Metal Finishing Guide Book


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A: My friend and colleague Rich Thelen of Global Finishing Systems has kindly agreed to answer this question...���Spray booth velocities are not cited in NFPA33. They are vestigial requirements from US 29 CFR 1910.107 (OSHA) that list velocities in certain circumstances, such as electrostatic painting or other painting. Some of these standards have worked their way into other codes in various fashions. The U.S. Military Guides (UFC-3-410) list different velocities for different spray techniques, including HVLP, electrostatic and airless. OSHA has deferred to NFPA-33 in the design of spray booths and has published its interpretation online at OSHA considers the use of velocities other than those listed in 1910.107 as ���de minimis��� violations. ���De minimis��� violations of standards that have no direct or immediate relationship to safety and health are not included in citations, but the velocities must conform to NFPA-33. The focus for air velocity is on NFPA-33, which simply states that the airflow shall be sufficient to keep the concentrations of solvents below 25% of the Lower Flammable Limit (LFL). NFPA is concerned with fire protection, whereas OSHA is concerned with human health and safety. Jointly, they cover health, safety and fire issues, and both agree that NFPA-33 is an acceptable rule. So much for legal issues! ���A spray booth must also remove overspray in a manner that gives a good paint finish, while at the same time providing high transfer efficiency. These are production issues that must be addressed by the owner. Regulations will not guarantee a good finish! As spray booth manufacturers, we recommend airflows of 75-125 fpm in cross-draft paint booths and 40-50 fpm in down-draft booths. These velocities consider cross-sections that are normal to the booth airflow. We have found that these airflow velocities are well above the 25% LFL requirement and they yield good paint jobs. ���Velocities below these numbers may also yield good paint jobs, but generally lower velocities are more likely to cause the airflow in the spray booth to stratify and lead to painting (and heating) problems. This happens particularly in large spray rooms. Stratification in downdraft booths occurs at lower velocities than in cross-draft booths. Because the direction of overspray particles is downward, and gravity contributes to particulate removal, down-draft booths are more efficient at removing overspray at low velocities than cross-draft booths. It can be costly to remedy stratification, because changing the exhaust and supply fans is a major project.��� REDUCING PAINT DEFECTS���AIRLESS SPRAY GUNS Q: Please give me advice about reducing paint overspray while using a fast-dry paint and an airless spray gun. A: The only method of which I am aware for reducing paint overspray while using an airless spray gun is to lower the hydraulic pressure. However, if you lower the pressure too much you will not get ���tails��� and you will not be happy with the finish. If your painter stands closer to the surface he is painting, then he might be able to lower the pressure sufficiently to make a difference. 613

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