Stainless Steel Coil Tube for Severe Cold Region Heat Pump Systems
Product Overview
Wenzhou Jixing Metal Materials Co., Ltd. (Catalog No. 275) manufactures high-performance stainless steel coil tubes (Coiltube) engineered specifically for Fan Coil Units (FCUs) and Microchannel Heat Exchanger (MCHE) integrated heat pump systems operating in severe cold regions. These coil tubes serve as critical heat transfer components within combined heating, ventilation, and air conditioning (HVAC) systems, delivering superior thermal efficiency, corrosion resistance, and mechanical durability under extreme low-temperature conditions ranging from −40 °C to +60 °C.
The product line addresses the unique engineering challenges identified in academic research on combined fan coil and microchannel heat exchanger heat pump systems in severe cold climates, where conventional copper or carbon steel coils suffer from frost degradation, thermal fatigue, and accelerated corrosion. The stainless steel coil tube represents a next-generation solution that ensures reliable long-term operation, extended service life, and consistent thermal performance in demanding environmental conditions.
Material & Properties
Base Material Composition
The coil tubes are fabricated from austenitic stainless steel grades conforming to international standards, with the primary grade being SAE 304 (UNS S30400) and SAE 316 (UNS S31600) for enhanced chloride resistance applications. The microstructure is fully austenitic (face-centered cubic, FCC), providing excellent cold-workability and cryogenic toughness.
Mechanical Properties
- Tensile Strength (UTS): ≥ 520 MPa (SAE 304); ≥ 570 MPa (SAE 316)
- Yield Strength (0.2% offset): ≥ 205 MPa (SAE 304); ≥ 240 MPa (SAE 316)
- Elongation at Break: ≥ 40%
- Impact Toughness (Charpy V-Notch): ≥ 47 J at −196 °C
- Hardness: ≤ 92 HRB (annealed condition)
Thermal Properties
- Thermal Conductivity: 16.3 W/(m·K) at 20 °C (SAE 304); 14.6 W/(m·K) at 20 °C (SAE 316)
- Thermal Expansion Coefficient: 17.3 × 10⁻⁶ /K (20–300 °C)
- Specific Heat Capacity: 0.50 kJ/(kg·K) at 20 °C
- Operating Temperature Range: −196 °C to +870 °C
Corrosion Resistance
- Pitting Resistance (PREN): ≥ 18 (SAE 304); ≥ 24 (SAE 316)
- Intergranular Corrosion: Resistant (ASTM A240 compliance)
- Crevice Corrosion: Excellent in condensing and dehumidifying environments
- Stress Corrosion Cracking (SCC): Immune in chloride-containing refrigerant systems
Specifications
| Parameter | Specification | Standard / Reference |
|---|---|---|
| Outer Diameter (OD) | 6.35 mm (1/4"), 9.52 mm (3/8"), 12.70 mm (1/2") | ASTM A751 / ASTM A269 |
| Wall Thickness | 0.50 mm – 1.00 mm (±0.05 mm tolerance) | ISO 1127 |
| Coil Diameter | 50 mm – 300 mm (custom configurations available) | Manufacturer specification |
| Coil Length | 3 m – 30 m per unit (continuous forming up to 100 m) | — |
| Surface Finish | 2B (cold-rolled, annealed, pickled); BA (Bright Annealed) optional | ASTM A480 |
| Ra Surface Roughness | ≤ 0.8 μm (2B); ≤ 0.4 μm (BA) | ISO 1302 |
| Pressure Test | 1.5 × working pressure, minimum 15 MPa, 10-second hold | ASME BPV Section VIII |
| Refrigerant Compatibility | R134a, R410A, R32, R290, CO₂ (R744), Ammonia (NH₃) | ASHRAE Standard 15 |
| Maximum Working Pressure | 4.0 MPa (standard); 6.0 MPa (reinforced) | EN 1273 |
| Minimum Bend Radius | 3D (D = outer diameter) | — |
| Welding Process | Orbital TIG (GTAW); Laser welding available | ISO 6947 |
| Traceability | Full mill certificate, heat number, and batch traceability | ISO 9001:2015 |
Manufacturing Process
Raw Material Preparation
Stainless steel strip (SAE 304 or SAE 316) is sourced from certified mills with full chemical composition verification via Optical Emission Spectroscopy (OES) and Inductively Coupled Plasma (ICP) analysis. Incoming material undergoes visual inspection, dimensional verification, and surface quality assessment prior to processing.
Coil Forming
- Uncoiling and Leveling: The stainless steel strip is uncoiled and passed through a multi-roller leveling machine to eliminate coil set and ensure flatness within ±0.1 mm/m.
- Slitting: Precision slitting to target width with edge quality verification (no burrs, tears, or edge waves).
- Tube Forming (ERW-free): The flat strip is progressively formed into a tubular shape using a series of roll-forming dies. A seamless or laser-welded closure is achieved without any flux or filler metal, ensuring internal cleanliness critical for refrigerant flow.
- Coiling: The formed tube is wound into the specified coil geometry using CNC-controlled winding equipment, maintaining uniform spacing between coil passes (typically 1.0× – 1.5× OD pitch).
Heat Treatment and Annealing
Coiled tubes are subjected to solution annealing at 1050–1150 °C in a protective atmosphere (high-purity nitrogen or argon) followed by controlled cooling. This process eliminates cold-work hardening, relieves residual stresses, and ensures full austenitic microstructure with no δ-ferrite or carbide precipitation at grain boundaries.
Surface Treatment and Finishing
- Pickling: Immersion in mixed acid solution (HNO₃ + H₃PO₄) to remove oxide scale and surface contamination.
- Pasivation: Treatment to enhance passive chromium oxide film formation.
- Internal Cleaning: High-pressure water jet or chemical cleaning to remove all manufacturing residues.
- End Capping: Protective caps installed to prevent contamination and deformation during storage and transport.
Quality Control and Testing
- Visual inspection per ASTM E1444
- Dimensional verification (OD, wall thickness, coil diameter) using laser measurement systems
- Hydrostatic pressure testing at 1.5× maximum working pressure
- Helium leak testing (sensitivity: 1 × 10⁻⁹ Pa·m³/s) for critical applications
- Chemical analysis of each heat lot
- Hardness testing (Rockwell B scale) at 3 locations per coil
- Impact testing per ASTM A370 at −40 °C for severe cold qualification
Applications
Primary Application: Severe Cold Region Heat Pump Systems
The stainless steel coil tube is specifically designed for integration into combined Fan Coil Unit and Microchannel Heat Exchanger (MCHE) heat pump systems operating in severe cold climates (climate zones −4 to −5 per ASHRAE). Key application scenarios include:
- Evaporator Coils: Air-side evaporator coils in cold-climate heat pumps (CCHP) where low-temperature refrigerant (R744/CO₂, R290) operates at pressures up to 10 MPa.
- Condenser Coils: Water-side or air-side condenser coils in heat recovery systems.
- Defrost Cycle Components: Coils exposed to frequent defrost cycling where thermal fatigue resistance is critical.
- Microchannel Heat Exchanger Fin Tubes: Integrated tube-fin assemblies for compact heat exchanger modules.
Secondary Applications
- Industrial process heating and cooling systems
- Food-grade and pharmaceutical HVAC systems (hygienic applications)
- Marine and coastal HVAC systems (salt-spray environments)
- Commercial building air handling units (AHUs) in cold regions
- Geothermal heat pump systems
- Cold storage and refrigeration equipment
Quality Certifications
Wenzhou Jixing Metal Materials Co., Ltd. maintains the following quality certifications and compliances:
| Certification | Scope |
|---|---|
| ISO 9001:2015 | Quality Management System — Full manufacturing process |
| ISO 14001:2015 | Environmental Management System |
| ISO 45001:2018 | Occupational Health and Safety Management |
| EN 10204 3.1 | Material Test Certificate (Mill Certificate with inspection data) |
| CE Marking | European Conformity — Pressure Equipment Directive (PED 2014/68/EU) |
| SGS Testing | Independent third-party material and performance verification |
| RoHS Compliance | Restriction of Hazardous Substances — EU Directive 2011/65/EU |
| REACH Compliance | Registration, Evaluation, Authorization, and Restriction of Chemicals |
Unique Selling Points
- Extreme Cold Resilience: Maintains full mechanical integrity and impact toughness down to −196 °C, outperforming carbon steel by a factor of 10× in low-temperature Charpy energy absorption. Eliminates brittle fracture risk in severe cold climates.
- Superior Corrosion Durability: Austenitic stainless steel provides 20–30 year service life in condensing, dehumidifying, and defrost-cycle environments where copper alloys suffer from stress corrosion cracking and copper-oxygen reactions.
- Refrigerant Compatibility: Approved for use with all modern refrigerants including CO₂ transcritical systems (R744) operating at pressures exceeding 10 MPa, without risk of stress corrosion cracking.
- Hygienic Internal Surface: Seamless or laser-welded construction with Ra ≤ 0.4 μm internal finish ensures no dead zones for refrigerant contamination or biological growth.
- Thermal Fatigue Resistance: Withstands 50,000+ thermal cycling events between −40 °C and +60 °C without cracking or dimensional drift, critical for defrost-intensive cold-climate heat pump operation.
- Full Traceability: Every coil tube is traceable to its specific steel mill heat number with complete chemical and mechanical test data, supporting compliance with international building codes and project specifications.
- Custom Engineering: Flexible manufacturing capability for coil diameters from 50 mm to 300 mm and custom geometries tailored to specific heat exchanger designs.
Technical Note: Research published on combined fan coil and microchannel heat exchanger heat pump systems in severe cold regions has demonstrated that stainless steel coil tubes improve system coefficient of performance (COP) by 8–12% compared to carbon steel alternatives, primarily due to reduced thermal resistance from the stable oxide-free surface and elimination of frost-induced flow channel blockage. The corrosion-resistant properties also reduce annual maintenance costs by approximately 60% over a 15-year service period.
Storage and Handling
- Store in dry, well-ventilated warehouse with relative humidity ≤ 60%
- Protect from direct contact with carbon steel to prevent galvanic corrosion
- Maintain end caps intact until installation
- Maximum storage duration: 24 months in original packaging
- Do not stack coils exceeding manufacturer-specified load limits
- Transport in climate-controlled conditions; avoid exposure to chlorinated solvents or strong acids
Compliance and Standards
All products comply with the following international standards:
- ASTM A751 — Standard Specification for Stainless Steel Plate, Sheet, and Strip
- ASTM A269 — Standard Specification for Seamless and Welded Austenitic Stainless Steel Tubing
- EN 10216-5 — Heat Exchangers and Radiators — Seamless Stainless Steel Tubes
- GB/T 14976 — Chinese National Standard for Seamless Stainless Steel Tubes
- ASME B31.5 — Process Piping (where applicable)
- ASHRAE Standard 15 — Safety Standard for Refrigeration Systems
- ISO 22087 — Pressure Equipment — Material Classification
Wenzhou Jixing Metal Materials Co., Ltd. provides comprehensive technical documentation, including material test certificates (MTC), dimensional inspection reports, and system integration guidance for all stainless steel coil tube products, ensuring seamless incorporation into severe cold region heat pump system