Stainless Steel Coil Tube (Coiltube) for Heat Pump & Fan Coil Applications
Product Overview
The Stainless Steel Coil Tube (Coiltube) is a precision-engineered continuous tube product supplied in coil form, specifically designed for integration into air source heat pump (ASHP) systems and small temperature-differential fan coil unit (FCU) assemblies. Manufactured by Wenzhou Jixing Metal Materials Co., Ltd., this product leverages the inherent corrosion resistance, thermal conductivity, and mechanical integrity of austenitic stainless steel grades to deliver superior performance in heating, ventilation, and air conditioning (HVAC) applications. The coil configuration facilitates seamless integration into automated coil-winding and brazing production lines, minimizing material waste and maximizing manufacturing efficiency.
This product represents a critical component in next-generation high-efficiency, comfort-oriented heating systems where small temperature-differential heat exchange (low ΔT operation) is essential for maximizing coefficient of performance (COP) and ensuring uniform thermal distribution across occupied spaces.
Material & Properties
Material Grades
- Austenitic Stainless Steel — Primary grades include SUS304 (ASTM A213 TP304), SUS316 (ASTM A213 TP316), and SUS316L (ASTM A213 TP316L) for enhanced molybdenum content and superior pitting corrosion resistance.
- Grade Selection Criteria — SUS304 is specified for standard indoor fan coil and heat exchanger applications; SUS316/SUS316L is recommended for coastal, industrial, or high-chloride environments where pitting and crevice corrosion risk is elevated.
Mechanical & Physical Properties
- Tensile Strength: ≥ 520 MPa (75 ksi) at room temperature
- Yield Strength: ≥ 205 MPa (30 ksi) at 0.2% offset
- Elongation: ≥ 40% in 50 mm gauge length
- Thermal Conductivity: 16.3 W/(m·K) at 100°C (SUS304)
- Thermal Expansion Coefficient: 17.3 × 10⁻⁶ /°C (20–100°C)
- Density: 8.00 g/cm³
- Operating Temperature Range: −196°C to +870°C (continuous service)
Corrosion Resistance
- Excellent resistance to atmospheric, industrial, and marine corrosion
- High resistance to oxidation at elevated temperatures up to 870°C
- SUS316/L grades provide superior resistance to chloride-induced pitting and stress corrosion cracking (SCC)
- Passive chromium oxide film (Cr₂O₃) provides self-healing surface protection
Specifications
| Parameter | Specification | Standard / Reference |
|---|---|---|
| Product Form | Continuous Coil Tube (Coiltube) | — |
| Outer Diameter (OD) | 6.35 mm (¼"), 9.53 mm (⅜"), 12.7 mm (½"), 15.88 mm (⅝"), 19.05 mm (¾") | ASTM A213 / GB/T 12771 |
| Wall Thickness (WT) | 0.50 mm – 1.50 mm (customizable) | ASTM A213 / GB/T 12771 |
| Coil Diameter | 200 mm – 600 mm (adjustable per production requirement) | — |
| Coil Length | 50 m – 300 m per coil (standard); custom lengths available | — |
| Surface Finish | No. 4 Brushed, Mirror Polished (Ra ≤ 0.4 μm), or Electropolished (Ra ≤ 0.25 μm) | ASTM A269 / GB/T 12771 |
| End Condition | Flat-cut, Beveled, or Flared (per customer specification) | — |
| Welding Process | Argon Arc Welding (TIG/PAW) — Weld Seam Type 1 (fully welded) or Type 2 (partially welded) | ASTM A269 / GB/T 12771 |
| Heat Treatment | Solution Annealed (1050–1150°C, water quenched) | ASTM A213 |
| Tolerance — OD | ±0.05 mm (precision grade); ±0.10 mm (standard grade) | GB/T 12771 |
| Tolerance — Wall Thickness | ±10% of nominal (standard); ±5% (precision grade) | GB/T 12771 |
| Pressure Rating | Up to 2.5 MPa (test pressure 3.75 MPa) — dependent on OD/WT combination | ASME B31.5 |
| Refrigerant Compatibility | R134a, R32, R410A, R407C, R290, CO₂ (R744) — all common ASHP refrigerants | — |
| Applicable Systems | Air Source Heat Pump, Fan Coil Unit, Chiller, Condenser, Evaporator | — |
| Coating / Passivation | Nitric acid passivation (ASTM A967); optional PTFE or ceramic internal coating | ASTM A967 |
Manufacturing Process
Process Flow
- Raw Material Preparation — Cold-rolled stainless steel strip (CRSS) is sourced from certified mills (e.g., POSCO, Outokumpu, Baowu) with full mill test certificates (MTC) conforming to ASTM A240 or equivalent.
- Coil Straightening & Feeding — The strip is unwound, straightened via roller or tension-based systems, and fed into the tube-forming die set at controlled feed rates (typically 15–30 m/min).
- Tube Forming (Roll Forming) — Multi-stage progressive forming dies convert the flat strip into a seamless circular tube blank. Precision roller alignment ensures uniform wall thickness and roundness.
- Welding — High-frequency induction welding (HF) or pulsed argon arc welding (PAW/TIG) is applied to close the longitudinal seam. PAW welding is preferred for Type 1 fully welded tubes requiring superior seam integrity. Weld parameters are controlled via automated feedback systems monitoring seam appearance and penetration.
- Piercing & Sizing — The welded tube blank is passed through a series of sizing dies (mandrel or floating mandrel systems) to achieve the final OD and wall thickness dimensions with tight tolerances.
- Coil Winding — The finished tube is wound into coils of specified diameter and length using a precision winding machine with tension control to prevent kinking, ovality, or surface damage.
- Heat Treatment (Solution Annealing) — Coils are solution-annealed at 1050–1150°C followed by rapid water quenching to restore full austenitic microstructure, eliminate residual stresses from cold working, and maximize corrosion resistance and ductility.
- Pickling & Passivation — Coils are treated with hydrofluoric/nitric acid blend to remove oxide scale, followed by nitric acid passivation per ASTM A967 to establish the protective chromium oxide passive film.
- Surface Finishing — Electropolishing, mechanical polishing, or brushing is applied to achieve the specified surface finish (Ra value), critical for minimizing fouling and maximizing heat transfer efficiency in heat exchanger applications.
- Quality Inspection & Testing — Each coil undergoes dimensional inspection (OD, WT, roundness), hydrostatic pressure testing (1.5× working pressure, 30-second hold), eddy current flaw detection (ASTM E709), and visual inspection of weld seams.
- Packaging & Marking — Coils are wrapped in protective film, placed on wooden or steel pallets, and marked with grade, dimensions, heat number, and compliance certificates.
Process Advantages
- Continuous Production — Coil form enables uninterrupted automated processing, reducing labor costs and improving throughput by 40–60% compared to straight-length tube fabrication.
- Material Efficiency — Minimal cutting waste (typically < 2%) compared to saw-cut straight tubes, contributing to lower material costs and reduced environmental impact.
- Consistent Quality — Automated forming and welding parameters ensure uniform tube geometry and weld quality throughout the coil length.
Applications
Primary Application: Air Source Heat Pump (ASHP) Systems
Stainless steel coil tubes are integral components in the evaporator and condenser heat exchangers of air source heat pump systems. In these applications, the tube serves as the refrigerant flow channel, with fins mechanically or brazed onto the tube surface to maximize the heat transfer area between the refrigerant and ambient air. The coil tube's corrosion resistance ensures long service life even in harsh outdoor environments, while its thermal conductivity enables efficient heat exchange at small temperature differentials (ΔT as low as 3–5°C), which is critical for achieving high COP values (typically 3.5–5.0) in modern ASHP systems.
Secondary Application: Small Temperature-Differential Fan Coil Units
In fan coil unit assemblies, the stainless steel coil tube forms the core of the air-side heat exchanger. The small temperature-differential design (supply/return water ΔT of 3–5°C, as opposed to conventional 7–10°C) enables:
- Higher flow rates at lower velocity, reducing pump energy consumption
- More uniform room temperature distribution and improved occupant comfort
- Reduced risk of condensation on coil surfaces, minimizing microbial growth
- Better compatibility with low-temperature district heating and radiant systems
Additional Applications
- Industrial process heat exchangers (chemical, pharmaceutical, food & beverage)
- Chiller and condenser tube bundles
- Marine and offshore HVAC systems
- Medical equipment cooling circuits
- Automotive thermal management systems
- Solar thermal energy collector tubes
Quality Certifications
Standards Compliance
- ASTM A213 — Standard Specification for Seamless and Welded Austenitic Stainless Steel Tubes for High-temperature Service
- ASTM A269 — Standard Specification for Seamless and Welded Austenitic Stainless Steel Tubing for Chemical and Similar Services
- GB/T 12771 — Chinese National Standard for Stainless Steel Seamless Tubes for Fluid Transport
- EN 10216-5 — European Standard for Welded Stainless Steel Tubes for Pressure Purposes
- ISO 1143 — Designation of Stainless Steel Grades
Quality Management Certifications
- ISO 9001:2015 — Quality Management System
- ISO 14001:2015 — Environmental Management System
- ISO 45001:2018 — Occupational Health and Safety Management System
- CE Marking — Conformity with European Union directives for pressure equipment (PED 2014/68/EU where applicable)
- TÜV / SGS Third-Party Inspection — Independent verification of material composition, mechanical properties, and dimensional accuracy
Testing & Inspection Protocols
- Hydrostatic Pressure Test — 100% inspection at 1.5× maximum working pressure, minimum 30-second hold, leak-free pass criteria
- Eddy Current Testing (ECT) — 100% inline inspection per ASTM E709 for weld seam integrity and wall thickness verification
- Chemical Composition Analysis — Spectrometric analysis (OES) per ASTM E415 for each heat lot
- Mechanical Testing — Tensile testing per ASTM E8/E8M for each heat lot
- Intergranular Corrosion Testing — ASTM A262 Practice 1E for sensitization resistance verification
- Dimensional Inspection — Optical or laser-based inline measurement of OD, WT, roundness, and straightness
Unique Selling Points
- Superior Corrosion Longevity — Austenitic stainless steel construction ensures 25+ year service life in HVAC applications without degradation, outperforming copper and carbon steel alternatives in corrosive environments.
- Optimized for Low-ΔT Operation — Precision wall thickness and surface finish engineering maximize heat transfer coefficients (U-values up to 180–220 W/m²·K) at small temperature differentials, directly contributing to higher system COP and energy savings of 15–25%.
- Hygienic Surface Quality — Electropolished finish (Ra ≤ 0.25 μm) minimizes bacterial adhesion and biofilm formation, critical for indoor air quality (IAQ) compliance in healthcare and commercial buildings.
- Full Refrigerant Compatibility — Compatible with all next-generation low-GWP refrigerants (R290, R32, R454B, CO₂) without risk of chemical interaction or tube degradation.
- Coil Form Efficiency — Continuous coil supply reduces installation labor time by up to 50% compared to cut-and-weld straight tube assemblies, with minimal material waste and lower carbon footprint.
- Full Traceability — Each coil is traceable to its mill heat number with complete documentation including MTC, NDE reports, and dimensional inspection certificates, ensuring full supply chain transparency.
Technical Notes & Recommendations
Installation Guidance: When integrating stainless steel coil tubes into heat exchanger assemblies, maintain bend radii of ≥ 3× OD to prevent wall thinning and ovality. Use stainless steel brazing alloys (e.g., Ni-Fe or Ag-Cu eutectic) compatible with the tube grade to avoid galvanic corrosion at joints. Ensure proper passivation treatment after any post-fabrication welding or mechanical processing.
Design Consideration: For small temperature-differential fan coil applications, recommend tube OD of 6.35–9.53 mm with wall thickness of 0.50–0.80 mm to optimize the balance between heat transfer area, flow resistance (pressure drop), and mechanical strength. Fin spacing of 2.0–2.5 mm provides optimal air-side performance for typical indoor air velocities of