For a PFA (Perfluoroalkoxy) Sheath Heater Used to Maintain 160°C in a 70% Sulfuric Acid Solution at 2 Bar Pressure, What Is the Maximum Allowable Wall Thickness to Prevent Thermal Degradation of the Polymer at the Resistance Wire Interface?

PFA (perfluoroalkoxy) sheath heaters in 70% sulphuric acid at 160°C and 2 bar pressure must be configured to keep the temperature at the resistance wire interface at or below the continuous service temperature of the polymer at 260°C. The PFA wall temperature drop is given by ΔT = (q × t) / k, where q is heat flow (W/m²), t is wall thickness (m), and k is PFA thermal conductivity (0.25 W/m·K). For a typical power density of 8 W/cm2 (80,000 W/m2) and a bulk acid temperature of 160 °C, the highest allowed wall thickness to maintain the wire temperature below 260 °C is 0.31 mm. Thicker walls would result in wire contact above 260°C and polymer softening and deterioration.

Mechanism of thermal degradation of PFA jacket

The heat generated by the resistance wire must be transferred through the PFA wall to the acid. Temperature at the wire interface T_wire = T_acid + ΔT. PFA starts to soften around 260°C and decomposes above 300°C. To achieve long-term reliability, the temperature of the wire interface should not exceed 260°C. The heat flux q is obtained from the power density (W/cm2) which is dependent on the power rating of the heater and the surface area.

Wall Thickness and Quantitative Wire Temperature

For 70% H2SO4 at 160C (8W/cm2 = 80,000 W/m2):

Wall Thickness (mm) ΔT Across PFA (°C) Wire Temperature (°C) PFA Condition 0.15 48 208 Safe 0.20 64 224 Safe 0.25 80 240 Safe 0.31 100 260 Maximum permitted 0.35 112 272 Degradation starts 0.40 128 288 Softening 0.50 160 320Huge deterioration
Maximum Permissible Wall Thickness for Different Power Densities

The following table shows the maximum allowed wall thickness for a PFA sheath heater in 70% H 2 SO 4 at 160°C vs. power density.

Power Density (W/cm²) ΔT per mm thickness (°C/mm)Maximum Wall Thickness for T_wire <260°C (mm) 5 20 0.50
6 24 0.42 7 28 0.36 8 32 0.31 9 36 0.28 10 40 0.25 12 48 0.21
Engineering Considerations

The wall thickness must be sufficient to withstand the 2 bar internal pressure (if the heater is a tube) and handling loads. For temperatures exceeding 1 bar, a minimum wall of 0.3 mm is recommended independent of thermal calculations. For this application (2 bar) a wall thickness of 0.3–0.35 mm is a balance between thermal and mechanical requirements. If the walls are thicker, then the power density will have to be reduced or a heat spreader added.

Alternative Choices

If the wall thickness required for mechanical integrity exceeds the thermal limit, the design can be adjusted by:

Decreasing power density (longer or larger diameter heater)

Use a more thermally conductive substance (PEEK has k = 0.32 W/m.K, a little better)

Inserting a metal heat spreader into the PFA sheath

Use a higher temperature polymer (none available above 260°C for this chemical environment)

A Knowledgeable Particulars

As an example, for a PFA sheath heater in 70% sulphuric acid at 160°C, with 2 bar pressure and 8 W/cm2 power density, specify a maximum wall thickness of 0.31 mm to maintain the wire contact below 260°C. If mechanical dependability is required, apply a reinforcing layer or reduce the power density to 6 W/cm² and wall can be 0.42 mm. The engineer controls the thickness of the wall and the power density to avoid the thermal degradation of the PFA at the interface with the resistance wire.