Stainless Steel Coil Tube (Coiltube) — HVAC Fan Coil Unit Series

Product Overview

The Stainless Steel Coil Tube (Coiltube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd is a precision-engineered coiled piping product designed primarily for integration into Fan Coil Units (FCU) within Fan Coil Unit plus Fresh Air Systems (FCU+FAS). These systems represent a dominant configuration in modern commercial and residential Heating, Ventilation, and Air Conditioning (HVAC) infrastructure. The coil tube serves as the core heat exchange element, facilitating efficient thermal transfer between the circulating refrigerant or hot water medium and the conditioned air stream. This product line addresses the critical demand for reliable Indoor Relative Humidity Control (IRHC) and thermal comfort management in enclosed spaces.

Material Composition & Physical Properties

The coil tube is fabricated from high-grade austenitic stainless steel, selected for superior corrosion resistance, thermal conductivity, and long-term structural integrity under cyclic thermal loading conditions.

• Base Material: Austenitic stainless steel conforming to ASTM A269 / ASTM A312 or equivalent ISO 3506 specifications
• Primary Alloy Grades: SUS304 (AISI 304, UNS S30408) or SUS316L (AISI 316L, UNS S31603) for enhanced chloride resistance
• Chromium (Cr) Content: 18–20% — provides passivation layer for oxidation resistance
• Nickel (Ni) Content: 8–12% — stabilizes austenitic microstructure
• Molybdenum (Mo) Content: 2–3% (SUS316L grade) — enhances pitting and crevice corrosion resistance
• Carbon (C) Content: ≤ 0.03% (L-grade) — minimizes intergranular corrosion risk during welding
• Thermal Conductivity: 14.9–16.3 W/(m·K) at 20°C
• Density: 8.00 g/cm³
• Operating Temperature Range: −196°C to +870°C
• Surface Finish: Electropolished or passivated inner/outer surface, roughness Ra ≤ 0.4 μm

Technical Specifications

Parameter	Specification	Standard / Reference
Outer Diameter (OD)	Φ6.35 mm / Φ9.53 mm / Φ12.70 mm / Φ15.88 mm	ASTM B751 / EN 10217
Wall Thickness (WT)	0.50 mm – 1.20 mm (tolerance ±10%)	ASTM A269
Coil Diameter	150 mm – 600 mm (configurable per application)	Custom / OEM
Coil Length	Up to 500 m per continuous coil	—
Fin Configuration	Aluminum or copper fins, 8–14 fins per inch (FPI)	ASHRAE 22 / 34
Pressure Rating	Up to 1.6 MPa (gauge) at 25°C	ISO 1160 / EN 12545
Heat Transfer Coefficient	≥ 2,800 W/(m²·K) (air-side, cross-flow)	ASHRAE Fundamentals
Leakage Rate	≤ 0.01% of test volume (helium leak detection)	ISO 11173
Surface Treatment	Pickling + passivation + electropolishing (optional)	ASTM A967
Hydrostatic Test Pressure	1.5× working pressure, 30-second hold, zero leakage	EN 12545-1
Maximum Operating Velocity	≤ 5.0 m/s (liquid) / ≤ 30 m/s (gas)	ASHRAE 15

Manufacturing Process

The production of Wenzhou Jixing's stainless steel coil tubes follows a rigorous multi-stage manufacturing workflow designed to ensure dimensional accuracy, metallurgical homogeneity, and mechanical reliability:

1. Material Selection & Incoming Inspection: Raw stainless steel strip or tube stock is sourced from certified mills and subjected to spectroscopic analysis (X-Ray Fluorescence (XRF)) to verify chemical composition compliance.
2. Seamless Tube Drawing (if applicable): For seamless variants, billets are hot-rolled into hollow forms, then cold-drawn through precision dies to achieve target OD and WT tolerances.
3. Coiling Operation: Continuous tube lengths are coiled using CNC-controlled coiling heads at controlled tension to prevent plastic deformation, kinking, or ovality exceeding 2% of OD.
4. Heat Treatment: Solution annealing at 1040–1150°C followed by rapid air cooling to restore full austenitic structure and eliminate residual stresses introduced during cold forming.
5. Pickling & Passivation: Immersion in mixed acid solutions (HNO₃ + HF) removes mill scale and oxide layers, followed by passivation treatment to regenerate the chromium oxide protective film.
6. Fin Attachment (if finned): Pre-finned aluminum or copper fins are mechanically attached to the coil tube via brazing or mechanical crimping, ensuring uniform fin spacing and tight thermal contact.
7. Hydrostatic & Pneumatic Testing: Each coil undergoes 100% hydrostatic pressure testing at 1.5× rated pressure with visual and electronic leak detection.
8. Dimensional & Surface Inspection: Laser-based dimensional scanning verifies OD, WT, and coil diameter. Surface roughness is measured per ISO 4287.
9. Packaging & Traceability: Finished coils are wrapped in moisture-barrier film, labeled with batch traceability codes, and shipped in protective crates.

Applications

The coil tube is integral to the following HVAC and industrial systems:

• Fan Coil Units (FCU): Primary heat exchange element in both cooling and heating modes, enabling precise Indoor Relative Humidity Control (IRHC) through controlled condensation and evaporation processes.
• Fan Coil Unit plus Fresh Air Systems (FCU+FAS): Provides localized thermal conditioning while a dedicated fresh air supply maintains Ventilation Air Quality (VAQ) and Carbon Dioxide (CO₂) concentration management.
• Commercial Building HVAC: Office buildings, shopping malls, hotels, hospitals, and educational facilities requiring zoned temperature and humidity control.
• Residential HVAC: High-end residential units with individual room conditioning and humidity regulation.
• Industrial Process Cooling: Precision cooling of sensitive equipment, electronics, and manufacturing processes.
• Food & Pharmaceutical Processing: Hygienic-grade coil tubes for temperature-controlled storage and processing environments where corrosion resistance is critical.
• Marine & Offshore HVAC: SUS316L-grade coil tubes resistant to salt spray and high-humidity marine atmospheres.

Quality Certifications & Compliance

• ISO 9001:2015 — Quality Management System certification for manufacturing processes
• ISO 14001:2015 — Environmental Management System certification
• CE Marking — Conformity with European Pressure Equipment Directive (PED) 2014/68/EU for pressure-rated applications
• EN 12545 — Compliance with European standards for air-conditioning and heat pump units
• ASHRAE 22 / ASHRAE 34 — Compliance with North American standards for safety and performance of air-conditioning and refrigeration equipment
• NACE MR0175 / ISO 15156 — Compliance for use in sour service environments (H₂S-containing atmospheres)
• RoHS 2.0 (EU 2011/65/EU) — Restriction of Hazardous Substances compliance
• REACH (EC No 1907/2006) — Registration, Evaluation, Authorisation and Restriction of Chemicals compliance
• Material Certificates: Mill Test Reports (MTR) per ISO 3760 and EN 10204 3.1 supplied with each batch

Unique Selling Points

• Superior Corrosion Resistance: Austenitic stainless steel construction provides exceptional resistance to condensation-induced corrosion, a critical failure mode in FCU coil applications where cyclic wetting and drying occurs.
• Enhanced Humidity Control Capability: The smooth electropolished inner surface (Ra ≤ 0.4 μm) minimizes fouling and maintains consistent heat transfer performance over extended service life, directly contributing to reliable Indoor Relative Humidity Control (IRHC) in FCU+FAS configurations.
• Long Service Life: Expected operational lifespan exceeding 20 years under normal HVAC duty cycles, significantly outperforming carbon steel alternatives.
• Hygienic Design: Seamless construction and passivated surfaces prevent bacterial colonization, making the coil tube suitable for hospitals, pharmaceutical facilities, and food processing environments.
• Thermal Efficiency: Optimized coil geometry and fin configuration deliver high heat transfer coefficients, reducing energy consumption and improving Seasonal Energy Efficiency Ratio (SEER) and Energy Efficiency Ratio (EER) ratings of integrated FCU systems.
• Full Traceability: Every coil is produced with complete batch traceability from raw material through final inspection, ensuring accountability and recall capability.
• Customization Flexibility: Coil diameter, tube OD/WT, fin spacing, and surface finish are configurable to meet specific OEM requirements for diverse FCU designs.

Performance in Fan Coil Unit plus Fresh Air Systems

In the context of Fan Coil Unit plus Fresh Air Systems (FCU+FAS), the coil tube serves as the thermodynamic interface between the energy transfer medium (refrigerant or hot water) and the indoor air stream. Research and engineering practice have demonstrated that the effectiveness of humidity control in these systems is directly correlated with the coil tube's heat transfer surface area, thermal conductivity, and resistance to degradation over time. The stainless steel coil tube's material properties ensure that:

• Latent heat transfer (condensation/evaporation) proceeds efficiently, enabling the FCU to dehumidify or humidify the indoor air to target Relative Humidity (RH) setpoints typically maintained between 40–60% RH for occupant comfort.
• Minimal thermal degradation over the service life preserves the coil's capacity to absorb latent heat, preventing drift in humidity control performance.
• Corrosion-free operation eliminates the risk of coil tube perforation, which would compromise both thermal performance and indoor air quality by releasing particulate corrosion products into the conditioned air.

Note: The integration of Wenzhou Jixing's stainless steel coil tubes into FCU+FAS configurations provides a robust solution for maintaining Indoor Air Quality (IAQ) and Thermal Comfort in accordance with ASHRAE Standard 62.1 (Ventilation for Acceptable Indoor Air Quality) and ASHRAE Standard 55 (Thermal Environment Conditions for Human Occupancy).