Stainless Steel Coil Tube (Coiltube) for HVAC Anti-Freeze Applications
Product Code: JX-CT-848 | Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd. (Wenzhou Jixing Jinshu Cailiao Youxian Gongsi)
1. Product Overview
The Stainless Steel Coil Tube (hereinafter referred to as "Coiltube") is a precision-engineered tubular component designed specifically for air conditioning unit (Air Conditioning Unit, ACU) coil assemblies. Unlike conventional straight-pipe configurations, the Coiltube is manufactured as a continuous spiral or helical winding, offering superior heat transfer efficiency, compact volumetric design, and enhanced resistance to thermal cycling and freeze-thaw degradation. This product addresses critical anti-freeze technical requirements (Anti-Freeze Technical Measures) inherent in HVAC systems operating in low-temperature or variable-climate environments.
The fundamental design philosophy behind the Coiltube product line is rooted in the metallurgical principle of minimizing stress concentration at pipe joints and bends. By eliminating soldered or brazed connections in favor of a continuous coil geometry, the product inherently reduces the risk of leakage under freeze-thaw cycling conditions—a common failure mode in conventional HVAC coil assemblies. The stainless steel material selection further ensures long-term corrosion resistance, mechanical integrity, and hygienic performance across diverse operating environments.
2. Material and Properties
The Coiltube is manufactured using austenitic stainless steel grades selected for their exceptional combination of ductility, corrosion resistance, and cryogenic toughness—properties essential for anti-freeze applications where the working fluid may reach sub-zero temperatures.
2.1 Primary Material Grades
- SAE 304 (UNS S30408): General-purpose austenitic stainless steel with 18% Chromium (Cr) and 8% Nickel (Ni). Offers excellent formability and resistance to atmospheric and mild chemical corrosion. Suitable for standard HVAC coil applications where ambient temperatures remain above −40°C.
- SAE 316 (UNS S31608): Enhanced austenitic grade incorporating 2–3% Molybdenum (Mo), providing superior resistance to pitting and crevice corrosion. Recommended for coastal installations, chemical processing environments, or applications involving chlorinated refrigerants.
- SAE 316L (UNS S31603): Low-carbon variant of 316 (Carbon ≤ 0.03%), designed to prevent intergranular corrosion during welding or heat treatment. Preferred for applications requiring post-manufacturing welding operations.
2.2 Key Metallurgical Properties
- Crystal Structure: Face-Centered Cubic (FCC), ensuring retained ductility at cryogenic temperatures (no ductile-to-brittle transition)
- Thermal Conductivity: 14–16 W/(m·K) at 20°C, adequate for heat exchange coil applications
- Linear Thermal Expansion: 17.3 × 10⁻⁶ /°C (20–100°C range)
- Specific Heat Capacity: ~478 J/(kg·K) at 25°C
- Impact Toughness: Retains full toughness down to −196°C (Liquid Nitrogen Temperature), critical for freeze-thaw cycling resistance
2.3 Anti-Freeze Performance Characteristics
The austenitic stainless steel matrix provides inherent resistance to freeze damage through the following mechanisms:
- Thermal Shock Resistance: The low thermal conductivity combined with high ductility allows the tube wall to accommodate volumetric expansion of trapped fluids during freezing without cracking or fracture.
- Corrosion Resistance Under Ice Formation: Unlike carbon steel, austenitic stainless steel does not suffer from stress corrosion cracking (Stress Corrosion Cracking, SCC) at grain boundaries during repeated freeze-thaw cycles, even in the presence of residual moisture and dissolved oxygen.
- Hydrostatic Pressure Tolerance: The continuous coil geometry distributes internal hydraulic pressure uniformly, preventing localized bulging or rupture at discrete points.
3. Technical Specifications
| Parameter | Specification / Range | Standard Reference |
|---|---|---|
| Outer Diameter (OD) | 6.0 mm – 25.4 mm (¼" – 1") | ASTM A213 / GB/T 14976 |
| Wall Thickness | 0.5 mm – 2.0 mm | ASTM A269 / GB/T 12771 |
| Coil Winding Diameter | 100 mm – 600 mm (configurable) | Manufacturer Specification |
| Coil Length (per Spool) | 5 m – 50 m (customizable) | Manufacturer Specification | Material Grade | SAE 304 / 316 / 316L | ASTM A213 TP304/316 |
| Surface Finish | Bright Annealed / Pickled & Passivated (Ra ≤ 0.8 μm) | ASTM A240 / EN 10088-2 |
| Hydrostatic Test Pressure | ≥ 1.5 × Working Pressure (minimum 1.0 MPa) | GB/T 12771 Clause 9 |
| Operating Temperature Range | −60°C to +200°C | ASTM A213 |
| Maximum Working Pressure | Up to 2.5 MPa (dependent on OD and wall thickness) | ASME B31.9 / GB 50235 |
| Minimum Bend Radius | ≥ 3D (where D = outer diameter) | ASTM A213 |
| Freeze-Thaw Cycle Endurance | ≥ 50,000 cycles without degradation | Internal Test Protocol |
| Chemical Composition (304) | Cr: 18.0–20.0%, Ni: 8.0–11.0%, C ≤ 0.08% | ASTM A213 / GB/T 1221 |
| Chemical Composition (316) | Cr: 16.0–18.0%, Ni: 10.0–14.0%, Mo: 2.0–3.0%, C ≤ 0.08% | ASTM A213 / GB/T 1221 |
| Deliverable Form | Continuous Coil (Spooled) or Cut-Length Sections | — |
4. Manufacturing Process
The production of the Coiltube follows a rigorous multi-stage manufacturing sequence, each step governed by strict quality control protocols to ensure metallurgical integrity and dimensional accuracy.
4.1 Raw Material Preparation
Hot-rolled stainless steel strip or seamless tube blanks conforming to ASTM A213 are received and subjected to incoming inspection, including Chemical Composition Analysis (via Optical Emission Spectroscopy, OES) and mechanical property verification (tensile testing per ASTM E8/E8M). Material traceability is maintained through heat number logging throughout the entire production chain.
4.2 Cold Forming and Coil Winding
The tube blank is fed through precision mandrel-guided forming rolls that incrementally bend the straight tube into a continuous helical coil geometry. The forming process is executed under controlled strain rates to prevent:
- Work hardening beyond acceptable limits (Hardness ≤ 200 HV)
- Wall thinning exceeding 5% of original thickness
- Residual stress accumulation that could compromise fatigue life
4.3 Solution Annealing and Stress Relief
Following coil formation, the product undergoes Solution Annealing at 1050°C – 1150°C in a controlled atmosphere furnace (Hydrogen or Vacuum), followed by rapid quenching. This treatment:
- Restores full austenitic microstructure (single-phase FCC)
- Eliminates cold-work-induced residual stresses
- Re-dissolves any precipitated chromium carbides (Chromium Carbide Precipitation)
- Maximizes corrosion resistance and ductility
4.4 Surface Treatment — Pickling and Passivation
The annealed coil is treated with a mixed acid pickling solution (Nitric Acid + Hydrofluoric Acid) to remove oxide scales, followed by passivation treatment to establish a uniform chromium oxide passive film (Chromium Oxide Passive Film, Cr₂O₃). This final layer provides the definitive corrosion barrier essential for long-term HVAC service.
4.5 Final Quality Inspection
- Dimensional Inspection: OD, wall thickness, and coil diameter verified using laser measurement systems
- Hydrostatic Pressure Testing: Each coil subjected to water pressure test per GB/T 12771
- Visual and Dye Penetrant Testing: Per ASTM E165/E165M for surface defect detection
- Spectroscopic Verification: Spot-check of chemical composition via portable XRF (X-Ray Fluorescence) analyzer
- Cold Bend Test: Verification of ductility post-manufacturing per ASTM A213
5. Applications
5.1 Primary Application — HVAC Coil Assemblies
The primary design application is within Air Conditioning Units (ACU), where the Coiltube serves as the heat exchange element in evaporator and condenser coils. The spiral geometry maximizes surface area per unit volume, enabling compact heat exchanger designs suitable for rooftop units, split-system air conditioners, and central air handling units (Air Handling Units, AHU).
5.2 Anti-Freeze Technical Measures Integration
In accordance with the technical framework outlined in Anti-Freeze Technical Measures for Air Conditioning Unit Coil Tubes (《空调机组盘管防冻技术措施》), the Coiltube is specifically engineered to mitigate freeze-related failures through:
- Continuous Tube Design: Elimination of brazed joints—common failure points during ice formation and thermal expansion
- Low-Temperature Ductility: Austenitic stainless steel retains full mechanical properties at sub-zero temperatures, preventing brittle fracture
- Corrosion-Free Interior Surface: Passivated inner surface prevents biofilm growth and scale formation that could reduce flow area and exacerbate freeze risk
- Uniform Wall Thickness: Precision manufacturing ensures no thin-wall weak points that could rupture under ice-induced hydraulic pressure
5.3 Secondary Applications
- Refrigeration system evaporators and condensers
- Heat pump coil assemblies (air-source and ground-source)
- Cold storage facility piping
- Food processing and pharmaceutical heat exchangers (hygienic-grade 316L)
- Automotive thermal management systems
- Industrial process cooling circuits requiring freeze protection
6. Quality Certifications and Standards Compliance
The Coiltube product line is manufactured and certified in accordance with the following international and national standards:
| Certification / Standard | Scope |
|---|---|
| ISO 9001:2015 | Quality Management System certification for manufacturing processes |
| ISO 14001:2015 | Environmental Management System |
| GB/T 14976 | Chinese National Standard for stainless steel seamless tubes |
| GB/T 12771 | Chinese National Standard for stainless steel welded tubes |
| ASTM A213 | American standard for seamless austenitic stainless steel tubes |
| EN 10217-7 | European standard for austenitic stainless steel tubes for pressure purposes |
| CE Marking | European Conformity for pressure equipment (PED 2014/68/EU) |
| RoHS Compliance | Restriction of Hazardous Substances (EU Directive 2011/65/EU) |
7. Unique Selling Points
"Engineered for Endurance: Where Freeze Meets Flow"
- Intrinsic Anti-Freeze Architecture: The continuous coil geometry eliminates the brazed joints and discrete connections that represent the primary failure points in conventional HVAC coils during freeze events. This represents a fundamental design philosophy shift from "protecting against freeze" to "inherently resisting freeze damage."
- Cryogenic Metallurgical Integrity: The austenitic stainless steel matrix maintains full ductility and toughness at temperatures as low as −196°C, ensuring zero risk of brittle fracture—a property unavailable in carbon steel or martensitic stainless steel alternatives.
- Superior Corrosion Endurance: The passivated surface provides a self-healing oxide barrier that resists pitting, crevice corrosion, and intergranular attack, extending service life beyond 25 years in typical HVAC operating environments.
- Optimized Heat Transfer Performance: The helical coil geometry induces secondary flow patterns (Dean Vortices) within the tube, enhancing convective heat transfer coefficients by 20–40% compared to straight-tube configurations of equivalent surface area.
- Compact Installation Footprint: The pre-formed coil design reduces on-site assembly time and eliminates field bending operations, minimizing installation errors and ensuring consistent performance from factory to field.
- Full Traceability and Quality Assurance: Every coil is traceable to its mill heat number, with complete documentation of chemical composition, mechanical properties, and all test results provided with delivery.
- Customization Capability: Wenzhou Jixing Metal Materials offers fully customizable coil diameter, OD, wall thickness, length, and material grade specifications to match specific HVAC system design parameters and anti-freeze technical requirements.
8. Technical Advisory Notes
From a materials science perspective, the selection of this Coiltube product for anti-freeze HVAC applications represents an optimal intersection of metallurgical performance and engineering practicality. The austenitic stainless steel provides the fundamental cryogenic toughness required for freeze-thaw survival, while the continuous coil geometry eliminates the structural vulnerabilities inherent in joint-based assemblies. Users should note that while the stainless steel itself provides excellent freeze resistance, proper system design—including adequate insulation, drainage provisions, and glycol-based heat transfer fluid where applicable—remains essential for comprehensive freeze protection strategy implementation.
For detailed anti-freeze system design integration, users are advised to reference the complete technical framework of Anti-Freeze Technical Measures