Stainless Steel Coil Tube (Coiltube) — Product No. 893
1. Product Overview
The Stainless Steel Coil Tube (hereinafter referred to as "Coiltube") manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered seamless or welded stainless steel tube product, continuously formed into a coiled configuration to facilitate efficient downstream processing, transportation, and installation. This product is specifically designed for applications in air-handling equipment, including fan coil units (FCUs), where acoustic performance, thermal conductivity, and long-term corrosion resistance are critical design parameters.
From a metallurgical standpoint, the Coiltube serves as a structural and functional component within heat exchange systems. The continuous coil form factor eliminates the need for field welding or brazing joints, thereby minimizing potential failure points and reducing the risk of low-frequency acoustic noise — a well-documented issue in fan coil unit assemblies caused by vibration-induced resonances at tube-to-header interfaces.
Key Design Philosophy: The elimination of discrete tube segments and their associated mechanical fasteners directly addresses the root causes of low-frequency noise generation in HVAC coil assemblies, as identified in industry fault analyses of fan coil unit acoustic performance degradation.
2. Material & Properties
2.1 Base Alloy Composition
The Coiltube is fabricated from austenitic stainless steel conforming to internationally recognized alloy specifications. The primary material grades include:
- AISI 304 / UNS S30400 (1.4301): Chromium (Cr) 18–20%, Nickel (Ni) 8–10.5%, Manganese (Mn) ≤2.0%, Carbon (C) ≤0.08%. Offers excellent general corrosion resistance and formability.
- AISI 316 / UNS S31600 (1.4401): Chromium (Cr) 16–18%, Nickel (Ni) 10–14%, Molybdenum (Mo) 2–3%. Enhanced resistance to pitting and crevice corrosion in chloride-containing environments.
- AISI 321 / UNS S32100 (1.4541): Chromium (Cr) 17–19%, Nickel (Ni) 9–12%, Titanium (Ti) ≥5×C. Stabilized grade for elevated-temperature service.
2.2 Mechanical Properties (Annealed Condition)
- Tensile Strength (UTS): 515–720 MPa (75,000–104,000 psi)
- Yield Strength (0.2% Offset): ≥205 MPa (29,700 psi)
- Elongation (in 50 mm): ≥40%
- Hardness (Rockwell B): ≤95 HRB
- Thermal Conductivity (at 20°C): 14.9–16.3 W/(m·K)
- Linear Expansion Coefficient (20–100°C): 17.3 × 10⁻⁶ /°C
2.3 Corrosion Resistance
The passive chromium oxide film (Cr₂O₃) formed on the austenitic stainless steel surface provides inherent resistance against atmospheric, freshwater, and mildly aggressive chemical environments. The material exhibits a Pitting Resistance Equivalent Number (PREN) of ≥19 for Grade 304 and ≥24 for Grade 316, calculated as: PREN = %Cr + 3.3 × %Mo + 16 × %N.
3. Specifications
| Parameter | Specification | Standard / Reference |
|---|---|---|
| Outer Diameter (OD) | Φ6.0 mm – Φ32.0 mm (custom available) | ASTM A213 / EN 10216-5 |
| Wall Thickness | 0.5 mm – 3.0 mm | ASME B36.19M |
| Inner Diameter (ID) | Calculated: ID = OD − 2 × Wall Thickness | — |
| Coil Length | Up to 500 m per coil (continuous) | Manufacturer Standard |
| Coil Inner Diameter | Φ300 mm – Φ600 mm | — |
| OD Tolerance | ±0.1 mm (precision grade) | ASTM A213 |
| Wall Thickness Tolerance | ±0.05 mm | ASTM A213 |
| Surface Finish | 2B, BA (Bright Annealed), or No. 4 Brushed | ASTM A666 |
| End Condition | Plain, Beveled, or Threaded | Custom |
| Operating Temperature Range | −196°C to +870°C (Grade 304); −196°C to +870°C (Grade 316) | ASTM A213 |
| Maximum Working Pressure | Up to 15 MPa (dependent on OD and wall thickness) | ASME B31.3 |
| Acoustic Vibration Damping | Continuous coil eliminates joint-borne low-frequency noise sources | ISO 3744 (acoustic reference) |
4. Manufacturing Process
4.1 Raw Material Selection and Inspection
Virgin stainless steel strip or pipe blank material is sourced from certified mills with traceable heat numbers. Incoming material undergoes spectroscopic verification (Optical Emission Spectrometry, OES) to confirm alloy composition within specified limits. Chemical analysis reports are retained for each production batch.
4.2 Tube Forming
- Seamless Route: Hot or cold piercing of solid billets followed by cold drawing and mandrel sizing to achieve precise dimensional tolerances.
- Welded Route: Precision cold-rolled strip is fed through forming rolls, longitudinally welded via Continuous TIG (Gas Tungsten Arc) welding or Flash Butt Welding, and subsequently annealed to restore full austenitic microstructure and eliminate residual weld stresses.
4.3 Cold Working and Dimensional Sizing
The formed tube passes through a series of calibrated mandrel-die stations for cold sizing, achieving the specified outer diameter and wall thickness tolerances (±0.1 mm OD, ±0.05 mm wall). This process also induces beneficial strain hardening at the surface layer, improving fatigue resistance.
4.4 Annealing and Surface Treatment
Post-forming annealing is conducted in a controlled atmosphere (argon or nitrogen) at temperatures between 1050°C and 1150°C, followed by rapid water quenching. This restores the fully austenitic (γ-phase) microstructure, eliminates carbide precipitation (sensitization), and achieves the desired mechanical properties. Surface finishing options include:
- 2B Finish: Cold-rolled with annealed and pickled surface.
- BA (Bright Annealed): Annealed in a protective atmosphere without pickling, yielding a smooth, reflective surface ideal for hygienic applications.
- No. 4 Brushed: Directional polish with 100–240 grit finish.
4.5 Coil Winding and Packaging
The finished tube is wound onto precision steel coils using a controlled tension winding system to prevent surface marking, deformation, or kinking. Each coil is protected with moisture-absorbent desiccant paper, polyethylene film wrapping, and shrink-wrap packaging with wooden pallets for intermodal transport.
5. Applications
5.1 Fan Coil Units (FCUs) — Primary Application
The Coiltube is the core heat transfer element in fan coil units used in commercial HVAC systems. The continuous coil configuration offers distinct advantages over segmented tube-and-fin coil assemblies:
- Acoustic Performance: Eliminates vibration-transmitting joints between discrete tube segments and headers, directly addressing the low-frequency noise (20–200 Hz) issue commonly diagnosed in fan coil unit fault analyses. The continuous metal path provides inherent vibration damping through distributed mass and geometric continuity.
- Thermal Efficiency: Uninterrupted tube geometry ensures uniform fluid flow distribution and maximizes the effective heat transfer surface area.
- Leak-Free Integrity: Absence of field welds or mechanical crimp joints eliminates the primary failure modes associated with refrigerant or water leakage.
5.2 Additional Applications
- Air Conditioning Heat Exchangers: Evaporator and condenser coils in split-system and packaged units.
- Process Cooling Systems: Chilled water piping in industrial facilities requiring sanitary-grade materials.
- Food and Beverage Industry: Beverage chiller coils, dairy processing heat exchangers (BA finish, 316 grade).
- Pharmaceutical Equipment: Clean-in-place (CIP) compatible cooling coils.
- Marine and Offshore: Seawater cooling loops (316L grade with Mo enhancement).
- Automotive Thermal Management: Battery cooling coils for electric vehicle thermal systems.
6. Quality Certifications and Compliance
Wenzhou Jixing Metal Materials Co., Ltd. maintains rigorous quality management systems and holds the following certifications and complies with international standards:
- ISO 9001:2015 — Quality Management System Certification
- ISO 14001:2015 — Environmental Management System
- EN 10204 — Type 3.1 Inspection Certificate (third-party witnessed testing)
- ASTM A213 — Standard Specification for Seamless Austenitic Stainless Steel Tube for High-temperature Service
- ASTM A269 — Standard Specification for Seamless and Welded Austenitic Stainless Steel Tubing for Chemical and Similar Severe Service
- EN 10216-5 — European Standard for Heat-Exchanger and Heat-Sink Tubes (Stainless Steel)
- GB/T 14976 — Chinese National Standard for Seamless Stainless Steel Tubes for Fluid Transport
- GB/T 12771 — Chinese National Standard for Welded Stainless Steel Tubes for Fluid Transport
- CE Marking — Compliance with European Pressure Equipment Directive (PED 2014/68/EU)
- NSF/ANSI 61 — Drinking Water System Components (available upon request)
6.1 Inspection and Testing Protocols
- Chemical Analysis: OES spectroscopy on every heat lot; supplementary wet chemistry verification for critical elements (C, S, P).
- Mechanical Testing: Tensile testing, hardness testing, and bend testing per ASTM A370 on each production batch.
- Dimensional Inspection: 100% online ultrasonic wall thickness gauging; dimensional verification via laser micrometry at intervals of ≤10 m.
- Non-Destructive Testing (NDT): Eddy current testing for surface and near-surface defect detection; hydrostatic pressure testing at 1.5× maximum working pressure for 30 seconds minimum hold time.
- Surface Quality: Visual inspection under controlled illumination (≥500 lux) for surface defects, scratches, and finish uniformity.
- Corrosion Testing: Salt spray testing per ASTM B117 (≥480 hours without pitting for Grade 316); potentiodynamic polarization testing for PREN verification.
7. Unique Selling Points
- Noise Reduction by Design: The continuous coil architecture inherently eliminates the discrete mechanical joints that serve as primary excitation sources for low-frequency structural-borne noise in fan coil unit assemblies — a well-documented failure mode in HVAC acoustic diagnostics.
- Full Traceability: Each coil is laser-marked with heat number, lot code, and production date, enabling complete material traceability from mill origin to end-of-life.
- Custom Dimensional Engineering: Flexible production capability accommodating OD ranges from 6 mm to 32 mm with wall thicknesses from 0.5 mm to 3.0 mm, tailored to specific heat exchanger design requirements.
- Extended Service Life: Austenitic stainless steel construction provides a design service life exceeding 25 years in HVAC applications, significantly outperforming copper or carbon steel alternatives in terms of corrosion fatigue resistance.
- Environmentally Responsible: 100% recyclable material content; manufacturing process adheres to ISO 14001 environmental management protocols with minimal waste generation.
8. Technical Advisory Note
For applications involving fan coil unit integration, it is recommended that the Coiltube be installed with appropriate vibration isolation mounts at header connections and that the coil assembly be acoustically decoupled from the fan housing structure. This practice, combined with the inherent joint-free design of the continuous coil tube, provides a comprehensive solution to the low-frequency noise (20–200 Hz) issue prevalent in conventional segmented-tube fan coil units, as documented in industry fault analysis literature.
Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd.
Product Reference Number: 893
Documentation Classification: Literature Study / Technical Reference