Stainless Steel Coil Tube (Coiltube) — Fan Coil Unit Metal Flexible Connector
Product Overview
The Stainless Steel Coil Tube (Coiltube), product reference No. 1229, is a precision-engineered flexible metal connector manufactured by Wenzhou Jixing Metal Materials Co., Ltd. This product is specifically designed for integration into Fan Coil Unit (FCU) HVAC systems, where it serves as a metal flexible joint connecting rigid piping infrastructure to the fan coil unit body. The coiltube design provides exceptional axial flexibility, lateral displacement accommodation, and vibration isolation — critical functions in reducing mechanical stress transmission in building services systems.
Unlike rigid pipe connections, this coil tube product allows controlled relative movement between connected components, thereby preventing fatigue failure at welding joints, minimizing noise propagation through the piping system, and extending the service life of the entire HVAC assembly. The product represents a convergence of metallurgical science, mechanical engineering, and thermal-fluid dynamics.
Material & Properties
Base Material
The coil tube is fabricated from austenitic stainless steel, predominantly conforming to the following grades:
- ASTM A269 / ASME SA-269 — Seamless and welded stainless steel tubing for high-temperature applications
- 304 (UNS S30400) — General-purpose grade with excellent corrosion resistance and formability
- 316 (UNS S31600) — Enhanced molybdenum-bearing grade for aggressive chemical environments
- 316L (UNS S31603) — Low-carbon variant for superior weldability and resistance to intergranular corrosion
Mechanical Properties (Typical for 304 Grade at Room Temperature)
- Tensile Strength (UTS): ≥ 515 MPa (74,700 psi)
- Yield Strength (0.2% offset): ≥ 205 MPa (29,700 psi)
- Elongation (at fracture, 50mm gauge): ≥ 40%
- Hardness (Brinell): ≤ 201 HB
- Density: 8.0 g/cm³
- Thermal Conductivity (at 100°C): 16.3 W/(m·K)
- Thermal Expansion Coefficient (20–100°C): 17.3 × 10⁻⁶ /°C
Corrosion Resistance Characteristics
The passive chromium oxide film (Cr₂O₃) on the austenitic stainless steel surface provides inherent resistance against atmospheric corrosion, dilute acids, and alkaline solutions. In the 316/316L grades, the addition of 2–3% Molybdenum (Mo) significantly enhances pitting and crevice corrosion resistance, making these grades suitable for coastal installations, chemical processing environments, and applications involving chlorinated water.
Specifications
| Parameter | Specification / Range |
|---|---|
| Nominal Diameter (DN) | DN15 to DN150 (½" to 6" NPS) |
| Outer Diameter (OD) | 14 mm – 168 mm (0.55" – 6.61") |
| Wall Thickness (WT) | 0.5 mm – 3.0 mm (0.020" – 0.118") |
| Coil Pitch (Helical Spacing) | 3 mm – 8 mm (adjustable per application) |
| Coil Length (As-Supplied) | Standard: 150 mm / 300 mm / 500 mm / 1000 mm |
| Connection Type | Flanged, Threaded (NPT/BSP), Welded (Socket/Butt), Grooved |
| Operating Temperature Range | −196°C to +600°C (−320°F to +1112°F) |
| Operating Pressure (PN) | PN6 / PN10 / PN16 / PN25 (per EN 1092-1) |
| Maximum Axial Compression | Up to 30% of nominal coil length |
| Maximum Axial Extension | Up to 15% of nominal coil length |
| Maximum Lateral Offset | Up to 10° angular misalignment |
| Surface Finish | Bright annealed, Ra ≤ 1.6 μm (internal); Ra ≤ 0.8 μm (polished option) |
| Applicable Standards | ASTM A269, ASME B31.3, EN 1092, GB/T 14976 |
Manufacturing Process
1. Raw Material Procurement & Incoming Inspection
Stainless steel tubing stock is procured from certified mills and subjected to rigorous incoming quality control, including spectroscopic analysis (OES) for chemical composition verification, mechanical testing per ASTM E8/E8M, and hydrostatic pressure testing per ASTM A269 Section 11.
2. Cutting & Beveling
Tubing is precision-cut to required lengths using CNC plasma or waterjet cutting systems. End faces are beveled at 37.5° ± 1.5° for butt-welded connections or machined to exact thread/flange dimensions per applicable standards.
3. Coiling / Helical Forming
The critical forming operation involves cold-working the straight tube into a helical coil geometry. This is accomplished using either:
- Rotary Draw Coiling: The tube is drawn around a rotating mandrel while being fed axially, producing uniform coil pitch and consistent wall thickness distribution.
- Progressive Die Coiling: Multi-station tooling progressively bends the tube through increasing angles until the helical shape is achieved.
During coiling, the metal undergoes plastic deformation with controlled strain hardening, which increases yield strength locally while maintaining adequate ductility for subsequent service movements. The coil pitch and diameter are maintained within ±0.2 mm tolerance to ensure uniform flexibility characteristics.
4. Annealing & Pickling (if required)
Post-forming annealing at 1050–1100°C (solution treatment) followed by rapid quenching restores full austenitic microstructure, eliminates residual stresses introduced during cold forming, and re-establishes the passive oxide film. Subsequent acid pickling (nitric/hydrofluoric acid blend) removes any surface scale and ensures the passive layer is regenerated.
5. End Connection Fabrication
Connection elements (flanges, threaded couplings, welding fittings) are welded or threaded onto the coil tube ends. TIG (GTAW) welding is the preferred method for stainless steel applications, using argon shielding gas to prevent oxidation of the weld zone. Back-purging with high-purity argon ensures a clean, oxide-free internal weld surface.
6. Final Quality Assurance
Each unit undergoes:
- Dimensional verification (caliper, micrometer, coordinate measuring machine)
- Hydrostatic pressure testing at 1.5× operating pressure for minimum 30 seconds
- Visual and dye penetrant inspection (DPI) of weld joints per ASTM E709
- Corrosion resistance spot testing (electrochemical potential measurement)
Applications
Primary Application: Fan Coil Unit Systems
The principal application domain is the connection between supply/return water piping and the coil section of Fan Coil Units in commercial and industrial HVAC systems. The metal flexible connector accommodates:
- Thermal expansion and contraction of piping systems (ΔT up to 60°C)
- Vibration isolation from fan motors and building structural dynamics
- Installation tolerance compensation during field assembly
- Seismic displacement in earthquake-prone regions
Secondary Applications
- Chilled Water Distribution Systems: Flexible connections in constant-volume and variable-flow chilled water loops
- Condenser and Evaporator Connections: Linking heat exchangers to piping manifolds in refrigeration plants
- Industrial Process Piping: Vibration-damping connections in chemical processing, pharmaceutical, and food-grade applications
- Marine HVAC Systems: Saltwater-resistant flexible joints in shipboard air conditioning systems
- Power Plant Auxiliary Systems: Flexible connections in cooling water and steam extraction systems
Quality Certifications & Standards Compliance
Wenzhou Jixing Metal Materials Co., Ltd. maintains a comprehensive quality management system with the following certifications:
| Certification / Standard | Scope |
|---|---|
| ISO 9001:2015 | Quality Management System — Design, manufacturing, and supply of stainless steel tubing products |
| ISO 14001:2015 | Environmental Management System |
| ISO 45001:2018 | Occupational Health and Safety Management |
| CE Marking (PED 2014/68/EU) | Pressure Equipment Directive compliance for categories I–III |
| EN 10204 3.1 | Material Test Certificate — Declaration of Compliance |
| EN 10204 3.2 | Material Test Certificate — Inspection Certificate (independent third-party inspection) |
| NACE MR0175 / ISO 15156 | Sour service resistance (H₂S-containing environments) |
| GB/T 19001-2016 | Chinese National Standard — Quality Management System |
| ASME Section IX | Welding procedure qualification for pressure-containing components |
Unique Selling Points
- Superior Fatigue Performance: The helical coil geometry distributes cyclic loading uniformly, achieving fatigue life exceeding 10⁶ cycles at design stress amplitude — significantly outperforming rigid connections in vibration-prone environments.
- Multi-Axis Flexibility: Unlike bellows-type flexible connectors, the coil tube design provides simultaneous accommodation of axial, lateral, and angular movements without requiring additional articulation joints.
- Full Temperature Compatibility: Maintains mechanical integrity from cryogenic temperatures (−196°C) to elevated service temperatures (600°C) without loss of flexibility or structural degradation.
- Hygienic Surface Finish: Electropolished internal surfaces (Ra ≤ 0.4 μm available) meet FDA and 3-A Sanitary standards, making the product suitable for pharmaceutical and food processing applications.
- Corrosion-Free Service Life: The austenitic stainless steel construction eliminates the risk of internal corrosion-induced failures that plague carbon steel flexible connectors, providing a design service life exceeding 25 years under normal operating conditions.
- Customizable Geometry: Coil pitch, length, and connection configurations can be tailored to specific installation constraints, enabling optimized performance for each unique application scenario.
Technical Note: The selection of coil pitch and wall thickness must be performed through finite element analysis (FEA) considering the specific thermal expansion loads, pressure ratings, and cyclic loading spectra of the target installation. Wenzhou Jixing Metal Materials Co., Ltd. provides application engineering support for custom design optimization.