In processes using heat transfer fluids, oxidation and thermal degradation can be corrected if you know what to look for.

This expansion tank was cut open to reveal a nasty surprise: excessive sludge buildup.

Heat transfer fluids are used in many applications that require heating or cooling, and sometimes both. Over the years, technology has greatly improved the service life of thermal fluids, but it is still important to understand how proper system design and operation play key roles in preventing fluid oxidation and degradation.


Oxidation is the most common form of fluid breakdown. It happens when a fluid comes in contact with air at high temperatures. It usually results in the formation of sludge within a system.

Oxidation can occur within various temperature ranges depending on the chemical makeup of a fluid. For most petroleum-based fluids, however, oxidation appears at temperatures above 200°F (93°C). It is generally accepted that once temperature exceeds 200°F (93°C), every 15°F (8.3°C) increase in temperature will double the rate of oxidation.

The total acid number (TAN) is a common measurement of oxidation in heat transfer fluids.

With respect to oxidization, weak points in a system easily can be found by looking for any point where the fluid contacts air. These usually are located in an expansion tank or reservoir. Once these points are identified, measure the average fluid temperature in this area during normal operation. If the fluid temperature is below 200°F, the system should not be prone to excessive oxidation. If 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 speaking, 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 the reservoir is connected via a T into the system rather than a part of the circulation loop.
  • If the expansion tank is not part of the circulation loop but it is still running hot, consider moving it further away from the main system, or add a nitrogen blanket to buffer the fluid from air contact.
Not all fluids are affected the same by oxidation. Check with your supplier to ensure that your fluid contains some type of defense against oxidation, particularly if your system is open to the atmosphere.

Both an electric heater chamber (top) and an electric heater element (bottom) are coated with coked hard carbon.

Thermal Degradation

Heating a fluid past its recommended maximum bulk temperature can cause what is known as thermal degradation, or thermal cracking. This refers to the breaking of carbon-to-carbon bonds in the fluid molecules. If the reaction stops when these bonds break, smaller molecules are formed. These are called low boilers, and they’re often indicated by a reduction in flashpoint. If instead the overheating continues, the fragments may react with each other to form polymeric molecules that are larger than the fluid’s original molecules. These are called high boilers.

Beyond ensuring your fluid is properly specified for your equipment and temperature requirements, there are a few things to be aware of that can contribute to thermal degradation.

Startup and Shutdown. Systems often are heated up too fast or shut down without cooling first. During startup, it is important to heat a system gradually, particularly with electrically heated systems. This helps reduce the risk of thermal degradation and ensures moisture or vapors are vented from the system gradually, without causing pump cavitation or having a geyser of vapor and fluid erupting from vent points.

Shutdown is equally important, particularly with electrically heated systems. If a system is not allowed to cool before fluid circulation stops, fluid can become trapped in the heater/boiler, which will likely expose it to temperatures much higher than the fluid is rated for.

System Modifications. Modifying a system design also can cause thermal degradation. A well-engineered system will use the heat transfer fluid as efficiently as possible without wasting valuable energy or fuel. This means pumps, valves, heater watt densities, user loads etc., are all engineered to work in harmony.

As systems get older or need change, some aspects of the system will likely need to be changed, modified or removed. If this is the case, work closely with system manufacturers, engineers and fluid suppliers to stay within the parameters of the system’s original design or make specific allowances with respect to the fluid’s capabilities.