Why Thermal Cracking Reduces Heat Transfer Oil Performance

Thermal cracking reduces heat transfer oil performance because every system and application behaves differently, and oil cannot last forever under high heat. When thermal fluids are continuously exposed to high temperatures, they slowly break down over time, which reduces their ability to transfer heat efficiently. This damage usually happens very slowly, so the change is not noticed for months or even years.

Because these systems often run with low maintenance, degradation products can build up without warning. Over time, this can lead to serious issues like blockages, leaks, or even system failure. In some cases, it may also increase safety risks such as overheating or fire. 

That is why understanding thermal cracking is important to prevent performance loss and keep the system running safely and smoothly. At Fubex Lubricants, we offer high-quality Heat Transfer Oil for all types of engines. Enjoy fast shipping, a price match guarantee, and no-questions-asked returns. Need help choosing the right oil? Call us at +971 50 544 9614 — our friendly team is ready to assist!

Awais I., Sales Director, says, “Thermal cracking slowly breaks down heat transfer oil under high temperatures, reducing its ability to move heat efficiently and causing loss in overall system performance, energy efficiency, and reliability.”

To learn more about how thermal cracking develops and how it can be controlled in real systems, keep reading and explore the key prevention methods and warning signs in detail.

Key Takeaways

• Thermal oil can break down when exposed to very high heat, which can damage its chemical structure and reduce performance.
• Overheating can cause the oil to form small or large unwanted particles, which may lower efficiency and block the system.
• Contact with air at high temperatures can lead to oxidation, making the oil thicker and creating harmful deposits over time.
• Dirt, water, or other contaminants can speed up oil damage and cause serious problems in machines if not properly removed.
• Proper temperature control, regular oil testing, and good system design help extend oil life and keep equipment running safely.

Thermal Degradation or Thermal Cracking 

Thermal degradation (thermal cracking) happens when a fluid gets too much heat energy and can’t handle it anymore. When this happens, the tiny bonds inside the fluid break apart.

In heat transfer fluids made from oil or chemicals, thermal cracking means the strong carbon bonds in the oil break due to very high heat. These bonds are usually very strong and need a lot of energy to break.

This process depends on how well the oil can handle heat and how much heat it is exposed to while it stays in a hot area. For example, a normal mineral oil used for heat transfer (ISO VG 32) has long carbon chains. When it gets too much heat, these long chains break into smaller parts. These smaller parts are called “low boilers” because they boil at lower temperatures.

As more of these small parts build up over time, the oil becomes more likely to evaporate. This also lowers its flash point, fire point, and the temperature at which it could catch fire on its own.

What is Heat Transfer Oil?

Heat transfer oil is a special liquid used in machines to move heat from one place to another. It helps carry heat safely and efficiently in industrial processes.

Sometimes, heat transfer fluids are also used for cooling. These fluids flow through machines to stop them from getting too hot or too cold. For example, glycol-based fluids can work in very cold conditions and are used in systems that need strong cooling or heating control. Some of these fluids can work in very low and very high temperatures, from about -90°C to 300°C.

Heat transfer oil is also called thermal oil, thermal fluid, thermic fluid, or heat transfer fluid. All these names mean the same type of liquid used for managing heat in machines.

How to Deal With Thermal Cracking

To control thermal cracking, it is important to follow some simple steps:

Use the Right Fluid

Always choose a heat transfer fluid that can handle high temperatures. Some special oils are made to stay stable even under strong heat. These are better than normal mineral oils. This helps reduce problems caused by sudden or uneven heating.

Release Trapped Gases Safely (Venting)

When the oil gets too hot, it can form light gases inside it. Venting means safely removing these gases from the system. This is usually done by moving some hot fluid to a tank where the gases can separate and escape or be collected safely. 

After venting, fresh oil must be added to keep the system at the right level. However, new oil should never be poured directly into very hot oil—it should go into a cooler tank first. Venting for too long is not safe because it can cause the oil to break down faster or create fire risks.

Follow Proper Start-Up and Shut-Down Steps

Thermal cracking often happens when machines heat up or cool down too quickly. Heating should be done slowly over several hours so the oil warms up evenly. If the temperature rises too fast, the oil can get damaged.

Shutting down is also important. Even after the heater is off, some parts of the machine stay very hot and can keep heating the oil. To prevent damage, the oil should keep circulating until it cools down to a safe temperature.

Extra Safety Tips

• Keep an eye on heat levels in the system
• Make sure pumps are working properly
• Check the oil regularly for early signs of damage
• Filter the oil to remove dirt and particles
• Always follow correct start-up and shut-down procedures

These steps help the oil last longer and keep the system working safely and efficiently.

What Happens During Thermal Degradation?

Thermal degradation, also called thermal cracking, happens when a fluid gets very hot and its chemical bonds start to break. The heat breaks the carbon bonds inside the fluid molecules, creating smaller pieces called free radicals.

These small pieces can do two things:

• They may stay small and form lighter molecules than before.
• Or they may join together and form larger, thicker molecules.

In heat transfer systems, these are called:

• Low boilers (small molecules)
• High boilers (large molecules)

If the temperature becomes extremely high, the damage gets worse. Not only do carbon bonds break, but hydrogen can also separate from carbon. This can create a hard carbon material called coke. Coke can quickly build up inside the system and block heat transfer, which can stop the machine from working properly.

Low boilers make the fluid easier to evaporate and lower its flash point and thickness (viscosity). High boilers make the fluid thicker. If too many build up, they can turn into solid particles, which can block and damage the heat transfer system.

Oxidation

Oxidation happens when heat transfer fluids react with air. This reaction slowly forms organic acids inside the fluid. At normal temperatures, this process is slow, but when the temperature goes up, oxidation becomes much faster. In fact, it can double for every 20°F (11°C) increase above 200°F (93°C).

These acids can change the fluid and make it thicker over time. They can also create sticky deposits that cause machine problems. Often, early signs of oxidation are seen in the expansion tank, like dirt buildup or corrosion, especially if the oil is not checked regularly.

To reduce oxidation, some simple steps can be followed:

• Keep the expansion tank temperature below 120°F (48°C)
• Reduce contact between hot oil and air
• Make sure the pump has enough pressure to work properly

If smoke is seen from the expansion tank vent, it usually means the tank is too hot. In that case, cooling methods like using a sealed system or nitrogen gas can help. Also, the expansion tank should not be fully insulated in vented systems because it needs proper cooling and airflow.

It is also important to prevent air from getting into the hot oil. Air can enter if there are blockages in the system, and this can damage pump seals and reduce system life.

Contamination

Contamination means unwanted dirt or substances get into the system. These contaminants can speed up oil damage and also cause serious equipment problems. They can enter during system setup, cleaning, or normal daily use.

• New Systems: Before starting a new system, all dirt, metal pieces, and welding leftovers must be removed. Never test the system with water. Instead, use heat transfer fluid or inert gas to check for leaks.
• System Cleaning: When cleaning, all liquids must be fully drained from the system. If full draining is not possible, fresh heat transfer fluid should be used to flush it. If water-based cleaners are used, they must be completely removed. Any leftover water should be dried out using hot nitrogen gas. Water should never be boiled off through the expansion tank because it can damage the fluid.
• Daily Operation: Always use fresh fluid when topping up the system. Any oil that leaks out or collects in trays should not be reused. Different types of fluids should never be mixed. Regular oil testing in a lab is also very important. It helps find early signs of damage like cracking, oxidation, or contamination so problems can be fixed early.

Oil samples should be taken from a running system, not from storage tanks. The system should be at normal working temperature when sampling. The sample should go directly into the bottle, without using another container. Most experts recommend checking the oil at least once a year or sooner if any issues are noticed.

How to Prevent Thermal Oil Damage

• Use Good-Quality Synthetic Oil: Synthetic heat transfer oil is better at handling high heat. It stays stable longer and does not break down easily, especially in hot systems.
• Keep the Right Temperature: Always run the system within the safe temperature range. If the temperature gets too high, the oil can get damaged or break down.
• Check the Oil Regularly: Testing the oil after some time helps find early signs of damage. This makes it easier to fix problems before they get serious.
• Use a Good System Design: A properly designed system helps the oil flow evenly. It also prevents very hot spots, which can damage the oil quickly.

Final Takeaways

Thermal cracking is one of the biggest reasons heat transfer oil loses its performance over time. When the oil is exposed to very high temperatures, its molecules start to break down, changing its natural properties. This leads to problems like lower efficiency, thicker fluid, deposits, and even system blockages. If not controlled, it can shorten the life of both the oil and the equipment. 

The good news is that with proper temperature control, regular monitoring, and the right type of oil, thermal cracking can be reduced. Taking these simple steps helps keep the system running smoothly, safely, and for a much longer time.

FAQs

Q1: What is heat transfer oil degradation?

Heat transfer oil degradation is when thermal oil slowly breaks down during use due to heat and operating conditions. This breakdown reduces the oil’s ability to move heat properly. Over time, it can also cause the oil to form sludge, carbon buildup, and other unwanted gases or compounds.

Q2: What are the signs of heat transfer oil degradation?

Heat transfer oil degradation can be seen when the oil becomes thicker and darker than usual. Sludge or carbon deposits may also form in the system. It can also cause poor heat transfer, higher fuel use, and unstable temperatures.