Stainless Steel Coil Tube (Coiltube) — Technical Specification

1. Product Overview

The Stainless Steel Coil Tube (Coiltube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered continuous-length tubular product formed into a helical coil configuration. This product is specifically designed for integration into HVAC (Heating, Ventilation, and Air Conditioning) systems, particularly ceiling-mounted fan coil units (e.g., the FWY-600 series residual-pressure ceiling fan coil unit), as well as industrial heat exchange, fluid conveyance, and process engineering applications. The coil tube is supplied in continuous coil form to facilitate automated downstream processing, including straightening, cutting, bending, and assembly into terminal heat exchanger cores.

The product designation Coiltube refers to the coil-form delivery state of the seamless or welded stainless steel tube, which offers logistical advantages over straight-length delivery for high-volume production environments. The material composition and dimensional tolerances are engineered to ensure long-term corrosion resistance, thermal efficiency, and mechanical integrity under continuous operational stress.

2. Material & Properties

2.1 Base Material Grades

• AISI 304 (UNS S30400): Austenitic stainless steel with 18% Chromium (Cr) and 8% Nickel (Ni); the most widely used grade for general corrosion resistance and food-grade applications.
• AISI 316 (UNS S31600): Enhanced austenitic stainless steel with 16–18% Cr, 10–14% Ni, and 2–3% Molybdenum (Mo); provides superior resistance to chlorides and acidic environments.
• AISI 316L (UNS S31603): Low-carbon variant of 316 (Carbon ≤ 0.03%) for improved weldability and intergranular corrosion resistance.

2.2 Mechanical Properties (Typical for AISI 304)

• Tensile Strength (UTS): ≥ 515 MPa (74,700 psi)
• Yield Strength (0.2% offset): ≥ 205 MPa (29,700 psi)
• Elongation (at fracture): ≥ 40%
• Hardness (HB): ≤ 217
• Thermal Conductivity (at 20°C): 16.3 W/(m·K)
• Thermal Expansion Coefficient: 17.3 × 10⁻⁶ /°C
• Density: 8.0 g/cm³

2.3 Surface Finish Options

• 2B Finish: Cold-rolled and annealed; smooth, matte appearance; suitable for general industrial applications.
• No. 4 Brushed Finish: Uniform unidirectional grain; decorative and corrosion-resistant.
• Mirror Polish (8K): Ra ≤ 0.2 μm; for hygienic and high-visibility applications.
• Pickled & Passivated: Removes decarburized layer and oxide scale; restores chromium-rich passive film.

3. Specifications

Parameter	Specification	Standard Reference
Outer Diameter (OD)	Φ6.0 mm – Φ50.8 mm (custom sizes available)	ASTM A554 / GB/T 14976
Wall Thickness (WT)	0.5 mm – 3.0 mm	ASTM A554 / GB/T 14976
Coil Diameter (CD)	Φ300 mm – Φ1,200 mm	ISO 1155
Coil Length (Continuous)	50 m – 200 m per coil	—
Dimensional Tolerance (OD)	±0.05 mm (precision grade); ±0.10 mm (standard)	ASTM A554
Dimensional Tolerance (WT)	±10% of nominal	ASTM A554
Roundness (Out-of-Round)	≤ 5% of OD tolerance	GB/T 14976
Surface Roughness (Ra)	≤ 0.8 μm (2B); ≤ 0.4 μm (polished)	ISO 1302
Maximum Operating Temperature	870°C (AISI 304); 870°C (AISI 316)	ASME BPVC Section II
Minimum Operating Temperature	−196°C (cryogenic capability)	—
Pressure Rating (PN)	PN16 – PN63 (size-dependent)	EN 10217
Welding Compatibility	TIG (GTAW), MIG (GMAW), orbital welding	ASME BPVC Section IX

4. Manufacturing Process

4.1 Process Flow

1. Raw Material Selection: Hot-rolled stainless steel strip or seamless tube blanks sourced from certified mills (e.g., Outokumpu, POSCO, Tsingshan). Incoming material undergoes spectroscopic verification (PMI — Positive Material Identification) to confirm chemical composition.
2. Forming (Seamless Tube): For seamless variants, hot-rolling or cold-drawing processes are employed. The billet is pierced and elongated through multiple reduction passes to achieve target OD and WT.
3. Forming (Welded Tube): For welded variants, cold-rolled strip is formed into a tubular shape and resistance-welded (ERW — Electric Resistance Welding) or TIG-welded along the longitudinal seam.
4. Heat Treatment (Annealing): Tubes are annealed at 1050–1150°C in a controlled atmosphere (vacuum or protective gas) to relieve residual stresses, refine grain structure, and restore full austenitic microstructure. This step is critical for achieving specified mechanical properties and corrosion resistance.
5. Pickling & Passivation: The annealed tube is immersed in a mixed acid bath (HNO₃ + HF) to remove oxide scale, followed by passivation in a nitric acid solution to restore the chromium oxide passive film (Cr₂O₃).
6. Dimensional Calibration: Precision sizing through cold-drawing or roll-calibration to achieve tight OD and roundness tolerances.
7. Coiling: The continuous tube is wound onto a mandrel to form the coil configuration. Tension control systems ensure uniform coil geometry without inducing cold-work distortion.
8. Final Inspection & Packaging: Each coil undergoes dimensional measurement, visual surface inspection, hydrostatic pressure testing, and non-destructive testing (NDT). Coils are packaged with moisture-absorbent desiccants and protective wrapping for shipment.

4.2 Key Quality Control Points

• Spectroscopic Analysis: Every heat lot undergoes optical emission spectrometry (OES) for full chemical composition verification against ASTM/EN standards.
• Hydrostatic Testing: Each coil is pressure-tested at 1.5× the maximum design pressure to verify wall integrity and absence of pinhole defects.
• Eddy Current Testing (ECT): Automated electromagnetic inspection detects subsurface cracks, inclusions, and wall thickness variations along the full coil length.
• Grain Structure Analysis: Metallographic examination confirms austenitic microstructure with controlled grain size (ASTM E112 Grain Size No. 6–10).

5. Applications

5.1 HVAC & Building Services

• Ceiling-Mounted Fan Coil Units (FCUs): The coil tube serves as the core heat exchange element in units such as the FWY-600 residual-pressure ceiling fan coil, where stainless steel tubing provides long-term resistance to condensation-induced corrosion in humid indoor environments.
• Air Handling Units (AHUs): Coil tubes are brazed into serpentine heat exchanger bundles for chilled water and hot water heating/cooling circuits.
• Heat Pump Systems: Used in evaporator and condenser coils for ground-source and air-source heat pumps.

5.2 Industrial & Process Applications

• Chemical Processing: Fluid conveyance in corrosive environments (acids, alkalis, halide-containing solutions) where AISI 316/316L grades provide essential corrosion protection.
• Food & Beverage Industry: Hygienic-grade coil tubing for CIP (Clean-In-Place) systems, pasteurization circuits, and dairy processing lines (FDA-compliant, 3-A Sanitary).
• Pharmaceutical Manufacturing: GMP-compliant fluid transfer tubing for sterile process lines, requiring electropolished interior surfaces (Ra ≤ 0.4 μm).
• Water Treatment & Desalination: Resistance to chlorinated water and high-salinity environments.

5.3 Specialty Applications

• Cryogenic Systems: Maintains ductility at temperatures as low as −196°C for LNG (Liquefied Natural Gas) transfer lines.
• Medical Equipment: Steam sterilization (autoclaving) compatible tubing for endoscope reprocessing and dialysis systems.
• Aerospace & Defense: High-temperature exhaust manifolds and hydraulic fluid lines.

6. Quality Certifications & Compliance

6.1 Product Certifications

• ISO 9001:2015 — Quality Management System certification for manufacturing processes.
• ISO 14001:2015 — Environmental Management System certification.
• CE Marking — Conformity with European Union directives (PED 2014/68/EU for pressure equipment).
• EN 10204 3.1 — Third-party inspection certificate for material compliance.
• Mill Test Reports (MTR) — Provided with each shipment, documenting chemical composition, mechanical properties, and heat treatment parameters.

6.2 Standards Compliance

• ASTM A554 — Standard Specification for Cold-Rolled or Cold-Drawn Seamless and Welded Austenitic Stainless Steel Tubing.
• ASTM A269 — Standard Specification for Seamless and Welded Austenitic Stainless Steel Tubing for Chemical and Similar Severely Corrosive Services.
• ASTM A270 — Standard Specification for Seamless and Welded Austenitic Stainless Steel Tubing for General Service.
• EN 10217-7 — Technical delivery conditions for non-alloy and stainless steel tubes for pressure purposes — austenitic steel tubes.
• GB/T 14976 — Chinese National Standard for seamless stainless steel tubes for general engineering and construction.
• GB/T 12771 — Chinese National Standard for welded stainless steel tubes for fluid transport.

6.3 Non-Destructive Testing (NDT) Compliance

• ASME BPVC Section V — Non-destructive examination procedures for eddy current and ultrasonic testing.
• ASME BPVC Section VIII Div. 1 — Pressure vessel code compliance for welded joint efficiency and allowable stress values.
• ASTM E2303 — Standard Practice for Eddy Current Examination of Seamless and Welded Tubular Products.

7. Unique Selling Points

• Continuous Coil Delivery: Eliminates butt-weld joints inherent in straight-length tube supply, reducing leak points and improving system integrity in heat exchanger assemblies.
• Precision Dimensional Control: Tight OD and wall thickness tolerances (±0.05 mm) ensure optimal fit in automated tube-bending and brazing operations, minimizing scrap rates.
• Full Traceability: Each coil is traceable to its mill heat number, enabling complete supply chain accountability and recall capability.
• Multi-Grade Availability: Single-source procurement of AISI 304, 316, 316L, and specialty grades (e.g., 321, 347) simplifies supply chain management.
• HVAC-Optimized Coiling: Coil geometry is engineered for seamless integration into automated fan coil unit assembly lines, compatible with standard tube-bending machinery.
• Superior Corrosion Resistance: The pickled and passivated surface ensures a minimum chromium oxide film thickness of 2–5 nm, providing decades of service life in aggressive environments.

Technical Note: The stainless steel coil tube product described herein has been evaluated and validated in the context of ceiling-mounted residual-pressure fan coil unit applications (FWY-600 series), demonstrating conformance with national technical appraisal standards in Shaoxing, Zhejiang Province, China. This validation confirms the material's suitability for continuous duty in HVAC terminal units subjected to condensation cycling, mechanical vibration, and thermal fatigue.