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Spray booth calculations4/24
Fan and motor requirement
I have a simply paint booth that requires an exhaust fan and 3 phase motor. Here are the details:
• The fan motor needs to be a 3 phase solution. I already have a variable frequency drive (VFD) controller. The maximum that this VFD can handle is a 2 HP motor.
My initial calculations.
• Airflow through the spray booth, by code, needs to be at least 100 FPM (feet per minute). This will roughly exchange all the air inside the booth four times every minute.
But this is confusing to me. The volume of the air in the chamber is a constant at any one time, all of which needs to be moved in the same time frame – roughly four times a minute..If anything, I would think that the CFM would have to increase using the smaller exhaust opening to make certain the all of the air in the paint chamber is exhausted.
Therefore, it would seem that the original calculation of 12,600 CFM would be a more likely number to use.
Frankly this is a theoretical calculation on my part and needs to be reviewed by more knowledgeable people. Given that the entire back wall is not being used by exhaust filters.
Up and above this, static pressure will need to be included in the final fan and motor decision. The fan opening is currently 24” in diameter. The idea is to keep this opening on the spray booth and use a 24” tube axial fan with 24” exhaust ductwork as specified above. The SP factor can only be calculated once the fan has been selected and the appropriate ductwork calculated for resistive flow.
Venturi, Two good sources with technical support for spray booth (tube axial) fans are globalfinishing.com and cincinnatifan.com. Typically, a spray booth is exhausted 6 feet vertically above roof line or peak of roof within 25 feet of exhaust. If exhaust pipe runs out a side wall, elbow up and bracket pipe to wall, still achieving the 6 foot mark. Weather canopy terminations push the exhaust fumes back to roof which can be sucked in by heating or make up air units. Also, avoids fumes from being blown into the face of maintenance people working on the roof. The vertical discharge with upblast stack cap sends the fume skyward. In addition, most spray booth package fans are designed for max of 25 feet vertical exhaust stack. One, 24" 90 degree elbow with a centerline radius of 1.5 x diameter which is the recommended minimum turning radius is equal to 57 feet of 24" straight pipe in equivalent resistance. When duct resistance is calculated, all bends must be converted to straight pipe and added to the straight pipe required. This condition will require more horse power than the package fan to achieve the minimum of 100 FPM per square feet of booth opening. Also, there is an explosion proof zone that extends to the front, sides, and back across the roof of spray booth. Double check with the fan people. The distance is reduced if the spray gun and exhaust fan are interlocked. Curt
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