Hot oil heaters
Systems with hot oil heaters are the most versatile systems. They can be used for virtually any type of plant, any size of plant and for any type of asphalt material including PMACs. Moreover, hot oil heaters rival direct-fired tanks in thermal efficiency.
However, this type of system may cost more than a direct-fired system for some plants. It is likely to cost more for a small portable plant that needs only one asphalt storage tank and little extra heating. Even a small hot oil heater is capable of heating several storage tanks along with several plant components. And because it is a more sophisticated system than a direct-fired tank it has more parts that are subject to maintenance. Even so, its maintenance is relatively low—not a matter of major concern.
The system may have several hot oil heating circuits, each controlled independently. Thus, it’s possible to shut off any component not in use. Moreover, the temperatures of each component can be varied independently.
More heat is possible from a hot oil heater than a direct-fired tank due to burner limitations of a direct-fired tank.
Direct-fired tanks employ fire tubes, which limit the size of the burners that can be used. Because hot oil heaters do not use fire tubes they can use larger burners. Moreover, the burner in a hot oil heater can raise the oil temperature higher than scavenger oil systems that are heated by the asphalt and limited by its temperature. The temperature of the oil in a hot oil heater is limited only by the properties of the oil, such as its flash point. For example, conventional hydrocarbon oil such as Exxon HT43 can be safely heated to 600 degrees F. Synthetic fluids such as Dowtherm or Therminol can be heated as high as 750 degrees F.
A hot oil heater can heat the oil to a temperature high enough to maintain the temperature of mix in storage silos. Its temperature needs to be at least 25 degrees F higher than the mix in the silos. The desired temperature can be preset on the temperature controller, which will automatically maintain the set temperature.
A hot oil heater system is always the one of choice where there is little or no difference in initial cost. And even when it costs more, its versatility may outweigh the additional cost.
Expansion tanks
Systems with hot oil heating circuits must have an expansion tank to allow for thermal expansion of the oil. This applies to systems using hot oil heaters as well as systems using direct-fired tanks with scavenger coils. The tank is usually incorporated on the heater or on the tank. The size of the tank varies according to the amount of heating oil in the system.
The tank should be filled with nitrogen to minimize oxidation of the hot oil. This significantly extends the life of the oil.
Electric heaters
It is possible to get electrically heated counterparts of fuel-fired systems. Thus, its possible to get an electric hot oil heater. And its possible to get an electrically heated asphalt storage tank. But electric heating systems are problematic. Heating with electricity significantly increases the power demand for a HMA plant. Increased demand may dramatically increase electricity rates (Figure 16). An electrically heated tank has virtually all the same limitations as a direct-fired tank, plus additional concerns. However, electric heating can be cost effective in a limited number of areas where electric power is available at extremely low rates.
Figure 16. Electrically Heated Tank.
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Some may favor using electric heating out of concern for conserving natural resources and reducing environmental pollution. But therein lies a myth. Using electrical energy doesn’t always achieve that goal. That’s because one or more power plants in a network that supplies your electrical energy may use fossil fuels and may cause pollution of their own. Thus, using electricity for heating may merely displace one’s concerns. And it may prove to be the most expensive means of heating.
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