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.