Stainless Steel Coil Tube (Coiltube) — Indirect Solar Water Heating System Heat Exchanger
Product Overview
The Stainless Steel Coil Tube (Coiltube) is a precision-engineered helical heat exchanger component manufactured by Wenzhou Jixing Metal Materials Co., Ltd. Specifically designed for integration into indirect-type solar water heating system storage tanks, this coil tube serves as the primary heat transfer interface between the solar collector loop (primary circuit) and the stored domestic hot water (secondary circuit). The helical geometry maximizes the effective heat exchange surface area within a compact cylindrical volume, ensuring efficient thermal energy transfer while minimizing thermal stratification disruption in the storage vessel.
As a critical component in solar thermal energy systems, the coil tube must withstand prolonged exposure to varying temperatures, cyclic thermal stress, and the corrosive environment of domestic water. Wenzhou Jixing Metal Materials Co., Ltd. delivers coil tubes engineered to meet rigorous international standards for both mechanical integrity and long-term corrosion resistance.
Material & Properties
Base Material
- Primary Grade: Austenitic stainless steel conforming to ASTM A269 (Standard Specification for Seamless Austenitic Stainless Steel Tubing for Chemical and Other Severely Corrosive Services) and GB/T 12771 (Chinese national standard for seamless stainless steel tubes for general fluid transport).
- Common Designations: SUS304 (UNS S30400), SUS316 (UNS S31600), and SUS316L (UNS S31603) — selected based on the corrosivity of the water medium and operating conditions.
Key Mechanical Properties
| Property | SUS304 (UNS S30400) | SUS316 (UNS S31600) | SUS316L (UNS S31603) |
|---|---|---|---|
| Tensile Strength (Minimum) | 515 MPa (74,700 psi) | 515 MPa (74,700 psi) | 485 MPa (70,300 psi) |
| Yield Strength (0.2% Offset) | 205 MPa (29,700 psi) | 205 MPa (29,700 psi) | 170 MPa (24,700 psi) |
| Elongation (in 50 mm / 2 in.) | ≥ 40% | ≥ 40% | ≥ 45% |
| Thermal Conductivity (at 300°C) | 16.3 W/(m·K) | 14.6 W/(m·K) | 14.6 W/(m·K) |
| Maximum Service Temperature | 870°C (1,598°F) | 870°C (1,598°F) | 870°C (1,598°F) |
| Chloride Pitting Resistance (PREN) | ~19 | ~24 | ~24 |
Chemical Composition (Typical, wt%)
| Element | C | Si | Mn | P | S | Cr | Ni | Mo | N |
|---|---|---|---|---|---|---|---|---|---|
| SUS304 | ≤ 0.08 | ≤ 1.00 | ≤ 2.00 | ≤ 0.045 | ≤ 0.030 | 18.0–20.0 | 8.0–10.5 | — | — |
| SUS316 | ≤ 0.08 | ≤ 1.00 | ≤ 2.00 | ≤ 0.045 | ≤ 0.030 | 16.0–18.0 | 10.0–14.0 | 2.00–3.00 | — |
| SUS316L | ≤ 0.03 | ≤ 1.00 | ≤ 2.00 | ≤ 0.045 | ≤ 0.030 | 16.0–18.0 | 10.0–14.0 | 2.00–3.00 | — |
Specifications
| Parameter | Specification / Range |
|---|---|
| Product Designation | Coiltube (Helical Coil Heat Exchanger) |
| Tube Outer Diameter (OD) | 12 mm – 32 mm (typical range); custom available |
| Tube Wall Thickness | 0.8 mm – 2.5 mm |
| Coil Pitch (Center-to-Center) | 15 mm – 50 mm (optimized for flow velocity and heat transfer coefficient) |
| Coil Overall Diameter | 100 mm – 600 mm (matched to storage tank inner diameter) |
| Coil Length (Total Tube Length) | 5 m – 200 m (depending on required heat exchange area) |
| Heat Transfer Surface Area | 0.5 m² – 15 m² (calculated per ASHRAE standard methods) |
| Operating Temperature Range | −40°C to +200°C (−40°F to +392°F) |
| Maximum Working Pressure | 1.0 MPa – 1.6 MPa (145–232 psi), hydrostatic tested at 1.5× working pressure |
| End Connection Type | Threaded (G, BSP, NPT), Flanged (PN10/PN16), or Socket Weld |
| Surface Finish | Electro-polished (Ra ≤ 0.4 μm) or Standard Pickled & Passivated (Ra ≤ 0.8 μm) |
| Manufacturing Standard | ASTM A269 / GB/T 12771 / EN 10216-5 |
| Heat Exchanger Design Reference | ASHRAE Handbook — Heating, Ventilating, and Air-Conditioning; TEMA Standards |
Manufacturing Process
The production of high-quality stainless steel coil tubes at Wenzhou Jixing Metal Materials Co., Ltd. follows a rigorous multi-stage manufacturing workflow:
- Material Selection & Incoming Inspection: Seamless stainless steel tubes are sourced from certified mills and subjected to spectroscopic analysis (OES) to verify chemical composition. Heat number traceability is maintained throughout production.
- Cutting & End Preparation: Tubes are cut to specified lengths using precision saws with burr removal to ensure clean, uniform ends suitable for forming and welding.
- Helical Coiling (Cold Forming): Using CNC-controlled coiling machines, straight tubes are wound into precise helical configurations. The coiling parameters — pitch, coil diameter, and number of turns — are calculated based on thermal design requirements to optimize the heat transfer coefficient (U-value) and pressure drop (ΔP).
- Stress Relief Annealing: Following cold coiling, the helical coils undergo controlled thermal treatment (typically 1050°C for SUS304/316, held for appropriate time, followed by controlled cooling) to relieve residual stresses introduced during forming and to restore full austenitic microstructure.
- Pickling & Passivation: The annealed coils are immersed in a citric acid or nitric acid solution to remove oxide scale and form a stable chromium oxide passive film, enhancing long-term corrosion resistance.
- End Fitting Attachment: Threaded fittings, flanges, or welding stubs are attached at the inlet and outlet ends of the coil using orbital TIG (GTAW) welding in a controlled inert gas (argon) atmosphere to prevent oxidation at weld zones.
- Hydrostatic Pressure Testing: Each coil is pressure-tested at 1.5 times the maximum working pressure for a minimum hold time of 30 seconds to verify leak-tight integrity.
- Non-Destructive Testing (NDT): Weld joints are inspected via dye penetrant testing (PT) or magnetic particle inspection (MT) to detect surface defects. Dimensional verification is performed using calibrated instruments.
- Final Inspection & Packaging: Completed coil tubes undergo visual inspection, dimensional audit, and are packaged with protective end caps and moisture barriers for shipping.
Applications
- Indirect Solar Water Heating Systems: The primary application — the coil tube is submerged within the storage tank of an indirect solar thermal system, transferring heat from the glycol-water mixture in the solar collector loop to the domestic hot water without mixing the two fluids.
- Domestic Hot Water Storage Tanks: Used in residential and commercial water heaters where sanitary-grade heat exchange is required.
- Heat Pump Water Heaters: Integrated as the evaporator or condenser coil within closed-loop heat pump systems for water heating.
- Geothermal Heating Systems: Employed as heat exchange elements in ground-source heat pump installations.
- Food & Beverage Industry: Where hygienic-grade heat exchange is required for process water heating (SUS316L grade with electropolished surface).
- Marine & Offshore Applications: SUS316/316L coil tubes provide superior chloride resistance for seawater-cooled systems.
- Industrial Process Cooling/Heating: Used as immersed heat exchangers in chemical processing, pharmaceutical, and biotechnology applications.
Design Analysis & Thermal Performance
Note: The design methodology referenced in the technical literature — "Design Analysis of Coil Heat Exchanger for Indirect Solar Water Heating System Storage Tank" — employs the following engineering framework:
- Heat Transfer Coefficient Calculation: The overall heat transfer coefficient (U) is determined using the Dittus-Boelter correlation for turbulent flow inside the coil (Reynolds number typically > 10,000) and natural convection correlations for the outer surface in the storage tank.
- Pressure Drop Analysis: The Darcy-Weisbach equation with the Fanning friction factor is applied to ensure that pump power requirements remain within acceptable limits (typically ΔP < 0.5 bar for solar loop systems).
- Thermal Stratification Optimization: The coil geometry (pitch, diameter, number of turns) is optimized to promote thermal stratification within the storage tank, maximizing the effective usable hot water volume and system efficiency.
- Fouling Factor Consideration: Design incorporates fouling factors (R_f = 0.0001–0.0002 m²·K/W) to account for long-term scale buildup, ensuring sustained performance over the system's design life (≥ 15 years).
Quality Certifications & Compliance
| Certification / Standard | Scope of Compliance |
|---|---|
| ISO 9001:2015 | Quality Management System — Manufacturing process control and continuous improvement |
| ISO 14001:2015 | Environmental Management System — Sustainable manufacturing practices |
| CE Marking (EU) | Compliance with EU Pressure Equipment Directive (PED 2014/68/EU) and Solar Keymark where applicable |
| Solar Keymark | European quality label for solar thermal products and systems |
| NSF/ANSI 61 | Drinking Water System Components — Health Effects (for potable water applications) |
| ASTM A269 | Seamless austenitic stainless steel tubing for chemical and corrosive services |
| GB/T 12771 | Chinese national standard for seamless stainless steel tubes for general fluid transport |
| EN 10 |