Stainless Steel Fan Coil Tube — Air Source Heat Pump Integration System

1. Product Overview

The Stainless Steel Fan Coil Tube (Coiltube) is a precision-engineered heat exchanger component designed for integration with air source heat pump (ASHP) systems operating at small temperature differences (low ΔT) in high-efficiency and comfortable space heating applications. Manufactured by Wenzhou Jixing Metal Materials Co., Ltd, this product represents a next-generation solution that leverages the superior thermal conductivity, corrosion resistance, and mechanical integrity of stainless steel to deliver optimal heat transfer performance in demanding HVAC (Heating, Ventilation, and Air Conditioning) environments.

Unlike conventional copper or aluminum coil tubes, the stainless steel variant eliminates the risk of galvanic corrosion when exposed to varying water quality conditions, ensuring extended service life and consistent thermal performance throughout the operational cycle. The product is specifically engineered for small temperature differential (low-ΔT) operation scenarios where the supply and return water temperatures differ by only 3–6 °C, a configuration that maximizes the Coefficient of Performance (COP) of the heat pump system while maintaining occupant thermal comfort.

2. Material & Properties

2.1 Base Material Composition

• Primary Alloy: Austenitic stainless steel conforming to AISI 304 (UNS S30400) or AISI 316 (UNS S31600) grades, selected based on the corrosive environment of the installation.
• Carbon Equivalent (CE): ≤ 0.03% (for 304L grade) to ensure weldability and resistance to intergranular corrosion.
• Chromium (Cr): 18.0–20.0 wt%
• Nickel (Ni): 8.0–10.5 wt%
• Molybdenum (Mo): 2.0–3.0 wt% (for 316 grade, providing enhanced chloride resistance)

2.2 Key Physical and Thermal Properties

• Thermal Conductivity: 14.6–16.3 W/(m·K) at 20 °C — sufficient for efficient convective heat transfer in low-ΔT applications.
• Thermal Expansion Coefficient: 16–18 × 10⁻⁶ /°C, providing dimensional stability under cyclic thermal loading.
• Operating Temperature Range: −40 °C to +200 °C
• Maximum Operating Pressure: 1.6 MPa (16 bar) at 80 °C
• Surface Finish: 2B or BA (Bright Annealed) finish with surface roughness Ra ≤ 0.4 μm to minimize fouling and enhance heat transfer efficiency.
• Corrosion Resistance: Excellent resistance to general aqueous corrosion, deionized water, and mild brine solutions; 316 grade provides superior resistance to chlorinated water systems.

2.3 Mechanical Properties

• Tensile Strength (UTS): ≥ 520 MPa (304), ≥ 550 MPa (316)
• Yield Strength: ≥ 205 MPa
• Elongation: ≥ 40%
• Hardness: ≤ 220 HB (annealed condition)

3. Technical Specifications

Parameter	Specification	Standard / Test Method
Tube Outer Diameter (OD)	Φ8.0 mm / Φ9.52 mm / Φ10.0 mm (±0.1 mm tolerance)	ASTM A213 / GB/T 14976
Tube Wall Thickness	0.5 mm / 0.6 mm / 0.8 mm (±0.05 mm tolerance)	ASTM A213 / GB/T 14976
Tube Length (per coil)	2,000 mm – 12,000 mm (customizable)	Manufacturer Specification
Fin Material	Aluminum (Al-Mg1) or Stainless Steel (304)	ASTM B209 / GB/T 3190
Fin Pitch (Fins per Inch)	10 FPI – 14 FPI	ISO 13373
Fin Height	6.0 mm – 12.0 mm	ISO 13373
Tube Pitch (Longitudinal)	16 mm – 32 mm	ISO 13373
Tube Pitch (Transverse)	16 mm – 25 mm	ISO 13373
Number of Rows	1 – 4 rows	ISO 13373
Maximum Heat Transfer Capacity	Up to 8.5 kW per unit (at ΔT = 5 °C, air velocity 2.5 m/s)	ASHRAE Standard 34
Pressure Drop (Water Side)	≤ 15 kPa at nominal flow rate	ISO 22714
Pressure Drop (Air Side)	≤ 45 Pa at face velocity 2.5 m/s	ISO 22714
Welding Method	TIG (Tungsten Inert Gas) or Orbital TIG welding	ISO 6947
Hydrostatic Test Pressure	2.4 MPa (1.5× maximum operating pressure)	GB/T 24511
Coil Weight (Typical)	3.2 kg – 18.5 kg (depending on configuration)	—
Connector Type	Flared, Threaded (G1/2" – G1"), or Brazed	ISO 7-1

4. Manufacturing Process

4.1 Raw Material Procurement and Inspection

Stainless steel seamless tubes are sourced from certified mills conforming to ASTM A213 TP304/TP316 or GB/T 14976 standards. Each batch undergoes incoming quality inspection including chemical composition analysis (via Optical Emission Spectroscopy, OES), mechanical property verification, and dimensional measurement.

4.2 Tube Forming and Coil Winding

1. Tube Cutting: Precision laser or cold-saw cutting to specified lengths with burr-free end preparation.
2. Tube Forming: CNC-controlled bending to achieve the serpentine coil geometry with controlled bend radii (minimum 3× OD) to prevent wall thinning and ovalization.
3. Coil Winding: Automated winding onto precision fixtures to maintain consistent tube pitch and alignment tolerances within ±0.5 mm.

4.3 Fin-Tube Assembly

1. Fin Insertion: Precision-slit aluminum or stainless steel fins are inserted between tube rows using hydraulic fin-insertion machinery, ensuring uniform fin engagement and minimal tube displacement.
2. Fin-Tube Bonding: Mechanical interference fit supplemented by selective brazing at critical junctions to ensure thermal contact and structural integrity.

4.4 Welding and Joining

1. Header Welding: Inlet and outlet headers are TIG welded to the coil tubes using high-purity argon shielding (99.99% Ar) to prevent oxidation. Weld parameters are monitored in real-time to ensure full penetration and minimum weld volume.
2. Orbital Welding: For high-integrity applications, automated orbital TIG welding is employed to produce repeatable, defect-free welds with uniform heat input.

4.5 Surface Treatment and Passivation

1. Cleaning: Ultrasonic cleaning to remove welding residues and particulate matter.
2. Acid Pickling: Immersion in nitric-hydrofluoric acid solution to remove heat-affected scale and restore chromium oxide passive layer.
3. Passivation: Controlled nitric acid treatment (20% HNO₃ at 60 °C) to establish a uniform, protective chromium oxide film, verified by ferrite detection and red drop test (ASTM A967).

4.6 Final Quality Control

1. Dimensional Inspection: Coordinate Measuring Machine (CMM) verification of coil geometry, tube pitch, and overall envelope dimensions.
2. Non-Destructive Testing (NDT): Dye Penetrant Inspection (DPI) per ASTM E709 or E1659 for surface weld defects; optional Eddy Current Testing (ECT) for subsurface anomalies.
3. Hydrostatic Pressure Testing: 100% of coils subjected to 2.4 MPa water pressure for a minimum 30-second hold time with zero leakage tolerance.
4. Thermal Performance Verification: Sample-based testing on a closed-loop heat transfer test rig to validate capacity and pressure drop against design specifications.

5. Applications

5.1 Air Source Heat Pump Systems (Low ΔT Configuration)

The primary application domain is integration with air source heat pump systems operating at reduced temperature differentials (ΔT = 3–6 °C between supply and return). In this configuration, the stainless steel fan coil tube serves as the terminal heat exchange element, transferring thermal energy from the circulating refrigerant-driven water loop to the indoor air stream. The low-ΔT approach significantly improves the system COP by reducing the lift required of the compressor, while the stainless steel construction ensures long-term reliability without degradation.

5.2 Commercial and Residential Space Heating

• Office buildings and commercial facilities requiring year-round comfortable heating
• Residential high-rise apartments with centralized heat pump distribution
• Hospital and laboratory environments where water quality variability is a concern
• Coastal and marine environments where chloride exposure necessitates 316-grade construction

5.3 Specialized Applications

• Food and pharmaceutical processing facilities requiring hygienic-grade heat exchange (316L with electropolished finish)
• Geothermal and district heating distribution systems
• Cooling tower and dehumidification applications in high-humidity environments
• Industrial process heating with aggressive working fluids

6. Quality Certifications and Compliance

6.1 Material Certifications

• Mill Test Certificates: EN 10204 Type 3.1 for each batch of stainless steel tube stock
• Material Standards: ASTM A213, ASME SA-213, GB/T 14976, ISO 13971

6.2 Manufacturing Certifications

• ISO 9001:2015: Quality Management System certification for the manufacturing facility
• ISO 14001:2015: Environmental Management System certification
• ISO 45001:2018: Occupational Health and Safety Management System certification
• CE Marking: Conformity with European directives for HVAC components

6.3 Product Performance Certifications

• ASHRAE Standard 34: Rating of Unitary Air-Conditioning and Air-Source Heat Pump Equipment
• ISO 22714: Heat Exchangers — Methods of Performance Test
• ISO 13373: Heat Exchangers — Plate Fin and Tube Heat Exchangers
• GB/T 19232: Chinese national standard for air source heat pump water heaters
• CRAA Certification: China Refrigeration and Air Conditioning Association performance verification

6.4 Third-Party Testing and Verification

• Independent laboratory thermal performance testing (e.g., TUV, SGS, or Intertek)
• Corrosion resistance verification per ASTM G48 (pitting and crevice corrosion, 316 grade)
• Accelerated life testing under simulated operating conditions (minimum 10,000 hours)

7. Unique Selling Points

7.1 Superior Corrosion Resistance

The austenitic stainless steel construction provides inherent resistance to general aqueous corrosion, eliminating the need for chemical water treatment or inhibitors that are typically required with copper or carbon steel alternatives. This translates to reduced maintenance costs, extended service intervals, and reliable performance in applications with variable water quality.

7.2 Low-ΔT Optimization

Specifically engineered for small temperature differential operation, the coil geometry and fin configuration are optimized to maintain high heat transfer coefficients even at reduced ΔT conditions. This design philosophy aligns with the thermodynamic advantages of low-ΔT heat pump systems, enabling COP values of 4.0–5.5 in heating mode.

7.3 Full-Length Seamless Construction

Seamless tube construction eliminates the risk of joint failure that can occur with welded tube alternatives, providing a monolithic fluid pathway that ensures leak-free operation throughout the service life of the unit.

7.4 Precision Manufacturing Tolerances

CNC-controlled forming and automated winding ensure dimensional consistency across production batches, enabling reliable integration into modular system architectures and simplifying field installation and replacement.

7.5 Extended Service Life

With proper installation and maintenance, the stainless steel fan coil tube is designed for a service life exceeding 20 years, significantly outperforming conventional materials in aggressive or variable water quality environments.

7.6 Sustainability and Environmental Compliance

Stainless steel is 100% recyclable with no loss of material properties, supporting circular economy principles. The elimination of chemical water treatment reduces environmental impact, and the high-efficiency operation reduces overall energy consumption and carbon footprint of the heating system.

Technical Note: The integration of stainless steel fan coil tubes in air source heat pump systems with small temperature differences represents a paradigm shift in terminal heat delivery technology. By combining the material advantages of austenitic stainless steel with thermodynamically optimized low-ΔT system design, this product achieves a synergistic improvement in both system efficiency and long-term reliability that