【Daily Question】Chemical Engineering Principles 347: Direct-Flow Boilers (March 24)

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, 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.

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, 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.

Answer: The feedwater for a direct-flow 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, 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.

Briefly describe the operating characteristics of the water and steam system in a direct-fired boiler. Answer: The feedwater for a direct-flow 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, 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.

A once-through boiler does not have a drum; the working fluid passes through all the heating surfaces in one go, and there are no fixed boundaries between these heating surfaces, which change as the boiler’s load and operating conditions vary. The structural features of a once-through boiler are mainly reflected in the evaporation heating surfaces and the steam-water system. The economizer, superheater, reheater, air preheater, and burner of a once-through boiler are similar to those of a natural circulation boiler. 2. Once-through boilers are suitable for boilers with higher pressure levels. 3. Once-through boilers can use small-diameter evaporation tubes that can be arranged freely. 4. Once-through boilers require high-quality feedwater. 5. Once-through boilers demand sophisticated automatic control systems

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, 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.

A once-through boiler does not have a drum; the working fluid passes through all the heating surfaces in one go, and there are no fixed boundaries between these heating surfaces, which change as the boiler’s load and operating conditions vary. The structural features of a once-through boiler are mainly reflected in the evaporation heating surfaces and the steam-water system. The economizer, superheater, reheater, air preheater, and burner of a once-through boiler are similar to those of a natural circulation boiler.

Briefly describe the operating characteristics of the water and steam system in a direct-fired boiler. Answer: The feedwater for a direct-flow 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, 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, 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.

Features of direct-flow boilers 1. Structural features of direct-flow boilers Direct-flow boilers do not have a steam drum; the working fluid passes through all the heating surfaces once, and there are no fixed boundaries between these surfaces, which change as the boiler’s load and operating conditions vary. The structural features of a once-through boiler are mainly reflected in the evaporation heating surfaces and the steam-water system. The economizer, superheater, reheater, air preheater, and burner of a once-through boiler are similar to those of a natural circulation boiler. 2. Once-through boilers are suitable for boilers with higher pressure levels. 3. Once-through boilers can use small-diameter evaporation tubes that can be arranged freely. 4. High quality of feedwater is required for once-through boilers. 5. High standards are required for the automatic control systems of once-through boilers. Since once-through boilers lack a drum and have small-diameter evaporation surface tubes, less metal is used, which results in a lower heat storage capacity for such boilers. When the load changes, its ability to mitigate steam pressure fluctuations by relying on the heat storage or release of the furnace water and metal is low. When the load changes, a once-through boiler must adjust both the water supply and the fuel supply simultaneously to maintain mass balance and energy balance, thereby keeping the steam pressure and temperature stable. Therefore, DC boilers require sophisticated automatic control systems for fuel and feedwater flow. 6. DC boilers have fast start-up and shutdown speeds as well as rapid load-changing capabilities. Since they lack a steam drum, DC boilers can reach higher temperatures more quickly during start-up, shutdown, or when changing load; as a result, the time required for starting up or shutting down the boiler is reduced, and its ability to change load increases. III. Basic types of once-through boilers 1. Types of early once-through boilers The structural characteristics of once-through boilers are mainly reflected in the evaporation heating surfaces and the steam-water system. Depending on the structure of the evaporation heating surfaces, early once-through boilers had three basic types: the horizontal surrounding rising tube arrangement (Ramsey type), the multiple series vertical rising tube arrays (Bunsen type), and the return tube arrangement (Sulzer type). (1) Ramses type: The evaporation heating surface of a Ramses-type direct-fired boiler is composed of multiple parallel horizontal or slightly inclined tubes that wind upward along the perimeter of the furnace. Ramsey-type direct-fired boilers are equipped with throttle orifice plates at the inlets of all the tubes in the evaporation surface; moreover, such boilers lack downcomers and intermediate headers. As a result, they offer advantages such as more stable hydrodynamics, smaller thermal gradients, less metal consumption, easier drainage of water vapor, and suitability for sliding pressure operation. However, due to the different structures of the water wall tubes, it is difficult to assemble them in advance, and because the water wall tubes expand in multiple directions, it is not possible to use simple tube-laying furnace walls; as a result, this approach has disadvantages such as heavy on-site assembly work, difficulties in addressing expansion issues, and challenges in providing support and suspension. (2) Bunsen type: The evaporation heating surface of a Bunsen-type direct-current boiler consists of multiple sets of vertically arranged tube banks. Each tube bank is made up of dozens of parallel rising tubes and headers at both ends; the width of each tube bank is 1.2 to 2 meters. The tube banks are connected to each other in series by 2 to 3 non-heating descending tubes. Bunsen-type direct-current boilers have a simple piping system, and the numerous intermediate headers help to balance the thermal variations caused by uneven heat absorption in the various tubes. As a result, they offer advantages such as a high installation efficiency, ease of manufacturing, and minimal thermal variations. However, since such boilers require external downcomers and a large number of header tanks, their metal consumption is high. Furthermore, this type of boiler has poor adaptability to sliding pressure operation. (3) Sulzer type: The evaporation heating surface of the Sulzer-type once-through boiler is composed of multi-stroke return tube banks. Based on the different methods of return flow, it can be divided into horizontal return flow and vertical return flow; horizontal return flow is generally used in the single-phase region, while vertical return flow is used in the two-phase region. Sulzer boilers do not have external downcomers, and intermediate mixing headers are rarely used, which gives them the advantages of easy installation and lower metal consumption. However, due to the long pipes between the boiler’s two header tanks, the thermal gradients between the pipes and between the pipe banks are very large ； It also has disadvantages such as difficulty in manufacturing, difficulty in achieving hydrophobic exhaust during vertical lifting and lowering, and poor hydrodynamic stability. 2. Types of modern direct-flow boilers: With the development of boiler technology, modern direct-flow boilers are becoming increasingly uniform in design. Modern direct-flow boilers come in three main types: the primary vertical rising tube panel type (UP type) ； Multiple risers at the lower part of the furnace, one riser at the upper part – tube bank type (FW type) ； The spiral wraps around the rising tube screen.