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Frequency converters on dryers enable flexibility and, ultimately, cost savings.

For nearly every asphalt mix recipe, it is possible to adjust the temperature of the exhaust gas to the optimum level. This is important, as there is only a slight difference between the dew point and wasting energy.

How do you make this adjustment?

First, use the drum rotation speed to adjust the curtain inside the dryer. This optimises the raw gas temperature. Remember: The slower the dryer turns, the less material will make its way inside the curtain. That means more energy will go to the filter, resulting in a higher filter inlet temperature.

Of course, the opposite is true as well: The faster the dryer turns, the more material moves inside the curtain. More energy will remain in the material, and less will go to the filter, resulting in a lower filter inlet temperature. You will increase dryer efficiency and reduce your fuel consumption.

The frequency converter and the technology it brings will save energy and increase the product range of your plant.

Let’s look at an example:

Let’s say a 240-tonne-per-hour plant produces an exhaust gas temperature of 110° C. A frequency converter on the dryer can reduce that temperature to 95° C. That’s a 2 percent increase in the dryer’s efficiency.

That 2 percent is a significant gain. With production of 100 000 tonnes, a plant can save 21500 litres of heating oil (diesel).


Always consider the dew point of the exhaust gas when making adjustments.

  • For exhaust gas without SO2 content, a temperature between 90º and 100° C is recommended.
  • For exhaust gas with SO2 content, a temperature of 120º and 130° C is recommended, depending on the concentration.

Sources for SO2 in the exhaust gas are fuels (e.g., heavy oil, lignite) or aggregates.