In the furnace of the water-tube boiler, the single-row parallel water pipes arranged against the wall are called water-cooled wall pipes. The water wall tube is a very important part in the boiler. It absorbs the heat of the high temperature flame and flue gas generated by burning coal and gas in the furnace mainly through thermal convection, radiation, etc., and then transfers the heat to the working fluid circulating in the tube, so that it is heated and evaporated to form a normal steam-water cycle.
The water-cooled wall tubes are arranged around the furnace to separate the flame from the furnace wall. As the water vapor in the water-cooled wall tubes continuously flows, it absorbs a large amount of heat radiated by the flame and cools the furnace wall, so that the temperature of the furnace wall is not too high. The role of the water-cooled wall and the cooling furnace wall of the water-tube boiler of a thermal power plant is secondary. When the shape of the furnace is different, the shape of the water wall is also different. If the furnace wall of the furnace is removed, the water wall is a cage composed of steel pipes that surrounds the furnace. The upper end of the water wall tube is connected to the boiler through the header or directly to the steam drum; the lower end of the water-cooling wall pipe is connected to the lower header, and a descending pipe is connected between the lower header and the steam drum to form a circulation loop, so that the water circulates continuously in the water-cooling wall pipe. The water-cooled wall pipe system has the characteristics of large number, large area and long single pipe.
According to the relationship between the adjacent single-row parallel water-cooled wall tubes, the water-cooled wall can be divided into two types: the plain tube water-cooled wall and the finned tube water-cooled wall. There is a certain gap between the adjacent tubes of the water-cooled wall of the light tube, and they do not touch each other. The furnace flame can pass through the gap between the tubes, and part of it is radiated to the furnace wall. The finned tube water wall is also called the membrane water wall. The adjacent tubes are connected by fins, so that the water-cooled wall tubes form a continuous metal wall film, which completely isolates the contact between the furnace fire and the furnace wall and increases the sealing performance of the furnace. Generally, the fins of small boilers are formed by welding a flat steel bar between two adjacent water wall tubes.
The material for making water wall tubes is generally carbon steel. When the boiler pressure is above 14MPa, some of the water wall tubes are also made of alloy steel. The outer diameter of the water wall pipes of natural circulation boilers is generally 51~83mm; the water walls of multiple forced circulation boilers and once-through boilers generally use 22~60mm pipes. The water wall of once-through boilers is not necessarily vertical like natural circulation boilers, but can also be horizontally surrounded or other forms. In order to prevent the deterioration of heat transfer in the high heat load area of the furnace, in high-parameter boilers, especially in once-through boilers, measures such as internally threaded tubes or turbulators are often installed in the tubes.
Working characteristics and functions of the water-cooled wall tubes
In the early stage of boiler development, the use of water-cooled wall tubes is mainly to protect the furnace wall. The water-cooled wall tubes arranged in front of the furnace wall absorb the radiant heat of a large number of high-temperature flames in the furnace, so as to reduce the temperature of the furnace wall and improve the mechanical strength to ensure reliable and safe operation of the boiler. With the development of boiler technology, the water wall has become one of the main heating surfaces of the boiler, with the following characteristics and functions:
①Strengthen heat transfer, reduce the heating surface area of the boiler, and save metal consumption. The flame and the water wall in the furnace transfer heat by radiation, and the heat transfer heat flow is proportional to the fourth power of the thermodynamic temperature of the flame. The temperature of the flame in the furnace is very high, while the temperature of the medium in the water wall is relatively low, and the radiation heat absorption of the water wall is very strong, which can absorb more heat with less heating surface area. Therefore, the total heating surface area can be reduced and the consumption of steel can be saved, so the water wall is an indispensable and important heating surface.
②Reduce the damaging effect of high temperature on the furnace wall and protect the furnace wall. After installing the water wall, the temperature of the inner wall of the furnace wall is greatly reduced, and the mechanical strength of the furnace wall is greatly enhanced, so the thickness and weight of the furnace wall can be reduced. In particular, the tube-laying furnace wall behind the membrane-type water-cooled wall does not need to use refractory materials with high density.
③It can effectively prevent the furnace wall from slagging. Once the high-temperature molten coke and ash collide with the lower temperature water wall tube, it will quickly cool down and solidify, lose its viscosity and slide down to the cold ash hopper, so as to prevent the molten ash from sticking to the high-temperature furnace wall and causing slagging.
④Suspended furnace wall. Modern medium and large power station boilers all use tube-laying furnace walls or tube-supporting furnace walls that are laid on water-cooled wall tubes. The entire weight of the furnace wall is supported by the suspended water wall, and at the same time, the heating surface expands downward together with the water wall.
⑤As the main evaporative heating surface of the boiler, it absorbs the radiant heat in the furnace, vaporizes the hot water in the water wall tube, and generates all or most of the saturated steam of the boiler. Usually, the average flow rate of high-temperature flue gas in the furnace is low, 5~10m/s, and the flow rate near the water wall tube is lower, so its convective heat transfer coefficient is very small; the temperature of the high temperature flame center is as high as 1500~1600℃, and the radiation heat exchange is very large, accounting for about 95% of the total heat exchange. Therefore, the water wall is the evaporative heating surface dominated by radiation heat exchange.