Stainless Steel Spiral Coil Tube for Heat Exchanger Applications
1. Product Overview
The Stainless Steel Spiral Coil Tube is a precision-engineered tubular product manufactured by Wenzhou Jixing Metal Materials Co., Ltd. This product is specifically designed for use as a heat exchange element within pressure-type split solar water heating systems, where it serves as the core component of the spiral coil heat exchanger installed inside the storage water tank. The coil tube facilitates efficient thermal energy transfer between the primary circulation fluid (typically a heat transfer medium such as water or glycol solution) and the secondary storage water, ensuring reliable and high-performance operation of the solar thermal system.
Based on rigorous analysis and design methodologies applied to spiral coil heat exchangers in pressure-vessel storage tanks, this product integrates advanced metallurgical properties with optimized geometric configurations to achieve superior heat transfer efficiency while maintaining structural integrity under sustained thermal cycling and pressure loading conditions.
2. Material & Properties
2.1 Base Material
The coil tube is fabricated from austenitic stainless steel, with the following grade options available:
- AISI 304 (UNS S30408) — General-purpose grade with excellent corrosion resistance and formability
- AISI 316 (UNS S31608) — Enhanced resistance to chloride-induced pitting and crevice corrosion, suitable for aggressive environments
- AISI 316L (UNS S31603) — Low-carbon variant of 316, offering improved weldability and resistance to intergranular corrosion
2.2 Key Material Properties
| Property | 304 Grade | 316 Grade | 316L Grade |
|---|---|---|---|
| Chromium (Cr, wt%) | 18.0–20.0 | 16.0–18.0 | 16.0–18.0 |
| Nickel (Ni, wt%) | 8.0–10.5 | 10.0–14.0 | 10.0–14.0 |
| Molybdenum (Mo, wt%) | — | 2.0–3.0 | 2.0–3.0 |
| Carbon (C, wt%) | ≤0.08 | ≤0.08 | ≤0.03 |
| Thermal Conductivity (W/m·K, 20°C) | 16.3 | 14.6 | 14.6 |
| Thermal Expansion Coefficient (×10⁻⁶/K, 20–100°C) | 17.3 | 16.8 | 16.8 |
| Maximum Service Temperature (°C) | 870 | 870 | 870 |
| Yield Strength (MPa, min.) | 205 | 205 | 170 |
| Tensile Strength (MPa, min.) | 515 | 515 | 485 |
| Elongation (% at break, min.) | 40 | 40 | 40 |
2.3 Surface Treatment
- Passivation: Nitric acid or citric acid passivation to enhance passive oxide layer stability
- Pickling: Removal of mill scale and welding discoloration to restore full corrosion resistance
- Surface Finish: Available in No. 1, 2B, BA (Bright Annealed), and No. 4 brushed finishes
3. Specifications
| Parameter | Specification |
|---|---|
| Tube Outer Diameter (OD) | Φ8 mm – Φ32 mm (customizable) |
| Wall Thickness | 0.5 mm – 1.5 mm |
| Coil Pitch (Spacing Between Turns) | 3.0 mm – 8.0 mm (optimized for heat transfer coefficient) |
| Coil Diameter (Mean) | Φ150 mm – Φ600 mm (dependent on tank diameter) |
| Coil Length / Number of Turns | Customized per thermal load requirement (typically 10–80 turns) |
| End Connection Type | Flanged, threaded (BSP/NPT), or welded stub connection |
| Operating Pressure | Up to 1.6 MPa (232 psi) at 100°C |
| Operating Temperature Range | -40°C to +180°C |
| Design Standards | ASME BPVC Section VIII, EN 12981, GB/T 151 |
| Applicable Standards for Material | ASTM A213/A213M, EN 10216-5, ISO 3506 |
| Applicable Standards for Heat Exchanger | EN 12981 (Heat exchangers for water heating installations), GB/T 26968 |
4. Manufacturing Process
4.1 Process Flow
- Raw Material Selection: Precision cold-rolled or seamless stainless steel tubing conforming to ASTM A213 Type 304/316/316L specifications, with full mill test certificates (MTC) traceable to heat number.
- Dimensional Inspection: Incoming tubes undergo ultrasonic testing (UT) for wall thickness verification and visual inspection for surface defects.
- Cutting: Precision cutting to required coil length using cold-cut or plasma-cut methods to avoid heat-affected zone (HAZ) degradation.
- End Preparation: End faces are deburred, chamfered, and prepared for the selected connection type (flange welding, threading, or socket welding).
- Spiral Coiling: The straight tube is formed into a precise spiral coil using CNC-controlled coiling machinery. The coiling process maintains constant pitch and coil diameter within tolerance of ±0.5 mm to ensure uniform heat transfer performance.
- Stress Relief: Post-coiling annealing at 400–500°C in a controlled atmosphere to relieve residual stresses introduced during the forming operation.
- Pickling and Passivation: Chemical treatment to remove any scale or discoloration and to restore the passive chromium oxide layer.
- Final Inspection: Dimensional verification (coil diameter, pitch, number of turns), leak testing (hydrostatic or pneumatic at 1.5× working pressure), and visual inspection.
4.2 Coiling Tolerance
- Coil diameter tolerance: ±0.5 mm
- Pitch tolerance: ±0.3 mm
- Tube straightness (pre-coiling): ≤1.5 mm/m
- End face perpendicularity: ±0.5°
5. Applications
5.1 Primary Application: Pressure-Type Split Solar Water Heating Systems
The spiral coil tube serves as the primary heat exchanger element within the storage water tank of a pressurized split solar thermal system. In this configuration:
- The coil tube is submerged inside the storage tank (secondary side)
- The primary circuit fluid (solar collector heat transfer fluid) circulates through the coil tube
- Thermal energy is transferred from the primary fluid to the stored domestic hot water via the stainless steel tube wall
- The pressure separation between primary and secondary circuits eliminates the risk of system contamination
5.2 Additional Applications
- Industrial Process Heating: Heat exchangers for chemical processing, food and beverage industry
- Geothermal Systems: Heat exchange coils for ground-source heat pump installations
- Boiler Auxiliary Heating: Feedwater preheating coils in low-pressure steam boilers
- Refrigeration and HVAC: Condenser and evaporator coil elements
- Pharmaceutical Equipment: Sanitary-grade heat exchange coils requiring high corrosion resistance
6. Quality Certifications
6.1 Product Certifications
- ISO 9001:2015 — Quality Management System certification for manufacturing operations
- ISO 14001:2015 — Environmental Management System certification
- CE Marking — Conformity with European Pressure Equipment Directive (PED 2014/68/EU)
- EN 12981 Compliance — Heat exchangers for water heating installations
- GB/T 26968 — Chinese national standard for solar water heating systems
6.2 Testing and Verification
- Hydrostatic Pressure Test: 1.5× maximum working pressure, held for minimum 10 minutes with zero leakage
- Dimensional Inspection: Coordinate Measuring Machine (CMM) verification of coil geometry
- Material Verification: Spectroscopic analysis (OES) confirming chemical composition
- Weld Inspection: Visual and dye penetrant testing (DPT) for all welded joints
- Corrosion Testing: Salt spray test (ASTM B117) for minimum 500 hours without pitting for 316/316L grades
6.3 Traceability
Each coil tube assembly is assigned a unique serial number with full traceability documentation including: heat number of the parent tube material, mill test certificate (EN 10204 3.1), manufacturing batch record, and final inspection report.
7. Unique Selling Points
- Optimized Heat Transfer Geometry: Spiral coil pitch and diameter are calculated based on thermal load requirements and fluid dynamics analysis (Nusselt number optimization), ensuring maximum heat transfer surface area within the available tank volume.
- Full Pressure Separation: The sealed coil design provides complete hydraulic separation between primary and secondary circuits, eliminating cross-contamination risk and enabling system pressurization up to 1.6 MPa.
- Thermal Cycling Durability: Austenitic stainless steel's low thermal expansion coefficient and excellent fatigue resistance ensure long service life under repeated heating/cooling cycles typical of solar thermal operation.
- Corrosion Resistance: The passive chromium oxide layer provides self-healing protection against internal water corrosion, eliminating the need for牺牲阳极 (sacrificial anode) protection and extending system maintenance intervals.
- Customization Capability: Coil dimensions, material grade, and connection types are fully customizable to match specific system design parameters and thermal performance targets.
- Comprehensive Quality Assurance: Multi-stage inspection protocol from raw material verification through final pressure testing ensures consistent product reliability and compliance with international standards.
8. Design Considerations for Heat Exchanger Integration
When integrating the spiral coil tube into a storage water tank heat exchanger, the following design parameters should be considered:
- Heat transfer area (A) calculated from required thermal duty (Q) and overall heat transfer coefficient (U): A = Q / (U × ΔTlm)
- Pressure drop through the coil tube should be minimized to reduce pumping power requirements
- Flow velocity within the tube (typically 1.0–2.5 m/s) should be sufficient to maintain turbulent flow (Re > 2300) for optimal heat transfer
- Thermal expansion of the coil assembly relative to the tank must be accommodated by flexible connections or expansion loops
- Anti-solar gain measures should be applied to the tank exterior to minimize parasitic heat loss