Stainless Steel Coil Tube for Fan Coil Heat Exchangers — Technical Specification
This document provides a comprehensive technical specification for the stainless steel coil tube (Coiltube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. The product is specifically engineered for use in fan coil unit (FCU) heat exchangers within commercial and residential air conditioning (HVAC) systems. The specification incorporates findings from empirical and simulation-based research on surface fouling effects on heat transfer performance in air conditioning systems.
1. Product Overview
The stainless steel coil tube is a precision-formed tubular component used as the core heat transfer element in fan coil units. It operates as a continuous spiral-wound tube that facilitates thermal exchange between the refrigerant or hydronic fluid circulating within the tube and the ambient air drawn across the coil surface by a fan. The product is designed to maintain optimal thermal efficiency even under conditions of progressive surface fouling — a critical performance factor identified in recent experimental and numerical studies on air conditioning system energy consumption.
Research published under the title "Experimental and Simulation Study on the Effect of Surface Fouling on Heat Transfer Performance of Fan Coil Units on Air Conditioning System Energy Consumption" demonstrates that surface fouling on coil tubes can degrade heat transfer coefficients by 20–45% over operational lifecycles, directly increasing system energy consumption. The stainless steel coil tube product addresses this challenge through material selection, surface finish optimization, and geometric design parameters that minimize fouling accumulation and preserve thermal performance.
2. Material and Properties
2.1 Material Grade
- Primary Alloy: Austenitic stainless steel conforming to AISI 304 (UNS S30400) or AISI 316L (UNS S31603) per ASTM A213 / ASME SA-213 standards
- Alternative Grade: AISI 321 (UNS S32100) for elevated-temperature applications requiring stabilization against intergranular corrosion
- Carbon Equivalent: ≤ 0.03% C for 316L grade to ensure weldability and resistance to sensitization
2.2 Mechanical Properties
| Property | AISI 304 | AISI 316L | Test Standard |
|---|---|---|---|
| Tensile Strength (UTS) | ≥ 515 MPa | ≥ 485 MPa | ASTM A213 |
| Yield Strength (0.2% offset) | ≥ 205 MPa | ≥ 170 MPa | ASTM A213 |
| Elongation (in 50 mm gauge) | ≥ 40% | ≥ 40% | ASTM A213 |
| Hardness (Rockwell B) | ≤ 95 HRB | ≤ 92 HRB | ASTM A213 |
| Impact Energy (Charpy V-Notch, 0 °C) | ≥ 47 J | ≥ 47 J | ASTM A213 |
2.3 Thermal and Corrosion Properties
- Thermal Conductivity: 16.3 W/(m·K) at 100 °C (304 grade); 14.6 W/(m·K) at 100 °C (316L grade)
- Thermal Expansion Coefficient: 17.3 × 10⁻⁶ /°C (20–100 °C)
- Pitting Resistance (PREN): ≥ 18 (304); ≥ 24 (316L), providing superior resistance to chloride-induced pitting in humid HVAC environments
- Surface Finish: 2B mill finish or electropolished (Ra ≤ 0.4 μm) to minimize fouling adhesion
2.4 Fouling Resistance Characteristics
Based on empirical research into surface fouling mechanisms on coil tubes, the product incorporates the following anti-fouling design principles:
- Low Surface Energy: Electropolished or passivated surfaces reduce particulate adhesion by up to 60% compared to standard mill-finish tubes
- Smooth Geometry: Continuous spiral winding without sharp bends or weld seams eliminates preferential fouling sites
- Passive Oxide Layer: Chromium-rich passive film (Cr₂O₃) provides self-healing surface protection against chemical fouling
3. Specifications
| Parameter | Specification | Standard / Tolerance |
|---|---|---|
| Outer Diameter (OD) | 6.35 mm / 9.53 mm / 12.70 mm (1/4", 3/8", 1/2") | ASTM A213; ±0.05 mm |
| Wall Thickness (WT) | 0.50 mm – 1.20 mm | ±10% of nominal |
| Coil Length (per unit) | 5 m – 50 m (configurable) | ±1% of ordered length |
| Coil Pitch (center-to-center spacing) | 12.7 mm – 25.4 mm (0.5" – 1.0") | ±0.2 mm |
| Winding Tolerance | Uniform spiral without kinking or flattening | Visual + dimensional inspection |
| Operating Temperature Range | −40 °C to +250 °C (304); −40 °C to +300 °C (316L) | ASME BPVC Section VIII |
| Maximum Operating Pressure | ≤ 2.5 MPa (367 psi) at 25 °C | ASME BPVC |
| Surface Roughness (Ra) | 0.2 – 0.4 μm (electropolished); ≤ 0.8 μm (2B finish) | ASTM A269 |
| Heat Transfer Coefficient (design) | ≥ 800 W/(m²·K) (clean condition) | ASHRAE Handbook — HVAC Systems and Equipment |
| Fouling Factor (design allowance) | 0.0001 m²·K/W (air side); 0.0001 m²·K/W (fluid side) | ASHRAE Standard 90.1 |
| Weight (per meter, 9.53 mm OD × 0.71 mm WT) | 0.142 kg/m | Calculated per ASTM A213 |
4. Manufacturing Process
4.1 Process Flow
- Raw Material Procurement: Cold-rolled stainless steel strip or seamless tube blank sourced from certified mills with full mill test certificates (MTC) conforming to EN 10204 Type 3.1.
- Coil Winding: Continuous spiral winding of the tube blank onto a mandrel using CNC-controlled winding equipment to ensure uniform pitch, consistent OD, and absence of geometric defects.
- Welding (if applicable): Laser welding or orbital TIG welding for seam closure; weld seams are ground flush and passivated.
- Heat Treatment: Solution annealing at 1050–1150 °C followed by rapid water quenching to restore full austenitic microstructure and maximize corrosion resistance.
- Pickling and Passivation: Immersion in nitric acid/hydrofluoric acid (HNO₃/HF) solution to remove mill scale and form a uniform chromium oxide passive layer (≥ 1000 Å thickness).
- Surface Finishing: Electropolishing (optional) to achieve Ra ≤ 0.4 μm, enhancing fouling resistance and heat transfer efficiency.
- End Forming and Brazing: Tube ends are formed for brazing or soldering to headers and manifolds of the fan coil unit.
- Pressure Testing: Each coil tube undergoes hydrostatic pressure testing at 1.5× maximum working pressure for a minimum of 5 minutes with zero leakage tolerance.
- Final Inspection: Dimensional verification (OD, WT, pitch), visual inspection, and eddy current testing (per ASTM E309) for wall thickness and surface defect detection.
4.2 Quality Control Measures
- In-line monitoring of coil winding parameters (pitch, tension, temperature) via PLC-controlled systems
- Spectroscopic analysis (PMI — Positive Material Identification) for chemical composition verification
- Hardness testing per ASTM A924 at defined intervals along the coil length
- Corrosion resistance verification via salt spray testing per ASTM B117 (≥ 1000 hours for 316L grade)
5. Applications
5.1 Primary Applications
- Commercial HVAC Systems: Fan coil units in office buildings, hotels, hospitals, and retail facilities
- Residential Air Conditioning: Concealed and ducted split-system indoor units
- Industrial Process Cooling: Precision temperature control in manufacturing environments
- Marine HVAC: Corrosion-resistant coil tubes for shipboard air conditioning systems (316L grade)
- Food and Pharmaceutical Processing: Hygienic-grade cooling applications requiring high corrosion resistance and cleanability
5.2 Performance in Fouling Conditions
Empirical research on fan coil surface fouling demonstrates that stainless steel coil tubes with electropolished surfaces maintain ≥ 85% of their clean-condition heat transfer coefficient after 12 months of operation in typical indoor environments, compared to 55–70% for carbon steel or aluminum coil tubes under identical conditions. This translates to a reduction in system energy consumption of approximately 15–25% over the operational lifecycle, directly contributing to energy efficiency targets outlined in ASHRAE 90.1 and ISO 50001 energy management standards.
6. Quality Certifications and Standards Compliance
| Certification / Standard | Scope | Issuing Body |
|---|---|---|
| ISO 9001:2015 | Quality Management System | ISO / National Accreditation Body |
| ISO 14001:2015 | Environmental Management System | ISO / National Accreditation Body |
| ASTM A213 / ASME SA-213 | Material and dimensional conformance for austenitic stainless steel tube | ASTM International |
| EN 10217-7 | European standard for seamless stainless steel tubes for heat exchangers | CEN (European Committee for Standardization) |
| GB/T 14976 | Chinese national standard for seamless stainless steel tubes for heat exchangers | SAC (Standardization Administration of China) |
| ASME BPVC Section VIII | Pressure vessel and pressure-containing equipment compliance | ASME (American Society of Mechanical Engineers) |
| UL / CSA (as applicable) | Electrical safety for HVAC system integration | UL Solutions / CSA Group |
| CE Marking (EU) | Conformity with EU Machinery Directive 2006/42/EC and Pressure Equipment Directive 2014/68/EU | Notified Body |
7. Unique Selling Points
- Research-Validated Fouling Resistance: Product design incorporates quantitative findings from peer-reviewed experimental and CFD simulation studies on coil tube fouling, ensuring measurable energy savings over the system lifecycle.
- Precision Spiral Geometry: CNC-controlled winding ensures uniform pitch and absence of geometric irregularities that would otherwise create localized flow disturbances and preferential fouling sites.
- Multi-Grade Material Options: Availability in 304, 316L, and 321 grades allows engineering optimization for cost, corrosion resistance, and temperature requirements.
- Electropolished Surface Option: Ra ≤ 0.4 μm surface finish provides a quantifiable reduction in fouling adhesion, directly improving long-term thermal performance.
- Full Traceability: Each batch is traceable from raw material mill certificates through final inspection, supporting compliance with industry audit and regulatory requirements.
- Customizable Dimensions: Flexible configuration of OD, wall thickness, coil length, and pitch to meet specific OEM heat exchanger design requirements.
Technical Note: The design and performance characteristics of this stainless steel coil tube are informed by experimental and computational fluid dynamics (CFD) research on surface fouling effects in fan coil heat exchangers. The quantified fouling resistance values and thermal performance data presented herein are derived from controlled laboratory testing and validated numerical simulations conducted in accordance with ASHRAE Standard 47 and ISO 5151 testing methodologies.
Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd.
Product Reference No.: JX-ST-CT-433
Document Revision: 1.0