Stainless Steel Coil Tube for Large Temperature Difference Chilled Water Systems — Fan Coil Unit Heat Exchanger
1. Product Overview
The Stainless Steel Coil Tube (Coiltube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered heat exchange component designed specifically for large temperature difference (LTD) chilled water systems employing Fan Coil Units (FCUs). This product addresses the critical challenge of maintaining efficient thermal transfer under high differential temperature conditions between supply water (进水) and return water (回水), where conventional carbon steel or copper tubing often suffers from accelerated corrosion, scaling, and thermal fatigue.
Unlike standard coil tubes, this product is engineered to withstand the thermal cycling and chemical stress inherent in LTD systems, where supply water temperatures may drop as low as 4–7°C while return water temperatures range from 15–22°C. The stainless steel construction provides superior resistance to electrochemical corrosion, pitting, and crevice corrosion caused by dissolved oxygen and chlorides present in circulating water.
2. Material & Properties
2.1 Base Material
- Primary Alloy: Austenitic Stainless Steel — Grade 304 (UNS S30400) and Grade 316L (UNS S31603) available upon specification
- Alternative Grades: 321 (UNS S32100) for elevated temperature applications; 316Ti for enhanced chloride resistance
- Standard Compliance: ASTM A213/A213M, GB/T 14976, JIS G3459, EN 10216-5
2.2 Mechanical Properties (Grade 304 — Typical)
- Tensile Strength: ≥ 515 MPa (75,000 psi)
- Yield Strength: ≥ 205 MPa (29,700 psi)
- Elongation: ≥ 40% (in 50 mm gauge length)
- Hardness: ≤ 220 HB (Brinell)
- Density: 7.93 g/cm³
- Thermal Conductivity: 16.3 W/(m·K) at 100°C
- Specific Heat Capacity: 0.50 J/(g·K) at 100°C
- Linear Thermal Expansion: 17.3 × 10⁻⁶ /K (20–100°C)
2.3 Chemical Composition (Grade 304 — Typical)
- Carbon (C): ≤ 0.08%
- Chromium (Cr): 18.0–20.0%
- Nickel (Ni): 8.0–10.5%
- Manganese (Mn): ≤ 2.0%
- Silicon (Si): ≤ 1.0%
- Sulfur (S): ≤ 0.03%
- Phosphorus (P): ≤ 0.045%
2.4 Corrosion Resistance Characteristics
- Excellent resistance to atmospheric, industrial, and marine environments
- Outstanding performance in deionized water, glycol-based heat transfer fluids, and treated circulating water
- Pitting Resistance Number (PREN): 19–20 (Grade 304); 24–26 (Grade 316L)
- Passive film (Cr₂O₃) self-healing capability under oxygenated conditions
3. Technical Specifications
| Parameter | Specification | Standard / Tolerance |
|---|---|---|
| Outer Diameter (OD) | 9.52 mm (⅜"), 12.70 mm (½"), 15.88 mm (⅝"), 19.05 mm (¾") | ASTM A213, ±0.10 mm |
| Wall Thickness | 0.50 mm – 1.50 mm (customizable) | ASTM A213, ±0.05 mm |
| Bend Radius (R) | 1.5D – 3.0D (D = OD) | GB/T 14976 |
| Coil Diameter | 150 mm – 600 mm (customizable) | ±2% of nominal |
| Tube Length per Coil | 1.0 m – 15.0 m | ±50 mm |
| Surface Finish | Electropolished (Ra ≤ 0.4 μm) or Brushed (Ra ≤ 0.8 μm) | ASTM A967 |
| Operating Temperature | −40°C to +150°C (304); −40°C to +200°C (316L) | Design condition |
| Operating Pressure | ≤ 1.6 MPa (design); ≤ 2.4 MPa (burst, minimum) | GB/T 14976 |
| Temperature Differential (ΔT) | Up to 20°C (supply–return) | LTD system design |
| End Connection | Straight-cut, flared, threaded (G/NPT), or welded (TIG) | Per customer specification |
| Minimum Order Quantity | 500 kg per grade | — |
4. Manufacturing Process
4.1 Raw Material Preparation
High-purity stainless steel strip or seamless tube stock is procured from certified mills (e.g., Outokumpu, POSCO, or domestic GB-grade suppliers). Incoming material undergoes spectroscopic verification (PMI — Positive Material Identification) to confirm chemical composition compliance.
4.2 Cold Forming & Coiling
- Hydroforming or Roll Coiling: Seamless tubes are cold-formed into precise helical or serpentine coil geometries using CNC-controlled coiling machines. Cold working induces beneficial strain hardening (work hardening), increasing yield strength by 15–25% while maintaining ductility.
- Springback Compensation: Advanced CNC programming accounts for elastic springback to achieve dimensional accuracy within ±0.5% of nominal geometry.
4.3 Heat Treatment (Solution Annealing)
Post-forming solution annealing is performed at 1050–1100°C followed by rapid quenching to restore full austenitic structure, eliminate residual stresses, and maximize corrosion resistance. This step is critical for maintaining the passive chromium oxide film integrity after cold working.
4.4 Surface Treatment & Finishing
- Acid Pickling & Passivation: Removal of oxide scale and contamination per ASTM A967 / AMS 2700, followed by citric or nitric acid passivation to enrich the surface in chromium oxide.
- Electropolishing (Optional): Electrochemical dissolution removes micro-roughness, achieving surface roughness Ra ≤ 0.4 μm, which minimizes fouling and biofilm adhesion in water-carrying applications.
- Internal Cleaning: Ultrasonic or mechanical brush cleaning ensures freedom from particulate contamination per ASME BPE (Bioprocessing Equipment) standards.
4.5 Quality Inspection
- Dimensional Inspection: Coordinate Measuring Machine (CMM) and laser scanning for coil geometry verification
- Pressure Testing: Hydrostatic test at 1.5× design pressure, held for 30 seconds with zero leakage
- Visual & Surface Inspection: 100% visual examination; dye penetrant testing (PT) or magnetic particle testing (MT) on welds per ASTM E709
- Corrosion Testing: Salt spray test (ASTM B117, 480 hours) and intergranular corrosion test (ASTM A262 Practice E) per batch
5. Applications
5.1 Primary Application Domain
This coil tube is purpose-designed for Large Temperature Difference (LTD) Chilled Water Systems in commercial and industrial HVAC installations. In such systems, the Fan Coil Unit (FCU) operates with a significantly higher ΔT between supply and return water compared to conventional systems (typically 5–7°C ΔT vs. 10–20°C ΔT in LTD systems). This enables:
- Reduced pumping power consumption (hydraulic power scales with flow rate cubed)
- Smaller pipe diameters and reduced installation space
- Enhanced thermal efficiency through optimized heat exchanger performance
5.2 Specific Application Scenarios
- Commercial Buildings: Office towers, hotels, shopping malls with centralized chilled water plants
- Industrial Facilities: Cleanroom HVAC, pharmaceutical manufacturing, semiconductor fabrication
- Food & Beverage Processing: Cold storage systems, fermentation temperature control
- Medical Facilities: Hospital operating rooms, MRI rooms requiring precise temperature and humidity control
- Data Centers: Precision cooling for server racks with high heat density
5.3 System Integration
The coil tube integrates into FCU heat exchangers as the water-side heat transfer element, typically paired with aluminum fins (fin-to-tube heat exchanger configuration) or used in plate-and-fin assemblies. The stainless steel construction ensures long-term reliability in closed-loop hydronic systems where water chemistry may vary.
6. Quality Certifications & Compliance
| Certification / Standard | Description |
|---|---|
| ISO 9001:2015 | Quality Management System — certified for manufacturing and supply chain |
| ISO 14001:2015 | Environmental Management System |
| CE Marking | Conformity with European Machinery Directive 2006/42/EC and Pressure Equipment Directive 2014/68/EU (Category I/II) |
| TÜV Inspection | Third-party quality assurance inspection report available per batch |
| MTC (Mill Test Certificate) | EN 10204 Type 3.1 material certification with full chemical and mechanical test data |
| GB/T 14976 | Chinese national standard for seamless stainless steel tubes |
| ASTM A213/A213M | American standard for seamless austenitic stainless steel boiler, heat exchanger, and heater tubes |
| ASME BPE | Bioprocessing Equipment standard for surface finish and cleanliness (optional) |
| RoHS / REACH | Compliance with EU Restriction of Hazardous Substances and Registration, Evaluation, Authorization of Chemicals directives |
7. Unique Selling Points
- Superior LTD System Compatibility: Engineered specifically for large temperature differential operation, with thermal fatigue resistance validated through cyclic testing (≥ 10,000 cycles without degradation)
- Extended Service Life: Stainless steel construction eliminates the corrosion-related failures common in carbon steel and copper coil tubes, achieving design life of 20–25 years in closed-loop systems
- Hygienic Design: Electropolished interior surface (Ra ≤ 0.4 μm) prevents biofilm formation and ensures sanitary compliance for food-grade and pharmaceutical applications
- Customization Capability: Full customization of coil diameter, tube geometry, end connections, and surface finish to match specific FCU design requirements
- Cost-Effective Lifecycle: While initial material cost is higher than carbon steel, the elimination of replacement cycles, reduced maintenance, and lower system downtime yield superior total cost of ownership (TCO)
- Traceability: Complete batch traceability from mill to finished product with digital material passports and digital twin documentation available
8. Technical Notes & Recommendations
Design Consideration: In LTD chilled water systems with ΔT > 12°C, the thermal expansion differential between the stainless steel coil tube and surrounding copper or aluminum components must be accommodated through flexible connections or expansion joints to prevent mechanical stress failure.
Water Quality: While stainless steel offers excellent corrosion resistance, the system water should be maintained at pH 7.0–9.0 with dissolved oxygen < 10 ppm and chloride concentration < 200 ppm to maximize passive film stability.
Welding: Field welding of stainless steel coil tube connections should use TIG (GTAW) with argon gas shielding (99.99% Ar) and ER308L or ER316L filler wire to maintain alloy integrity and prevent sensitization.