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In actual production, in addition to the two main types of partitioned heat transfer evaporators mentioned above—cyclic and single-pass types—evaporators with direct contact heat transfer are sometimes also used. It involves directly injecting the high-temperature flue gases generated by burning fuel (usually gas or heavy oil) mixed with air into the solution that is being evaporated; the direct contact between these high-temperature flue gases and the solution causes the solution to boil and vaporize rapidly. The evaporated water is discharged directly from the top of the evaporator along with the flue gas. Typically, the depth of the combustion chamber of such evaporators in the solution is 200–600 mm, and the temperature of the hot flue gases inside the combustion chamber can exceed 1000°C. However, due to the fast heat transfer rate when gas comes into direct contact with the liquid, the gas leaves the liquid surface at a temperature only 2–4°C higher than that of the solution. The nozzles in the combustion chamber are prone to damage due to operation at high temperatures; therefore, they should be made from materials that can withstand high temperatures and corrosion, and their design should facilitate replacement. The submerged combustion evaporator is characterized by its simple structure and high heat transfer efficiency. This evaporator is particularly suitable for the evaporation of materials that are prone to crystallization, scaling, or are corrosive. Currently, this type of evaporator is widely used in the treatment of waste acid and the evaporation of sulfuric acid sulfate solutions. However, it is not suitable for the treatment of materials that cannot be contaminated by smoke, and its secondary steam is also difficult to utilize.