The purpose of this paper is to provide guidance in choosing the right heat transfer fluid. Every system has its own specific requirements, but the information below should give you enough context to simplify the decision-making process.

Today’s market offers a wide range of high-temperature heat transfer fluids, giving you many choices to evaluate. Some are suited for open systems, while others are not. Certain products can operate effectively up to 398°C (750°F), whereas others are only rated for temperatures as low as 232°C (450°F).

All claim to transfer heat efficiently, but what other considerations should you take into account before deciding?

For systems larger than about 950 liters or 250 gallons, design features are typically in place to protect the heat transfer fluid from oxidation and thermal breakdown. Expansion tanks are often shielded with inert or buffer gases to minimize oxidation, and additional safeguards such as low-level or low-flow shutoffs are commonly included to further protect the fluid.

Understanding how fluids break down

Oxidation

In chemical terms, oxidation is when oxygen in the air reacts with the fluid through a free radical mechanism. This process generates larger molecules that become polymers or solids. These compounds increase the fluid’s viscosity, making pumping more difficult and reducing heat transfer efficiency. At the same time, the fluid’s acidity or TAN (Total Acid Number) rises, which raises the likelihood of coke forming inside the system.

As with many reactions, the rate of oxidation accelerates with higher temperatures. At room temperature it is almost negligible, but as the fluid gets hotter, oxidative degradation increases rapidly unless protective measures, such as inert gas blanketing in the expansion tank, are used.

In simple terms, oxidation happens whenever hot fluid is exposed to air. Evidence of oxidation usually appears as sludge, particularly in low-circulation areas like expansion tanks or reservoirs.

Thermal Degradation

Thermal degradation, also called thermal cracking, occurs when heat breaks apart carbon–carbon bonds within fluid molecules. This generates smaller fragments known as free radicals. Sometimes the reaction stops there, but in other cases these fragments recombine to form larger polymeric molecules.

In heat transfer systems, the results of this process are commonly referred to as “low boilers” and “high boilers.”

Low Boilers: Their presence is indicated by a drop in flash point, a reduction in viscosity, and a rise in vapor pressure. Elevated vapor pressure can reduce system efficiency and cause pump cavitation, potentially leading to premature equipment failure. Lower flash points also raise safety and operating risks.

High Boilers: At very high temperatures – above 400°C (752°F) – carbon bonds break apart and hydrogen atoms separate, forming coke. High boilers increase the fluid’s viscosity while dissolved, but once their solubility limit is exceeded, they form solids that coat heat transfer surfaces. This fouling can escalate quickly and cause the system to shut down.

In short, thermal degradation happens when fluid is overheated beyond its boiling point. This produces lighter fractions, often as vapors. Repeated overheating or cracking lowers viscosity and reduces flash point, fire point, and auto-ignition temperatures — all of which raise safety concerns.

Four fluid groups

Mineral Oils: Typically produced by large refineries, these fluids are inexpensive and versatile but contain few additives for extra protection. Because they are lightly refined, they may still contain petroleum distillates or aromatic hydrocarbons such as naphthalene, xylene, toluene, or benzene. They can also retain sulfur, waxes, and other compounds that shorten their service life, especially at elevated temperatures.

White/Paraffinic Oils: Refining technology has greatly advanced over the past two decades, resulting in highly purified paraffinic and white oils that are virtually free of aromatics. Some grades have proven effective in heat transfer systems, and certain engineered blends with additives offer improved protection and longer lifespans for demanding applications.

Synthetics (PAOs and Silicones): These are generally the most expensive fluids. PAOs (similar to those in synthetic motor oils) provide inherent resistance to oxidation and thermal degradation up to about 287°C (550°F). Silicones, though costly and relatively new to the market, are highly resistant to thermal and oxidative breakdown. However, they may interfere with finishing processes such as painting or coating if vapors come in contact with surfaces.

Chemical/Synthetic Aromatics: Built on benzene-based molecular structures, these fluids cover a wide temperature range and are often usable up to 398°C (750°F). They deliver strong thermal performance but tend to be costly, less environmentally friendly, and more hazardous to workers. They are also generally unsuitable for open systems.

Ready to choose? Here’s how to start!

The first step is to determine your operating temperature range. Consider both the maximum temperature and any low-temperature conditions that could affect pumping or processing at ambient conditions.

Systems operating below 315°C (600°F) have the widest fluid selection.

Systems between 315°C and 332°C (600°F to 630°F) have fewer options, while systems requiring above 343°C (650°F) are even more restricted. In general, the higher the temperature, the fewer the options and the greater the cost.

For applications below 315°C (600°F), petroleum-based fluids are often the most practical in terms of cost, performance, and environmental impact.

Between 315°C (600°F) and 332°C (630°F), viable options include high-grade petroleum fluids, chemical aromatics, and certain silicones.

Above 332°C (630°F), options are essentially limited to chemical aromatics and some silicone-based products.

It’s worth noting that high-temperature fluids rated at 343°C (650°F) are sometimes used in applications as low as 204°C (400°F). While building in a safety buffer can be useful, over-specifying may result in lost performance, higher costs, or unnecessary environmental trade-offs.

Once you established your operating temperature range there are a few more things to consider.

How long is the overall process or the system’s life expectancy?

If the system will only run for a short period — a few years, for example — then fluid longevity is less important, making cost the deciding factor. But if the system will operate long-term, fluid stability and replacement costs become critical.

Chemical aromatics often have high vapor pressures, particularly near their upper temperature limits. In systems that aren’t sealed or pressurized, vapors escape through the expansion tank, requiring frequent top-ups.

Mineral oils perform poorly at their maximum use temperatures. When run near their limits, they degrade quickly and oxidize if exposed to air due to residual distillates and a lack of stabilizing additives.

White and paraffinic oils generally perform well up to their recommended maximums, but if those limits are exceeded, they also degrade. The lighter fractions produced must be vented, creating similar issues to chemical aromatics regarding fluid replenishment. Additionally, these oils vary in their susceptibility to oxidation, which should be considered for systems not sealed with nitrogen.

What processes require a food grade fluid?

Only a small number of heat transfer fluids are rated for food grade use (USDA, USP, H1, etc.).

While this simplifies the selection process, food grade fluids often have performance trade-offs compared to non-food grade options. Their resistance to degradation, especially oxidation, should be evaluated carefully.

Food grade fluids are sometimes specified unnecessarily. Because manufacturing them involves certain restrictions, their performance and lifespan can be limited compared to conventional fluids.

Do you have specific environmental concerns?

Environmental and safety considerations are key when selecting a fluid.

Regulatory agencies such as the EPA, OSHA, or local bodies should be consulted regarding handling, disposal, and reporting requirements. If your system is open to atmosphere, requires frequent human contact with the fluid, or carries a risk of leaks, potential health and environmental impacts must be weighed before selection.

Chemical aromatics typically emit strong odors and carry clear health risks.

White/paraffinic oils and most mineral oils are generally the cleanest, easiest to use, and simplest to dispose of.

Synthetics such as PAOs and silicones are also usually considered environmentally friendly.

What about disposal?

Eventually, all fluids will need to be removed, whether small amounts during maintenance or the entire system volume at end of service life. Disposal costs should therefore be factored in from the start.

Chemical aromatics usually require segregation and may need to be treated as hazardous waste, which is expensive.

Silicones also require separation but are not considered hazardous. PAOs and petroleum-based fluids are often easier and cheaper to dispose of, as they can typically be mixed with other waste oils.