Abstract: A thermal system includes a plurality of heat exchangers in series and the same number of flash tanks in series is interposed between two subsequent effects of a multiple effect evaporating system for sulfate black liquor. The liquor is indirectly heated in the heat exchangers by vapor produced by step-by-step expansion of the same liquor in the flash tanks and additionally by steam from an external source. To decrease the viscosity of the black liquor it is heated to a temperature of 190.degree.-200.degree. C. and retained in a reactor vessel for 10-20 minutes.Preferably the flash tanks and the heat exchange elements are positioned on top of each other within a shell to form an integral construction in which the vapor compartment of each flash tank is directly connected to the vapor compartment of a corresponding one of the heat exchange elements.