Stainless Steel Coil Tube (Coil Tube) — HVAC Fan Coil Unit Replacement Series
1. Product Overview
The Stainless Steel Coil Tube (hereinafter referred to as "Coil Tube") manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered, continuous-length tubular product formed through cold-rolling and coiling processes. Specifically designed as a high-performance alternative to traditional copper and carbon steel piping in Fan Coil Unit (FCU) systems, this product addresses the growing industry demand for corrosion-resistant, thermally efficient, and environmentally sustainable heat exchanger tubing.
The Coil Tube series eliminates the need for traditional brazed or welded joints commonly found in conventional fan coil unit piping assemblies. By utilizing a continuous coil form, the product ensures hydraulic integrity, minimizes leakage risks, and significantly reduces on-site installation labor. This innovation represents a paradigm shift in HVAC piping solutions, transitioning from segmented assembly to continuous-flow architecture.
2. Material & Properties
The Coil Tube is fabricated from high-purity austenitic stainless steel grades, selected to optimize the balance between corrosion resistance, thermal conductivity, mechanical strength, and cost-effectiveness.
2.1 Primary Alloy Grades
- 304 (UNS S30408) — The standard-grade austenitic stainless steel offering excellent general corrosion resistance and formability. Suitable for non-aggressive HVAC fluid environments.
- 316 (UNS S31608) — Enhanced with 2.0–3.0% Molybdenum (Mo), providing superior resistance to chloride-induced pitting corrosion. Recommended for coastal or chemically aggressive environments.
- 321 (UNS S32108) — Stabilized with Titanium (Ti), offering improved resistance to intergranular corrosion at elevated temperatures above 425°C.
2.2 Key Mechanical & Physical Properties
| Property | Grade 304 | Grade 316 | Test Standard |
|---|---|---|---|
| Tensile Strength (Rm) | ≥ 515 MPa | ≥ 515 MPa | ASTM A269 / EN 10216-5 |
| Yield Strength (Rp0.2) | ≥ 205 MPa | ≥ 205 MPa | ASTM A269 |
| Elongation (A50) | ≥ 40% | ≥ 40% | ASTM A269 |
| Thermal Conductivity (λ, 20°C) | 16.3 W/(m·K) | 14.6 W/(m·K) | ASTM E1530 |
| Specific Heat (Cp, 20°C) | 502 J/(kg·K) | 500 J/(kg·K) | ASTM E1530 |
| Linear Expansion (α, 20–100°C) | 16.8 × 10-6 /°C | 16.6 × 10-6 /°C | ASTM E228 |
| Corrosion Potential (Ecorr, 3.5% NaCl) | −0.25 V vs. SCE | −0.10 V vs. SCE | ASTM G5 |
| Maximum Service Temperature | 870°C | 870°C | ASM Section IX |
2.3 Surface Finish
The inner and outer surfaces are finished to a 2B mill finish (Ra ≤ 0.8 μm) or, upon request, a electropolished finish (Ra ≤ 0.4 μm). The electropolished variant provides enhanced hygienic properties and further reduces scaling tendency in water-carrying applications.
3. Specifications
| Parameter | Specification |
|---|---|
| Outer Diameter (OD) | 9.52 mm – 34.93 mm (3/8" – 1-1/4" nominal) |
| Wall Thickness (WT) | 0.50 mm – 3.00 mm |
| Coil Diameter (CD) | 150 mm – 600 mm (adjustable per application) |
| Coil Length per Spool | 25 m – 100 m (standard); custom lengths available |
| Dimensional Tolerance (OD) | ±0.05 mm (fine tolerance); ±0.10 mm (standard) |
| Dimensional Tolerance (WT) | ±10% of nominal wall thickness |
| Roundness Tolerance | ≤ 0.5% of OD |
| Maximum Operating Pressure | 2.5 MPa (at 80°C, per ASME B31.9) |
| Hydrostatic Test Pressure | 1.5 × Maximum Operating Pressure |
| Operating Temperature Range | −196°C to +425°C (continuous) |
| Standard Length Options | 5 m, 10 m, 15 m, 25 m, 50 m, 100 m (straight sections) |
| Available Grades | 304, 304L, 316, 316L, 321, 310S |
| Surface Treatment | 2B, BA (Bright Annealed), Electropolished, Passivated |
| Packaging | Wooden pallet with moisture-proof wrapping; export-grade containers |
| Minimum Order Quantity (MOQ) | 500 kg per grade |
4. Manufacturing Process
The production of the Coil Tube follows a rigorous, multi-stage manufacturing workflow compliant with international quality management systems. Each stage incorporates in-process inspection to ensure dimensional accuracy and metallurgical integrity.
4.1 Process Flow
- Raw Material Selection & Incoming Inspection: Cold-rolled stainless steel strip (coil form) is procured from certified mills. Each batch undergoes spectroscopic analysis (OES) to verify chemical composition against ASTM/EN standards. Heat treatment certificates (MTC) are reviewed.
- Surface Cleaning & Pickling: The incoming strip is subjected to acid pickling (HNO3/HF mixture) to remove oxide scales, mill scale, and surface contaminants. This ensures a clean substrate for subsequent forming operations.
- Coil Forming & Cold Rolling: The flat strip is precisely formed into the target circular cross-section using a series of sequential roller stations. Each roller station progressively bends the strip, with controlled reduction rates to prevent surface cracking and ensure uniform wall thickness distribution.
- Seam Welding (if applicable): For welded coil tube configurations, the formed tube undergoes TIG (Tungsten Inert Gas) welding using Argon (Ar) shielding gas. The welding parameters are optimized to produce a full-penetration, defect-free seam with minimal heat-affected zone (HAZ).
- Heat Treatment & Annealing: The formed coil tube is solution-annealed at 1010–1150°C in a controlled atmosphere furnace, followed by rapid quenching. This restores full austenitic microstructure, eliminates residual stresses, and maximizes corrosion resistance.
- Passivation: Post-annealing, the tube is passivated using a citric acid or nitric acid solution to form a stable chromium oxide (Cr2O3) passive film on the surface.
- Coiling & Sizing: The continuous tube is wound onto precision-mandrel reels at the target coil diameter. Final sizing ensures roundness and dimensional consistency.
- Final Inspection & Testing: Each spool undergoes hydrostatic pressure testing, dimensional verification (OD, WT, roundness), visual inspection, and optional eddy current or ultrasonic flaw detection.
4.2 Quality Control Milestones
| Inspection Stage | Test Method | Acceptance Criteria |
|---|---|---|
| Chemical Composition | Optical Emission Spectroscopy (OES) | Within ASTM A269 / EN 10216-5 limits |
| Mechanical Properties | Tensile Testing (ASTM A370) | Meets specified grade requirements |
| Hydrostatic Testing | Water pressure at 1.5 × Poperating | No leakage or permanent deformation |
| Dimensional Accuracy | Micrometer & bore gauge measurement | Within ±0.05 mm (OD), ±10% (WT) |
| Surface Quality | Visual + Profilometer (Ra) | No cracks, laps, or inclusions; Ra ≤ 0.8 μm |
| Corrosion Resistance | ASTM A967 Passivity Test + Salt Spray | No red rust; ≥ 500 h (ASTM B117) for 316 |
5. Applications
The Coil Tube series is engineered for a broad spectrum of industrial and commercial applications, with particular emphasis on HVAC systems where it serves as a direct replacement for traditional fan coil unit piping.
5.1 Primary Application Domains
- HVAC Fan Coil Unit Systems: The primary design intent. The Coil Tube replaces conventional copper or carbon steel piping in FCU units, providing superior corrosion resistance against condensate water, glycol-based heat transfer fluids, and ambient moisture. The continuous coil form eliminates brazed joints, reducing failure points by up to 80%.
- Commercial & Residential Building Services: Used in chilled water distribution, condenser water loops, and domestic hot water recirculation systems in multi-story buildings.
- Industrial Process Cooling: Suitable for closed-loop cooling circuits in manufacturing facilities, data centers, and cleanroom environments.
- Food & Pharmaceutical Processing: The electropolished variant meets sanitary requirements (3-A Sanitary Standards) for potable water, beverage processing, and pharmaceutical fluid transfer.
- Marine & Offshore Applications: Grade 316 Coil Tube provides reliable performance in seawater-cooled heat exchangers and bilge water systems.
- Solar Thermal & Geothermal Systems: The wide operating temperature range and resistance to scaling make the Coil Tube suitable for solar water heating collectors and geothermal heat exchange loops.
5.2 Advantages Over Conventional Alternatives
| Performance Criterion | Stainless Steel Coil Tube | Copper Tubing | Carbon Steel Tubing |
|---|---|---|---|
| Corrosion Resistance (Condensate) | Excellent | Good (but susceptible to acid condensate) | Poor (requires lining) |
| Service Life | 30–50+ years | 15–25 years | 10–15 years |
| Joint Leakage Risk | Minimal (continuous coil) | Moderate (brazed joints) | High (welded/flanged joints) | Thermal Conductivity | Moderate (16 W/m·K) | High (390 W/m·K) | High (50 W/m·K) |
| Installation Speed | Fast (pre-formed coil) | Moderate | Slow |
| Recyclability | 100% recyclable | 100% recyclable | 100% recyclable |
| Cost per Meter (Relative) | Moderate | High | Low |
6. Quality Certifications & Standards Compliance
Wenzhou Jixing Metal Materials Co., Ltd. maintains a comprehensive quality assurance framework ensuring full traceability and compliance with international standards.
6.1 Management System Certifications
- ISO 9001:2015 — Quality Management System
- ISO 14001:2015 — Environmental Management System
- ISO 45001:2018 — Occupational Health and Safety Management System
- ISO 3834-2 — Quality Requirements for Welding of Metallic Constructions (applicable to welded variants)
6.2 Product Standards Compliance
- ASTM A269/A269M — Standard Specification for Seamless and Welded Austenitic Stainless Steel Tubing for Chemical and Similar Services
- ASTM A312/A312M — Standard Specification for Seamless, Welded, and Heavy-Walled Austenitic Stainless Steel Pipes
- EN 10216-5 — Non-Alloy and Alloy