An improved thermal model for the solid conveying section of a twin-screw extrusion cooker

A new analysis of mass and heat transfer in the partially fielled conveying section of a twin-screw cooker extruder is presented. An original hypothesis, suggested by observations made during dead-stop experiments, was introduced into the thermal model. A layer of material is assumed to be located in the clearance between the barrel and the tip of the screw flight. This layer is heated by conduction from the temperature-controlled barrel, and releases some energy to the solid powdery material transported in the screw channel. In parallel, the screws are assumed to be the origin of a large fraction of the heat supplied to the product. Complete temperature profiles along the solid converying section were computed. The influence of different process parameters was evaluated. The results prove to be qualitatively satisfactory and in agreement with measurements. The limits of this model come from the fact that more complex phenomena (including internal friction between granules) have to be taken into account as soon as the screw channel is completely filled.