Abstract: A unit for the preparation of fermenting gas by fermenting viscous media with mixing paddles that can be moved continuously back and forth and in the same direction in the tank to mix the medium to be fermented. Alternately, the mixing paddles hang from a rotating roller in the tank and the roller is driven such that the mixing paddles are moved up and down. The unit may be fitted with several mixing paddles arranged in series and in the direction of the mixing effect, i.e., the suspended mixing paddles are arranged above each other in the event of a vertical tank, in which case a joint drive unit is provided for each mixing paddle group, thus moving the mixing paddles simultaneously in such a manner that each individual mixing paddle executes only a limited stroke, but the mixing paddles as a whole achieve a complete mixing of the medium to be fermented in the chamber, in which are installed the mixing paddles. The heat exchange is improved, when a heat exchange wall is installed between the two chambers.