Operation
Rotary cascade dryers have the feed materials pass through a rotating cylinder together with a stream of hot gas. Internal lifters or flights elevate the feed and drop it in a curtain from the top to the bottom cascading along the length of the dryer. Flights need to be carefully designed to prevent asymmetry of the curtain. Material moves from one end of the dryer to the other by the motion of the material falling due to the angle of inclination of the drum. Some rotary dryers are double and triple pass units where each drum is nested inside the previous drum.
Rotary louvre dryers have the feed materials supported and moving over a set of louvres mounted to an external rotating drum. The hot gas is introduced into a tapered bustle below the feed ring. The air passes through the louvres and then the product before being exhausted from
the dryer. The rotation of the drum causes the material to roll and mix providing intimate contact with the drying gas. Material moves from one end of the dryer to the other by the motion of rolling, some fluidization and due to the angle of inclination of the drum.
Feeding /discharge
Automatic and continuous. Vibrators, screw feeders, rotary airlocks, double flap valves, gravity chutes
Feed Characteristics
Powders, granules, non friable agglomerates, large solid particles. Low to high feed rates
Heat Source
Steam, electrical, coal, liquid fuels, or gas
Exposure
Mostly Direct. Flow is co-current or countercurrent for cascade dryers. Flow is a combination of through the bed, cross, co-current or countercurrent for rotary louvre. Residence time Controllable. Residence time can be altered by varying the speed of rotation of the drum and the drum angle. Fill can be varied. Fills are typically between 7% and 25% of the cross sectional area of the drum.
Extraction
Normally induced. Systems can have combustion, forced and induced draft fans for operation.
Dust Collection
Cyclones, bag houses, scrubbers, Electrostatic precipitators. Systems require dust collection.
Control
PLC, infrequently solid state controllers. System controls on discharge temperature. Can modulate energy and feed.
Benefits
High throughput
Forgiving Operation
Low cost - Higher for louvre
Gentle handling for louvre dryer
Very intimate product contact for louvre dryer
Robust
Can handle variations in feed although the product may be inconsistent
High temperature operation - can be refractory lined.
Unit can have integral cooling section.
Limitations
Clogging although chains and knockers will reduce this May cause size reduction due to attrition and impact Inconsistent dried product characteristics
Poor energy efficiency - high losses in cascade units
Poor heat transfer in cascade units
Seals are problematic on high temperature applications (hot side)
High maintenance
Large real estate required
Many wear components
No implicit dust separation |
