How to Measure the In-Situ Thermal Conductivity Degradation of a PFA Sheath After 5,000 Hours in Oxidizing Acids?

Over time, oxidising acids such as nitric acid (HNO₃), chromic acid (H₂CrO₄), and concentrated sulphuric acid (H₂SO₄) breakdown PFA by incorporating polar functional groups (carbonyl, carboxyl, sulfonate) into the polymer backbone and amorphous areas. This chemical degradation changes the thermal transport properties of the PFA, with the thermal conductivity (k) typically decreasing 10-30% after 5000 hours at elevated temperature. A lower k leads to a larger temperature drop across the sheath for the same heat flux, which increases the PFA inner surface temperature and further accelerates the degradation in a positive feedback loop. A comparison approach for measuring in-situ thermal conductivity degradation does not need removal of the heater from service. The surface temperature of the heater is measured at a known heat flux and fluid temperature and the apparent k value is calculated from the heat conduction equation. Degradation is indicated by a decreasing computed k with time. Regular measurements (every 500 to 1,000 hours) allow you to observe the sheath degeneration before it fails visually.

In-Situ Measurement Method
For a PFA heater in steady state operation, the heat flux q (W/m^2) through the sheath is determined from the electrical power input and heater surface area (q = P_elec*η / A, where η is the heating efficiency, normally 0.95–0.98 for submerged heaters). The fluid temperature T fluid is measured with a calibrated thermocouple. The PFA outer surface temperature, T surface , can be measured by one of the following methods: (1) a thin-film thermocouple bonded to the sheath with high temperature tape and thermal paste (point measurement); (2) an infrared camera (requires transparent liquid and access); or (3) a resistance temperature detector (RTD) clamped to the sheath . The temperature difference across the PFA wall is ΔT_PFA = T_surface_inner - T_surface_outer However, the temperature of the inner surface is not readily quantifiable. Using the entire temperature drop from the metal core (T_core) to the fluid: T_core is not reachable either. The practical technique takes use of the fact that the major obstacle is the PFA wall itself. The equation of heat transfer is re-arranged as q = (T_surface_outer - T_fluid) × h where h is convective heat transfer coefficient. But h is unknown and is vary. Instead, measure T_surface_outer at two distinct power levels (q1 and q2) under identical flow conditions. The difference cancels h: ΔT_surface = (q2 - q1) * (t_PFA/k) therefore k = (q2 - q1) * t_PFA / ΔT_surface. The differential technique does not require the knowledge of T_core or h.

Run a heater with t_PFA = 2.0 mm at q1 = 2.0 W/cm² (20,000 W/m²) and q2 = 3.0 W/cm² (30,000 W/m²). Measure T_surface for each power, T1 = 95°C, T2 = 115°C, T_fluid = 80°C (constant) ΔT_surface = 20C Then k = (30000-20000) * .002 / 20 = 10000 * .002 / 20 = 20 / 20 = 1.0 W/m.K. This is technically impossible (PFA k is 0.19-0.22). The difference reveals that the measured $\Delta T_{surface}$ incorporates the variations in h with temperature and flow conditions. The differential technique requires constant h. This is rarely so. A more precise way is to utilise a reference heater with known, stable k (e.g., a fresh PFA heater or a metal heater) put next to the degraded heater. Measure T_surface of both heaters at identical q and same fluid conditions . The greater T surface of the deteriorated heater implies the higher thermal resistance. k_degraded = k_reference * (T_surface_reference - T_fluid) / (T_surface_degraded - T_fluid) The accuracy of this comparative procedure is ±10–15 per cent.

Expected k Degradation in Oxidising Acids
Oxidising Acid Concentration Temperature (°C) Exposure Time (hours) Initial k (W/m·K) Degraded k (W/m·K) Reduction (%) Mechanism
Nitric acid 30% 90 5,000 0.20 0.17-0.18 10-15% Carbonyl formation
Nitric acid 70 % 100 5,000 0.20 0.14-0.16 20-30 %Intense surface oxidation
Chromic acid 50 g/L 60 5,000 0.20 0.16-0.18 10-20%Sulfonation of surfaces
Sulphuric acid 96% 120 5,000 0.20 0.15-0.17 15-25%Sulfonation + dehydratation
Sulphuric acid 96% 140 2000 0.20 0.12–0.14 30–40%Severe deterioration (not advised for 5000 hr)
Mixed acid (HNO₃/HF) 20/5% 80 5,000 0.20 0.16–0.18 10–20% Synergistic assault
Piranha (H 2 SO 4 /H 2 O 2 ) 3:1 120 1,000 (short life) 0.20 0.10-0.12 40-50%Severe oxidation
Stepwise Measurement Procedure
Baseline Measurement: For a new heater, measure k with the manufacturer's approved value or laboratory measurement (ASTM E1530) on a sample from the same extrusion batch. If possible, install a reference (new) heater in parallel to the service heater.

In-situ setup: 3 thermocouples are attached to the PFA surface at the hottest predicted point (usually 10-20 cm from the cold end), mid-length, and near the bottom. Use thermal paste for a good contact. The thermocouple junction should be insulated from the fluid by a small silicone rubber cap in order to measure the true surface temperature (not influenced by the fluid flow). Alternatively, a non-contact measurement may be made using an infrared camera through a sight glass.

Steady state circumstances are: Run the tank at normal set point and flow for a minimum of 1 hour to achieve thermal equilibrium. 3. Record PFA surface temperature at 3 points, fluid temperature (multiple locations), electrical power (voltage x current) Calculate the real heat flux: q=(V×I×η)/A, where η is the efficiency factor that takes into account heat losses through the walls of the tank (usually 0.90–0.95 for well-insulated tanks, 0.80–0.85 for uninsulated). A = π × D × L_sub

Calculate the apparent k: t_PFA * (T_core - T_surface) = q / k As T_core is unknown, the reference heater approach should be used: T_surface_ref = T_fluid + q*(t_PFA/k_ref + 1/h) for reference heater (known k_ref, same q, same T_fluid). T_surface_degraded = T_fluid + q × (t_PFA/k_degraded + 1/h) for the degraded heater. Subtract the two equations (same q and h): T_surface_degraded - T_surface_ref = q x t_PFA x (1/k_degraded - 1/k_ref) Solve for k_degraded = 1 / (1/k_ref + (T_surface_degraded - T_surface_ref) / (q * t_PFA))

Example calculation: k_ref=0.20 W/m·K, t_PFA=0.002 m, q=30,000 W/m2 (3 W/cm2). T_surface_ref=95°C, T_surface_degraded=108°C, T_fluid=80°C. Then $1/k_{degraded} = 1/0.20 + (108-95)/(30,000 \times 0.002) = 5 + 13/60 = 5.217$ $k_{degraded} = 1/5.217 = 0.192$ W/m·K Reduction 4%-minimal degradation. If T_surface_degraded=125°C then 1/k_degraded=5+(125-95)/(60)=5+30/60=5.5, k_degraded=0.182 W/m·K, reduction=9%

What To Do With The Results
The decrease of k of 10–15% implies substantial deterioration. The heater can continue to be used but should be inspected every three months. A 15-25% drop suggests a considerable deterioration. In 6 to 12 months, it is recommended to replace the heater. And 25% reduction means serious degradation. The PFA could be near failure, therefore replace the heater right away. The reduction k indicates that the inner surface temperature is higher than planned and degradation is accelerated. If the measured k continues to decline at an accelerating pace (e.g., 10% decline in first 5,000 hours, 15% decline in next 2,000 hours), the heater is in a failure spiral and should be replaced preventively.

If the laboratory or facility does not have a reference heater, a viable option is to measure the PFA surface temperature at a set power and compare it to the temperature when the heater was new. A continuous increase in surface temperature with time (at constant power, fluid temperature and flow) illustrates the k deterioration. For a 2.0 mm wall at 3 W/cm2 , a 10°C increase in T surface leads to a k reduction of about 0.01–0.015 W/m·K (5–7%). 20°C increase equals to 10-15% reduction. Replacement of heaters is required at an increase of 30°C or greater.

Summary: Regular in-situ k measurement detects hidden degradation
The thermal conductivity of a PFA sheath is reduced by 10-30% after 5,000 hours of exposure to oxidising acids (nitric, chromic, concentrated sulphuric) at elevated temperatures. This degradation can be assessed in-situ using a comparison approach with a reference heater with known stable k or by monitoring the increase of PFA surface temperature over time at constant operating circumstances. If the drop is >15%, it signals serious degradation and should trigger a scheduled replacement within 6-12 months. If reduction > 25%, quick replacement required. Early warning of sheath degradation before apparent cracking or blistering appears will be provided by regular measurement (every 500-1,000 hours in vigorous oxidising service). For important processes, place a reference heater next to each service heater to allow for continuous monitoring. The cost of a reference heater ($200-500) is trivial compared to the cost of an unexpected failure resulting in bath contamination and process disruption. Engineers without access to reference heaters shall periodically determine the apparent surface temperature rise and replace the heaters when the temperature rise reaches 20 °C above the baseline value at the same operating conditions. Thermal conductivity degradation is an early warning of approaching PFA failure-do not overlook it.