Hood Capture Velocity
How is capture velocity related to duct velocity, and why does distance matter?
The OSHA Technical Manual shows that at one duct diameter from a plain hood, capture velocity is about 0.1 times duct velocity; at two diameters, it drops to about 0.01 times. This exponential decay means effective LEV requires keeping the source close to the hood opening, often within 1.5 duct diameters, and using enclosing or flanged hoods to improve capture efficiency.
How does WorkSafeBC's face velocity requirement relate to capture velocity for laboratory hoods?
WorkSafeBC requires average face velocity of 0.4–0.6 m/s (80–120 fpm) across the operational face opening, with no point <80% or >120% of the average. For partially enclosed hoods, capture velocity at the hood opening is effectively face velocity, so this standard ensures contaminants generated just inside the hood are drawn in.
What capture velocities are recommended for different industrial processes on a shop floor?
For non-heated process tanks (e.g., plating baths), maintain capture velocity 65–100 fpm immediately above the bath. For vapour release via evaporation, minimum capture velocity 100 fpm at the generation point. For general LEV with minimal room currents, about 0.5 m/s (100 fpm) is typically adequate. Higher velocities (125–150 fpm) are recommended for highly hazardous materials.
Hood capture velocity is the air speed in front of or at the opening of a local exhaust hood, sufficient to overcome competing air currents and pull contaminants into the hood. It is distinct from duct velocity and face velocity, though in partially enclosed hoods, capture velocity at the opening is often called face velocity. Typical design ranges for lab hoods are 0.4–0.6 m/s (80–120 fpm).
On a shop floor, hood capture velocity is a design, commissioning, and compliance parameter for local exhaust ventilation (LEV) systems used in welding, grinding, solvent tanks, and paint booths. Designers select a target capture velocity at the contaminant source based on contaminant type and room air currents, then compute required airflow. The source must be kept close to the hood opening, as capture velocity decays rapidly with distance—at one duct diameter away, it is about 0.1 times duct velocity. Operators must position work pieces within the effective capture zone, control cross-drafts, and maintain proper sash or door positions to ensure effective contaminant capture and compliance with regulations like WorkSafeBC.
Source too far from hood: Work performed outside the effective capture zone causes contaminants to escape despite nominal fan performance, as capture velocity falls by a factor of 10 from one to two duct diameters away.
Adequate fan flow but poor control due to cross-drafts: Competing air currents from fans, supply diffusers, or open doors/windows reduce effective capture even when face velocity meets criteria.
Non-compliant or unstable face/capture velocity: Lab or process hoods fail WorkSafeBC face velocity criteria (average 0.4–0.6 m/s, no point <80% or >120% of average), often due to single-point readings below 80% or alarms not calibrated.