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The structure of the central circulation tubular evaporator is as shown in the diagram. Its heating chamber consists of a vertical array of heating tubes (boiling tubes), and at the center of this tube array there is a tube with a larger diameter, known as the central circulation tube. The cross-sectional area of this tube typically accounts for 40% to 100% of the total cross-sectional area of the heating tube array. When a heating medium is passed through the tubes for heating, since the heating area per unit volume of liquid inside the heating tubes is larger than that of the liquid in the central circulation tube, the liquid inside the heating tubes has a lower relative density. This creates a density difference between the liquid in the heating tubes and that in the central circulation tube, and it is this density difference that enables a natural convection flow: the solution flows downward from the central circulation tube and upward through the heating tubes. The circulation speed of the solution depends on the density difference created by the solution and the length of the tube; the greater the density difference and the longer the tube, the higher the circulation speed of the solution. However, due to the limitation of total height, these types of evaporators have relatively short heating tubes, typically 1–2 meters in length, with a diameter of 25–75 mm, and an aspect ratio of 20–40. The central circulation tube evaporator boasts advantages such as a compact structure, easy manufacturing, and reliable operation, which makes it widely used in industry; it is thus known as the \"standard evaporator\". But in reality, due to structural limitations, its circulation speed is low (usually below 0.5 m/s) ； Furthermore, as the solution continuously circulates within the heating tube, its concentration remains close to that of the finished solution; as a result, the boiling point of the solution is high and the effective temperature difference decreases. Furthermore, the cleaning and maintenance of the equipment are also not convenient enough.