The central circulation tube evaporator is also known as a standard evaporator. Its heating chamber is composed of vertical tube bundles, with a central circulation tube with a large diameter in the middle, and the other heating tubes with smaller diameters are called boiling tubes. Due to the large size of the central circulation tube, the heat transfer surface occupied by a unit volume of solution in the central circulation tube is smaller than that occupied by a unit volume of solution in the boiling tube. This means that the heating degree of the solution in the central circulation tube and other heating tubes is different, resulting in a lower density of the vapor liquid mixture in the boiling tube compared to the solution in the central circulation tube. In addition, the upward suction effect of the rising steam will cause the solution in the evaporator to form a solution that falls from the central circulation tube Circulating flow rising from a boiling tube. This cycle is mainly caused by the density difference of the solution, hence it is called a natural cycle. This effect is beneficial for improving the heat transfer effect inside the evaporator.

Direct heat source heating refers to the evaporation process of mixing fuel with air, causing the high-temperature flame and smoke generated by its combustion to be directly sprayed into the evaporated solution through a nozzle to heat the solution and vaporize the solvent.

Evaporators are divided into three types according to operating pressure: normal pressure, pressurized, and depressurized. According to the movement of the solution in the evaporator, it can be divided into: ① circulating type. Boiling solution passes through the heating surface multiple times in the heating chamber, such as central circulation tube type, hanging basket type, external heating type, Levin type, and forced circulation type One way type. The boiling solution passes through the heated surface in the heating chamber at once without circulating flow, and the concentrated solution is discharged, such as rising film, falling film, stirring film, and centrifugal film Direct contact type. The heating medium directly contacts the solution for heat transfer, such as an immersed combustion evaporator. During the operation of the evaporation device, a large amount of heating steam needs to be consumed. To save heating steam, a multi effect evaporation device and a steam recompression evaporator can be used. Evaporators are widely used in departments such as chemical and light industries.