Stainless Steel Coil Tube (Coiltube) — Low-Temperature Geothermal Fan Coil Heating System
1. Product Overview
The Stainless Steel Coil Tube (hereinafter referred to as "Coiltube") manufactured by Wenzhou Jixing Metal Materials Co., Ltd is a precision-engineered continuous coil product designed specifically for low-temperature geothermal fan coil heating systems (Low-Temperature Geothermal Fan Coil Heating System). This product represents the integration of metallurgical expertise in stainless steel pipe manufacturing with advanced thermal-fluid engineering principles, delivering superior heat exchange efficiency, corrosion resistance, and long-term structural integrity in demanding geothermal and HVAC (Heating, Ventilation, and Air Conditioning) applications.
The Coiltube is supplied in continuous coil form, enabling seamless integration into heat exchanger assemblies, fan coil units (FCU), radiant floor heating loops, and geothermal energy distribution networks. The product has been studied and optimized within the context of low-temperature geothermal fan coil heating system operational parameter design, ensuring that its geometric and material properties align with the thermodynamic requirements of low-enthalpy geothermal energy extraction and distribution.
2. Material Composition & Properties
2.1 Base Material Grades
The Coiltube is fabricated from high-purity austenitic and duplex stainless steel grades, selected to ensure optimal performance under low-temperature geothermal conditions where chloride-induced pitting and crevice corrosion may be present in groundwater or brine media. Common grades include:
- AISI 304 / EN 1.4301 (UNS S30400) — General-purpose austenitic stainless steel with excellent corrosion resistance and formability.
- AISI 316L / EN 1.4404 (UNS S31603) — Low-carbon, molybdenum-bearing austenitic stainless steel offering enhanced resistance to chloride pitting and crevice corrosion.
- AISI 316Ti / EN 1.4571 (UNS S31635) — Titanium-stabilized variant providing improved resistance to intergranular corrosion during welding operations.
- AISI 321 / EN 1.4541 (UNS S32100) — Columbium-stabilized austenitic grade suitable for elevated temperature service.
- Duplex 2205 / EN 1.4462 (UNS S31803) — 22% Cr–5% Ni–3% Mo duplex stainless steel offering high yield strength and superior resistance to chloride stress corrosion cracking (CSCC).
2.2 Key Mechanical Properties
| Property | AISI 304 | AISI 316L | Duplex 2205 | Test Standard |
|---|---|---|---|---|
| Tensile Strength (UTS) | ≥ 515 MPa | ≥ 485 MPa | ≥ 550 MPa | ASTM A240 / EN 10204 |
| Yield Strength (0.2% Proof Stress) | ≥ 205 MPa | ≥ 170 MPa | ≥ 450 MPa | ASTM A240 / EN 10204 |
| Elongation (at Break) | ≥ 40% | ≥ 35% | ≥ 25% | ASTM A240 |
| Hardness (HBW) | ≤ 217 | ≤ 217 | ≤ 350 | ASTM E10 |
| Density (g/cm³) | 7.93 | 8.03 | 7.80 | — |
| Thermal Conductivity at 20 °C (W/m·K) | 16.3 | 14.6 | 18.0 | ASTM E153 |
| Thermal Expansion Coefficient (10⁻⁶/K, 20–100 °C) | 17.3 | 16.0 | 13.0 | ASTM E228 |
2.3 Corrosion Resistance Characteristics
The selection of stainless steel grade is governed by the corrosion potential of the geothermal fluid and the operating environment. The following table summarizes the relative corrosion resistance indices:
| Corrosion Type | AISI 304 | AISI 316L | Duplex 2205 |
|---|---|---|---|
| Atmospheric Corrosion | Excellent | Excellent | Excellent |
| Chloride Pitting (ASTM G48) | Moderate | Excellent | Superior |
| Crevice Corrosion | Limited | Excellent | Superior |
| Stress Corrosion Cracking (CSCC) | Vulnerable | Resistant | Highly Resistant |
| Intergranular Corrosion | Good (with stabilizing element) | Excellent (L-grade) | Excellent |
| Sulfidation (Geothermal Brine) | Limited | Good | Excellent |
3. Product Specifications
3.1 Geometric Dimensions
| Parameter | Specification Range | Standard Reference |
|---|---|---|
| Outer Diameter (OD) | 6.0 mm – 32.0 mm (typical); up to 50.8 mm on request | ASTM A778 / EN 10217 |
| Wall Thickness (WT) | 0.5 mm – 3.0 mm (typical) | ASTM A778 / EN 10217 | Coil Inner Diameter (CID) | 300 mm – 1,200 mm (standard); custom available | Manufacturer Specification |
| Coil Length (per Spool) | 50 m – 300 m (standard); up to 1,000 m for specialized applications | Manufacturer Specification |
| Diameter Tolerance | ±0.1 mm (≤ 12 mm OD); ±0.15 mm (> 12 mm OD) | ASTM A778 |
| Wall Thickness Tolerance | ±10% of nominal | ASTM A778 / EN 10217 |
| Coil Roundness | ≤ 0.5% of OD | Manufacturer Specification |
| Surface Finish | 2B (cold-rolled and annealed); BA (Bright Annealed); No. 4 Brushed; Mirror (on request) | ASTM A480 |
| Surface Roughness (Ra) | ≤ 0.8 μm (2B); ≤ 0.4 μm (BA); ≤ 0.2 μm (Mirror) | ISO 4287 |
3.2 Geometric Tolerances and Conformity
| Dimension | OD 6–12 mm | OD 12–25 mm | OD 25–50 mm |
|---|---|---|---|
| OD Tolerance (mm) | ±0.08 | ±0.10 | ±0.15 |
| WT Tolerance (mm) | ±0.05 | ±0.07 | ±0.10 |
| Wall Thickness Variation (max) | ±10% | ±10% | ±10% |
| Roundness (mm) | 0.05 | 0.07 | 0.10 |
3.3 Thermal Performance Parameters
In the context of low-temperature geothermal fan coil heating systems, the thermal performance of the Coiltube is characterized by the following parameters:
- Heat Transfer Coefficient (U-value): Typically 150–400 W/m²·K for water-to-air heat exchangers incorporating this Coiltube, depending on fluid velocity, fin geometry, and operating conditions.
- Operating Temperature Range: −40 °C to +250 °C (AISI 304/316L); −50 °C to +300 °C (Duplex 2205).
- Operating Pressure: Up to 1.6 MPa (PN16) standard; up to 6.3 MPa (PN63) for thickened-wall configurations.
- Maximum Flow Velocity: ≤ 3.0 m/s (to prevent erosion-corrosion in geothermal brine service).
- Design Life: ≥ 25 years under continuous low-temperature geothermal service conditions.
4. Manufacturing Process
4.1 Process Flow
The manufacturing of the Stainless Steel Coil Tube follows a rigorous multi-stage process chain adhering to international quality standards:
- Raw Material Procurement: Sourcing of certified stainless steel strip (cold-rolled coil) from accredited mills with full chemical and mechanical property traceability (Mill Test Certificate — MTC, per EN 10204 Type 3.1).
- Strip Inspection and Preparation: Incoming strip undergoes visual inspection, ultrasonic testing (UT) for subsurface defects, and chemical composition verification via Optical Emission Spectroscopy (OES).
- Forming (Cold Drawing / Cold Forming): The strip is cold-formed into a tubular geometry using precision forming dies. The forming ratio (strip width / pipe OD) is optimized to minimize work hardening and ensure dimensional accuracy.
- Welding: The formed tube is longitudinally welded using one of the following methods:
- TIG (Tungsten Inert Gas) Welding — ASTM A249, with argon shielding on both inside and outside to prevent oxidation.
- Laser Welding — High-energy-density welding producing narrow heat-affected zones (HAZ) and superior weld integrity.
- Flash Butt Welding (FBW) — For larger diameters, producing seamless-equivalent joints.
- Post-Weld Heat Treatment (PWHT): Solution annealing at 1010–1150 °C followed by rapid water quenching to restore full austenitic microstructure, eliminate residual stresses, and ensure uniform corrosion resistance. For Duplex 2205, the annealing temperature is controlled at 1020–1100 °C to maintain the ferrite-austenite balance (target: 40–60% ferrite).
- Pickling and Passivation: The annealed tube is pickled in a mixed acid bath (HNO₃/HF or H₂SO₄/HNO₃) to remove surface scale and oxides, followed by passivation to restore the chromium oxide passive film (minimum Cr₂O₃ layer thickness: 5–10 nm).
- Dimensional Correction and Sizing: The tube passes through a series of sizing rollers to achieve final OD and WT tolerances.
- Coiling: The finished tube is wound into a continuous coil on a precision pay-off reel, with the coil inner diameter controlled to ensure consistent curvature and prevent kinking or ovality.
- Final Quality Inspection: Comprehensive testing as detailed in Section 5.
- Packaging and Dispatch: Coils are wrapped in protective film, placed in wooden or metal crates, and palletized for transport.
4.2 Welding and Joining Considerations
For applications requiring field installation or system integration, the Coiltube may be joined using the following methods:
- Orbitron Orbital TIG Welding: Automated, high-precision welding for seamless pipe joints with full internal and external argon shielding.
- Threaded Connections: For smaller diameters (≤ 12 mm OD), threaded fittings with PTFE (Polytetrafluoroethylene) or PTFE-lined gaskets.
- Compression Fittings: For rapid installation in fan coil unit assemblies.
- Brazing (with compatible filler metals): Silver-based filler alloys (e.g., CuAg 72) for dissimilar metal joints, with careful control of the heat input to avoid sensitization.
5. Quality Certifications and Testing
5.1 Applicable Standards
| Standard | Description | Scope |
|---|---|---|
| ASTM A778 | Standard Specification for Stainless Steel Seamless and Wrought Straight Pipe and Tube | Material and dimensional requirements |
| EN 10217-7 | Technical Delivery Conditions for Stainless Steel Tubes for Pressure Purposes | Pressure vessel applications |
| ASME B31.3 | Process Piping | System design and installation |
| ISO 1143 | Steel and Iron — Designations | Nomenclature |