A device for making polybutylene terephthalate includes (1) a slurry paste vessel; (2) a tower reactor to which a mixture of 1,4-butane diol and terephthalic acid from vessel (1) is supplied, the tower reactor having a plurality of reactor zones wherein the lower third of the tower reactor is in the form of a hydrocyclone with attached heat exchanger, and the hydrocyclone has a supply line from vessel (1), the hydrocyclone being connected to the top side of the tower reactor; (3) a first continuously stirred tank reactor to which the product from tower reactor (2) is supplied; (4) an optional second continuously stirred tank reactor to which the product from (3) is supplied; (5) a dual shaft ring reactor to which the product from stirred tank reactor (3) or (4), is supplied; and (6) a pelletizer where the product from dual shaft ring reactor (5) is continuously fed and pelletized.