Regardless of the heat transfer fluid type, thermal degradation typically occurs when the fluid molecules receive more thermal energy than they can absorb and carry away at a particular time. The excess energy causes molecules to break down, or crack.
In organic-based heat transfer fluids – such as petroleum oils or chemical aromatics, for example – thermal cracking is the breaking of the covalent carbon-carbon or carbonhydrogen bonds, which are normally very stable and need a high amount of energy to degrade. Thermal cracking involves a number of steps: initiation, hydrogen abstraction, radical decomposition, and then termination.
This type of degradation is a function of both the oil’s inherent ability to absorb heat and the heat flux inside the heat source (i.e. the amount of energy the fluid receives during its residence time in the presence of heat).
Figure 2a illustrates an example of what happens to the molecule in thermal cracking; in this case, the molecule is a typical mineral-oil-based heat transfer fluid. Excess energy breaks the long hydrocarbon molecule (comprising 26 carbon atoms) into two shorter molecules of 12 and 14 carbons. These short molecules are called low boilers, because they have lower boiling points than the 26-carbon molecule. As the concentration of low boilers increases over time, the volatility (i.e. vapor pressure) of the fluid increases and that translates directly to a reduction in properties such as the flash point, fire point and possibly auto-ignition temperature.
Figure 2a/b: A hydrocarbon in a mineral-based heat transfer fluid undergoes thermal degradation, which creates lighter hydrocarbons
with lower viscosities and flash points, and heavy carbon deposits (FIGURE 2b).
In an open system, where the hot operating fluid is directly in contact with air, a reduction in fire point and flash point could pose a significant safety hazard, and venting or even fluid replacement will be required.
Another concern with thermal cracking is the formation of coke-like residue in the system (as shown in Figure 2). This occurs when thermal cracking forms high boilers, which are high-carbon, low-hydrogen molecules that can polymerize or agglomerate. As these coke-like molecules grow in size and accumulate, it can deposit in the system to obstruct lines and elbows. An abrasive and carbonaceous-type residue can damage pump seals. In systems with electrical heat, the residues will coat the electrical elements; in a furnace, they will form layers inside the heater coil. In both cases, this will act as an insulator.
The residue becomes a problem when the heater, set to a certain temperature, must then produce more thermal energy to pass through not only the pipe wall, but also the carbonaceous layer to get to the fluid. The additional heat raises the Tfilm of the system, causing the gap between Tfilm and bulk to widen. This creates a cycle of thermal degradation (see Figure 3), where excessive heat causes thermal cracking of the heat transfer fluid, which causes formation of high boilers and build-up of residues on heating surfaces, which in turn forces the heater to produce more energy to maintain the fluid’s Tbulk.
Figure 3: Thermal cracking of the heat transfer fluid that occurs at the heat source can create high boilers – long molecules that agglomerate and bake on the hot surface of the heat source or pipe wall. Over time, the carbonaceous residue forms a layer on the heat source that acts as an insulator. The heater must then produce more energy to raise the temperature of the fluid to the set point temperature, which in turn causes more thermal cracking. Thus, a cycle of thermal degradation occurs.
Even when systems operate at temperatures that are considered relatively mild (well below the fluid’s maximum rated bulk-oil temperature), the fluid is not exempt from thermally degrading, thus shortening its useful life.