If you own or plan to purchase HMA (hot mix asphalt) production equipment the information in this paper can help you understand many issues you face. It will give you insight into important issues that are often overlooked or little understood when planning the heating and storage of asphalt at a HMA plant.
Figure 1. Conservation of energy is a major concern.
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Heat conservation
Perhaps the most fundamental issue concerned with heating and storage of asphalt is the conservation of energy (Figure 1). In recent years conservation of all natural resources—especially energy resources—ranks high. It is a major concern of governments and citizens alike, not only in the United States, but also in many other countries of the world. Fortunately, it is possible to conserve energy and save money at the same time when heating and storing asphalt. Any higher initial costs are quickly recovered and turned into savings.
Figure 2. We need to look ahead.
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Considerations
There are many important considerations whether replacing old equipment or purchasing an entirely new system. While some issues—such as the thermal efficiency of asphalt heating—may seem less important than considerations for drying aggregate, all are important in the long run. In just a few years what may have initially appeared to be insignificant may either result in significant savings or significant waste, depending on how the issue was initially addressed. So, it makes sense to take a close look at all issues that could in time have a positive or negative impact (Figure 2). Details are important!
There are now a number of new issues that have come about recently from the use of PMACs (polymer-modified asphalt cements). These asphalts require heating and storage significantly different from virgin or neat asphalts. They must usually be maintained at higher temperatures and must be continuously agitated or mixed to keep the polymers from separating. Moreover, they normally have a higher viscosity, which may affect piping and pumping needs.
For additional information on PMACs, please refer to Technical Paper T-133 entitled, Heating, Mixing and Storing of Modified Asphalt. This paper is available from Heatec.
Figure 3. Asphalt is usually delivered by tanker truck.
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Basic needs
Heating and storing asphalt is a basic function of all HMA facilities. Asphalt is normally delivered to the HMA facility in a liquid state, usually by tanker truck (Figure 3). The viscosity of the asphalt must be low enough to allow it to be pumped from the delivery truck into the storage tanks. Viscosity is a measure of a fluid’s resistance to flow and is related to its temperature. The higher the temperature the lower the viscosity or resistance to flow. Viscosity is commonly expressed in units known as SSU (Saybolt Second Universal), poise and centipoise.
The actual temperature of the liquid asphalt at time of delivery may vary somewhat, depending upon the arrangements with the supplier. But most asphalt terminals deliver asphalt to HMA plants at a temperature suitable for making hot mix or slightly higher.
Figure 4. A barrel melter is used for asphalt shipped in barrels.
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Some HMA facilities—mainly those in remote locations and on small islands—buy solidified asphalt in barrels. Thus, the solidified material must be melted into a liquid before it can be pumped into the asphalt storage tanks. This requires special equipment known as barrel melters (Figure 4). In some instances asphalt is transported aboard ships in insulated shipping containers (Figure 5). The containers usually have heating coils or provisions for a burner so the asphalt can be re-heated in the container after it reaches its destination.
Figure 5. Shipping containers may be used to transport asphalt.
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