A system for the solid phase polymerization of polymers wherein cold amorphous polymer is introduced to a crystallizer (12) and heated to crystallize the polymer, the crystallized polymer is discharged to a reactor to achieve polymerization of the polymer, and the hot polymer product of the reactor is discharged to a fluid bed (12) cooler for cooling of the polymerized product. The fluid bed (12) cooler includes an inlet for the hot polymer and an inlet for the cooling gas, and the cooled polymer and the heated gas are discharged from the cooler (34) after contact of the gas with the polymer. The cooler includes a bed portion through which the polymer moves while in contact with the cooling gas, and an upper chamber which collects the heated gas. The temperature of the polymer gradually decreases from the location of the inlet (52) for the polymer to the location of the discharge for the polymer, and the gas in the upper chamber is hotter in the area of the upper chamber adjacent the inlet (52) for the hot polymer than in the area of the upper chamber adjacent the discharge location of the cooled polymer. At least two outlets (3 and 15) are defined by the upper chamber and a weir baffle is used to separate an upper chamber zone containing the hotter gas from the balance of the upper chamber. Hotter gas (20 and 22) is then discharged to the reactor (32) through an outlet communicating with the upper chamber zone containing the hotter gas.