Mineral or synthetic? How to choose a heat transfer fluid
In any process that requires indirect heat transfer, it’s essential to select the appropriate thermal fluid for the application — whether it’s for heating processes in power generation or cooling products in food processing. But with so many heat transfer fluids on the market, reviewing options can be a time-consuming task. Here Clive Jones, managing director of thermal fluid expert Global Heat Transfer, explains the properties of mineral and synthetic heat transfer fluids, when to use each and how to maintain the fluid accordingly.
Taking the time to select the best thermal fluid for an application will improve a variety of factors, such as heat transfer efficiency, system performance and maintenance costs. So where should manufacturers start?
The right temperature
Matching the fluid to the application’s temperature is important because it will determine safety and efficiency — fluid choice can also impact heat transfer reaction speed, improving productivity. Thermal fluids must reach and maintain specific temperatures for prolonged periods, so choosing a fluid that offers thermal stability at the outlined temperature is crucial to maintaining uptime.
Manufacturers should look at both the highest and lowest operating temperatures outlined on the technical data sheet. Considering the heat transfer reaction speed could also impact productivity — comparing fluids and selecting a product with faster heat flow can reduce start-up time.
Chemical composition
A thermal fluid’s chemical composition will often dictate its operating temperature range. Thermal fluids fall into two categories — mineral and synthetic. Mineral-based thermal fluids contain complex hydrocarbon molecules and additives that enhance thermal stability and other properties that impact performance.
Synthetic thermal fluids, on the other hand, are specially formulated using carefully selected chemical compounds, such as glycol-based and silicone-based fluids. These fluids are chemically engineered to provide enhanced performance and stability at both high and low temperatures.
Making the choice
When deciding between mineral or synthetic fluids, thermal fluid experts such as Global Heat Transfer, can advise on the best fluid based on reviewing several factors, including temperature range and chemical compatibility with the system. Mineral thermal fluids are popular because of their cost-effectiveness, reliability and availability, and have been on the market for decades. Mineral thermal fluids offer excellent thermal stability at moderate temperatures. However, when heated to elevated temperatures for prolonged periods, they may degrade at a faster rate, negatively impacting heat transfer efficiency.
For applications that demand thermal stability and efficiency at both higher and lower temperatures, synthetic fluids can improve performance. Synthetic thermal fluids are engineered to operate effectively in more demanding industrial heating applications because of their wide operating temperature range, exceptional heat transfer efficiency and excellent resistance to degradation, corrosion, and fouling.
While mineral fluids may seem the more cost-effective option, they do not always offer the heat transfer efficiency required and can increase long-term maintenance costs. Synthetic fluids on the other hand will maintain thermal stability, efficiency and performance across a broader temperature range, maximising uptime.
Maintaining synthetic fluids
Proactive maintenance is important when using any heat transfer fluid to increase productivity and reduce maintenance costs, but engineers should adapt procedures based on the fluid they use.
Because of their limited temperature range, as mineral thermal fluids near the end of their lifespan thermal cracking and oxidation cause the fluid to degrade. As the fluid approaches the very end of its practical lifespan, there is a gradual curve that drops off quickly — the degradation curve. This gradual curve before the drop is a grey area where engineers can still manage the fluid. To mitigate for by-products of degradation, such as sludge and carbon deposits, plot condition and anticipate the degradation curve, engineers should regularly sample fluid to understand its condition. Synthetic heat transfer fluids are designed to be more resistant to degradation, but engineers will still benefit from routine representative fluid sampling and analysis.
By drawing a sample when the system is hot, closed and circulating, engineers can send it for analysis and gain an accurate representation of what’s happening inside. Engineers can trend condition over time, monitoring for any signs of change in thermal fluid quality that may impact production and plan maintenance accordingly.
When choosing a thermal fluid, manufacturers can narrow down the list of options by first understanding if they need a mineral or synthetic thermal oil. By carefully assessing the process requirements, temperature range and potential maintenance costs, manufacturers can make an informed decision on what fluid will deliver the thermal stability and performance required.