What is a heat transfer fluid and what is it used for?
A heat transfer fluid is a liquid circulated in a closed loop to carry heat between a heater or chiller and process equipment. Industry uses it to heat reactors, tanks, dryers and dies and to cool HVAC, food and beverage loops (Canter, 2022; Dow, 2008); some synthetic aromatic fluids also run as vapor (FRAGOL, accessed 2026).
ISO 6743-12:1989 classifies heat transfer fluids as family Q of industrial oils and was last reviewed and confirmed in 2026 (ISO 6743-12:1989).
DIN 51522:1998-11, Heat transfer fluids Q, specifications and test, is the current German edition and replaced the December 1995 edition (DIN Media, accessed 2026).
FRAGOL, a thermal oil maker, states that a 10 °C rise in temperature doubles the thermal decomposition rate of a thermal oil and halves its operating life (FRAGOL, accessed 2026).
Below -18 °C (0 °F), viscosity above 1,000 mPa·s can make Dow's propylene glycol fluids impractical unless larger pumps are installed (Dow, 2008).
Secondary loop supermarket refrigeration systems generally need half as much refrigerant as comparable direct expansion systems (Katz and Walker, 2008).
How does a heat transfer fluid move heat in a closed loop?
A heat transfer fluid moves heat in a closed loop by being pumped through a heater or chiller, where it gains or loses heat, and then through the equipment that uses that heat or cold. Edward Cass of Paratherm describes the usual layout: the fluid is circulated through a heating or cooling source and then through a process loop with one or more thermal process units, such as tanks, reactors, kettles, rollers, dryers, ovens and dies (Canter, 2022). Thiago Alonso of Dow adds that heat transfer fluid systems run at lower pressures compared to steam (Canter, 2022). In a liquid-phase loop, the fluid carries heat as sensible heat, so its temperature rises or falls without boiling. Two temperatures limit the loop. The bulk temperature is the average fluid temperature, and the film temperature is the higher value at the hot heater wall. Dow, for example, recommends a maximum bulk temperature of 121 °C (250 °F) for its DOWFROST propylene glycol fluid and a film temperature no higher than 149 °C (300 °F) (Dow, 2008). An expansion tank absorbs the volume change as the fluid heats. In thermal oil systems, FRAGOL recommends nitrogen blanketing of the expansion tank or a cold oil trap to keep air and moisture away from hot oil, because atmospheric oxygen is the most common cause of oxidative decomposition (FRAGOL, accessed 2026).
What are the main types of heat transfer fluids?
The main types of heat transfer fluids are water, glycol and water blends, mineral oils, synthetic aromatic fluids and silicone fluids, with polyglycols and specialty fluids in narrower roles. Cass groups them into aqueous, mineral oil and synthetic categories, and calls water the most common heat transfer fluid because its properties make it efficient for heating and cooling but limit its practical operating range (Canter, 2022). Glycol and water blends extend that range downward. Dow states that its propylene glycol fluids are widely used for secondary cooling and heating, for freeze and burst protection of pipes, and for deicing, defrosting and dehumidifying (Dow, 2008). Mineral oils, distilled from crude oil, are the largest group of thermal oils according to FRAGOL; white oils, a more refined type, can carry food grade classifications such as NSF HT1 (FRAGOL, accessed 2026). Synthetic aromatic fluids contain benzene-type rings, which FRAGOL describes as thermally very stable, and some can run in both the vapor and liquid phase (FRAGOL, accessed 2026). Silicone fluids such as dimethyl polysiloxane cover a wide span from low to high temperature (Dow, 2001). For oil-based fluids, ISO 6743-12:1989 defines the product categories of family Q, heat transfer fluids, for their main applications (ISO 6743-12:1989).
What temperature range does each heat transfer fluid cover?
Each heat transfer fluid class covers a different temperature band: glycol and water blends sit roughly between -46 °C and 163 °C in Dow's propylene glycol products, mineral oils up to about 290 °C to 305 °C, and synthetic aromatic and silicone fluids up to about 400 °C (Dow, 2008; FRAGOL, accessed 2026; Dow, 2001). The table below shows which fluid class fits a given operating temperature, with the typical range and source in each row.
Fluid class | Typical temperature range | Where it is used |
|---|---|---|
Water | Above 0 °C; practical range limited, glycol blends and brines extend it (Canter, 2022) | Heating and cooling loops above freezing |
Propylene glycol and water | -46 °C to 121 °C for DOWFROST, -46 °C to 163 °C for DOWFROST HD (Dow, 2008) | HVAC freeze and burst protection, food and beverage cooling, defrosting (Dow, 2008) |
Mineral oil | Supply temperature up to 290 °C for ordinary oils, up to 305 °C for specially refined oils (FRAGOL, accessed 2026) | Process heating loops below 300 °C supply temperature (FRAGOL, accessed 2026) |
Synthetic aromatic | Bulk temperatures up to 400 °C (FRAGOL, accessed 2026) | Supply temperatures above 300 °C, liquid or vapor phase (FRAGOL, accessed 2026) |
Silicone | -40 °C to 400 °C for SYLTHERM 800 (Dow, 2001) | High temperature liquid phase systems |
Polyglycol | Up to about 200 °C to 250 °C, depending on product (FRAGOL, accessed 2026) | Tempering units in contact with air |
Dielectric (hydrocarbon) | Low temperature; EV immersion fluids will need a freezing point below -40 °C (Canter, 2022) | Direct immersion cooling of batteries and electronics (Canter, 2022) |
The key row is the mineral oil row: FRAGOL's general guideline is that mineral oils are used as a rule when the maximum supply temperature is below 300 °C, and synthetic aromatic or silicone fluids when it is above 300 °C (FRAGOL, accessed 2026). At the cold end, Alonso of Dow notes that certain silicone and organic fluids can be used below -25 °C and down to -100 °C, where glycol and water fluids are not practical (Canter, 2022). The bands are typical maker values, not limits for every product; the datasheet for the specific fluid sets the real limit.
How is heat transfer fluid condition measured?
Heat transfer fluid condition is measured by sampling the circulating fluid and testing the properties that change as it degrades: acid number, viscosity and flash point for oils, and glycol concentration and inhibitor level for glycol and water fluids. Michael Bates of Duratherm names acid number, viscosity and flash point as the parameters that assess thermal oil condition, and advises keeping a history of analyses for each system (Canter, 2022). FRAGOL notes that oxidative decomposition can be detected by an oil analysis measuring the acid or neutralization number (FRAGOL, accessed 2026). Alonso adds that low and high boiling compounds need to be monitored in hot fluids, while for glycol and water fluids the focus is the glycol concentration and the inhibitor package (Canter, 2022). For glycol, Dow's guide calls for checking concentration with a refractometer after mixing and for keeping pH between 8 and 10 to limit corrosion and glycol degradation (Dow, 2008). Trends matter more than one result, because Bates notes there are typically no visible signs of degradation until actual failure (Canter, 2022). When test results and the fluid's datasheet point in different directions, Lubinpla can organize the case from your own TDS, SDS and site history and return one cited document for your review.
Heat transfer fluid vs refrigerant: what is the difference?
A heat transfer fluid carries heat from a heater or chiller to the equipment without a compression cycle of its own, while a refrigerant carries heat by boiling in the evaporator and condensing after a compressor (Katz and Walker, 2008). In a direct expansion refrigeration system, Katz and Walker of Hill Phoenix explain, the liquid refrigerant entering the evaporator boils as it absorbs heat from the air in the case (Katz and Walker, 2008). In a secondary loop system, the glycol coolant absorbs heat through the secondary heat exchangers, which raises its temperature but does not cause it to boil (Katz and Walker, 2008). The two fluids meet at the chiller: the chiller serves as the evaporator for the refrigerant side and cools the secondary coolant, which then circulates to the loads (Katz and Walker, 2008). Reference books treat the two separately. The 2025 ASHRAE Handbook, Fundamentals, has one chapter on thermophysical properties of refrigerants and another, chapter 31, on physical properties of secondary refrigerants (brines) (ASHRAE, 2025). In practice, a chilled glycol loop in an HVAC plant is a heat transfer fluid loop, and the refrigerant stays sealed inside the chiller. A secondary loop also reduces refrigerant charge: in supermarket medium temperature systems, Katz and Walker report that secondary loop systems generally require half as much refrigerant as comparable direct expansion systems (Katz and Walker, 2008).
Related terms
Thermal oil: a heat transfer fluid based on mineral or synthetic oil, used for process heating.
Secondary coolant: a heat transfer fluid, such as glycol and water or brine, cooled by a refrigerant in a chiller.
Bulk temperature: the average temperature of the circulating fluid.
Film temperature: the fluid temperature at the heated wall, higher than the bulk temperature.
Family Q: the ISO 6743 class L family for heat transfer fluids.
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Sources checked 2026-10-14.
Sources
ISO. ISO 6743-12:1989, Lubricants, industrial oils and related products (class L), Classification, Part 12: Family Q (Heat transfer fluids). Edition 1, 1989, confirmed 2026. https://committee.iso.org/standard/13221.html
DIN Media. DIN 51522:1998-11, Heat transfer fluids Q, Specifications, test. 1998. https://www.dinmedia.de/en/standard/din-51522/6519061
ASHRAE. Description, 2025 ASHRAE Handbook, Fundamentals. 2025. https://www.ashrae.org/technical-resources/ashrae-handbook/description-2025-ashrae-handbook-fundamentals
Dow. Engineering and Operating Guide for DOWFROST and DOWFROST HD Inhibited Propylene Glycol-based Heat Transfer Fluids, Form No. 180-01286-0208. February 2008. https://cdn.shoplightspeed.com/shops/621040/files/64538514/dowfrost-inhibited-propylene-glycol-heat-transfer.pdf
Dow. SYLTHERM 800 Silicone Heat Transfer Fluid, Product Information, Form No. 176-01469-1101. 2001. https://www.npl.washington.edu/TRIMS/sites/sand.npl.washington.edu.TRIMS/files/manuals-documentation/syltherm-800-technical-data-sheet.pdf
FRAGOL. Heat Transfer Fluids: Information. Accessed 2026. https://www.fragol.de/en/heat-transfer-fluids/heat-transfer-fluids/informations.html
Canter, N. Heat-transfer fluids: Growing in visibility and importance. Thermal Processing, reprinted from STLE TLT, December 2021. February 15, 2022. https://thermalprocessing.com/heat-transfer-fluids-growing-in-visibility-and-importance/
Katz, B. and Walker, R. (Hill Phoenix). Refrigeration 101: Traditional and Secondary Supermarket Refrigeration Systems. Contracting Business, December 2008. https://www.hillphoenix.com/wp-content/uploads/2017/05/White-Paper-Refrigeration-Systems-Refrigeration-101-Traditional-and-Secondary-Refrigeration-Systems-June-2008-1.pdf
Written by the Lubinpla team from the sources listed and checked against them before publication.