The key aspect of any industrial kiln is that it creates an immensely hot space within the rotating drum. However, how the inner chamber becomes hot can differ between kilns. There are two primary types of firing methods: direct-fired and indirect-fired. These have substantially different outcomes, and it’s important to match the firing method to the process you’re attempting to implement.
These are the major differences between direct- and indirect-fired industrial kilns.
In a direct-fired industrial kiln, the heat source is directly within the kiln itself. This means the kiln must be lined with strong refractory bricks, or a similar inner surface, to trap the heat and reflect it inwards.
Direct-fired kilns are used when maximum heat is called for, such as pyroprocessing. They are useful for destroying organic matter to reclaim inorganic components, such as separating metals during material recycling.
However, in many cases, the materials being fired inside a kiln should not come directly into contact with the heat source. In that case, an indirect-fired kiln should be used.
With an indirect-fired kiln, the heat source is located outside the kiln, typically either gas flames or electric burners. The heat is transferred through the skin of the kiln to heat up the interior. As such, indirect-fired kilns typically have much thinner inner linings, so that it’s easy for heat to transfer through.
These are most typically used when the materials inside cannot withstand direct contact with the heat source.
However, indirect-fired kilns have other advantages as well. For example, it is much easier to maintain and shape heat throughout the full length of the kiln when the heat source is located outside. The heat source can be more easily controlled, allowing for highly precise shaping of the heat envelope.
Along the same lines, this higher degree of control also makes it simpler to maintain specific atmospheres within the kiln.
Also, indirect-fired kilns tend to release less gas from within the kiln. This, in turn, means that there’s less need for treatment, redirecting, or otherwise dealing with these hot gasses. In some cases, this can lead to more-efficient heating with less energy waste.
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