Stainless Steel Coil Tube (Coiltube) — Product Technical Specification
1. Product Overview
The Stainless Steel Coil Tube (Coiltube), manufactured by Wenzhou Jixing Metal Materials Co., Ltd., is a precision-engineered continuous stainless steel tubing product supplied in coiled form for downstream processing, heat exchanger fabrication, and refrigeration system integration. Unlike discrete-length seamless or welded pipes, coil tubes are delivered in continuous lengths wound onto spools, enabling uninterrupted feeding into automated forming, bending, and welding lines. This product is particularly suited for applications in fan coil unit (FCU) refrigeration systems, where the coil tube serves as the primary heat transfer medium within evaporator and condenser assemblies.
From a metallurgical standpoint, the coil tube represents a cold-worked or hot-rolled stainless steel tubing product whose mechanical properties, surface finish, and dimensional tolerances are optimized for thermal exchange efficiency, corrosion resistance, and long-term structural integrity under cyclic thermal loading conditions inherent to HVAC and refrigeration environments.
2. Material & Properties
2.1 Material Grades
The coil tube is available in the following standard stainless steel grades, each selected based on the specific service environment and corrosion exposure:
- AISI 304 (UNS S30408) — The most widely used austenitic stainless steel, offering excellent general corrosion resistance, formability, and weldability. Contains approximately 18% Chromium (Cr) and 8% Nickel (Ni).
- AISI 304L (UNS S30403) — A low-carbon variant of 304 designed to mitigate intergranular corrosion (IGC) during welding and heat treatment operations.
- AISI 316 (UNS S31608) — Enhanced austenitic grade with 2–3% Molybdenum (Mo) addition, providing superior resistance to pitting corrosion, crevice corrosion, and chloride-induced degradation.
- AISI 316L (UNS S31603) — Low-carbon version of 316 with improved weldability and resistance to sensitization.
- AISI 321 (UNS S32100) — Titanium-stabilized austenitic grade with excellent resistance to oxidation at elevated temperatures and resistance to intergranular corrosion in the 450–850 °C range.
- AISI 310S (UNS S31008) — High-temperature austenitic grade with elevated Chromium and Nickel content, suitable for high-temperature service up to 1150 °C.
2.2 Mechanical Properties
| Property | AISI 304 / 304L | AISI 316 / 316L | AISI 321 |
|---|---|---|---|
| Tensile Strength (UTS) | ≥ 515 MPa (74.7 ksi) | ≥ 515 MPa (74.7 ksi) | ≥ 515 MPa (74.7 ksi) |
| Yield Strength (0.2% Offset) | ≥ 205 MPa (29.7 ksi) | ≥ 205 MPa (29.7 ksi) | ≥ 205 MPa (29.7 ksi) |
| Elongation (A50mm) | ≥ 40% | ≥ 40% | ≥ 40% |
| Density | 7.93 g/cm³ | 8.00 g/cm³ | 7.95 g/cm³ |
| Thermal Conductivity (20 °C) | 16.3 W/(m·K) | 14.6 W/(m·K) | 16.3 W/(m·K) |
| Thermal Expansion (20–100 °C) | 17.3 × 10⁻⁶ /°C | 16.6 × 10⁻⁶ /°C | 17.3 × 10⁻⁶ /°C |
| Maximum Service Temperature | 870 °C (oxidation) | 870 °C (oxidation) | 1093 °C (oxidation) |
| Corrosion Resistance (ASTM G48 Pitting) | Good | Excellent | Good |
2.3 Surface Finish Options
- 2B Finish — Cold-rolled, annealed, and pickled surface; matte appearance; standard for general industrial applications.
- No. 4 Brushed Finish — Fine linear grinding marks; aesthetic appeal for visible installations.
- Mirror (8K) Finish — Polished to high reflectivity; suitable for food-grade and pharmaceutical applications.
- Pickle & Passivate (P&P) — Acid-cleaned and chemically passivated surface to remove scale and form a protective chromium oxide layer.
3. Specifications
| Parameter | Specification / Range | Standard Reference |
|---|---|---|
| Outer Diameter (OD) | Φ6.0 mm – Φ159.0 mm (0.24" – 6.26") | ASTM A213 / A269 |
| Wall Thickness | 0.5 mm – 6.0 mm (0.020" – 0.236") | ASTM A213 / A269 |
| OD Tolerance | ±0.05 mm (±0.002") for OD ≤ 25 mm; ±0.10 mm for OD > 25 mm | ASTM A213 |
| Wall Thickness Tolerance | ±10% of nominal wall thickness | ASTM A213 |
| Coil Length (Continuous) | 50 m – 500 m (164 ft – 1,640 ft) per coil | Manufacturer standard |
| Coil Weight | Up to 2,000 kg (4,409 lbs) per coil | Manufacturer standard |
| End Condition | Open ends; optionally beveled, plugged, or cap-sealed | Per customer specification |
| Heat Treatment Condition | Annealed and Pickled (A&P); Solution Annealed | ASTM A213 |
| Manufacturing Method | Seamless (Pierceless / Rotary Piercing); EFW (Electric Resistance Welded) | ASTM A213 / A269 |
| Standards Compliance | ASTM A213, ASTM A269, EN 10216-5, GB/T 12771, JIS G3459 | Multi-standard |
| Packaging | Wooden reel or steel drum; desiccant-packed; VCI (Vapor Corrosion Inhibitor) coated | ISO 4711 |
4. Manufacturing Process
The production of stainless steel coil tube involves a multi-stage metallurgical and mechanical processing sequence, each stage critically controlled to ensure dimensional accuracy, microstructural integrity, and surface quality:
4.1 Raw Material Preparation
- Steel Billet Casting: Stainless steel ingots are produced via Electric Arc Furnace (EAF) or Vacuum Arc Remelting (VAR) processes, followed by casting into billets of controlled chemistry. Chemical composition is verified by Optical Emission Spectroscopy (OES) and Inductively Coupled Plasma (ICP) analysis.
- Desulfurization & Deoxidation: Sulfur (S) and phosphorus (P) content is minimized to ≤ 0.030% and ≤ 0.045% respectively to prevent hot shortness and embrittlement.
4.2 Hot Working
- Reheating: Billets are heated to 1100–1250 °C in controlled-atmosphere furnaces to achieve austenitization and reduce yield strength for subsequent deformation.
- Rotary Piercing / Mannesmann Process: The heated billet is pierced to form a hollow tube (tube blank) with an initial wall thickness significantly greater than the final product. The piercing process is controlled to minimize centerline segregation and internal defects.
- Hot Rolling / Elongation: The tube blank is elongated through a series of hot reduction stands to achieve the desired OD and wall thickness ratio.
4.3 Cold Working
- Cold Drawing / Cold Pilgering: The hot-rolled tube is drawn through precision-die tooling to achieve tight dimensional tolerances, improved surface finish, and enhanced mechanical properties through strain hardening. Drawing lubricants (e.g., graphite-based or calcium soap) are applied to the inner surface.
- Cold Sizing: Final dimensional accuracy is achieved through a cold sizing pass using precision rollers or dies.
4.4 Heat Treatment & Surface Processing
- Solution Annealing: The cold-worked tube is heated to 1050–1150 °C and rapidly quenched to dissolve carbides, restore full austenitic microstructure, and eliminate residual stresses. This process is critical for preventing sensitization and intergranular corrosion.
- Pickling: The annealed tube is immersed in a mixed acid solution (typically HNO₃ + HF or HNO₃ + HCl) to remove oxide scale and discoloration formed during annealing.
- Passivation: A final treatment with dilute nitric acid or citric acid promotes the formation of a thin, adherent chromium oxide (Cr₂O₃) passive film (typically 2–5 nm thick), which provides the primary corrosion protection mechanism.
4.5 Coiling & Packaging
- Coiling: The continuous tube is wound onto a wooden or steel reel under controlled tension to prevent deformation, kinking, or creasing. Coiling tension is calibrated based on tube OD, wall thickness, and material grade.
- Inspection & Testing: Each coil undergoes visual inspection, dimensional measurement (OD, wall thickness via ultrasonic thickness gauging), hydrostatic testing, and spectrographic verification of chemical composition.
- Packaging: Coils are wrapped in protective film, placed in desiccant-treated packaging, and secured on export-grade wooden reels or steel drums for shipment.
5. Applications
5.1 Fan Coil Unit (FCU) Refrigeration Systems
The primary application domain for this coil tube product is the fabrication of heat exchanger coils within Fan Coil Units (FCUs). In an FCU refrigeration system, the stainless steel coil tube is bent into serpentine or zigzag configurations to maximize heat transfer surface area within a compact envelope. The tube serves as the conduit for refrigerant flow (e.g., R410A, R32, R134a) while fins are mechanically or brazed onto the tube outer surface to enhance convective heat transfer with the air stream.
Key Design Considerations for FCU Coil Tubes: The coil tube must exhibit adequate thermal conductivity, resistance to condensation corrosion (particularly in high-humidity environments), and sufficient mechanical strength to withstand refrigerant pressure cycling (typically 1.0–4.0 MPa operating pressure). The austenitic stainless steel microstructure provides excellent resistance to stress corrosion cracking (SCC) in the presence of chloride ions from ambient air or cleaning agents.
5.2 Additional Application Domains
- Automotive Heat Exchangers: Radiator cores, intercooler tubes, and condenser assemblies where lightweight, corrosion-resistant tubing is required.
- Food & Beverage Processing: Sanitary-grade tubing for dairy, brewing, and pharmaceutical heat exchange systems (FDA-compliant surfaces).
- Chemical Processing: Reactor cooling coils, distillation column internals, and heat transfer loops handling aggressive chemical media.
- Power Generation: Boiler tubes, superheater elements, and condenser tubes in thermal power plants.
- Oil & Gas: Downhole heat exchanger tubing, wellhead instrumentation lines, and offshore platform heat transfer systems.
- Marine Engineering: Seawater cooling circuits, desalination plant heat exchangers, and ballast water system components.
- Medical Equipment: Autoclave heating coils, MRI cooling circuits, and sterilization equipment tubing.
6. Quality Certifications & Testing Protocols
6.1 Product Certifications
| Certification / Standard | Scope of Compliance |
|---|---|
| ISO 9001:2015 | Quality Management System — design, production, and delivery |
| ISO 14001:2015 | <