Hot air ducts with combined refractory masonry are usually constructed with standard refractory masonry, using clay or high-alumina bricks for the inner layer. Insulating bricks form the outer layer. Hot air ducts are prone to accidents such as red burning and wind channeling. Complex structures such as the hot air duct outlet, the connection of the hot blast branch and main pipes, the connection of the main pipe with the ring pipe and expansion telescopic pipes can cause brick joints cracking, expansion joints that are not fully sealed once heated and upper bricks falling off. For a temporary solution, grouting can be used during the windbreak. However, this could also lead to problems like mud leaking into the pipe and refractory materials peeling off. Henan Refractory Materials Factory suggests that in order to solve these problems effectively, we should do research on masonry designs and masonry methods.
Experts have carried out experiments with refractory brick masonry for the hot blast furnace ducts. The test was performed on a model that had a steel shell with an inner diameter 2500mm and a pipe with an inner diameter 1500mm. It also had a length 5400mm. Two plans A and B for the combined bricks of the hot-air duct were created during the test. The original plan was to build circumferentially in a through joint configuration. In the 120deg range, the refractory blocks are constructed with pipe bricks that have been keyed. All expansion joints above 120deg should be filled with buffer mud and laid in layers starting from the inside. Plan B is staggered-ring masonry. After the refractory blocks are laid, the insulation layer is first pounded using castables in the 120-degree range and the expansion joints are then filled with buffer mud.
The test was conducted with a heating and cooling rate of 50degC/h. The temperature was then raised to an inner surface temperature between 1300degC and 1450degC over a period of 200 hours. After 60-70h, the maximum expansion, which is 100-110h heating, is seen, then it gradually decreases. The measured expansion is 1.2 to 1.4 times that calculated for simple circumferential expansion and 0.74 to 0.83 times calculated value based upon the top fan shape protrusion into an elliptical form.
The deformation of the rings after expansion will also be different due to the differences in the settings of the joints. Plan A's expansion is smaller even though the temperature is higher. In the hot state it appears that Plan A has an elliptical deformation.
The first insulation layer in the Plan B hot air duct does not expand properly when the temperature increases. According to the results of the disassembly, the upper part was damaged and bent. Cracks appeared at the brick joints of the insulated brickwork for the hot air duct.
The expansion rate of refractory bricks slows down when the temperature inside the refractory wall is higher than 800-900degC. The expansion joint does not have enough expansion because the buffer mud is only 30-50% compressible. Replace the buffer mud with cotton that is fire resistant and has a better compressibility.
The expansion joints should be installed in sections, at intervals between 2-4m along the length the hot air duct. The linear expansion coefficient (LEC) of refractory blocks can be used to determine the size of the expansion joints. Generally, 20mm is the recommended choice.
To prevent the hotblast valve from being damaged by thermal expansion and to not affect the replacement of a hotblast valve, the masonry must not extend beyond the flange on the hotblast branch pipe. The direction of the masonry also needs to be controlled. Expansion in the blue-colored direction. The flange is constructed with refractory bricks in a special shape, and the expansion joints have been encrypted to reduce the distance between them.
Refractory bricks can be easily removed from the connections between the hot-air branch pipe and hot-air stove and between the main hot-air pipe and hot-air main pipe. The furnace shell is also reddened by burning the bricks. Special-shaped refractory blocks were used in addition to the thicker spray coatings and insulating bricks. The masonry methods have also been improved.
Corner bricks can easily crack or break at pipe connections. The corners of the bricks are also peeling away in the branch pipes. The entire brick will eventually fall off. The branch pipe connection is damaged by excessive load. The branch pipe will be affected by thrust when air is provided. The change in pressure is equal to the change of axial thrust. This has a large impact on the damage done to the masonry.
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