Stainless Steel Coil Tube — High-Temperature Fan Coil Unit Application Series

Product Overview

The Stainless Steel Coil Tube (SSCT) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered coiled piping product designed specifically for heat exchange applications within fan coil unit (FCU) systems. This product series has been rigorously validated under high-temperature operating conditions, with performance parameters derived from extensive experimental analysis of traditional fan coil unit thermal behavior. The coil tube serves as the primary heat transfer medium in air-handling systems, where it interfaces with refrigerant or hot water circuits to achieve efficient thermal energy exchange between the fluid medium and ambient air streams.

Based on comprehensive operational data analysis of conventional fan coil units under elevated temperature scenarios, this product line is engineered to maintain structural integrity and thermal conductivity performance at sustained operating temperatures exceeding standard design limits, making it suitable for demanding industrial and commercial HVAC (Heating, Ventilation, and Air Conditioning) environments.

Material and Properties

Base Material Composition

• Primary Alloy Grades: Austenitic stainless steel conforming to AISI 304 (UNS S30400) and AISI 316L (UNS S31603) specifications, with selective availability in duplex grades such as 2205 (UNS S31803) for high-temperature service.
• Carbon Equivalent: ≤ 0.08% (304 grade) / ≤ 0.03% (316L grade), ensuring resistance to intergranular corrosion during welding and thermal cycling.
• Chromium (Cr): 18.0–20.0% — provides passive oxide layer formation and general corrosion resistance.
• Nickel (Ni): 8.0–11.0% — stabilizes austenitic microstructure at elevated temperatures.
• Molybdenum (Mo): 2.0–3.0% (316L grade) — enhances pitting and crevice corrosion resistance in chloride-containing environments.

Key Mechanical and Thermal Properties

• Tensile Strength: ≥ 515 MPa at room temperature; retains ≥ 420 MPa at 300°C operating conditions.
• Yield Strength (0.2% Offset): ≥ 205 MPa at ambient; ≥ 160 MPa at 250°C.
• Elongation: ≥ 40% (as-rolled condition), ensuring adequate ductility for coil forming operations.
• Thermal Conductivity: 16.3 W/(m·K) at 100°C; 19.2 W/(m·K) at 300°C — favorable for heat exchanger duty performance.
• Thermal Expansion Coefficient: 17.3 × 10⁻⁶ /K (20–300°C range).
• Maximum Continuous Service Temperature: 870°C (304 grade) / 870°C (316L grade).
• Cold Formability: Excellent — permits tight bend radii without cracking or excessive work hardening.

Technical Specifications

Parameter	Specification	Standard / Test Method
Outer Diameter (OD)	6.0 mm – 12.7 mm (standard); 15.9 mm – 33.4 mm (special order)	ASME B36.19M
Wall Thickness	0.5 mm – 2.0 mm (tolerance ±10%)	ASTM A213 / A269
Coil Length (Continuous)	50 m – 500 m per coil (configurable)	Manufacturer standard
Coil Inner Diameter	300 mm – 600 mm (standard); up to 1200 mm (custom)	—
Bend Radius (Minimum)	≥ 3D (three times outer diameter)	ASTM A213
Surface Finish	2B (cold-rolled, annealed, pickled); BA (Bright Annealed) available	ASTM A480
Surface Roughness (Ra)	≤ 0.4 μm (BA finish); ≤ 0.8 μm (2B finish)	ISO 4287
Heat Treatment Condition	Solution annealed at 1040–1120°C, followed by water quench	ASTM A213
Pressure Rating (Hydrostatic Test)	2.0 – 6.4 MPa (depending on wall thickness and diameter)	ASME B31.3
Operating Temperature Range	−196°C to +450°C (continuous); up to 870°C (intermittent)	ASME BPVC Section VIII
Corrosion Resistance	Pitting Resistance Equivalent Number (PREN) ≥ 19 (304) / ≥ 24 (316L)	ISO 11445
Coating / Passivation	Passivated per ASTM A967; optional PTFE or ceramic internal coating for high-temperature service	ASTM A967 / A380

Manufacturing Process

1. Raw Material Procurement and Incoming Inspection: Stainless steel strip or seamless tube stock is sourced from certified mill suppliers. Each batch undergoes spectrographic verification (OES — Optical Emission Spectroscopy) for chemical composition conformity, along with mechanical property testing per ASTM E8/E8M.
2. Coil Forming (Cold Bending): Continuous tube stock is fed through precision multi-roll bending machines at controlled feed rates. The bending process is executed at ambient temperature with the minimum bend radius maintained at ≥ 3D to prevent ovalization exceeding 5% and to avoid microstructural damage. Hydraulic tension control ensures uniform curvature throughout the coil length.
3. Post-Forming Annealing: For applications requiring restored ductility after forming, the coiled tube undergoes solution annealing at 1040–1120°C in a protective atmosphere (endothermic gas or vacuum) followed by controlled cooling to relieve residual stresses introduced during cold forming.
4. Pickling and Passivation: The coil tube is subjected to acid pickling (mixed nitric-hydrofluoric acid solution) to remove scale and surface contamination, followed by passivation treatment per ASTM A967 to establish a chromium-rich oxide passive film that enhances corrosion resistance.
5. Hydrostatic Pressure Testing: Each coil is pressure-tested at 1.5 times the maximum design operating pressure using deionized water. The test duration is maintained for a minimum of 60 seconds with zero leakage permitted. Test records are retained for traceability.
6. Dimensional Inspection and Non-Destructive Testing (NDT): Outer diameter, wall thickness, and coil geometry are verified using laser scanning and ultrasonic thickness gauging. Eddy current testing (ET) per ASTM E309 is performed to detect surface and near-surface defects including cracks, pits, and inclusions.
7. Packaging and Dispatch: Coils are wrapped with moisture-barrier packaging, secured on wooden or steel pallets, and labeled with full traceability documentation including heat number, certification, and dimensional data.

Applications

Primary Application Domains

• Fan Coil Units (FCU) — High-Temperature Operation: The coil tube serves as the heat exchange element in commercial and industrial fan coil units where supply water temperatures may reach 85–95°C in heating mode. Analysis of traditional FCU high-temperature operating parameters demonstrates that stainless steel coil tubes maintain heat transfer efficiency within acceptable degradation limits (< 5% reduction) over extended service periods, outperforming carbon steel alternatives in terms of corrosion resistance and thermal stability.
• Chilled Water Distribution Systems: Used in secondary loop piping for air conditioning systems where anti-corrosion performance is critical, particularly in systems utilizing glycol-based heat transfer fluids.
• Process Heating and Cooling: Applicable in food and beverage processing, pharmaceutical manufacturing, and chemical processing where sanitary requirements and high-temperature operation (up to 450°C continuous) are mandated.
• Marine and Coastal HVAC Systems: 316L grade coil tubes provide superior resistance to chloride-induced pitting corrosion in coastal environments where HVAC units are exposed to salt-laden air.
• Geothermal Heat Exchange Systems: Suitable for ground-source heat pump applications where coil tubes are embedded in geothermal boreholes and subjected to long-term thermal cycling.
• Refrigeration Condenser and Evaporator Coils: Stainless steel coil tubes offer extended service life in ammonia-based and CO₂ (transcritical) refrigeration systems where copper alternatives suffer from stress corrosion cracking.

Performance Under High-Temperature Conditions

Experimental analysis of traditional fan coil units operating under elevated temperature conditions (supply water temperature ≥ 75°C, ambient return air ≥ 30°C) confirms that stainless steel coil tubes exhibit stable thermal performance with minimal fouling tendency compared to bare copper or carbon steel alternatives. The passive oxide layer formed on the stainless steel surface acts as a self-healing barrier, reducing scale formation rates by approximately 60–70% relative to unprotected ferrous surfaces. This translates to sustained heat transfer coefficients (U-values) within 95% of initial design values even after 10,000+ hours of continuous operation.

Quality Certifications and Standards Compliance

Product Certifications

• ISO 9001:2015 — Quality Management System certification for manufacturing processes.
• ISO 14001:2015 — Environmental Management System certification.
• EN 10204 3.1 — Mill Test Certificate (MTC) issued with each batch, providing full chemical and mechanical property documentation.
• CE Marking — Conformity with European directives for pressure equipment (PED 2014/68/EU) where applicable.
• ASME Stamp — Manufacturer's registration for pressure-containing components (Section VIII, Division 1).

Applicable International Standards

• ASTM A213 / A213M — Standard Specification for Seamless Austenitic Stainless Steel Tubing for High-temperature Service.
• ASTM A269 / A269M — Standard Specification for Seamless and Welded Austenitic Stainless Steel Pipe, Tubing, and Rod for Chemical and Similar Severely Corrosive Uses.
• EN 10216-5 — Non-alloy and alloy steel tubes for technical uses — austenitic stainless steel tubes for pressure purposes.
• ASME B36.19M — Welded and Seamless Wrought Stainless Steel Pipe.
• GB/T 14976 (Chinese National Standard) — Seamless stainless steel tubes for general use.
• JIS G3459 — Standard for seamless and welded austenitic stainless steel tubing.

Unique Selling Points

• Validated High-Temperature Performance: Backed by comprehensive operational parameter analysis of fan coil units under extreme thermal conditions, providing quantified performance data for engineering design confidence.
• Superior Corrosion Resistance: The austenitic stainless steel matrix with chromium-molybdenum alloying provides outstanding resistance to scaling, pitting, and general corrosion in both heating and cooling water circuits.
• Extended Service Life: Designed for a minimum service life of 25 years in HVAC applications without degradation of thermal efficiency, significantly outperforming carbon steel coil alternatives (typical service life: 8–12 years).
• Hygienic Surface Finish: Available in BA (Bright Annealed) finish with Ra ≤ 0.4 μm, suitable for food-grade and pharmaceutical applications without additional passivation requirements.
• Customizable Configuration: Flexible coil length, diameter, and wall thickness options to accommodate diverse system designs and installation constraints.
• Full Traceability: Every coil is accompanied by complete mill documentation including heat analysis, mechanical test results, and NDT reports, ensuring full material traceability from mill to installation.

Technical Support and After-Sales Service

Wenzhou Jixing Metal Materials Co., Ltd. provides comprehensive technical support including: thermal performance modeling for specific application scenarios, installation guidance documentation, and periodic performance verification recommendations. The engineering team is available for consultation on system design optimization, material selection for specific operating conditions, and troubleshooting of thermal efficiency degradation in existing installations.