The local exhaust hood capture efficiencies of 15 vapor degreasers were examined during normal operation and compared to the American Conference of Governmental Industrial Hygienists (ACGIH) design criteria. Solvent samples were collected on charcoal tubes placed in the duct and at specified distances from the degreaser. A standard pitot tube and manometer were used to measure local exhaust hood flow rates in the duct. Air velocity through building and room openings was determined with an anemometer. An Alnor Balometer was used to assess ventilation supply and exhaust air flows. The completely mixed space mass balance model was applied to the calculation of fugitive emission rates. The measured capture efficiency was computed as the ratio of the duct emission rate to the total emission rate. Degreaser operation ranged from 36% to 163% of design, with an average of 104% of design. At most sites, the air change rate exceeded eight changes per hour. The average fugitive emission rates varied from 0.002 grams per second (g/s) to 0.972g/s, with an overall average of 0.200g/s. The average capture efficiency ranged from 9% to 99%, with an overall average of 63%. Capture efficiency was not correlated with the hood exhaust flow rate, with a coefficient of only 0.073. Capture efficiency was most correlated with the hood face velocity, with a coefficient of 0.449. Hood flow was negatively correlated with workspace emissions, with a coefficient of -0.299. Solvent emissions and total solvent removed in the exhaust systems were correlated, with a coefficient of 0.238. The authors conclude that the ACGIH recommended flow rate of 0.25 cubic meters per second per meter squared is adequate for some, but not all conditions. The ACGIH design criteria can not predict the variable hood performance. Fugitive emissions are related to factors other than hood flow rate.
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