Organic heat transfer oils are employed as heating and cooling media in the temperature range from approximately -55 °C to +400 °C. With regards to sealing aspects it is possible to classify heat transfer oils in 4 groups:

• Heat transfer oils with a vapour head <1 bar at the respective maximum inlet temperature.
• Heat transfer oils with a vapour head from 1 to 3.5 bar at the respective maximum inlet temperature.
• The eutectoid mixture of 26.5 standard % biphenyl ("diphenyl") and 73.5 standard % diphenyl ether (diphenyl oxide), known under various trade names such as e.g. Diphyl^®, Dowtherm^®A, Gilotherm^®DO, Santotherm^® (Therminol^®) VP-1, Therm-S^®300, Thermex^®, with a vapour head of approximately 11.1 bar at the maximum inlet temperature of 400 °C.
• Silicon organic heat transfer media such as e.g. the dimethyl polysiloxane Syltherm^® 800, which must be placed under equilibrium pressure to preserve its composition (Syltherm 800: 13.7 bar at 400 °C).

One attribute common to all heat transfer oils is their sensitivity to oxygen, which is negligible up to around 80 °C, but from a temperature of roughly 100 °C increases by approximately double with every 10 °C rise of temperature. The oxidative impairment of the oil results in hard residues. Consequently, the sealing of heat transfer oils at single mechanical seals is possible only up to temperatures of around 100 °C at the mechanical seal. Given higher temperatures at the mechanical seal, it is necessary to use either a double mechanical seal or a single-acting mechanical seal with quench, depending on the vapour head or equilibrium pressure. Cold heat transfer oil circulated via a cooled thermosyphon vessel is used as quench liquid or buffer liquid.