Every heat transfer fluid (also known as hot oil) has a finite service life in an extruder. The biggest factor in determining how long it lasts is the type of fluid being used, though process temperatures and production rates also play a role.

While there are many fluids available, only a select group of premium-grade products contain the necessary additive systems that boost performance and greatly extend the time between required oil changes.

To optimize oil life, it is also recommended to use a fluid analysis program. Regular analysis helps establish the proper change-out interval for your operation. Most reputable suppliers provide this service at no cost or for a nominal fee.

Maximizing Fluid Life

All fluids eventually degrade, and if they are not replaced as needed, they will leave behind deposits. The key is to balance production requirements with maintenance intervals.

To put this into perspective, the same fluid that lasts just 2,500 hours in an extruder could last 12 to 15 years in a larger system where it is not exposed to air. This shows how demanding extruder applications can be on thermal fluids.

How Fluids Break Down

Oxidation

Oxidation occurs when fluid at temperatures above 93°C (200°F) comes in contact with air. At these conditions, oxidation accelerates rapidly, especially in reservoirs where oil is not cooled before returning. Heat exchangers are essential in preventing this by maintaining reservoir oil temperatures below 93°C.

The simplest way to confirm heat exchanger performance is to monitor reservoir or tank temperature with a probe. A properly operating system should keep this temperature under 93°C. If temperatures are higher, fouling or reduced efficiency may be the cause.

Thermal Degradation

Thermal degradation happens when oil is heated beyond its maximum rated temperature. While this generally does not occur under normal conditions, certain operating practices can increase the risk.

During idle periods, extruders are sometimes left in “heat only” mode with barrel heater bands maintaining temperature. If circulation is minimal or stopped altogether, oil in the barrel wraps may overheat.

Shutdown procedures can also contribute. If circulation pumps are turned off before cooling the oil, stagnant fluid becomes trapped near hot surfaces, causing localized overheating.

As fluid continues to age, it darkens, develops acids through oxidation, and eventually leads to sludge formation. Regular oil changes are essential to prevent these issues, much like scheduled maintenance with a vehicle.

Proper Draining Techniques

When changing oil, it is critical to remove as much of the used fluid as possible. This includes not just the reservoir but also circulation lines, filter housings, heat exchangers, and other areas where oil can remain trapped. Leaving too much old oil behind will contaminate the new charge, causing immediate darkening and faster degradation.

One way to evaluate how effectively the system was drained is to compare Total Acid Number (TAN) values from samples taken before and after the change. This helps estimate how much used fluid remained in the system.

What to Do if Oil Changes Are Delayed

If the oil life has been stretched but deposits have not yet formed, a flushing fluid from the supplier can help reduce the amount of residual oil left after draining. This lowers the risk of contamination when refilling with fresh fluid.

If deposits are present, or suspected, specialized cleaning products are available to help remove them. Some cleaners allow production to continue during use, while others require downtime. In any case, deposits and residual oil should be removed carefully before refilling so that the new charge provides maximum service life and performance.