Stainless Steel Coil Tube for Low Temperature Difference Fan Coil Systems
Product Overview
The Stainless Steel Coil Tube (Catalog No. 648) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered tubular product specifically designed for use in low temperature difference (LTD) fan coil unit systems within the Heating, Ventilation, and Air Conditioning (HVAC) industry. Unlike conventional coil tubes that operate with large temperature differentials, this product is optimized for applications where the temperature difference (ΔT) between supply and return fluid is minimal—typically 3–6°C—thereby demanding superior thermal conductivity, corrosion resistance, and dimensional stability.
Coil tubes serve as the primary heat exchange elements in fan coil units, chiller condensers, heat recovery systems, and industrial process cooling loops. The coil configuration maximizes surface area within a compact footprint, making material selection and manufacturing precision critical to overall system performance and longevity.
Material Composition and Properties
The coil tube is fabricated from austenitic stainless steel, selected for its exceptional balance of mechanical strength, corrosion resistance, and thermal performance under low-temperature-differential operating conditions.
Grade Options
- 304 Stainless Steel (AISI 304 / UNS S30400) — The standard grade offering excellent general-purpose corrosion resistance, formability, and weldability. Suitable for non-chloride environments.
- 316L Stainless Steel (AISI 316L / UNS S31603) — Enhanced molybdenum (Mo) content (2.0–2.5%) provides superior resistance to pitting and crevice corrosion in chloride-rich or marine environments.
- 321 Stainless Steel (AISI 321 / UNS S32100) — Titanium-stabilized grade offering improved resistance to intergranular corrosion at elevated temperatures.
Mechanical and Physical Properties
| Property | 304 Grade | 316L Grade | 321 Grade |
|---|---|---|---|
| Tensile Strength (MPa) | ≥520 | ≥485 | ≥520 |
| Yield Strength (MPa) | ≥205 | ≥170 | ≥205 |
| Elongation (%) | ≥40 | ≥45 | ≥40 |
| 16.3 | 14.6 | 16.3 | |
| Density (kg/m³) | 8.0 | 8.0 | 8.0 |
| Operating Temperature Range (°C) | −196 to +870 | −196 to +870 | −196 to +900 |
Technical Specifications
| Parameter | Specification | Tolerance |
|---|---|---|
| Outer Diameter (OD) | 6.0 mm, 8.0 mm, 9.53 mm (⅜"), 12.7 mm (½") | ±0.05 mm |
| Wall Thickness | 0.4 mm, 0.5 mm, 0.6 mm, 0.8 mm, 1.0 mm | ±0.03 mm |
| Coil Pitch (Turn Spacing) | 10 mm to 30 mm (customizable) | ±0.2 mm |
| Coil Diameter | 50 mm to 500 mm (customizable) | ±1.0 mm |
| Tube Length (Per Coil) | 1 m to 50 m | ±0.5% |
| Surface Finish | Bright Annealed (BA), No. 4 Brushed, or Mirror Polish (Ra ≤ 0.4 μm) | — |
| End Forming | Flat, Beveled, Flared, or Threaded (as specified) | — |
| Pressure Rating | Up to 1.6 MPa (standard); 4.0 MPa (reinforced) | — |
| Minimum Bend Radius | 3× OD (cold-formed) | — |
| Standard Length (Straight Section) | Up to 12,000 mm | ±10 mm |
Manufacturing Process
The production of high-precision stainless steel coil tubes involves a multi-stage manufacturing sequence designed to ensure dimensional accuracy, metallurgical integrity, and surface quality:
- Hot-Rolled Strip Production — Virgin or recycled stainless steel ingots are cast, hot-rolled into strip, and cold-rolled to the required thickness. This establishes the base material with controlled grain structure and mechanical properties.
- Tube Forming (Seamless or Welded) —
- Seamless method: Hot-rolled blanks are pierced and drawn through dies to produce seamless tube stock, eliminating weld lines and maximizing pressure resistance.
- Welded method: Cold-rolled strip is roll-formed and joined via continuous TIG (Tungsten Inert Gas) welding under argon shielding to prevent oxidation.
- Heat Treatment (Solution Annealing) — Tubes are annealed at 1050–1150°C followed by rapid quenching (water or air) to dissolve carbide precipitates, restore full austenitic microstructure, and achieve maximum corrosion resistance and ductility.
- Precision Coiling — Straight tube stock is cold-coiled using CNC-controlled coiling machines. The coiling process maintains constant tension and feed rate to ensure uniform pitch and prevent ovality or stress concentration at bend points.
- Pickling and Passivation — Tubes undergo hydrochloric and nitric acid pickling to remove scale and surface contaminants, followed by passivation to form a protective chromium oxide (Cr₂O₃) layer on the surface.
- Hydrostatic Pressure Testing — Each coil is subjected to hydrostatic testing at 1.5× the rated working pressure to verify leak-tightness and structural integrity.
- Final Inspection and Packaging — Dimensional verification, visual inspection, and surface quality assessment are performed before protective packaging.
Applications
The coil tube is engineered for demanding thermal management applications where reliability and efficiency are paramount:
- Low Temperature Difference (LTD) Fan Coil Units — Primary application in precision HVAC systems where small ΔT (3–6°C) demands high thermal transfer efficiency and minimal pressure drop. Experimental studies have demonstrated that stainless steel coil tubes in LTD configurations achieve up to 18% improvement in overall heat transfer coefficient (U-value) compared to standard copper-tube alternatives when properly designed.
- Commercial and Industrial Chiller Systems — Evaporator and condenser coils in water-cooled and air-cooled chillers operating at elevated pressures.
- Heat Recovery Units — Plate-fin and tube-fin heat exchangers in energy recovery ventilation (ERV) systems.
- Marine HVAC Systems — 316L grade coils deployed in shipboard cooling systems exposed to saltwater spray and humid marine atmospheres.
- Food and Pharmaceutical Processing — Sanitary-grade coils for pasteurization, refrigeration, and temperature-controlled processing loops requiring FDA-compliant materials.
- Solar Thermal Collectors — Vacuum tube and flat-plate solar collectors where corrosion resistance and thermal stability under cyclic loading are essential.
- Industrial Process Cooling — Heat exchange coils in chemical, petrochemical, and semiconductor manufacturing processes involving aggressive media.
Quality Certifications and Standards
Wenzhou Jixing Metal Materials Co., Ltd. maintains rigorous quality management systems and adheres to international standards throughout the manufacturing process:
| Standard / Certification | Description |
|---|---|
| ISO 9001:2015 | Quality Management System certification ensuring consistent product quality and process control. |
| ISO 14001:2015 | Environmental Management System certification for sustainable manufacturing practices. |
| ASTM A269 / A270 | American material standards for seamless and welded austenitic stainless steel tubing. |
| EN 10217-7 | European standard for cold-worked seamless and welded tubes for pressure purposes. |
| JIS G3447 | Japanese Industrial Standard for stainless steel tubes for general purposes. |
| GB/T 12771 | Chinese National Standard for austenitic stainless steel tubes for general purposes. |
| CE Marking | European Conformity marking for products intended for the European Economic Area. |
| RoHS Compliance | Restriction of Hazardous Substances compliance for environmentally responsible manufacturing. |
Unique Selling Points
- Superior Low-ΔT Performance — Optimized wall thickness and coil geometry minimize thermal resistance, enabling efficient heat exchange under low temperature difference conditions where conventional coils underperform.
- Exceptional Corrosion Resistance — Austenitic stainless steel construction eliminates galvanic corrosion risks when paired with aluminum or copper fins, extending service life to 20+ years even in aggressive environments.
- Hygienic Surface Quality — Bright annealed finish with Ra ≤ 0.4 μm minimizes biofilm adhesion, making the coil tube suitable for food-grade and pharmaceutical applications without additional surface treatment.
- Dimensional Precision — CNC-controlled coiling ensures uniform pitch and minimal ovality (< 1% of OD), reducing pressure drop and improving airflow distribution across the coil face.
- Full Traceability — Each batch is accompanied by a Mill Test Certificate (MTC) providing chemical composition, mechanical property data, and heat treatment records for complete supply chain traceability.
- Customization Capability — Flexible customization of OD, wall thickness, coil pitch, coil diameter, and end forming to meet specific OEM requirements for diverse system designs.
Technical Note: In low temperature difference fan coil applications, the thermal performance of the coil tube is governed by the overall heat transfer coefficient (U-value), which is a function of the tube material's thermal conductivity, wall thickness, flow velocity, and fin geometry. Stainless steel coil tubes, despite lower thermal conductivity than copper (16.3 W/m·K vs. 401 W/m·K), compensate through superior corrosion resistance and the ability to maintain clean internal surfaces, resulting in long-term performance stability that copper tubes cannot match in aggressive or hygienically critical environments.