Heating systems
Two basic types of heating systems are commonly used at HMA plants. Both use heating fuels. One system employs what is known as a direct-fired tank (Figure 12). The other system employs a hot oil heater and tanks with heating coils (Figure 13). An electric heating system (which does not use heating fuels) is sometimes used.
Figure 12. Plant With Direct-Fired Tank.
 |
|
|
Figure 13. Large Stationary Plant With Hot Oil Heater And Tanks.
 |
|
|
Suppose your asphalt is delivered to your plant at a temperature that is 25 degrees F lower than needed to make hot mix. Consider the energy required to heat a 30,000 gallon tank of asphalt to increase its temperature 25 degrees F (from 275 to 300 degrees F) over a period of 24 hours. Then compare that with the energy required to maintain a temperature of 300 degrees F over the same period. In both cases assume that the tank has 3 inches of insulation. It takes 4,127,810 Btu to increase the temperature 25 degrees F and to make up for losses that will occur during a 24 hour period. But it only takes 1,127,810 Btu to maintain a temperature of 300 degrees F for 24 hours, a difference of 3,000,000 Btu. Thus, raising the temperature requires more than three times the amount of fuel needed to maintain the temperature (Figure 9).
The direct-fired tank is an asphalt storage tank that has a burner mounted on one end of the tank. The burner fires directly into a fire tube or heating chamber located inside the tank (Figure 14). One type of fire tube, known as a single-pass fire tube, runs the full length of the tank and exhausts on the end away from the burner. Another type, known as a two-pass fire tube, runs the full length of the tank and doubles back toward the burner. Its exhausts on the same end as the burner.
Figure 14. Cross-section Of Direct-Fired Tank With Scavenger Coils.
 |
|
|
The burner heats the fire tube, which in turn heats the asphalt that surrounds it. If anything in addition to the asphalt needs to be heated, the tank can be equipped with scavenger coils. The coils are totally independent from the burner and fire tube. They are positioned above the fire tube so they are immersed in the asphalt, enabling them to scavenge heat from the asphalt. Oil is pumped through the coils, carrying the scavenged heat to other plant components.
The other type of heating widely used is an indirect system. It employs a hot oil heater and tanks equipped with heating coils The heater heats oil (thermal fluid) as it is pumped through the heater. The hot oil is piped to the asphalt storage tank where it heats the tank coils. And the coils heat the asphalt or contents of the tank. By simply using additional piping circuits the hot oil can be used to heat additional tanks and virtually any plant component. It is important to match the capacity of the heater with the heat load.
Systems with hot oil heaters often have a single hot oil heater and one or more coil tanks, all as separate units (Figure 15). Some systems have a trailer-mounted coil tank with the hot oil heater mounted on the gooseneck of the trailer.
Figure 15. Cross-section Of Coil Tank With Hot Oil Heater.
 |
|
|
The system using electric heat is very similar to the one using the direct-fired tank. The tank has an electric heating coil instead of the burner and fire tube. It can employ scavenger coils the same way as the direct-fired tank.
Direct-fired tanks
A heating system using a direct-fired tank has the advantage of lower initial cost than most other systems. The thermal efficiency of the tank is also very high, especially if it has a two-pass fire tube. Moreover, it has relatively few parts and requires very little maintenance. The direct-fired tank is well-suited for small portable asphalt plants using a surge bin and for small batch plants. These are plants that usually need only one asphalt storage tank. And they don’t usually require heat for other plant components.
Direct-fired tanks are not well-suited for larger plants and for plants that use PMACs. Larger plants usually need more than one asphalt tank. Moreover, large plants usually need to heat a number of plant components such as drag conveyors, asphalt silos, and fuel tanks. The only way a direct-fired tank can heat other components is by use of scavenger coils or by use of a booster heater.
Scavenger coils are very limited in their ability to heat other components. Because the hot asphalt is the source of heat for the scavenger coil, the temperature of the oil flowing through it is limited by the temperature of the asphalt. This limits the amount of heat available for heating other equipment.
The maximum scavenged temperature of the thermal oil will be range from about 250 to 300 degrees F, or about 25 to 50 degrees below the asphalt temperature. Oil at this temperature is suitable for heating jacketed asphalt lines, metering systems, drag conveyors, emulsion tanks and heated fuel tanks. However, oil at this temperature is not suitable for maintaining the heat of an asphalt storage silo because the oil temperature is lower than the temperature of the hot mix in the silo. Moreover, oil at a temperature lower than that of the hot mix would actually steal heat from the hot mix. However, some plants use scavenged heat to pre-heat the cone of a surge bin prior to its use.
Thus, if the direct-fired tank is unable to provide adequate heating for other purposes it becomes necessary to add a booster heater. At this point it may be more feasible to use a system with a hot oil heater in lieu of the direct-fired tank.
Another disadvantage of a direct-fired tank is a low rate of heat transfer because the fire tube has a limited amount of heating surface. An asphalt temperature recovery rate of 1 to 3 degrees F per hour is typical, depending on the volume stored in the tank. Accordingly, the direct-fired tank is better suited to maintain temperature rather than to raise temperature.
Another disadvantage of a direct-fired tank is the need to always retain enough asphalt in the tank to cover the fire tube and scavenger coils. This requires retaining anywhere from 12 to 20 percent of the tank’s total volume. Allowing the level to drop below the fire tube and scavenger coils exposes their surfaces to air and will cause the film of asphalt on their surfaces to oxidize or coke. Layers of coke have insulating properties similar to fiberglass insulation. Accordingly, a build-up of coke on heating coils significantly reduces thermal efficiency or the ability of the heating surfaces to heat the asphalt.
Direct-fired tanks are not well suited for heating PMACs because of unfavorable film temperatures and mixing limitations. The outside surfaces of a fire tube are significantly hotter than the outside surfaces of coils heated by hot oil. The hotter surfaces cause the asphalt in contact with them to have a higher film temperature. The film temperature is the temperature of the thermal boundary layer in the asphalt or material surrounding the fire tube. When the film temperature exceeds the maximum recommended film temperature for the material, the material breaks down. This is not a problem with virgin asphalt because of its higher film temperature rating. However, polymers have lower film temperature ratings that would be exceeded when heated by a fire tube. (Please refer to Technical Paper T-133.)
← previous page next page →
