Face velocity is the average amount of air pulled through a hood's face. If velocities are too low, chemical fumes escape into the lab. If too high, energy costs increase significantly. The balance between safety and efficiency depends on proper airspeed.
Face Velocity Requirements
Traditional hoods should operate between 60 and 100 FPM according to many agencies, potentially increasing to 120 FPM when working with highly toxic chemicals. Note: Faster flow doesn't guarantee better performance and can actually create less efficient, more dangerous conditions through turbulent air.
Average Face Velocity Calculations
Divide the sash opening into a grid of approximately 1 ft² squares. Take air measurements at each square's center using a velometer, anemometer, or similar device for roughly 10 seconds per reading. If any measurement is 20% over or under standard, turbulent or nonlaminar airflow may exist.
Analyzing Results
Velocities greater than 0.8 m/s indicate face velocity is too fast and should decrease. Velocities below 0.4 m/s indicate face velocity is too low and should increase.
Smoke Tests: Visualizing Fume Hood Airflow
Acquire a smoke pen, puffer, or stick. Move it around the hood's bottom and inside edges to ensure smoke exhausts properly. Place the device just outside the fume hood to determine if outside environment negatively affects airflow.
Additional Safety Checks
Diffuser Check
Air supply diffusers on ceilings compensate for exhausted air. If diffuser air currents reach the hood face at sufficient velocity, they can blow fumes into the lab. Regular diffuser balancing is critical.
Foot Traffic Analysis
Walking at 3 mph produces a small air vortex behind a person, potentially pulling chemical fumes from hoods into the laboratory. Analyze foot traffic to ensure it's not directed in front of operating hoods.