Thermic Fluid Selection

The primary function of a Thermic Fluid(Heat Transfer Fluid) system is to transfer heat from a heat source to process facilities. Whether you are an existing user looking for an upgrade, facing technical issues, or trying to select a suitable brand and grade of thermic fluid, you will be faced with the complex task of collecting information and data from many sources. Based on the information you will then be required to make a sound and effective long-term decision having technical and financial implications.

With the availability of many makes and grades of thermic fluid you need to list out your required operating parameters correctly to select the most suitable grade of thermic fluid. After you have shortlisted the most suitable grades, the following are the thermic fluid characteristics which are to be critically looked into for the selection


  1. OXIDATION STABILITY: The property of the fluid to resist oxidation
  2. THERMAL STABILITY: The property of the fluid to resist thermal cracking
  3. HEAT TRANSFER COEFFICIENT: Better heat transfer coefficient gives an overall better heat transfer and efficiency
  4. VISCOSITY: Lower viscosity leads to low energy requirements for circulation in the system

Except for the Heat transfer co-efficient and viscosity, other parameters such as oxidation stability and thermal stability cannot be quantified in the material data sheets since these are dependent on application.


For proper selection of fluid or for proper maintenance of existing fluid, we need to understand the different types of degradation that takes place in a thermic fluid system. The two most critical properties of the heat transfer medium which are the prime requirements for a reliable system – Oxidation Stability and Thermal Stability have not been quite appreciated or quantified.

Oxidative Degradation (Common in small systems)

Oxidative degradation is the reaction of oxygen (in air) with the fluid to form polymers or solids. These thicken the fluid and increase its viscosity. A more viscous fluid will be need more energy to pump and circulate, have poorer heat transfer characteristics as well as an increased affinity for coke formation. Oxidation is also accompanied by an increase in the acidity (TAN) of the fluid.

With exposure to ambient air, high temperature systems are more prone to faster rates of oxidation than low temperature systems. At room temperature, the reaction rate is hardly measurable.  In industries such as plastics extrusion and die-casting which use high temperature systems, it is observed that oxidation is the main cause of fluid degradation and sludge formation in reservoirs and piping.

 Thermal Degradation (Heating above 240°C in a manual coal / wood / husk fired heaters and above 280°C in Diesel fired heaters)

Thermal degradation or thermal cracking is the break-down of carbon-carbon bonds in the fluid molecules by heat in excess of the recommended maximum bulk temperature of the fluid. The reaction may either stop at that point, in which case formation of molecules occur which are smaller than which existed previously, or, the smaller molecules may react with each other to form polymeric molecules larger than those which existed previously in the fluid. In heat transfer parlance, these two types of products of degradation are known as low boilers and high boilers.

Low Boilers The effect of presence of low boilers in the thermic fluid is to decrease its flash point and viscosity of the fluid as well as to increase its vapor pressure. The increased vapor pressure can affect overall system efficiency and can cause cavitation in the pumps. The reduction in the flash point could also be cause for safety concerns.

High Boilers If thermal degradation occurs at higher operational temperatures , the effect is not only to break carbon – carbon bonds but also to separate hydrogen atoms from carbon atoms and formation of coke. The effect of high boilers is to increase the viscosity of the fluid as long as they remain in solution. However, once their solubility limit is exceeded, they begin to form solids which can foul the heat transfer surfaces. In this case, fouling of the heat transfer surfaces is very rapid and the system will soon choke and cease to operate.

 Total Acid Number (n/t-H)

The common measurement of oxidative degradation is called TAN. An increase in TAN indicates higher oxidative degradation of the thermic fluid. These acids will promote sludge and resin formation. TAN values above the range of 1.0 to 1.5 mg KOH/g are usually a cause for concern. It is important to note that with smaller systems and systems with less efficient draining these acids can remain behind and contaminate new and replaced fluids. It is extremely important to ensure maximum evacuation of the spent fluid prior to refilling if the TAN number is greater than 1.0

Flash Point

Flash point value is important from the viewpoint of safety; usually, it is not a concern unless it falls below 120°C(248°F). It is quite common for heat transfer systems to be operated at temperatures above the flash point of the fluid.


We need to understand the causes of the above and the possible solutions :

  1. Oxidation: Oxidation happens when the hot thermic fluid comes in contact with atmospheric oxygen. In a typical closed system the only possible source of oxidation can be within the expansion tank. Care should be taken during system design to ensure that the fluid temperature in the expansion tank does not exceed 60°C
  2. Thermal Cracking: Thermal cracking can happen due to-
  • The chemistry of the product itself
  • Operating at higher bulk temperatures than specified


Due to Oxidation 

  • TAN increases – sludge formation in the Expansion tank and system.
  • System corrosion
  • Fouling of heat transfer areas with carbon tar
  • Increase in viscosity

Due to Thermal Cracking 

  • Low boilers formed to be vented out if it crosses more than 5%
  • Low boilers have the effect if reducing the flash point of the thermic fluid. Once vented out the flash point can recover.
  • Formation of high boilers shall result in increase of viscosity and resulting in lower heat transfer efficiency of the system.
  • Fouling of heat transfer areas leading to lower efficiency, drop in production and complete choking, if unchecked.

Due to Insolubles 

  • Most of the times, it is observed that when thermic fluid is spilled, operators refill the system with the same fluid, without proper filtration. These practices increase the level of insolubles in the system.


1.Viscosity at 40°CLess than 20 cStWarning Level 40 cSt
Critical Level 100 cSt
( The increase in viscosity could be because of increase in high boilers or due to oxidation or due to the increase of insolubles.)
2.Percentage of Low BoilersNilVent out Fluid if more than 5%
( The venting of the fluid is supposed to be done online in a combined expansion cum DA tank. If the levels increase in-spite of that then there could be some problem in the expansion tank. Contact OEM / Consultant)
3.Percentage of High BoilersNilDiscard the Fluid if more than 10%
( A reasonably well designed system operating on agro-waste/ coal and working at 280 °C with a top up of 10 to 15 % per annum should give a life of about 10 years or more. If the fluid life is less kindly review the system design / piping design and layout / heat and mass balance.)
4.Flash PointWarning level
Less than 110 °C
( The primary cause of fall of flash point is due to the formation of low boilers. The venting of low boilers can improve the flash point value. )
Mg KOH/g
0.3 to 0.7Less than 0
More than 0.7
( The increase in acidity is due to oxidation and chemistry of the product)



Myth 1 : High Flash Point-High Safety.

  • The assumption that higher flash point leads to high safety is a pure myth. We need to consider the fact that in many installations, the operating temperatures are always higher than the flash point.
  • If fire safety is a concern; we need to understand that fire can happen only if there is source of oxygen, a source of ignition and a source of leakage. Sometimes fire happens, due to poor system design, poor operational control or poor system maintenance, if all the above three conditions occur simultaneously. Then, it does not make a difference whether the flash point of that particular thermic fluid is 140 °C or 300 °C.
  • A well engineered system / Efficient and reliable pump / nil leakages in glands / flanges / joints and regular maintenance and good safety practices will ensure that a relatively lower flash point of thermic fluid by itself is not a safety hazard.

Myth 2 : MSDS – Hence fluid is impossible to handle

  • MSDS means MATERIAL SAFETY DATA SHEET. It is a mandatory requirement for any chemical / petroleum product. In fact, end users must demand MSDS rather than being misguided by the misconception that since MSDS is provided it is banned chemistry.


  • Better Oxidation Stability can be achieved by adopting a good system design. User should prefer a supplier who knows the product and its applications thoroughly.
  • Better Thermal Stability can be achieved by selecting the fluid by its chemistry. User must cross-reference the history of the particular brand and grade with existing users of the fluid in various industry / application.
  • Long-term Availability is an important factor in ensuring continuity and avoiding service disruptions. This will also ensure that there are no mismatch between different grades and brands during top-ups/refills.

For Service Back-up, users must ensure that selected brand and grade comes with system engineering guidance / start up guidance / fluid maintenance / application knowledge.

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