The screw conveyor dryer (SCD) finds varied applications in the
process industries either as a pre-dryer or for post-drying operation.
In certain cases, it can be used as the main dryer. The full potential
of SCD has not been exploited for the lack of its understanding.
The conveying of paste or particles through a screw conveyor may fol-low a near plug flow behavior; however, it is essential to ascertain a
desired level of inter-particle mixing during the course of drying/
conveying so as to obtain the desired drying conditions. The effects
of feed rate (15–176 kg/h) and screw speed (10.8–28 rpm) on the
RTD were studied for the conveying of sand in a pilot-scale SCD using
pulse input of a tracer (dye-coated sand). Two new parameters defining the relative stagnancy and screw effectiveness were introduced to
study the flow performance of SCD. The flow in the SCD approaches
plug flow with an increase in feed rate or by decrease in the screw
speed. A low value of the holdback, a parameter of relative stagnancy,
indicates the flow in SCD to be near plug flow. Mean residence time
was found to be 3 to 4 times longer than the linear residence time due
to lower values of screw effectiveness (0.24–0.32).