The following is a quick reference guide to help you understand how fluids break down. You can also download this information in pdf format.
Oxidative degradation is the reaction of oxygen (in air) with the fluid, which creates polymers or solids. These thicken the fluid and increase its viscosity. A more viscous fluid will be more difficult to pump, have poorer heat transfer characteristics as well as an increased chance of coke formation. Oxidation is also accompanied by an increase in the acidity (TAN) of the fluid.
Oxidation occurs more rapidly as the temperature is increased. At room temperature, the reaction rate is hardly measurable. As system temperatures rise, however, so too does the rate of oxidation. In industries like plastics extrusion and die casting, oxidation is the main cause of fluid degradation and sludge formation commonly found in reservoirs and piping.
Thermal degradation or thermal cracking is the breaking of carbon/carbon bonds in fluid molecules. This occurs when the maximum bulk temperature of the fluid is higher than recommended. The reaction may either stop at this point, in which case smaller molecules are formed. The fragments may also read with each other to form polymeric molecules larger than previously existed in the fluid. In heat transfer terminology, the two types of degradation products are known as low boilers and high boilers.
Low boilers decrease the flash point and viscosity of the fluid, while also increasing its vapor pressure. The increased vapor pressure can affect overall system efficiency and can cause pump cavitation. The reduction in the flash point could also be cause for safety concerns.
If thermal degradation occurs at extreme temperatures greater than 400°C (752°F), the effect is not only to break carbon/carbon bonds but to separate hydrogen atoms from carbon atoms and form coke. The effect of the high boilers is to increase the viscosity of the fluid as long as they remain in solution. However, once their solubility limit is exceeded, they begin to form solids which can foul the heat transfer surfaces. In this case, fouling of the heat transfer surfaces is very rapid and the system will soon cease to operate.
The common measurement of oxidative degradation is TAN. TAN will increase as fluids experience oxidative degradation. These acids will promote sludge and resin formation. TAN values above the range of 1.0 to 1.5 mg KOH/g are usually a cause for concern. It is important to note that with smaller, less efficient draining systems these acids can remain behind and contaminate new fluids. It is extremely important, especially if the TAN number is greater than 1.0, to ensure maximum evacuation of the spent fluid prior to refilling.
The flash point is important from the viewpoint of safety. However, it is not a concern unless it falls below 120°C (248°F). It is quite common for heat transfer systems to be operated at temperatures above the flash point of the fluid.