For a PFA (Perfluoroalkoxy) Sheath Heater Used to Maintain 160°C in a 90% Sulfuric Acid Solution at Atmospheric Pressure, What Is the Maximum Allowable Wall Thickness to Keep the Polymer Below Its Glass Transition Temperature at the Resistance Wire Interface?

The glass transition temperature (Tg) of PFA is about 160°C in 90% sulfuric acid at 160°C. The temperature of the wire interface is higher than that of the bulk acid because of heat flux through the PFA wall. With a power density of 8 W/cm² the maximum permissible wall thickness to maintain the contact below 160°C is 0.25 mm. In thick walls the wire contact overcomes Tg, resulting in polymer softening, creep and eventual failure. The computation is based on ΔT = q × t / k, with k = 0.25 W/m·K. Thermal degradation at the interface of the wire The PFA transforms from a glassy to rubbery condition at or above the Tg of the wire contact. The polymer loses its mechanical strength and creeps under the strain of the resistance wire. The wire can sink into the softened polymer, leading to a short circuit or exposing the wire to the corrosive acid. Once Tg is exceeded, the degradation is rapid. Quantitative Wire Temperature vs. Wall Thickness For 90% H₂SO₄ at 160 °C, 8 W/cm² power density Wall Thickness (mm) ΔT over PFA (°C)Temperature of wire (°C)PFA Condition 0.15 48 208 Above Tg, softening 0.20 64 224 Above Tg, creep begins 0.25 80 240 Above Tg, rapid creep 0.30 96 256 Near melt, failure imminent 0.35 112 272 Melting, immediate failure Summary: PFA Not Suitable at 160°C with Large Heat Flux The interface temperature of the wire (208 °C) is higher than the Tg (160 °C) even at a wall thickness of 0.15 mm. Thus PFA is not suitable as heater sheath at 160°C with any reasonable power density. Other materials such as PEEK (Tg = 143°C, but greater continuous usage temperature), polyimide, or metal sheathings (tantalum, titanium) should be investigated. Writing a Good Specification For 90% H2SO4 at 160C, PFA heater should not be used. Specify tantalum or silicon carbide heater. The engineer chooses a material which can withstand high temperatures to provide for a reliable operation.