Metal Finishing Guide Book


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flammable gases, vapors, and dusts are handled. Most industrial booths are Class I. Class I, Division 1 areas are the inside of the spray booth and the inside of the ductwork. Class I, Division 2 is any area within a 10-ft radius of the open face of a spray booth when the spray gun is not interlocked with the exhaust fan to prevent spraying unless the fan is operating. When the spray gun and fan are interlocked, the Class I, Division 2 area extends five feet back from the open face. This area also extends three feet from a conveyor opening and includes the area above the ceiling of the booth (see Figs.1 to 3). Equipment located in the Class I, Division 1 atmosphere must be classified as explosion proof. In practice there should be no electrical items inside a spray booth. Electrical equipment in the Class I, Division 2 atmosphere must be thirdparty listed (such as UL, ETL, ER) and must not produce sparks under normal operating conditions. MEASURING BOOTH EFFICIENCY By design, a spray booth collects solids known as particulate emissions. Efficiency factors, specifically grain count, measure how effectively a spray booth and filter system will be in trapping these particulate emissions. The following formula is used to determine the relative efficiency of a specific system. The grain count, or relative efficiency, can be altered by making changes in equipment (transfer efficiency), coating material (percent of solids in paint), and the air flow (cfm), rather than changes only in booth design. For example, if a painter switched from conventional air spray equipment Fig. 3. Spray area hazardous limits. to HVLP equipment, the higher transfer efficiency possible with HVLP would lower the grain count. Because of its ability to trap particulate matter, a spray booth can help the end user meet EPA requirements. Unfortunately efficiency factors have at times been misrepresented as providing an assurance that a spray booth will meet EPA requirements. 773

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