With thousands of applications for heat transfer fluids (HTF), it can be a near-impossible task for an HTF manufacturer to suggest a specific oil maintenance schedule or change interval. Each usage has defined characteristics that will contribute to how and when the HTF degrades. Additionally, each HTF will often react differently in various operating environments.
To understand maintenance and change intervals, it is important to look at how HTF becomes degraded and the different types of system used. There are two main types of degradation – thermal and oxidative.
Thermal degradation
Sometimes referred to as thermal cracking, the process of thermal degradation is defined as when heat breaks down carbon-to-carbon bonds in fluid molecules to create smaller fragments. As these bonds break down, hydrogen atoms are separated from carbon atoms and coke is formed which can foul the HTF, causing systems to cease operation.
In simple terms, thermal degradation happens when oil is overheated beyond its boiling point. Excessive overheating causes reduced viscosity, which not only leads to HTF contamination but increases the flash and fire point of fluids to unsafe levels.
Oxidative degradation
This type of degradation is caused by how the HTF reacts with air. Larger molecules form, increasing the viscosity of the HTF, making it more difficult to pump and reducing its heat transfer capabilities. Oxidation also increases the chance of coke creation but also spikes the HTF’s acidity levels. Put simply, oxidation happens when HTF is exposed to air. This can cause sludge build-up that reduces flow and operational efficiency.
Maintenance for closed systems
Closed systems commonly use an inert gas buffer at points where HTF comes into contact with oxygen to minimise degradation via oxidisation. These systems are larger by necessity and, in some cases, feature proprietary plumbing that can eliminate issues of oxidisation degradation.
These systems typically never run at high temperatures and the inert gas or plumbing negates oxidation. However, these systems can still suffer from thermal degradation issues.
The first maintenance priority is to consult the original equipment manufacturer (OEM) and HTF vendor before making changes to a system. A system is built on user needs and the HTF in use. While most HTFs share the same range of physical properties, changes unaccounted for in a system’s functionality or design can negatively affect the fluid.
The second maintenance rule is to adhere to an established fluid analysis program and track the HTF’s overall health. Usually offered as a complimentary service from HTF vendors, a complete fluid analysis program helps detect changes to fluids early. A proactive approach while using your fluid vendor’s analysis program provides time to implement system corrections that mitigate fluid degradation, improving its condition.
Changing HTFs in closed loop systems is infrequent, but a complete changeover will be required at some point. This can be a multi-stage process involving either a flushing or cleaning fluid.
Understanding the condition of both the system and the HTF before changing fluids is vital, whether users are swapping to a new HTF or refilling with the same fluid. For this reason, both a fluid analysis and general systems inspection is critical, including inspections of the boiler and its pipework.
It is important to completely drain any old fluid before refilling, as degraded HTF will quickly contaminate your new supply.
Maintenance for open systems
Open systems, however, involve HTFs making contact with air, making them susceptible to oxidisation. Smaller systems, by design, are often employed in manufacturing processes for the plastic and diecast industries, among others.
Open systems run at temperatures far below recommended bulk temperatures. While this reduces the chance of thermal breakdown, oxidation is always a problem. Fluid analysis is the best maintenance tool to achieve peak efficiency. When using an HTF in an open system, the fluid must be monitored regularly to understand when it requires changing. Equipment manufacturers can provide guidelines on fluid life expectancy, although not all HTFs are made equal. Differences in operating environments and usage can impact oil life; detailed fluid analysis is the only answer.
As with a closed system, all spent fluid must be extracted before refilling. An HTF that has degraded will contains acidic compounds which can increase the degradation rate of the new supply.
Open systems often have many areas where HTF can become trapped. Points like filter housings, heat exchangers, and horizontal piping runs must be examined for any residue. Blowing nitrogen or dry air through lines can help remove old fluid. When this approach is not possible, fluid manufacturers provide a light and affordable flushing agent that can also be used to get the same result.
Finally, maintaining a strict fluid change-out schedule will ensure the internal components of your system stay clean and run at maximum efficiency for longer. Users who neglect maintenance schedules will typically find that their lack of care leads to both a total system failure and extensive and expensive repair work.
