Specialty Duct Materials for Demanding Applications

Overview

Specialty duct materials address applications where standard galvanized steel or aluminum ducts fail due to corrosive environments, extreme temperatures, hygiene requirements, or unique installation constraints. Material selection requires analysis of chemical compatibility, thermal limits, structural strength, and long-term durability under operating conditions.

Stainless Steel Ductwork

Material Properties

Stainless steel ducts provide superior corrosion resistance and temperature capability for demanding environments.

Thermal expansion coefficient: $$\alpha_{ss} = 17.3 \times 10^{-6} \text{ m/m°C}$$

Allowable stress for 304 stainless: $$\sigma_{allow} = \frac{\sigma_{yield}}{SF} = \frac{215 \text{ MPa}}{1.5} = 143 \text{ MPa}$$

Applications

• Commercial kitchen exhaust systems (grease-laden air)
• Laboratory fume exhaust (chemical exposure)
• Pharmaceutical cleanrooms (hygiene and cleaning requirements)
• High-temperature process exhaust (up to 816°C for Type 309)
• Coastal installations (salt air corrosion)

Key considerations:

• Type 304 for general corrosion resistance
• Type 316 for chloride environments
• Type 430 for cost-sensitive non-structural applications
• Welded seams required for grease ducts per NFPA 96
• Surface finish affects cleanability (2B standard, electropolished for pharmaceutical)

Fiberglass and FRP Ductwork

Fiberglass Reinforced Plastic (FRP)

FRP ducts combine corrosion resistance with structural strength through fiber reinforcement in thermosetting resin matrices.

Flexural modulus: $$E_f = \frac{FL^3}{48I\delta}$$

Where:

• F = applied load (N)
• L = span length (m)
• I = moment of inertia (m⁴)
• δ = deflection (m)

Chemical resistance factor: $$CRF = \frac{\sigma_{exposed}}{\sigma_{control}}$$

A CRF greater than 0.8 indicates acceptable chemical resistance.

Applications

• Acid fume exhaust systems
• Wastewater treatment plants
• Industrial scrubber connections
• Coastal or marine environments
• Underground duct runs (corrosion immunity)

PVC and CPVC Ductwork

Material Comparison

Temperature Derating

PVC and CPVC duct pressure ratings decrease with temperature:

$$P_{rated}(T) = P_{rated,20°C} \times \left(1 - \frac{T - 20}{T_{max} - 20} \times 0.5\right)$$

Applications

PVC ducts:

• Laboratory fume exhaust (acids, solvents)
• Plating shop exhaust
• Battery charging rooms
• Semiconductor fab exhaust

CPVC ducts:

• Higher temperature chemical exhaust
• Hot acid fume applications
• Chlorine gas handling

Polypropylene Ductwork

Polypropylene offers excellent chemical resistance with higher temperature capability than PVC.

Properties:

• Maximum temperature: 100°C continuous
• Density: 0.90-0.91 g/cm³
• Tensile strength: 32-40 MPa
• Outstanding resistance to acids, bases, and organic solvents

Joining methods:

• Butt fusion welding (heated plate method)
• Extrusion welding for field joints
• Flanged connections with gaskets

Fabric Duct Systems

Fabric ducts provide unique advantages for specific applications through permeable or semi-permeable textile construction.

Airflow Distribution

Discharge area calculation: $$A_{discharge} = \frac{Q}{V_{discharge}}$$

Pressure drop through fabric: $$\Delta P = \frac{8\mu LQ}{\pi r^4} + \frac{\rho V^2}{2C_d^2}$$

Where Cd represents the discharge coefficient of the fabric material.

Applications and Benefits

graph TD
    A[Fabric Duct Applications] --> B[Food Processing]
    A --> C[Cold Storage]
    A --> D[Sports Facilities]
    A --> E[Temporary Installations]

    B --> B1[Washable/sanitizable]
    B --> B2[No condensation drip]

    C --> C1[Eliminates condensation]
    C --> C2[Lightweight installation]

    D --> D1[Aesthetic appearance]
    D --> D2[Even air distribution]

    E --> E1[Quick deployment]
    E --> E2[Reusable/relocatable]

Performance characteristics:

• Air velocity: 5-15 m/s internal
• Operating pressure: 50-400 Pa static
• Materials: polyester, nylon, fiberglass
• Cleaning: machine washable or in-place vacuum
• Fire ratings: Class 1 or A available per ASTM E84

Material Selection Framework

Selection Criteria Matrix

flowchart TD
    A[Define Requirements] --> B{Corrosive<br/>Environment?}
    B -->|Yes| C{Chemical<br/>Type?}
    B -->|No| D{Temperature<br/>Range?}

    C -->|Acids| E[PVC/CPVC/FRP]
    C -->|Bases| F[FRP/Polypropylene]
    C -->|Organic| G[Stainless Steel]
    C -->|Salt/Marine| H[Stainless 316]

    D -->|High >200°C| I[Stainless/Black Iron]
    D -->|Moderate| J[Standard Materials]
    D -->|Cold <5°C| K[Fabric/Insulated]

    E --> L[Check Temp Limits]
    F --> L
    G --> L
    H --> L
    I --> M[Verify Codes]
    J --> M
    K --> M
    L --> M
    M --> N[Final Selection]

Decision Factors

Environmental exposure:

• Chemical concentration and temperature
• Continuous vs. intermittent exposure
• Maintenance accessibility

Structural requirements:

• Static pressure class per SMACNA
• Span lengths and support spacing
• Seismic and wind loads

Code compliance:

• IMC Chapter 6 requirements
• NFPA 96 for grease ducts
• Local amendments and approvals

Economic analysis:

• Material cost per linear meter
• Installation labor requirements
• Expected service life
• Maintenance costs

Installation and Joining

Specialty Material Connections

Support Spacing

Maximum hanger spacing varies by material stiffness:

$$L_{max} = \sqrt[4]{\frac{384EI}{5w_{max}\delta_{allow}}}$$

Where:

• E = elastic modulus (Pa)
• I = moment of inertia (m⁴)
• w = distributed load (N/m)
• δallow = allowable deflection (typically L/200)

Standards and References

SMACNA Standards:

• HVAC Duct Construction Standards
• Rectangular Industrial Duct Construction
• Round Industrial Duct Construction

ASHRAE Guidelines:

• ASHRAE Handbook - HVAC Systems and Equipment
• Chapter 19: Duct Design

Material Standards:

• ASTM A480: Stainless steel
• ASTM D1784: PVC compounds
• ASTM F441: CPVC pipe and fittings
• ASTM E84: Surface burning characteristics

Selection of specialty duct materials requires thorough analysis of operating conditions, chemical exposure, temperature extremes, and structural demands to ensure long-term system performance and code compliance.