Many fuels (oil, wood or process gases) contain components which can be found as acid-forming compounds in the flue gas after combustion and which are absorbed to a small degree in the condensate when the dew point is reached. This acid, and therefore highly corrosive condensate, increases the demands on the materials used for the areas affected by condensation in condensing boilers. The concepts described in the article "Design of Scrubbers for Condensing Boilers" are based upon use of the condensate for washing the flue gas. To achieve this, the flue gas is cooled below the dew point in contact with the already neutralized condensate. This process step allows the wet separation of noxious matter and avoids acid corrosion of the materials in the area of condensation, raising the choice of possible materials decisively. The review article also exemplifies the state-of-the-art for condensing boiler technology, as it gives a view of the fundamental principles of two-phase flows, of absorption of acid-forming gases and their neutralization. Using the examples of sulfuric oxides SO, and nitrous oxides NO,, the effects of different characteristic features of the gases on the reaction steps are described and possible process steps of the wet separation in the condensate are discussed. To achieve as high a separation degree as possible between the flue gas and the condensate, good heat and mass transfer conditions must be guaranteed between the two phases-flue gas and condensate. Dispersing one of the two phases leads to a strong increase of the interphase. Generally, fluid vaporizers (flue gas as coherent phase) and gas bubble washers (condensate as coherent phase) can be taken into consideration. Advantages and disadvantages of these absorbers are worked out for use as washers in combination with condensing boiler technology and the fluid-mechanical principles necessary for the design of a gas bubble washer. The sometimes contrary influences of constructive parameters on pressure changes in the flue gas, as pressure loss and mass transfer conditions between hue gas and condensate, are discussed.