Heat transfer fluids explained

Heat transfer systems are crucial for transferring thermal energy to or away from a specific location. Each heat transfer system is designed based on the properties of a new heat transfer fluid, so it’s vital for operators to understand the impact of the fluid on the system’s efficiency. For example, how does the fluid’s degradation affect the system performance? And when should the heat transfer fluid be topped up or replaced?

This Q&A will explain everything operators need to know about heat transfer fluids and their maintenance.

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A heat transfer fluid carries thermal energy away from a heat source to another location. In most processes, the heat source is either a furnace operating on liquid fuel, natural gas or wood products, or submerged electrical elements heating the circulating fluid.

Not all uses, however, are this traditional. For example, in solar power plants the fluid is heated by sunlight and carries the heat to boil water, which in turn powers steam turbines. In waste heat recovery applications, the fluid is heated by hot tubes in which recovered hot exhaust gas is flowing.

There are also cases where the chemical reaction or manufactured part itself is the heat source, while the fluid circulation carries the heat away to cool or maintain the reaction temperature.

For any heat transfer system, the components, line sizes, pump sizing and heat exchange calculations are all based on new fluid properties to ensure it operates at maximum efficiency.

But as the fluid degrades, its viscosity can increase or even decrease under severe thermal cracking and can begin to carry solids or gases from degradation or contamination. For equipment operators it’s important to be aware of the three modes of degradation:

  • Contamination – either external, such as rain or dust entering the system when a bolted cover is missing, or internal, such as a pinhole leak allowing process material to enter the hot oil stream, or component wear.
  • Oxidation – it’s the hot fluid reacting with oxygen in the air. It starts with darkening and thickening of the fluid, but if left unattended it will lead to the formation of sludge and build-up of acidic residues in low flow and low disturbance areas, such as the bottom of reservoirs or expansion tanks.
  • Thermal cracking – typically occurs when fluid molecules receive more thermal energy than they can absorb and carry away. It can lead to reduction in the flash point (and boiling point) of the fluid and fouling of heat receiving surfaces. The carbon layer depositing acts as an insulator, forcing operators to increase heat flux to maintain the same operating temperature.

Once degradation occurs, the original heat exchange calculations become less accurate, and the system will need to be used more harshly to compensate and produce similar results. This will directly impact business finances, as a need for increased temperatures due to oxidized fluids affects fuel and electrical costs, while insoluble materials collecting in low disturbance areas increases the need for costly maintenance.

There are two key elements that need to be considered when looking at how often a heat transfer fluid should be replaced. The first is the type of application, for example in plastic extrusion, engineers can expect oil to last between 4,000 – 6,000 hours, while in asphalt storage there are instance where operators can use the same oil change for up to 25 years, if its properly maintained.

Secondly, the care that is taken over maintenance and testing will inevitably play a role in the longevity of a heat transfer fluid. Those companies which do regular fluid analysis and take advantage of their fluid supplier’s expertise will understand the rate of degradation and can act accordingly to reduce or eliminate the need for a total system shutdown just to replace fluids. For example, replacing just 10 percent of the fluid on a regular basis can extend the lifespan of the system fluid without the need for a complete fluid replacement.

On the other hand, those who forgo regular testing typically act reactively when production is affected, at which point maintenance becomes much more time-consuming and expensive. For example, if a system traditionally works at 550°F, operators may see their thermostat creeping up to 580°F over the years, as the system is forced to work harder due to the fouling of heat exchange surfaces or increasing fluid viscosity.

In the worst-case scenarios of degradation, the fluid will not circulate well enough to maintain production, or cracking will cause the boiling point of the fluid to fall dangerously close to the operating temperature, which causes pump cavitation and creates a significant safety concern. After the point of no return is reached, a total unplanned shut down is necessary to drain, clean, flush and recharge the system with fresh oil. This has a direct negative impact on the businesses’ bottom line with up to four days of lost production added to expensive repair costs.

Used oil analysis programs are vital to maintaining the efficiency and effectiveness of heat transfer fluids. For operators considering incorporating used heat transfer fluid analysis into their maintenance schedule, or expanding their normal industrial asset oil analysis program to the heat transfer fluid, it’s important to look into the testing offered, as unfortunately, some of the tests performed on conventional industrial lubricants are either irrelevant or lacking for heat transfer fluids.

For example, ISO particle counting is useful for gear and hydraulic oils but totally useless for heat transfer fluids due to the darkening that usually occurs during use. On the other hand, gas chromatography, flash point testing and measuring solids content aren’t normally part of the regular testing regimen but provide a useful insight into the fluid’s condition.

It’s also important to note that results from common testing methods can change depending on the mode of fluid degradation. For example, oxidation thickens the oil while thermal cracking thins the oil, so it’s plausible for oil to appear to have a normal viscosity while the system suffers from both issues simultaneously. To avoid these issues being overlooked, a gas chromatography analysis can provide a clear and accurate picture of the heat transfer fluid.

When selecting a heat transfer fluid, one of the widespread misconceptions is that the more expensive it is, the higher quality of fluid you’ll receive. Although this is true to an extent, it doesn’t tell the whole story. Some heat transfer fluids are expensive by design, yet they oxidize rapidly due to the lack of anti-oxidant protection. Technical data sheets can be useful and provide information about the fresh fluid properties, however, they offer little information on its resistance to degradation, which is the aspect that delivers an extended life.

To avoid confusion or potentially selecting the wrong fluid, operators should partner with a lubricant expert that has a unique insight into the formulation of the fluid and shares its technical support.

Beyond this, it’s important to look for customer testimonials which offer honest insight into experiences using the fluids. Operations, maintenance and safety departments should also be involved in the discussions alongside procurement to find a supplier eager to provide a product that fits the specific needs of the business with minimal impact to the environment and workers’ health. Industry best practice means that this should also include a dedicated thermal fluid testing program, as well as local, readily available technical support to help increase or maintain productivity to reduce the likelihood of costly shutdowns and unplanned downtime.



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