Tyres and rollers of Rotary kiln

A tire, sometimes called a ring, usually consists of a single annular steel casting,machined to a smooth cylindrical surface,loosely attached to the kiln shell by various "chair" arrangements.These all required some clever design, as the tire had to fit snugly against the casing, but also allow thermal movement.The tires ride on pairs of steel rollers that are also machined to a smooth cylindrical surface and are spaced about half a kiln diameter apart.The drum must support the kiln and allow rotation as friction-free as possible.A well designed kiln, when the power is cut, will swing like a pendulum many times before it stops.The mass of a typical 6 x 60 meter kiln (including refractories and feed) is approximately 1100 tonnes and will be carried by three sets of tires and rollers distributed along the length of the kiln.The longest kilns may have 8 sets of drums, while very short and small kilns may not.The kiln typically rotates at a speed of 0.5 to 2 rpm.The kiln in a modern cement plant runs at 4 to 5 rpm.The bearings of the rollers must be able to withstand the large static and live loads involved and must be carefully protected from the ingress of furnace heat and dust. Since the kiln is sloped, it also needs support to keep it from walking off the support rolls.Usually the upper and lower "holding (or thrust) roller" bearings rest against the side of the tire to keep the kiln off the support rollers.

Drive gear 

The kiln is usually turned by a single girth around the cooler part of the kiln tube, but sometimes driven rollers are also turned.The gears are connected to the variable speed motor through a gear train.Eccentric load kiln with large starting torque. A 6 x 60 m kiln needs around 800 kW to turn at 3 rpm.The speed at which material flows through the kiln is directly proportional to the rotational speed; a variable speed drive is required to control it.When driven by rollers, hydraulic drive can be used.These have the advantage of producing extremely high torque.In many processes,it is dangerous to let the heat press stand still if the drive power fails.The bottom of the kiln will cause warping of the kiln body and damage to the refractory material.Therefore, it is common practice to provide auxiliary drives for use during power outages.This could be a small electric motor with its own power supply, or it could be a diesel engine.This makes the kiln turn very slowly, but enough to prevent damage.

Internal heat exchangers

Heat exchange in a rotary kiln can be done by conduction, convection, and radiation, with high to low efficiencies.In low temperature processes, and in the cooler sections of long kilns without a preheater, the kiln is usually equipped with internal heat exchangers to facilitate heat exchange between the gas and the feed.These may include scoops or "risers" which cascade the feed through the air flow, or possibly metal inserts which are heated in the upper part of the kiln and transfer heat to the feed as it dips below the feed surface, as The kiln rotates. The latter is favored where the jack would cause excessive dust pickup.The most common heat exchanger consists of chains suspended in a curtain in the airflow.

Other equipment

The kiln is connected with the discharge hood at the lower end and the exhaust gas pipeline.This requires a hermetic seal at both ends of the kiln.The exhaust gas may be wasted, or it may enter a preheater where it further exchanges heat with the incoming feed.Gas must be drawn into the kiln and preheater (if installed) by means of a fan located on the exhaust side.In preheater units, which may have high pressure drops, considerable fan power may be required, and the fan drive is usually the largest drive in the kiln system. Exhaust air contains dust and may contain undesirable components such as sulfur dioxide or hydrogen chloride.Install equipment to remove these substances from the air stream before entering the atmosphere.