Stainless Steel Coil Tube for HVAC Fan Coil Unit Applications
Product Overview
Wenzhou Jixing Metal Materials Co., Ltd (Product No. 1589) manufactures precision stainless steel coil tubes (Coil Tube) engineered specifically for use in HVAC (Heating, Ventilation, and Air Conditioning) fan coil unit assemblies. These coil tubes serve as the primary heat exchange media within fan coil systems, facilitating efficient thermal transfer between circulating water/air and the ambient environment. The product line addresses the critical engineering requirements of condensate management, thermal performance optimization, and long-term corrosion resistance in demanding indoor air quality environments.
The coil tube product family is designed with particular attention to the interplay between tube geometry, condensate generation rates, and condensate drain pipe sizing — a subject of extensive engineering study documented in the technical literature on fan coil unit performance. Proper coil tube specification directly influences condensate volume calculations and the appropriate selection of condensate drain pipe diameters, ensuring system reliability and preventing water damage in commercial and residential installations.
Material & Properties
Material Composition
- Primary Alloy: Austenitic stainless steel conforming to ASTM A269/A269M and ASTM A312/A312M standards
- Common Grades: Type 304 (UNS S30400), Type 316L (UNS S31603), and Type 321 (UNS S32100) depending on application severity
- Chromium Content: 18.0–20.0 wt% for passive oxide layer formation
- Nickel Content: 8.0–12.0 wt% ensuring austenitic microstructure stability
- Molybdenum (316L): 2.0–3.0 wt% for enhanced chloride resistance
Mechanical Properties
- Tensile Strength: ≥ 515 MPa (75,000 psi) minimum per ASTM A269
- Yield Strength: ≥ 205 MPa (29,750 psi) minimum
- Elongation: ≥ 35% in 50 mm (2 in) gauge length
- Hardness: ≤ 92 HRB (annealed condition)
- Working Temperature Range: −196 °C to +870 °C (−320 °F to +1,600 °F)
Thermal & Corrosion Performance
- Thermal Conductivity: 14–16 W/(m·K) at 20 °C
- Corrosion Resistance: Superior resistance to atmospheric corrosion, condensate water, and mildly aggressive HVAC fluids
- Surface Finish: Bright annealed (BA) or electropolished with Ra ≤ 0.4 μm for reduced fouling
- Crevice Corrosion Resistance: Critical for condensate drain junction areas where stagnant water accumulates
Specifications
| Parameter | Specification | Standard/Reference |
|---|---|---|
| Tube Outer Diameter (OD) | 3.18 mm (1/8"), 6.35 mm (1/4"), 9.53 mm (3/8"), 12.70 mm (1/2") | ASTM A269 |
| Tube Wall Thickness | 0.50 mm – 1.20 mm (0.020" – 0.047") | ASTM A269 Table 3 |
| Coil Winding Diameter | 50 mm – 250 mm (2" – 10") | Manufacturer specification |
| Coil Length per Spool | 10 m – 100 m (33 ft – 328 ft) | Custom order available |
| Fins per Inch (FPI) | 10 FPI, 12 FPI, 14 FPI (when finned) | ASHRAE Standard 15 |
| Fin Material | Aluminum (Al 1100) or Stainless Steel | — |
| Pressure Rating | ≤ 1.6 MPa (232 psi) operating; 2.4 MPa (348 psi) test | EN 10217-7 |
| Condensate Flow Capacity | 0.5 – 8.0 L/min depending on coil size and AHU configuration | ASHRAE Handbook – HVAC Systems |
| Drain Pipe Diameter (Recommended) | 15 mm (1/2") minimum for single unit; 20 mm (3/4") for multi-unit manifolds | ASHRAE 90.1; Local plumbing codes |
| Surface Roughness (Ra) | 0.2 – 0.8 μm | ISO 1302 |
| Dimensional Tolerance | ±0.05 mm (OD); ±0.02 mm (wall thickness) | ASTM A213 |
Manufacturing Process
1. Raw Material Preparation
High-purity stainless steel wire rod (typically Grade 304 or 316L) is sourced from certified mills with full chemical composition documentation. Incoming material undergoes spectroscopic verification (OES analysis) to confirm alloy chemistry within specification limits.
2. Seamless Tube Forming
The coil tube is produced via the seamless drawing process:
- Piercing: Hot-rolled steel billets are pierced to create hollow blanks
- Reduction: Multiple passes through reduction mills achieve target OD and wall thickness
- Drawing: Cold drawing through precision dies ensures dimensional accuracy and improved surface finish
- Pilot Removal: Internal mandrel (pilot) is removed post-drawing for seamless internal bore
3. Annealing & Pickling
Tubes undergo solution annealing at 1,050–1,100 °C followed by controlled cooling to restore full austenitic structure and eliminate residual stresses from cold working. Subsequent acid pickling (HNO₃/HF mixture) removes oxide scale, establishing the passive chromium oxide film essential for corrosion resistance.
4. Coil Winding & Finning
Formed tubes are wound into precise helical coils using CNC-controlled winding machinery. When finned configurations are required, aluminum or stainless steel fins are mechanically bonded to the tube OD via a process that ensures thermal contact resistance below 0.001 m²·K/W.
5. Brazing & Assembly
Header connections are fabricated using phosphorus-copper brazing alloy (e.g., BAg-8) at 650–750 °C under nitrogen protection atmosphere. This ensures leak-tight joints compatible with the stainless steel substrate without galvanic degradation.
6. Hydrostatic Testing & Inspection
Each coil assembly undergoes 100% hydrostatic pressure testing at 1.5× operating pressure for a minimum dwell time of 30 seconds. Non-destructive testing (NDT) methods including dye penetrant inspection and, where applicable, eddy current testing are employed on random samples.
Applications
HVAC Fan Coil Units (Primary Application)
The coil tube is the core heat exchange element within fan coil units used in commercial buildings, hospitals, hotels, and residential HVAC systems. Proper coil tube design directly impacts:
- Condensate Generation Rate: Determined by the coil's effective surface temperature relative to the dew point of incoming air. Engineering calculations must account for coil geometry (rows, FPI, tube count) to predict condensate volume accurately.
- Condensate Drain Sizing: The calculated condensate volume (typically 0.5–8.0 L/min per unit) dictates minimum drain pipe diameter. Undersized drains lead to water backup, mold growth, and equipment damage. Industry practice follows the principle that drain pipe diameter should provide a minimum slope of 1:100 with adequate flow capacity at the maximum condensate rate.
- System Efficiency: Optimized coil tube geometry maximizes COP (Coefficient of Performance) while minimizing condensate-related maintenance issues.
Additional Applications
- Industrial process heating and cooling heat exchangers
- Food and beverage industry heat transfer coils (FDA-compliant grades)
- Pharmaceutical clean-room HVAC systems
- Marine HVAC and refrigeration systems (316L grade)
- Commercial refrigeration evaporator coils
- District heating and cooling distribution loops
Quality Certifications & Standards Compliance
- ISO 9001:2015 — Quality Management System certification
- ISO 14001:2015 — Environmental Management System
- CE Marking — Conformity with European standards (EN 10217-7 for heat exchanger tubes)
- ASTM A269/A269M — Seamless austenitic stainless steel tube for heat exchangers
- ASTM A312/A312M — Seamless austenitic stainless steel tubes
- EN 10217-7 — Heat exchanger and radiator tubes
- GB/T 17705 — Chinese national standard for heat exchanger tubes
- ASME B31.5 — Refrigeration piping and heat transfer piping
- ASHRAE Standard 15 — Safety standard for refrigeration systems
Unique Selling Points
- Integrated Condensate Engineering: Product specifications are developed with explicit reference to condensate volume calculations and drain pipe sizing requirements, enabling turnkey system design from coil selection through condensate management — reducing engineering errors and field failures.
- Precision Coil Geometry: CNC winding ensures uniform pitch and consistent coil diameter, guaranteeing predictable airflow distribution and thermal performance across the full operating envelope.
- Corrosion-Free Condensate Environment: Stainless steel construction eliminates the corrosion and biofilm growth risks associated with carbon steel alternatives, extending service life beyond 20 years in typical HVAC applications.
- Traceability & Documentation: Full material traceability from mill certificate through final inspection report, including chemical analysis, mechanical test data, and dimensional verification records.
- Custom Engineering Support: Technical consultation available for condensate load calculations, drain routing optimization, and coil selection matching specific fan coil unit configurations.
Technical Notes on Condensate Management
The selection of appropriate coil tube specifications must be integrated with condensate drainage system design. Key engineering considerations include:
- Condensate Volume Calculation: Based on coil effective surface area, air-side heat transfer coefficient, dew point depression, and system operating conditions. Typical formula: Q_condensate = m_air × (ω_in − ω_out) × 3600, where ω represents humidity ratio.
- Drain Pipe Diameter Selection: Minimum 15 mm (1/2") for individual fan coil units handling up to 4.0 L/min condensate; upgrade to 20 mm (3/4") for flows exceeding 4.0 L/min or multi-unit header connections.
- Slope Requirements: Condensate drain lines must maintain a minimum slope of 0.5% (1:200) toward the discharge point to ensure gravity drainage without air locking.
Wenzhou Jixing Metal Materials Co., Ltd provides complete technical documentation and engineering support to ensure that coil tube selection, condensate prediction, and drainage design are harmonized for optimal system performance and long-term reliability.