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Spray booth calculations

4/24/17       
Venturi Member

Fan and motor requirement

I have a simply paint booth that requires an exhaust fan and 3 phase motor. Here are the details:

Paint booth
• Size of booth is 24 feet deep by 9 feet wide by 14 feet high
• It is a cross flow style booth
• Two front doors are entirely covered by air filters
• The rear part of the booth although 14ft high by 9ft wide only has exhaust filters rising from the floor to a height of 6 ft. (not the entire 14 ft height of the back wall, but they do span the entire 9 foot width)
• There is another chamber behind the exhaust filters that interfaces to the hole in the ceiling.
• The fan opening in the exhaust chamber on the ceiling is 24” in diameter

Exhaust ducting
• The ducting will require one 90 degree elbow and a straight run of ductwork to the exhaust opening in the shop wall. The straight ducting is 10 feet in length.
• The exhaust ducting should be the maximum diameter permitted to interface with the selected round 24” fan.

Fan Motor

• 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.
• The motor should not be in the exhaust stream of the system. This will avoid having to use an explosion proof motor.
• It ideally should be mounted to the fan assembly externally, and drive the fan via a belt.
• Fan blades should not create sparks; typically they should not be made of steel or any ferrous metal.

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.
• I am uncertain how to calculate the exhaust area. The actual area of the booth at the end is 14’ x 9’ = 126 sq. ft.
However the actual exhaust part of the rear wall is only 14’ x 6’ = 84 sq. ft.
• Conventional theory to calculate the CFM is vertical square footage of the exhaust spray booth wall times the velocity of the air movement (FPM). This can yield two different results:
If the entire square footage of the back wall is used, then the CFM (cubic feet per minute) number would be 126 x 100 = 12600 CFM.
• If the more logically figure of the exhaust opening (in the back wall) is used the total CFM would be 84 x 100 = 8400 CFM.
It would then appear that the smaller exhaust opening would require a smaller fan based strictly on the CFM flow.

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.

SP calculation.

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.

4/26/17       #2: Spray booth calculations ...
Curt Corum

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

Click the link below to download the file included with this post.

Spray_Booth.pdf


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