Stainless Steel Coil Tube (Coiltube) — HVAC Fan Coil Unit Heat Exchanger Tubing
1. Product Overview
The Stainless Steel Coil Tube (hereinafter referred to as "Coiltube") is a precision-engineered continuous tubular product manufactured by Wenzhou Jixing Metal Materials Co., Ltd. (Product No. 960). This product is specifically designed for use in fan coil unit (FCU) heat exchangers and related thermal-fluid exchange applications. The Coiltube is formed through continuous coil winding of high-grade stainless steel tubing, producing a helical or serpentine coil configuration that maximizes surface area within a compact volumetric envelope — a critical design parameter for efficient heat transfer in HVAC (Heating, Ventilation, and Air Conditioning) systems.
Unlike conventional straight-length tubing that requires secondary bending and brazing operations, the Coiltube is delivered as a pre-formed continuous coil, thereby eliminating the risks associated with post-fabrication deformation, joint integrity failure, and thermal cycling fatigue at welded connections. This continuous, seamless coil architecture represents a significant advancement in heat exchanger manufacturing methodology.
2. Material & Properties
2.1 Base Material Grades
- AISI 304 (UNS S30400) — The standard grade offering excellent corrosion resistance, formability, and weldability. Suitable for general HVAC and water-handling applications where chloride exposure is minimal.
- AISI 316 (UNS S31600) — Enhanced grade with molybdenum (Mo) addition (2.0–2.5% by weight), providing superior resistance to pitting and crevice corrosion in aggressive environments, including coastal and industrial settings.
- AISI 316L (UNS S31603) — Low-carbon variant of 316 (C ≤ 0.03%), optimized for welding applications where sensitization resistance is paramount.
2.2 Mechanical Properties
| Property | 304 Grade | 316 Grade | Test Standard |
|---|---|---|---|
| Tensile Strength (UTS) | ≥ 520 MPa (75,400 psi) | ≥ 515 MPa (74,700 psi) | ASTM A213 / ASME SA-213 |
| Yield Strength (0.2% offset) | ≥ 205 MPa (29,700 psi) | ≥ 170 MPa (24,700 psi) | ASTM A213 |
| Elongation (in 50 mm gauge) | ≥ 40% | ≥ 35% | ASTM A213 |
| Hardness (HB) | ≤ 200 HB | ≤ 200 HB | ASTM E10 |
| Density | 8.0 g/cm³ | 8.0 g/cm³ | — |
| Thermal Conductivity (at 100°C) | 16.3 W/(m·K) | 16.3 W/(m·K) | ASTM A213 |
2.3 Chemical Composition (Typical)
| Element | 304 (Max/Min %) | 316 (Max/Min %) |
|---|---|---|
| Carbon (C) | ≤ 0.08 | ≤ 0.08 |
| Chromium (Cr) | 18.0 – 20.0 | 16.0 – 18.0 |
| Nickel (Ni) | 8.0 – 10.5 | 10.0 – 14.0 |
| Molybdenum (Mo) | — | 2.0 – 2.5 |
| Iron (Fe) | Balance | Balance |
| Phosphorus (P) | ≤ 0.045 | ≤ 0.045 |
| Sulfur (S) | ≤ 0.030 | ≤ 0.030 |
3. Specifications
3.1 Dimensional Parameters
| Parameter | Specification Range | Tolerance | Standard Reference |
|---|---|---|---|
| Outer Diameter (OD) | 6.0 mm – 25.4 mm (1/4" – 1") | ±0.10 mm | ASTM A213 / EN 10216-5 |
| Wall Thickness (WT) | 0.50 mm – 2.00 mm | ±0.05 mm | ASTM A213 |
| Coil Pitch (Helical Spacing) | 10 mm – 30 mm (adjustable) | ±0.5 mm | Custom / OEM |
| Coil Diameter (Helix) | 200 mm – 800 mm | ±1.0 mm | Custom / OEM |
| Coil Length (Straight Segment) | 1.0 m – 6.0 m (continuous) | ±10 mm | Custom / OEM |
| Surface Finish | No. 4 Brushed / 2B / Mirror (Ra ≤ 0.2 μm) | — | ASTM A480M |
3.2 Manufacturing Standards
- ASTM A213 / ASME SA-213 — Standard Specification for Seamless Austenitic Stainless Steel Tubing for High-Temperature and High-Pressure Boiler-Fired Heaters
- EN 10216-5 — Non-alloy and Alloy Steels — Seamless Tubes for Mechanical and Thermal Uses — Part 5: Technical Delivery Conditions for Austenitic Tubes
- JIS G4304 — Seamless Stainless Steel Tubes for General Purposes
- GB/T 14976 — Chinese National Standard for Seamless Stainless Steel Tubes for Fluid Transportation
4. Manufacturing Process
4.1 Process Flow
- Raw Material Procurement & Inspection: Certified stainless steel bar stock or seamless pipe blanks are sourced from approved suppliers and subjected to incoming quality control including spectroscopic analysis (OES — Optical Emission Spectroscopy) to verify chemical composition compliance.
- Hot Rolling & Annealing: Billet material is hot-rolled into seamless tube blanks and subsequently annealed at 1050–1150°C to achieve a fully austenitic microstructure with optimal grain uniformity and ductility.
- Precision Cold Drawing: The annealed tube blanks undergo multi-pass cold drawing through calibrated tungsten carbide (WC) dies to achieve the target OD and wall thickness with tight dimensional tolerances (±0.05 mm). This process simultaneously induces beneficial strain hardening while maintaining surface integrity.
- Intermediate Annealing: Periodic solution annealing treatments are performed between cold-drawing passes to relieve accumulated residual stresses and restore ductility for subsequent forming operations.
- Continuous Coil Winding: The precision-drawn tubing is fed into a CNC-controlled (Computer Numerical Control) coil winding machine that forms the helical or serpentine coil geometry. The winding parameters — including pitch, diameter, and number of turns — are programmatically controlled to ensure geometric accuracy and consistency across production batches.
- Final Annealing & Pickling: The formed coil undergoes a final annealing cycle followed by chemical pickling (using a mixed acid solution of HNO₃ and HF) to remove any oxidation scale, surface contaminants, and residual carbon deposits. This step ensures a clean, uniform, and corrosion-resistant surface finish.
- Precision Cutting & End Preparation: The coil is cut to the required length using high-precision shearing or plasma cutting equipment. Tube ends are deburred and chamfered to facilitate downstream assembly and brazing/welding operations.
- Quality Inspection & Testing: Each batch undergoes comprehensive non-destructive testing (NDT) including:
- Visual surface inspection (100% coverage)
- Dimensional verification via coordinate measuring machine (CMM)
- Hydrostatic pressure testing at 1.5× working pressure (minimum 30 minutes hold)
- Eddy current testing (ECT) for internal and external defect detection
- X-ray radiographic testing (RT) for weld joint integrity (where applicable)
- Packaging & Dispatch: Qualified coils are wrapped in protective polyethylene film, secured with anti-corrosion paper, and packed into export-grade wooden crates or steel containers with desiccant packs to prevent moisture ingress during transit.
5. Applications
5.1 Primary Application — Fan Coil Unit (FCU) Heat Exchangers
The Coiltube serves as the primary heat transfer element in fan coil unit evaporators and condensers. In an FCU system, conditioned air is circulated by a fan across the coil tube array, where heat exchange occurs between the air stream and the refrigerant or hot/cold water flowing through the coil. The helical coil geometry provides a significantly larger heat transfer surface area compared to straight-tube configurations of equivalent volume, directly translating to improved thermal efficiency (COP — Coefficient of Performance).
5.2 Key Application Areas
- Commercial HVAC Systems: Residential and commercial air conditioning units, including ducted and ductless mini-split systems
- Industrial Process Heating & Cooling: Heat exchangers in chemical processing, food and beverage manufacturing, and pharmaceutical production lines
- Refrigeration Systems: Evaporators and condensers in commercial refrigeration equipment, cold storage facilities, and supermarket refrigeration systems
- Automotive & Transportation: Auxiliary heating and cooling coils in commercial vehicles, rail transit systems, and marine HVAC installations
- Water Heating Systems: Immersion heating elements and heat exchanger coils in domestic and industrial water heating applications
- Medical Equipment: Sterilization chambers, incubators, and temperature-controlled laboratory equipment
5.3 Design Considerations for FCU Installation
Based on engineering practice literature regarding fan coil unit modeling and installation, the following critical considerations apply to Coiltube integration:
- Condensate Drainage: The coil tube layout must incorporate adequate slope (minimum 1:100 gradient) toward the condensate drain pan to prevent water accumulation and microbial growth. The coil geometry should be designed to avoid condensate bridging between coil rows.
- Airflow Distribution: Coil tube pitch and row configuration must be optimized to ensure uniform airflow distribution across the entire heat exchanger face, minimizing bypass air and ensuring consistent temperature control.
- Vibration Isolation: The coil tube assembly must be mounted with appropriate vibration-damping supports to prevent fatigue cracking at tube-to-header connections, particularly in high-fan-speed configurations.
- Access for Maintenance: Coil tube arrangement should permit access for cleaning, inspection, and replacement of individual coil sections without requiring complete unit disassembly.
- Anti-Corrosion Protection: In environments with elevated humidity or corrosive contaminants, 316-grade Coiltube is recommended with additional passivation treatment to extend service life.
6. Quality Certifications
6.1 Product Certifications
- ISO 9001:2015 — Quality Management System certification ensuring consistent product quality and process control throughout the manufacturing lifecycle
- ISO 14001:2015 — Environmental Management System certification demonstrating commitment to sustainable manufacturing practices and environmental stewardship
- CE Marking (EU Conformity) — Compliance with European Union machinery and pressure equipment directives (PED 2014/68/EU)
- ASME "S" Stamp — American Society of Mechanical Engineers certification for seamless austenitic stainless steel tubing
- NACE MR0175 / ISO 15156 — Resistance to sulfide stress cracking in oil and gas environments (for applicable grades)
6.2 Testing & Inspection Certifications
- Mill Test Certificate (MTC / EN 10204 3.1) — Full chemical composition and mechanical property data provided for each heat lot
- Third-Party Inspection (TPI) — Available upon request through SGS, Bureau Veritas, or TÜV certification bodies
- Hydrostatic Test Report — Pressure test results documenting integrity verification at 1.5× maximum operating pressure
- NDT Report — Eddy current and ultrasonic testing documentation confirming absence of internal and external defects