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1. How to reduce the excess air coefficient? (1) Seal all leaks thoroughly – seal up any areas that are not airtight in burners that are not in use, manholes, inspection ports, explosion-proof doors, as well as convective and radiative elbow boxes – in order to minimize air leakage inside the furnace. (2) Control the chimney dampers to ensure that neither positive pressure nor excessive negative pressure occurs inside the furnace. Generally, heating furnaces are not designed and constructed for positive pressure conditions. Positive pressure inside the furnace can cause high-temperature flue gases to leak out, increasing heat loss; it may also overheat or damage the steel structures outside the furnace, leading to other accidents. The greater the negative pressure inside the furnace, the more air leaks in. (3) Improve the burner by using ones that are properly manufactured and have good performance; the position of the oil gun should be adjusted appropriately in all directions to ensure that the air entering the burner mixes thoroughly with the fuel. By using forced-air burners to increase the inlet air velocity and improve the air supply method, it is also possible to reduce the excess air coefficient. (4) Ensure that the viscosity of the fuel oil is within an appropriate range. 2. What is generally used as a basis for determining the flue gas temperature of a heating furnace? Generally, it is determined based on the temperature of the medium flowing through the pipes entering the furnace. If the temperature of the medium entering the furnace is 250°C, then the exhaust temperature of the furnace must be higher than 250°C in order to transfer the heat from the flue gases to the medium. The higher the flue gas exhaust temperature of the furnace, the greater the temperature difference between the flue gas and the medium; as a result, fewer convection tubes are needed in the furnace, but the thermal efficiency is lower. In the past, the temperature difference between the exhaust gas temperature and the temperature of the medium fed into the furnace was mostly above 100°C. Due to the rising fuel prices, and in order to reduce fuel consumption, this temperature difference is being continuously reduced. Some designs use pinched tubes or finned tubes to bring this value down to 50°C. When a waste heat recovery system is used, the minimum flue gas temperature is determined based on low-temperature corrosion conditions. 3. What impact does the heat loss due to incomplete chemical combustion have on the thermal efficiency of a heating furnace? The heat loss resulting from incomplete chemical combustion is manifested in the presence of CO and H2 in the exhaust gases. Since methane (CH4) is more difficult to burn than other higher-carbon hydrocarbons, small amounts of CH4 may still be present in some cases. Since analyzing H2 in flue gas is difficult, only CO is generally analyzed. According to tests conducted by relevant domestic institutions, when flue gas contains CO, it necessarily also contains H2. When CO < 1%, CO/H2 = 3.6 ； When CO > 1%, CO/H2 = 2. According to calculations, when the excess air coefficient α = 1.2, if the volume fraction of CO in the flue gas is 0.2%, the thermal efficiency of the furnace decreases by 1%. In the past, some petrochemical plants found that when the CO content was 1.5%, the thermal efficiency of the furnace would decrease by about 7%. Therefore, the heat loss caused by incomplete combustion in chemistry is considerable, and sufficient attention must be paid to it during operations. 4. What is the impact of heat losses due to mechanical incomplete combustion on the thermal efficiency of a heating furnace? Heat losses resulting from mechanical incomplete combustion are manifested in the presence of carbon black particles in the exhaust gases. The heat loss due to incomplete combustion in oil-fired heating furnaces is generally small, and it was previously considered negligible. However, recent studies suggest that under abnormal combustion conditions, this value cannot be ignored under certain circumstances. For example, when a factory’s power plant burned Renqiu residue oil, the carbon black concentration in the flue gas was 1616 mg/Nm3, with heat losses of 15%; after adopting water-emulsified combustion, the carbon black concentration was reduced by over 90%. Additionally, incompletely burned carbon black particles may contaminate the heat transfer surfaces, affecting heat transfer. The release of carbon black particles into the atmosphere also pollutes the environment and causes various harms. 5. What is the impact of heat loss through the furnace walls on the thermal efficiency of a heating furnace? It is quite difficult to calculate the heat loss due to the furnace walls accurately; for simplicity, such calculations are generally not carried out, and instead a fixed value of 2% to 3% is assumed. The heat loss from the furnace wall of a cylindrical furnace is generally assumed to be 2%, and it does not exceed 3% in the case of a waste heat recovery system. Lowering the furnace wall temperature can reduce heat loss and ensure safety, but it increases material costs; therefore, it is necessary to determine an appropriate temperature for the furnace’s outer wall in order to strike a balance between these two factors. 6. What are the advantages of using pinhead tubes in the convection chamber? Using pinhead tubes in the convection chamber allows for an increase in the heating surface area, thereby improving the thermal efficiency of the furnace. The surface area of a finned tube is generally 2-3 times that of a smooth tube. The quality of the nail-head tube is 1.5 to 2 times that of the plain tube, while its heat resistance is 2 to 3 times that of the plain tube. Another advantage of using nail-head pipes is that it can increase the pipe wall temperature, thereby reducing low-temperature dew point corrosion. When refining crude oil with a high sulfur content, alloy materials must be used for the furnace tubes, but carbon steel studs can be used on the outside of the tubes (when the temperature of the studs is below 470°C). 7. What does low-temperature dew point corrosion mean? What factors are related to it? When fuel burns, hydrogen (H2) and oxygen (O2) in it combine to form water vapor (H2O), and since most burners use steam for atomization, the flue gases in the furnace contain a large amount of water vapor. Furthermore, sulfur (S) in the fuel generates sulfur dioxide (SO2) upon combustion; a small amount of this SO2 is further oxidized to sulfur trioxide (SO3), which combines with water vapor in the flue gases to form sulfuric acid (H2SO4). The dew point of flue gas containing sulfuric acid vapor rises significantly; when the wall temperature of the heating surface is below this dew point, the sulfuric acid-containing vapor condenses on the heating surface to form a sulfuric acid-containing liquid, causing severe corrosion to the heating surface. Since it occurs on the heated surface at lower temperatures, it is called low-temperature corrosion. Since this type of corrosion occurs only after dew forms on the heated surface, it is also known as dew point corrosion. The level of dew point temperature is related not only to the sulfur content in the fuel, but also to factors such as the excess air coefficient and the amount of sulfur trioxide generated. The higher the furnace temperature and the less excess air, the smaller the proportion of SO2 generated from sulfur during combustion that is oxidized to SO3, resulting in a lower dew point temperature. This is why the relationship between dew point temperature and fuel sulfur content provided in general data is not exactly the same. Based on the sulfur content of fuels in our country, the dew point temperature generally ranges from 105 to 130°C. When possible, it is best to use the dew point temperature for actual measurements on-site. During operation, if the wall temperature at the surface in contact with soot becomes lower than the dew point due to heating, in addition to corrosion, it will cause the soot to adhere to that heated surface; such sticky ash buildup is difficult to remove using conventional soot-blowing methods. The presence of ash not only affects heat transfer efficiency and increases the flow resistance on the flue gas side, but it also accelerates corrosion; in severe cases, metal corrosion products and ash can block the passages. Therefore, when burning sulfur-containing fuels, it is very important to take measures to keep the temperature of the metal in contact with the flue gas above the dew point. Additionally, the factors that affect the corrosion rate include the concentration of sulfuric acid and the wall temperature. The corrosion rate of steel by concentrated sulfuric acid is very low, while the corrosion rate of carbon steel by sulfuric acid is highest at a concentration of around 50%. Regarding wall temperature, at higher temperatures, the chemical reaction rate increases, which in turn accelerates corrosion. Therefore, due to the differences in sulfuric acid concentration and wall temperature at various low-temperature areas, the corrosion rates vary. To reduce corrosion caused by low-temperature dew points, it is most important to keep the wall temperature of the tubes or heating elements above the dew point, or to use corrosion-resistant materials. Raising the wall temperature can be achieved by increasing the temperature of the medium outside or inside the tube; for example, the inlet temperature of the low-temperature oil to the furnace should be above 100°C. Heat air circulation should be used in air preheaters, or other media can be employed to raise the inlet air temperature to above 60°C. Additionally, reducing excess air and using detachable structures in areas with low temperatures are also effective measures that are often applied. V. Furnace Tubes 8. What are the common diameters of furnace tubes? The furnace tubes used in heating furnaces typically have the following diameters (in millimeters): ϕ60, ϕ89, ϕ102, ϕ114, ϕ127, ϕ152, ϕ219. For ϕ219 furnace tubes, an economic comparison should be conducted before use to determine their suitability; under normal circumstances, they are not recommended for use. 9. What are the common lengths of furnace tubes? The common lengths (in mm) for furnace tubes in heating furnaces are: 2000, 2500, 3000, 3500, 4000, 4500, 6000, 8000, 9000, 10000, 12000, 14000, 15000, 16000, 18000. 10. What are the requirements regarding the typical length of furnace tubes? The maximum effective length of the radiant tubes in a cylindrical furnace should generally not exceed 18 m, while 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.