Automotive plants use solvent-based paint in spray booths. When solvent vapors exceed LFL, explosion occurs. NFPA 33 specifies 0.5 m/s booth face velocity, 25 percent LEL shutdown and Class I Div 1 (Zone 1) electrical classification. OEM paint shops and body repair centers follow the same rules.
Booth Airflow Design
- Face velocity: 0.5 m/s (100 fpm) min
- Downdraft booth: floor grating exhaust
- Semi-downdraft: asymmetric flow
- Cross-draft: legacy, not recommended
- Plenum: makeup air above, positive pressure
LEL Monitoring
- Solvent: toluene, xylene, MIBK
- LFL: toluene 1.2 percent, xylene 1.0 percent
- Setpoint: 10 percent LEL alarm, 25 percent shutdown
- Detector: catalytic bead, exhaust duct
- Calibration: 6-month span gas (methane)
Class I Div 1 (Zone 1) Area Classification
- Inside booth: Class I Div 1 (Zone 1)
- 1 m outside booth: Div 2 (Zone 2)
- Machinery: Ex d/Ex e motors
- Lighting: explosion-proof LED
- Static: grounding + bonding, 10^6 ohm max
Fire Protection
- Sprinkler: K=80, Extra Hazard 2
- Dry chemical: 30 kg ABC inside booth
- Exhaust stack: 3 m spaced flame arrestors
- Filter bank: overspray burn panel
- Manual alarm: push station outside booth
Waterborne Paint Transition
- Water-based paint: low VOC, no LEL
- Solvent reduction: EPA/REACH compliance
- Booth change: longer flash/drying zone
- Electrostatic bell applicator: ESD precaution
- Energy: 50 percent lower ventilation after switch
Spray booths with MEP Calc
MEP Calc computes spray-booth flow (0.5 m/s face velocity), LEL setpoint, Class I classification and NFPA 33 compliance.
Download MEP Calc on the App Store
Sources & Further Reading
NFPA 33, NFPA 86, ANSI/OSHA 1910.107, EN 12215, IEC 60079-10-1. NFPA 33 Spray Application standard.