Stainless Steel Coil Tube for Fan Coil Unit Heat Exchange Systems
1. Product Overview
The Stainless Steel Coil Tube (Product No. 1295) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered coiled tubing product designed for high-performance heat exchange applications, particularly within Fan Coil Units (FCU) and hydronic air conditioning systems. This product represents a critical component in the thermal management architecture of modern building climate control infrastructure.
The coil tube serves as the primary heat transfer medium in fan coil systems, where hot or chilled water circulates through the coiled stainless steel tubing to exchange thermal energy with ambient air. The design and material properties of the coil tube directly influence the supply and return water temperature difference (ΔT) — a key performance indicator in HVAC system efficiency. By optimizing the thermal conductivity, wall thickness, and coil geometry of the stainless steel tube, engineers can achieve enhanced heat transfer rates, reduced pumping energy consumption, and improved overall system thermodynamic efficiency.
Wenzhou Jixing Metal Materials Co., Ltd. leverages advanced metallurgical processing and precision forming technologies to produce coil tubes that meet the rigorous demands of commercial and industrial HVAC applications worldwide.
2. Material & Properties
2.1 Base Material
The coil tubes are fabricated from austenitic stainless steel conforming to recognized international standards. The primary grade utilized is:
- SUS304 / AISI 304 / EN 1.4301 — General-purpose austenitic stainless steel with excellent corrosion resistance and formability
- SUS316 / AISI 316 / EN 1.4401 — Molybdenum-enhanced grade offering superior resistance to chlorides and aggressive environments
2.2 Mechanical Properties
| Property | Specification (SUS304) | Specification (SUS316) | Test Standard |
|---|---|---|---|
| Tensile Strength (Rm) | ≥ 520 MPa | ≥ 515 MPa | ASTM A270 / EN 10217 |
| Yield Strength (Rp0.2) | ≥ 205 MPa | ≥ 205 MPa | ASTM A270 / EN 10217 |
| Elongation (A50mm) | ≥ 35% | ≥ 35% | ASTM A270 |
| Hardness (HBW) | ≤ 223 | ≤ 223 | ASTM E10 |
2.3 Thermal Properties
| Property | Value | Relevance to FCU Application |
|---|---|---|
| Thermal Conductivity (λ) | 14.6 – 16.3 W/(m·K) | Directly affects heat transfer coefficient (U-value) of the coil bundle |
| Specific Heat Capacity (Cp) | 500 – 540 J/(kg·K) | Influences thermal response time during load cycling |
| Linear Thermal Expansion (α) | 16.0 – 17.3 × 10⁻⁶ /K | Critical for dimensional stability across operating temperature range (5°C to 90°C) |
2.4 Corrosion Resistance
- Pitting Corrosion Resistance: SUS316 grade achieves PREN (Pitting Resistance Equivalent Number) ≥ 24, suitable for coastal and chemically aggressive environments
- Intergranular Corrosion: Carbon content controlled ≤ 0.08% (304L equivalent) to prevent chromium carbide precipitation during welding
- Crevice Corrosion: Resistant at chloride concentrations up to 500 ppm at temperatures below 60°C
3. Technical Specifications
| Parameter | Specification | Tolerance | Standard Reference |
|---|---|---|---|
| Outer Diameter (OD) | 6.0 / 8.0 / 9.5 / 12.0 mm | ±0.10 mm | ASTM A270 / EN 10217-7 |
| Wall Thickness (WT) | 0.50 / 0.60 / 0.70 / 0.80 / 1.00 mm | ±0.05 mm | ASTM A270 |
| Coil Winding Diameter | 150 – 600 mm (customizable) | ±5 mm | ISO 13071 |
| Coil Length (per bundle) | Up to 50,000 mm continuous | ±100 mm | — |
| Surface Finish | 2B (Cold Rolled & Annealed) / BA (Bright Annealed) | Ra ≤ 0.4 μm | ASTM A403 |
| Surface Roughness (ID) | Ra ≤ 0.8 μm | — | ISO 4287 |
| Maximum Operating Temperature | 800°C (SUS304) / 900°C (SUS316) | — | — |
| Minimum Operating Temperature | -196°C (cryogenic capable) | — | — |
| Maximum Operating Pressure | 1.6 – 4.0 MPa (depending on WT) | — | EN 10217-7 |
| Water Flow Velocity (recommended) | 1.0 – 2.5 m/s | — | ASHRAE 90.1 |
| Recommended ΔT (Supply – Return) | 5 – 10 K (optimized for enhanced heat transfer) | — | ASHRAE Handbook – HVAC Systems & Equipment |
4. Manufacturing Process
4.1 Process Flow
- Raw Material Procurement: Hot-rolled stainless steel strip (coil form) sourced from certified mill producers (e.g., POSCO, Outokumpu, Taigang), verified via mill test certificates (MTC) per EN 10204 Type 3.1
- Decoiling & Straightening: Strip is decoiled and fed through a leveling mill to achieve flatness tolerance ≤ 0.1% of width
- Slitting: Precision slitting to target coil tube blank dimensions with edge quality inspection
- Forming (Tube Making): Multi-roll progressive forming converts flat strip into seamless round tubes via the ERW-free seamless forming process (roll-forming method), ensuring no weld seam weakness
- Coil Winding: Formed tubes are wound onto precision mandrels to achieve target coil diameter with controlled inter-pass spacing (typically 1.2 – 1.5× OD)
- Heat Treatment (Annealing): Solution annealing at 1050 – 1100°C followed by controlled cooling to relieve forming stresses and restore full austenitic microstructure
- Pickling & Passivation: Immersion in nitric acid/hydrofluoric acid blend to remove oxide scale, followed by passivation to form protective chromium oxide film
- Dimensional Inspection: CMM (Coordinate Measuring Machine) verification of OD, WT, coil diameter, and straightness
- Pressure Testing: Hydrostatic pressure test at 1.5× maximum working pressure for a minimum dwell time of 60 seconds
- Final Packaging: Protective wrapping, moisture-barrier packaging, and labeling per ISO 128 for shipment
4.2 Key Process Advantages
- Seamless Forming: The roll-forming process eliminates welded seams, eliminating potential leak paths and stress concentration points — critical for long-term reliability in hydronic systems
- Precision Dimensional Control: ±0.05 mm wall thickness tolerance ensures uniform flow distribution across the coil bundle, preventing channeling and optimizing the heat transfer surface area utilization
- Controlled Surface Finish: BA (Bright Annealed) finish reduces fluid friction losses and minimizes fouling propensity, maintaining the designed ΔT performance over the system's operational lifetime
5. Applications
5.1 Primary Applications
- Fan Coil Units (FCU): Core heat exchange element in both cooling and heating modes for commercial HVAC systems
- Chilled Water Coil Systems: Integration with central plant cooling towers and chiller systems
- Underfloor Heating Systems: Low-pressure hydronic radiant heating networks
- Industrial Process Cooling: Precision temperature control in manufacturing environments
5.2 Performance Optimization — Enhanced ΔT Strategy
A critical engineering consideration in fan coil system design is the optimization of the supply and return water temperature difference (ΔT). Research and field experience indicate that increasing ΔT from the conventional 5 K to 8–10 K yields significant system-level benefits:
- Reduced Water Flow Rate: For a fixed thermal load (Q), increasing ΔT proportionally reduces the required volumetric flow rate (Q = ṁ × Cp × ΔT), decreasing pumping energy by 30–50%
- Smaller Pipe Sizing: Reduced flow velocities permit smaller distribution piping, lowering material costs and installation complexity
- Enhanced Heat Transfer Coefficient: Stainless steel coil tubes with optimized wall thickness and surface geometry maintain effective heat transfer even at reduced flow rates, due to the high thermal conductivity of austenitic stainless steel
- Improved Part-Load Performance: Wider ΔT provides greater design flexibility during part-load conditions, reducing cycling losses and improving COP (Coefficient of Performance)
Wenzhou Jixing's coil tube specifications are engineered to support high-ΔT operating strategies without degradation of heat transfer efficiency, making them particularly suitable for energy-efficient HVAC system designs aligned with ASHRAE 90.1 and LEED sustainability standards.
5.3 Industry Sectors
- Commercial Office Buildings and Retail Spaces
- Healthcare Facilities (hospitals, laboratories, clean rooms)
- Hospitality Industry (hotels, resorts)
- Education and Public Assembly Buildings
- Light Industrial and Data Center Cooling
6. Quality Certifications & Standards Compliance
6.1 Product Certifications
| Certification / Standard | Scope | Applicability |
|---|---|---|
| ISO 9001:2015 | Quality Management System | Full manufacturing process |
| ISO 14001:2015 | Environmental Management System | Production facility |
| EN 10217-7 | Welded and seamless stainless steel tubes for pressure purposes | Product dimensional and mechanical requirements |
| ASTM A270 | Standard Specification for Seamless and Wrought Stainless Steel Tube | Material and product verification |
| EN 10204 Type 3.1 | Test report based on inspection results | Mill Test Certificate (MTC) documentation |
| CE Marking (PED 2014/68/EU) | Pressure Equipment Directive | Systems operating above 0.5 bar |
| NSF/ANSI 61 | Drinking Water System Components | Where coil tubes are in contact with potable water |
6.2 Quality Assurance Measures
- Incoming Material Inspection: Spectroscopic analysis (OES) of every incoming coil to verify chemical composition against ASTM/EN specifications
- In-Process Monitoring: Real-time dimensional monitoring during forming with automated rejection of out-of-tolerance product
- Non-Destructive Testing (NDT): Eddy current testing (ECT) for surface and near-surface defect detection; dye penetrant testing (DPT) for critical joints
- Hydrostatic Pressure Testing: 100% batch testing at 1.5× maximum working pressure with automated logging
- Traceability: Full batch traceability from raw material heat number through to finished product shipment documentation
7. Unique Selling Points
Wenzhou Jixing Metal Materials Co., Ltd. distinguishes its stainless steel coil tube product through the following competitive advantages:
- High-ΔT Optimized Design: Coil tube geometry and wall thickness profiles are specifically engineered to maintain high heat transfer efficiency at elevated supply-return temperature differences (8–10 K), delivering superior energy savings compared to conventional coil designs
- Seamless Roll-Formed Construction: Elimination of longitudinal weld seams ensures zero leak risk and eliminates stress-corrosion cracking susceptibility at the weld zone — a common failure mode in welded stainless steel tubing
- Extended Service Life: Superior corrosion resistance (SUS316 option) and controlled surface finish minimize fouling and scaling, extending maintenance intervals by 2–3× compared to carbon steel alternatives
- Precision Dimensional Consistency: Tight manufacturing tolerances (±0.05 mm WT, ±0.10 mm OD) ensure uniform flow distribution and predictable thermal performance across the entire coil bundle
- Full Traceability & Documentation: Complete EN 1