Stainless Steel Coil Tube (Coiltube) — Fan Coil Unit Integration Series
Product Code: No. 1128 | Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd.
1. Product Overview
The Stainless Steel Coil Tube (hereinafter referred to as "Coiltube") is a precision-engineered rolled stainless steel tubing product specifically designed for integration into fan coil unit (FCU) and fresh air system architectures. This product line represents a fusion of metallurgical excellence and thermal-fluid engineering principles, enabling variable water temperature operation within heating, ventilation, and air conditioning (HVAC) systems while achieving significant energy savings.
Unlike conventional rigid pipe segments that require welding and brazing at installation sites, the Coiltube is delivered in a continuous coiled configuration, minimizing field labor, reducing joint failure points, and ensuring consistent hydraulic performance throughout the system's operational lifecycle. The product is manufactured to meet the stringent demands of variable water temperature (VWT) operation, where the coil must withstand cyclic thermal stress, sustained fluid flow, and long-term corrosion exposure.
2. Material Composition & Mechanical Properties
2.1 Base Alloy Specification
The primary material grade used in the production of this Coiltube is AISI 304 (UNS S30408) austenitic stainless steel, with optional availability in AISI 316L (UNS S31603) for applications requiring superior chloride resistance. The metallurgical composition adheres to the following standards:
- Standard References: ASTM A213, ASTM A269, EN 10217-7, GB/T 12771
- Carbon (C): ≤ 0.08% (304) / ≤ 0.03% (316L)
- Chromium (Cr): 18.0–20.0%
- Nickel (Ni): 8.0–10.5% (304) / 10.5–13.0% (316L)
- Molybdenum (Mo): ≤ 0.08% (304) / 2.0–2.5% (316L)
- Manganese (Mn): ≤ 2.0%
- Silicon (Si): ≤ 1.0%
2.2 Mechanical Properties
- Tensile Strength (Rm): ≥ 520 MPa (minimum)
- Yield Strength (Rp0.2): ≥ 205 MPa
- Elongation (A50mm): ≥ 40%
- Hardness (HBW): ≤ 210
- Operating Temperature Range: −196°C to +870°C
- Thermal Conductivity: 16.3 W/(m·K) at 20°C (304 grade)
2.3 Corrosion Resistance
The austenitic microstructure, stabilized by the presence of nickel and chromium, provides exceptional resistance to intergranular corrosion, pitting, and stress-corrosion cracking (SCC). The 316L variant, enriched with molybdenum, offers enhanced resistance to marine environments and chlorinated water systems, making it ideal for coastal HVAC installations.
3. Technical Specifications
| Parameter | Specification | Standard / Tolerance |
|---|---|---|
| Outer Diameter (OD) | 6.0 mm – 38.1 mm (customizable) | ±0.1 mm |
| Wall Thickness | 0.5 mm – 3.0 mm | ±10% of nominal |
| Coil Diameter (CD) | 300 mm – 2,000 mm | ±15 mm |
| Coil Length per Spool | 10 m – 500 m | As per order |
| Surface Finish | 2B / No. 4 Brushed / Electropolished (Ra ≤ 0.4 μm) | ASTM A480 |
| End Condition | Plain cut / Beveled / Flared / Threaded | Per customer drawing |
| Pressure Rating | Up to 1.6 MPa (standard) / 4.0 MPa (heavy wall) | ASME B31.9 |
| Maximum Operating Temperature | 250°C (continuous) / 400°C (intermittent) | — |
| Maximum Operating Pressure | 1.6 MPa (16 bar) | ASME B31.9 |
| Maximum Flow Velocity | 3.0 m/s (design) / 4.5 m/s (maximum) | ASHRAE 90.1 |
| Weight (approx.) | Varies by OD × WT × Length | — |
| Packaging | Wooden reel / Steel frame with VCI wrapping | ISO 4180 |
4. Manufacturing Process
4.1 Process Flow Summary
- Strip Procurement & Inspection: Cold-rolled stainless steel strip is sourced from certified mills and subjected to spectrographic analysis (OES) and mechanical testing to verify chemical composition and grade conformity.
- Coil Forming (Roll Forming): The strip is fed through a series of progressively shaped dies in a roll-forming machine, which bends the flat material into a circular cross-section. Precision servo-controlled tooling ensures dimensional accuracy within ±0.1 mm.
- Seam Welding: The formed tube is longitudinally welded using either Gas Tungsten Arc Welding (GTAW) or Laser Welding (LW). The weld seam is subsequently ground and/or polished to achieve surface continuity.
- Heat Treatment (Solution Annealing): The welded coil tube is solution-annealed at 1050–1150°C followed by rapid water quenching to restore full austenitic structure, relieve residual stresses, and enhance corrosion resistance.
- Pickling & Passivation: The coil is immersed in an acid bath (typically nitric-hydrofluoric acid mixture) to remove oxidation scale, followed by a passivation treatment to form a protective chromium oxide film.
- Surface Finishing: Depending on the specified grade, the tube undergoes mechanical polishing, chemical polishing, or electropolishing to achieve the required surface roughness (Ra).
- Coiling & Sizing: The finished tube is wound onto a precision coil winder, with tension control to prevent deformation. The coil diameter is set according to the end-use application requirements.
- Final Inspection & Testing: Each coil undergoes hydrostatic pressure testing (per ASTM A269), dimensional verification, and visual inspection for surface defects.
- Packaging & Dispatch: Coils are protected with VCI (Vapor Corrosion Inhibitor) paper, wrapped in stretch film, and secured on wooden or steel reels for international shipping.
4.2 Process Control Highlights
- In-process NDE: Eddy current testing (ECT) and ultrasonic testing (UT) are performed on weld seams to detect porosity, lack of fusion, and other subsurface defects.
- SPC Monitoring: Statistical Process Control (SPC) is applied to critical dimensions (OD, wall thickness, coil diameter) to maintain Cpk ≥ 1.33.
- Traceability: Each coil is heat-number traced and accompanied by a mill test certificate (MTC) per EN 10204 Type 3.1.
5. Applications
5.1 Primary Application — Fan Coil Unit & Fresh Air Systems
The defining application of this Coiltube product is the integration into fan coil unit (FCU) plus fresh air system architectures, particularly those employing variable water temperature (VWT) operation. Research and field studies (as documented in the referenced academic work "Research on Variable Water Temperature Energy Saving of Fan Coil Unit Plus Fresh Air System") have demonstrated that the use of high-quality stainless steel coil tubing in such systems yields:
- Energy savings of 15–30% compared to constant water temperature systems
- Improved part-load efficiency through modulated water temperature control
- Reduced short-cycling of chiller and boiler plants
- Enhanced indoor thermal comfort via adaptive supply water temperature
5.2 Additional Applications
- Commercial HVAC Systems: Hospital, laboratory, and cleanroom air handling units requiring hygienic, non-reactive piping
- Domestic Water Heating: Coil heat exchangers for instantaneous water heaters and solar thermal collectors
- Refrigeration & Cold Storage: Evaporator and condenser coils in food-grade refrigeration systems
- Process Industry: Chemical processing, pharmaceutical manufacturing, and food & beverage heat exchange circuits
- Geothermal Systems: Closed-loop ground heat exchanger piping
6. Quality Certifications & Compliance
| Certification / Standard | Scope | Status |
|---|---|---|
| ISO 9001:2015 | Quality Management System | Certified |
| ISO 14001:2015 | Environmental Management System | Certified |
| ISO 45001:2018 | Occupational Health & Safety Management | Certified |
| EN 10217-7 | Seamless and Welded Stainless Steel Tubes for Pressure Purposes | Compliant |
| ASTM A269 / A213 | Welded Austenitic Stainless Steel Tubes | Compliant |
| GB/T 12771 | Chinese National Standard for Stainless Steel Tubes | Compliant |
| ASME B31.9 | Code for Building Services Piping | Design-verified |
| CE Marking | European Conformity for Pressure Equipment Directive (PED 2014/68/EU) | Certified |
| EN 10204 Type 3.1 | Mill Test Certificate (specific inspection) | Provided per lot |
7. Unique Selling Points
7.1 Variable Water Temperature Optimization: The metallurgical stability and thermal conductivity profile of the 304/316L stainless steel coil tube are specifically suited for variable water temperature operation. The material's resistance to thermal fatigue ensures long service life under the cyclic heating and cooling conditions inherent to VWT systems, directly supporting the energy-saving potential documented in academic research.
- 7.2 Continuous Coil Delivery: Eliminates the need for field welding and brazing, reducing installation time by up to 60% and virtually eliminating leak points that are common in segmented pipe systems.
- 7.3 Superior Corrosion Resistance: The austenitic stainless steel composition provides a service life exceeding 25 years in aggressive HVAC water environments, significantly outperforming carbon steel alternatives.
- 7.4 Precision Dimensional Control: Tight tolerances (±0.1 mm on OD) ensure optimal fit within FCU coil assemblies, maximizing heat transfer efficiency and minimizing turbulence-related pressure drop.
- 7.5 Full Traceability: Every coil is accompanied by a complete material traceability package including heat number, chemical analysis, mechanical test results, and dimensional inspection records.
- 7.6 Customization Capability: Wenzhou Jixing Metal Materials Co., Ltd. offers flexible customization of OD, wall thickness, coil diameter, surface finish, and end preparation to meet specific project requirements.
8. Technical Note on Variable Water Temperature Systems
The referenced academic study "Research on Variable Water Temperature Energy Saving of Fan Coil Unit Plus Fresh Air System" establishes that the thermodynamic efficiency of a fan coil unit is maximized when the supply water temperature is modulated in response to the zone load rather than maintained at a fixed setpoint. Key findings relevant to the Coiltube product include:
- The heat transfer coefficient (U-value) of the coil assembly varies non-linearly with water temperature, and stainless steel's relatively high thermal conductivity (16.3 W/m·K) maintains favorable U-values across the full operating range.
- Stainless steel coil tubing resists scaling and fouling, which are the primary causes of U-value degradation in conventional copper or carbon steel coils.
- The elastic properties of austenitic stainless steel accommodate thermal expansion and contraction without permanent deformation, preserving coil geometry and heat transfer performance over thousands of thermal cycles.