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Fan Choice and Air Speed Calculations for a Spray Booth

Figuring the fan capacity for a spray booth is tricky, because you need to estimate the air speed across the working space. December 29, 2008

Question
Is a totally enclosed motor different from an "explosion proof" motor, other than the label and the $200 + difference in price? What do you use in your finishing rooms? What is a good brand and good deal on a 16 inch exhaust fan? My finishing room is 12' x 12' x 10' high, with a window in it that I would like to exhaust out as opposed to mounting a fan through the wall. I shoot mostly WB, but I also shoot a good bunch of alchohol base shellac.

Forum Responses
(Finishing Forum)
From contributor R:
Totally enclosed fan cooled (TEFC) motors are quite different than explosion proof. You would not use a TEFC motor to replace an explosion proof. That said, the motor needed depends on the fan you have. If the motor will be in the exhaust air stream, you need an explosion proof motor. These are usually direct drive fans with the blade mounted on the motor shaft. If your fan is belt driven with the belt inside a tube that extends to the outside of the fan, a TEFC motor will be fine. These fans are tube axial.

The speed of the air that you should move through the booth is 100 feet per minute (FPM). Your booth would be 12 x 12 x 100. A 16" fan will not move that kind of air. The fan should be about twice that size and have about a 15,000 CFM at 1" SP rating. The less air moved, the more likely over spray will land on your product.



From the original questioner:
Thanks for the info. The room is only 12 x 12 x 10 (not 100). So to exchange the air 4 times a minute, I need to move about 6000 CFM (am I'm figuring it right?). What brand would you recommend, and where is a good place to get a total fan package?


From contributor A:
Have a look at Dayton, Greenheck, and Cincinnati Fan. These are some of the leaders in the business and have lots of distributors. If you're in Canada, check out canadianblower.com.



From contributor J:
Contributor R is there on his calculations. You must move 100 linear feet per minute across the cross section of the booth. 12' W x 10' H is your cross section. 120 sq. ft x 100 ft/min = 12000 CF/M. Changeover in a room is not the proper design for a booth. Drying rooms and mixing rooms use changeover.


From contributor B:
I was going to suggest that you look into a company by the name of Grainger. They have quite a few fans to choose from that will satisfy your requirements. Correct me if I'm wrong, but I think that Dayton just might be one of the brands that Grainger handles. I would look for a fan that has aluminum blades (they won't spark!) as well as one that has a motor whose innards are enclosed (the carbon brushes). If you do any on site work you can easily transfer the fan to the job site.


From contributor R:
To the original questioner: You missed my explanation. Contributor J has my formula corrected. Look for a fan that will deliver 12,000 CFM. Size your filters for no more than 200 FPM or you'll be changing them too often.


From contributor A:
Something is wrong with someone's calculations, I think. The air movement should be 100 FPM in the working area, yes, but you do not do the calculations like that. The calculations of a booth are based upon cubic feet of air in the booth and CFM of the exhaust system. The airflow of the FMP in the work area is a direct result of how the airstream flows through the work area. Additionally the closer you are to the filterbank, the faster the fpm will be in most booths.

For example, if you take an automotive booth that is 26' long, 15' wide and 10' high, the 100 FPM would suggest that you need 15,000 CFM for this booth. Once you stick a car in the booth the cubic volume of air is greatly reduced and the airflow rate is at a higher FPM than if the booth was empty. This is why many booths have a frequency control unit for the fan speed. If you stick a minivan in a car booth you have to turn the fan down or it's too windy. The minivan reduces the opening by 65-70 sq ft which would result in 170+FPM with a 15000 CFM fan.

In a parts booth or woodwork booth you need to change the air over 3-4 times per minute including losses. If you have a 20' long 10' wide and 10' high booth you have 2000 cubic feet of air. For this application you want a fan that will move 8000 CFM at a full load of static pressure that includes ductwork and filters. If you shop around for a factory made booth of this nature you will see that most have fans from 7500 to 12,000 CFM at 1/2" SP.

My booth (home made) is 20'L x 12'W x 9'6"H and I run at 6000 CFM @ 1" SP for crossflow or semi-downdraft. Since I do mainly cabinets, flat pack and other basic wood stuff I don't need a mega fan like the 28,000 CFM one from my old truck booth.

Find some specs for a UL listed booth and follow what they have as a baseline. Also study the dynamics of the airflow to ensure that you have 100 FPM in the main working area. If you are spraying parts and stand directly in front of the filter bank you shouldn't get overspray on your finished parts.



From the original questioner:
Thank you very much! When I first read100 FPM, I thought wow - that will be like a wind tunnel. Then I did some quick elementary school math and realized it's only 1 mph! I think I'm going to lean towards a 24 inch 5000 + cfm fan. I want to exhaust out my existing window (24" from floor). Most of the time I am spraying directly in front of the fan, so overspray should go right into the airstream. Next question is how big should my air make-up holes be? I was thinking 2 of them across the room from the fan, each 24" square. Is this enough, or too much? This website is as useful of a tool as my tape measure.
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