Stainless Steel Coil Tube (Coiltube) — Technical Specification
1. Product Overview
The Stainless Steel Coil Tube (Coiltube) is a precision-engineered tubular product manufactured by Wenzhou Jixing Metal Materials Co., Ltd., designed for integration into HVAC (Heating, Ventilation, and Air Conditioning) systems, particularly as a replacement component for traditional single-pipe parallel-flow radiator assemblies. This coil tube serves as a high-efficiency heat exchange element within fan coil unit (FCU) systems, offering superior thermal performance, corrosion resistance, and long-term reliability compared to conventional carbon steel alternatives.
The product is categorized under technical literature and engineering study materials, reflecting its role as a subject of academic and industrial analysis in thermal-hydraulic system optimization. The coil tube geometry enables compact installation footprints while maximizing surface-area-to-volume ratios, making it an ideal solution for space-constrained commercial and residential applications.
2. Material & Properties
The coil tube is fabricated from high-grade austenitic stainless steel, selected for its exceptional combination of mechanical strength, chemical resistance, and thermal conductivity.
2.1 Material Grades
- 304 Stainless Steel (UNS S30400): Primary grade offering excellent general corrosion resistance, weldability, and formability. Suitable for most indoor HVAC applications with non-aggressive water treatment chemicals.
- 316 Stainless Steel (UNS S31600): Enhanced grade with added molybdenum (2–3%), providing superior resistance to chloride-induced pitting and crevice corrosion. Recommended for coastal environments or systems utilizing aggressive chemical treatments.
- 316L Stainless Steel (UNS S31603): Low-carbon variant of 316, offering improved resistance to intergranular corrosion during welding and heat treatment processes.
2.2 Mechanical Properties
| Property | 304 Grade | 316 Grade | 316L Grade |
|---|---|---|---|
| Tensile Strength (MPa) | ≥ 520 | ≥ 520 | ≥ 485 |
| Yield Strength (MPa) | ≥ 205 | ≥ 205 | ≥ 170 |
| Elongation (%) | ≥ 40 | ≥ 40 | ≥ 40 |
| Thermal Conductivity (W/m·K) at 20°C | 16.3 | 14.6 | 14.6 |
| Thermal Expansion Coefficient (×10⁻⁶/K) | 17.3 | 17.8 | 17.8 |
2.3 Chemical Composition (wt%)
| Element | 304 | 316 | 316L |
|---|---|---|---|
| Carbon (C) | ≤ 0.08 | ≤ 0.08 | ≤ 0.03 |
| Chromium (Cr) | 18.0–20.0 | 16.0–18.0 | 16.0–18.0 |
| Nickel (Ni) | 8.0–10.5 | 10.0–14.0 | 10.0–14.0 |
| Molybdenum (Mo) | — | 2.0–3.0 | 2.0–3.0 |
| Manganese (Mn) | ≤ 2.0 | ≤ 2.0 | ≤ 2.0 |
| Silicon (Si) | ≤ 1.0 | ≤ 1.0 | ≤ 1.0 |
3. Specifications
| Parameter | Specification | Notes |
|---|---|---|
| Tube Outer Diameter (OD) | Φ6.0 mm – Φ16.0 mm | Standard sizes; custom diameters available upon request |
| Wall Thickness | 0.3 mm – 1.0 mm | Selected based on operating pressure and temperature |
| Coil Length | 0.5 m – 50.0 m | Continuous coil or cut-to-length |
| Bend Radius | ≥ 3 × Tube OD (minimum) | Ensures no cross-sectional distortion |
| Operating Pressure | 0.1 MPa – 1.6 MPa | Dependent on material grade and wall thickness |
| Operating Temperature | −40°C to +450°C | Continuous operation range |
| Surface Finish | Ba ≤ 0.4 μm (polished) or 2B (cold-rolled) | Polished finish recommended for high-flow applications |
| End Forming | Plain, flanged, threaded, or soldered | Custom end configurations available |
| Tolerance (OD) | ±0.05 mm (precision grade) | ISO 1128 tolerance class |
| Coil Pitch | 8 mm – 25 mm (adjustable) | Determined by heat exchange requirements |
4. Manufacturing Process
The production of stainless steel coil tubes follows a rigorous multi-stage process ensuring dimensional accuracy, metallurgical integrity, and surface quality:
4.1 Raw Material Preparation
- Hot Rolling: Stainless steel ingots are hot-rolled into strip or sheet form at temperatures between 1100°C and 1250°C to achieve initial thickness reduction and homogenization of microstructure.
- Pickling and Annealing: The hot-rolled strip undergoes acid pickling to remove oxide scale, followed by solution annealing at 1050°C–1150°C in a protective atmosphere (argon or vacuum) to dissolve carbides and restore ductility.
- Cold Rolling: The annealed strip is cold-rolled to the final wall thickness, inducing work hardening and improving surface finish. Intermediate annealing cycles may be inserted to prevent excessive strain hardening.
4.2 Tube Forming
- Seam Welding: The cold-rolled strip is formed into a tube using high-frequency induction welding (HFW) or orbital TIG (Tungsten Inert Gas) welding. HFW is preferred for production efficiency, achieving weld speeds up to 100 m/min with weld penetration exceeding 95% of wall thickness.
- Post-Weld Annealing: The welded tube is solution-annealed to relieve welding residual stresses and homogenize the weld zone microstructure, ensuring consistent mechanical properties throughout the tube cross-section.
- Piercing and Sizing: The tube is pierced to the final outer diameter and passed through cold-drawing or cold-rolling dies to achieve precise dimensional tolerances.
4.3 Coil Forming and Heat Exchanger Assembly
- Coil Winding: The straight tube is wound into a helical coil configuration using CNC (Computer Numerical Control) coil-forming machinery. The winding process maintains constant bend radius and pitch, preventing ovality, kinking, or wall thinning at bend locations.
- Tube End Finishing: Tube ends are cut, deburred, and formed into the required connection geometry (flanges, threaded fittings, or plain ends for brazing/soldering).
- Surface Treatment: Optional electropolishing or passivation treatment is applied to enhance corrosion resistance and reduce surface roughness, improving fluid flow characteristics and heat transfer efficiency.
4.4 Quality Control
- Dimensional Inspection: Laser micrometers and coordinate measuring machines (CMM) verify OD, wall thickness, coil pitch, and overall length to ±0.05 mm tolerance.
- Non-Destructive Testing (NDT): Eddy current testing (ECT) and ultrasonic testing (UT) inspect weld seams for defects such as porosity, incomplete fusion, or cracks. Radiographic testing (RT) is available for critical applications.
- Hydrostatic Pressure Testing: Each coil tube is pressure-tested at 1.5 times the maximum operating pressure, held for a minimum of 60 seconds, to verify leak-tightness and structural integrity.
- Chemical Analysis: Spectrometric analysis (OES — Optical Emission Spectroscopy) confirms chemical composition compliance with specified material grades.
5. Applications
5.1 HVAC Systems — Fan Coil Unit Replacement
The primary application domain of this coil tube product is the replacement of traditional single-pipe parallel-flow radiators within fan coil unit systems. In conventional single-pipe parallel-flow radiator configurations, thermal imbalance between the supply and return loops leads to uneven heat distribution and reduced system efficiency. The stainless steel coil tube, when integrated into an FCU-based system, addresses these limitations through:
- Enhanced Heat Transfer Coefficients: The helical coil geometry promotes turbulent flow within the tube, increasing the internal convective heat transfer coefficient by 20–40% compared to straight-tube radiators of equivalent surface area.
- Uniform Temperature Distribution: The compact coil design minimizes thermal stratification, ensuring consistent outlet temperatures across all connected terminals.
- Reduced Hydraulic Imbalance: The inherent pressure drop characteristics of the coil tube facilitate more balanced flow distribution in multi-terminal systems, reducing the need for balancing valves.
5.2 Additional Application Domains
- Industrial Heat Exchangers: Compact coil tubes are used as tube bundles in shell-and-tube heat exchangers for process heating and cooling in chemical, pharmaceutical, and food processing industries.
- Refrigeration Systems: Coil tubes serve as evaporators and condensers in commercial refrigeration units, leveraging the corrosion resistance of stainless steel in ammonia-based or CO₂-based refrigerant circuits.
- Domestic Hot Water Systems: Coil tubes are employed in indirect water heaters and solar thermal collectors where long-term corrosion resistance is critical.
- Process Engineering: Custom coil configurations are used in chemical reactors, distillation columns, and heat recovery systems requiring high-temperature and high-pressure operation.
6. Quality Certifications and Standards Compliance
Wenzhou Jixing Metal Materials Co., Ltd. maintains a comprehensive quality management system ensuring full traceability and compliance with international standards:
6.1 Certifications
- ISO 9001:2015: Quality Management System certification covering design, manufacturing, and testing processes.
- ISO 14001:2015: Environmental Management System certification demonstrating commitment to sustainable manufacturing practices.
- ISO 45001:2018: Occupational Health and Safety Management System certification ensuring worker safety and regulatory compliance.
- CE Marking: Conformity with European directives for products intended for the European Economic Area (EEA) market.
- TUV Certification: Third-party verification by TUV Rheinland or TUV SUD for product safety and quality assurance.
6.2 Applicable Standards
| Standard | Title | Relevance |
|---|---|---|
| ASTM A213/A213M | Standard Specification for Seamless and Welded Austenitic Stainless Steel Tubing for High-Temperature Service | Material and dimensional requirements for heat exchanger tubing |
| ASTM A778/A778M | Standard Specification for Seamless and Welded Austenitic Stainless Steel Tubing for Heat-Transfer Equipment in Non-Corrosive Service | Heat exchanger coil tube specifications |
| EN 10216-5 | Non-Alloy and Alloy Steel Tubes — Technical Delivery Conditions — Part 5: Welded Tubes of Austenitic Steels | European dimensional and mechanical requirements |
| GB/T 14976 | Seamless Steel Tubes for Fluids | Chinese national standard for fluid transport tubing |
| ASME BPV Section VIII | Boiler and Pressure Vessel Code | Pressure vessel and heat exchanger design codes |
| EN 12828 | Heating Systems in Buildings — Design of Hot-Water Heating Systems | HVAC system design requirements |
6.3 Traceability and Documentation
Each production lot is accompanied by a comprehensive Mill Test Certificate (MTC) in accordance with EN 10204 Type 3.1, providing:
- Chemical composition analysis results
- Mechanical property test data (tensile, hardness, impact)
- NDT inspection reports
- Hydrostatic test records
- Heat number traceability from raw material through final product
7. Unique Selling Points
Superior Corrosion Resistance: Unlike carbon steel radiators that require protective coatings and are susceptible to internal corrosion from dissolved oxygen and aggressive water chemistry, stainless steel coil tubes maintain structural integrity over extended service lifetimes (25–50 years), significantly reducing maintenance costs and system downtime.
Optimized Thermal Performance: The helical coil geometry inherently promotes turbulent flow at lower Reynolds numbers compared to straight tubes, enhancing heat transfer efficiency without requiring additional surface treatments or fin attachments. This results in 15–30% improved thermal performance per unit volume compared to conventional single-pipe parallel-flow radiator designs.
Compact Installation Footprint: The coil configuration achieves high heat exchange surface area within a minimal envelope, enabling installation in space-constrained environments such as retrofitted buildings, commercial offices, and residential apartments where traditional radiator panels are impractical.
System Compatibility: Designed as a direct replacement for single-pipe parallel-flow radiator assemblies, the coil tube integrates seamlessly with existing piping infrastructure, minimizing installation labor and system modification costs during upgrades or retrofits.