HOW DOSE COLD AGGREGATE STORAGE AND FEEDING WORK?
The cold aggregate feed is the first major component of the mixing plant. The cold feeder may be charged by one or a combination of three methods: 1) Open top bins with several compartments. Materials are usually fed by a front-end loader 2) Tunnels under stockpiles separated by bulkheads. Materials are stockpiled over the tunnel by belt conveyor, or front-end loader 3) Bunker or large bins. Materials are usually fed by trucks, car unloaders, or bottom dump freight cars emptying directly into the bunkers 3-5 When charging the cold bins (Figure 3-3), segregation and degradation of the aggregate are problems that may occur. These problems may be prevented by taking the same precautions outlined for proper stockpiling. Enough materials are required to be maintained in all bins to provide a constant and uniform flow.
When a front-end loader is used to charge the bins, the operator should not pick up material from the storage stockpile at ground level. The scoop is held high enough above the ground to prevent contamination. When trucks are used to charge the bins, the aggregate is deposited directly above the feeder. When the stockpile is replenished by overhead belts or elevated conveyors, the free falling materials is controlled by baffles. Aggregate feeder units are located beneath storage bins or stockpiles, or in positions that ensure a uniform flow of aggregates. Openings located at the bottom of the bins deposit the different aggregates on a belt conveyor, and/or bucket lines, which carry the aggregates to the dryer. Feeder controls regulate the amount of aggregate flowing from each bin, thereby providing a continuous, uniform flow of properly-graded aggregate to the plant. 3-6 The most common type of cold feeder used in a plant is the continuous belt type (Figure 3-4).
The key element in the continuous belt feeder is how to control or regulate the flow of material from each bin. Every manufacturer has a different control method. Typical control variations are: 1) Gate opening a. Fixed b. Adjustable 2) Belt a. One speed (on or off) b. Adjustable speed The most common configuration is the adjustable gate with an adjustable belt speed. 3-7 Ensuring Proper Feeder Functions Because a uniform flow of proper-sized aggregates is important to asphalt mixture production, the Technician is required to check before and during production to be certain that the feeder system is functioning properly. Conditions that help ensure proper feeder functions include: 1) Correct sizes of aggregates in stockpiles and cold bins 2) No segregation of aggregates 3) No intermixing of aggregate stockpiles 4) Accurately calibrated, set, and secured feeder gates 5) No obstruction in feeder gates or in cold bins 6) Correct speed control settings Calibrating and Setting Feeders The cold aggregate feeder is calibrated, set, and secured to ensure a uniform flow of aggregate. This calibration is the responsibility of the Producer. The feeder is calibrated for each type and size of aggregate. Manufacturers often furnish approximate calibrations for their equipment, but the only accurate way to set a cold feed is to prepare a calibration chart for each of the aggregates to be used in the asphalt mixture. The Technician is required to examine the calibration charts of the cold feed systems to be aware of the production rate settings and how adjustments are made during production. Calibration is simply determining the "Flow Rate" of a material graphed against the "Control" used by the particular system. Each material is calibrated for three to four control settings spanning the working production range anticipated for the material. Control Setting Each manufacturer has a method to control the flow of material from the cold feeds. The variable speed short belt feeder under each cold feed is the most common. The operator may adjust the RPM of the belt from the control room. Therefore, control is expressed as RPM or a percentage of the belt's total speed potential (Figure 3-4). This same concept is used with vibrating units. The vibrator may be adjusted from the control room and expressed as a percent of maximum vibration potential. 3-8 Adjustable gates are employed on most cold feeds. The gate height is measured by the height of the opening. This gate height is required to not change when using the variable speed control. There may be variations and modifications of these concepts. Each plant is unique; however, the plants are required to have some means to control the cold feeder. The system is required to be completely understood and controlled in a positive way to provide a uniform flow of material. Flow Rate Flow rate may be determined by a variety of methods that are basically predetermined by the configuration of the plant. The most common and accurate method of determining flow rate is to physically weigh the material delivered at a specific control setting over a measured period of time. A divert chute on the intake of the dryer is the simplest, most accurate, and quickest method to do the calibration. Material may be weighed on a weigh bridge, if available, or completely processed through the plant and weighed on the plant scales. The flow rate is then converted to tons per hour. Moisture content is required to be considered in this procedure. The degree of accuracy is only as good as the method used to determine the flow rate for each control setting. Therefore, the larger the sample measured, the more accurate the data received. Using an entire truck load of material provides dependable numbers. Calibration Chart After understanding the plant "Control" system and determining the best method to obtain a "Flow Rate", a calibration is required to be done. This process determines a flow rate at four different control settings for each cold feed. The process may be time consuming but the benefits are worth much more than the time spent. (Figure 3-49) illustrates a typical calibration chart of each bin. After multiple calculations have been done for each bin used during production, the calibration chart is prepared. On the chart, control settings are plotted on a horizontal scale, and the flow rate is plotted on the vertical scale. For larger production plants, more than one bin is required to be calibrated for each material. This back-up cold feed calibration allows continuation of production if a cold feed bin fails mechanically. Another common practice for large production rates is to use two cold feeders to supply the same size of material. This practice allows for slower machinery rates, and tends to reduce segregation.