Stainless Steel Coil Tube for HVAC Air Purification Systems
1. Product Overview
The Stainless Steel Coil Tube (also referred to as Coil Tube or Helical Coil) is a precision-engineered tubular component manufactured by Wenzhou Jixing Metal Materials Co., Ltd. Specifically designed for integration into central air conditioning air handling units (AHUs) and fan coil units (FCUs), these coil tubes serve as critical structural and functional elements within air purification assemblies installed at return air ducts. The product is engineered to provide a durable, corrosion-resistant conduit through which air flows during the purification and conditioning process, ensuring long-term operational reliability in demanding indoor air quality (IAQ) environments.
From a metallurgical perspective, the coil tube represents a specialized form of cold-formed tubing where a straight stainless steel pipe is mechanically coiled into a continuous helical geometry without compromising the metallurgical integrity of the base material. This configuration maximizes heat transfer surface area per unit volume while maintaining structural rigidity under cyclic thermal and pressure loading conditions typical of HVAC applications.
2. Material Composition and Properties
2.1 Base Material Grades
The coil tubes are fabricated from austenitic stainless steel grades conforming to international standards. The primary grades utilized include:
- SAE 304 (UNS S30400) — The most commonly specified grade for general HVAC applications, offering excellent corrosion resistance, formability, and weldability.
- SAE 316 (UNS S31600) — Selected for applications requiring superior resistance to chlorides and aggressive environmental contaminants, incorporating 2.0–2.5% molybdenum.
- SAE 321 (UNS S32100) — Tantalum-stabilized variant employed where elevated operating temperatures or thermal cycling resistance is required.
2.2 Chemical Composition (SAE 304 Reference)
| Element | Composition (%) | Function |
|---|---|---|
| Carbon (C) | ≤ 0.08 | Controls precipitation hardening; low carbon prevents sensitization |
| Chromium (Cr) | 18.0 – 20.0 | Primary corrosion-resistant element; forms passive Cr₂O₃ film |
| Nickel (Ni) | 8.0 – 10.5 | Stabilizes austenitic microstructure; enhances ductility |
| Manganese (Mn) | ≤ 2.0 | Partial nickel substitute; improves hot workability |
| Silicon (Si) | ≤ 0.75 | Deoxidizer; enhances high-temperature strength |
| Phosphorus (P) | ≤ 0.045 | Impurity control; limits intergranular corrosion susceptibility |
| Sulfur (S) | ≤ 0.030 | Impurity control; maintains weldability |
| Iron (Fe) | Balance | Matrix element |
2.3 Mechanical Properties
| Property | Specification (SAE 304, Annealed) | Test Standard |
|---|---|---|
| Tensile Strength (UTS) | 515 – 710 MPa | ASTM A270 / GB/T 24511 |
| Yield Strength (0.2% Offset) | ≥ 205 MPa | ASTM A270 |
| Elongation (in 50 mm gauge) | ≥ 40% | ASTM A270 |
| Hardness (Rockwell B) | ≤ 95 HRB | ASTM A270 |
| Thermal Conductivity (at 25°C) | 16.3 W/(m·K) | ASTM E153 |
| Thermal Expansion Coefficient (20–100°C) | 16.9 × 10⁻⁶ /°C | ASTM E228 |
2.4 Surface Finish Characteristics
- Standard Finish: No. 4 Brushed (2B Mill Finish) with surface roughness Ra ≤ 0.8 μm
- Available Options: No. 8 Mirror Polish (Ra ≤ 0.25 μm), Electropolished (Ra ≤ 0.1 μm), Matte Passivated
- Passivation Treatment: Nitric acid or citric acid passivation per ASTM A967 to enhance passive film stability
3. Technical Specifications
| Parameter | Specification | Tolerance |
|---|---|---|
| Outer Diameter (OD) | φ6.0 mm – φ25.0 mm (custom sizes available) | ±0.10 mm |
| Wall Thickness | 0.4 mm – 1.5 mm | ±10% |
| Coil Diameter (Mean) | φ50 mm – φ300 mm | ±2.0 mm |
| Coil Length (Uncoiled) | 1.0 m – 50.0 m (continuous) | ±50 mm |
| Coil Pitch (Spacing) | 8 mm – 25 mm (adjustable) | ±0.5 mm |
| Number of Turns | 10 – 500 turns | ±1 turn |
| End Forming | Flat, flared, threaded, or brazed connection | Per drawing |
| Operating Temperature Range | −196°C to +870°C (SAE 304) | — |
| Maximum Working Pressure | ≤ 2.5 MPa (dependent on geometry) | — |
| Surface Roughness (ID) | Ra ≤ 1.6 μm (standard); Ra ≤ 0.4 μm (electropolished) | — |
| Weight (Approximate) | 0.03 – 0.15 kg/m (depending on OD and wall) | — |
4. Manufacturing Process
4.1 Process Flow
- Raw Material Selection: Hot-rolled stainless steel strip or seamless tube blanks conforming to ASTM A269/A270 are selected and inspected for chemical composition and mechanical properties.
- Cold Drawing / Seamless Forming: Straight tube is produced via cold-drawing or cold-rolling processes to achieve precise dimensional tolerances and enhanced mechanical properties through work hardening.
- Annealing: Solution annealing at 1050–1100°C followed by rapid quenching to restore full austenitic ductility and eliminate residual stresses from forming operations.
- Coil Forming (Mechanical Coiling): The straight tube is fed through precision-guided tooling to form a continuous helical coil. Computer Numerical Control (CNC) systems ensure consistent coil pitch, diameter, and turn count. The coiling process is performed in a cold-forming manner to preserve the metallurgical properties of the austenitic structure.
- Heat Treatment (Optional): Stress-relief annealing at 300–400°C for coil tubes subjected to significant forming deformation, to prevent springback and dimensional instability.
- Surface Treatment: Acid pickling and passivation treatment to remove scale, contamination, and restore the passive chromium oxide layer. Electropolishing is available for critical applications requiring ultra-smooth internal surfaces.
- End Processing: Cutting, flaring, threading, or brazing of end connections as specified by the application requirements.
- Quality Inspection: Comprehensive dimensional verification, visual inspection, hydrostatic pressure testing, and surface finish measurement.
4.2 Critical Manufacturing Considerations
The coiling of stainless steel tubes presents unique metallurgical challenges. The cold-forming process introduces plastic deformation that can lead to work hardening, particularly at the outer fiber of the coil where tensile strain is concentrated. For SAE 304 austenitic stainless steel, the strain hardening exponent (n-value) of approximately 0.5–0.6 provides adequate formability, but careful control of the bend radius relative to tube diameter (R/D ratio ≥ 5:1 recommended) is essential to prevent cracking or ovalization. The non-magnetic character of the annealed austenitic structure is partially compromised by cold working, with induced ferromagnetism proportional to the degree of deformation — a consideration for applications involving electromagnetic compatibility (EMC).
5. Applications
5.1 Primary Application: HVAC Air Purification Systems
The primary application domain for this product is the installation within central air conditioning fan coil units, specifically at return air vents where air purification devices are deployed. The coil tube configuration provides:
- Extended Air Path Length: The helical geometry significantly increases the effective air contact time with purification media (activated carbon filters, HEPA filters, photocatalytic surfaces), enhancing particulate removal efficiency and volatile organic compound (VOC) adsorption.
- Thermal Regulation: Stainless steel's moderate thermal conductivity allows the coil tube to function as a passive heat exchange element, stabilizing air temperature as it passes through the purification zone.
- Structural Support: The coiled configuration provides inherent rigidity while accommodating thermal expansion and contraction of the surrounding ductwork.
5.2 Secondary Applications
- Industrial process heating and cooling coils
- Refrigeration evaporator and condenser tubes
- Medical gas delivery systems
- Food and beverage processing equipment
- Chemical processing heat exchangers
- Automotive exhaust aftertreatment systems
6. Quality Certifications and Standards Compliance
| Certification / Standard | Scope | Relevance |
|---|---|---|
| ISO 9001:2015 | Quality Management System | Ensures consistent manufacturing quality and process control |
| ISO 14001:2015 | Environmental Management System | Environmental sustainability in production |
| ASTM A270 / A269 | Stainless Steel Tubing Standards | Material specification and mechanical property verification |
| GB/T 12771 | Chinese National Standard for Stainless Steel Seamless Tubes | Domestic compliance for Chinese market applications |
| EN 10216-5 | European Standard for Heat Exchanger Tubes | International export compliance |
| NSF/ANSI 61 | Drinking Water System Components | Applicable for potable water system integration |
| CE Marking | European Conformity | Market access for European HVAC installations |
7. Unique Selling Points
- Superior Corrosion Resistance: The austenitic stainless steel matrix provides exceptional resistance to atmospheric corrosion, condensation-induced degradation, and chemical attack from purification agents — significantly extending service life compared to carbon steel alternatives in humid HVAC environments.
- Hygienic Surface Integrity: Electropolished variants achieve Ra ≤ 0.1 μm surface roughness, eliminating micro-crevices where microbial biofilm can accumulate — critical for maintaining indoor air quality (IAQ) standards per ASHRAE 62.1.
- Thermal Stability: Operating capability from cryogenic temperatures (−196°C) to elevated temperatures (870°C) ensures dimensional stability and mechanical integrity across the full thermal range encountered in HVAC systems.
- Non-Magnetic Character: The austenitic microstructure maintains non-magnetic properties in the annealed condition, preventing interference with electromagnetic components within air handling units.
- Full Traceability: Each batch is accompanied by a Mill Test Certificate (MTC) documenting chemical composition, mechanical properties, and heat treatment history per EN 10204 Type 3.1 requirements.
- Custom Engineering Capability: Wenzhou Jixing Metal Materials Co