Stainless Steel Coil Tube for Air-Source Heat Pump Fan Coil Systems
Product Code: 330 | Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd. | Classification: Technical Literature & Engineering Reference
1. Product Overview
The stainless steel coil tube (盘管, Coiltube) is a precision-engineered heat exchange component manufactured by Wenzhou Jixing Metal Materials Co., Ltd. Specifically designed for integration into Air-Source Heat Pump (ASHP) systems and Fan Coil Units (FCUs), this product serves as the critical thermal transfer medium within closed-loop refrigerant or water circulation circuits. The coil tube is formed into helical, serpentine, or spiral geometries to maximize surface-area-to-volume ratios, thereby optimizing heat exchange efficiency under variable operating conditions.
Grounded in advanced research on fan coil system optimization strategies, this product line reflects state-of-the-art understanding of thermodynamic performance in air-source heat pump architectures. The coil tube design accounts for variable refrigerant flow rates, pressure drop minimization, and thermal conductivity maximization across a wide operating temperature spectrum.
2. Material & Properties
2.1 Base Material Composition
The coil tube is manufactured from high-grade Austenitic Stainless Steel, conforming to international metallurgical standards. The primary alloy designations include:
- AISI 304 (UNS S30400): Chromium (18–20%), Nickel (8–10.5%), with excellent general corrosion resistance and formability.
- AISI 316 (UNS S31600): Chromium (16–18%), Nickel (10–14%), Molybdenum (2–3%), offering superior resistance to chlorides and aggressive chemical environments.
- AISI 316L (UNS S31603): Low-carbon variant of 316, ideal for welded assemblies requiring enhanced intergranular corrosion resistance.
2.2 Key Physical Properties
| Property | Specification | Relevance to Application |
|---|---|---|
| Thermal Conductivity (λ) | 14.9–16.3 W/(m·K) | Ensures efficient heat transfer between refrigerant/water and ambient air |
| Specific Heat Capacity (cp) | 500 J/(kg·K) | Contributes to thermal mass stability during transient load cycles |
| Density (ρ) | 7,900–8,000 kg/m³ | Determines structural weight and mechanical load on mounting brackets |
| Thermal Expansion Coefficient (α) | 16–18 × 10⁻⁶ /K | Must be accommodated in system design to prevent thermal stress fatigue |
| Operating Temperature Range | −196 °C to +870 °C (304); −196 °C to +870 °C (316) | Covers full ASHP operating envelope including defrost cycles |
| Maximum Operating Pressure | 1.6–4.0 MPa (depending on wall thickness) | Supports high-pressure refrigerant circuits (R410A, R32, R134a) |
2.3 Corrosion Resistance
The austenitic stainless steel matrix provides exceptional resistance to:
- Atmospheric Corrosion: Suitable for outdoor ASHP installations exposed to humid and polluted environments.
- Chloride-Ion Pitting: 316/316L grades resist pitting in coastal or de-icing salt environments.
- Intergranular Corrosion: Low-carbon grades prevent sensitization during welding and high-temperature service.
- Galvanic Corrosion: Non-magnetic austenitic structure eliminates galvanic coupling with aluminum fins.
3. Technical Specifications
| Parameter | Specification / Range | Standard Reference |
|---|---|---|
| Outer Diameter (OD) | 6.0 mm – 12.7 mm (¼" – ½") | ASME B36.19M |
| Wall Thickness (WT) | 0.5 mm – 1.5 mm | ASTM A269 / ASTM A213 |
| Inner Diameter (ID) | 4.0 mm – 10.7 mm (derived) | — |
| Coil Pitch | 15 mm – 50 mm (adjustable per system design) | Manufacturer-specific |
| Coil Diameter | 100 mm – 600 mm | Manufacturer-specific |
| Tube Length (per coil) | 3 m – 30 m (continuous) | — |
| Surface Finish | Bright Annealed (BA) or Pickled & Passivated | ASTM A478 |
| Tolerance on OD | ±0.1 mm (precision grade) | GB/T 14976 |
| Formability | ≥ 30% elongation (tensile test) | ASTM A370 |
| Pressure Test | Hydrostatic test at 1.5× working pressure, 30 min hold | ASME BPV Code Section VIII |
4. Manufacturing Process
The production of stainless steel coil tubes follows a rigorously controlled multi-stage process to ensure dimensional accuracy, metallurgical integrity, and surface quality:
- Raw Material Procurement: Cold-rolled stainless steel strip is sourced from certified mills with full mill test certificates (MTC) per EN 10204 Type 3.1.
- Tube Forming (Seam Welding): Strip is formed into round cross-section tubes via high-frequency induction welding (HF welding) or orbital TIG welding. Weld seam quality is verified by ultrasonic testing (UT) per ASTM E165.
- Pipe Drawing & Sizing: Tubes undergo cold drawing through precision dies to achieve exact OD and wall thickness tolerances. Multi-pass drawing ensures uniform microstructure and eliminates residual stresses.
- Coil Forming (Bending): The straight tube is coiled using CNC-controlled roll forming machines. Bending parameters (die radius, reduction rate) are optimized to prevent ovality, wrinkling, or micro-cracking at the bend apex. Minimum bend radius is maintained at ≥ 3× OD.
- Heat Treatment (Solution Annealing): Coiled tubes are solution-annealed at 1,050–1,100 °C followed by controlled cooling to restore full austenitic structure, relieve bending-induced residual stresses, and maximize corrosion resistance.
- Pickling & Passivation: Surface oxidation and residual contaminants are removed via controlled acid pickling (nitric/hydrofluoric acid blend). A passive chromium oxide layer (Cr₂O₃) is then established to enhance corrosion resistance.
- Non-Destructive Testing (NDT): 100% inline inspection includes eddy current testing (ECT) for wall thickness verification, hydrostatic pressure testing, and visual inspection of weld seams.
- Final Coiling & Packaging: Tubes are wound onto protective cores and packaged in moisture-barrier wrapping with desiccant to prevent post-processing contamination.
5. Applications
5.1 Air-Source Heat Pump Systems
The primary application domain for this coil tube is the evaporator and condenser heat exchangers within air-source heat pump units. In these systems, the coil tube circulates refrigerant (R410A, R32, or R134a) through a helical coil wrapped with aluminum fins. The stainless steel construction provides:
- Resistance to corrosion from outdoor air pollutants (SO₂, NOₓ, chloride aerosols)
- Long-term dimensional stability under repeated freeze-thaw cycles during defrost operation
- Compatibility with aluminum fin stacks without galvanic degradation
5.2 Fan Coil Units (FCUs)
In fan coil system architectures, the coil tube serves as the indoor heat exchange element. Optimization research on ASHP-FCU systems has demonstrated that stainless steel coil tubes outperform traditional copper tubes in the following respects:
- Corrosion-Free Longevity: Elimination of copper oxidation products that can contaminate indoor air quality (IAQ).
- Reduced Maintenance: No scaling or pitting in closed-loop hydronic systems using treated water.
- Lightweight Construction: Lower overall unit weight facilitates easier installation and reduces structural load on ceiling mounts.
5.3 Additional Applications
- Domestic hot water (DHW) heat exchangers in heat pump water heaters
- Ground-source heat pump (GSHP) borehole heat exchanger coils
- Industrial process cooling loops requiring hygienic-grade materials
- Food and beverage processing equipment (316L grade)
- Marine and coastal HVAC systems (316L grade for chloride resistance)
6. Quality Certifications & Standards Compliance
| Certification / Standard | Description |
|---|---|
| ISO 9001:2015 | Quality Management System certification for manufacturing processes |
| ISO 14001:2015 | Environmental Management System certification |
| CE Marking | Compliance with EU Machinery Directive (2006/42/EC) and Pressure Equipment Directive (PED 2014/68/EU) |
| ASTM A269 | Standard specification for welded austenitic stainless steel tubing |
| ASTM A213 | Standard specification for austenitic stainless steel tube for heat transfer equipment |
| EN 10216-5 | European standard for welded stainless steel tubes for technical uses |
| GB/T 14976 | Chinese national standard for cold-rolled stainless steel seamless tubes |
| NACE MR0175 / ISO 15156 | Suitability for sour service environments (H₂S-containing atmospheres) |
| NSF/ANSI 61 | Health effects standard for materials in contact with potable water (304/316L grades) |
| ASME BPV Code Section VIII | Pressure vessel code compliance for high-pressure heat exchanger applications |
7. Unique Selling Points
- Extended Service Life: Stainless steel coil tubes deliver 2–3× the service life of equivalent copper or carbon steel alternatives in corrosive outdoor environments, reducing total cost of ownership (TCO).
- Hygienic & Food-Safe: 316L grade provides a non-toxic, non-leaching surface suitable for food-grade and potable water applications, eliminating concerns about heavy metal contamination.
- Thermal Optimization: Designed with reference to ASHP-FCU system optimization research, the coil geometry and material selection are calibrated for maximum coefficient of performance (COP) across the full operating range.
- Galvanic Compatibility: Austenitic stainless steel (non-magnetic, low electrical conductivity) prevents galvanic corrosion when in contact with aluminum fins, unlike copper-based alternatives.
- Environmental Sustainability: Stainless steel is 100% recyclable with no loss of material properties, aligning with circular economy principles and green building certifications (LEED, BREEAM).
- Precision Manufacturing: CNC-controlled coil forming ensures ±0.1 mm dimensional accuracy, critical for automated fin-tube assembly lines in high-volume heat exchanger production.
Technical Note: When selecting coil tube specifications for ASHP-FCU system integration, engineers should refer to the optimization parameters established in "Air-Source Heat Pump – Fan Coil System Optimization Strategy Research" (Wenzhou Jixing Metal Materials Co., Ltd. Technical Literature, Ref. No. 330). This document provides validated correlations between coil geometry, tube material properties, and system-level COP performance under variable climate conditions.
Document prepared by Wenzhou Jixing Metal Materials Co., Ltd. — Technical Literature Division. All specifications are subject to revision without notice. Consult the manufacturer for project-specific engineering data and custom fabrication requirements.