The “working” layer and insulating layer refer to a rotary kiln’s multiple layers of refractory. Refractory is what protects the kiln from high internal temperatures, and maintains heat retention. Sometimes it is desirable to use multiple layers of refractory, especially when higher efficiency is preferred, or very high temperatures are involved.
The “working” layer is in direct contact with the material being processed. It consists of a dense lining that can withstand the high temperatures within the rotary kiln and the constant abrasion from the material.
When it comes to refractory, the denser it is, the less insulating capabilities it has. This means that even though there may be a durable, thick working layer in place, the heat can easily pass through to the shell of the kiln. Consequently, an insulating layer is needed beneath the working layer, to insulate the kiln shell so the high temperatures cannot reach the shell and damage it.
The working layer and insulating layer are often made up of the same material (i.e. brick or castable), with varying chemistries. The working layer tends to be a higher density, and stronger material that is more conductive. The insulating later tends to be softer, lighter, and less conductive.
When insulation is especially critical, an optional third layer of ceramic fiber backing can be used. This thin layer is similar to fiberglass insulation found in a house, but more compressed.
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