Stainless Steel Coil Tube (Coiltube) — Wenzhou Jixing Metal Materials Co., Ltd.
Product Catalog No.: 1583 | Classification: Technical Reference & Performance Documentation
1. Product Overview
The Stainless Steel Coil Tube (hereinafter referred to as "Coiltube") is a precision-engineered continuous-length tubular product manufactured by Wenzhou Jixing Metal Materials Co., Ltd. This product is supplied in a coiled configuration to facilitate downstream processing such as cutting, forming, and assembly into heat exchanger components, particularly for Fan Coil Units (FCUs) and air-conditioning systems. The coil form factor eliminates the need for butt welding during coil-to-heat-exchanger integration, thereby reducing manufacturing defects, improving thermal efficiency, and extending service life under continuous cyclic thermal loading.
This product documentation is informed by rigorous thermal performance testing methodologies, specifically the Room Heat Balance Method (RHB) as defined in international standards for testing fan coil unit thermal performance. The Coiltube is designed to meet the demanding thermal and structural requirements of modern HVAC (Heating, Ventilation, and Air Conditioning) applications.
2. Material & Properties
2.1 Base Material
The Coiltube is fabricated from austenitic stainless steel conforming to recognized international material specifications. The primary material grades utilized are:
- AISI 304 (UNS S30408) — General-purpose grade with excellent corrosion resistance, forming characteristics, and weldability. Suitable for most HVAC and commercial refrigeration applications.
- AISI 316L (UNS S31603) — Low-carbon variant with added molybdenum (Mo), providing superior resistance to pitting and crevice corrosion. Recommended for coastal, marine, or chemically aggressive environments.
- AISI 304L (UNS S30403) — Low-carbon version of 304, offering enhanced resistance to intergranular corrosion during welding operations.
2.2 Mechanical Properties
| Property | Specification (AISI 304) | Specification (AISI 316L) |
|---|---|---|
| Tensile Strength (Rm) | ≥ 520 MPa | ≥ 485 MPa |
| Yield Strength (Rp0.2) | ≥ 205 MPa | ≥ 170 MPa |
| Elongation (A50mm) | ≥ 40% | ≥ 40% |
| Hardness (HBW) | ≤ 217 | ≤ 217 |
| Density (ρ) | 7.93 g/cm³ | 8.00 g/cm³ |
| Thermal Conductivity (λ) @ 100°C | 16.3 W/(m·K) | 16.3 W/(m·K) |
| Specific Heat (cp) @ 100°C | 0.503 kJ/(kg·K) | 0.503 kJ/(kg·K) |
| Thermal Expansion Coefficient (α) 0–100°C | 17.3 × 10⁻⁶ /K | 17.3 × 10⁻⁶ /K |
2.3 Chemical Composition
| Element | AISI 304 (wt%) | AISI 316L (wt%) |
|---|---|---|
| Carbon (C) | ≤ 0.08 | ≤ 0.03 |
| Chromium (Cr) | 18.0 – 20.0 | 16.0 – 18.0 |
| Nickel (Ni) | 8.0 – 10.5 | 10.0 – 14.0 |
| Molybdenum (Mo) | — | 2.0 – 3.0 |
| Manganese (Mn) | ≤ 2.0 | ≤ 2.0 |
| Silicon (Si) | ≤ 1.0 | ≤ 1.0 |
| Sulfur (S) | ≤ 0.03 | ≤ 0.03 |
2.4 Corrosion Resistance
The austenitic microstructure of the Coiltube provides inherent resistance to atmospheric corrosion, aqueous corrosion, and oxidation at elevated temperatures. The chromium content facilitates the formation of a self-healing passive oxide film (Cr₂O₃), which acts as a primary barrier against environmental degradation. In the case of AISI 316L, the molybdenum addition significantly enhances resistance to chloride-induced pitting corrosion, making it suitable for applications involving de-icing salts or marine air exposure.
3. Specifications
3.1 Geometric Dimensions
| Parameter | Specification | Tolerance |
|---|---|---|
| Outer Diameter (OD) | 3.0 mm – 12.7 mm (customizable) | ± 0.05 mm |
| Wall Thickness (WT) | 0.3 mm – 1.5 mm | ± 0.03 mm |
| Coil Inner Diameter (CID) | 300 mm – 1000 mm | ± 10 mm |
| Coil Length per Spool | 50 m – 500 m (configurable) | ± 2 m |
| Surface Finish | Bright Annealed (BA) / No. 4 Brushed | — |
| End Condition | Plain / Flared / Threaded | — |
3.2 Performance Specifications
| Parameter | Value / Range |
|---|---|
| Maximum Operating Temperature | −196°C to +870°C (AISI 304); −196°C to +870°C (AISI 316L) |
| Maximum Operating Pressure | Up to 2.5 MPa (design-dependent) |
| Cyclic Thermal Fatigue Endurance | ≥ 10⁶ cycles (−20°C to +150°C) |
| Formability (Cold Bending Radius) | ≥ 3D (where D = outer diameter) |
| Hydrostatic Test Pressure | 1.5 × maximum working pressure |
4. Manufacturing Process
The production of the Stainless Steel Coil Tube follows a tightly controlled multi-stage manufacturing workflow designed to ensure dimensional accuracy, material integrity, and consistent surface quality:
- Raw Material Procurement & Incoming Inspection — Stainless steel strip or rod material is sourced from certified mills and subjected to spectrographic analysis (Optical Emission Spectrometry, OES) to verify chemical composition compliance. Mill test certificates are cross-referenced with purchase specifications.
- Strip Coiling & Pickling — The raw strip is coiled and undergoes acid pickling (HNO₃ + HF blend) to remove mill scale, oxide layers, and surface contaminants, exposing the clean base metal surface.
- Forming & Seaming — The strip is fed through a progressive roll-forming die set. Precision rollers incrementally shape the strip into a tubular cross-section. The longitudinal seam is closed using either a double-lock seam or a resistance-welded (butt-welded) configuration, depending on the pressure rating requirements.
- Pneumatic or Hydraulic Expansion (Mandrel Drawing) — The formed tube passes through a series of calibrated mandrel-die stations that expand the tube to the target outer diameter and wall thickness. This step corrects any ovality introduced during forming and ensures uniform wall thickness around the circumference.
- Continuous Annealing — The tube is passed through a controlled-atmosphere annealing furnace (H₂ or N₂ atmosphere) at temperatures between 1050°C and 1150°C, followed by controlled cooling. This process relieves internal stresses, restores full austenitic microstructure, and achieves the specified mechanical properties.
- Surface Treatment & Passivation — Post-annealing, the tube is subjected to a final pickling and passivation treatment to remove any furnace scale and to establish the protective chromium oxide passive film. Surface roughness (Ra) is maintained at ≤ 0.8 μm for bright annealed finish.
-
Non-Destructive Testing (NDT) — Every coil is inspected using:
- Electromagnetic Eddy Current Testing (ECT) for wall thickness uniformity and seam integrity.
- Visual Inspection (VI) for surface defects, scratches, and discoloration.
- Dimensional Verification using laser diameter gauges and ultrasonic thickness gauges at intervals not exceeding 10 meters.
- Coiling & Packaging — The finished tube is coiled onto a precision-wound coil with controlled tension to prevent deformation. Each coil is wrapped in moisture-proof polyethylene film, secured with steel strapping, and packaged on wooden pallets for shipment.
5. Thermal Performance Testing — Room Heat Balance Method
As documented in the referenced technical literature, the thermal performance of Fan Coil Units (FCUs) incorporating this Coiltube is validated using the Room Heat Balance Method (RHB), a methodology aligned with ISO 13256 and ASHRAE Standard 16 (formerly AHRI Standard 400).
5.1 Methodology Overview
The RHB method evaluates the total thermal capacity of a fan coil unit by measuring the net heat gain or loss of the entire test room containing the unit under test. The approach involves:
- Constructing a thermally insulated test room with minimal heat leakage through walls, floors, and ceilings.
- Installing the fan coil unit (containing the Coiltube-based heat exchanger) inside the test room.
- Simultaneously measuring the electrical power input to all room loads (heating/cooling coils, fans, lighting) and the room's heat exchange with the ambient environment.
- Calculating the net thermal capacity as the difference between total heat input and room envelope heat transfer.
5.2 Governing Equation
The total capacity (Q) of the fan coil unit is determined by:
Q = Σ Pelectrical − Qenvelope + Qfiltration
Where:
- Pelectrical = Sum of all electrical power inputs within the test room (W)
- Qenvelope = Heat transfer through the room envelope (W), calculated as: Qenvelope = U × A × (Troom − Tambient)
- Qfiltration = Heat transfer through air infiltration (W), calculated as: Qfiltration = cp × ρ × n × V × (Troom − Tambient)
5.3 Relevance to Coiltube Performance
The Coiltube serves as the primary heat transfer surface within the fan coil unit's coil assembly. Its thermal conductivity, surface finish, and geometric configuration directly influence the overall heat exchanger efficiency. The RHB testing validates that the Coiltube-based coil assembly achieves the rated capacity under standardized conditions, ensuring that the product meets or exceeds manufacturer-published performance claims. Key performance indicators derived from RHB testing include:
- Total Capacity (W) — Combined sensible and latent heat transfer rate.
- Sensible Heat Ratio (SHR) — Ratio of sensible to total heat transfer, indicating dehumidification capability.
- Temperature Approach (°C) — Difference between air leaving the coil and the refrigerant/water coil temperature.
- Air-Side Pressure Drop (Pa) — Energy efficiency indicator for fan motor sizing.
6. Applications
The Stainless Steel Coil Tube from Wenzhou Jixing Metal Materials Co., Ltd. is deployed across a broad spectrum of industries where corrosion resistance, thermal efficiency, and mechanical durability are paramount:
- HVAC & Air Conditioning Systems — Primary application. Used as the coil tube in fan coil units, air handlers, rooftop units, and split-system air conditioners. The coil tube forms the heat exchange circuit that transfers thermal energy between refrigerant (or chilled/hot water) and process air.
- Commercial