When you have a need for process heating that requires heat transfer fluid at medium-high temperature — between the temperatures of 400 and 600°F (200 and

315°C) — different products may be used. One is a fluid designed to work specifically as a heat transfer fluid, which includes mineral-oil-based fluids or synthetic oils such as polyalphaolefin (PAO). Mineral-oil based fluids often are referred to as “hot oils” or “heat transfer oil.” Another is lubricating oil, which can be rated at those temperatures as well.

These products can share physical characteristics as well as basic chemistries. Both are oils with good lubricating qualities, and they may have close specifications for flashpoint, specific gravity, viscosity and other fluid characteristics. And lubricating oils sometimes are used in heat transfer fluid systems for other applications.

But, as the temperature requirement goes up, it pays — in real dollars — to know which specific fluid characteristics are better at which function. The operating temperature of 400°F (200°C) can be considered a magic number. Beyond that temperature, the molecular bonds of oils start to lose their ability to withstand the energy transferred by heat, leading to fluid breakdown. A fluid’s ability to withstand the physical changes brought about by heat is defined by its thermal stability.

So why not just use the lowest priced fluid labeled as heat transfer fluid from a local source? If you operate at temperatures greater than 400°F, you need the expertise of companies that specialize in high temperature fluids and understand thermal stability well. A specialty chemical formulated to be a heat transfer fluid is going to cost more than a basic lubricant. Understanding what the extra cost is about is important.

The Role of Additives

Additives often are used in lubricating oils and advertised as enhancing their operation in heat transfer fluid applications. Additives provide corrosion resistance, anti-wear and other characteristics not required by heat transfer fluid. They also can provide enhanced oxidative stability, which is useful. But at high temperatures, it is better to prevent heat transfer fluids from contact with air or oxidizers as part of the system design.

Oxidative stability is the ability of the fluid molecules to resist the degrading effects of exposure to oxidizers such as air. Remember the magic number of 400°F? Beyond that temperature, additives are exhausted so quickly as to be ineffective. So, at high temperature, the fluid must be formulated to have inherent oxidative stability.

What is the basis of that formulation? Heat transfer fluids are designed for the specific application of transferring heat while minimizing degradation. In mineral-oil-based fluids, the manufacturing process involves more severe hydrogenation and a different catalytic reaction than a lubricant may require. This is so that an extremely high level of purity and bond strength can be attained. In synthetic oils as well as mineral oils, this high-purity fluid is combined in a proper blend to give the final heat transfer fluid the best possible flow characteristics for maximum heat transfer.

This more severe processing increases the price of the final product, and it is not economical if it is not needed. In lubricating oils, it may not even be desirable. For instance, what might be considered an impurity in a heat transfer fluid may add to the lubricating characteristics of a lubricating oil.

In other words, heat transfer fluids are formulated for a specific end-use application rather than a general-purpose application. This specialized processing is required when the threshold of 400°F is reached.

Fluid Characteristics for Medium-High Temperature Processes

Among the fluid characteristics of heat transfer fluids are heat transfer efficiency, which is calculated as the heat transfer coefficient, and thermal stability, which is

measured as a comparative test between fluids under the standard ASTM D6743‑11. Of all of the characteristics that define heat transfer fluid, these are the most basic.

•  Heat transfer efficiency determines the performance — as well as cost of operation — of the fluid and the entire system.

•  Thermal stability determines the life of the fluid. Paying more for fluids with high values in these areas pays off.

When heat transfer fluids are compared to general-purpose fluids not specifically designed for heat transfer applications, the results are measurable. Figure 1 compares the efficiency and stability from typical fluids in the market. In this example, the specifically designed heat transfer fluid is about 6 percent more efficient at 550°F (288°C). In real terms, depending on the system design, that can mean producing 6 percent more parts per hour or costing 6 percent less energy to run the system. The end user can calculate this from the published characteristics that are provided by all heat transfer fluid manufacturers. It is a combination of viscosity, specific heat, density and thermal conductivity. If these characteristics are not available from a manufacturer, I recommend you do not even consider it as a heat transfer fluid, no matter the price. Combined with flow rate, the calculation can be completed.

The comparative test of thermal stability shown in figure 1 traditionally has been done using a two-week test with nitrogen-purged ampules. A new ASTM standard includes a three-week test using similar methods. The nitrogen purge is used to eliminate oxygen and test the fluid based only upon its heat resistance. This test is valid because on systems operating at temperature higher than 400°F, a nitrogen or other inert gas blanket should be used on the expansion tank, or a cool expansion tank design should be used. These approaches eliminate the exposure of the hot fluid to atmosphere.

Fluid Testing

The differences between heat transfer fluid and lubricating oil technology can be made clearer by looking at the type of testing done to detect fluid degradation. Just like the oil in your car, fluids degrade. In industrial fluid applications, you are dealing with a lot more fluid than the five quarts in your car, so a frequent oil change is not practical or economical. Common practice is to test the fluid for wear and change it as required.

The primary degradation factors of heat transfer fluids are:

•  Total acid number (TAN).

•  Insolubles, which create system sludge.

•  High and low boilers, which are molecules outside of the standard boiling range of the original formulation of the fluid.

Testing lubricating oils for degradation can get complicated because they need to be tested for some of the same things as a heat transfer fluid — like TAN — but also other criteria:

•  Fine particle contaminants.

•  Trace metals.

•  Other items not pertinent to heat transfer fluids.

For these reasons, considering the two oils as separate products allows for proper maintenance of each kind.

 Knowing the heat transfer fluid characteristics can help you choose the best fluid for the application and justifies the extra cost that accompanies a heat transfer fluid. The right fluid provides a relatively quick payback in terms of performance and long life.  

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