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Rotary Kiln Refractory: The Working Layer vs. The Insulating Layer

Written by:Carrie Carlson

After a material is chosen for yourrotary kilnrefractory, you must decide the desired heat loss. Whenefficiency is of concern, or very high temperatures are involved, often it is desirable to use multiple layers of

refractory: a working layer, and an insulating layer. The working layer is what is in direct contact with the

material being processed. Because of this, this working layer is a dense lining that can withstand the high

temperatures within the rotary kiln, and the constant abrasion from the material. However, when it comes to

refractory, the denser it is, the less insulating capabilities it has. This means that even though there may be a

tough, durable, thick working layer in place, the heat can easily pass through it to the shell of the rotary kiln. For

this reason, an insulating layer is needed beneath the working layer (See Figures 1 and 2). The insulating layer

does just that; it insulates the shell of the rotary kiln so the high temperatures cannot reach the shell and

damage it, or the nearby components.

Typically the working layer and the insulating layer are made of the same material (ie. brick or castable), with

varying chemistries. The working layer tends to be a higher density, stronger material that is more conductive.The insulating layer does not need these qualities, and tends to be softer, lighter, and less conductive,

therefore more insulating. These two layers often vary in thicknesses, and these are determined from the

needs of the rotary kiln and what material is being processed. Although, sometimes temperatures are low

enough, or efficiency is of little concern, it is only necessary to use one working layer. For these reasons,

refractory in a rotary kiln is often a very custom part of the design.

Sometimes when insulation is extremely critical, an optional third layer of ceramic fiber backing is used (See

Figure 1). Though there are various kinds of this backing, this thin, but very efficient layer is similar to fiberglass

insulation found in a house, but it is much more compressed. However, the decision to employ this layer comes

with some responsibility. Should a crack in the refractory occur and go unnoticed, it is possible for the high heat

inside the rotary kiln to reach this backing and actually burn it up. This would create a gap between the

refractory itself, and the shell of the rotary kiln, which could cause disastrous problems. Due to this potential of

increased risk, this third layer is not always appropriate.

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FIGURE 1.

This is an example of castable

refractory. Anchors help to

give the refractory its strength.

Note the working and

insulating layers, with the

optional ceramic fiber backing.

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FIGURE 2.

This figure shows an example

of brick refractory with a

working layer and insulating

layer.