ASPHALT DISTRIBUTOR

The asphalt distributor (fig. 16-33) is a unit consisting of an insulated storage and heating tank, an open flame heating system, an asphalt pump, a low-pressure air blower, and a circulating and spraying system. Power to operate the components is PTO driven.

NOTE: The operation of this truck requires the absolute need for experienced personnel only. Mishaps, resulting in loss of man-days and equipment, are a direct result of this factor being overlooked.

Heating System

The air blower provides low-pressure air to atomize fuel for the burners. The burners heat the tubes, located in the tank. An asphalt covering must be maintained over the fire tubes to prevent them from overheating and causing a fire or explosion. Because the distributor is mobile, care must be taken to ensure that heating is performed in a level area that is well-ventilated and that the distributor truck is not moving at all and is at a COMPLETE HALT. Whenever you are heating cutbacks, the asphalt must be circulating at all times. This is a must to prevent any chance of volatile liquids overheating around the flues, which can be very dangerous.

Spray System

The spray system consists of necessary piping, a series of hand-operated valves to control the flow of bitumen, and an adjustable length spray bar, capable of providing coverage from 4 to 14 feet wide. The spray bar may be the full-circulating or the noncirculating type, depending on the model of the distributor. The spray bar may be equipped with either 1/8-inch nozzles or 3/8-inch nozzles; the 1/8-inch nozzles are used for most applications. The application rate is controlled by the length of the spray bar, the pump output, and the forward speed of the distributor truck.

Spray Bar Adjustments

For normal use, the spray bar of the distributor should be adjusted, so the vertical axes of the nozzles are perpendicular to the roadway. Also, each nozzle on the spray bar should be set at the same angle. The angle set for each should be between 15 degrees to 30 degrees of the horizontal axis of the spray bar (fig. 16-34) or according to manufacturer's specifications. This action prevents the fan-shaped spray patterns of the nozzles from interfering with each other.

Another adjustment that is essential for uniform prime or tack coat coverage is the adjustment of the height of the spray bar. As shown in figure 16-35, the fan-shaped spray patterns from the nozzles overlap to different degrees, depending on the distance between the spray bar and the surface to be covered. The spray bar should be set high enough, usual] y 10 to 12 inches above the roadway for the surface to receive triple

Figure 16-33.-Asphalt distributor.

Figure 16-34.-Proper angling of nozzles.

_coverage. This height varies according to the nozzle spacing of the spray bar. Under heavy wind conditions or depending on the nozzle spacing, it maybe necessary to lower the bar farther down so that the surface only receives double coverage.

On some distributors, as asphalt is sprayed (and the load lightens), the truck rear springs of the truck rise, raising the distributor and changing the height of the spray bar. Mechanical devices are usually available that automatically correct the height of the spray bar, as this change occurs.

The uniform application of asphalt prime and tack coat is essential. Transverse spread should not be allowed to vary more than 15 percent, and the longitudinal spread should not vary more than 10 percent. To ensure the correct application, you must calibrate the distributor before it is used. Then the transverse and longitudinal spread rate variations should be checked periodically to determine when the distributor is operating within these limits. A procedure for checking these spread variations in the field has been standardized by ASTM D 2995, published by the American Society for Testing and Materials.