10.2 LEV Hood Taxonomy and Capture Velocity Selection
Key Takeaways
- Air pulled into an exhaust opening is drawn from all directions, so centreline velocity falls to under 10% of face velocity within about one duct diameter; blowing jets carry ten times farther, which is why push-pull systems work.
- Enclosing hoods are always preferred over capturing hoods, which are preferred over receiving hoods, because enclosure removes reliance on capture velocity entirely.
- Capture velocity is selected from the contaminant release energy and the ambient air motion: roughly 50 to 100 fpm for release into quiet air, up to 2,000 fpm or more for release with high initial velocity into rapid air motion.
- Capture velocity is the velocity at the point of contaminant generation, not at the hood face — confusing the two is the most common LEV design error.
LEV Hood Taxonomy and Capture Velocity Selection
Local Exhaust Ventilation (LEV) is the primary engineering control methodology utilized in industrial hygiene to capture airborne contaminants—including toxic dusts, metal fumes, mists, vapors, and gases—at or near their emission source before they disperse into the general workroom atmosphere and enter worker breathing zones. Unlike dilution ventilation, which merely dilutes already-dispersed airborne contaminants, a properly engineered LEV system captures contaminants at high localized concentrations using minimal air volumes, thereby optimizing worker protection, reducing capital HVAC energy costs, and minimizing air cleaning equipment sizing.
1. Aerodynamic Principles and Velocity Decay
The fundamental physical challenge of LEV hood design is governed by the laws of fluid dynamics: air under suction has virtually no directional momentum compared to air under positive pressure (blowing).
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| SUCTION VELOCITY DECAY VS. BLOWING JETS |
+-------------------------------------------------------------------------+
| |
| BLOWING (Positive Jet): |
| Air pushed out of a duct forms a focused, high-velocity core jet. |
| Centerline velocity remains ~10% of duct face velocity at 30 Diameters!|
| |
| Duct =====> [ >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> ] |
| 0D 10D 20D 30D |
| |
| SUCTION (Exhaust Hood): |
| Air pulled into an exhaust opening is drawn radially from ALL directions|
| (a spherical 360° intake field). Suction velocity decays precipitously!|
| Centerline velocity drops to < 10% of face velocity at just 1 Diameter!|
| |
| \ | / |
| \ v / |
| <==== [ DUCT ] <==== |
| / ^ \ |
| / | \ |
| x = 1D (Velocity <= 10% V_face) |
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Because suction velocity decays inversely with the square of distance (x²) for point and round openings, hood placement close to the contaminant generation point is the single most critical determinant of LEV capture efficacy.
2. Comprehensive LEV Hood Taxonomy
Industrial exhaust hoods are categorized into three primary structural and aerodynamic classifications: Enclosing Hoods, Receiving Hoods, and Exterior Hoods.
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| CLASSIFICATION OF LEV HOODS |
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| |
| 1. ENCLOSING HOODS |
| - Complete Enclosures: Gloveboxes, sealed abrasive blast cabinets |
| - Partial Enclosures: Laboratory chemical fume hoods, paint booths |
| |
| 2. RECEIVING HOODS |
| - Thermal Canopy Hoods: Capture rising hot buoyant plumes |
| - Grinding Wheel Hoods: Capture projectile momentum of particles |
| |
| 3. EXTERIOR HOODS |
| - Plain Open Ducts: Freestanding round/rectangular suction mouths |
| - Flanged Exterior Hoods: Flange plates eliminate rear air draw |
| - Slot Hoods / Plenums: Uniform linear velocity along tank edges |
| - Downdraft Tables: Suction pulled downward through perforated grid |
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Enclosing Hoods
Enclosing hoods surround the contaminant generation source completely or partially. They represent the most aerodynamically efficient hood design because they isolate the process from cross-drafts and require the lowest volumetric airflow (Q) per unit process area.
- Complete Enclosures (Gloveboxes, Isolators):
- Process is 100% sealed under negative static pressure (typically -0.5 to -1.0 in. w.g.). Air enters only through HEPA intake filters, and workers manipulate materials through sealed, impermeable elastomeric gloves.
- Applications: Highly potent pharmaceutical compounds (OEB 4/5), radiochemistry, handling pyrophoric or infectious pathogens (BSL-3/4).
- Partial Enclosures (Laboratory Chemical Fume Hoods):
- Provide an open frontal access area (face) with an adjustable sliding sash (vertical, horizontal, or combination).
- Face Velocity Standards (ANSI/ASSP Z9.5 & OSHA 1910.1450):
- Standard operating face velocity: 80 to 120 fpm (0.4 to 0.6 m/s) across the active sash opening (typically verified at a standard operating sash height of 18 inches / 45 cm).
- Face Velocity Upper Limit: Operating a chemical fume hood above 150 fpm (0.75 m/s) creates internal turbulent eddy currents behind the sash lip and around the worker's torso, causing wake vortices that pull contaminants out of the hood into the breathing zone.
- Bypass Design: Modern constant air volume (CAV) bypass fume hoods incorporate an upper bypass grille that opens as the sash is lowered, maintaining a relatively constant face velocity rather than allowing dangerous velocity spikes at low sash heights.
Receiving Hoods
Receiving hoods are positioned to intercept a contaminant stream that possesses an intrinsic initial directional velocity or buoyant momentum generated by the process itself.
- Canopy Hoods: Positioned directly above hot processes (e.g., molten metal furnaces, commercial deep fryers, hot dye vats). The hot process heats ambient air, creating a rising thermal convective plume. Canopy hoods must be sized larger than the process footprint to encompass the expanding conical plume (30° spread angle).
- Grinding and Buffing Hoods: Positioned directly in line with the tangential projectile trajectory of sparks, grit, and metallic dust thrown off by high-speed rotating wheels, utilizing the kinetic energy of the particles to deliver them into the hood mouth.
Exterior Hoods
Exterior hoods are located adjacent to an emitting process without enclosing it. Because the contaminant is released outside the physical hood boundary with little or no directional momentum toward the hood, the hood must generate an active air current across the intervening space to capture and draw the contaminant into the duct.
- Flanged Exterior Hoods: Adding a flat, perpendicular flange around the perimeter of an exterior hood opening redirects air inflow, eliminating unwanted airflow from behind the hood and increasing capture velocity in front of the hood by 25% to 33% for the same volumetric flow rate.
- Slot Hoods on Open Surface Tanks: Narrow horizontal or lateral slots attached to an exhaust plenum provide uniform linear capture across broad liquid surfaces (e.g., electroplating baths, degreasing tanks, pickling tanks) while keeping ductwork unobstructed above the tank.
- Downdraft Tables: Draw air downward through a perforated workbench grid, utilizing gravity to assist in capturing dense solvent vapors or heavy grinding/sanding dusts away from the operator's breathing zone.
3. Capture Velocity (Vc) Selection Criteria
Capture velocity (Vc) is the minimum air velocity required at the most distant point of contaminant generation to overcome cross-drafts, ambient room air currents, and contaminant momentum, successfully drawing the contaminant into the exhaust hood.
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| CAPTURE VELOCITY SELECTION HIERARCHY |
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| Condition / Contaminant Release Profile | Capture Velocity|
+-------------------------------------------------------+-----------------+
| Quiet evaporation; gas/vapor release with practically| 50 - 100 fpm |
| no velocity into still room air (e.g., solvent tanks)| (0.25 - 0.5 m/s)|
+-------------------------------------------------------+-----------------+
| Low to moderate release velocity into moderate room | 100 - 200 fpm |
| cross-drafts (e.g., welding, barrel filling, plating)| (0.5 - 1.0 m/s) |
+-------------------------------------------------------+-----------------+
| Active generation; high-speed release into rapid room| 200 - 500 fpm |
| air motion (e.g., conveyor loading, crushers, paint) | (1.0 - 2.5 m/s) |
+-------------------------------------------------------+-----------------+
| High initial mechanical velocity; extreme momentum | 500 - 2,000 fpm |
| (e.g., grinding, abrasive blasting, fast spinning) | (2.5 - 10.0 m/s)|
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Key Determinants of Capture Velocity Selection
When selecting the design capture velocity within a given range (e.g., selecting 100 fpm vs. 200 fpm for welding), the industrial hygienist must apply the higher end of the range if:
- Room cross-drafts are significant (e.g., cross-drafts > 50 fpm from pedestrian traffic, supply diffusers, or open bay doors).
- Contaminant toxicity is high (low OEL / TLV, carcinogen, sensitizer).
- Process generation rate is intermittent or surging.
- Worker position is between the hood and the source (to avoid pulling contaminants across the breathing zone).
Which of the following face velocity ranges is specified by ANSI/ASSP Z9.5 and OSHA guidelines as standard for general laboratory chemical fume hoods under normal operating conditions?
When designing a local exhaust ventilation hood for a high-speed dry abrasive grinding wheel producing heavy metallic particulates, which capture velocity range is recommended by ACGIH?