Stainless Steel Coil Tube (Coiltube) — HVAC & Radiant Cooling System Application
1. Product Overview
The Stainless Steel Coil Tube (Coiltube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. (Catalog No. 681) is a precision-engineered tubular product specifically designed for integration into Fan Coil Unit (FCU) and Floor Radiant Combined Systems (FCS). These systems represent a hybrid cooling architecture that leverages both convective and radiant heat transfer mechanisms to achieve superior thermal comfort and energy efficiency in commercial and residential HVAC (Heating, Ventilation, and Air Conditioning) applications.
The coil tube serves as the primary heat exchange medium within the system, facilitating efficient thermal transfer between the refrigerant or chilled water circuit and the conditioned air or radiant floor surface. Its geometry—characterized by a continuous helical winding configuration—maximizes the surface-area-to-volume ratio, thereby enhancing convective heat exchange coefficients and minimizing pressure drop across the coil circuit.
Key Insight from System Analysis: Research on the cooling capacity of Fan Coil Unit and Floor Radiant Combined Systems has identified that the geometric configuration, material thermal conductivity, and surface finish of the coil tube are among the most significant influencing factors on overall system performance. The selection of high-grade stainless steel as the coil tube material directly addresses the requirements for thermal efficiency, corrosion resistance, and long-term structural integrity.
2. Material & Properties
2.1 Base Material Composition
The coil tube is fabricated from austenitic stainless steel conforming to international metallurgical standards. The primary grades utilized include:
- SUS304 / AISI 304 (UNS S30400): Chromium-Nickel austenitic stainless steel with minimum 18% Cr and 8% Ni, offering excellent general corrosion resistance and formability.
- SUS316 / AISI 316 (UNS S31600): Molybdenum-bearing austenitic stainless steel with 2–3% Mo addition, providing enhanced resistance to pitting and crevice corrosion in aggressive environments.
- SUS321 / AISI 321 (UNS S32100): Titanium-stabilized austenitic stainless steel suitable for elevated-temperature applications exceeding 450°C.
2.2 Physical & Mechanical Properties
| Property | Unit | SUS304 Grade | SUS316 Grade | Relevance to Application |
|---|---|---|---|---|
| Density | g/cm³ | 7.93 | 8.00 | Weight budgeting in HVAC units |
| Thermal Conductivity (at 20°C) | W/(m·K) | 16.3 | 14.6 | Heat transfer efficiency in coil |
| Specific Heat Capacity | J/(kg·K) | 500 | 500 | Thermal mass and response time |
| Tensile Strength (Rm) | MPa | ≥520 | ≥520 | Structural integrity under pressure |
| Yield Strength (Rp0.2) | MPa | ≥205 | ≥205 | Deformation resistance |
| Elongation (A50mm) | % | ≥40 | ≥40 | Formability during coil winding |
| Maximum Service Temperature | °C | 870 | 870 | Thermal cycling durability |
| Corrosion Resistance (Pitting) | PREN | 19 | 25 | Longevity in humid HVAC environments |
2.3 Surface Treatment Options
- Mill Finish (No. 2B): Standard cold-rolled surface with moderate reflectivity; suitable for general HVAC applications.
- Pickled & Passivated: Acid-etched surface removing decarburization layer; enhances passive oxide film stability.
- Electropolished: Mirror-grade surface (Ra ≤ 0.4 μm); reduces fouling tendency and facilitates cleaning in hygienic applications.
- PVD Coated (Physical Vapor Deposition): Optional hydrophobic or anti-bacterial coating for enhanced maintenance intervals.
3. Specifications
| Parameter | Specification / Range | Standard Reference |
|---|---|---|
| Outer Diameter (OD) | 3.0 mm – 25.0 mm | Custom per customer drawing |
| Wall Thickness (WT) | 0.30 mm – 1.20 mm | Custom per customer drawing |
| Coil Winding Diameter | 50 mm – 300 mm (helical pitch configurable) | GB/T 14976, ASTM A213 |
| Coil Length (per unit) | 1.0 m – 12.0 m | Application-dependent |
| Surface Roughness (Ra) | 0.4 μm – 1.6 μm | ISO 1302 |
| Pressure Rating (Hydrostatic Test) | Up to 2.5 MPa (configurable) | GB/T 24511 |
| Operating Temperature Range | −196°C to +450°C (continuous) | Material-dependent |
| Refrigerant Compatibility | R134a, R410A, R32, R407C, Ammonia (NH₃), CO₂ (R744) | ASHRAE Standard 15 |
| Chilled Water Flow Rate | 0.5 – 15.0 L/min (per coil unit) | ASHRAE 90.1 |
| Dimensional Tolerance (OD) | ±0.05 mm (precision grade) | GB/T 14976, EN 10216 |
| Dimensional Tolerance (WT) | ±10% (standard) / ±5% (precision) | ASTM A269 |
| Minimum Bend Radius | ≥ 3× OD (without wrinkling) | Internal manufacturing standard |
| Weight (per linear meter, OD 6.0 × WT 0.8 mm) | ≈ 102.9 g/m | Calculated |
| Delivery Form | Coiled (on spool), cut-to-length, or assembled sub-unit | Customer specification |
4. Manufacturing Process
4.1 Process Flow
- Raw Material Selection & Incoming Inspection: Cold-rolled stainless steel strip or seamless tube stock is procured from certified mills. Each batch undergoes spectrographic analysis (OES), mechanical testing, and visual inspection to verify compliance with specified grade and quality requirements.
- Strip Preparation / Tube Forming: For formed coil tubes, cold-rolled strip is fed through a rotary-draw bending machine (RDB) or tube-forming mill to produce seamless tubular geometry. For drawn tubing, seamless tube stock is cold-drawn to precise OD and WT dimensions.
- Coil Winding (Helical Forming): The formed tube is wound onto a precision mandrel using CNC-controlled helical winding equipment. Winding parameters (pitch, diameter, number of turns, entry/exit angles) are programmed to meet exact customer specifications. Constant tension control ensures uniform coil geometry without kinking or ovality.
- Welding & Joining: Entry and exit manifolds are welded to the coil tube using TIG (Tungsten Inert Gas) welding with argon shielding. Weld seams undergo 100% visual inspection and are subject to leak testing.
- Heat Treatment (if applicable): Solution annealing at 1050–1100°C followed by rapid quenching may be performed to relieve residual stresses from forming and welding operations, restoring full austenitic microstructure and maximum corrosion resistance.
- Pickling & Passivation: The welded assembly is subjected to pickling (HNO₃ + HF mixture) to remove welding scale and oxidation, followed by passivation (HNO₃ or citric acid treatment) to restore and stabilize the chromium oxide passive film.
- Hydrostatic Pressure Testing: Each coil tube unit is pressure-tested at 1.5× the rated operating pressure for a minimum of 30 seconds to verify weld integrity and absence of leaks.
- Final Inspection & Dimensional Verification: Completed units undergo dimensional measurement (OD, WT, coil diameter, pitch, total length), surface finish verification, and leak testing (helium mass spectrometry for critical applications).
- Packaging & Dispatch: Units are wrapped in protective film, placed in wooden crates or custom fixtures to prevent deformation during transit, and labeled with batch traceability information.
4.2 Quality Control Milestones
| Stage | Inspection Method | Acceptance Criteria |
|---|---|---|
| Incoming Material | OES Spectroscopy, Tensile Test, PMI | Grade compliance per ASTM/GB |
| Tube Forming | Caliper, Micrometer, Ovality Gauge | OD ±0.05 mm, Ovality < 0.5% |
| Coil Winding | CMM (Coordinate Measuring Machine) | Pitch ±0.5 mm, Diameter ±1.0 mm |
| Welding | Visual (VT), Dye Penetrant (PT) | No cracks, porosity, undercut |
| Pressure Test | Hydrostatic Test Rig | No pressure drop for 30 s at 1.5× P |
| Final | Helium Leak Test (if required) | Leak rate < 1×10⁻⁶ mbar·L/s |
5. Applications
5.1 Fan Coil Unit (FCU) Systems
In standalone Fan Coil Unit systems, the stainless steel coil tube serves as the evaporator or condenser coil element. Chilled water or refrigerant flows through the coil while a fan forces ambient air across the coil surface, achieving sensible and latent heat removal. The stainless steel material ensures resistance to condensation-induced corrosion—a common failure mode in copper-alloy coils exposed to humid indoor air.
5.2 Floor Radiant Combined Systems (FCS)
In the Fan Coil Unit and Floor Radiant Combined System, the coil tube is embedded within the floor radiant panel or ceiling radiant panel. The system operates on the principle of simultaneous convective cooling (via the FCU coil) and radiant cooling (via the floor surface). The stainless steel coil tube embedded in the floor slab provides:
- Uniform radiant surface temperature distribution due to the helical coil geometry ensuring even heat absorption across the panel.
- Elimination of cold spots and thermal stratification by maintaining the surface temperature above the dew point, preventing condensation.
- Reduced fan energy consumption compared to conventional all-air systems, as radiant cooling reduces the required air velocity by 30–50%.
- Improved thermal comfort through the elimination of drafts and localized temperature gradients.
5.3 Additional Application Domains
- Commercial HVAC: Office buildings, hotels, hospitals, and retail spaces requiring high indoor air quality (IAQ).
- Residential HVAC: High-end residential radiant floor heating/cooling systems.
- Industrial Process Cooling: Precision temperature control for manufacturing processes.
- Food & Pharmaceutical: Hygienic-grade coil tubes for cleanroom and food processing applications (electropolished finish).
- Refrigeration Systems: Evaporator and condenser coils in commercial refrigeration units.
6. Quality Certifications & Standards Compliance
| Certification / Standard | Scope | Applicability |
|---|---|---|
| ISO 9001:2015 | Quality Management System | Manufacturing process control |
| ISO 14001:2015 | Environmental Management System | Environmental compliance |
| ISO 45001:2018 | Occupational Health & Safety | Workplace safety |
| GB/T 14976 | Stainless Steel Seamless Tubes for Fluid Transport | Product dimensional & mechanical compliance |
| GB/T 12771 | Stainless Steel Welded Tubes for Fluid Transport | Welded coil tube compliance |
| ASTM A213 | Seamless Austenitic Stainless Steel Tubes for Heat-Transfer | Material grade verification |
| ASTM A269 | Welded Austenitic Stainless Steel Tubes | Welded product compliance |
| EN 10216-5 | Welded Circular Stainless Steel Tubes — Technical Delivery Conditions | European market compliance |
| EN 10217-7 | Seamless Stainless Steel Tubes — Technical Delivery Conditions | European seamless tube compliance |
| AS |