Heat transfer fluids (HTF) are essential to the sustained operation of plants at high temperature. They are also required in a number of processes including the generation of energy from concentrated solar power plants and the production of processed foods.

HTFs and their systems are extremely expensive and valuable assets, so monitoring their condition is imperative because it provides insight into normal or abnormal changes in the health of a system. When new facilities are built, a heat transfer system can be contaminated, and flushing and cleaning protocols may be an effective way to remove contaminants prior to filling with a new HTF. This article explores the findings of new research on the use of these protocols and discusses why the detection of water is so important.

Global Industry News recently reported that Europe became the largest market for HTFs in 2015 with a 33.6 percent share in terms of HTF consumption. One of the key factors for this growth has been the expansion of concentrated solar power (CSP) plants. This increase presents an opportunity to standardize and establish best practices when new plants are built or when old fluids, which thermally degrade after prolonged use at high temperature, are replaced.

The most commonly used solar HTF is the eutectic mixture of biphenyl and diphenyl oxide, such as Dowtherm A, Therminol VP-1 and Globaltherm Omnitech. Such fluids can operate up to 400oC. Solar thermal fluids were estimated to be worth $2.8 billion in 2015 and projected to grow by 6.8 percent over the next five years.1 CSP plants are an important factor in the growth of HTFs and more specifically in the growth of solar HTFs. A solar HTF is needed in both newly built CSP plants and when replacing an old one.

Data analysis

A research paper published in Applied Thermal Engineering2 highlights the importance of defined protocols to effectively flush and clean newly built facilities before filling an HTF system with a costly fluid, such as eutectic mixture of biphenyl and diphenyl oxide HTF.

The current case concerns a company based in Scandinavia that built a new 100-metric-ton HTF system. The system was flushed with an industrial flushing and cleaning fluid (Globaltherm C1). After flushing around the system, this fluid was drained and chemically analyzed to assess the extent of contamination from water and environmental particulates, such as iron, silicon, aluminum, zinc and calcium. The flushing and cleaning protocol used in this study incorporated a fine filter with 15-micron pores and laboratory analysis to assess the cleanliness of the fluid.

HTF flushing, thermal plants, heat transfer fluid

Figure 1. Relationship between water content (high = red, low = blue) and particle contaminants in a flushing and cleaning fluid.

Figure 1 highlights the key findings from this work. Data were divided based on water content: low and high water which was defined as less than 100 or equal/greater than 100 parts per million. The particles contaminating the fluid were then analyzed according to International Organization of Standards 4406:1999 cleanliness coding. Particles were assessed based on their size (4, 6 and 14 microns, respectively) and their quantity in the flushing and cleaning fluid, with a higher cleanliness score representing a higher quantity of particles and contamination (see the y-axis in Figure 1).

In Figure 1, the high water content group had a statistically higher quantity of larger (14-micron) particles than the low water content group. This association between water and environmental contaminants suggests that the presence of water could be a strong indicator of environmental contamination in newly built systems.

The authors concluded: “…the flushing and cleaning protocol described herein is effective in the removal of contaminants during a HTF system build. This protocol has been shown to be effective by subsequent laboratory analysis. The presence of water may be an early sign of environmental contamination and the formation of rust.”

Practical implications

The first point to make is that the findings clearly show the need to monitor more than one parameter during flushing and cleaning. Simply monitoring water is not sufficient because it does not fairly reflect the presence of all particles in the fluid. With that said, however, if water is detected in a fluid during flushing and cleaning of the HTF system, then it likely also contains other contaminants.

HTF flushing, thermal plants, heat transfer fluid

Image 1. HTF filter system

Ensuring that the HTF system is protected and not left exposed for long periods prior to the flushing and filling with a HTF is also important. The presence of foreign particles in a fluid accelerates the aging of an HTF. Flushing is, therefore, imperative to remove the particles and maximize the life of the HTF. Any contaminants should raise alarm bells that water, rust or other contaminants are present in the system. The current protocol works in combination with a HTF system filter (see Image 1) to ensure the removal of contaminants that can and cannot be detected visually.

Relationship between water & environmental contaminants

Analysis revealed an association between water, level and particle size in the low water content group (see Figure 1) but not the high water content group. This suggests the presence of water had an effect that was seen evident in the 14 microns group. The reason seems to relate to rust forming when the internal iron pipework is exposed to air and water. The formation of rust particles with a larger size (14 microns) would then explain the findings reported in Figure 1 in which the high water content group also had a high large particle count.

The effectiveness of flushing & cleaning protocols

This research shows that flushing and cleaning protocols are effective, with industrial flushing and cleaning fluids working to physically suspend and remove contaminants from the HTF system. These contaminants are detected when the flushing and cleaning fluid is drained. The overall effectiveness of this process can be enhanced by incorporating a filter with a suitable pore size. In this case, particles equal or greater than 15 microns in size were removed from the circulating fluid.

The importance of flushing & cleaning protocols

On-site engineers should follow a set protocol to ensure the same results are repeatable and the process is effective. The intention is to ensure that the virgin HTF used to fill the HTF system does not become contaminated by water or particulates in the system and that the physical properties of the HTF in the system are identical to those recorded prior to filling. This is important because the engineer and client need to be confident that the protocol works and the aging of the virgin HTF will not be artificially accelerated by the presence of unwanted contaminants.


This is the first study to demonstrate the effectiveness of a flushing fluid in the removal of contaminants from an HTF system and to show the benefits of flushing an HTF system according to a set protocol to remove contaminants that could accelerate the aging of a virgin HTF that is costly to replace.

The relationship between water and environmental contaminants suggests that the presence of water may be an indicator of HTF system contamination and an early indicator of environmental contamination during the process of building and filling a new HTF system. The advantage of monitoring water is that it is easily detected, both visually and using permittivity sensors, by on-site engineers and the client.

Editor’s Note: The author would like to acknowledge the writing support provided by Red Pharm Communications, which is part of the Red Pharm company (@RedPharmCo on Twitter). Contact the author at ciwright26@hotmail.com for reference materials cited in ermaleis article.


  1. Global Industry News, published March 18, 2016. globalindustrynews.org/2016/03/18/europe-became-largest-market-for-heat-transfer-fluids-in-2015-with-33-6-share-in-terms-of-htfs-consumption/.
  2. Wright, C. I., The use of a flushing and cleaning protocol to remove foreign contaminants – a study from a newly built heat transfer plant with a capacity of 100 metric tonnes. Applied Thermal Engineering 2016 (In Press). Accessed March 20, 2016. sciencedirect.com/science/article/pii/S1359431116300916.


Christopher Wright is a research scientist who graduated from the University of Leeds in the U.K. with a BSc and Ph.D. 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.