Several rotary kiln designs have evolved, each specific to the process
application it is intended for. They also come in several forms and
shapes. Although the majority consist of straight, cylindrical vessels,
dumbbell-shaped designs (Figure 1.4) take advantage of the benefits
that variable drum sizes can bring to process application. With regard
to internal kiln fixtures, most direct fired kilns are lined with refractory
materials for several reasons but the primary purposes are to insulate
and protect the outer shell, in high temperature applications, from
thermal damage and to save energy. Kilns may also be equipped with
dams to increase the material dwell time or with lifters and tumblers (Figure 1.5) to aid the materials to flow axially and in some
cases to improve particle mixing achieved through surface renewal.
Table 1.2 presents some of the energy-saving advantages of using lifters
in various applications and processes. Some of these savings can be
substantial.