Ductwork Sizing Fundamentals
Proper ductwork design is critical for dust collection system performance. Undersized ducts create excessive pressure drop and reduce airflow, while oversized ducts waste energy and increase costs. The key to optimal ductwork design is understanding air velocity requirements and calculating friction losses.
Air Velocity Requirements
Minimum air velocity in main ducts must be sufficient to keep dust particles suspended and prevent settling. Different duct sections have different requirements:
| Duct Type |
Minimum Velocity (FPM) |
Typical Range (FPM) |
| Main duct from booth |
3,500-4,000 |
3,500-5,000 |
| Primary collection duct |
3,000-3,500 |
3,000-4,000 |
| Branch ducts |
2,500-3,000 |
2,500-3,500 |
| Return air duct |
1,500-2,000 |
1,500-2,500 |
Note: Velocities below minimum values result in dust settling and duct blockage. Velocities above typical range significantly increase friction losses and fan power requirements.
Duct Sizing Calculation
The process for determining duct diameter is straightforward using CFM and desired velocity:
Duct Diameter Formula
Diameter (inches) = √(CFM × 144 / (Velocity × π)) × 2
Step-by-Step Example:
Problem: Size main duct for 5,000 CFM sandblasting booth with 4,000 FPM desired velocity
Step 1: Use formula: Diameter = √(5,000 × 144 / (4,000 × 3.14159)) × 2
Step 2: Calculate area needed: 5,000 / 4,000 = 1.25 sq ft = 180 sq inches
Step 3: Diameter = √(180 / π) × 2 = √57.3 × 2 = 7.57 × 2 = 15.1 inches
Result: Use 16-inch duct (nearest standard size)
Actual Velocity: 5,000 / (π × 16² / 4) = 5,000 / 201 = 24.9 FPM... wait, that's wrong. Let me recalculate.
Correct: Area = π × (8)² = 201.06 sq inches = 1.397 sq ft. Velocity = 5,000 / 1.397 = 3,579 FPM ✓
Friction Loss Calculation
Friction loss is the pressure drop caused by air moving through ducts. This loss must be overcome by the fan and affects total system design pressure.
Darcy-Weisbach Friction Loss Formula
Friction Loss (in WC) = f × (L / D) × (V² / 2g) / 1,097
Where:
- f = friction factor (typically 0.015-0.025 for ductwork)
- L = duct length in feet
- D = duct diameter in inches
- V = air velocity in FPM
- g = gravitational constant (32.174)
Simplified Friction Loss Table:
| Duct Size |
CFM @3,500 FPM |
Friction per 100 ft |
| 8" |
1,357 |
0.18" WC |
| 10" |
2,120 |
0.09" WC |
| 12" |
3,053 |
0.05" WC |
| 14" |
4,164 |
0.03" WC |
| 16" |
5,359 |
0.02" WC |
Fittings and Elbows Pressure Loss
Duct fittings, elbows, and transitions create additional pressure losses beyond straight duct friction. These are calculated using equivalent duct length.
Equivalent Duct Length for Common Fittings
| Fitting Type |
Equivalent Length Factor |
| 45° elbow |
0.4 × diameter |
| 90° elbow (smooth) |
0.9 × diameter |
| 90° elbow (mitered) |
1.3 × diameter |
| Tee (branch) |
1.8 × diameter |
| Transition (reduction) |
0.5 × diameter |
| Gate valve (open) |
0.2 × diameter |
Example: A 90° smooth elbow in a 12" duct = 0.9 × 12 = 10.8 feet of equivalent length
Branch Duct Design Strategy
When multiple collection points feed into a main duct, proper branch design prevents backpressure and ensures balanced airflow.
Blast Room with Multiple Booth Ducts
Design Approach:
- Equal friction path: Design branch ducts so total friction loss to collector is similar from each booth, preventing pressure imbalance
- Main duct velocity: Increase velocity in main duct as more branches join (CFM increases), maintaining 3,000-4,000 FPM
- Branch entry: Use smooth transitions where branches enter main duct to minimize losses
- Slope: Maintain slight downward slope (1" per 10') to help dust flow
- Access cleanouts: Install cleanout ports at low points and elbows for maintenance
Example - Three Booth System:
- Booth 1 (25' away): 2,000 CFM, 8" duct = 1,370 FPM
- Booth 2 (35' away): 2,000 CFM, 8" duct = 1,370 FPM
- Booth 3 (15' away): 2,000 CFM, 8" duct = 1,370 FPM
- Main duct to collector: 6,000 CFM, 14" duct = 4,000 FPM ✓
Total System Pressure Drop Calculation
The fan must overcome all pressure drops in the system to achieve desired CFM.
Pressure Drop Summary Example
Scenario: 5,000 CFM sandblasting booth system
| Component |
Pressure Drop (in WC) |
| Booth capture hood |
0.25" |
| Main duct (100' @ 4,000 FPM) |
0.28" |
| 3 × 90° elbows + fittings |
0.18" |
| Dust collector inlet |
0.10" |
| Clean filter (new cartridge) |
0.50" |
| Dirty filter (maintenance level) |
3.00" |
| TOTAL SYSTEM PRESSURE |
4.31" WC |
Fan Selection: Requires fan rated for 5,000 CFM @ 4.5" WC static pressure (includes safety margin)
Common Ductwork Design Mistakes
- Undersizing main duct: Most common error; leads to high pressure drop and inadequate capture velocity. Check actual CFM at system flow rate.
- Using 90° elbows instead of 45°: Smooth 45° transitions significantly reduce pressure loss. Use long-radius elbows when possible.
- Sharp velocity changes: Sudden transitions cause turbulence and energy loss. Use gradual tapers and smooth transitions.
- Horizontal ductwork without slope: Dust settles in level sections. Minimum 1" drop per 10' of horizontal run is recommended.
- Ignoring equivalent lengths: Fittings create substantial pressure losses. Don't use actual duct length alone in calculations.
- Oversizing for "safety": Creates poor velocity, dust settling, maintenance problems. Design to optimal velocity range.
- No cleanout access: Inevitably leads to clogs. Install cleanouts at bends and low points.
Ductwork Material Selection
Material choice affects durability, pressure loss, and maintenance requirements.
Common Ductwork Materials
Mild Steel (Carbon Steel)
- Most common for industrial dust collection
- Cost-effective, strong, easy to install
- Can rust if moisture is present - paint or galvanize for protection
- Typical thickness: 16-18 gauge for main ducts
Galvanized Steel
- Zinc-coated for corrosion resistance
- Better for humid environments or outdoor installations
- Higher cost than painted mild steel
- Standard for systems in coastal areas or damp facilities
Stainless Steel
- Excellent corrosion resistance
- Higher cost, primarily for specialized applications
- Used when dust is corrosive (chemical blasting, certain abrasives)
PVC or Plastic
- Non-corrosive, good for certain applications
- Lower pressure ratings than metal
- Limited to low-pressure systems and return air ducting
- Not suitable for hot or abrasive dust
Ductwork Installation Checklist
- ☐ All ducts properly sized per velocity requirements
- ☐ Total system pressure drop calculated and verified
- ☐ Horizontal runs sloped at least 1" per 10'
- ☐ No sharp velocity transitions - use gradual tapers
- ☐ Elbows are smooth-sweep or long-radius type (avoid 90° street elbows)
- ☐ All duct connections sealed and supported
- ☐ Cleanout access points installed at low points and after each turn
- ☐ Flexible connectors used at fan to isolate vibration
- ☐ All exterior seams sealed with duct sealant
- ☐ System pressure drop verified with manometer after installation
- ☐ Fan discharge ducted away from work areas
- ☐ Ductwork supported every 6-8 feet to prevent sagging