The evaporator is a very important component among the four major components of refrigeration. The low-temperature condensed liquid passes through the evaporator, exchanges heat with the outside air, vaporizes and absorbs heat, and achieves the refrigeration effect. The evaporator is mainly composed of a heating chamber and an evaporation chamber. The heating chamber provides the heat required for evaporation to the liquid, causing the liquid to boil and vaporize; the evaporation chamber completely separates the gas and liquid phases.
Evaporators are divided into three types according to operating pressure: normal pressure, pressurized and reduced pressure. According to the movement of the solution in the evaporator, it is divided into:
1.Circulation type. The boiling solution passes through the heating surface multiple times in the heating chamber, such as central circulation tube type, suspended basket type, external heating type, Levin type and forced circulation type, etc.
2.One-way type. The boiling solution passes through the heating surface in the heating chamber once, and the concentrated liquid is discharged without circulating flow, such as rising film type, falling film type, stirring film type and centrifugal film type.
3.Direct contact type. The heating medium is in direct contact with the solution for heat transfer, such as a submerged combustion evaporator. During the operation of the evaporation device, a large amount of heating steam is consumed. In order to save heating steam, a multi-effect evaporation device and a steam recompression evaporator can be used. Evaporators are widely used in chemical industry, light industry and other departments.
1. According to evaporation method:
Natural evaporation: that is, the solution evaporates at a temperature lower than the boiling point, such as seawater salt. In this case, because the solvent only vaporizes on the surface of the solution, the solvent vaporization rate is low.
Boiling evaporation: Heating a solution to its boiling point causes it to evaporate in a boiling state. Industrial evaporation operations are basically of this type.
2. According to heating method:
Direct heat source heating is an evaporation process in which fuel is mixed with air and the high-temperature flame and smoke produced by combustion are directly injected into the evaporated solution through a nozzle to heat the solution and vaporize the solvent.
Indirect heat source heating is transferred to the evaporated solution through the walls of the container. That is, the heat transfer process carried out in the dividing wall heat exchanger.
3. According to operating pressure:
It can be divided into normal pressure, pressurized and reduced pressure (vacuum) evaporation operations. Obviously, heat-sensitive materials such as antibiotic solutions, fruit juices, etc. should be processed under reduced pressure. High-viscosity materials should be evaporated using pressurized high-temperature heat sources (such as thermal oil, molten salt, etc.)
4. Score by effectiveness:
It can be divided into single-effect and multiple-effect evaporation. If the secondary steam generated by evaporation is directly condensed and no longer used, it is called single-effect evaporation. If the secondary steam is used as the next effect heating steam and multiple evaporators are connected in series, the evaporation process is called multiple effect evaporation.
Evaporation is a unit operation that uses heating to heat a solution containing non-volatile solutes to a boiling state so that part of the solvent is vaporized and removed, thereby increasing the concentration of the solute in the solvent. There are the following occasions when evaporation operations are applied in industrial production:
1. Concentrate dilute solutions to directly produce products or reprocess concentrated solutions (such as cooling and crystallization) to produce solid products, such as the concentration of electrolytic caustic soda solution, the concentration of sugar aqueous solutions, and the concentration of various fruit juices.
2. Concentrate the solution and recover the solvent at the same time, such as the concentration and debenzenization of organophosphorus pesticide benzene solution, the evaporation of alcohol leachate in the production of traditional Chinese medicine, etc.
3. In order to obtain pure solvents, such as seawater desalination, etc.
In short, evaporation operations are widely used in chemical industry, food industry, pharmaceutical industry, etc.
