Stainless Steel Coil Tube — Dual-Coil Cold Source Temperature and Humidity Independent Control System Application
Product Overview
The stainless steel coil tube (Coiltube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered continuous tube product designed specifically for use in Dual-Coil Temperature and Humidity Independent Control Systems (DTUICS). These systems represent the next generation of Heating, Ventilation, and Air Conditioning (HVAC) technology, enabling decoupled control of indoor temperature and relative humidity through the coordinated operation of two independent coil circuits — a Chilled Water Coil (CWC) and a Hot Water Coil (HWC).
Unlike conventional single-coil air handling units that rely on mechanical dehumidification or reheat strategies, the dual-coil approach leverages the thermodynamic separation of sensible and latent heat removal. The stainless steel coil tube serves as the primary heat transfer conduit, carrying chilled or hot water through serpentine configurations to facilitate precise thermal exchange with the supply air stream. The product is engineered to withstand continuous thermal cycling, chemical exposure from water treatment additives, and mechanical stresses inherent in large-scale commercial and industrial HVAC installations.
Key Innovation: The dual-coil architecture eliminates the need for energy-intensive mechanical dehumidification (e.g., desiccant wheels or compressor-based dehumidifiers), achieving energy savings of 30–50% compared to conventional Variable Air Volume (VAV) systems while maintaining superior indoor air quality (IAQ).
Material & Properties
Base Material Composition
The coil tube is fabricated from Austenitic Stainless Steel, specifically conforming to the following alloy designations:
- ASTM A269 / ASTM A312 — Type 304 (UNS S30400): The standard grade offering excellent general corrosion resistance, formability, and weldability. Suitable for most HVAC water circuits with pH 6.5–9.5.
- ASTM A269 / ASTM A312 — Type 316L (UNS S31603): Enhanced grade with 2.0–2.5% Molybdenum (Mo) content, providing superior resistance to chloride-induced pitting and crevice corrosion. Recommended for coastal environments, aggressive water chemistries, or high-chloride condensate conditions.
- ASTM A269 — Type 321 (UNS S32100): Stabilized grade with Titanium (Ti) addition, suitable for applications involving elevated temperatures or exposure to sulfur-containing compounds.
Mechanical Properties
| Property | Type 304 (Annealed) | Type 316L (Annealed) | Test Standard |
|---|---|---|---|
| Tensile Strength (Rm) | ≥ 515 MPa (74,700 psi) | ≥ 485 MPa (70,300 psi) | ASTM A269 / GB/T 249 |
| Yield Strength (Rp0.2) | ≥ 205 MPa (29,700 psi) | ≥ 170 MPa (24,700 psi) | ASTM A269 / GB/T 249 |
| Elongation (A50) | ≥ 40% | ≥ 40% | ASTM A269 / GB/T 249 |
| Hardness (HV) | ≤ 210 HV | ≤ 200 HV | ASTM A269 / GB/T 13914 |
| Density (ρ) | 7.93 g/cm³ | 8.00 g/cm³ | — |
| Thermal Conductivity (λ) at 20°C | 16.3 W/(m·K) | 16.3 W/(m·K) | ASTM E1530 |
| Specific Heat Capacity (cp) at 20°C | 0.50 J/(g·K) | 0.50 J/(g·K) | — |
| Thermal Expansion (α) 20–100°C | 17.3 × 10⁻⁶ /K | 17.3 × 10⁻⁶ /K | ASTM E228 |
Corrosion Resistance
The austenitic microstructure, stabilized by Chromium (Cr ≥ 18%) and Nickel (Ni ≥ 8%), provides a passive oxide film (Cr₂O₃) that self-heals in the presence of oxygen. Type 316L additionally benefits from Molybdenum-enhanced pitting resistance equivalent number (PREN):
- Type 304 PREN: 19–20 (adequate for standard HVAC water)
- Type 316L PREN: 24–26 (excellent for chloride-rich or aggressive environments)
Specifications
| Parameter | Specification | Standard / Reference |
|---|---|---|
| Outer Diameter (OD) | φ9.53 mm (3/8"), φ12.70 mm (1/2"), φ15.88 mm (5/8"), φ19.05 mm (3/4"), φ25.40 mm (1") | ASTM A269 / GB/T 12771 |
| Wall Thickness (WT) | 0.80 mm, 1.00 mm, 1.20 mm, 1.50 mm, 2.00 mm (custom available) | ASTM A269 / GB/T 12771 |
| Coil Diameter | 1,200 mm – 2,500 mm (standard); custom diameters available | Manufacturer specification |
| Coil Length | 50 m – 200 m per coil (standard); up to 500 m on request | Manufacturer specification |
| Surface Finish | No.1 Annealed & Pickled; 2B Cold-Rolled; BA Bright Annealed; Custom (Ra ≤ 0.4 μm available) | ASTM A480 / GB/T 3280 |
| Tolerance — OD | ±0.05 mm (precision grade); ±0.10 mm (standard grade) | GB/T 12771 |
| Tolerance — WT | ±10% (standard); ±5% (precision grade) | GB/T 12771 |
| Operating Temperature Range | −196°C to +800°C (continuous); cryogenic service qualified | ASTM A269 |
| Maximum Operating Pressure | Up to 2.5 MPa (365 psi) at 90°C (schedules-dependent) | ASME B31.9 |
| Water Quality Compatibility | pH 6.5–9.5; Cl⁻ < 200 ppm (Type 304); Cl⁻ < 1,000 ppm (Type 316L); TDS < 1,000 ppm | ASME A269 / GB/T 12771 |
| Hydrostatic Test Pressure | 1.5 × maximum operating pressure (minimum 0.5 MPa) | GB/T 12771 |
| Weldability | Excellent — TIG (GTAW), MIG (GMAW), Resistance Spot Welding; no preheating required | ASME IX / AWS D1.6 |
| Formability | Excellent — capable of multi-pass bending to radii as small as 2× OD without cracking | ASTM A269 |
| Packaging | Wooden pallet with stretch film wrapping; moisture-proof inner lining; shipping marks per ISO 780 | ISO 780 |
| Traceability | Full heat-number traceability; mill test certificates (MTC) per EN 10204 Type 3.1 | EN 10204 |
Manufacturing Process
The production of stainless steel coil tube at Wenzhou Jixing Metal Materials Co., Ltd. follows a rigorously controlled multi-stage process aligned with ISO 9001:2015 quality management principles:
- Raw Material Procurement & Inspection: Virgin or certified recycled stainless steel ingots are sourced from accredited mills (e.g., Outokumpu, POSCO, Baosteel). Incoming materials undergo spectrographic analysis (OES — Optical Emission Spectroscopy) to verify chemical composition within ASTM/GB tolerances.
- Hot Rolling: Billets are reheated to 1,150–1,250°C and hot-rolled into seamless tube blanks with controlled reduction ratios to achieve the desired outer diameter and wall thickness. The hot-rolled coil is subsequently pickled and annealed to remove scale and restore ductility.
- Cold Drawing / Cold Rolling: For precision-grade tubes, the hot-rolled coil undergoes cold drawing through calibrated dies to achieve tight dimensional tolerances (±0.05 mm OD) and improved mechanical properties (up to 2× cold-work strengthening). Alternatively, cold rolling reduces wall thickness while maintaining or increasing OD.
- Intermediate Annealing: Periodic annealing at 1,050–1,100°C in protective atmosphere (Nitrogen — N₂, or Argon — Ar) relieves accumulated cold-work stresses and restores full annealed condition (HV ≤ 210) before further forming operations.
- Coil Winding & Straightening: The finished tube is wound into coils of specified diameter using precision coil winding machines. Tension-controlled winding prevents kinking, ovality, or surface marking. Straightening sections are available upon request.
- Surface Treatment: Final surface conditioning includes acid pickling (HNO₃/HF blend) to remove residual scale, followed by passivation (HNO₃ 20–30%) to maximize chromium oxide film integrity. For BA finish, annealing is performed in a reducing atmosphere (H₂/N₂) to produce a bright, smooth surface suitable for hygienic applications.
- Non-Destructive Testing (NDT): 100% of production undergoes electromagnetic (Eddy Current) testing for internal and external wall defects. Hydrostatic testing at 1.5× operating pressure verifies pressure integrity. Visual inspection confirms surface quality and dimensional compliance.
- Final Inspection & Packaging: Each coil receives a unique identification tag with heat number, grade, dimensions, and production date. Full documentation (Mill Test Certificate, Certificate of Conformance) accompanies shipment.
Applications
Primary Application: Dual-Coil Temperature and Humidity Independent Control Systems
The coil tube is purpose-designed for integration into Dual-Coil Cold Source Temperature and Humidity Independent Control Systems, a research-validated HVAC architecture that achieves precise indoor climate control through thermodynamic decoupling:
- Chilled Water Coil Circuit: Stainless steel coil tubes carry chilled water (typically 7–12°C) to provide both sensible cooling and latent heat removal (dehumidification) from the supply air. The high thermal conductivity and corrosion resistance of stainless steel ensure long-term efficiency without fouling or degradation.
- Hot Water Coil Circuit: A second set of stainless steel coil tubes carries hot water (typically 35–55°C) for reheat or temperature correction, enabling independent modulation of supply air temperature without affecting the dehumidification performance of the chilled coil.
- Fresh Air Supply Configuration: As studied in the referenced literature ("Research on Fresh Air Supply Method of Dual Coiltube Cold Source Temperature and Humidity Independent Control System"), the coil tubes are arranged in serpentine or cross-flow configurations within fresh air handling units to optimize air-side heat transfer coefficients while minimizing pressure drop.
Extended Applications
- Commercial HVAC: Large-scale air handling units (AHUs), rooftop units (RTUs), and fan coil units (FCUs) in office buildings, hospitals, laboratories, and data centers.
- Industrial Process Cooling: Precision temperature control in pharmaceutical manufacturing, semiconductor cleanrooms, food processing, and chemical plants where stainless steel purity and corrosion resistance are critical.
- Marine & Offshore HVAC: Type 316L coil tubes for shipboard and offshore platform HVAC systems exposed to high-chloride marine atmospheres.
- Geothermal & Heat Pump Systems: Heat exchanger tubes for ground-source heat pump systems requiring long-term durability in geothermal brine environments.
- Refrigeration & Chilled Beam Systems: Perforated stainless steel coil tubes for active chilled beam HVAC systems used in high-performance commercial buildings.