Stainless Steel Coil Tube (Coiltube) — Model No. 1216
1. Product Overview
The Stainless Steel Coil Tube (Coiltube), catalog reference No. 1216, is a precision-engineered continuous coil product manufactured by Wenzhou Jixing Metal Materials Co., Ltd. This product is specifically designed for applications in air-cooled heat exchangers, fan coil units (FCUs), and large temperature-differential (LTD) cooling systems where thermal efficiency, corrosion resistance, and long-term structural integrity are paramount.
Unlike discrete straight tubes, coil tubes are produced in continuous helical or serpentine configurations, enabling seamless integration into heat exchanger cores, refrigeration coils, and HVAC (Heating, Ventilation, and Air Conditioning) assemblies. The coil form factor minimizes welding points, reduces thermal stress concentrations, and optimizes fluid dynamics within the tube passages, making it an ideal choice for systems operating under significant cold-water temperature differentials (ΔT > 8°C).
This product line has been studied and validated in the context of large cold-water temperature difference scenarios, where conventional coil designs may suffer from reduced heat transfer coefficients, increased pressure drop, and accelerated corrosion. The No. 1216 coil tube addresses these challenges through optimized geometry, superior material selection, and advanced manufacturing processes.
2. Material & Properties
2.1 Base Material
The coil tube is fabricated from austenitic stainless steel grades conforming to international standards. The primary material specifications include:
- Grade 304 (UNS S30400) — Chromium-Nickel austenitic stainless steel with 18% Cr and 8% Ni, offering excellent general corrosion resistance and weldability.
- Grade 316 (UNS S31600) — Molybdenum-bearing austenitic stainless steel with 2–3% Mo addition, providing superior resistance to pitting and crevice corrosion in chloride-rich environments.
- Grade 321 (UNS S32100) — Stabilized austenitic stainless steel with titanium (Ti) stabilization, suitable for elevated-temperature service conditions.
2.2 Mechanical Properties
| Property | Grade 304 | Grade 316 | Test Standard |
|---|---|---|---|
| Tensile Strength (UTS) | ≥ 515 MPa | ≥ 515 MPa | ASTM A270 / GB/T 12771 |
| Yield Strength (0.2% Offset) | ≥ 205 MPa | ≥ 205 MPa | ASTM A270 / GB/T 12771 |
| Elongation (A50mm) | ≥ 40% | ≥ 40% | ASTM A270 / GB/T 12771 |
| Density | 8.00 g/cm³ | 8.00 g/cm³ | — |
| Thermal Conductivity (25°C) | 16.3 W/(m·K) | 16.3 W/(m·K) | ASTM E1461 |
| Thermal Expansion Coefficient (20–100°C) | 17.3 × 10⁻⁶ /°C | 17.3 × 10⁻⁶ /°C | ASTM E228 |
| Specific Heat Capacity (25°C) | 500 J/(kg·K) | 500 J/(kg·K) | — |
2.3 Chemical Composition (Typical, wt%)
| Element | Grade 304 | Grade 316 |
|---|---|---|
| Carbon (C) | ≤ 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 |
| Manganese (Mn) | ≤ 2.0 | ≤ 2.0 |
| Silicon (Si) | ≤ 1.0 | ≤ 1.0 |
| Phosphorus (P) | ≤ 0.045 | ≤ 0.045 |
| Sulfur (S) | ≤ 0.030 | ≤ 0.030 |
2.4 Corrosion Resistance Performance
Under large cold-water temperature differential (LTD) conditions — where the coil tube is exposed to aggressive aqueous environments with pH fluctuations, dissolved oxygen, and potential chloride ingress — the No. 1216 coil tube demonstrates the following corrosion resistance characteristics:
- Pitting Resistance (Grade 316): PREN (Pitting Resistance Equivalent Number) ≥ 24, ensuring robust resistance to localized corrosion in chlorinated water systems.
- Intergranular Corrosion Resistance: Fully solution-annealed microstructure with low carbon content (≤ 0.08%) minimizes chromium carbide precipitation at grain boundaries.
- Stress Corrosion Cracking (SCC) Resistance: Austenitic crystal structure provides inherent resistance to chloride-induced SCC, critical for long-term service in HVAC condensate environments.
3. Specifications
3.1 Dimensional Specifications
| Parameter | Specification | Tolerance | Standard |
|---|---|---|---|
| Outer Diameter (OD) | 6.35 mm (¼"), 9.52 mm (⅜"), 12.70 mm (½"), 15.88 mm (⅝"), 19.05 mm (¾") | ±0.13 mm | ASTM A270 / GB/T 12771 |
| Wall Thickness (WT) | 0.50 – 2.00 mm | ±10% | ASTM A270 / GB/T 12771 |
| Coil Winding Diameter | 150 – 500 mm | ±3 mm | Manufacturer Specification |
| Coil Length (Per Roll) | Up to 500 m (continuous) | — | — |
| Coil Length (Custom) | Up to 1000 m (upon request) | — | — |
| Surface Finish | Bright Annealed (BA), No. 4 Brushed, or Mirror (Ra ≤ 0.4 μm) | — | ASTM A478 |
| End Condition | Plain, Beveled, or Threaded (upon request) | — | — |
3.2 Performance Under Large Temperature Differential Conditions
Based on engineering studies of large cold-water temperature difference effects on air coolers and fan coil units, the No. 1216 coil tube has been optimized for the following thermal-hydraulic performance parameters:
| Performance Parameter | Design Target | Test Condition |
|---|---|---|
| Heat Transfer Coefficient (h) | ≥ 3500 W/(m²·K) | Water velocity 1.5 m/s, ΔT = 12°C |
| Pressure Drop (ΔP) | ≤ 80 kPa | Water velocity 1.5 m/s, 50 m equivalent length |
| Fouling Factor (Water Side) | ≤ 0.0001 m²·K/W | Domestic water quality, continuous operation |
| Operating Temperature Range | −40°C to +200°C | Continuous service |
| Operating Pressure | ≤ 1.6 MPa (standard); ≤ 2.5 MPa (thick-wall) | Hydrostatic test at 1.5× working pressure |
| Design Life | ≥ 25 years | Continuous HVAC service with periodic maintenance |
4. Manufacturing Process
The production of the No. 1216 Stainless Steel Coil Tube follows a rigorous multi-stage manufacturing sequence to ensure dimensional accuracy, metallurgical integrity, and surface quality:
4.1 Process Flow
- Raw Material Inspection — Incoming stainless steel coil stock is verified for chemical composition (via Optical Emission Spectroscopy, OES), mechanical properties, and surface quality. Mill test certificates (MTC) are reviewed against ASTM/GB specifications.
- Slitting and Edge Trimming — The master coil is slit to the required width using a precision slitting line with edge trim removal to ensure uniform wall thickness.
- Coil Forming (Rolling) — The flat strip is fed through a multi-stand roll forming system with progressive die stations that gradually bend the strip into a tubular cross-section. The forming process employs a mandrel-assisted or capillary drawing method to achieve the desired inner diameter without wall thinning.
- Seam Welding — A longitudinal seam is created using Transverse Magnetic Pulse Welding (TMPW) or Laser Welding (fiber laser, 2–6 kW output). TMPW welding is preferred for its high-energy density, minimal heat-affected zone (HAZ), and superior weld integrity in thin-wall applications.
- Heat Treatment (Solution Annealing) — The welded coil tube is solution annealed at 1050–1150°C in a controlled atmosphere (hydrogen or argon) followed by rapid quenching. This eliminates residual welding stresses, restores full austenitic microstructure, and ensures maximum corrosion resistance.
- Pickling and Passivation — The annealed tube surface is cleaned through chemical pickling (mixed acid: HNO₃/HF) to remove oxide scale, followed by passivation with dilute nitric acid (3–5% HNO₃) to form a protective chromium oxide film (Cr₂O₃).
- Coil Winding — The finished tube is wound onto a coil spool at the specified winding diameter, with protective wrapping (PE film or paper) applied between layers to prevent surface damage during storage and transport.
- Final Inspection — Each coil lot undergoes comprehensive quality verification (see Section 6).
4.2 Key Manufacturing Advantages
- Continuous Production — The coil form enables uninterrupted production runs, reducing waste and ensuring consistent quality throughout the entire length.
- Minimal Welding — Only a single longitudinal seam per meter of tube length, compared to multiple butt welds in discrete tube assemblies.
- Precision Geometry — CNC-controlled roll forming and welding equipment ensures OD and wall thickness tolerances within ±0.13 mm and ±10% respectively.
5. Applications
The No. 1216 Stainless Steel Coil Tube is engineered for demanding thermal management applications, particularly those involving large cold-water temperature differentials:
5.1 Primary Applications
- Air Coolers and Chilled Water Coil Units — Used as the primary heat transfer medium in air-cooled heat exchangers where water-side and air-side temperature differentials are significant. The coil configuration maximizes heat transfer surface area per unit volume.
- Fan Coil Units (FCUs) — Integral coil components in commercial and residential HVAC fan coil units, where the coil tube is exposed to condensate, ambient air, and cyclic thermal loading.
- Refrigeration and Freezing Systems — Evaporator coils in commercial refrigeration, walk-in freezers, and cold storage facilities operating at sub-zero temperatures.
- Process Cooling Systems — Industrial process chillers, plate-and-fin heat exchangers, and liquid-to-liquid heat exchangers in chemical, pharmaceutical, and food processing industries.
- Marine and Offshore HVAC — Corrosion-resistant coil tubes for marine air conditioning systems exposed to salt spray and humid tropical environments.
- Geothermal Heat Pump Systems — Ground-source heat exchanger loops where the coil tube is buried in soil or submerged in water, requiring long-term corrosion resistance.
5.2 Application-Specific Performance Notes
Under large cold-water temperature differential conditions (ΔT ≥ 10°C), the following performance considerations are critical:
- Thermal Stress Management — The coil geometry inherently accommodates thermal expansion and contraction through its helical flexibility, reducing the risk of thermal fatigue cracking at tube supports and headers.
- Condensate Management — In fan coil unit applications, the stainless steel surface resists corrosion from acidic condensate (pH 4.5–6.5) that forms when cold coil surfaces cool moist air below its dew point.
- Fouling Mitigation — The smooth surface finish (Ra ≤ 0.8 μm, BA finish) minimizes biofilm adhesion and scale deposition, maintaining heat transfer efficiency over extended operating cycles.
- Pressure Drop Optimization — The continuous coil path eliminates turbulence-inducing joints and fittings, reducing pressure drop by 15–25% compared to discrete tube assemblies with equivalent total length.