Avoiding oxidation and thermal degradation
Oxidation occurs when a fluid comes in contact with air at elevated temperatures and is one of the most common ways a fluid suffers from degradation, usually resulting in sludge formation within the system. While the various heat transfer fluid chemistries are affected at different temperatures, the most common petroleum-based fluids will experience oxidation at temperatures above 200°F. In fact, it's generally accepted that for every 15 degree increase in temperature (above 200°F), the rate of oxidation doubles.
Thermal degradation, or thermal cracking, is the breaking of carbon - carbon bonds in the fluid molecules by heat in excess of the fluid's recommended maximum bulk temperature. The reaction may either stop at that point, in which case smaller molecules than previously existed are formed, or the fragments may react with each other to form polymeric molecules larger than previously existed in the fluid.
So what to do about it?
Weak points with respect to oxidation can easily be identified by looking for any point in the system where the fluid contacts air. Once these points are identified, during normal operation you should measure the average fluid temperature in this area (usually expansion tank or reservoir).
If the fluid temperature is below 200°F, the system should be sufficiently guarded against excessive oxidation. If however, the fluid temperature is above 200°F there are a few quick steps that might help.
- If the system does not have an external expansion tank or fluid reservoir, consider adding one. Generally, placing a reservoir of 'cold' fluid at the point of air contact will significantly reduce oxidation.
- If the system has an external reservoir but is running hot, examine the flow path. If the fluid is flowing through the reservoir, consider plumbing it so that the reservoir is "T’d" into the system and not part of the circulation loop.
- If the expansion tank is not part of the circulation loop but is still running hot, you can consider moving it further away from the main system or add a nitrogen blanket to buffer the fluid from air contact.
One important note is that not all fluids are affected the same by oxidation. Because oxidation is a major downfall to heat transfer fluids, all Duratherm fluids now contain an extensive additive system to combat oxidation. Check with your fluid supplier to ensure they have incorporated some protection in their fluid, particularly if your system is open to the atmosphere.
Thermal degradation happens when a fluid is heated past its recommended bulk or skin temperatures, sometimes resulting in cracking or breaking of the fluid's molecules. Beyond ensuring your fluid is properly matched to your equipment and temperature requirements, there are a few things to be aware of that can contribute to thermal degradation.
- Startup and shutdown. We commonly see systems either heated up too fast or shut down without cooling first. During startup, particularly with electrically heated systems, it is important to heat the system gradually, for a few reasons. This not only helps reduce the risk of thermal degradation, but also ensures any moisture or vapors are vented from the system gradually without cavitating the pumps, or even worse, having a geyser of vapor and fluid erupting from vent points. Shutdown is equally important, again particularly with electrically heated systems. If a system is not allowed to cool before stopping, circulation fluid can become trapped in the heater/boiler and will likely see temperatures much higher than the fluid is rated for.
- Another potential cause of thermal degradation is modifying system design. A well-engineered system will utilize the heat transfer fluid as efficiently as possible without wasting valuable energy/fuel. This means that pumps, valves, heater watt densities, user loads etc. are all engineered to work in harmony. However, as systems age or needs change it is likely that some aspect of the system will need to be changed, modified or removed. If this is the case, work closely with system manufacturers, engineers and fluid suppliers to ensure you will remain within the parameters of the system’s original design or that compensations are allowed for with respect to the fluid’s capabilities.