Stainless Steel Coil Tube (Coiltube) — Spiral-Wound Refrigeration Evaporator Tube
Product Code: 1011 | Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd. | Category: Refrigeration & Heat Exchanger Components
1. Product Overview
The Stainless Steel Coil Tube (Coiltube) is a precision-engineered, continuously wound tubular product designed primarily for use in small-capacity refrigeration units, particularly as the core heat exchange element in falling-film evaporators (Falling Film Evaporator, FFE). This product is manufactured by winding a seamless or welded stainless steel tube into a helical coil geometry, providing an exceptionally high surface-area-to-volume ratio that maximizes thermal transfer efficiency in compact refrigeration systems.
Based on rigorous numerical simulation and experimental validation research — including studies on "Numerical Calculation and Experimental Investigation of Falling-Film Evaporators in Small Refrigeration Units" — this coil tube has been optimized for laminar-to-turbulent transition flow regimes within the falling-film heat transfer environment, ensuring uniform refrigerant distribution and minimal pressure drop across the evaporator assembly.
Unique Selling Points
- Superior Thermal Performance: The helical coil geometry promotes turbulent flow characteristics at low Reynolds numbers, enhancing the heat transfer coefficient by up to 40% compared to straight-tube configurations.
- Compact Footprint: The coiled design achieves maximum heat exchange surface area within a minimal envelope, ideal for space-constrained refrigeration applications.
- Corrosion Resistance: Fabricated from austenitic stainless steel grades, providing long-term resistance to refrigerant-induced pitting, intergranular corrosion, and stress corrosion cracking.
- Uniform Refrigerant Distribution: The continuous spiral winding ensures even refrigerant flow distribution, eliminating dry spots and dead zones within the evaporator.
- Low Pressure Drop: Smooth inner surface finish and optimized coil pitch minimize frictional pressure losses, improving overall system Coefficient of Performance (COP).
- Hygienic & Food-Grade Compliance: Suitable for food processing, pharmaceutical, and dairy refrigeration applications requiring sanitary-grade heat exchangers.
2. Material & Properties
2.1 Base Material Grades
| Property | Grade 304 (UNS S30408) | Grade 316L (UNS S31603) | Grade 321 (UNS S32100) |
|---|---|---|---|
| Standard Reference | ASTM A213 / ASME SA-213 | ASTM A213 / ASME SA-213 | ASTM A213 / ASME SA-213 |
| Chromium (Cr) Content | 18.0–20.0% | 16.0–18.0% | 17.0–19.0% |
| Nickel (Ni) Content | 8.0–10.5% | 10.0–14.0% | 9.0–12.0% |
| Molybdenum (Mo) Content | — | 2.0–3.0% | — |
| Titanium (Ti) Content | — | — | 5×C to 0.70% |
| Carbon (C) Maximum | 0.08% | 0.03% | 0.08% |
| Density (g/cm³) | 8.0 | 8.0 | 8.0 |
| Thermal Conductivity (W/m·K) at 20°C | 16.3 | 14.6 | 16.3 |
| Thermal Expansion Coefficient (μm/m·K) | 17.3 | 16.0 | 17.3 |
| Maximum Operating Temperature | 870°C | 870°C | 900°C |
| Corrosion Resistance Rating | Good (general environments) | Excellent (chloride-rich environments) | Good with high-temperature stabilization |
2.2 Mechanical Properties (Solution-Annealed Condition)
| Mechanical Property | Specification |
|---|---|
| Tensile Strength (UTS) | ≥ 515 MPa |
| Yield Strength (0.2% offset) | ≥ 205 MPa |
| Elongation at Break | ≥ 40% |
| Hardness (Rockwell B) | ≤ 95 HRB |
| Cold Bending Performance | No cracking or cracking at 180° bend (bend diameter = 2× tube OD) |
| Impact Strength (Charpy V-Notch) | ≥ 47 J at −196°C (liquid nitrogen temperature) |
2.3 Surface Finish
- Outer Surface: Bright annealed (BA) or electropolished finish with surface roughness Ra ≤ 0.4 μm.
- Inner Surface: Passivated and pickled finish with Ra ≤ 0.8 μm; available in sanitary electropolished grade (Ra ≤ 0.25 μm) upon request.
- Surface Treatment: Chemical passivation per ASTM A967 or AMS 2700 to enhance passive oxide layer integrity.
3. Technical Specifications
| Parameter | Specification / Range | Tolerance |
|---|---|---|
| Tube Outer Diameter (OD) | 6.35 mm – 38.1 mm (1/4" – 1-1/2") | ±0.05 mm |
| Tube Wall Thickness | 0.5 mm – 3.0 mm | ±10% of nominal |
| Coil Mean Diameter (CMD) | 50 mm – 300 mm | ±1.0 mm |
| Coil Pitch (Center-to-Center Spacing) | 1.2× OD – 3.0× OD | ±0.3 mm |
| Number of Turns | 10 – 500 turns (customizable) | ±1 turn |
| Coil Length (Total Unwound Length) | 2 m – 120 m | ±2% |
| End Preparation | Flat cut, beveled (37.5°), or flanged ends | Per ASME B31.3 |
| Operating Pressure (Design) | Up to 4.0 MPa (580 psi) | — |
| Operating Temperature Range | −196°C to +450°C | — |
| Refrigerants Compatible | R134a, R404A, R407C, R410A, R290, R600a, NH₃ (anhydrous) | — |
| Maximum Allowable Velocity (Inner Flow) | ≤ 3.0 m/s (liquid); ≤ 30 m/s (vapor) | — |
| Weight | Varies per configuration (approx. 0.05 – 15.0 kg) | — |
4. Manufacturing Process
4.1 Production Workflow
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Raw Material Inspection & Incoming Quality Control:
Stainless steel tube stock is received from certified mills (e.g., Outokumpu, Sandvik, Nippon Steel, or domestic equivalents). Each batch undergoes spectrographic verification (PMI — Positive Material Identification) to confirm alloy composition compliance, followed by hydrostatic testing per ASTM A269.
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Tube Preparation & Straightening:
Coiled tube stock is straightened using a multi-roller straightening machine to achieve straightness tolerance of ≤ 0.5 mm/m. Ends are cut to length with a precision tube cutting saw and deburred to eliminate internal burrs that could impede refrigerant flow.
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Helical Coiling (Spiral Winding):
The straightened tube is fed through a precision CNC coil winding machine equipped with programmable die sets. The winding parameters — coil diameter, pitch, number of turns, and feed rate — are controlled by a computerized Numerical Control (NC) system to ensure geometric accuracy. A tension-controlled feed mechanism prevents tube distortion, ovalization, or work hardening during the forming process.
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Intermediate Heat Treatment (Stress Relief):
Post-winding stress relief annealing is performed at 1050°C in a controlled atmosphere (argon or vacuum) furnace to relieve residual stresses introduced during cold coiling. This restores full austenitic microstructure and prevents stress corrosion cracking (SCC) susceptibility.
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End Forming & Brazing/Welding:
Tube ends are prepared for connection to headers or manifolds. Options include:
- Orbital TIG Welding (Gas Tungsten Arc Welding, GTAW): Per ASME Section IX, using matching filler metal (e.g., ER308L for Grade 304, ER316L for Grade 316L).
- Brazing: Silver-tin alloy brazing for smaller diameters, with backing gas (argon) to prevent oxidation.
- Flanged Connection: Flanges forged and welded to tube ends for bolted assembly.
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Surface Finishing & Passivation:
Completed coils undergo acid pickling (nitric-hydrofluoric acid blend per ASTM A967) followed by water quenching and chemical passivation to restore the protective chromium oxide film. Electropolishing is available for sanitary applications requiring Ra ≤ 0.25 μm surface roughness.
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Final Quality Inspection & Testing:
- Dimensional Inspection: Coordinate Measuring Machine (CMM) verification of coil diameter, pitch uniformity, and total length.
- Pressure Testing: Hydrostatic pressure test at 1.5× design pressure for a minimum hold time of 30 minutes, with no visible leakage.
- Leak Testing: Helium leak detection (detection limit ≤ 1×10⁻⁶ Pa·m³/s) for critical refrigeration applications.
- Visual & Dye Penetrant Inspection: Per ASTM E165/E166 for surface and weld integrity.
- Eddy Current Testing: Per ASTM E309 for subsurface defect detection in the coil body.
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Packaging & Dispatch:
Coils are wrapped in anti-corrosion VCI (Vapor Corrosion Inhibitor) film, secured in wooden crates with desiccant packs, and labeled with batch traceability documentation including Material Test Reports (MTR), Certificate of Compliance (CoC), and heat treatment records.
5. Applications
5.1 Primary Applications
- Small Refrigeration Units: Core component in commercial refrigeration systems (1–20 kW capacity), including display refrigerators, ice cream freezers, and walk-in coolers.
- Falling-Film Evaporators (FFE): Designed for refrigerant falling-film heat exchange where uniform liquid film distribution across the coil surface is critical for efficient evaporation.
- Heat Exchangers & Condensers: Used in air-cooled condensers, evaporators, and intermediate heat exchangers in refrigeration and air conditioning systems.
- Chiller Units: Integrated into water-cooled and air-cooled chiller systems for industrial process cooling.
5.2 Industry-Specific Applications
| Industry | Application | Preferred Grade |
|---|---|---|
| Food & Beverage Processing | <