Short dry kilns are usually accompanied by an external preheater or pre-calciner (Figure 1.7) in which the feed is dried, preheated, or even partially calcined prior to entering the main reactor (kiln). As a result the thermal load on the kiln proper is reduced. Hence kilns equipped with preheaters or precalciners tend to be short, on the order of 15–75 m (about 50–250 ft) depending on the process. The shorter kilns are those in which the entering feed material is almost calcined.
Applications include cement and some lime kilns. Because of the large feed particle size encountered in limestone calcination, modern lime kilns are equipped with preheaters which function as a packed bed of stone with a countercurrent flow of kiln exhaust gas rather than the typical cyclone preheaters in cement kiln systems.