Heat transfer fluid (HTF) presents a system pump with numerous mechanical, thermal and chemical stresses — all with the potential to cause pump wear and ultimately failures. HTF is a chemical substance that operates at high temperatures for long periods of time and is continuously in contact and interacting with the pump. Incorporating a filter removes the large particles, but small particles/chemical substances can pass through filter pores. This article discusses the risks they pose to the pump and suggests that monitoring the HTF condition provides a holistic approach to maintenance of a fluid and also the pump.
The global heat transfer fluids market was valued at an estimated $2,811 million in 2015, according to the Your Petrochemical News website. It is expected to grow at a compound annual growth rate of 6.8 percent from 2016 to 2021.1 One of the reasons for this growth is the consumption of HTFs in Europe in areas such as concentrated solar power plants. These plants carry out complex processes and in combination with an HTF need to be correctly designed, operated and maintained.2 Pumps are critical in any HTF system. They need to be able to work with a wide range of temperatures and rates of flow, and at the same time, they need to remain reliable and leak-tight.
A quick search on Google for "pump wear and heat transfer fluids" yields around 506,000 search results. Focusing on just the first search results page, companies focus on the HTF, the maintenance of the HTF (including sampling, analysis and filtration), pump life and pump efficiency. The article "Hot oil filtration improves pump life and efficiency in heat transfer system" opens by stating: "Implementation of a hot oil filtration system from Liquid Process Systems Inc. of Charlotte, North Carolina, has resulted in improving the efficiency of heat transfer systems. Bottom line benefits for a Minneapolis die casting company include reduced downtime and costs while tripling the life of its heat pumps."
This suggests that filtering an HTF is the principal way to maintain the condition of an HTF system. This perspective concerns the presence of solid materials that can lead to accelerated pump wear. However, this perspective is quite limited in its outlook because it only deals with the removal of larger particles/materials from the circulation and dismisses the importance of monitoring the condition or health of the fluid.
Indeed, filtering works to remove particles — including insoluble elements such as iron and certain byproducts like carbon, which forms as the fluid thermally degrades when operating at high temperatures for prolonged periods — but particles below a certain size will pass through the filter. These include water and light-ends (also known as light-chain hydrocarbons), which form as the fluid degrades and need to be removed from the system. Water and light-chain hydrocarbons have the potential to damage an HTF system pump. A filter can also block with prolonged use and render it ineffective.
Water presents a significant risk during the building of new HTF systems and when starting systems from cold, since water can ingress and contaminate a system.3 In some cases, large amounts of water can lead to problems with vapor locking and subsequent over-pressurization of the HTF system. In the worst-case scenario, the pumps can vapor lock and quickly lead to overheating of the furnace coil.2 This is also true for light-ends. Like water vapor, if the system vents cannot cope with the vapor venting rate needed to remove light-end vapors, they will accumulate and can lead to vapor locking.
This can also occur following flushing and cleaning of the system with a solvent. According to Ennis,2 "after solvent flushing the system must be thoroughly drained and flushed with water to remove any remaining traces of the solvent before recommissioning." Here the water and solvent have the potential to form vapors and over-pressurize the system and pump.
Oxidation is another process by which an HTF degrades. In the presence of oxygen, an HTF will become more acidic, which will lead to corrosion of the pump and internal pipework.
The choice of HTF is important because it will have a bearing on the rate that an HTF thermally degrades and oxidizes. A well-designed HTF will be thermally stable and have good heat transfer efficiency and a high purity (see Figure 1).4 The purity is important because the presence of impurities can catalyze the thermal degradation of an HTF.
As a general rule, synthetic-based HTFs can operate at higher temperatures than mineral-based HTFs. They also offer better resistance to thermal degradation and oxidation with less carbon formation.5
The condition of an HTF should be maintained2 by regularly sampling and chemically analyzing the fluid.3 Routine tests include measuring carbon formed, the presence of particles (wear and contamination from elements such as silicon), water contamination, light-ends and acidity.
Pump wear can be avoided by incorporating a filter into an HTF system, but this does not remove all harmful materials from the HTF. Water, light-ends and acids can all pass through a filter. Water and light-ends can lead to vapor locks that can over-pressurize a system, which in turn stresses the pump and can lead to overheating of the heater coil. The presence of acids works to corrode the internal components of an HTF system.
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). This work was initiated while advising the Global Group of Companies, Cold Meece Estate, Cold Meece, Staffordshire, United Kingdom.
- P&S Market Research. "Global heat transfer fluids market expected to grow at 6.8% CAGR during 2016-2021." Accessed March 25, 2016, from http://www.yourpetrochemicalnews.com.
- Ennis, T. (2009). "Safety in design of thermal fluid heat transfer systems." Hazards XXI. Symposium series number 155 (2009), 162-169. Accessed March 25, 2016, from https://www.icheme.org/~/media/Documents/Subject%20Groups/Safety_Loss_Prevention/Hazards%20Archive/XXI/XXI-Paper-025.pdf.
- Wright, C. (2016). "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 (In Press). Accessed from http://www.sciencedirect.com/science/article/pii/S1359431116300916.
- Wright, C. (2016). "What to consider when making the buying decision about a heat transfer fluid for your system — A report of the webinar hosted by Process Heating." Journal of Applied Mechanical Engineering, 5: 191. Accessed from http://www.omicsgroup.org/journals/what-to-consider-when-making-the-buying-decision-about-a-heattransfer-fluid-for-your-system–a-report-of-the-webinar-hosted-byproc-2168-9873-1000191.pdf.
- Wright, C. "Comparing the thermal stability and oxidative state of mineral and biphenyl diphenyl oxide based heat transfer fluids." Journal of Applied Mechanical Engineering 2015, 4: 187. Accessed from http://www.omicsgroup.org/journals/comparing-the-thermal-stability-and-oxidative-state-of-mineral-andbiphenyl-diphenyl-oxide-based-heat-transfer-fluids-2168-9873-1000187.pdf.
Chris Wright is a research scientist for Global Group of Companies. He graduated from the University of Leeds in the U.K. with Bachelor of Science and doctorate degrees. His research focuses on the use and maintenance of heat transfer fluids in manufacturing and processing, which includes food, pharmaceutical, specialist chemical and solar sectors. Please contact the author for reference materials cited in this article. He may be reached at [email protected].