Heating fuels
Asphalt heating systems using direct-fired tanks and hot oil heaters work with a variety of fuels. The asphalt heating system often uses the same fuel as the aggregate dryer. And because the dryer uses significantly more fuel than the heating system, fuel choice is usually based on dryer usage.
Two key concerns are availability of the fuel and cost per Btu. However, the amount of emissions it produces when burned can be an overriding factor. Fuels that produce high levels of emissions are not well-suited for asphalt heating, even though those same fuels may be satisfactory for a dryer. That’s because there are some important differences in the ability of aggregate dryers to use such fuels and the ability of asphalt heaters to use those fuels.
Natural gas, LP gas, No. 1 fuel oil and No. 2 fuel oil are all highly appropriate fuels for both aggregate dryers and asphalt heaters. Natural gas produces very low emissions. Most LP gases produce low emissions, but some LPs may produce a higher level of NOX than others. No. 1 and 2 fuel oils generally produce fairly low emissions. Federal, state and local ordinances governing allowable emissions may affect the choice between these fuels.
Other fuels, such as No. 4 fuel oil, heavy oil and waste oils are difficult to burn and produce high levels of emissions. (Heavy oil is also known as No. 6 fuel oil or Bunker C.) Consequently, these fuels are not suited for small (3 million Btu or less) asphalt heaters. Moreover, they are problematic even for large asphalt heaters. The first problem is getting a burner that will burn these fuels. Special burners are available. But the fuel must be preheated and the burners are subject to reliability and maintenance problems. Furthermore, these fuels leave heavy residues in the heater, requiring the heater to have a special lining to facilitate clean-out. The special provisions are not usually cost-effective.
Heavy fuel preheaters
A heavy fuel preheater is used to pre-heat heavy fuel oil so it can be burned by a fuel oil burner. Preheaters are typically used to heat No. 5 or No. 6 fuel oils for aggregate dryers. The pre-heating lowers the viscosity of the oil so it can be atomized by the burner.
A preheater is usually connected in the fuel supply line between the fuel tank and dryer. (Figure 17). Or, it can be mounted inside the fuel tank. It may be used to pre-heat the fuel flowing directly to the burner when a single pass through the heater raises its temperature adequately for atomization. The fuel may be recirculated to the fuel tank if additional heating is needed.
Figure 17. Heavy fuel preheater.
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The preheater usually consists of a shell-and-tube heat exchanger with the capacity to increase the temperature of the fuel 100 degrees F at a rate of 1,000 gph. The fuel oil is heated as it passes through the shell surrounding the tubes. The tubes are heated by thermal fluid (hot oil) from a hot oil heater as it circulates through the tubes.
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