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Three strategies for maximising the performance of heat transfer fluids

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Heat transfer fluid (HTF) systems must be maintained properly to avoid any unplanned downtime. HTF systems often remain problem-free for extended periods, but this can lead them to become neglected with issues only identified after a sudden failure. Maintaining system performance is largely reliant on using a fluid-management programme and protecting HTFs from anticipated degradation. Below, we examine three strategies to optimise fluid performance.

1. Select HTFs that exceed requirements

Today, hundreds of HTFs are available but fluids should only be ordered from reputable suppliers. As experts in HTFs, they can determine exact operating limits by combining up-to-date performance data with thermal stress analyses. Thermal attributes across the complete operating range of the HTF should be provided to find a suitable match for system specifications.

HTF purity is also key. Even lower levels of impurity can have a bearing on coking tendency and unwanted surface deposits. Users should be aware that using an inferior fluid can cause considerable harm. In particular, cheap darker-coloured HTFs are commonly associated with delivering poor performance.

Operating temperatures must be considered. Where temperatures exceed 300°C, synthetic aromatic solutions are suitable. Aromatic HTFs are more specialised but offer greater flexibility and more extreme temperature abilities compared to mineral oil-based fluids.

2. Always manage threats to seamless performance

Among the threats to continuous performance of an HTF are contaminants, oxidative degradation and thermal degradation.

Contamination is usually caused by operational errors and leaks. Common mistakes include using the wrong HTF, poor cleaning procedures and sharing process equipment. The results of contamination vary depending on the contaminant type. For example, water manifests swiftly, causing pump cavitation, mechanical knocking sounds and unexpected venting. In other instances, contaminants can degrade quickly, causing increased acidity, carbon creation, generation of low and high boilers, and fouling of surfaces. Contaminants can have various side effects like reduced heat transfer, mechanical failure and a reduction in operational safety.

The process of oxidative degradation happens when hot HTF reacts with oxygen inside areas of the system like reservoirs and vented expansion tanks. Fluid oxidation has a significant impact on HTF performance. As oxidation produces organic acids, the HTF degrades into carbon sludge and soot if left uncorrected. As oxidation continues, the total acid number of the HTF increases.

Additionally, oxidation causes substantial increases in HTF viscosity and precipitation of soot and sludge, leading to plugged lines and fouled surfaces. In most HTF systems, the risk of oxidation is eliminated via blanketing expansion tanks with inert gasses like nitrogen. When blanketing isn’t an option, thermal buffer tanks and cold-seal pots are alternatives to consider. While it is easily fixed, oxidation is behind 90 percent of all cases of premature fluid degradation.

Thermal degradation involves fluid molecules absorbing more heat energy than they can release. The thermal stress breaks the molecular bonds, resulting in permanent damage to the HTF and changes to its physical attributes.

Thermal degradation rate is impacted by operating conditions and fluid stability. As fluid molecules crack, they form low boilers (lower-boiling fragments) that recombine to create higher boiling polymers. Excessive low boiler build-up decreases thermal efficiency and reduces the HTFs’ flashpoint, and can cause issues like venting unexpectedly.

When higher-boiling polymers accumulate in a system, HTF viscosity can increase, affecting thermal efficiency and pumpability. When solubility limits are exceeded, these high boilers present themselves as sludge, fouling lines and heat exchange surfaces. In severe cases, this can result in heavy coking and heater tube failure.

Thermal degradation rate is minimised by choosing an HTF based on application temperature range requirements and never exceeding the original equipment manufacturer’s (OEM’s) recommended maximum temperatures. Providing the adequate HTF flow through the heater is also vital, as is using a preventative maintenance programme.

To manage the above threats that lead to potential fluid degradation, regular system reviews are critical. Operators should walk the system at least once a week and note down any leaks, unusual sounds, venting smoke, or unexpected temperature and pressure readings. It’s worth remembering that small issues can swiftly grow into bigger problems, so proactively addressing abnormalities immediately is always the best practice.

Furthermore, both proper start-up and shutdown of the HTF system are key to long-term performance. At start-up, the system heater should stay in low fire right up until turbulent flow is well established to stop overheating. At shutdown, operators should ensure the main circulation pumps remain running up until the heater’s outlet temperature drops to 82°C or lower.

3. Implement a fluid maintenance programme

Finally, condition monitoring of the HTF is crucial to achieve sustained performance. Any unseen alterations in HTF quality can impact the system’s ability to answer production demands and can result in unplanned mechanical downtime. By using routine analysis, end users are alerted early to any changes in the fluid’s condition, which in turn can help them detect potential equipment malfunctions and resolve them before they become catastrophes.

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