The present invention relates to a reactor apparatus for free-flowing media, especially for polymers, for polycondensation of polyesters, comprising a rotor which rotates in a reactor casing having an inlet and outlet, said rotor being supported horizontally via a support apparatus by means of stub shaft arrangements which reach beyond the end walls of the reactor casing, wherein annular film-forming elements and, between the annular film-forming elements, strippers are arranged on the inner surfaces of the reactor casing on the rotor, such that the cylindrical rotor has at least one partly heated section.