Stainless Steel Coil Tube (盘管 Coiltube) — Technical Specification

1. Product Overview

The Stainless Steel Coil Tube (also known as Coil Pipe or Coil Tubing) is a continuous-length tubular product supplied in the form of a helical coil, manufactured by Wenzhou Jixing Metal Materials Co., Ltd. Unlike conventional straight-length seamless or welded pipes, coil tubes are produced in uninterrupted runs, which eliminates the need for field welding and significantly reduces installation labor and joint failure points. This product category falls under the classification of long-length tubular products and is widely utilized in HVAC (Heating, Ventilation, and Air Conditioning), hydronic heating, refrigeration, chemical processing, and industrial heat exchange systems.

From a metallurgical perspective, the coil tube represents a unique combination of material integrity and geometric continuity. The continuous winding process imposes specific metallurgical and mechanical constraints on the material, requiring precise control of work-hardening behavior, dimensional tolerances, and surface finish to maintain uniform mechanical properties throughout the entire coil length.

2. Material & Properties

2.1 Common Material Grades

• SUS304 / AISI 304 / 06Cr19Ni10 — General-purpose austenitic stainless steel with excellent corrosion resistance and formability. The most widely specified grade for coil tube applications.
• SUS316L / AISI 316L / 022Cr17Ni12Mo2 — Molybdenum-enhanced austenitic grade offering superior resistance to pitting and crevice corrosion, suitable for aggressive chemical and marine environments.
• SUS321 / AISI 321 / 06Cr18Ni11Ti — Titanium-stabilized austenitic grade providing enhanced resistance to intergranular corrosion at elevated temperatures (450–850°C).
• SUS310S / AISI 310S / 06Cr25Ni20 — High-temperature austenitic grade with excellent oxidation resistance, suitable for high-temperature heat exchange and furnace applications.
• Duplex Grades (S31803 / S32205) — Duplex stainless steels offering high yield strength (≥450 MPa) and excellent chloride stress corrosion cracking (Cl-SCC) resistance.

2.2 Key Material Properties

• Chemical Composition: Compliant with ASTM A213, GB/T 14976, and EN 10216 standards. Carbon content is tightly controlled (typically ≤0.08% for 304, ≤0.03% for 316L) to minimize sensitization risk.
• Mechanical Properties: Tensile strength (Rm) typically 520–720 MPa; yield strength (Rp0.2) typically 205–310 MPa; elongation (A5) ≥35% for annealed condition.
• Microstructure: Fully annealed austenitic structure (FCC, face-centered cubic) with minimal or no ferrite content for 304/316L grades. Grain size typically ASTM No. 5–8.
• Corrosion Resistance: Passive film formation via chromium oxide layer (Cr₂O₃) provides self-healing corrosion protection. 316L grade exhibits PREN (Pitting Resistance Equivalent Number) ≥24.
• Thermal Conductivity: Approximately 16.3 W/(m·K) for 304 grade at 25°C, making it suitable for heat exchange applications where moderate thermal transfer is required.
• Operating Temperature Range: -196°C (cryogenic) to +870°C (304/316L); up to +1150°C for 310S grade.

3. Specifications

Parameter	Specification
Product Name	Stainless Steel Coil Tube (盘管 Coiltube)
Product Code	1244
Outer Diameter (OD)	Φ6.0 mm – Φ50.8 mm (1/4" – 2" NPS)
Wall Thickness (WT)	0.5 mm – 3.0 mm (Schedules 5S – 40S)
Coil Length	Continuous lengths up to 200 m (standard); custom lengths available upon request
Coil Diameter	Typically 1.0 m – 3.0 m (adjustable based on application requirements)
End Condition	Cut to length, beveled, or pre-formed (elbow, U-bend) as specified
Surface Finish	2B (bright annealed), No. 4 (brushed), BA (Bright Annealed), or custom per client specification
Applicable Standards	ASTM A213, ASTM A269, GB/T 14976, GB/T 14975, EN 10216-5, JIS G3459
Heat Treatment	Solution annealing (1050–1150°C) followed by water or air quenching
Tolerance (OD)	±0.05 mm (precision grade); ±0.10 mm (commercial grade)
Tolerance (WT)	±10% of nominal wall thickness (commercial); ±5% (precision)
Straightness	≤1.5 mm per meter (for straight-cut sections)
Coating/Passivation	Acid pickling and passivation (citric acid or nitric acid based); optional PTFE or ceramic coating
Packaging	Steel banding, wooden pallet, or custom coil crate; moisture-proof inner wrapping

4. Manufacturing Process

The production of stainless steel coil tubes involves a multi-stage metallurgical and mechanical process chain, each stage critical to ensuring the final product meets stringent dimensional, mechanical, and surface quality requirements.

4.1 Process Flow

1. Raw Material Preparation: Cold-rolled stainless steel strip (coil) is sourced from certified mills. Incoming material undergoes spectroscopic analysis (OES) to verify chemical composition and visual/NDT inspection for surface defects.
2. Coil Forming (Pipe Forming): The flat strip is fed through a series of progressive roll-forming stations that gradually shape the strip into a tubular profile. The number of forming stations (typically 10–20) is determined by the target OD, wall thickness, and material grade. The bending radius at each station is carefully calculated to avoid excessive localized work hardening and to maintain uniform cross-sectional geometry.
3. Welding (if applicable): For welded coil tubes, longitudinal welding is performed using TIG (Tungsten Inert Gas) or orbital welding processes with high-purity argon shielding gas (≥99.998% Ar). The weld seam is back-purged with argon to prevent internal oxidation. Weld parameters (current, voltage, speed) are precisely controlled to achieve full penetration with minimal heat-affected zone (HAZ) distortion.
4. Heat Treatment (Solution Annealing): The formed coil tube is passed through a continuous annealing furnace at temperatures of 1050–1150°C for sufficient dwell time to dissolve carbides and restore full solution-annealed microstructure. This step is critical for achieving optimal corrosion resistance and ductility, particularly after cold-forming work hardening.
5. Pickling and Passivation: The annealed tube undergoes acid pickling (typically HNO₃/HF or citric acid based) to remove mill scale and oxide layers formed during annealing. This is followed by passivation to restore the protective chromium oxide passive film. The passivation process is verified using ferri-ferrocyanide spot testing or redox potential measurement.
6. Dimensional Inspection and Calibration: The coil tube passes through precision calibrating rolls to ensure dimensional accuracy of OD and wall thickness within specified tolerances. Online ultrasonic thickness gauging and laser diameter measurement systems provide real-time quality feedback.
7. Coiling and Packaging: The finished product is wound onto a coil reel with controlled tension to prevent ovality and surface damage. Each coil is identified with traceability markings (heat number, grade, dimensions, date code) and packaged for shipment.

4.2 Critical Process Parameters

• Forming Reduction Ratio: Typically 15–25% per pass to control strain hardening and prevent cracking in austenitic grades.
• Interpass Temperature: Maintained below 200°C to avoid sensitization (carbide precipitation at grain boundaries).
• Annealing Atmosphere: Controlled atmosphere (reducing or vacuum) to prevent re-oxidation during high-temperature exposure.
• Weld Dilution Ratio: Controlled to maintain weld metal composition within acceptable range (typically 15–20% base metal dilution for 304/316L).

5. Applications

The stainless steel coil tube finds extensive application across multiple industrial sectors due to its combination of corrosion resistance, mechanical strength, continuous length, and hygienic surface properties.

5.1 HVAC and Fan Coil Units (风机盘管)

In the context of fan coil unit (FCU) equipment selection, stainless steel coil tubes serve as the primary heat exchange elements. The continuous coil configuration maximizes heat transfer surface area within a compact footprint, making it ideal for:

• Chilled water cooling coils in commercial and residential HVAC systems
• Hot water heating coils for radiant heating and space conditioning
• Condenser and evaporator tubes in packaged rooftop units
• Secondary heat exchangers in district heating/cooling distribution systems

The selection of coil tube for fan coil units requires careful consideration of the following engineering parameters:

• Tube Diameter and Pitch: Typically Φ6.35 mm (1/4") or Φ9.52 mm (3/8") OD with 12–16 FPI (fins per inch) for optimal air-side and water-side heat transfer balance.
• Row Configuration: 2-row to 6-row arrangements depending on required capacity and temperature differential (ΔT).
• Flow Velocity: Water-side velocity maintained at 1.5–3.0 m/s to ensure turbulent flow and minimize fouling.
• Material Selection: SUS316L is preferred for applications involving treated water, brines, or humid coastal environments to prevent galvanic and crevice corrosion.

5.2 Other Industrial Applications

• Refrigeration and Cryogenics: Evaporator and condenser coils in industrial refrigeration systems operating at temperatures as low as -40°C.
• Chemical Processing: Heat exchanger tubes, condensers, and reboilers in chemical plants handling corrosive media.
• Food and Pharmaceutical: Sanitary-grade coil tubes (electropolished surface, Ra ≤0.25 μm) for dairy processing, beverage pasteurization, and pharmaceutical manufacturing.
• Power Generation: Feedwater heaters, condensers, and economizer tubes in thermal power plants.
• Automotive: Heat exchanger tubes in engine cooling systems and fuel cell vehicle thermal management.
• Marine and Offshore: Seawater cooling coils and heat exchangers in shipboard systems and offshore platforms.

6. Quality Certifications and Standards Compliance

Wenzhou Jixing Metal Materials Co., Ltd. maintains a comprehensive quality management system ensuring that all coil tube products meet or exceed international standards and regulatory requirements.

6.1 Quality Management System

• ISO 9001:2015 — Quality Management Systems certification ensuring consistent product quality and continuous improvement.
• ISO 14001:2015 — Environmental Management Systems certification for sustainable manufacturing practices.
• ISO 45001:2018 — Occupational Health and Safety Management Systems certification.

6.2 Product-Specific Certifications

• CE Marking — Compliance with European Union directives for pressure equipment and construction products.
• EN 10204 3.1 — Material Test Reports (MTR) issued by qualified technical personnel, certifying compliance with specified standards.
• EN 10204 3.2 — Third-party witnessed inspection available upon request.
• ASTM A213 / A269 Compliance — Full compliance with American Society for Testing and Materials standards for seamless and welded austenitic stainless steel tubes.
• GB/T 14976 / GB/T 14975 Compliance — Compliance with Chinese National Standards for fluid transport and boiler tubes.
• NSF/ANSI 61 — Certification for potable water contact applications (food-grade stainless steel).
• PED 2014/68/EU — Pressure Equipment Directive compliance for applications involving pressure vessels and piping.

6.3 Testing and Inspection Protocols

• Chemical Analysis: Optical Emission Spectroscopy (OES) and wet chemistry verification for each heat.
• Mechanical Testing: Tensile testing (ASTM A370), hardness testing (Rockwell/Brinell), and impact testing (Charpy V-Notch) per batch.
• Non-Destructive Testing (NDT): Hydrostatic pressure testing (1.5× working pressure), eddy current inspection (ECI) for weld seam integrity, and visual inspection for surface defects.
• Dimensional Inspection: Caliper measurement, ultrasonic wall thickness gauging, and straightness verification.
• Corrosion Testing: Salt spray testing (ASTM B117, ≥500 hours for 316L), intergranular corrosion testing (ASTM A262 Practice E),