Stainless Steel Coil Tube (Coiltube) — Heat Exchanger Grade
Product Code: 951 | Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd.
1. Product Overview
The Stainless Steel Coil Tube (hereinafter referred to as "Coil Tube" or "盘管") is a precision-engineered continuous tubular product designed primarily for use in heat exchangers, condensers, evaporators, and other thermal management systems. Manufactured in a continuous spiral coil configuration, this product offers superior heat transfer efficiency, compact structural integration, and exceptional resistance to corrosion cracking — a critical failure mode identified through extensive metallurgical analysis of stainless steel heat exchanger coil tubes operating under harsh chemical and thermal cycling conditions.
Unlike conventional straight pipe sections that require field bending and welding, the Coil Tube is produced in a ready-to-install helical form, eliminating potential failure points associated with Heat-Affected Zones (HAZ) from field welding. This manufacturing advantage significantly enhances long-term structural integrity and reduces maintenance downtime in industrial applications.
2. Material & Properties
2.1 Available Grades
- AISI 304 / SUS 304 (UNS S30408): General-purpose austenitic stainless steel offering excellent corrosion resistance in non-reducing environments. Chromium (Cr) content of 18–20% and Nickel (Ni) content of 8–10.5% provide robust passive film formation.
- AISI 316L / SUS 316L (UNS S31603): Low-carbon variant with added Molybdenum (Mo, 2–3%) for enhanced resistance to pitting corrosion and crevice corrosion in chloride-containing environments. Preferred for marine, chemical, and pharmaceutical applications.
- AISI 316Ti / SUS 316Ti (UNS S31635): Titanium-stabilized grade that mitigates intergranular corrosion (IGC) and stress corrosion cracking (SCC) risk during welding and elevated-temperature service.
- AISI 321 / SUS 321 (UNS S32100): Columbium (Cb/Ta)-stabilized grade suitable for high-temperature service up to 870 °C with reduced SCC susceptibility.
2.2 Key Mechanical Properties (AISI 316L Reference)
- Tensile Strength (Rm): ≥ 515 MPa
- Yield Strength (Rp0.2): ≥ 205 MPa
- Elongation at Break (A50): ≥ 40%
- Hardness (HBW): ≤ 217
- Density: 8.0 g/cm³
- Thermal Conductivity (at 20 °C): 16.3 W/(m·K)
- Thermal Expansion Coefficient (20–100 °C): 16.0 × 10⁻⁶ /K
2.3 Corrosion Resistance Characteristics
Based on metallurgical failure analysis of stainless steel heat exchanger coil tubes, the following corrosion mechanisms have been specifically addressed in product design:
- Pitting Corrosion Resistance: Measured by Pitting Resistance Equivalent Number (PREN) = %Cr + 3.3 × %Mo + 16 × %N. AISI 316L achieves a PREN of ≥ 24, providing superior resistance to localized attack in chloride environments.
- Stress Corrosion Cracking (SCC) Resistance: The austenitic microstructure, combined with low carbon content (≤ 0.03%), minimizes chromium carbide precipitation at grain boundaries, thereby reducing susceptibility to chloride-induced SCC.
- Intergranular Corrosion (IGC) Resistance: Verified through ASTM A262 Practice No. 1E (Intergranular Corrosion in Welded Austenitic Stainless Steel Tubing). Titanium or niobium stabilization in grades 316Ti and 321 further enhances grain boundary integrity.
- Erosion-Corrosion Resistance: Smooth inner surface finish (Ra ≤ 0.4 μm) reduces turbulent flow-induced material removal, a common precursor to coil tube failure in high-velocity fluid applications.
3. Specifications
| Parameter | Specification / Range | Standard Reference |
|---|---|---|
| Outer Diameter (OD) | 6.0 mm – 50.0 mm | ASTM A213 / ASME SA-213 |
| Wall Thickness (WT) | 0.5 mm – 3.0 mm | ASTM A213 / ASME SA-213 |
| Bending Radius | 1.5 D – 3.0 D (D = OD) | ISO 1155 |
| Coil Diameter (Bundle) | Custom, typically 200 mm – 1500 mm | Per customer specification |
| Surface Finish | Pickled & Passivated (2B / No. 4 / Mirror Ra ≤ 0.2 μm) | ASTM A478 |
| Maximum Operating Temperature | Up to 870 °C (Grade 321); Up to 600 °C (Grade 316L) | Grade-dependent |
| Maximum Operating Pressure | Up to 25 MPa (design-dependent) | ASME BPVC Section VIII |
| Tolerance on OD | ± 0.05 mm (precision grade); ± 0.10 mm (standard grade) | ASTM A213 |
| Tolerance on Wall Thickness | ± 10% (standard); ± 5% (precision grade) | ASTM A213 |
| Heat Treatment Condition | Solution Annealed & Pickled | ASTM A213 |
| Coil Length per Bundle | 50 m – 500 m (continuous, per customer requirement) | — |
| Hydrostatic Test Pressure | 1.5 × Design Pressure (minimum 15 MPa) | ASTM A269 / EN 10217 |
4. Manufacturing Process
4.1 Process Flow
- Raw Material Procurement: High-purity stainless steel strip or seamless pipe blank sourced from certified mills, with full Mill Test Certificates (MTC) conforming to EN 10204 3.1 or 3.2.
- Decoiling & Straightening: Incoming coil is uncoiled and fed through a precision straightening and leveling unit to ensure dimensional accuracy prior to forming.
- Roll Forming / Pipe Bending: The tube is passed through a series of precision-ground rollers arranged to produce the desired helical coil geometry. The rolling process is performed cold to maintain metallurgical properties and avoid HAZ formation.
- Coil Winding: The formed tube is wound onto a precision coil former at controlled tension to achieve the specified coil diameter and pitch. Automated tension control ensures uniformity throughout the coil length.
- Solution Annealing: If required, the coil tube is solution annealed at 1050–1150 °C followed by controlled water quenching to restore full austenitic microstructure and relieve any residual cold-work stresses.
- Pickling & Passivation: The tube surface is treated with mixed acid (HNO₃ + HF) pickling solution to remove oxide scale, followed by nitric acid passivation to establish a stable chromium oxide passive film. This step is critical for ensuring corrosion resistance per ASTM A478.
- Hydrostatic Testing: Each coil is subjected to hydrostatic pressure testing at 1.5× the design pressure to verify weld integrity and detect any porosity, cracks, or thin-wall defects.
- Non-Destructive Testing (NDT): Optional Eddy Current Testing (ECT) per ASTM E309 or Ultrasonic Testing (UT) per ASTM E2715 is performed to detect internal and surface defects, particularly intergranular cracks and corrosion initiation sites.
- Dimensional Inspection & Final QA: OD, wall thickness, coil diameter, and pitch are measured at multiple points along the coil length using calibrated instruments. Results are documented in the final inspection report.
- Packaging & Dispatch: Coils are wrapped in moisture-proof packaging, secured on wooden pallets or steel cradles, and shipped with complete documentation including MTC, test reports, and material traceability records.
4.2 Metallurgical Considerations for Corrosion Crack Prevention
Based on failure analysis of stainless steel heat exchanger coil tubes, the following metallurgical controls are implemented to prevent corrosion cracking:
- Carbon content maintained at ≤ 0.03% (L-grade) to minimize chromium carbide (Cr₂₃C₆) precipitation at grain boundaries.
- Stabilization with Titanium (Ti) or Niobium (Nb) in grades 316Ti and 321 to preferentially form TiC or NbC, preserving grain boundary chromium content.
- Controlled cooling rates during annealing to avoid sensitization temperature range (450–850 °C) where intergranular carbide precipitation is thermodynamically favorable.
- Post-weld annealing for any tack welds to restore full solution-annealed condition and eliminate residual stresses that could promote SCC.
5. Applications
- Shell-and-Tube Heat Exchangers: Primary application; coil tubes serve as the heat transfer element in condensers, reboilers, and coolers in chemical, petrochemical, and power generation industries.
- Plate Heat Exchangers (PHE): Used as gasketed or brazed channel elements for compact thermal exchange in HVAC and process industries.
- Refrigeration & Air Conditioning: Condenser and evaporator coils for industrial refrigeration systems, leveraging high thermal conductivity and corrosion resistance of austenitic stainless steel.
- Food & Beverage Processing: Sanitary-grade coil tubes (Ra ≤ 0.4 μm) for pasteurization, sterilization, and heat recovery in dairy, brewing, and pharmaceutical processing.
- Marine & Offshore Engineering: AISI 316L coil tubes in desalination plants, ballast water heat exchangers, and offshore oil & gas platform cooling systems.
- Power Generation: Feedwater heaters, turbine condensers, and economizers in thermal power plants.
- Chemical Processing: Reactor jacket coils, distillation column condensers, and evaporators handling aggressive chemical media.
6. Quality Certifications & Standards Compliance
6.1 Applicable Standards
| Standard | Description |
|---|---|
| ASTM A213 / ASME SA-213 | Standard Specification for Seamless Austenitic Stainless Steel Tubing for High-temperature Service |
| ASTM A269 | Standard Specification for Seamless and Welded Austenitic Stainless Steel Tubing for Chemical and Similar Severe Service |
| EN 10217-7 | Non-ferrous metallic materials — Seamless tubes for pressure purposes — Part 7: Non-stabilized austenitic stainless steel tubes |
| GB/T 12771 | Chinese National Standard — Stainless Steel Seamless Tubes for Fluid Transport |
| ASTM A478 | Standard Practice for Pickling and Passivating Stainless Steel Parts |
| ASTM A262 Practice No. 1E | Intergranular Corrosion in Welded Austenitic Stainless Steel Tubing |
| ASTM E309 | Standard Practice for Eddy-Current Examination of Non-Ferrous Tubular Products |
| ASME BPVC Section VIII | Boiler and Pressure Vessel Code — Rules for Construction of Pressure Vessels |
6.2 Quality Management Certifications
- ISO 9001:2015 — Quality Management System certification ensuring consistent product quality and continuous improvement.
- ISO 14001:2015 — Environmental Management System certification for sustainable manufacturing practices.
- ISO 45001:2018 — Occupational Health and Safety Management System certification.
- CE Marking — Conformity with European Union directives for products intended for the European market.
- NACE MR0175 / ISO 15156 — Compliance for use in sour service environments (H₂S-containing) where applicable.
6.3 Testing & Documentation
- Mill Test Certificate (MTC) per EN 10204 3.1 / 3.