What Is the Difference Between a PTFE Heat Exchanger with a Carbon Steel Shell and One with a Stainless Steel Shell?

In a shell and tube heat exchanger with PTFE the process fluid never sees the shell – just the PTFE tubes and tubesheets. But the choice of shell material, usually carbon or stainless steel, is critical for cost, lifespan and compatibility with the installation environment. Understanding these variances can help the heat exchanger as a whole, not just the wetted sections, to give reliable service throughout its intended life.

PTFE's Shell Role Shell and Tube Exchangers
In a PTFE shell and tube heat exchanger, the shell is the pressure-containing vessel which contains the service fluid (cooling water, steam, heating medium) around the PTFE tube bundle. The shell does not see the corrosive process fluid, it is all handled by the PTFE tubes, tubesheets and any interior fluoropolymer liners. Accordingly, the shell material is chosen on the basis of considerations other than the internal corrosion resistance:

Mechanical strength to resist the pressure of the service fluid.

Compatibility with external environment (plant atmosphere, humidity, chemical vapours, washdown techniques).

Cost and availability

Thermal expansion and weight properties.

The two most popular shell materials for PTFE heat exchangers are carbon steel and stainless steel, usually Types 304 or 316. They each have their perks and disadvantages.

Carbon Steel Shell: Lowest Initial Cost, Requires Outside Protection
Carbon steel is the most used material for pressure vessels in regular industrial service. For PTFE heat exchangers a carbon steel shell provides:

Lowest upfront cost - Carbon steel costs considerably less than stainless steel, per kilogram. For big diameter shells (e.g. 300 mm to 1 m) the cost difference could be significant.

High strength - Carbon steel has good mechanical qualities thus you can use thinner walls compared to other alternative materials for the same pressure rating .

Good availability - Carbon steel pipe, plate and flanges are commonly stocked by most sources.

The humid air, moisture, or chemical vapours act on the carbon steel, causing it to corrode from the outside. The heat exchanger has the shell on the outside. The process fluid does not come into touch with the shell, but the plant environment does. The atmosphere in many chemical process and surface finishing plants contains:

Acidic fumes (HCl, H₂SO₄, HNO₃)

Chlorides or chlorine

Damp and condensation

These chemicals damage exposed carbon steel over time creating rust and pitting. The PTFE tube bundle and process fluid are not exposed to external corrosion, but the shell may be impaired in its pressure integrity, may leak the service fluid (e.g., cooling water or steam), and can be a safety concern.

To reduce external corrosion, carbon steel shells are commonly coated with a protective coating or paint system. Typical alternatives are:

Epoxy or polyurethane paints (two-component systems)

Coating: Powdercoated

Hot dip galvanising of tiny components

Coatings nevertheless require frequent examination and touch-up, especially in hostile situations. If the coating gets destroyed (e.g. from impact or abrasion) the steel underneath may quickly start to corrode.

Stainless Steel Shell: More expensive, better external corrosion resistance
Stainless steel shells, most commonly Type 304 or Type 316, provide enhanced resistance to air corrosion, chemical vapours and washdown conditions. The chromium concentration (at least 10.5% for 304, and nickel and molybdenum for 316) creates a passive oxide coating that prevents the metal from rusting.

Key properties of stainless steel shells are:

Excellent external corrosion resistance - Stainless steel can be left unpainted or passivated. It withstands humid, coastal or chemically demanding plant atmospheres without the need for regular coating maintenance.

Higher initial cost - On a material basis, stainless steel is typically 3 to 5 times more expensive than carbon steel. Fabrication costs are also greater due to the requirement for cautious welding methods (to avoid chromium carbide precipitation) and possible post-weld treatment.

Strength of stainless steel is similar or slightly lower - Stainless steel has similar tensile strength to carbon steel (e.g. 304 has yield strength ~205MPa, carbon steel ~250MPa). The walls may have to be a little thicker for the same pressure rating, but the difference is usually negligible.

Weight? – Stainless steel has a density in the same range as carbon steel (7.9–8.0 g/cm³) so weight will be equivalent for the same sizes.

Stainless steel shells are generally left unpainted with a pickled and passivated polish to improve the passive layer. Stainless steel's smooth, non-porous surface also makes it easy to clean and sanitise in food, pharmaceutical or cleanroom applications.

Carbon Steel versus Stainless Shell PTFE Exchanger: No Effect on Inner Corrosion Resistance
It is important to note that the choice of carbon steel vs stainless steel shell for the PTFE exchanger does not affect the corrosion resistance of the wetted portions. The PTFE tubes, tubesheets and any fluoropolymer liners are totally protected from the process fluid. The internal fluid only sees PTFE whether the shell is carbon steel or stainless steel. The choice is therefore completely determined by the external environment and cost.

Sometimes this distinction is confused. Choosing a stainless steel shell for a PTFE exchanger does not enhance internal chemical resistance. It simply enhances the shell's capacity to withstand external corrosion. However, a decent coating on a carbon steel shell can do rather well in a benign indoor environment.

Other Considerations: Weight, Thermal Expansion and Inspection
Weight - Carbon steel and stainless steel are almost equivalent in density. For shells of the same thickness and diameter the difference in weight is minimal. But if a carbon steel shell requires a thick external coating (e.g. heavy duty epoxy) the overall weight can be a little greater. In practical terms weight is rarely an issue in the choice between the two materials.

Thermal expansion - Carbon steel and stainless steel have similar thermal expansion coefficients (carbon steel ~12×10-6/°C, stainless steel ~17×10-6/°C). The change is insignificant compared to the expansion of the interior PTFE parts (which is compensated for by the mounting techniques as detailed in previous articles). Shell material expansion is not a primary design factor.

Inspection and maintenance - Stainless steel shells can be visually inspected for corrosion without removing the paint Cracks or pitting are readily apparent on the exposed metal surface. Carbon steel shells with opaque coatings are more difficult to evaluate for the state of the underlying metal. Rust staining might be the first indication of coating deterioration.

Decision Guidance: Shell in Carbon Steel or Stainless Steel
The main differences are summarised in the below table:

Feature Carbon Steel Shell 304/316 Stainless Steel Casing
Initial cost (relative)Baseline (bottom)Higher (3–5× cost of material)
Resistance to external corrosionUncoated, alone is poor, needs paint or other protection.Good. May be left unpainted.
Coating / upkeep needPeriodic checking of painting or coating necessaryMinimal. Occasional cleaning/passivation.
Perfect for humid or coastal locationsVery poor unless adequately coated and consistently maintained Very good
Suitable for chemical plant atmospheres (acidic vapours)No good without good coating systemGood to good (316 suggested for chlorination)
Indoor, dry and controlled environment suitabilityGood at basic paintOver-specification (unneeded cost)
Good for washdown areas or outdoor exposed locationsLimited (needs heavy duty coating and frequent inspection)Selected
Effect on the corrosion resistance withinNone None
Typical applications Protected skid installations Indoor process plants with pure dry airOutdoor installations, seaside facilities, chemical factories with severe vapours, food/cleanroom environments 
Suggestions for Practice
Carbon steel shell painted with conventional industrial epoxy or polyurethane paint offers a cost-effective solution for indoor, regulated situations where the exchanger is shielded from rain, moisture and chemical vapours. Such conditions are present in many surface finishing businesses and chemical industries. With good coating maintenance, the carbon steel shell will survive for 15 to 20+ years.

Stainless steel shell is suitable where the installation is outdoors, at the coast, or in a facility where the atmosphere contains acid vapours (such as pickling lines, electroplating shops or waste acid treatment areas). The extra expense of stainless makes sense when the alternative would be periodic repainting, risk of hidden corrosion, or possible downtime for shell replacement.

For highly harsh external situations, such as marine atmospheres or regions with high chloride concentrations (such as near hydrochloric acid storage or pickling tanks), use Type 316 stainless steel instead of Type 304. Type 316 contains molybdenum, which enhances its resistance to chloride-induced pitting and crevice corrosion.

A less typical alternative is to specify a carbon steel shell with a heavy-duty chemical-resistant liner on the outside (e.g., a two-part epoxy or even a PTFE-based coating). Such coatings are however expensive, and can be close to the cost of a stainless shell, but still subject to mechanical damage.

Conclusion: Decision Based on External Environment
The only difference between a PTFE heat exchanger with a carbon steel shell and one with a stainless steel shell is determined by the external installation environment and budget constraints. The process fluid chemistry is not essential to this decision as the PTFE wetted parts provide complete internal corrosion protection independent of the shell material.

Carbon steel shells are the least expensive to start with, but must be coated externally and maintained regularly to avoid rusting in humid or chemically aggressive atmospheres. Stainless steel shells are more expensive upfront, but offer better external corrosion resistance, generally removing the need for continued coating maintenance. Holistic equipment specification takes into account internal and external service circumstances. Selecting the correct shell material for a PTFE heat exchanger guarantees that the entire assembly, not just the fluoropolymer components, can reach its full intended life.