smaller diameter one. He can also calculate the energy and capital cost for changing to a higher HP motor. Depending on how much more air velocity you require, one option might be more cost effective than the other.

If you are interested, please visit for information about a class I'm giving on this topic.

Best wishes,

Ron Joseph

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