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1. What factors are related to the thermal intensity of a convection tube? The thermal intensity of a convection tube is calculated based on various data inside and outside the tube. The factors affecting the convective heat transfer coefficient are as follows. Outside the tube: The greater the weight flow rate of the flue gas, the higher the external film heat transfer coefficient ； The higher the convectively averaged flue gas temperature, the greater the outer film heat transfer coefficient ； The smaller the outer diameter of the convection tube, the higher the external film heat transfer coefficient. Inside the pipe: the greater the mass flow rate of the medium, the higher the heat transfer coefficient of the inner wall ； The lower the viscosity of the medium, the greater the heat transfer coefficient of the inner membrane. Although the thermal intensity of convective tubes is influenced by various factors both inside and outside the tubes, when the interior contains a liquid, it is primarily the external conditions that govern the overall heat transfer coefficient; the internal conditions have little effect. 2. What does the height-to-diameter ratio of a cylindrical furnace mean? The height-to-diameter ratio of a cylindrical furnace is the ratio of the effective length of the radiation tubes (excluding the length of the sharp bends) to the diameter of the coil’s torus. The height-to-diameter ratio of conventional cylindrical furnaces is generally no more than 2.75. The height-to-diameter ratio is too large, resulting in uneven heat transfer up and down the furnace tube ； If the height-to-diameter ratio is too small, the floor area increases, the volume of the furnace chamber grows, and the volumetric heat intensity in the radiation chamber decreases, which is unfavorable for heat transfer. 3. What are the requirements for a proper design of a heating furnace? A properly designed heating furnace must meet the following requirements: (1) The heat intensity on the surface of the radiant tubes should be at an optimal level. (2) The furnace volume should be small. (3) The heat transfer distribution ratio between the radiation zone and the convection zone should be appropriate, that is, the direct radiation coefficient should meet the recommended values. When the furnace structure differs, at the same flue gas exit temperature in the radiation zone, a more reasonable furnace structure can yield a higher direct radiation coefficient. At the same flue gas exit temperature in the radiation range, the direct radiation coefficient of a cylindrical furnace is approximately 0.5 to 0.55. (4) The structures inside the furnace, other than the furnace tubes, should be simple. (5) The selection of the furnace tube’s dimensions, wall thickness, material, etc., should be reasonable. 4. What are the principles for designing the radiation chamber of a heating furnace? When designing a cylindrical furnace, the dimensions of its radiation chamber should meet the following requirements: (1) The ratio of the furnace height to the theoretical flame length should be between 1.5 and 2. ①Large furnace, large burner; this ratio is often close to 1.5. ②Small furnace, small burner; this ratio is often close to 2. The flame lengths of various burners are shown in Table 3-4. Table 3-4 Flame length of various burners: Fuel consumption/(kg/h), Flame length/m; Fuel consumption/(kg/h), Flame length/m: 60, 100, 200 – 2.2, 2.5, 3.8; 300, 400, 500 – 6, 10, 12.5. (2) The height-to-diameter ratio is within the ideal range. Based on the designed thermal load of the heating furnace, the ratio L/D of the effective length of the radiant tubes to the pitch diameter of the coiled tubes is selected with reference to Table 3-5. Table 3-5: Ratio of the effective length of the radiant tubes in a cylindrical furnace to the diameter of the coil’s torus, L/D; Design heat load of the heating furnace / MWL/D: Maximum, Minimum ≤33～6; >6: 2.0, 2.5, 2.75, 1.5, 1.5, 1.5. (3) The distance between the centerline of the burner and the centerline of the furnace tubes is greater than the safe value. The distance between the center of the burner and the center of the furnace tube must not be less than the values shown in Table 3-6. Table 3-6 Minimum distances between various burners and the center of the furnace tube. Fuel consumption/(kg/h) Minimum distance between the burner center and the furnace tube center/(mm) Fuel consumption/(kg/h) Minimum distance between the burner center and the furnace tube center/(mm) 60 100 200 1000 1250 1400 300 400 500 1500 1600 1700 (4) It is advisable that the number of burners and the number of furnace tube channels be in an integer multiple relationship to each other. (5) Under the same tube array area, it has the smallest total external surface area of the radiation chamber compared to other solutions. (6) The number of radiation tubes should be even to ensure that the raw material inlets and outlets are above the furnace top. (7) The actual number of furnace tubes should be an integer multiple of the number of tube passes. 5. Why are the inlets and outlets of the radiant chamber furnace tubes generally located above the furnace top rather than at the bottom? In the heating furnaces used in petrochemical plants, the inlets and outlets of the radiant chamber furnace tubes are in most cases placed above the furnace top; in a few cases, they are located at the bottom for various reasons. The reason is that the space above the furnace top is larger than that below the furnace bottom, which facilitates the installation and maintenance of pipelines. If the inlets and outlets of the furnace tubes in the radiation chamber are placed at the furnace bottom, it not only makes the installation and maintenance of pipelines difficult, but more importantly, it also hinders operation. 6. What are the design principles for the convection chamber of a heating furnace? When designing the convection chamber of a heating furnace, the following requirements must be met: (1) The heat intensity on the surface of the convection tubes should be at an optimal level. (2) The volume of the convection chamber should be small. (3) The selection of the dimensions, wall thickness, material, etc., of the convection tube should be reasonable. (4) The number of passes in the convection chamber tubes is the same as that in the radiation tubes. (5) When determining the length of the convection tube, the following should be considered: ① Select it in accordance with the regulations regarding the length of the furnace tube. ② It must not prevent the radiation tubes from being pulled upward; an access hole should be provided at the top of the radiation chamber to facilitate the maintenance of the radiation tubes. (6) When determining the width of the convection chamber, the following should be taken into account: ① The number of tubes in each row should be an integer multiple of the number of convection passes. ② The width of the convection chamber must not exceed the side length of the square inscribed in the pitch circle diameter D of the radiation chamber (i.e., less than 0.707D), in order to keep the convection chamber rectangular. (7) When determining the height of the convection chamber, it should be taken into account that the distance between the center of the top row of furnace tubes and the lining at the top of the convection chamber must not be less than the outer diameter of the furnace tubes. (8) Convection tubes are generally arranged in an equilateral triangle pattern. (9) The center-to-center distance between convection tubes shall be selected according to the following requirements: ① When smooth tubes are used, the center-to-center distance should be 1.5–2d_out (where d_out is the outer diameter of the convection tube). ②When nail-head tubes or finned tubes are used, the tube center distance is 2d. 7. What are the principles for designing coil structures? The design of coil structures should comply with the following principles: (1) In multi-pass piping systems, the hydraulic and thermal parameters of each layer should be balanced. (2) The calculated temperature of the inner wall in the radiation section, shielding section, or convection section shall not exceed the maximum allowable temperature for the medium being heated. (3) The ratio of the cross-sectional area inside the manifold to the total cross-sectional areas of the branch pipes should be 1.2–1.5 when the medium flowing through the pipes is a liquid ； For gases, it must not be less than 1. (4) The furnace tubes in the shielding section should be made of transparent tubes. (5) The design of the coil should be determined based on the expansion amount during normal operation and when steam-air burning occurs during shutdown, with sufficient expansion space provided ； For furnace tubes operating at high temperatures, measures should be taken to prevent them from bending. (6) The center-to-center distance of the coiled tubes shall comply with the provisions in Table 3-7. For special needs, other tube center distances can also be used. Table 3-7 Center-to-center distance between coils: Outer diameter of furnace tubes / m, Center-to-center distance / mm. Outer diameter of furnace tubes / mm, Center-to-center distance / mm: 60, 76, 89, 102, 114, 127, 120; 150, 130; 152, 150; 178, 172; 203, 203; 230, 215; 250, 141, 152, 168, 180, 219, 273, 254; 282, 275; 304, 304; 336, 324; 360, 372; 438, 478; 546. (7) The minimum distance from the centerline of the radiant tubes to the inner surface of the furnace wall should generally be 1.5 times the outer diameter of the tubes. (8) When the same medium is present inside both the shielding tube and the radiation tube, the design requirements for the shielding tube shall not be lower than those for the radiation tube ； If another medium is present inside the shielding tube, detailed calculations should be carried out. (9) The maximum effective length of the radiant vertical tubes in a cylindrical furnace should generally not exceed 18 m; for horizontal tube-type furnaces with burners installed on the end walls, the maximum effective length of the radiant tubes should not exceed 12 m. (10) When rolled tubes are used for the radiant tubes and shielding tubes, welding to increase the length is permitted if the designed tube length exceeds the maximum normal length specified in the relevant standards for furnace tubes. However, the number of joints should be minimized, and they should be located in the cooler areas of the furnace. (11) The distance at which the ends of the convection tubes protrude from the outer surfaces of the tube sheets may be 150 mm, but under no circumstances shall it be less than the dimensions specified in Table 3-8. Table 3-8 Distance A at both ends of the furnace tubes beyond the outer surface of the tube sheets; Wall thickness of furnace tubes / mm: 68, 10, 12, 14, 16, 18, 20; A / mm: 110, 120, 130, 140, 150, 160, 165, 170. (12) The ratio of the effective length of the radiant tubes in a cylindrical furnace (excluding the length of the sharp bends) to the diameter of the coil’s pitch circle should generally not be greater than 2.75. 8. What is the typical smoke flow velocity in the convection chamber? When calculating the cross-sectional area of the convection section, the mass flow velocity of the smoke in this section should not exceed the range of 1.5–4 kg/(m2·s). When using light pipes, a value of 1.5–2 kg/(m2·s) is generally adopted ； When nail-head or finned tubes are used, a value of 2–4 kg/(m2·s) is generally adopted. The greater the mass flow rate of flue gas in the convection section, the higher the convective heat transfer coefficient, and the less convective area is required. However, as the flow velocity increases, the smoke resistance in the convection section rises, and a taller chimney is required for natural ventilation; therefore, the flow velocity of the smoke stream should be chosen appropriately without compromising one aspect at the expense of another. If high chimneys are used to prevent pollution, there is sufficient draft in the chimney, allowing the convection chamber to use higher flow velocities. 9. What are the principles for designing the chimney of a heating furnace? When designing a chimney, its height must meet the following requirements: (1) Under the designed excess air coefficient and maximum heat load conditions, the negative pressure at any point inside the furnace should not be less than 20 Pa. (2) It shall comply with environmental protection regulations. (3) The minimum height of the chimney shall be at least 3 m higher than the highest operating platform or building within a 15-m radius around it. 10. What is the typical value for the flue gas velocity inside a chimney? The usual values for the flue gas velocity inside a chimney are as follows: (1) In cases of natural ventilation, it generally should not exceed 5–8 m/s. (2) During forced ventilation, it should generally not exceed 10–20 m/s.