Diluting thermal fluid

How to dilute heat transfer oil

When thermal fluid degrades, contaminants will build up in the system and reduce the heat transfer efficiency of the fluid. Diluting thermal fluid allows manufacturers to remove these contaminants, improve heat transfer efficiency and extend the lifespan of their fluid.


Fluid degradation

While thermal fluids are designed to operate at and maintain high temperatures for long periods of time, they will begin to degrade over their lifespan, particularly if they are left unattended.

Thermal oil will break down by both oxidation and thermal cracking. All systems are designed to run using Ph neutral fluid to avoid excessive corrosion. Oxidation occurs when heated fluid above 60°C is exposed to air with no nitrogen blanket and once acid (TAN) levels start to increase. When the fluid oxidises, heavy ends, or carbon, instantly start to form, which will adversely affect pipework, the heater coil, the expansion tank and general system efficiency and safety.

Degradation can be a gradual process, so may go unnoticed until it negatively impacts production. For example, if food products cook inconsistently or chemicals do not blend, it is likely that the fluid has degraded. When this occurs, manufacturers should cease production and take action to maintain the fluid.

Once it enters the system, heat transfer oil is no longer visible. Taking regular samples enables manufacturers to monitor the condition of the thermal fluid. Engineers will get a representative sample if they draw it using the closed method, and while the thermal oil is hot and circulating. Manufacturers can also collate data from thermal fluid analysis to plot fluid changes in fluid condition (trending) over time, allowing them to intervene before degradation impacts production.

Thermal fluid does not go from being fit for use to needing replacing overnight. There is a grey area where fluids can be managed against what specialists call ‘the degradation curve’. Degradation is steady if a system is operated properly. It is only as the fluid approaches the end of its practical working life that there is a gradual curve, which eventually drops off very sharply. This sudden change in the quality of thermal fluids is one of the reasons why regular and preventative condition monitoring and maintenance is so vital.

Heat transfer fluid analysis will show where thermal fluid condition is in terms of degradation. A specialist laboratory will analyse oil samples to determine its condition, carrying out multiple tests. If any one or combination of the eleven tests carried out is outside the acceptable limits, based on typical data for the fluid sample, it is an indication that the fluid is beginning to degrade. The specialist can then recommend dilution.


Engineers can dilute thermal fluid with virgin thermal fluid that is miscible. Topping up the existing fluid extends the useable life of the heat transfer oil and increase its heat transfer efficiency.

Extending the lifespan of existing thermal oil will reduce operational and capital expenditure costs associated with shutting down production, cleaning the system and replacing the fluid. Intervening at this time will also prevent downtime, reduce the risk of associated production losses, and reduce the environmental impact of the heat transfer fluid.

Replacing a fluid is reactive management — engineers will only act when a fluid is degraded and no longer fit for use. Dilution is a proactive response, enabling engineers to intervene and maintain fluid before it impacts production.

The cost of dilution is favourable when compared to the cost of complete fluid replacement. Fluid replacement is a last resort because it also means a loss of production. To replace fluid, engineers should drain and flush the system before refilling it with virgin fluid.

As well as reducing costs, diluting fluid to extend its lifespan has environmental benefits. Once diluted, fluid can operate efficiently at its intended temperature, removing the need to turn up the temperature to maintain production. As a result, manufacturers may see a reduction in energy usage and therefore costs. While dilution reduces light ends, it does not address the cause. Instead, engineers should consider a long-term solution, such as installing a Light Ends Removal Kit (LERK), to manage the ongoing production of light ends.

Choosing to maintain existing fluid rather than replacing it also reduces the environmental impact and cost associated with removing, transporting, and processing waste.

Diluting the thermal fluid by topping it up is a cost-effective and durable option, but only while the fluid is in the early part of the degradation curve.

After the condition of the fluid has significantly deteriorated, dilution is no longer a viable alternative. Diluting a degraded fluid would be a very short-term solution because the system would not operate at optimum capacity.

The best thing to do when thermal fluid reaches the end of its lifespan is to flush and clean the system before refilling it with fresh heat transfer fluid.

Once a manufacturer has decided that fluid dilution is the best maintenance option, they should consult a thermal fluid maintenance specialist.

Thermal fluid maintenance engineers will consider a variety of parameters when calculating the ratio of dilution. Engineers will consider the site, application, fluid type, system type and the rate of fluid degradation over time during these calculations.

Heat transfer fluid maintenance

Regular sampling enables manufacturers to understand fluid condition and anticipate its degradation, planning in maintenance before issues negatively impact production. Engineers can implement a preventative maintenance programme, such as Thermocare®, for sampling and maintenance support.

Manufacturers can also introduce remote condition monitoring systems to further improve maintenance. Thermocare® 24/7 Live Condition Monitoring is a cloud-based remote monitoring system that uses live real-time analytics to assess the condition of thermal oil and diagnose potential issues. As soon as the platform detects an anomaly, it warns maintenance personnel by sending an alert to their smart devices.

Where can I get more support with diluting thermal fluid?

For more advice on diluting thermal fluid, other methods that can extend the lifespan of heat transfer oil or other heat transfer system management services, contact Global Heat Transfer