Conditions Required for Designing Electric Heating Thermal Oil Furnace
1 Design heat load. A certain margin should be left between the heat load of the thermal oil furnace and the effective heat load, generally 10%~15%.
2 Design temperature. The design temperature of the thermal oil furnace is determined by its operating temperature and should be designed in accordance with the relevant provisions of GB9222 "Strength Calculation of Water Tube Boiler Components".
3 Design pressure. The design pressure of the thermal oil should be slightly higher than the maximum working pressure and should not be less than the opening pressure of the safety valve. The design pressure of the vapor phase furnace is 1.2~1.5 times the working pressure; the design pressure of the liquid phase furnace should be 1.05~1.2 times the pressure; the pressure difference between the inlet and outlet of the thermal oil in the liquid phase furnace should be greater than 0.15MPa (1.5kgf/cm2).
4 Inlet and outlet temperature of thermal oil. The design should be economical and safe, providing a suitable temperature difference for the thermal oil in the system, which should be less than 30°C.
5 Flow velocity of thermal oil in the tube. The design should ensure a certain flow velocity of thermal oil in the tube to avoid coking due to local overheating. Generally, the flow velocity in the radiation section tube is 2~4m/s, and in the convection section tube is 1.5~2.5m/s. When determining this parameter, the resistance of hot oil in the tube and the factor ensuring turbulent flow of hot oil in the tube should also be considered. Larger tube diameter requires higher flow velocity; smaller tube diameter requires lower flow velocity.
6 Average heat intensity of furnace tube. The design requires that the average heat intensity of the furnace tube be within a certain range, so that the thermal oil does not overheat and the heat transfer area of the furnace tube is fully utilized. Generally, the average heat intensity of the radiation section furnace tube is 0.084~0.167GJ/(m2.h), and for the six-section furnace tube, the average heat intensity is 0.033~0.047GJ/(m2.h).
7 Exhaust gas temperature. According to the operating temperature of the thermal oil, the difference between the exhaust gas temperature and the thermal oil temperature should be controlled at 80~120°C, and the exhaust gas temperature should be suitable at 350~400°C, so that the convection heating surface is not too large. To fully utilize thermal energy, the heat from the relatively high exhaust gas temperature discharged by the thermal oil furnace should be recovered and reused by installing a waste heat recovery device, especially for larger thermal oil furnaces, which should be considered and emphasized.
8 All pipes and fittings in contact with thermal oil are strictly prohibited from being made of non-ferrous metals and cast iron. Flanges and valves should use cast steel valves with a nominal pressure of 2.5MPa (about 25kgf/cm2) or above, and seals should be made of high-temperature and oil-resistant materials. For biphenyl mixture type thermal oil, tongue-and-groove or concave-convex face flanges should be used for connection.
9 The thermal oil furnace must be equipped with a low-level drain valve, and it is required to be able to drain the material completely, ensuring no residual liquid remains.
