The present invention relates to a reactor apparatus (25) for free-flowing media, especially for polymers, for polycondensation of polyesters, comprising a rotor which rotates in a reactor casing (1) having an inlet and outlet (31, 32), said rotor being supported horizontally via a support apparatus (4) by means of stub shaft arrangements which reach beyond the end walls of the reactor casing (1), wherein annular film-forming elements (8) and, between the annular film-forming elements (8), 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.