What is a once-through boiler?

A once-through boiler is a type of boiler in which feedwater passes through all the heating surfaces once to be converted into steam; its circulation ratio is 1.0. Driven by the pressure from the water pump, the feedwater passes sequentially through the economizer, the evaporation heating surfaces, and the superheater, where it is completely converted into superheated steam. In medium-capacity DC boilers, there are three basic types depending on the arrangement of the evaporative heating surfaces in the furnace: horizontal surrounding tube banks, vertical rising tube banks, and return tube banks. Compared with natural circulation boilers, the advantages of once-through boilers include greater flexibility in the layout of the evaporation heating surfaces; they can be used at supercritical and critical pressures; the absence of a drum simplifies manufacturing and saves metal; furthermore, they start up and shut down quickly and allow for flexible adjustment. Its disadvantages include high requirements for water supply quality and automatic control, high resistance in the evaporation heating surfaces, and high power consumption of the feedwater pumps. Relevant textbooks such as the \"Boiler Manual\" contain information on direct-flow boilers as well as their structural layout diagrams; you can find one of these books to take a look.

Let’s see if it works. http://www.zepc.edu.cn/jpkc/dcgl/web/szkt/dcgl9/9.ppt This post was last edited by Oxygen on 2009-3-15 21:53]

DC boiler: A boiler in which, under the pressure of the feed water pump, the feed water passes sequentially through the economizer, the evaporation heating surfaces (water wall), and the superheater, turning entirely 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 for steam-water separation during the heating process. Therefore, it does not have a drum like a natural circulation boiler. There is no fixed boundary between the economizer heating surfaces, the evaporation heating surfaces, and the superheater heating surfaces; it changes as the boiler load varies.

Boilers are classified according to the driving force of the water or steam cycle into natural circulation boilers, once-through boilers, and forced circulation boilers. In a once-through boiler, the flow of water or steam through the heating tubes is driven by a water pump. Unlike natural circulation boilers and forced circulation boilers, a once-through boiler does not have a steam drum; the water turns into saturated steam or superheated steam after passing through the heating tubes just once, which is why it is called a once-through boiler.

Compared to natural circulation boilers, once-through boilers do not have a steam drum. Its working principle is that water enters the boiler under the pressure exerted by the feed pump. Heat it first in the hot water section. The temperature is gradually rising. After reaching the saturation temperature, it enters the evaporation stage; upon reaching the superheat point, it is completely vaporized to become dry saturated steam, after which it enters the superheating stage where the temperature rises gradually, resulting in superheated steam with a certain degree of superheat. Since all the feedwater passes through each section of the heating surface in sequence. Therefore, the circulation ratio K in the evaporation zone is equal to one. Furthermore, there is no fixed boundary between the evaporation section and the superheating section as in natural circulation boilers; rather, it changes depending on the operating conditions. Due to their different operating principles, once-through boilers and natural circulation boilers exhibit many differences in structure and operation, each with its own characteristics. Main advantages and disadvantages of DC boilers: 1. Less metal consumption, allowing for a 20-30% savings in steel usage ； 2. Not restricted by the operating pressure of the boiler; suitable for pressures above ultra-high pressure ； 3. Since there is no drum, it is convenient for manufacturing, installation, and transportation ； 4. The layout of the evaporation heating surfaces is relatively flexible ； 5. Fast start-up and shutdown of the furnace. The downside is that it has high requirements for the quality of the feed water ； The boiler consumes a large amount of energy for its own use ； Control and regulation are complex ； Starting and stopping are complex, with high heat loss ； The safety conditions of the water wall are poor.

No air pockets; available in large boilers, used in power plants

DC boilers do not have a steam drum; they are generally used in large-scale supercritical and ultra-supercritical power plants these days

Without a steam drum, there is also no steam separation device; the boiler feed water is converted directly into steam through the water wall, with no water circulation inside the furnace

In general, a once-through boiler does not have a steam drum; it cannot rely on pressure differences to achieve water circulation as a natural circulation boiler does. Instead, it requires a pump to provide the power for water circulation, so that the water turns directly into saturated or superheated steam after being heated :)

Supercritical and ultra-supercritical boilers, whose steam pressure is higher than the critical pressure, both adopt a straight-flow structure, and this will become the dominant design for power station boilers in China in the future.