Rotary kiln is indispensable core equipment in modern dry process cement plants. The design process of a rotary kiln covers the calculation and formulation of various parameters, which is a very complex process. Here we only briefly introduce the determination of some fundamental parameters in the design of rotary kilns.
Shapes of Kiln
Kiln shapes are determined based on the variation of the diameter of each section of the kiln shell. Generally, the shape of rotary kiln can be divided into six types:
a. Straight rotary kiln; b. Rotary kiln with enlarged burning zone; c. Rotary kiln with enlarged calcining zone; d. Rotary kiln with enlarged calcining and burning zone; e. Rotary kiln with enlarged drying, calcining and burning zone (wet process kiln); f. Rotary kiln with enlarged drying or preheating zone (long dry process or wet process kiln)
The enlargement of each zone has its own function. For example, rotary kilns with enlarged drying zones increase the heating area of materials during drying, accelerate the evaporation of slurry water and reduce the heat consumption; rotary kilns with enlarged calcining zones increase the amount of fuel and heat transfer per unit time and have higher outputs.
However, the zone enlargement also causes the zones of a kiln to have different transportation times, which leads to the irregular material passage, thus impairing the kiln’s operation.
After years of practice and application, it is generally believed that the straight rotary kiln is the most suitable rotary kiln for cement dry process production.
In the design process of a rotary kiln, the length-diameter ratio is the parameter that should be first determined.
The designed length-diameter ratio of a rotary kiln should be able to ensure the smooth completion of the material reaction process in the kiln. Apart from that, it also needs to be able to meet the requirements of kiln tail water evaporate and electric dust collector operation, and to ensure an appropriate kiln tail gas temperature.
The inner diameter of an industrial rotary kiln ranges from 1.0 to 4.5 meters, and the length-diameter ratio is generally 12:1 to 20:1.
Number of Tyres
A cement rotary kiln usually has 2~4 tyres to support the kiln shell. The exact number of tyres is related to the diameter and the length-diameter ratio of the rotary kiln. The table below shows the number of tyres under different length-diameter ratios.
Length of Each Section
In the design process, we can divide the rotary kiln body into four sections, which are kiln head overhanging section, kiln tail overhanging section, preheating section, and calcining section.
The length of each part of a rotary kiln is designed based on the inner diameter (D) of the kiln shell.
1) The length of the kiln head overhanging section (l1): l1= (1.2~1.5) D
As the first section of a rotary kiln, l1 is better not to be designed too long, or the length of the effective parts of the kiln will have to be shortened.
2) The length of the preheating section (l2): l2= (5~5.5) D
3) The length of the calcining section (l3): l3= (5~6) D
Because of the high temperature of the kiln body in the calcining zone, the steel plates and refractory lining bricks of this part of the kiln are heavily loaded, so the l4 is not appropriate to be too long.
4) The length of the kiln tail overhanging section (l4): l4= (2~3) D
Due to the low temperature and thin kiln lining of the kiln tail overhanging section, l4 can be designed relatively long, which is conducive to make the reaction moments of the middle sections of the rotary kiln close.
Distribution of Fulcrums
The surface temperature and additional bending stress of the rotary kiln shell are two major factors that must be considered in the design of fulcrum distribution.
When there are three fulcrums in the design of a rotary kiln, the positions of the first and the third fulcrum at the kiln head and kiln tail are basically fixed, so the position of the second fulcrum is particularly important.
Nowadays, in the precalciner kiln used in the dry process cement production line, the decomposition rate of the material entering the kiln is usually above 90%, and the temperature of the material out of the kiln is between 1350 ~ 1400 ℃. The length of the calcining zone is about 50% of the total length of the kiln body, which also makes the high-temperature area of the kiln body long. And the length of the kiln skin used to protect the kiln shell from high temperature is 5.5 ~ 6D.
In the actual operation, the surface temperature of the kiln shell after the kiln skin is about 400 ℃. If the position of the second fulcrum is in the high-temperature area, it is easy to cause the improper gap between the tyre and the rollers, or the kiln shell will be deformed, which will seriously affect the normal operation of the rotary kiln.