TECHNOLOGY
Traditional burners applied to asphalt plants use the air from the environment( sucked in by an exhaust fan with constant turning speed) and a mechanical fan( called a blower) to provide the amount of air needed for combustion inside the dryer drum.
turning speed) and a mechanical fan( called a blower) to provide the amount of air needed for combustion inside the dryer drum. The control over the airflow for combustion is carried out by a component that is called a damper, placed in the gases’ outlet to the environment, which controls the pressure of the plant by manipulating the flow of the gases. There is an optimum combustion ratio of 13:1( thirteen volumes of air for one volume of fuel) and the airflow variation is dependent on the position of the damper, since the exhaust flow is constant.
The latest combustion technology in plants accurately and automatically control the optimum air-to-fuel ratio. In this technology, all the combustion air is mechanically supplied by the plant( without air suction from the
environment) through two fans( a blower and an axial fan). Called‘ total air’, it works in a closed loop from an internal pressure input of the dryer and automatic variation of the airflow by the variation of the rotational speed of the exhauster, which transports the gases from the dryer to the filter and later to the atmosphere. This way, only the air used for combustion is heated, resulting in fuel economy. Electric power is also saved, as the largest plant engine, the exhauster, turns according to the demand.
Control over mixing time
During the design of an asphalt concrete project in the laboratory, the mixing time between aggregates and binder depends on the characteristics of the materials. If the plant is a large-scale laboratory, the mixing time should also vary according to the properties of the materials. Therefore, an external pug mill mixer has been developed, which automatically controls the mixing time according to the requirements of the materials and as determined by the operator before or during production.
The technology is based on the control over the material outlet area between the mixer and the drag elevator. When this outlet area is reduced, the blend is trapped within the mixer, increasing the material volume and the mixing time, keeping the production of the plant constant. If the outlet area is open, the asphalt mix leaves the mixer more freely, decreasing the volume of material in the mixer and the mixing time, while still maintaining constant production.
In the past, asphalt plants had to adapt to materials and project characteristics, with variable productivity. With these new technologies, productivity is constant, regardless of inputs and projects, and the quality is reached according to the parameters established in the project. n
About the author
Aggregates and binder mixing inside the asphalt plant. During the design of an asphalt concrete project in the laboratory, the mixing time between aggregates and binder depends on the characteristics of the materials.
Marcelo Zubaran is a product and application engineer at Ciber Equipamentos Rodoviários, a Brazilbased member of the Wirtgen Group that specialises in developing and manufacturing equipment for building, maintaining, and repairing roads.
26 _ QUARRY SA | NOVEMBER 2017