Stainless Steel Coil Tube (Coiltube) — Solar Biogas Pond Heat Exchange Series
1. Product Overview
The Stainless Steel Coil Tube (Coiltube), manufactured by Wenzhou Jixing Metal Materials Co., Ltd., is a precision-engineered helical tubing solution designed for thermal energy transfer applications, particularly in Solar Biogas Pond Heat Exchange Systems. This product is engineered to maximize heat transfer efficiency within confined spatial geometries, enabling optimal temperature regulation in anaerobic digestion environments where biogas production is thermally dependent.
Unlike conventional straight-pipe heat exchangers, the coiltube configuration leverages the inherent turbulence enhancement of helical flow paths to achieve superior convective heat transfer coefficients. This makes it an ideal solution for applications requiring compact footprint, high surface-area-to-volume ratios, and resistance to the corrosive environments typical of biogas systems (containing H₂S, CO₂, and moisture).
2. Material & Properties
2.1 Base Material Grades
- 304 (UNS S30400 / AISI 304) — General-purpose austenitic stainless steel offering excellent corrosion resistance, formability, and weldability. Suitable for non-aggressive biogas environments.
- 316 (UNS S31600 / AISI 316) — Enhanced molybdenum-bearing austenitic grade providing superior resistance to chlorides, sulfides, and acidic environments. Recommended for high-H₂S biogas applications.
- 316L (UNS S31603 / AISI 316L) — Low-carbon variant of 316, offering improved weldability and resistance to intergranular corrosion in sensitized conditions.
- 321 (UNS S32100 / AISI 321) — Titanium-stabilized grade providing exceptional resistance to intergranular corrosion at elevated temperatures (up to 870°C).
2.2 Mechanical Properties
| Property | 304 | 316 | 316L | 321 |
|---|---|---|---|---|
| Tensile Strength (MPa) | ≥ 520 | ≥ 515 | ≥ 485 | ≥ 515 |
| Yield Strength (MPa) | ≥ 205 | ≥ 205 | ≥ 170 | ≥ 205 |
| Elongation (%) | ≥ 40 | ≥ 40 | ≥ 45 | ≥ 40 |
| Hardness (HRB) | ≤ 92 | ≤ 92 | ≤ 92 | ≤ 92 |
| Maximum Service Temperature (°C) | 870 | 870 | 870 | 870 |
| Thermal Conductivity (W/m·K, 25°C) | 16.3 | 14.6 | 14.6 | 16.3 |
2.3 Chemical Composition (Wt%)
| Grade | C (max) | Cr | Ni | Mo | Ti | Si (max) | Mn (max) | P (max) | S (max) |
|---|---|---|---|---|---|---|---|---|---|
| 304 | 0.08 | 18.0–20.0 | 8.0–10.5 | — | — | 1.00 | 2.00 | 0.045 | 0.030 |
| 316 | 0.08 | 16.0–18.0 | 10.0–14.0 | 2.00–3.00 | — | 1.00 | 2.00 | 0.045 | 0.030 |
| 316L | 0.03 | 16.0–18.0 | 10.0–14.0 | 2.00–3.00 | — | 1.00 | 2.00 | 0.045 | 0.030 |
| 321 | 0.08 | 17.0–19.0 | 9.0–12.0 | — | 5×(C–0.02)–0.70 | 1.00 | 2.00 | 0.045 | 0.030 |
3. Specifications
| Parameter | Specification | Tolerance | Standard Reference |
|---|---|---|---|
| Outer Diameter (OD) | 6.0 – 50.8 mm (¼" – 2") | ±0.15 mm | ASTM A213 / EN 10217 |
| Wall Thickness | 0.5 – 3.0 mm | ±10% | ASTM A213 |
| Coil Pitch (Helix Spacing) | 15 – 100 mm (configurable) | ±1.0 mm | Custom / Project-Specific |
| Coil Diameter (Mean Helix Diameter) | 50 – 600 mm | ±2.0 mm | Custom / Project-Specific |
| Number of Turns | 3 – 50 turns | ±1 turn | Custom / Project-Specific |
| Tube Length (Uncoiled) | 1.0 – 120.0 m | ±0.5% | ASTM A213 |
| Surface Finish | Mirror Polish (Ra ≤ 0.2 μm) / Pickled & Passivated (Ra ≤ 1.6 μm) / Mill Finish | — | ASTM A478 / ASTM A967 |
| End Forming | Plain / Beveled / Threaded / Flanged / Welding Neck | Per drawing | ASME B16.11 / B16.25 |
| Pressure Rating | Up to 4.0 MPa (design dependent) | — | ASME BPVC Section VIII |
| Coil Density (Turns per Meter) | 10 – 67 turns/m | ±5% | Custom |
| Minimum Bend Radius | 3D (where D = OD) | — | ASTM A213 |
| Delivery Condition | Coiled / Straightened & Coiled / Custom Configured | — | — |
4. Manufacturing Process
4.1 Process Flow
- Raw Material Inspection — Incoming stainless steel seamless tubes are verified against mill certificates (MTC) for chemical composition, mechanical properties, and dimensional conformity per ASTM A213 / EN 10217.
- Cutting & Preparation — Tubes are cut to specified lengths with precision cold sawing to minimize burr and deformation. End surfaces are deburred and cleaned.
- Coiling / Bending — Precision CNC coiling machines form the helical geometry using mandrel-supported bending to prevent ovalization, wrinkling, or wall thinning. The bending process maintains dimensional accuracy within ±1.0 mm pitch tolerance.
- Welding (if required) — Butt welds or TIG (Tungsten Inert Gas) welds are applied at connection points using matching filler wire (e.g., ER308L for 304, ER316L for 316). Weld quality is verified per ASME Section IX.
- Surface Treatment — Pickling and passivation per ASTM A967 to remove surface oxides and contaminants, establishing a protective chromium oxide layer. Mirror polishing is performed using multi-stage abrasive finishing for high-purity applications.
- Quality Control & Testing — Dimensional inspection (OD, wall thickness, pitch, helix diameter), hydrostatic pressure testing (1.5× design pressure), and visual/NDT inspection of welds.
- Packaging & Dispatch — Coils are wrapped in protective film, secured on wooden pallets or crates, and labeled with product identification, heat number, and test certificates.
4.2 Key Manufacturing Advantages
- Controlled Ovalization — CNC mandrel bending ensures ovalization remains below 2% of OD, critical for maintaining uniform flow and heat transfer.
- Stress Relief — Optional post-bending stress relief annealing eliminates residual stresses that could lead to deformation during service or welding assembly.
- Custom Configurability — Full parametric control over pitch, turns, helix diameter, and end forming enables tailored designs for specific heat exchange geometries.
5. Applications
5.1 Primary Application: Solar Biogas Pond Heat Exchange
In solar biogas digesters, the coiltube serves as the primary heat exchange element for maintaining optimal anaerobic digestion temperatures (typically 35–38°C for mesophilic digestion). The helical coil configuration provides:
- Enhanced Heat Transfer — Secondary Dean vortices generated in helical flow increase the Nusselt number by 30–50% compared to straight tubes, improving thermal efficiency.
- Space Optimization — Compact helical geometry maximizes heat transfer surface area within the limited volume of biogas digesters.
- Corrosion Resistance — Stainless steel (particularly 316L) withstands the aggressive biogas environment containing hydrogen sulfide (H₂S), carbon dioxide (CO₂), and moisture without degradation.
- Thermal Stability — Even heat distribution prevents localized hot spots that could inhibit methanogenic bacteria or cause thermal shock.
5.2 Additional Applications
- Industrial Heat Exchangers — Shell-and-coil heat exchangers for chemical processing, pharmaceuticals, and food processing.
- Refrigeration & HVAC — Evaporator and condenser coils for refrigeration systems requiring corrosion-resistant tubing.
- Automotive & Aerospace — Fuel system lines, hydraulic circuits, and cooling loops where weight reduction and durability are critical.
- Medical & Laboratory — Precision fluid delivery coils in analytical instruments, chromatography systems, and medical devices.
- Power Generation — Heat recovery steam generators (HRSG) and waste heat recovery systems.
- Marine & Offshore — Seawater heat exchange systems where chloride resistance is paramount.
6. Quality Certifications & Standards Compliance
| Standard / Certification | Scope | Description |
|---|---|---|
| ASTM A213 | Material Specification | Standard Specification for Seamless Austenitic Stainless Steel Tubing for High-Temperature Service |
| EN 10217-7 | Material Specification | Non-alloy and Alloy Steels for Tubes — Seamless Heat Exchanger Tubes — Part 7: Stainless Steel |
| ASTM A |