How Does the Thermal Conductivity of a Polypropylene (PP) Exchanger Compare to PTFE in Low-Temperature Acid Service?

PTFE is the king of chemical resistance and hence the default choice for cooling a stream of cold, dilute hydrochloric acid might be a PTFE heat exchanger. But PTFE costs a lot. Polypropylene (PP) is another one, a commodity plastic that costs a fraction as much for this particular low-temperature, non-oxidizing service. It is in the same slow low range for thermal conductivity hence it is not a thermal enhancement. The choice of PP is simply economic, sacrificing a wide temperature range and universal chemical protection for a significantly lower initial cost.

Thermal Conductivity: Almost Perfect Match
The thermal conductivity of polypropylene (PP) at room temperature is about 0.2 W/m·K. The thermal conductivity of PTFE (polytetrafluoroethylene) is slightly higher at about 0.25 W/m·K. In practice, this 20 percent differential makes little impact in most heat exchanger sizing calculations. Both are thermal insulators compared to metals (e.g. stainless steel ~15 W/m·K). Therefore, a PP heat exchanger will require about the same amount of heat transfer surface area as a PTFE unit for the same duty, fluid velocities and temperatures. There is no reason to pick PP over PTFE, or vice versa, on the basis of raw thermal performance.

The final conclusion of a PP vs PTFE exchanger thermal conductivity low temperature acid study is clear: selection does not lead thermal conductivity. The two materials fall into the same low conductivity class and any design changes are not significant.

Why PP? The Economic Case
The major benefit of polypropylene is that it is cheap. PP is a commodity thermoplastic produced in large quantities. Raw material costs are typically 5-10 times lower than PTFE. Also cheaper methods of fabrication such as hot gas welding, extrusion and injection moulding are available in comparison to the specialised sintering and compression moulding of PTFE. A shell-and-tube or coil-type PP heat exchanger is made at a quarter of the cost of a PTFE device of the same size. This cost save can be large for a non-aggressive acid service at low temperature.

Resistance to Temperature and Chemicals: The Limits of PP
PP is economically attractive but has a narrow working window. Polypropylene can only be used continually at temperatures up to roughly 80 °C (176 °F). At temperatures above this PP becomes soft, and loses its mechanical strength and starts to crawl under pressure. Conversely, PTFE can run continuously at 200 °C (392 °F) and intermittently up to 260 °C. This constraint is acceptable in low‐temperature acid service (i.e. below 60 °C); nevertheless, any process upset or steam cleaning could exceed the limit of PP .

From a chemical point of view PP is resistant to many dilute acids, bases and aqueous salt solutions. It is affected by powerful oxidising acids, such as concentrated nitric acid, sulphuric acid above 70% and chromic acid. It also expands or dissolves in many organic solvents e.g. aromatic hydrocarbons, chlorinated solvents and ketones. However, PTFE is resistant to nearly all substances except molten alkali metals and elemental fluorine at high temperatures. PP is hence a specialist for cool, clean, non-oxidizing acids (e.g. dilute HCl, dilute H 2 SO 4 , acetic acid). PTFE is the generalist immune, high temperature and safe to everybody.

Further Technical Considerations
Brittleness and UV sensitivity
Polypropylene is not resistant to ultraviolet (UV) light. If the exchanger is situated outdoors or in a highly lighted region, UV exposure causes photo-oxidation that can lead to surface cracking and loss of impact strength. PP exchangers should be protected or sprayed with UV-resistant coatings for outdoor usage. PTFE is naturally UV resistant and will not deteriorate in sunshine.

Fire Resistance
PP is a combustible substance, with a Limiting Oxygen Index (LOI) about 18% . Once ignited, it burns easily with copious smoke and runs of molten polymer. Fire safety issues include sprinklers, fire barriers, and electrical area classification. PTFE is non‑flammable with a LOI of 95%, it will not support combustion and will self-extinguish when the flame source is removed. This makes PTFE the safer option in fire risk areas.

Mechanical strength at low temperatures
At low temperatures (e.g. below 10 °C), PP becomes more brittle, more susceptible to impact damage from water hammer or ice formation. PTFE is robust and flexible to cryogenic temperatures (-200 °C). For cold acid services such as chilled water or refrigerated brines, PP exchangers must be carefully protected from mechanical shock.

Selection Guidance Selecting the Right Material for the Service
The choice of a PP or PTFE exchanger for a low temperature acid is not a thermal choice, but merely an economic and chemical compatibility choice. When to use PP:

The acid is dilute, non-oxidizing and clean (HCl < 20%, H2SO4 < 50%, acetic acid).

Operating temperature below 60 °C (with safety margin below 80 °C).

It contains no organic solvents or strong oxidants.

The exchanger is put indoors or in a UV shielded zone.

Fire risk is low or well controlled.

Lowest. Driving factor is initial capital cost.

Use PTFE for:

The acid is hot (above 80 °C), concentrated or oxidising (e.g. nitric, chromic or hot sulphuric).

Aggressive organic solvents in the fluid.

Process upsets may briefly exceed PP's temperature limit.

Fire safety or UV exposure is a worry.

The increased initial cost is justified by a long service life and low maintenance.

The cheap, no-nonsense workhorse for the cold, gentle acid is polypropylene. The high-end, invincible warrior for the hotter, wilder and more unpredictable chemistry is PTFE. Neither is always better, the right decision is completely process envelope dependent.

Conclusion: thermal conductivity is not the determining factor
In low-temperature acid application, the thermal conductivity of PP and PTFE is so close that heat exchanger size and performance are practically comparable. So it comes down to chemical compatibility, temperature restrictions, safety and cost. PP provides substantial cost savings and reliable service for a well defined mild cool acid service. No other plastic can match PTFE's resistance to any difficult, heated or aggressive chemical and that is why it costs more. The best material for the job is the most economical one that can be depended on-and for low-temperature, non-oxidizing acids, that material may well be polypropylene.