Analysis and comparison of the advantages and disadvantages of falling film evaporators

Falling film evaporator is a kind of high-efficiency evaporation equipment, which is collectively called thin film evaporator together with rising film evaporator and scraped film evaporator.

Thin film evaporators have significant advantages and are widely used in chemical, light industry and food processing industries to achieve the purpose of liquid concentration. The structure of the falling film evaporator is actually not complicated. It can be simply understood as a vertical tube and tube heat exchanger with a set of film distribution devices, but there is another obvious difference from ordinary heat exchangers. Its shell side is fixed as The fluid is heated, and the tube side is fixed as the material to be evaporated. The material in the tube pass of the falling film evaporator enters the heat exchange tube from the top of the tube, and flows down the inner wall of the tube by its own gravity. Before entering the heat exchange tube, it needs to pass through the film distributor to spread the film, so when it enters the heat exchange tube It will be in the form of a thin film, and the moisture will evaporate out of the material during the top-down process, so as to achieve the purpose of material concentration. If the shell side is heated by steam condensation or heat conduction oil, if steam is used, it will go in and out. If heat conduction is used The oil should be bottom-up. It can be seen that the flow direction of the medium inside and outside the tube is not necessarily reverse flow. But this is not important, because the pressure in the tube is basically unchanged and will not cause the fluid temperature to change. The change in the temperature of the medium in the tube is only due to the change in the concentration of the medium which causes the boiling point to rise (this is only a slight increase).

Analysis of the advantages of falling film evaporator:

(1) The residence time of the materials in the heat exchange tube is short, which will not affect the chemical properties of the heat-sensitive materials;

(2) The material in the heat exchange tube is in the form of a thin film with a fast flow rate and a large evaporation heat transfer coefficient;

(3) The pressure drop is almost zero, and the material in the pipe side is in a normal pressure environment, and it completely flows down the inner wall of the pipe by its own gravity, which can save energy;

(4) Since the materials in the pipe side flow down the inner wall of the pipe by their own gravity, instead of being driven by pressure difference, low temperature difference can be selected;

(5) The amount of material retained in the pipe pass space is very small;

(6) The material in the tube side shows convection boiling after heating, so the surface state of the heat exchange tube hardly affects the material boiling.

Analysis of the disadvantages of falling film evaporator:

(1) The evaporation effect of the equipment is directly related to the distributor, and the requirements for the design, manufacture and installation of the distributor are very high;

(2) Equipment installation requirements are high, mainly manifested as verticality requirements, assuming that the length of the heat exchange tube is 9m, the verticality of the equipment is required to be ≤6.4mm;

(3) It is necessary to ensure that the inner wall of the heat exchange tube of the equipment is completely wet, but the thickness of the cloth film is not too large. It is necessary to perform very accurate design calculations on the material input and the length of the heat exchange tube to meet such requirements;

(4 Due to the high design and manufacturing requirements, the equipment procurement cost is very high;

(5) Not applicable to materials that are easy to scale or contain suspended solids.

Yanjialong Machinery Technology Co., Ltd.


Post time: Sep-09-2020