How to Select a PTFE Heater for Use in a Pressurized Tank (up to 5 bar)?

PTFE immersion heaters are mostly used in open tanks. But when the process requires a sealed, pressurised vessel running at a few bar, then the heater becomes a pressure boundary and has to be constructed as such. A conventional open-tank heater just is not going to cut it. Selecting the right unit involves a grasp of the interaction of pressure, temperature and material qualities – and a willingness to work with the manufacturer for a tailored solution.

Why Standard PTFE Heaters Fail in Pressurised Tanks
The PTFE sheath is only subject to the ambient pressure and the static head of the liquid in an open tank. The heater's main role is to transport heat without electrical leaks or corrosion. The external pressure is applied to the entire wetted surface of the heater in a pressurised tank (up to 5 bar or 72 psi). If the PTFE sheath is too thin or not well maintained, it will collapse under the differential pressure. On the other hand, if the tank is pressurised by a gas blanket and the inside of the heater is vented to atmosphere, the sheath has to resist outward ballooning.

The penetration point where the heater goes through the tank wall or flange is also a potential leak path. A normal compression fitting or open gland will not hold pressure. The complete assembly-sheath, seal, flange and electrical termination-must be considered as a pressure containing component.

Major Design Challenges for Pressurised Service
1. Reinforced sheath of PTFE
PTFE is a rather soft polymer having low mechanical strength, particularly at elevated temperatures. The tensile strength of PTFE at 20°C is around 20–30 MPa, but decreases dramatically with increased temperature. In the usual processing temperatures of 80–100 °C, the material loses more than half of its strength. Therefore, the pressure rating of a PTFE heater is a function of both temperature and sheath geometry.

For service up to 5 bar the sheath must be strengthened occasionally by 50–100% compared with an open tank design. Sometimes, a metal mesh or perforated metal liner is included with the PTFE to offer mechanical support while still maintaining chemical resistance, this is called a reinforced sheath. The PTFE provides corrosion prevention and the reinforcement takes the load of the pressure.

2. High Pressure Terminal Seal
The seal between the PTFE sheath and the mounting flange shall be hermetic and pressure rated . A basic O-ring or gasket will not work with cyclic pressure and temperature fluctuations. Two common solutions are:

Spring-energized PTFE seals: PTFE jacket with internal metallic spring providing consistent sealing force against flange bore through thermal expansion and contraction.

Metal reinforced gaskets: A composite gasket (e.g., PTFE coated stainless steel) that combines chemical inertness and mechanical robustness.

If the seal material limit is exceeded, the seal must be positioned so that it is not subjected to the full process temperature. In many configurations the cool zone is formed by extending the unheated area of the sheath above the liquid level and away from the flange.

3. Rated Mounting Flange Pressure
The flange that attaches to the tank nozzle must be constructed to a suitable pressure vessel standard such ASME B16.5 for raised face or ring joint flanges. However, a typical metal flange cannot be overmolded directly with PTFE for a pressurised heater. Instead, the manufacturer generally employs a metal flange (stainless steel or carbon steel with a PTFE liner or coating) that is machined to receive the PTFE sheath and seal arrangement. The flange pressure class shall be equal to or greater than the design pressure of the tank at the maximum operating temperature.

In the case of pressurised tanks, the heater is a component of pressure equipment and comes under the same rules as the vessel. A flanged connection that leaks or blows under pressure is a severe safety issue, especially with hot, corrosive fluids.

4. Junction Box and Electrical Entrance
If the heater's wiring junction box is in direct connection with the process fluid, it may need to be rated for the internal pressure of the tank. A long cold zone, a long unheated stretch of the PTFE sheath extending above the tank nozzle, is commonly used to isolate the junction box from the wetted area. This frigid region works as a thermal and pressure barrier. The electrical wires go via a hermetic seal (glass to metal or ceramic) at the top of the heater assembly, where the pressure has been lowered.

The manufacturer may also provide a pressure-rated electrical feedthrough, rated for the specific pressure and temperature range. The entry of wire must maintain the pressure rating of the tank, i.e., any conduit connection must also be sealed.

The Selection Process – A Step by Step Guide
Choosing a PTFE heater for use in a pressurised tank is not a DIY activity. This information must be submitted to the manufacturer in order for a safe, compliant design to be created.

Specifications Required
Parameter Details to be given
Maximum operating pressure e.g. 5 bar (gauge)
Max. operating temperature e.g. 95°C
Process fluid | Composition, pH, solvents present
Size of tank nozzles and flange standard e.g. DN80 PN16, ANSI 3″ Class 150
Required power and watt density kW needed, W/cm2
Control method On/off, SCR or thermostatic
Typical response from manufacturer
According to the specs, the manufacturer will:

Calculate the needed sheath thickness – thicker PTFE for greater pressures; a reinforced design might be advised.

Choose a seal type – Spring-energized PTFE or metal reinforced gasket, depending on the pressure and temperature.

Specify the flange – A suitable metal flange (typically stainless steel) with a raised face or RTJ groove drilled to accommodate the PTFE sheath and seal.

Design the cold zone - Provide a minimum of unheated sheath length above the maximum liquid level to keep the flange and seal cool enough for reliable operation.

Provide pressure test certification: Most manufacturers will hydrostatically test the heater assembly to 1.5 times the rated pressure before shipment.

Decision Selection Checklist for PTFE Heater Pressurised Tank
The following points should be checked while evaluating a supplier or developing a buy specification:

Simultaneous Pressure and Temperature State - Never presume a heater rated 5 bar at room temperature would work at 90 deg. C.

Request a reinforced sheath - If the pressure is above 2 bar or the temperature above 80 °C, particularly enquire whether the sheath has a metal reinforcement or an enhanced wall thickness.

Confirm pressure rated flange and seal – What is the flange pressure class (Class 150, Class 300, etc.) and the seal type? Ask for test documentation.

Check electrical penetration - Verify that wiring entry is pressure boundary maintained. All installs may not be enough for a single chilly zone.

Consult the manufacturer - Do not attempt to convert an open-tank heater to pressurised service. Only select a source experienced in pressure rated PTFE heaters.

The term PTFE heater pressurised tank selection is an ultimate recognition that the heater has to be designed as a pressure vessel component, not a simple immersion element.

Summary: Specialisation Is Not Optional
A PTFE heater for use in a pressurised tank (up to 5 bar) is a custom order and the process conditions need to be fully disclosed to the manufacturer so that a safe, compliant design can be produced. When the boundary conditions are changed standard components have to be changed. Anything that will work in an open tank will bust collapse or leak, under pressure. With the right specifications of pressure, temperature, fluid chemistry and flange details, plus a qualified manufacturer, a dependable, long-lasting heating solution is achievable. The additional technical work is a minimal price to pay for process safety and conformity to the regulations.