Stainless Steel Coil Tube (Coiltube) for Fan Coil Unit Applications
Product Overview
The Stainless Steel Coil Tube (Coiltube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered coiled tubing product designed primarily for use in Fan Coil Units (FCUs) within HVAC (Heating, Ventilation, and Air Conditioning) systems. This product represents a critical heat exchange component that facilitates efficient thermal transfer between refrigerant/cooling media and air streams. The coil tube is available in pre-formed spiral configurations, eliminating on-site bending labor and ensuring consistent geometry for optimal airflow distribution.
As a specialized component in hydronic and refrigerant piping systems, this coil tube integrates seamlessly with rubber expansion joints and flexible connectors to accommodate thermal expansion, vibration isolation, and differential movement between rigid pipe sections and equipment housings.
Material Composition and Properties
Primary Material Grades
- AISI 304 (UNS S30400) — General-purpose austenitic stainless steel with excellent corrosion resistance and formability
- AISI 316 (UNS S31600) — Enhanced corrosion resistance grade containing molybdenum, suitable for aggressive chemical environments
- AISI 316L (UNS S31603) — Low-carbon variant of 316 for superior weldability and resistance to intergranular corrosion
- AISI 321 (UNS S32100) — Titanium-stabilized grade for elevated temperature service applications
Mechanical Properties
| Property | Specification | Test Standard |
|---|---|---|
| Tensile Strength (UTS) | ≥ 520 MPa (75,400 psi) | ASTM A213 / EN 10216-5 |
| Yield Strength (0.2% offset) | ≥ 205 MPa (29,700 psi) | ASTM A213 / EN 10216-5 |
| Elongation at Break | ≥ 40% | ASTM A213 / EN 10216-5 |
| Hardness (Rockwell B) | ≤ 92 HRB | ASTM E18 |
| Thermal Conductivity (at 20°C) | 14.6 – 16.3 W/(m·K) | ASTM E1461 |
| Thermal Expansion Coefficient | 16.0 – 18.0 × 10⁻⁶ /°C | ASTM E228 |
Chemical Composition (AISI 304 — Typical)
| Element | Minimum (%) | Maximum (%) |
|---|---|---|
| Chromium (Cr) | 18.0 | 20.0 |
| Nickel (Ni) | 8.0 | 10.5 |
| Carbon (C) | — | 0.08 |
| Manganese (Mn) | — | 2.0 |
| Silicon (Si) | — | 1.0 |
| Phosphorus (P) | — | 0.045 |
| Sulfur (S) | — | 0.030 |
Dimensions and Specifications
| Parameter | Specification / Range | Notes |
|---|---|---|
| Outer Diameter (OD) | 6.35 mm – 34.93 mm (¼" – 1⅜") | Custom sizes available upon request |
| Wall Thickness (WT) | 0.50 mm – 2.00 mm (0.020" – 0.079") | Selected per pressure rating requirements |
| Coil Diameter (CD) | 150 mm – 600 mm (5.9" – 23.6") | Adaptable to FCU housing dimensions |
| Coil Length (Uncoiled) | 1.0 m – 12.0 m (3.3 ft – 39.4 ft) | Configurable per unit design |
| Number of Turns | 2 – 12 turns | Determined by heat exchange duty |
| Operating Temperature Range | −40°C to +350°C (−40°F to +662°F) | AISI 304; extended range with 321 |
| Maximum Working Pressure | ≤ 2.5 MPa (363 psi) | Dependent on OD and WT combination |
| Surface Finish | Mirror polish / Electropolished / Brushed | Ra ≤ 0.4 μm for mirror finish |
| End Preparation | Flat cut, beveled, or threaded (BSP/NPT) | Compatible with rubber joint couplings |
| Tolerance on OD | ± 0.10 mm (± 0.004") | Per EN 10216-5 Class B |
| Tolerance on Wall Thickness | ± 10% of nominal | Per ASTM A213 |
Manufacturing Process
- Raw Material Selection and Inspection: High-purity austenitic stainless steel wire rod or seamless pipe blanks are sourced from certified mills. Incoming material undergoes spectroscopic analysis (OES — Optical Emission Spectroscopy) to verify chemical composition compliance with ASTM A213 / EN 10216-5 requirements.
- Tubing Forming: Seamless tubing is produced via cold drawing or hot-rolled and cold-finished processes to achieve precise dimensional tolerances. For smaller diameters, seamless tube is cold-drawn through precision dies to ensure uniform wall thickness and surface integrity.
- Coil Forming (Spiral Bending): The straight tubing is fed through a CNC-controlled (Computer Numerical Control) coil bending machine with precision die sets. The bending process employs controlled strain rates to prevent work hardening, cracking, or ovality exceeding 5% of nominal OD. Multi-axis forming ensures consistent coil diameter and inter-turn spacing.
- Heat Treatment (Annealing): Post-forming annealing is conducted at 1050°C – 1150°C followed by controlled water quenching to restore full solution-annealed condition, relieve residual stresses introduced during coiling, and ensure maximum ductility and corrosion resistance.
- Surface Treatment: Depending on application requirements, the coil tube receives electropolishing, passivation (per ASTM A967), or mirror polishing to achieve surface roughness values as low as Ra 0.2 μm. Passivation removes surface contaminants and enhances the passive chromium oxide layer.
- End Processing and Fitting Preparation: Coil ends are cut to specified lengths with clean, burr-free edges. Threaded ends are machined to BSPP (British Standard Pipe Parallel) or NPT (National Pipe Thread) standards for compatibility with rubber expansion joints and flexible connectors.
- Quality Inspection: Each coil tube undergoes hydrostatic pressure testing (per ASTM A269), dimensional verification via laser micrometry, visual inspection of surface finish, and optional eddy current testing (ECT) for wall thickness uniformity and defect detection.
- Packaging and Traceability: Finished coils are protected with anti-corrosion packaging, labeled with unique batch identification codes, and accompanied by Mill Test Certificates (MTC) for full material traceability.
Applications
Primary Application: Fan Coil Units (FCUs)
The stainless steel coil tube serves as the primary heat exchange element in Fan Coil Units used in commercial and residential HVAC systems. The coiled configuration maximizes heat transfer surface area within a compact envelope, enabling efficient cooling and heating of air streams passing over the coil. Integration with rubber expansion joints at the coil tube inlet and outlet connections accommodates thermal expansion, absorbs mechanical vibration from the fan assembly, and prevents stress transmission to rigid piping.
Additional Applications
- Air Conditioning Heat Exchangers: Evaporator and condenser coils in split-system and packaged air conditioning units
- Refrigeration Systems: Evaporator coils in commercial refrigeration equipment and cold storage units
- Process Cooling: Heat exchange coils in food processing, pharmaceutical, and chemical manufacturing equipment
- Condensing Coils: Refrigerant condensing coils in industrial heat pump systems
- Hydronic Heating Systems: Hot water circulation coils in radiator and underfloor heating applications
- Marine HVAC: Corrosion-resistant coil assemblies for shipboard air conditioning systems (AISI 316 grade)
Integration with Rubber Expansion Joints
The coil tube is designed for seamless integration with rubber expansion joints (flexible connectors) at connection points. This combination provides:
- Vibration Isolation: Damping of fan-induced and mechanical vibrations to reduce noise transmission
- Thermal Expansion Accommodation: Compensation for differential thermal movement between coil tube and rigid piping (expansion up to ±25 mm axial, ±15 mm lateral)
- Alignment Tolerance: Accommodation of installation misalignment without inducing residual stresses
- Pressure Wave Damping: Attenuation of hydraulic hammer and pressure surges in hydronic circuits
Unique Selling Points
- Superior Corrosion Resistance: Austenitic stainless steel construction provides exceptional resistance to oxidation, scaling, and corrosion from condensation, refrigerants, and humid air — eliminating the need for protective coatings and ensuring service life exceeding 25 years in typical HVAC environments.
- Precision Coil Geometry: CNC-controlled forming ensures consistent coil diameter, uniform turn spacing, and minimal ovality — resulting in optimized airflow distribution and maximum heat transfer efficiency compared to hand-formed alternatives.
- Full Traceability: Each coil tube is accompanied by a complete Mill Test Certificate (MTC) with chemical analysis, mechanical test data, and heat treatment records, satisfying the most stringent quality assurance requirements.
- Custom Configurability: Flexible production capabilities allow customization of OD, wall thickness, coil diameter, number of turns, end preparation, and surface finish to meet specific OEM design requirements.
- Hygienic Surface Finish: Electropolished and mirror-finished surfaces with Ra ≤ 0.4 μm prevent fouling, facilitate cleaning, and comply with sanitary requirements for food-grade and pharmaceutical applications.
- Integrated System Compatibility: Designed for direct compatibility with standard rubber expansion joints, threaded fittings, and FCU mounting brackets — reducing installation complexity and field assembly time.
Quality Certifications and Compliance Standards
Product Standards
| Standard | Description | Scope |
|---|---|---|
| ASTM A213 | Standard Specification for Seamless Austenitic Stainless Steel Tubing for High-temperature Service | Material and mechanical requirements |
| ASTM A269 | Standard Specification for Seamless and Welded Austenitic Stainless Steel Tubing for Chemical and Other Severely Corrosive Environments | Corrosion resistance and hydrostatic testing |
| EN 10216-5 | Non-alloy and Alloy Steel Tubes — Technical Delivery Conditions — Part 5: Non-alloy, Fine Grain and Alloy Cold-Drawn Tubes | Dimensional tolerances and delivery condition |
| ISO 3506 | Stainless Steel Tubing — Dimensions and Tolerances | Dimensional compliance |
| GB/T 12771 | Stainless Steel Seamless Tubes for Fluid Transport (Chinese National Standard) | Domestic market compliance |