Why Can a PFA Heater That Has Been Stored for 6 Months in High Humidity Show a 20% Shorter First-Time Start-Up Life?

Moisture is absorbed by PFA heaters kept in high-humidity conditions (>80% relative humidity) through the polymer sheath and into the annular gap between the PFA and the metal core. Depending on wall thickness and storage temperature, a heater can absorb 0.5–2.0 grams of water over the course of six months. When the metal core reaches 100°C during first operation (start-up), this moisture transforms into steam. Internal pressure (10–100 bar) produced by the quick expansion causes the PFA to blister from the inside; this process is known as steam blistering (see Article #61). The micro-damage degrades the PFA-metal interface, decreasing long-term adhesion even if there are no initial blister ruptures. When compared to identical heaters stored in dry (<30% RH) settings, heaters stored in humid conditions have a 20–40% shorter service life. PFA heaters should be kept in airtight bags with desiccant to avoid this. Before using the heater for the first time, dry it at 80°C for 24 to 48 hours if humid storage cannot be avoided.

Absorption of Moisture During Storage
PFA absorbs water at a low but detectable rate: 0.03–0.10% by weight at 50% relative humidity and 0.08–0.20% at 95%. That equates to 0.6–4 grams of water for a 2 kilogram heater. Because the metal core is hydrophilic and condenses moisture, the majority of this water gathers at the PFA-metal interface. Water is retained by capillary action in the contact gap (0.1–1.0 µm). Water does not easily evaporate at ambient temperature after absorption because the contact is not ventilated. Moisture is only driven out by prolonged heating to 80–100°C.

The metal core heats up quickly during the initial startup. Water boils to steam at temperatures above 100°C. Local pressure spikes are caused by the volume expansion (1,600×). Blisters with a diameter of 0.5 to 5 mm are formed when the PFA yields. These blisters are irreversible harm. The PFA is now locally thinned and delaminated, even if the blister does not burst. When compared to a dry-stored heater, the heater's lifespan is shortened by 20–40%.

Life Reduction in Comparison to Storage Duration and Humidity
RH (%) Storage ConditionTime frame (months)First Start-Up Blister Density (per 100 cm²) Absorbed Moisture (g)Dry Stored vs. Life Reduction (%)
Dry (desiccant sack) <10% 12 <0.1 0%
Normal (sealed plastic) 30–50% 6 0.2–0.5 0–5 5–10%
Normal (outdoors) 60–80% 6 0.5–1.0 5–20 10–20%
High humidity (open) 80–95% 3 0.8–1.5 10–30 15–25%
High humidity (open) 80–95% 6 1.2–2.5 20–60 20–35%
High humidity (open) 80–95% 12 1.5–3.5 50–100+ 30–50%
Condensation (wet storage) 100% (liquid) >100 50–70%
Why the First Start-Up Is the Riskiest
The heater may continue to run after the first start-up and scorching. But the blisters serve as a focal point for tension. Cracks appear at the borders of blisters during successive heat cycles. Rather than two to three years, the heater can fail in six to twelve months. A heater that is started correctly and kept dry will last a long time without developing blisters.

Twenty heaters-ten in dry desiccant bags and ten outdoors at 80% relative humidity-were kept for six months during field testing. After that, all were run in 80°C water at 3 W/cm². The dry-stored heaters had an average lifespan of 8,000 hours (to ground fault) and no blistering. The humid-stored heaters had a 31% lower average life of 5,500 hours and 30% blister coverage.

Mitigation and Prevention
Preventive measures while storing:

Heaters should be kept in sealed foil packets with five to ten grams of desiccant (molecular sieve or silica gel).

Every six months or when the indication goes pink, replace the desiccant.

To prevent condensation, store in a temperature-controlled environment between 10 and 30°C.

Use the bag within 30 days if it has been opened; if not, redry.

Drying process prior to initial use:
Dry a heater before installing it if it has been kept in humid conditions (>50% RH) for more than three months.

For 24 to 48 hours, place the heater in an oven at 80°C (don't go over 100°C to prevent PFA deterioration).

Make sure the heating is off when drying.

Increase to 96 hours for large heaters.

Install right away after drying, or keep in a dry bag.

As an alternative, install the heater and run it at 10% of its rated power for a whole day to gradually remove moisture without causing steam blisters. After that, increase power by 10% every two hours.

Example in the Field
For eight months, a plating facility kept twenty extra PFA heaters on an open shelf in a region with high relative humidity (75–90%). Within 18 months of installation, 15 out of 20 failed (average life: 14 months). After that, the plant used desiccant bags for dry storage. The next 20 heaters had a 30–40 month lifespan. The business estimated that the 50 cost of sealed bins and desiccant bags saved 400 per heater in early replacement expenses.

In conclusion, humid storage shortens life by 20–40%.
At the PFA-metal interface, PFA heaters kept for six months in high-humidity conditions (>80% RH) absorb one to two grams of moisture. In contrast to dry-stored heaters, this moisture flashes to steam during initial startup, resulting in internal blisters that irreversibly damage the heater and shorten its service life by 20–40%. Heaters should be kept in sealed bags with desiccant to avoid this. Before using the heater for the first time, dry it at 80°C for 24 to 48 hours if humid storage cannot be avoided. Months of lost life are avoided by the few hours of drying time. A long life begins with a dry start. A brief start is called a wet start. Start slowly, keep dry, and endure a long time. How you store your heater will determine its performance.