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1. What should be the typical gas flow velocity in the flue? Under forced ventilation, the gas flow velocity in the flue should generally not exceed 12–15 m/s. 2. What is the typical gas flow velocity in the hot air duct? In forced ventilation, the gas flow velocity in the hot air duct should generally not exceed 12–15 m/s. 3. What is the typical gas flow velocity in the cold air duct? The typical values for the gas flow velocity in such ducts are as follows: (1) In the case of natural ventilation, it should generally not exceed 6–8 m/s. (2) During forced ventilation, it should generally not exceed 10–12 m/s. 4. What is the design flow rate of the burner in relation to its rated flow rate? The operating parameters of the burner, such as the properties and pressure of the fuel oil, as well as the pressure and temperature of the atomizing steam, all vary within certain ranges. To ensure that the burner can achieve the rated flow rate when operating at lower parameters, its design flow rate should be increased by 25% based on the rated flow rate; this way, even if individual burners are shut down for maintenance, the furnace can still maintain its normal operating load. 5. What is the significance of the furnace heat intensity? In a tubular furnace, the furnace not only serves as a combustion chamber but also houses tubes within it to meet certain requirements related to heat transfer in the processing process. Since the heat intensity in the furnace chamber is limited by the heat intensity on the surface of the furnace tubes, the heat intensity in the furnace chamber of a heating furnace ranges from 60 to 120 kW/m³, which is much lower than that of boilers (1000 to 1400 kW/m²). Therefore, there is no issue of incomplete combustion due to excessively high heat intensity in the furnace chamber. However, an excessively low heat intensity in the furnace indicates a large furnace volume and low temperatures, which are unfavorable for both combustion and heat transfer. 6. Why is the diameter of the radiation outlet tube in the decompression furnace increased? The two furnace tubes at the radiation outlet of the decompression furnace have a diameter of φ 19x10 mm, while the other radiation furnace tubes have a diameter of φ 52x10 mm; all of them are made of Cr5Mo material. The purpose of expanding the outlet pipe is to increase the yield and the gasification rate at the furnace outlet. To achieve this, a considerable amount of steam must be injected into the furnace tube to reduce the partial pressure of oil and gas, thereby ensuring a high gasification rate at lower furnace outlet temperatures. A high gasification rate and a large steam injection volume will in turn cause the flow velocity and pressure drop inside the furnace to increase rapidly. Furthermore, excessive flow velocities can cause vibration and noise, and may even lead to damage to the components, and the flow velocity within the pipe is limited by the critical velocity (i.e., the noise velocity). Therefore, it is necessary to increase the diameter of the radiation outlet pipe in order to keep the flow rate and pressure drop within appropriate ranges. However, if the pipe diameter is increased too much and the flow velocity drops excessively, the flow pattern will deteriorate, leading to localized overheating as well as the cracking of the oil and subsequent coking. The correct solution is to gradually increase the diameter of the furnace tubes, ensuring that the oil remains in a favorable flow pattern throughout the gasification section for isothermal gasification. IV. Thermal efficiency of heating furnaces 7. What is the thermal efficiency of a heating furnace? file:///C:\Users\Blackfox\AppData\Local\Temp\ksohtml\wpsFB2A.tmp.jpg The effective heat absorbed represents the thermal load of the furnace ； The total heat release is generally the calorific value of the fuel. When the thermal load of the furnace remains constant, the higher the thermal efficiency, the less fuel is required. 8. What is the significance of improving the thermal efficiency of heating furnaces? The average total energy consumption per ton of oil processed nationwide is around (0.9–1) MW, accounting for approximately 8%–9% of the crude oil processed ； The specific fuel consumption for fuel oil is approximately 30–40 kg per ton of crude oil, and heating furnaces account for around 35% of the total fuel consumption in refineries. Therefore, improving the thermal efficiency of these heating furnaces can lead to significant savings in fuel usage, which is both necessary and highly effective for reducing energy consumption. 9. What requirements should the designed thermal efficiency meet? The designed thermal efficiency shall be calculated in accordance with the provisions of the \"Method for Calculating Thermal Efficiency of Petrochemical Tubular Furnaces\", and the assumptions underlying this calculation shall be as follows: (1) The heat load to be used is the designed heat load. (2) The design excess air coefficient shall be adopted according to Table 4-1. Table 4-1 Design excess air coefficient: Excess air coefficient by burner type – for oil or gas combustion with natural ventilation: 1.25; 1.20. For forced ventilation: 1.20; 1.15. (3) Heat loss is determined as a percentage of the total heat supply; in the absence of a waste heat recovery system, it should not exceed 2% ； With a waste heat recovery system, it is not more than 3%. (4) The reference temperature is 15.6°C, (5) the ambient temperature is the annual average temperature of the location where the plant is built. 10. What are the advantages and disadvantages of using the forward and reverse balance methods to calculate the thermal efficiency of a heating furnace? Forward balance method: file:///C:\Users\Blackfox\AppData\Local\Temp\ksohtml\wpsFB3A.tmp.jpg Reverse balance method: file:///C:\Users\Blackfox\AppData\Local\Temp\ksohtml\wpsFB3B.tmp.jpg When using the forward balance method to calculate thermal efficiency, it is necessary to determine the heat load of the furnace as well as the calorific value of the fuel. If the heat load has an error of 1% for various reasons, the error in thermal efficiency will also be around 1% (assuming that the calorific value of the fuel is accurate). However, when calculating thermal efficiency using the reverse balance method, since various heat losses account for 10% to 20% of the total heat, even a measurement error of 1% results in an impact on thermal efficiency of only 0.1% to 0.2%. From this perspective, the reverse balance method is highly accurate for calculating thermal efficiency; moreover, it is also convenient to use, which is why it is widely adopted nowadays.