Welcome back to the second half of our series on utilizing rotary dryers as pre-heaters within your industrial kiln setup. In the previous article, we covered the benefits of pre-heating, as well as why industrial dryers are well-suited for this role. In this second part, we’ll look into the specifics of preheating dryer designs and how they could affect the outcome.
Choices In Rotary Dryer Design When Being Used for Pre-Heating
1 – Direct vs Indirect Heating
As with kilns, rotary driers can potentially be heated either through direct or indirect heat. Direct-heated driers include the heat source within the drum, while indirect models place the heat source outside the drum.
In most situations, direct heating is the best option here. Since dryers run at much lower temperatures than industrial kilns, there’s less chance of the heat source damaging materials within. Meanwhile, these offer much more efficient heat transfer and reduce fuel use, which is one of the main benefits of using a dryer for pre-heating.
However, if the materials or feedstock cannot come into contact with combustion gasses, indirect heating is still an option. You could also incorporate a combustion chamber to isolate the heat source within a direct-heating system.
2 – Co-Current vs Counter-Current Air Flow
Again, as with kilns, rotary dryers can be configured for co-current airflow, where the heat moves along with the materials, or counter-current where it blows against the material movement.
Generally, counter-current is the better option here. This arrangement guarantees that the hottest point in the dryer is immediately before the materials exit into the kiln, so they enter the kiln at maximum possible heat. This reduces thermal shock. Otherwise, the material could potentially cool significantly as they transfer from the pre-heater and into the kiln drum.
Note: when utilizing a rotary dryer for pre-heating, it typically requires a lower air volume than for drying operations. This offers another opportunity for cost savings.
3 – Flights
Rotary dryers utilize a flight as a material lifter for moving it through the drum. Flights are typically customized to the needs of the user, to best transport the materials being heated. You should consult with your supplier on the best flight design to properly move your feedstock while exposing it to maximum amounts of airflow.
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