Refrigerant Piping Materials

Refrigerant piping material selection directly impacts system reliability, efficiency, and compliance with safety codes. Material properties must accommodate refrigerant chemistry, operating pressures, temperature extremes, and environmental exposure while maintaining leak-tight integrity throughout the system lifecycle.

Copper Tubing for Refrigerant Service

Copper dominates refrigerant piping applications due to superior thermal conductivity (391 W/m·K at 20°C), excellent formability, proven compatibility with traditional and HFC refrigerants, and established installation practices. Material specifications distinguish between standard plumbing grades and specialized refrigeration service.

ACR Tubing (Air Conditioning and Refrigeration)

ACR tubing represents copper manufactured specifically for refrigeration applications with critical distinctions from plumbing grades:

Manufacturing Standards:

• ASTM B280 specification governs composition, dimensions, and cleanliness
• Dehydrated during manufacturing (moisture content <15 ppm)
• Factory sealed with nitrogen charge or plastic end caps
• Internally cleaned to remove oils, particulates, and oxidation
• Outer diameter sizing matches nominal dimensions (unlike plumbing copper)

Mechanical Properties:

• Drawn temper (hard) for smaller sizes (≤7/8" OD typical)
• Annealed temper (soft) for field bending and vibration resistance
• Ultimate tensile strength: 36,000-42,000 psi (annealed), 58,000-68,000 psi (drawn)
• Yield strength: 10,000-15,000 psi (annealed), 45,000-55,000 psi (drawn)
• Elongation: 30-45% in 2" (annealed), <10% (drawn)

Wall Thickness and Pressure Ratings:

Temperature Performance:

• Service range: -320°F to +400°F
• Annealing temperature: 700-1200°F (material softening occurs)
• Coefficient of thermal expansion: 9.3 × 10⁻⁶ in/in·°F
• No brittle transition at low temperatures (suitable for cascade systems)

Type L Copper for Refrigerant Applications

Type L represents heavier-wall plumbing copper occasionally employed in refrigeration for specific conditions:

Specification Comparison:

Application Scenarios for Type L:

• Ammonia refrigeration systems (industrial applications)
• High-pressure CO₂ transcritical systems (>1,800 psi)
• Underground refrigerant piping (mechanical protection)
• Jurisdictions requiring plumbing-grade materials by code
• Retrofit projects matching existing Type L installations

Field Preparation Requirements:

• Nitrogen purge during brazing (prevent internal oxidation)
• Acid cleaning or flushing to remove mill scale
• Pressure testing to verify integrity
• Evacuation to <500 microns before charging

Type K Copper (Heavy Wall)

Type K provides maximum wall thickness for extreme service conditions:

Critical Applications:

• Ammonia systems in industrial refrigeration plants
• Underground refrigerant mains (corrosion allowance)
• High-pressure receiver vessels and accumulators
• Seismic zones requiring enhanced mechanical strength
• Process cooling with aggressive secondary coolants

Design Considerations:

• Higher material cost vs. marginal pressure benefit for standard HVAC
• Increased weight requires additional support spacing
• Heavier wall reduces internal volume (affects refrigerant charge)
• Brazing requires increased heat input (potential joint quality issues)

Steel Refrigerant Piping

Steel piping serves large-tonnage industrial refrigeration and specific refrigerant combinations incompatible with copper.

Carbon Steel Applications

Material Specifications:

• ASTM A53 or A106 seamless or welded pipe
• Schedule 40 or Schedule 80 based on pressure requirements
• Black iron (standard) or galvanized (ammonia-resistant coating)

Refrigerant Compatibility:

• Ammonia (R-717): Preferred material, no reaction with steel
• CO₂ (R-744): Acceptable for transcritical systems
• Incompatible with halogenated refrigerants (moisture/corrosion concerns)

Pressure-Temperature Ratings (Schedule 40):

Installation Requirements:

• Welded connections (TIG or MIG preferred for refrigeration)
• Threaded connections limited to <2" and low-pressure applications
• Internal cleaning critical (mill scale removal)
• Dry nitrogen purge during welding
• Pressure testing at 1.5× design pressure minimum

Stainless Steel for Specialized Service

Alloy Selection:

• 304 stainless: General refrigerant service, moderate corrosion resistance
• 316 stainless: Enhanced corrosion resistance (coastal/chemical environments)
• Compatibility with all common refrigerants including HFOs

Advantages:

• Superior corrosion resistance in harsh environments
• Compatible with ammonia and halogenated refrigerants
• Clean internal surface (minimal contamination risk)
• High strength-to-weight ratio

Limitations:

• Material cost 4-6× carbon steel equivalent
• Welding requires inert gas shielding (argon purge)
• Lower thermal conductivity than copper (heat transfer penalty)
• Specialized fabrication skills required

Aluminum Refrigerant Piping

Aluminum offers weight advantages for specific applications with compatibility limitations.

Aluminum Alloy Properties

Common Alloys:

• 3003-H14: General purpose, good formability
• 5052-H32: Higher strength, marine applications
• 6061-T6: Structural applications, weldable

Performance Characteristics:

• Density: 169 lb/ft³ (35% of copper weight)
• Thermal conductivity: 205 W/m·K (53% of copper)
• Coefficient of expansion: 13.1 × 10⁻⁶ in/in·°F (41% higher than copper)
• Pressure ratings comparable to copper ACR with equivalent wall thickness

Refrigerant Compatibility:

• Compatible: R-22, R-410A, R-134a, R-404A, R-507A
• Incompatible: Ammonia (forms aluminum nitride, catastrophic failure risk)
• Conditional: Some HFO blends (verify manufacturer data)

Application Limitations:

• Galvanic corrosion risk when coupled with dissimilar metals
• Brazing requires specialized aluminum filler alloys
• Lower ductility than copper (vibration fatigue concerns)
• Primarily used in factory-assembled equipment vs. field piping

Material Compatibility with Refrigerants

Refrigerant chemistry determines material selection through mechanisms including chemical reaction, moisture interaction, and oil solubility effects.

Halogenated Refrigerants (HFCs, HFOs)

Copper/Brass Compatibility:

• Excellent with dry refrigerants (R-134a, R-410A, R-32, R-454B, R-1234yf)
• Moisture catalyzes acid formation (hydrofluoric/hydrochloric acid)
• Polyol ester (POE) oils stabilize modern HFC systems
• Maximum moisture content: <50 ppm in system

Material Restrictions:

• Magnesium content in alloys must be <0.5% (galvanic corrosion)
• Zinc in brass limited to <15% (dezincification with moisture)
• Elastomers must be HFC-compatible (nitrile, EPDM unsuitable)

Ammonia (R-717) Systems

Material Requirements:

• Carbon steel or stainless steel mandatory
• Copper and copper alloys strictly prohibited (stress corrosion cracking)
• Aluminum prohibited (aluminum nitride formation)
• Iron and steel provide cathodic protection to ammonia

Ammonia-Metal Reactions:

• Copper forms copper nitride, becomes brittle
• Failure occurs at stress concentrations without warning
• Reaction accelerates with moisture and contamination
• IIAR 2 standard forbids copper in ammonia systems

Carbon Dioxide (R-744) Systems

Transcritical System Requirements:

• Operating pressures exceed 1,800 psi (discharge side)
• Copper, steel, or stainless steel acceptable
• Wall thickness calculated for high-pressure service
• Brazed or welded joints (mechanical fittings limited)

Material Selection Criteria:

• Copper ACR adequate for low-side (<1,000 psi)
• Steel Schedule 80 or heavy-wall copper for high-side
• All joints designed for full system pressure (no low-pressure relief)

Material Compatibility with Refrigeration Oils

Oil chemistry interacts with piping materials affecting system performance and longevity.

Mineral Oil Systems

Compatibility:

• Universal compatibility with ferrous and non-ferrous metals
• Used with CFC and HCFC refrigerants (R-12, R-22)
• Immiscibility with refrigerant simplifies oil return
• Moisture absorption minimal (<100 ppm saturation)

Material Considerations:

• Copper oxide formation inhibited by mineral oil lubrication
• Steel surfaces protected from corrosion
• Elastomer selection: Nitrile, neoprene compatible

Polyol Ester (POE) Oils

HFC System Standard:

• Required for R-134a, R-410A, R-404A, R-407C refrigerants
• Highly hygroscopic (absorbs moisture rapidly)
• Miscible with refrigerant across temperature range
• Aggressive moisture limits: <30 ppm in system

Material Interactions:

• Attacks paints, varnishes, and some plastics
• Compatible with copper, steel, aluminum
• Requires synthetic elastomers (HNBR, FKM)
• Filter drier sizing critical (moisture removal capacity)

Polyalkylene Glycol (PAG) Oils

Automotive AC Applications:

• Primary oil for R-1234yf mobile systems
• Extremely hygroscopic (moisture absorption >POE)
• Not miscible with mineral oil (never mix systems)

Handling Requirements:

• Minimize atmospheric exposure during service
• Copper and aluminum compatible with proper system dryness
• Dedicated service equipment (no cross-contamination)

Brazing Alloys and Joining Methods

Joint integrity determines system leak tightness and long-term reliability.

Copper Phosphorus (Silfos/Phos-Copper)

Alloy Composition:

• BCuP-3: 93% Cu, 7% P (1,190°F liquidus)
• BCuP-5: 85% Cu, 15% Ag, 5% P (1,190°F liquidus, improved flow)
• BCuP-6: 92% Cu, 8% P (1,300°F liquidus)

Application Requirements:

• Copper-to-copper joints only (self-fluxing with phosphorus content)
• Prohibited for ferrous metals (brittle phosphide formation)
• Capillary gap: 0.001-0.003" for optimal strength
• Joint strength: 25,000-40,000 psi tensile

Advantages:

• No flux required (clean joints, no residue)
• Lower brazing temperature than silver alloys
• Economical for copper refrigeration systems
• Established track record in HVAC applications

Silver Brazing Alloys (BAg Series)

Common Formulations:

• BAg-1: 45% Ag, 15% Cu, 16% Zn, 24% Cd (1,125°F liquidus) - restricted Cd content
• BAg-5: 45% Ag, 30% Cu, 25% Zn (1,370°F liquidus) - cadmium-free
• BAg-7: 56% Ag, 22% Cu, 17% Zn, 5% Sn (1,205°F liquidus) - higher strength

Application Specifications:

• Required for dissimilar metal joints (copper-to-brass, copper-to-steel)
• Flux necessary except for copper-phosphorus alloys
• Higher joint strength than phos-copper (40,000-70,000 psi)
• Improved ductility for vibration resistance

Flux Considerations:

• AWS Type 3A flux for copper alloys (1,050-1,600°F range)
• Complete flux removal after brazing (potential corrosion)
• Water quench prohibited (thermal shock risk)
• Nitrogen purge during heating prevents internal oxidation

Nitrogen Purge During Brazing

Nitrogen displacement of atmospheric oxygen prevents internal oxidation (scale formation) that contaminates refrigeration systems.

Procedure Requirements:

• Flow rate: 3-5 CFM through tube during heating
• Pressure: 1-3 psig (minimal positive pressure, not zero flow)
• Duration: Start before heating, continue until joint cools below 300°F
• Monitoring: Visual confirmation of nitrogen discharge at far end

Oxidation Consequences:

• Copper oxide scale breaks loose during operation
• Particulates damage compressor valves and bearings
• TXV orifice plugging causes hunting and instability
• Filter drier rapid saturation reduces service life

Economic Justification:

• Nitrogen cost nominal (<$5/joint typical)
• Prevents costly callbacks and compressor failures
• Required by ASHRAE Standard 183 for refrigeration quality
• Many manufacturers void warranty without nitrogen purge documentation

Cleanliness Requirements for Refrigeration Piping

Contamination tolerance in refrigeration systems measures in parts per million, demanding rigorous cleanliness protocols.

Particulate Contamination Limits

ASHRAE Standard 63.2 Requirements:

• Maximum particulate: <100 mg per 100 ft² internal surface area
• Particle size distribution: >95% smaller than 100 microns
• Method: Visual inspection (white cloth test) or extraction testing

Contamination Sources:

• Manufacturing residues (drawing compounds, cutting oils)
• Installation debris (metal shavings, brazing flux residue)
• Environmental exposure (dust, dirt during open-pipe storage)
• Corrosion products (iron oxide, copper oxide)

Cleaning Methods:

• Factory cleaning (ACR tubing pre-cleaned, sealed)
• Nitrogen blow-out for field-cut tubes (minimum 100 psig)
• Solvent flushing for contaminated systems (R-11 substitute, recovery required)
• Mechanical cleaning prohibited (introduces additional debris)

Moisture Control

Maximum Moisture Levels by Refrigerant:

Drying Procedures:

• Nitrogen sweep (continuous flow >4 hours, limited effectiveness)
• Vacuum evacuation to <500 microns absolute pressure
• Triple evacuation method (vacuum-break-vacuum cycles)
• Holding test: Vacuum rises <300 microns in 1 hour (leak-tight verification)

Moisture Measurement:

• Electronic moisture indicators (real-time monitoring)
• Liquid refrigerant sight glass with moisture indicator
• Filter drier condition monitoring (pressure drop trending)

Oil Residue Limits

Acceptable Contamination:

• Mineral oil: <50 ppm in HFC systems (miscibility issues)
• POE oil: System-compatible, cleanliness for particulate only
• Cutting oils and lubricants: Complete removal required (incompatibility)

Removal Methods:

• Solvent flushing with compatible refrigerant or approved substitute
• Nitrogen blow-dry after solvent flush
• Filter drier installation to capture residual contamination

Pressure Testing and System Integrity Verification

Leak detection before refrigerant charging prevents costly callbacks and environmental releases.

Pressure Test Requirements

Test Pressure Determination:

• Low-pressure systems (<50 psig): 1.5× design pressure or 150 psig minimum
• High-pressure systems: 1.5× design pressure, not to exceed component ratings
• Ammonia systems: IIAR 2 requires specific test pressure schedules
• Local codes may mandate higher values (verify jurisdiction requirements)

Test Medium Selection:

• Dry nitrogen: Universal acceptance, inert, non-combustible
• Compressed air: Prohibited for final test (moisture and combustion risk)
• Refrigerant/nitrogen mix: Leak detection enhancement (10% refrigerant maximum)

Test Duration:

• Initial pressurization: 24 hours minimum for large systems
• Standing pressure test: Pressure loss <1% indicates acceptable leak rate
• Temperature compensation: Correct pressure readings for ambient temperature change

Leak Detection Methods

Sensitivity Comparison:

Leak Repair Acceptance:

• Zero tolerance for refrigerant leaks in new construction
• EPA Section 608 limits leak rates for existing equipment
• Repair before charging (nitrogen test proves integrity)

Design Considerations for Refrigerant Piping Systems

Material selection integrates with system design for optimal performance and code compliance.

Support Spacing and Expansion Compensation

Copper Tubing Support Intervals:

Thermal Expansion Calculation:

• ΔL = L × α × ΔT
• Where: L = length (in), α = 9.3 × 10⁻⁶ in/in·°F (copper), ΔT = temperature change (°F)
• Example: 100 ft run, 100°F change = 1.1" expansion
• Expansion loops, offsets, or flexible connections required for runs >50 ft

Insulation Material Selection

Closed-Cell Elastomeric (Primary Choice):

• Thermal conductivity: 0.27 BTU·in/hr·ft²·°F at 75°F mean temperature
• Water vapor permeability: <0.05 perm·in (prevents condensation)
• Temperature range: -297°F to +220°F
• Required thickness per ASHRAE 90.1 energy code

Alternative Insulation Types:

• Polyisocyanurate (PIR): Higher R-value, rigid applications
• Phenolic foam: Fire-rated applications, lower permeability
• Aerogel composites: Space-constrained installations (premium cost)

Code Compliance and Standards References

Applicable Codes and Standards:

• ASHRAE Standard 15: Safety Standard for Refrigeration Systems
• IIAR 2: Ammonia refrigeration system design and installation
• ASME B31.5: Refrigeration piping and heat transfer components
• IMC/UMC: International/Uniform Mechanical Code refrigeration chapters
• NFPA 1: Fire code provisions for refrigerant system installation

Material Approval Requirements:

• UL listing for refrigeration service (fittings, valves)
• Pressure vessel code compliance (ASME Section VIII for receivers)
• Factory test reports for steel pipe (mill test certificates)
• Third-party certification for critical components

Installation Inspection Points:

• Material verification (grade, specification compliance)
• Joint quality (visual inspection, radiography for welded steel)
• Pressure test documentation (test pressure, duration, results)
• Cleanliness verification (white cloth test, moisture measurement)
• Insulation continuity (vapor barrier integrity, no gaps)

Summary of Material Selection Decision Factors

Refrigerant piping material selection balances technical requirements, economic constraints, and regulatory compliance:

1. Refrigerant compatibility governs base material choice (copper vs. steel vs. aluminum)
2. Pressure requirements determine wall thickness and specification grade
3. Temperature extremes verify material performance across operating envelope
4. Installation environment addresses corrosion, mechanical damage, and accessibility
5. Code compliance confirms approved materials and testing protocols
6. Economic optimization balances first cost against lifecycle reliability

Proper material selection, combined with quality installation practices and rigorous testing, ensures refrigerant system integrity, efficiency, and compliance throughout the design service life.