The thermal oil is the heat transfer fluid used in countless applications for the development of industrial processes. From the standpoint of the final process or from heat consumer, the thermal fluid is the primary fluid. Opposite, if the volume of control or analysis is limited to the boiler, the thermal fluid is the secondary fluid, since the heat exchange is performed from the primary (which may be a flame, combustion gases or electric energy transformed into heat) to the secondary fluid (thermal oil). Since the temperatures of the primary can be very high, it is essential to ensure a sufficient flow of oil to prevent it exceeds its maximum operating temperature or that overheating points in the boiler appear.
The nominal rated power of the standard range of PIROBLOC double coil boilers for fossil fuels (natural gas, oil, hfo) starts on 100,000 kcal / h (116kW) up to 5,000,000 kcal / h (5814kW). In all them, there is a flow control below which, the supply of heat stops to prevent overheating that can crack the oil and / or can damage to the boiler. Such flow control is performed by the differential pressure between the inlet and outlet oil collectors.
The nominal values for differential pressure in the mentioned range of boilers normally are between 1,8bar and 2,8bar depending on the boiler and the project. Typical values of set point for such alarm are between 1 bar and 1,8bar depending on the case. It allows include a flow alarm in the system for low differential pressure measurement, by a differential pressure switch (DPSL) or by the difference in readings of two pressure transmitters (PT), placed one in the inlet collector and another in the outlet one.
Thermal fluid electric boilers
The thermal fluid electric boilers from the hydraulic point of view, reasonably resemble to a shell and tube heat exchanger, wherein the tube bundle is replaced by the set of forks which are electrically feed to heat the oil circulating in the shell.
The rated power of PIROBLOC thermal fluid electric heaters standard range starts at 24kW up to 1000kW. The differential pressure nominal values for the mentioned boilers are between 0.1 bar and 0,3bar. This low differential pressure is due to the short path of the oil in an electric heater in comparison with the path in a coil boiler, and also to the slower speed. This would cause that the flow alarm set point by differential pressure was between 0.05bar and 0,15bar. Such situation would be a problem for standard differential pressure sensors (taking into account that the static pressure can be 15bar_g or higher).
The solution adopted by PIROBLOC has been to include at the boiler inlet, an orifice plate (restriction orifice) calculated according to standard ISO_5167-2 (Measurement of fluid flow by Means of pressure differential devices). This calculation is made individually for each client and project according to their working conditions: Characteristics of fluid used (density, viscosity) and nominal flow. This element ensures that the differential pressure of the skid boiler and plate is set around 1,2bar, which allows the use of standard instruments for measuring differential pressure. To this advantage we must add that the boiler behaves like the way indicated in ISO_5167-2 standard, allowing us to know with good accuracy the circulating flow at all times.