In a PFA (Perfluoroalkoxy) Sheath Heater Used to Heat a 20% Hydrochloric Acid Solution at 85°C, How Does the Surface Roughness (Ra 0.2 µm vs. 1.0 µm) Change the Bubble Nucleation Density and the Critical Heat Flux for Nucleate Boiling?
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PFA sheath heaters in 20% hydrochloric acid at 85°C are in the nucleate boiling regime. The rough surface (Ra 1.0 microm) provides a bubble nucleation site density of 10,000-50,000 sites/cm 2 , a bubble departure diameter of 0.2-0.5 mm and a critical heat flux (CHF) of 150-200 kW/m 2 . Surface Smooth (Ra 0.2 µm) Nucleation Site Density 500-2,000 sites/cm 2 Larger Bubble Departure Diameter 0.8-1.5 mm Lower CHF 80-120 kW/m 2 The rough surface increases heat transport by 30-50% and allows for increased power density before film boiling occurs.
Bubble Nucleation Mechanism on Surface Roughness
Surface cavities are locations of nucleation. The number of active sites increases with increasing roughness The bubble departure diameter decreases with the site density due of merging and easier detachment of bubbles. CHF scales with site density since the surface is wetted for a longer time.
Quantitative Heat Transfer and Ra
Surface Roughness Ra (µm) Nucleation Site Density (sites/cm²) Bubble Departure Diameter (mm) CHF (kW/m²) Heat Transfer Coefficient (W/m²·K) 0.1 100-500 1.0-2.0 50-80 3,000-5,000 0.2 500-2,000 0.8-1.5 80-120 5,000-8,000 0.5 2,000-10,000 0.5-1.0 120-160 8,000-12,000 1.0 10,000-50,000 0.2-0.5 150-200 12,000-18,000 2.0 50,000-200,000 0.1-0.3 180-220 15,000-22,000
Surface Roughness Selection Guide
Expected CHF (kW/m2) Recommended Ra (µm) Desired Heat Transfer PerformanceRecommended for Boiled HCl
Low (smooth, easy to clean) 0.2 80-120 No
Medium 0.5 120-160 Yes
High (optimised) 1.0 150-200 Yes
Very high (possible fouling) 2.0 180-220 Limited
Specifying with confidence
A surface roughness of Ra 0.5-1.0 µm is recommended for a PFA heater in boiling HCl at 85°C to improve the nucleate boiling heat transmission. Engineer controls heater efficiency by manipulating surface roughness.
