【Daily Question】Chemical Engineering Principles 414: Direct-Flow Boilers (August 25)

1. A once-through boiler is a type of boiler in which, under the pressure of the feedwater pump, the feedwater passes sequentially through the economizer, the evaporation heating surfaces (water wall), and the superheater, where it is completely converted into superheated steam. 2. A once-through boiler is a type of boiler in which, under the pressure generated by the feed water pump, the feed water passes sequentially through the economizer, the evaporation heating surfaces (water wall), and the superheater, where it is completely converted into superheated steam. Since, after the feedwater enters the boiler, the heating of the water, its evaporation, and the superheating of the steam all take place continuously within the heating surfaces, there is no need to separate steam from water during the heating process. Therefore, it does not have a drum like a natural circulation boiler. A power plant boiler in which there is no fixed boundary between the economizer heating surfaces, the evaporation heating surfaces, and the superheater heating surfaces; this boundary changes as the boiler load varies. It lacks a steam drum, and it is the pressure from the feedwater pump that causes the feedwater to be preheated, evaporated, and superheated, flowing through each stage of heating surfaces to produce steam with the specified parameters and capacity. The recirculation ratio of its evaporation zone is 1. 3. In DC boilers, since there is no clear distinction between the evaporation and superheating surfaces, issues such as flow instability and pulsations can occur in the evaporation surfaces. To address this, throttle orifice plates are installed at the inlet of the tube bank, and breather tanks are placed between the inlet and outlet headers as corrective measures. Furthermore, film boiling occurs in the evaporating heating surface (that is, the bubbles formed on the heating surface stick together to form a vapor film due to not having enough time to separate), which hinders heat conduction and leads to overheating of the tube walls, affecting the safe operation of the boiler. To address this, turbulators can be installed in the water wall of areas with high heat loads, or internally threaded tubes can be used to delay the onset of film boiling. Measures such as flue gas recirculation or adjusting the burner arrangement can also be employed to reduce the maximum heat load inside the furnace. In a once-through boiler, the flow of the working fluid does not have self-balancing properties; that is, in tubes where more heat is absorbed, the increase in the specific volume of the fluid leads to increased resistance, which in turn reduces the flow rate through those tubes, resulting in a thermal imbalance. Therefore, UP-type direct-flow boilers often use intermediate mixers to draw the fluid from the water wall tubes out of the furnace at an intermediate stage and into the mixer, allowing the enthalpy, temperature, and pressure of the fluids in each tube to be thoroughly mixed before they flow back into the water wall, thereby eliminating thermal differences between the various tubes. The number of mixings, the location, and the method depend on the capacity and characteristics of the boiler. As operating conditions place increasing demands on the peak-shaving capabilities of boilers, the adaptability of once-through boilers to load fluctuations has come to be emphasized. DC boilers equipped with spirally ascending tube coils are the least sensitive to thermal imbalances within the furnace; there is no issue of two-phase fluid distribution between the tubes, nor is it necessary to install orifice plates at the tube ends, thereby offering excellent performance at low load conditions. However, it requires high standards for water supply quality and automatic control, the steam-water system has high resistance, and the water supply pumps consume a lot of electricity.

In a DC boiler, the feed water flows sequentially through heat exchange surfaces such as the economizer, water wall, and superheater, driven by the pressure from the make-up water pump. During this single pass, the water undergoes heating, evaporation, and superheating, eventually turning into superheated steam that is then discharged from the boiler. A DC furnace does not have a steam drum, and the water does not need to be circulated multiple times to complete the evaporation process. Unlike drum boilers, it is not possible to remove impurities from the boiler water through the boiler; nor can scale prevention and removal be carried out by treating the water inside the boiler. If feedwater containing impurities enters a once-through boiler, these impurities either form deposits within the boiler tubes or are carried by steam to the turbine, where they cause corrosion or the formation of deposits, thereby directly affecting the safety and efficiency of the unit’s operation. Therefore, the feedwater purity requirement for DC boilers is very high.

Briefly describe the operating characteristics of the water and steam system in a once-through boiler. Answer: In a once-through boiler, the feedwater flows sequentially through heat exchange surfaces such as the economizer, water wall, and superheater, driven by the pressure from the make-up water pump. The water passes through these surfaces once, undergoing heating, evaporation, and superheating processes, and ends up as superheated steam that is then discharged from the boiler. A DC furnace does not have a steam drum, and the water does not need to be circulated multiple times to complete the evaporation process. Unlike drum boilers, it is not possible to remove impurities from the boiler water through the boiler; nor can scale prevention and removal be carried out by treating the water inside the boiler. If feedwater containing impurities enters a once-through boiler, these impurities either form deposits inside the boiler tubes or are carried by steam to the turbine, where they cause corrosion or the formation of deposits, thereby directly affecting the safety and efficiency of the unit’s operation. Therefore, the feedwater purity requirement for DC boilers is very high.

Most importantly, a DC furnace does not have a steam drum; all the water is converted into steam, and there is no water circulation in the furnace. The requirements for water quality are also higher. Specifically as follows: 1. High requirements are placed on the quality of the feedwater. Since direct-flow boilers do not have a drum, they cannot carry out blowdown or treat the water inside the boiler in the same way as drum boilers; as a result, the salts in the feedwater have no way to be removed, and all of these salts will accumulate on the heating surfaces of the boiler or be carried into the turbine along with the superheated steam. Therefore, to prevent salt deposits from accumulating over time on various heat-exchanging surfaces as a result of long-term operation of the unit, which could affect heat transfer and in severe cases even lead to tube rupture, higher standards must be set for the quality of the feedwater. 2. Automatic control requires high precision. DC boilers lack a steam drum, and thus cannot use one to store water, steam, or heat; as a result, steam pressure fluctuates significantly when external loads change. Furthermore, there are no fixed boundaries between the heating, evaporation, and superheating processes of the feedwater across various heating surfaces; disturbances in the feedwater or fuel will both cause fluctuations in steam temperature. Therefore, to ensure good static and dynamic control characteristics, DC boilers require a superior automatic control system. 1.3 The resistance in the steam system is high, which results in high power consumption by the feed water pump. In a once-through boiler, it is the pressure generated by the feed water pump that is used to overcome the flow resistance of the working fluid in the evaporation surfaces, thereby ensuring stable flow of the working fluid within the water wall. To ensure the safe operation of the water wall, the weight flow rate γw of the working fluid in this wall must be high; as a result, the hydraulic resistance increases, leading to higher power consumption by the feed water pump. 1.4 Fast start-up and shutdown speed: DC boilers do not have a drum, so they are not subject to the limitations imposed by thick walls in a drum-type boiler, which could lead to bending deformation and excessive thermal stress during rapid start-up and shutdown. Therefore, it can start and stop quickly. Its start-up and shutdown times are limited only by cylinder expansion and vibration. 1.5 The main method for controlling the superheated steam temperature in a DC boiler is to maintain a constant ratio between the feedwater volume and the fuel volume, that is, a constant coal-to-water ratio. Because a DC furnace does not have a drum, there is no fixed boundary between the heating, evaporation, and superheating processes of the feedwater within the economizer, water wall, and superheater surfaces; rather, this boundary changes depending on the operating conditions. When the thermal load of the boiler and other conditions remain unchanged, reducing the feedwater volume causes the evaporation point and superheating point of the working fluid within the heating surfaces to advance, resulting in an increase in the steam temperature at the outlet of the superheater ； Conversely, increasing the water supply will cause the superheated steam temperature to drop. Multi-stage water spray cooling merely makes fine adjustments to the steam temperature on this basis. 1.6 Film boiling on the evaporation surfaces In the evaporation surfaces of once-through boilers, as the vapor fraction X of the working fluid increases from 0 to 1, the vapor fraction of the steam-water mixture inside the tubes rises to a certain value; at this point, the water film attached to the inner wall of the tube is torn apart by the steam flow or dried up, and is replaced by a layer of steam film. Since the heat release coefficient of steam is much lower than that of water, the tube wall temperature rises sharply due to impaired heat transfer, and in severe cases the tubes can be damaged. This type of boiling in the evaporation surfaces of once-through boilers is known as film boiling. Obviously, to ensure the safe operation of the boiler’s evaporation heating surfaces, measures should be taken to prevent the occurrence of this phenomenon. 1.7 Separate start-up bypass system: Since there is no fixed boundary between the various heating surfaces in a DC boiler, hot water is produced during the initial stages of startup; as the boiler’s combustion rate increases, wet steam, saturated steam, and superheated steam are gradually generated. To provide a path for the hot water, water-steam mixture, saturated steam, and superheated steam at a lower degree of superheat generated in the early stages of startup ； Recovering working fluid and heat during startup ； To meet the requirement that the main steam must have a certain degree of superheat (usually above 50°C) during the turbine startup process and to shorten the startup time as much as possible, a start-up bypass system must be installed in DC boilers.

In a DC boiler, the feed water flows sequentially through heat exchange surfaces such as the economizer, water wall, and superheater, driven by the pressure from the make-up water pump. During this single pass, the water undergoes heating, evaporation, and superheating, eventually turning into superheated steam that is then discharged from the boiler. A DC furnace does not have a steam drum, and the water does not need to be circulated multiple times to complete the evaporation process. Unlike drum boilers, it is not possible to remove impurities from the boiler water through the boiler; nor can scale prevention and removal be carried out by treating the water inside the boiler. If feedwater containing impurities enters a once-through boiler, these impurities either form deposits inside the boiler tubes or are carried by steam to the turbine, where they cause corrosion or the formation of deposits, thereby directly affecting the safety and efficiency of the unit’s operation. Therefore, the feedwater purity requirement for DC boilers is very high.

DC boilers are mostly used under pressures greater than 19.2 MPa. At such high pressures, the density difference between steam and water is nearly zero, and thus the upward force resulting from this density difference is also zero. Therefore, a circulation pump must be connected in series in the downcomer to pump the working fluid directly into the superheater, where preheating, vaporization, and superheating take place all at once; hence, this type of boiler is also known as a forced-circulation boiler

In a DC boiler, the feed water flows sequentially through heat exchange surfaces such as the economizer, water wall, and superheater, driven by the pressure from the make-up water pump. During this single pass, the water undergoes heating, evaporation, and superheating, eventually turning into superheated steam that is then discharged from the boiler. A DC furnace does not have a steam drum, and the water does not need to be circulated multiple times to complete the evaporation process. Unlike drum boilers, it is not possible to remove impurities from the boiler water through the boiler; nor can scale prevention and removal be carried out by treating the water inside the boiler. If feedwater containing impurities enters a once-through boiler, these impurities either form deposits inside the boiler tubes or are carried by steam to the turbine, where they cause corrosion or the formation of deposits, thereby directly affecting the safety and efficiency of the unit’s operation. Therefore, the feedwater purity requirement for DC boilers is very high.

In a DC boiler, the feed water flows sequentially through heat exchange surfaces such as the economizer, water wall, and superheater, driven by the pressure from the make-up water pump. During this single pass, the water undergoes heating, evaporation, and superheating, eventually turning into superheated steam that is then discharged from the boiler. A DC furnace does not have a steam drum, and the water does not need to be circulated multiple times to complete the evaporation process. Unlike drum boilers, it is not possible to remove impurities from the boiler water through the boiler; nor can scale prevention and removal be carried out by treating the water inside the boiler. If feedwater containing impurities enters a once-through boiler, these impurities either form deposits inside the boiler tubes or are carried by steam to the turbine, where they cause corrosion or the formation of deposits, thereby directly affecting the safety and efficiency of the unit’s operation. Therefore, the feedwater purity requirement for DC boilers is very high.

Answer: The feedwater for a DC boiler flows, in sequence, through heat exchange surfaces such as the economizer, water wall, and superheater, driven by the pressure from the make-up water pump. During this single pass, the water undergoes heating, evaporation, and superheating, and ends up as superheated steam that is then discharged from the boiler. A DC furnace does not have a drum, and the water does not need to be circulated multiple times to complete the vaporization process. It is unable to remove impurities from the water as a drum furnace can, nor can it carry out descaling treatments of the water inside the furnace and remove the scale. High requirements are placed on the purity of the feed water.

In a DC boiler, the feed water flows sequentially through heat exchange surfaces such as the economizer, water wall, and superheater, driven by the pressure from the make-up water pump. During this single pass, the water undergoes heating, evaporation, and superheating, eventually turning into superheated steam that is then discharged from the boiler. A DC furnace does not have a steam drum, and the water does not need to be circulated multiple times to complete the evaporation process. Unlike drum boilers, it is not possible to remove impurities from the boiler water through the boiler; nor can scale prevention and removal be carried out by treating the water inside the boiler. If feedwater containing impurities enters a once-through boiler, these impurities either form deposits inside the boiler tubes or are carried by steam to the turbine, where they cause corrosion or the formation of deposits, thereby directly affecting the safety and efficiency of the unit’s operation. Therefore, the feedwater purity requirement for DC boilers is very high.