Stainless Steel Coil Tube (Coiltube) — Model No. 622
Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd.
Product Category: Literature Reference & Technical Application — Evaporative Cooling Systems
Reference Document: Discussion on the System Design of Evaporative Cooling Fresh Air Units with Fan Coil Units in Arid Regions
1. Product Overview
The Stainless Steel Coil Tube (Coiltube) is a precision-engineered tubular component manufactured in continuous coil form, designed for integration into evaporative cooling fresh air units and fan coil systems operating in arid and semi-arid climatic zones. As a critical heat-exchange element, the coil tube facilitates efficient thermal transfer between the process air stream and the refrigerant or chilled water medium circulating within the tube interior.
Unlike conventional copper or carbon steel tubing, the stainless steel coil tube offers superior corrosion resistance, mechanical durability, and long-term hygienic performance — attributes that are particularly advantageous in evaporative cooling environments where condensation, mineral deposition, and variable humidity levels impose severe demands on material integrity.
This product has been specifically evaluated and documented in the context of fan coil system design for arid-region applications, where the thermodynamic efficiency of evaporative cooling is maximized due to the large wet-bulb depression between dry and saturated air conditions.
2. Material & Properties
2.1 Base Material Specification
| Property | Specification |
|---|---|
| Material Grade | ASTM A270 Type 304 / Type 316L Stainless Steel (UNS S30400 / S31603 / S31603) |
| Chromium (Cr) Content | ≥ 18.0% (Type 304); ≥ 16.0% (Type 316L) |
| Nickel (Ni) Content | ≥ 8.0% (Type 304); ≥ 10.0% (Type 316L) |
| Molybdenum (Mo) Content | — (Type 304); 2.0%–3.0% (Type 316L) |
| Carbon (C) Maximum | ≤ 0.08% (Type 304); ≤ 0.03% (Type 316L) |
| Tensile Strength (UTS) | ≥ 515 MPa (annealed condition) |
| Yield Strength (0.2% offset) | ≥ 205 MPa |
| Elongation (A₅₀) | ≥ 40% |
| Thermal Conductivity (at 20°C) | 14.9–16.3 W/(m·K) |
| Density | 7.93 g/cm³ |
| Maximum Service Temperature | 870°C (oxidation resistance limit) |
2.2 Key Material Advantages
- Exceptional Corrosion Resistance: The passive chromium oxide layer (Cr₂O₃) self-repairs in the presence of oxygen, providing outstanding resistance to chlorides, condensation-induced pitting, and atmospheric pollutants prevalent in arid environments.
- Hygienic Surface: The smooth, non-porous surface of annealed stainless steel inhibits bacterial adhesion and biofilm formation — critical for fresh air handling units where indoor air quality (IAQ) is paramount.
- Weldability: Type 304 and 316L grades exhibit excellent weldability under both Gas Metal Arc Welding (GMAW) and TIG (GTAW) processes, enabling reliable integration with fan coil headers and manifolds.
- Thermal Cycling Resistance: Maintains mechanical properties and dimensional stability across repeated heating and cooling cycles inherent to evaporative cooling operations.
3. Specifications
| Parameter | Specification | Notes |
|---|---|---|
| Outer Diameter (OD) | 6.35 mm, 9.53 mm, 12.70 mm (1/4", 3/8", 1/2") | Standard HVAC refrigeration tube sizes |
| Wall Thickness (WT) | 0.50 mm – 1.00 mm | Selected based on pressure rating |
| Coil Length | Up to 200 meters per coil | Continuous production capability |
| Coil Inner Diameter (ID) | 300 mm – 600 mm | Adaptable to downstream forming equipment |
| Surface Finish | 2B (Cold Rolled & Annealed) | Optional: No. 4 brushed, Electro-polished (EP) |
| Surface Roughness (Ra) | ≤ 0.4 μm (2B finish); ≤ 0.1 μm (EP finish) | EP finish for food-grade / medical applications |
| Tolerance (OD) | ± 0.05 mm | Per ASTM A270 / ASME B76.5 |
| Tolerance (WT) | ± 10% of nominal wall thickness | — |
| Working Pressure Rating | Up to 2.5 MPa (363 psi) | Dependent on OD and WT combination |
| Formability | Excellent — cold-bending radius ≥ 1.5 × OD | Enables tight serpentine coil configurations |
| Coating / Treatment | None (bare stainless steel); optional de-oxidized pickled | Passivation treatment available on request |
4. Manufacturing Process
4.1 Production Workflow
- Raw Material Preparation: Stainless steel strip (coil) of specified grade is procured from certified mills (e.g., Outokumpu, POSCO, Baowu). Incoming material undergoes spectroscopic verification (OES — Optical Emission Spectroscopy) to confirm chemical composition compliance with ASTM A270 requirements.
- Slitting & Leveling: The master coil is slit to the required strip width using a precision slitting line. A roll leveling machine (typically 21-roll or 23-roll configuration) ensures flatness within ± 0.1 mm/m, eliminating coil set and springback.
- Forming (Roll Forming): The leveled strip passes through a series of progressively curved rollers (typically 15–25 stations) that incrementally deform the flat strip into a seamless circular tube. This roll-forming process produces a seamless tube without the need for welding, preserving metallurgical homogeneity and eliminating weld-induced stress corrosion cracking susceptibility.
- Heat Treatment (Solution Annealing): Formed tubing is subjected to solution annealing at 1050°C–1150°C followed by controlled cooling (air or furnace cool). This step dissolves carbon carbides, restores full austenitic microstructure, relieves residual forming stresses, and maximizes corrosion resistance and ductility.
- Pickling & Passivation: The annealed tube is pickled in a mixed acid solution (HNO₃ / HF) to remove scale and oxide. Subsequent passivation with nitric acid (HNO₃, 20–30% concentration) establishes a uniform chromium oxide passive film, enhancing long-term corrosion protection.
- Coiling: The finished tube is coiled onto a pay-off reel at controlled tension to prevent kinking, ovality, or springback. Each coil is labeled with heat number, grade, dimensions, and production date for full traceability.
4.2 Quality Control Checkpoints
- Dimensional Inspection: OD and wall thickness measured at every 10-meter interval using ultrasonic thickness gauges and laser micrometers.
- Ovality Check: Cross-sectional roundness verified at intervals to ensure ≤ 2% ovality, critical for downstream coil forming and heat exchanger assembly.
- Hydrostatic Pressure Testing: Each coil segment tested at 1.5× working pressure for minimum 30 seconds to verify weld-free integrity and absence of pinholes or micro-cracks.
- Corrosion Testing: Salt spray testing (ASTM B117, 96-hour minimum) and intergranular corrosion testing (ASTM A262 Practice E) conducted on periodic samples.
- Surface Quality: Visual and profilometric inspection for scratches, dents, or contamination that could impair heat transfer performance.
5. Applications
5.1 Primary Application: Evaporative Cooling Fresh Air Systems in Arid Regions
The stainless steel coil tube serves as the heat exchanger core in evaporative cooling fresh air units deployed in arid and semi-arid climates (e.g., Northwest China, Middle East, Southwest United States). In these environments, the large difference between dry-bulb and wet-bulb temperatures enables highly efficient evaporative cooling with minimal mechanical refrigeration input.
Within the system architecture, the coil tube performs the following functions:
- Primary Heat Exchange: Chilled water or refrigerant circulates through the coil tube while ambient air passes over the external surface. The high thermal conductivity and corrosion-resistant properties of stainless steel ensure sustained heat transfer efficiency over the equipment's service life.
- Condensate Management: In evaporative cooling configurations, moisture-laden air contacts the cooled tube surface, inducing condensation. The corrosion-resistant surface prevents pitting and scaling that would otherwise degrade heat transfer and contaminate the air stream.
- Integration with Fan Coil Units (FCU): The coil tube is formed into serpentine or zigzag configurations and finned to create the air-side heat exchanger within the fan coil unit. The seamless construction eliminates leak paths at tube-to-fintube joints.
5.2 Secondary Applications
- Chilled Water Distribution Loops: Secondary piping in commercial HVAC systems where corrosion resistance and hygiene are required.
- Process Cooling in Food & Beverage Industry: Cooling coils for fermentation tanks, pasteurization lines, and cold storage systems.
- Industrial Heat Recovery: Waste heat recovery systems where fluid streams contain chlorides or acidic components.
- Condenser & Evaporator Coils: Refrigeration and air conditioning heat exchangers operating in corrosive environments (coastal, chemical processing).
6. Quality Certifications & Standards Compliance
| Standard / Certification | Applicable Requirement |
|---|---|
| ASTM A270 / A270M | Standard Specification for Seamless and Welded Austenitic Stainless Steel Tubing for General Piping and Heat Exchanger Service |
| ASME B76.5 | Standard for Refrigeration and Air-Conditioning Tube |
| GB/T 12771 (China National Standard) | Stainless Steel Tubes for General Purposes |
| ISO 9001:2015 | Quality Management System Certification — Wenzhou Jixing Metal Materials Co., Ltd. |
| ISO 14001:2015 | Environmental Management System Certification |
| EN 10216-5 | European Standard for Heat Exchanger and Heat Transfer Component Tubes — Part 5: Non-ferrous Tubes |
| EN 10217-7 | Product Specification for Heat Exchanger Tubes — Part 7: Non-ferrous Tubes |
| Third-Party Material Test Reports (MTR) | Provided with each shipment — includes chemical analysis, mechanical test results, and hydrostatic test records |
7. Unique Selling Points
- Seamless Roll-Formed Construction: Unlike welded stainless steel tubing, the roll-forming process produces a truly seamless tube with no longitudinal weld seam — eliminating the primary failure mode (stress corrosion cracking along the weld) in chloride-containing evaporative cooling environments.
- Arctic-Region Proven Performance: The product has been specifically studied and validated for use in evaporative cooling systems designed for arid-region deployment, where the combination of low humidity, high mineral content in water, and extreme temperature swings creates a uniquely demanding operating environment.
- Full Traceability: Each coil is accompanied by a comprehensive Material Test Report (MTR) linking the product back to the original mill heat number, ensuring complete supply chain transparency and compliance with international procurement standards.
- Customizable Dimensions: Wenzhou Jixing Metal Materials Co., Ltd. offers flexible OD, wall thickness, and coil length configurations to meet specific fan coil unit design requirements, enabling optimized heat transfer surface area and pressure drop characteristics.
- Sustained Thermal Performance: The non-scaling, non-oxidizing surface of properly passivated stainless steel maintains near-original heat transfer coefficients throughout the service life, unlike copper tubing which is susceptible to oxidation layer formation and fouling