This type of evaporator, the solution is in the evaporator in the circulation flow. Due to the different causes of circulation, it can be divided into natural circulation and forced circulation.
1. Central Circulation Tube Evaporator: This kind of evaporator is also called the standard type evaporator. Its heating chamber consists of a vertical tube bundle, in the middle of a large diameter central circulation tube, the rest of the smaller diameter of the heating tube is called boiling pipe. Because the central circulation tube is larger, the heat transfer surface of the unit volume solution occupies, compared with the boiling Tube Unit solution is smaller, that is, the central circulation tube and other heating pipe solution heating degree is different, so that the boiling pipe of the vapor-liquid mixture density than the central circulation tube in the solution of the density of small, coupled with the rise of the upward suction action of steam, Will cause the solution in the evaporator to be formed by the central circulation tube down, by the boiling pipe upward circulation flow. This cycle, mainly caused by the density difference of solution, is called the natural cycle. This effect is beneficial to the improvement of heat transfer effect in evaporator.
In order to make the solution have good circulation, the cross-sectional area of the central circulating pipe is generally the 40~100% of the total sectional area of the other heating tubes, the heating tube is generally meters and the heating pipe is in the 25~75mm. The evaporator is widely used because of its advantages such as compact structure, convenient manufacture, good heat transfer and reliable operation. However, due to structural constraints, the cycle is not very fast. In addition, the solution in the heating chamber is constantly circulating, so that its concentration is always close to the completion of the liquid concentration, so the solution is high boiling point, the effective temperature difference is reduced. This is the common shortcoming of the circulation evaporator. In addition, equipment cleaning and maintenance is not convenient, so this evaporator is difficult to fully meet the production requirements.
2. Suspended Basket type evaporator: In order to overcome the shortcoming of the evaporation liquid in the circulating evaporator, which is easy to be crystallized and not easy to clean, the structure of the standard evaporator is improved reasonably, which is the suspending-basket evaporator. The heating chamber is 4 like a basket, which is suspended in the lower part of the evaporator shell and replaces the central circulation tube by heating the annular channel between the outer wall and the evaporator inner wall. The solution rises in the center of the heating tube and then flows downward through the annular gap between the outside wall of the suspended-basket heating and the inner wall of the evaporator. Because the annular area is about the 100至150% of the total sectional area of the heating pipe, the solution cycle speed is larger than the standard evaporator, and can reach 1./. In addition, the evaporator's heating chamber can be removed from the top for overhaul or replacement, and the heat loss is also small. Its main disadvantage is the complex structure, the unit heat transfer area of metal consumption more.
3. Levin-Type Evaporator: The above-mentioned natural circulation evaporator, the cycle speed is not large enough, generally in 1./below. In order to make the evaporator more suitable for evaporation viscosity of large, easy to crystallization or scaling serious solution, and increase the circulation speed of the solution to extend the operating cycle and reduce the number of cleaning.
The structure characteristic is to add boiling room on the heating room. The solution in the heating chamber is not boiling in the heating tube because of the static pressure attached to the liquid column of the boiling chamber. Until it rises to the boiling chamber when the pressure is lowered to start boiling, the boiling vaporization of the solution is shifted from the heating chamber to the boiling chamber without the heat transfer surface, thus avoiding the formation of the crystallization or fouling in the heating tube. In addition, the section of the evaporator is about the $number times of the total cross-sectional area of the heating pipe, the solution cycle speed is up to 2.5 to 3 m/s, so the total heat transfer coefficient is also larger. The main disadvantage of this kind of evaporator is the temperature difference loss caused by static pressure head effect of liquid column (see 6.3.1) larger, in order to maintain a certain effective temperature difference requires heating steam has higher pressure. In addition, the equipment is huge, consumes the material many, needs the tall plant and so on. In addition to the above-mentioned natural circulation evaporator, in the evaporation viscosity of large, easy crystallization and scaling materials, but also the use of forced circulation evaporator. In this evaporator, the circulation of the solution mainly relies on the additional power, and the pump forces it to flow in a certain direction to produce a cycle. The size of the cycle speed can be controlled by the flow rate of the pump, generally in 2./above. The heat transfer coefficient of forced circulation evaporator is also larger than that of normal natural circulation. But its obvious drawback is the energy consumption is big, each square metre heats the area to need approximately 0.4~0.8kW.