Heat transfer fluid—Q&A

Compiled by our team of technical experts, this Q&A section contains answers to frequently asked questions and commonly used definitions in the heat transfer industry.


Generally, a heat transfer fluid is selected based on its ability to provide long-term, reliable system operation and performance. Desired operating conditions of the heat transfer fluid system, particularly the maximum operating temperature, will typically narrow the field of choices. If the system is used for heating and cooling or is operated outdoors in extremely cold climates then you will need to consider the low-temperature properties of the heat transfer fluid as well. Other criteria that may factor into your decision are environmental friendliness of the fluid, specific system design criteria, and cost. Your Therminol specialist is always available for consultation in fluid selection. Selecting a supplier is at least as important as the decision of which fluid to buy. When you select Therminol, you are choosing the world leader in heat transfer fluids. Therminol fluids are produced on four continents and stocked in strategic locations around the world. Therminol representatives with unmatched capabilities can be found in nearly every country in the world. Eastman’s Therminol team will work to make you successful from the earliest conceptual design phases of the project right through detailed design, start-up, and routine operation. We are with you all the way, helping you achieve your goal of safe, reliable, and cost-effective operation.
Standardized methods for measuring thermal stability of organic heat transfer fluids exist in Europe and the Americas. DIN 51528/51435 and ASTM D6743/7213 provide the test methods needed for thermally stressing the fluids and analysis of degradation products formed by simulated distillation (chromatography). The output from both of these methods is a quantification of low-boiling (low boilers) and high-boiling (high boilers) thermal degradation products, which have boiling points below and above the initial and final boiling points of the unstressed heat transfer fluid.
Therminol heat transfer fluids are synthetics and are nonfouling by nature. In systems which are properly designed and operated, they generally do not form coke or insoluble tars and sludges. They are much more resistant to solids formation caused by oxidation than other types of heat transfer fluids, such as mineral oils.
After use, most Therminol heat transfer fluids will meet the criteria for nonpetroleum used oil under the U.S. EPA Standards for Management of Used Oil (40 CFR 279), which govern recycling and disposal of used oil. Contact your Therminol representative for details on assistance in the proper disposal of used heat transfer fluid under these regulations with Eastman’s used heat transfer fluid trade-in program. Should any organic heat transfer fluid be spilled, efforts should be taken to prevent it from entering sewers or drainage pathways into the environment. More information can be found on the specific safety data sheets.

Heat transfer fluid life is time and temperature dependent. Most Therminol heat transfer fluids will last for many years. Actual fluid life is quite dependent on the total system design and operation and can vary from system to system. Significant overheating and fluid contamination will also affect the performance of a heat transfer fluid system. An in-depth analysis of your fluid in the Therminol laboratory gives excellent guidance on the condition of the fluid and likelihood of continued reliable operation. The most common reasons for suggesting fluid replacement are:

  • Elevated viscosity resulting in impaired low-temperature fluid performance
  • Elevated quantities of high-molecular-weight degradation products approaching (or, in the worst cases, exceeding) their limit of solubility in the subject fluid
  • Contamination of the heat transfer fluid with foreign compounds

Eastman recommends regular sampling and analysis of heat transfer fluid systems. By taking advantage of our complementary sampling program, you can monitor your system and plan ahead for fluid replacement before you experience a loss in system performance.

Color is the least reliable indicator of fluid condition. Although Therminol heat transfer fluids are clear or slightly tinted when new, it is normal for them to darken in color when heated and exposed to minor contaminants such as iron oxide or mill scale in a heat transfer system. This change in color has no effect on their operational performance or overall fluid life.
Of course! We offer several services that will help when it comes time to change your fluid. Eastman prides itself on being an environmental leader, and in North America, we offer a trade-in program for used heat transfer fluids. Depending on the fluid and its condition, we can take back your old fluid and in return offer you a credit towards the purchase of your new heat transfer fluid for your system. The trade-in program eliminates your disposal costs of spent heat transfer fluid. Deposits from oxidized, thermally degraded, or contaminated fluid can foul interior surfaces, making it necessary to clean the system prior to charging with new fluid. Based on the condition of your system, this may be recommended. If so, you may want to consider using Therminol FF. Therminol FF is the first and best flushing fluid for liquid-phase heat transfer systems. In some areas, Eastman will even take back your used Therminol FF at no additional cost. Our cradle-to-grave approach eliminates expensive disposal costs.
A synthetic heat transfer fluid is one that is manufactured as a first-intent heat transfer fluid which meets precise purity and performance specifications. Mineral oils, on the other hand, are by-products of lube oil production.
All heat transfer fluids thermally degrade to what are called low and high boilers. These are chemical compounds of different molecular weights that boil at lower or higher temperatures than the original fluid. Therminol fluids have been lab and field tested at elevated temperatures to measure the rate of formation of these compounds. This is usually expressed as percent degradation vs. time. The maximum operating temperature is recommended for each fluid.

Glossary of terms

The bulk fluid temperature is the average temperature of the fluid at a specified point in the heat transfer system. It is usually measured at the exit from a heater. A close relationship exists between the highest bulk temperature and the degradation rate of a fluid. From an economic standpoint, a critical degradation rate is usually considered to be 5% per year but also depends on the proportions of low- and high-boiling components formed. This degradation rate should only be reached at the highest temperature recommended as the bulk temperature. In general, Eastman Therminol® heat transfer fluids can give long service life if the maximum bulk and film temperatures of the system do not exceed the recommended maximum limits for the fluid and if no contamination or exposure to oxygen occurs. The recommended maximum bulk temperatures of Therminol heat transfer fluids have been determined by degradation rate measurements made at high temperatures for each fluid. These measurements are made by a controlled thermal aging on a standard volume of fluid at fixed temperatures. Dynamic and static aging tests have been performed.

The film temperature is the maximum temperature of the thin layer of fluid in contact with the metal wall in tubes or pipes. The fluid in this layer is not in turbulent flow and, in a heater, often has a temperature 20°–30°C (30°–50°F) higher than the bulk fluid temperature. Although very little fluid is present in the film, if the film temperature exceeds the maximum recommended, the contribution to the degradation of that fluid volume can be high and can be estimated for individual cases.

Film temperature can be calculated by the ratio of the maximum total heat flux density of a system to the heat transfer coefficient.

The autoignition temperature is the minimum temperature for a substance to initiate self-combustion in air in the absence of a spark or flame. It permits grouping combustible liquids with respect to their behavior in contact with hot surfaces. This provides a basis for determining protective measures for explosion-proof electrical or nonelectrical apparatus.

Turbulent flow inside commercial tubes and pipes is assumed. Heat transfer is computed from the HTRI correlation:

Nu = 0.025 * (Re^0.79) * (Pr^0.42) * phi where

Nu = Nusselt number = h * D/k

h = Heat transfer coefficient, W/(m²•K)

D = Inside diameter, m

k = Thermal conductivity, W/(m•K)

Re = Reynolds number = ρ * V * D/µ

ρ = Fluid density, kg/m³

V = Bulk fluid velocity, m/s

µ = Fluid viscosity, Pa•s

muw = Fluid viscosity at the wall, Pa•s

Pr = Prandtl number = cp * µ/k

cp = Fluid heat capacity, kJ/(kg•K)

and the factor phi = (µ/muw)^0.11 is given the fixed value of 1.023, which corresponds to a film temperature difference of about 30°C (50°F) for liquids at common use temperatures.

Pressure drop is computed from:

Delta P = f * (L/D) * (ρ * V²)/2

1/√f = -0.86 * ln (e/(3.7 * D) + 2.51/(Re * √f))


Delta P = Pressure drop, Pa/m

f = Friction factor, from Colebrook

L = Pipe length, m

√f= Square root of f

e = Wall roughness, m

ln = Natural logarithm

In the transition region, for 2,000 < Re < 10,000, an average Nusselt number is computed from HTRI correlations:

Nu = θ * Nu2 + (1 – θ) * Nu10

where Nu2 is the laminar-based Nu computed at Re = 2,000, and Nu10 is the turbulent-based Nu computed at Re = 10,000:

Nu2 = 2 + 20 * (1/3)^4 + 1.45 * ( (3.14/4) * 2,000 * Pr /(L/D) )^(1/3)

Nu10 = 0.025 * (10,000^0.79) * (Pr^0.42) * 1.023

θ = 1.25 – Re/8,000

Negligible natural convection, negligible entrance effect, and negligible viscosity gradient correction are assumed, and L/D = 100 is taken as typical.

High acidity generally indicates contamination from material added to the system inadvertently or leaked from the process side. High acidity may also indicate severe fluid oxidation if the system is not protected with inert gas in the expansion tank vapor space.

If the acid condition becomes excessive, the system expansion tank is at increased risk of corrosion and failure. Corrosive products form sludge and deposits that decrease the heat transfer rate. Contamination or oxidation of this nature may require removing the fluid for disposal, system flushing to remove acidic or contaminant residues, and refilling with new heat transfer fluid while ensuring the correction of the identified root cause of acidity.

Viscosity changes generally indicate contamination, thermal stress, or oxidation degradation. Viscosity is related to the molecular weights of the fluid components. Generally, lower-molecular-weight components decrease viscosity and higher-molecular-weight components increase viscosity.

Contamination from leaked process streams, incorrect fluid added to the system, solvents from system cleanout, thermal stress, or oxidation may be the source of materials that increase or decrease viscosity. Operational problems may result from either high or low viscosity. If the viscosity is high, the circulating system may have difficulty starting up, resulting in heater burnout. Heat transfer rates may be reduced. High-viscosity fluid generally requires draining and replacement with fresh fluid. Extended use of high-viscosity fluid may contribute to fouling, thereby requiring a system flush before refilling. Sometimes, however, the problem may be corrected by significant dilution with fresh fluid.

If viscosity is low, low-boiling components will be more volatile and can result in pump cavitation and reduced flow. To remove low-boiling components, the heated fluid should be circulated through the expansion tank with an inert gas purge of the vapor space. The cause of viscosity changes should be determined no matter what action is taken.

Moisture generally indicates that either there is a system leak on the process side or wet fluid has been added to the system. New systems or systems cleaned using aqueous solutions can contain residual water. Water can also infiltrate through open vents, expansion tanks, or storage tanks. Moisture can cause corrosion, high system pressures, pump cavitation, and vapor lock. If hot fluid contacts a water pocket, steam may develop which can cause fluid from the system to erupt and its components to fail.

Corrective action includes careful and gradual start-up of a potentially wet system with circulation through the expansion tank where the vapor space is slowly purged with inert gas to sweep moisture from the system. If a large amount of water contamination is present, it may be necessary to remove the fluid for external drying. Leaks from the process side should be corrected, and new heat transfer fluid should be stored to minimize water entry. When stored outside, new sealed drums should be turned on their sides and adequately covered to prevent moisture contamination from rain.

The flash point is the lowest temperature at which a fluid gives off sufficient vapor to burn when ignited; however, the rate of evolution of vapor at the flash point is insufficient to maintain a flame.

The flash point is determined by two techniques:

1. In an open crucible (Cleveland open-cup (COC) method), ASTM D92 or DIN ISO 2592

2. In a closed crucible (Pensky-Martens closed-cup method), ASTM D93 or DIN 22719

The values found by the Pensky-Martens method are about 20°–30°C (30°–50°F) lower* than by the COC method because the gases are kept together and are not diluted by air addition.

*For fresh product; in-service tested fluid difference may be larger.

Low- and high-boiling components are measured by a gas chromatography technique. This method, based on ASTM D7213 (DIN 51435), determines the boiling range distribution or distillation curve of organic heat transfer fluids.

This technique helps to assess the thermal stability of heat transfer fluids and is offered as a routine test as part of the testing service.

Heat transfer system equipment—descriptions

Here you will find a list of equipment descriptions providing recommendations for equipment and components suitable for use with Therminol products within their respective acceptable design limitations.

Usually, the expansion tank is installed at the highest point of the system and is connected to the suction side of the pump. It may also be connected to the main circulating loop at the lowest pressure point. It should serve as the main venting point of the system as well as provide for fluid expansion, which can be 25%-30% of the total system volume. Actual fluid expansion volume depends on the physical properties of the fluid selected and the operating temperature range.

All expansion tank vent lines must be routed, preferably via a cooled condenser, to a safe external location so that vapor may not enter working areas. The normal design choice will be a double-leg expansion tank which provides higher flexibility in normal operation than a single-leg expansion tank with degassing tank and temperature buffer tank. With careful attention to design, particularly to venting systems for noncondensable and water, both single-leg and double-leg designs may be used and can provide satisfactory service.

Low boilers and moisture should be collected in either a vent condensate or cold-seal trap and should be periodically discarded as part of routine operating procedures.

An effective way to minimize fluid oxidation is to blanket the system with an inert gas such as nitrogen. In small systems, the nitrogen may be replaced by a cold-seal trap or an expansion leg filled with system fluid maintained at a low temperature.

Before a new system is started, a wire mesh strainer should be installed in the pump section. These strainer baskets may be removed after debris removal from the start-up.

When operating where solids or contaminants might enter or be generated in the heat transfer system, it is advisable to install a high-temperature filter bypass line that can be positively isolated with valves for periodic cleaning or replacement.

Filter elements are commonly glass fiber string-wound cartridges or sintered metal filters in the 5–20 micron range. These filters require a significant pressure drop between the inlet and outlet of the bypass.

For high-temperature heat transfer fluid systems, spiral-wound or graphite types of flange gasketing conforming to API 601 and DIN 4754 specifications are recommended.

Standard materials for spiral-wound flange gaskets are type 304 stainless steel and pure graphite. To avoid leaking with spiral-wound gaskets, it is important to use raised-face flanges, allowing steel bolting and even compression of the gasket during bolt tightening. Graphite gaskets are an acceptable alternative for many applications.

Generally, sheet gasketing with various binders is unacceptable for Therminol 66 and some other fluids because of incompatibility of the binders with these fluids.

The heater may be electrical, fuel-oil, or gas-fired and is the most critical component in designing a heat transfer system for use with Therminol fluids. With the proper balance of heating capacity, temperatures, and fluid velocity, the service life of the heat transfer fluid is increased to an optimum level. Another important factor for good life is that systems must be protected from contamination by foreign materials.

Two basic designs of fired heaters for use with Therminol fluids are liquid tubes and fired tubes. In liquid-tube heaters, the heat transfer fluid is pumped through the tubes as it is heated. The hot gases pass outside the tubes. In fired-tube heaters, the fluid flows through the heater shell with hot gases passing through the tubes.

When bulk fluid temperatures higher than about 240°C (460°F) are required, a liquid-tube heater must be used unless a specific heater design is devised to force a uniformly steady turbulent flow of liquid over the fired-tube surfaces.

Most Therminol fluids are liquid when transferring heat. To avoid hot spots in the heater, the fluid should be pumped over or through the heating surfaces at sufficient velocity so that no area of fluid stagnation occurs. Since heating is not perfectly uniform in fired-tube heaters, the maximum heat stress conditions must be calculated to determine what film temperatures will be encountered.

Fluid velocities over heat transfer surfaces must be relatively high to develop turbulent flow. This helps to avoid excessive film temperatures that may be detrimental to heat transfer surfaces and the fluid. The heater manufacturer should be consulted for the required flow velocities.

Organic heat transfer fluids, such as Therminol fluids, have a slow oxidation reaction with air in the presence of insulation materials when the fluid temperature is above 260°C (500°F). Porous insulation, such as calcium silicate, offers a larger reaction surface with poor heat dissipation which, along with possible catalysis from the insulation material, can cause a temperature buildup. This temperature rise may result in ignition of the fluid when the saturated insulation is exposed to air, such as for repairs.

This phenomenon is not fully understood but appears not to occur with cellular glass, possibly because of its closed cell structure. Cellular glass should be used in all areas where leakage is a possibility. The principal leakage areas are usually near instrument connections, valve packing glands, flanges, and other sealed surfaces. As a precaution, eliminate any source of leakage promptly. Replace leaky gaskets and oil-soaked insulation, and repack valve stems. Cover insulation where leaks might occur with metal covers. Where possible, install valves with the stems in a horizontal position so that leaks will drip away from the insulation.

The piping layout for systems using Therminol heat transfer fluids should be sized to provide the normal required flow rate at an economical pressure drop.

Because the system will undergo temperature changes, adequate flexibility to relieve thermal expansion and contraction stresses is essential. Schedule 40 carbon steel pipe or equivalent should be used throughout the system. The tendency to leak through joints and fittings is a characteristic of most organic fluids unless the fittings are very tight.

The best way to prevent piping leakage is to weld all connections. Where access is necessary, raised-face flanges with weld neck joints are recommended.

To help insure good seating and sealing of the spiral-wound gaskets recommended for Therminol fluid piping, the following procedure should be followed:

  • Clean flange faces free of loose rust and dirt. Remove any weld spatter. Assure that the flange faces have no gouges or grooves and are aligned properly, since gaskets cannot correct for these problems.
  • Check alloy stud bolts and nuts to assure they are clean and free of rust and thread shavings, and lubricate the threads. The bolting stress and torque are defined by the supplier of the gasket. The torque is also a function of the diameter and the thickness of the gasket.
  • Torquing is performed by tightening opposite studs to the required torque values using small increments. Tighten studs in the sequence 9, 3, 6, and 12 o'clock and repeat with adjoining studs.

Pumps must have enough capacity and pressure head to circulate the fluid at the required rate through the system. Pumps are generally centrifugal, seal-canned, glandless, or magnetically driven. They must conform to appropriate standards. The pump housing may be cast steel for most systems but may be made of other appropriate materials for very low or high temperatures.

For temperatures higher than 200°C (390°F), pump manufacturers usually specify either water-cooled ring seals or, preferably, fluid-cooled stuffing or air-cooled, extended-shaft seal and bearing.

On pumps with a stuffing box, at least five rings of laminar graphite packing should be provided. Inert blanketing of the seal with steam or nitrogen eliminates deposit formation from oxidation, which can lead to seal leakage. A secondary seal provides additional safety in case of sudden seal failure.

Regardless of the type of pump selected, the flow rate should be checked regularly against the pump characteristic performance curve originally supplied. To prevent alignment problems and seal leakage, it is important to avoid pipe support stresses on the body of the pump. Each pump should be fitted with a control device to switch off the heat source in case of pump failure. If expansion loops are used in the pump section piping, they should be horizontal or vertically downward. Loops should not be vertically upward because this forms a trap which can collect air and vapor that seriously hampers pump performance.

Forged-steel valves with deep stuffing boxes are satisfactory for systems utilizing Therminol fluids. Gate and globe valves with an outside screw should be used throughout the heat transfer system. Gate valves do not always provide an absolutely tight shutoff.

Various types of packing are used to seal valve stems on high-temperature systems, and generally five rings are specified on valve stems to assure a good seal. Valve stem bellows will provide virtually leak-free operation.