Common Heat Transfer Fluid Problems: Oxidative Degradation
Problem: Oxidative Degradation
Oxidative degradation occurs when oxygen reacts with a heat transfer fluid. This is a reaction that eventually leads to the formation of weak acids and sludge. As a result, the viscosity of the fluid increases. Because of the consequent decrease in turbulence and the coating of system components by varnish, the fluid’s thermal efficiency reduces.
Cause: Exposure of heat transfer fluid to air leads to an oxidation reaction that degrades the fluid over time. In addition, absence of blanketing with an inert gas (e.g N2, Ar).
Consequence:
- Increase in Total Acid Number (TAN).
- Leads to corrosion problems if moisture is present
- Fouling/formation of sludge and varnish. This decreases heat transfer efficiency due to the coating of heating surfaces and increases maintenance costs
- Fluid discoloration
Solution:
Antioxidants help impede oxidation and its harmful effects, thereby prolonging a lubricant’s service life and providing better equipment protection. A high quality one, like Caldera Heat Transfer Fluids, inhibit oxidation with both primary and secondary antioxidants.
When used in high-temperature applications, Caldera’s proprietary 2-stage antioxidant package ensures greater resistance to the destructive oxidation processes which can lead to carbon and varnish deposits. A full suite of corrosion inhibitors and wear protection additives help to extend the life of system componentry as well.
Another effective method of inhibiting fluid oxidation is to blanket the expansion tank with an inert gas such as nitrogen or carbon dioxide, or with natural gas. The purpose of inert gas blanketing is to maintain an oxygen-free atmosphere in the expansion tank, and one of positive pressure to prevent air entry. A regulated supply of inert gas with a backpressure regulator on the vent outlet line is necessary to obtain this protection. A pressure relief valve also is required to protect the expansion tank from overpressure due to regulator failure, fire and other causes.
In open bath systems, oxidative degradation is more drastic because of the exposure to air. It is very important to select a fluid that mitigates oxidation. Caldera 6 is a polyalkylene glycol based heat transfer fluid that is more resistant to oxidation compared to other mineral oil based thermal fluids. Caldera 7 is a silicone-based heat transfer fluid that eliminates the problem of oxidation.