The evaporator is composed of a heating chamber and a separation chamber. The heating chamber is equivalent to a heat exchange device, the heating medium is steam, and the separation chamber is equivalent to a gas-liquid separation device. The material liquid enters from the middle of the heating chamber. During the process of passing through the entire heating chamber, the material liquid receives heat, the water vaporizes, and the concentrated liquid material is discharged from the bottom of the heating chamber. The gas vaporized from the material liquid is called the secondary steam. The secondary steam enters the condenser, directly condensed by the cooling water, the cooling water is discharged from the bottom, and the non-condensable gas such as entrained air is discharged from the top.