Stainless Steel Coil Tube — Floating Coil Heat Exchanger & U-Tube Heat Exchanger Grade
Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd (温州吉兴金属材料有限公司)
Product Reference No.: 985
Category: Literature Study / Technical Reference
1. Product Overview
The Stainless Steel Coil Tube (盘管 / Coiltube) is a precision-engineered tubular product designed for use in floating coil heat exchangers and U-tube heat exchangers within building hot water supply systems. These heat exchanger configurations have gained widespread adoption in commercial, institutional, and residential HVAC (Heating, Ventilation, and Air Conditioning) applications due to their superior thermal efficiency, compact footprint, and resistance to scaling and fouling.
Wenzhou Jixing Metal Materials Co., Ltd supplies high-grade austenitic stainless steel coil tubes that meet the stringent demands of hydronic heating systems. The coil tubes are manufactured to conform with international standards and are specifically engineered to withstand cyclic thermal loading, hydrostatic pressure, and long-term exposure to potable water and heating media without degradation of mechanical or corrosion-resistant properties.
The fundamental distinction between floating coil and U-tube configurations lies in their thermal exchange mechanism:
- Floating Coil Heat Exchanger (浮动盘管换热器): The coil tube is suspended within a cylindrical shell, allowing free thermal expansion and contraction. This design minimizes thermal stress and facilitates easy coil replacement.
- U-Tube Heat Exchanger (U型管换热器): The coil tube is bent into a continuous U-shaped configuration, providing a larger heat transfer surface area per unit volume and enhanced resistance to vibration-induced fatigue.
2. Material & Properties
2.1 Base Alloy Composition
The primary material grade employed is SAE 304 (UNS S30400) and SAE 316 (UNS S31600) austenitic stainless steel, selected for their excellent corrosion resistance, formability, and weldability. The following table summarizes typical chemical composition and mechanical properties:
| Property | SAE 304 (UNS S30400) | SAE 316 (UNS S31600) |
|---|---|---|
| Carbon (C) max (%) | 0.08 | 0.08 |
| Chromium (Cr) (%) | 18.0 – 20.0 | 16.0 – 18.0 |
| Nickel (Ni) (%) | 8.0 – 10.5 | 10.0 – 14.0 |
| Molybdenum (Mo) (%) | — | 2.0 – 3.0 |
| Tensile Strength (MPa) min | 515 | 515 |
| Yield Strength (MPa) min | 205 | 205 |
| Elongation (%) min | 40 | 40 |
| Thermal Conductivity (W/m·K) @ 100°C | 16.3 | 16.3 |
| Thermal Expansion Coefficient (μm/m·K) @ 20–100°C | 17.3 | 17.3 |
2.2 Surface Finish
The coil tube interior surface is supplied in electropolished or passivated condition, achieving a surface roughness (Ra) of ≤ 0.4 μm. This ultra-smooth finish minimizes biofilm adhesion, reduces scaling accumulation, and ensures hygienic compliance for potable water applications in accordance with NSF/ANSI 61 (Drinking Water System Components – Health Effects) and WRAS (Water Regulations Advisory Scheme) guidelines.
3. Specifications
| Parameter | Specification |
|---|---|
| Outer Diameter (OD) | Φ6 mm, Φ8 mm, Φ10 mm, Φ12 mm, Φ16 mm (custom sizes available) |
| Wall Thickness | 0.5 mm – 2.0 mm |
| Coil Diameter (Bending Radius) | Φ150 mm – Φ500 mm (configurable per heat exchanger design) |
| Coil Length | Up to 50 m per continuous coil (U-tube configurations up to 200 m) |
| Operating Temperature Range | −20 °C to +250 °C |
| Maximum Operating Pressure | 1.6 MPa (standard); 2.5 MPa (reinforced) |
| Thermal Efficiency | ≥ 95% heat transfer effectiveness in floating coil configurations |
| Design Life | ≥ 25 years under normal operating conditions |
| Applicable Standards | ASTM A213/A213M, GB/T 12771, EN 10216-5, ISO 1143 |
4. Manufacturing Process
The production of high-quality stainless steel coil tubes involves a multi-stage process chain designed to ensure dimensional accuracy, metallurgical integrity, and surface quality:
- Hot Rolling & Annealing: Stainless steel strip or pipe blanks are hot-rolled to approximate dimensions and subsequently annealed to relieve residual stresses and achieve a fully austenitic microstructure.
- Cold Drawing & Sizing: The pipe undergoes cold drawing through precision dies to achieve exact OD and wall thickness tolerances (± 0.05 mm). This process enhances mechanical strength through work hardening while maintaining ductility.
- Precision Coiling (Coil Forming): The straight tube is fed through a CNC-controlled bending machine that forms the coil or U-tube geometry. The bending process is performed at controlled speeds to prevent ovalization, wrinkling, or thinning at the bend apex. The minimum bend radius is maintained at ≥ 3× OD to avoid stress concentration.
- Post-Form Annealing: After coiling, the tube is annealed at 1050–1100 °C followed by rapid quenching to restore full softness, eliminate bending-induced residual stresses, and re-establish the protective chromium oxide passive layer.
- Surface Treatment & Passivation: The tube undergoes acid pickling (HNO₃/HF mixture) to remove scale and impurities, followed by passivation to form a uniform chromium oxide film (≥ 10 nm thickness) that provides long-term corrosion resistance.
- Non-Destructive Testing (NDT): Each coil tube lot is subjected to 100% hydrostatic pressure testing at 1.5× the maximum operating pressure for a minimum dwell time of 30 seconds. Additional eddy current testing is performed to detect surface and near-surface defects.
- Dimensional Inspection: OD, wall thickness, coil diameter, and straightness are verified using laser measurement systems and coordinate measuring machines (CMM) to ensure compliance with tolerance specifications.
5. Applications
5.1 Building Hot Water Supply Systems
Stainless steel coil tubes are the core heat transfer element in indirect water heating systems for commercial buildings, hotels, hospitals, schools, and residential complexes. The floating coil configuration allows the coil to expand and contract freely within the storage tank, accommodating thermal cycling without inducing mechanical failure. This is particularly advantageous in systems where water temperature fluctuates between 40 °C (distribution) and 65 °C (storage) on a daily cycle.
5.2 Specific Application Scenarios
- Central Heating Systems: Integration with district heating networks for space heating and domestic hot water (DHW) preparation.
- Commercial Kitchen Hot Water: High-throughput instant hot water delivery for food service establishments requiring hygiene compliance.
- Hospital Sanitation Systems: Legionella-resistant hot water distribution (≥ 60 °C at all outlets) with low biofilm formation.
- Industrial Process Heating: Preheating of process water in textile, food processing, and pharmaceutical manufacturing.
- Renewable Energy Integration: Coupling with solar thermal collectors and heat pump systems for hybrid heating solutions.
- Swimming Pool Heating: Efficient heat transfer in chlorinated or saltwater pool circulation systems, leveraging the corrosion resistance of SAE 316 grade.
6. Quality Certifications & Compliance
Wenzhou Jixing Metal Materials Co., Ltd maintains a comprehensive quality management system and holds the following certifications and compliances:
- ISO 9001:2015 — Quality Management Systems (质量管理体系)
- ISO 14001:2015 — Environmental Management Systems (环境管理体系)
- CE Marking — Conformity with European Directive 2014/68/EU (Pressure Equipment Directive, PED)
- EN 10204 3.1 — Material Test Reports (MTR) with third-party inspection
- NSF/ANSI 61 — Drinking Water System Components Health Effects Certification
- GB/T 12771-2008 — Chinese National Standard for Welded Stainless Steel Tubes for General Fluid Transport
- ASTM A213/A213M — Standard Specification for Seamless and Welded Austenitic Stainless Steel Tubing
7. Unique Selling Points
Thermal Cycling Resilience: The floating coil design combined with fully annealed austenitic stainless steel eliminates thermal fatigue failure, ensuring reliable operation over 100,000+ thermal cycles without performance degradation.
Hygienic Interior Surface: Electropolished interior (Ra ≤ 0.4 μm) provides an inherently clean surface that resists bacterial adhesion and biofilm formation, critical for potable water and sanitary applications.
Scalability & Customization: Available in multiple OD, wall thickness, and coil diameter configurations, with custom engineering support for non-standard heat exchanger designs. Minimum order quantities accommodate both large-scale projects and prototype development.
Corrosion Resistance in Aggressive Environments: SAE 316 grade with molybdenum addition provides superior resistance to chloride-induced pitting and crevice corrosion, extending service life in coastal, industrial, or high-chloride water environments.
Sustainability: Stainless steel is 100% recyclable with no loss of material properties. The long service life (≥ 25 years) significantly reduces lifecycle carbon footprint compared to copper or carbon steel alternatives.
This technical specification is provided as a reference document based on literature study and technical analysis of floating coil heat exchanger and U-tube heat exchanger applications in building hot water systems. For project-specific requirements, please consult Wenzhou Jixing Metal Materials Co., Ltd engineering team for detailed design support and material selection guidance.