Stainless Steel Coil Tube (Helical Coil Tube) — Product No. 720
1. Product Overview
The Stainless Steel Coil Tube (Helical Coil Tube) is a precision-engineered tubular component manufactured from austenitic and duplex stainless steel grades, designed for integration into helical coil-type electric heating elements, heat exchangers, and low-temperature dust collection systems. This product is supplied in continuous coiled form (Coil Tube), enabling downstream fabricators to cut, form, and assemble coils of varying pitch, diameter, and length to meet specific thermal and mechanical design requirements.
The coil tube serves as the primary conduit for electrical resistance heating or fluid circulation in industrial thermal management systems. Its helical geometry maximizes heat transfer surface area within a compact volumetric envelope, making it particularly advantageous in space-constrained installations such as low-temperature and ultra-low-temperature (LT/ULT) bag filter dust collection systems, where pre-heating of flue gas is critical to prevent condensation and acid dew-point corrosion.
2. Material & Properties
2.1 Common Material Grades
- Austenitic Stainless Steel: AISI 304 (UNS S30400), AISI 316L (UNS S31603), AISI 310S (UNS S31008) — offering excellent corrosion resistance, formability, and thermal stability.
- Duplex Stainless Steel: UNS S31803 (2205), UNS S32750 (2507) — providing superior mechanical strength and resistance to chloride-induced stress corrosion cracking (SCC).
- Super-Austenitic Stainless Steel: Alloy 20 (UNS N08825), Alloy C-276 (UNS N10276) — for extreme environments with aggressive sulfuric or hydrochloric acid exposure.
- High-Nickel Alloys: Inconel 600 (UNS N06600), Incoloy 800HT (UNS N08810) — for ultra-high-temperature service exceeding 1000 °C.
2.2 Key Physical and Mechanical Properties (Representative — AISI 316L)
- Density (Density): 8.0 g/cm³
- Melting Point (Melting Range): 1400–1450 °C
- Thermal Conductivity (Thermal Conductivity): 14.9 W/(m·K) at 20 °C
- Specific Heat Capacity (Specific Heat Capacity): 0.50 J/(g·K) at 20 °C
- Tensile Strength (Tensile Strength): ≥ 515 MPa (annealed condition)
- Yield Strength (Yield Strength): ≥ 205 MPa (0.2% offset)
- Elongation (Elongation): ≥ 40% (in 50 mm gauge length)
- Maximum Continuous Service Temperature: 870 °C (oxidation resistance limit)
- Corrosion Resistance: Excellent resistance to atmospheric, aqueous, and mild chemical environments; superior to 304 in chloride-containing media due to molybdenum (Mo) content of 2.0–2.5%.
2.3 Chemical Composition (AISI 316L — Representative)
| Element | Symbol | Content (wt%) |
|---|---|---|
| Carbon | C | ≤ 0.030 |
| Chromium | Cr | 17.00 – 19.00 |
| Nickel | Ni | 10.00 – 14.00 |
| Molybdenum | Mo | 2.00 – 2.50 |
| Manganese | Mn | ≤ 2.00 |
| Silicon | Si | ≤ 1.00 |
| Phosphorus | P | ≤ 0.035 |
| Sulfur | S | ≤ 0.030 |
| Iron | Fe | Balance |
3. Technical Specifications
| Parameter | Specification / Range | Standard Reference |
|---|---|---|
| Product Name | Stainless Steel Coil Tube (Helical Coil Tube) | — |
| Product Number | 720 | Internal Catalog |
| Outer Diameter (OD) | Φ 3.0 mm – Φ 32.0 mm (standard); custom up to Φ 50.0 mm | ASTM A269 / GB/T 12771 |
| Wall Thickness (WT) | 0.30 mm – 2.50 mm | ASTM A269 / GB/T 12771 |
| Coil Inner Diameter (Coil ID) | Φ 100 mm – Φ 500 mm (as supplied) | Supplier Standard |
| Coil Length (per spool) | 50 m – 500 m (negotiable) | Supplier Standard |
| Surface Finish | Pickled & Passivated (2B); Electropolished; Brushed | ASTM A240 / GB/T 4237 |
| Heat Treatment Condition | Solution Annealed (SA); Cold Worked (CW) | ASTM A269 |
| Dimensional Tolerance (OD) | ± 0.05 mm (precision); ± 0.10 mm (standard) | ASTM A269 |
| Dimensional Tolerance (WT) | ± 10% of nominal (minimum ± 0.02 mm) | ASTM A269 |
| Helical Pitch (as-formed) | Custom: 15 mm – 150 mm | Project-Specific |
| Helical Coil Diameter | Custom: Φ 50 mm – Φ 2000 mm | Project-Specific |
| Maximum Operating Temperature | Up to 1000 °C (grade-dependent) | Grade Data Sheet |
| Maximum Operating Pressure | Up to 6.3 MPa (316L, Φ 12 mm × 1.0 mm, CW condition) | ASME BPVC VIII |
| Electrical Resistance (per unit length, 316L Φ 6 × 0.5) | ≈ 0.085 Ω/m (at 20 °C) | Calculated per ASTM A269 |
| Minimum Bend Radius | 3 × OD (cold bending without filler) | Supplier Standard |
| Welding Compatibility | TIG (GTAW), MIG (GMAW), Laser Welding | ASME Section IX |
4. Manufacturing Process
4.1 Raw Material Procurement
High-purity stainless steel billets or wire rods are sourced from certified mills (e.g., Outokumpu, Sandvik, POSCO, Taigang) with full mill test certificates (MTC) conforming to EN 10204 Type 3.1. Incoming materials undergo spectroscopic verification (Optical Emission Spectrometry, OES) and ultrasonic flaw detection (Ultrasonic Testing, UT) to ensure compositional accuracy and absence of internal defects.
4.2 Seamless Tube Production
- Hot Rolling / Hot Extrusion: Billets are heated to 1200–1300 °C and pierced to produce seamless tube blanks.
- Cold Drawing / Cold Pilgering: The tube blanks are cold-worked through progressively reducing dies to achieve precise OD and WT tolerances. This process enhances surface finish and mechanical properties through work hardening.
- Solution Annealing: Tubes are reheated to 1050–1150 °C and rapidly quenched in water to dissolve carbides, restore full austenitic microstructure, and achieve maximum corrosion resistance.
- Pickling & Passivation: Tubes are immersed in a mixed acid bath (HNO₃/HF or HNO₃/HCl) to remove oxide scale, followed by passivation in nitric acid to form a stable chromium oxide (Cr₂O₃) passive film.
4.3 Coil Winding
The straight, annealed tubes are wound onto precision-guided reels to produce coil tube spools. The winding process employs tension-controlled mechanisms to prevent ovalization or kinking. Each spool is labeled with material grade, heat number, dimensions, and batch identification for full traceability.
4.4 Helical Coil Forming (Downstream — Reference)
In downstream fabrication, the coil tube is formed into helical coils using CNC-controlled bending machines. The forming process involves:
- Induction pre-heating (if required for cold-worked material) to 400–500 °C to prevent cracking during bending.
- Multi-axis CNC bending to achieve precise helical pitch and coil diameter.
- Post-forming stress relief annealing at 850–950 °C to eliminate residual stresses introduced during forming.
4.5 Quality Control (Quality Control)
- Chemical Analysis: Spectrometric verification of every heat lot.
- Mechanical Testing: Tensile testing (Tensile Testing) and hardness testing (Hardness Testing) per ASTM A269.
- Hydrostatic Testing: Each tube length is pressure-tested to verify watertight integrity (Hydrostatic Pressure Test).
- Dimensional Inspection: Caliper measurement of OD, WT, and straightness; laser scanning for coil geometry verification.
- Surface Inspection: Visual and eddy current testing (Eddy Current Testing, ECT) for surface defects and wall thickness uniformity.
- Corrosion Testing: Salt spray testing (Salt Spray Test, ASTM B117) — minimum 200 hours without pitting for 316L grade.
5. Applications
5.1 Low and Ultra-Low Temperature (LT/ULT) Dust Collection Systems
This is the primary application domain highlighted in the referenced technical literature. In cement, metallurgical, and power generation industries, flue gas temperatures can drop below the acid dew point (Acid Dew Point, typically 120–180 °C for SOₓ/HCl-containing gases). Helical coil electric heaters, fabricated from this coil tube, are installed upstream of bag filters to pre-heat the flue gas, preventing condensation of sulfuric acid (H₂SO₄) and hydrochloric acid (HCl) on filter bags. This eliminates:
- Acidic corrosion of filter media (Acidic Corrosion of Filter Media).
- Blindness and agglomeration of filter bags (Blindness and Agglomeration of Filter Bags).
- Reduced filter service life and unplanned shutdowns.
The helical coil geometry provides uniform radial heat distribution and high heat flux density, ensuring rapid and consistent gas temperature elevation. Stainless steel grades such as 310S, Alloy 20, or Inconel 600 are typically specified for the heating element wire, while 316L or 304 coil tube is used for the containment sheath and manifold piping.
5.2 Additional Application Domains
- Industrial Process Heating: Helical coil heat exchangers for chemical reactors, oil refineries, and pharmaceutical processing where high heat transfer coefficients are required.
- Steam Generator Systems: Boiler tube coils for low-pressure steam generation in marine and mobile power applications.
- Automotive Thermal Management: Compact heat exchanger coils for electric vehicle (Electric Vehicle, EV) battery thermal control and cabin HVAC systems.
- Food & Beverage Industry: Sanitary-grade coil heat exchangers for pasteurization, sterilization, and CIP (Clean-In-Place) systems, utilizing electropolished 316L tubing.
- Marine & Offshore: Seawater-cooled heat exchanger coils in shipboard HVAC and process cooling systems, requiring 2205 or 2507 duplex steel for chloride resistance.
- Renewable Energy: Geothermal heat exchanger coils and solar thermal collector tubes.
- Aerospace: Fuel pre-heating coils and hydraulic system heat exchangers using Inconel or Monel alloys.