Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 212 of 331

sures and consequently high fluid stream velocity at the tip of the spray gun. This can cause overspray tore bound and may expose the operator to toxic materials pre- sent in the paint. Velocity should always be sufficient to carry the overspray away from the operator and into the exhaust chamber. The velocity possible in a booth depends on the fan size. Most standard booths of- fered in the market come equipped with fan and motor packages sized to deliver the necessary draft. Draft requirements take into account real-world static pressures in- cluding resistance to air flow from entry losses, stack filters, and duct work. Static pressure is the amount of resistance air must overcome while moving from point A to point B. Static pres- sure in a spray booth is encountered in two areas: intake and exhaust filters and intake and exhaust duct work. The static pressure of any filter is deter- mined by how much air will pass through that filter. Air-intake filters for downdraft spray booths are denser and pass less air than air-intake filters for either cross-draft or semi downdraft booths. Consequently, air-intake filters for downdraft spray booths have a high- er static pressure rating than the air-in- take filters for other booths. When intake or exhaust filters become clogged with dirt or material overspray, the amount of air that can pass through the filter decreases. When air flow is re- stricted, the filter's static pressure or re- sistance to air flow increases. Air intake and exhaust ducts also influence static pressure. Fig. 11. To determine the size of the booth in cubic feet per minute, multiply the cross-sectional area of the booth in square feet by the velocity of the air through the booth in feet per minute (i.e., 10 ft x 12 ft = 120 ft2 ; 120 ft2 ; 120 ft2 x 100 fpm = 12,000 cfm). Air volume and velocity are decreased when elbows, reducers, transitions, and long runs are added to ducts. Elbows introduce angles and increase resistance to air flow. Re- ducers and transitions also increase the static pressure in duct work. The ideal situation is to keep duct work to a minimum. Static pressure is also a factor when choosing an air replacement unit. Because of the similarities to an exhaust booth, pressure drops in and out of the unit must be considered. Tables I and II give recommended spray booth velocities covering average conditions. The figures are all based on empty booths and include the face opening plus any conveyor openings. These are recommendations only, and are not meant to replace local or state regulations on minimum air velocity. In NFPA-33 (section 5–2) air velocity requirements are defined. According to the guidelines, a booth needs to "provide adequate ventilation to maintain the concen- Fig. 12. Paint-arrestor spray booth. 211

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