Stainless Steel Coil Tube — HVAC Fan Coil Unit Application
Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd.
Product Category: Stainless Steel Coil Tube for Chilled Water Heat Exchange Systems
Reference Document: 《冷冻水大温差对风机盘管性能影响的研究》 (Research on the Impact of Large Chilled Water Temperature Differential on Fan Coil Unit Performance)
1. Product Overview
The Stainless Steel Coil Tube (盘管) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered coiled piping component designed primarily for use in Fan Coil Units (FCUs) within commercial and industrial HVAC (Heating, Ventilation, and Air Conditioning) systems. These coil tubes serve as the critical heat exchange element through which chilled water or hot water circulates, transferring thermal energy to or from the surrounding air stream.
The product is engineered to withstand the demanding operating conditions associated with large temperature differential (ΔT) chilled water systems, where supply water temperatures may range from as low as −2 °C to as high as 18 °C, and return water temperatures may reach up to 30 °C. This creates significant thermal cycling stress on the coil tube material, necessitating high-performance stainless steel grades with excellent fatigue resistance, corrosion resistance, and thermal conductivity.
Key Insight from Research: Large chilled water temperature differentials (ΔT ≥ 10 °C) significantly enhance the heat transfer efficiency of fan coil units but simultaneously impose greater mechanical and thermal stress on the coil tubes, accelerating potential degradation mechanisms such as thermal fatigue cracking, corrosion pitting, and loss of dimensional tolerance.
2. Material & Properties
2.1 Material Grade
The coil tubes are fabricated from austenitic stainless steel conforming to international material standards. The primary grades utilized are:
- SUS304 / AISI 304 (1.4301) — General-purpose grade with excellent corrosion resistance and formability
- SUS316L / AISI 316L (1.4404) — Low-carbon grade with superior resistance to chloride-induced pitting and crevice corrosion, recommended for high-ΔT and aggressive water quality conditions
- SUS321 / AISI 321 (1.4541) — Titanium-stabilized grade offering enhanced resistance to intergranular corrosion at elevated temperatures
2.2 Mechanical Properties
| Property | SUS304 (1.4301) | SUS316L (1.4404) | SUS321 (1.4541) |
|---|---|---|---|
| Tensile Strength (Rm) | ≥ 520 MPa | ≥ 500 MPa | ≥ 520 MPa |
| Yield Strength (Rp0.2) | ≥ 205 MPa | ≥ 170 MPa | ≥ 205 MPa |
| Elongation (A50mm) | ≥ 40 % | ≥ 40 % | ≥ 40 % |
| Hardness (HV) | ≤ 200 | ≤ 190 | ≤ 200 |
| Thermal Conductivity (20 °C) | 16.3 W/(m·K) | 14.6 W/(m·K) | 16.3 W/(m·K) |
| Thermal Expansion Coefficient (20–100 °C) | 17.3 × 10−6 /K | 15.8 × 10−6 /K | 17.3 × 10−6 /K |
| Operating Temperature Range | −196 °C to +870 °C | −196 °C to +870 °C | −196 °C to +1000 °C |
2.3 Corrosion Resistance Characteristics
- Pitting Resistance Number (PREN): SUS304 ≥ 18; SUS316L ≥ 26; SUS321 ≥ 18
- Resistance to Deaerated Water: Excellent in all grades; critical for closed-loop chilled water systems
- Resistance to Chloride Ions: SUS316L recommended for systems with water conductivity exceeding 1,500 µS/cm
- Resistance to Thermal Cycling: All grades maintain mechanical integrity through ≥ 10,000 thermal cycles (−2 °C to +30 °C)
3. Specifications
3.1 Geometric Dimensions
| Parameter | Specification | Tolerance |
|---|---|---|
| Outer Diameter (OD) | 6.35 mm (¼") / 9.52 mm (⅜") / 12.70 mm (½") | ±0.10 mm |
| Wall Thickness | 0.40 mm / 0.50 mm / 0.65 mm / 0.80 mm / 1.00 mm | ±0.05 mm |
| Inner Diameter (ID) | 5.55 mm / 8.52 mm / 11.40 mm / 10.90 mm | ±0.05 mm |
| Coil Pitch (Turn Spacing) | 6.0 mm / 8.0 mm / 10.0 mm / 12.0 mm | ±0.20 mm |
| Coil Length (Uncoiled) | 1.5 m to 30.0 m (customizable) | ±50 mm |
| Coil Diameter (Rolled) | Φ200 mm / Φ300 mm / Φ400 mm / Φ500 mm | ±5 mm |
| End Straight Length | 50 mm to 150 mm (both ends) | ±2 mm |
| Surface Finish | 2B / BA (Bright Annealed) / Electropolished | Ra ≤ 0.8 µm (Electropolished) |
3.2 Performance Parameters (FCU Application)
| Parameter | Typical Value | Notes |
|---|---|---|
| Maximum Operating Pressure | 1.6 MPa (16 bar) | Hydrostatic test at 2.4 MPa |
| Design Water Flow Rate | 0.5 to 15.0 m³/h | Dependent on coil configuration |
| Chilled Water Supply Temp. | −2 °C to +18 °C | Large ΔT operation supported |
| Chilled Water Return Temp. | +15 °C to +30 °C | — |
| Temperature Differential (ΔT) | 5 °C to 15 °C | Optimized for ΔT ≥ 10 °C |
| Heat Transfer Coefficient | 800 to 1,500 W/(m²·K) | Water-side U-value |
| Cooling Capacity (per m² coil) | 150 to 350 W/m² | Dependent on air velocity and ΔT |
| Service Life | ≥ 25 years | Under proper water treatment conditions |
4. Manufacturing Process
The production of high-precision stainless steel coil tubes follows a rigorous multi-stage manufacturing process designed to ensure dimensional accuracy, mechanical integrity, and surface quality:
- Raw Material Selection: Cold-rolled stainless steel strip is sourced from certified mills, with each batch subjected to spectrographic analysis (OES) to verify chemical composition against ASTM A240 / EN 10088-2 specifications.
- Slitting and Edge Trimming: The strip is slit to the required width and edges are trimmed to eliminate burrs and surface defects that could propagate during forming.
- Tube Forming (Tube Mill): The strip is passed through a series of progressively reducing rollers in a tube mill to form a seamless round tube. The forming process is controlled to maintain uniform wall thickness and minimize residual stress.
- Welding (if applicable): For welded tubes, a high-purity argon gas-shielded TIG (Tungsten Inert Gas) welding process is employed, with the weld seam subjected to visual and dye-penetrant inspection.
- Coil Forming (Coiling): The formed tube is precisely coiled using CNC-controlled coiling equipment to achieve the specified pitch, coil diameter, and turn uniformity. Coiling parameters are optimized to prevent tube flattening, kinking, or excessive plastic deformation.
- Stress Relief Annealing: The coiled tubes undergo a controlled annealing cycle (typically 800–1050 °C for austenitic grades) to relieve residual stresses introduced during forming and coiling, restoring full mechanical properties and enhancing corrosion resistance.
- Pickling and Passivation: The surface is cleaned via acid pickling (nitric/hydrofluoric acid blend) to remove scale and discoloration, followed by passivation to restore the chromium oxide passive film (Cr₂O₃) critical for corrosion protection.
- End Finishing: Tube ends are deburred, chamfered, and trimmed to specified lengths to facilitate brazing or welding into heat exchanger headers.
- Quality Inspection: Each coil tube undergoes dimensional verification (OD, wall thickness, coil pitch), pressure testing, and surface inspection. Statistical process control (SPC) is maintained throughout production.
- Packaging and Delivery: Coils are wrapped in protective film, placed on wooden or metal spools, and packaged for transport to prevent mechanical damage and moisture exposure.
5. Applications
5.1 Primary Application — Fan Coil Units (FCUs)
The stainless steel coil tube is the core heat exchange component in fan coil units, which are widely deployed in commercial buildings, hospitals, hotels, and industrial facilities. The research referenced in the product documentation specifically addresses the performance implications of large temperature differential (大温差) chilled water operation on FCU coil tubes, establishing critical design and material selection criteria:
- Large ΔT Chilled Water Systems: Systems operating with supply water at 5–8 °C and return water at 15–20 °C (ΔT = 10–12 °C) achieve higher cooling efficiency per unit water flow. The coil tube material must accommodate the resulting thermal cycling without degradation of heat transfer performance or structural integrity.
- Free Cooling Applications: In variable-temperature-differential systems, coil tubes may experience intermittent exposure to near-freezing water, requiring materials with excellent resistance to low-temperature embrittlement and thermal shock.
5.2 Secondary Applications
- Air Handling Units (AHUs): Large-scale coil assemblies for central air handling systems
- Chilled Beam Systems: Indirect evaporative cooling systems requiring high-corrosion-resistant coil tubes
- Process Cooling Heat Exchangers: Industrial applications involving aggressive process fluids
- Refrigeration Condensers and Evaporators: Closed-circuit refrigeration systems
- Pharmaceutical and Food Processing: Hygienic-grade heat exchangers requiring electropolished surfaces (Ra ≤ 0.4 µm)
- Marine HVAC Systems: Shipboard air conditioning systems exposed to salt-laden air and seawater-cooled chillers
6. Unique Selling Points
- Large ΔT Performance Validation: The product is specifically engineered and validated for operation under large chilled water temperature differentials (ΔT ≥ 10 °C), addressing a critical gap in conventional coil tube specifications that typically assume smaller ΔT operating conditions.
- Superior Thermal Fatigue Resistance: Material selection and manufacturing process are optimized to withstand ≥ 10,000 thermal cycles without measurable degradation in heat transfer coefficient or dimensional stability.
- Corrosion-Resistant Material Options: Availability in SUS304, SUS316L, and SUS321 grades allows precise matching of material properties to water quality and operating conditions, optimizing cost-performance ratio.
- Precision Coiling Accuracy: CNC-controlled coiling ensures uniform pitch and turn geometry, which is critical for consistent air-side heat transfer performance and prevents uneven flow distribution within the coil assembly.
- Extended Service Life: Design life exceeding 25 years under proper maintenance, reducing lifecycle costs and minimizing replacement frequency in commercial HVAC installations.
- Customization Capability: Flexible customization of OD, wall thickness, coil pitch, coil length, and surface finish to meet specific OEM and project requirements.
7. Quality Certifications & Standards Compliance
| Standard / Certification | Scope | Applicable Requirement |
|---|---|---|
| ISO 9001:2015 | Quality Management System | Comprehensive QMS covering design, manufacturing, inspection, and delivery |
| ISO 14001:2015 |