Duct Velocities 4000 FPM for Lint Transport

Minimum Transport Velocity Requirements

Textile lint particles require minimum duct velocities to prevent settling and accumulation within exhaust systems. The 4000 FPM threshold represents the industry-standard minimum transport velocity for lint-laden air streams in textile processing facilities.

Critical Velocity Determination

Transport velocity must overcome gravitational settling forces acting on lint particles. The minimum velocity depends on particle characteristics:

Settling Velocity Relationship:

$$V_{transport} = \frac{V_{settling}}{\sin(\theta)} \times SF$$

Where:

• $V_{transport}$ = Minimum duct transport velocity (FPM)
• $V_{settling}$ = Particle terminal settling velocity (FPM)
• $\theta$ = Duct angle from horizontal (degrees)
• $SF$ = Safety factor (typically 1.5-2.0)

ACGIH Recommended Velocities

Velocity Maintenance Principles

Pressure Drop Considerations

Higher velocities increase system pressure drop exponentially. The Darcy-Weisbach equation governs friction losses:

$$\Delta P = f \times \frac{L}{D} \times \frac{\rho V^2}{2 \times 144}$$

Where:

• $\Delta P$ = Pressure drop (inches w.g.)
• $f$ = Friction factor (dimensionless)
• $L$ = Duct length (feet)
• $D$ = Duct diameter (feet)
• $\rho$ = Air density (lb/ft³)
• $V$ = Air velocity (FPM)

Key Relationship: Doubling velocity increases pressure drop by factor of four.

Energy Consumption Analysis

Fan power requirement scales with velocity cubed:

$$HP = \frac{Q \times \Delta P}{6356 \times \eta_{fan}}$$

Where:

• $HP$ = Fan brake horsepower
• $Q$ = Airflow rate (CFM)
• $\Delta P$ = Total system pressure (inches w.g.)
• $\eta_{fan}$ = Fan total efficiency (decimal)

Duct Sizing Methodology

Design Procedure

graph TD
    A[Determine Airflow Rate CFM] --> B[Select Transport Velocity 4000-4500 FPM]
    B --> C[Calculate Duct Area Required]
    C --> D{Round or Rectangular?}
    D -->|Round| E[Diameter = √4A/π]
    D -->|Rectangular| F[Select Aspect Ratio ≤4:1]
    E --> G[Round to Standard Size]
    F --> H[Calculate Dimensions]
    G --> I[Verify Actual Velocity]
    H --> I
    I --> J{Velocity Within 10% of Target?}
    J -->|No| B
    J -->|Yes| K[Calculate Pressure Drop]
    K --> L[Size Fan and Motor]

Sizing Calculations

Required Duct Area:

$$A = \frac{Q}{V}$$

Where:

• $A$ = Cross-sectional area (ft²)
• $Q$ = Airflow rate (CFM)
• $V$ = Design velocity (FPM)

Round Duct Diameter:

$$D = \sqrt{\frac{4A}{\pi}} = \sqrt{\frac{4Q}{\pi V}}$$

Rectangular Duct Dimensions:

For aspect ratio $AR = \frac{W}{H}$:

$$W = \sqrt{A \times AR}$$

$$H = \frac{A}{W}$$

Standard Duct Sizes

System Design Considerations

Velocity Profile Management

graph LR
    A[Branch 1<br/>4000 FPM] --> D[Main Duct<br/>4200 FPM]
    B[Branch 2<br/>4000 FPM] --> D
    C[Branch 3<br/>4000 FPM] --> D
    D --> E[Collector<br/>4500 FPM]

    style A fill:#e1f5ff
    style B fill:#e1f5ff
    style C fill:#e1f5ff
    style D fill:#ffe1e1
    style E fill:#ffe1cc

Design Rules:

• Never reduce velocity in direction of airflow
• Increase velocity 5-10% at each junction
• Maintain minimum 4000 FPM throughout system
• Design collector inlet for 4500 FPM minimum

Lint Settling Prevention

Critical factors preventing lint accumulation:

1. Horizontal Run Limitations: Minimize horizontal ductwork length. Vertical or sloped sections preferred.

2. Elbow Design: Use large radius elbows (R/D ≥ 2.0) to prevent settling at direction changes.

3. Velocity Monitoring: Install velocity sensors at critical locations to verify design conditions.

4. Cleanout Access: Provide access doors every 20 feet of horizontal run and at all low points.

Energy Optimization Strategies

Balancing Transport and Energy Costs

Operating at 4000 FPM minimum versus 4500 FPM recommended:

Energy Comparison:

*Based on $0.12/kWh, 6000 hours/year operation, 60% fan efficiency

Design Recommendations

ASHRAE Industrial Ventilation Guidelines:

• Design for 4500 FPM when energy costs permit
• Accept 4000 FPM minimum only with rigorous maintenance program
• Consider variable speed control to maintain velocity as filters load
• Monitor actual velocities quarterly to verify design conditions

System Verification:

• Commission systems with Pitot tube traverse measurements
• Document baseline velocities at all measurement points
• Establish maintenance procedures for velocity monitoring
• Replace or clean ducts when velocity cannot be maintained

References

• ACGIH Industrial Ventilation Manual, 30th Edition
• ASHRAE Handbook - HVAC Applications, Chapter 31: Industrial Ventilation
• NFPA 654: Standard for Prevention of Fire and Dust Explosions from Manufacturing, Processing, and Handling of Combustible Particulate Solids