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1. What are the main process parameters of a heating furnace? (1) Heat load: It indicates the production capacity of the heating furnace. (2) Furnace temperature: The furnace temperature of a heating furnace should not be too high; it is generally kept around 800°C, but this is not absolute. A high furnace temperature is favorable for radiation heat transfer, but if it becomes too high, the heat stress on the furnace tubes increases, making it easy for them to coking and get damaged. Furthermore, the temperature of the flue gas entering the convection chamber will also be too high, causing the convection tubes to burn out easily. Therefore, the furnace temperature is an important indicator for ensuring the long-term safe operation of the heating furnace. Increasing the area of the radiation tubes can lower the furnace temperature, but uniform and appropriate heating is required. Increasing the number of radiant tubes too much does not result in a proportional increase in processing capacity; instead, it wastes steel. (3) Furnace heat intensity (or volume heat intensity): Once the dimensions of the furnace are determined, burning more fuel will inevitably increase the furnace heat intensity; accordingly, the furnace temperature will rise as well, resulting in an increased heat load on the furnace tubes. The furnace heat intensity for typical tubular heating furnaces is: it should be less than 124 kW/m3 when burning oil ； It should be less than 165 kW/m3 during gas burning. (4) Surface heat intensity of the furnace tubes: The higher the surface heat intensity of the furnace tubes, the fewer tubes are required under a given heat load. This allows the size of the furnace to be reduced and investment costs to be lowered; therefore, it is necessary to increase the surface heat intensity of the furnace tubes as much as possible. However, increasing the surface heat strength of the furnace tubes is also subject to certain limitations, because: ① As the heat strength of the furnace tubes increases, the wall temperature also rises, and the oil near the tube walls will decompose and coking due to overheating; in severe cases of coking, this may lead to the rupture of the furnace tubes. ②Inside the furnace, the heating across the furnace tubes is uneven; the sides of the tubes that face the flame are exposed directly to the flame’s radiation, while those facing away from the flame receive only the reflected heat from the furnace walls. As a result, the heating intensity on the sides facing the flame is higher than that on the sides facing away from the flame ； In the direction of the tube length, the area closer to the flame receives more radiant heat than the area further away ； Viewed as a whole furnace, the distance of each furnace tube from the flame varies, resulting in different amounts of radiant heat being absorbed by each tube. To ensure that the furnace tubes in areas with high heat intensity do not suffer from coking or damage, it is necessary to select an appropriate heat intensity for these tubes, so as to achieve as uniform heating as possible throughout the tubes. To make the heat intensity on the surface of the radiant furnace tube relatively uniform, the following methods can generally be adopted: ① Use furnace tubes that are irradiated from both sides as much as possible. When heat is applied to both sides of a single-row furnace tube, the heating is much more uniform compared to when heat is applied to only one side ； When heat is applied to both sides of double-row furnace tubes, the heating is more uniform compared to when heat is applied to only one side of single-row furnace tubes. ②In a cylindrical furnace, in order to reduce uneven heating along the length of the furnace tubes, it is necessary to select an appropriate height-to-diameter ratio for the radiation chamber. Additionally, a suitable burner must be chosen so that the flame length of the burner does not differ too much from the length of the furnace tubes. For example, if the length of the radiation tubes is 15 meters, a burner with a flame length of 12–13 meters should be used, thereby ensuring more uniform heating at the upper and lower parts of the furnace tubes. ③In vertical furnaces, some use multiple nozzles on the side of the furnace ； Some have decorative walls added between the two rows of nozzles ； Nozzles are also installed at the upper part of the furnace; all these measures are taken to improve the uniform heating of the furnace tubes. (5) Thermal efficiency of the heating furnace: Thermal efficiency is an indicator of fuel consumption, and it is also one of the indicators of the operational quality of the heating furnace. The higher the thermal efficiency, the greater the effective utilization rate of the fuel oil, and the lower the fuel consumption. (6) Flow rate and pressure drop of oil in the pipe: The flow rate of oil within the pipe should not be too low, otherwise it can cause coking of the oil in the pipe, leading to damage to the furnace tubes. Because when the flow velocity is too low, the boundary layer thickness inside the tube is large, heat transfer is slow, the tube wall temperature rises, and the oil droplets stay inside the tube for a longer time. The flow rate of the oil inside the pipe cannot be too high either, because if it is too high, the pressure drop will be large. Since the pressure drop is limited by the pump’s head, the flow rate is determined based on the allowable pressure drop. The approximate ranges of fluid velocity and pressure drop of the oil in the furnace tube are shown in Table 3-1. Table 3-1 Flow rate and pressure drop of oil in furnace tubes. Sequence Number, Furnace Purpose, Oil Quality, Flow Rate/(kg/m²·s), Pressure Drop/Pa: 1, 2, 3, 4, 5, 6. Atmospheric pressure furnace, Vacuum furnace, Asphalt furnace, Viscosity-reduction furnace, Rich-oil heating furnace, Light fraction reboiler. Flow rates range from 1000–1500; pressure drops range from 1000–1500 (before gasification), 1200–1500, 1400–2000, 1200–1700, and 1200–1700 respectively. The pressure drop in the heating furnace is also an important indicator to determine whether there is coking in the furnace tubes. If the cold oil flow rate remains unchanged while the pressure drop increases, it is a sign of coking in the furnace tubes. Because after coking, the inner diameter of the furnace tube decreases, the actual flow rate of the oil increases, and thus the pressure drop also increases. 2. How are the characteristic parameters of a heating furnace expressed? The name of the diagram showing the overall layout of the heating furnace usually indicates its characteristic parameters. Its meaning is illustrated by the following example. Cylindrical vertical tube-type heating furnace file:///C:\Users\Blackfox\AppData\Local\Temp\ksohtml\wpsD2BE.tmp.jpg file:///C:\Users\Blackfox\AppData\Local\Temp\ksohtml\wpsD2BF.tmp.jpg 3. How is the thermal load for the design of a heating furnace determined? The design heat load of the heating furnace should be based on the calculated heat load under normal design conditions. 4. How is the heat load distributed between the radiation chamber and the convection chamber? The heat load distribution in a radiation-convection type heating furnace is as follows. (1) Cylindrical vertical tube furnace: When plain tubes are used in the convection section, radiation accounts for 75%–80%, while convection accounts for 20%–25%. When nail-head tubes or finned tubes are used in the convection section, radiation accounts for 70%–75%, while convection accounts for 25%–30%. (2) Vertical furnace with horizontal tubes: radiation accounts for 70%–75%, while convection accounts for 25%–30%. 5. How is the heat generated by fuel combustion transferred to the oil inside the tube? When the heater is in operation, the heat produced by fuel combustion is transferred through the tube walls to the oil inside the tube, providing the heat needed for the oil to warm up and vaporize. The flames emitted by the burner in the radiation chamber (including luminous and non-luminous flames) exert radiative heat transfer on the furnace tubes ； Meanwhile, the high-temperature flue gas, as it moves from the outlet of the radiation chamber into the convection chamber, washes against the furnace tubes, thereby facilitating convective heat transfer to them. The wall of the furnace tube serves to conduct heat, transferring it from the outer wall of the tube to the inner wall and then to the oil. As can be seen from the above analysis, there are three ways of heat transfer in the furnace tubes inside the radiation chamber: radiation, convection, and conduction. In different areas, one or several heat transfer mechanisms are at work. In situations where several modes of heat transfer are at work, one mode must play a dominant role. On the outer wall of the furnace tube, radiation heat transfer from the flame, flue gas, and furnace wall plays the main role, with convective heat transfer from the flue gas acting as a supplementary mechanism. In the furnace tube of the radiation chamber, heat transfer is primarily by radiation, with convection playing a secondary role. Inside the convection chamber, convective heat transfer is the dominant mechanism. 6. What is the significance of the temperature of the flame wall? In heat transfer calculations for heating furnaces, the flame wall temperature was traditionally considered to be the average temperature of the flue gas in the radiation chamber. The higher the firewall temperature, the greater the heat intensity of the radiant tubes, and the more heat is absorbed by the radiation chamber. From another perspective, the higher the firewall temperature, the higher the wall temperature of the furnace tubes, and the easier it is for the oil inside the tubes to coker. Therefore, the firewall temperature is used as an important indicator for control at the site. It should be noted, however, that the required firewall temperature varies depending on the type of furnace and the medium inside the tubes. It is unscientific to control the flame wall temperature at a fixed level (such as 800°C) in a general manner, without taking into account the equipment or furnace type. For example, even among pressure reduction furnaces, vertical furnaces with horizontal tubes allow a furnace wall temperature that is about 100°C higher than that of cylindrical furnaces. Of course, it is appropriate and possible to establish specific control indicators for various types of furnaces used for different purposes, based on one’s own operational experience on site. 7. Should the higher or lower heating value of fuel be used in process calculations? Why? In process calculations, the lower heating value of fuel is generally used, because the higher heating value includes the heat released when the water produced by the combustion of hydrogen condenses from a gaseous state to a liquid state. In industrial applications, since the water produced by combustion is discharged from the chimney in vapor form, the heat released by the condensation of water vapor cannot be utilized; therefore, low-calorific value fuels are generally used instead of those with high calorific value. 8. What factors are related to the thermal intensity of a radiant tube? The thermal intensity of a radiant tube is the amount of heat transferred per hour to the outer surface of the radiant tube per square meter, with the unit being W/m2. The factors that affect the thermal intensity of the radiant tube are as follows. (1) Uneven heating along the circumference of the furnace tubes: In a row of furnace tubes arranged along the furnace wall, each tube on the fire-facing side is primarily exposed to the radiant heat from the flames and hot flue gases, while the back side receives mainly reflected heat from the furnace wall. The heat intensity is highest at the very front point on the fire-facing side, decreasing gradually at other points. If the heat intensity at the highest point is 1, the average heat intensity across the entire circumference is only 0.562. If a point with a heat intensity of 1 represents the maximum utilization of the furnace tube, then for the entire circumference, the surface utilization rate of the tube is only around 56.2%. (2) Uneven heating along the length of the furnace tube: In a vertical tube heating furnace, when heating occurs only at the lower part, the heat intensity is generally highest in the lower and middle sections of the furnace tube. The degree of unevenness up and down the furnace tube is related to factors such as the length of the tube and the flame, as well as the distance between the burner and the furnace tube; generally, the unevenness coefficient (maximum/average) ranges from 1.2 to 1.5. (3) Effect of heating medium temperature: When the furnace temperature remains constant, different temperatures of the medium inside the tubes result in varying thermal intensities of the furnace tubes. The higher the temperature of the medium inside the tube, the smaller the temperature difference between the flue gas outside the tube and the medium inside it, resulting in less heat being transferred in – that is, the heat intensity is low ； Conversely, the lower the temperature of the medium inside the pipe, the higher the heat intensity. During design, the thermal intensity is usually determined based on the average temperature of the medium entering and exiting the radiation tube. (4) Local dead zones: The relative position of each furnace tube to the flame directly affects the heat transfer rate of each furnace tube. The furnace tubes of the cylindrical furnace are arranged in a circular pattern along the furnace wall, with the burners located in the center; it can be assumed that the heat transfer rate for each furnace tube is uniform. In a box-type furnace, the heat transfer rate of the furnace tubes at the corners is lower than that of the tubes in the middle, resulting in local dead zones; therefore, the heat transfer rates between the furnace tubes vary. 9. What are the methods to address uneven heating of furnace tubes? (1) Solutions to uneven heating along the circumference of the furnace tubes: For various arrangements of furnace tubes, the average surface utilization rate is shown in Table 3-2. Table 3-2 Average surface utilization tube center distance under radiation conditions for different numbers of tube rows ； 2×outer diameter of the tube: For a single row of tubes, radiation occurs on one side while reflection takes place on the other, resulting in 56.2% efficiency for radiation on one side and 83.8% for radiation on both sides. For a double row of tubes, radiation occurs on one side and reflection on the other, giving 31.15% efficiency for radiation on one side and 54.4% for radiation on both sides. As can be seen from the table above, the single row of tubes achieves the highest efficiency in terms of radiation on both sides; thus, this arrangement is the best way to address the issue of uneven heating along the circumference of the furnace tubes. Of course, other methods such as increasing the pipe center distance and using oval tubes are also effective, but they are rarely used in petrochemical plants; these measures are only adopted in chemical plants where the cost of furnace tubes is high in order to reduce the amount of piping required. Depending on the specific conditions, the average heat intensity of the radiant tubes in the heating furnace can be increased by adopting single-row double-sided radiation (fully or partially). (2) Solutions to uneven heating along the length of the furnace tube: Installing radiation cones on cylindrical furnaces, using flameless combustion, wall-attached flames, stepped furnaces, adding nozzles to the upper part of the radiant chamber in bottom-fired furnaces – these are all different measures adopted to address this issue. Bottom-fired circular furnaces or vertical tube furnaces require the flame length to be approximately 60% of the length of the radiant tubes, which is also intended to reduce unevenness along the furnace tubes. (3) Solutions to the uneven heating of the furnace tubes caused by the temperature of the heating medium: When designing a heating furnace, it is of course impossible to maintain the same temperature across all the tube walls; in other words, it is not possible to supply different amounts of heat based on the temperature of the medium inside each tube. However, if the radiant tube can be divided into several sections and adjusted separately using burners, the average heat intensity can be increased. This is why there has been partitioned computing and partitioned adjustment recently. (4) Solutions for local dead zones: Methods to reduce local dead zones should be considered from two aspects: the relative position between the furnace tubes and the burner, as well as the distribution of the flue gas flow field. In old-style box furnaces, the heat transfer through the furnace tubes at the corners is less than that in the middle; therefore, furnace designers removed these dead corners, turning the box furnace into a sloped-roof furnace. The upper part of the radiation chamber in the vertical furnace with horizontal tubes is designed with an inclined shoulder, also to increase the radiant and convective heat from the upper tubes. In addition to reducing dead zones, to prevent localized overheating, the flame should not be too close to the furnace tubes; preventing the flame from touching the tubes is also a necessary measure to increase the average heat intensity. 10. How should the average designed heat intensity on the surface of radiant tubes in heating furnaces for various applications be selected? The average designed heat intensity on the surface of the radiation tube should be determined based on existing design experience; for single-sided radiation with a single row of tubes, the values in Table 3-3 can be used ； For double-sided radiation and single-tube arrangements, 1.5 times the values in Table 3-3 can be used. Table 3-3 Average Design Heat Intensity on the Surface of Single-Sided Radiation, Single-Tube Array Radiation Tubes. Name of Tubular Furnace – Average Design Heat Intensity/(W/m2): All Vertical Tube Furnaces; Vertical Furnaces or Horizontal Box Furnaces; Atmospheric Pressure Furnaces; Reduced Pressure Furnaces; Catalytic Cracking Furnaces; Catalytic Reforming Furnaces: 30,000–37,000; 24,000–31,000; 240D–31,000; 25,000–32,000; 36,000–44,000; 29,000–37,000; 29,000–37,000; 29,000–37,000. Table 3-3 Average Design Heat Intensity on the Surface of Single-Sided Radiation, Single-Tube Array Radiation Tubes (Continued). Name of Tubular Furnace – Average Design Heat Intensity/(W/m2): All Vertical Tube Furnaces; Vertical Furnaces or Horizontal Box Furnaces; Coking Furnaces; Pre-Hydrogenation Furnaces; Viscosity-Reduction Heating Furnaces; Hydrocracking Furnaces; Dewaxing Furnaces; Propane Deasphalting Furnaces; Oxidized Asphalt Furnaces; Phenol Refining Furnaces; Furfural Refining Furnaces; Steam Superheating Furnaces: —— 24,000–35,000; 23,000–27,000; 23,000–31,000; 23,000–31,000; 18,000–23,000; 16,000–20,000; 17,000–23,000; 17,000–23,000; 28,000–35,000, 29,000–32,000; —— 28,000–31,000