Pipe and Duct Materials

Material selection for piping and ductwork distribution systems directly impacts system performance, longevity, installation cost, and maintenance requirements. Each material exhibits distinct physical properties that govern its application boundaries, including thermal conductivity, expansion characteristics, pressure capacity, and corrosion resistance. Understanding these properties enables proper material specification for specific operating conditions.

Piping Material Properties

Steel Pipe

Steel pipe remains the dominant choice for commercial hydronic systems due to its strength, pressure capacity, and temperature tolerance. Steel exhibits excellent mechanical properties but requires corrosion protection in most applications.

Carbon Steel Properties:

• Density: 7,850 kg/m³ (490 lb/ft³)
• Thermal conductivity: 54 W/(m·K) at 20°C
• Coefficient of thermal expansion: 11.7 × 10⁻⁶ /°C (6.5 × 10⁻⁶ /°F)
• Specific heat capacity: 486 J/(kg·K)
• Modulus of elasticity: 200 GPa (29 × 10⁶ psi)
• Maximum service temperature: 427°C (800°F) for Schedule 40

Schedule Designations and Pressure Ratings:

Black and galvanized steel pipe follows ASME B36.10M standards for wall thickness. The schedule number approximates the relation: Schedule = 1000 × (P/S), where P is internal pressure (psi) and S is allowable stress (psi).

Pressure ratings for carbon steel at 100°F; derating applies at elevated temperatures

Thermal Expansion Considerations:

Steel pipe expands 0.0065 inches per 100 feet per 100°F temperature change. A 200-foot heating main with a 180°F temperature swing (70°F to 250°F) experiences 2.34 inches of linear expansion, requiring expansion compensation through loops, offsets, or mechanical expansion joints.

Copper Tube

Copper tubing offers superior corrosion resistance, ease of installation, and extended service life in potable water and refrigerant applications. Copper’s thermal conductivity exceeds steel by 7:1, reducing heat loss when insulated but increasing unwanted heat transfer when uninsulated.

Copper Properties:

• Density: 8,940 kg/m³ (558 lb/ft³)
• Thermal conductivity: 401 W/(m·K) at 20°C
• Coefficient of thermal expansion: 16.6 × 10⁻⁶ /°C (9.2 × 10⁻⁶ /°F)
• Specific heat capacity: 385 J/(kg·K)
• Modulus of elasticity: 117 GPa (17 × 10⁶ psi)
• Maximum service temperature: 204°C (400°F) for Type L

Type Designations and Wall Thickness:

Pressure ratings for annealed copper at 100°F per ASTM B88

Type K provides the heaviest wall thickness for underground service and high-pressure applications. Type L serves as the standard for above-ground plumbing and HVAC systems. Type M offers minimum wall thickness for residential applications where codes permit.

Copper’s thermal expansion coefficient of 9.2 × 10⁻⁶ /°F exceeds steel by 42%, producing 0.0092 inches of expansion per 100 feet per 100°F change. This higher expansion rate requires more frequent expansion compensation in long runs.

PVC and CPVC Pipe

Polyvinyl chloride (PVC) and chlorinated polyvinyl chloride (CPVC) provide corrosion-proof, lightweight alternatives for specific applications. PVC suits cold water and condensate drain service, while CPVC extends temperature capability into heating system territory.

PVC Properties (Schedule 40):

• Density: 1,380 kg/m³ (86 lb/ft³)
• Thermal conductivity: 0.19 W/(m·K)
• Coefficient of thermal expansion: 54 × 10⁻⁶ /°C (30 × 10⁻⁶ /°F)
• Specific heat capacity: 1,050 J/(kg·K)
• Maximum service temperature: 60°C (140°F)
• Pressure rating: 480 psi at 73°F (1" diameter)

CPVC Properties (Schedule 80):

• Density: 1,520 kg/m³ (95 lb/ft³)
• Thermal conductivity: 0.14 W/(m·K)
• Coefficient of thermal expansion: 68 × 10⁻⁶ /°C (38 × 10⁻⁶ /°F)
• Specific heat capacity: 1,000 J/(kg·K)
• Maximum service temperature: 93°C (200°F)
• Pressure rating: 400 psi at 73°F (1" diameter)

Temperature Derating:

Plastic pipe pressure ratings decrease substantially with temperature. A CPVC system rated for 400 psi at 73°F drops to 100 psi at 200°F. This 4:1 derating factor mandates conservative sizing for hot water applications.

The thermal expansion coefficient of CPVC (38 × 10⁻⁶ /°F) exceeds steel by 5.8:1 and copper by 4.1:1. A 100-foot CPVC run experiences 0.38 inches of expansion per 10°F temperature change, requiring expansion compensation every 30-40 feet in hot water service.

Ductwork Material Properties

Galvanized Steel Ductwork

Galvanized steel dominates commercial ductwork applications, providing structural rigidity, fire resistance, and long-term durability. The zinc coating delivers corrosion protection in typical HVAC environments.

Galvanized Steel Properties:

• Base material: Low-carbon steel (A653 or A924)
• Zinc coating weight: G60 (0.60 oz/ft²) or G90 (0.90 oz/ft²)
• Thermal conductivity: 52 W/(m·K)
• Coefficient of thermal expansion: 11.7 × 10⁻⁶ /°C
• Maximum service temperature: 150°C (300°F) continuous

SMACNA Gauge Requirements:

Sheet Metal and Air Conditioning Contractors National Association (SMACNA) standards specify minimum gauges based on duct dimensions and pressure class:

Rectangular duct with 13-foot maximum transverse joint spacing

Gauge selection directly impacts system leakage, structural integrity, and acoustical performance. Undersized gauge selection leads to duct deflection, increased leakage, and drumming noise generation.

Aluminum Ductwork

Aluminum ductwork provides corrosion resistance in coastal or high-humidity environments while maintaining formability and structural properties. Aluminum costs 2-3 times galvanized steel but eliminates corrosion concerns in aggressive atmospheres.

Aluminum Properties (3003-H14 Alloy):

• Density: 2,730 kg/m³ (170 lb/ft³)
• Thermal conductivity: 193 W/(m·K)
• Coefficient of thermal expansion: 23.2 × 10⁻⁶ /°C (12.9 × 10⁻⁶ /°F)
• Specific heat capacity: 893 J/(kg·K)
• Maximum service temperature: 200°C (400°F)

Aluminum’s lower density (35% of steel) reduces hanger loading and facilitates installation. The thermal expansion coefficient exceeds steel by 10%, requiring attention to expansion joint spacing in systems experiencing temperature swings.

Stainless Steel Ductwork

Stainless steel serves specialized applications requiring maximum corrosion resistance: kitchen exhaust, laboratory fume exhaust, and industrial process ventilation. Type 304 suits most HVAC applications, while Type 316 provides enhanced corrosion resistance.

Type 304 Stainless Properties:

• Density: 8,000 kg/m³ (500 lb/ft³)
• Thermal conductivity: 16.2 W/(m·K)
• Coefficient of thermal expansion: 17.3 × 10⁻⁶ /°C (9.6 × 10⁻⁶ /°F)
• Maximum service temperature: 870°C (1,600°F)
• Cost multiplier vs. galvanized: 8-12×

Flexible Ductwork

Flexible ductwork consists of wire helix-supported polymer film or metalized film with external insulation jacket. Flex duct provides final connection to air terminals and accommodates minor layout adjustments.

Construction:

• Inner core: Polyester, metalized polyester, or aluminum
• Wire helix: Spring steel at 1-3" pitch
• Insulation: Fiberglass (R-4.2 to R-8.0)
• Vapor barrier: Metalized polyester or vinyl
• Maximum static pressure: 2-4" wg depending on diameter
• Maximum air velocity: 1,200-1,500 fpm

Friction loss in flexible ductwork exceeds rigid ductwork by 40-100% depending on installation quality. Each 90° bend adds equivalent length of 15-20 diameters. Compressed or sagging flex duct dramatically increases friction and reduces system performance.

Material Selection Criteria

Operating Temperature Range

Material temperature limits define application boundaries:

Pressure Requirements

System operating pressure establishes minimum wall thickness. Safety factors of 2:1 to 4:1 apply depending on application criticality and pressure vessel code requirements.

Pressure Classification:

• Low pressure: 0-150 psi (hydronic), 0-2" wg (air)
• Medium pressure: 150-300 psi (hydronic), 2-6" wg (air)
• High pressure: 300-600 psi (hydronic), 6-10" wg (air)
• Extra high: >600 psi (hydronic), >10" wg (air)

Corrosion Environment

Water chemistry, soil conditions, and atmospheric exposure determine corrosion vulnerability:

Piping Applications:

• Closed-loop hydronic (treated water): Steel, copper
• Open-loop cooling (untreated): Copper, stainless steel
• Condensate drain (pH 3-5): Copper, PVC, CPVC
• Underground (corrosive soil): Type K copper, PE, HDPE
• Seawater/brackish: Copper-nickel, titanium

Ductwork Applications:

• Standard HVAC: Galvanized steel
• Coastal/marine: Aluminum, stainless steel
• Kitchen exhaust: Stainless steel Type 304
• Chemical fume: Stainless steel Type 316, PVC-coated
• Underground: Concrete, fiberglass

Thermal Expansion Management

Compare linear expansion rates to determine expansion joint requirements:

Expansion per 100 ft per 100°F ΔT:

• Steel: 0.78 inches
• Copper: 1.10 inches
• Aluminum: 1.55 inches
• PVC: 3.60 inches
• CPVC: 4.56 inches

Systems using materials with expansion coefficients >20 × 10⁻⁶ /°C require expansion compensation at closer intervals. CPVC hot water systems typically need expansion joints or offsets every 30 feet, while steel systems accommodate 80-100 foot straight runs before requiring expansion devices.

Cost-Performance Analysis

Initial material cost represents one component of lifecycle economics:

Relative Cost Index (Steel = 1.0):

• Carbon steel pipe: 1.0
• Copper Type L: 3.5-4.5
• PVC Schedule 40: 0.3-0.4
• CPVC Schedule 80: 0.6-0.8
• Galvanized steel duct: 1.0
• Aluminum duct: 2.0-3.0
• Stainless steel duct: 8.0-12.0

Installation labor costs modify material cost differentials. Copper’s lighter weight and brazed connections may offset higher material costs through reduced installation time compared to threaded steel. PVC’s solvent welding produces fastest installation but limits applications through temperature restrictions.

Expected service life factors into lifecycle cost calculations. Copper’s 50+ year expectancy in proper water chemistry justifies premium over steel’s 25-35 year typical lifespan. Maintenance costs—including water treatment, system flushing, and component replacement—accumulate over decades of operation.

Sections