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Preheater with Modulating Control vs. Preheater with On-Off Control
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Graph from field tests shows temperature variations of two preheaters heated by thermal fluid. Preheater with modulating control is represented in red, versus preheater with on-off control represented in blue. Note how the modulating control maintains temperature at setpoint with great accuracy. Note how the preheater with on-off control has temperature swings 25 degrees higher and lower than setpoint.
Significance of precise temperature control
It's a well-known fact that heavy fuel at optimum temperature burns more efficiently than fuel at higher or lower temperatures. Thus, the burner of a dryer / drum mixer uses less fuel when its fuel is at the right temperature. And since dryers use relatively large amounts of fuel, the savings can be significant.
Moreover, fuel at temperatures much higher or lower than optimum produce adverse side effects on hot mix quality, bag house filters, stack emissions and burner operation.
Here's what happens: Fuel at a temperature several degrees lower than optimum does not atomize properly. Consequently some fuel droplets do not burn, so combustion is incomplete. Unburned fuel can contaminate bag house filters. Unburned fuel also produces unwanted emissions that pollute the atmosphere.
Fuel heated to a temperature higher than optimum can cause a different problem. If its temperature goes high enough for the fuel to vaporize it will cause the burner of the dryer / drum mixer to shut down.
