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.

It can also refer to coolants — fluids that flow through a device to prevent overheating. For example, glycol-based thermal 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 environments.

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

The chemical composition of a thermal 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 also more resistant to fouling, which means they tend to form less coke on the internal pipework and heater.

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.

Factors like the highest and lowest operating temperatures must be considered to ensure optimal performance during startup and sustained operation.

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.

While all thermal fluids degrade over time, 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 specific applications.

Heat transfer oils are designed to last for many years and all heat transfer fluids can provide good service over extended periods, even when operating at high temperatures.

Routine testing and analysis and thermal fluid management is key to extending fluid life.

The lifespan also depends on operating conditions, system design, and adherence to maintenance schedules.

There are two main types of fluid degradation:

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, also known as thermal cracking. When a thermal fluid is heated above the maximum film temperature specified by the manufacturer, it will start to degrade rapidly. 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 formation of carbon in thermal fluid also increases the viscosity. When the concentration of carbon reaches a certain level, it starts to form sludge on the insides of pipework, in a process known as 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 chain length decreases, so does the weight of the molecules, meaning less energy is required to accelerate them to a velocity where they will escape liquid phase. 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 thermal oil. This increases the risk of fire, putting the workforce and facility at risk.

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

The light ends are either drained automatically or manually from the system. 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.

A preventative maintenance programme, such as Thermocare from Global Heat Transfer, can help to extend the life of the thermal fluid, slowing degradation and maintaining efficiency.

Regular sampling and analysis helps detect issues early and protect components. Engineers can then intervene and carry out maintenance tasks before thermal fluid degradation impacts production, reducing the risk of downtime while also maintaining safe operations.

If the system cannot maintain required operating temperatures, this means the fluid has degraded substantially. The degradation will likely begin to impact productivity and product quality, leading to wasted product 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 found in the system — up to 25 per cent of a systems’ fluid volume can remain after draining. So, you must flush and clean a system using a cleaning and flushing fluid such as Globaltherm C1 before introducing any new oil.

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

Working with thermal oils involves safety considerations due to the potential risks of fire and explosion. Many countries enforce regulations to ensure the safe handling, storage and use of flammable or combustible substances in industrial environments.

While specific legislation varies by region, employers generally have a legal and ethical responsibility to assess risk, implement safety controls and maintain through documentation. In the UK and Europe, for example, regulations like the Dangerous Substances and Explosive Atmospheres Regulations (DSEAR) of 2002 and the Explosive Atmosphere Directive (ATEX 137) are mandatory requirements for minimising safety risks and protecting workers from fire and explosion where flammable or explosive materials, such as thermal oils, are present.

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.

Global Heat Transfer also offers Thermocare® 24/7 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 if it detects an anomaly.

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 also needed to prevent corrosion, which leads to downtime and production loss.

In contrast, thermal fluid systems are safer, more efficient and offer more precise temperature control due to their excellent thermal stability.

With condition monitoring and a preventive maintenance programme applied, they ensure reliable system availability.