When looking for a rotary kiln for sale, there are several key considerations to make – but one of the most important is its airflow configuration. That is, whether air flowing through the kiln is moving in the same direction as the heated material (co-current airflow) or whether it moves in the opposite direction (counter-current).
Airflow is crucial because it is one of the main determining factors in how heat is distributed throughout the kiln’s interior. The air carries heat, so understanding the flow of air is necessary to create a proper heat envelope within your kiln. In some cases, airflow may even need to be adjusted and controlled to produce desired effects based on the materials being fired. Whether your kiln uses counter-current or co-current airflow will have a huge impact on its capabilities and types of material which can be fired.
Kilns can only be designed to utilize one type of airflow, so it’s vital to understand their differences and which processes are best supported by each type. In this article, we’ll briefly cover both counter-current and co-current airflows, as well as their advantages and disadvantages.
Understanding Counter-Current vs Co-Current Kiln Airflow
I. Counter-Current Flow
When the airflow is moving in the opposite direction from the flow of material, the primary result is that it causes the material to be heated up relatively slowly and evenly.
This can be highly advantageous in situations where you need the material to heat up in a highly controlled way, or if you need to prevent it from exceeding a certain temperature at any point. As such, counter-current kilns are often used in situations where a chemical change is being induced. Heat hardening is another process which benefits from this airflow.
Additionally, such kilns are also extremely heat-efficient. However, they do require a higher initial temperature to heat the material as desired. In turn, this causes them to be at higher risk of overheating, which can damage the refractory lining or cause warping in the shell. Either condition can seriously harm your kiln, leading to costly repairs. Monitoring the temperature is crucial when using counter-current airflow.
II. Co-Current Flow
In a co-current flow kiln, both the heated air and the material are introduced at the same end and move together across the kiln. This means the coldest material will be put directly into contact with the hottest gases and creates an extremely rapid temperature change. However, this change is also somewhat uncontrolled, compared to the control granted by a counter-current flow kiln.
Co-current flow is most commonly seen in kilns intended for organic combustion processes since those benefit from the rapid heating and -therefore- rapid combustion. Likewise, there is no need for substantial fine control over the heating process. So, such kilns can speed up any process where rapid heating is key.
However, many materials and processes do not benefit from such rapid, semi-controlled heating. As such, co-current flow kilns are less commonly seen than counter-current models and have more specialized usage.
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