10.4 Particulate Transport Aerodynamics and Duct Sizing
Key Takeaways
- Minimum transport velocity prevents settling: roughly 2,000 to 2,500 fpm for vapours, gases, and fumes; 3,500 to 4,000 fpm for average industrial dust; and 4,500 fpm or more for heavy or moist material.
- Settling velocity follows Stokes law for particles below about 50 µm, scaling with the square of particle diameter and with particle density.
- Continuity (Q = V·A) links duct area to velocity, so undersizing a duct raises velocity and friction loss while oversizing it drops velocity below transport velocity and lets material settle out.
- Round duct is preferred over rectangular because it resists collapse under negative pressure and has a lower perimeter-to-area ratio, reducing friction for the same flow.
Particulate Transport Aerodynamics and Duct Sizing
The ductwork system in Local Exhaust Ventilation (LEV) serves as the pneumatic conveying pipeline connecting the intake hoods to the air cleaning device and exhaust fan. Proper duct design must balance two competing aerodynamic requirements: maintaining sufficient air velocity to convey solid particulates without settling (preventing duct plugging and combustible dust hazards) while minimizing fluid friction and dynamic fitting losses to conserve fan electrical power.
1. Particulate Transport Aerodynamics & Settling Physics
When particulate-laden air flows through a horizontal duct, two opposing forces act upon every suspended particle:
- Gravitational Sedimentation: Gravitational force accelerates particles downward at their terminal settling velocity (Vs), governed by Stokes' Law.
- Turbulent Upward Diffusion: Turbulent eddies generated by high-velocity shear stress near the duct walls impart upward velocity vectors that keep particles entrained in the fluid stream.
+-------------------------------------------------------------------------+
| PARTICULATE DEPOSITION IN A DUCT RUN |
+-------------------------------------------------------------------------+
| |
| A. INSUFFICIENT VELOCITY (V_duct < V_tr): |
| Gravitational settling overwhelms turbulent mixing. Dust falls out |
| and accumulates along the bottom of horizontal ducts. |
| |
| Duct Wall -----------------------------------------------+ |
| ===> ===> ===> (Low Velocity Air Stream) | |
| . . . (Particles settle downward) | |
| Dust Bed -> ==============================================| |
| Duct Wall -----------------------------------------------+ |
| * Consequence: Duct plugging, reduced hood Q, combustible fire/boom!|
| |
| B. PROPER TRANSPORT VELOCITY (V_duct >= V_tr): |
| Turbulent eddies maintain continuous pneumatic suspension. |
| |
| Duct Wall -----------------------------------------------+ |
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~| |
| ====> * ===> * ====> * ====> * ====| |
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~| |
| Duct Wall -----------------------------------------------+ |
+-------------------------------------------------------------------------+
If the duct velocity (Vd) drops below the minimum transport velocity (Vtr), particles settle out along the duct invert (bottom). This causes three severe failures:
- Progressive Duct Choking: Accumulated dust reduces the effective cross-sectional area, increasing system resistance and starving intake hoods of design capture airflow.
- Catastrophic Structural Collapse: The weight of accumulated solid dust (e.g., sand, lead dust, steel shot) can exceed duct hanger weight ratings, causing physical duct collapse.
- Combustible Dust Deflagration: A thick dust bed inside ductwork provides fuel for explosive deflagration propagation (NFPA 652 / NFPA 654) if an ignition spark travels through the duct.
2. Minimum Particulate Transport Velocities (Vtr)
The ACGIH Industrial Ventilation Manual specifies standardized minimum transport velocities based on contaminant physical state, particle size, and specific gravity:
| Contaminant Classification | Examples & Industrial Processes | Minimum Transport Velocity (Vtr) |
|---|---|---|
| Gases, Vapors, Smoke, Fumes | Solvent vapors, acid mists, paint fumes, welding smoke (non-condensing) | 1,000 to 2,000 fpm (5.0 to 10.0 m/s) (Governed by economics/noise) |
| Light Particulates & Dusts | Cotton lint, wood flour, grain dust, tobacco, fine textile fibers | 2,000 to 2,500 fpm (10.0 to 12.5 m/s) |
| Standard Industrial Dusts | Silica sand, cement dust, limestone, dry sawdust, metal grinding dust, foundry shakeout | 3,500 to 4,000 fpm (18.0 to 20.0 m/s) |
| Heavy or Moist Dusts | Lead oxide dust, heavy foundry sand, spray paint mist, wet sawdust, sticky buffing lint | 4,000 to 4,500 fpm (20.0 to 23.0 m/s) |
| Extremely Heavy / Settling Particulates | Metal turnings, lead shot, cast iron chips, wet foundry sand, brass shavings | 4,500+ fpm (23.0+ m/s, often 5,000 fpm) |
Exam Rule of Thumb: For standard mineral and abrasive dusts (silica, grinding, dry wood, cement), the standard CIH design transport velocity is 3,500 to 4,000 fpm (4,000 fpm is standard). For gases, vapors, and clean air, duct velocity is sized at 1,000 to 2,000 fpm to minimize acoustic noise and fan energy consumption.
3. Duct Sizing Mechanics: Continuity and Geometry
Duct Sizing Calculation
Duct cross-sectional area (Ad, in ft²) is calculated from volumetric flow rate (Q, in cfm) and target transport velocity (Vd, in fpm):
For a circular (round) duct of diameter D (in inches):
Round vs. Rectangular Ductwork
In industrial LEV design, round ductwork is strictly preferred over rectangular ductwork for particulate conveying:
+-------------------------------------------------------------------------+
| ROUND VS. RECTANGULAR DUCT GEOMETRY |
+-------------------------------------------------------------------------+
| |
| ROUND DUCT (Standard LEV): RECTANGULAR DUCT (HVAC Only): |
| |
| /-------\ +-----------------------+ |
| / \ | * Dead corner * | |
| | Uniform | | (Velocity -> 0 fpm) | |
| | Velocity | | Core Flow | |
| \ Profile / | | |
| \-------/ +-----------------------+ |
| * Dust deposits in 4 corners! |
| - Maximum structural hoop strength - High collapse risk under SP |
| - Zero dead-velocity corners - Particulate fallout corners |
| - Lowest sheet-metal weight per area - Higher friction per volume |
+-------------------------------------------------------------------------+
- Aspiration Hoop Strength: Under negative static pressures (e.g., -5 to -15 in. w.g.), round ducts maintain structural rigidity. Flat-sided rectangular ducts experience high bending moments and readily collapse inward unless heavily reinforced.
- Velocity Profile Uniformity: Rectangular ducts possess four 90° corners where boundary layer wall friction drops fluid velocity toward zero. Particulates deposit into these dead corners, progressively filling the duct.
- Frictional Economy: Round ducts offer the minimum perimeter (wetted wall area) per unit cross-sectional area, minimizing surface drag and sheet metal material costs.
When designing a local exhaust ventilation ductwork system conveying lead oxide dust, heavy foundry sand, or metallic machining turnings, what minimum transport velocity is required by ACGIH guidelines to prevent particulate settling and duct plugging?
An LEV branch must exhaust Q = 1,600 cfm of standard industrial silica dust requiring a minimum transport velocity of 3,800 fpm. What is the maximum theoretical duct diameter that maintains transport velocity, and which standard commercial duct size must be specified?