Frequently asked questions about heat transfer oil

Here, our experts answer some frequently asked questions when choosing, using and maintaining heat transfer oil.

A heat transfer oil is a gas or liquid specifically manufactured to transmit heat from one application to another.

This branch of science and engineering involves the transfer of heat via a liquid or vapour — temperature, flow rate, phase transition, pressure, volume, and density all play an important role.

It can also refer to coolants — fluids that flow through a device to prevent overheating. For example, we use water cooling to prevent car engines from overheating.

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

Steam heat transfer systems operate at very high operating pressures of about 85 bars or 8,500 kPa. If the steam reaches critical pressure and the system has no way to vent, it can cause pipes and valves to burst. In steam-based systems, manufacturers must also regularly maintain pipes to prevent corrosion which can result in downtime and loss of production.

Thermal fluid systems are often better than steam-based systems because they are more efficient, safer and offer more precise temperature control. System availability is also reliable when condition monitoring and a preventative maintenance programme is applied.


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 commonly used in indirect heat transfer processes. These are synthetic oils specially formulated to have high thermal stability and can operate at elevated temperatures for prolonged periods, making them suitable for various processes and heat exchangers.

Though all thermal fluids will degrade over time, matching the fluid choice to the system and the operating temperature can help to preserve fluid life. Heat transfer fluids are specifically designed to offer unique properties for specific applications.

Important factors to consider when choosing a thermal oil include operating temperature — you should consider both the highest and lowest operating temperatures to ensure it operates effectively during start up and can maintain the required temperatures for long periods.

The application can also dictate fluid choice, because many manufacturers need to adhere to industry specific regulations in their country. For example, manufacturers in food, beverage and pharmaceutical processing should use a certified food-grade thermal oil.

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.

The exact lifespan depends on several factors such as oil quality, operating temperature, system design and maintenance. Routine testing and analysis and thermal fluid management is key to extending fluid life.

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 begin to degrade as a result of oxidation and thermal 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 atmosphere as a hot expansion tank would be, which protects the oil against oxidation ageing.

Implementing a continuous preventative maintenance programme, such as Thermocare from Global Heat Transfer, can help to slow degradation and improve efficiency.

Regular fluid sampling and analysis enables engineers to get an accurate representation of the condition of the fluid in the system to anticipate what could happen in the near future. 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.

Engineers should sample the fluid when the system is hot, closed and circulating to get an accurate representation of what is happening inside the system. Engineers should then send the sample to a lab for testing, to track degradation and decide if maintenance is necessary to extend fluid lifespan which will protect system components and minimise impact on maintenance and energy costs.

As part of its analysis service, Global Heat Transfer completes an eleven-point test. This includes the Ramsbottom carbon residue test (RCR), open and closed flash point measurement and acidity level tests.

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 will indicate 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.

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.

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.

If you have any other questions about heat transfer systems or heat transfer oils, contact Global Heat Transfer.

On our contact page you can find contact details for our offices in the UK, US and worldwide.