Stainless Steel Coil Tube for HVAC Fan Coil Unit Water Systems

Product Overview

The stainless steel coil tube manufactured by Wenzhou Jixing Metal Materials Co., Ltd. (Product No. 1499) is a precision-engineered heat exchanger tube designed specifically for air conditioning fan coil unit (FCU) water systems. These coil tubes serve as the core heat transfer component within hydronic air distribution systems, facilitating efficient thermal exchange between circulating water and ambient air. The product is engineered to meet the rigorous demands of modern building services engineering, addressing critical issues in FCU water system design and construction including water hammer, air locking, thermal stratification, and pressure balance.

Coil tubes in fan coil unit applications are subject to continuous cyclic thermal loading, hydrodynamic stress, and chemical exposure from treated water systems. The stainless steel coil tube from Wenzhou Jixing Metal Materials is formulated to provide exceptional resistance to corrosion, scaling, and mechanical fatigue under these demanding operating conditions, ensuring long-term reliability and energy efficiency in HVAC installations.

Material and Properties

The coil tube is manufactured from high-grade austenitic stainless steel, selected for its superior combination of mechanical strength, corrosion resistance, and thermal conductivity. The material composition and resulting properties are as follows:

• Base Material: Austenitic stainless steel conforming to GB/T 4237 (Chinese National Standard) and ASTM A213/A269 (American Society for Testing and Materials)
• Primary Alloy Grades: Type 304 (UNS S30400), Type 304L (UNS S30403), Type 316L (UNS S31603)
• Chromium Content: 18.0%–20.0% minimum
• Nickel Content: 8.0%–10.5% minimum
• Molybdenum Content (Type 316L): 2.0%–3.0% minimum
• Carbon Content: ≤0.08% (standard grade); ≤0.03% (L-grade for enhanced intergranular corrosion resistance)
• Thermal Conductivity: 16.3 W/(m·K) at 100°C
• Thermal Expansion Coefficient: 17.3 × 10⁻⁶ /°C (20–100°C)
• Yield Strength: ≥205 MPa (0.2% offset)
• Tensile Strength: ≥520 MPa
• Elongation: ≥40%
• Operating Temperature Range: −196°C to +870°C
• Corrosion Resistance: Excellent resistance to pitting, crevice, and intergranular corrosion in chlorinated water systems

Technical Specifications

Parameter	Specification	Standard Reference
Tube Outer Diameter (OD)	9.52 mm / 12.70 mm / 15.88 mm / 19.05 mm	GB/T 14976; ASTM A269
Tube Wall Thickness	0.50 mm / 0.65 mm / 0.80 mm / 1.00 mm	GB/T 14976
Coil Pitch (Center-to-Center)	15.88 mm / 25.40 mm / 31.75 mm	Manufacturer Specification
Coil Length (Standard)	3.0 m / 4.0 m / 5.0 m / 6.0 m / Custom	—
Number of Rows	2-row / 4-row / 6-row / 8-row configurations	—
Surface Finish	Polished (Ra ≤ 0.4 μm) / Bright Annealed	ASTM A403
End Condition	Plain / Flared / Threaded / Brazed Joint	GB/T 12771
Maximum Operating Pressure	1.6 MPa (standard) / 2.5 MPa (reinforced)	GB/T 12771; EN 10216
Hydrostatic Test Pressure	1.5× working pressure (minimum 2.5 MPa)	ISO 4427
Heat Transfer Efficiency	U-value ≥ 50 W/(m²·K) (air-side / water-side)	ASHRAE Standard 90.1
Water Flow Rate Capacity	0.5–20.0 L/min per circuit (depending on configuration)	—
Refrigeration Capacity Range	0.8 kW – 12.0 kW (single unit)	GB/T 19232
Seam Type	Welded (TIG / Resistance Welded) / Seamless	GB/T 14976; ASTM A213
Tolerance on OD	±0.10 mm (welded); ±0.05 mm (seamless)	ISO 4427
Tolerance on Wall Thickness	±10% of nominal	GB/T 14976

Manufacturing Process

The production of stainless steel coil tubes at Wenzhou Jixing Metal Materials follows a multi-stage precision manufacturing process designed to ensure dimensional accuracy, surface integrity, and metallurgical consistency:

1. Raw Material Selection and Inspection: Cold-rolled stainless steel strip is procured from certified mills and subjected to spectroscopic analysis (OES) to verify chemical composition compliance with the specified grade. Incoming material undergoes visual inspection, hardness testing, and surface quality assessment.
2. Strip Slitting and Edge Preparation: The cold-rolled strip is slit to precise width tolerances using high-speed slitting lines. Edges are inspected for defects including cracks, laps, and roll marks that could propagate during forming.
3. Coil Forming (Cold Bending): The flat strip is fed through a series of precision roll-forming stations that progressively bend the material into the required helical coil geometry. Forming forces are carefully controlled to prevent work hardening, cracking, or springback beyond acceptable tolerances. The forming process is performed at ambient temperature to maintain the metallurgical properties of the austenitic stainless steel.
4. Welding (for Welded Tubes): For welded configurations, the formed coil is continuously welded using Transverse TIG (Tungsten Inert Gas) welding or resistance welding under argon gas shielding. Weld parameters are computer-controlled to ensure consistent penetration, bead profile, and absence of porosity or undercut.
5. Heat Treatment (if applicable): Seamless tubes produced via piercing and cold drawing undergo solution annealing at 1050°C–1150°C followed by controlled cooling to restore full austenitic structure and eliminate residual stresses.
6. Surface Finishing: Tubes undergo mechanical polishing or electrolytic polishing to achieve the required surface roughness (Ra ≤ 0.4 μm for high-efficiency heat transfer configurations). This step is critical for minimizing fouling resistance and maintaining optimal heat transfer coefficients over the service life.
7. End Processing: Tube ends are cut to length using precision shearing or abrasive cutting, followed by deburring, chamfering, and (where specified) flaring or threading. End faces are inspected for squareness and cleanliness.
8. Quality Control and Testing: Each batch undergoes comprehensive testing including hydrostatic pressure testing, dimensional verification using coordinate measuring machines (CMM), ultrasonic flaw detection (UT) for internal defects, and random chemical spot-checking.
9. Coating and Packaging: Tubes may receive protective end caps or desiccant-filled packaging to prevent moisture ingress during storage and transport. Product identification labels and traceability documentation are affixed prior to dispatch.

Applications

The stainless steel coil tube from Wenzhou Jixing Metal Materials is primarily deployed in the following HVAC and building services engineering applications:

• Fan Coil Units (FCUs): As the primary heat exchanger element in both chilled water and hot water fan coil units used for precise temperature and humidity control in commercial buildings, hotels, hospitals, and institutional facilities.
• Air Handling Units (AHUs): Integrated as coil sections within large air handling units serving central air conditioning systems in high-rise buildings, shopping centers, and industrial facilities.
• Radiant Floor Heating Systems: Adapted for use in hydronic radiant floor and wall heating systems where stainless steel's corrosion resistance provides long-term durability in direct water contact applications.
• Industrial Process Cooling/Heating: Utilized in process air conditioning systems for clean rooms, electronics manufacturing, pharmaceutical production, and food processing facilities requiring hygienic-grade heat exchange surfaces.
• Refrigeration and Chilled Water Distribution: Employed in secondary chilled water loops and refrigerant-to-water heat exchangers in district cooling and heating systems.
• Marine and Offshore HVAC: Where Type 316L grade coil tubes provide enhanced resistance to chloride-induced corrosion in marine atmospheric environments.

Quality Certifications and Compliance

Wenzhou Jixing Metal Materials Co., Ltd. maintains rigorous quality management systems and holds the following certifications and standards compliance:

• ISO 9001:2015 — Quality Management System Certification, ensuring consistent product quality and continuous improvement processes throughout the manufacturing lifecycle.
• ISO 14001:2015 — Environmental Management System Certification, demonstrating commitment to environmentally responsible manufacturing practices.
• ISO 45001:2018 — Occupational Health and Safety Management System Certification.
• GB/T 14976 — Chinese National Standard for seamless stainless steel tubes for general use, ensuring dimensional accuracy and mechanical property compliance.
• GB/T 12771 — Chinese National Standard for welded stainless steel tubes for general use.
• GB/T 19232 — Chinese National Standard for air conditioning and heat pump water-to-air units, ensuring thermal performance compliance.
• CE Marking — European Conformity marking, certifying compliance with applicable European directives including the Pressure Equipment Directive (PED 2014/68/EU) for pressure-bearing components.
• RoHS Compliance — Restriction of Hazardous Substances, ensuring the product is free from specified hazardous materials.
• NSF/ANSI 61 (for potable water contact applications) — Ensuring materials do not leach harmful substances into water supplies.

Unique Selling Points

• Superior Corrosion Resistance: The austenitic stainless steel construction provides exceptional resistance to corrosion from treated water systems containing biocides, corrosion inhibitors, and dissolved oxygen, significantly extending service life compared to copper or carbon steel alternatives.
• Hygienic Surface Quality: Electrolytically polished surfaces with Ra ≤ 0.4 μm minimize biofilm formation and fouling, ensuring sustained heat transfer efficiency and compliance with hygienic requirements in pharmaceutical and food-grade applications.
• Thermal Stability: Maintains mechanical integrity and dimensional stability across the full operating temperature range of −196°C to +870°C, eliminating concerns related to thermal cycling fatigue.
• Non-Toxic and Food-Safe: Stainless steel coil tubes are inherently non-toxic, making them suitable for applications involving potable water and food processing environments without risk of metallic taste or contamination.
• Custom Configuration Capability: Wenzhou Jixing Metal Materials offers flexible customization of coil pitch, row count, tube diameter, and end conditions to match specific FCU design parameters and system pressure requirements.
• Full Traceability: Each production batch is accompanied by a mill test certificate (MTC) with full chemical composition and mechanical property data, enabling complete supply chain traceability.

Design and Construction Considerations for FCU Water Systems

In the context of air conditioning fan coil unit water system design and construction, the following engineering considerations are critical to ensuring optimal performance of the stainless steel coil tube heat exchanger:

• Water Hammer Prevention: The coil tube's robust wall thickness and material ductility provide resistance to transient pressure surges. System design should incorporate proper valve sequencing, slow-closing valves, and air venting provisions to minimize water hammer risk.
• Air Locking Mitigation: The coil tube geometry should be designed with appropriate flow direction and slope to facilitate automatic air discharge. Automatic air vents should be installed at high points in the coil circuit to prevent air locking that reduces heat transfer capacity.
• Pressure Balance: The coil tube's pressure drop characteristics must be accounted for in system hydraulic design. Balancing valves should be installed to ensure proper flow distribution across parallel FCU circuits.
• Water Quality Management: Although stainless steel offers superior corrosion resistance, proper water treatment is essential. System water should be maintained at pH 7.0–9.5, with appropriate biocide and corrosion inhibitor dosing. Chloride ion concentration should be kept below 200 ppm to prevent pitting in Type 304 grades (below 1000 ppm for Type 316L).
• Installation Orientation: Coil tubes should be installed in the recommended orientation (typically horizontal for FCU applications) to ensure uniform water distribution across all tube rows and prevent stratification that could lead to localized overheating or freezing.
• Drainage Provisions: Adequate drain connections must be provided at the lowest point of the coil circuit to facilitate complete drainage during maintenance and to prevent water stagnation that could promote corrosion or biological growth.

Note: This technical specification is provided for informational and procurement reference purposes. For specific project requirements, Wenzhou Jixing Metal Materials Co., Ltd. recommends consultation with their engineering team to confirm material grade selection, dimensional specifications, and system compatibility prior to finalization of design and procurement.