The rotary dryer is a type of industrial dryer employed to reduce or minimize the liquid moisture content of the material it is handling by bringing it into direct contact with a heated gas. The dryer is made up of a large, rotating cylindrical tube, usually supported by concrete columns or steel beams. The dryer slopes slightly so that the discharge end is lower than the material feed end in order to convey the material through the dryer under gravity. Material to be dried enters the dryer, and as the dryer rotates, the material is lifted up by a series of internal fins lining the inner wall of the dryer. When the material gets high enough to roll back off the fins, it falls back down to the bottom of the dryer, passing through the hot air stream as it falls. This air stream can either be moving toward the discharge end from the feed end (known as co-current flow), or toward the feed end from the discharge end (known as counter-current flow). Humid air carrying moisture, and in some cases dust, is transferred from the material being and removed in a cyclone, a cyclone / wet scrubber combination or bag filter.
Efficient drying of materials with high moisture contents.
Handles a wide size range of materials with extended residence times.
Design permits highest possible drying temperatures.
High thermal efficiency.
There are two basic configurations of Direct Fired Dryers:
Co-current (also referred to as parallel flow)
Counter-current (also referred to as counter flow)
Co-Current Rotary Dryer
wet material is introduced into the drum on the same end that the hot air enters the dryer. Conversely, dry material discharges on the same end of the dryer as the exhaust vapor. The co-current dryer offers two advantages:
The wettest material comes in contact with the hottest air as they enter the dryer. This results in a rapid evaporation and rapid cooling of the hot process gas. This typically allows the dryer shell to operate slightly cooler than the counter-flow configuration, which extends the life cycle and decreases maintenance of the dryer.
Since the dry product and vapor exit the dryer on the same end, a correlation exists between the exhaust air temperature and the condition of the dried product
Counter-Current Rotary Dryer
With the counter-current or counter flow dryer, wet material is introduced into the drum on the opposite end that the hot air enters the dryer and dry material discharges on the opposite end of the dryer as the exhaust vapor. The counter-current dryer is usually considered for processes such as:
The material is going to be heated in addition to drying. In some aggregate and ore processing applications, it is desired that the material be heated. In those applications, material enters the cold end of the drum and travels towards the hot end. Since the exhaust vapor exits opposite the material, there is no relationship between the temperatures. The product temperature can be hundreds of degrees hotter than the exhaust vapor providing lower stack loss and greater thermal efficiency.
Material has internal moisture. Some materials contain a high percentage of internal moisture. These materials require sufficient time for the water to migrate to the surface of the particle for evaporation.