Stainless Steel Coil Tube for Solar Cavity Air Receiver Systems
Product Overview
The stainless steel coil tube (盘管 / Coil Tube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered tubular component designed primarily for solar thermal energy collection systems, particularly cavity-type air receiver applications. These coil tubes serve as critical heat exchange elements within concentrated solar thermal (CST) systems, where they absorb, transport, and transfer thermal energy efficiently under high-temperature and high-radiation environments. The product is engineered to meet the demanding requirements of solar cavity receivers, where heat transfer performance, thermal fatigue resistance, and long-term structural integrity are paramount.
Based on extensive experimental research and numerical simulation studies conducted on solar cavity coil tube air receivers, the design philosophy emphasizes optimal thermal-hydraulic performance, minimized pressure drop across the coil assembly, and maximized solar-to-thermal conversion efficiency. The coil tube geometry is specifically optimized to enhance convective heat transfer between the working fluid (typically air) and the tube surface while maintaining mechanical stability under cyclic thermal loading conditions.
Material & Properties
Material Grade Selection
The primary material grade for this coil tube product is austenitic stainless steel, with the following grades available based on application requirements:
- SAE 304 / UNS S30408 — General-purpose solar thermal applications; excellent corrosion resistance and formability
- SAE 316L / UNS S31603 — Enhanced molybdenum content for superior resistance to pitting and crevice corrosion in aggressive environments
- SAE 321 / UNS S32100 — Titanium-stabilized grade for elevated-temperature service (up to 870°C) with excellent resistance to intergranular corrosion
- SAE 310S / UNS S31008 — High-temperature grade for concentrated solar applications exceeding 1000°C operating temperatures
Mechanical Properties
| Property | Grade 304 | Grade 316L | Grade 321 | Grade 310S |
|---|---|---|---|---|
| Tensile Strength (ASTM A213) [MPa] | ≥ 515 | ≥ 485 | ≥ 515 | ≥ 515 |
| Yield Strength (0.2% Offset) [MPa] | ≥ 205 | ≥ 170 | ≥ 205 | ≥ 205 |
| Elongation (50 mm gauge) [%] | ≥ 40 | ≥ 35 | ≥ 40 | ≥ 40 |
| Thermal Conductivity at 100°C [W/(m·K)] | 16.3 | 14.6 | 16.3 | 15.7 |
| Thermal Conductivity at 500°C [W/(m·K)] | 24.8 | 23.4 | 24.8 | 24.1 |
| Thermal Expansion (20–500°C) [×10⁻⁶/K] | 16.5 | 16.0 | 16.5 | 17.5 |
| Maximum Service Temperature [°C] | 870 | 870 | 870 | 1150 |
Chemical Composition (Typical)
| Element | 304 | 316L | 321 | 310S |
|---|---|---|---|---|
| C [%] | ≤ 0.08 | ≤ 0.03 | ≤ 0.08 | ≤ 0.08 |
| Cr [%] | 18.0–20.0 | 16.0–18.0 | 18.0–20.0 | 24.0–26.0 |
| Ni [%] | 8.0–10.5 | 10.0–14.0 | 9.0–12.0 | 19.0–22.0 |
| Mo [%] | — | 2.0–3.0 | — | — |
| Ti [%] | — | — | 5×(C–0.02) to 0.70 | — |
| Si [%] | ≤ 0.75 | ≤ 0.75 | ≤ 0.75 | ≤ 1.50 |
| Mn [%] | ≤ 2.00 | ≤ 2.00 | ≤ 2.00 | ≤ 2.00 |
| P [%] | ≤ 0.045 | ≤ 0.045 | ≤ 0.045 | ≤ 0.045 |
| S [%] | ≤ 0.030 | ≤ 0.030 | ≤ 0.030 | ≤ 0.030 |
Specifications
| Parameter | Specification | Standard Reference |
|---|---|---|
| Product Type | Coil Tube (盘管) for Solar Cavity Air Receiver | — |
| Tube Outer Diameter (OD) [mm] | 12 – 50 (customizable) | ASTM A213 / GB/T 12771 |
| Tube Wall Thickness [mm] | 0.5 – 3.0 (customizable) | ASTM A213 / GB/T 12771 |
| Coil Pitch (Turn Spacing) [mm] | 15 – 60 (application-dependent) | Custom / Engineering Design |
| Coil Helix Diameter [mm] | 80 – 500 (customizable) | Custom / Engineering Design |
| Number of Turns | 10 – 200+ (project-specific) | Custom / Engineering Design |
| Total Tube Length [m] | 5 – 150 (customizable) | — |
| Surface Finish | Mirror polish (Ra ≤ 0.4 μm), matte, or selective absorber coating | — |
| End Preparation | Beveled ends, plain ends, or threaded connections | ASME B1.20.1 / ISO 7 |
| Pressure Rating | Up to 6.0 MPa (design-dependent) | ASME BPV Code Section VIII |
| Maximum Operating Temperature [°C] | Up to 1150 (grade-dependent) | — |
| Minimum Operating Temperature [°C] | -196 (cryogenic capability) | — |
| Tolerance on OD | ±0.1 mm (precision grade) | ASTM A213 |
| Tolerance on Wall Thickness | ±10% of nominal | ASTM A213 |
Manufacturing Process
Raw Material Preparation
High-purity stainless steel wire rod is sourced from certified mills with full traceability documentation. Incoming materials undergo spectroscopic analysis (OES) to verify chemical composition compliance with ASTM A213 or equivalent standards. The wire rod is annealed and pickled to ensure uniform microstructure and surface cleanliness prior to forming operations.
Pipe Forming
The seamless tube is produced via either:
- Hot-rolled (HR) process — For larger diameter tubes (OD > 25 mm), involving piercing, rolling, and sizing operations
- Cold-drawn (CD) process — For precision tubes (OD ≤ 25 mm), providing superior dimensional accuracy, surface finish, and mechanical properties
Coil Forming (Spiral Bending)
The coil forming process is a critical step that directly impacts the product's geometric accuracy and mechanical integrity:
- Pre-forming setup — The straight tube is loaded onto a CNC-controlled spiral forming machine with precisely calibrated die sets
- Progressive bending — The tube is incrementally bent around the helical axis with controlled feed rates to prevent ovalization, wrinkling, or plastic deformation at the bend radius
- Spring-back compensation — Real-time adjustment of the bending tooling accounts for elastic recovery (spring-back) inherent in austenitic stainless steels, ensuring the final coil geometry matches design specifications within ±0.5 mm tolerance
- Uniformity verification — In-process dimensional checks at regular intervals confirm coil pitch consistency, helix diameter accuracy, and tube cross-section circularity
Heat Treatment
Following coil forming, the product undergoes solution annealing (1050–1150°C) in a protective atmosphere (vacuum or argon) to:
- Relieve residual stresses introduced during cold forming
- Restore full austenitic microstructure and ductility
- Ensure uniform mechanical properties throughout the coil assembly
Surface Treatment
Depending on the application, the coil tube surface may undergo:
- Pickling and passivation — Removal of surface oxides and formation of a protective chromium oxide layer
- Mechanical polishing — Achieving mirror finish (Ra ≤ 0.4 μm) for optical solar absorber applications
- Selective absorber coating — Application of high-selectivity coatings (e.g., Al₂O₃, CrN, or ceramic-based coatings) to maximize solar absorptance (α ≥ 0.95) while minimizing thermal emittance (ε ≤ 0.10)
Quality Inspection
Comprehensive non-destructive testing (NDT) protocols are applied:
- Visual inspection — 100% inspection of coil surface for defects, distortion, or coating irregularities
- Dimensional verification — Coordinate measuring machine (CMM) or laser scanning for geometric accuracy
- Hydrostatic pressure testing — Performed at 1.5× design pressure to verify weld integrity and structural soundness
- Eddy current testing (ECT) — For subsurface defect detection in the tube wall
- Rockwell hardness testing — Verification of annealed condition (HRB ≤ 95)
Applications
Primary Applications
- Solar Cavity Air Receivers — As the primary heat exchange element in cavity-type solar air receivers, where concentrated solar radiation heats the working fluid (air) flowing through the coil tubes. Numerical simulations and experimental studies confirm that optimized coil geometry achieves thermal efficiency of 45–65% depending on solar concentration ratio and ambient conditions.
- Solar Air Heating Systems — For domestic and industrial solar air heating, providing clean, dry heated air for space heating, drying processes, and agricultural applications.
- Concentrated Solar Power (CSP) Receivers — In parabolic trough and dish systems where coil tubes serve as heat transfer surfaces in secondary heat exchange circuits.
Secondary Applications
- Industrial heat recovery systems — Utilizing exhaust heat from furnaces and kilns
- Chemical process heat exchangers — For corrosive medium service requiring stainless steel construction
- Automotive exhaust heat exchangers — Compact coil designs for waste heat recovery
- Food processing equipment — Sanitary-grade coil assemblies for pasteurization and sterilization
Quality Certifications & Standards Compliance
| Certification / Standard | Description |
|---|---|
| ISO 9001:2015 | Quality Management System certification for manufacturing processes |
| ISO 14001:2015 | Environmental Management System certification |
| ASTM A213 / A269 | American standard for seamless austenitic stainless steel tubing |
| GB/T 12771 | Chinese national standard for welded austenitic stainless steel tubes |
| EN 10217-7 | European standard for seamless heat exchanger tubes |
| ASME BPV Section VIII | Pressure vessel code compliance for pressure-rated applications |
| CE Marking | European conformity marking for pressure equipment directive (PED 2014/68/EU) |
| Mill Test Certificate (MTC) | EN 10204 3.1 certified material traceability documentation |