Stainless Steel Internal-Threaded Coil Tube for HVAC Applications — Wenzhou Jixing Metal Materials Co., Ltd.

1. Product Overview

The Stainless Steel Internal-Threaded Coil Tube (SS-ITCT) is a precision-engineered coiled tubing product developed and manufactured by Wenzhou Jixing Metal Materials Co., Ltd. This product represents a significant advancement in heat exchanger tube technology, specifically designed for air conditioning (HVAC — Heating, Ventilation, and Air Conditioning) systems. The coil tube features a helical internal thread (Internal Threaded Tube, ITT) structure formed through a proprietary coil drawing and threading process, which dramatically enhances heat transfer efficiency by increasing the effective internal surface area and promoting turbulent fluid flow within the tube bore.

The successful development of the Internal Threaded Air Conditioner Coil Tube Drawing Machine — a specialized production line capable of simultaneously coiling and internally threading stainless steel tubes — marks a milestone in the company's manufacturing capabilities. This integrated manufacturing approach eliminates the need for separate coiling and threading operations, thereby reducing dimensional tolerance accumulation, improving product consistency, and achieving higher throughput rates.

2. Material and Properties

2.1 Base Material

The primary material used for the SS-ITCT product is austenitic stainless steel, with the following grades available:

• SUS304 (AISI 304): The most commonly used grade, offering excellent corrosion resistance, formability, and weldability. It contains approximately 18% Chromium (Cr) and 8% Nickel (Ni), providing robust resistance to oxidation and pitting in typical HVAC operating environments.
• SUS316 (AISI 316): Enhanced with 2–3% Molybdenum (Mo) for superior resistance to chloride-induced pitting and crevice corrosion, suitable for coastal or high-humidity applications.
• SUS321 (AISI 321): Titanium (Ti) stabilized grade, offering improved resistance to intergranular corrosion at elevated temperatures, suitable for high-temperature heat exchanger applications.

2.2 Mechanical Properties

• Tensile Strength (UTS): ≥ 520 MPa (for SUS304, annealed condition)
• Yield Strength: ≥ 205 MPa
• Elongation: ≥ 40% (at fracture, 50 mm gauge length)
• Hardness: 140–200 HV (annealed condition)
• Density: 7.93 g/cm³ (SUS304)

2.3 Thermal Properties

• Thermal Conductivity: 16.3 W/(m·K) at 20°C (SUS304)
• Specific Heat Capacity: 0.50 J/(g·K) at 20°C
• Thermal Expansion Coefficient: 17.3 × 10⁻⁶ /°C (20–100°C)
• Operating Temperature Range: −196°C to +870°C (continuous service)

2.4 Corrosion Resistance

• Excellent resistance to atmospheric corrosion, acidic condensates, and refrigerant fluids
• Resistance to Stress Corrosion Cracking (SCC) at temperatures below 60°C
• Passivation capability through natural chromium oxide (Cr₂O₃) film formation

3. Product Specifications

Parameter	Specification	Standard Reference
Outer Diameter (OD)	Φ6.0 mm – Φ12.7 mm (0.236" – 0.500")	Customizable
Wall Thickness	0.30 mm – 0.80 mm	ASTM A213 / ASTM A269
Internal Thread Pitch	0.5 mm – 1.5 mm (adjustable)	Proprietary design
Internal Thread Depth	0.05 mm – 0.20 mm	Proprietary design
Coil Outer Diameter	Φ300 mm – Φ800 mm	Customer requirement
Coil Length per Piece	3 m – 15 m (continuous coiling available)	Customizable
Straight Length Option	1 m – 6 m (straightened post-coiling)	ASTM A213
Surface Finish	Mill finish / Electro-polished / Passivated	ASTM A967 (Passivation)
End Condition	Plain end / Flared end / Threaded end (per customer spec)	ASME B1.20.1
Pressure Rating	Up to 2.5 MPa (design pressure, depending on OD and wall thickness)	ASME BPVC Section VIII
Weight Tolerance	±5% of theoretical weight	ISO 1461
Dimensional Tolerance (OD)	±0.05 mm	ASTM A269
Dimensional Tolerance (Wall)	±10% of nominal wall thickness	ASTM A269
Coil Pitch Uniformity	±0.5 mm	Internal standard

4. Manufacturing Process

4.1 Process Flow

The SS-ITCT is manufactured through a proprietary multi-stage process that integrates coil forming and internal threading in a single continuous operation:

1. Raw Material Inspection: Incoming stainless steel seamless tube or welded tube undergoes rigorous inspection for chemical composition (via Optical Emission Spectroscopy, OES), dimensional accuracy, and surface quality. Material certification per mill test reports (MTR) is verified against ASTM A213/A269 specifications.
2. Pre-Treatment and Annealing: Tubes are annealed at 1050°C – 1100°C followed by controlled cooling to achieve a fully austenitic microstructure, ensuring optimal formability and eliminating residual stresses from prior cold working.
3. Pickling and Passivation: Tubes are subjected to acid pickling (using a mixture of nitric acid and hydrofluoric acid) to remove oxide scale, followed by passivation treatment per ASTM A967 to restore the protective chromium oxide passive film.
4. Coil Drawing and Internal Threading (Integrated Process): The tube is fed into the proprietary Internal Threaded Coil Tube Drawing Machine. In this critical step:
   • The tube is guided through a precision-drawn die set that progressively reduces the outer diameter and wall thickness while simultaneously forming the coil geometry.
   • Internal threading rollers or a precision broaching tool simultaneously form the helical internal thread structure within the tube bore.
   • The integrated design ensures that thread geometry, coil pitch, and dimensional tolerances are maintained with high accuracy throughout the continuous process.
   • Draw speed is controlled at 5 m/min – 20 m/min depending on tube dimensions and thread configuration.
5. Post-Coiling Stress Relief: Coiled tubes undergo stress relief annealing at 300°C – 400°C for 1 hour to eliminate residual stresses introduced during the drawing and coiling process, preventing spring-back and dimensional instability.
6. Quality Inspection: Comprehensive inspection including:
   • Dimensional measurement (OD, wall thickness, thread depth, pitch) using coordinate measuring machine (CMM) and optical microscopy
   • Visual inspection for surface defects, thread integrity, and coil uniformity
   • Hydrostatic pressure testing at 1.5× design pressure for a minimum of 10 seconds
   • Non-Destructive Testing (NDT) including eddy current testing for internal defect detection and dye penetrant testing for surface discontinuities
   • Chemical composition verification via X-Ray Fluorescence (XRF) spectrometry
   • Corrosion testing (Salt Spray Test per ASTM B117, minimum 480 hours without red rust)
7. Final Treatment and Packaging: Tubes are cleaned with deionized water, dried, and packaged in protective packaging (wooden crates or moisture-proof wrapping) to prevent surface damage and corrosion during transit.

4.2 Key Process Innovations

The proprietary Internal Threaded Coil Tube Drawing Machine represents a significant engineering achievement. By integrating the coil forming and internal threading operations into a single synchronized process, the system achieves:

• Elimination of separate handling and transport between coiling and threading stations, reducing product damage risk
• Improved dimensional consistency due to the single-process approach, with coil pitch uniformity maintained within ±0.5 mm
• Increased production throughput by approximately 40% compared to conventional sequential processes
• Reduced energy consumption per unit length due to the elimination of intermediate heating and handling steps
• Real-time process monitoring with automated feedback control for thread depth and coil geometry

5. Applications

5.1 Primary Applications

• Air Conditioning Heat Exchangers: The internal thread structure increases the internal heat transfer coefficient by 30% – 60% compared to plain tubes, enabling more compact and efficient evaporators and condensers for residential, commercial, and industrial HVAC systems.
• Refrigeration Systems: Suitable for use in refrigeration evaporators, condensers, and heat exchangers operating with R-410A, R-32, and other modern refrigerants, providing superior corrosion resistance compared to copper tubes.
• Automotive Air Conditioning: Lightweight stainless steel coil tubes offer an alternative to aluminum tubes in automotive HVAC systems, providing enhanced durability and resistance to coolant corrosion.
• Heat Pump Systems: Ideal for heat pump evaporator and condenser applications, where resistance to both freezing and high-temperature operation is required.

5.2 Secondary Applications

• Industrial Process Heat Exchangers: Shell-and-tube heat exchangers in chemical processing, food processing, and pharmaceutical manufacturing where sanitary and corrosion-resistant requirements apply.
• Water Heating Systems: Solar water heaters and electric water heater heat exchangers, leveraging the corrosion resistance and durability of stainless steel.
• Marine HVAC Systems: Resistance to seawater corrosion makes SS-ITCT suitable for marine air conditioning and refrigeration applications.

5.3 Performance Advantages

• Enhanced Heat Transfer: The internal helical thread promotes secondary flow (Dean vortices) and increases turbulence, enhancing the overall heat transfer coefficient (U-value) by 30% – 60% over smooth-wall tubes.
• Reduced Fouling: The threaded internal surface provides a self-cleaning effect due to increased wall shear stress, reducing fouling accumulation and maintaining long-term performance.
• Compact Design: The enhanced heat transfer allows for smaller heat exchanger volumes, contributing to lighter and more compact system designs.
• Extended Service Life: Superior corrosion resistance ensures a service life of 15 – 25 years in typical HVAC applications, significantly exceeding the lifespan of copper alternatives.

6. Quality Certifications and Standards Compliance

6.1 International Standards

• ASTM A213: Standard Specification for Seamless and Welded Austenitic Stainless Steel Tubing for High-Pressure and High-Temperature Boilers and Heat Exchanger Tubing
• ASTM A269: Standard Specification for Seamless and Welded Austenitic Stainless Steel Tubing for General Application
• ASTM A967: Standard Practice for Chemical Cleaning and Passivation of Stainless Steel Parts
• ASME BPVC Section VIII: Rules for Construction of Pressure Vessels
• ISO 1461: Non-alloy Steel Tubes and Profiles — Dimensional Tolerances
• EN 10216-5: Technical Delivery Conditions for Seamless Heat Exchanger Tubes of Non-Alloy and Alloy Steels — Part 5: Austenitic Stainless Steel Tubes

6.2 Quality Management System

• ISO 9001:2015: Quality Management Systems — Requirements
• ISO 14001:2015: Environmental Management Systems — Requirements
• ISO 45001:2018: Occupational Health and Safety Management Systems — Requirements

6.3 Product Certifications

• Mill Test Reports (MTR) provided with each batch, including chemical composition analysis, mechanical test results, and dimensional verification data
• Third-party inspection available per customer requirement (e.g., SGS, Bureau Veritas, TÜV)
• Material traceability maintained throughout the entire manufacturing process from raw material to finished product

6.4 Testing and Verification

• Hydrostatic Test: 100% of production tested at 1.5× design pressure, minimum hold time 10 seconds
• Dimensional Inspection: 100% inspection of OD, wall thickness, and coil pitch; sampling inspection of thread geometry
• Corrosion Testing: Batch sampling for Salt Spray Test (ASTM B117) — minimum 480 hours without red rust formation
• Mechanical Testing: Tensile testing per ASTM A370, minimum 1 specimen per heat
• NDT: Eddy current testing for internal defect detection on 100% of production; dye penetrant testing for surface defects on 100% of production

7. Unique Selling Points

1. Proprietary Integrated Manufacturing: The world's first commercially viable integrated coil drawing and internal threading machine, enabling superior dimensional accuracy and process efficiency.
2. Significant Heat Transfer Enhancement: 30% – 60% improvement in heat transfer coefficient over plain-wall tubes, enabling more compact and energy-efficient HVAC system designs.
3. Superior Corrosion Resistance: Stainless steel construction provides unmatched resistance to corrosion from condensate, refrigerants, and atmospheric exposure, ensuring 15 – 25 year service life.
4. Environmentally Friendly: Stainless steel is 100% recyclable, and the enhanced heat transfer efficiency reduces energy consumption in HVAC systems, contributing to lower carbon emissions.
5. Customization Capability: Full range of