For a PFA (Perfluoroalkoxy) Sheath Heater Used to Maintain 130°C in a 50% Potassium Hydroxide Solution at Atmospheric Pressure, What Is the Maximum Allowable Wall Thickness to Keep the Polymer Below Its Melting Point at the Resistance Wire Interface?

PFA has a melting point of 260°C in 50% potassium hydroxide at 130°C. The temperature at the wire interface is T acid + ΔT where ΔT = q × t / k with k = 0.25 W/m.K. The maximum permitted wall thickness is 0.41 mm for a representative power density of 8 W/cm² (80,000 W/m²) and bulk temperature of 130°C to ensure the wire contact remains below 260°C. If the walls were even thicker the interface would surpass 260°C, melting the polymer and causing system catastrophic failure.

The process of surface melting of a wire

The resistance wire becomes hot. The temperature difference across the PFA wall is proportional to the wall thickness. When the interface temperature surpasses 260°C the PFA melts and the wire is exposed and the electrical insulation fails. The caustic solution may potentially react with the molten polymer.

Quantitative Wire Temperature Versus Wall Thickness

For 50% KOH at 130°C and 8 W/cm2:

Wall thickness (mm) ΔT (°C) Wire temperature (°C) PFA condition 0.2 64 194 Safe 0.3 96 226 Safe 0.4 128 258 Near melting 0.41 131 261 Melting starts 0.5 160 290 Melted, failed Maximum allowable wall thickness guide

Power Density (W/cm 2 ) Max Wall Thickness for T wire <260 o C (mm) 5 0.65 6 0.54 7 0.46 8 0.41 10 0.32 12 0.27
Developing an Informed Specification

For a PFA heater in 50% KOH at 130°C, provide wall thickness 0.35-0.40 mm at 8 W/cm². Reduce thickness for increased power density. The engineer adjusts the wall thickness so as not to melt the PFA sheath.