Stainless Steel Coil Tube — Chiller Coil Application Grade
Product Code: No. 402 | Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd. | Classification: Technical Literature & Application Reference
1. Product Overview
The Stainless Steel Coil Tube (Coiltube) produced by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered continuous coil-form tubing designed for demanding thermal exchange applications, particularly in large-capacity air handling units (AHUs) and chillers used in urban metro transit systems and industrial HVAC infrastructure. This product is supplied in coil (continuous roll) format to facilitate seamless, high-speed automated forming into serpentine coil configurations for chiller coils, condenser coils, and evaporator coils.
The product has been specifically validated through post-service failure analysis on metro Line 10 Phase 2 large chiller coil assemblies, where the tube demonstrated critical performance characteristics under sustained high-pressure differential cycling, thermal fatigue, and condensate corrosion environments. The lessons derived from this field analysis have informed the enhanced metallurgical specifications and dimensional tolerances of the current production standard.
2. Material & Properties
2.1 Base Alloy Composition
The coil tube is manufactured from austenitic stainless steel conforming to the following grades, selected based on application-specific corrosion and mechanical requirements:
- AISI 304 (UNS S30400): General-purpose thermal exchange tubing with excellent formability and moderate corrosion resistance. Chromium (Cr) 18–20%, Nickel (Ni) 8–10.5%.
- AISI 316L (UNS S31603): Enhanced pitting and crevice corrosion resistance for aggressive condensate environments. Chromium (Cr) 16–18%, Nickel (Ni) 10–14%, Molybdenum (Mo) 2–3%.
- AISI 321 (UNS S32100): Stabilized grade with Titanium (Ti) addition for elevated-temperature resistance in condenser applications. Titanium (Ti) 5×(C−0.02)%.
2.2 Mechanical Properties (Annealed Condition)
| Property | Specification | Test Standard |
|---|---|---|
| Tensile Strength (UTS) | ≥ 515 MPa (304); ≥ 530 MPa (316L) | ASTM A213 / GB/T 12771 |
| Yield Strength (0.2% offset) | ≥ 205 MPa | ASTM A213 |
| Elongation (in 50 mm gauge length) | ≥ 40% | ASTM A213 |
| Hardness (Rockwell B) | ≤ 95 HRB | ASTM A213 |
| Impact Energy (−40°C) | ≥ 47 J (Charpy V-notch) | GB/T 229 |
2.3 Thermal & Corrosion Properties
- Thermal Conductivity: 14.5–16.3 W/(m·K) at 25°C (grade-dependent)
- Maximum Service Temperature: 870°C (304); 870°C (316L); 900°C (321)
- Corrosion Resistance: Excellent resistance to condensate corrosion, chlorided cooling water, and atmospheric oxidation; 316L grade offers superior pitting resistance (PREN ≥ 24)
- Creep Resistance: Maintains dimensional stability under cyclic thermal loading (−20°C to +80°C) as validated in metro chiller service
3. Specifications
| Parameter | Specification | Notes |
|---|---|---|
| Outer Diameter (OD) | 6.35 mm (¼") / 9.53 mm (⅜") / 12.7 mm (½") | Custom OD available upon request |
| Wall Thickness (WT) | 0.81 mm (32 GA) / 1.09 mm (28 GA) / 1.65 mm (22 GA) | ±0.05 mm tolerance |
| Coil Length | Continuous; standard cut lengths 50–200 m per coil | Custom lengths available |
| Coil Inner Diameter (ID) | 200 mm / 300 mm / 400 mm | Dependent on OD and WT |
| Coil Weight | Up to 500 kg per coil | — |
| Surface Finish | 2B (Cold Rolled & Annealed) / No. 4 (Brushed) | Surface roughness Ra ≤ 0.8 μm |
| Internal Surface | Smooth; Ra ≤ 0.4 μm (for flow efficiency) | Pipe drawing process |
| End Condition | Plain / Beveled / Flanged (custom) | — |
| Pressure Rating (Design) | Up to 1.6 MPa (gauge) at 80°C | Verified per GB/T 12771 |
| Dimensional Tolerance (OD) | ±0.05 mm (≤12.7 mm OD); ±0.10 mm (>12.7 mm OD) | GB/T 12771 Class A |
| Dimensional Tolerance (WT) | ±10% of nominal | GB/T 12771 Class A |
| Standard Conformance | GB/T 12771-2019; ASTM A213/A213M; EN 10216-5 | Multi-standard certified |
4. Manufacturing Process
4.1 Process Flow
- Raw Material Selection: Sourcing of certified austenitic stainless steel plate or strip from primary mills (Baosteel, Outokumpu, POSCO) with full mill test certificates (MTC) verifying chemical composition and mechanical properties.
- Slitting & Leveling: Precision slitting of coil strip to target width with edge beveling (2–3 mm chamfer) to facilitate seamless tube forming without edge cracking.
- Seamless Tube Forming: Continuous roll-forming of slit strip into tubular geometry via progressive mandrel dies, followed by resistance welding (ERW) or orbital TIG welding at the longitudinal seam.
- Pipe Drawing (Cold Drawing): Multi-stage cold drawing through tungsten carbide dies to achieve precise OD, wall thickness, and internal surface finish. This process enhances mechanical properties and eliminates residual weld imperfections.
- Solution Annealing: Controlled furnace annealing at 1050–1100°C followed by rapid water quench to restore full austenitic microstructure, eliminate work hardening, and maximize corrosion resistance.
- Pickling & Passivation: Chemical cleaning to remove scale, oxidation, and contaminants; passivation to form a stable chromium oxide protective layer.
- Precision Coiling: Automated coiling onto specified diameter cores with controlled tension to prevent surface marking, deformation, or inter-coil contact damage.
- Quality Inspection: 100% ultrasonic (UT) flaw detection of weld seams; hydrostatic pressure testing; dimensional verification; and spectroscopic chemical analysis.
4.2 Key Process Parameters
| Process Step | Critical Parameter | Control Range |
|---|---|---|
| Welding (TIG) | Shielding Gas Flow | 10–15 L/min Argon (99.995% purity) |
| Cold Drawing | Reduction per Pass | 15–25% cross-sectional area |
| Annealing | Temperature | 1050–1100°C ± 10°C |
| Quench | Cooling Rate | ≥ 50°C/s from 800°C to 400°C |
| Hydrostatic Test | Test Pressure | 1.5× design pressure, hold 5 min |
5. Applications
5.1 Primary Application Domains
- Urban Transit HVAC Systems: Chiller coils for metro station air handling units (AHUs), validated through post-failure analysis on metro Line 10 Phase 2 large chiller coil assemblies. The tube withstands continuous cycling between refrigerant-side (−10°C to +15°C) and air-side (+25°C to +45°C) temperature differentials.
- Industrial Chiller & Condenser Coils: Serpentine tube coils for process cooling, refrigeration compressors, and heat exchanger banks in chemical, pharmaceutical, and food processing industries.
- Commercial HVAC: Air-cooled condenser coils, evaporator coils, and radiant cooling panels in commercial building systems.
- Marine & Offshore: Seawater-cooled condenser tubing where chloride resistance is paramount (316L grade).
- Food & Pharmaceutical Equipment: Sanitary-grade heat exchangers requiring smooth internal surfaces and full corrosion resistance.
5.2 Metro Transit Chiller Coil — Failure Analysis Insights
Post-service investigation of large chiller coil assemblies in metro Line 10 Phase 2 identified critical failure modes including: (1) weld seam fatigue cracking under cyclic pressure differential; (2) intergranular corrosion at heat-affected zones (HAZ); and (3) mechanical tube-to-tube fretting at coil bend supports. These findings directly informed the enhanced manufacturing specifications adopted in this product line, including: increased minimum wall thickness, improved weld seam integrity through pipe drawing, optimized annealing schedules to prevent sensitization, and tighter dimensional tolerances to reduce assembly stress.
6. Quality Certifications & Compliance
6.1 Standards Compliance
- GB/T 12771-2019: Non-alloy steel and alloy steel seamless and welded steel tubes for heat exchangers and general heat transfer purposes
- ASTM A213/A213M-21: Standard Specification for Seamless and Welded Austenitic Stainless Steel Tubes for Heat-Transfer Exchangers and Similar Service
- EN 10216-5:2014: Heat exchangers and refrigerating equipment — Tubes of austenitic and austenitic-ferritic stainless steels — Part 5: Seamless tubes
- ISO 3506: Fastener materials — Stainless steel (for associated fittings)
6.2 Factory Certifications
- ISO 9001:2015: Quality Management System certification for manufacturing operations
- ISO 14001:2015: Environmental Management System certification
- ISO/TS 22163: Quality management systems for rail transport industry (relevant for metro transit supply chain)
- ASME Stamp: Authorized manufacturer for pressure-containing heat exchanger components (where applicable)
- Material Traceability: Full heat-lot traceability from primary mill to finished coil, with digital material passport documentation
6.3 Inspection & Testing Protocol
| Inspection Method | Frequency | Acceptance Criteria |
|---|---|---|
| Chemical Analysis (OES/Spectroscopy) | Per heat lot | Within ASTM A213 composition limits |
| Ultrasonic Flaw Detection (UT) | 100% of production | No indications exceeding 0.5 mm equivalent |
| Hydrostatic Pressure Test | 100% of production | No leakage at 1.5× design pressure |
| Dimensional Verification | Per coil (AQL sampling) | Within GB/T 12771 Class A tolerances |
| Surface Inspection | 100% visual + automated | No scratches, pits, or oxidation exceeding 0.1 mm depth |
| Pitting Corrosion Test (ASTM G48) | Per heat lot (incoming) | No pitting at 30 g/L NaCl, 60°C, 24 h |
| Intergranular Corrosion Test (ASTM A262 Practice E) | Per heat lot | No intergranular attack |
7. Unique Selling Points
- Field-Validated Metallurgy: Product specifications directly informed by real-world failure analysis of large-scale metro transit chiller coil assemblies, ensuring proven reliability under extreme cyclic thermal and pressure loading conditions.
- Precision Pipe Drawing: Post-weld cold drawing eliminates weld-induced microstructural heterogeneity, ensuring uniform mechanical properties throughout the tube cross-section and superior fatigue resistance at coil bends.
- Multi-Grade Availability: Single-source procurement of 304, 316L, and 321 grades with consistent quality control, enabling application-specific material selection without supply chain complexity.
- Rail Industry Quality System: ISO/TS 22163 alignment ensures the product meets the elevated quality and traceability standards required for critical infrastructure applications in urban transit.
- Optimized Coil Format: Precision coiling with controlled tension eliminates surface damage and dimensional distortion, enabling direct integration into automated coil-fin and coil-tube manufacturing lines with minimal off-line processing.
- Comprehensive Documentation: Full digital material passport with heat-lot traceability, chemical analysis, mechanical test data, and non