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1. What is heat absorption? Heat absorption refers to the total amount of heat absorbed by the coil, including the heat used to preheat the combustion air. 2. What is heat release? Heat release refers to the total amount of heat released by a certain fuel, calculated based on its lower calorific value. 3. What is temperature margin? Temperature margin refers to the amount of extra temperature that needs to be taken into account when selecting the design metal temperature. It usually includes factors such as uneven distribution of process fluid or flue gas flow, unknown operating factors, and design errors. The designed metal temperature typically includes the highest calculated tube wall metal temperature or the equivalent tube wall metal temperature plus a temperature margin. 4. What is fouling margin? Fouling margin refers to the residual layer resulting from the accumulation of soot and fouling substances on the inner surface of the furnace tubes, which causes an increase in pressure drop and an elevation in wall temperature. It can be expressed in terms of thickness or thermal resistance, and this value should be used when calculating the pressure drop after fouling occurs and when designing the wall temperatures. 5. What is corrosion margin? Corrosion margin refers to the thickness allowed for corrosion; it is the product of the corrosion rate and the design life of the pipe, and is expressed in \"millimeters (mm)\“. 6. What is the wear rate? The wear rate refers to the amount of thinning of the pipe wall thickness caused by the mechanical erosion of the process fluid, and it is expressed in “mm/year”. II. Furnace Types 7. How are heating furnaces classified? At present, there is no unified classification method for heating furnaces either domestically or internationally; conventionally, two methods are most commonly used: one is to classify them based on their external shape, such as box furnaces, inclined-roof furnaces, cylindrical furnaces, vertical furnaces, etc ； Another classification is based on their industrial applications, such as atmospheric furnaces, reduced-pressure furnaces, catalytic furnaces, coking furnaces, hydrogen production furnaces, asphalt furnaces, etc. In addition to the above classifications, there are also those categorized by the number of chambers, such as double-chamber furnaces, three-in-one furnaces, multi-chamber furnaces, etc ； Pure radiation furnaces, pure convection furnaces, convection-radiation furnaces, etc., classified by heat transfer method ； Single-sided radiant furnaces and double-sided radiant furnaces, etc., classified according to different heating methods. 8. A brief overview of the development of tubular heating furnaces and their future development direction. Box-type furnaces (Figure 2–1) and inclined-roof furnaces (Figure 2–2) were commonly used in petroleum chemical plants in China during the 1950s. Due to their large floor space, complex structure, high metal consumption, and high cost, these types of furnaces have been gradually phased out. After the 1970s, the furnace types widely used in China’s petrochemical plants were vertical furnaces (Figures 2–3, 2–4) and cylindrical furnaces (Figure 2–5). The future development direction of tubular heating furnaces is toward larger size and higher efficiency; the use of various types of waste heat recovery systems to improve the furnace’s thermal efficiency, the adoption of tall chimneys to prevent environmental pollution, as well as the use of high-power burners and long operating cycles. 9. What are the common types of heating furnaces in use today? The commonly used heating furnaces include cylindrical furnaces, vertical furnace with vertical tubes, and vertical furnace with horizontal tubes. 10. What principles and requirements should the design of tubular heating furnaces meet? The design of tubular heating furnaces should be based on the requirements of the processing operations, the need for long-term operation, ease of maintenance, and low investment costs. It should also take into account the site conditions and the type of waste heat recovery system. Additionally, such furnaces must meet the following requirements: (1) When the designed heat load is less than 1 MW, a pure radiation cylindrical furnace is preferred. (2) When the design heat load is (1–30) MW, a radiant-convection type cylindrical furnace should be given priority. (3) When the designed heat load is greater than 30 MW, cylindrical furnaces, vertical furnaces, box-type furnaces, or other furnace types with intermediate tubes in the furnace chamber should be selected through comparison. (4) When the medium to be heated tends to coking, a horizontal-tube vertical furnace is advisable. (5) When the flow rate of the medium to be heated is low and a small pressure drop is required, a spiral-tube cylindrical furnace is suitable. (6) When the flow rate of the medium to be heated is high and a low pressure drop is required (such as in reformers), a gate-type tube box furnace is suitable. (7) When using furnace tubes with expensive material costs, furnace types with double-sided radiant tube arrays should be given priority.