Engineering Controls and Ventilation
Engineering Controls and Ventilation
This domain tests whether you can specify, measure, and troubleshoot engineering controls, especially local exhaust ventilation (LEV). Expect hood design, airflow measurement, dilution versus capture, and maintenance issues.
Core concepts
- LEV has five parts: hood, duct, fan, air cleaner, and exhaust stack. A weak hood cannot be fixed by a larger fan if capture is poor.
- Capture velocity is the air speed needed at the contaminant source to pull it into the hood. Transport velocity is the speed in the duct needed to keep dust from settling.
- Enclosing hoods are more reliable than external hoods; external hoods must be close to the source because capture velocity falls rapidly with distance.
- Dilution ventilation mixes contaminated air with clean air; it is not a substitute for capture of toxic or dust contaminants.
- Make-up air must replace exhausted air, or building pressure and hood performance will drop.
- Measure with a velometer or pitot tube, check static pressure, and use smoke tubes to visualize airflow. OSHA ventilation rules include 1910.94 for abrasive blasting, spray finishing, and grinding.
Worked example
A welding bench has a movable hood 12 inches from the arc. Smoke drifts to the welder's face. Check whether the hood is too far, capture velocity is insufficient, the filter is clogged, or make-up air is closed. Move the hood closer, verify airflow, clean or replace the filter, and retest with smoke. Then confirm with personal sampling.
Traps
- Measuring duct velocity and calling it hood capture velocity.
- Adding duct runs or closing dampers without rebalancing.
- Using dilution ventilation for high-toxicity contaminants.
- Assuming a fan curve replaces field measurement.