It is generally accepted that a lower carbon level in a heat transfer fluid (HTF) is better as carbon build-up on the internal surfaces of pipework, heaters and pumps and can lead to reduced plant efficiency and increased operating costs. However, it must be realized that a HTF will thermally degrade as it operates at high temperatures for prolonged periods of time. Indeed, eutectic mixtures of biphenyl and diphenyl oxide (examples including Dowtherm A and Therminol VP-1) operate at temperatures up to 400º C and whilst being more resistant to thermal cracking and oxidation, they will degrade over time. In April's edition of World Pumps there was an article entitled ‘The stability of thermal fluids’, which outlined some of the properties of synthetic-based biphenyl diphenyl oxide HTFs in order to explain why they are commonly used as a heat transfer media and why they have such good thermal stability compared with other fluids, specifically mineral-based HTF. 1

 Factory equipment. (Courtesy of hramovnick/Shutterstock)

Carbon accumulates in a HTF as it thermally degrades or oxidizes, or both, at high operating temperature. A recent webinar hosted by Process Heating2 touched on the impact of carbon on HTF plant pumps highlighting the impact of increased viscosity and the development and build-up of tar / sludge (i.e., carbon formations) as well as the impact of oxidation and reduced pH (with carbon deposits formed on contact with air).

 

 

 
In the case of viscosity it was highlighted that a HTF at ambient temperature can become extremely viscous or solid, indeed the freezing point for biphenyl and diphenyl oxide HTFs is around 15ºC. In the case of thermal degradation there is also the possibility that carbon particulates will settle out of the HTF and accumulate in low pressure or no flow areas of the HTF system. This can cause havoc with the system but in terms of the pump this can lead to pump cavitation and may cause seal wear.2
 
In terms of oxidation, the formation of carbon deposits will have the same effect on pumps, but the multiplier in terms of degradation is the reduced pH / acidic environment in the HTF system. This was also discussed in the Process Heating webinar where it was stated that more acidic HTF “…eats away at the weakest components of the system”, which again includes pump seals as well as gaskets, weldments and metalwork found in the expansion tank.
 
The impact of carbon build-up
 
The build-up of carbon on internal surfaces must not be ignored. One way in which this can be done is by taking regular samples of the HTF and analyzing its properties including carbon residue and acidity.
In mineral-based HTFs, carbon formation has been shown to be linearly related to acidity. So as carbon increases, so does acidity. Although this is not a one-to-one relationship as carbon generally forms at twice the rate it acidifies.3 In the event that carbon starts to rise there is a chance that the acidity of the fluid will also start to increase and lead to pump seal wear.

 

The association of carbon residue with other test parameters has been tested and included kinematic viscosity (Figure 1, top panel), wear debris (Figure 1, middle panel) and soluble elements in the HTF (Figure 1, bottom panel). This new analysis shows no relationship between carbon residue and wear debris (i.e., the linear r-value was near to zero and the statistical ‘P-value’ was greater than an acceptable level of 0.05).
 
The same comparison with soluble elements, such as is found when a system is contaminated by environmental elements, showed a statistical relationship but the linear relationship was not meaningful as it is also close to zero (r-value was 0.09). Interestingly, however, there was a moderate relationship (r-value was between 0.3 and 0.7) between carbon residue and kinematic viscosity which was significant (the P-value was >0.0001; see Figure 1, bottom panel).
 
Therefore revealing that as carbon builds-up in a HTF it may also be associated with the fluid becoming thicker with progressive thermal degradation of the HTF. The consequence being that to be as efficient as a virgin HTF, a pump will have to work slightly harder to maintain the same level of flow and as wear stress increases. Another unforeseen consequence on the pump is that as carbon bakes on to the internal surfaces of a system, i.e. the heater, the energy required to achieve the optimal bulk temperature with a higher viscosity fluid further increases the stress on the pump.
 
Figure 1. The relationship between carbon residue and kinematic viscosity (top panel), wear debris (middle panel) and soluble elements (bottom panel) of a mineral-based HTF.
 
 
 
 

 

Modern thermal power plant. (Courtesy of momente/Shutterstock)

 
Conclusions
 
The current article discusses the potential impact of raised carbon levels from the perspective of the HTF system pump. The article explains the association between carbon formations, as occurs during thermal degradation, and the potential for increased acidity and also kinematic viscosity. As a HTF becomes more acidic it can lead to internal destruction of pump seals. Increases in kinematic viscosity can lead to the requirement for an increased pump speed and thus higher energy consumption. Lastly, as carbon forms and potentially bakes on to internal surfaces of the heater it would act as an insulator which would require further energy to achieve the optimal bulk temperature and further stress the pump which would need to pump a hotter fluid with an increased kinematic viscosity.
 
 
Acknowledgements
 
The author would like to acknowledge the writing support provided by Red Pharm communications, which is part of the Red Pharm company.
 
Please contact the author for reference materials cited in this article.