Stainless Steel Coil Tube for Fan Coil Unit Applications
1. Product Overview
The Stainless Steel Coil Tube (盘管, Coiltube) is a precision-engineered tubular component manufactured by Wenzhou Jixing Metal Materials Co., Ltd (Product No. 789). This product is specifically designed for use in fan coil unit (FCU) assemblies, as governed by the Chinese national standard GB/T 19232—2003 (Specifications for Fan Coil Units). The coil tube serves as the primary heat exchange element within FCU systems, facilitating efficient thermal transfer between refrigerant and air streams in commercial and residential HVAC (Heating, Ventilation, and Air Conditioning) installations.
As a specialist in stainless steel pipe and tube manufacturing, Wenzhou Jixing Metal Materials Co., Ltd. applies advanced metallurgical processing techniques to produce coil tubes that meet or exceed the performance, durability, and dimensional accuracy requirements stipulated in GB/T 19232—2003, while addressing known deficiencies identified in subsequent standard revision studies.
2. Material & Properties
2.1 Base Material
The coil tube is fabricated from austenitic stainless steel grades conforming to international standards, with the following primary options:
- AISI 304 (UNS S30408 / 06Cr19Ni10) — General-purpose grade offering excellent corrosion resistance, formability, and weldability. Suitable for most HVAC applications involving water-glycol mixtures and ambient atmospheric conditions.
- AISI 316 (UNS S31608 / 06Cr17Ni12Mo2) — Enhanced grade with added molybdenum (2.0–3.0% Mo), providing superior resistance to pitting and crevice corrosion in chloride-rich environments. Recommended for coastal installations and chemical processing HVAC systems.
- AISI 321 (UNS S32108 / 06Cr18Ni11Ti) — Titanium-stabilized grade suitable for high-temperature applications (up to 870°C) where sensitization resistance is critical.
2.2 Mechanical Properties
| Property | Specification (AISI 304) | Specification (AISI 316) | Test Standard |
|---|---|---|---|
| Tensile Strength (Rm) | ≥ 520 MPa | ≥ 515 MPa | ASTM A262 / GB/T 228 |
| Yield Strength (Rp0.2) | ≥ 205 MPa | ≥ 205 MPa | ASTM A262 / GB/T 228 |
| Elongation (A50mm) | ≥ 40% | ≥ 40% | ASTM A262 / GB/T 228 |
| Hardness (HBW) | ≤ 217 | ≤ 217 | GB/T 231 |
| Density (ρ) | 7.93 g/cm³ | 8.00 g/cm³ | — |
| Thermal Conductivity (λ, 20°C) | 16.3 W/(m·K) | 14.6 W/(m·K) | ASTM E1461 |
| Thermal Expansion (α, 100°C) | 17.3 × 10⁻⁶ /°C | 17.3 × 10⁻⁶ /°C | ASTM E228 |
2.3 Corrosion Resistance
The austenitic microstructure (face-centered cubic, FCC) of the selected stainless steel grades provides inherent resistance to uniform corrosion, intergranular corrosion, and stress corrosion cracking (SCC). The passive chromium oxide film (Cr₂O₃), formed spontaneously upon exposure to atmospheric oxygen, acts as a self-healing protective barrier. In accordance with the revision recommendations for GB/T 19232—2003, the coil tubes are subjected to accelerated corrosion testing including:
- Pitting Resistance — Measured via PREN (Pitting Resistance Equivalent Number): PREN = %Cr + 3.3×%Mo + 16×%N
- Intercrystalline Corrosion — 100-hour boil-acid test (ASTM A262 Practice E)
- Crevice Corrosion — Ferric chloride immersion test (ASTM G48)
3. Specifications
| Parameter | Specification | Tolerance | Reference Standard |
|---|---|---|---|
| Outer Diameter (OD) | 6.35 mm (1/4"), 9.52 mm (3/8"), 12.70 mm (1/2"), 15.88 mm (5/8") | ±0.10 mm | GB/T 19232—2003; ASTM B880 |
| Wall Thickness (WT) | 0.50 mm – 1.50 mm | ±0.05 mm | GB/T 19232—2003 |
| Coil Diameter (CD) | 200 mm – 800 mm (adjustable per design) | ±5 mm | GB/T 19232—2003 |
| Coil Turns | 2 – 12 turns | ±0.5 turns | Design specification |
| Tube Length (Total Developed) | 1.5 m – 15.0 m | ±25 mm | GB/T 19232—2003 |
| Bend Radius (Minimum) | ≥ 2.5 × OD | — | ASTM B880 |
| Surface Finish | Mirror polish (Ra ≤ 0.25 μm) or Commercial finish (Ra ≤ 0.8 μm) | — | ASTM B880 |
| End Condition | Plain, Beveled (37.5°), or Flared | Per drawing | ASME B31.9 |
| Operating Pressure | ≤ 1.6 MPa (232 psi) | — | GB/T 19232—2003 |
| Hydrostatic Test Pressure | 2.4 MPa (348 psi) for 30 seconds | — | GB/T 19232—2003 |
| Operating Temperature | −40°C to +150°C | — | Material datasheet |
| Heat Exchange Efficiency | ≥ 92% (design condition) | — | GB/T 19232—2003 |
4. Manufacturing Process
4.1 Raw Material Preparation
Hot-rolled stainless steel strip is sourced from certified mills and subjected to incoming inspection including chemical composition analysis (optical emission spectrometry, OES), grain size verification, and visual surface examination. The strip is then cold-rolled to the required thickness with tight dimensional control (±0.02 mm), followed by annealing at 1050°C – 1100°C in a controlled atmosphere (dew point ≤ −60°C) to restore ductility and achieve a uniform austenitic microstructure free of carbide precipitation.
4.2 Cold Forming and Coiling
The annealed strip is fed into a precision rotary-draw bending (RDB) machine equipped with servo-controlled tooling. The coiling process follows these critical parameters:
- Forming Die Selection — Matched die radius to target bend radius to minimize wall thinning (maximum allowable thinning: 12% of original wall thickness).
- Bend Speed Control — Programmable speed reduction at bend initiation and completion to prevent springback and ovality.
- Support Mandrel — Internal mandrel support during bending to maintain circularity (ovality ≤ 2% of OD).
- Interpass Annealing — For multi-pass coiling of thick-wall tubes, intermediate annealing at 850°C restores formability.
4.3 Surface Treatment and Finishing
- Pickling and Passivation — Immersion in a mixed acid solution (HNO₃ + HF, 20–30%) followed by passivation in 20–30% HNO₃ to remove scale and restore the passive chromium oxide layer.
- Mechanical Polishing — Multi-stage polishing (grit 240 → 600 → 1200 → 2000) using abrasive belts for mirror finish applications.
- Electrolytic Polishing — Optional finishing step achieving Ra ≤ 0.1 μm for hygienic or high-flow-velocity applications.
4.4 Welding and Assembly
Where coil tube assemblies require connection to headers or manifolds, TIG (Tungsten Inert Gas) welding per ASME B31.9 is employed using argon shielding (99.99% purity) with internal back-purging. Weld parameters are optimized to minimize heat input and prevent sensitization in the heat-affected zone (HAZ). Post-weld passivation is mandatory to restore corrosion resistance at weld regions.
4.5 Quality Control and Testing
- Dimensional Inspection — Coordinate Measuring Machine (CMM) verification of OD, wall thickness, coil diameter, and total length.
- Hydrostatic Pressure Test — 100% testing at 2.4 MPa for 30 seconds with zero leakage tolerance.
- Visual Inspection — 100% examination for surface defects, weld quality, and geometric accuracy.
- Material Verification — Positive Material Identification (PMI) via XRF (X-Ray Fluorescence) for every production lot.
- Performance Testing — Sample-based heat exchange efficiency verification per GB/T 19232—2003 test procedures.
5. Applications
5.1 Primary Applications
- Commercial HVAC Systems — Fan coil units in office buildings, shopping malls, hospitals, and hotels. The coil tube serves as the refrigerant-side heat exchanger, transferring thermal energy between the circulating refrigerant and the air stream moved by the FCU fan.
- Residential Air Conditioning — Ducted and ductless split-system indoor units where space-constrained coil geometries are required.
- Process Cooling — Precision temperature control in semiconductor manufacturing, pharmaceutical cleanrooms, and laboratory environments.
5.2 Technical Advantages of Stainless Steel Coil Tubes
Note on Standard Revision: The GB/T 19232—2003 standard, while foundational, has been identified to contain ambiguities in thermal performance rating, material specification requirements, and long-term durability testing protocols. Wenzhou Jixing Metal Materials Co., Ltd. addresses these gaps by exceeding baseline standard requirements and adopting international best practices.
- Corrosion Durability — Unlike carbon steel coil tubes requiring internal coatings, stainless steel provides intrinsic corrosion resistance, eliminating concerns about coating degradation and internal fouling.
- Hygienic Performance — Smooth surface finish minimizes biofilm formation and particulate accumulation, critical for pharmaceutical and food-processing HVAC.
- Thermal Stability — Austenitic stainless steel maintains mechanical integrity across the full operating temperature range without phase transformation or significant property degradation.
- Weldability — Excellent weldability facilitates on-site fabrication and repair without loss of corrosion resistance in properly executed joints.
- Long Service Life — Design life exceeding 25 years under normal operating conditions, significantly outperforming copper coil alternatives in aggressive environments.
6. Quality Certifications
| Certification / Standard | Scope | Applicability |
|---|---|---|
| ISO 9001:2015 | Quality Management System | Enterprise-wide manufacturing process |
| ISO 14001:2015 | Environmental Management System | Sustainable manufacturing practices |
| GB/T 19232—2003 | Fan Coil Unit Specifications | Product performance compliance |
| ASTM B880 | Stainless Steel Seamless and Welded Pipe | Material and dimensional conformance |
| ASME B31.9 | Process Piping | Pressure vessel and piping code compliance |
| EN 10217-7 | Stainless Steel Tubes for Mechanical Uses | European market conformance |
| CE Marking (PED 2014/68/EU) | Pressure Equipment Directive | European regulatory compliance for pressure-rated assemblies |
| NSF/ANSI 61 | Drinking Water System Components | Health effects verification for water-contact applications |
6.1 Traceability and Documentation
Each production batch is accompanied by a comprehensive Quality Certificate (Mill Test Certificate, MTC) documenting:
- Chemical