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1. What are the advantages and disadvantages of the commonly used cylindrical furnaces and horizontal-tube vertical furnaces? (1) Cylindrical furnaces: At present, cylindrical furnaces are the most widely used in petroleum chemical plants in China; they account for about 65% of all heating furnaces. The advantages and disadvantages of this type of furnace are as follows: ① Advantages: Small floor space required； It has a simple structure, and its design, manufacturing, and installation are all relatively convenient ； The smaller the thermal load of the furnace, the greater the advantages of this type of furnace, which is why cylindrical furnaces are more commonly used in small and medium-sized furnaces. ②Disadvantage: Not suitable for heating furnaces with high heat loads ； Since the furnace tubes are vertical, heat transfer is uneven between the top and bottom, resulting in a lower average heat intensity of the radiant furnace tubes compared to those in vertical furnaces with horizontal tubes. (2) Horizontal tube vertical furnace: This type of furnace is currently commonly used in coking plants. Its advantages and disadvantages are as follows: ① Advantages: Since the furnace tubes are placed horizontally, heat transfer is relatively uniform, and the average heat intensity on the radiant furnace tubes is greater than that in a cylindrical furnace ； The flue gas flows upward with minimal resistance loss, **which reduces the height of the chimney and eliminates the need to build a chimney outside the furnace. ②Disadvantage: The structure is more complex than that of a cylindrical furnace ； The furnace chamber is small, making tempering likely to occur ； Radiant tube heaters have a small heating area and low thermal efficiency; alloy tube frames are commonly used, resulting in higher costs compared to cylindrical furnaces. 2. What criteria are used to determine the size of a heating furnace? The size of a heating furnace is not determined by its diameter and height, but rather by the amount of heat load it can handle. 3. Why is a heating furnace divided into a radiation chamber and a convection chamber? (1) The radiation chamber of a heating furnace serves two purposes: one is to act as a combustion chamber ； Secondly, the flame ejected by the burner, the high-temperature flue gas, and the radiant heat from the furnace wall are transferred to the medium through the furnace tubes. This type of furnace relies mainly on radiation heat transfer within the radiation chamber ； A small portion relies on convective heat transfer from the convection chamber, which accounts for only about 10% of the total heat transfer. (2) The main function of the convection chamber is to transfer heat from the high-temperature flue gas inside it to the medium within the furnace tubes through convection. There is also a very small amount of heat transfer due to flue gas and furnace wall radiation inside the convection chamber. If a heating furnace has only a radiation chamber and no convection chamber, the flue gas temperature becomes very high, resulting in energy waste, increased operating costs, and reduced economic efficiency. Therefore, when designing heating furnaces, a convection chamber is usually included to allow for the effective recovery of heat from the flue gases. 4. Why are radiation-convection type furnaces commonly used, while pure convection furnaces are generally not employed? Pure convection furnaces require a large heat transfer area and a large amount of tubing; the tubes located near the furnace chamber are exposed not only to radiant heat but also to high-temperature convective heat, which can cause the tubes to get damaged easily. Additionally, the oil inside the tubes tends to coking, so pure convection furnaces are not typically used, in favor of radiation-convection type furnaces. In this type of furnace, the flue gas first receives a large amount of radiant heat in the radiation chamber, which reduces its temperature to some extent; afterward, it enters the convection chamber, where it absorbs the remaining heat from the flue gas, thereby improving the furnace’s thermal efficiency. 5. Why are pure radiation furnaces more commonly used in small cylindrical furnaces? When the design heat load is less than 1 MW, a pure radiation cylindrical furnace is suitable. Because the small pure-radiation cylindrical furnace has the simplest structure and requires only a low chimney. If a convection chamber is used, although the tubes are arranged in a furnace, the convection tubes are too short, resulting in a small heating area; yet the pressure drop inside the tubes increases significantly ； Furthermore, due to the low thermal load of the furnace, the amount of heat carried away by the exhaust smoke is limited, resulting in little waste; therefore, pure radiation furnaces are commonly used in small cylindrical furnaces. With rising fuel prices and increasing energy-saving requirements, the commonly adopted method nowadays is to collect the flue gases from small furnaces in order to recover waste heat. 6. What are the differences between vertical furnaces with vertical radiation tubes and those with horizontal radiation tubes? In vertical furnaces with vertical radiation tubes, the radiation tubes are upright; they are generally quite long. The burner is located at the bottom, similar to the radiation tubes in cylindrical furnaces. Heat transfer up and down along the furnace tubes is not uniform, and the average heat intensity of the radiation tubes is relatively low. For heating furnaces with high heat loads, in order to reduce the floor space required, they are designed to be taller; generally, vertical tube vertical-type furnaces are used. The radiant tubes in a horizontal tube vertical furnace are horizontal; generally, the height of the radiant chamber is lower than that in a vertical tube vertical furnace. Burners can be of bottom-firing type or side-firing type. This type of furnace has a low radiation chamber, with an upper section designed in a sloping shoulder shape; it features uniform radiation heat transfer, a high average heat intensity of the furnace tubes, and a high temperature at the fire wall. 7. What are the advantages and disadvantages of using vertical pipes and horizontal pipes in the radiation chamber of a heating furnace? The advantage of vertical pipes is that they require less high-alloy steel for construction and occupy less space ； The drawback is gas-liquid separation; some plants have found that when the medium flows downward, the furnace tubes are prone to damage due to this reason. That’s why horizontal tubes are generally used in coking furnaces and viscosity-reduction furnaces. The advantage of horizontal tubes is that the gas and liquid flow within the furnace tube is uniform, making separation difficult ； The disadvantages are: a large amount of intermediate pipe supports are required for high-alloy steels; space must be reserved outside the furnace for tube extraction, resulting in a large footprint. 8. What type of furnace is commonly used for heating furnaces with high heat loads? For heating furnaces with a heat load greater than 58 MW, vertical tube vertical furnaces should be employed; the furnace tubes in the radiation chamber are not only arranged along the furnace walls but also placed appropriately in the middle of the furnace chamber as well, so as to make full use of the furnace space and reduce the size of the furnace. Currently, the largest heating furnace abroad has a thermal load of over 232 MW, and it uses a vertical tube vertical furnace design. The largest heating furnace that has been built and put into operation in our country has a heat load of 52 MW; it is a cylindrical furnace with furnace tubes arranged in two circles around a radiation chamber. 9. What is the function of the radiation cone in a cylindrical furnace? Radiation cones were once used at the upper part of the radiation chamber in cylindrical furnaces; their purpose was to increase the heat transfer rate in the upper section of the radiation tubes, thereby reducing the unevenness in heat transfer between the upper and lower parts of these tubes. Later, as the thermal load on rotary kilns continued to increase, the use of radiation cones proved disadvantageous in terms of structure, investment, and the amount of high-alloy steel required. Moreover, the benefits brought about by radiation cones were not clearly quantifiable. Consequently, in recent years, newly designed rotary kilns no longer employ radiation cones. 10. What is the difference between single-sided radiant furnace tubes and double-sided radiant furnace tubes? Single-sided radiant furnace tubes are those located against the furnace wall inside the radiation chamber; they are exposed to the radiant heat from the flame and high-temperature flue gases on one side, as well as the reflected heat from the furnace wall on the other side. Double-sided radiant furnace tubes are arranged in the middle of the furnace chamber (in one or two rows), being exposed to radiation from flames and hot flue gases on both sides. A single-row tube with dual-side radiation achieves more uniform heat transfer compared to one with single-sided radiation; it requires fewer tubes, but the furnace is larger in size and demands more structural steel. It is only used when the furnace tubes are expensive. The uniformity of heat transfer in double-row tubes with dual radiation and single-row tubes with single radiation is almost the same; it should not be assumed that dual radiation in double-row tubes is superior to single radiation in single-row tubes.