Stainless Steel Coil Tube — Fan Coil Unit (FCU) Networked Energy-Saving Application
Wenzhou Jixing Metal Materials Co., Ltd. (Product No. 850) manufactures high-performance stainless steel coil tubes specifically engineered for integration into Fan Coil Unit (FCU) networked energy-saving control systems. These coil tubes serve as critical heat exchange conduits within HVAC (Heating, Ventilation, and Air Conditioning) distribution networks, delivering superior thermal conductivity, corrosion resistance, and long-term structural integrity under continuous cyclic operating conditions.
Product Overview
The stainless steel coil tube is a continuous-length tubing product formed into a coil configuration for ease of handling, storage, and on-site installation. Unlike seamless or welded straight pipe sections, coil tubes are produced through cold drawing or cold rolling processes that yield uniform wall thickness, precise dimensional tolerances, and enhanced mechanical properties due to work hardening. The coil form factor eliminates the need for field welding or mechanical joining, significantly reducing installation labor and minimizing potential leak points in hydronic distribution loops.
This product is specifically designed for use in networked fan coil unit systems where centralized energy management controllers regulate flow rates, supply temperatures, and zone-level thermal loads in real time. The coil tube's metallurgical consistency and fatigue resistance make it particularly suitable for the repeated thermal expansion and contraction cycles inherent in modulating HVAC operation.
Material & Properties
Base Alloys
- AISI 304 / UNS S30408 — Austenitic stainless steel (18% Cr, 8% Ni); general-purpose grade with excellent weldability and moderate corrosion resistance.
- AISI 316L / UNS S31603 — Low-carbon austenitic grade with 2–3% Molybdenum addition; superior resistance to pitting and crevice corrosion in chlorinated environments.
- AISI 321 / UNS S32108 — Stabilized austenitic grade (Ti-stabilized); recommended for elevated-temperature applications exceeding 450 °C.
Mechanical & Physical Properties (Typical for AISI 304)
| Property | Symbol | Value | Test Method |
|---|---|---|---|
| Tensile Strength | σb | ≥ 515 MPa | ASTM A269 |
| Yield Strength (0.2% offset) | σ0.2 | ≥ 205 MPa | ASTM A269 |
| Elongation | δ5 | ≥ 40% | ASTM A269 |
| Hardness (Brinell) | HB | ≤ 217 | ASTM A269 |
| Thermal Conductivity (at 100 °C) | λ | 16.3 W/(m·K) | ASTM E153 |
| Linear Expansion Coefficient (20–100 °C) | α | 17.3 × 10-6 /K | ASTM E228 |
| Density | ρ | 7.93 g/cm3 | — |
| Maximum Service Temperature | Tmax | 870 °C | ASME B31.3 |
Corrosion Resistance Characteristics
- Passive chromium oxide film (Cr2O3) self-repairs in oxidizing environments, providing inherent resistance to atmospheric, freshwater, and mild industrial corrosion.
- AISI 316L grade demonstrates pitting resistance equivalent number (PREN) of ≥ 24, suitable for coastal and chlorinated water applications.
- Resistance to stress corrosion cracking (SCC) in chloride environments at temperatures up to 60 °C (AISI 316L).
Specifications
| Parameter | Specification | Standard Reference |
|---|---|---|
| Outer Diameter (OD) | 6.0 mm – 50.8 mm (1/4" – 2") | ASTM A269 / ASME SA-269 |
| Wall Thickness (WT) | 0.5 mm – 3.0 mm | ASTM A269 |
| OD Tolerance | ± 0.10 mm (for OD ≤ 12.7 mm); ± 0.15 mm (for OD > 12.7 mm) | ASTM A269 |
| Wall Thickness Tolerance | ± 10% of nominal | ASTM A269 |
| Coil Length | 5 m – 100 m (customizable) | — |
| Surface Finish | No. 4 Brushed / Mirror (Ra ≤ 0.1 μm) / Matte | ASTM A403 |
| End Condition | Plain / Beveled / Threaded (BSP, NPT, G-thread) | — |
| Heat Treatment | Solution Annealed (1050–1150 °C, water quench) | ASTM A269 |
| Hydrostatic Test Pressure | 30 bar (minimum) | ASME B31.3 |
| Minimum Bend Radius | 1.5 × OD (cold bend, no filler) | — |
| Packaging | Wooden coil stand with VCI (Vapor Corrosion Inhibitor) wrap | ISO 11607 |
Manufacturing Process
Production Workflow
- Raw Material Ingot Casting — Electro-slag remelting (ESR) or Vacuum Arc Remelting (VAR) to ensure low inclusion content and homogeneous microstructure.
- Hot Rolling & Annealing — Billet reduction to seamless tube blanks; intermediate annealing to relieve residual stresses.
- Piercing & Elongation — Mannesmann piercing or plug piercing to produce seamless tube billets; multi-stage elongation to achieve target OD/WT ratios.
- Cold Drawing / Cold Rolling — Precision reduction through multi-pass drawing dies to achieve final dimensions within ± 0.10 mm tolerance. Cold working induces beneficial work hardening, increasing yield strength by 40–60% over the annealed condition.
- Intermediate Annealing — Periodic solution annealing (1050–1150 °C, controlled cooling) to restore ductility and prevent cracking during subsequent drawing passes.
- Surface Finishing — Pickling (HNO3/HF mixture) to remove oxide scale; passivation treatment to establish a uniform chromium oxide passive layer (minimum Cr2O3 film thickness of 2–5 nm).
- Coiling — Continuous tube wound onto precision coil stands with controlled coil diameter (typically 500–1200 mm) to prevent kinking and deformation.
- Quality Inspection — Ultrasonic testing (UT) per ASTM E164 for internal defects; eddy current testing (ET) per ASTM E309 for surface discontinuities; hydrostatic testing per ASME B31.3.
- Packaging & Dispatch — VCI-impregnated wrapping, moisture-absorbent desiccant placement, and wooden stand crating for export-grade shipment.
Key Process Parameters
- Reduction ratio per cold drawing pass: 15–25% cross-sectional area reduction (optimal balance of dimensional accuracy and mechanical property enhancement).
- Maximum total cold working reduction: 60–70% from hot-rolled blank to final product.
- Annealing cycle: 1050–1150 °C, hold time 30–60 minutes, water quench or controlled air cooling.
- Surface roughness (Ra): 0.4–0.8 μm (standard); ≤ 0.1 μm (mirror finish).
Applications
Primary Application: Fan Coil Unit (FCU) Networked Energy-Saving Control Systems
The coil tube is specifically engineered for integration into modern building HVAC systems employing networked fan coil unit architectures. In these systems, individual FCU terminals are connected to a centralized Building Management System (BMS) via digital communication protocols (Modbus, BACnet, KNX), enabling real-time optimization of thermal load distribution, flow rate modulation, and energy consumption monitoring.
The stainless steel coil tube serves as the hydronic distribution conduit connecting the central chiller/boiler plant to distributed FCU terminals. Its advantages in this application include:
- Elimination of Field Welding — Coil form factor allows continuous-length installation without joints, reducing leak probability by up to 90% compared to straight pipe with welded connections.
- Thermal Cycling Fatigue Resistance — Austenitic stainless steel retains ductility at both cryogenic and elevated temperatures, accommodating the 5–70 °C supply temperature range typical of FCU systems without risk of brittle fracture.
- Chemical Compatibility — Resistance to glycol-based antifreeze solutions (ethylene glycol, propylene glycol at concentrations up to 50 vol%) used in freeze-protection applications.
- Long Service Life — Design life exceeding 30 years under normal operating conditions, significantly outperforming carbon steel alternatives (10–15 years).
Secondary Applications
- Commercial and residential HVAC hydronic distribution
- Industrial process heating and cooling circuits
- Food and beverage processing (CIP/SIP compatible)
- Pharmaceutical clean-in-place systems
- Domestic hot water recirculation loops
- Geothermal heat exchanger piping
Quality Certifications & Standards Compliance
| Certification / Standard | Scope | Issuing Body |
|---|---|---|
| ISO 9001:2015 | Quality Management System | SGS / Bureau Veritas |
| ISO 14001:2015 | Environmental Management System | SGS |
| ISO 45001:2018 | Occupational Health & Safety Management | Bureau Veritas |
| EN 10217-7 | Stainless Steel Tubes for General Purposes | CEN (European Committee) |
| ASTM A269 / ASME SA-269 | Seamless and Welded Austenitic Stainless Steel Tubes | ASTM International |
| JIS G3446 | Stainless Steel Tubes for General Purposes | Japan Industrial Standards Committee |
| GB/T 12771 | Stainless Steel Tubes for General Purposes (China) | SAC (Standardization Administration of China) |
| CE Marking (PED 2014/68/EU) | Pressure Equipment Directive Compliance | Notified Body (EU) |
| Mill Test Certificates (MTC) | EN 10204 Type 3.1 / Type 3.2 | Independent Third-Party Inspector |
Unique Selling Points
- Seamless Continuity — Zero-joint coil configuration eliminates the primary failure mode (leakage at connections) inherent in traditional pipe systems.
- Superior Thermal Conductivity — Austenitic stainless steel thermal conductivity (16.3 W/m·K) exceeds carbon steel (45 W/m·K at higher temperatures but with inferior corrosion resistance), enabling efficient heat transfer in FCU applications.
- Work-Hardened Mechanical Properties — Cold drawing process imparts 40–60% yield strength enhancement over annealed condition, providing superior resistance to mechanical deformation during installation and operation.
- Full Traceability — Heat-number traceability from ingot to finished coil, with complete chemical and mechanical test documentation per EN 10204 Type 3.2.
- Customization Capability — Flexible OD, WT, coil length, surface finish, and end-condition configurations