T-140: Heat requirements

Heat requirements
The amount of heat (Btu/hr) needed for heating asphalt and components at an asphalt plant is known as heat demand. An estimate of the heat demand should be made when purchasing new asphalt heating equipment or upgrading old equipment. The estimate can provide important information for selecting new equipment. Heat demand is the key to choosing the appropriate capacity of the heating equipment and can enable you to closely estimate fuel costs.

Your estimate should include every heated component (including asphalt piping, and hot oil piping) along with the Btu required to maintain them at a specific temperature. Be sure to include an appropriate amount of extra heat demand for raising the temperature of asphalt that you buy at a temperature lower than required for the mix. (This assumes that you must raise its temperature in order to use it.) Also allow for asphalt tanks and storage silos you plan to add in the foreseeable future. Add a safety margin of about 20 percent to the estimated demand to allow for unknowns and contingencies.

Figures 38, 39 and 40 show hourly and daily Btu usage for typical stationary, relocatable and portable plants. The total hourly usage can be used as a starting point for estimating heat demand for such plants. But remember that the totals in those figures is only a sum of the heat required for the components listed. Moreover, the total hourly usage does not include the extra heat needed during the first hour of a cold start, before heat from hot aggregate supplements heat of the hot oil system.

So, the heat demand for your plant should start with the hourly usage for all of its specific components. To that you should add extra heat for startup plus all of the factors mentioned earlier. It would be virtually impossible to accurately calculate all those factors because of unknowns. So, it is appropriate to make a judgement based on experience.

We recommend increasing the total hourly usage by a factor of about 4 times. In some cases the factor could be up to 10 times.

Thus, we recommend a heater with an output of 2 million Btu/hour for the stationary plant described in Figure 38. We recommend a 1.2 milion Btu/hour heater for the relocatable plant described in Figure 39. And we recommend a 1million Btu/hour heater for the portable plant described in Figure 40.

Figure 41 shows heat demands for key components of drum mix plants and batch plants. Figure 42 shows the heat demands for various other components of a plant. Heat demands are shown for pre-startup and run. In both cases this is the heat loss that the hot oil system must replace.

Figure 41. Heat Demands For Key Components.
Equipment	Btu per Hour
Equipment	Pre-Startup	Run
Double Barrel Drum Mixer 7 x 35 ft (hot oil jacket)	213,091	35,200
Double Barrel Drum Mixer 8 x 39 ft (hot oil jacket)	215,929	38,640
Double Barrel Drum Mixer 9 x 46 ft (hot oil jacket)	349,270	43,520
Metering Pkg, Drum Mix Plant, 100–300 tph	38,725	19,363
Metering Pkg, Drum Mix Plant, 400–600 tph	50,171	25,085
Weigh Bucket & Pugmill, 4,000 lb Batch Plant	117,000	90,962
Weigh Bucket & Pugmill, 6,000 lb Batch Plant	135,000	93,184
Weigh Bucket & Pugmill, 8,000 lb Batch Plant	168,000	101,884
Weigh Bucket & Pugmill, 10,000 lb Batch Plant	214,000	111,230
Weigh Bucket & Pugmill, 12,000 lb Batch Plant	275,000	120,752
Weigh Bucket & Pugmill, 14,000 lb Batch Plant	351,000	131,156
Weigh Bucket & Pugmill, 16,000 lb Batch Plant	442,000	142,618

Figure 42. Heat Demands For Other Components.
Equipment	Btu per Hour
Equipment	Pre-Startup	Run
Asphalt Storage Silo, 100-Ton	100,000	17,735
Asphalt Storage Silo, 150-Ton	150,000	21,635
Asphalt Storage Silo, 200-Ton	200,000	25,719
Asphalt Storage Silo, 250-Ton	250,000	30,590
Asphalt Storage Silo, 300-Ton	300,000	33,216
Traverse conveyor, 24 inches x 14 ft-8 inches	24,911	5,720
Traverse conveyor, 36 inches x 16 ft-8 inches	34,465	9,750
Drag conveyor, 300 tph, 24 Inch width (per foot)	3,736	390
Drag conveyor, 400 tph, 36 Inch width, (per foot)	4,377	585
Drag conveyor, 500 tph, 42 Inch width, (per foot)	4,698	683
Heavy fuel preheater 1000 gal/hour	3,593	336,709
Silos have heated cones and gates, 6-inch sidewall insulation, 8-inch cone insulation.

Pre-startup means that the plant is not running HMA. Thus, the hot oil system replaces virtually all of the heat lost. Run means that the plant is running HMA. Consequently, heat from the HMA replaces much of the heat lost, thereby reducing the load on the hot oil system.

The goal of carefully estimating the heat demand is to get a heater that is neither too small nor too large. Correctly estimating the demand will enable you to pick a heater of the optimum size. A properly sized heater runs about 75 percent of the time while the plant is operating. If the demand is significantly underestimated you will get a heater that is too small to handle the plant temperature requirements. Moreover, heaters that are too small will cause over-firing, an undesirable operating condition. Over-firing is when the burner operates at a higher rate than the design capacity of the heater for extended periods in an attempt to maintain needed temperatures. Over-firing decreases efficiency and increases the film temperature of the thermal fluid, which shortens its life.

The reverse will happen when the heater is too large. It may constantly cycle on and off. Its run cycle may be too short for maximum efficiency. However, aside from the extra cost of the heater, moderate over-sizing is not a problem for hot oil heaters with fully-modulating burners. Fully-modulating burners can normally fire at a rate low enough to reduce frequent on-off cycling. Even so, hot oil heaters should not be grossly oversized.

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