The double effect rising film evaporator utilizes secondary steam once and can use the secondary steam evaporated from the first effect as the heat source for the second effect. Each evaporator enters from the bottom of the evaporator, and due to the heating of the material, a continuous liquid film is formed on the pipe wall.
The formation process of liquid film in a long tube is divided into the following processes:
1. The temperature of the material entering the evaporator is lower than its boiling point. There is a heating tube in the evaporator as a preheating zone, and the heat transfer method is natural convection to maintain the normal operation of the evaporator. The liquid level in the heating tube is generally 1/4 to 1/5 of the tube length.
2. The material is heated to its boiling point, and the solution begins to boil. The resulting steam bubbles are dispersed in a continuous liquid phase. Due to the low density of steam bubbles, they rise through the liquid surface. The specific gravity of the fluid decreases.
3. The liquid continues to be heated, and as the amount of bubbles increases, small bubbles combine to form larger bubbles, accelerating the upward velocity of the gas. The liquid phase experiences a decrease in hydrostatic pressure due to the presence of steam bubbles.
4. The bubbles continue to grow and form a columnar shape. Occupying most of the space in the middle of the pipe, the gas rises at a high speed, while the liquid slides down along the edge of the bubble due to gravity.
5. The gas column continues to increase, the liquid film between them disappears, and steam occupies the middle of the entire tube, forming a continuous phase. Liquids can only be distributed on the pipe wall. And form a climbing membrane
6. The gas velocity further increases, and the liquid forms a liquid film
The rising film evaporator is suitable for materials that are prone to foaming and will not precipitate crystals during the evaporation process.