Stainless Steel Coil Tube — HVAC Fresh Air Unit Heating Application
Product Overview
The Stainless Steel Coil Tube (catalog reference No. 333) is a precision-engineered heat exchange component manufactured by Wenzhou Jixing Metal Materials Co., Ltd., designed for integration into Fresh Air Handling Units (FAHU) and related HVAC systems. This coil tube assembly serves as the core heating element within air distribution systems, facilitating efficient thermal energy transfer between the circulating heat medium and the incoming outdoor air stream.
Unlike conventional copper or carbon steel coil tubes, the stainless steel variant offers superior corrosion resistance, long-term structural integrity, and hygienic performance, making it particularly suitable for applications in cleanroom environments, food processing facilities, pharmaceutical manufacturing, and commercial ventilation systems where both thermal efficiency and material durability are critical.
Key Distinction: This product addresses a critical engineering challenge — anti-freezing protection of heating coil tubes in fresh air units — by leveraging the thermal conductivity, mechanical strength, and chemical stability of stainless steel to prevent low-temperature damage during winter operations or system shutdown periods.
Material Composition & Properties
The coil tube is fabricated from high-grade austenitic stainless steel, with the following material specifications:
- Primary Alloy: Stainless Steel Grade 304 (AISI 304 / SUS304) — the industry-standard austenitic grade offering excellent formability and corrosion resistance.
- Alternative Grade: Stainless Steel Grade 316L (AISI 316L / SUS316L) — available for aggressive chemical environments or marine/near-coastal installations where chloride-induced pitting corrosion is a concern.
- Chromium (Cr) Content: 18.0% – 20.0% (Grade 304) / 16.0% – 18.0% (Grade 316L)
- Nickel (Ni) Content: 8.0% – 10.5% (Grade 304) / 10.0% – 14.0% (Grade 316L)
- Molybdenum (Mo) Content: ≤ 0.08% (Grade 304) / 2.0% – 3.0% (Grade 316L)
- Carbon (C) Content: ≤ 0.08% (Grade 304) / ≤ 0.03% (Grade 316L — "L" denotes low-carbon variant)
Mechanical & Thermal Properties
| Property | Grade 304 | Grade 316L | Test Standard |
|---|---|---|---|
| Tensile Strength (抗拉强度) | ≥ 520 MPa | ≥ 485 MPa | ASTM A270 / ASTM A313 |
| Yield Strength (屈服强度) | ≥ 205 MPa | ≥ 170 MPa | ASTM A270 / ASTM A313 |
| Elongation (延伸率) | ≥ 40% | ≥ 40% | ASTM A270 / ASTM A313 |
| Thermal Conductivity (导热系数, 20°C) | 16.3 W/(m·K) | 16.3 W/(m·K) | ASTM A967 |
| Thermal Expansion Coefficient (热膨胀系数, 20–100°C) | 17.3 × 10⁻⁶ /°C | 17.3 × 10⁻⁶ /°C | ASTM A967 |
| Operating Temperature Range (工作温度范围) | −196°C to +870°C | −196°C to +870°C | ISO 15510 |
| Specific Heat Capacity (比热容) | 500 J/(kg·K) | 500 J/(kg·K) | ISO 15510 |
| Density (密度) | 8.0 g/cm³ | 8.0 g/cm³ | — |
Corrosion Resistance Characteristics
- Pitting Resistance (点蚀抗性): Grade 316L achieves a PREN (Pitting Resistance Equivalent Number) of ≥ 24, significantly outperforming Grade 304 (PREN ≥ 18).
- Intergranular Corrosion Resistance (晶间腐蚀抗性): Both grades pass the ASTM A270 intergranular corrosion test (5% sulfuric acid + copper sulfate solution).
- Resistance to Condensate Corrosion: The passive chromium oxide film (Cr₂O₃) provides self-healing protection against acidic condensate formed in HVAC environments with high humidity and trace pollutants.
- Chloride Ion Tolerance: Grade 316L withstands chloride concentrations up to 2,000 ppm at ambient temperatures without pitting initiation.
Technical Specifications
| Parameter | Specification | Notes |
|---|---|---|
| Tube Outer Diameter (OD) (管外径) | 6.35 mm, 9.53 mm, 12.70 mm, 15.88 mm | Standard imperial sizes: ¼", ⅜", ½", ⅝" |
| Tube Wall Thickness (壁厚) | 0.50 mm – 1.50 mm | Customizable per application requirements |
| Tube Length (管长) | 1.0 m – 12.0 m (continuous coil) | Coil form; straight sections available on request |
| Coil Pitch / Row Spacing (排间距) | 15 mm – 50 mm (adjustable) | Determines heat transfer area density |
| Coil Turn Diameter (盘绕直径) | 50 mm – 300 mm | Adaptable to FAHU casing dimensions |
| Surface Finish (表面光洁度) | 2B (cold-rolled) / No. 4 (brushed) / BA (bright annealed) | BA finish recommended for hygienic applications |
| Surface Roughness (表面粗糙度) | Ra ≤ 0.8 μm (standard) / Ra ≤ 0.4 μm (hygienic) | Low roughness reduces fouling and biofilm attachment |
| Heat Transfer Coefficient (传热系数) | 80 – 150 W/(m²·K) | Depends on flow velocity, fin configuration, and medium |
| Maximum Operating Pressure (最大工作压力) | 1.6 MPa – 4.0 MPa | Hydrostatic test at 1.5× working pressure |
| Heat Medium (热媒介质) | Hot water (40°C – 90°C), steam (≤ 0.6 MPa), thermal oil | Anti-freeze glycol mixtures compatible |
| Anti-Freeze Protection (防冻保护) | Integrated low-flow recirculation + trace heating compatible | Prevents ice formation and hydraulic hammering |
| Connection Method (连接方式) | Flanged, threaded, brazed, or welded (TIG / orbital) | Weld-free crimp connections available |
| Fin Configuration (翅片配置) | Plain tube / L-fin / V-fin / microchannel | Fin material: aluminum or stainless steel |
Anti-Freeze Protection Engineering — Technical Approach
A critical engineering consideration for stainless steel coil tubes deployed in fresh air handling units is the prevention of freezing during winter operation or system standby periods. The following multi-layered anti-freezing strategy is integrated into the product design:
- Low-Flow Recirculation Design (低流量循环设计): The coil tube geometry is optimized to maintain a minimum volumetric flow rate that prevents stagnation of the heat medium. Stagnant fluid in coil tubes is the primary cause of localized freezing and subsequent thermal stress cracking.
- Stainless Steel Thermal Inertia (不锈钢热惯性): The high specific heat capacity and thermal conductivity of stainless steel ensure that the coil tube wall retains residual heat longer than carbon steel or copper alternatives during system shutdown, delaying the onset of freezing conditions.
- Glycol-Antifreeze Compatibility (乙二醇防冻液兼容性): The stainless steel surface is fully compatible with ethylene glycol (EG) and propylene glycol (PG) antifreeze solutions at concentrations up to 60% by volume, without risk of galvanic corrosion or material degradation.
- Trace Heating Integration (伴热带集成): The coil tube support structure is designed to accommodate external electrical trace heating cables (typically 10–25 W/m), providing supplemental thermal protection during extended non-operational periods.
- Drainage & Venting Provisions (排水与排气配置): The coil tube assembly includes integrated drain ports and automatic air vent valves to facilitate complete drainage before seasonal shutdown, eliminating trapped water that could freeze and expand within the tube bore.
- Insulation Coordination (保温协调): The coil tube external surface is designed for seamless integration with closed-cell elastomeric or mineral wool insulation systems (R-value ≥ 2.5 m²·K/W), minimizing conductive heat loss to the ambient environment.
Manufacturing Process
The coil tube is produced through a multi-stage manufacturing process adhering to international quality standards:
- Material Sourcing & Incoming Inspection (材料采购与来料检验): Stainless steel seamless or welded tubes are procured from certified mills and subjected to chemical composition analysis (spectroscopy), mechanical testing, and dimensional verification per ASTM A270 / A313.
- Tube Cutting & Deburring (管材切割与去毛刺): Precision plasma or laser cutting to specified lengths, followed by mechanical deburring to ensure clean, burr-free tube ends suitable for brazing or welding.
- Forming & Coil Winding (成型与盘绕): Tubes are cold-formed into the required coil geometry using CNC-controlled winding machines. The forming process maintains controlled strain rates to avoid work-hardening-induced reduction in ductility.
- Heat Treatment (热处理): Solution annealing at 1050°C – 1150°C followed by rapid water quenching to restore full austenitic microstructure, eliminate residual stresses from forming, and maximize corrosion resistance. Stress-relief annealing at 425°C – 550°C is available for post-forming applications.
- Surface Treatment (表面处理): Acid pickling and passivation per ASTM A967 to remove surface contaminants and establish a stable chromium oxide passive film. Electropolishing is available for hygienic-grade applications to achieve Ra ≤ 0.4 μm.
- Fin Installation (翅片安装): Aluminum or stainless steel fins are mechanically attached to the coil tube via crimping, welding, or brazing. Fin density and geometry are optimized for the target heat transfer coefficient.
- Assembly & Header Fabrication (组装与集管制造): Individual coil tubes are brazed or welded to inlet/outlet headers (集管), forming the complete coil tube assembly. Brazing is performed using silver-bearing filler alloys (e.g., Cu-Ag 5% or Cu-Ag 30%) under argon protection to prevent oxidation.
- Quality Control & Testing (质量控制与测试): Each assembly undergoes hydrostatic pressure testing (1.5× maximum working pressure, held for 30 minutes), dimensional inspection, visual examination of welds, and optional eddy current testing for wall thickness verification.
Applications
The stainless steel coil tube is applicable across a broad spectrum of HVAC and industrial thermal management systems:
- Commercial Fresh Air Handling Units (商用新风机组): Primary heating element in FAHU systems for office buildings, shopping malls, hospitals, and educational facilities.
- Cleanroom HVAC Systems (洁净室暖通系统): Temperature control in ISO Class 5–8 cleanrooms in semiconductor fabrication, biotechnology, and pharmaceutical manufacturing.
- Food & Beverage Processing (食品加工): Hygienic air heating in food production lines, cold storage ventilation, and bakery systems where corrosion resistance and cleanability are paramount.
- Pharmaceutical & Medical Facilities (医药与医疗设施): GMP-compliant air handling in sterile manufacturing environments and hospital air distribution systems.
- Industrial Ventilation (工业通风): Pre-heating of outdoor air for industrial exhaust systems, boiler air supply, and process air conditioning.
- Agricultural & Greenhouse Climate Control (农业与温室气候控制): Temperature regulation in controlled-environment agriculture where high humidity and chemical treatments create corrosive conditions.
- Marine & Offshore Applications (海洋与海上应用): HVAC systems on vessels and offshore platforms where saltwater corrosion resistance is essential (Grade 316L recommended).
Quality Certifications & Compliance
Wenzhou Jixing Metal Materials Co., Ltd. maintains the following certifications and complies with the listed standards:
| Certification / Standard | Description |
|---|---|
| ISO 9001:2015 | Quality Management System (QMS) certification for manufacturing processes |
| ISO 14001:2015 | Environmental Management System (EMS) certification |
| ISO 45001:2018 | Occupational Health and Safety Management System certification |
| EN 10204 3.1 | Material test certificates conforming to European standard for product inspection |
| ASTM A270 / A313 | Compliance with American Society for Testing and Materials standards for stainless steel tubes |
| ASME B31.9 | Pressure Piping |