Spiral Coil Tube (Coiltube) — Stainless Steel Heat Exchange Coil System
Product Overview
The Spiral Coil Tube (Coiltube) product line developed by Wenzhou Jixing Metal Materials Co., Ltd represents a high-performance stainless steel heat exchange solution engineered for integration into water tank and thermal storage systems. These coils are manufactured from premium austenitic and duplex stainless steel grades, designed to deliver superior thermal transfer efficiency while maintaining long-term resistance to corrosion, scaling, and mechanical fatigue under continuous or intermittent operating conditions.
Extensive research into the heat exchange characteristics of spiral coil tanks under different operating modes has informed the design optimization of these products. Studies have demonstrated that spiral coil configurations exhibit distinct thermal performance profiles depending on flow regime, inlet temperature differential, tank geometry, and operational duty cycles. Wenzhou Jixing Metal Materials Co., Ltd leverages these findings to engineer coils that maximize convective heat transfer coefficients while minimizing thermal stratification losses within the enclosed water volume.
The coil tube architecture—formed by continuous helical winding of precision-drawn stainless steel tubing—creates a compact, high-surface-area heat exchange element that can be submerged within a water storage tank or integrated into a closed-loop thermal system. This configuration enables rapid and uniform heating or cooling of the tank water with minimal pressure drop across the coil circuit.
Material & Properties
The coil tubes are manufactured using a range of stainless steel grades selected to meet the specific demands of thermal cycling, chemical exposure, and structural loading encountered in water tank applications:
Primary Material Grades
- Austenitic Stainless Steel — 304 (UNS S30408 / 1.4301): The standard grade for general-purpose heat exchange coils. Contains approximately 18% Chromium (Cr) and 8% Nickel (Ni), providing excellent general corrosion resistance, formability, and weldability. Suitable for potable water applications and moderate-temperature thermal cycling up to approximately 425°C in non-oxidizing environments.
- Austenitic Stainless Steel — 316 (UNS S31608 / 1.4401): Enhanced with 2–3% Molybdenum (Mo) to improve resistance to pitting and crevice corrosion in chloride-containing environments. Recommended for applications involving seawater, brackish water, or chemical-laden process fluids.
- Austenitic Stainless Steel — 316L (UNS S31603 / 1.4435): Low-carbon variant of 316 with carbon content ≤ 0.03%, minimizing the risk of intergranular corrosion during welding operations. Preferred for thick-walled coils requiring post-fabrication welding without sensitization concerns.
- Duplex Stainless Steel — 2205 (UNS S32205 / 1.4462): A 22% Cr, 5% Ni, 3% Mo duplex phase alloy offering approximately double the yield strength of 304/316 grades while maintaining excellent resistance to chloride stress corrosion cracking (Cl-SCC). Ideal for high-pressure or high-temperature applications.
- Super Duplex Stainless Steel — 2507 (UNS S32750 / 1.4501): Contains 25% Cr, 7% Ni, 3% Mo, and 1.5% Manganese (Mn), providing exceptional strength (yield strength ≥ 550 MPa) and outstanding resistance to aggressive marine and chemical environments.
Key Mechanical and Physical Properties
| Property | 304 (1.4301) | 316L (1.4435) | 2205 (1.4462) | 2507 (1.4501) |
|---|---|---|---|---|
| Density (g/cm³) | 7.93 | 7.98 | 7.80 | 7.80 |
| Thermal Conductivity at 20°C (W/m·K) | 16.3 | 14.6 | 13.5 | 14.0 |
| Thermal Conductivity at 100°C (W/m·K) | 21.6 | 19.0 | 18.0 | 19.5 |
| Coefficient of Thermal Expansion at 20–100°C (×10⁻⁶/K) | 17.3 | 16.0 | 13.0 | 13.3 |
| Yield Strength (MPa, min.) | 205 | 170 | 450 | 550 |
| Tensile Strength (MPa, min.) | 515 | 485 | 620 | 730 |
| Maximum Operating Temperature (°C) | 870 | 870 | 350 | 350 |
Specifications
| Parameter | Specification Range |
|---|---|
| Tube Outer Diameter (OD) | 12 mm – 108 mm |
| Tube Wall Thickness | 0.5 mm – 3.0 mm |
| Coil Pitch (Helical Spacing) | 25 mm – 150 mm (configurable) |
| Coil Diameter (Overall) | 300 mm – 3,000 mm |
| Coil Length (Developed Tube Length) | 5 m – 500 m (custom) |
| End Connection Types | Threaded (G/BSP/NPT), Flanged (ANSI/DIN/JIS), Welded, Brazed |
| Surface Finish | 2B (Cold Rolled), No. 4 Brushed, No. 8 Mirror, Electropolished |
| Maximum Working Pressure | Up to 25 bar (dependent on grade and wall thickness) |
| Operating Temperature Range | −196°C to +350°C (duplex); −196°C to +870°C (austenitic) |
| Standard Reference | ASTM A213, ASTM A269, EN 10216-5, ISO 3506 |
| Available Configurations | Single-pass spiral, Multi-pass spiral, U-tube return, Serpentine variant |
Manufacturing Process
The production of high-quality spiral coil tubes follows a rigorous multi-stage manufacturing sequence designed to ensure dimensional accuracy, metallurgical integrity, and surface quality:
- Material Selection and Incoming Inspection: Stainless steel seamless or welded tube stock is procured from certified mills with full traceability documentation (mill certificates per EN 10204 3.1/3.2). Incoming material undergoes chemical composition analysis (spectroscopy), mechanical property verification (tensile testing), and visual/dimensional inspection.
- Tubing Preparation: The tube stock is cut to the required developed length using precision sawing or laser cutting. End faces are deburred and chamfered to facilitate subsequent forming and connection welding. Surface contamination (oils, oxides) is removed via alkaline cleaning and pickling.
- Helical Coiling (Cold Forming): The straight tube is wound into the specified spiral geometry using a CNC-controlled coiling machine. The machine parameters—spool diameter, pitch, tension, and winding speed—are precisely calibrated to achieve the target coil diameter and pitch without inducing excessive residual stresses, ovality, or wall thinning. Cold forming may also introduce beneficial work-hardening in austenitic grades, increasing yield strength by 15–30%.
- Welding and End Connection Fabrication: End connections (flanges, threaded fittings, or U-bend return sections) are welded to the coil terminations using Gas Tungsten Arc Welding (GTAW/TIG) or Metal Active Gas Welding (MAG/MIG) with appropriate filler metals (e.g., ER308L for 304, ER316L for 316L, ER2209 for 2205). Weld procedures are qualified per ASME Section IX or ISO 15614.
- Stress Relief Annealing (if required): For applications sensitive to residual stress or for coils fabricated from stabilized grades, a solution annealing or stress-relief heat treatment may be performed at 1050–1150°C followed by controlled cooling (water quench or furnace cool).
- Pickling and Passivation: Post-welding oxide scales are removed via acid pickling (HNO₃/HF blend or citric acid solution), and a chromium oxide passive film is established through nitric acid passivation to ensure optimal corrosion resistance.
- Surface Finishing: Depending on the application requirements, the coil may undergo mechanical polishing, electropolishing, or chemical brightening to achieve the specified surface roughness (Ra values as low as 0.2 µm for electropolished surfaces).
- Pressure Testing and Leak Detection: Each coil assembly undergoes hydrostatic pressure testing at 1.5× the maximum design pressure, held for a minimum duration to verify structural integrity. Helium leak testing or ultrasonic testing may be employed for critical applications.
- Final Inspection and Documentation: Comprehensive dimensional verification, visual inspection, and non-destructive testing (NDT) are performed. Full traceability documentation, including material certificates, weld records, test reports, and dimensional drawings, is compiled for each production batch.
Applications
Spiral coil tubes from Wenzhou Jixing Metal Materials Co., Ltd are deployed across a broad spectrum of industrial, commercial, and institutional applications where reliable heat transfer within a water storage or thermal buffer system is critical:
- Domestic and Commercial Hot Water Systems: Immersion coils installed within water storage tanks to provide indirect heating from a boiler, heat pump, or solar thermal collector. The spiral configuration maximizes the heat transfer surface area within a compact volume, enabling rapid tank heating with minimal thermal stratification.
- Industrial Process Heating and Cooling: Integration into process water tanks, cooling water loops, and thermal buffer systems in chemical processing, food and beverage manufacturing, pharmaceutical production, and textile dyeing operations.
- Solar Thermal Water Heating Systems: Coils serve as the heat exchanger element connecting solar collectors to the domestic hot water storage tank, transferring thermal energy from the collector loop fluid to the stored water.
- Heat Pump Systems: Used as evaporator or condenser coils within buffer tanks for air-source or ground-source heat pump installations, providing efficient thermal coupling between the refrigerant circuit and the water storage volume.
- Marine and Offshore Applications: Duplex and super duplex coil tubes provide corrosion-resistant heat exchange solutions in marine ballast water systems, shipboard hot water supply, and offshore platform thermal management.
- Food and Beverage Industry: Electropolished 316L coils meet hygienic requirements (EHEDG, 3-A Sanitary Standards) for CIP (Clean-in-Place) compatible water heating and cooling in breweries, dairies, and beverage processing plants.
- Geothermal and District Heating Systems: Coils integrated into large-volume thermal storage tanks for geothermal energy extraction systems and district heating/cooling networks.
- Research and Laboratory Applications: Custom coil configurations for thermal engineering research, including studies on heat transfer characteristics under varying flow regimes, natural convection patterns, and transient thermal response in stratified water tanks.
Heat Exchange Performance Characteristics
Research on the thermal performance of spiral coil tanks under different operating modes has revealed several key findings that directly inform the engineering of these products:
- Natural Convection Dominance: In passive (no-forced-circulation) operating modes, buoyancy-driven natural convection currents within the tank water govern the heat transfer rate. The coil's spiral geometry promotes the formation of secondary flow patterns that enhance mixing and reduce thermal stratification compared to straight-tube configurations.
- Forced Circulation Enhancement: When external circulation pumps are employed, the overall heat transfer coefficient (U-value) increases significantly, typically by 40–80% compared to natural convection alone. The coil's low-flow-resistance design minimizes the parasitic pumping power required.
- Temperature Differential Sensitivity: The logarithmic mean temperature difference (LMTD) between the coil fluid and the tank water is the primary driving force for heat transfer. Coil configurations with uniform pitch and consistent tube-to-tank-wall spacing provide more predictable and uniform heat flux distribution.
- Transient Response: Spiral coils exhibit faster transient thermal response compared to plate heat exchangers of equivalent surface area, owing to the lower thermal mass of the tube structure and the direct volumetric immersion in the tank water.
- Stratification Management: Proper coil placement (axial centering and vertical positioning) within the tank is critical to managing thermal stratification. Wenzhou Jixing provides engineering guidance on optimal coil positioning to maximize the effective hot water volume available for draw-off.
Quality Certifications
Wenzhou Jixing Metal Materials Co., Ltd maintains a comprehensive quality management and product certification framework ensuring compliance with international standards and regulatory requirements:
Management System Certifications
- ISO 9001:2015 — Quality Management Systems (QMS)
- ISO 14001:2015 — Environmental Management Systems (EMS)
- ISO 45001:2018 — Occupational Health and Safety Management Systems
Product and Material Certifications
- EN 10204 Type 3.1 / 3.2 — Material test certificates with certified chemical composition and mechanical property