Stainless Steel Coil Tube (盘管) — Evaporative Condenser Application Series

1. Product Overview

The Stainless Steel Coil Tube (盘管) manufactured by Wenzhou Jixing Metal Materials Co., Ltd. is a precision-engineered continuous helical or serpentine tubing product specifically designed for use in evaporative condensers and other high-efficiency heat exchange systems. The coil tube is formed into continuous helical or serpentine configurations that maximize the heat transfer surface area within a compact envelope, thereby enhancing thermal performance in evaporative cooling applications.

Based on rigorous numerical simulation studies — particularly the research titled "Numerical Simulation of the Effect of Coil Cross-Sectional Shape on Heat Transfer in Evaporative Condensers" (《蒸发式冷凝器的盘管截面形状对传热影响的数值模拟》) — the cross-sectional geometry of the coil tube has been systematically optimized. Computational Fluid Dynamics (CFD) analysis and conjugate heat transfer simulations have demonstrated that the cross-sectional profile of the coil tube exerts a decisive influence on the overall heat transfer coefficient, flow distribution uniformity, and pressure drop characteristics within the evaporative condenser unit.

This product line represents the culmination of material science expertise combined with thermal-hydraulic engineering optimization, delivering superior thermal performance, corrosion resistance, and long-term operational reliability in demanding industrial and commercial cooling environments.

2. Material Composition and Properties

The coil tube is fabricated from high-grade austenitic stainless steel, selected for its exceptional combination of corrosion resistance, thermal conductivity, mechanical strength, and formability. The primary material grades offered include:

Key Physical Properties (Typical for SAE 316 Grade):

3. Technical Specifications

Parameter Specification / Range Standard Reference
Outer Diameter (OD) (Outer Diameter): 12.7 mm – 114.3 mm (1/2" – 4") ASTM A213 / ASME B36.19M
Wall Thickness (Wall Thickness): 0.50 mm – 6.35 mm (0.020" – 0.250") ASTM A213 / ASME B36.19M
Coil Pitch (Coil Pitch): 15 mm – 150 mm (customizable) Manufacturer's specification
Coil Diameter (Coil Diameter): 200 mm – 3000 mm (customizable) Manufacturer's specification
Tube Length per Coil (Tube Length per Coil): 5 m – 500 m (customizable) Customer requirement
Cross-Sectional Shape (Cross-Sectional Shape): Circular, Elliptical, Flat-oval, Rectangular, or Custom Profile Based on CFD-optimized design
Surface Finish (Surface Finish): Mirror polished (Ra ≤ 0.4 μm), Satin finish, or Pickled & Passivated ASTM A478
Operating Pressure (Operating Pressure): Up to 25 bar (373 psi) (design-dependent) ASME BPV Code Section VIII
Operating Temperature (Operating Temperature): −196 °C to +450 °C Material grade dependent
Heat Transfer Coefficient Enhancement (Heat Transfer Coefficient Enhancement): 15% – 40% improvement over plain circular tubes (CFD-validated) Numerical simulation results
Hydrostatic Test Pressure (Hydrostatic Test Pressure): 1.5 × design pressure, minimum 10 bar ASTM A213 / ASME BPV Code
Dimensional Tolerance (Dimensional Tolerance): ±0.1 mm (OD), ±0.1 mm (wall thickness) ASTM A213
Standards Compliance (Standards Compliance): ASTM A213, ASME B36.19M, EN 10216-5, GB/T 14976 Multinational standards

4. Cross-Sectional Geometry Optimization — Numerical Simulation Basis

A distinguishing feature of this coil tube product line is its cross-sectional geometry optimization, grounded in extensive numerical simulation research. The following key findings from the CFD-based study on evaporative condenser coil tubes are incorporated into the product design:

  1. Circular vs. Non-Circular Cross-Sections: Numerical simulations comparing circular, elliptical, flat-oval, and rectangular coil cross-sections revealed that non-circular profiles can significantly alter the secondary flow patterns within the coil. Elliptical and flat-oval cross-sections, when oriented with the major axis perpendicular to the coil curvature, enhance secondary vortex formation, thereby increasing the local Nusselt number (Nusselt Number) and improving heat transfer by up to 35% compared to circular tubes of equivalent cross-sectional area.
  2. Deformation Parameter (Deformation Parameter) Effect: The degree of cross-sectional flattening (deformation parameter, defined as the ratio of minor axis to major axis) was systematically varied. An optimal deformation parameter of approximately 0.5–0.7 was identified, balancing enhanced heat transfer against acceptable pressure drop increments. Beyond a deformation parameter of 0.8, the heat transfer benefit diminishes while the pressure drop increases disproportionately.
  3. Curvature Effect (Curvature Effect): The Dean number (Dean Number), defined as De = Re × √(d/D), where Re is the Reynolds number (Reynolds Number), d is the tube diameter, and D is the coil diameter, was shown to be the primary dimensionless parameter governing secondary flow intensity. The simulation results confirmed that the secondary flow strength increases with the square root of the Dean number, leading to enhanced mixing and boundary layer disruption.
  4. Reynolds Number Regime (Reynolds Number Regime): The heat transfer enhancement due to non-circular cross-sections is most pronounced in the transitional flow regime (Re = 2000–10,000), which is the typical operating range for evaporative condensers. In fully turbulent flow (Re > 10,000), the relative enhancement decreases but remains significant.
  5. Pressure Drop Penalty (Pressure Drop Penalty): The simulations quantified that the pressure drop increase due to non-circular cross-sections ranges from 8% to 25% relative to circular tubes, depending on the deformation parameter and flow regime. The overall thermal-hydraulic performance, evaluated using the Colburn j-factor (Colburn j-factor) ratio and the overall performance ratio, confirms a net positive benefit for the optimized geometries.

Design Principle: The coil tube cross-sectional profiles are selected based on the specific evaporative condenser operating conditions — including flow rate, working fluid properties, inlet temperature, and allowable pressure drop — to achieve the optimal balance between heat transfer enhancement and pumping power consumption.

5. Manufacturing Process

The manufacturing of the stainless steel coil tube follows a rigorous multi-stage process to ensure dimensional accuracy, material integrity, and surface quality:

  1. Raw Material Procurement (Raw Material Procurement): High-purity stainless steel wire rod or seamless tube stock is sourced from certified mills, with each batch accompanied by a mill test certificate (Mill Test Certificate) verifying chemical composition (Chemical Composition) and mechanical properties (Mechanical Properties).
  2. Heat Treatment (Heat Treatment): The tube stock undergoes solution annealing (Solution Annealing) at temperatures between 1050 °C and 1150 °C, followed by controlled cooling, to achieve a fully austenitic microstructure with optimal corrosion resistance and ductility. For 321 grade, titanium stabilization treatment is performed to prevent chromium carbide precipitation.
  3. Piercing and Cross-Rolling (Piercing and Cross-Rolling): For seamless tube production, solid billets are heated and pierced to form hollow tubes, followed by cross-rolling to achieve the required outer diameter and wall thickness with tight tolerances.
  4. Stretch Reducing / Cold Drawing (Stretch Reducing / Cold Drawing): The hot-rolled or cross-rolled tube is cold-drawn through precision dies to achieve the final dimensions with high accuracy. This process also introduces beneficial compressive residual stresses on the inner surface, enhancing fatigue resistance.
  5. Coil Forming (Coil Forming): The straight tube is cold-coiled onto a mandrel of the specified coil diameter using a precision coiling machine. The coiling process is controlled to maintain uniform pitch and prevent cross-sectional deformation beyond the designed profile. For non-circular cross-sections, specialized forming tools are employed to achieve the target profile with high repeatability.
  6. Welding (Welding) (for Coil-to-Coil or Coil-to-Header Connections): Where necessary, coil-to-coil or coil-to-header connections are made using orbital TIG welding (Orbital TIG Welding) or automatic orbital welding, ensuring consistent weld quality. Welding parameters are optimized for each material grade to minimize the heat-affected zone (Heat-Affected Zone) and prevent sensitization.
  7. Pickling and Passivation (Pickling and Passivation): The finished coil tube assembly undergoes pickling in a mixed acid solution (typically nitric acid + hydrofluoric acid) to remove mill scale, oxide films, and surface contaminants. Passivation treatment in a dilute nitric acid solution (typically 20–30% by volume) forms a stable chromium oxide passive film on the surface, enhancing corrosion resistance.
  8. Surface Polishing (Surface Polishing): For applications requiring low surface roughness (e.g., food and pharmaceutical processing), the tube interior and exterior are polished to a mirror finish (Ra ≤ 0.4 μm) using electrochemical polishing (Electrochemical Polishing) or mechanical polishing techniques.
  9. Quality Inspection (Quality Inspection): Every coil tube undergoes comprehensive inspection including:

6. Applications

The Stainless Steel Coil Tube is applicable in a wide range of industries and systems where efficient heat transfer, corrosion resistance, and hygienic design are critical requirements: