Heat Transfer Fluids and Oxidation

Hydrocarbon liquids tend to oxidize when placed in areas where there is nonstop supply of fresh air. Several of them will oxidize at temperatures considerably below the regular operating temperature of thermal fluid systems. Beginning at the smoke point of the fluid, the rate of oxidation roughly doubles with each 20°F increase in fluid temperature. The rate of degradation will be more when there is more mixing of air and fluid.

Oxidation Products

Hydrocarbon liquids smoke as they start to oxidize – similar to overheated vegetable oil in a skillet on a stove. The same, usual products of combustion are generated, generally carbon monoxide, carbon dioxide, and water vapor.

When the oxidation continues, organic acids start to form in the liquid and concentrate. The fluid becomes highly viscous. This decreases the film coefficient and enhances the pressure drop. Higher pressure drop decreases the flow rate which, when integrated with lower film coefficients, greatly raises the film temperature inside the heater and speeds up fluid degradation.

When it starts to degrade, which is mostly at an accelerating rate, the fluid starts to become less capable of efficiently transporting heat. The fluid loses its ability to handle high temperatures, and becomes even more likely to degrade.

Oxidation Symptoms

The heat transfer fluid will start to darken and emit a pungent smell, as acidic carbonaceous sludge is generated. Gradually, the sludge deposits on the whole system’s surface. Inside the heater, these deposits solidify and permanently decrease heat transfer. In the heat user, these deposits can plug up lines.

Cleaning the System

This method will help eliminate the degraded fluid that comprises solid contaminants and acids. When the system is cleaner, it will function longer and better. Also, the longevity of the fluid will be high.

  • Drain the current fluid as much as possible
  • Eliminate solid matter from system. This may need chemical cleaning with the help of oxidizing agents, manual scraping of surfaces or circulating hydrocarbon solvents
  • Systematically flush with heat transfer fluid that matches type used to operate in the system
  • Soon after the system is started, deliver a fluid sample to the thermal fluid supplier for testing

System Operation

To avoid oxidation, fluid present in the expansion tank has to be maintained cool. If this cannot be performed, think about “padding” the system using inert gas. Nitrogen is cheap and easily available. Extend a line from user’s nitrogen source to the head space of the expansion tank. The gas must flow from the source via an alarmed flow meter, check valve, regulator, and into the expansion tank.

Fix a back-pressure control valve on the vent line of the tank along with a relief valve. The back pressure valve will ease system pressure when system is started. The relief valve should be appropriately sized to handle an unexpected large pressure increase in the tank, such as water turning to steam. It is recommended to not use the relief valve to control the back pressure in the tank as the valve may not reseat correctly.

Apart from protecting the fluid from oxidation, the inert gas will thwart water from condensing in the fluid due to raised ambient temperature and dew point alterations. Oxidation inhibitors present in some heat transfer fluids are sacrificial materials that help prevent fluid oxidation upon incidental exposure to air. They are not intended to take the place of better system design, operation and maintenance.

This information has been sourced, reviewed and adapted from materials provided by Paratherm.

For more information on this source, please visit Paratherm.

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