Thermal degradation occurs when fluid molecules receive more thermal energy than they can absorb and carry away. This excess energy causes the bonds between the atoms of that molecule to break.
In organic-based heat transfer fluids – such as petroleum oils or chemical aromatics – thermal cracking is the breaking of the covalent carbon-carbon or carbon- hydrogen bonds, which are normally very stable and require high amounts of energy to degrade.
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 – the amount of energy the fluid receives during its residence time in the presence of heat.
Figure 2a illustrates a simplistic example of what happens to a typical ISO Viscosity Grade 32 mineral-oil-based heat transfer fluid in thermal cracking,. Excess energy breaks the long hydrocarbon molecule, comprised predominantly of 26 carbon atoms long, 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 of the fluid increases and that translates directly to a reduction in 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 safe 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.. As these abrasive, coke-like molecules keep on forming and accumulate, they contribute to fouling of surfaces in the heat source, obstructing lines and elbows, and damaging pump seals. In systems with electrical heat, the residue will coat the electrical elements and grow thicker over time; 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 Tbulk to widen. This creates a cycle of thermal degradation (see Figure 3) –excessive heat causes thermal cracking of the heat transfer fluid, which causes the formation of high boilers and build-up of residue on heating surfaces, forcing 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, the fluid is not exempt from thermally degrading or its useful lifespan shortening.