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January 2021 / 57 ically conveys wet feed material in heated air at 4,000 to 6,000 fpm from the dryer's feedpoint to a material receiver, such as a cyclone or baghouse. Heat transfers by convection from the air to the material to evaporate moisture, and the residence time in the dryer is typically from 1 to 2 seconds. When the dryer is used with a cage mill — similar to a rotating squir- rel cage, with two facing circular plates connected at their circum- ference with spacer bars — the mill is located at the dryer's feed inlet after the air heater. The wet feed material enters through the rotating cage mill's at side, and the heated air passes through the mill's cylindrical periphery and into the dryer duct. As particles are reduced in size in the cage mill, they're entrained in the airstream and are swept out of the cage mill material toward the heated vessel wall, where a thin layer forms and travels along the wall in a spiral ow path. Heat transfers through the wall into the material while moisture is evaporated into the dryer's vapor space (called annu- lar space, which is between the inner vessel wall and the rotor assembly's outer surface). An air sweep owing countercurrent to the material ow path removes the evaporated moisture from the dryer's inlet end as the nal dried product exits from the dryer's opposite end. This dryer is suitable for delumping material and, when the paddles' tip speed and pitch are properly adjusted, can coarse- grind material to the granular particle size range. Flash dryer with cage mill. The ash dryer consists of a round or rectangular duct that pneumat- uously metered onto the porous belt in a thin layer at the housing's inlet end and exits from the hous- ing's opposite end. The convection dryer circulates heated air down through the material layer and through the belt to heat the mate- rial and remove the evaporated moisture. Because the heated air passes through the material rather than passing over its surface, as in the batch tray dryer, the mate- rial's moisture content is uniform throughout the entire layer. The material remains stationary on the moving belt, eliminating particle attrition and allowing the dryer to handle shaped or formed products such as cereals and snack foods without damage, preserving their geometry or structural integrity. Dryers that reduce particle size Several continuous dryers can reduce your material's particle size during drying. Which one is right for your application depends on the particle size you need to achieve. The following dryers are typical choices for drying applications that require particle size reduction. High-speed paddle dryer. The high-speed paddle dryer consists of a horizontal, cylindrical vessel with a longitudinal rotor-and- paddle assembly. The paddles' tip-to-wall clearance is typically between 0.25 and 0.75 inches, and the paddles' pitch can be adjusted to set the material's residence time in the dryer. Wet feed material is continuously fed into the vessel's inlet end as the rotor-and- paddle assembly rotates, producing a paddle tip speed typically between 1,500 and 2,500 fpm. The dryer uses conduction heating: Steam, hot water, or hot oil circulates through external jacket sections on the vessel to transfer heat through the vessel wall. The rotating pad- dle's centrifugal action throws the PROCESSING THE INGREDIENTS FOR SUCCESS MICRO INGREDIENT SYSTEMS & CONTROLS www.apecusa.com 616.374.1000