Stainless Steel Coil Tube — HVAC Chilled Water System Application
Product Overview
The Stainless Steel Coil Tube (盘管 / Coil Tube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. (Product No. 394) is a precision-engineered coiled tubular product designed for use in heating, ventilation, and air conditioning (HVAC) systems. This product is specifically optimized for deployment in fan coil units (FCU, Fan Coil Unit) operating within large temperature difference (LTD, Large Temperature Difference) chilled water systems, where supply and return water temperature differentials significantly exceed conventional design parameters.
In conventional chilled water systems, the supply water temperature typically ranges from 6–7 °C to a return water temperature of 12–13 °C, yielding a temperature differential (ΔT) of approximately 5–6 °C. However, in large temperature difference systems, the supply water temperature can be reduced to 2–4 °C, with return water temperatures reaching 16–18 °C, producing a ΔT of 12–16 °C. This dramatic increase in temperature differential imposes unique thermal stress, corrosion resistance, and flow efficiency requirements on the coil tube components. The coil tube described herein is engineered to withstand these demanding operational conditions while maintaining long-term structural integrity and heat transfer performance.
Coil tubes are formed by winding straight stainless steel tubing into a continuous helical or serpentine configuration. The resulting geometry maximizes heat exchange surface area within a compact volume, making them ideal for evaporator and condenser applications in fan coil units, air handlers, and unitary air conditioning systems.
Material & Properties
Material Composition
The coil tube is fabricated from austenitic stainless steel, with the following primary grade options:
- SUS 304 / AISI 304 (UNS S30408): The most widely used grade, containing 18% chromium (Cr) and 8% nickel (Ni). Offers excellent general corrosion resistance, good formability, and weldability. Suitable for standard chilled water applications with moderate chemical exposure.
- SUS 316 / AISI 316 (UNS S31608): Enhanced grade with the addition of 2–3% molybdenum (Mo), providing superior resistance to pitting and crevice corrosion. Recommended for applications involving deionized water, glycol-based heat transfer fluids, or environments with elevated chloride concentrations.
- SUS 321 / AISI 321 (UNS S32108): Titanium-stabilized grade offering improved resistance to intergranular corrosion, suitable for applications involving periodic high-temperature exposure during maintenance or system flushing.
Key Material Properties
| Property | SUS 304 | SUS 316 | Test Standard |
|---|---|---|---|
| Tensile Strength (抗拉强度) | ≥ 520 MPa | ≥ 520 MPa | ASTM A270 / ASTM A269 |
| Yield Strength (屈服强度) | ≥ 205 MPa | ≥ 205 MPa | ASTM A270 |
| Elongation (延伸率) | ≥ 40% | ≥ 40% | ASTM A270 |
| Thermal Conductivity (导热系数, 20 °C) | 16.3 W/(m·K) | 16.3 W/(m·K) | ASTM A270 |
| Corrosion Resistance — Pitting (点蚀) | Moderate | Excellent | ASTM G48 |
| Operating Temperature Range (工作温度范围) | −196 °C to +870 °C | −196 °C to +870 °C | — |
| Chilled Water ΔT Tolerance (温差耐受) | Up to 18 °C | Up to 18 °C | Manufacturer Specification |
Microstructural Characteristics
The fully austenitic microstructure (face-centered cubic, FCC) ensures the following advantageous characteristics for coil tube applications:
- Non-magnetic (无磁性): Remains essentially non-magnetic in the annealed condition, eliminating electromagnetic interference concerns in HVAC control systems.
- Excellent cold workability (优异冷加工性能): The austenitic matrix accommodates significant plastic deformation without cracking, enabling tight bending radii during coil forming.
- Low thermal expansion coefficient (低热膨胀系数): At approximately 17.3 × 10⁻⁶ /°C, the coefficient minimizes thermal expansion stresses during the large ΔT cycles inherent in LTD systems.
- Passivation capability (钝化能力): The chromium oxide (Cr₂O₃) passive film self-heals in the presence of dissolved oxygen, providing continuous corrosion protection even under dynamic flow conditions.
Specifications
Dimensional Specifications
| Parameter (参数) | Specification (规格) | Tolerance (公差) | Standard (标准) |
|---|---|---|---|
| Outer Diameter (外径, OD) | 6.35 mm, 9.53 mm, 12.70 mm (1/4", 3/8", 1/2") | ±0.10 mm | ASTM A269 / GB/T 12771 |
| Wall Thickness (壁厚, WT) | 0.40 mm, 0.50 mm, 0.80 mm, 1.00 mm | ±10% | ASTM A269 |
| Coil Inner Diameter (盘管内径) | 150 mm – 400 mm | ±5 mm | Manufacturer Specification |
| Bend Radius (弯曲半径) | ≥ 3 × OD (minimum) | — | ASTM A270 |
| Tube Length per Coil (单盘管长度) | 3 m – 50 m (configurable) | ±0.5% | Manufacturer Specification |
| Number of Circuits (回路数) | 1 – 4 (typical) | — | Manufacturer Specification |
| Surface Finish (表面光洁度) | Commercial (光亮) / Pickled & Passivated (酸洗钝化) | Ra ≤ 1.6 μm | ASTM A967 |
| End Preparation (端部处理) | Flat cut / Flared / Grooved (as specified) | — | ASME B16.17 |
Performance Specifications for Large Temperature Difference Systems
| Performance Parameter (性能参数) | Value (数值) | Condition (工况) |
|---|---|---|
| Supply Water Temperature (供水温度) | 2 °C – 7 °C | LTD Mode / Standard Mode |
| Return Water Temperature (回水温度) | 12 °C – 18 °C | LTD Mode / Standard Mode |
| Temperature Differential (温差, ΔT) | 5 °C – 16 °C | System Operating Range |
| Design Pressure (设计压力) | 1.0 MPa – 1.6 MPa | Hydrostatic Test |
| Flow Velocity (流速) | 1.5 m/s – 2.5 m/s | Optimal Heat Transfer |
| Heat Transfer Coefficient (传热系数) | 1,200 – 2,500 W/(m²·K) | Water-side, turbulent flow |
| Pressure Drop (压降) | ≤ 50 kPa (per circuit) | Design flow rate |
Manufacturing Process
The manufacturing of stainless steel coil tubes involves a multi-stage process designed to ensure dimensional accuracy, metallurgical integrity, and corrosion resistance suitable for demanding HVAC applications.
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Raw Material Selection & Inspection (原材料选择与检验):
Stainless steel wire or tube blanks are sourced from certified suppliers with documented chemical composition (符合 ASTM A276 化学成分要求). Incoming material undergoes spectroscopic analysis (光谱分析) to verify Cr, Ni, and Mo content, along with mechanical property verification via tensile testing.
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Tubing Forming (制管):
Stainless steel wire or strip is formed into seamless tubing through cold drawing (冷拔) or cold rolling processes. The drawing process ensures uniform wall thickness and tight dimensional tolerances. For OD ≤ 12.7 mm, the cold drawing route is standard, producing seamless tubes conforming to ASTM A269.
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Heat Treatment & Annealing (热处理与退火):
Tubes undergo solution annealing (固溶退火) at 1,050–1,150 °C followed by controlled cooling to relieve internal stresses introduced during cold working. This restores full austenitic microstructure and optimizes corrosion resistance. The annealing cycle is controlled to prevent grain growth (晶粒长大) and sigma phase (σ相) formation.
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Pickling & Passivation (酸洗钝化):
Following annealing, tubes are pickled in a mixed acid solution (typically nitric acid + hydrofluoric acid) to remove oxide scale. Subsequently, a passivation treatment per ASTM A967 establishes a uniform chromium oxide passive film (钝化膜) on the surface, enhancing corrosion resistance. This step is critical for chilled water service where dissolved oxygen and trace chlorides are present.
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Coil Winding (盘绕成型):
Pre-formed tubing is wound into helical coils using precision coil winding machinery. The winding process maintains a constant bend radius (≥ 3 × OD) to prevent material thinning (减薄) and cracking at the bend apex. Computer-controlled winding ensures uniform pitch (螺距) and consistent coil geometry. For serpentine (蛇形) configurations, the tubing is bent at precise angles using mandrel-assisted bending (芯棒弯管) to maintain cross-sectional shape.
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Hydrostatic Testing (水压试验):
Each coil tube undergoes hydrostatic pressure testing at 1.5 times the design pressure (per ASTM A270) for a minimum hold time of 30 seconds. This verifies the absence of leaks, porosity, and manufacturing defects. Non-conforming units are rejected and scrapped.
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Dimensional Inspection & Non-Destructive Testing (尺寸检验与无损检测):
Finished coils are inspected for dimensional accuracy (OD, wall thickness, coil diameter, pitch) using calibrated gauges and ultrasonic thickness gauges. Visual inspection for surface defects, kinks, and distortion is performed. For critical applications, eddy current testing (涡流检测) or dye penetrant testing (渗透检测) may be applied to detect surface and near-surface flaws.
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End Preparation & Connection Fitting (端部加工与接头装配):
Tube ends are prepared according to the connection method specified by the system designer — including flaring (扩口), grooving (沟槽), threading (螺纹), or brazing preparation (钎焊准备). Stainless steel fittings and transition components are assembled as required.
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Packaging & Shipping (包装与运输):
Finished coil tubes are packaged with protective end caps, wrapped in moisture-barrier film (防潮膜), and secured on wooden pallets (木托盘) or in steel crates (钢箱) to prevent mechanical damage and moisture exposure during transit.
Applications
Primary Application — Large Temperature Difference Chilled Water Fan Coil Units
The primary application domain for this coil tube is within fan coil units (风机盘管机组, FCU) integrated into large temperature difference chilled water systems. In such systems, the chilled water is produced at significantly lower temperatures (2–4 °C) compared to conventional systems (6–7 °C), with correspondingly higher return temperatures (16–18 °C). Key advantages of LTD systems include:
- Reduced pump energy consumption (降低水泵能耗): The increased ΔT allows for lower volumetric flow rates at the same cooling capacity, reducing pump power consumption by 30–50%.
- Smaller pipe diameters (减小管径): Reduced flow rates permit smaller piping, decreasing material costs and installation space.
- Enhanced system efficiency (提高系统能效): The combined effect of lower pumping power and optimized chiller operation yields improved coefficient of performance (COP, 性能系数).
The stainless steel coil tube must withstand the following conditions in this application:
- Thermal cycling (热循环) between 2 °C and 18 °C water temperatures
- Potential for localized low-temperature spots near supply