Screw dryers are used for drying pourable products at low temperatures. Large surfaces of heat transfer are obtained due to the heated vertical mixing screw. Even at low temperatures, large quantities of energy are transmitted by the surfaces resulting in gentle drying of products sensitive to temperatures. Conical screw dryers consist of a hopper-type container and a rotating mixing tool arranged vertically. By means of this mixing tool, the product is conveyed upward in the center and slides down at the walls. Due to this intensive exchange, a high heat transfer is obtained. The product is discharged at the bottom of the container. The advantage of the screw dryer is the large heat-exchanging surface with low space requirements. Good residual emptying and easy cleaning of the machine are additional advantages. The dryers can be additionally equipped with injection devices, pressure and temperature measuring units as well as sampling devices. In the case of special executions of the mixing tool it is also possible to provide knife heads.