How Do All-Fluoropolymer Heat Exchangers Compare to Metal Exchangers with Fluoropolymer Linings?

For corrosive duty when pressure or temperature exceed the limits of solid fluoropolymers, a metal heat exchanger lined internally with fluoropolymer is a compromise. The trade-offs in dependability, cost and performance of this technique over an all-fluoropolymer exchanger are discussed. These discrepancies are of great importance for process engineers who design heat transfer equipment for very aggressive acids, mixed acid pickling lines or challenging chemical reactors.

Types of Construction: How to Prevent Fluoropolymer Corrosion in 2 Ways
All-fluoropolymer heat exchangers are made entirely of fluoropolymer materials such as PTFE (polytetrafluoroethylene), PFA (perfluoroalkoxy alkane) or FEP (fluorinated ethylene propylene). Solid fluoropolymer is used for the shell (if any), tube bundle, tube sheets and all wetted internal surfaces. The most common configurations are shell-and-tube (PTFE or PFA tubing) and immersion coils. No metal parts touch the corrosive process fluid.

Fluoropolymer-lined metal exchangers consist of a metal shell that holds pressure and tube sheets produced from carbon steel, stainless steel, or other alloys. Then the internal surfaces that are in touch with the corrosive fluid are lined with a fluoropolymer, usually PTFE or PFA. Lining Techniques:

Sheet lining: PTFE sheets are welded or glued to the metallic substrate.

Spray or electrostatic coating: Thinner fluoropolymer coatings are applied and sintered.

Loose lining: A removable PTFE sock located within the metal shell.

Lining thickness is typically 0.5 to 1.5 millimetres, according on the application and liner type.

Pressure Rating: The Big Benefit of Lined Metal Exchangers
The most important difference between the two systems is their pressure containment ability.

The pressure of all-fluoropolymer exchangers is limited by the mechanical strength of the fluoropolymer itself. PTFE and PFA have low tensile strength (usually 20–30 MPa) and, more critically, show creep (cold flow) under sustained load. At higher pressures, the fluoropolymer will deform, causing tube collapse, tube sheet leaking or shell rupture. Typical practical maximum operating pressures of allfluoropolymer shell-and-tube exchangers are 3–5 bar (45–75 psi) on the fluoropolymer side. Some specifically reinforced designs get to 7 bar, although these are rare.

Lined metal exchangers, on the other hand, depend on the metal shell and tubesheets to contain pressure. Carbon steel can securely handle pressures of 10 bar, 20 bar or greater, limited only by the design code (e.g. ASME Section VIII) and flange ratings. The fluoropolymer liner is not depended upon for structural integrity, it is a corrosion barrier only. In high pressure acid service such as refineries, chemical plants or high pressure hydrochloric acid scrubbers lined exchangers are generally the only non-metallic option.

Temperature Limits; Important Differences and Similarities
The maximum continuous temperature of the fluoropolymer itself is similar for all-fluoropolymer and lined fluoropolymer exchangers. Above roughly 200°C (392°F) for PFA and 260°C (500°F) for PTFE, both PTFE and PFA begin to soften and creep considerably in short exposures. But practical continuous service temperatures are lower:

All fluoropolymer exchangers: Generally restricted to 110-120°C for PTFE (because to creep under tube pressure differentials) and 150-180°C for PFA (due to wall thickness and support design).

Lined metal exchangers: The liner is supported by the metal substrate, so extensive deformation is avoided. They can work at greater temperatures. The PTFE-lined steel exchangers are normally utilised at 180–200°C (continuous) and up to 230°C for short periods. The liner permeation and liner adherence to the metal becomes the limiting element.

It should be noted that at elevated temperatures all fluoropolymers show enhanced permeability to some gases and tiny molecules (e.g. chlorine, hydrogen chloride). Lined exchangers have the extra risk of vapour permeation through the liner condensing at the metal contact which could result in hidden corrosion.

Corrosion Reliability: Uniform Protection or Liner Failure Risk
All-fluoropolymer exchangers give uniform, continuous corrosion resistance over the whole wetted surface. There are no seams, pinholes or interfaces of metal exposed to the process fluid. The fluoropolymer is uniform in the thickness of the wall. Even if the surface is scraped or abraded, the material underneath is still chemically resistant. This design avoids the danger of "hidden corrosion" under a liner.

Lined metal exchangers have a number of failure scenarios that are not present in all-fluoropolymer designs:

Pinholes or voids in the lining - even very small imperfections (less than 0.1 mm) allow corrosive liquids to access the metal substrate. Corrosion products may cause blistering or debonding of the liner after the metal starts corroding, which can lead to accelerated failure.

Liner debonding - The bond between the fluoropolymer liner and metal substrate is never perfect. Thermal cycling can induce differential expansion (PTFE expands ~7× more than steel as stated in prior sections) which can cause the liner to pull away from the wall. The resultant space may entrap permeating acid and cause localised attack.

Permeation and condensation - Some acids (particularly HCl at elevated temperatures) infiltrate the thin liner. If the metal shell is cooler than the process side (as is often the case in cooling jobs), the penetrated acid condenses on the metal surface producing corrosion from the 'inside out'. This is a common failure mode in PTFE lined steel equipment.

Damage in service or during installation or maintenance - Liners can easily be cut, scratched or dented by tools. One deep scratch can be a moment of beginning of failure.

In actuality a major problem with lined exchangers is that a little unnoticeable pinhole might lead to catastrophic collapse within a matter of weeks or months, whereas an all fluoropolymer exchanger will be able to continue operation safely. Alternatively, a professionally manufactured and properly maintained lined exchanger can give many years of service at pressures and temperatures not possible with solid fluoropolymer systems.

Cost comparison Initial investment and life-cycle considerations
For low pressure, low temperature jobs, all-fluoropolymer heat exchangers are often cheaper to produce. Materials (PTFE or PFA tubes, sheets) are generally inexpensive cost. Fabrication requires heat sealing or welding of fluoropolymer components.

Metal exchangers with tubing are more expensive to construct. The metal shell and tube sheets must be made to pressure vessel codes and then lined with fluoropolymer – a labour-intensive process involving expert workers, quality control (spark testing for pinholes) and specialised welding of fluoropolymer sheets. As a result, the initial purchase price of a lined exchanger is 30–100% greater than an all-fluoropolymer exchanger of same heat transfer area.

However, for high pressure applications (e.g. 10 bar or higher) an all-fluoropolymer exchanger may not be practical at all. In these instances the sole alternative is a lined exchanger- or a metallic alloy exchanger. The cost comparison then moves on to lined fluoropolymer against exotic alloys (titanium, tantalum, Hastelloy). That comparison generally shows lined fluoropolymer as a major cost savings.

Summary Comparison - Metal Exchanger With All-Fluoropolymer and Fluoropolymer Lining FeaturesAll Fluoropolymer ExchangerFluoropolymer-lined metal heat exchanger
Typical pressure rating 3-5 bar (limited by fluoropolymer creep)10-20 bar+ (restricted by metal shell design code)
Maximum continuous temperature 110-120°C (PTFE); 150-180°C (PFA) 180-200°C (PTFE liner with metal support)
Corrosion resistance Excellent, consistent protection, no metal to corrodeGood but possibility of pinholes, delamination and permeation corrosion
In case of liner problem, failure moden/a (no liner)Fast metal corrosion Possible catastrophic leaks
Testing and InspectionVisual and electrical continuity (simple) Need spark testing (pinhole identification). Will need frequent relining.
First cost Lower for low pressure duties More (pressure vessel + lining labour)
Maintenance risk Low; no liner replacement Moderate to high; liner may be damaged during maintenance
Typical applications Heating/cooling of electroplating, atmospheric acid scrubbers, low pressure pickling bathsHigh-pressure HCl coolers, heat exchangers for concentrated acid in refineries, chemical reactors.
Decision Guideline: How to Choose the Right Technology
The decision to use an all-fluoropolymer or a lined metal exchanger should be based on the pressure and temperature requirement and the acceptable level of failure risk.

Choose an all-fluoropolymer heat exchanger if:

Operating pressure less than 5 bar (75 psi).

Temperature is below 120°C (PTFE) or 180°C (PFA).

Absolute corrosion reliability is essential. No pinhole or liner failure is tolerated.

The process fluid is oxidising acids or mixed acids and slight contact with metals will result in quick failure.

A lower initial capital cost is wanted.

The exchanger is located in a place where leaking is not acceptable (e.g. above sensitive equipment or in a cleanroom environment).

When selecting a metal heat exchanger with a fluoropolymer lining:

Operating pressure is above 5 bar and can be above 20 bar or higher.

Temperature is less than 200°C and greater than 120°C

The process fluid is adequately characterised and does not contain species which increase liner permeability (e.g. hot HCl gas).

There is a stringent inspection and maintenance programme involving periodic spark testing of the liner.

Exotic metal alloys (titanium, tantalum) are too expensive, therefore lined steel is a cost effective option.

Some chance of liner failure is permitted if redundant safety devices are installed (leak detection, secondary containment).

Lined PTFE exchangers are a traditional solution for high pressure acid services typical of refineries and big chemical facilities, such as hydrochloric acid coolers at 10 bar and 150°C. Most, however, install leak detection devices (conductivity probes between liner and shell) to give early warning of liner breach.

Conclusion: Reliability and safety requirements Direct the Choice
All-fluoropolymer heat exchangers exclude metal from the wetted path to ensure maximum corrosion dependability. No debond lining, no corrosion pinhole and no concealed metal substrate to breakdown. But their pressure and temperature ranges are small.

Fluoropolymer-lined metal exchangers are provided with a metal shell containment which permits operation at higher pressures and temperatures. The tradeoff is that it is more complex, more expensive to start up, and introduces new failure modes-pinholes, debonding, permeation-that are not present in all-fluoropolymer designs.

Process engineers have to consider these trade-offs carefully. All-fluoropolymer exchangers are generally the best solution for low pressure, high reliability applications. Rigorous quality control, inspection and leak detection are necessary for high pressure, high temperature jobs where lined fluoropolymer is the only non metallic choice. This key design decision should be driven by safety and reliability criteria, ensuring that the selected heat exchanger technology is suitable for the particular hazards and operating circumstances of the chemical process.