Stainless Steel Coil Tube — HVAC Fan Coil Unit Series (Wenzhou Jixing)
Product Code: 1076 | Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd. | Category: Coil Tube / Coiltube for HVAC Applications
1. Product Overview
The Stainless Steel Coil Tube (盘管, Coiltube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered tubular component designed for integration into Fan Coil Units (FCU) and other HVAC (Heating, Ventilation, and Air Conditioning) heat exchanger systems. This product addresses critical performance challenges arising from large cold water temperature differentials (大温差, Large Temperature Difference) in chilled water distribution systems, ensuring consistent thermal efficiency and mechanical integrity under demanding operating conditions.
Coil tubes serve as the core heat transfer medium within fan coil units, facilitating the exchange of thermal energy between circulating refrigerant water and ambient air. The design and material properties of the coil tube directly determine the heat transfer coefficient, pressure drop, corrosion resistance, and long-term service life of the entire system. Wenzhou Jixing's stainless steel coil tube is engineered to mitigate performance degradation caused by thermal cycling and water-side scaling, particularly in systems where inlet chilled water temperatures may reach as low as 4°C (39°F) while outlet temperatures rise to 12°C–18°C (54°F–64°F), creating significant thermal gradients.
2. Material & Properties
2.1 Base Material Composition
The coil tube is fabricated from Stainless Steel Grade 304 (UNS S30400 / AISI 304) and optionally Grade 316 (UNS S31600 / AISI 316), depending on the corrosive environment of the installation. The following table summarizes the key material properties:
| Property | Grade 304 | Grade 316 | Test Standard |
|---|---|---|---|
| Chromium (Cr) | 18.0–20.0% | 16.0–18.0% | ASTM A270 |
| Nickel (Ni) | 8.0–10.5% | 10.0–14.0% | ASTM A270 |
| Molybdenum (Mo) | — | 2.0–3.0% | ASTM A270 |
| Carbon (C) Max | 0.08% | 0.08% | ASTM A270 |
| Tensile Strength (UTS) | 515–730 MPa | 515–730 MPa | ASTM A270 |
| Yield Strength (0.2% Offset) | ≥205 MPa | ≥205 MPa | ASTM A270 |
| Thermal Conductivity (20°C) | 16.3 W/(m·K) | 14.6 W/(m·K) | ASTM E153 |
| Thermal Expansion (20–100°C) | 17.3 × 10⁻⁶ /°C | 17.0 × 10⁻⁶ /°C | ASTM E228 |
| Pitting Resistance (PREN) | 19–20 | 24–26 | ASTM G48 |
2.2 Performance Under Large Cold Water Temperature Difference
Research documented in the study "Effect of Large Cold Water Temperature Difference on Fan Coil Unit Performance Parameters" (《冷水大温差对风机盘管性能参数的影响》) highlights several critical findings relevant to this product:
- Thermal Stress Management: Large temperature differentials (ΔT ≥ 8°C) between inlet and outlet water generate cyclic thermal stresses within the coil tube walls. The elastic modulus of stainless steel (≈193 GPa) combined with its superior fatigue resistance ensures dimensional stability over thousands of thermal cycles.
- Condensation and Corrosion Mitigation: When coil surface temperatures drop below the dew point of ambient air, condensation forms on the tube outer surface. The passive chromium oxide layer (Cr₂O₃) on stainless steel provides inherent resistance to chloride-induced pitting and crevice corrosion, even in humid environments.
- Heat Transfer Stability: Unlike copper-alloy tubes, stainless steel coil tubes exhibit minimal change in thermal conductivity over the operating temperature range of 2°C–25°C, maintaining consistent heat transfer performance regardless of supply water temperature fluctuations.
- Scale Formation Resistance: The smooth, non-porous surface of stainless steel coil tubes reduces mineral scale deposition, which is a common performance-degrading mechanism in large ΔT systems where localized boiling or supersaturation may occur.
3. Technical Specifications
| Parameter | Specification | Standard / Reference |
|---|---|---|
| Tube Outer Diameter (OD) | 9.52 mm (3/8"), 12.70 mm (1/2"), 15.88 mm (5/8") | ASME B36.19M |
| Tube Wall Thickness | 0.50 mm – 1.20 mm (customizable) | ASTM A270 |
| Coil Diameter | Φ200 mm – Φ800 mm (configurable) | Manufacturer's Design |
| Coil Length / Turns | Up to 500 m continuous coil | — |
| Surface Finish | Bright Annealed (BA) / No. 4 Brushed / Mirror Polished | ASTM A480 |
| Operating Temperature Range | −40°C to +200°C (−40°F to +392°F) | — |
| Maximum Operating Pressure | 1.6 MPa (232 psi) standard; up to 2.5 MPa (363 psi) on request | ASME B31.9 |
| Heat Transfer Coefficient (U-value) | 150–250 W/(m²·K) depending on configuration | ASHRAE Handbook—HVAC Components |
| Pressure Drop (Water Side) | ≤ 30 kPa at rated flow rate | ASHRAE 90.1 |
| Welding Type | Argon Arc Welding (TIG / GTAW) | ASME B31.9 |
| Thread Standard (Fittings) | BSPP (G), NPT, or ISO 7-1 | ISO 7 / ASME B1.20.1 |
| Design Service Life | ≥ 25 years | — |
4. Manufacturing Process
4.1 Process Flow
- Raw Material Procurement & Inspection: Stainless steel strip (coil) is sourced from certified mills and subjected to incoming quality control, including spectroscopic analysis (OES) for chemical composition verification and mechanical property testing per ASTM A270.
- Coil Forming: High-precision coiling machines wind the flat stainless steel strip into continuous helical tube shapes. The forming process employs multi-stage rolling with controlled reduction ratios to achieve uniform wall thickness and dimensional accuracy within ±0.05 mm tolerance.
- Longitudinal Welding: The formed strip edges are fused using Gas Tungsten Arc Welding (GTAW/TIG) with argon shielding gas to prevent oxidation. Weld quality is verified through Helium Leak Testing at 10 kPa test pressure with a detection sensitivity of ≤1×10⁻⁶ Pa·m³/s.
- Heat Treatment (Solution Annealing): The welded coil tube is annealed at 1050°C–1100°C followed by rapid water quenching to dissolve intergranular carbides and restore full solution-hardened properties. This step is critical for preventing intergranular corrosion (IGC) and ensuring uniform microstructure.
- Pickling & Passivation: Post-annealing, the tube surface is treated with a nitric acid–hydrofluoric acid (HNO₃/HF) pickling solution to remove oxide scale, followed by passivation with a 20%–30% HNO₃ solution to regenerate the protective chromium oxide passive film.
- Dimensional Inspection & Non-Destructive Testing (NDT): Each batch undergoes ultrasonic testing (UT) for wall thickness uniformity, dimensional gauging for OD and coil diameter, and visual inspection for surface defects. Critical weld seams may be subjected to Penetrant Testing (PT) per ASTM E165.
- Hydrostatic Pressure Testing: Finished coil tubes are pressure-tested at 1.5× the maximum operating pressure (typically 2.4 MPa) for a minimum dwell time of 30 seconds to verify weld integrity and absence of leaks.
- Coil Assembly & Fin Attachment (if applicable): For fan coil unit applications, the coil tube may be assembled with aluminum or stainless steel fins using mechanical clamping or brazing to maximize the heat transfer surface area.
- Final Packaging & Traceability: Each unit is tagged with a unique batch number, material certificate, and test report for full traceability through the supply chain.
4.2 Key Manufacturing Controls
- Coil forming accuracy is maintained within ±0.1 mm for tube OD and ±1% for coil diameter, ensuring uniform flow distribution across the heat exchanger.
- Weld seam quality is controlled to achieve a weld strength ≥95% of the base metal tensile strength, eliminating the weld zone as a fatigue crack initiation site.
- Surface roughness of the coil tube inner wall is maintained at Ra ≤ 0.8 μm to minimize frictional pressure drop and reduce fouling potential.
5. Applications
5.1 Primary Applications
- Fan Coil Units (FCU): The coil tube serves as the water-side heat exchanger element within FCUs used in commercial buildings, hospitals, hotels, and residential HVAC systems. It transfers thermal energy between chilled/hot water and supply air.
- Chilled Water Systems with Large Temperature Differential: Specifically optimized for systems where supply water temperatures may vary significantly (e.g., 4°C supply / 12°C return), the stainless steel construction maintains stable performance without degradation of heat transfer coefficients.
- Industrial Process Cooling: Used in process heat exchangers for food and beverage processing, pharmaceutical manufacturing, and chemical plants where corrosion resistance and sanitary requirements are paramount.
- Marine & Coastal HVAC Systems: Grade 316 coil tubes are recommended for installations in high-salinity coastal environments where chloride-induced corrosion of copper or carbon steel tubes would be unacceptable.
- Underfloor Air Distribution (UFAD) Systems: Coil tubes integrated into underfloor plenum systems for precise zone temperature control in office and institutional buildings.
5.2 Performance Advantages in Large ΔT Applications
Based on the research findings documented in the study referenced by Wenzhou Jixing (Product Code 1076), stainless steel coil tubes demonstrate superior thermal stability under large cold water temperature differentials compared to traditional copper or aluminum coil tubes. Key performance advantages include:
- Up to 15% lower pressure drop at equivalent heat transfer capacity due to smoother internal surfaces and reduced scaling tendency.
- Zero intergranular corrosion after solution annealing, ensuring long-term structural integrity in chloride-containing water systems.
- Thermal cycling endurance exceeding 100,000 cycles without measurable change in heat transfer performance, validated per ASTM E290 thermal fatigue testing.
6. Quality Certifications & Standards Compliance
6.1 Certifications
- ISO 9001:2015 — Quality Management System certification for manufacturing and quality assurance processes.
- ISO 14001:2015 — Environmental Management System certification, ensuring environmentally responsible production practices.
- CE Marking — Compliance with European Economic Area (EEA) health, safety, and environmental protection standards.
- SGS Testing Report — Independent third-party material composition and mechanical property verification.
- EN 10204 3.1 Certificate — Mill test certificates with full chemical analysis and mechanical property data for each production batch.
6.2 Applicable Standards
| Standard | Scope |
|---|---|
| ASTM A270 / A270M | Standard Specification for Seamless and Welded Austenitic Stainless Steel Tubing for Chemical and General Line Piping |
| ASME B31.9 | Pressure Piping—Refrigeration and Air-Conditioning |
| ASME B36.19M | Welded and Seamless Wrought Stainless Steel Pipe |
| ASTM A480 | Standard Specification for Chromium and Chromium-Nickel Stainless Steel Plate, Sheet, and Strip for General Application |
| ASTM G48 | Standard Practices for Conducting Pitting and Crevice Corrosion Resistance Testing on Stainless Steel Alloys |
| ASTM E165 | Standard Practice for Liquid Penetrant Inspection |
| ASHRAE 90.1 | Energy Standard for Buildings Except Low-Rise Residential Buildings |
| GB/T 14976 | Chinese |