Stainless Steel Coil Tube (Coiltube) — Technical Specification
Product Code: 1132 | Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd
Classification: Literature Study & Technical Reference | Reference Document: Analysis of Freezing Failure in Fan Coil Units During the Initial Heating Phase of a Building in Xi'an
1. Product Overview
The Stainless Steel Coil Tube (Coiltube) is a precision-formed tubular product supplied in continuous coiled configuration, engineered for applications requiring high corrosion resistance, thermal efficiency, and structural integrity under variable temperature conditions. Manufactured by Wenzhou Jixing Metal Materials Co., Ltd, this product is designed to meet the rigorous demands of HVAC (Heating, Ventilation, and Air Conditioning) systems, industrial heat exchangers, process piping, and building services infrastructure.
The coiled form factor offers significant advantages over straight-length pipe sections, including reduced installation time, minimized welding joints, enhanced flexibility during layout, and lower material waste. The product has been extensively studied in the context of building heating system reliability, with particular attention given to failure modes such as those documented in the Xi'an Building Fan Coil Unit Freezing Incident, where inadequate coil tube material selection and insufficient antifreeze protection led to catastrophic system failure during the initial commissioning phase of a commercial high-rise.
2. Material & Properties
2.1 Base Material Composition
The coil tube is fabricated from austenitic stainless steel conforming to international standards, with the following grade options available:
- Grade 304 (UNS S30400): General-purpose corrosion-resistant alloy with balanced mechanical properties and excellent formability. Chromium (Cr) content of 18–20%, Nickel (Ni) content of 8–10%.
- Grade 316 (UNS S31600): Enhanced chloride resistance due to Molybdenum (Mo) addition of 2–3%. Recommended for coastal, chemical, and high-humidity environments.
- Grade 316L (UNS S31603): Low-carbon variant (C ≤ 0.03%) of Grade 316, offering superior resistance to intergranular corrosion (IGC) after welding.
2.2 Key Physical & Mechanical Properties
| Property | Grade 304 | Grade 316 | Grade 316L |
|---|---|---|---|
| Density (kg/m³) | 8.00 | 8.03 | 8.03 |
| Tensile Strength, Rm (MPa) | ≥ 520 | ≥ 520 | ≥ 520 |
| Yield Strength, Rp0.2 (MPa) | ≥ 205 | ≥ 205 | ≥ 205 |
| Elongation, A5 (%, min) | ≥ 40 | ≥ 40 | ≥ 40 |
| Thermal Conductivity, λ (W/m·K, at 100°C) | 16.3 | 16.3 | 16.3 |
| Specific Heat, cp (J/kg·K, at 100°C) | 500 | 500 | 500 |
| Linear Thermal Expansion, α (×10⁻⁶/°C, 100–500°C) | 17.3 | 17.3 | 17.3 |
| Maximum Service Temperature (°C) | 870 | 870 | 870 |
| Corrosion Resistance (Pitting Resistance Equivalent Number, PREN) | 19–20 | 24–26 | 24–26 |
2.3 Critical Performance Characteristics
- Freezing Resistance: The non-magnetic, non-sparking austenitic microstructure provides inherent resistance to brittle fracture at sub-zero temperatures. However, as documented in the Xi'an case study, the water within the coil tube — not the tube itself — is the primary freezing vulnerability. System-level antifreeze measures are essential.
- Oxidation Resistance: The passive chromium oxide (Cr₂O₃) film self-heals in the presence of oxygen, providing long-term protection against atmospheric and aqueous corrosion.
- Weldability: All grades exhibit excellent weldability with minimal risk of intergranular sensitization, particularly Grade 316L.
3. Specifications
3.1 Dimensional Parameters
| Parameter | Specification | Standard Reference |
|---|---|---|
| Outer Diameter (OD), d | 3.0 mm – 50.8 mm (custom sizes available) | ASTM A213 / GB/T 14976 |
| Wall Thickness, t | 0.3 mm – 3.0 mm | ASTM A213 / GB/T 14976 |
| OD Tolerance | ±0.10 mm (precision grade); ±0.15 mm (standard grade) | EN 10217-7 |
| Wall Thickness Tolerance | ±10% of nominal (standard); ±7% (precision) | EN 10217-7 |
| Coil Inner Diameter (ID) | 300 mm – 1200 mm | Manufacturer standard |
| Coil Length per Spool | 50 m – 500 m (typical) | Manufacturer standard |
| Straightness (per 1000 mm) | ≤ 1.0 mm | EN 10217-7 |
| End Condition | Plain end / Beveled end (optional) | Per customer specification |
3.2 Surface Finish Options
| Finish Type | Surface Roughness Ra (μm) | Application |
|---|---|---|
| Pickled & Passivated (P&P) | 0.5 – 1.0 | General industrial, HVAC |
| Mechanically Polished (MP) | 0.2 – 0.5 | Food & beverage, pharmaceutical |
| Electro-polished (EP) | 0.1 – 0.2 | Semiconductor, ultra-high purity |
| Hairline Brushed | 0.4 – 0.8 | Architectural, decorative |
3.3 Heat Treatment & Delivery Condition
- Solution Annealed: Heated to 1050–1150°C followed by rapid water quench to achieve uniform austenitic microstructure and maximum corrosion resistance.
- Cold Work Hardened (Optional): Up to 30% cold reduction for enhanced yield strength and fatigue resistance in high-cycle applications.
- Stress Relieved: Post-coiling stress relief at 300–400°C to minimize residual stresses introduced during the coiling process.
4. Manufacturing Process
4.1 Process Flow
- Raw Material Inspection: Ingot certification verification, chemical composition analysis (spectrometry), and mechanical property testing per ASTM A240 / GB/T 4237.
- Hot Rolling: Billet heating and multi-stand hot rolling to produce seamless or welded tube blanks with controlled dimensional tolerances.
- Hot Extrusion (for seamless tubes): High-pressure extrusion through precision dies to achieve seamless wall integrity, particularly for smaller ODs (< 20 mm).
- Intermediate Annealing: Periodic annealing at 1050–1100°C with water quench to restore ductility and prevent work hardening during successive cold reduction passes.
- Cold Drawing / Cold Pilgering: Precision cold forming to achieve final OD and wall thickness dimensions with tight tolerances. Multiple drawing passes may be employed for high-precision grades.
- Final Solution Annealing & Pickling: Continuous or batch solution annealing followed by acid pickling (HNO₃/HF blend) to remove scale and restore the passive oxide layer.
- Passivation: Immersion in nitric acid (HNO₃) solution to ensure maximum chromium enrichment of the surface film, enhancing corrosion resistance.
- Coiling: Continuous coiling onto calibrated spools with controlled tension to prevent tube distortion, kinking, or ovality.
- Non-Destructive Testing (NDT): 100% eddy current (EC) inspection for wall thickness verification and surface defect detection; hydrostatic pressure testing for leak integrity.
- Packaging & Dispatch: Protective end caps, moisture-absorbent desiccant packaging, and wooden or steel palletization for transport.
4.2 Process Quality Control Checkpoints
| Stage | Inspection Method | Acceptance Criteria |
|---|---|---|
| Raw Material | Spectrometric Analysis (OES) | Chemical composition within ASTM A213 limits |
| Post Cold Drawing | Dimensional Gauge & Optical Comparator | OD & WT within specified tolerance |
| Post Annealing | Metallographic Examination | ≥ 95% austenite; no delta ferrite in weld zones |
| Post Pickling | Water Drop Test (ASTM A967) | No rust spots after 2-hour test |
| Post Coiling | Eddy Current Testing (ASTM E165) | No wall thinning, cracks, or inclusions |
| Final | Hydrostatic Pressure Test | No leakage at 1.5× design pressure for 30 seconds |
5. Applications
5.1 HVAC & Building Services Systems
The coil tube is extensively used in fan coil units (FCUs), chilled water coils, heating coils, and condenser/evaporator assemblies. The Xi'an building case study provides critical lessons for this application domain:
Key Lesson from Xi'an Freezing Incident: During the initial heating commissioning of a commercial building, fan coil units experienced catastrophic freezing failure due to (a) delayed activation of the heating system allowing ambient cold to penetrate unheated coil circuits, (b) inadequate antifreeze fluid concentration, and (c) poor insulation of exposed coil tube runs. The resulting ice expansion exceeded the tube's pressure tolerance, causing rupture and extensive water damage. This underscores the critical importance of selecting coil tubes with adequate burst pressure margins and implementing robust antifreeze protocols during system commissioning.
- Chilled Water Coil Tubes: OD 9.52 mm (3/8"), WT 0.81 mm — standard for FCU evaporator sections
- Heating Coil Tubes: OD 9.52 mm (3/8"), WT 0.89 mm — for hot water heating circuits
- Condenser Tubes: OD 12.7 mm (1/2"), WT 1.0 mm — for air-cooled and water-cooled condensers
5.2 Industrial Heat Exchange
- Shell-and-tube heat exchangers for chemical processing, petroleum refining, and power generation
- Plate-and-fin heat exchanger tubing for compact thermal management
- Refrigeration and cryogenic process coils operating at temperatures as low as −196°C (LNG applications)
5.3 Process Piping & Fluid Transfer
- Food and beverage processing lines requiring sanitary-grade tubing
- Pharmaceutical CIP (Clean-In-Place) and SIP (Sterilization-In-Place) compatible fluid circuits