Downflow type means that the flow direction of the hot air flow is the same as that of the material, both moving from the feed end to the discharge end. When the material just enters the drum, the drying driving force is the largest and the free water content is high. The drying speed of the front part of the flow type is the fastest, and then as the material moves to the discharge port, the temperature of the material increases, the drying driving force becomes smaller, the free moisture content decreases, and the drying speed also slows down. Therefore, the drying of the down-flow drying drum is more uneven than the counter-flow type.

The counter-flow type is that the flow direction of the hot air flow is opposite to the movement direction of the material, and the temperature of the drum is the highest at the material outlet end, and the temperature is lower at the material inlet end. The temperature of the material is the lowest when it first enters the drum, and the temperature is the highest at the outlet end, which is in the same direction as the high and low temperature of the drum. Because the highest temperature of the drum is on the same side as the highest temperature of the material, and the lowest temperature of the drum is on the same side as the lowest temperature of the material, so the driving force of countercurrent drying is more uniform than that of downstream drying.

In general, the heating of the drum is mainly carried out by heat convection. The down-flow type means that the combustion chamber and the feed inlet are installed on the same side, and the flow direction of the hot air flow is the same as that of the material. Otherwise, it is a counter-flow type.

When the counter-flow drum is drying and heating, the interior of the drying drum can be divided into three areas according to the change of material temperature: dehumidification area, drying area and heating area. Because the material contains moisture when it first enters the drum, the moisture in the material will be removed in the first zone, the aggregate will be dried in the second zone, and the drum will be at the highest temperature in the third zone Contact with the dried material to raise the temperature. Generally speaking, as the temperature of the material increases in the counter-current drum, the drying medium also increases, so the drying force is relatively uniform, the average temperature difference between the hot air flow and the material is large, and the efficiency of the counter-current drying is relatively smooth. flow high.
 
On the drum-type asphalt mixing plant, the drum has two functions, drying and mixing; while on the batch asphalt mixing plant and the continuous asphalt mixing plant, the drum only plays the role of heating. Because the mixing in the batch and continuous asphalt mixing plants is carried out through the mixing pot, there is no need to add asphalt to the drum for mixing, so the countercurrent drying drum with high drying efficiency is used.