High temperature heat transfer oil at 300C: mineral or synthetic?

This answer covers liquid-phase, indirect heat transfer systems running organic fluids near 300 C bulk. It does not cover vapor-phase systems, water or glycol loops, or silicone fluids, and it does not replace the heater maker's film temperature calculation or the fluid maker's written rating for your system.

Which conditions decide a heat transfer fluid for 300 C?

Four conditions decide a heat transfer fluid for 300 C: the maximum bulk temperature, the maximum film temperature at the heater wall, how the expansion tank meets air, and the lowest temperature at which the system must start and pump. Bulk temperature is the number most people quote, but it is only the first filter. A fluid rated to exactly 300 C bulk has no headroom when the heater runs hotter than planned.

Film temperature is the second filter. The thin layer of fluid on the heater tube is not in turbulent flow and often runs 20 to 30 C above the bulk fluid temperature (Eastman, 2026). Each data sheet states its own maximum film temperature, and that limit, not bulk, often decides whether a mineral oil can run at 300 C.

Tank type is the third filter and changes the rating of the same product. One mineral heat transfer liquid is rated 40 to 300 C in closed systems but 40 to 230 C in open systems (LAUDA, 2026). The fourth condition is start-up: the same fluid needs 40 C at the low end, while one synthetic fluid lists a minimum use temperature of minus 3 C (Eastman, 2025). Record all four for your system before comparing any data sheets.

Mineral or synthetic heat transfer fluid by bulk, film and tank type

The table below turns those four conditions into a fluid class for a 300 C system, with the published limit and its condition in each row. Values come from individual product data sheets and apply to those products only; your fluid's own sheet decides.

The key row is the open tank: at 300 C bulk, no fluid class found is rated for an open expansion tank, so the tank design changes before the fluid does. The open ratings found run from 190 C to 230 C for mineral oils (Phillips 66, 2016; LAUDA, 2026) and 220 C bath temperature for one synthetic sold for open-type units (Idemitsu, 2023).

To use the table, start from the top row and stop at the first row whose conditions all match your system. If two rows match, take the more demanding fluid class. A row that says a mineral oil can work is a shortlist, not a choice: the heat transfer fluid still has to show, on its own data sheet, a closed-system bulk rating at or above your set point and a film rating above your heater's calculated film temperature at reduced flow.

How much margin does a 300 C heat transfer fluid need between bulk and film temperature?

A 300 C heat transfer fluid needs a maximum film temperature at least as far above 300 C as your heater's actual film rise, which Eastman puts at often 20 to 30 C above bulk in a heater (Eastman, 2026). Published mineral oil ratings show how little room is left. One mineral oil lists a 300 C maximum working temperature and a 330 C maximum film temperature (Shell, 2022), so at 300 C bulk a 30 C film rise uses the whole margin. Another mineral oil lists 320 C maximum flow and 350 C maximum film temperature for closed heating systems (Ravenol, 2026), which leaves 50 C of film headroom at 300 C bulk.

Synthetic fluids widen the gap. One synthetic fluid lists 345 C maximum bulk and 375 C maximum film temperature (Eastman, 2025), so at 300 C bulk it has 75 C to its film limit. The film rise in your system depends on heat flux and flow velocity, and Eastman states that film temperature can be calculated from the ratio of maximum heat flux density to the heat transfer coefficient (Eastman, 2026). Ask the heater maker for that calculation at design flow and at reduced flow, because a partly closed valve or a weak pump raises film temperature without changing the bulk reading on the control panel.

Is a mineral heat transfer oil enough at 300 C, or do you need a synthetic?

A mineral heat transfer oil is enough at 300 C when the system is closed or blanketed, the calculated film temperature stays under the oil's published film limit, and the bulk rating is at or above 300 C. Several mineral products meet the bulk part: one mineral series is recommended up to 300 C in closed systems (Idemitsu, 2023), and another mineral liquid is rated to 300 C closed (LAUDA, 2026). Not all do. One mineral heat transfer oil family recommends a maximum bulk oil temperature of 288 C in closed systems with expansion tanks and relief valves (Phillips 66, 2016), which rules it out at 300 C.

Choose a synthetic when the film limit is the problem, when the bulk set point may climb above the mineral fluid's own bulk rating, or when the plant needs low-temperature start-up. Synthetic is not a fix for an open tank: one synthetic heat transfer oil for open-type units is rated 0 to 220 C bath temperature (Idemitsu, 2023). Mineral oil at 300 C also runs well above its flash point. One mineral oil lists a 220 C flash point (COC) and a 332 C autoignition temperature (Shell, 2022), so check the fluid's SDS and the installation's leak and insulation-fire controls under DIN 4754-1:2015-03 or your local rule before choosing.

Why does an open expansion tank lower the heat transfer fluid temperature limit?

An open expansion tank lowers the heat transfer fluid temperature limit because contact with ambient air accelerates the aging of hot oil (Ravenol, 2026), so makers rate the same fluid far lower for open systems. The drop is large. One mineral liquid falls from 300 C in closed systems to 230 C in open systems (LAUDA, 2026). One mineral series falls from 300 C closed to 200 C open (Idemitsu, 2023). One mineral oil family allows 288 C bulk in closed systems but 190 C in open systems with cold-oil sealed expansion tanks (Phillips 66, 2016).

The open-system figures differ by product, from 190 C to 230 C, because each maker tests and rates its own formulation; they are different fluids, not conflicting sources. The same maker states that hot oil should never contact air and recommends eliminating air before bringing the oil up to operating temperature (Phillips 66, 2016). Eastman notes that high acidity may indicate severe fluid oxidation when the expansion tank vapor space is not protected with inert gas (Eastman, 2026). For a 300 C process, an open tank is the design limit to change first: no open-system rating found reaches 300 C for either fluid class.

When does a nitrogen blanket make sense for a high temperature heat transfer fluid?

A nitrogen blanket makes sense for a high temperature heat transfer fluid whenever the expansion tank would otherwise let hot fluid touch air, which at 300 C means in practice every system. Makers of both fluid classes recommend it. For a mineral oil family, Phillips 66 recommends an inert gas such as nitrogen under positive pressure in the expansion tank at all times during operation (Phillips 66, 2016). For one synthetic fluid, Eastman recommends that systems be blanketed with an inert gas atmosphere to protect against oxidation (Eastman, 2025).

Small systems have an alternative. Eastman states that in small systems the nitrogen may be replaced by a cold-seal trap or an expansion leg filled with fluid kept at a low temperature (Eastman, 2026), and Phillips 66 lists a cold-oil seal pot on the expansion tank vent as an accepted closed-system design (Phillips 66, 2016). A blanket also helps with water and light ends: Eastman describes purging the expansion tank vapor space with inert gas to sweep out moisture and low-boiling components (Eastman, 2026). Low-boiling compounds should be vented to a safe location away from personnel and sources of ignition (Eastman, 2025), so plan the vent with the SDS and DIN 4754-1:2015-03 or your local pressure equipment rule.

What should you ask a heat transfer fluid supplier before running at 300 C?

Ask a heat transfer fluid supplier five things before running at 300 C: the maximum bulk and film temperatures and the system type each applies to, the minimum start-up temperature, the flash point and autoignition temperature, thermal stability data and the method used, and whether the rating assumes a nitrogen blanket. Ask for the classification too. Heat transfer fluids sit in family Q of ISO 6743-12:1989 (ISO, 1989), and DIN 51522:1998-11 sets requirements and tests for family Q fluids (DIN, 1998). One mineral oil states it is classified as ISO 6743-12 family Q and meets DIN 51522 (Shell, 2022).

For thermal stability, Eastman names DIN 51528 with DIN 51435, and ASTM D6743 with ASTM D7213, as the methods that thermally stress a fluid and quantify low and high boilers (Eastman, 2026). Ask which method and temperature the supplier used, because a stability claim without its test condition cannot be compared. For how a full product recommendation should lay out requirements, candidates and evidence, see what a chemical product recommendation should include. Call a heat transfer specialist when the film calculation is missing or the tank is open at 300 C. Lubinpla treats this as a product recommendation that fixes operating temperature and system type as requirements and compares data sheet maximum use temperatures with their source and measurement condition.