T-140: Increasing temperature vs maintaining it

When asphalt is used to make hot mix it must be within a specified temperature range, usually from about 300 to 325 degrees F. However, as already noted, higher temperatures are usually required for PMACs. Accordingly, when asphalt is delivered to the plant at a temperature suitable for use, the role of the asphalt heating and storage equipment is to maintain that temperature until the material is used. Maintaining its temperature requires only enough heat to replace that lost during storage and pumping.

But when the asphalt is delivered to the plant at temperatures lower than required for making hot mix, the heating equipment must increase its temperature to meet that specified for use, in addition to replacing all heat lost during storage and pumping. This calls for more heating capacity, which may affect the choice of equipment.

Moreover, heating equipment for some types of plants nearly always performs additional roles. On relocatable and stationary plants heating is needed to maintain the temperature of the hot mix after it leaves the mixer. This usually requires heating drag conveyors, traverse conveyors and the cones of hot mix storage silos. And sometimes it is necessary to heat hot mix additives and heavy fuel oil. Some of these added roles may require greater heating capacity. Again, this may affect the choice of equipment.

Whenever stored asphalt is to be left unused for an extended period, some operators allow it to cool down. This conserves heating energy and minimizes oxidation of the asphalt. This practice is especially appropriate for PMACs that have been heated to higher temperatures than used for virgin asphalt. The heating system should be capable of restoring the temperature within a short period of time.

A typical heating and storage system for a small HMA plant is shown in Figure 6. A typical heating and storage system for a large HMA plant that uses PMACs is shown in Figure 7.

Figure 6. Typical heating and storage for a small HMA plant.

Figure 7. Typical heating and storage for a large HMA plant.

Increasing temperature Vs maintaining it
It costs less to maintain the temperature of asphalt than to increase its temperature. Accordingly, it pays to buy your asphalt at the temperature at which it can be used and to use it before it loses much of its heat. And it pays to do everything possible to keep it from losing heat. Using well-insulated tanks and piping is the most cost-effective way of minimizing heat loss. The amount of heat required to raise the temperature of various quantities of asphalt is shown in Figure 8 (and does not include any additional heat to make up for losses during heat up).

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).

Figure 8. Heat Required To Raise Asphalt Temperatures
Quantity (Gallons)	Heat Required (Btu)
Quantity (Gallons)	5 Degree F Increase	10 Degree F Increase	15 Degree F Increase	25 Degree F Increase
1,000	20,000	40,000	60,000	100,000
5,000	100,000	200,000	300,000	500,000
6,750	135,000	270,000	405,000	675,000
10,000	200,000	400,000	600,000	1,000,000
15,000	300,000	600,000	900,000	1,500,000
20,000	400,000	800,000	1,200,000	2,000,000
25,000	500,000	1,000,000	1,500,000	2,500,000
30,000	600,000	1,200,000	1,800,000	3,000,000
35,000	700,000	1,400,000	2,100,000	3,500,000
Heat required = weight x specific heat x temperature difference. Weight = 8.0 pounds per gallon. Specific heat (degrees F) = 0.5 Btu per pound per degree F. Heat required does not include extra heat to make up for heat lost while heating.

Figure 9. Energy to Raise Temperature Vs. Maintain Temperature Over 24 Hours
Energy Needed	Raise Temperature 25 Degrees F	Maintain Temperature @ 300 Degrees F
Btu	4,127,810	1,127,810
Gallons No. 2 Fuel	31.3	8.5
Temperature is raised and maintained for 24 hours. Heat to raise temperature includes heat to make up for heat loss during the 24 hours. 30,000 gallons AC @ 8 pounds per gallon = 240,000 lbs. Heat required to raise one pound of AC one degree = 0.5 Btu. Heat required to raise 240,000 pounds of AC one degr = 120,000 Btu. One gallon No. 2 fuel produces 132,000 Btu LHV. Tank has 3-inch insulation.

Now, consider the energy needed to maintain a 30,000 gallon tank of asphalt at 300 degrees F for a month. Compare the energy needed to maintain a non-insulated tank for that length of time with a tank that has 3-inch insulation and one that has 6-inch insulation (Figure 10). It takes 33,834,296 Btu to maintain the tank with 3-inch insulation. It takes only 18,475,382 Btu for the tank with 6-inch insulation. But it takes over one U.S. billion (1,034,653,248) Btu to maintain the non-insulated one. These are significant differences.

Figure 10. Energy Used By Insulated Vs. Non-insulated Tanks
Energy Needed	Tank With No Insulation	Tank With 3-inch Insulation	Tank With 6-inch Insulation
Btu	1,034,653,248	33,834,296	18,475,382
Gallons No. 2 Fuel	9,798	320	175
30,000 gallon tank. Maintain AC temperature at 300 deg F for 30 days

Having well-insulated tanks and piping also makes it feasible to shut-off heating equipment over week ends or for other extended periods of non-operation. Figure 11 shows the amount of heat required to restore the temperature. A 30,000 gallon tank of asphalt with 6-inches of insulation requires only 1,367,283 Btu to restore the temperature to 300 degrees F after a 60-hour shut-down. It will take about 1.5 hours, depending upon the size of the heater and how much of its heat is needed for other equipment. The same tank with 3 inches of insulation takes 2,463,091 Btu to restore the temperature and likewise takes about 2.7 hours. But the same tank with no insulation takes 126,418,650 Btu to restore the temperature and will take about 140 hours. An automatic timer can be used to start up the heating system to restore asphalt temperature in time for plant re-start. You may have noted that the table shows the bulk or average temperatures of the three tanks range from 252 to 289 after being off for 60 hours.

Figure 11. Energy Used To Restore Heat After Shutdown
Temperature & Energy	Tank With No Insulation	Tank With 3-inch Insulation	Tank With 6-inch Insulation
Bulk Temperature After 60-Hours	252 Degrees F	281 Degrees F	289 Degrees F
Btu To Restore After Shutdown	126,418,650	2,463,091	1,367,283
Gallons No. 2 Fuel To Restore	1197.1	23.3	12.9
Shutdown is for 60 hours. Assumes a 30,000 gallon tank of AC at a temperature of 300 deg F before shutdown. Assumes a 1.2 million Btu heater using 75 percent of its output to reheat the tank. Heat to restore temperature includes heat to make up for heat loss during restoration.

You may have expected a greater temperature drop for the uninsulated tank. But an interesting phenomenon occurs as the asphalt cools. Asphalt in contact with tank inner surfaces solidifies, creating a highly effective insulation. This solidified layer retards heat loss to a major extent. Moreover, asphalt in the uninsulated tank solidifies sooner than in the insulated ones.

Nevertheless, it takes considerably more heat to restore the temperature of both the uninsulated one and the one with 3 inches of insulation because they will lose more heat while being re-heated than the one with 6 inches of insulation.

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