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1. What is a natural circulation boiler? A natural circulation boiler is a type of boiler in which the circulation of the working fluid within the evaporation system occurs naturally, driven solely by the difference in density between steam and water. 2. What is a boiler’s circulation loop? The closed loop composed of the boiler’s drum, downcomer, header, water wall, and steam-water ducts is called the boiler’s circulation loop. 3. What equipment makes up the evaporation system of a natural circulation boiler? It is mainly composed of a drum, downcomer tubes, water wall tubes, headers, and ducts. 4. Why does the water wall need to be divided into multiple circulation circuits? Due to the uneven distribution of heat load along the width and depth of the furnace, the water-cooled wall tubes on each side are exposed to uneven heating; as a result, the tubes in the middle part are heated the most, while those at the edges are heated less. If the water-cooled walls covering an entire wall form only one circulation loop, then among the parallel water-cooled walls, the tubes subjected to greater heat absorption have a higher circulation flow rate, while those with less heat absorption have a lower flow rate; this results in poor cooling of the tube walls. To reduce uneven heating among the parallel water wall tubes and improve the safety of water circulation in these tubes, the water walls on each side of the boiler are usually divided into several circulation circuits. 5. How many circulation circuits does the SG400/140-50410 type boiler have? How to arrange it? This type of boiler has a total of 14 circulation circuits, with 4 on each of the front and rear walls, and 3 on each of the side walls. 6. What are the main functions of a steam drum? The main functions of the drum are: (1) it serves as a connecting point for the three processes of heating, evaporation, and superheating of the working fluid; simultaneously, it acts as a pressure equilibrium vessel to maintain the head pressure required for the flow of the steam-water mixture in the water wall. (2) It can hold a certain amount of water and steam, and due to the large mass of the drum itself, it possesses considerable heat storage capacity. This allows it to act as a buffer and stabilize steam pressure when the conditions of the boiler change. (3) Install a steam-water separation and steam purification device to ensure the quality of saturated steam. (4) Instrumentation and safety accessories of the device, such as pressure gauges, level gauges, safety valves, etc. 7. What are the main devices inside the steam drum of a power plant boiler? What are their placement locations and functions? Depending on the parameter capacity, the internal components of power plant boilers vary. Here, taking the drums of high-pressure and ultra-high-pressure boilers as examples, their internal components, their arrangement, and their main functions are discussed. Several cyclone separators are installed on both sides along the length of the drum. At the top of each cyclone separator’s cylinder, there is a louvered (wavy plate) separator; their main function is to carry out preliminary separation of steam and water in the steam-water mixture introduced from the rising pipe. In the middle and upper part of the drum, steam cleaning orifice plates are installed horizontally, with a layer of cleaning feedwater above them. When steam passes through this water layer, some of the salts dissolved in the steam or carried by it are transferred into the water, thereby reducing the salt content in the steam. A perforated plate is located near the top of the drum; it distributes the steam rising within the drum evenly and further separates the moisture from the steam. An emergency drain outlet is located at approximately 150 mm below the centerline of the drum ； A continuous blowdown pipe opening is located about 200 mm below the normal water level, with a chemical dosing pipe arranged further below. A cross baffle is also installed at the inlet of the downcomer to prevent vortices from forming at the downcomer outlet, which could cause vapor to enter the downcomer. 8. What is the structure and working principle of a cyclone separator? A cyclone separator consists of components such as a cylinder, an inlet pipe, a top cap, an overflow ring, bottom guide vanes, and a base plate. A cyclone separator is a vapor-water separation device with excellent separation performance. Its working principle and process are as follows: A mixture of steam and water at a high flow rate enters the cylinder tangentially through the inlet pipe, thereby generating rotational motion; under the effect of centrifugal force, the water droplets are thrown against the cylinder wall, allowing for a preliminary separation of steam from water. The separated water flows into the drum water volume through the guide vanes around the bottom of the drum. Saturated steam flows upward inside the cylinder, entering the gaps between the corrugated plates of the top cap where it moves in a tortuous path. Under the influence of centrifugal and inertial forces, small water droplets are thrown onto the corrugated plates; due to adhesion, a water film is formed that flows downward and enters the water volume in the drum, thereby facilitating further separation of steam from water. Meanwhile, saturated steam enters the steam space in the drum from above or around the top cap. 9. What is the structure and working principle of the louver (wavy plate) separator? The louver separator is composed of many parallel wavy thin steel plates; the thickness of these wavy plates ranges from 0.8 to 1.2 mm, the distance between adjacent plates is 10 mm, and they are secured by steel frames that are 2–3 mm thick. The working principle and process are as follows: The steam, after preliminary separation, enters the louver separator and flows in a meandering path between the corrugated plates. The small water droplets in the steam are thrown against the plate wall under the influence of centrifugal force, inertial force, and gravity; due to adhesion forces, these droplets stick to the corrugated plate, forming a water film. Under the effect of gravity, the water film flows downward into the drum water volume, enabling further separation of steam and water. Due to its excellent effectiveness in separating fine water droplets from steam using adhesion, the louver separator is widely used as a fine separation device. 10. How should left-rotating and right-rotating cyclone separators be arranged inside the drum? Why is it arranged this way? Although the cyclone separator enables the separated water to flow smoothly into the drum water volume through the inclined guide vanes at the bottom of the drum, it cannot eliminate its rotational kinetic energy; the rotational motion of the water may cause an imbalance in the drum water level. Therefore, by using an arrangement with left-handed and right-handed cyclone separators placed alternately, the rotational movements of the drainage can be offset against each other, thereby maintaining a stable water level in the steam drum. 11. What is the function and structure of the cleaning device? This device uses a economizer to clean the feed water, and it cleans the steam that has been mechanically separated, causing some of the salts in the steam to dissolve back into the water, thereby reducing the salt content in the steam. There are various types of such cleaning devices, but modern boilers mostly use flat-hole plate-type steam stratum cleaning devices, whose structure consists of multiple flat-hole plates. Each flat orifice plate is drilled with many small holes of 5–6 mm in diameter. U-shaped clamps are installed between adjacent orifice plates, and angle irons are placed between the end plates of the cleaning device and the flat orifice plates to create a reliable water seal and prevent steam from bypassing. 12. What is the working principle of the flat-orifice plate cleaning device? About 50% of the feed water is evenly distributed over the orifice plate through a distribution device; steam passes upward through the small holes in the orifice plate and exits via a cleaning water layer 40–50 mm thick, allowing some of the dissolved salts in the steam to diffuse into the water. The water then overflows the baffle and flows into the water volume. The water layer on the orifice plate is held in place by the pressure difference across the orifice plate, which is caused by the resistance to steam penetration. The recommended speed for steam piercing is 1.3–1.6 m/s, to prevent the formation of a dry orifice area at low loads or excessive carryover of cleaning water at high loads. 13. Where are continuous sewage discharge outlets generally installed? Why? What is the pollution discharge rate? The continuous discharge pipe outlet is generally installed 200–300 mm below the normal water level of the steam drum (i.e., at the “0” position). The water in the pot gradually becomes more concentrated due to continuous evaporation, resulting in the highest salt concentration near the water surface. Therefore, the continuous discharge outlet should be installed in the area where the boiler water concentration is highest, in order to continuously discharge the high-concentration boiler water and replace it with clean feedwater, thereby improving the quality of the boiler water. The wastewater discharge rate is generally around 1% of the evaporation amount. 14. What is the purpose of chemical treatment of the water in the boiler drum? To prevent scaling in the boiler, if hardness in the feed water is to be removed solely through external water treatment of the boiler, a large amount of equipment is required, which significantly increases the investment costs ； Increasing the drainage of water from the boiler not only increases heat loss of the working fluid but also fails to eliminate the residual hardness in the boiler water. Therefore, in addition to external water treatment of the boiler feedwater, chemicals are also added to the boiler water to remove any residual hardness and prevent scaling in the boiler. The method involves adding phosphates to the boiler water, causing the phosphate ions to combine with the calcium and magnesium ions present in the water, thereby forming insoluble precipitates that can be removed regularly. This helps maintain an appropriate level of phosphates in the boiler water, preventing scaling and corrosion while ensuring the quality of the steam. 15. What is the purpose of regular sewage discharge? Where should the sewage outlet be installed? Since the boiler water contains impurities such as rust and sedimentary residues formed by chemical treatment, which settle at the bottom of the water circulation circuit, the purpose of regular drainage is to remove these sedimentary impurities on a regular basis in order to improve the quality of the boiler water. Regular sewage outlets are generally located at the lower header of the water wall or at the bottom of the collective downcomers. 16. What are the main types of water wall? The main types of boiler water wall tubes are as follows: (1) Plain tube water wall ； (2) Membrane-type water wall ； (3) Inner wall spiral groove water-cooled wall ； (4) Pin-type water wall (also known as stud tube water wall). This type of water wall features circular steel bars of a certain length welded to the surface of the bare tubes, to facilitate the laying and fixing of refractory materials. It is mainly used in the burner areas of liquid slag discharge furnaces, cyclone furnaces, and certain solid slag discharge furnaces. 17. What are the advantages of using a membrane-type water wall? There are two types of membrane water walls: one is formed by welding finned tubes that have been rolled into shape, and the other is created by welding flat steel strips between bare tubes. The main advantages are: ① The membrane-type water wall tightly encloses the furnace chamber, providing thorough protection for the furnace walls; as a result, only insulation materials and sealing coatings are required for these walls, rather than refractory materials. This simplifies the structure of the furnace walls and reduces the overall weight of the boiler ； ②The furnace has good airtightness, resulting in minimal air leakage, which reduces heat loss through exhaust gases and improves the boiler’s thermal efficiency ； ③It is easy to manufacture into large assemblies for water wall panels, thus enabling fast and convenient installation. 18. How is the flame bend angle formed? What is its structure? The flame deflection angle is formed by the water-cooled wall on the rear wall bending inward into the furnace at a certain elevation and in a specific shape (commonly known as the flame deflection nose). There are two types of structural designs, as shown in the attached drawings. One method involves using branching tubes to divide each water wall tube into two paths, with one path bending inward to form a certain shape. The other path is a vertical short tube, which serves to suspend and transfer the weight of the water wall components. The steam-water mixtures in both pipelines flow into the upper header on the rear water wall, and then are introduced into the steam drum through ducts. To allow most of the working fluid to pass through the heavily heated flame bend tubes, throttle orifice plates are installed at the connection between the vertical short tubes and the header to limit the flow rate in the vertical tubes. Another configuration involves the upper part of the rear wall water-cooling tubes bending inward to form a flame deflection corner; behind this corner, one out of every three tubes extends vertically upward as a suspension tube for the rear wall water-cooling walls, while the other two continue extending backward to form the inclined bottom of the horizontal flue, before bending upward again to enter the upper header. The vertical water wall pointing upward transfers the weight of the rear wall water wall assembly, which is part of the flame deflection structure, through the tie rods that connect the vertical water wall tubes before and after the flame deflection angle, and directs it upward to the upper header. (a) Old furnace type flame deflection angle ； (b) Burner tip angle for new furnace type 1 – Steam-water mixture outlet pipe ； 2 – Intermediate header ； 3 – Throttling orifice plate ； 4 – Vertical short tube 5 – Branching tube ； 6. Flame tube at 10% discount 19. What are the functions of the flame deflection angle? (1) The length of the horizontal flue can be increased to facilitate the layout of the heating surfaces in high-pressure and ultra-high-pressure large-capacity boilers (such as screen superheaters, etc.). (2) The flue gas flow path was added to enhance flue gas mixing, resulting in a more uniform distribution of the flue gas along the height of the flue. (3) It can improve the scouring characteristics of flue gas on the screen superheater, thereby enhancing heat transfer efficiency. 20. How are cold flues formed? What is its function? In the combustion chamber of solid slag discharge boilers, a cold ash hopper is formed by the lower parts of the water wall on the front and rear walls bending inward. Its main function is to gather, cool, and automatically discharge ash, and it also facilitates the connection and sealing between the lower header and the ash pit. 21. What is the structure of the bottom steam heating device? In this device, steel pipes are placed inside the lower header along its length; small holes with a diameter of 5 mm are made in these pipes (the number of holes corresponds to the number of water wall tubes), and they are connected to the steam pipes coming from outside. When in use, the amount of steam entering is controlled by valves. 22. What is the principle of natural circulation? When the boiler is at cold conditions, the water filled within the evaporation system below the water level of the drum remains stationary. When the rising tube is heated inside the boiler, some of the water turns into steam, resulting in a vapor-water mixture with lower density. The downcomer is not exposed to heat outside the furnace, and the water inside it has a higher density. As a result of the difference in densities between these two fluids, a driving force is generated. The steam-water mixture flows upward within the water wall, passing through the upper header and tubes to reach the steam drum. Meanwhile, the water coming from the steam drum flows downward through the lower header and is supplied back into the water wall. This continuous cycle of flow constitutes natural circulation, as shown in the accompanying diagram. 23 What is the cycle multiplier? The ratio of the circulating water volume G entering the rising tube in the circulation loop to the steam volume D at the outlet of the rising tube is called the circulation ratio. It is denoted by the symbol K: K=24. What is circulating water velocity? The circulating water velocity refers to the flow velocity in the circulation loop, at the inlet section of the rising pipe, converted to the density of saturated water under the operating pressure. 25. What is the self-compensation ability of natural circulation? Within a certain range of circulation rates, the property whereby, as the heat absorption by the water wall in a natural circulation loop increases, the circulation volume increases correspondingly along with the steam production to compensate for this ； It is a self-compensating ability called natural circulation. 26. What are the main faults in natural circulation? The main faults in natural circulation include circulation stagnation, backflow, vapor-liquid stratification, steam in the downcomer, and deterioration of boiling heat transfer. 27. Under what conditions does the water cycle come to a standstill? What are the harms? Stagnation of the water circulation tends to occur in some water-cooled wall tubes that are less heated. When the hydrostatic head in these tubes is equal to or close to the common pressure difference throughout the circuit, water hardly flows within them; only a small number of bubbles formed rise slowly upward through the water and into the steam drum. When the amount of water entering the rising tubes is equal to the amount of steam exiting, circulation stagnation occurs. When the water circulation stops, the flow rate of water in the lower water-cooling wall tubes approaches or becomes zero; as a result, heat transfer relies mainly on conduction, even when the heat load is low. Since heat cannot be removed in time, the tube walls may still overheat and get damaged. Furthermore, as the water continues to \"evaporate,\" the salt concentration in the water increases, which can lead to salt deposition and corrosion on the pipe walls. When circulation stops in the riser introducing steam into the drum space. A \"free water level\" will form inside the rising pipe; the part of the pipe above the water surface is filled with steam, and deteriorating cooling conditions can lead to overheating and pipe explosion ； At the water-vapor interface, due to fluctuations in water level, the pipe wall is prone to fatigue damage under alternating thermal stresses. 28. Under what conditions does backflow in the water cycle occur? What are the harms? The backflow phenomenon in the water cycle occurs when the rising pipe leads directly into the drum water space, and the pipe is heated so little that its hydrostatic pressure difference exceeds the common pressure difference of the circuit. When the upward flow velocity of the steam bubbles in the reverse flow tube approaches the velocity of the backflowing fluid, the bubbles cannot be carried away. The bubbles that remain stationary or move slowly gradually accumulate and grow, forming a vapor plug. This causes the temperature of the pipe wall to rise or to fluctuate, leading to overheating or fatigue damage. 29. Under what conditions does vapor-liquid separation occur? Why? Steam separation tends to occur in pipes that are horizontal or have a low slope, as well as in pipes where the flow velocity of the steam-water mixture is too low. This is because of the different densities of steam and water: steam tends to flow in the upper part of the pipe, while water, with its higher density, flows in the lower part. If the flow rate of the soda mixture is too low, the mixing effect caused by disturbances is less than the separation effect, resulting in a stratification of the soda. Therefore, the water wall of natural circulation boilers should avoid a layout with little horizontal and inclination. 30. What are the advantages of large-diameter downcomers? Using large-diameter downcomers can reduce flow resistance, which is beneficial for water circulation. Furthermore, it simplifies the layout, saves steel, and reduces the number of openings in the steam drum. 31. What are the reasons for steam in the downcomer? (1) The distance between the steam-water mixture inlet in the drum and the downcomer inlet is too small, or the position of the downcomer inlet is too high. (2) When the boiler water enters the downcomer, an excessive pressure drop is generated due to the inlet flow resistance and the acceleration of the water flow, causing the boiler water to vaporize on its own. (3) A vortex funnel is formed above the inlet section of the downcomer to draw in steam. (4) The drum water chamber contains steam, which enters the downcomer along with water. (5) The downcomer heats up and generates steam. 32. What are the hazards of steam in the downcomer? When the water in the downcomer contains vapor, it reduces the average density of the working fluid in the downcomer, thereby lowering the circulation head. At the same time, the average volumetric flow rate of the working fluid increases along with its flow velocity, which leads to an increase in flow resistance. As a result, the ability to overcome the resistance in the rising pipe is reduced, the circulation water speed decreases, and the likelihood of failures such as circulation stagnation and backflow increases. 33. What are the measures to prevent vapor from entering the downcomer? Mainly, during structural design, certain measures are taken to address the issue of steam formation, such as installing cross baffles or grids at the inlet of the large-diameter downcomer ； Increase the sub-humidity of the feedwater and direct the feedwater with sub-humidity to the inlet of the downcomer (or nearby). Prevent the downcomer from heating up ； Specify the distance between the soda mixture and the inlet of the downcomer ； The downcomer leads out from the very bottom of the drum, etc. During operation, it is also necessary to maintain the drum water level to prevent vapor from entering the downcomer when it becomes too low. 34. What are the types of superheaters classified by heat transfer method? Classified by the heat transfer method, superheaters come in three types: (1) radiant superheater. Such as the front screen (full large screen), ceiling, wall-mounted superheaters, etc. (2) Semi-radiant superheater. Such as the rear screen superheater. (3) Convection superheater. Such as high-temperature convection, low-temperature convection superheaters, etc. 35. Classification by medium flow direction. What are the types of convective superheaters? Based on the direction of medium flow, there are several types of superheaters such as co-current, counter-current, double counter-current, and mixed flow. 36. What are the types of superheaters classified by their arrangement? Based on their arrangement, superheaters are classified into vertical and horizontal types. 37. What are the flow-temperature (i.e., thermodynamic) characteristics of a convective superheater? The convective superheater is installed in the convective flue; it is a type of superheater that primarily functions to absorb the heat released by the convection of flue gases. In this type of superheater, the steam temperature increases as the boiler load rises. This is because when the load increases, fuel consumption rises as well, and the amount of flue gas flowing through the heat exchanger increases, thereby increasing the heat transfer coefficient from the flue gas to the tube walls ； Furthermore, as the fuel amount increases, the flue gas temperature at the furnace outlet rises, thereby increasing the average temperature difference. Although the steam flow rate has increased, more heat is still absorbed per unit mass of steam, raising the temperature of the outlet steam. Conversely, when the boiler load decreases, the steam temperature in the convective superheater will drop. The relationship between boiler load and superheated steam temperature is shown in the attached diagram. 38. What are the flow-temperature (i.e., thermodynamic) characteristics of a radial superheater? A radial superheater is a type of superheater that primarily absorbs the radiant heat from flames or flue gas; in this type of superheater, the temperature of the steam at its outlet decreases as the boiler load (steam flow rate) increases. This is because the heat absorbed by the radiant superheater is primarily determined by the temperature of the flames and flue gases in the furnace, while the radiant exitance is proportional to the fourth power of their absolute temperature. When the boiler load increases, although the furnace temperature and flue gas temperature rise, the increase is not significant. As a result, although radiation heat transfer increases, the steam flow rate through the superheater also increases; the effect of this increased steam volume is greater than the effect of the increased radiation heat absorption, which leads to less heat being absorbed per unit mass of steam. Therefore, the outlet steam temperature decreases. Conversely, when the load decreases, the outlet temperature of the radiant superheater increases. 39. What are the flow-temperature (i.e., thermodynamic) characteristics of a semi-radiant superheater? A semi-radiant superheater absorbs both the radiant heat from the flames and flue gas, as well as the convective heat release from the flue gas. Therefore, the variation in its exhaust steam temperature is less affected by changes in boiler load (steam flow), falling between the radiant and convective types. However, tests have shown that the thermal characteristics of this type of superheater are similar to those of a convective superheater, with only a smaller impact and a more stable change in steam temperature. 40. What is a combined superheater? What are its thermal properties? Modern high-parameter, large-capacity boilers require a large amount of steam superheating heat, thus demanding a large heating surface area for the superheater. To maintain a relatively stable outlet steam temperature as the load changes, radiant, semi-radiant, and convective superheaters are employed, resulting in a combined superheater. Its thermal characteristics are determined by the proportion of heat transfer from various types of superheaters, and it generally exhibits thermal characteristics similar to those of a convective superheater. That is, as the boiler load increases or decreases, the outlet steam temperature also increases or decreases slightly accordingly. 41. What are the characteristics of a vertically arranged superheater? The superheater arranged vertically is easy and safe to support, with a low likelihood of dust accumulation or slag formation during operation; it is generally located above the flame deflection angle and within the horizontal flue. The downside is that the water accumulated inside the coiled tubes is difficult to drain when the furnace is shut down, and poor steam flow through the tubes during startup can cause them to overheat. 42. What are the characteristics of a horizontally arranged superheater? The horizontal superheater installed in the vertical flue does not accumulate water easily inside its coiled tubes, making it convenient to drain water through steam vents. However, support and suspension are difficult; placing all the supporting components inside the flue can lead to their damage, so higher-quality steel is required. Therefore, modern boilers commonly use heat-exchanging tubes cooled by a working fluid (such as economizers) as suspension tubes. Additionally, it tends to accumulate dust, affecting heat transfer. 43. What is a heat exchanger? What are the different types of heat exchangers? A device used to exchange heat between hot and cold fluids is called a heat exchanger. Based on their working principle, heat exchangers come in three types: (1) Surface-type heat exchangers: In these heat exchangers, the cold and hot fluids do not come into contact with each other during the heat transfer process; instead, heat is transferred between them through the metal walls. These are the most widely used in thermal power plants. Such as superheaters, reheaters, economizers, oil coolers, etc. (2) Hybrid heat exchanger: This type of heat exchanger is used during the heat exchange process. It is achieved through the direct contact and mixing of hot and cold fluids. Heat transfer is accompanied by the exchange and mixing of mass. Such as spray-type steam temperature reducers, etc. (3) Regenerative heat exchanger: The heat exchange process in this type of heat exchanger is carried out through a medium, namely the heat transfer element. Alternate hot and cold fluids flow through the heat transfer element. When hot fluid flows through, it transfers heat to the heat transfer element and stores it there ； When a cold fluid flows through. The heat stored in the heat transfer element is then transferred to the cold fluid, which carries it away, thereby enabling heat exchange. Such as rotary air preheaters. 44. What is the co-current arrangement of heat exchangers? What are its features? The arrangement in which the cold and hot fluids flowing inside and outside the tubes of a surface-type heat exchanger move in the same direction is called co-current arrangement (see figure). Its feature is that the high-temperature end of the hot fluid coincides with the low-temperature end of the cold fluid, resulting in a lower wall temperature of the heat exchanger and greater safety ； However, the heat transfer temperature difference is small, resulting in poor heat transfer efficiency. When a certain amount of heat needs to be transferred, a larger heat transfer area is required, resulting in a relatively larger volume for the heat exchanger. 45. What is the counterflow arrangement of a heat exchanger? What are its features? Schematic diagram of co-current arrangement: A configuration in which the flow directions of the cold and hot fluids inside and outside the heat exchanger tubes are opposite to each other (see figure). Its characteristic is that the high-temperature end of the hot fluid in the heat exchanger coincides with the high-temperature end of the cold fluid, resulting in higher tube wall temperatures and reduced safety. However, the heat transfer temperature difference in a counterflow arrangement is larger; therefore, less heat exchange area is required to transfer a certain amount of heat, resulting in heat exchangers with relatively smaller dimensions in a counterflow arrangement. Schematic diagram of counterflow arrangement 46. What are the characteristics of heat exchangers with double counterflow and mixed flow arrangements? A heat exchanger with a double counterflow and mixed flow arrangement (see figure) is a type of heat exchanger that combines the advantages of counterflow and co-current flows while overcoming their disadvantages. It not only maintains a large heat transfer temperature difference and high heat transfer efficiency between the cold and hot fluids, but also offsets the high-temperature ends of these fluids, thereby ensuring the safety of the heat exchanger tube walls. 47. What is the steam flow pattern in the superheater of the SG400/140‑50410 type boiler? Steam drum — Header at the inlet of the ceiling superheater — Ceiling superheater and rear wall encircling tubes — Lower header of the encircling tubes — Encircling tubes on the side walls — Headers on the side walls — Front screen superheater — Primary desuperheater (left and right cross connections) — Rear screen superheater — Secondary desuperheater (left and right cross connections) — Convection superheater — Steam collection header — High-pressure cylinder of the turbine. 48. What is a screen-type superheater? How does it work? The superheater coils are arranged in a screen format and suspended parallel to the width of the furnace chamber above the combustion chamber or at its outlet. The one generally arranged right above the combustion chamber is called the front shield, while the one arranged at the outlet is called the rear shield. A large distance is maintained between adjacent screens of the screen-type superheater, which helps to reduce the temperature of the flue gas at the furnace outlet and prevents slag formation on the subsequent heating surfaces. At the same time, it is also a major component of the heating surfaces in modern large-scale boiler superheaters. 49. What are the functions of a header? In the layout of the heating surfaces, the header serves to collect, mix, and distribute the working fluid, acting as the connecting hub for the arrangement of these heating surfaces. Additionally, some header tanks are also used to suspend the heating surfaces and to install steam traps or drain devices. 50. Why is cross-connection from left to right necessary in the superheated steam process? Crossing from left to right within the superheated steam flow helps to mitigate the uneven heat load caused by uneven flue gas temperatures across the width of the furnace; it is also an effective measure to reduce the thermal differences between the left and right sides of the superheater.