Thermal liquid heating is a specialized form of process heating that harnesses the forced circulation of a special heating medium, called thermal fluid, as a liquid. In many types of process heating, high temperature rather than high pressure is essential where heat — not vapor or steam — is required. 

Since the days of the Roman Empire, hot water and hot gases have been circulated as heating media. Some time around 1920, a new concept in industrial and process heating was first successfully tested in Europe.