Stainless Steel Coil Tube for HVAC Fan Coil Unit Systems
Product Code: 1315 | Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd
1. Product Overview
The Stainless Steel Coil Tube (SSCT) is a precision-engineered coiled tubing product designed specifically for integration into heating, ventilation, and air conditioning (HVAC) systems, with particular emphasis on Fan Coil Unit (FCU) applications. Manufactured to stringent metallurgical standards, this coil tube serves as the core heat exchange medium within distributed air conditioning systems, facilitating efficient thermal transfer between refrigerant or water-based heat transfer fluids and the ambient air stream.
In the context of FCU selection and system design, the coil tube geometry, material composition, and surface characteristics directly influence the overall thermal performance, pressure drop characteristics, and long-term service reliability of the installed system. Wenzhou Jixing Metal Materials Co., Ltd produces coil tubes that address the critical engineering challenges identified in system design literature, including flow distribution uniformity, corrosion resistance under variable humidity conditions, and dimensional stability during thermal cycling.
2. Material Composition and Properties
2.1 Base Alloy Designation
The coil tube is fabricated from austenitic stainless steel conforming to the following standard grades:
- 304 (UNS S30408): General-purpose grade with excellent corrosion resistance, suitable for standard HVAC duty cycles with moderate humidity exposure.
- 316 (UNS S31608): Molybdenum-enhanced grade offering superior resistance to chlorid-induced stress corrosion cracking, recommended for coastal or chemically aggressive environments.
- 321 (UNS S32100): Titanium-stabilized grade providing enhanced resistance to intergranular corrosion at elevated operating temperatures.
2.2 Mechanical Properties (Annealed Condition)
| Property | 304 Grade | 316 Grade | 321 Grade | Test Standard |
|---|---|---|---|---|
| Tensile Strength (MPa) | ≥ 520 | ≥ 515 | ≥ 515 | ASTM A213 / A269 |
| Yield Strength (0.2% Offset, MPa) | ≥ 205 | ≥ 170 | ≥ 170 | ASTM A213 / A269 |
| Elongation (A50mm, %) | ≥ 40 | ≥ 40 | ≥ 40 | ASTM A213 / A269 |
| Hardness (HRB) | ≤ 95 | ≤ 95 | ≤ 95 | ASTM E10 |
| Thermal Conductivity at 100°C (W/m·K) | 16.3 | 14.6 | 16.3 | ASTM E1225 |
| Linear Expansion Coefficient (10⁻⁶/K, 20–100°C) | 17.2 | 16.9 | 17.2 | ASTM E228 |
2.3 Chemical Composition (wt%)
| Element | 304 | 316 | 321 |
|---|---|---|---|
| Carbon (C) | ≤ 0.08 | ≤ 0.08 | ≤ 0.08 |
| Chromium (Cr) | 18.0 – 20.0 | 16.0 – 18.0 | 17.0 – 19.0 |
| Nickel (Ni) | 8.0 – 10.5 | 10.0 – 14.0 | 9.0 – 13.0 |
| Molybdenum (Mo) | — | 2.0 – 3.0 | — |
| Titanium (Ti) | — | — | 5 × (C × 5) – 0.70 |
| Manganese (Mn) | ≤ 2.0 | ≤ 2.0 | ≤ 2.0 |
| Silicon (Si) | ≤ 1.0 | ≤ 1.0 | ≤ 1.0 |
| Phosphorus (P) | ≤ 0.045 | ≤ 0.045 | ≤ 0.045 |
| Sulfur (S) | ≤ 0.030 | ≤ 0.030 | ≤ 0.030 |
2.4 Surface Finish Options
- 2B Finish: Standard cold-rolled and annealed surface with a smooth, matte appearance. Suitable for general HVAC applications.
- No. 4 Finish: Fine ground surface providing enhanced aesthetic quality for visible installations.
- Mirror Finish (8K): High-polish surface for premium architectural applications.
- Passivated Surface: Chemical treatment to remove free iron and enhance passive film integrity.
3. Technical Specifications
| Parameter | Specification | Applicable Standard |
|---|---|---|
| Tube Outer Diameter (OD) | 6.35 mm (⅛"), 9.53 mm (⅜"), 12.70 mm (½"), 15.88 mm (⅝"), 19.05 mm (¾") | ASTM A269 |
| Tube Wall Thickness (WT) | 0.40 – 1.20 mm (customizable) | ASTM A269 |
| Coil Diameter (CD) | 200 mm – 1200 mm (standard); up to 2000 mm (special order) | Manufacturer Specification |
| Coil Length (per bundle) | 3 m – 60 m (continuous); adjustable per application requirement | Manufacturer Specification |
| Coil Pitch / Spacing | 6.0 – 12.0 mm (typical for FCU coil assemblies) | ASHRAE Standard 15 |
| Number of Rows | 2 – 6 rows (configurable for capacity requirements) | ASHRAE Standard 15 |
| Operating Temperature Range | −40°C to +250°C (304/321); −50°C to +250°C (316) | ASTM A213 |
| Operating Pressure (Max) | 2.5 MPa (367 psi) at 25°C | ASME BPVC Section VIII |
| Tolerance on OD | ±0.05 mm (precision grade); ±0.10 mm (standard grade) | ASTM A269 |
| Tolerance on Wall Thickness | ±0.025 mm (precision grade); ±0.05 mm (standard grade) | ASTM A269 |
| Roundness Deviation | ≤ 5% of nominal wall thickness | ASTM A269 |
| Surface Roughness (Ra) | ≤ 0.8 μm (internal); ≤ 0.4 μm (external, polished) | ASTM E19 |
| Coating (Optional) | Hydrophilic / Hydrophobic fluoropolymer coating for enhanced heat transfer | Manufacturer Specification |
4. Manufacturing Process
The production of high-quality stainless steel coil tubes for HVAC applications involves a multi-stage metallurgical and forming process. Each stage is critical to ensuring dimensional accuracy, mechanical integrity, and corrosion resistance:
- Raw Material Selection and Inspection: Virgin or certified remelted stainless steel slabs are procured from approved mills. Incoming material undergoes spectroscopic verification (OPT or LECO analysis) to confirm chemical composition within specified limits. Heat traceability is maintained throughout the supply chain.
- Hot Rolling and Pickling: Steel slabs are hot-rolled into strip or plate form at temperatures exceeding the austenitization range (1100–1200°C). The resulting scale is removed through acid pickling (HNO₃/HF mixture) and subsequent passivation to establish the initial chromium oxide passive film.
- Cold Rolling (Reduction): The pickled strip is cold-rolled to the target wall thickness. Cold working introduces beneficial work hardening while maintaining dimensional precision. Interpass annealing may be performed if reduction exceeds 60% per pass to prevent excessive strain hardening.
- Seamless Tube Forming (Capillary Method): For seamless coil tubes, the capillary piercing and rolling method is employed. A pilot rod forces the heated billet through a piercing die, creating a hollow tube. Subsequent rolling passes reduce the OD and WT to specification while maintaining roundness and straightness.
- Continuous Annealing and Pickling: The cold-worked tube is continuously annealed at 1050–1150°C in a controlled atmosphere (dissociated ammonia or vacuum) to relieve residual stresses, restore ductility, and homogenize the microstructure. This is followed by acid pickling and passivation.
- Coiling and Dimensional Verification: The annealed tube is coiled to the specified coil diameter using precision coiling equipment. Each coil is inspected for dimensional accuracy using laser-based OD and WT measurement systems, and roundness is verified via coordinate measuring machines (CMM).
- Final Surface Treatment: Optional surface treatments including electropolishing, hydrophilic coating application, or bead blasting are performed to enhance heat transfer characteristics and corrosion resistance as required by the application.
- Non-Destructive Testing (NDT): 100% eddy current testing is performed to detect surface and near-surface defects. Hydrostatic pressure testing is conducted at 1.5× the maximum operating pressure to verify weld integrity (for welded tubes) or structural soundness (for seamless tubes).
5. Applications in HVAC Fan Coil Unit Systems
The coil tube serves as the fundamental heat exchange element in Fan Coil Unit (FCU) systems. Based on engineering analysis of FCU selection and system design, the following application considerations are critical:
5.1 Chilled Water FCU Applications
- Residential and commercial space cooling where water-side heat exchange is employed
- Operating temperatures typically 7–12°C (supply) and 17–22°C (return)
- 304 grade stainless steel recommended for standard indoor environments
- Hydrophilic coating enhances condensation management and reduces fouling resistance
5.2 Heating FCU Applications
- Hot water heating circuits in temperate and cold climates
- Operating temperatures typically 45–70°C (supply) and 30–55°C (return)
- 321 grade recommended for sustained elevated temperature service to prevent sensitization
- Thermal expansion management requires flexible loop configurations
5.3 Refrigerant Direct-Expansion (DX) Applications
- Direct refrigerant evaporation/condensation within the coil
- Higher pressure differentials require enhanced wall thickness and material grade
- 316 grade preferred where refrigerant oil degradation products may cause localized corrosion
- Internal surface roughness directly impacts pressure drop and capacity
5.4 Special Environment Applications
- Coastal/Marine Environments: 316L grade with enhanced molybdenum content for chloride resistance
- Food and Pharmaceutical Processing: 304L or 316L with electropolished internal surfaces (Ra ≤ 0.25 μm) for sanitary compliance
- Chemical Processing: Grade selection based on specific chemical exposure; 316 or duplex grades for aggressive media
- Cleanroom Applications: Ultra-low carbon grades with passivated surfaces to minimize particulate generation
6. System Design Considerations
Based on established engineering literature regarding FCU selection and system design, the following parameters must be addressed during coil tube integration:
6.1 Heat Transfer Performance
- The overall heat transfer coefficient (U-value) of the coil assembly depends on tube material thermal conductivity, fouling factors, and flow velocity within the tube
- Stainless steel's lower thermal conductivity compared to copper (~16 W/m·K vs. ~400 W/m·K) necessitates larger surface area or higher flow velocities to achieve equivalent performance
- Enhanced internal geometry (helical corrugations, dimples) may be employed to increase turbulent mixing and improve convective heat transfer coefficients
6.2 Pressure Drop and Flow Distribution
- Internal surface roughness directly impacts frictional pressure drop (Darcy-Weisbach relationship)
- Uniform flow distribution across all tube passes is critical for balanced performance; header design and coil tube entrance geometry must be optimized
- Pressure drop should not exceed 50 kPa for water-side circuits and 35 kPa for air