Stainless Steel Coil Tube — Product No. 813
1. Product Overview
The Stainless Steel Coil Tube (Coiltube), Product No. 813, manufactured by Wenzhou Jixing Metal Materials Co., Ltd, is a precision-engineered continuous-length tubular product formed into a compact coil configuration. This product is specifically designed for thermal exchange, fluid transport, and structural applications in high-performance HVAC (Heating, Ventilation, and Air Conditioning) systems, with particular emphasis on direct evaporative fresh air dehumidification systems employing dry coil tube technology.
The coil tube is supplied as a continuous helical coil, enabling efficient spool storage, easy field installation, and minimal joint requirements. Its inherent resistance to corrosion, high thermal conductivity, and mechanical durability make it an ideal choice for demanding industrial, commercial, and residential applications where long service life and hygienic performance are critical.
Industry Context: Research published in the field of HVAC engineering, specifically the study titled "Experimental Study on the Performance of Direct Evaporative Fresh Air Dehumidification with Dry Coil Tube Air Conditioning System", has demonstrated that dry coil tube configurations significantly improve dehumidification efficiency by eliminating condensation water on the coil surface, thereby reducing microbial growth risk and maintaining superior indoor air quality (IAQ). The stainless steel coil tube is a core component in such advanced systems.
2. Material & Properties
2.1 Base Material
- Primary Alloy: Austenitic stainless steel grades conforming to ASTM A213 / ASME SA-213 standards for seamless austenitic stainless steel tubing.
- Available Grades:
- 304 (UNS S30400): General-purpose grade with excellent corrosion resistance and formability. Chromium (Cr) content ≥18%, Nickel (Ni) content ≥8%.
- 316 (UNS S31600): Enhanced corrosion resistance grade with 2–3% Molybdenum (Mo) addition, suitable for chloride-rich and aggressive chemical environments.
- 316L (UNS S31603): Low-carbon variant of 316 (C ≤ 0.03%) for superior weldability and resistance to intergranular corrosion.
- 321 (UNS S32100): Titanium-stabilized grade offering excellent resistance to high-temperature oxidation and stress corrosion cracking.
2.2 Mechanical Properties (Typical for Grade 304)
| Property | Specification | Test Standard |
|---|---|---|
| Tensile Strength (UTS) | ≥ 515 MPa (74,700 psi) | ASTM A380 |
| Yield Strength (0.2% Offset) | ≥ 205 MPa (29,700 psi) | ASTM A380 |
| Elongation at Break | ≥ 35% (min. 40% for cold-worked) | ASTM A380 |
| Hardness (Rockwell B) | ≤ 95 HRB (annealed) | ASTM A380 |
| Impact Strength | Non-ductile (austenitic, no ductile-to-brittle transition) | ASTM A380 |
2.3 Thermal & Physical Properties
| Property | Value (Grade 304) | Value (Grade 316) |
|---|---|---|
| Thermal Conductivity (at 20°C / 68°F) | 16.3 W/(m·K) | 14.6 W/(m·K) |
| Thermal Conductivity (at 500°C / 932°F) | 22.3 W/(m·K) | 21.3 W/(m·K) |
| Specific Heat Capacity (at 20°C) | 500 J/(kg·K) | 500 J/(kg·K) |
| Density | 8.00 g/cm³ | 8.03 g/cm³ |
| Linear Thermal Expansion (20–100°C) | 17.3 × 10⁻⁶ /°C | 16.0 × 10⁻⁶ /°C |
| Maximum Service Temperature | 870°C (1,598°F) | 870°C (1,598°F) |
2.4 Corrosion Resistance
- Pitting Resistance (PREN): Grade 304: PREN ≈ 19; Grade 316: PREN ≈ 24–26
- Crevice Corrosion: Resistant in most atmospheric and aqueous environments; Grade 316 recommended for chloride concentrations > 200 ppm
- Intergranular Corrosion: Grade 316L and 321 offer superior resistance due to low carbon content and titanium stabilization, respectively
- Stress Corrosion Cracking (SCC): Susceptibility increases above 60°C in chloride-rich environments; Grade 321 provides enhanced SCC resistance
- Galvanic Compatibility: Compatible with aluminum, copper, and titanium; proper isolation recommended when joined with carbon steel
3. Specifications
3.1 Dimensional Specifications
| Parameter | Range / Value | Standard Reference |
|---|---|---|
| Outer Diameter (OD) | 3.18 mm (1/8") – 50.80 mm (2") | ASTM A213 / ASME SA-213 |
| Wall Thickness | 0.50 mm – 6.35 mm (0.020" – 0.250") | ASTM A213 |
| Inner Diameter (ID) | 2.00 mm – 40.00 mm (typical) | Derived from OD and wall thickness |
| Coil Diameter (Major) | 100 mm – 1,500 mm (4" – 60") | Custom per application |
| Coil Length per Spool | 50 m – 500 m (164 ft – 1,640 ft) | Custom per order |
| Coil Pitch | 15 mm – 100 mm (0.6" – 4.0") | Application-dependent |
| Number of Turns per Coil | 10 – 200 turns | Custom |
| OD Tolerance | ±0.10 mm (±0.004") | ASTM A213 |
| Wall Thickness Tolerance | ±10% of nominal | ASTM A213 |
| Roundness | ≤ 0.1% of OD | ISO 4427 |
| Surface Finish | Mill finish, polished (Ra ≤ 0.4 μm), or electropolished (Ra ≤ 0.2 μm) | ASTM A967 |
3.2 Performance Specifications (Dry Coil Application)
| Performance Parameter | Typical Value | Test Conditions |
|---|---|---|
| Heat Transfer Coefficient (U-value) | 200 – 400 W/(m²·K) | Air-side vs. water-side, cross-flow |
| Pressure Drop (Air Side) | 50 – 200 Pa | Air velocity 2.0 – 4.0 m/s |
| Pressure Drop (Fluid Side) | 10 – 50 kPa | Water flow velocity 1.0 – 2.5 m/s |
| Dehumidification Efficiency (ε) | 70% – 90% | Direct evaporative system, 25°C / 60% RH |
| Maximum Operating Pressure | 1.5 – 3.0 MPa (218 – 435 psi) | Grade-dependent; per ASME B31.3 |
| Operating Temperature Range | -196°C to +870°C (-321°F to +1,598°F) | Cryogenic to high-temperature |
| Service Life | ≥ 25 years (indoor); ≥ 15 years (outdoor, marine) | Based on corrosion rate < 0.05 mm/year |
4. Manufacturing Process
The production of stainless steel coil tubes follows a rigorous multi-stage manufacturing process to ensure dimensional accuracy, metallurgical integrity, and surface quality.
- Raw Material Selection:
- Virgin stainless steel ingots or continuously cast slabs sourced from certified mills
- Chemical composition verified by Spectroscopic Analysis (OES — Optical Emission Spectroscopy)
- Inclusion content and grain structure evaluated per ASTM A923
- Hot Rolling / Seamless Tube Forming:
- Slabs hot-rolled into perforated plates and then formed into seamless tubes via Piercing (e.g., Mannesmann process or Plug Piercing)
- Alternatively, Autoclaved Welded Tube (AWT) production for larger diameters with equivalent seamless properties
- Hot rolling temperature maintained between 1,100°C and 1,200°C to ensure proper austenitic microstructure
- Pilot Mill & Cold Drawing:
- Hot-rolled tubes passed through pilot mills to reduce OD and achieve intermediate dimensions
- Cold drawing through precision dies to achieve final OD, wall thickness, and roundness tolerances
- Work hardening achieved through controlled cold reduction (typically 15–40% reduction ratio)
- Heat Treatment (Solution Annealing):
- Heating to 1,010°C – 1,150°C (1,850°F – 2,094°F) in a controlled atmosphere furnace
- Followed by rapid quenching (water or air) to dissolve carbides and restore full austenitic structure
- Eliminates residual stresses from cold working and restores ductility
- Per ASTM A319 for solution heat treatment of austenitic stainless steel
- Pickling & Passivation:
- Acid pickling using mixed acid solutions (nitric acid + hydrofluoric acid) to remove scale and oxidation
- Passivation treatment per ASTM A967 to restore chromium oxide passive film
- Chloride contamination tested via ASTM A967 Appendix to ensure < 50 ppm surface chloride
- Coil Forming:
- Continuous-length tube fed through precision coil forming equipment
- Controlled bending radius (≥ 5× OD) to prevent ovality and wall thinning
- Coil pitch and diameter precisely controlled by CNC-driven forming machinery
- Post-forming inspection for dimensional accuracy and surface integrity
- Quality Inspection & Testing:
- Hydrostatic Testing: Per ASTM A213 — pressure test at 1.5× maximum allowable working pressure for minimum 10 seconds
- Non-Destructive Testing (NDT):
- Eddy Current Testing (ECT) — per ASTM E709 for internal and external defect detection
- Ultrasonic Testing (UT) — for wall thickness verification and subsurface defect identification
- Visual Inspection (VT) — 100% inspection of surface finish and coil geometry
- Chemical Analysis: Full spectral analysis per ASTM E415
- Mechanical Testing: Tensile, hardness, and impact tests per ASTM A380
- Dimensional Verification: Optical comparator and laser measurement for OD, wall thickness, roundness, and coil geometry
- Packaging & Shipping:
- Coils wrapped in protective plastic film and moisture-absorbing desiccant
- Secured on wooden pallets or steel racks for shipping
- Material test certificates (MTC) and certificates of compliance provided per EN 10204 3.1