The Introduction to Evaporator（Three）

In the previous article, we already have a certain understanding of the evaporator. This article will continue the partition heat transfer evaporator.

 

Scraper evaporator

 

The evaporator shell is equipped with a heating steam jacket, which is equipped with a rotatable blade or scraper. There are two types of squeegee: fixed and rotor. The gap between the former and the inner wall of the housing is 0.5 to 1.5 mm, and the gap between the latter and the wall changes with the number of revolutions of the rotor. The feed liquid is added from the upper part of the evaporator along the tangential direction (also added to the material throwing tray coaxial with the scraper). Due to the effects of gravity, centrifugal force and the scraping belt of the rotating scraper, the solution forms a thin film on the inner wall of the device, and is evaporated and concentrated in the process, and the completed liquid is discharged at the bottom. This evaporator is a single-pass type evaporator that uses external power to form a film. Its outstanding advantages are its strong adaptability to materials and short residence time, generally a few seconds or tens of seconds. Therefore, it can be adapted to materials with high viscosity (such as tannin extract, honey, etc.) and easy to crystallize, scale, and heat-sensitive. However, its structure is complicated and its power consumption is large. It takes about 1.5～3kW per square meter of heat transfer surface. In addition, its handling capacity is very small and manufacturing and installation requirements are high.

 

Direct contact heat transfer evaporator

 

In actual production, evaporators that directly contact heat transfer are sometimes used. It is a fuel (usually gas and oil) mixed with air and burned in a combustion chamber immersed in a solution. The resulting high-temperature flame and flue gas are directly sprayed into the evaporated solution through a nozzle in the lower part of the combustion chamber. The high-temperature gas and the solution are in direct contact, and heat transfer is performed to evaporate and vaporize the water. The generated water vapor and waste smoke are discharged from the top of the evaporator. The immersion depth of the combustion chamber in the solution is generally 0.2 to 0.6m, and the temperature of the gas exiting the combustion chamber can reach more than 1000°C. Because of direct contact heat transfer, it has a good heat transfer effect and high heat utilization rate. Because there is no need for a fixed heat transfer wall, the structure is simple, and it is especially suitable for easy crystallization, scaling and evaporation of corrosive materials. It has been widely used in waste acid treatment and evaporation of ammonium sulfate solution. However, if the evaporated liquid is not allowed to be contaminated by flue gas, this type of evaporator is generally not suitable. And because of the existence of a large amount of flue gas, the use of secondary steam is limited. In addition, the nozzle is easily damaged because it is immersed in the high-temperature liquid.

 

It can be seen from the above introduction that there are many types of evaporators, each with its own advantages and disadvantages and applicable occasions. When selecting a model, we must first see whether it can adapt to the process characteristics of the evaporated material, including the viscosity of the material, heat sensitivity, corrosion, and whether it is easy to crystallize or scale. Then it is required to have a simple structure, easy manufacturing, low metal consumption, convenient maintenance, good heat transfer effect and so on.