Ask the Expert Question-and-Answer Archive by Ron Joseph May, 2005 Calculating Air Flow Rate (Velocity) in a Spray Booth Q. What should the airflow be in a semi down draught booth with heated air replacement, that is 6.6m long x 4.4m wide x 2.2m high? Can the size of the air inlet filter be reduced? What are the calculations? A. Acording to OSHA Table G10 of 29 CFR 1910.94(c)6)(i) and NFPA-33 you should have a design velocity of 100 ft/min of air passing across the areas were the painters are working in a cross draft booth using manual air spray guns. In a large paint spray booth you are permitted to lower this to 50 ft/min. Many spray equipnment companies regard a downdraft booth to be a large booth and they design for the 50 ft/min target. From your question is it not clear what you mean by "semi-downdraft" booth. Some booths are designed so that air entering at the back of the booth but from the ceiling moves to the far end of the booth where exhaust filters are located. This is a modification of a conventional cross draft booth. Other booths are designed with air entering from the ceiling and ehausting through filters that are located immediately above the floor but along two side walls. In other words, the air entering at the ceiling splits with half moving towards the filters on the one side, and the other half moving toward the filters on the other side of the booth. This is a modification of a downdraft booth. Scenario #1 - Air moving from ceiling at back of booth to the exhaust filters at far end of booth. Based on these assumptions: Surface area (ft2) = 4.4m (wide) x 2.2 m (high) x 10.76 (m2/ft2)                     = 102.4 ft2 Air velocity = 102.4 ft2 x 100 ft/min                     = 10,240 ft3/min Scenario #2: Air entering at ceiling and moving downward toward the filters immediately above the floor along the sides of the booth. Based on these assumptions: Surface area (ft2) = 6.6m (long) x 4.4m (wide) x 10.76 (m2/ft2)                      = 312.5 ft2 Air velocity = 312.5 ft2 x 50 ft/min                     = 15,625 ft3/min If you want uniform air distribution throughout the booth you should size the air intake filters accordingly. If you make the filter section too small, you can expect turbulence in the booth and this will cause paint overspray to settle on the walls, ceiling, lamps, etc. In addition, turbulence lowers transfer efficiency. While these calculations might be useful to you, I recommend that you consult a spray booth company, such as Global Finishing Systems for the final drawings and the air flow requirements. Best wishes, Ron Joseph

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