Frequently asked questions about heat transfer oil

A heat transfer oil is a gas or liquid specifically manufactured to transfer thermal energy from one application to another, using its high heat capacity to efficiently store and transfer energy in industrial processes.

It can also refer to coolants used in heating and cooling applications — fluids that flow through a device to prevent overheating or freezing. For example, glycol-based heat transfer fluids, like our low temperature thermal fluids, are engineered to perform in extreme cold conditions. With a service temperature range from -90°C — 300°C (-130°F — 572°F), they’re ideal for cooling systems in demanding industrial processes.

Oil can also be referred to as thermal oil, thermal fluid, heat transfer fluid, thermic fluid, thermal fluid or thermic oil.

 

The chemical composition of a heat transfer fluid can be organic or synthetic. Synthetic heat transfer fluids, such as a silicone or terphenyl, have a lower propensity to form carbon than mineral based oils, offering better heat transfer efficiency and thermal stability. They are more resistant to fouling, as they tend to form less coke on the internal pipework and heater, helping maintain energy efficient operations at higher temperatures.

Heat transfer oil, or thermal oil, is widely used in indirect heat transfer processes. Synthetic oils are designed for high thermal stability and prolonged operation at elevated temperatures, making them ideal for various processes and heat exchangers.

Application-specific needs also influence fluid choice. For instance, manufacturers in food, beverage and pharmaceutical industries should use a certified food-grade thermal oil to meet regulatory standards.

Selecting the right fluid for the system and operating temperature can extend its lifespan. Heat transfer fluids are specifically designed to offer unique properties for each application.

1. Oxidation: A thermal oil oxidises when it reacts with oxygen in the air by a free radical mechanism, causing carbon to form. The rate of oxidation increases with temperature.
2. Thermal degradation (thermal cracking): When a heat transfer oil exceeds its maximum film temperature, it begins to degrade. During cracking, the bonds between hydrocarbon chains start to break, producing shorter chained light ends. This leads to vaporisation and the formation of volatiles.

The buildup of carbon in the thermal fluid also increases viscosity. When carbon concentration reaches a certain level, sludge starts to form on the insides of pipework, resulting in fouling. The sludge accumulates, particularly in low flow areas such as reservoirs and expansion tanks and reduces the efficiency of heat exchange.

At high operating temperatures, fluids degrade because of oxidation and cracking. As hydrocarbon chains shorten, the boiling point decreases, allowing light molecules to vaporise. Hydrocarbon chains can also recombine to form heavy ends that usually cause fouling of the heat transfer system.

Light ends will lower the flash point, or the ignition point of the heat transfer oil, increasing fire and safety risks.

Engineers can install a light ends removal kit (LERK) to remove volatile light ends. Hot thermal fluid flows through a distillation vessel where gaseous light ends are collected in the liquid phase of the condenser.

During the process, the system is not open to the atmosphere as a hot expansion tank would be, which protects the oil against oxidation ageing.

If the system cannot maintain required operating temperatures, this means the fluid has degraded substantially. The degradation will likely impact productivity and product quality, leading to wasted batches.

Thermal fluid analysis measures a specific range of fluid parameters including total acid number (TAN) and carbon residue data. If any suspended material in a heat transfer oil reaches over five per cent of the total system volume, the fluid is no longer suitable for use.

Draining the system will remove the degraded fluid but may not remove all the contaminants — up to 25 per cent of a system’s fluid volume can remain. So, you must flush and clean a system using a cleaning and flushing fluid such as Globaltherm C1 before.

Some thermal fluid suppliers offer a drain, flush and refill service to help safely drain the system and dispose of the waste product.

While specific legislation varies by region, employers generally have a legal and ethical responsibility to assess risk and implement safety controls.

In the UK and Europe, regulations like the Dangerous Substances and Explosive Atmospheres Regulations (DSEAR) of 2002 and the Explosive Atmosphere Directive (ATEX 137) are mandatory requirements for protecting workers from flammable or explosive materials like thermal oils.

Employers have a legal obligation to not only comply with this legislation but to prepare and maintain documentary evidence. Similar standards exist globally. It’s essential to consult local safety authorities or regulatory bodies to understand and comply with applicable requirements in your region.

Manufacturers can work with thermal oil suppliers to implement an effective preventative maintenance programme.

As part of Thermocare®, for example, Global Heat Transfer’s engineers offer both on-site and remote technical support that will help to extend fluid lifespan, reduce environmental impact and maintain regulatory compliance.

 



Thermocare also offers live condition monitoring. This cloud-based remote monitoring system continuously monitors fluid condition, sharing real-time data with the cloud that engineers can access from any location. The platform can determine the presence of degradation factors and warn maintenance personnel with an alert to smart devices.

Steam heat transfer systems operate at high pressures, around 85 bars or 8,500 kPa. Without proper venting, critical pressure can cause pipes and valves to burst. Regular maintenance is essential to prevent corrosion.

Thermal fluid systems are safer because they provide better temperature control thanks to their great thermal stability. With condition monitoring and a preventive maintenance programme applied, they ensure reliable system availability.