Stainless Steel Coil Tube — Fan Coil Unit (FCU) Application Grade
Product Overview
The Stainless Steel Coil Tube (盘管) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. (温州吉兴金属材料有限公司) is a precision-engineered coiled tubing product specifically designed for use in Fan Coil Units (FCU) within Heating, Ventilation, and Air Conditioning (HVAC) systems. As a critical heat-exchange component in commercial and residential building infrastructure, this coil tube serves as the primary conduit for circulating refrigerant or hot water, enabling efficient thermal transfer between the air stream and the working fluid.
Unlike conventional copper or aluminum coil tubes, the stainless steel variant offers superior corrosion resistance, mechanical durability, and long-term structural integrity, making it particularly suitable for environments where water quality variability, chemical exposure, or prolonged operational cycles demand enhanced material performance. The product addresses a well-documented industry challenge — fan coil water leakage — by leveraging the inherent oxidation resistance of austenitic stainless steel to eliminate the micro-pitting and intergranular corrosion that typically leads to tube failure and subsequent condensate leakage in HVAC installations.
Material & Properties
Base Alloy Composition
The coil tube is fabricated from austenitic stainless steel, conforming to the following grade designations:
- ASTM A213 TP304 — General-purpose corrosion-resistant grade with excellent formability and weldability
- ASTM A213 TP316 — Enhanced molybdenum-bearing grade for superior resistance to pitting and crevice corrosion in chloride-containing environments
- ASTM A213 TP316L — Low-carbon variant of TP316, optimized for welding applications where sensitization resistance is critical
Chemical Composition (Typical — TP316L)
| Element | Specification (%) | Function |
|---|---|---|
| Carbon (C) | ≤ 0.030 | Low carbon content prevents chromium carbide precipitation at weld zones |
| Chromium (Cr) | 16.00 – 18.00 | Primary corrosion-resistant element; forms passive oxide layer |
| Nickel (Ni) | 10.00 – 14.00 | Stabilizes austenitic microstructure; enhances ductility |
| Molybdenum (Mo) | 2.00 – 3.00 | Improves pitting and crevice corrosion resistance |
| Iron (Fe) | Balance | Base matrix element |
| Silicon (Si) | ≤ 1.00 | Deoxidizer; minor contribution to high-temperature strength |
| Sulfur (S) | ≤ 0.030 | Limited to prevent hot-shortness during forming |
Mechanical Properties
| Property | Value | Test Standard |
|---|---|---|
| Tensile Strength (UTS) | ≥ 515 MPa (74,700 psi) | ASTM A213 |
| Yield Strength (0.2% offset) | ≥ 205 MPa (29,700 psi) | ASTM A213 |
| Elongation (in 50 mm / 2 in.) | ≥ 40% | ASTM A213 |
| Hardness (Rockwell B) | ≤ 95 HRB | ASTM E140 |
| Thermal Conductivity | 16.3 W/(m·K) at 20°C | — |
| Coefficient of Thermal Expansion | 16.0 × 10⁻⁶ /°C (20–300°C) | — |
| Operating Temperature Range | −196°C to +870°C | — |
Specifications
| Parameter | Specification | Standard / Tolerance |
|---|---|---|
| Outer Diameter (OD) | 6.35 mm (1/4") – 12.70 mm (1/2") | ±0.05 mm |
| Wall Thickness | 0.50 mm – 1.00 mm | ±0.03 mm |
| Coil Diameter (Pitch) | 12 mm – 25 mm (configurable) | ±0.5 mm |
| Tube Length (Coiled) | Up to 100 m continuous coil | Custom order available |
| Surface Finish | Pickled & Passivated (2B / No. 4) | Ra ≤ 0.8 μm |
| Heat Treatment | Solution Annealed (1050–1150°C) | ASTM A213 |
| Pressure Test | Hydrostatic at 2.5× working pressure | ASTM A213 |
| End Condition | Plain / Beveled / Threaded (BSP / NPT) | Per customer specification |
| Delivery Form | Coiled (on spool or rack) | ISO 2230 packaging |
| Standard Length Options | 3 m, 6 m, 12 m (straight cut); Coiled | ±10 mm |
Manufacturing Process
The production of the stainless steel coil tube follows a rigorously controlled multi-stage manufacturing sequence to ensure dimensional accuracy, metallurgical integrity, and surface quality:
- Coil Uncoiling & Straightening — Cold-rolled stainless steel strip (slit coil) is uncoiled and fed through a series of straightening rollers to eliminate residual curvature and set-up stress.
- Tube Forming (ERW / Weldless) — The strip is formed into a tubular cross-section via either Electric Resistance Welding (ERW) or seamless pressing methods, depending on the required wall thickness and pressure rating.
- Weld Seaming & Heat-Affected Zone (HAZ) Treatment — For ERW tubes, the weld seam undergoes internal and external grinding to achieve a flush surface, followed by localized heat treatment to restore metallurgical homogeneity in the HAZ.
- Coiling — The formed tube is precisely wound into a spiral coil using automated coiling equipment that maintains consistent pitch diameter and inter-turn spacing, critical for uniform heat exchange performance in FCU applications.
- Solution Annealing — The coiled tube is heated to 1050–1150°C in a protective atmosphere furnace, then rapidly cooled to dissolve any precipitated chromium carbides and restore full austenitic corrosion resistance.
- Pickling & Passivation — The tube surface is treated with a mixed acid solution (HNO₃ + H₂SO₄) to remove scale, oxide, and contaminants, followed by passivation to establish a stable chromium oxide passive film.
- Dimensional Inspection & Non-Destructive Testing (NDT) — Each batch undergoes ultrasonic testing (UT) for wall thickness verification, eddy current testing (ECT) for seam integrity, and hydrostatic pressure testing per ASTM A213.
- End Processing & Packaging — Tube ends are cut, deburred, and optionally threaded or beveled. Coils are secured on wooden spools or steel racks with protective wrapping to prevent mechanical damage and moisture ingress during transit.
Applications
Primary Application: Fan Coil Units (FCU) in Commercial HVAC
The stainless steel coil tube is the core heat-exchange element in fan coil units installed in office buildings, commercial complexes, hospitals, hotels, and residential high-rises. In these systems, the coil tube circulates chilled water (for cooling) or hot water (for heating), transferring thermal energy to or from the air passing over the coil fins via the associated fan assembly.
Key Application Scenarios
- Office Building HVAC Systems — Addressing documented water leakage issues in fan coil units by replacing conventional copper coils with stainless steel alternatives that resist electrochemical corrosion from variable water chemistry.
- Medical & Pharmaceutical Facilities — Where hygiene standards mandate corrosion-resistant materials and resistance to chemical cleaning agents (e.g., quaternary ammonium compounds, hydrogen peroxide-based sanitizers).
- Coastal & Marine Environments — TP316L-grade coil tubes withstand chloride-laden air and condensate, preventing the accelerated pitting corrosion that degrades copper or carbon steel coils in humid coastal climates.
- High-Cycle Commercial Operations — Continuous 24/7 operation in retail, data center, and industrial HVAC installations where tube fatigue and micro-cracking are primary failure modes for lower-grade materials.
- Refrigerant Piping — Low-temperature coil tube applications in refrigeration systems where the material maintains ductility and fracture toughness down to cryogenic temperatures.
Advantages Over Conventional Materials in FCU Applications
| Performance Criterion | Stainless Steel (TP316L) | Copper (Cu-ETP) | Aluminum (1060-H) |
|---|---|---|---|
| Corrosion Resistance (Water) | Excellent | Moderate | Poor |
| Galvanic Compatibility | Passive (no galvanic coupling) | Galvanic with steel components | Galvanic with copper/steel |
| Service Life (HVAC) | 25+ years | 10–15 years | 5–10 years |
| Thermal Conductivity | 16.3 W/(m·K) | 398 W/(m·K) | 237 W/(m·K) |
| Chloride Resistance | Excellent | Poor | Poor |
| Cost (Relative) | High | Medium | Low |
Quality Certifications & Standards Compliance
The stainless steel coil tube products from Wenzhou Jixing Metal Materials Co., Ltd. are manufactured and tested in accordance with the following international and national standards:
Material & Product Standards
- ASTM A213 / A213M — Standard Specification for Seamless and Welded Austenitic Stainless Steel Tubing for High-Pressure and High-Temperature Service
- ASTM A778 / A778M — Standard Specification for Welded Austenitic Stainless Steel Tubing for General Applications
- GB/T 12771 — Chinese National Standard for Stainless Steel Seamless Tubes for General Purposes
- EN 10216-5 — European Standard for Non-Alloy and Alloy Steel Tubes for Pressure Purposes (Stainless Steel)
- JIS G3447 — Japanese Industrial Standard for Austenitic Stainless Steel Tubes for Pressure Vessels and Heat Exchangers
Quality Management & Testing Standards
- ISO 9001:2015 — Quality Management Systems certification for manufacturing processes
- ISO 14001:2015 — Environmental Management Systems certification
- ISO 45001:2018 — Occupational Health and Safety Management Systems
- EN 10204 — Type 3.1 Material Test Certificates provided per shipment
- NDT Compliance — Ultrasonic testing per ASTM E164, Eddy current testing per ASTM E309, Hydrostatic testing per ASTM A213
Third-Party Verification
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