Does the Use of a Double O-Ring Sealing System on the Mounting Flange Eliminate Leakage in PFA Heaters for High-Vacuum Applications (0.01 mbar)?

Due to penetration through the elastomer and micro-leakage at the contact, a single O-ring seal on a PFA heater mounting flange is sometimes insufficient for high-vacuum applications needing pressures as low as 0.01 mbar (10⁻¹ bar). Leakage rates can be lowered from 10⁻³–10⁻² mbar·L/s (single seal) to 10⁻◦–10⁻² mbar·L/s (acceptable for most high-vacuum applications) with a double O-ring sealing system with an intermediate vacuum port (a "pumped" double seal). A vacuum pump removes any gas that seeps past the first O-ring before it can reach the second O-ring, creating a guarded volume. A metal-sealed flange (ConFlat type) is necessary for the highest vacuum levels (10⁻² mbar and below). The two O-ring system is a significant reduction rather than a total elimination. For medium to high vacuum (10⁻³ to 10⁻◦ mbar), it works quite well. The extra complexity for PFA heaters is only warranted when leak rates are less than 10⁻² mbar·L/s.

Leakage Routes and the Double Seal Mechanism
Three pathways lead to leakage in a single O-ring seal (Viton or FFKM): (1) gas permeation through the elastomer (10⁻³–10⁻³ mbar·L/s per cm of seal length); (2) micro-channels at the O-ring-flange interface (10⁻◦–10⁻³ mbar·L/s for a 100 mm flange); and (3) the PFA-metal interface itself (the heater sheath passing through the flange). Usually, a single seal may attain a leak rate of 10⁻³–10⁻² mbar·L/s, which is too high for high vacuum.

A second O-ring and a vacuum port are added in a twin O-ring system. A tiny auxiliary vacuum pump evacuates the area between the O-rings. Any gas that seeps past the first O-ring is pumped out right away and never makes it to the second. The product of (leak rate of first seal) × (leak rate of second seal) / (pumping speed) is the effective leak rate. Leak rates between 10⁻² and 10⁻² mbar·L/s are feasible in practice.

Comparison of Leak Rates for 50 mm PFA Heater Flanges
Materials, Seal Configuration, Achievable Leak Rate (mbar·L/s), Appropriate Vacuum Level, and Cost Increase (%)
Compression single O-ring FKM 10⁻³–10⁻¹ 1–10 mbar (rough vacuum) 0%
Single O-ring (FFKM, Kalrez) FFKM 10⁻◦–10⁻¹ 0.1–1 mbar +10%
FKM + grease 10⁻◦–10⁻◦ 0.01–0.1 mbar + 15% single O-ring with groove sealant (vacuum grease)
Non-pumped double O-ring FKM + FKM 10⁻²–10⁻◦ 0.1–1 mbar + 20%
Double O-ring (pumped, rough pump) FKM + FKM 10⁻◦–10⁻¹ mbar +35%
Double O-ring (pumped, turbo pump) FFKM + FFKM 10⁻²–10⁻◦ mbar +50%
Metal seal (copper gasket, ConFlat)Aluminum or copper <10⁻¹ +100% Extremely high vacuum
PFA welded to metal flange <10⁻¹ (through PFA) Welded PFA-to-metal transition (hermetic)PFA permeation limit of +200%
Installing a PFA Heater Practically
To install a double O-ring seal for high-vacuum duty on a PFA heater flange:

Flange design: The tank side of the PFA flange needs to have two concentric O-ring grooves that are 15–25 mm apart. There is a port (1/4 inch or 6 mm) in the space between the O-rings that is connected to a vacuum pump (rotary vane, 10⁻² mbar).

O-ring material: For chemical compatibility with hostile media, use FFKM (Kalrez, Chemraz). For non-aggressive vapors, FKM (Viton) is appropriate.

Torque: To compress O-rings by 20–25% of their cross-sectional diameter, uniformly tighten flange bolts in a star pattern.

Pumping: Throughout operation, the inter-seal volume needs to be continually pumped. It is adequate to have a specialized little vacuum pump (10 L/min).

Heater sheath: Another possible leak path is the PFA-to-metal core exit point at the cold end. Use a bellows seal made of welded metal or a silicone overmold (see Article #87).

Example in the Field
A PFA heater was needed to keep a semiconductor research chamber at 80°C at a high vacuum (10⁻¹ mbar). Leakage around the heater flange generated pressure spikes (10⁻³ mbar) using a single O-ring seal. A pumped double O-ring seal (two FKM O-rings with an intermediate port linked to a tiny diaphragm pump) was installed in place of the previous arrangement. For six months, the chamber pressure stayed constant at 2×10⁻◦ mbar. Several deposition runs were accomplished with the heater. The pump and flange upgrade came at an extra cost of $500.

PFA's Restrictions in High Vacuum
Even with flawless sealing, the PFA sheath itself is gas-permeable and outgasses, releasing absorbed water and leftover solvents. Outgassing from the PFA will limit the background pressure at pressures below 10⁻² mbar (usually 10⁻◦–10⁻² mbar). Regardless of sealing, PFA cannot be used at ultra-high vacuum (UHV, <10⁻¹ mbar). PFA with a double O-ring seal is suitable for medium to high vacuum (10⁻³ to 10⁻◦ mbar).

In conclusion, using a double O-ring and pumping reduces leakage to 10⁻²–10⁻² mbar·L/s.
A twin O-ring sealing system on a PFA heater mounting flange, with the inter-seal volume evacuated by a pump, reduces leakage rates from 10⁻³–10⁻² mbar·L/s (single seal) to 10⁻◦–10⁻² mbar·L/s for high-vacuum applications down to 0.01 mbar (10⁻¹ bar). For the majority of medium and high-vacuum procedures, this is enough. Although leakage is reduced by two to four orders of magnitude, it is not entirely eliminated by the technology. Metal sealing or hermetic PFA-to-metal transitions should be considered for pressures less than 10⁻◦ mbar. For vacuum ovens, coating chambers, and degassing systems, the double seal is a workable alternative. It turns a troublesome flange into a dependable barrier. Nearly flawless, but not quite. It is sufficient for the majority of high-vacuum requirements.