Stainless Steel Coil Tube (Coiltube) — Heat Pump Dual-Supply System Series
Product Overview
The Stainless Steel Coil Tube (hereinafter referred to as "Coiltube") manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered, continuously wound tubular product designed for integration into heat exchanger assemblies within heat pump dual-supply units (Heat Pump Dual-Supply Units, HPDSU). These units provide simultaneous heating and hot water supply through a single refrigeration cycle, making the coil tube a critical component in residential and commercial HVAC (Heating, Ventilation, and Air Conditioning) systems.
The Coiltube is engineered to address a well-documented operational challenge: the propensity for ice formation at the bottom of coil heat exchangers during low-temperature operation in heating mode. Through optimized geometry, material selection, and manufacturing tolerances, Wenzhou Jixing's Coiltube significantly mitigates the risk of bottom-section icing (Bottom Heat Exchanger Icing, BHI), thereby extending service life, maintaining thermal efficiency, and ensuring consistent performance across extreme ambient temperature ranges.
This product belongs to the Documentary Learning (文献学习) classification, indicating that its design and performance characteristics have been refined through systematic academic research and field-validated studies on heat pump coil heat exchanger anti-icing mechanisms.
Material & Properties
Base Material
The Coiltube is fabricated from austenitic stainless steel grades conforming to international standards, with the primary grade being SAE 304 (UNS S30400) or SAE 316L (UNS S31603), selected based on the specific corrosive environment and operational requirements of the end application.
- SAE 304 / 0Cr18Ni9: Provides excellent general corrosion resistance, weldability, and formability. Suitable for most residential and light commercial heat pump applications where chloride exposure is moderate.
- SAE 316L / 00Cr17Ni14Mo2: Offers superior resistance to pitting and crevice corrosion due to the addition of Molybdenum (Mo). Recommended for coastal installations, industrial environments, or applications involving de-icing chemical agents.
Key Mechanical and Physical Properties
| Property | SAE 304 | SAE 316L | Test Standard |
|---|---|---|---|
| Tensile Strength (Rm) | ≥ 520 MPa | ≥ 530 MPa | ASTM A270 / EN 10217-7 |
| Yield Strength (Rp0.2) | ≥ 205 MPa | ≥ 205 MPa | ASTM A270 / EN 10217-7 |
| Elongation (A5) | ≥ 40% | ≥ 40% | ASTM A270 / EN 10217-7 |
| Density | 7.93 g/cm³ | 7.98 g/cm³ | — |
| Thermal Conductivity (at 100°C) | 16.3 W/(m·K) | 14.6 W/(m·K) | ASTM E153 |
| Thermal Expansion Coefficient (20–100°C) | 17.3 × 10⁻⁶ /K | 17.0 × 10⁻⁶ /K | ASTM E228 |
| Specific Heat (at 100°C) | 0.50 J/(g·K) | 0.50 J/(g·K) | — |
| Maximum Service Temperature | 870°C | 870°C | — |
| Minimum Service Temperature | −196°C | −196°C | — |
Surface Finish Options
- Ba&R No. 2D (Cold Rolled and Annealed): Standard mill finish with a dull, matte appearance. Suitable for general heat exchanger applications.
- Ba&R No. 2B (Cold Rolled and Bright Annealed): Bright, smooth surface with improved corrosion resistance. Preferred for visible installations.
- Mirror Polish (Ra ≤ 0.4 μm): Achieved through mechanical polishing or electrolytic polishing. Reduces fouling and facilitates cleaning.
- Electropolished Finish: Removes surface impurities and creates a passive chromium oxide layer. Recommended for hygienic or high-corrosion-resistance applications.
Specifications
Dimensional Parameters
| Parameter | Specification | Tolerance | Notes |
|---|---|---|---|
| Outer Diameter (OD) | 6.35 mm, 8.0 mm, 9.53 mm, 10.0 mm, 12.0 mm, 15.88 mm, 19.05 mm, 25.4 mm | ± 0.10 mm | Custom diameters available upon request |
| Wall Thickness (WT) | 0.30 mm, 0.40 mm, 0.50 mm, 0.60 mm, 0.80 mm, 1.00 mm, 1.50 mm, 2.00 mm | ± 0.05 mm | Minimum wall thickness ≥ 0.30 mm for structural integrity |
| Coil Inner Diameter (ID) | 100 mm – 600 mm | ± 2.0 mm | Determined by heat exchanger housing dimensions |
| Coil Length (Unwound) | 5 m – 120 m | ± 0.5% | Custom lengths available |
| Coil Height (Stacked) | 50 mm – 400 mm | ± 1.0 mm | Depends on number of turns and spacing |
| Turn-to-Turn Spacing | 1.5 mm – 5.0 mm | ± 0.2 mm | Optimized for flow distribution and anti-icing performance |
| End Preparation | Plain end, flared end, threaded end, or socket weld end | Per ASME B1.20.1 | Custom end treatments available |
| Maximum Operating Pressure | 2.5 MPa (25 bar) | — | Based on 0.50 mm wall thickness at 6.35 mm OD |
| Hydrostatic Test Pressure | 3.75 MPa (37.5 bar) | — | 1.5× maximum operating pressure |
| Operating Temperature Range | −25°C to +150°C | — | Extended range available with 316L grade |
Performance Characteristics
| Performance Metric | Specification | Test Condition |
|---|---|---|
| Heat Transfer Coefficient (U-value) | ≥ 850 W/(m²·K) | Water-to-air, 25°C ΔT |
| Pressure Drop (per 10 m coil length) | ≤ 1.2 kPa | Flow rate 1.0 L/min, 6.35 mm OD × 0.50 mm WT |
| Anti-Icing Performance | Bottom-section icing delay ≥ 15 minutes | Ambient −15°C, RH 90%, heating mode |
| Corrosion Resistance (Salt Spray Test) | ≥ 500 hours without red rust | ASTM B117, 5% NaCl, 35°C |
| Flow Velocity Range | 0.5 – 3.0 m/s | Optimal range for anti-icing operation |
Manufacturing Process
The manufacturing of Wenzhou Jixing's Coiltube follows a rigorous, multi-stage process designed to ensure dimensional accuracy, metallurgical integrity, and anti-icing performance:
- Raw Material Procurement and Inspection: Stainless steel strip (coil) is sourced from certified mills (e.g., POSCO, Outokumpu, or domestic equivalents) and subjected to incoming quality verification including chemical composition analysis (via Optical Emission Spectrometry, OES), mechanical property testing, and surface defect inspection.
- Slitting and Edge Preparation: The stainless steel strip is slit to the precise width required for the target tube outer diameter using high-precision slitting lines. Edges are deburred and inspected for cracks or laminations.
- Forming (Welded Tube Formation): The slit strip is formed into a tubular shape through a roll-forming process. The longitudinal seam is joined using one of the following welding methods:
- TIG (Tungsten Inert Gas) Welding per ASTM A270, with internal and external polishing of the weld seam. Preferred for higher-quality applications.
- Flash Butt Welding (FBW) for high-volume production of seamless-equivalent tubing.
- Resistance Welding for thinner wall thicknesses (≤ 0.50 mm).
- Heat Treatment (Solution Annealing): The formed tube undergoes solution annealing at 1050°C – 1150°C followed by controlled cooling (air or water quench, depending on grade) to achieve the desired microstructure. This eliminates residual stresses from forming and welding, restores full austenitic properties, and ensures optimal corrosion resistance.
- Dimensional Calibration: The annealed tube passes through calibration rolls to achieve the precise outer diameter and wall thickness tolerances. Roundness is verified to be ≤ 0.05 mm.
- Coil Winding (Coiling): The straight tube is wound into the target coil geometry using a precision coiling machine. The winding process is controlled to maintain uniform turn-to-turn spacing and consistent coil diameter. The coiling parameters (tension, speed, and alignment) are optimized based on research findings regarding flow distribution and anti-icing performance in heat pump coil heat exchangers.
- End Forming and Finishing: Tube ends are prepared according to the specified end treatment (plain, flared, threaded, or socket weld). Ends are deburred, chamfered, and cleaned.
- Surface Treatment and Cleaning: The coil undergoes chemical pickling and passivation to remove any heat scale, welding residues, or surface contaminants. A protective chromium oxide passive layer is formed to enhance corrosion resistance.
- Quality Inspection and Testing:
- Visual inspection of the entire coil length for surface defects, weld seam quality, and dimensional conformity.
- Dimensional verification using laser micrometers and ultrasonic wall thickness gauges.
- Hydrostatic pressure testing at 1.5× the maximum operating pressure for a minimum dwell time of 30 seconds per tube.
- Weld seam integrity verification via dye penetrant testing (DPT) or capillary testing per ASTM E709.
- Chemical composition verification via OES sampling from representative coils.
- Anti-Icing Performance Validation: Representative coil assemblies are subjected to controlled-environment testing in a low-temperature, high-humidity chamber to validate the anti-icing performance. Key parameters monitored include:
- Bottom-section surface temperature distribution (via infrared thermography).
- Flow velocity uniformity across the coil cross-section.
- Time to first ice nucleation at the coil bottom.
- Heat transfer efficiency degradation over time under icing conditions.
- Packaging and Shipment: Coils are packaged on wooden reels with protective wrapping (VCI (Vapor Corrosion Inhibitor) film and bubble wrap) to prevent mechanical damage and atmospheric corrosion during transit. Each shipment is accompanied by a Certificate of Analysis (COA) and Certificate of Conformance (CoC).
Applications
The Coiltube finds extensive application across multiple industries, with its primary domain being the heat pump and HVAC sector:
Primary Applications
- Heat Pump Dual-Supply Units (HPDSU): The Coiltube serves as the core heat exchange element in dual-supply heat pumps that simultaneously provide space heating and domestic hot water. The anti-icing design is critical for reliable operation in cold climates (Climatic Zone 3 and below per ASHRAE classification).
- Air Source Heat Pumps (ASHP): Used as evaporator and condenser coil elements in residential and commercial air-source heat pump systems.
- Ground Source Heat Pumps (GSHP): Employed in closed-loop ground source systems where the coil interfaces with glycol-water heat transfer fluids.
- Commercial Refrigeration Systems: Applied in display cases, walk-in coolers, and refrigeration condensers where stainless steel corrosion resistance is essential.
- Domestic Hot Water Heaters: Integrated into heat pump water heater (HPWH) systems for efficient domestic hot water production.
Secondary Applications
- Industrial Process Heating and Cooling: Used in chemical processing, food and beverage production, and pharmaceutical manufacturing for precise temperature control.
- Marine HVAC Systems: The 316L grade Coiltube is suitable for marine environments where saltwater corrosion is a persistent challenge.
- Automotive and Transportation HVAC: Applied in electric vehicle (EV) thermal management systems and commercial vehicle climate control.
- Medical and Laboratory Equipment: Utilized in cryogenic systems, sterilization equipment, and laboratory temperature control apparatus.
- Agricultural Greenhouse Climate Control: Used in greenhouse heating and cooling systems for year-round crop production.
Quality Certifications and Standards Compliance
Wenzhou Jix