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 have to be designed for 70 % sulfuric acid at 160°C and 2 bar pressure so that the temperature at the interface to the resistance wire stays below the continuous service temperature of the polymer of 260°C. The temperature drop across the PFA wall is given by the equation ΔT = (q × t) / k, where q is the heat flux (W/m²), t is the wall thickness (m), and k is the thermal conductivity of PFA (0.25 W/m·K). For a typical power density of 8 W/cm2 (80000 W/m2) and a bulk acid temperature of 160°C the highest permissible wall thickness to ensure the wire temperature <260°C is 0.31 mm. If the walls are made thicker, the wire interface would be above 260°C, at which point the polymer will soften and degrade

Thermal decomposition mechanism of PFA sheaths

The heat generated by the resistance wire must conduct through the PFA wall to the acid. T_wire = T_acid + ΔT is the temperature at the interface of the wire PFA begins to soften at 260°C and starts to degrade above 300°C. For long term reliability the wire interface temperature should not exceed 260°C. The heat flux q is given in power density (W/cm2) and is a function of the heater 's power rating and surface area .

Wire Temperature Vs Wall Thickness Quantitative

At 160°C, power density of 8 W/cm² (80,000 W/m²) for 70% H₂SO₄:

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 allowable 0.35 112 272 Degradation begins 0.40 128 288 Softening 0.50 160 320Severe deterioration of
Maximum Permissible Wall Thickness at Various Power Densities

The following table shows the maximum allowed wall thickness for PFA sheath heaters in 70% H2SO4 at 160°C as a function of power density.

Power Density (W/cm2) ΔT per mm thickness (°C/mm)Max Wall Thickness for T_wire <260°C (mm) 5 20 0.5
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 1.

The wall thickness must also be able to resist the 2 bar internal pressure (if the heater is a tube) and handling loads and give the required mechanical strength. For pressures higher than 1 bar, a minimum wall of 0.3 mm is recommended, regardless of thermal calculations. For this application (2 bar) a wall thickness of 0.3 – 0.35 mm is a compromise between thermal and mechanical requirements. If the walls are thicker, the power density needs to be reduced or a heat spreader added.

Other Methods

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

Reducing the power density (longer or larger diameter heater)

Using a more thermally conductive material (PEEK has k = 0.32 W/m·K, a little better)

Inserting metal heat spreader into PFA sheath

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

A Well Informed Specification

To keep the wire contact below 260°C, the maximum wall thickness of a PFA sheath heater with a power density of 8 W/cm2 in 70% sulfuric acid at 160°C and 2 bar pressure is 0.31 mm. To guarantee mechanical dependability, either a reinforcing layer should be used or the power density should be reduced to 6 W/cm², which allows a wall thickness of 0.42 mm. The engineer controls wall thickness and power density to avoid heat degradation of the PFA at the interface with the resistance wire.