Aggregate Heating and Drying , Dryers are inclined rotating steel drums ranging in size from 1 m to 3 m in diameter and from 5 m to 16 m in length depending on the capacity of the plant and the moisture content of the materials being handled.
A dryer may not be required in a plant producing cold mix but it is an essential component of a hot mix plant.
In a hot mix plant, the dryer performs two functions: it removes moisture from the aggregate and heats it to the required mix temperature.
Cold aggregate is fed in one end of the dryer and is continually lifted by lifting flights which line the cylinder and dropped in veils through the burning flame and hot gases. The slope of the cylinder, its speed of rotation, diameter, length and arrangement of flights all control the time taken by the aggregate to pass through the dryer.
The amount of aggregate entering the dryer should be such that it can be dried and heated to a temperature slightly higher than mixing temperature, before passing out at the opposite end of the drum. The hot aggregate is carried to the screen deck by means of a bucket elevator.
A temperature control in the exhaust line is generally used to adjust the burner (or burners) to maintain a reasonably uniform temperature in the dried aggregate.
A check should be kept on the temperature of the discharged aggregate at all times to ensure that it is within the specified range because overheating of the aggregate may result in damage to the binder by oxidation. This description about aggregate heating and drying will be part of the process of asphalt batch and continuous plants.