In the production of plastic granules, centrifugal dryers are used for separating the granulation water from the granules. These known centrifugal dryers essentially consist of a rotor surrounded by a strainer basket arranged vertically in a housing. The wet or moist granules are fed into the bottom of the dryer and delivered at the top at a product outlet by conveyor blades arranged on the rotor. In the region of the product outlet is arranged a fan provided with ejector blades for assisting the discharge of dry plastic granules. The water is separated by the strainer of the strainer basket. An upwardly directed current of air is produced by the conveyer blades by rotating the rotor and this is further intensified by the suction effect of the uniform size ejector blades of the fan. In order to reduce the suction effect of the fan, it is known from CH-PS 620 859, for the drying of light foil strips, to adjust the magnitude of air aspirated from the top by the fan by means of adjustable air slots, depending on the type of material to be dried.
In this way only the quantity of air aspirated at the top for the discharge fan is controlled; different pressure stages for considerably longer or shorter residence times of the granules in the centrifugal dryer cannot be realized in this way.
The residence time of the granules in the dryer can be adjusted according to DE-OS 31 20 792 by changing the angle of the conveyor blades, by providing a stepwise or conical configuration of the rotor and/or by changing the rotor speed. For this purpose, of course, considerable modifications to the dryer are necessary. A high abrasion of the drying material is produced at the transition regions of the individual steps in the case of the stepwise configuration.
By improving the preliminary separation of water after the granulating process the centrifugal dryer often no longer must assume the primary task of separating large quantities of water, but only of separating the residual moisture from the plastic granules. If the plastic granules have been substantially dewatered in advance, problems occur in introducing the granules into the centrifugal dryer, since the transport water is now absent.