Generally, before the dilute black liquor enters the evaporation system, it must pass through the preheater to get a higher temperature of black liquor.

The reasons are as follows:

①The black liquor that is preparing to enter the evaporator. Due to the different equipment and process conditions for pulping and extraction of black liquor, the temperature difference of dilute black liquor is large. The temperature of black liquor in wood pulp mills generally does not exceed 75~80℃.

The temperature of the black liquor in a straw pulp mill may be lower. Meanwhile, after a certain storage time, the temperature of the dilute black liquor is reduced.

According to the process of black liquor in the evaporation, if there is large scale of temperature difference between the inlet temperature of black liquor and the boiling temperature of the black liquor in evaporation, especially for the rising film evaporator, it is necessary to use a black liquor preheater.

②Preheat the black liquor which is in the low temperature condition before entering the evaporation process in high temperature . When the mixed flow or countercurrent liquid supply process is adopted, in order to heat the black liquor to the boiling point temperature, the black liquor is made to be close to the temperature under the high temperature working pressure. Black liquor preheater is generally essential equipment.