Slowing degradation – High temperature heat transfer fluids
High temperature heat transfer fluids degrade when used at high temperature for prolonged periods, so what can be done to slow this process?
Heat transfer fluids (HTFs) can be used at very high temperatures (around 400 degrees Celsius is the standard in concentrated solar power plants). With prolonged use these fluids will start to degrade and the by-products that form need to be actively managed to help maintain a safe operating environment and to simply extend the fluid’s operating life. Indeed, few companies understand that regularly servicing a HTF and HTF systems can help sustain their longevity. Global Heat Transfer examines how the build-up of short-chain hydrocarbons, a by-product of HTF thermal degradation, can be managed.
Ennis [1] described the process of thermal degradation as “When a heat transfer fluid degrades, it forms a combination of ‘lights’ and ‘heavies.’ The lights are short chain molecules formed by thermal cracking and may also include hydrogen gas. These have the effect of reducing the flashpoint of the HTF as well as being boiled off from the liquid as a flammable vapour.”
Flammable vapours are effectively removed using batch venting or through installation of a portable or permanently installed ‘light-ends removal kit’, commonly referred to as a LERK [2].
Batch venting is the process of intermittently heating a circulating HTF enough to vaporise the light-ends and reduce their build-up. However, this is only an intermittent solution and may need to be repeated at a later date [2].
A LERK, on the other hand, can be installed intermittently, if a portable unit is deployed, or permanently fitted to a HTF system and operational whilst the system is running (see image). Therefore, it offers the potential to continuously remove flammable light-end by-products through the process of distillation and prevent their build-up whilst the HTF thermally degrades. Indeed, a recent article showed that a permanently installed LERK still remained effective 5 years after its installation. Results showed that after this time, flammable vapours still remained constant [3] and the plant continues to be monitored to assess its effectiveness over the longer-term.
In summary, light-ends present a potential hazard that need to be managed to ensure the safe operation of a thermal system. A LERK can be extremely effective, offering the potential to prevent the build-up of light-ends by removing them during HTF thermal degradation.
The current article highlights the approaches used to manage the build-up of light-ends, which can be offered as part of a well-designed maintenance plan, such as a Thermocare programme, which can help to sustain the life of a HTF and a HTF system.
Find out more about light-end removal kits and the management of flash point temperatures.
References
