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There are essentially three critical product features of a well-designed high temperature heat transfer fluid – high thermal stability, high heat transfer efficiency and high purity. Dr Chris I Wright examines why purity is so beneficial to a pump.

A heat transfer fluid operating at high temperature for a long time will thermally degrade and the by-products of thermal degradation can potentially damage a pump and lead to its failure. The rate of thermal degradation is influenced by temperature and exposure to oxygen, but also the impurities in the fluid which catalyse the breakdown of the fluid.

In 2015, Lang and Lee assessed the working life of different grades (varying levels of impurity) of fluids commonly used in concentrated solar power plants and operating at temperatures between 350 and 430 degrees Celsius. Essentially a branded fluid (99.9% purity) was compared with an unbranded fluid (99.5% purity).

 At the same operating conditions the unbranded fluid degraded 1.7 to 2.4 times faster than branded fluid with a higher purity and that unbranded fluids would require more annual top-ups, and sampling, at an estimated cost of $2 per kilogram of fluid over a 25-year life span. Thus the choice of an unbranded fluid would be more favourable based on cost when selecting a fluid but over the life of the fluid and plant this would result in higher overall operating costs.

It is no surprise that a high purity fluid will be more expensive per kilogram than an equivalent lower purity fluid. The initial cost of a low purity fluid may seem to appealing when a new system is being filled, especially if it is a large system with a significant volume. However, over the life-time of a fluid the cost of maintaining the low purity fluid will be higher due to the principle effect of the impurities potentially accelerating the rate of thermal degradation.