Stainless Steel Coil Tube (Coiltube) — Fan Coil Unit Application Grade
Product Overview
The Stainless Steel Coil Tube (Coiltube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered continuous coil tubing product designed specifically for use in Fan Coil Units (FCUs) and air handling systems. This product is engineered to withstand the demanding conditions of both wet and dry operating modes within HVAC (Heating, Ventilation, and Air Conditioning) systems. The coil tube is formed into a serpentine or helical configuration that maximizes heat transfer surface area while minimizing pressure drop, making it an ideal heat exchanger element for commercial and industrial ventilation applications.
Based on extensive research into dry-operation performance of fan coil units, this product is optimized to maintain structural integrity and thermal efficiency under low-humidity, condensation-free conditions, as well as traditional wet-bulb operation scenarios. The continuous coil form factor eliminates the need for field welding, reducing installation complexity and improving long-term reliability.
Material & Properties
Material Composition
The coil tube is fabricated from high-grade austenitic stainless steel, primarily utilizing the following grades:
- AISI 304 (UNS S30408) — General-purpose corrosion-resistant grade with excellent formability
- AISI 316 (UNS S31608) — Enhanced corrosion resistance with molybdenum addition for aggressive environments
- AISI 321 (UNS S32108) — Titanium-stabilized grade for elevated-temperature applications
Mechanical Properties
- Tensile Strength (UTS): 515–720 MPa (depending on grade and condition)
- Yield Strength (0.2% offset): ≥ 205 MPa
- Elongation: ≥ 40% (annealed condition)
- Hardness: HRB 70–95 (annealed)
Thermal Properties
- Thermal Conductivity: 16.3 W/(m·K) at 20°C (AISI 304)
- Specific Heat Capacity: 500 J/(kg·K) at 20°C
- Thermal Expansion Coefficient: 17.3 × 10⁻⁶ /°C (20–100°C)
- Maximum Operating Temperature: 870°C (continuous), 1150°C (intermittent)
Corrosion Resistance
The austenitic stainless steel composition provides superior resistance to atmospheric corrosion, condensation water attack, and chemical degradation from refrigerants. The passive chromium oxide film (Cr₂O₃) self-heals under normal oxidizing conditions, ensuring service life exceeding 25 years in typical HVAC environments.
Specifications
| Parameter | Specification |
|---|---|
| Outer Diameter (OD) | 6.35 mm / 7.94 mm / 9.53 mm / 12.70 mm (¼" / ⅜" / ⅜" / ½") |
| Wall Thickness | 0.50 mm / 0.65 mm / 0.80 mm / 1.00 mm |
| Bend Radius (Minimum) | ≥ 3× OD |
| Coil Length (Per Roll) | 50 m – 200 m (configurable) |
| Coil Bundle Height | 150 mm – 600 mm |
| Number of Rows | 2 – 8 rows | Row Spacing (Pitch) | 19.05 mm – 25.40 mm (¾" – 1") |
| Tube Pitch (Lateral) | 12.70 mm – 19.05 mm (½" – ¾") |
| Surface Treatment | Polished (Ra ≤ 0.4 μm) / Passivated / Brushed |
| Heat Transfer Medium | Water / Glycol-Water Solution / Refrigerant (R-134a, R-410A) |
| Design Pressure | 1.0 – 2.5 MPa (16 – 40 bar) |
| Operating Temperature Range | −40°C to +150°C |
| Standard Reference | GB/T 14976, ASTM A269/A270, EN 10217 |
Manufacturing Process
1. Raw Material Preparation
High-purity stainless steel strip (cold-rolled and annealed) is sourced from certified mill producers. Incoming material undergoes spectroscopic verification (OES — Optical Emission Spectroscopy) to confirm chemical composition compliance with ASTM and GB standards.
2. Precision Coil Forming
The flat strip is fed through a multi-stage progressive die forming system. Rollers with matched radii incrementally bend the strip into the desired circular cross-section. The forming process is performed in a controlled environment to prevent work hardening and maintain dimensional accuracy within ±0.05 mm tolerance.
3. Seam Welding (where applicable)
For welded coil tubes, the seam is closed using high-frequency induction welding (HFIR — High-Frequency Induction Resistance welding) at a weld speed of 2–8 m/min. The weld zone is subsequently annealed in a protective nitrogen atmosphere to restore ductility and prevent intergranular corrosion.
4. Heat Exchanger Bundle Assembly
Individual coil tubes are mounted onto stamped sheet metal headers (U-bends or return headers) using precision punch-press forming. The coil bundle is arranged in staggered or in-line configurations to optimize airflow distribution and heat transfer coefficient.
5. Surface Finishing & Passivation
The completed coil tube assembly undergoes:
- Mechanical polishing or chemical polishing to achieve Ra ≤ 0.4 μm surface roughness
- Acid pickling and passivation (nitric acid-based, 5–10% concentration) to remove scale and activate the passive film
- Final rinse with deionized water and drying under forced warm air
6. Quality Inspection
- Dimensional verification via laser measurement systems
- Helium leak testing at 0.1 MPa differential pressure (leak rate < 1×10⁻⁶ mbar·L/s)
- Hydrostatic pressure testing at 1.5× design pressure for 30 minutes minimum
- Visual inspection under 10× magnification for surface defects
- Corrosion testing per ASTM G48 (pitting resistance) for 316 grade
Applications
Primary Applications
- Fan Coil Units (FCUs): Central air conditioning systems in commercial buildings, hotels, hospitals, and office complexes
- Air Handling Units (AHUs): Heating and cooling coils in large-scale ventilation systems
- Chilled Water Systems: Terminal cooling in district cooling networks
- Heat Recovery Units: Energy-efficient heat exchangers in ventilation systems
Dry-Operation Optimization
Research into the dry-operation performance of fan coil units has demonstrated that stainless steel coil tubes maintain superior thermal performance when the evaporating temperature exceeds the dew point of incoming air. Key advantages in dry-operation mode include:
- Elimination of condensate-related corrosion and microbial growth (Legionella risk reduction)
- Reduced fan power consumption due to lower coil surface fouling
- Improved indoor air quality (IAQ) through absence of moisture transfer
- Enhanced dehumidification control via separate latent and sensible heat management
Specialized Applications
- Pharmaceutical cleanroom HVAC systems (GMP-compliant environments)
- Food processing and dairy industry refrigeration
- Marine and offshore HVAC (AISI 316 grade for saltwater resistance)
- Semi-conductor manufacturing facility temperature control
Quality Certifications & Standards Compliance
Product Certifications
- ISO 9001:2015 — Quality Management System
- ISO 14001:2015 — Environmental Management System
- ISO 45001:2018 — Occupational Health and Safety
- CE Marking — European Conformity (EN 10217-7)
- ASME B31.9 — Pressure Piping, Refrigeration, Air-Conditioning
Material Certifications
- Mill Test Reports (MTR) — EN 10204 Type 3.1 for each batch
- EN 10204 — Delivery Documents for Products
- GB/T 4237 — Chinese national standard for stainless steel plate/strip
- NORSOK M-640 — For offshore applications (316L grade)
Performance Certifications
- AHRI Standard 340/360 — Performance rating for air-conditioning equipment
- EN 14511 — Water-to-air heat exchangers for ventilation and air conditioning
- ASHRAE 90.1 — Energy efficiency compliance
Unique Selling Points
- Full-Length Continuous Coil Construction: Eliminates field joints and potential leak points, providing factory-sealed reliability with zero on-site welding requirements
- Dual-Mode Optimization: Engineered for both wet-bulb and dry-operation conditions, offering flexibility in HVAC system design and seasonal adaptation
- Superior Surface Finish: Mirror-polished interior (Ra ≤ 0.4 μm) reduces flow resistance and prevents scale deposition, maintaining heat transfer efficiency over the product lifecycle
- Customizable Geometry: Variable row count, pitch, and bundle dimensions allow precise thermal capacity matching to specific design loads
- Extended Service Life: 25+ year expected lifespan in HVAC applications with no replacement required, supported by inherent corrosion resistance of austenitic stainless steel
- Environmentally Sustainable: 100% recyclable material, zero CFC contribution, and reduced maintenance chemicals compared to copper-tube alternatives
- Lightweight Construction: 30% lighter than equivalent copper coil assemblies, reducing structural load on mounting brackets and building infrastructure
Technical Notes on Dry-Operation Performance
Studies on fan coil unit dry-operation conditions (风机盘管干工况运行) have revealed that when the coil surface temperature remains above the dew point of the supply air, latent heat transfer is eliminated, and the unit operates purely on sensible heat exchange. The stainless steel coil tube, with its high thermal conductivity and smooth surface finish, maintains a sensible heat ratio (SHR) exceeding 0.95 under dry conditions. This characteristic is particularly advantageous in climates with high relative humidity, where conventional wet-coil operation leads to excessive dehumidification and subsequent reheat energy consumption. Wenzhou Jixing Metal Materials Co., Ltd. leverages this research to engineer coil geometries that optimize both modes of operation, providing system designers with a versatile heat exchange solution.
Supplier Information
Wenzhou Jixing Metal Materials Co., Ltd. is a specialized manufacturer of stainless steel coil tubes and heat exchanger components based in Wenzhou, Zhejiang Province, China. The company maintains in-house capabilities for coil forming, welding, passivation, and precision assembly, supported by a dedicated R&D team focused on HVAC thermal performance optimization.