The main characteristics of this type of evaporator are: the solution in the evaporator only through the heating chamber once, do not make the circulation flow is to become concentrated liquid eduction. When the solution passes through the heating chamber, it is film-like flow on the tube wall, so it is used to be called liquid film evaporator. According to the different flow of materials in the evaporator, one-way evaporator is divided into the following several.
1. Rising film type evaporator: Its heating chamber consists of many vertical long tubes. The commonly used heating pipe diameter is 25~50mm, pipe length and pipe diameter ratio is about $number. After preheating, the material liquid is introduced by the bottom of the evaporator, heated in the heating tube and quickly vaporized, the steam generated in the heating tube is rising at high speed, the suitable outlet steam speed is 20~50m/s when operating under normal pressure, and the steam speed can reach 100 to 160m/s or greater under decompression. The solution is driven by the rising steam, which rises along the wall of the tube and continues to evaporate, the vapor and liquid mixture is separated in the separator 2, the complete liquid is discharged from the bottom of the separator, and two times the steam is exported at the top. It should be noted that if the water from the liquid evaporation is not much, it is difficult to meet the above requirements of the steam speed, that is, the rising film evaporator is not suitable for the evaporation of concentrated solution; it is very large in viscosity, easy to crystallize or easy to scale the material also does not apply.
2. Falling film Evaporator: The difference between the falling film evaporator and the rising film type evaporator is that the liquid is added from the top of the evaporator, and the film descends along the wall of the tube under the action of gravity, and in the process the evaporation is thickened and the concentrated liquid is obtained at the bottom. Because the film-forming mechanism is different from the film-type evaporator, the falling film evaporator can evaporate the high concentration and the viscosity is larger (for example, in 0.05~0.45ns/m2 range), heat-sensitive materials. However, due to the uneven distribution of liquid film in the tube, the heat transfer coefficient is smaller than that of the film-type evaporator, it is still not suitable for easy crystallization or scaling of materials.
Because the solution in the one-way evaporator is film-like flow, so the heat transfer coefficient is greatly increased, so that the solution can be in the heating chamber through no longer cycle to achieve the required concentration, therefore than the circulation of the evaporator has a greater advantage. The solution does not circulate the benefits are: (1) The solution in the evaporator in a short time, it is particularly suitable for the evaporation of heat-sensitive materials; (2) The concentration of the whole solution is not always close to the concentration of the finished liquid, so the effective temperature difference of the evaporator is larger. Its main disadvantage is: The feed load fluctuation is quite sensitive, when the design or operation is not easy to film, at this time, the convective heat transfer coefficient will be significantly reduced.
3. Scraper Evaporator Evaporator: The casing is provided with a heated steam jacket, which is fitted with a rotatable blade that is a scraper. Scraper has a fixed and rotor type, the former with the shell of the gap between the walls of 0.5~1.5mm, the latter and the clearance of the wall with the rotor number of changes. The liquid is added along the tangential direction of the upper part of the evaporator (also added to the flip plate on the same shaft as the scraper). Due to gravity, centrifugal force and rotary scraper scraping, the solution in the inner wall of the device to form a spinning film, and in the process of evaporation condensed, the completion of the liquid at the bottom of the discharge. This kind of evaporator is a one-way evaporator, which utilizes the external dynamic film. Its outstanding advantage is the material adaptability is very strong, and stay for a short period of time, generally a few seconds or a few 10 seconds, it can be adapted to high viscosity (such as tannin, honey, etc.) and easy crystallization, scaling, heat-sensitive materials. But its structure is complex, power consumption is big, the heat transfer surface of each square metre needs about 1.5~3kw. In addition, it has a small processing capacity and high manufacturing and installation requirements.