A fume hood operating at 60 FPM may still allow 20% of contaminants to escape into the lab — that's a fact from a NIOSH/CDC tracer study. Yet many labs rely on a single velocity measurement without accounting for sash opening, leading to false confidence.
The Formula
Face velocity is the average speed of air entering the hood opening. It's calculated as:
Imperial:
Face Velocity (FPM) = Exhaust Airflow (CFM) / Opening Area (ft²)
Metric:
Face Velocity (m/s) = Exhaust Airflow (m³/s) / Opening Area (m²)
Opening Area = Width × Height (both in consistent units). For a hood with sash partially open, this is the actual open area — not the total hood face. The physics is simple: airflow divided by area. But each term carries engineering significance.
Exhaust Airflow is the volumetric flow rate the exhaust system pulls. In metric, it's often given in m³/h, which must be converted to m³/s (divide by 3600). The hood's ability to capture contaminants depends on maintaining adequate velocity across the open area.
Why the formula works: It's a direct application of the continuity equation for incompressible flow. The hood acts as a duct; the average velocity is the flow rate per unit area. This is a screening metric, not a guarantee of containment — ASHRAE Standard 110 is the full performance test.
Worked Example 1
Scenario: A 6 ft wide, 2.5 ft tall sash opening with 600 CFM exhaust.
Step 1: Compute opening area
Area = 6 ft × 2.5 ft = 15.0 ft²
Step 2: Compute face velocity
Face Velocity = 600 CFM / 15.0 ft² = 40.0 FPM
Interpretation: 40 FPM is below typical control velocities (80-100 FPM). This is a low face velocity — likely inadequate for safe containment. The engineer should increase exhaust airflow or reduce sash opening.
Worked Example 2
Scenario: A 1.2 m wide, 0.8 m high sash opening with 0.5 m³/s exhaust (converted from 1800 m³/h).
Step 1: Compute opening area
Area = 1.2 m × 0.8 m = 0.96 m²
Step 2: Compute face velocity
Face Velocity = 0.5 m³/s / 0.96 m² ≈ 0.521 m/s
Convert to FPM for comparison: 0.521 m/s × 196.85 ≈ 102.5 FPM
Interpretation: This velocity is within the 80-100 FPM range recommended by NIOSH/CDC. It's a practical face velocity, but the engineer must still consider room drafts and hood design.
What Engineers Often Miss
1. Face velocity alone is not a safety metric. ASHRAE 110 testing reveals that even at 100 FPM, leakage can occur due to turbulence. Use face velocity as a screening tool, not a pass/fail criterion.
2. Sash position changes everything. A partially closed sash reduces opening area, increasing face velocity for the same airflow. But a closed sash can create dead zones. Always measure with the sash at the typical operating position.
3. Room airflow patterns matter more than you think. Cross-drafts from HVAC diffusers or open doors can disrupt the hood's capture ability. NIOSH/CDC studies show that at 60 FPM, room AC operation caused significant leakage. At 100 FPM, leakage was eliminated regardless of AC.
Try the Calculator
Estimate face velocity instantly with the Fume Hood Face Velocity Calculator. Input your exhaust airflow and sash dimensions to get face velocity and opening area — then compare against the recommended ranges.
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