Stainless Steel Coil Tube — Deep-Sea Manned Chamber Fan Coil System Application
1. Product Overview
The Stainless Steel Coil Tube (Coiltube) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered tubular component designed for integration into closed-loop thermal management systems, particularly those deployed in deep-sea manned chamber environments. These coil tubes serve as critical heat exchange elements within fan coil units (FCU) that regulate cabin temperature, humidity, and atmospheric composition under extreme hydrostatic pressure conditions.
Derived from simulation studies of deep-sea manned chamber fan coil tube systems, the coil tube geometry and material selection have been optimized to withstand cyclic thermal loading, high-pressure differentials, and prolonged exposure to corrosive marine atmospheres. The product represents a convergence of metallurgical science, fluid dynamics, and structural mechanics tailored for extreme-environment reliability.
2. Material & Properties
2.1 Base Alloy Composition
The coil tube is fabricated from austenitic stainless steel conforming to the following grade designations:
- SAE 316L (UNS S31603) — Low-carbon variant with molybdenum addition for enhanced pitting and crevice corrosion resistance in chloride-rich environments
- SAE 316Ti (UNS S31635) — Titanium-stabilized variant offering superior resistance to sensitization during welding and elevated-temperature service
2.2 Mechanical Properties
| Property | 316L Specification | 316Ti Specification |
|---|---|---|
| Tensile Strength (Rm) | ≥ 515 MPa | ≥ 515 MPa |
| Yield Strength (Rp0.2) | ≥ 205 MPa | ≥ 205 MPa |
| Elongation (A50) | ≥ 40% | ≥ 40% |
| Hardness (HV30) | ≤ 217 | ≤ 217 |
| Impact Energy (KV2, -196°C) | ≥ 47 J | ≥ 47 J |
2.3 Corrosion Resistance
- Pitting Resistance Equivalent Number (PREN): ≥ 24 for 316L; ≥ 26 for 316Ti
- Resistance to intergranular corrosion per ASTM A262 Practice E (acid solution test)
- Resistance to stress corrosion cracking (SCC) in high-chloride, high-temperature aqueous environments
2.4 Thermal Properties
- Thermal conductivity at 20°C: 14.6 W/(m·K)
- Thermal conductivity at 100°C: 17.0 W/(m·K)
- Linear thermal expansion coefficient (20–100°C): 17.3 × 10⁻⁶ /K
- Specific heat capacity at 20°C: 500 J/(kg·K)
3. Specifications
| Parameter | Specification |
|---|---|
| Product Name | Stainless Steel Coil Tube (Coiltube) |
| Product Code | No. 434 |
| Material Grade | SAE 316L / SAE 316Ti |
| Outer Diameter (OD) | 6.35 mm / 8.00 mm / 9.53 mm / 12.70 mm (customizable) |
| Wall Thickness | 0.50 mm – 1.20 mm |
| Coil Pitch | 3.00 mm – 6.00 mm (adjustable per system design) |
| Coil Diameter (Bend Radius) | 15 mm – 50 mm |
| Coil Length (Developed) | Up to 5,000 mm (continuous coil) |
| Surface Finish | Mirror polish (Ra ≤ 0.4 μm) / Satin finish (Ra ≤ 0.8 μm) |
| Operating Temperature Range | -196°C to +350°C |
| Maximum Operating Pressure | Up to 150 bar (gauge) |
| Hydrostatic Test Pressure | 1.5 × Maximum Operating Pressure |
| Applicable Standards | ASTM A269, ASTM A213, EN 10216-5, ASME B31.3 |
| Delivery Condition | Solution annealed (1050°C – 1150°C, water quench) |
4. Manufacturing Process
4.1 Raw Material Preparation
Hot-rolled stainless steel strip is procured from certified mill sources and subjected to incoming inspection including chemical composition analysis (OES/SEM), mechanical testing, and surface quality verification. The strip is solution-annealed to achieve the desired austenitic microstructure prior to cold working.
4.2 Cold Forming & Coiling
- Strip Slitting: The hot-rolled coil is slit to the required strip width with precision edge trimming to ensure dimensional accuracy.
- Cold Roll Forming: The flat strip is passed through a series of progressive forming dies to achieve the target tube cross-section (circular or finned profile).
- Continuous Coiling: The formed tube is wound into a helical coil using CNC-controlled bending equipment with synchronized feed and rotation axes to maintain uniform coil pitch and bend radius.
- Welding: Where continuous coil length is insufficient, longitudinal butt welds are performed using TIG (Tungsten Inert Gas) welding with helium shielding gas to minimize oxidation and preserve metallurgical integrity.
4.3 Post-Forming Heat Treatment
Following cold forming, the coil tube undergoes solution annealing at 1050°C–1150°C followed by rapid water quenching to:
- Eliminate cold work hardening and residual stresses
- Restore full austenitic ductility
- Homogenize grain structure
- Maximize corrosion resistance
4.4 Surface Treatment
- Pickling: Removal of oxide scale using a mixed acid solution (HNO₃ + H₂SO₄) to achieve a clean, passive surface.
- Pasivation: Application of citric acid or nitric acid passivation treatment to enhance the chromium oxide passive film.
- Precision Polishing: Mechanical or electrochemical polishing to achieve mirror surface finish (Ra ≤ 0.4 μm) for reduced flow resistance and biofilm resistance.
4.5 Quality Control & Testing
- Dimensional Inspection: Coordinate Measuring Machine (CMM) verification of OD, wall thickness, coil pitch, and bend radius at intervals not exceeding 500 mm.
- Hydrostatic Pressure Testing: Each coil tube is pressure-tested at 1.5× the maximum operating pressure for a minimum dwell time of 60 seconds.
- Non-Destructive Testing (NDT): Eddy current testing (ECT) and dye penetrant inspection (PT) for detection of surface and near-surface defects.
- Corrosion Testing: Salt spray testing per ASTM B117 (minimum 1,000 hours without pitting) and ferric chloride pitting test per ASTM G48.
- Intergranular Corrosion Testing: ASTM A262 Practice E to verify resistance to sensitization.
5. Applications
5.1 Deep-Sea Manned Chamber Systems
The primary application domain for this coil tube is in the thermal regulation subsystems of deep-sea manned submersibles and hyperbaric chambers. In these environments, the coil tube functions as the heat exchange core within fan coil units that maintain cabin temperature stability despite extreme external hydrostatic pressures (up to 1,000+ bar at operational depths exceeding 6,000 meters). Simulation studies have demonstrated that the coil tube geometry optimizes the balance between heat transfer coefficient and pressure drop, ensuring efficient thermal management with minimal parasitic power consumption.
5.2 Additional Applications
- Offshore Platform HVAC Systems: Temperature control in topside living quarters and control rooms exposed to marine atmospheric corrosion.
- Submarine Life Support Systems: Thermal conditioning of breathable atmosphere in closed-loop environmental control and life support systems (ECLSS).
- Chemical Processing Heat Exchangers: Service in aggressive chemical environments where carbon steel or aluminum alloys would suffer rapid degradation.
- Pharmaceutical & Food Processing: Hygienic heat exchange applications requiring mirror-finish surfaces and resistance to caustic cleaning solutions.
- Cryogenic Systems: Heat exchanger components operating at temperatures as low as -196°C (liquid nitrogen) where austenitic toughness is essential.
- Nuclear Industry: Secondary loop heat exchange components requiring radiation resistance and long-term structural integrity.
6. Quality Certifications & Standards Compliance
| Certification / Standard | Description |
|---|---|
| ISO 9001:2015 | Quality Management System certification for manufacturing processes |
| ISO 14001:2015 | Environmental Management System certification |
| ISO 3834 | Quality requirements for welding of metallic materials |
| EN 10204 3.1 | Inspection documents — Test report certifying conformity to specified requirements |
| ASTM A269 | Standard specification for seamless austenitic stainless steel tube |
| ASTM A213 | Standard specification for seamless austenitic stainless steel boiler, heat exchanger, and heat-treating tubing |
| ASME B31.3 | Process piping code compliance for pressure-containing applications |
| NACE MR0175 / ISO 15156 | Materials for use in H₂S-containing environments in oil and gas production |
| PED 2014/68/EU | European Pressure Equipment Directive compliance (Category II) |
| ABS / DNV / LR Classification | Marine classification society approval for submarine and offshore applications |
7. Unique Selling Points
- Deep-Sea Proven Design: Coil geometry validated through computational fluid dynamics (CFD) and thermal simulation specifically for manned deep-sea chamber environments, ensuring reliable performance at extreme pressures and temperatures.
- Superior Corrosion Resistance: Molybdenum-enhanced austenitic microstructure provides exceptional resistance to pitting, crevice corrosion, and intergranular corrosion in chloride-laden marine atmospheres.
- Full Traceability: Each coil tube is accompanied by a complete material traceability package including mill certificates, heat number tracking, and third-party test reports.
- Cryogenic Capability: Maintains ductility and toughness down to -196°C, making it suitable for dual-service applications involving both cryogenic and elevated-temperature thermal cycles.
- Customization Flexibility: Coil pitch, bend radius, tube diameter, and surface finish are fully customizable to match specific system design requirements, enabling optimized thermal-hydraulic performance.
- Hygienic Surface Quality: Mirror-polished finish (Ra ≤ 0.4 μm) minimizes fluid friction losses and prevents biofilm accumulation, extending maintenance intervals in demanding operational environments.
Note: This product specification is informed by simulation studies of deep-sea manned chamber fan coil tube systems, integrating findings from thermal-fluid analysis, structural fatigue modeling, and materials degradation prediction to deliver a product specification that meets the most demanding requirements of deep-ocean exploration and exploitation.