Propellent plate evaporators can be divided into the following three categories according to the evaporation process:

    (1) Atmospheric pressure evaporation, pressure evaporation and reduced pressure evaporation

    According to the different evaporation operating pressures, it can be divided into normal pressure, pressurized and reduced pressure (vacuum) evaporation. For most solutions without special requirements, normal pressure, pressurized or reduced pressure can be used. However, the evaporation of heat-sensitive liquids, such as antibiotic solutions, fruit juices, etc., needs to be carried out under reduced pressure in order to ensure product quality.

    The following are the advantages of reduced pressure evaporation.

    ① The boiling point of the solution decreases. Under the condition of a certain heating steam temperature, the average temperature difference of the evaporator heat transfer increases, so the heat transfer area decreases;

    ②As the boiling point of the solution decreases, low-pressure steam or waste heat steam can be used as heating steam;

    ③The solution has a low boiling point, which can prevent the denaturation or decomposition of heat-sensitive materials;

    ④ Due to the low temperature, the heat loss of the system is small. But on the other hand, due to the lower boiling point and the higher viscosity of the solution, the heat transfer coefficient of evaporation is reduced. At the same time, vacuum evaporation requires additional equipment and power when evaporating under reduced pressure.

     (2) Single-effect evaporation and multi-effect evaporation

    According to whether the secondary steam is used as heating steam for another evaporator, it can be divided into single-effect evaporation and multi-effect evaporation. If the secondary steam of the previous effect is directly condensed and is no longer used, it is called single-effect evaporation. If the secondary steam is introduced to the next evaporator as heating steam, the evaporation process in which multiple evaporators are connected in series to use the heating steam for multiple times is called multi-effect evaporation.

    (3) Intermittent evaporation and continuous evaporation

    According to the process mode of evaporation, it can be divided into batch evaporation and continuous evaporation. Batch evaporation refers to the evaporation operation of batch feeding or discharging. The characteristic of intermittent operation is that during the whole process, the concentration and boiling point of the solution in the evaporator change with time, so intermittent evaporation is an unsteady operation. Usually intermittent evaporation is suitable for small-scale and multi-variety occasions, while continuous evaporation is suitable for large-scale production processes.