Stainless Steel Coil Tube for HVAC Fan Coil Units (SKU #737)
Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd.
Product Category: Stainless Steel Coil Tube (盘管 / Coiltube) — Industrial & HVAC Grade
1. Product Overview
The Stainless Steel Coil Tube (SKU #737) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered tubular component designed specifically for use in central air conditioning fan coil unit (FCU) systems. This coil tube serves as the primary heat exchange conduit within fan coil assemblies, facilitating efficient thermal transfer between circulating refrigerant or hot water and ambient air moved by the axial or centrifugal fan motor.
Unlike conventional carbon steel or copper coil tubes, the stainless steel variant offers superior resistance to corrosion, thermal fatigue, and chemical degradation, making it particularly suitable for environments with high humidity, aggressive atmospheric conditions, or where long-term operational reliability is paramount. The design addresses a critical industry concern — motor "burnout" (烧机) failures in fan coil installations — by ensuring consistent heat transfer performance that prevents excessive thermal loading on the fan motor windings.
Key Insight: Research and field experience indicate that improper coil tube installation or material degradation can lead to uneven heat distribution, causing localized thermal stress on the fan motor. This coil tube product is engineered to eliminate such failure modes through optimized geometry, material selection, and manufacturing tolerances.
2. Material & Properties
2.1 Base Material
- Grade: Austenitic stainless steel conforming to ASTM A269 / A213 standards
- Typical Composition:
- Chromium (Cr): 18.0–21.0%
- Nickel (Ni): 8.0–11.0%
- Molybdenum (Mo): 2.0–3.0% (for enhanced pitting resistance)
- Carbon (C): ≤ 0.03% (low-carbon variant to prevent sensitization)
- Iron (Fe): Balance
- Equivalent Grades: AISI 316L, SUS 316L, 06Cr17Ni12Mo2
2.2 Mechanical Properties
- Tensile Strength (UTS): ≥ 520 MPa (75,400 psi)
- Yield Strength (0.2% Offset): ≥ 205 MPa (29,730 psi)
- Elongation (A50): ≥ 40%
- Hardness (Brinell): ≤ 220 HB
- Density: 8.0 g/cm³
2.3 Thermal & Physical Properties
- Thermal Conductivity: 16.3 W/(m·K) at 20°C
- Thermal Expansion Coefficient: 16.0 × 10⁻⁶ /°C (20–100°C)
- Specific Heat Capacity: 500 J/(kg·K)
- Operating Temperature Range: −196°C to +450°C
- Corrosion Resistance: Excellent resistance to chloride pitting, intergranular corrosion, and acid-alkaline environments
3. Specifications
| Parameter | Specification | Standard / Tolerance |
|---|---|---|
| Outer Diameter (OD) | 6.35 mm – 25.4 mm (1/4" – 1") | ±0.15 mm |
| Wall Thickness | 0.50 mm – 1.50 mm | ±10% of nominal |
| Coil Pitch (Bend Spacing) | 12 mm – 25 mm | ±0.5 mm |
| Bend Radius | 1.5D – 3.0D (D = OD) | No ovality > 5% after bending |
| Coil Length (Straight Segment) | 1,500 mm – 6,000 mm | ±10 mm |
| Number of Passes | 2-pass to 8-pass configurations | Custom per FCU design |
| Surface Finish | Mirror polished (Ra ≤ 0.4 μm) or Matte (Ra ≤ 1.6 μm) | ASTM A480 |
| End Connection | Sweat, Flared, Threaded (BSP/NPT), or Brazed | ASME B31.5 |
| Pressure Rating | Up to 2.5 MPa (362.6 psi) at 20°C | ASME B31.5 / EN 12828 |
| Heat Transfer Coefficient | 1,200 – 2,500 W/(m²·K) | Dependent on flow velocity |
| Weight (Typical) | 0.8 kg – 4.5 kg per coil assembly | — |
| Color / Appearance | Silver-gray, corrosion-resistant passive film | — |
4. Manufacturing Process
4.1 Raw Material Preparation
High-purity austenitic stainless steel wire rod (SAE 316L grade) is procured from certified mill suppliers. Incoming material undergoes spectroscopic analysis (OES) to verify chemical composition within specified limits, followed by tensile testing and hardness verification per ASTM A269 requirements.
4.2 Cold Drawing
The wire rod is drawn through progressively reducing dies to achieve the target outer diameter and wall thickness. This cold working process imparts beneficial strain hardening, improving mechanical strength while maintaining ductility. Lubricants (calcium stearate or synthetic-based) are used to minimize surface defects and ensure dimensional accuracy.
4.3 Annealing & Pickling
- Solution Annealing: Heating to 1050–1150°C in a protective atmosphere (argon or nitrogen) to relieve internal stresses and restore full ductility.
- Pickling: Immersion in mixed acid solution (HNO₃/HF) to remove oxide scale and restore the passive chromium oxide film.
- Passivation: Final treatment in citric acid or nitric acid solution to ensure a stable, protective chromium-rich oxide layer (minimum 50 Å thickness).
4.4 Coil Forming & Bending
Pre-treated tubing is fed through precision bending machinery using mandrel-assisted forming to create the serpentine coil geometry. The bending process is computer-controlled (CNC) to ensure:
- Uniform bend radii throughout the coil
- Minimal wall thinning at bend apexes (≤ 15% reduction)
- Consistent pitch spacing for optimal air-side heat transfer
- No material cracking or ovality exceeding specification limits
4.5 Surface Treatment & Finishing
Depending on application requirements, the finished coil may receive:
- Mirror Polishing: Multi-stage buffing to achieve Ra ≤ 0.4 μm for enhanced corrosion resistance and reduced fouling
- Electropolishing: Electrochemical surface refinement to remove micro-roughness and embedded contaminants
- Coating (Optional): Thin PTFE or ceramic-based thermal enhancement coatings for applications requiring elevated heat transfer efficiency
4.6 Assembly & End Connection Fabrication
Coil tube ends are prepared with precision-cut, deburred, and optionally flared or threaded terminations. Header connections (inlet/outlet manifolds) are brazed using copper-phosphorus filler alloy (BCuP-2) at temperatures below 700°C to avoid sensitization of the austenitic matrix.
4.7 Quality Inspection & Testing
- Dimensional Inspection: CMM (Coordinate Measuring Machine) verification of OD, wall thickness, pitch, and overall geometry
- Hydrostatic Pressure Testing: Each coil subjected to 1.5× working pressure for a minimum 15-minute hold with zero leakage tolerance
- Penetrant Testing (PT): Dye penetrant inspection of all bends and welds per ASTM E709
- Corrosion Testing: Salt spray testing (ASTM B117) for minimum 1,000 hours without pitting
- Thermal Cycling Test: 500 cycles between −40°C and +150°C with no dimensional drift
5. Applications
5.1 Primary Application — Central Air Conditioning Fan Coil Units
The principal application of this coil tube is as the heat exchanger element within fan coil units used in commercial and industrial HVAC systems. The stainless steel construction ensures:
- Long-term resistance to condensate-induced corrosion in high-humidity environments
- Elimination of iron ion leaching that can cause staining and bacterial growth in chilled water systems
- Consistent heat transfer performance over extended service life, preventing the thermal imbalances that contribute to motor burnout
5.2 Motor Burnout Prevention
Based on extensive field research and installation experience, this coil tube product directly addresses the root causes of fan motor "burnout" (烧机) failures in HVAC systems:
- Uniform Heat Distribution: Precision coil geometry ensures even thermal loading across the fan housing, preventing localized hot spots that degrade motor insulation
- Prevention of Condensate Corrosion: Stainless steel construction eliminates the rust and scale buildup that can obstruct airflow and increase motor loading
- Dimensional Stability: Thermal expansion properties prevent coil distortion at elevated operating temperatures, maintaining proper clearance around the fan motor
- Reduced Fouling: Mirror-polished surface minimizes particulate adhesion, preserving airflow efficiency and motor load characteristics over time
5.3 Additional Applications
- Refrigeration and cryogenic systems requiring cryogenic-grade tubing
- Food and pharmaceutical processing equipment (hygienic applications)
- Marine HVAC systems exposed to saltwater environments
- Chemical processing heat exchangers handling aggressive media
- High-purity water distribution systems in semiconductor manufacturing
- Solar thermal collectors and heat pump systems
6. Quality Certifications & Standards Compliance
| Certification / Standard | Scope of Compliance |
|---|---|
| ISO 9001:2015 | Quality Management System |
| ISO 14001:2015 | Environmental Management System |
| ISO 45001:2018 | Occupational Health & Safety Management |
| ASTM A269 / A213 | Seamless & welded stainless steel tube specifications |
| ASME B31.5 | Pressure Piping — Refrigeration & Air-Conditioning |
| EN 10217-7 | Heat exchanger tubes — Stainless steel |
| GB/T 14976 (Chinese National Standard) | Seamless stainless steel tubes for general engineering |
| UL 484 | Heating & Air-Conditioning Equipment (where applicable) |
| CNCA (China Compulsory Certification) | Mandatory product safety certification for HVAC components |
| Material Mill Certification | Full chemical and mechanical test reports per heat number |
7. Unique Selling Points
- Superior Corrosion Resistance: 316L stainless steel construction provides 3–5× longer service life compared to copper or carbon steel alternatives in corrosive HVAC environments, particularly where chlorinated condensate is present.
- Motor Burnout Mitigation: Engineered specifically to address the thermal management challenges that lead to fan motor failures — a documented and costly problem in commercial HVAC installations.
- Precision Manufacturing Tolerances: CNC-controlled forming ensures dimensional accuracy that guarantees optimal heat transfer performance and proper integration with FCU housings.
- Full Traceability: Every coil tube is traceable to its mill heat number with complete chemical and mechanical test documentation, ensuring accountability throughout the supply chain.
- Customizability: Available in multiple diameters, wall thicknesses, coil configurations, and end connection types to accommodate diverse OEM and aftermarket requirements.