Rotary kilns are classified in long dry kilns, short dry kilns, coolers, and dryers, and indirectly fired kilns.
Wet kilns are those typically fed with materials in suspension. These are typically long, with kiln lengths of 150 meters to 180 meters. The Inlet side is typically fitted with chains that serve as a heat “flywheel” by recovering heat in the off gas for use in preheating the inlet and supporting the drying process. Chains are also used to break up lumps that the material may form during the transition phase of changing from suspension to solids during drying. In the cement industry, these kilns are not very efficient and are considered somewhat obsolete, being replaced by long dry kilns. However, there are certain applications that are not suitable for the alternative use of long dry kilns, for example lime slurry kilns found in the pulp and paper industry and some other applications.
Long dry kilns are shorter kilns than wet kilns, with lengths of 90 meters to 120 meters. In these long dry kilns, drying, preheating, and calcining occur in one place. However, these kilns work well when the feed particles are large. The reason for the relatively shorter length is that the feed is dry with a moisture content equal to that of the granular solids rather than the pulp. Applications include lime kilns and lightweight aggregate kilns where mined stones are crushed to around 120 mm to 400 mm before being introduced into the kiln.
Short drying kilns are accompanied by an external preheater or pre-calciner in which the feed material is dried, preheated or even partially calcined before entering the main reactor (kiln). As a result, the thermal load on the kiln itself is reduced. Therefore, kilns equipped with preheaters or precalciners are usually short,15 to 75 meters, depending on the process. The shortest kilns are those where the incoming feed material is nearly calcined. Applications include cement kilns and some lime kilns.
Regarding coolers and dryers some may be in the form of contactors, like the rotary kiln itself, although some are fixed-bed contactors, such as grate coolers. Rotary coolers may be in-line or coupled. However, coupled coolers impose an additional mechanical load that must be accounted for in the design calculations. They also may have maintenance challenges. Rotary coolers and dryers must typically be equipped with lifters, which cause the material to cascade well above its angle of repose to take advantage of better solid-gas contact.
Indirectly fired kilns are those that are externally heated. These kilns are typically designed for applications where direct contact between the material and the gas providing the heat source is undesirable. In this case, the heat source is external to the kiln. Any internally flowing gas in the free space is used to purge any volatile compounds or gases arising from the bed because of chemical or physical reactions. Due to their low thermal efficiency, externally heated indirectly fired kilns are small up to 1.3 meters in diameter and are used for specific applications such as calcining specialty materials.
All these types of kilns will benefit from an external lamella seal because this type of seal helps maintain thermal and energy efficiency by reducing false air intake and material loss. The external lamellas create a flexible and adjustable barrier that adapts to the movement and expansion of the kiln due to high temperatures, which is key to preventing unwanted false air intake and heat loss. In addition, they improve operational safety by reducing the possibility of toxic or hazardous gas emissions into the work environment, thus maintaining a more controlled and safer environment for the process.
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