The purpose of this paper is to provide baseline industry knowledge to help you make an informed decision when selecting a heat transfer fluid. While every system has its own unique requirements, this guide should provide enough context to make fluid selection for your application more straightforward.

A wide variety of high-temperature heat transfer fluids are available today. Some are suitable for open-to-air systems, while others are not. Their operating ranges also differ significantly — some can withstand up to 398°C (750°F), while others are limited to around 232°C (450°F). All of them move heat effectively, but efficiency alone isn’t enough to make the right choice.

In systems below a few hundred gallons, designs are often open to atmosphere with no nitrogen blanketing at the expansion tank or reservoir. This exposure changes the considerations when selecting a fluid.

The 4 Primary Fluid Types

Mineral Oils: Readily available and inexpensive, these multipurpose oils are usually lightly refined and contain minimal additives. Because they often retain petroleum distillates and aromatic hydrocarbons such as benzene, xylene, or toluene — along with sulfur and waxes — their service life is shorter, especially at elevated temperatures.

White/Paraffinic Oils: Refining improvements in the last two decades have produced highly purified paraffinic oils nearly free of aromatics. Many are effective for heat transfer use, and some have been engineered with additives to extend service life and resist degradation in demanding applications.

Synthetics (PAOs and Silicones): The most expensive category. PAOs, similar to those used in synthetic motor oils, resist oxidation and thermal breakdown up to about 287°C (550°F). Silicones are newer to this market; though costly, they are extremely resistant to degradation. However, silicone vapors can interfere with finishing processes such as painting or coating.

Chemical/Synthetic Aromatics: Built on benzene-based chemical structures, these fluids cover a broad temperature range up to 398°C (750°F). While thermally effective, they are expensive, less environmentally safe, and typically not recommended for open systems.

Fluids Break Down in essentially 2 categories:

Oxidation — the most common issue

Oxidative degradation occurs when oxygen reacts with the fluid through a free radical chain reaction, producing polymers and solids that increase viscosity. Pumping becomes harder, heat transfer declines, acidity rises, and the risk of coke formation grows.

The reaction rate is minimal at room temperature but increases rapidly with heat. In systems open to air, oxidation is most evident as sludge deposits in low-flow areas such as reservoirs or expansion tanks.

Thermal Degradation — heat damage

Thermal degradation, also called cracking, occurs when high heat splits carbon–carbon bonds into smaller radicals. These may remain as lighter fractions (“low boilers”) or recombine into heavier molecules (“high boilers”).

Low Boilers: Lead to lower flash point, reduced viscosity, and increased vapor pressure, which can lower efficiency, cause pump cavitation, and create safety risks.

High Boilers: At extreme temperatures (above 400°C / 752°F), bonds break further and hydrogen separates, forming coke. This increases viscosity until solids precipitate, fouling surfaces and shutting the system down.

In short, overheating fluid past its boiling point produces a lighter component usually in the form of vapors and lowers critical safety thresholds such as flash point and autoignition temperature, posing serious safety issues.

Key Factors Beyond Temperature

While matching your fluid to the required temperature is critical, other factors also influence the decision:

1. Cost versus service life
2. Thermal stability and its effect on longevity
3. Resistance to oxidation and its effect on performance
4. Environmental and worker safety considerations

Balancing Cost and Longevity

Low-cost fluids ($12–$15 per gallon): Lightly refined mineral oils with minimal additives. Suitable for closed, low-temperature, or high-loss systems where fluids are regularly replaced. In open systems, however, they degrade quickly, producing sludge and carbon that can cause failures.

Mid-priced fluids ($15–$20 per gallon): More refined paraffinic or white oils, sometimes hydrotreated and often supplemented with antioxidants. Cleaner and longer lasting than mineral oils, with some blends designed specifically for open systems.

Synthetic oils (generally over $35 per gallon): PAO-based fluids stable at low temperatures but often with narrower high-temperature limits than paraffinic oils. While resistant to oxidation, their high cost makes them impractical in systems prone to leaks. Some paraffinic fluids with advanced additives may outperform PAOs.

Silicones ($50–$90 per gallon): Costly but extremely resistant to oxidation and thermal degradation. When systems are designed for silicone fluids, they can deliver very long life cycles.

Chemical aromatics (over $25 per gallon): Typically avoided in open systems due to environmental concerns and high vapor pressures.

Resistance to Oxidation

This is arguably the most critical factor for systems exposed to the atmosphere. In any open system, there will always be a point where the fluid encounters air. The higher the temperature at that point, the faster the oxidation rate. Oxidation leads to the buildup of sludge and carbon within the system, which, if ignored, can cause total system failure through clogged lines, fouled heaters, or restricted circulation.

Heat transfer fluids generally resist oxidation at temperatures below 93°C (200°F). Once the temperature climbs above that threshold, the rate of oxidation doubles with every 8°C (15°F) increase. The hotter the system runs, the more protection is required. For this reason, system design and the degree of exposure must be carefully evaluated when selecting a fluid.

Petroleum-based fluids provide some natural resistance to oxidation, though performance varies—some last longer and remain cleaner than others. Advances in additive technology have greatly improved both durability and cleanliness. Synthetic PAO fluids deliver comparable oxidation resistance to well-formulated petroleum products, while silicone fluids, when used correctly, can offer near-complete immunity to oxidation.

Chemical aromatics, however, are better suited to closed systems, as they generally provide little protection against oxidation.

Thermal Stability

It is essential to choose a fluid with a maximum operating temperature rating higher than the system’s intended operating temperature.

Petroleum-based fluids can run up to 332°C (630°F), but the effects of elevated heat—particularly in smaller, electrically heated units—must be considered. Even if the overall operating temperature is well below the fluid’s maximum limit, electric heaters can create impingement points that reach several hundred degrees hotter than the average system temperature.

When exposed to excessive heat, petroleum fluids will thermally degrade (crack or boil), producing lighter fractions. In most cases, incidental breakdown is manageable as light ends are vented from the system. However, persistent overheating can cause longer-term damage.

PAO fluids provide good thermal stability but typically have a lower maximum use temperature than petroleum-based options. As such, they are not advisable for sustained operation above 260°C to 288°C (500°F to 550°F).

Silicone fluids boast extremely high maximum temperature limits, but their naturally high viscosity index can reduce efficiency, as they do not thin out significantly as temperature rises.
While chemical aromatics appeal with their very high temperature capacity, the other drawbacks described in this document must be carefully considered before selecting them.

Environmental Impact/Worker Safety

Petroleum-based fluids (mineral, white, or paraffinic) are typically the most environmentally straightforward, easy to work with, and simple to dispose of. They usually don’t need special handling and can be discarded alongside other waste oils.

Synthetic PAOs offer nearly the same environmental advantages as petroleum fluids, though disposal should be verified with your waste service provider, as segregation from mixed oils may be necessary.

Chemical aromatics, on the other hand, may contain compounds that, under high temperatures, can generate carcinogenic byproducts. Leak management and disposal can also present reporting and cost challenges in the workplace.

Another issue with chemical aromatics is their high vapor pressure. At operating temperatures, they can release vapors up to 15 psia. In open systems without proper containment, this can result in significant fluid loss and pose health risks to workers. Additionally, vapor loss necessitates ongoing fluid replacement to maintain system levels.