Stainless Steel Heating Coil Tube for Sewage Sedimentation Tank Applications
Product Overview
The Stainless Steel Heating Coil Tube (also referred to as Heat Tracing Coil Tube or Jacketed Coil Pipe) is a specialized process piping component manufactured by Wenzhou Jixing Metal Materials Co., Ltd. (Product No. 355). This coil tube is specifically engineered for integration into sewage sedimentation tanks (secondary clarifiers) within wastewater treatment plants, where it serves as an embedded heating element to maintain optimal process temperatures and prevent biological fouling, sediment freezing, and microbial die-off in low-temperature operating environments.
Unlike conventional straight pipe sections, the coil tube geometry maximizes the surface area-to-volume ratio within a confined footprint, enabling superior heat exchange efficiency in tank-bottom and tank-wall installations. The product is designed based on rigorous thermal-hydraulic analysis of heat transfer characteristics and influencing factors in sewage sedimentation environments, incorporating lessons learned from extensive field performance studies.
Material & Properties
Base Material
- Primary Alloy: Austenitic Stainless Steel conforming to ASTM A213 / ASME SA-213 (Tube for Boilers, Heaters, and Exchangers)
- Typical Grades:
- Type 304 / 304L (UNS S30400 / S30403) — General corrosion resistance in mild wastewater environments
- Type 316 / 316L (UNS S31600 / S31603) — Enhanced chloride resistance for high-chloride sewage streams
- Type 316L (UNS S31603) — Low-carbon variant for welded coil assemblies requiring superior weld-zone corrosion resistance
- Type 321 (UNS S32100) — Titanium-stabilized grade for elevated temperature service above 425°C (intergranular corrosion prevention)
- Alternative Grades (upon request): Duplex 2205 (UNS S32205), Super Duplex 2507 (UNS S32750) for aggressive industrial wastewater
Mechanical Properties (at 20°C / 68°F)
- Tensile Strength (UTS): ≥ 515 MPa (Type 304); ≥ 550 MPa (Type 316L)
- Yield Strength (0.2% Offset): ≥ 205 MPa (Type 304); ≥ 240 MPa (Type 316L)
- Elongation: ≥ 40% (Type 304); ≥ 35% (Type 316L)
- Impact Energy (Charpy V-Notch at -40°C): ≥ 47 J (Type 304L); ≥ 40 J (Type 316L)
- Hardness: ≤ 220 HV (Type 304); ≤ 230 HV (Type 316L)
Thermal Properties
- Thermal Conductivity: 16.3 W/(m·K) at 20°C (Type 304); 14.6 W/(m·K) at 20°C (Type 316L)
- Specific Heat Capacity: 500 J/(kg·K) at 20°C
- Linear Expansion Coefficient: 17.3 × 10⁻⁶ /°C (Type 304); 16.6 × 10⁻⁶ /°C (Type 316L)
- Maximum Operating Temperature: 870°C (Type 304); 870°C (Type 316L)
- Cryogenic Service: Suitable down to -196°C (Liquid Nitrogen temperature) without loss of ductility
Corrosion Resistance
- Pitting Resistance (PREN): 18–20 (Type 304); 24–26 (Type 316L)
- Resistance to: Dilute sulfuric acid, dilute phosphoric acid, organic acids, atmospheric corrosion, chloride-containing wastewater, ammonia solutions
- Crevice Corrosion Resistance: Superior to carbon steel and duplex grades in chloride environments below 50°C
Specifications
| Parameter | Specification | Standard / Reference |
|---|---|---|
| Product Name | Stainless Steel Heating Coil Tube (盘管 Coiltube) | — |
| Product No. | 355 | Wenzhou Jixing Catalog |
| Tube Outer Diameter (OD) | Φ12.7 mm, Φ15.9 mm, Φ19.1 mm, Φ21.3 mm, Φ25.4 mm, Φ31.8 mm, Φ38.1 mm (custom sizes available) | ASTM A213 / ASME SA-213 |
| Tube Wall Thickness | 1.0 mm – 3.0 mm (standard); up to 6.0 mm (special order) | ASTM A213 |
| Coil Bend Diameter | 150 mm – 1200 mm (configurable per tank geometry) | ASME B31.3 |
| Coil Length | Up to 3000 m per continuous coil (welded assembly available) | — |
| End Connection | Threaded (NPT / BSP), Flanged (ANSI 150#/300#), Socket Weld, Butt Weld (BW) | ASME B1.20.1 / ASME B16.5 |
| Surface Finish | Pickled & Passivated (2B finish); Electropolished (Ra ≤ 0.4 μm upon request) | ASTM A967 |
| Pressure Rating | Up to 1.6 MPa (standard); up to 4.0 MPa (thick-wall variant) | ASME B31.3 |
| Operating Temperature Range | -40°C to +200°C (standard); up to +400°C (321 grade) | — |
| Heat Medium | Hot water (60–90°C), Steam (0.2–0.6 MPa), Thermal oil | — |
| Coating / Cladding | Bare (standard); PTFE-lined; EPDM rubber jacket (thermal insulation variant) | — |
| Welding Process | ASME Section IX | |
| Tolerance (OD) | ±0.05 mm (precision coil); ±0.1 mm (standard coil) | ASTM A213 |
| Length Tolerance | ±50 mm | — |
| Coil Pitch | 50 mm – 150 mm (adjustable for heat flux optimization) | — |
| Weight | Varies by specification; typical density 8.0 g/cm³ | — |
Manufacturing Process
Process Flow
- Raw Material Inspection: Mill certificates (EN 10204 Type 3.1) verified; chemical composition confirmed via OES (Optical Emission Spectroscopy) and PMI (Positive Material Identification).
- Tube Cutting: Precision laser or mechanical saw cutting to specified coil segment lengths, with burr-free end preparation.
- Cold Coil Forming: Continuous tube fed through CNC-controlled coiling machine with hydraulic mandrel support to achieve precise bend diameters without wall thinning, ovality, or collapse. Minimum bend radius maintained at ≥ 3× tube OD.
- Welding: Orbital TIG (Gas Tungsten Arc Welding) for spiral coil joints and end connections. Inert gas (Argon) shielding on both inside and outside of weld seam. Weld parameters optimized per ASME Section IX qualified procedures (WPQ).
- Heat Treatment (if required): Solution annealing at 1050–1150°C followed by rapid water quench to restore full austenitic structure and maximize corrosion resistance.
- Pickling & Passivation: Immersion in nitric-hydrofluoric acid bath to remove mill scale and weld discoloration; followed by passivation treatment to establish protective chromium oxide film (ASTM A967).
- Hydrostatic Testing: 100% pressure test at 1.5× maximum working pressure for 30 seconds minimum dwell time. Zero-leak criterion enforced.
- Non-Destructive Testing (NDT): 100% radiographic examination (RT) of all weld seams; eddy current testing (ECT) for tube body wall thickness verification; dye penetrant testing (PT) for surface defect detection.
- Dimensional Inspection: Coordinate measuring machine (CMM) verification of coil geometry, pitch accuracy, and end alignment.
- Final Packaging: Protective end caps, moisture-absorbing desiccant, shrink-wrap packaging with product identification labels and traceability documentation.
Quality Control Milestones
- Incoming material inspection with full traceability to mill heat numbers
- In-process dimensional checks at coiling station
- Post-weld visual and radiographic inspection
- Post-passivation surface quality audit
- Final hydrostatic pressure test with digital data logging
Applications
Primary Application: Sewage Sedimentation Tank Heating
The coil tube is designed for installation in secondary clarifiers (sedimentation tanks) within municipal and industrial wastewater treatment plants. The heating coil maintains the tank effluent temperature above the minimum threshold required for effective biological treatment (typically 10–15°C for activated sludge processes), thereby ensuring:
- Prevention of sludge freezing in cold-climate regions
- Maintenance of microbial activity in biological treatment stages
- Prevention of organic matter decomposition causing odor issues
- Optimization of sludge settling velocity and effluent quality
Heat Transfer Performance Analysis
Based on the study referenced in the product knowledge base ("Heat Transfer Characteristics and Influencing Factors of Heating Coil Tubes in Sewage Sedimentation Tanks"), the following key performance factors have been identified and optimized in the coil tube design:
- Convective Heat Transfer Coefficient: Enhanced by coil geometry-induced secondary flow (Dean vortices), achieving 1.3–1.8× the heat transfer rate of equivalent straight tubes
- Influencing Factors:
- Coil diameter and pitch ratio (D/d) — optimized for minimum pressure drop with maximum heat flux
- Reynolds number (Re) of the heat transfer fluid — turbulent regime (Re > 10,000) preferred
- Fluid viscosity and thermal conductivity of the sewage medium
- Temperature differential between heat medium and tank effluent
- Fouling factor accumulation on coil surface — mitigated by smooth electropolished finish
- Heat Loss Minimization: Insulated coil variants with EPDM or PTFE jacketing reduce radiative and convective heat losses to ambient by up to 60%
Additional Applications
- Industrial wastewater treatment (chemical, pharmaceutical, food processing)
- Process tank heating in chemical manufacturing
- Heat tracing for buried pipelines in cold regions
- Bioreactor and fermentation tank temperature control
- Marine and offshore platform heating systems
- Pharmaceutical cleanroom process piping
- Food and beverage industry sanitary heating coils
- Geothermal energy extraction systems
Quality Certifications
Compliance Standards
- Material Standards: ASTM A213 / ASME SA-213; EN 10216-5; GB/T 14976 (China National Standard)
- Piping Code: ASME B31.3 (Process Piping); ASME B31.5 (Institutional Piping)
- Welding Standards: ASME Section IX; AWS D10.9 (Stainless Steel Welding)
- Surface Treatment: ASTM A967 (Pickling and Passivation of Stainless Steel Parts)
- Pressure Vessel Components: PED 2014/68/EU (EU Pressure Equipment Directive); ASME BPVC Section VIII
- Sanitary Applications: 3-A Sanitary Standards; EHEDG guidelines; FDA-compliant materials
Management System Certifications
- ISO 9001:2015 — Quality Management System
- ISO 14001:2015 — Environmental Management System
- ISO 45001:2018 — Occupational