Pre-evaporation systems utilize low-level waste heat to evaporate weak liquor prior to the main evaporator train. Three heat sources are most often utilized in a pre-evaporator system: heat from a blow heat system, heat from a stand-alone concentrator or heat from a continuous digester extraction liquor flash tank. The amount of evaporation achievable is dependent upon both the quantity of heat available and the number of effects in the system. Whenever a pre-evaporator is installed, its impact on the existing multiple effect evaporator system must be considered, in particular the effect of the higher feed solids.

Aside from using waste heat, one of the main advantages of a pre-evaporation system is condensate segregation. The ideal scenario for a mill with a pre-evaporation system is to use separate liquor storage tanks for the virgin weak liquor and liquor that has been through the pre-evaporator. The pre-evaporator strips the bulk of the TRS and VOC's from the liquor resulting in an evaporator system condensate that is substantially cleaner.

A blow heat pre-evaporator system is two or three effects. Because of the limited delta T available in this system, a three-effect system would likely use falling film technology while a two-effect system could be either falling film or LTV technology. Condensate from the blow heat system is flashed one or more times externally to the pre-evaporator to provide the heat source to drive the system.

The vapor from a stand-alone concentrator system can also be utilized for pre-evaporation of liquor. Since the quantity of heat is fixed, the evaporation rate is dependent upon the number of effects. The number of effects possible is dependent upon both the concentrator vapor pressure and whether falling film or LTV technology is utilized in the pre-evaporator system.

A continuous digester pre-evaporator system, also commonly known as a flash evaporator, is usually a co-current multiple effect system using either falling film or LTV technology. Extraction liquor from the digester flashes in a flash tank to drive the first effect, evaporating liquor from the flash tank. The liquor then flashes and evaporates in subsequent effects, cascading down in pressure and temperature so that the liquor exits the system at a temperature suitable for the multiple effect evaporator system. Both vapor for the steaming vessel and chip bin can be provided by the flash evaporator system. The recovery of turpentine at a high efficiency rate is also possible in this system. In addition, hot water at an appropriate temperature for process use can be generated without loss of evaporation.