Ventilation Design Considerations for Occupant Health in Aircraft Painting Facilities Under OSHA Requirements

摘要

Reducing exposures of aircraft painters to hazardous metals and organics motivates design and operation of hangar ventilation systems in purpose-built facilities. Facilities are often repurposed for aircraft painting even when the ventilation system has been designed for thermal comfort or general dilution. Contaminant exposures under cross-flow, ceiling diffuser, and hybrid ventilation configurations were evaluated. Occupational Safety and Health Administration (OSHA) regulations require 100 fpm (0.508 m/s) through spray booths/ rooms, and this condition is difficult to achieve with most ceiling diffuser installations. Cross-flow designs provided lower contaminant exposures, with decreased residence times and efficient flow paths. CFD modeling tracer gas testing and exposure monitoring examined contaminant exposure vs. crossflow ventilation velocity. RANS CFD modeling (RNG k-Є) showed exposures to simulated methyl isobutyl ketone of 294 and 83.6 ppm, as a spatial average of five worker locations, for velocities of 0.508 and 0.381 m/s (100 and 75 fpm), respectively. In tracer gas experiments, observed supply/exhaust velocities of 0.706/ 0.503 m/ s (136/99 fpm) were termed full-flow, and reduced velocities were termed 3/4-flow and half-flow. Half-flow showed higher tracer gas concentrations than 3 / 4-flow, which had the lowest time-averaged concentration, with difference in log means significant at the 95% confidence level. Half-flow compared to full-flow and 3/4-flow compared to full-flow showed no statistically significant difference. CFD modeling using these ventilation conditions agreed closely with the tracer results for the full-flow and 3/4-flow comparison, yet not for the 3/4-flow and half-flow comparison. Full-flow conditions at the painting facility produced a velocity of 0.528 m/s (104 jpm) midway between supply and exhaust locations, with the supply rate of 94.4 m~3/s (200,000 cfm) exceeding the exhaust rate of 68.7 m~3/s (146,000 cfm). Ventilation modifications to correct this imbalance created a midhangar velocity of 0.406 m/s (80.0 fpm). Personal exposure monitoring for two worker groups—sprayers and sprayer helpers ("hosemen")—compared process duration means for the two velocities. Hexavalent chromium (Cr[VI]) exposures were 500 vs. 360 μg/1 m~3 for sprayers and 120 vs. 170 μg/m~3 for hosemen, for 0.528 m/s (104 fpm) and 0.406 m/s (80.0 fpm), respectively. Hexamethylene diisocyanate (HDI) monomer means were 32.2 vs. 13.3 μg/ m~3 for sprayers and 3.99 vs. 8.42 μg/m~3 for hosemen. Crossflow velocities affected exposures inconsistently, and local work zone velocities were much lower. Aircraft painting contaminant control is accomplished better with the unidirectional crossflow ventilation presented here than with other observed configurations. Exposure limit exceedances for this ideal condition reinforce continued use of personal protective equipment.