An installation for treating a plastic melt, in particular a polycondensate melt, and setting the intrinsic viscosity thereof, having a reactor which comprises a reactor housing, with at least one first reactor housing part with an upper end region and a lower end region, and a first chamber part extending between the upper and the lower end region (6, 7), wherein the first chamber part has a vertical height extent, and the reactor housing has, in the region of the lower end region of the at least one first reactor housing part, at least one second reactor housing part, with a second chamber part, directly adjoining said first reactor housing part, wherein the at least one second reactor housing part comprises an approximately horizontally extending longitudinal extent with a first end region and a second end region arranged at a distance from said first end region, and the chamber parts (8, 9) are flow-connected to one another and formed to be sealed off with respect to the external surroundings, and in the region of the upper end region of the at least one first reactor housing part at least one feed line for the plastic melt opens into the at least one first reactor housing part at at least one inlet opening, and at least one outlet opening for the plastic melt is arranged in the at least one second reactor housing part, and wherein the at least one outlet opening for the plastic melt is arranged in the region of the second end region of the at least one second reactor housing part, said second end region being arranged at a distance from the at least one first reactor housing part, and the chamber parts (8, 9) of the first and second reactor housing parts (4, 5) that are flow-connected to one another are flow-connected to a negative pressure generator via at least one connection opening and at least one extraction line, and having at least one mixing element which is arranged in the at least one second reactor housing part, said mixing element being mounted in the at least one second reactor housing part so as to be rotatable about an axis of rotation, wherein the reactor housing comprises two first reactor housing parts and two second reactor housing parts, characterized in that - the two horizontally arranged second reactor housing parts are arranged so as to face one another on their second end regions and are connected to form a unit on the second end regions, - the at least one outlet opening is arranged in a floor region of the second reactor housing parts, and - the at least one mixing element respectively extends over the longitudinal extent of the second chamber parts between the first and second end regions (15, 16) of the second reactor housing parts arranged at a distance from one another and is entirely arranged in each of the second chamber parts. - the two horizontally arranged second reactor housing parts are arranged so as to face one another on their second end regions and are connected to form a unit on the second end regions, - the at least one outlet opening is arranged in a floor region of the second reactor housing parts, and - the at least one mixing element respectively extends over the longitudinal extent of the second chamber parts between the first and second end regions (15, 16) of the second reactor housing parts arranged at a distance from one another and is entirely arranged in each of the second chamber parts.