Stainless Steel Coil Tube for Fan Coil Units — Anti-Freeze Grade
Product Overview
The Stainless Steel Coil Tube (Coiltube) produced by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered tubular product designed specifically for the coil assemblies within Fan Coil Units (FCUs) used in heating, ventilation, and air conditioning (HVAC) systems. This product addresses the critical challenge of freeze protection in northern regions where ambient temperatures can drop significantly below freezing during winter months. The coil tube serves as the core heat exchange element, facilitating efficient thermal transfer between circulating refrigerant or heating medium and the surrounding air stream.
Drawn from extensive industry experience regarding the anti-freezing challenges encountered in northern climates, this coil tube is engineered with material properties and geometric configurations that minimize the risk of freezing damage, ensuring reliable operation even under extreme cold conditions. The product represents a convergence of metallurgical expertise, thermal engineering, and practical field knowledge accumulated through real-world deployment scenarios.
Material & Properties
Base Material
The coil tube is manufactured from austenitic stainless steel, primarily conforming to ASTM A213 TP304 or ASTM A213 TP316 specifications. The TP316 grade incorporates molybdenum (Mo) content of 2.0–3.0% by weight, providing enhanced resistance to chloride-induced pitting and crevice corrosion — a critical advantage in humid environments where condensation accumulates on coil surfaces.
Key Material Properties
- Chemical Composition (TP304): Carbon (C) ≤ 0.08%, Chromium (Cr) 18.0–20.0%, Nickel (Ni) 8.0–10.5%, Manganese (Mn) ≤ 2.0%, Silicon (Si) ≤ 0.75%
- Chemical Composition (TP316): Carbon (C) ≤ 0.08%, Chromium (Cr) 16.0–18.0%, Nickel (Ni) 10.0–14.0%, Molybdenum (Mo) 2.0–3.0%
- Tensile Strength: ≥ 515 MPa (min.)
- Yield Strength: ≥ 205 MPa (min.)
- Elongation: ≥ 40% (at break)
- Thermal Conductivity: 16.3 W/(m·K) at 20°C
- Operating Temperature Range: −196°C to +870°C (continuous)
- Cryogenic Performance: Retains full ductility and mechanical integrity at temperatures as low as −196°C (Liquid Nitrogen Temperature), making it inherently resistant to freeze-induced embrittlement
Anti-Freeze Design Characteristics
The austenitic stainless steel matrix exhibits a face-centered cubic (FCC) crystal structure that does not undergo a ductile-to-brittle transition at low temperatures. Unlike carbon steel alternatives, this coil tube maintains full toughness and formability even at sub-zero temperatures, eliminating the risk of catastrophic brittle fracture during freeze events. This intrinsic material property is the primary defense mechanism against the freeze damage problems documented in northern regional installations.
Specifications
| Parameter | Specification | Standard / Reference |
|---|---|---|
| Outer Diameter (OD) | 6.35 mm (1/4"), 9.53 mm (3/8"), 12.7 mm (1/2"), 15.88 mm (5/8"), 19.05 mm (3/4") | ASTM B801 / EN 10217 |
| Wall Thickness | 0.3 mm – 1.5 mm (configurable) | ASTM B619 |
| Coil Length | Up to 300 m per continuous coil (spool) | — | Surface Finish | No. 1 (hot-rolled), 2B (cold-rolled), BA (Bright Annealed), or Polished (Ra ≤ 0.4 μm) | ASTM A480 |
| Coil Winding | Spiral-wound or serpentine-formed configurations | — |
| Tube Pitch | Configurable: 12 mm – 25 mm center-to-center | ASHRAE Standard 15 |
| Hydrostatic Test Pressure | 1.5 × Maximum Operating Pressure (MOP) | ASME B31.5 |
| Maximum Operating Pressure | Up to 2.5 MPa (362.5 psi) | — |
| Refrigerant Compatibility | R-410A, R-32, R-134a, R-407C, Ammonia (NH₃), Glycol solutions | — |
| Heat Transfer Coefficient | 1,800 – 2,500 W/(m²·K) (depending on flow velocity and configuration) | — |
| Design Service Life | ≥ 25 years in HVAC applications | — |
| Freeze Resistance | Operational integrity maintained down to −40°C ambient | ISO 23946 (Cryogenic piping) |
Manufacturing Process
1. Raw Material Selection and Milling
Virgin stainless steel wire rod or strip is sourced from certified mills (e.g., Outokumpu, POSCO, or domestic equivalents) with full chemical composition traceability. Incoming material undergoes spectroscopic verification (OES) to confirm compliance with specified grades.
2. Drawing and Cold Forming
The tube is cold-drawn through precision tungsten carbide dies to achieve dimensional tolerances within ±0.05 mm for outer diameter and ±10% for wall thickness. This cold-working process enhances surface finish and mechanical properties through work hardening.
3. Annealing and Pickling
Periodic solution annealing is performed at 1050°C – 1100°C in a controlled atmosphere (nitrogen or argon) to relieve residual stresses introduced during drawing. Subsequent pickling and passivation removes surface oxides and restores the passive chromium oxide film essential for corrosion resistance.
4. Coiling and Forming
The straight tube is mechanically formed into the required spiral or serpentine coil geometry using CNC-controlled coiling equipment. Forming parameters are optimized to prevent wall thinning beyond 15% of original thickness and to maintain consistent coil pitch throughout the assembly.
5. Brazing and Header Assembly
Tube ends are joined to inlet and outlet headers using phosphorus-free copper brazing alloys (e.g., BAg-19Cu per AWS A5.8) in a vacuum or inert gas atmosphere. This ensures contamination-free joints that maintain the full corrosion resistance of the stainless steel system.
6. Quality Inspection
- Dimensional verification via laser micrometry and coordinate measuring machines (CMM)
- Hydrostatic pressure testing at 1.5× MOP with 30-minute hold time
- Helium leak detection for vacuum integrity (detection limit: 1×10⁻⁹ Pa·m³/s)
- Visual and dye penetrant inspection (PT) of all welds and brazed joints per ASTM E709
- Corrosion testing via salt spray (ASTM B117) — minimum 500 hours without pitting
Applications
Primary Applications
- Fan Coil Units (FCUs) in Northern Regions: Core heat exchange coil for unitary air handling systems deployed in cold climates where freeze events are a documented operational risk
- Commercial HVAC Systems: Chilled water and hot water coil assemblies in office buildings, hotels, hospitals, and retail facilities
- Industrial Process Cooling: Precision temperature control in pharmaceutical, food processing, and electronics manufacturing
- Automotive HVAC: Condenser and evaporator coil tubing for vehicle air conditioning systems
- Refrigeration Systems: Evaporator and condenser coils in walk-in coolers, freezers, and cold storage facilities
Anti-Freeze Application Context
In northern regions (particularly those with design temperatures below −20°C), conventional carbon steel coils in fan coil units are susceptible to freeze damage when heating circuits are inadvertently shut off or when glycol concentration falls below the protection threshold. The stainless steel coil tube's cryogenic toughness, combined with its low thermal expansion coefficient, provides a robust solution that withstands repeated freeze-thaw cycles without degradation of mechanical integrity or dimensional accuracy.
Quality Certifications
- ISO 9001:2015 — Quality Management System Certification
- ISO 14001:2015 — Environmental Management System Certification
- ASME Stamp — Pressure Vessel and Piping Components (where applicable)
- EN 10204 3.1 — Material Test Certificate (Mill Test Report) for every production batch
- CE Marking — Conformity with European Machinery Directive (2006/42/EC) and Pressure Equipment Directive (2014/68/EU)
- UL / CSA Listing — Component-level safety certification for HVAC applications in North American markets
- RoHS / REACH Compliance — Restriction of Hazardous Substances and Chemical Registration for global market access
Unique Selling Points
- Intrinsic Cryogenic Toughness: Austenitic stainless steel retains full ductility below −40°C, fundamentally eliminating the brittle fracture risk that plagues carbon steel coils in northern freeze events
- Corrosion-Resistant Lifecycle: Superior resistance to condensation-induced corrosion extends service life beyond 25 years, reducing total cost of ownership (TCO) by 30–40% compared to copper tube alternatives
- Field-Validated Design: Product specifications derived from documented case studies of freeze failures in northern fan coil installations, incorporating lessons learned into preventive design
- Full Traceability: Every coil tube spool is accompanied by a complete material pedigree including mill certificate, heat number, chemical analysis, and mechanical test data
- Configurability: Custom coil geometry, tube diameter, wall thickness, and pitch configurations available to meet specific heat exchange load calculations
Technical Notes for Northern Region Deployment
Recommendation: When deploying stainless steel coil tubes in fan coil units operating in northern climates, the following measures should be implemented in conjunction with the coil tube's inherent freeze resistance:
- Maintain glycol solution concentration at or above 30% for design temperatures below −20°C
- Install low-water-flow and freeze-stat protection devices upstream of the coil assembly
- Ensure proper insulation of connecting piping to minimize heat loss and condensation formation
- Implement seasonal commissioning protocols that verify coil integrity after each freeze-season cycle
- Design coil configuration to allow complete drainage (drip-leg arrangement) to eliminate residual water that could freeze during shutdown