Stainless Steel Coil Tube — Fan Coil Unit Heat Exchanger Tube
Product Overview
The Stainless Steel Coil Tube (Coiltube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered tubular component specifically designed for integration into Fan Coil Unit (FCU) heat exchanger assemblies. As a critical element within HVAC (Heating, Ventilation, and Air Conditioning) systems, this coil tube serves as the primary medium for thermal energy transfer between refrigerant or hydronic fluid and ambient air. The product is engineered to meet the stringent performance requirements of modern air handling systems, ensuring efficient heat exchange, long service life, and resistance to corrosion under demanding operational conditions.
This product represents a specialized form of stainless steel tubing that has been formed into a continuous coil configuration, optimized for serpentine heat exchanger layouts. The coil geometry maximizes surface area within a compact envelope, a design imperative for modern FCU systems that demand high thermal performance in space-constrained installations.
Material Composition and Properties
Base Material
The coil tube is fabricated from austenitic stainless steel conforming to internationally recognized material specifications. The primary alloy grades employed include:
- ASTM A213 TP304 / TP304L — General-purpose austenitic stainless steel offering excellent corrosion resistance, formability, and weldability. The "L" (low carbon) variant minimizes intergranular corrosion susceptibility during welding operations.
- ASTM A213 TP316 / TP316L — Enhanced corrosion resistance grade incorporating molybdenum (Mo 2.0–2.5% by weight), providing superior resistance to pitting and crevice corrosion in chloride-containing environments.
- ASTM A213 TP316Ti — Titanium-stabilized variant for applications requiring resistance to sensitization at elevated temperatures.
Mechanical Properties
- Tensile Strength (UTS): ≥ 515 MPa (75 ksi) minimum
- Yield Strength (0.2% offset): ≥ 205 MPa (30 ksi) minimum
- Elongation: ≥ 40% in 50 mm (2 in) gauge length
- Hardness: ≤ 92 HRB (annealed condition)
- Impact Resistance: Conforms to ASTM A999 Charpy V-notch requirements at −40°C
Physical and Thermal Properties
- Thermal Conductivity: 16.3 W/(m·K) at 100°C
- Thermal Expansion Coefficient: 17.3 × 10⁻⁶ /°C (20–100°C)
- Density: 8.0 g/cm³
- Operating Temperature Range: −196°C to +870°C (non-oxidizing atmosphere)
Technical Specifications
| Parameter | Specification | Standard Reference |
|---|---|---|
| Tube Outer Diameter (OD) | 6.35 mm (¼") to 15.88 mm (⅝") | ASTM A213 / ASME B36.19M |
| Tube Wall Thickness | 0.50 mm to 1.20 mm | ASTM A213 |
| Dimensional Tolerance — OD | ±0.05 mm (±0.002 in) | ASTM A213 / ASME B36.19M |
| Dimensional Tolerance — Wall Thickness | ±10% of nominal | ASTM A213 |
| Bend Radius (Minimum) | ≥ 2D (where D = tube OD) | Manufacturer specification |
| Coil Length (Continuous) | Up to 100 m per coil (configurable) | — |
| Surface Finish | 2B Annealed / Pickled, Ra ≤ 0.8 μm | ASTM A478 |
| Surface Roughness (Internal) | Ra ≤ 1.6 μm (standard) / Ra ≤ 0.4 μm (premium) | ASTM A478 |
| Heat Treatment Condition | Fully Annealed (Solution Annealed) | ASTM A213 |
| End Preparation | Plain / Beveled / Flared / Threaded (custom) | Per customer specification |
| Maximum Working Pressure | 1.6 MPa (standard) / 4.0 MPa (reinforced) | ASME BPVC Section VIII |
| Hydrostatic Test Pressure | 1.5× Maximum Working Pressure (minimum 2.4 MPa) | ASME BPVC / EN 10216-5 |
| Coating (Optional) | Passivation treatment per ASTM A967 | ASTM A967 / ASTM A380 |
| Standards Compliance | ASTM A213, ASME B36.19M, EN 10216-5, GB/T 14976 | Multinational |
Manufacturing Process
Stage 1 — Raw Material Procurement and Inspection
High-purity stainless steel strip or wire rod is sourced from certified mills meeting ASTM A240 / EN 10088-2 chemical composition requirements. Incoming material undergoes rigorous spectral analysis (PMI — Positive Material Identification) and mechanical testing to verify grade conformance prior to processing.
Stage 2 — Cold Drawing and Sizing
The raw material is subjected to precision cold drawing through a series of calibrated dies to achieve the required outer diameter and wall thickness with tight dimensional tolerances. This process enhances mechanical properties through work hardening while maintaining material integrity.
Stage 3 — Solution Annealing
The cold-drawn tube is heat-treated at 1050–1150°C in a protective atmosphere (argon or vacuum) to restore full ductility, relieve residual stresses, and ensure uniform microstructure. Controlled cooling rates prevent carbide precipitation at grain boundaries.
Stage 4 — Pickling and Passivation
Following annealing, the tube surface undergoes acid pickling to remove oxide scale, followed by passivation treatment per ASTM A967 to establish a chromium-rich oxide film that enhances corrosion resistance.
Stage 5 — Coil Forming and Bending
The straight tube is mechanically formed into the required coil geometry using CNC-controlled bending equipment. Precision bending parameters ensure uniform curvature, absence of wall thinning, and prevention of cross-sectional distortion (ovalization ≤ 5% of OD). The minimum bend radius is maintained at ≥ 2D to prevent material failure.
Stage 6 — Quality Inspection and Testing
- Dimensional Inspection: Laser-based OD and wall thickness measurement at intervals of ≤ 1 meter along coil length.
- Hydrostatic Testing: Each coil is pressure-tested at 1.5× the maximum working pressure with a minimum hold time of 30 seconds. No leakage or permanent deformation is permitted.
- Visual Inspection: Full-length examination for surface defects, dents, scratches, or discoloration.
- Chemical Analysis: Spectrographic verification of alloy composition against ASTM A213 requirements.
- Microstructural Examination: Metallographic analysis to confirm fully austenitic structure with no ferrite phase (for 304/316 grades).
- Corrosion Testing: Pitting resistance verification via ASTM G48 (ferric chloride) or ASTM G110 (potentiodynamic) testing for 316-grade material.
Stage 7 — Packaging and Dispatch
Finished coil tubes are individually wrapped in protective film, secured on wooden or steel spools, and labeled with material traceability documentation including mill certificates, heat numbers, and test reports.
Applications
Primary Application — Fan Coil Units (FCU)
The primary design intent of this coil tube is for use in Fan Coil Unit heat exchangers, which are the terminal heat exchange devices in commercial and residential HVAC systems. The stainless steel coil tube replaces traditional copper tubing in applications where:
- Corrosion resistance is paramount (coastal environments, industrial settings)
- Long-term durability without replacement is required (20+ year service life)
- Hygienic requirements mandate non-leaching materials (hospital, pharmaceutical, food processing)
- Refrigerant compatibility demands inert materials (ammonia-based systems)
Additional Applications
- Chilled Water Systems: Heat exchanger tubes in constant temperature water loops
- Refrigeration Systems: Condenser and evaporator tube assemblies
- Industrial Process Cooling: Heat exchangers in chemical, pharmaceutical, and food processing plants
- Marine HVAC: Saltwater-resistant air handling units for vessels and offshore platforms
- Commercial Kitchen Equipment: Integrated cooling systems in food service installations
Quality Certifications and Standards Compliance
The manufacturing facility and product line maintain the following certifications and conform to the following international standards:
| Certification / Standard | Scope |
|---|---|
| ISO 9001:2015 | Quality Management System |
| ISO 14001:2015 | Environmental Management System |
| ISO 45001:2018 | Occupational Health and Safety Management |
| EN 10204 3.1 | Material Test Certificate (Third-party inspection available) |
| ASTM A213 | Material specification for austenitic stainless steel tube |
| ASME B36.19M | Dimensional standards for welded and seamless stainless steel tube |
| EN 10216-5 | European standard for seamless stainless steel tubes |
| GB/T 14976 | Chinese national standard for seamless stainless steel tubes |
| NSF/ANSI 61 | Drinking water system components (where applicable) |
Unique Selling Points
Superior Corrosion Resistance: The austenitic stainless steel construction provides inherent resistance to oxidation, pitting, and crevice corrosion, eliminating the need for protective coatings or cathodic protection systems. This translates to a service life exceeding 25 years in most HVAC applications, significantly outperforming carbon steel alternatives and matching or exceeding copper in non-chloride environments.
Precision Dimensional Accuracy: Tight manufacturing tolerances (±0.05 mm OD) ensure optimal fit within heat exchanger headers and expansion joints, minimizing installation labor and reducing the risk of mechanical stress points that could lead to premature failure.
Hygienic Design: The smooth internal surface finish (Ra ≤ 0.4 μm on premium grade) prevents biofilm accumulation and facilitates easy cleaning, making this product ideal for healthcare, pharmaceutical, and food-grade HVAC applications where microbial contamination is a critical concern.
Thermal Efficiency: The optimized coil geometry combined with the inherent thermal conductivity of austenitic stainless steel (16.3 W/m·K) delivers competitive heat transfer performance. The continuous coil formation eliminates joint resistance, providing uniform thermal distribution throughout the heat exchanger assembly.
Sustainability and Recyclability: Stainless steel is 100% recyclable without degradation of material properties. The product lifecycle generates minimal environmental impact, and end-of-life material can be fully recovered through established recycling streams, supporting circular economy objectives.
Customization Capability: Wenzhou Jixing Metal Materials Co., Ltd. offers configurable specifications including custom coil lengths, bend radii, end preparations, and surface treatments to meet specific OEM requirements for Fan Coil Unit manufacturers and HVAC system integrators.
Technical Notes and Recommendations
- Installation must maintain the minimum bend radius specification; field re-bending is not recommended as it may compromise material properties and dimensional tolerances.
- Welding to stainless steel coil tube requires inert gas shielding (argon or argon-helium mixture) to prevent oxidation and maintain corrosion resistance at weld zones.
- Post-weld passivation is recommended to restore the protective oxide film at heat-affected zones.
- For systems utilizing water containing chlorides above 200 ppm, the 316/316L grade is strongly recommended over 304/304L to prevent pitting corrosion.
- Storage conditions should maintain relative humidity below 65% to prevent atmospheric corrosion on the as-delivered surface finish.