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.


Table 1. Comparison of low purity unbranded and high purity branded fluids.


Low purity fluid

High purity fluid

↑ rate of thermal degradation

→ rate of thermal degradation

↑ annual fluid top-up and fluid sampling

→ annual fluid top-up and fluid sampling

↑ risk of pump wear

→ risk of pump wear

↑ annual operating costs and costs from due to repair or replacement

→ annual operating costs and costs from due to repair or replacement


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.

The knock-on effects to the pump and system are caused by the by-products of thermal degradation, such as carbon, which accumulate on the surfaces of the internal pipework and longer-term will lead to the need to replace the fluid but also potentially expensive equipment. This has been discussed in a previous article in World Pumps. So when considering a new fluid also consider its purity and the additional ongoing and longer-term costs that may associated with its use at high temperature for prolonged periods of time. 


Christopher Wright

Global Heat Transfer Ltd,

Cold Meece Estate,

Cold Meece,


United Kingdom




Author biography



Chris Wright is a research scientist, who graduated from the University of Leeds in the UK with a BSc and PhD. His research focuses on the use and maintenance of heat transfer fluids in manufacturing and processing, which includes food, pharmaceutical, specialist chemicals and solar sectors. For more information follow @RedPharmCo on Twitter.