Copyright © Thomas Fetter
Here is the design of the paint booth ventilation system.  I chose to use real paint booth components for safety reasons.   It's important to ensure the proper airflow needed to keep the booth free of a buildup of fumes, to remove the airborne paint particles before they exit the booth, and to ensure the fan does not ignite the highly flammable paint vapors. Please - do NOT use an open frame motor that is exposed to the paint fumes!!!

My booth is set up in my garage, with the air exit and fan pointed out the open garage door.  There is a large open window opposite the garage door which supplies the air to the booth.  In operation, I do not notice any smell of paint outside the booth in the garage, which means my arrangement for venting the  booth works well, and that none of the exhaust air is circulating back through the garage.

Paint Booth - Ventilation Design
This is a "cross-flow" paint booth design.  The air enters on the top on one side of the booth, across the entire width of the booth, and exits at the bottom of the opposite side of the booth, picking up the paint over-spray along the way.  Several references for spray booth operation recommend a linear flow rate of 100 fpm to be enough to pick up the spray particles while not so fast as to disrupt the spraying operation.  Fans are specified in terms of volume of flow (as a combination of diameter, speed, and motor size) pre unit time, or cfm.  To calculate the linear flow rate, I assumed a 2' tall by 10' wide column of air moving across the booth, which, at 100 fpm, would be 2000 cfm, the flow rate required of the fan.

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I purchased my fan and motor from Grainger.  They have a wide selection of fans and motors.  This table is the starting point.  I knew I needed 2000 cfm.  From this table, a 16"diameter fan, driven by a 1hp motor at 2547 rpm is capable of around 3000 cfm.  I decided to go with this combination which is more than I calculated I needed, in case my calculation underestimated the flow at the center of the booth.  I knew I could then slow the fan speed as needed using different sized pulleys, to ensure I achieved a 100 fpm flow rate. 

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The fan is a 16" propeller diameter tubaxial fan.  This is a real paint booth fan intended to be installed in a round duct at the outlet of the spray booth. The motor mounts on top of the fan, outside the airflow, to keep it out of the air flow, which includes flammable vapors.  The motor drives the fan with a v-belt.  This makes it easy to change the fan speed by changing the pulley diameters.

The fan should be mounted on the spray booth outlet, draw air from the booth, so  that the booth is at a negative pressure relative to the outside.  This way, air is drawn in through the cracks, and all the air and volatile paint vapors exit through the paint filters are directed away from the booth.  If the fan were mounted on the inlet of the booth, the booth would be at a positive pressure, and paint fumes would exit out the cracks as well as out the filter.

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For the motor, I purchased a totally enclosed, explosion proof, 1hp motor.  Despite being out of the direct airflow, its still a good idea to take this precaution, despite being more expensive.

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For the exit filters, I used 2" progressively layered fiberglass filter material, pre-cut to 20"x20" squares.  It does a good job of filtering out the paint over-spray particles.

After completing the booth, I used a wind speed meter to measure the flow.  The flow exceeded the 100 fpm goal, and I adjusted the fan speed by putting a smaller pulley on the motor.

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