Stainless Steel Coil Tube (盘管) — HVAC Fan Coil Unit Application
Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd. (温州吉兴金属材料有限公司)
Product Catalog No.: 1132
Classification: Technical Literature & Application Case Study
Reference Document: "Analysis of Frozen Fan Coil Tube Failures During Initial Heating Season in a Xi'an Commercial Building"
1. Product Overview
The Stainless Steel Coil Tube (盘管 / Coil Tube) is a precision-engineered tubular component manufactured by Wenzhou Jixing Metal Materials Co., Ltd. It is primarily designed for use in HVAC (Heating, Ventilation, and Air Conditioning) systems, specifically within Fan Coil Units (FCUs, 风机盘管) that serve commercial, residential, and industrial buildings. These coil tubes function as the primary heat exchange medium, transferring thermal energy between the circulating fluid (water or refrigerant) and the surrounding air stream.
Wenzhou Jixing Metal Materials Co., Ltd. specializes in the production of high-grade stainless steel tubing and pipe products with strict adherence to international quality standards. The coil tube product line is engineered to withstand demanding thermal cycling, corrosive environments, and prolonged service life, making it an ideal solution for building services engineering.
The associated technical case study — "Analysis of Frozen Fan Coil Tube Failures During Initial Heating Season in a Xi'an Commercial Building" — provides critical engineering insight into failure modes encountered during the commissioning phase of heating systems in cold climate regions. This analysis underscores the importance of proper material selection, installation methodology, and system design in preventing catastrophic coil tube failures.
2. Material Composition & Mechanical Properties
2.1 Base Material
The coil tubes are fabricated from Austenitic Stainless Steel (奥氏体不锈钢), with the following primary grades available:
- 304 Stainless Steel (UNS S30400): General-purpose grade offering excellent corrosion resistance and formability; the most widely used grade in HVAC applications.
- 316 Stainless Steel (UNS S31600): Enhanced molybdenum content provides superior resistance to pitting and crevice corrosion, particularly suitable for coastal or chemically aggressive environments.
- 321 Stainless Steel (UNS S32100): Titanium-stabilized grade offering improved resistance to intergranular corrosion at elevated temperatures.
2.2 Chemical Composition (Typical for Grade 304)
| Element | Content (%) | Element | Content (%) |
|---|---|---|---|
| Carbon (C) | ≤ 0.08 | Chromium (Cr) | 18.0 – 20.0 |
| Nickel (Ni) | 8.0 – 10.5 | Manganese (Mn) | ≤ 2.0 |
| Silicon (Si) | ≤ 1.0 | Phosphorus (P) | ≤ 0.045 |
| Sulfur (S) | ≤ 0.030 | Nitrogen (N) | ≤ 0.030 |
| Iron (Fe) | Balance | — | — |
2.3 Mechanical Properties (Annealed Condition)
| Property | Specification | Test Standard |
|---|---|---|
| Tensile Strength (抗拉强度) | ≥ 520 MPa | GB/T 228 / ASTM A380 |
| Yield Strength (屈服强度) | ≥ 205 MPa | GB/T 228 / ASTM A380 |
| Elongation (延伸率) | ≥ 40% | GB/T 228 / ASTM A380 |
| Hardness (硬度) | ≤ 217 HB | GB/T 231 |
| Density (密度) | 7.93 g/cm³ | — |
| Thermal Conductivity (导热系数) | 16.3 W/(m·K) at 100°C | — |
| Thermal Expansion Coefficient (热膨胀系数) | 17.3 × 10⁻⁶ /K (20–100°C) | — |
2.4 Corrosion Resistance
- Pitting Resistance (点蚀抗力): PREN (Pitting Resistance Equivalent Number) ≥ 19 for Grade 304; ≥ 24 for Grade 316.
- Intergranular Corrosion Resistance (晶间腐蚀抗力): Passes ASTM A262 Practice E (acid solution test) without pre-treatment for Grade 321.
- Chloride Environment Tolerance: Suitable for indoor HVAC applications with water quality up to 200 ppm Cl⁻ (Grade 304); up to 1000 ppm Cl⁻ (Grade 316).
3. Technical Specifications
| Parameter | Specification | Standard Reference |
|---|---|---|
| Product Type | Coil Tube / 盘管 (Spiral-wound heat exchange tube) | — |
| Material Grade | SUS 304 / 316 / 321 (Austenitic Stainless Steel) | ASTM A269 / GB/T 12771 |
| Outer Diameter (OD) | φ 6.0 mm – φ 22.0 mm (standard range) | ASTM A269 |
| Wall Thickness (WT) | 0.50 mm – 2.00 mm | ASTM A269 |
| Tube Length (per coil) | 3 m – 30 m (configurable) | — |
| Coil Winding Diameter | φ 150 mm – φ 600 mm | — |
| Surface Finish | Mirror Polish (Ra ≤ 0.4 μm) / Brushed / Matte | GB/T 8170 |
| Operating Temperature Range | −196°C to +870°C | — |
| Maximum Operating Pressure | 1.6 MPa (standard); up to 4.0 MPa (thick-wall) | GB 150 |
| Hydrostatic Test Pressure | 1.5 × Maximum Operating Pressure | GB/T 24597 |
| Tolerance on OD | ± 0.05 mm (precision grade) | ASTM A269 |
| Tolerance on Wall Thickness | ± 10% of nominal | ASTM A269 |
| Delivery Form | Coiled / Straight lengths / Pre-formed coil assemblies | — |
| Packaging | Wooden pallet with moisture-proof wrapping; export-grade packaging available | ISO 4180 |
4. Manufacturing Process
4.1 Production Flow
- Raw Material Procurement (原材料采购): Sourcing of certified stainless steel coils from approved mills (e.g., Baosteel, Outokumpu, POSCO) with full mill test certificates (MTC/EN 10204 Type 3.1).
- Slitting & Leveling (分条与平整): Coils are slit to required width and leveled to ensure flatness and dimensional accuracy.
- Tube Forming (制管): Continuous cold-rolling process to form seamless tubes from flat strip; or butt-welded tube production for larger diameters.
- Heat Treatment (热处理): Solution annealing at 1050–1150°C followed by controlled cooling to achieve full austenitic microstructure and maximum corrosion resistance.
- Pickling & Passivation (酸洗钝化): Removal of oxide scale followed by passivation treatment to form a protective chromium oxide layer (Cr₂O₃).
- Coil Winding (盘绕成型): Precision winding machines form tubes into specified coil geometries with uniform pitch and tight winding tolerance.
- End Fitting & Brazing (端部加工与钎焊): Installation of headers, end caps, and brazing joints using silver-based filler metals (e.g., BAg-17C per GB/T 10044) for leak-tight connections.
- Quality Inspection (质量检测): Hydrostatic pressure testing, dimensional verification, visual inspection, and non-destructive testing (NDT) as applicable.
- Packaging & Dispatch (包装发货): Export-grade packaging with product identification labels, material certificates, and shipping documentation.
4.2 Key Process Controls
- In-line Optical Inspection: Automated detection of surface defects, wall thickness variations, and dimensional deviations during tube forming.
- Positive Material Identification (PMI): X-ray fluorescence (XRF) spectrometry verification of chemical composition at each production lot.
- Process Capability Index (Cpk): Cpk ≥ 1.33 maintained for critical dimensions (OD, wall thickness, coil pitch).
5. Applications
5.1 Primary Applications
- HVAC Fan Coil Units (HVAC 风机盘管): Core heat exchange component in commercial building air conditioning systems. The coil tube transfers heat between the water circuit and the air stream, enabling precise temperature control in offices, hotels, hospitals, and shopping centers.
- Chilled Water Cooling Coils (冷冻水盘管): Used in cooling mode where chilled water circulates through the coil tubes to dehumidify and cool supply air.
- Heating Coils (热水盘管): Used in heating mode where hot water circulates through the coil tubes to warm indoor spaces.
- VRF/VAV Systems (变风量/变制冷剂流量系统): Integral component of variable refrigerant flow and variable air volume terminal units.
5.2 Secondary Applications
- Industrial process heat exchangers
- Refrigeration evaporator and condenser coils
- Automotive air conditioning heat exchangers
- Food processing and pharmaceutical clean-room HVAC systems
- Marine and offshore HVAC installations (Grade 316 recommended)
5.3 Engineering Case Study — Xi'an Commercial Building Heating Commissioning
The reference document "Analysis of Frozen Fan Coil Tube Failures During Initial Heating Season in a Xi'an Commercial Building" documents a critical failure event during the initial heating commissioning of a commercial high-rise in Xi'an, Shaanxi Province, China. Key findings include:
- Failure Mode: Multiple fan coil units experienced tube rupture due to ice formation and subsequent hydraulic expansion pressure exceeding the burst pressure of the coil tubes.
- Root Cause Analysis: Inadequate antifreeze protection during commissioning, improper system flushing procedures, and insufficient insulation on exposed coil sections in unheated zones.
- Material Assessment: The stainless steel coil tubes themselves demonstrated adequate mechanical integrity; failures were attributed to design and installation deficiencies rather than material degradation.
- Recommendations: Use of properly specified stainless steel coil tubes with verified pressure ratings, implementation of comprehensive commissioning protocols including pressure testing and antifreeze verification, and adherence to GB 50243 (Code for Acceptance of Construction Quality of HVAC Engineering).
This case study reinforces the importance of selecting high-quality stainless steel coil tubes from certified manufacturers and following rigorous installation and commissioning procedures to prevent premature failures in cold climate heating systems.
6. Quality Certifications & Compliance
6.1 Manufacturer Certifications
- ISO 9001:2015 — Quality Management System (质量管理体系)
- ISO 14001:2015 — Environmental Management System (环境管理体系)
- ISO 45001:2018 — Occupational Health and Safety Management System (职业健康安全管理体系)
- CE Marking — European Conformity for relevant product categories
6.2 Product Standards Compliance
| Standard | Description | Scope |
|---|---|---|
| ASTM A269 / A269M | Standard Specification for Seamless and Welded Austenitic Stainless Steel Tubing | Material & Dimensions |
| GB/T 12771 | Seamless Stainless Steel Tubes for General Purposes (中国国家标准) | Material & Dimensions |
| GB/T 24597 | Hydrostatic Test Methods for Steel Tubes | Pressure Testing |
| GB 50243 | Code for Acceptance of Construction Quality of HVAC Engineering | Installation & Commissioning |
| EN 10217-7 | Heat Exchanger Tubes — Technical Delivery Conditions — Austenitic Steels |