The primary circuit consists of a heating boiler, pump(s), piping, storage tank, expansion tank, possibly a heat exchanger, etc. that is constructed as a circuit. Of course equipped with the necessary pressure and temperature protection units.
The boiler heats the thermal oil, the pump circulates the oil in the primary circuit.
Thermal oil installations can be equipped with so-called economizers. These are units that are installed in the exhaust gas system of the boiler and that recover heat and improve efficiency by a few percent.
In the secondary circuits, i.e. the parts of the thermal installation that are connected to the users (this can be a reactor vessel, a drying tunnel, or whatever), will release the heat required there via controlled valves.
The cooled oil is pumped back into the primary circuit to be reheated by the boiler. It will be clear that the capacity of the boiler depends on the size of the secondary circuits and the heat required per circuit.
In every thermal oil installation, all circuits are protected against overpressure, too high and/or too low temperatures, and the liquid flow is constantly measured. When the pump(s) fails, the boiler is automatically switched off to prevent overheating and/or combustion of the thermal oil.
The vast majority of thermal oil installations up to 350°C are virtually pressureless (only the pump pressure) and operate in the liquid phase of the thermal oil. At temperatures above 350°C, the pressure can rise up to 10 bar.
Thermal oil installations operating above 350°C can operate in the gas phase of the thermal fluid in addition to the liquid phase, depending on the customer’s wishes or the design of the installation.